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font-size:1.2em; font-weight:bold;'>The Project Gutenberg eBook of Radio-Activity, by Ernest Rutherford</div> -<div style='display:block; margin:1em 0'> -This eBook is for the use of anyone anywhere in the United States and -most other parts of the world at no cost and with almost no restrictions -whatsoever. You may copy it, give it away or re-use it under the terms -of the Project Gutenberg License included with this eBook or online -at <a href="https://www.gutenberg.org">www.gutenberg.org</a>. If you -are not located in the United States, you will have to check the laws of the -country where you are located before using this eBook. -</div> -<div style='display:block; margin-top:1em; margin-bottom:1em; margin-left:2em; text-indent:-2em'>Title: Radio-Activity</div> -<div style='display:block; margin-top:1em; margin-bottom:1em; margin-left:2em; text-indent:-2em'>Author: Ernest Rutherford</div> -<div style='display:block;margin:1em 0'>Release Date: March 04, 2021 [eBook #64693]<br> -[Most recently updated: September 17, 2023]</div> -<div style='display:block;margin:1em 0'>Language: English</div> -<div style='display:block;margin:1em 0'>Character set encoding: UTF-8</div> -<div style='display:block; margin-left:2em; text-indent:-2em'>Produced by: Richard Tonsing, David King, and the Online Distributed Proofreading Team at http://www.pgdp.net. (This file was produced from images generously made available by The Internet Archive.)</div> -<div style='margin-top:2em;margin-bottom:4em'>*** START OF THE PROJECT GUTENBERG EBOOK RADIO-ACTIVITY ***</div> - - -<div class='figcenter id001'> -<span class='pageno' id='Page_on'>on</span> -<img src='images/cover.jpg' alt='' class='ig001'> -</div> -<div class='pbb'> - <hr class='pb c000'> -</div> -<div> - <h1 class='c001'>Radio-Activity</h1> -</div> - -<div class='nf-center-c1'> -<div class='nf-center c002'> - <div><span class='pageno' id='Page_i'>i</span>CAMBRIDGE PHYSICAL SERIES.</div> - <div class='c000'><span class='sc'>General Editors:—F. H. Neville, M.A., F.R.S.</span></div> - <div><span class='sc'>and W. C. D. Whetham, M.A., F.R.S.</span></div> - <div class='c003'>RADIO-ACTIVITY</div> - </div> -</div> - -<div class='nf-center-c1'> -<div class='nf-center c002'> - <div><span class='pageno' id='Page_ii'>ii</span>CAMBRIDGE UNIVERSITY PRESS WAREHOUSE</div> - <div>C. F. CLAY, <span class='sc'>Manager</span>.</div> - <div>London: FETTER LANE, E.C.</div> - <div>Glasgow: 50, WELLINGTON STREET.</div> - <div class='c000'>ALSO</div> - <div class='c000'>London: H. K. LEWIS, 136, GOWER STREET, W.C.</div> - <div>Leipzig: F. A. BROCKHAUS.</div> - <div>New York: THE MACMILLAN COMPANY.</div> - <div>Bombay and Calcutta: MACMILLAN AND CO., <span class='sc'>Ltd.</span></div> - <div class='c003'>[<i>All Rights reserved.</i>]</div> - </div> -</div> - -<div class='nf-center-c1'> -<div class='nf-center c002'> - <div><span class='pageno' id='Page_iii'>iii</span><span class='xxlarge'><b>RADIO-ACTIVITY</b></span></div> - <div class='c003'><span class='large'><b>BY</b></span></div> - <div class='c000'><span class='xxlarge'><b>E. RUTHERFORD, D.Sc., F.R.S., F.R.S.C.</b></span></div> - <div><span class='large'><b>MACDONALD PROFESSOR OF PHYSICS, McGILL UNIVERSITY, MONTREAL</b></span></div> - <div class='c003'>SECOND EDITION</div> - <div class='c003'>CAMBRIDGE</div> - <div>AT THE UNIVERSITY PRESS</div> - <div>1905</div> - </div> -</div> - -<div class='nf-center-c1'> -<div class='nf-center c002'> - <div><span class='pageno' id='Page_iv'>iv</span><i>First Edition 1904</i></div> - <div><i>Second Edition 1905</i></div> - </div> -</div> - -<div class='nf-center-c1'> -<div class='nf-center c002'> - <div><span class='pageno' id='Page_v'>v</span>J. J. THOMSON</div> - <div class='c000'>A TRIBUTE OF MY RESPECT AND ADMIRATION</div> - </div> -</div> - -<div class='chapter'> - <span class='pageno' id='Page_vii'>vii</span> - <h2 class='c004'>PREFACE TO THE FIRST EDITION.</h2> -</div> -<p class='c005'>In this work, I have endeavoured to give a complete and -connected account, from a physical standpoint, of the properties -possessed by the naturally radio-active bodies. Although the -subject is comparatively a new one, our knowledge of the properties -of the radio-active substances has advanced with great -rapidity, and there is now a very large amount of information on -the subject scattered throughout the various scientific journals.</p> - -<p class='c006'>The phenomena exhibited by the radio-active bodies are -extremely complicated, and some form of theory is essential in -order to connect in an intelligible manner the mass of experimental -facts that have now been accumulated. I have found the -theory that the atoms of the radio-active bodies are undergoing -spontaneous disintegration extremely serviceable, not only in -correlating the known phenomena, but also in suggesting new -lines of research.</p> - -<p class='c006'>The interpretation of the results has, to a large extent, been -based on the disintegration theory, and the logical deductions to -be drawn from the application of the theory to radio-active -phenomena have also been considered.</p> - -<p class='c006'>The rapid advance of our knowledge of radio-activity has -been dependent on the information already gained by research -into the electric properties of gases. The action possessed by the -radiations from radio-active bodies of producing charged carriers -or ions in the gas, has formed the basis of an accurate quantitative -method of examination of the properties of the radiations and of -<span class='pageno' id='Page_viii'>viii</span>radio-active processes, and also allows us to determine with considerable -certainty the order of magnitude of the different -quantities involved.</p> - -<p class='c006'>For these reasons, it has been thought advisable to give a brief -account of the electric properties of gases, to the extent that is -necessary for the interpretation of the results of measurements -in radio-activity by the electric method. The chapter on the -ionization theory of gases was written before the publication -of J. J. Thomson’s recent book on “Conduction of Electricity -through Gases,” in which the whole subject is treated in a -complete and connected manner.</p> - -<p class='c006'>A short chapter has been added, in which an account is given -of the methods of measurement which, in the experience of the -writer and others, are most suitable for accurate work in radio-activity. -It is hoped that such an account may be of some service -to those who may wish to obtain a practical acquaintance with the -methods employed in radio-active measurements.</p> - -<p class='c006'>My thanks are due to Mr W. C. Dampier Whetham, F.R.S., -one of the editors of the Cambridge Physical Series, for many -valuable suggestions, and for the great care and trouble he has -taken in revising the proof sheets. I am also much indebted to -my wife and Miss H. Brooks for their kind assistance in correcting -the proofs, and to Mr R. K. McClung for revising the index.</p> - -<div class='lg-container-l c007'> - <div class='linegroup'> - <div class='group'> - <div class='line'>E. R.</div> - </div> - <div class='group'> - <div class='line'><span class='sc'>Macdonald Physics Building,</span></div> - <div class='line'><span class='sc'>Montreal</span>,</div> - <div class='line'><i>February, 1904</i>.</div> - </div> - </div> -</div> - -<div class='chapter'> - <span class='pageno' id='Page_ix'>ix</span> - <h2 class='c004'>PREFACE TO THE SECOND EDITION.</h2> -</div> -<p class='c005'>I feel that some apology is due to my readers for bringing -out at such an early date a new edition which includes so -much new material, and in which the rearrangement is so extensive -as to constitute almost a new work. Though only a year has -passed since the book first made its appearance, the researches -that have been carried out in that time have been too numerous -and of too important a character to permit the publishing of a -mere reprint, unless the author were to relinquish his purpose -of presenting the subject as it stands at the present moment.</p> - -<p class='c006'>The three new chapters which have been added possibly constitute -the most important change in the work. These chapters -include a detailed account of the theory of successive changes and -of its application to the analysis of the series of transformations -which occur in radium, thorium, and actinium.</p> - -<p class='c006'>The disintegration theory, which was put forward in the first -edition as an explanation of radio-active phenomena, has in these -later researches proved to be a most powerful and valuable method -of analysing the connection between the series of substances which -arise from the transformation of the radio-elements. It has disclosed -the origin of radium, of polonium and radio-tellurium, and -of radio-lead, and now binds together in one coherent whole the -large mass of apparently heterogeneous experimental facts in -radio-activity which have been accumulating since 1896. The -theory has received a remarkable measure of verification in the -past year, and, in many cases, has offered a quantitative as well -<span class='pageno' id='Page_x'>x</span>as a qualitative explanation of the connection between the various -properties exhibited by the radio-active bodies. In the light of -this evidence, radio-activity may claim to have assumed the -position of an independent subject, though one with close affinities -to physics on the one hand and to chemistry on the other.</p> - -<p class='c006'>The present edition includes a large amount of new material -relating to the nature and properties of the radiations and the -emanations. In the limits of this book, it would have been found -impossible, even had it been thought desirable, to include more -than a brief sketch of the physiological effects of the rays. The -literature on this subject is already large, and is increasing rapidly. -For reasons of space, I have not been able to refer more than -briefly to the mass of papers that have appeared dealing with the -examination of various spring and well waters, sediments, and soils, -for the presence of radio-active matter.</p> - -<p class='c006'>In order to make the book more self-contained, a short account -has been given in Chapter <a href='#chap02'>II</a> of the magnetic field produced by -an ion in motion, of the action of an external magnetic and -electric field upon it, and of the determination of the velocity and -mass of the particles constituting the cathode stream.</p> - -<p class='c006'>Two appendices have been added, one giving an account of -some work upon the α rays which was completed too late for -inclusion in the subject matter of the book, and the other containing -a brief summary of what is known in regard to the -chemical constitution of the various radio-active minerals, the -localities in which they are found, and their probable geologic -age. For the preparation of the latter, I am indebted to my -friend Dr Boltwood of New Haven, who, in the course of his -researches, has had occasion to analyse most of these minerals -in order to determine their content of uranium and radium. I -hope that this account of radio-active minerals will prove of value -to those who are endeavouring to elucidate the connection between -the various radio-active substances and the inactive products which -arise from their transformation.</p> - -<p class='c006'><span class='pageno' id='Page_xi'>xi</span>For the convenience of those who have read the first edition, -a list of the sections and chapters which contain the most -important additions and alterations is added below the table of -contents.</p> - -<p class='c006'>The writing of a complete account of a subject like radio-activity, -in which so much new work is constantly appearing, has -been a matter of no little difficulty. Among other things it has -involved a continuous revision of the work while the volume was -passing through the press.</p> - -<p class='c006'>I wish to express my thanks to my colleague Professor Harkness -for the care and trouble he has taken in revising the proofs -and for many useful suggestions; also to Mr R. K. McClung for -his assistance in correcting some of the proofs and in preparing -the index.</p> - -<div class='lg-container-l c007'> - <div class='linegroup'> - <div class='group'> - <div class='line'>E. R.</div> - </div> - <div class='group'> - <div class='line c003'><span class='sc'>McGill University,</span></div> - <div class='line'><span class='sc'>Montreal</span>,</div> - <div class='line'><i>9 May, 1905</i>.</div> - </div> - </div> -</div> - -<div class='chapter'> - <span class='pageno' id='Page_xii'>xii</span> - <h2 class='c004'>ERRATA.</h2> -</div> -<div class='tnotes'> - -<div class='nf-center-c1'> -<div class='nf-center c003'> - <div>Transcriber’s Note:</div> - </div> -</div> - -<p class='c006'>These corrections have been applied to the text in the book.</p> - -</div> -<div class='lg-container-l c007'> - <div class='linegroup'> - <div class='group'> - <div class='line'>page 48, line 24 section 218 should read section 284</div> - <div class='line in2'>„ 77, last line „ 263 „ „ „ 270</div> - <div class='line in2'>„ 123, 5th line from bottom „ 254 „ „ „ 261</div> - <div class='line in2'>„ 124, 10th „ „ „ „ 246 „ „ „ 253</div> - <div class='line in2'>„ 151, line 3 „ 228 „ „ „ 229</div> - <div class='line in2'>„ 156, 13th line from bottom „ 261 „ „ „ 268</div> - <div class='line in2'>„ 200, line 9 „ 246 „ „ „ 253</div> - <div class='line in2'>„ 216, line 3 „ 260 „ „ „ 267</div> - <div class='line in2'>„ 184, at the top of 5th column of table the letter γ should be inserted.</div> - </div> - </div> -</div> - -<div class='chapter'> - <span class='pageno' id='Page_xiii'>xiii</span> - <h2 class='c004'>TABLE OF CONTENTS.</h2> -</div> - -<p class='c005'><a href='#chap01'>I. Radio-active Substances</a> <a href='#Page_1'>1</a></p> - -<p class='c006'><a href='#chap02'>II. Ionization Theory of Gases</a> <a href='#Page_31'>31</a></p> - -<p class='c006'><a href='#chap03'>III. Methods of Measurement</a> <a href='#Page_82'>82</a></p> - -<p class='c006'><a href='#chap04'>IV. Nature of the Radiations</a> <a href='#Page_108'>108</a></p> - -<p class='c006'><a href='#chap05'>V. Properties of the Radiations</a> <a href='#Page_201'>201</a></p> - -<p class='c006'><a href='#chap06'>VI. Continuous Production of Radio-active Matter</a> <a href='#Page_218'>218</a></p> - -<p class='c006'><a href='#chap07'>VII. Radio-active Emanations</a> <a href='#Page_238'>238</a></p> - -<p class='c006'><a href='#chap08'>VIII. Excited Radio-activity</a> <a href='#Page_295'>295</a></p> - -<p class='c006'><a href='#chap09'>IX. Theory of Successive Changes</a> <a href='#Page_325'>325</a></p> - -<p class='c006'><a href='#chap10'>X. Transformation Products of Uranium, Thorium and Actinium</a> <a href='#Page_346'>346</a></p> - -<p class='c006'><a href='#chap11'>XI. Transformation Products of Radium</a> <a href='#Page_371'>371</a></p> - -<p class='c006'><a href='#chap12'>XII. Rate of Emission of Energy</a> <a href='#Page_418'>418</a></p> - -<p class='c006'><a href='#chap13'>XIII. Radio-active Processes</a> <a href='#Page_437'>437</a></p> - -<p class='c006'><a href='#chap14'>XIV. Radio-activity of the Atmosphere and of Ordinary Materials</a> <a href='#Page_501'>501</a></p> - -<p class='c006'> <a href='#appa'>Appendix A. Properties of the α Rays</a> <a href='#Page_543'>543</a></p> - -<p class='c006'> <a href='#appb'>Appendix B. Radio-active Minerals</a> <a href='#Page_554'>554</a></p> - -<p class='c006'> <a href='#index'>Index</a> <a href='#Page_559'>559</a></p> - -<p class='c006'>Plate (Fig. 46<span class='fss'>A</span>: Spectrum of Radium Bromide) <i>to face p.</i> <a href='#Page_206'>206</a></p> - -<hr class='c008'> - -<p class='c006'>For the convenience of the reader, the sections and chapters which -contain mostly new matter, or have been either partly or wholly rewritten, -are appended below.</p> - -<div class='lg-container-l c007'> - <div class='linegroup'> - <div class='group'> - <div class='line'>Chap. I. Sections 18, 20–23.</div> - <div class='line in1'>„ II. „ 48–52.</div> - <div class='line in1'>„ III. „ 69.</div> - <div class='line in1'>„ IV. „ 83–85, 92, 93, 103, 104, 106–108, 111, 112.</div> - <div class='line in1'>„ V. „ 115, 117, 119, 122.</div> - <div class='line in1'>„ VII. „ 171–173.</div> - <div class='line in1'>„ VIII. „ 182–184, 190.</div> - <div class='line in1'>„ IX-XIV. Mostly rewritten.</div> - </div> - </div> -</div> - -<p class='c009'><span class='pageno' id='Page_xiv'>xiv</span>ABBREVIATIONS OF REFERENCES TO SOME OF THE JOURNALS.</p> -<p class='c010'><i>Ber. d. deutsch. Chem. Ges.</i> Berichte der deutschen chemischen Gesellschaft. -Berlin.</p> - -<p class='c011'><i>C. R.</i> Comptes Rendus des Séances de l’Académie des Sciences. Paris.</p> - -<p class='c011'><i>Chem. News.</i> Chemical News. London.</p> - -<p class='c011'><i>Drude’s Annal.</i> Annalen der Physik. Leipzig.</p> - -<p class='c011'><i>Phil. Mag.</i> Philosophical Magazine and Journal of Science. London.</p> - -<p class='c011'><i>Phil. Trans.</i> Philosophical Transactions of the Royal Society of London.</p> - -<p class='c011'><i>Phys. Rev.</i> Physical Review. New York.</p> - -<p class='c011'><i>Phys. Zeit.</i> Physikalische Zeitschrift.</p> - -<p class='c011'><i>Proc. Camb. Phil. Soc.</i> Proceedings of the Cambridge Philosophical Society. -Cambridge.</p> - -<p class='c011'><i>Proc. Roy. Soc.</i> Proceedings of the Royal Society of London.</p> - -<p class='c011'><i>Thèses-Paris.</i> Thèses présentées à la Faculté des Sciences de l’Université -de Paris.</p> - -<p class='c011'><i>Wied. Annal.</i> Annalen der Physik. Leipzig.</p> -<div class='chapter'> - <span class='pageno' id='Page_1'>1</span> - <h2 id='chap01' class='c004'>CHAPTER I. <br> RADIO-ACTIVE SUBSTANCES.</h2> -</div> -<p class='c005'><b>1. Introduction.</b> The close of the old and the beginning -of the new century have been marked by a very rapid increase of -our knowledge of that most important but comparatively little -known subject—the connection between electricity and matter. -No study has been more fruitful in surprises to the investigator, -both from the remarkable nature of the phenomena exhibited and -from the laws controlling them. The more the subject is examined, -the more complex must we suppose the constitution of matter in -order to explain the remarkable effects observed. While the -experimental results have led to the view that the constitution of -the atom itself is very complex, at the same time they have -confirmed the old theory of the discontinuous or atomic structure -of matter. The study of the radio-active substances and of the -discharge of electricity through gases has supplied very strong -experimental evidence in support of the fundamental ideas of the -existing atomic theory. It has also indicated that the atom itself -is not the smallest unit of matter, but is a complicated structure -made up of a number of smaller bodies.</p> - -<p class='c006'>A great impetus to the study of this subject was initially -given by the experiments of Lenard on the cathode rays, and -by Röntgen’s discovery of the X rays. An examination of the -conductivity imparted to a gas by the X rays led to a clear view -of the mechanism of the transport of electricity through gases -by means of charged ions. This ionization theory of gases has -been shown to afford a satisfactory explanation not only of the -passage of electricity through flames and vapours, but also of the -<span class='pageno' id='Page_2'>2</span>complicated phenomena observed when a discharge of electricity -passes through a vacuum tube. At the same time, a further -study of the cathode rays showed that they consisted of a stream -of material particles, projected with great velocity, and possessing -an apparent mass small compared with that of the hydrogen atom. -The connection between the cathode and Röntgen rays and the -nature of the latter were also elucidated. Much of this admirable -experimental work on the nature of the electric discharge has -been done by Professor J. J. Thomson and his students in the -Cavendish Laboratory, Cambridge.</p> - -<p class='c006'>An examination of natural substances, in order to see if they -gave out dark radiations similar to X rays, led to the discovery of -the radio-active bodies which possess the property of spontaneously -emitting radiations, invisible to the eye, but readily detected by -their action on photographic plates and their power of discharging -electrified bodies. A detailed study of the radio-active bodies has -revealed many new and surprising phenomena which have thrown -much light, not only on the nature of the radiations themselves, -but also on the processes occurring in those substances. Notwithstanding -the complex nature of the phenomena, the knowledge of -the subject has advanced with great rapidity, and a large amount -of experimental data has now been accumulated.</p> - -<p class='c006'>In order to explain the phenomena of radio-activity, Rutherford -and Soddy have advanced a theory which regards the atoms of the -radio-active elements as suffering spontaneous disintegration, and -giving rise to a series of radio-active substances which differ in -chemical properties from the parent elements. The radiations -accompany the breaking-up of the atoms, and afford a comparative -measure of the rate at which the disintegration takes place. This -theory is found to account in a satisfactory way for all the known -facts of radio-activity, and welds a mass of disconnected facts into -one homogeneous whole. On this view, the continuous emission of -energy from the active bodies is derived from the internal energy -inherent in the atom, and does not in any way contradict the law -of the conservation of energy. At the same time, however, it -indicates that an enormous store of latent energy is resident in the -radio-atoms themselves. This store of energy has not been observed -previously, on account of the impossibility of breaking up -<span class='pageno' id='Page_3'>3</span>into simpler forms the atoms of the elements by the action of the -chemical or physical forces at our command.</p> - -<p class='c006'>On this theory we are witnessing in the radio-active bodies a -veritable transformation of matter. This process of disintegration -was investigated, not by direct chemical methods, but by means -of the property possessed by the radio-active bodies of giving out -specific types of radiation. Except in the case of a very active -element like radium, the process of disintegration takes place so -slowly, that hundreds if not thousands of years would be required -before the amount transformed would come within the range of -detection of the balance or the spectroscope. In radium, however, -the process of disintegration takes place at such a rate that it -should be possible within a limited space of time to obtain definite -chemical evidence on this question. The recent discovery that -helium can be obtained from radium adds strong confirmation to -the theory; for helium was indicated as a probable disintegration -product of the radio-active elements before this experimental -evidence was forthcoming. Several products of the transformation -of the radio-active bodies have already been examined, and the -further study of these substances promises to open up new and -important fields of chemical enquiry.</p> - -<p class='c006'>In this book the experimental facts of radio-activity and the -connection between them are interpreted on the disintegration -theory. Many of the phenomena observed can be investigated in -a quantitative manner, and prominence has been given to work of -this character, for the agreement of any theory with the facts, -which it attempts to explain, must ultimately depend upon the -results of accurate measurement.</p> - -<p class='c006'>The value of any working theory depends upon the number of -experimental facts it serves to correlate, and upon its power of -suggesting new lines of work. In these respects the disintegration -theory, whether or not it may ultimately be proved to be correct, -has already been justified by its results.</p> -<p class='c005'><b>2. Radio-active Substances.</b> The term “radio-active” is -now generally applied to a class of substances, such as uranium, -thorium, radium, and their compounds, which possess the property -of <i>spontaneously</i> emitting radiations capable of passing through -<span class='pageno' id='Page_4'>4</span>plates of metal and other substances opaque to ordinary light. -The characteristic property of these radiations, besides their -penetrating power, is their action on a photographic plate and -their power of discharging electrified bodies. In addition, a -strongly radio-active body like radium is able to cause marked -phosphorescence and fluorescence on some substances placed near -it. In the above respects the radiations possess properties -analogous to Röntgen rays, but it will be shown that, for the -major part of the radiations emitted, the resemblance is only -superficial.</p> - -<p class='c006'>The most remarkable property of the radio-active bodies is -their power of radiating energy spontaneously and continuously at -a constant rate, without, as far as is known, the action upon them -of any external exciting cause. The phenomena at first sight -appear to be in direct contradiction to the law of conservation of -energy, since no obvious change with time occurs in the radiating -material. The phenomena appear still more remarkable when it -is considered that the radio-active bodies must have been steadily -radiating energy since the time of their formation in the earth’s -crust.</p> - -<p class='c006'>Immediately after Röntgen’s discovery of the production of -X rays, several physicists were led to examine if any natural -bodies possessed the property of giving out radiations which could -penetrate metals and other substances opaque to light. As the -production of X rays seemed to be connected in some way with -cathode rays, which cause strong fluorescent and phosphorescent -effects on various bodies, the substances first examined were those -that were phosphorescent when exposed to light. The first observation -in this direction was made by Niewenglowski<a id='r1' href='#f1' class='c012'><sup>[1]</sup></a>, who found -that sulphide of calcium exposed to the sun’s rays gave out some -rays which were able to pass through black paper. A little later -a similar result was recorded by H. Becquerel<a id='r2' href='#f2' class='c012'><sup>[2]</sup></a> for a special -calcium sulphide preparation, and by Troost<a id='r3' href='#f3' class='c012'><sup>[3]</sup></a> for a specimen of -hexagonal blend. These results were confirmed and extended in -a later paper by Arnold<a id='r4' href='#f4' class='c012'><sup>[4]</sup></a>. No satisfactory explanations of these -<span class='pageno' id='Page_5'>5</span>somewhat doubtful results have yet been given, except on the -view that the black paper was transparent to some of the light -waves. At the same time Le Bon<a id='r5' href='#f5' class='c012'><sup>[5]</sup></a> showed that, by the action of -sunlight on certain bodies, a radiation was given out, invisible to -the eye, but active with regard to a photographic plate. These -results have been the subject of much discussion; but there seems -to be little doubt that the effects are due to short ultra-violet light -waves, capable of passing through certain substances opaque to -ordinary light. These effects, while interesting in themselves, are -quite distinct in character from those shown by the radio-active -bodies which will now be considered.</p> -<p class='c005'><b>3. Uranium.</b> The first important discovery in the subject of -radio-activity was made in February, 1896, by M. Henri Becquerel<a id='r6' href='#f6' class='c012'><sup>[6]</sup></a>, -who found that a uranium salt, the double sulphate of uranium -and potassium, emitted some rays which gave an impression on a -photographic plate enveloped in black paper. These rays were -also able to pass through thin plates of metals and other substances -opaque to light. The impressions on the plate could not have -been due to vapours given off by the substances, since the same -effect was produced whether the salt was placed directly on the -black paper or on a thin plate of glass lying upon it.</p> - -<p class='c006'>Becquerel found later that all the compounds of uranium as -well as the metal itself possessed the same property, and, although -the amount of action varied slightly for the different compounds, -the effects in all cases were comparable. It was at first natural to -suppose that the emission of these rays was in some way connected -with the power of phosphorescence, but later observations showed -that there was no connection whatever between them. The uranic -salts are phosphorescent, while the uranous salts are not. The uranic -salts, when exposed to ultra-violet light in the phosphoroscope, -give a phosphorescent light lasting about ·01 seconds. When the -salts are dissolved in water, the duration is still less. The amount -of action on the photographic plate does not depend on the particular -compound of uranium employed, but only on the amount of -uranium present in the compound. The non-phosphorescent are -<span class='pageno' id='Page_6'>6</span>equally active with the phosphorescent compounds. The amount -of radiation given out is unaltered if the active body be kept -continuously in darkness. The rays are given out by solutions, -and by crystals which have been deposited from solutions in the -dark and never exposed to light. This shows that the radiation -cannot be due in any way to the gradual emission of energy stored -up in the crystal in consequence of exposure to a source of light.</p> -<p class='c005'><b>4.</b> The power of giving out penetrating rays thus seems to be -a specific property of the element uranium, since it is exhibited by -the metal as well as by all its compounds. These radiations from -uranium are persistent, and, as far as observations have yet gone, -are unchanged, either in intensity or character, with lapse of time. -Observations to test the constancy of the radiations for long -periods of time have been made by Becquerel. Samples of uranic -and uranous salts have been kept in a double box of thick lead, -and the whole has been preserved from exposure to light. By a -simple arrangement, a photographic plate can be introduced in a -definite position above the uranium salts, which are covered with a -layer of black paper. The plate is exposed at intervals for 48 hours, -and the impression on the plate compared. No perceptible -weakening of the radiation has been observed over a period of -four years. Mme Curie<a id='r7' href='#f7' class='c012'><sup>[7]</sup></a> has made determinations of the activity of -uranium over a space of five years by an electric method described -later, but found no appreciable variation during that period.</p> - -<p class='c006'>Since the uranium is thus continuously radiating energy from -itself, without any known source of excitation, the question arises -whether any known agent is able to affect the rate of its emission. -No alteration was observed when the body was exposed to ultra-violet -light or to ultra-red light or to X rays. Becquerel states -that the double sulphate of uranium and potassium showed a -slight increase of action when exposed to the arc light and to -sparks, but he considers that the feeble effect observed was -another action superimposed on the constant radiation from -uranium. The intensity of the uranium radiation is not affected -by a variation of temperature between 200° C. and the temperature -of liquid air. This question is discussed in more detail later.</p> -<p class='c005'><span class='pageno' id='Page_7'>7</span><b>5.</b> In addition to these actions on a photographic plate, -Becquerel showed that uranium rays, like Röntgen rays, possess the -important property of discharging both positively and negatively -electrified bodies. These results were confirmed and extended by -Lord Kelvin, Smolan and Beattie<a id='r8' href='#f8' class='c012'><sup>[8]</sup></a>. The writer made a detailed -comparison<a id='r9' href='#f9' class='c012'><sup>[9]</sup></a> of the nature of the discharge produced by uranium -with that produced by Röntgen rays, and showed that the discharging -property of uranium is due to the production of charged -ions by the radiation throughout the volume of the gas. The -property has been made the basis of a qualitative and quantitative -examination of the radiations from all radio-active bodies, and is -discussed in detail in <a href='#chap02'>chapter <span class='fss'>II</span></a>.</p> - -<p class='c006'>The radiations from uranium are thus analogous, as regards -their photographic and electrical actions, to Röntgen rays, but, -compared with the rays from an ordinary X ray tube, these -actions are extremely feeble. While with Röntgen rays a strong -impression is produced on a photographic plate in a few minutes -or even seconds, several days’ exposure to the uranium rays is -required to produce a well-marked action, even though the uranium -compound, enveloped in black paper, is placed close to the plate. -The discharging action, while very easily measurable by suitable -methods, is also small compared with that produced by X rays -from an ordinary tube.</p> -<p class='c005'><b>6.</b> The rays from uranium show no evidence of direct reflection, -refraction, or polarization<a id='r10' href='#f10' class='c012'><sup>[10]</sup></a>. While there is no direct reflection -of the rays, there is apparently a diffuse reflection produced -where the rays strike a solid obstacle. This is due in reality -to a secondary radiation set up when the primary rays impinge -upon matter. The presence of this secondary radiation at first -gave rise to the erroneous view that the rays could be reflected -and refracted like ordinary light. The absence of reflection, refraction, -or polarization in the penetrating rays from uranium -necessarily follows in the light of our present knowledge of the -rays. It is now known that the uranium rays, mainly responsible -for the photographic action, are deviable by a magnetic field, and -<span class='pageno' id='Page_8'>8</span>are similar in all respects to cathode rays, <i>i.e.</i> the rays are composed -of small particles projected at great velocities. The absence of the -ordinary properties of transverse light waves is thus to be expected.</p> -<p class='c005'><b>7.</b> The rays from uranium are complex in character, and, in -addition to the penetrating deviable rays, there is also given off -a radiation very readily absorbed by passing through thin layers -of metal foil, or by traversing a few centimetres of air. The -photographic action due to these rays is very feeble in comparison -with that of the penetrating rays, although the discharge of -electrified bodies is mainly caused by them. Besides these two -types of rays, some rays are emitted which are of an extremely -penetrating character and are non-deviable by a magnetic field. -These rays are difficult to detect photographically, but can readily -be examined by the electric method.</p> -<p class='c005'><a id='section008'></a> -<b>8.</b> The question naturally arose whether the property of -spontaneously giving out penetrating radiations was confined to -uranium and its compounds, or whether it was exhibited to any -appreciable extent by other substances.</p> - -<p class='c006'>By the electrical method, with an electrometer of ordinary -sensitiveness, any body which possesses an activity of the order of -¹⁄₁₀₀ of that of uranium can be detected. With an electroscope of -special construction, such as has been designed by C. T. R. Wilson -for his experiments on the natural ionization of air, a substance -of activity ¹⁄₁₀₀₀₀ and probably ¹⁄₁₀₀₀₀₀ of that of uranium can -be detected.</p> - -<p class='c006'>If an active body like uranium be mixed with an inactive body, -the resulting activity in the mixture is generally considerably less -than that due to the active substance alone. This is due to the -absorption of the radiation by the inactive matter present. The -amount of decrease largely depends on the thickness of the layer -from which the activity is determined.</p> - -<p class='c006'>Mme Curie made a detailed examination by the electrical -method of the great majority of known substances, including the -very rare elements, to see if they possessed any activity. In cases -where it was possible, several compounds of the elements were -examined. With the exception of thorium and phosphorus, none -<span class='pageno' id='Page_9'>9</span>of the other substances possessed an activity even of the order of -¹⁄₁₀₀ of uranium.</p> - -<p class='c006'>The ionization of the gas by phosphorus does not, however, -seem to be due to a penetrating radiation like that found in the -case of uranium, but rather to a chemical action taking place at -its surface. The compounds of phosphorus do not show any -activity, and in this respect differ from uranium and the other -active bodies.</p> - -<p class='c006'>Le Bon<a id='r11' href='#f11' class='c012'><sup>[11]</sup></a> has also observed that quinine sulphate, if heated and -then allowed to cool, possesses for a short time the property of -discharging both positively and negatively electrified bodies. It -is necessary, however, to draw a sharp line of distinction between -phenomena of this kind and those exhibited by the naturally radio-active -bodies. While both, under special conditions, possess the -property of ionizing the gas, the laws controlling the phenomena -are quite distinct in the two cases. For example, only one compound -of quinine shows the property, and that compound only -when it has been subjected to a preliminary heating. The action -of phosphorus depends on the nature of the gas, and varies with -temperature. On the other hand, the activity of the naturally -radio-active bodies is spontaneous and permanent. It is exhibited -by all compounds, and is not, as far as is yet known, altered by -change in the chemical or physical conditions.</p> -<p class='c005'><b>9.</b> The discharging and photographic action alone cannot be -taken as a criterion as to whether a substance is radio-active or -not. It is necessary in addition to examine the radiations, and to -test whether the actions take place through appreciable thicknesses -of all kinds of matter opaque to ordinary light. For example, a -body giving out short waves of ultra-violet light can be made to -behave in many respects like a radio-active body. As Lenard<a id='r12' href='#f12' class='c012'><sup>[12]</sup></a> has -shown, short waves of ultra-violet light will ionize the gas in their -path, and will be absorbed rapidly in the gas. They will produce -strong photographic action, and may pass through <i>some</i> substances -opaque to ordinary light. The similarity to a radio-active body is -thus fairly complete as regards these properties. On the other -<span class='pageno' id='Page_10'>10</span>hand, the emission of these light waves, unlike that of the radiations -from an active body, will depend largely on the molecular state -of the compound, or on temperature and other physical conditions. -But the great point of distinction lies in the nature of the radiations -from the bodies in question. In one case the radiations behave -as transverse waves, obeying the usual laws of light waves, while in -the case of a naturally active body, they consist for the most part -of a continuous flight of material particles projected from the -substance with great velocity. Before any substance can be called -“radio-active” in the sense in which the term is used to describe -the properties of the natural radio-active elements, it is thus -necessary to make a close examination of its radiation; for it is -unadvisable to extend the use of the term “radio-active” to -substances which do not possess the characteristic radiating -properties of the radio-active elements which we have described, -and the active products which can be obtained from them. Some -of the pseudo-active bodies will however be considered later in -<a href='#chap09'>chapter <span class='fss'>IX</span></a>.</p> -<p class='c005'><b>10. Thorium.</b> In the course of an examination of a large -number of substances, Schmidt<a id='r13' href='#f13' class='c012'><sup>[13]</sup></a> found that thorium, its compounds, -and the minerals containing thorium, possessed properties similar -to those of uranium. The same discovery was made independently -by Mme Curie<a id='r14' href='#f14' class='c012'><sup>[14]</sup></a>. The rays from thorium compounds, like those -from uranium, possess the properties of discharging electrified -bodies and acting on a photographic plate. Under the same -conditions the discharging action of the rays is about equal in -amount to that of uranium, but the photographic effect is -distinctly weaker.</p> - -<p class='c006'>The radiations from thorium are more complicated than those -from uranium. It was early observed by several experimenters -that the radiation from thorium compounds, especially the oxide, -when tested by the electrified method, was very variable and -uncertain. A detailed investigation of the radiations from thorium -under various conditions was made by Owens<a id='r15' href='#f15' class='c012'><sup>[15]</sup></a>. He showed that -thorium oxide, especially in thick layers, was able to produce -<span class='pageno' id='Page_11'>11</span>conductivity in the gas when covered with a large thickness of -paper, and that the amount of this conductivity could be greatly -varied by blowing a current of air over the gas. In the course of -an examination<a id='r16' href='#f16' class='c012'><sup>[16]</sup></a> of this action of the air current, the writer -showed that thorium compounds gave out a material emanation -made up of very small particles <i>themselves radio-active</i>. The -emanation behaves like a radio-active gas; it diffuses rapidly -through porous substances like paper, and is carried away by -a current of air. The evidence of the existence of the emanation -and its properties, is considered in detail later in <a href='#chap08'>chapter <span class='fss'>VIII</span></a>. In -addition to giving out an emanation, thorium behaves like uranium -in emitting three types of radiation, each of which is similar in -properties to the corresponding radiation from uranium.</p> -<p class='c005'><b>11. Radio-active minerals.</b> Mme Curie has examined -the radio-activity of a large number of minerals containing -uranium and thorium. The electrical method was used, and the -current measured between two parallel plates 8 cms. in diameter -and 3 cms. apart, when one plate was covered with a uniform -layer of the active matter. The following numbers give the order -of the saturation current obtained in amperes.</p> - -<table class='table0' > -<colgroup> -<col class='colwidth20'> -<col class='colwidth20'> -</colgroup> - <tr> - <td class='c013'>Pitchblende from Johanngeorgenstadt</td> - <td class='c014'>8·3 × 10<sup>-11</sup></td> - </tr> - <tr> - <td class='c013'>„ Joachimsthal</td> - <td class='c014'>7·0 „</td> - </tr> - <tr> - <td class='c013'>„ Pzibran</td> - <td class='c014'>6·5 „</td> - </tr> - <tr> - <td class='c013'>„ Cornwall</td> - <td class='c014'>1·6 „</td> - </tr> - <tr> - <td class='c013'>Cleveite</td> - <td class='c014'>1·4 „</td> - </tr> - <tr> - <td class='c013'>Chalcolite</td> - <td class='c014'>5·2 „</td> - </tr> - <tr> - <td class='c013'>Autunite</td> - <td class='c014'>2·7 „</td> - </tr> - <tr> - <td class='c013'>Thorite</td> - <td class='c014'>from 0·3 to 1·4 „</td> - </tr> - <tr> - <td class='c013'>Orangite</td> - <td class='c014'>2·0 „</td> - </tr> - <tr> - <td class='c013'>Monazite</td> - <td class='c014'>0·5 „</td> - </tr> - <tr> - <td class='c013'>Xenotine</td> - <td class='c014'>0·03 „</td> - </tr> - <tr> - <td class='c013'>Aeschynite</td> - <td class='c014'>0·7 „</td> - </tr> - <tr> - <td class='c013'>Fergusonite</td> - <td class='c014'>0·4 „</td> - </tr> - <tr> - <td class='c013'>Samarskite</td> - <td class='c014'>1·1 „</td> - </tr> - <tr> - <td class='c013'>Niobite</td> - <td class='c014'>0·3 „</td> - </tr> - <tr> - <td class='c013'>Carnotite</td> - <td class='c014'>6·2 „</td> - </tr> -</table> - -<p class='c006'>Some activity is to be expected in these minerals, since they all -contain either thorium or uranium or a mixture of both. An -<span class='pageno' id='Page_12'>12</span>examination of the action of the uranium compounds with the -same apparatus and under the same conditions led to the following -results:</p> - -<table class='table1' > -<colgroup> -<col class='colwidth26'> -<col class='colwidth13'> -</colgroup> - <tr> - <td class='c013'>Uranium (containing a little carbon)</td> - <td class='c014'>2·3 × 10<sup>-11</sup> amperes</td> - </tr> - <tr> - <td class='c013'>Black oxide of uranium</td> - <td class='c014'>2·6 „</td> - </tr> - <tr> - <td class='c013'>Green „ „</td> - <td class='c014'>1·8 „</td> - </tr> - <tr> - <td class='c013'>Acid uranic hydrate</td> - <td class='c014'>0·6 „</td> - </tr> - <tr> - <td class='c013'>Uranate of sodium</td> - <td class='c014'>1·2 „</td> - </tr> - <tr> - <td class='c013'>Uranate of potassium</td> - <td class='c014'>1·2 „</td> - </tr> - <tr> - <td class='c013'>Uranate of ammonia</td> - <td class='c014'>1·3 „</td> - </tr> - <tr> - <td class='c013'>Uranous sulphate</td> - <td class='c014'>0·7 „</td> - </tr> - <tr> - <td class='c013'>Sulphate of uranium and potassium</td> - <td class='c014'>0·7 „</td> - </tr> - <tr> - <td class='c013'>Acetate</td> - <td class='c014'>0·7 „</td> - </tr> - <tr> - <td class='c013'>Phosphate of copper and uranium</td> - <td class='c014'>0·9 „</td> - </tr> - <tr> - <td class='c013'>Oxysulphide of uranium</td> - <td class='c014'>1·2 „</td> - </tr> -</table> - -<p class='c006'>The interesting point in connection with these results is that -some specimens of pitchblende have four times the activity of the -metal uranium; chalcolite, the crystallized phosphate of copper -and uranium, is twice as active as uranium; and autunite, a -phosphate of calcium and uranium, is as active as uranium. From -the previous considerations, none of the substances should have -shown as much activity as uranium or thorium. In order to be -sure that the large activity was not due to the particular chemical -combination, Mme Curie prepared chalcolite artificially, starting -with pure products. This artificial chalcolite had the activity to -be expected from its composition, viz. about 0·4 of the activity of -the uranium. The natural mineral chalcolite is thus five times as -active as the artificial mineral.</p> - -<p class='c006'>It thus seemed probable that the large activity of some of -these minerals, compared with uranium and thorium, was due to -the presence of small quantities of some very active substance, -which was different from the known bodies thorium and uranium.</p> - -<p class='c006'>This supposition was completely verified by the work of M. and -Mme Curie, who were able to separate from pitchblende by purely -chemical methods two active bodies, one of which in the pure state -is over a million times more active than the metal uranium.</p> - -<p class='c006'>This important discovery was due entirely to the property -of radio-activity possessed by the new bodies. The only guide -<span class='pageno' id='Page_13'>13</span>in their separation was the activity of the products obtained. In -this respect the discovery of these bodies is quite analogous to the -discovery of rare elements by the methods of spectrum analysis. -The method employed in the separation consisted in examining -the relative activity of the products after chemical treatment. In -this way it was seen whether the radio-activity was confined to one -or another of the products, or divided between both, and in what -ratio such division occurred.</p> - -<p class='c006'>The activity of the specimens thus served as a basis of rough -qualitative and quantitative analysis, analogous in some respects -to the indication of the spectroscope. To obtain comparative -data it was necessary to test all the products in the dry state. -The chief difficulty lay in the fact that pitchblende is a very -complex mineral, and contains in varying quantities nearly all the -known metals.</p> -<p class='c005'><b>12. Radium.</b> The analysis of pitchblende by chemical -methods, using the procedure sketched above, led to the discovery -of two very active bodies, polonium and radium. The name polonium -was given to the first substance discovered by Mme Curie -in honour of the country of her birth. The name radium was -a very happy inspiration of the discoverers, for this substance in -the pure state possesses the property of radio-activity to an -astonishing degree.</p> - -<p class='c006'>Radium is extracted from pitchblende by the process used -to separate barium, to which radium is very closely allied in -chemical properties<a id='r17' href='#f17' class='c012'><sup>[17]</sup></a>. After the removal of other substances, the -radium remains behind mixed with barium. It can, however, be -partially separated from the latter by the difference in solubility of -the chlorides in water, alcohol, or hydrochloric acid. The chloride -of radium is less soluble than that of barium, and can be separated -from it by the method of fractional crystallization. After a large -number of precipitations, the radium can be freed almost completely -from the barium.</p> - -<p class='c006'>Both polonium and radium exist in infinitesimal quantities in -pitchblende. In order to obtain a few decigrammes of very active -radium, it is necessary to use several tons of pitchblende, or the -<span class='pageno' id='Page_14'>14</span>residues obtained from the treatment of uranium minerals. It is -thus obvious that the expense and labour involved in preparation -of a minute quantity of radium are very great.</p> - -<p class='c006'>M. and Mme Curie were indebted for their first working -material to the Austrian government, who generously presented -them with a ton of the treated residue of uranium materials from -the State manufactory of Joachimsthal in Bohemia. With the -assistance of the Academy of Science and other societies in France, -funds were given to carry out the laborious work of separation. -Later the Curies were presented with a ton of residues from the -treatment of pitchblende by the Société Centrale de Produits -Chimiques of Paris. The generous assistance afforded in this -important work is a welcome sign of the active interest taken in -these countries in the furthering of purely scientific research.</p> - -<p class='c006'>The rough concentration and separation of the residues was -performed in the chemical works, and there followed a large amount -of labour in purification and concentration. In this manner, -the Curies were able to obtain a small quantity of radium which -was enormously active compared with uranium. No definite results -have yet been given on the activity of pure radium, but the Curies -estimate that it is about one million times that of uranium, -and may possibly be still higher. The difficulty of making a -numerical estimate for such an intensely active body is very great. -In the electric method, the activities are compared by noting the -relative strength of the maximum or saturation current between -two parallel plates, on one of which the active substance is spread. -On account of the intense ionization of the gas between the plates, -it is not possible to reach the saturation current unless very high -voltages are applied. Approximate comparisons can be made by -the use of metal screens to cut down the intensity of the radiations, -if the proportion of the radiation transmitted by such a screen has -been determined by direct experiment on impure material of easily -measurable activity. The value of the activity of radium compared -with that of uranium will however vary to some extent according to -which of the three types of rays is taken as a basis of comparison.</p> - -<p class='c006'>It is thus difficult to control the final stages of the purification -of radium by measurements of its activity alone. Moreover the -activity of radium immediately after its preparation is only about -<span class='pageno' id='Page_15'>15</span>one-fourth of its final value; it gradually rises to a maximum after -the radium salt has been kept in the dry state for about a month. -For control experiments in purification, it is advisable to measure -the initial rather than the final activity.</p> - -<p class='c006'>Mme Curie has utilized the coloration of the crystals of radiferous -barium as a means of controlling the final process of purification. -The crystals of salts of radium and barium deposited from -acid solutions are indistinguishable by the eye. The crystals of -radiferous barium are at first colourless, but, in the course of a few -hours, become yellow, passing to orange and sometimes to a beautiful -rose colour. The rapidity of this coloration depends on the amount -of barium present. Pure radium crystals do not colour, or at any -rate not as rapidly as those containing barium. The coloration is a -maximum for a definite proportion of radium, and this fact can be -utilized as a means of testing the amount of barium present. When -the crystals are dissolved in water the coloration disappears.</p> - -<p class='c006'>Giesel<a id='r18' href='#f18' class='c012'><sup>[18]</sup></a> has observed that pure radium bromide gives a beautiful -carmine colour to the Bunsen flame. If barium be present in any -quantity, only the green colour due to barium is observed, and a -spectroscopic examination shows only the barium lines. This -carmine coloration of the Bunsen flame is a good indication of the -purity of the radium.</p> - -<p class='c006'>Since the preliminary announcement of the discovery of -radium, Giesel<a id='r19' href='#f19' class='c012'><sup>[19]</sup></a> has devoted a great deal of attention to the -separation of radium, polonium and other active bodies from pitchblende. -He was indebted for his working material to the firm -of P. de Haen, of Hanover, who presented him with a ton of pitchblende -residues. Using the method of fractional crystallization of -the bromide instead of the chloride, he has been able to prepare -considerable quantities of pure radium. By this means the labour -of final purification of radium has been much reduced. He states -that six or eight crystallizations with the bromide are sufficient to -free the radium almost completely from the barium.</p> -<p class='c005'><b>13. Spectrum of radium.</b> It was of great importance to -settle as soon as possible whether radium was in reality modified -<span class='pageno' id='Page_16'>16</span>barium or a new element with a definite spectrum. For this -purpose the Curies prepared some specimens of radium chloride, -and submitted them for examination of their spectrum to -Demarçay, an authority on that subject. The first specimen of -radium chloride examined by Demarçay<a id='r20' href='#f20' class='c012'><sup>[20]</sup></a> was not very active, but -showed, besides the lines due to barium, a very strong new line in -the ultra-violet. In another sample of greater activity, the line -was still stronger and others also appeared, while the intensity of -the new lines was comparable with those present due to barium. -With a still more active specimen which was probably nearly pure, -only three strong lines of barium appeared, while the new spectrum -was very bright. The following table shows the wave-length of -the new lines observed for radium. The wave lengths are expressed -in Ångström units and the intensity of each ray is denoted by a -number, the ray of maximum intensity being 16.</p> - -<table class='table2' > -<colgroup> -<col class='colwidth26'> -<col class='colwidth23'> -<col class='colwidth26'> -<col class='colwidth23'> -</colgroup> - <tr> - <th class='c015'>Wave length</th> - <th class='c015'>Intensity</th> - <th class='c015'>Wave length</th> - <th class='c016'>Intensity</th> - </tr> - <tr> - <td class='c015'>4826·3</td> - <td class='c015'>10</td> - <td class='c015'>4600·3</td> - <td class='c016'>3</td> - </tr> - <tr> - <td class='c015'>4726·9</td> - <td class='c015'>5</td> - <td class='c015'>4533·5</td> - <td class='c016'>9</td> - </tr> - <tr> - <td class='c015'>4699·6</td> - <td class='c015'>3</td> - <td class='c015'>4436·1</td> - <td class='c016'>6</td> - </tr> - <tr> - <td class='c015'>4692·1</td> - <td class='c015'>7</td> - <td class='c015'>4340·6</td> - <td class='c016'>12</td> - </tr> - <tr> - <td class='c015'>4683·0</td> - <td class='c015'>14</td> - <td class='c015'>3814·7</td> - <td class='c016'>16</td> - </tr> - <tr> - <td class='c015'>4641·9</td> - <td class='c015'>4</td> - <td class='c015'>3649·6</td> - <td class='c016'>12</td> - </tr> -</table> - -<p class='c006'>The lines are all sharply defined, and three or four of them -have an intensity comparable with any known lines of other -substances. There are also present in the spectrum two strong -nebulous bands. In the visible part of the spectrum, which has -not been photographed, the only noticeable ray has a wave -length 5665, which is, however, very feeble compared with that of -wave length 4826·3. The general aspect of the spectrum is similar -to that of the alkaline earths; it is known that these metals have -strong lines accompanied by nebulous bands.</p> - -<p class='c006'>The principal line due to radium can be distinguished in -impure radium of activity 50 times that of uranium. By the -electrical method it is easy to distinguish the presence of radium -in a body which has an activity only ¹⁄₁₀₀ of uranium. With a -more sensitive electrometer ¹⁄₁₀₀₀₀ of the activity of uranium -<span class='pageno' id='Page_17'>17</span>could be observed. For the detection of radium, the examination -of the radio-activity is thus a process nearly a million times more -sensitive than spectrum analysis.</p> - -<p class='c006'>Later observations on the spectrum of radium have been made by -Runge<a id='r21' href='#f21' class='c012'><sup>[21]</sup></a>, Exner and Haschek<a id='r22' href='#f22' class='c012'><sup>[22]</sup></a>, with specimens of radium prepared -by Giesel. Crookes<a id='r23' href='#f23' class='c012'><sup>[23]</sup></a> has photographed the spectrum of radium -in the ultra-violet, while Runge and Precht<a id='r24' href='#f24' class='c012'><sup>[24]</sup></a>, using a highly purified -sample of radium, observed a number of new lines in the spark -spectrum. It has been mentioned already that the bromide of -radium gives a characteristic pure carmine-red coloration to the -Bunsen flame. The flame spectrum shows two broad bright bands -in the orange-red, not observed in Demarçay’s spectrum. In -addition there is a line in the blue-green and two feeble lines in -the violet.</p> -<p class='c005'><b>14. Atomic weight of radium.</b> Mme Curie has made -successive determinations of the atomic weight of the new element -with specimens of steadily increasing purity. In the first observation -the radium was largely mixed with barium, and the atomic -weight obtained was the same as that of barium, 137·5. In -successive observations with specimens of increasing purity the -atomic weights of the mixture were 146 and 175. The final value -obtained recently was 225, which may be taken as the atomic -weight of radium on the assumption that it is divalent.</p> - -<p class='c006'>In these experiments about 0·1 gram of pure radium chloride -was obtained by successive fractionations. The difficulty involved -in preparing a quantity of pure radium chloride large enough to -test the atomic weight may be gauged from the fact that only a -few centigrams of fairly pure radium, or a few decigrams of less -concentrated material, are obtained from the treatment of about -2 tons of the mineral from which it is derived.</p> - -<p class='c006'>Runge and Precht<a id='r25' href='#f25' class='c012'><sup>[25]</sup></a> have examined the spectrum of radium in -a magnetic field, and have shown the existence of series analogous -to those observed for calcium, barium, and strontium. These series -<span class='pageno' id='Page_18'>18</span>are connected with the atomic weights of the elements in question, -and Runge and Precht have calculated by these means that the -atomic weight of radium should be 258—a number considerably -greater than the number 225 obtained by Mme Curie by means of -chemical analysis. Marshall Watts<a id='r26' href='#f26' class='c012'><sup>[26]</sup></a>, on the other hand, using another -relation between the lines of the spectrum, deduced the value -obtained by Mme Curie. Runge<a id='r27' href='#f27' class='c012'><sup>[27]</sup></a> has criticised the method of -deduction employed by Marshall Watts on the ground that the -lines used for comparison in the different spectra were not homologous. -Considering that the number found by Mme Curie agrees -with that required by the periodic system, it is advisable in the -present state of our knowledge to accept the experimental number -rather than the one deduced by Runge and Precht from spectroscopic -evidence.</p> - -<p class='c006'>There is no doubt that radium is a new element possessing -remarkable physical properties. The detection and separation of -this substance, existing in such minute proportions in pitchblende, -has been due entirely to the characteristic property we are considering, -and is the first notable triumph of the study of radio-activity. -As we shall see later, the property of radio-activity can -be used, not only as a means of chemical research, but also as an -extraordinarily delicate method of detecting chemical changes of a -very special kind.</p> -<p class='c005'><b>15. Radiations from radium.</b> On account of its enormous -activity, the radiations from radium are very intense: a screen of -zinc sulphide, brought near a few centigrams of radium bromide, -is lighted up quite brightly in a dark room, while brilliant -fluorescence is produced on a screen of platino-barium cyanide. -An electroscope brought near the radium salt is discharged almost -instantly, while a photographic plate is immediately affected. -At a distance of one metre, a day’s exposure to the radium -rays would produce a strong impression. The radiations from -radium are analogous to those of uranium, and consist of three -types of rays: easily absorbed, penetrating, and very penetrating. -Radium also gives rise to an emanation similar to that of thorium, -<span class='pageno' id='Page_19'>19</span>but with a very much slower rate of decay. The radium emanation -retains its activity for several weeks, while that of thorium lasts -only a few minutes. The emanation obtained from a few centigrams -of radium illuminates a screen of zinc sulphide with -great brilliancy. The very penetrating rays of radium are able to -light up an X ray screen in a dark room, after passage through -several centimetres of lead and several inches of iron.</p> - -<p class='c006'>As in the case of uranium or thorium, the photographic action -is mainly due to the penetrating or cathodic rays. The radiographs -obtained with radium are very similar to those obtained -with X rays, but lack the sharpness and detail of the latter. The -rays are unequally absorbed by different kinds of matter, the -absorption varying approximately as the density. In photographs -of the hand the bones do not stand out as in X ray photographs.</p> - -<p class='c006'>Curie and Laborde have shown that the compounds of radium -possess the remarkable property of always keeping their temperature -several degrees above the temperature of the surrounding -air. Each gram of radium radiates an amount of energy corresponding -to 100 gram-calories per hour. This and other properties -of radium are discussed in detail in chapters <a href='#chap05'><span class='fss'>V</span></a> and <a href='#chap12'><span class='fss'>XII</span></a>.</p> -<p class='c005'><b>16. Compounds of radium.</b> When first prepared in the -solid state, all the salts of radium—the chloride, bromide, acetate, -sulphate, and carbonate—are very similar in appearance to the -corresponding salts of barium, but in time they gradually become -coloured. In chemical properties the salts of radium are practically -the same as those of barium, with the exception that the -chloride and bromide of radium are less soluble in water than the -corresponding salts of barium. All the salts of radium are naturally -phosphorescent. The phosphorescence of impure radium -preparations is in some cases very marked.</p> - -<p class='c006'>All the radium salts possess the property of causing rapid -colorations of the glass vessel which contains them. For feebly -active material the colour is usually violet, for more active material -a yellowish-brown, and finally black.</p> -<p class='c005'><a id='section017'></a> -<b>17. Actinium.</b> The discovery of radium in pitchblende gave -a great impetus to the chemical examination of uranium residues, -and a systematic search early led to the detection of several -<span class='pageno' id='Page_20'>20</span>new radio-active bodies. Although these show distinctive radio-active -properties, so far none of them have been purified sufficiently -to give a definite spectrum as in the case of radium. -One of the most interesting and important of these substances -was discovered by Debierne<a id='r28' href='#f28' class='c012'><sup>[28]</sup></a> while working up the uranium -residues, obtained by M. and Mme Curie from the Austrian -government, and was called by him actinium. This active substance -is precipitated with the iron group, and appears to be very -closely allied in chemical properties to thorium, though it is many -thousand times more active. It is very difficult to separate from -thorium and the rare earths. Debierne has made use of the following -methods for partial separation:</p> - -<p class='c006'>(1) Precipitation in hot solutions, slightly acidulated with -hydrochloric acid, by excess of hyposulphite of soda. The active -matter is present almost entirely in the precipitate.</p> - -<p class='c006'>(2) Action of hydrofluoric acid upon the hydrates freshly -precipitated, and held in suspension in water. The portion -dissolved is only slightly active. By this method titanium may -be separated.</p> - -<p class='c006'>(3) Precipitation of neutral nitrate solutions by oxygenated -water. The precipitate carries down the active body.</p> - -<p class='c006'>(4) Precipitation of insoluble sulphates. If barium sulphate, -for example, is precipitated in the solution containing the active -body, the barium carries down the active matter. The thorium -and actinium are freed from the barium by conversion of the -sulphate into the chloride and precipitation by ammonia.</p> - -<p class='c006'>In this way Debierne has obtained a substance comparable -in activity with radium. The separation, which is difficult and -laborious, has not yet been carried far enough to bring out any -new lines in the spectrum.</p> -<p class='c005'><a id='section018'></a> -<b>18.</b> After the initial announcement of the discovery of -actinium, several years elapsed before any definite results upon it -were published by Debierne. In the meantime, Giesel<a id='r29' href='#f29' class='c012'><sup>[29]</sup></a> had -independently obtained a radio-active substance from pitchblende -which seemed similar in many respects to the actinium of Debierne. -<span class='pageno' id='Page_21'>21</span>The active substance belongs to the group of cerium earths and is -precipitated with them. By a succession of chemical operations, -the active substance is separated mixed with lanthanum. While -intensely active in comparison with thorium, the new active -substance closely resembles it in radio-active properties, although, -from the method of separation thorium cannot be present except -in minute quantity. Giesel early observed that the substance gave -off a radio-active emanation. On account of the intensity of the -emanation it emits, he termed it the “emanating substance.” -Recently this name has been changed to “emanium,” and under -this title preparations of the active substance prepared by Giesel -have been placed on the market.</p> - -<p class='c006'>Giesel found that the activity of this substance was permanent -and seemed to increase during the six months’ interval after separation. -In this respect it is similar to radium compounds, for the -activity of radium, measured by the electric method, increases -in the course of a month’s interval to four times its initial value -at separation.</p> - -<p class='c006'>There can be no doubt that the “actinium” of Debierne -and the “emanium” of Giesel contain the same radio-active constituent, -for recent work<a id='r30' href='#f30' class='c012'><sup>[30]</sup></a> has shown that they exhibit identical -radio-active properties. Each gives out easily absorbed and -penetrating rays, and emits a characteristic emanation of which -the rate of decay is the same for both substances. The rate of -decay of the emanation is the simplest method of distinguishing -actinium from thorium, which it resembles so closely in radio-active -as well as in chemical properties. The emanation of -actinium loses its radiating power far more rapidly than that of -thorium, the time taken for the activity to fall to half value being -in the two cases 3·7 seconds and 52 seconds respectively.</p> - -<p class='c006'>The rapid and continuous emission of this short-lived emanation -is the most striking radio-active property possessed by actinium. -In still air, the radio-active effects of this emanation are confined -to a distance of a few centimetres from the active material, as it is -only able to diffuse a short distance through the air before losing -its radiating power. With very active preparations of actinium, -<span class='pageno' id='Page_22'>22</span>the material appears to be surrounded by a luminous haze produced -by the emanation. The radiations produce strong luminosity in -some substances, for example, zinc sulphide, willemite and platinocyanide -of barium. The luminosity is especially marked on screens -of zinc sulphide. Much of this effect is due to the emanation, -for, on gently blowing a current of air over the substance, the -luminosity is displaced at once in the direction of the current. -With a zinc sulphide screen, actinium shows the phenomena of -“scintillations” to an even more marked degree than radium itself.</p> - -<p class='c006'>The preparations of emanium are in some cases luminous, -and a spectroscopic examination of this light has shown a number -of bright lines<a id='r31' href='#f31' class='c012'><sup>[31]</sup></a>.</p> - -<p class='c006'>The distinctive character of the emanation of actinium, as well -as of the other radio-active products to which it gives rise, coupled -with the permanence of its activity, renders it very probable that -actinium will prove to be a new radio-active element of very great -activity. Although very active preparations of actinium have -been obtained, it has not yet been found possible to free it from -impurities. Consequently, no definite observations have been -made on its chemical properties, and no new spectrum lines have -been observed.</p> - -<p class='c006'>A more complete discussion of the radio-active and other -properties of actinium is given in later chapters.</p> -<p class='c005'><b>19. Polonium.</b> Polonium was the first of the active substances -obtained from pitchblende. It has been investigated in -detail by its discoverer Mme Curie<a id='r32' href='#f32' class='c012'><sup>[32]</sup></a>. The pitchblende was dissolved -in acid and sulphuretted hydrogen added. The precipitated -sulphides contained an active substance, which, after separation -of impurities, was found associated with bismuth. This active -substance, which has been named polonium, is so closely allied in -chemical properties to bismuth that it has so far been found -impossible to effect a complete separation. Partial separation of -polonium can be made by successive fractionations based on one -of the following modes of procedure:</p> - -<p class='c006'>(1) Sublimation in a vacuum. The active sulphide is more -<span class='pageno' id='Page_23'>23</span>volatile than that of bismuth. It is deposited as a black substance -at those parts of the tube, where the temperature is between 250 -and 300° C. In this way polonium of activity 700 times that of -uranium was obtained.</p> - -<p class='c006'>(2) Precipitation of nitric acid solutions by water. The -precipitated sub-nitrate is much more active than the part that -remains in solution.</p> - -<p class='c006'>(3) Precipitation by sulphuretted hydrogen in a very acid -hydrochloric acid solution. The precipitated sulphides are much -more active than the salt which remains in solution.</p> - -<p class='c006'>For concentration of the active substance Mme Curie<a id='r33' href='#f33' class='c012'><sup>[33]</sup></a> has made -use of method (2). The process is, however, very slow and tedious, -and is made still more complicated by the tendency to form -precipitates insoluble either in strong or weak acids. After a -large number of fractionations, a small quantity of matter was -obtained, enormously active compared with uranium. On examination -of the substance spectroscopically, only the bismuth lines -were observed. A spectroscopic examination of the active bismuth -by Demarçay and by Runge and Exner has led to the discovery of -no new lines. On the other hand Sir William Crookes<a id='r34' href='#f34' class='c012'><sup>[34]</sup></a> states that -he found one new line in the ultra-violet, while Berndt<a id='r35' href='#f35' class='c012'><sup>[35]</sup></a>, working -with polonium of activity 300, observed a large number of new -lines in the ultra-violet. These results await further confirmation.</p> - -<p class='c006'>The polonium prepared by Mme Curie differs from the other -radio-active bodies in several particulars. In the first place the -radiations include only very easily absorbable rays. The two -penetrating types of radiation given out by uranium, thorium, -and radium are absent. In the second place the activity does -not remain constant, but diminishes continuously with the time. -Mme Curie states that different preparations of polonium had -somewhat different rates of decay. In some cases, the activity -fell to half value in about six months, and in others, about half -value in eleven months.</p> -<p class='c005'><b>20.</b> The gradual diminution of the activity of polonium with -time seemed at first sight to differentiate it from such substances -<span class='pageno' id='Page_24'>24</span>as uranium and radium, the activity of which appeared fairly -permanent. This difference in behaviour is, however, one of degree -rather than of kind. We shall show later that there is present in -pitchblende a number of radio-active substances, the activity of -which is not permanent. The time taken for these bodies to lose -half of their activity varies in different cases from a few seconds to -several hundreds of years. In fact, this gradual loss of activity is -an essential feature of our theory of regarding the phenomena -of radio-activity. No radio-active substance, left to itself, can -continue to radiate indefinitely; it must ultimately lose its -activity. In the case of bodies like uranium and radium, the -loss of activity is so slow that no sensible alteration has been -observed over a period of several years, but it can be deduced -theoretically that the activity of radium will eventually decrease -to half value in a period of about 1000 years, while in the case -of a feebly radio-active substance like uranium, more than a -100 million years must elapse before the diminution of the -activity becomes appreciable.</p> - -<p class='c006'>It may be of interest here to consider briefly the suggestions -advanced at various times to account for the temporary character -of the activity of polonium. Its association with bismuth led -to the view that polonium was not a new active substance, but -merely radio-active bismuth, that is, bismuth which in some way -had been made active by admixture with radio-active bodies. It -was known that a body placed in the vicinity of thorium or radium -became temporarily active. The same action was supposed to take -place when inactive matter was in solution with active matter. -The non-active matter was supposed to acquire activity by “induction,” -as it was called, in consequence of its intimate contact with -the active material.</p> - -<p class='c006'>There is no proof, however, that such is the case. The -evidence points rather to the conclusion that the activity is due, -not to an alteration of the inactive body itself, but to an admixture -with it of a very small quantity of intensely active matter. This -active matter is present in pitchblende and is separated with the -bismuth but differs from it in chemical properties.</p> - -<p class='c006'>The subject cannot be considered with advantage at this stage, -but will be discussed later in detail in <a href='#chap11'>chapter <span class='fss'>XI</span></a>. It will -<span class='pageno' id='Page_25'>25</span>there be shown that polonium, that is, the radio-active constituent -mixed with the bismuth, is a distinct chemical substance, which -is allied in chemical properties to bismuth, but possesses some -distinct analytical properties which allow of a partial separation -from it.</p> - -<p class='c006'>The polonium, if obtained in a pure state, should initially be -several hundred times as active as pure radium. This activity, -however, is not permanent; it decays with the time, falling to half -value in about six months.</p> - -<p class='c006'>The absence of any new lines in the spectrum of radio-active -bismuth is to be expected, for, even in the most active bismuth -prepared, the active matter exists in a very small proportion.</p> -<p class='c005'><a id='section021'></a> -<b>21.</b> The discussion of the nature of polonium was renewed by -the discovery of Marckwald<a id='r36' href='#f36' class='c012'><sup>[36]</sup></a> that a substance similar to polonium -can be separated from pitchblende; the activity of this substance, -he stated, did not decay appreciably with the time. The method -of separation from the bismuth chloride solution, obtained from -uranium residues, was very simple. A rod of bismuth or antimony, -dipped in the active solution, rapidly became coated with a black -deposit which was intensely active. This process was continued -until the whole of the activity was removed from the solution. -The active deposit gave out only easily absorbed rays, and in that -respect resembled the polonium of Mme Curie.</p> - -<p class='c006'>The active substance was found to consist mainly of tellurium, -and for this reason Marckwald gave it the name of radio-tellurium. -In later work, however, Marckwald<a id='r37' href='#f37' class='c012'><sup>[37]</sup></a> has shown that the active -constituent has no connection with tellurium, but can always be -separated completely from it by a simple chemical process.</p> - -<p class='c006'>In order to obtain a large amount of the active substance, -2000 kilos. of pitchblende were worked up. This yielded 6 kilos. -of bismuth oxychloride, and from this was separated 1·5 grams of -radio-tellurium. The tellurium present was precipitated from a -hydrochloric acid solution by hydrazine hydrochloride. The precipitated -tellurium still showed some activity, but this was -removed by repeating the process. The active matter then -<span class='pageno' id='Page_26'>26</span>remained in the filtrate, and, after evaporation, the addition of a -few drops of stannous chloride caused a small quantity of a dark -precipitate which was intensely active. This was collected on a -filter and weighed only 4 milligrams.</p> - -<p class='c006'>When plates of copper, tin or bismuth were dipped into an -hydrochloric acid solution of this active substance, the plates were -found to be covered with a very finely divided deposit. These -plates were intensely active, and produced marked photographic -and phosphorescent action. As an illustration of the enormous -activity of this deposit, Marckwald stated that a precipitate of -¹⁄₁₀₀ milligram on a copper plate, 4 square centimetres in area, -illuminated a zinc sulphide screen so brightly that it could be seen -by an audience of several hundred people.</p> - -<p class='c006'>The active substance of Marckwald is very closely allied in -chemical and radio-active properties to the polonium of Mme -Curie. Both active substances are separated with bismuth and -both give out only easily absorbed rays. The penetrating rays, -such as are given out by uranium, radium or thorium, are completely -absent.</p> - -<p class='c006'>There has been a considerable amount of discussion as to -whether the active substance obtained by Marckwald is identical -with that present in the polonium of Mme Curie. Marckwald -stated that his active substance did not sensibly diminish in -activity in the course of six months, but it is doubtful whether -the method of measurement used was sufficiently precise.</p> - -<p class='c006'>The writer has found that radio-tellurium of moderate activity, -prepared after Marckwald’s method and sold by Dr Sthamer of -Hamburg, undoubtedly loses its activity with time. The radio-tellurium -is obtained in the form of a thin radio-active deposit on -a polished bismuth rod or plate. A bismuth rod was found to -have lost half its activity in about 150 days, and a similar result -has been recorded by other observers.</p> - -<p class='c006'>The two substances are thus similar in both radio-active and -chemical properties, and there can be no reasonable doubt that the -active constituent present in each case is the same. The evidence -is discussed in detail in <a href='#chap11'>chapter <span class='fss'>XI</span></a> and it will there be shown that -the active substance present in the radio-tellurium of Marckwald is -a slow transformation product of radium.</p> -<p class='c005'><span class='pageno' id='Page_27'>27</span><a id='section022'></a> -<b>22. Radio-active lead.</b> Several observers early noticed -that the lead separated from pitchblende showed strong radio-active -properties, but considerable difference of opinion was -expressed in regard to the permanence of its activity. Elster and -Geitel<a id='r38' href='#f38' class='c012'><sup>[38]</sup></a> found that lead sulphate obtained from pitchblende was -very active, but they considered that the activity was probably due -to an admixture of radium or polonium with the lead, and, by -suitable chemical treatment, the lead sulphate was obtained in an -inactive state. Giesel<a id='r39' href='#f39' class='c012'><sup>[39]</sup></a> also separated some radio-active lead but -found that its activity diminished with the time. On the other -hand, Hofmann and Strauss<a id='r40' href='#f40' class='c012'><sup>[40]</sup></a> obtained lead from pitchblende whose -activity seemed fairly permanent. They state that the radio-active -lead resembled ordinary lead in most of its reactions, but -showed differences in the behaviour of the sulphide and sulphate. -The sulphate was found to be strongly phosphorescent. These -results of Hofmann and Strauss were subjected at the time of their -publication to considerable criticism, and there is no doubt that -the lead itself is not radio-active but contains a small quantity of -radio-active matter which is separated with it. In later work<a id='r41' href='#f41' class='c012'><sup>[41]</sup></a>, it -has been shown that radio-lead contains several radio-active constituents -which can be removed temporarily from it by suitable -chemical methods.</p> - -<p class='c006'>There can be no doubt that the lead separated from pitchblende -by certain methods does show considerable activity and that this -activity is fairly permanent. The radio-active changes occurring in -radio-lead are complicated and cannot be discussed with advantage -at this stage, but will be considered in detail in chapter <span class='fss'>XI</span>. It -will there be shown that the primary constituent present in lead -is a slow transformation product of radium. This substance then -slowly changes into the active constituent present in polonium, -which gives out only easily absorbed rays.</p> - -<p class='c006'>This polonium can be separated temporarily from the lead by -suitable chemical methods, but the radio-lead still continues to -produce polonium, so that a fresh supply may be obtained -<span class='pageno' id='Page_28'>28</span>from it, provided an interval of several months is allowed to -elapse.</p> - -<p class='c006'>It will be calculated later that in all probability the radio-lead -would lose half of its activity in an interval of 40 years.</p> - -<p class='c006'>The constituent present in radio-lead has not yet been separated, -but it will be shown that, in the pure state, it should have an -activity considerably greater than that of radium itself. Sufficient -attention has not yet been paid to this substance, for, separated -in a pure state, it should be as useful scientifically as radium. In -addition, since it is the parent of polonium, it should be possible to -obtain from it at any time a supply of very active polonium, in the -same way that a supply of the radium emanation can be obtained -at intervals from radium.</p> - -<p class='c006'>Hofmann and Strauss have observed a peculiar action of the -cathode rays on the active lead sulphate separated by them. They -state that the activity diminishes with time, but is recovered by -exposure of the lead for a short time to the action of cathode rays. -No such action is shown by the active lead sulphide. This effect -is due most probably to the action of the cathode rays in causing a -strong phosphorescence of the lead sulphate and has nothing to do -with the radio-activity proper of the substance.</p> -<p class='c005'><a id='section023'></a> -<b>23. Is thorium a radio-active element?</b> The similarity -of the chemical properties of actinium and thorium has led to the -suggestion at different times that the activity of thorium is not -due to thorium itself, but to the presence of a slight trace of -actinium. In view of the difference in the rate of decay of the -emanations of thorium and actinium, this position is not tenable. -If the activity of thorium were due to actinium, the two emanations, -as well as the other products obtained from these substances, -should have identical rates of decay. Since there is not the -slightest evidence that the rate of decay of activity of the various -products can be altered by chemical or physical agencies, we may -conclude with confidence that whatever radio-active substance is -responsible for the activity of thorium, it certainly is not actinium. -This difference in the rate of decay of the active products is of far -more weight in deciding the question whether two bodies contain -the same radio-active constituent than differences in chemical -<span class='pageno' id='Page_29'>29</span>behaviour, for it is quite probable that the active material in each -case may exist only in minute quantity in the matter under -examination, and, under such conditions, a direct chemical examination -in the first place is of little value.</p> - -<p class='c006'>Recent work of Hofmann and Zerban and of Baskerville, -however, certainly tends to show that the element thorium is itself -non-radio-active, and that the radio-activity observed in ordinary -thorium compounds is due to the admixture with it of an unknown -radio-active element. Hofmann and Zerban<a id='r42' href='#f42' class='c012'><sup>[42]</sup></a> made a systematic -examination of the radio-activity of thorium obtained from different -mineral sources. They found generally that thorium, obtained -from minerals containing a large percentage of uranium, were more -active than those obtained from minerals nearly free from uranium. -This indicates that the radio-activity observed in thorium may -possibly be due to a transformation product of uranium which is -closely allied chemically to thorium and is always separated with -it. A small quantity of thorium obtained from the mineral gadolinite -was found by Hofmann to be almost inactive, whether tested -by the electric or by the photographic method. Later Baskerville -and Zerban<a id='r43' href='#f43' class='c012'><sup>[43]</sup></a> found that thorium obtained from a Brazilian mineral -was practically devoid of activity.</p> - -<p class='c006'>In this connection the recent work of Baskerville on the complexity -of ordinary thorium is of interest. By special chemical -methods, he succeeded in separating two new and distinct -substances from thorium, which he has named carolinium and -berzelium. Both of these substances are strongly radio-active, and -it thus seems probable that the active constituent observed in -ordinary thorium may be due to one of these elements.</p> - -<p class='c006'>If, as we have suggested, thorium itself is not active, it is -certainly a matter of surprise that ordinary commercial thorium -and the purest chemical preparations show about the same activity. -Such a result indicates that the methods of purification have not -removed any of the radio-active constituent originally present.</p> - -<p class='c006'>Whatever the radio-active constituent in thorium may ultimately -prove to be, it is undoubtedly not radium nor actinium nor -any of the known radio-active substances.</p> - -<p class='c006'><span class='pageno' id='Page_30'>30</span>In later chapters, the radio-activity of thorium will, for simplicity, -be discussed on the assumption that thorium is itself a radio-active -element. The analysis of the changes which occur will thus -not refer to thorium itself but to the primary radio-active -substance usually found associated with it. The conclusions to be -drawn from an examination of the radio-active processes are for -the most part independent of whether thorium is itself radio-active -or whether the radio-activity is due to an unknown element. If -thorium is not radio-active itself, it is not possible to draw any -conclusions upon the question of the duration of the primary radio-activity -associated with it. Such a deduction cannot be made -until the quantity of the radio-active element present in thorium -has been definitely determined.</p> -<p class='c005'><b>24.</b> If elements heavier than uranium exist, it is probable that -they will be radio-active. The extreme delicacy of radio-activity -as a means of chemical analysis would enable such elements to -be recognized even if present in infinitesimal quantities. It is -probable that considerably more than the three or four radio-elements -at present recognized exist in minute quantity, and that -the number at present known will be augmented in the future. -In the first stage of the search, a purely chemical examination is -of little value, for it is not probable that the new element should -exist in sufficient quantity to be detected by chemical or spectroscopic -analysis. The main criteria of importance are the existence -or absence of distinctive radiations or emanations, and the permanence -of the radio-activity. The discovery of a radio-active emanation -with a rate of decay different from those already known would -afford strong evidence that a new radio-active body was present. -The presence of either thorium or radium in matter can very -readily be detected by observing the rate of decay of the emanations -given out by them. When once the existence of a new -radio-element has been inferred by an examination of its radio-active -properties, chemical methods of separation can be devised, -the radiating or emanating property being used as a guide in -qualitative and quantitative analysis.</p> - -<div class='chapter'> - <span class='pageno' id='Page_31'>31</span> - <h2 id='chap02' class='c004'>CHAPTER II. <br> IONIZATION THEORY OF GASES.</h2> -</div> -<p class='c005'><b>25. Ionization of gases by radiation.</b> The most important -property possessed by the radiations from radio-active bodies is -their power of discharging bodies whether positively or negatively -electrified. As this property has been made the basis of a method -for an accurate quantitative analysis and comparison of the -radiations, the variation of the rate of discharge under different -conditions and the processes underlying it will be considered in -some detail.</p> - -<p class='c006'>In order to explain the similar discharging power of Röntgen -rays, the theory<a id='r44' href='#f44' class='c012'><sup>[44]</sup></a> has been put -forward that the rays produce -positively and negatively -charged carriers throughout -the volume of the gas surrounding -the charged body, and -that the rate of production is -proportional to the intensity -of the radiation. These carriers, -or ions<a id='r45' href='#f45' class='c012'><sup>[45]</sup></a> as they have been termed, move with a uniform velocity -through the gas under a constant electric field, and their velocity -varies directly as the strength of the field.</p> - -<div id='fig001' class='figcenter id002'> -<img src='images/fig-001.png' alt='Fig. 1.' class='ig001'> -<div class='ic002'> -<p>Fig. 1.</p> -</div> -</div> - -<p class='c006'>Suppose we have a gas between two metal plates <i>A</i> and <i>B</i> -(<a href='#fig001'>Fig. 1</a>) exposed to the radiation, and that the plates are kept -at a constant difference of potential. A definite number of ions -will be produced per second by the radiation, and the number -<span class='pageno' id='Page_32'>32</span>produced will depend in general upon the nature and pressure of -the gas. In the electric field the positive ions travel towards the -negative plate, and the negative ions towards the positive, and -consequently a current will pass through the gas. Some of the -ions will also recombine, the rate of recombination being proportional -to the square of the number present. For a given intensity -of radiation, the current passing through the gas will increase at -first with the potential difference between the plates, but it will -reach a limit when all the ions are removed by the electric field -before any recombination occurs.</p> - -<p class='c006'>This theory accounts also for all the characteristic properties of -gases made conducting by the rays from active substances, though -there are certain differences observed between the conductivity -phenomena produced by active substances and by <i>X</i> rays. These -differences are for the most part the result of unequal absorption -of the two types of rays. Unlike Röntgen rays, a large proportion -of the radiation from active bodies consists of rays which are -absorbed in their passage through a few centimetres of air. The -ionization of the gas is thus not uniform, but falls off rapidly with -increase of distance from the active substance.</p> -<p class='c005'><b>26. Variation of the current with voltage.</b> Suppose that -a layer of radio-active matter is spread uniformly on the lower of -two horizontal plates <i>A</i> and <i>B</i> (<a href='#fig001'>Fig. 1</a>). The lower plate <i>A</i> is -connected with one pole of a battery of cells the other pole of which -is connected with earth. The plate <i>B</i> is connected with one pair of -quadrants of an electrometer, the other pair being connected with -earth.</p> - -<p class='c006'>The current<a id='r46' href='#f46' class='c012'><sup>[46]</sup></a> between the plates, determined by the rate of -movement of the electrometer needle, is observed at first to increase -rapidly with the voltage, then more slowly, finally reaching -a value which increases very slightly with a large increase in the -voltage. This, as we have indicated, is simply explained on the -ionization theory.</p> - -<p class='c006'>The radiation produces ions at a constant rate, and, before the -electric field is applied, the number per unit volume increases -<span class='pageno' id='Page_33'>33</span>until the rate of production of fresh ions is exactly balanced by the -recombination of the ions already produced. On application of a -small electric field, the positive ions travel to the negative electrode -and the negative to the positive.</p> - -<p class='c006'>Since the velocity of the ions between the plates is directly -proportional to the strength of the electric field, in a weak field -the ions take so long to travel between the electrodes that most of -them recombine on the way.</p> - -<p class='c006'>The current observed is consequently small. With increase of -the voltage there is an increase of speed of the ions and a smaller -number recombine. The current consequently increases, and will -reach a maximum value when the electric field is sufficiently -strong to remove all the ions before appreciable recombination has -occurred. The value of the current will then remain constant even -though the voltage is largely increased.</p> - -<p class='c006'>This maximum current will be called the “saturation” current, -and the value of the potential difference required to give this -maximum current, the “saturation <span class='fss'>P.D.</span>”<a id='r47' href='#f47' class='c012'><sup>[47]</sup></a></p> - -<p class='c006'>The general shape of the current-voltage curve is shown in -<a href='#fig002'>Fig. 2</a>, where the ordinates represent current and the abscissae -volts.</p> - -<div id='fig002' class='figcenter id006'> -<img src='images/fig-002.png' alt='Fig. 2.' class='ig001'> -<div class='ic002'> -<p>Fig. 2.</p> -</div> -</div> - -<p class='c006'><span class='pageno' id='Page_34'>34</span>Although the variation of the current with voltage depends -only on the velocity of the ions and their rate of recombination, -the full mathematical analysis is intricate, and the equations, -expressing the relation between current and voltage, are only -integrable for the case of uniform ionization. The question is complicated -by the inequality in the velocity of the ions and by the -disturbance of the potential gradient between the plates by the -movement of the ions. J. J. Thomson<a id='r48' href='#f48' class='c012'><sup>[48]</sup></a> has worked out the case -for uniform production of ions between two parallel plates, and has -found that the relation between the current <i>i</i> and the potential -difference <i>V</i> applied is expressed by</p> - -<div class='calc'> - -<div class='lg-container-b c017'> - <div class='linegroup'> - <div class='group'> - <div class='line'><i>Ai<sup>2</sup> + Bi = V</i></div> - </div> - </div> -</div> - -</div> - -<p class='c018'>where <i>A</i> and <i>B</i> are constants for a definite intensity of radiation -and a definite distance between the plates.</p> - -<div id='fig003' class='figcenter id006'> -<img src='images/fig-003.png' alt='Fig. 3.' class='ig001'> -<div class='ic002'> -<p>Fig. 3.</p> -</div> -</div> - -<p class='c006'>In certain cases of unsymmetrical ionization, which arise in the -study of the radiations from active bodies, the relation between -current and voltage is very different from that expressed by -<span class='pageno' id='Page_35'>35</span>the above equation. Some of these cases will be considered in -<a href='#section047'>section <b>47</b></a>.</p> -<p class='c005'><b>27.</b> The general shape of the current-voltage curves for gases -exposed to the radiations from active bodies is shown in <a href='#fig003'>Fig. 3</a>.</p> - -<p class='c006'>This curve was obtained for ·45 grams of impure radium -chloride, of activity 1000 times that of uranium, spread over an -area of 33 sq. cms. on the lower of two large parallel plates, -4·5 cms. apart. The maximum value of the current observed, -which is taken as 100, was -1·2 × 10<sup>-8</sup> -amperes, the current for low -voltages was nearly proportional to the voltage, and about 600 -volts between the plates was required to ensure approximate -saturation.</p> - -<p class='c006'>In dealing with slightly active bodies like uranium or thorium, -approximate saturation is obtained for much lower voltages. -Tables I. and II. show the results for the current between two -parallel plates distant 0·5 cms. and 2·5 cms. apart respectively, when -one plate was covered with a thin uniform layer of uranium oxide.</p> - -<div class='nf-center-c1'> -<div class='nf-center c007'> - <div><span class='sc'>Table I.</span></div> - <div class='c000'>0·5 cms. apart</div> - </div> -</div> - -<table class='table3' > -<colgroup> -<col class='colwidth41'> -<col class='colwidth58'> -</colgroup> - <tr> - <th class='c015'>Volts</th> - <th class='c016'>Current</th> - </tr> - <tr> - <td class='c015'>·125</td> - <td class='c016'>18</td> - </tr> - <tr> - <td class='c015'>·25</td> - <td class='c016'>36</td> - </tr> - <tr> - <td class='c015'>·5</td> - <td class='c016'>55</td> - </tr> - <tr> - <td class='c015'>1</td> - <td class='c016'>67</td> - </tr> - <tr> - <td class='c015'>2</td> - <td class='c016'>72</td> - </tr> - <tr> - <td class='c015'>4</td> - <td class='c016'>79</td> - </tr> - <tr> - <td class='c015'>8</td> - <td class='c016'>85</td> - </tr> - <tr> - <td class='c015'>16</td> - <td class='c016'>88</td> - </tr> - <tr> - <td class='c015'>100</td> - <td class='c016'>94</td> - </tr> - <tr> - <td class='c015'>335</td> - <td class='c016'>100</td> - </tr> -</table> - -<div class='nf-center-c1'> -<div class='nf-center c007'> - <div><span class='sc'>Table II.</span></div> - <div class='c000'>2·5 cms. apart</div> - </div> -</div> - -<table class='table3' > -<colgroup> -<col class='colwidth41'> -<col class='colwidth58'> -</colgroup> - <tr> - <th class='c015'>Volts</th> - <th class='c016'>Current</th> - </tr> - <tr> - <td class='c015'>·5</td> - <td class='c016'>7·3</td> - </tr> - <tr> - <td class='c015'>1</td> - <td class='c016'>14</td> - </tr> - <tr> - <td class='c015'>2</td> - <td class='c016'>27</td> - </tr> - <tr> - <td class='c015'>4</td> - <td class='c016'>47</td> - </tr> - <tr> - <td class='c015'>8</td> - <td class='c016'>64</td> - </tr> - <tr> - <td class='c015'>16</td> - <td class='c016'>73</td> - </tr> - <tr> - <td class='c015'>37·5</td> - <td class='c016'>81</td> - </tr> - <tr> - <td class='c015'>112</td> - <td class='c016'>90</td> - </tr> - <tr> - <td class='c015'>375</td> - <td class='c016'>97</td> - </tr> - <tr> - <td class='c015'>800</td> - <td class='c016'>100</td> - </tr> -</table> - -<p class='c006'>The results are shown graphically in <a href='#fig004'>Fig. 4</a>.</p> - -<div id='fig004' class='figcenter id006'> -<img src='images/fig-004.png' alt='Fig. 4.' class='ig001'> -<div class='ic002'> -<p>Fig. 4.</p> -</div> -</div> - -<p class='c006'>From the above tables it is seen that the current at first increases -nearly in proportion to the voltage. There is no evidence -of complete saturation, although the current increases very slowly -for large increases of voltage. For example, in Table I. a change of -voltage from ·125 to ·25 volts increases the current from 18 to -36% of the maximum, while a change of voltage from 100 to 335 -volts increases the current only 6%. The variation of the current -per volt (assumed uniform between the range of voltages considered) -is thus about 5000 times greater for the former change.</p> - -<p class='c006'><span class='pageno' id='Page_36'>36</span>Taking into consideration the early part of the curves, the -current does not reach a practical maximum as soon as would be -expected on the simple ionization theory. It seems probable that -the slow increase with the large voltages is due either to an action -of the electric field on the rate of production of ions, or to the -difficulty of removing the ions produced near the surface of the -uranium before recombination. It is possible that the presence -of a strong electric field may assist in the separation of ions which -otherwise would not initially escape from the sphere of one -another’s attraction. From the data obtained by Townsend for -the conditions of production of fresh ions at low pressures by the -movement of ions through the gas, it seems that the increase of -current cannot be ascribed to an action of the moving ions in the -further ionization of the gas.</p> -<p class='c005'><a id='section028'></a> -<b>28.</b> The equation expressing the relation between the current -and the voltage is very complicated even in the case of a uniform -rate of production of ions between the plates. An approximate -<span class='pageno' id='Page_37'>37</span>theory, which is of utility in interpreting the experimental results, -can however be simply deduced if the disturbance of the potential -gradient is disregarded, and the ionization assumed uniform between -the plates.</p> - -<p class='c006'>Suppose that the ions are produced at a constant rate <i>q</i> per -cubic centimetre per second in the gas between parallel plates -distant <i>l</i> cms. from each other. When no electric field is applied, -the number <i>N</i> present per c.c., when there is equilibrium between -the rates of production and recombination, is given by</p> -<div class='calc'> - -<div class='lg-container-b c017'> - <div class='linegroup'> - <div class='group'> - <div class='line'><i>q</i> = α<i>N</i><sup>2</sup>,</div> - </div> - </div> -</div> - -</div> -<p class='c018'>where α is a constant.</p> - -<p class='c006'>If a small potential difference <i>V</i> is applied, which gives only a -small fraction of the maximum current, and consequently has not -much effect on the value of <i>N</i>, the current <i>i</i> per sq. cm. of the -plate, is given by</p> - -<div class='calc'> - -<div class='lg-container-b c017'> - <div class='linegroup'> - <div class='group'> - <div class='line in6'><i>NeuV</i></div> - <div class='line'><i>i</i> = -----</div> - <div class='line in7'><i>l</i></div> - </div> - </div> -</div> - -</div> - -<p class='c018'>where <i>u</i> is the sum of the velocity of the ions for unit potential -gradient, and <i>e</i> is the charge carried by an ion.</p> - -<div class='calc'> - -<div class='lg-container-b c017'> - <div class='linegroup'> - <div class='group'> - <div class='line'> <i>uV</i></div> - <div class='line'>-----</div> - <div class='line'> <i>l</i></div> - </div> - </div> -</div> - -</div> - -<p class='c018'>is the velocity -of the ions in the electric field of strength</p> - -<div class='calc'> - -<div class='lg-container-b c017'> - <div class='linegroup'> - <div class='group'> - <div class='line'> <i>V</i></div> - <div class='line'>----</div> - <div class='line'> <i>l</i></div> - </div> - </div> -</div> - -</div> - -<p class='c018'>The number of ions produced per second in a prism of length <i>l</i> -and unit area of cross-section is <i>ql</i>. The maximum or saturation -current <i>I</i> per sq. cm. of the plate is obtained when all of these -ions are removed to the electrodes before any recombination has -occurred.</p> - -<p class='c006'>Thus</p> -<div class='calc'> - -<div class='lg-container-b c017'> - <div class='linegroup'> - <div class='group'> - <div class='line'><i>I</i> = <i>q . l . e</i>,</div> - </div> - </div> -</div> - -</div> - -<p class='c018'>and</p> - -<div class='figcenter id010'> -<img src='images/form-001.png' alt='Formula.' class='ig001'> -</div> - -<p class='c006'>This equation expresses the fact previously noted that, for small -voltages, the current <i>i</i> is proportional to <i>V</i>.</p> - -<p class='c006'>Let</p> - -<div class='calc'> - -<div class='lg-container-b c017'> - <div class='linegroup'> - <div class='group'> - <div class='line'><i>i/I</i> = ρ,</div> - </div> - </div> -</div> - -</div> - -<p class='c018'>then</p> - -<div class='figcenter id010'> -<img src='images/form-002.png' alt='Formula.' class='ig001'> -</div> - -<p class='c006'><span class='pageno' id='Page_38'>38</span>Now the greater the value of <i>V</i> required to obtain a given -value of ρ (supposed small compared with unity), the greater the -potential required to produce saturation.</p> - -<p class='c006'>It thus follows from the equation that:</p> - -<p class='c006'>(1) For a given intensity of radiation, the saturation <span class='fss'>P.D.</span> -increases with the distance between the plates. In the equation, -for small values of ρ, <i>V</i> varies as -<i>l</i><sup>2</sup>. -This is found to be the case -for uniform ionization, but it only holds approximately for non-uniform -ionization.</p> - -<p class='c006'>(2) For a given distance between the plates, the saturation -<span class='fss'>P.D.</span> is greater, the greater the intensity of ionization between the -plates. This is found to be the case for the ionization produced -by radio-active substances. With a very active substance like -radium, the ionization produced is so intense that very large -voltages are required to produce approximate saturation. On the -other hand, only a fraction of a volt per cm. is necessary to produce -saturation in a gas where the ionization is very slight, for example, -in the case of the natural ionization observed in a closed vessel, -where no radio-active substances are present.</p> - -<p class='c006'>For a given intensity of radiation, the saturation <span class='fss'>P.D.</span> decreases -rapidly with the lowering of the pressure of the gas. This is due -to two causes operating in the same direction, viz. a decrease in -the intensity of the ionization and an increase in the velocity of -the ions. The ionization varies directly as the pressure, while the -velocity varies inversely as the pressure. This will obviously have -the effect of causing more rapid saturation, since the rate of -recombination is slower and the time taken for the ions to travel -between the electrodes is less.</p> - -<p class='c006'>The saturation curves observed for the gases hydrogen and -carbon dioxide<a id='r49' href='#f49' class='c012'><sup>[49]</sup></a> are very similar in shape to those obtained for air. -For a given intensity of radiation, saturation is more readily -obtained in hydrogen than in air, since the ionization is less than -in air while the velocity of the ions is greater. Carbon dioxide on -the other hand requires a greater <span class='fss'>P.D.</span> to produce saturation than -does air, since the ionization is more intense and the velocity of -the ions less than in air.</p> -<p class='c005'><span class='pageno' id='Page_39'>39</span><a id='section029'></a> -<b>29.</b> Townsend<a id='r50' href='#f50' class='c012'><sup>[50]</sup></a> has shown that, for low pressures, the variation -of the current with the voltage is very different from that observed -at atmospheric pressure. If the increase of current with the voltage -is determined for gases, exposed to Röntgen rays, at a pressure of -about 1 mm. of mercury, it is found that for small voltages the -ordinary saturation curve is obtained; but when the voltage -applied increases beyond a certain value, depending on the pressure -and nature of the gas and the distance between the electrodes, the -current commences to increase slowly at first but very rapidly as -the voltage is raised to the sparking value. The general shape of -the current curve is shown in <a href='#fig005'>Fig. 5</a>.</p> - -<div id='fig005' class='figcenter id007'> -<img src='images/fig-005.png' alt='Fig. 5.' class='ig001'> -<div class='ic002'> -<p>Fig. 5.</p> -</div> -</div> - -<p class='c006'>The portion <i>OAB</i> of the curve corresponds to the ordinary -saturation curve. At the point <i>B</i> the current commences to -increase. This increase of current has been shown to be due to -the action of the negative ions at low pressures in producing fresh -ions by collision with the molecules in their path. The increase of -current is not observed in air at a pressure above 30 mms. until -the <span class='fss'>P.D.</span> is increased nearly to the value required to produce a -spark. This production of ions by collision is considered in more -detail in <a href='#section041'>section 41</a>.</p> -<p class='c005'><span class='pageno' id='Page_40'>40</span><a id='section030'></a> -<b>30. Rate of recombination of the ions.</b> A gas ionized -by the radiation preserves its conducting power for some time -after it is removed from the presence of the active body. A -current of air blown over an active body will thus discharge an -electrified body some distance away. The duration of this after -conductivity can be examined very conveniently in an apparatus -similar to that shown in <a href='#fig006'>Fig. 6</a>.</p> - -<div id='fig006' class='figcenter id004'> -<img src='images/fig-006.png' alt='Fig. 6.' class='ig001'> -<div class='ic002'> -<p>Fig. 6.</p> -</div> -</div> - -<p class='c006'>A dry current of air or any other gas is passed at a constant -rate through a long metal tube <i>TL</i>. After passing through a -quantity of cotton-wool to remove dust particles, the current of air -passes over a vessel <i>T</i> containing a radio-active body such as -uranium, which does not give off a radio-active emanation. By -means of insulated electrodes <i>A</i> and <i>B</i>, charged to a suitable -potential, the current between the tube and one of these electrodes -can be tested at various points along the tube.</p> - -<p class='c006'>A gauze screen, placed over the cross-section of the tube at <i>D</i>, -serves to prevent any direct action of the electric field in abstracting -ions from the neighbourhood of <i>T</i>.</p> - -<p class='c006'>If the electric field is sufficiently strong, all the ions travel -in to the electrodes at <i>A</i>, and no current is observed at the electrode -<i>B</i>. If the current is observed successively at different distances -along the tube, all the electrodes except the one under consideration -being connected to earth, it is found that the current diminishes -with the distance from the active body. If the tube is of fairly -wide bore, the loss of the ions due to diffusion is small, and the -decrease in conductivity of the gas is due to recombination of the -ions alone.</p> - -<p class='c006'>On the ionization theory, the number <i>dn</i> of ions per unit volume -which recombine in the time <i>dt</i> is proportional to the square of -the number present. Thus</p> - -<div class='calc'> - -<div class='lg-container-b c017'> - <div class='linegroup'> - <div class='group'> - <div class='line'> <i>dn</i></div> - <div class='line in1'>--- = α<i>n²</i>,</div> - <div class='line'> <i>dt</i></div> - </div> - </div> -</div> - -</div> - -<p class='c018'>where α is a constant.</p> - -<p class='c006'><span class='pageno' id='Page_41'>41</span>Integrating this equation,</p> - -<div class='calc'> - -<div class='lg-container-b c017'> - <div class='linegroup'> - <div class='group'> - <div class='line in1'>1 1</div> - <div class='line'>--- – --- = α<i>t</i>,</div> - <div class='line'> <i>n</i> <i>N</i></div> - </div> - </div> -</div> - -</div> - -<p class='c018'>if <i>N</i> is the initial number of ions, and <i>n</i> the number after a time <i>t</i>.</p> - -<p class='c006'>The experimental results obtained<a id='r51' href='#f51' class='c012'><sup>[51]</sup></a> have been shown to agree -very well with this equation.</p> - -<p class='c006'>In an experiment similar to that illustrated in <a href='#fig006'>Fig. 6</a>, using -uranium oxide as a source of ionization, it was found that half the -number of ions present in the gas recombined in 2·4 seconds, and -that at the end of 8 seconds one-fourth of the ions were still -uncombined.</p> - -<p class='c006'>Since the rate of recombination is proportional to the square of -the number present, the time taken for half of the ions present in -the gas to recombine decreases very rapidly with the intensity of -the ionization. If radium is used, the ionization is so intense that -the rate of recombination is extremely rapid. It is on account of -this rapidity of recombination that large voltages are necessary to -produce saturation in the gases exposed to very active preparations -of radium.</p> - -<p class='c006'>The value of α, which may be termed the <i>coefficient of recombination</i>, -has been determined in absolute measure by Townsend<a id='r52' href='#f52' class='c012'><sup>[52]</sup></a>, -McClung<a id='r53' href='#f53' class='c012'><sup>[53]</sup></a> and Langevin<a id='r54' href='#f54' class='c012'><sup>[54]</sup></a> by different experimental methods but -with very concordant results. Suppose, for example, with the -apparatus of <a href='#fig006'>Fig. 6</a>, the time <i>T</i>, taken for half the ions to recombine -after passing by the electrode <i>A</i>, has been determined experimentally. -Then</p> - -<div class='calc'> - -<div class='lg-container-b c017'> - <div class='linegroup'> - <div class='group'> - <div class='line in2'>1</div> - <div class='line'>---- = α<i>T</i>,</div> - <div class='line in2'><i>N</i></div> - </div> - </div> -</div> - -</div> - -<p class='c018'>where <i>N</i> is the number of ions per c.c. -present at <i>A</i>. If the saturation current <i>i</i> is determined at the -electrode <i>A</i>, <i>i = NVe</i>, where <i>e</i> is the charge on an ion and <i>V</i> is the -volume of uniformly ionized gas carried by the electrode <i>A</i> per -second. Then</p> - -<div class='calc'> - -<div class='lg-container-b c017'> - <div class='linegroup'> - <div class='group'> - <div class='line in4'> <i>Ve</i></div> - <div class='line'>α = ---- .</div> - <div class='line in4'> <i>iT</i></div> - </div> - </div> -</div> - -</div> - -<p class='c018'>The following table shows the value of α obtained for different -gases.</p> - -<div class='nf-center-c1'> -<div class='nf-center c007'> - <div><span class='pageno' id='Page_42'>42</span><i>Value of</i> α.</div> - </div> -</div> - -<table class='table4' > -<colgroup> -<col class='colwidth28'> -<col class='colwidth23'> -<col class='colwidth23'> -<col class='colwidth23'> -</colgroup> - <tr> - <th class='c013'>Gas</th> - <th class='c013'>Townsend</th> - <th class='c013'>McClung</th> - <th class='c014'>Langevin</th> - </tr> - <tr> - <td class='c013'> </td> - <td class='c013'> </td> - <td class='c013'> </td> - <td class='c014'> </td> - </tr> - <tr> - <td class='c013'>Air</td> - <td class='c013'>3420 × <i>e</i></td> - <td class='c013'>3384 × <i>e</i></td> - <td class='c014'>3200 × <i>e</i></td> - </tr> - <tr> - <td class='c013'>Carbon Dioxide</td> - <td class='c013'>3500 × <i>e</i></td> - <td class='c013'>3492 × <i>e</i></td> - <td class='c014'>3400 × <i>e</i></td> - </tr> - <tr> - <td class='c013'>Hydrogen</td> - <td class='c013'>3020 × <i>e</i></td> - <td class='c013'> </td> - <td class='c014'> </td> - </tr> -</table> - -<p class='c006'>The latest determination of the value of <i>e</i> (see <a href='#section036'>section 36</a>) is -3·4 × 10<sup>-10</sup> -<span class='fss'>E.S.</span> units; thus -α = 1·1 × 10<sup>-6</sup>.</p> - -<p class='c006'>Using this value, it can readily be shown from the equation of -recombination that, if -10<sup>6</sup> -ions are present per c.c., half of them -recombine in about 0·9 sec. and 99% in 90 secs.</p> - -<p class='c006'>McClung (<i>loc. cit.</i>) showed that the value of α was approximately -independent of the pressure between ·125 and three atmospheres. -In later observations, Langevin has found that the value of α -decreases rapidly when the pressure is lowered below the limits -used by McClung.</p> -<p class='c005'><a id='section031'></a> -<b>31.</b> In experiments on recombination it is essential that the -gas should be free from dust or other suspended particles. In -dusty air, the rate of recombination is much more rapid than in -dust-free air, as the ions diffuse rapidly to the comparatively large -dust particles distributed throughout the gas. The effect of the -suspension of small particles in a conducting gas is very well -illustrated by an experiment of Owens<a id='r55' href='#f55' class='c012'><sup>[55]</sup></a>. If tobacco smoke is -blown between two parallel plates as in <a href='#fig001'>Fig. 1</a>, the current at once -diminishes to a small fraction of its former value, although a <span class='fss'>P.D.</span> -is applied sufficient to produce saturation under ordinary conditions. -A much larger voltage is then necessary to produce -saturation. If the smoke particles are removed by a stream of air, -the current returns at once to its original value.</p> -<p class='c005'><b>32. Mobility of the ions.</b> Determinations of the mobility -of the ions, <i>i.e.</i> the velocity of the ions under a potential gradient -of 1 volt per cm., have been made by Rutherford<a id='r56' href='#f56' class='c012'><sup>[56]</sup></a>, Zeleny<a id='r57' href='#f57' class='c012'><sup>[57]</sup></a>, and -Langevin<a id='r58' href='#f58' class='c012'><sup>[58]</sup></a> for gases exposed to Röntgen rays. Although widely -different methods have been employed, the results have been very -concordant, and fully support the view that the ions move with a -<span class='pageno' id='Page_43'>43</span>velocity proportional to the strength of the field. On the application -of an electric field, the ions almost instantly attain the -velocity corresponding to the field and then move with a uniform -speed.</p> - -<p class='c006'>Zeleny<a id='r59' href='#f59' class='c012'><sup>[59]</sup></a> first drew attention to the fact that the positive and -negative ions had different velocities. The velocity of the negative -ion is always greater than that of the positive, and varies with the -amount of water vapour present in the gas.</p> - -<p class='c006'>The results, previously discussed, of the variation of the current -with voltage and of the rate of recombination of the ions do not of -themselves imply that the ions produced in gases by the radiations -from active bodies are of the same size as those produced by -Röntgen rays under similar conditions. They merely show that -the conductivity under various conditions can be satisfactorily -explained by the view that charged ions are produced throughout -the volume of the gas. The same general relations would be -observed if the ions differed considerably in size and velocity from -those produced by Röntgen rays. The most satisfactory method -of determining whether the ions are identical in the two cases is -to determine the velocity of the ions under similar conditions.</p> - -<p class='c006'>In order to compare the velocity of the ions<a id='r60' href='#f60' class='c012'><sup>[60]</sup></a>, the writer has -used an apparatus similar to that shown in <a href='#fig006'>Fig. 6</a> on p. <a href='#Page_40'>40</a>.</p> - -<p class='c006'>The ions were carried with a rapid constant stream of air -past the charged electrode <i>A</i>, and the conductivity of the gas tested -immediately afterwards at an electrode <i>B</i>, which was placed close -to <i>A</i>. The insulated electrodes <i>A</i> and <i>B</i> were fixed centrally in -the metal tube <i>L</i>, which was connected with earth.</p> - -<p class='c006'>For convenience of calculation, it is assumed that the electric -field between the cylinders is the same as if the cylinders were -infinitely long.</p> - -<p class='c006'>Let <i>a</i> and <i>b</i> be the radii of the electrode <i>A</i>, and of the tube <i>L</i> -respectively, and let <i>V</i> = potential of <i>A</i>.</p> - -<p class='c006'>The electromotive intensity <i>X</i> (without regard to sign) at a -distance <i>r</i> from the centre of the tube is given by</p> - -<div class='figcenter id010'> -<img src='images/form-003.png' alt='Formula.' class='ig001'> -</div> - -<p class='c006'><span class='pageno' id='Page_44'>44</span>Let -<i>u</i><sub>1</sub> and <i>u</i><sub>2</sub> -be the velocities of the positive and negative -ions for a potential gradient of 1 volt per cm. If the velocity is -proportional to the electric force at any point, the distance <i>dr</i> -traversed by the negative ion in the time <i>dt</i> is given by</p> - -<p class='c006'><i>dr</i> = <i>Xu</i><sub>2</sub> <i>dt</i>,</p> - -<p class='c006'>or</p> - -<div class='figcenter id010'> -<img src='images/form-004.png' alt='Formula.' class='ig001'> -</div> - -<p class='c006'>Let -<i>r</i><sub>2</sub> -be the greatest distance measured from the axis of the -tube from which the negative ion can just reach the electrode <i>A</i> -in the time <i>t</i> taken for the air to pass along the electrode.</p> - -<p class='c006'>Then</p> - -<div class='figcenter id005'> -<img src='images/form-005.png' alt='Formula.' class='ig001'> -</div> - -<p class='c006'>If -ρ<sub>2</sub> -be the ratio of the number of the negative ions that reach -the electrode <i>A</i> to the total number passing by, then</p> - -<div class='figcenter id010'> -<img src='images/form-006.png' alt='Formula.' class='ig001'> -</div> - -<p class='c006'>Therefore</p> - -<div id='equation001' class='figcenter id005'> -<img src='images/equation-001.png' alt='Equation 1.' class='ig001'> -<div class='ic002'> -<p>Equation 1.</p> -</div> -</div> - -<p class='c006'>Similarly the ratio -ρ<sub>1</sub> -of the number of positive ions that give -up their charge to the external cylinder to the total number of -positive ions is given by</p> - -<div class='figcenter id009'> -<img src='images/form-007.png' alt='Formula.' class='ig001'> -</div> - -<p class='c006'>In the above equations it is assumed that the current of air is -uniform over the cross-section of the tube, and that the ions are -uniformly distributed over the cross-section; also, that the movement -of the ions does not appreciably disturb the electric field. -Since the value of <i>t</i> can be calculated from the velocity of the -current of air and the length of the electrode, the values of the -velocities of the ions under unit potential gradient can at once be -determined.</p> - -<p class='c006'>The <a href='#equation001'>equation (1)</a> shows that -ρ<sub>2</sub> -is proportional to <i>V</i>,—<i>i.e.</i> that -<span class='pageno' id='Page_45'>45</span>the rate of discharge of the electrode <i>A</i> varies directly as the -potential of <i>A</i>, provided that the value of <i>V</i> is not large enough to -remove all the ions from the gas as it passes by the electrode. -This was found experimentally to be the case.</p> - -<p class='c006'>In the comparison of the velocities, the potential <i>V</i> was adjusted -to such a value that -ρ<sub>2</sub> -was about one half, when uranium oxide -was placed in the tube at <i>L</i>. The active substance was then -removed, and an aluminium cylinder substituted for the brass -tube. X rays were allowed to fall on the centre of this aluminium -cylinder, and the strength of the rays adjusted to give about the -same conductivity to the gas as the uranium had done. Under -these conditions the value of -ρ<sub>2</sub> -was found to be the same as for -the first experiment.</p> - -<p class='c006'>This experiment shows conclusively that the ions produced -by Röntgen rays and by uranium move with the same velocity -and are probably identical in all respects. The method described -above is not very suitable for an accurate determination of the -velocities, but gave values for the positive ions of about 1·4 cms. -per second per volt per centimetre, and slightly greater values for -the negative ions.</p> -<p class='c005'><b>33.</b> The most accurate determinations of the mobility of the -ions produced by Röntgen rays have been made by Zeleny<a id='r61' href='#f61' class='c012'><sup>[61]</sup></a> and -Langevin<a id='r62' href='#f62' class='c012'><sup>[62]</sup></a>. Zeleny used a method similar in principle to that -explained above. His results are shown in the following table, -where -<i>K</i><sub>1</sub> -is the mobility of the positive ion and -<i>K</i><sub>2</sub> -that of the -negative ion.</p> - -<table class='table1' > -<colgroup> -<col class='colwidth25'> -<col class='colwidth15'> -<col class='colwidth15'> -<col class='colwidth23'> -<col class='colwidth19'> -</colgroup> - <tr> - <th class='c013'>Gas</th> - <th class='c015'><i>K</i><sub>1</sub></th> - <th class='c015'><i>K</i><sub>2</sub></th> - <th class='c015'><i>K</i><sub>2</sub>/<i>K</i><sub>1</sub></th> - <th class='c016'>Temperature</th> - </tr> - <tr> - <td class='c013'>Air, dry</td> - <td class='c015'>1·36</td> - <td class='c015'>1·87</td> - <td class='c015'>1·375</td> - <td class='c016'>13°·5 C.</td> - </tr> - <tr> - <td class='c013'>„ moist</td> - <td class='c015'>1·37</td> - <td class='c015'>1·51</td> - <td class='c015'>1·10</td> - <td class='c016'>14°</td> - </tr> - <tr> - <td class='c013'>Oxygen, dry</td> - <td class='c015'>1·36</td> - <td class='c015'>1·80</td> - <td class='c015'>1·32</td> - <td class='c016'>17°</td> - </tr> - <tr> - <td class='c013'>„ moist</td> - <td class='c015'>1·29</td> - <td class='c015'>1·52</td> - <td class='c015'>1·18</td> - <td class='c016'>16°</td> - </tr> - <tr> - <td class='c013'>Carbon dioxide, dry</td> - <td class='c015'>0·76</td> - <td class='c015'>0·81</td> - <td class='c015'>1·07</td> - <td class='c016'>17°·5</td> - </tr> - <tr> - <td class='c013'>„ „ moist</td> - <td class='c015'>0·81</td> - <td class='c015'>0·75</td> - <td class='c015'>0·915</td> - <td class='c016'>17°</td> - </tr> - <tr> - <td class='c013'>Hydrogen, dry</td> - <td class='c015'>6·70</td> - <td class='c015'>7·95</td> - <td class='c015'>1·15</td> - <td class='c016'>20°</td> - </tr> - <tr> - <td class='c013'>„ moist</td> - <td class='c015'>5·30</td> - <td class='c015'>5·60</td> - <td class='c015'>1·05</td> - <td class='c016'>20°</td> - </tr> -</table> - -<p class='c006'><span class='pageno' id='Page_46'>46</span>Langevin determined the velocity of the ions by a direct method -in which the time taken for the ion to travel over a known distance -was observed.</p> - -<p class='c006'>The following table shows the comparative values obtained for -air and carbon dioxide.</p> - -<table class='table1' > -<colgroup> -<col class='colwidth16'> -<col class='colwidth16'> -<col class='colwidth13'> -<col class='colwidth13'> -<col class='colwidth13'> -<col class='colwidth13'> -<col class='colwidth13'> -</colgroup> - <tr> - <th class='c013'></th> - <th class='c013'>Air <i>K</i><sub>1</sub></th> - <th class='c013'>Air <i>K</i><sub>2</sub></th> - <th class='c013'>Air <i>K</i><sub>2</sub>/<i>K</i><sub>1</sub></th> - <th class='c013'>CO<sub>2</sub> <i>K</i><sub>1</sub></th> - <th class='c013'>CO<sub>2</sub> <i>K</i><sub>2</sub></th> - <th class='c014'>CO<sub>2</sub> <i>K</i><sub>2</sub>/<i>K</i><sub>1</sub></th> - </tr> - <tr> - <td class='c013'>Direct method (Langevin)</td> - <td class='c013'>1·40</td> - <td class='c013'>1·70</td> - <td class='c013'>1·22</td> - <td class='c013'>0·86</td> - <td class='c013'>0·90</td> - <td class='c014'>1·05</td> - </tr> - <tr> - <td class='c013'>Current of gas (Zeleny)</td> - <td class='c013'>1·36</td> - <td class='c013'>1·87</td> - <td class='c013'>1·375</td> - <td class='c013'>0·76</td> - <td class='c013'>0·81</td> - <td class='c014'>1·07</td> - </tr> -</table> - -<p class='c006'>These results show that for all gases except -CO<sub>2</sub>, -there is a -marked increase in the velocity of the negative ion with the dryness -of the gas, and that, even in moist gases, the velocity of the -negative ions is always greater than that of the positive ions. The -velocity of the positive ion is not much affected by the presence -of moisture in the gas.</p> - -<p class='c006'>The velocity of the ions varies inversely as the pressure of the -gas. This has been shown by Rutherford<a id='r63' href='#f63' class='c012'><sup>[63]</sup></a> for the negative ions -produced by ultra-violet light falling on a negatively charged surface, -and later by Langevin<a id='r64' href='#f64' class='c012'><sup>[64]</sup></a> for both the positive and negative ions -produced by Röntgen rays. Langevin has shown that the velocity -of the positive ion increases more slowly with the diminution of -pressure than that of the negative ion. It appears as if the negative -ion, especially at pressures of about 10 mm. of mercury, -begins to diminish in size.</p> -<p class='c005'><a id='section034'></a> -<b>34. Condensation experiments.</b> Some experiments will -now be described which have verified in a direct way the theory -that the conductivity produced in gases by the various types -of radiation is due to the production of charged ions throughout -the volume of the gas. Under certain conditions, the ions form -nuclei for the condensation of water, and this property allows us -to show the presence of the individual ions in the gas, and also to -count the number present.</p> - -<p class='c006'>It has long been known that, if air saturated with water-vapour -be suddenly expanded, a cloud of small globules of water is formed. -These drops are formed round the dust particles present in the gas, -<span class='pageno' id='Page_47'>47</span>which act as nuclei for the condensation of water around them. -The experiments of R. von Helmholtz and Richarz<a id='r65' href='#f65' class='c012'><sup>[65]</sup></a> had shown that -chemical reactions, for example the combustion of flames, taking -place in the neighbourhood, affected the condensation of a steam-jet. -Lenard showed that a similar action was produced when ultra-violet -light fell on a negatively charged zinc surface placed near -the steam-jet. These results suggested that the presence of electric -charges in the gas facilitated condensation.</p> - -<p class='c006'>A very complete study of the conditions of condensation of -water on nuclei has been made by C. T. R. Wilson<a id='r66' href='#f66' class='c012'><sup>[66]</sup></a>. An apparatus -was constructed which allowed a very sudden expansion of the air -over a wide range of pressure. The amount of condensation was -observed in a small glass vessel. A beam of light was passed -into the apparatus which allowed the drops formed to be readily -observed by the eye.</p> - -<p class='c006'>Preliminary small expansions caused a condensation of the -water round the dust nuclei present in the air. These dust nuclei -were removed by allowing the drops to settle. After a number of -successive small expansions, the air was completely freed from -dust, so that no condensation was produced.</p> - -<p class='c006'>Let -<i>v</i><sub>1</sub> -= initial volume of the gas in the vessel, -<i>v</i><sub>2</sub> -= volume after expansion.</p> - -<p class='c006'>If -<i>v</i><sub>2</sub>/<i>v</i><sub>1</sub> < -1·25 no condensation is produced in dust-free air. If -however -<i>v</i><sub>2</sub>/<i>v</i><sub>1</sub> > -1·25 and < 1·38, a few drops appear. This number is -roughly constant until -<i>v</i><sub>2</sub>/<i>v</i><sub>1</sub> -= 1·38, when the number suddenly increases -and a very dense cloud of fine drops is produced.</p> - -<p class='c006'>If the radiation from an X ray tube or a radio-active substance -is now passed into the condensation vessel, a new series of phenomena -is observed. As before, if -<i>v</i><sub>2</sub>/<i>v</i><sub>1</sub> < -1·25 no drops are formed, but if -<i>v</i><sub>2</sub>/<i>v</i><sub>1</sub> -= 1·25 there is a sudden production of a cloud. The water drops -of which this cloud is formed are finer and more numerous the -<span class='pageno' id='Page_48'>48</span>greater the intensity of the rays. The point at which condensation -begins is very marked, and a slight variation of the amount -of expansion causes either a dense cloud or no cloud at all.</p> - -<p class='c006'>It now remains to be shown that the formation of a cloud by -the action of the rays is due to the productions of ions in the -gas. If the expansion vessel is provided with two parallel plates -between which an electric field can be applied, it is seen that the -number of drops, formed by the expansion with the rays acting, -decreases with increase of the electric field. The stronger the -field the smaller the number of drops formed. This result is to be -expected if the ions are the centres of condensation; for in a strong -electric field the ions are carried at once to the electrodes, and thus -disappear from the gas. If no electric field is acting, a cloud can -be produced some time after the rays have been cut off; but if a -strong electric field is applied, under the same conditions, no cloud -is formed. This is in agreement with experiments showing the -time required for the ions to disappear by recombination. In -addition it can be shown that each one of the fine drops carries -an electric charge and can be made to move in a strong uniform -electric field.</p> - -<p class='c006'>The small number of drops produced without the action of the -rays when -<i>v</i><sub>2</sub>/<i>v</i><sub>1</sub> -> 1·25 is due to a very slight natural ionization of -the gas. That this ionization exists has been clearly shown by -electrical methods (<a href='#section284'>section 284</a>).</p> - -<p class='c006'>The evidence is thus complete that the ions themselves serve -as centres for the condensation of water around them. These experiments -show conclusively that the passage of electricity through -a gas is due to the presence of charged ions distributed throughout -the volume of the gas, and verify in a remarkable way the -hypothesis of the discontinuous structure of the electric charges -carried by matter.</p> - -<p class='c006'>This property of the ions of acting as nuclei of condensation -gives a very delicate method of detecting the presence of ions in -the gas. If only an ion or two is present per c.c., their presence -after expansion is at once observed by the drops formed. In this -way the ionization due to a small quantity of uranium held a yard -away from the condensation vessel is at once made manifest.</p> -<p class='c005'><span class='pageno' id='Page_49'>49</span><b>35. Difference between the positive and negative ions.</b> -In the course of experiments to determine the charge carried by -an ion, J. J. Thomson<a id='r67' href='#f67' class='c012'><sup>[67]</sup></a> observed that the cloud formed under the -influence of X rays increased in density when the expansion was -about 1·31, and suggested in explanation that the positive and -negative ions had different condensation points.</p> - -<div id='fig007' class='figcenter id007'> -<img src='images/fig-007.png' alt='Fig. 7.' class='ig001'> -<div class='ic002'> -<p>Fig. 7.</p> -</div> -</div> - -<p class='c006'>This difference in behaviour of the positive and negative ions -was investigated in detail by C. T. R. Wilson<a id='r68' href='#f68' class='c012'><sup>[68]</sup></a> in the following way. -X rays were made to pass in a narrow beam on either side of a -plate <i>AB</i> (<a href='#fig007'>Fig. 7</a>) dividing the condensation vessel into two equal -parts. The opposite poles of a battery of cells were connected -with two parallel plates <i>C</i> and <i>D</i>, placed symmetrically with regard -to <i>A</i>. The middle point of the battery and the plate <i>A</i> were connected -with earth. If the plate <i>C</i> is positively charged, the ions in -the space <i>CA</i> at a short distance from <i>A</i> are all negative in sign. -Those to the right are all positive. It was found that condensation -occurred only for the negative ions in <i>AC</i> when -<i>v</i><sub>2</sub>/<i>v</i><sub>1</sub> -= 1·25 but did -not occur in <i>AD</i> for the positive ions until -<i>v</i><sub>2</sub>/<i>v</i><sub>1</sub> -= 1·31.</p> - -<p class='c006'><span class='pageno' id='Page_50'>50</span>Thus the negative acts more readily than the positive ion -as a centre of condensation. The greater effect of the negative -ion in causing condensation has been suggested as an explanation -of the positive charge always observed in the upper atmosphere. -The negative ions under certain conditions become centres for the -formation of small drops of water and are removed to the earth by -the action of gravity, while the positive ions remain suspended.</p> - -<p class='c006'>With the apparatus described above, it has been shown that -the positive and negative ions are equal in number. If the expansion -is large enough to ensure condensation on both ions, the -drops formed on the right and left of the vessel in <a href='#fig007'>Fig. 7</a> are -equal in number and fall at the same rate, <i>i.e.</i> are equal in -size.</p> - -<p class='c006'>Since the ions are produced in equal numbers from a gas -electrically neutral, this experiment shows that the charges on -positive and negative ions are equal in value but opposite in sign.</p> -<p class='c005'><a id='section036'></a> -<b>36. Charge carried by an ion.</b> For a known sudden expansion -of a gas saturated with water vapour, the amount of water -precipitated on the ions can be calculated readily. The size of the -drops can be determined by observing the rate at which the cloud -settles under the action of gravity. From Stokes’ equation, the -terminal velocity <i>u</i> of a small sphere of radius <i>r</i> and density <i>d</i> falling -through a gas of which the coefficient of viscosity is μ is given by</p> - -<div class='calc'> - -<div class='lg-container-b c017'> - <div class='linegroup'> - <div class='group'> - <div class='line in6'>2 <i>dgr<sup>2</sup></i></div> - <div class='line'><i>u</i> = --------</div> - <div class='line in6'>9 μ</div> - </div> - </div> -</div> - -</div> - -<p class='c018'>where <i>g</i> is the acceleration due to gravity. The radius of the drop -and consequently the weight of water in each drop can thus be -determined. Since the total weight of water precipitated is known, -the number of drops present is obtained at once.</p> - -<p class='c006'>This method has been used by J. J. Thomson<a id='r69' href='#f69' class='c012'><sup>[69]</sup></a> to determine the -charge carried by an ion. If the expansion exceeds the value 1·31, -both positive and negative ions become centres of condensation. -From the rate of fall it can be shown that approximately the -drops are all of the same size.</p> - -<p class='c006'><span class='pageno' id='Page_51'>51</span>The condensation vessel was similar to that employed by -C. T. R. Wilson. Two parallel horizontal plates were fitted in the -vessel and the radiation from an X ray tube or radio-active substance -ionized the gas between them. A difference of potential <i>V</i>, -small compared with that required to saturate the gas, was applied -between the parallel plates distant <i>l</i> cms. from each other. The -small current <i>i</i> through the gas is given (<a href='#section028'>section 28</a>) by</p> - -<div class='calc'> - -<div class='lg-container-b c017'> - <div class='linegroup'> - <div class='group'> - <div class='line in6'><i>NuVe</i></div> - <div class='line'><i>i</i> = ------</div> - <div class='line in6'><i>l</i></div> - </div> - </div> -</div> - -</div> - -<p class='c018'>where</p> - -<div class='calc'> - -<div class='lg-container-b c017'> - <div class='linegroup'> - <div class='group'> - <div class='line'><i>N</i> = number of ions present in the gas,</div> - <div class='line'><i>e</i> = charge on each ion,</div> - <div class='line'><i>u</i> = sum of the velocities of the positive and negative ions.</div> - </div> - </div> -</div> - -</div> - -<p class='c018'>Since the value of <i>N</i> is the same as the number of drops, and the -velocity <i>u</i> is known, the value of <i>e</i> can be determined.</p> - -<p class='c006'>In his last determination J. J. Thomson found that</p> - -<div class='calc'> - -<div class='lg-container-b c017'> - <div class='linegroup'> - <div class='group'> - <div class='line'><i>e</i> = 3·4 × 10<sup>-10</sup> electrostatic units.</div> - </div> - </div> -</div> - -</div> - -<p class='c018'>A very concordant value, namely, -3·1 × 10<sup>-10</sup>, -has been obtained by -H. A. Wilson<a id='r70' href='#f70' class='c012'><sup>[70]</sup></a>, by using a modified method of counting the drops. -A check on the size of the drops, determined by their rate of fall, -was made by observing the rate at which the drops moved in -a strong electric field, arranged so as to act with or against gravity.</p> - -<p class='c006'>J. J. Thomson found that the charge on the ions produced in -hydrogen and oxygen is the same. This shows that the nature -of the ionization in gases is distinct from that occurring in the -electrolysis of solutions where the oxygen ion always carries twice -the charge of the hydrogen ion.</p> -<p class='c005'><a id='section037'></a> -<b>37. Diffusion of the ions.</b> Early experiments with ionized -gases showed that the conductivity was removed from the gas by -passage through a finely divided substance like cotton-wool, or by -bubbling through water. This loss of conductivity is due to the -fact that the ions in passing through narrow spaces diffuse to the -sides of the boundary, to which they either adhere or give up their -charge.</p> - -<p class='c006'>A direct determination of the coefficient of diffusion of the ions -<span class='pageno' id='Page_52'>52</span>produced in gases by Röntgen rays or by the rays from active -substances has been made by Townsend<a id='r71' href='#f71' class='c012'><sup>[71]</sup></a>. The general method -employed was to pass a stream of ionized gas through a diffusion -vessel made up of a number of fine metal tubes arranged in parallel. -Some of the ions in their passage through the tubes diffuse to the -sides, the proportion being greater the slower the motion of the -gas and the narrower the tube. Observations were made of the -conductivity of the gas before and after passage through the tubes. -In this way, correcting if necessary for the recombination during -the time taken to pass through the tubes, the proportion <i>R</i> of -either positive or negative ions which are abstracted can be -deduced. The value of <i>R</i> can be expressed mathematically by -the following equation in terms of <i>K</i>, the coefficient of diffusion -of the ions into the gas with which they are mixed<a id='r72' href='#f72' class='c012'><sup>[72]</sup></a>,</p> - -<div class='figcenter id002'> -<img src='images/form-008.png' alt='Formula.' class='ig001'> -</div> - -<p class='c006'>where</p> - -<div class='calc'> - -<div class='lg-container-b c017'> - <div class='linegroup'> - <div class='group'> - <div class='line'><i>a</i> = radius of the tube,</div> - <div class='line'><i>Z</i> = length of the tube,</div> - <div class='line'><i>V</i> = mean velocity of the gas in the tube.</div> - </div> - </div> -</div> - -</div> - -<p class='c018'>Only the first two terms of the series need be taken into -account when narrow tubes are used.</p> - -<p class='c006'>In this equation <i>R</i>, <i>V</i>, and <i>a</i> are determined experimentally, -and <i>K</i> can thus be deduced.</p> - -<p class='c006'>The following table shows the results obtained by Townsend -when X rays were used. Almost identical results were obtained -later, when the radiations from active substances replaced the -X rays.</p> - -<div class='nf-center-c1'> -<div class='nf-center c007'> - <div><i>Coefficients of diffusion of ions into gases.</i></div> - </div> -</div> - -<table class='table5' > -<colgroup> -<col class='colwidth38'> -<col class='colwidth15'> -<col class='colwidth15'> -<col class='colwidth15'> -<col class='colwidth15'> -</colgroup> - <tr> - <th class='c013'>Gas</th> - <th class='c013'><i>K</i> for + ions</th> - <th class='c013'><i>K</i> for – ions</th> - <th class='c013'>Mean value of <i>K</i></th> - <th class='c014'>Ratio of values of <i>K</i></th> - </tr> - <tr> - <td class='c013'>Air, dry</td> - <td class='c013'>·028</td> - <td class='c013'>·043</td> - <td class='c013'>·0347</td> - <td class='c014'>1·54</td> - </tr> - <tr> - <td class='c013'>„ moist</td> - <td class='c013'>·032</td> - <td class='c013'>·035</td> - <td class='c013'>·0335</td> - <td class='c014'>1·09</td> - </tr> - <tr> - <td class='c013'>Oxygen, dry</td> - <td class='c013'>·025</td> - <td class='c013'>·0396</td> - <td class='c013'>·0323</td> - <td class='c014'>1·58</td> - </tr> - <tr> - <td class='c013'>„ moist</td> - <td class='c013'>·0288</td> - <td class='c013'>·0358</td> - <td class='c013'>·0323</td> - <td class='c014'>1·24</td> - </tr> - <tr> - <td class='c013'>Carbonic acid, dry</td> - <td class='c013'>·023</td> - <td class='c013'>·026</td> - <td class='c013'>·0245</td> - <td class='c014'>1·13</td> - </tr> - <tr> - <td class='c013'>„ „ moist</td> - <td class='c013'>·0245</td> - <td class='c013'>·0255</td> - <td class='c013'>·025</td> - <td class='c014'>1·04</td> - </tr> - <tr> - <td class='c013'>Hydrogen, dry</td> - <td class='c013'>·123</td> - <td class='c013'>·190</td> - <td class='c013'>·156</td> - <td class='c014'>1·54</td> - </tr> - <tr> - <td class='c013'>„ moist</td> - <td class='c013'>·128</td> - <td class='c013'>·142</td> - <td class='c013'>·135</td> - <td class='c014'>1·11</td> - </tr> -</table> - -<p class='c006'><span class='pageno' id='Page_53'>53</span>The moist gases were saturated with water vapour at a temperature -of 15° C.</p> - -<p class='c006'>It is seen that the negative ion in all cases diffuses faster than -the positive. Theory shows that the coefficients of diffusion should -be directly proportional to the velocities of the ions, so that this -result is in agreement with the observations on the greater velocity -of the negative ion.</p> - -<p class='c006'>This difference in the rate of diffusion of the ions at once -explains an interesting experimental result. If ionized gases are -blown through a metal tube, the tube gains a negative charge -while the gas itself retains a positive charge. The number of -positive and negative ions present in the gas is originally the same, -but, in consequence of the more rapid diffusion of the negative ions, -more of the negative ions than of the positive give up their charges -to the tube. The tube consequently gains a negative and the gas -a positive charge.</p> -<p class='c005'><b>38.</b> A very important result can be deduced at once when the -velocities and coefficients of diffusion of the ions are known. -Townsend (<i>loc. cit.</i>) has shown that the equation of their motion -is expressed by the formula</p> - -<div class='calc'> - -<div class='lg-container-b c017'> - <div class='linegroup'> - <div class='group'> - <div class='line in1'>1 <i>dp</i></div> - <div class='line'>---- <i>pu</i> = – ---- + <i>nXe</i> ,</div> - <div class='line'> <i>K</i> <i>dx</i></div> - </div> - </div> -</div> - -</div> - -<p class='c018'>where <i>e</i> is the charge on an ion,</p> - -<div class='calc'> - -<div class='lg-container-b c017'> - <div class='linegroup'> - <div class='group'> - <div class='line'><i>n</i> = number of ions per c.c.,</div> - <div class='line'><i>p</i> = their partial pressure,</div> - </div> - </div> -</div> - -</div> - -<p class='c018'>and <i>u</i> is the velocity due to the electric force <i>X</i> in the direction -of the axis of <i>x</i>. When a steady state is reached,</p> - -<div class='calc'> - -<div class='lg-container-b c017'> - <div class='linegroup'> - <div class='group'> - <div class='line in1'><i>dp</i> <i>nXeK</i></div> - <div class='line'>---- = 0 and <i>u</i> = ---- ,</div> - <div class='line in1'><i>dx</i> <i>p</i></div> - </div> - </div> -</div> - -</div> - -<p class='c018'>Let <i>N</i> be the number of molecules in a cubic centimetre of -gas at the pressure <i>P</i> and at the temperature 15° C., for which -the values of <i>u</i> and <i>K</i> have been determined. Then <i>N</i>/<i>P</i> may be -substituted for <i>n</i>/<i>p</i>, and, since <i>P</i> at atmospheric pressure is -10<sup>6</sup>,</p> - -<p class='c006'><span class='pageno' id='Page_54'>54</span>then</p> - -<div class='calc'> - -<div class='lg-container-b c017'> - <div class='linegroup'> - <div class='group'> - <div class='line in6'>3 × 10<sup>8</sup><i>u<sub>1</sub></i></div> - <div class='line'><i>Ne</i> = ---------- electrostatic units,</div> - <div class='line in8'><i>K</i></div> - </div> - </div> -</div> - -</div> - -<p class='c018'>where -<i>u</i><sub>1</sub> -is the velocity for 1 volt (<i>i.e.</i> ¹⁄₃₀₀ <span class='fss'>E. S.</span> unit) per cm.</p> - -<p class='c006'>It is known that one absolute electromagnetic unit of -electricity in passing through water liberates 1·23 c.c. of hydrogen -at a temperature of 15° C. and standard pressure. The number of -atoms in this volume is 2·46<i>N</i>, and, if <i>e´</i> is the charge on the -hydrogen atom in the electrolysis of water,</p> - -<div class='calc'> - -<div class='lg-container-b c017'> - <div class='linegroup'> - <div class='group'> - <div class='line'>2·46 <i>Ne´</i> = 3 × 10<sup>10</sup> <span class='fss'>E. S.</span> units,</div> - <div class='line in5'><i>Ne´</i> = 1·22 × 10<sup>10</sup> <span class='fss'>E. S.</span> units.</div> - </div> - </div> -</div> - -</div> - -<div class='calc'> - -<div class='lg-container-b c017'> - <div class='linegroup'> - <div class='group'> - <div class='line in7'><i>e</i> <i>u<sub>1</sub></i></div> - <div class='line'>Thus --- = 2·46 × 10<sup>-2</sup> ---</div> - <div class='line in7'><i>e´</i> <i>K</i></div> - </div> - </div> -</div> - -</div> - -<p class='c018'>For example, substituting the values of <i>u</i><sub>1</sub> and <i>K</i> determined -in moist air for the positive ion,</p> - -<div class='calc'> - -<div class='lg-container-b c017'> - <div class='linegroup'> - <div class='group'> - <div class='line in1'><i>e</i> 2·46 1·37</div> - <div class='line'>--- = ----- × ----- = 1·04.</div> - <div class='line in1'><i>e´</i> 100 ·032</div> - </div> - </div> -</div> - -</div> - -<p class='c018'>Values of this ratio, not very different from unity, are obtained -for the positive and negative ions of the gases hydrogen, oxygen, -and carbon dioxide. Taking into consideration the uncertainty in -the experimental values of -<i>u</i><sub>1</sub> -and <i>K</i>, these results indicate that the -<i>charge carried by an ion in all gases is the same and is equal to -that carried by the hydrogen ion in the electrolysis of liquids</i>.</p> -<p class='c005'><a id='section039'></a> -<b>39. Number of the ions.</b> We have seen that, from experimental -data, Townsend has found that <i>N</i>, the number of molecules -present in 1 c.c. of gas at 15° C. and standard pressure, is given by</p> - -<div class='calc'> - -<div class='lg-container-b c017'> - <div class='linegroup'> - <div class='group'> - <div class='line'><i>Ne</i> = 1·22 × 10<sup>10</sup>.</div> - </div> - </div> -</div> - -</div> - -<p class='c018'>Now <i>e</i>, the charge on an ion, is equal to -3·4 × 10<sup>-10</sup> -<span class='fss'>E. S.</span> units;</p> - -<div class='calc'> - -<div class='lg-container-b c017'> - <div class='linegroup'> - <div class='group'> - <div class='line'>thus <i>N</i> = 3·6 × 10<sup>19</sup>.</div> - </div> - </div> -</div> - -</div> - -<p class='c018'>If <i>I</i> is the saturation current through a gas, and <i>q</i> the total -rate of production of ions in the gas,</p> - -<div class='calc'> - -<div class='lg-container-b c017'> - <div class='linegroup'> - <div class='group'> - <div class='line in5'><i>I</i></div> - <div class='line'><i>q</i> = ---.</div> - <div class='line in5'><i>e</i></div> - </div> - </div> -</div> - -</div> - -<p class='c018'><span class='pageno' id='Page_55'>55</span>The saturation current through air was found to be -1·2 × 10<sup>-8</sup> -ampères, <i>i.e.</i> 36 <span class='fss'>E.S.</span> units, for parallel plates 4·5 cms. apart, when ·45 -gramme of radium of activity 1000 times that of uranium was spread -over an area of 33 sq. cms. of the lower plate. This corresponds to a -production of about -10<sup>11</sup> -ions per second. Assuming, for the purpose -of illustration, that the ionization was uniform between the plates, -the volume of air acted on by the rays was about 148 c.c., and the -number of ions produced per c.c. per second about -7 × 10<sup>8</sup>. -Since -<i>N</i> = 3·6 × 10<sup>19</sup>, -we see that, if one molecule produces two ions, -the proportion of the gas ionized per second is about -10<sup>-11</sup> -of the -whole. For uranium the fraction is about -10<sup>-14</sup>, -and for pure radium, -of activity one million times that of uranium, about -10<sup>-8</sup>. -Thus -even in the case of pure radium, only about one molecule of gas is -acted on per second in every 100 millions.</p> - -<p class='c006'>The electrical methods are so delicate that the production of -one ion per cubic centimetre per second can be detected readily. -This corresponds to the ionization of about one molecule in every -10<sup>19</sup> -present in the gas.</p> -<p class='c005'><b>40. Size and nature of the ions.</b> An approximate estimate -of the mass of an ion, compared with the mass of the molecule of -the gas in which it is produced, can be made from the known data -of the coefficient <i>K</i> of inter-diffusion of the ions into gases. The -value of <i>K</i> for the positive ions in moist carbon dioxide has been -shown to be ·0245, while the value of <i>K</i> for the inter-diffusion of -carbon dioxide with air is ·14. The value of <i>K</i> for different gases -is approximately inversely proportional to the square root of the -products of the masses of the molecules of the two inter-diffusing -gases; thus, the positive ion in carbon dioxide behaves as if its -mass were large compared with that of the molecule. Similar -results hold for the negative as well as for the positive ion, and for -other gases besides carbon dioxide.</p> - -<p class='c006'>This has led to the view that the ion consists of a charged -centre surrounded by a cluster of molecules travelling with it, -which are kept in position round the charged nucleus by electrical -forces. A rough estimate shows that this cluster consists of about -30 molecules of the gas. This idea is supported by the variation -in velocity, <i>i.e.</i> the variation of the size of the negative ion, in the -<span class='pageno' id='Page_56'>56</span>presence of water vapour; for the negative ion undoubtedly has a -greater mass in moist than in dry gases. At the same time it is -possible that the apparently large size of the ion, as determined -by diffusion methods, may be in part a result of the charge carried -by the ion. The presence of a charge on a moving body would -increase the frequency of collision with the molecules of the gas, -and consequently diminish the rate of diffusion. The ion on this -view may not actually be of greater size than the molecule from -which it is produced.</p> - -<p class='c006'>The negative and positive ions certainly differ in size, and this -difference becomes very pronounced for low pressures of the gas. -At atmospheric pressure, the negative ion, produced by the action -of ultra-violet light on a negatively charged body, is of the -same size as the ion produced by X rays, but at low pressures -J. J. Thomson has shown that it is identical with the corpuscle or -electron, which has an apparent mass of about ¹⁄₁₀₀₀ of the mass -of the hydrogen atom. A similar result has been shown by -Townsend to hold for the negative ion produced by X rays at a -low pressure. It appears that the negative ion at low pressure -sheds its attendant cluster. The result of Langevin, that the -velocity of the negative ion increases more rapidly with the -diminution of pressure than that of the positive ion, shows that -this process of removal of the cluster is quite appreciable at a -pressure of 10 mms. of mercury.</p> - -<p class='c006'>We must suppose that the process of ionization in gases -consists in a removal of a negative corpuscle or electron from -the molecule of the gas. At atmospheric pressure this corpuscle -immediately becomes the centre of an aggregation of molecules -which moves with it and <i>is</i> the negative ion. After removal of -the negative ion the molecule retains a positive charge, and probably -also becomes the centre of a cluster of new molecules.</p> - -<p class='c006'>The terms electron and ion as used in this work may therefore -be defined as follows:—</p> - -<p class='c006'>The <i>electron</i> or <i>corpuscle</i> is the body of smallest mass yet -known to science. It carries a negative charge of value 3·4 × -10<sup>-10</sup> -electrostatic units. Its presence has only been detected when in -rapid motion, when, for speeds up to about -10<sup>10</sup> -cms. a second, it has -an apparent mass <i>m</i> given by <i>e</i>/<i>m</i> = 1·86 × -10<sup>7</sup> -electromagnetic -<span class='pageno' id='Page_57'>57</span>units. This apparent mass increases with the speed as the velocity -of light is approached (see <a href='#section082'>section 82</a>).</p> - -<p class='c006'>The ions which are produced in gases at ordinary pressure have -an apparent size, as determined from their rates of diffusion, large -compared with the molecule of the gas in which they are produced. -The negative ion consists of an electron with a cluster of molecules -attached to and moving with it; the positive ion consists of a -molecule from which an electron has been expelled, with a cluster -of molecules attached. At low pressures under the action of an -electric field the electron does not form a cluster. The positive ion -is always atomic in size, even at low pressures of the gas. Each of -the ions carries a charge of value 3·4 × -10<sup>-10</sup> -electrostatic units.</p> -<p class='c005'><a id='section041'></a> -<b>41. Ions produced by collision.</b> The greater part of the -radiation from the radio-active bodies consists of a stream of charged -particles travelling with great velocity. In this radiation, the α -particles, which cause most of the ionization observed in the gas, -consist of positively charged bodies projected with a velocity about -one-tenth the velocity of light. The β rays consist of negatively -charged particles, which are identical with the cathode rays -generated in a vacuum tube, and travel with a speed about one-half -the velocity of light (<a href='#chap04'>chapter <span class='fss'>IV.</span></a>). Each of these projected -particles, in virtue of its great kinetic energy, sets free a large -number of ions by collision with the gas molecules in its path. -No definite experimental evidence has yet been obtained of the -number of ions produced by a single particle, or of the way in -which the ionization varies with the speed, but there is no doubt -that each projected body gives rise to many thousand ions in its -path before its energy of motion is destroyed.</p> - -<p class='c006'>It has already been mentioned (<a href='#section029'>section 29</a>) that at low pressures -ions moving under the action of an electric field are able to produce -fresh ions by collision with the molecules of the gas. At low -pressures the negative ion is identical with the electron set free -in a vacuum tube, or emitted by a radio-active substance.</p> - -<p class='c006'>The mean free path of the ion is inversely proportional to the -pressure of the gas. Consequently, if an ion moves in an electric -field, the velocity acquired between collisions increases with diminution -of the pressure. Townsend has shown that fresh ions are -<span class='pageno' id='Page_58'>58</span>occasionally produced by collision when the negative ion moves -freely between two points differing in potential by 10 volts. If -the difference be about <i>V</i> = 20 volts, fresh ions arise at each -collision<a id='r73' href='#f73' class='c012'><sup>[73]</sup></a>.</p> - -<p class='c006'>Now the energy <i>W</i>, acquired by an ion of charge <i>e</i> moving -freely between two points at a difference of potential <i>V</i>, is given by</p> - -<div class='calc'> - -<div class='lg-container-b c017'> - <div class='linegroup'> - <div class='group'> - <div class='line'><i>W</i> = <i>V</i><i>e</i>.</div> - </div> - </div> -</div> - -</div> - -<p class='c018'>Taking <i>V</i> = 20 volts = ²⁰⁄₃₀₀ <span class='fss'>E. S.</span> units, and <i>e</i> = 3·4 × -10<sup>-10</sup>, -the -energy <i>W</i> required in the case of a negative ion to produce an ion -by collision is given by</p> - -<div class='calc'> - -<div class='lg-container-b c017'> - <div class='linegroup'> - <div class='group'> - <div class='line'><i>W</i> = 2·3 × 10<sup>-11</sup> ergs.</div> - </div> - </div> -</div> - -</div> - -<p class='c018'>The velocity <i>u</i> acquired by the ion of mass <i>m</i> just before a -collision is given by</p> - -<div class='calc'> - -<div class='lg-container-b c017'> - <div class='linegroup'> - <div class='group'> - <div class='line in1'>1</div> - <div class='line'>--- <i>mu<sup>2</sup></i> = <i>Ve</i>,</div> - <div class='line in1'>2</div> - </div> - </div> -</div> - -</div> - -<p class='c018'>and</p> - -<div class='figcenter id010'> -<img src='images/form-009.png' alt='Formula.' class='ig001'> -</div> - -<p class='c006'>Now <i>e</i>/<i>m</i> = 1·86 × -10<sup>7</sup> -electromagnetic units for the electron at -slow speeds (<a href='#section082'>section 82</a>).</p> - -<p class='c006'>Taking <i>V</i> = 20 volts, we find that</p> - -<div class='calc'> - -<div class='lg-container-b c017'> - <div class='linegroup'> - <div class='group'> - <div class='line'><i>u</i> = 2·7 × 10<sup>8</sup> cms. per sec.</div> - </div> - </div> -</div> - -</div> - -<p class='c018'>This velocity is very great compared with the velocity of -agitation of the molecules of the gas.</p> - -<p class='c006'>In a weak electric field, the negative ions only produce ions -by collision. The positive ion, whose mass is at least 1000 times -greater than the electron, does not acquire a sufficient velocity to -generate ions by collision until an electric field is applied nearly -sufficient to cause a spark through the gas.</p> - -<p class='c006'>An estimate of the energy required for the production of an -ion by X rays has been made by Rutherford and McClung. The -energy of the rays was measured by their heating effect, and the -total number of ions produced determined. On the assumption -that <i>all</i> the energy of the rays is used up in producing ions, it -<span class='pageno' id='Page_59'>59</span>was found that <i>V</i> = 175 volts—a value considerably greater than -that observed by Townsend from data of ionization by collision. -The ionization in the two cases, however, is produced under very -different conditions, and it is impossible to estimate how much of -the energy of the rays is dissipated in the form of heat.</p> -<p class='c005'><b>42.</b> Variations are found in the saturation current through gases, -exposed to the radiations from active bodies, when the pressure -and nature of the gas and the distance between the electrodes are -varied. Some cases which are of special importance in measurements -will now be considered. With unscreened active material -the ionization of the gas is, to a large extent, due to the α rays, which -are absorbed in their passage through a few centimetres of air. -In consequence of this rapid absorption, the ionization decreases -rapidly from the surface of the active body, and this gives rise to -conductivity phenomena different in character from those observed -with Röntgen rays, where the ionization is in most cases uniform.</p> -<p class='c005'><b>43. Variation of the current with distance between the -plates.</b> It has been found experimentally<a id='r74' href='#f74' class='c012'><sup>[74]</sup></a> that the intensity of -the ionization, due to a large plane surface of active matter, falls -off approximately in an exponential law with the distance from the -plate. On the assumption that the rate of production of ions at -any point is a measure of the intensity <i>I</i> of the radiation, the -value of <i>I</i> at that point is given by</p> - -<div class='calc'> - -<div class='lg-container-b c017'> - <div class='linegroup'> - <div class='group'> - <div class='line'><i>I</i>/<i>I</i>₀ = <i>e</i><sup>–λ<i>x</i></sup>,</div> - </div> - </div> -</div> - -</div> - -<p class='c018'>where λ is a -constant, <i>x</i> the distance from the plate, and -<i>I</i>₀ -the intensity of the -radiation at the surface of the plate.</p> - -<p class='c006'>While the exponential law, in some cases, approximately represents -the variation of the ionization with distance, in others the -divergence from it is wide. The ionization, due to a plane surface -of polonium, for example, falls off more rapidly than the exponential -law indicates. The α rays from an active substance like radium -are highly complex; the law of variation of the ionization due -to them is by no means simple and depends upon a variety of -conditions. The distribution of ionization is quite different according -as a thick layer or a very thick film of radio-active matter -is employed. The question is fully considered at the end of -<span class='pageno' id='Page_60'>60</span><a href='#chap04'>chapter <span class='fss'>IV.</span></a>, but for simplicity, the exponential law is assumed in -the following calculations.</p> - -<p class='c006'>Consider two parallel plates placed as in <a href='#fig001'>Fig. 1</a>, one of which is -covered with a uniform layer of radio-active matter. If the distance -<i>d</i> between the plates is small compared with the dimensions of the -plates, the ionization near the centre of the plates will be sensibly -uniform over any plane parallel to the plates and lying between -them. If <i>q</i> be the rate of production of ions at any distance <i>x</i> -and -<i>q</i>₀ -that at the surface, then -<i>q</i> = <i>q</i>₀<i>e</i><sup>-λ<i>x</i></sup>. -The saturation current -<i>i</i> per unit area is given by</p> - -<div class='figcenter id002'> -<img src='images/form-010.png' alt='Formula.' class='ig001'> -</div> - -<p class='c006'>hence, when λ<i>d</i> is small, <i>i.e.</i> when the ionization between the -plates is nearly constant,</p> - -<div class='calc'> - -<div class='lg-container-b c017'> - <div class='linegroup'> - <div class='group'> - <div class='line'><i>i</i> = <i>q</i>₀<i>e´</i><i>d</i>.</div> - </div> - </div> -</div> - -</div> - -<p class='c018'>The current is thus proportional to the distance between the -plates. When λ<i>d</i> is large, the saturation current -<i>i</i>₀ -is equal to -<i>q</i>₀<i>e´</i>/λ, -and is independent of further increase in the value of <i>d</i>. In such -a case the radiation is completely absorbed in producing ions -between the plates, and</p> - -<div class='figcenter id010'> -<img src='images/form-011.png' alt='Formula.' class='ig001'> -</div> - -<p class='c006'>For example, in the case of a thin layer of uranium oxide spread -over a large plate, the ionization is mostly produced by rays the -intensity of which is reduced to half value in passing through -4·3 mms. of air, <i>i.e.</i> the value of λ is 1·6. The following table is an -example of the variation of <i>i</i> with the distance between the plates.</p> - -<table class='table6' > -<colgroup> -<col class='colwidth33'> -<col class='colwidth66'> -</colgroup> - <tr> - <th class='c015'>Distance</th> - <th class='c016'>Saturation Current</th> - </tr> - <tr> - <td class='c015'> </td> - <td class='c016'> </td> - </tr> - <tr> - <td class='c015'>2·5 mms.</td> - <td class='c016'>32</td> - </tr> - <tr> - <td class='c015'>5 „</td> - <td class='c016'>55</td> - </tr> - <tr> - <td class='c015'>7·5 „</td> - <td class='c016'>72</td> - </tr> - <tr> - <td class='c015'>10 „</td> - <td class='c016'>85</td> - </tr> - <tr> - <td class='c015'>12·5 „</td> - <td class='c016'>96</td> - </tr> - <tr> - <td class='c015'>15 „</td> - <td class='c016'>100</td> - </tr> -</table> - -<p class='c006'>Thus the increase of current for equal increments of distance -between the plates decreases rapidly with the distance traversed -by the radiation.</p> - -<p class='c006'><span class='pageno' id='Page_61'>61</span>The distance of 15 mms. was not sufficient to completely absorb -all the radiation, so that the current had not reached its limiting -value.</p> - -<p class='c006'>When more than one type of radiation is present, the saturation -current between parallel plates is given by</p> - -<div class='figcenter id005'> -<img src='images/form-012.png' alt='Formula.' class='ig001'> -</div> - -<p class='c006'>where <i>A</i>, -<i>A</i><sub>1</sub> -are constants, and λ, -λ<sub>1</sub> -the absorption constants of -the radiations in the gas.</p> - -<p class='c006'>Since the radiations are unequally absorbed in different gases, -the variation of current with distance depends on the nature of the -gas between the plates.</p> -<p class='c005'><b>44. Variation of the current with pressure.</b> The rate -of production of ions by the radiations from active substances is -directly proportional to the pressure of the gas. The absorption of -the radiation in the gas also varies directly as the pressure. The -latter result necessarily follows if the energy required to produce -an ion is independent of the pressure.</p> - -<p class='c006'>In cases where the ionization is uniform between two parallel -plates, the current will vary directly as the pressure; when however -the ionization is not uniform, on account of the absorption of the -radiation in the gas, the current does not decrease directly as the -pressure until the pressure is reduced so far that the ionization -is sensibly uniform. Consider the variation with pressure of the -saturation current <i>i</i> between two large parallel plates, one of which -is covered with a uniform layer of active matter.</p> - -<p class='c006'>Let -λ<sub>1</sub> -= absorption constant of the radiation in the gas for -unit pressure.</p> - -<p class='c006'>For a pressure <i>p</i>, the intensity <i>I</i> at any point <i>x</i> is given by</p> - -<div class='figcenter id010'> -<img src='images/form-013.png' alt='Formula.' class='ig001'> -</div> - -<p class='c006'>The saturation current <i>i</i> is thus proportional to</p> - -<div class='figcenter id007'> -<img src='images/form-014.png' alt='Formula.' class='ig001'> -</div> - -<p class='c006'>If <i>r</i> be the ratio of the saturation currents for the pressures -<i>p</i><sub>1</sub> -and -<i>p</i><sub>2</sub>,</p> - -<div class='figcenter id009'> -<img src='images/form-015.png' alt='Formula.' class='ig001'> -</div> - -<p class='c006'><span class='pageno' id='Page_62'>62</span>The ratio is thus dependent on the distance <i>d</i> between the -plates and the absorption of the radiation by the gas.</p> - -<p class='c006'>The difference in the shape of the pressure-current curves<a id='r75' href='#f75' class='c012'><sup>[75]</sup></a> is -well illustrated in <a href='#fig008'>Fig. 8</a>, where curves are given for hydrogen, air, -and carbonic acid for plates 3·5 cms. apart.</p> - -<div id='fig008' class='figcenter id006'> -<img src='images/fig-008.png' alt='Fig. 8.' class='ig001'> -<div class='ic002'> -<p>Fig. 8.</p> -</div> -</div> - -<p class='c006'>For the purpose of comparison, the current at atmospheric -pressure and temperature in each case is taken as unity. The -actual value of the current was greatest in carbonic acid and -least in hydrogen. In hydrogen, where the absorption is small, -the current over the whole range is nearly proportional to the -pressure. In carbonic acid, where the absorption is large, the -current diminishes at first slowly with the pressure, but is nearly -proportional to it below the pressure of 235 mms. of mercury. -The curve for air occupies an intermediate position.</p> - -<p class='c006'><span class='pageno' id='Page_63'>63</span>In cases where the distance between the plates is large, the -saturation current will remain constant with diminution of pressure -until the absorption is so reduced that the radiation reaches -the other plate.</p> - -<p class='c006'>An interesting result follows from the rapid absorption of -radiation by the gas. If the current is observed between two -fixed parallel plates, distant -<i>d</i><sub>1</sub> -and -<i>d</i><sub>2</sub> -respectively from a large -plane surface of active matter, the current at first increases with -diminution of pressure, passes through a maximum value, and -then diminishes. In such an experimental case the lower plate -through which the radiations pass is made either of open gauze or -of thin metal foil to allow the radiation to pass through readily.</p> - -<p class='c006'>The saturation current <i>i</i> is obviously proportional to</p> - -<div class='figcenter id002'> -<img src='images/form-016.png' alt='Formula.' class='ig001'> -</div> - -<p class='c006'>This is a function of the pressure, and is a maximum when</p> - -<div class='figcenter id005'> -<img src='images/form-017.png' alt='Formula.' class='ig001'> -</div> - -<p class='c006'>For example, if the active matter is uranium, -<i>p</i>λ<sub>1</sub> = 1·6 -for the -α rays at atmospheric pressure. If -<i>d</i><sub>2</sub> = 3, -and -<i>d</i><sub>1</sub> = 1, -the saturation -current reaches a maximum when the pressure is reduced to about -⅓ of an atmosphere. This result has been verified experimentally.</p> -<p class='c005'><a id='section045'></a> -<b>45. Conductivity of different gases when acted on by -the rays.</b> For a given intensity of radiation, the rate of production -of ions in a gas varies for different gases and increases -with the density of the gas. Strutt<a id='r76' href='#f76' class='c012'><sup>[76]</sup></a> has made a very complete -examination of the relative conductivity of gases exposed to the -different types of rays emitted by active substances. To avoid -correction for any difference of absorption of the radiation in the -various gases, the pressure of the gas was always reduced until -the ionization was directly proportional to the pressure, when, as -we have seen above, the ionization must everywhere be uniform -throughout the gas. For each type of rays, the ionization of -air is taken as unity. The currents through the gases were -determined at different pressures, and were reduced to a common -<span class='pageno' id='Page_64'>64</span>pressure by assuming that the ionization was proportional to the -pressure.</p> - -<p class='c006'>With unscreened active material, the ionization is almost -entirely due to α rays. When the active substance is covered with -a layer of aluminium ·01 cm. in thickness, the ionization is mainly -due to the β or cathodic rays, and when covered with 1 cm. of lead, -the ionization is solely due to the γ or very penetrating rays. -Experiments on the γ rays of radium were made by observing the -rate of discharge of a special gold-leaf electroscope filled with the -gas under examination and exposed to the action of the rays. -The following table gives the relative conductivities of gases -exposed to various kinds of ionizing radiations.</p> - -<table class='table7' > -<colgroup> -<col class='colwidth33'> -<col class='colwidth17'> -<col class='colwidth11'> -<col class='colwidth11'> -<col class='colwidth11'> -<col class='colwidth15'> -</colgroup> - <tr> - <th class='c013'>Gas</th> - <th class='c013'>Relative Density</th> - <th class='c013'>α rays</th> - <th class='c013'>β rays</th> - <th class='c013'>γ rays</th> - <th class='c014'>Röntgen rays</th> - </tr> - <tr> - <td class='c013'> </td> - <td class='c013'> </td> - <td class='c013'> </td> - <td class='c013'> </td> - <td class='c013'> </td> - <td class='c014'> </td> - </tr> - <tr> - <td class='c013'>Hydrogen</td> - <td class='c013'>0·0693</td> - <td class='c013'>0·226</td> - <td class='c013'>0·157</td> - <td class='c013'>0·169</td> - <td class='c014'>0·114</td> - </tr> - <tr> - <td class='c013'>Air</td> - <td class='c013'>1·00</td> - <td class='c013'>1·00</td> - <td class='c013'>1·00</td> - <td class='c013'>1·00</td> - <td class='c014'>1·00</td> - </tr> - <tr> - <td class='c013'>Oxygen</td> - <td class='c013'>1·11</td> - <td class='c013'>1·16</td> - <td class='c013'>1·21</td> - <td class='c013'>1·17</td> - <td class='c014'>1·39</td> - </tr> - <tr> - <td class='c013'>Carbon dioxide</td> - <td class='c013'>1·53</td> - <td class='c013'>1·54</td> - <td class='c013'>1·57</td> - <td class='c013'>1·53</td> - <td class='c014'>1·60</td> - </tr> - <tr> - <td class='c013'>Cyanogen</td> - <td class='c013'>1·86</td> - <td class='c013'>1·94</td> - <td class='c013'>1·86</td> - <td class='c013'>1·71</td> - <td class='c014'>1·05</td> - </tr> - <tr> - <td class='c013'>Sulphur dioxide</td> - <td class='c013'>2·19</td> - <td class='c013'>2·04</td> - <td class='c013'>2·31</td> - <td class='c013'>2·13</td> - <td class='c014'>7·97</td> - </tr> - <tr> - <td class='c013'>Chloroform</td> - <td class='c013'>4·32</td> - <td class='c013'>4·44</td> - <td class='c013'>4·89</td> - <td class='c013'>4·88</td> - <td class='c014'>31·9</td> - </tr> - <tr> - <td class='c013'>Methyl iodide</td> - <td class='c013'>5·05</td> - <td class='c013'>3·51</td> - <td class='c013'>5·18</td> - <td class='c013'>4·80</td> - <td class='c014'>72·0</td> - </tr> - <tr> - <td class='c013'>Carbon tetrachloride</td> - <td class='c013'>5·31</td> - <td class='c013'>5·34</td> - <td class='c013'>5·83</td> - <td class='c013'>5·67</td> - <td class='c014'>45·3</td> - </tr> -</table> - -<p class='c006'>With the exception of hydrogen, it will be seen that the ionization -of gases is approximately proportional to their density -for the α, β, γ rays of radium. The results obtained by Strutt -for Röntgen rays are quite different; for example, the relative -conductivity produced by them in methyl iodide was more than -14 times as great as that due to the rays of radium. The relative -conductivities of gases exposed to X rays has been recently -re-examined by McClung<a id='r77' href='#f77' class='c012'><sup>[77]</sup></a> and Eve<a id='r78' href='#f78' class='c012'><sup>[78]</sup></a>, who have found that the -conductivity depends upon the penetrating power of the X rays -employed. The results obtained by them will be discussed later -(<a href='#section107'>section 107</a>).</p> - -<p class='c006'><span class='pageno' id='Page_65'>65</span>This difference of conductivity in gases is due to unequal -absorptions of the radiations. The writer has shown<a id='r79' href='#f79' class='c012'><sup>[79]</sup></a> that the -total number of ions produced by the α rays for uranium, when -completely absorbed by different gases, is not very different. The -following results were obtained:</p> - -<table class='table8' > -<colgroup> -<col class='colwidth75'> -<col class='colwidth25'> -</colgroup> - <tr> - <td class='c013'>Gas</td> - <td class='c016'>Total Ionization</td> - </tr> - <tr> - <td class='c013'>Air</td> - <td class='c016'>100</td> - </tr> - <tr> - <td class='c013'>Hydrogen</td> - <td class='c016'>95</td> - </tr> - <tr> - <td class='c013'>Oxygen</td> - <td class='c016'>106</td> - </tr> - <tr> - <td class='c013'>Carbonic acid</td> - <td class='c016'>96</td> - </tr> - <tr> - <td class='c013'>Hydrochloric acid gas</td> - <td class='c016'>102</td> - </tr> - <tr> - <td class='c013'>Ammonia</td> - <td class='c016'>101</td> - </tr> -</table> - -<p class='c006'>The numbers, though only approximate in character, seem to -show that the energy required to produce an ion is probably not -very different for the various gases. Assuming that the energy -required to produce an ion in different gases is about the same, -it follows that the relative conductivities are proportional to the -relative absorption of the radiations.</p> - -<p class='c006'>A similar result has been found by McLennan for cathode rays. -He proved that the ionization was directly proportional to the -absorption of the rays in the gas, thus showing that the same -energy is required to produce an ion in all the gases examined.</p> -<p class='c005'><b>46. Potential Gradient.</b> The normal potential gradient -between two charged electrodes is always disturbed when the gas -is ionized in the space between them. If the gas is uniformly -ionized between two parallel plates, Child and Zeleny have shown -that there is a sudden drop of potential near the surface of both -plates, and that the electric field is sensibly uniform for the intermediate -space between them. The disturbance of the potential -gradient depends upon the difference of potential applied, and is -different at the surface of the two plates.</p> - -<p class='c006'>In most measurements of radio-activity the material is spread -over one plate only. In such a case the ionization is to a large -extent confined to the volume of the air close to the active plate. -The potential gradient in such a case is shown in <a href='#fig009'>Fig. 9</a>. The -<span class='pageno' id='Page_66'>66</span>dotted line shows the variation of potential at any point between -the plates when no ionization is produced between the plates; -curve <i>A</i> for weak ionization, such as is produced by uranium, -curve <i>B</i> for the intense ionization produced by a very active -substance. In both cases the potential gradient is least near the -active plate, and greatest near the opposite plate. For very -intense ionization it is very small near the active surface. The -potential gradient varies slightly according as the active plate is -charged positively or negatively.</p> - -<div id='fig009' class='figcenter id006'> -<img src='images/fig-009.png' alt='Fig. 9.' class='ig001'> -<div class='ic002'> -<p>Fig. 9.</p> -</div> -</div> -<p class='c005'><a id='section047'></a> -<b>47. Variation of current with voltage for surface ionization.</b></p> - -<p class='c006'>Some very interesting results, giving the variation of the -current with voltage, are observed when the ionization is intense, -and confined to the space near the surface of one of two parallel -plates between which the current is measured.</p> - -<p class='c006'>The theory of this subject has been worked out independently -by Child<a id='r80' href='#f80' class='c012'><sup>[80]</sup></a> and Rutherford<a id='r81' href='#f81' class='c012'><sup>[81]</sup></a>. Let <i>V</i> be the potential difference -<span class='pageno' id='Page_67'>67</span>between two parallel plates at a distance <i>d</i> apart. Suppose that -the ionization is confined to a thin layer near the surface of the -plate <i>A</i> (see <a href='#fig001'>Fig. 1</a>) which is charged positively. When the electric -field is acting, there is a distribution of positive ions between the -plates <i>A</i> and <i>B</i>.</p> - -<div class='calc'> - -<div class='lg-container-b c017'> - <div class='linegroup'> - <div class='group'> - <div class='line'>Let</div> - <div class='line'><i>n</i><sub>1</sub></div> - <div class='line'>= number of positive ions per unit volume at a distance <i>x</i> from the plate <i>A</i>,</div> - </div> - <div class='group'> - <div class='line'><i>K</i><sub>1</sub></div> - <div class='line'>= mobility of the positive ions,</div> - </div> - <div class='group'> - <div class='line'><i>e</i> = charge on an ion.</div> - </div> - </div> -</div> - -</div> - -<p class='c018'>The current -<i>i</i><sub>1</sub> -per square centimetre through the gas is -constant for all values of <i>x</i>, and is given by</p> - -<div class='figcenter id009'> -<img src='images/form-018.png' alt='Formula.' class='ig001'> -</div> - -<p class='c006'>By Poisson’s equation</p> - -<div class='figcenter id010'> -<img src='images/form-019.png' alt='Formula.' class='ig001'> -</div> - -<p class='c006'>Then</p> - -<div class='figcenter id009'> -<img src='images/form-020.png' alt='Formula.' class='ig001'> -</div> - -<p class='c006'>Integrating</p> - -<div class='figcenter id005'> -<img src='images/form-021.png' alt='Formula.' class='ig001'> -</div> - -<p class='c006'>where <i>A</i> is a constant. Now <i>A</i> is equal to the value of</p> - -<div class='calc'> - -<div class='lg-container-b c017'> - <div class='linegroup'> - <div class='group'> - <div class='line'> <i>dV</i></div> - <div class='line'>----</div> - <div class='line'> <i>dx</i></div> - </div> - </div> -</div> - -</div> - -<p class='c018'>when -<i>x</i> = 0. By making the ionization very intense, the value of</p> - -<div class='calc'> - -<div class='lg-container-b c017'> - <div class='linegroup'> - <div class='group'> - <div class='line'> <i>dV</i></div> - <div class='line'>----</div> - <div class='line'> <i>dx</i></div> - </div> - </div> -</div> - -</div> - -<p class='c018'>can be made extremely small.</p> - -<p class='c006'>Putting <i>A</i> = 0, we see that</p> - -<div class='figcenter id009'> -<img src='images/form-022.png' alt='Formula.' class='ig001'> -</div> - -<p class='c006'>This gives the potential gradient between the plates for different -values of <i>x</i>.</p> - -<p class='c006'>Integrating between the limits 0 and <i>d</i>,</p> - -<div class='figcenter id005'> -<img src='images/form-023.png' alt='Formula.' class='ig001'> -</div> - -<p class='c006'>or</p> - -<div class='figcenter id009'> -<img src='images/form-024.png' alt='Formula.' class='ig001'> -</div> - -<p class='c006'><span class='pageno' id='Page_68'>68</span>If -<i>i</i><sub>2</sub> -is the value of the current when the electric field is -reversed, and -<i>K</i><sub>2</sub> -the velocity of the negative ion,</p> - -<div class='figcenter id009'> -<img src='images/form-025.png' alt='Formula.' class='ig001'> -</div> - -<p class='c006'>and</p> - -<div class='calc'> - -<div class='lg-container-b c017'> - <div class='linegroup'> - <div class='group'> - <div class='line'> <i>i<sub>1</sub></i> <i>K<sub>1</sub></i></div> - <div class='line'>--- = ---- .</div> - <div class='line'> <i>i<sub>2</sub></i> <i>K<sub>2</sub></i></div> - </div> - </div> -</div> - -</div> - -<p class='c018'>The current in the two directions is thus directly proportional -to the velocities of the positive and negative ions. The current -should vary directly as the square of the potential difference -applied, and inversely as the cube of the distance between the -plates.</p> - -<p class='c006'>The theoretical condition of surface ionization cannot be fulfilled -by the ionization due to active substances, as the ionization extends -some centimetres from the active plate. If, however, the distance -between the plates is large compared with the distance over which -the ionization extends, the results will be in rough agreement with -the theory. Using an active preparation of radium, the writer has -made some experiments on the variation of current with voltage -between parallel plates distant about 10 cms. from each other<a id='r82' href='#f82' class='c012'><sup>[82]</sup></a>.</p> - -<p class='c006'>The results showed</p> - -<p class='c006'>(1) That the current through the gas for small voltages -increased more rapidly than the potential difference applied, but -not as rapidly as the square of that potential difference.</p> - -<p class='c006'>(2) The current through the gas depended on the direction of -the electric field; the current was always smaller when the active -plate was charged positively on account of the smaller mobility of -the positive ion. The difference between -<i>i</i><sub>1</sub> -and -<i>i</i><sub>2</sub> -was greatest -when the gas was dry, which is the condition for the greatest -difference between the velocities of the ions.</p> - -<p class='c006'>An interesting result follows from the above theory. For given -values of <i>V</i> and <i>d</i>, the current cannot exceed a certain definite -value, however much the ionization may be increased. In a -similar way, when an active preparation of radium is used as a -source of surface ionization, it is found that, for a given voltage -<span class='pageno' id='Page_69'>69</span>and distance between the plates, the current does not increase -beyond a certain value however much the activity of the material -is increased.</p> -<p class='c005'><a id='section048'></a> -<b>48. Magnetic field produced by an ion in motion.</b> It -will be shown later that the two most important kinds of rays -emitted by radio-active substances consist of electrified particles, -spontaneously projected with great velocity. The easily absorbed -rays, known as α rays, are positively electrified atoms of matter; -the penetrating rays, known as β rays, carry a negative charge, -and have been found to be identical with the cathode rays produced -by the electric discharge in a vacuum tube.</p> - -<p class='c006'>The methods adopted to determine the character of these rays -are very similar to those first used by J. J. Thomson to show that -the cathode rays consisted of a stream of negatively electrified -particles projected with great velocity.</p> - -<p class='c006'>The proof that the cathode rays were corpuscular in character, -and consisted of charged particles whose mass was very small compared -with that of the hydrogen atom, marked an important epoch -in physical science: for it not only opened up new and fertile fields -of research, but also profoundly modified our previous conceptions -of the constitution of matter.</p> - -<p class='c006'>A brief account will accordingly be given of the effects produced -by a moving charged body, and also of some of the experimental -methods which have been used to determine the mass and velocity -of the particles of the cathode stream<a id='r83' href='#f83' class='c012'><sup>[83]</sup></a>.</p> - -<p class='c006'>Consider an ion of radius <i>a</i>, carrying a charge of electricity <i>e</i>, -and moving with a velocity <i>u</i>, small compared with the velocity of -light. In consequence of the motion, a magnetic field is set up -around the charged ion, which is carried with it. The charged ion -in motion constitutes a current element of magnitude <i>eu</i>, and the -magnetic field <i>H</i> at any point distant <i>r</i> from the sphere is given by</p> - -<div class='calc'> - -<div class='lg-container-b c017'> - <div class='linegroup'> - <div class='group'> - <div class='line in4'><i>eu</i> sin θ</div> - <div class='line'><i>H</i> = -----</div> - <div class='line in6'><i>r<sub>2</sub></i></div> - </div> - </div> -</div> - -</div> - -<p class='c018'><span class='pageno' id='Page_70'>70</span>where θ is the angle the radius vector makes with the direction of -motion. The lines of magnetic force are circles around the axis -of motion. When the ion is moving with a velocity small compared -with the velocity of light, the lines of electric force are nearly -radial, but as the speed of light is approached, they tend to leave -the axis of motion and to bend towards the equator. When the -speed of the body is very close to that of light, the magnetic and -electric field is concentrated to a large extent in the equatorial -plane.</p> - -<p class='c006'>The presence of a magnetic field around the moving body -implies that magnetic energy is stored up in the medium surrounding -it. The amount of this energy can be calculated very simply -for slow speeds.</p> - -<p class='c006'>In a magnetic field of strength <i>H</i>, the magnetic energy stored -up in unit volume of the medium of unit permeability is given by</p> - -<div class='calc'> - -<div class='lg-container-b c017'> - <div class='linegroup'> - <div class='group'> - <div class='line in1'><i>H<sup>2</sup></i></div> - <div class='line'>----</div> - <div class='line in1'>8π</div> - </div> - </div> -</div> - -</div> - -<p class='c018'>Integrating the value of this expression over the region -exterior to a sphere of radius <i>a</i>, the total magnetic energy due to -the motion of the charged body is given by</p> - -<div class='figcenter id007'> -<img src='images/form-026.png' alt='Formula.' class='ig001'> -</div> - -<p class='c006'>The magnetic energy, due to the motion, is analogous to kinetic -energy, for it depends upon the square of the velocity of the body. -In consequence of the charge carried by the ion, additional kinetic -energy is associated with it. If the velocity of the ion is changed, -electric and magnetic forces are set up tending to stop the change -of motion, and more work is done during the change than if the -ion were uncharged. The ordinary kinetic energy of the body is</p> - -<div class='calc'> - -<div class='lg-container-b c017'> - <div class='linegroup'> - <div class='group'> - <div class='line'>1</div> - <div class='line'>-- <i>mu</i><sup>2</sup></div> - <div class='line'>2</div> - </div> - </div> -</div> - -</div> - -<p class='c018'>In consequence of its charge, the kinetic energy associated -with it is increased by</p> - -<div class='calc'> - -<div class='lg-container-b c017'> - <div class='linegroup'> - <div class='group'> - <div class='line in1'><i>e<sup>2</sup>u<sup>2</sup></i></div> - <div class='line'>----</div> - <div class='line in1'>3<i>a</i></div> - </div> - </div> -</div> - -</div> - -<p class='c018'>It thus behaves as if it possessed a -mass -<i>m</i> + <i>m</i><sub>1</sub> -where -<i>m</i><sub>1</sub> -is <i>the electrical mass</i>, with the value</p> - -<div class='calc'> - -<div class='lg-container-b c017'> - <div class='linegroup'> - <div class='group'> - <div class='line'>2<i>e</i><sup>2</sup></div> - <div class='line'>---</div> - <div class='line'>3<i>a</i></div> - </div> - </div> -</div> - -</div> - -<p class='c018'><span class='pageno' id='Page_71'>71</span>We have so far only considered the electrical mass of a charged -ion moving with a velocity small compared with that of light. As -the speed of light is approached, the magnetic energy can no -longer be expressed by the equation already given. The general -values of the electrical mass of a charged body for speed were first -worked out by J. J. Thomson<a id='r84' href='#f84' class='c012'><sup>[84]</sup></a> in 1887. A more complete examination -was made in 1889 by Heaviside<a id='r85' href='#f85' class='c012'><sup>[85]</sup></a>, while Searle<a id='r86' href='#f86' class='c012'><sup>[86]</sup></a> worked out -the case for a charged ellipsoid. Recently, the question was again -attacked by Abraham<a id='r87' href='#f87' class='c012'><sup>[87]</sup></a>. Slightly different expressions for the -variation of electrical mass with speed have been obtained, depending -upon the conditions assumed for the distribution of the -electricity on the sphere. The expression found by Abraham, -which has been utilized by Kaufmann to show that the mass of the -electron is electromagnetic in origin, is given later in <a href='#section082'>section 82</a>.</p> - -<p class='c006'>All the calculations agree in showing that the electrical mass -is practically constant for slow speeds, but increases as the speed -of light is approached, and is theoretically infinite when the speed -of light is reached. The nearer the velocity of light is approached, -the greater is the resisting force to a change of motion. An infinite -force would be required to make an electron actually attain the -velocity of light, so that, according to the present theory, it would -be impossible for an electron to move faster than light, <i>i.e.</i> faster -than an electromagnetic disturbance travels in the ether.</p> - -<p class='c006'>The importance of these deductions lies in the fact that an -electric charge in motion, quite independently of any material -nucleus, possesses an apparent mass in virtue of its motion, and -that this mass is a function of the speed. Indeed, we shall see -later (see section 82) that the apparent mass of the particles constituting -the cathode stream can be explained in virtue of their -charge, without the necessity of assuming a material body in which -the charge is distributed. This has led to the suggestion that all -mass may be electrical in origin, and due purely to electricity in -motion.</p> -<p class='c005'><b>49. Action of a magnetic field on a moving ion.</b> Let us -consider the case of an ion of mass <i>m</i> carrying a charge <i>e</i> and -<span class='pageno' id='Page_72'>72</span>moving freely with a velocity <i>u</i>. If <i>u</i> is small compared with the -velocity of light, the ion in motion corresponds to a current -element of magnitude <i>eu</i>. If the ion moves in an external -magnetic field of strength <i>H</i>, it is acted on by a force at right -angles both to the direction of motion, and to that of the magnetic -force and equal in magnitude to <i>Heu</i> sin θ, where θ is the angle -between the direction of the magnetic force and the direction of -motion. Since the force due to the magnetic field is always -perpendicular to the direction of motion, it has no effect upon -the velocity of the particle, but can only alter the direction of its -path.</p> - -<p class='c006'>If ρ is the radius of curvature of the path of the ion, the force -along the normal is equal to</p> - -<div class='calc'> - -<div class='lg-container-b c017'> - <div class='linegroup'> - <div class='group'> - <div class='line in2'><i>mu<sup>2</sup></i></div> - <div class='line in1'>--- ,</div> - <div class='line in2'>ρ</div> - </div> - </div> -</div> - -</div> - -<p class='c018'>and this is balanced by the force -<i>Heu</i> sin θ.</p> - -<p class='c006'>If</p> - -<div class='calc'> - -<div class='lg-container-b c017'> - <div class='linegroup'> - <div class='group'> - <div class='line in5'>π</div> - <div class='line'>θ = --- ,</div> - <div class='line in5'>2</div> - </div> - </div> -</div> - -</div> - -<p class='c018'><i>i.e.</i> if the ion is moving at right angles to the direction -of the magnetic field</p> - -<div class='calc'> - -<div class='lg-container-b c017'> - <div class='linegroup'> - <div class='group'> - <div class='line in8'><i>mu<sup>2</sup></i></div> - <div class='line'><i>Heu</i> = ----</div> - <div class='line in8'>ρ</div> - </div> - <div class='group'> - </div> - </div> -</div> - -</div> - -<p class='c018'>or</p> - -<div class='calc'> - -<div class='lg-container-b c017'> - <div class='linegroup'> - <div class='group'> - <div class='line in8'><i>m</i></div> - <div class='line'><i>H</i>ρ = ----- <i>u</i></div> - <div class='line in8'><i>e</i></div> - </div> - </div> -</div> - -</div> - -<p class='c018'>Since <i>u</i> is constant, -ρ is also constant, <i>i.e.</i> the particle describes a circular orbit of -radius ρ. The radius of the circular orbit is thus directly -proportional to <i>u</i>, and inversely proportional to <i>H</i>.</p> - -<p class='c006'>If the ion is moving at an angle θ with the direction of the -magnetic field, it describes a curve which is compounded of a -motion of a particle of velocity <i>u</i> sin θ perpendicular to the field and -<i>u</i> cos θ in the direction of the field. The former describes a circular -orbit of radius ρ, given by</p> - -<div class='calc'> - -<div class='lg-container-b c017'> - <div class='linegroup'> - <div class='group'> - <div class='line in7'><i>m</i></div> - <div class='line'><i>H</i>ρ = --- <i>u</i> sin θ ;</div> - <div class='line in7'><i>e</i></div> - </div> - </div> -</div> - -</div> - -<p class='c018'>the latter is unaffected -by the magnetic field and moves uniformly in the direction -of the magnetic field with a velocity <i>u</i> cos θ. The motion of -the particle is in consequence a helix, traced on a cylinder of -radius</p> - -<div class='calc'> - -<div class='lg-container-b c017'> - <div class='linegroup'> - <div class='group'> - <div class='line in5'><i>mu</i> sin θ</div> - <div class='line'>ρ = --------- ,</div> - <div class='line in8'><i>eH</i></div> - </div> - </div> -</div> - -</div> - -<p class='c018'>whose axis is in the direction of the magnetic -field. Thus an ion projected obliquely to the direction of a -uniform magnetic field always moves in a helix whose axis is -parallel to the lines of magnetic force<a id='r88' href='#f88' class='c012'><sup>[88]</sup></a>.</p> -<p class='c005'><span class='pageno' id='Page_73'>73</span><a id='section050'></a> -<b>50. Determination of e/m for the cathode stream.</b> The -cathode rays, first observed by Varley, were investigated in detail -by Crookes. These rays are projected from the cathode in a -vacuum tube at low pressure. They travel in straight lines, and -are readily deflected by a magnet, and produce strong luminosity -in a variety of substances placed in their path. The rays are -deflected by a magnetic field in the same direction as would be -expected for a negatively charged particle projected from the -cathode. In order to explain the peculiar properties of these rays -Crookes supposed that they consisted of negatively electrified -particles, moving with great velocity and constituting, as he -appropriately termed it, “a new or fourth state of matter.” The -nature of these rays was for twenty years a subject of much -controversy, for while some upheld their material character, others -considered that they were a special form of wave motion in the -ether.</p> - -<p class='c006'>Perrin and J. J. Thomson showed that the rays always carried -with them a negative charge, while Lenard made the important -discovery that the rays passed through thin metal foil and other -substances opaque to ordinary light. Using this property, he sent -the rays through a thin window and examined the properties of -the rays outside the vacuum tube in which they were produced.</p> - -<p class='c006'>The absorption of the rays by matter was shown to be nearly -proportional to the density over a very wide range, and to be -independent of its chemical constitution.</p> - -<p class='c006'>The nature of these rays was successfully demonstrated by -J. J. Thomson<a id='r89' href='#f89' class='c012'><sup>[89]</sup></a> in 1897. If the rays consisted of negatively -electrified particles, they should be deflected in their passage -through an electric as well as through a magnetic field. Such an -experiment had been tried by Hertz, but with negative results. -J. J. Thomson, however, found that the rays were deflected by an -electric field in the direction to be expected for a negatively -charged particle, and showed that the failure of Hertz to detect -the same was due to the masking of the electric field by the strong -ionization produced in the gas by the cathode stream. This effect -was got rid of by reducing the pressure of the gas in the tube.</p> - -<p class='c006'><span class='pageno' id='Page_74'>74</span>The experimental arrangement used for the electric deflection -of the rays is shown in <a href='#fig010'>Fig. 10</a>.</p> - -<p class='c006'>The cathode rays are generated at the cathode <i>C</i>, and a narrow -pencil of rays is obtained by passing the rays through a perforated -disc <i>AB</i>. The rays then passed midway between two parallel -insulated plates <i>D</i> and <i>E</i>, <i>d</i> centimetres apart, and maintained at -a constant difference of potential <i>V</i>. The point of incidence of the -pencil of rays was marked by a luminous patch produced on a -phosphorescent screen placed at <i>PP´</i>.</p> - -<p class='c006'>The particle carrying a negative charge <i>e</i> in passing between the -charged plates, is acted on by a force <i>Xe</i> directed towards the positive -plate, where <i>X</i>, the strength of the electric field, is given by</p> - -<div class='calc'> - -<div class='lg-container-b c017'> - <div class='linegroup'> - <div class='group'> - <div class='line in1'><i>V</i></div> - <div class='line'>--- .</div> - <div class='line in1'><i>d</i></div> - </div> - </div> -</div> - -</div> - -<div id='fig010' class='figcenter id004'> -<img src='images/fig-010.png' alt='Fig. 10.' class='ig001'> -<div class='ic002'> -<p>Fig. 10.</p> -</div> -</div> - -<p class='c018'>The application of the electric field thus causes the luminous -patch to move in the direction of the positive plate. If now a -uniform magnetic field is applied at the plates <i>D</i> and <i>E</i>, perpendicular -to the pencil of rays, and parallel to the plane of the plates, -and in such a direction that the electric and magnetic forces are -opposed to one another, the patch of light can be brought back to its -undisturbed position by adjusting the strength of the magnetic field. -If <i>H</i> is the strength of the magnetic field, the force on the particle -due to the magnetic field is <i>Heu</i>, and when a balance is obtained</p> - -<div class='calc'> - -<div class='lg-container-b c017'> - <div class='linegroup'> - <div class='group'> - <div class='line'><i>Heu</i> = <i>Xe</i>,</div> - </div> - <div class='group'> - <div class='line'>or</div> - <div class='line in6'><i>X</i></div> - <div class='line'><i>u</i> = --- (1).</div> - <div class='line in6'><i>H</i></div> - </div> - </div> -</div> - -</div> - -<p class='c018'>Now if the magnetic field <i>H</i> is acting alone, the curvature ρ of the -path of the rays between the plates can be deduced from the -deflection of the luminous patch. But we have seen that</p> - -<div class='calc'> - -<div class='lg-container-b c017'> - <div class='linegroup'> - <div class='group'> - <div class='line in6'><i>mu</i></div> - <div class='line'><i>H</i> = ---- (2).</div> - <div class='line in6'><i>e</i></div> - </div> - </div> -</div> - -</div> -<p class='c018'><span class='pageno' id='Page_75'>75</span>From equations (1) and (2), the value of <i>u</i> and <i>e</i>/<i>m</i> for the particle -can be determined.</p> - -<p class='c006'>The velocity <i>u</i> is not constant, but depends upon the -potential difference between the electrodes, and this in turn -depends upon the pressure and nature of the residual gas in the -tube.</p> - -<p class='c006'>By altering these factors, the cathode particles may be made -to acquire velocities varying between about 10<sup>9</sup> and 10<sup>10</sup> cms. per -second. This velocity is enormous compared with that which can -be impressed ordinarily upon matter by mechanical means. On the -other hand, the value of <i>e</i>/<i>m</i> for the particles is sensibly constant for -different velocities.</p> - -<p class='c006'>As a result of a series of experiments the mean value -<i>e</i>/<i>m</i> = -7·7 × 10<sup>6</sup> -was obtained. The value of <i>e</i>/<i>m</i> is independent of the -nature or pressure of the gas in the vacuum tube and independent -of the metal used as cathode. A similar value of <i>e</i>/<i>m</i> was obtained -by Lenard<a id='r90' href='#f90' class='c012'><sup>[90]</sup></a> and others.</p> - -<p class='c006'>Kaufmann<a id='r91' href='#f91' class='c012'><sup>[91]</sup></a> and Simon<a id='r92' href='#f92' class='c012'><sup>[92]</sup></a> used a different method to determine -the value of <i>e</i>/<i>m</i>. The potential difference <i>V</i> between the terminals -of the tube was measured. The work done on the charged particle -in moving from one end of the tube to the other is <i>Ve</i>, and this -must be equal to the kinetic energy</p> - -<div class='calc'> - -<div class='lg-container-b c017'> - <div class='linegroup'> - <div class='group'> - <div class='line'>1</div> - <div class='line'>-- <i>mu<sup>2</sup></i></div> - <div class='line'>2</div> - </div> - </div> -</div> - -</div> - -<p class='c018'>acquired by the moving -particle. Thus</p> - -<div class='calc'> - -<div class='lg-container-b c017'> - <div class='linegroup'> - <div class='group'> - <div class='line'> <i>e</i> <i>u<sup>2</sup></i></div> - <div class='line in1'>--- = --- (3).</div> - <div class='line'> <i>m</i> 2<i>V</i></div> - </div> - </div> -</div> - -</div> - -<p class='c018'>By combination of this equation with (2) obtained by measurement -of the magnetic deflexion, both <i>u</i> and <i>e</i>/<i>m</i> can be determined.</p> - -<p class='c006'>Simon found by this method that</p> - -<div class='calc'> - -<div class='lg-container-b c017'> - <div class='linegroup'> - <div class='group'> - <div class='line'><i>e</i></div> - <div class='line'>-- = 1·865 × 10<sup>7</sup>.</div> - <div class='line'><i>m</i></div> - </div> - </div> -</div> - -</div> - -<p class='c018'><span class='pageno' id='Page_76'>76</span>It will be seen later (<a href='#section082'>section 82</a>) that a similar value was deduced -by Kaufmann for the electrons projected from radium.</p> - -<p class='c006'>These results, which have been based on the effect of a -magnetic and electric field on a moving ion, were confirmed by -Weichert, who determined by a direct method the time required -for the particle to traverse a known distance.</p> - -<p class='c006'>The particles which make up the cathode stream were termed -“corpuscles” by J. J. Thomson. The name “electron,” first employed -by Johnstone Stoney, has also been applied to them and -has come into general use<a id='r93' href='#f93' class='c012'><sup>[93]</sup></a>.</p> - -<p class='c006'>The methods above described do not give the mass of the -electron, but only the ratio of the charge to the mass. A direct -comparison can, however, be made between the ratio <i>e</i>/<i>m</i> for the -electron and the corresponding value for the hydrogen atoms set -free in the electrolysis of water. Each of the hydrogen atoms -is supposed to carry a charge <i>e</i>, and it is known that 96,000 -coulombs of electricity, or, in round numbers, -10<sup>4</sup> -electromagnetic -units of quantity are required to liberate one gram of hydrogen. -If <i>N</i> is the number of atoms in one gram of hydrogen, then -<i>Ne</i> = -10<sup>4</sup>. -But if <i>m</i> is the mass of a hydrogen atom, then <i>Nm</i> = 1. -Dividing one by the other <i>e</i>/<i>m</i> = -10<sup>4</sup>. -We have seen already that a -gaseous ion carries the same charge as a hydrogen atom, while -indirect evidence shows that the electron carries the same charge -as an ion, and consequently the same charge as the atom of -hydrogen. Hence we may conclude that the apparent mass -of the electron is only about ¹⁄₁₀₀₀ of the mass of the hydrogen -atom. The electron thus behaves as the smallest body known to -science.</p> - -<p class='c006'>In later experiments J. J. Thomson showed that the negative -ions set free at low pressures by an incandescent carbon filament, -and also the negative ions liberated from a zinc plate exposed to -the action of ultra-violet light, had the same value for <i>e</i>/<i>m</i> as the -<span class='pageno' id='Page_77'>77</span>electrons produced in a vacuum tube. It thus seemed probable -that the electron was a constituent of all matter. This view -received strong support from measurements of quite a different -character. Zeeman in 1897 found that the lines of the spectrum -from a source of light exposed in a strong magnetic field were -displaced and doubled. Later work has shown that the lines in -some cases are trebled, in others sextupled, while, in a few cases, -the multiplication is still greater. These results received a general -explanation on the radiation theories previously advanced by -Lorenz and Larmor. The radiation, emitted from any source, was -supposed to result from the orbital or oscillatory motion of the -charged parts constituting the atom. Since a moving ion is acted -on by an external magnetic field, the motion of the charged ions -is disturbed when the source of light is exposed between the poles -of a strong magnet. There results a small change in the period -of the emitted light, and a bright line in the spectrum is, in -consequence, displaced by the action of the magnetic field. According -to theory, the small change in the wave-length of the emitted -light depends upon the strength of the magnetic field and on the -ratio <i>e</i>/<i>m</i> of the charge carried by the ion to its mass. By comparison -of the theory with the experimental results, it was deduced -that the moving ion carried a negative charge, and that the value -of <i>e</i>/<i>m</i> was about -10<sup>7</sup>. -The charged ion, responsible for the radiation -from a luminous body, is thus identical with the electron set -free in a vacuum tube.</p> - -<p class='c006'>It thus seems reasonable to suppose that the atoms of all -bodies are complex and are built up, in part at least, of electrons, -whose apparent mass is very small compared with that of the -hydrogen atom. The properties of such disembodied charges has -been examined mathematically among others by Larmor, who sees -in this conception the ultimate basis of a theory of matter. -J. J. Thomson and Lord Kelvin have investigated mathematically -certain arrangements of a number of electrons which are stable for -small disturbances. This question will be discussed more in detail -in <a href='#section270'>section 270</a>.</p> -<p class='c005'><span class='pageno' id='Page_78'>78</span><a id='section051'></a> -<b>51. Canal rays.</b> If a discharge is passed through a vacuum -tube provided with a perforated cathode, within certain limits of -pressure, luminous streams are observed to pass through the holes -and to emerge on the side of the cathode remote from the anode. -These rays were first observed by Goldstein<a id='r94' href='#f94' class='c012'><sup>[94]</sup></a> and were called by -him the “Canal-strahlen.” These rays travel in straight lines and -produce phosphorescence in various substances.</p> - -<p class='c006'>Wien<a id='r95' href='#f95' class='c012'><sup>[95]</sup></a> showed that the canal rays were deflected by strong -magnetic and electric fields, but the amount of deflection was very -small compared with that of the cathode rays under similar conditions. -The deflection was found to be opposite in direction to -the cathode rays, and this indicates that the canal rays consist -of positive ions. Wien determined their velocity and the ratio -<i>e</i>/<i>m</i>, by measuring the amount of their magnetic and electric -deflection. The value of <i>e</i>/<i>m</i> was found to be variable, depending -upon the gas in the tube, but the maximum value observed was 10<sup>4</sup>. -This shows that the positive ion, in no case, has a mass less than -that of the hydrogen atom. It seems probable that the canal rays -consist of positive ions, derived either from the gas or the electrodes, -which travel towards the cathode, and have sufficient -velocity to pass through the holes of the cathode and to appear in -the gas beyond.</p> - -<p class='c006'>It is remarkable that, so far, no case has been observed where -the carrier of a positive charge has an apparent mass less than -that of the hydrogen atom. Positive electricity always appears to -be associated with bodies atomic in size. We have seen that the -process of ionization in gases is supposed to consist of the expulsion -of an electron from the atom. The corresponding positive -charge remains behind on the atom and travels with it. This -difference between positive and negative electricity appears to -be fundamental, and no explanation of it has, as yet, been forthcoming.</p> -<p class='c005'><a id='section052'></a> -<b>52. Radiation of energy.</b> If an electron moves uniformly -in a straight line with constant velocity, the magnetic field, which -<span class='pageno' id='Page_79'>79</span>travels with it, remains constant, and there is no loss of energy -from it by radiation. If, however, its motion is hastened or -retarded, the magnetic field is altered, and there results a loss of -energy from the electron in the form of electromagnetic radiation. -The rate of loss of energy from an accelerated electron was first -calculated by Larmor<a id='r96' href='#f96' class='c012'><sup>[96]</sup></a> and shown to be</p> - -<div class='calc'> - -<div class='lg-container-b c017'> - <div class='linegroup'> - <div class='group'> - <div class='line'>2<i>e<sup>2</sup></i></div> - <div class='line'>---- × (acceleration)<sup>2</sup> ,</div> - <div class='line'>3<i>V</i></div> - </div> - </div> -</div> - -</div> - -<p class='c018'>where -<i>e</i> is the charge on the electron in electromagnetic units, and <i>V</i> the -velocity of light.</p> - -<p class='c006'>Any alteration in the velocity of a moving charge is thus -always accompanied by a radiation of energy from it. Since the -electron, set free in a vacuum tube, increases in velocity in passing -through the electric field, energy must be radiated from it during -its passage from cathode to anode. It can, however, readily be -calculated that, in ordinary cases, this loss of energy is small compared -with the kinetic energy acquired by the electron in passing -through the electric field.</p> - -<p class='c006'>An electron moving in a circular orbit is a powerful radiator of -energy, since it is constantly accelerated towards the centre. An -electron moving in an orbit of radius equal to the radius of an -atom (about -10<sup>-8</sup> -cms.) would lose most of its kinetic energy of -motion in a small fraction of a second, even though its velocity was -originally nearly equal to the velocity of light. If, however, a -number of electrons are arranged at equal angular intervals on the -circumference of a circle and move with constant velocity round -the ring, the radiation of energy is much less than for a single -electron, and rapidly diminishes with an increase in the number of -electrons round the ring. This result, obtained by J. J. Thomson, -will be discussed in more detail later when the stability of systems -composed of rotating electrons is under consideration.</p> - -<p class='c006'>Since the radiation of energy is proportional to the square of the -acceleration, the proportion of the total energy radiated depends -upon the suddenness with which an electron is started or stopped. -Now some of the cathode ray particles are stopped abruptly when -they impinge on the metal cathode, and, in consequence, give up a -fraction of their kinetic energy in the form of electromagnetic -radiation. Stokes and Weichert suggested that this radiation -<span class='pageno' id='Page_80'>80</span>constituted the X rays, which are known to have their origin at -the surface on which the cathode rays impinge. The mathematical -theory has been worked out by J. J. Thomson<a id='r97' href='#f97' class='c012'><sup>[97]</sup></a>. If the motion of -an electron is suddenly arrested, a thin spherical pulse in which -the magnetic and electric forces are very intense travels out from -the point of impact with the velocity of light. The more suddenly -the electron is stopped, the thinner and more intense is the pulse. -On this view the X rays are not corpuscular like the cathode rays, -which produce them, but consist of transverse disturbances in the -ether, akin in some respects to light waves of short wave-length. -The rays are thus made up of a number of pulses, which are non-periodic -in character, and which follow one another at irregular -intervals.</p> - -<p class='c006'>On this theory of the nature of the X rays, the absence of -direct deflection, refraction, or polarization is to be expected, if -the thickness of the pulse is small compared with the diameter of -an atom. It also explains the non-deflection of the path of the -rays by a magnetic or electric field. The intensity of the electric -and magnetic force in the pulse is so great that it is able to cause -a removal of an electron from some of the atoms of the gas, over -which the pulse passes, and thus causes the ionization observed.</p> - -<p class='c006'>The cathode rays produce X rays, and these in turn give rise -to a secondary radiation whenever they impinge on a solid body. -This secondary radiation is emitted equally in all directions, -and consists partly of a radiation of the X ray type and also of -electrons projected with considerable velocity. This secondary -radiation gives rise to a tertiary radiation and so on.</p> - -<p class='c006'>Barkla<a id='r98' href='#f98' class='c012'><sup>[98]</sup></a> has shown that the secondary radiation emitted from -a gas through which the rays pass consists in part of scattered -X rays of about the same penetrating power as the primary rays -as well as some easily absorbed rays.</p> - -<p class='c006'>Part of the cathode rays is diffusely reflected on striking the -cathode. These scattered rays consist in part of electrons of the -same speed as in the primary beam, but also include some others of -much less velocity. The amount of diffuse reflection depends upon -the nature of the cathode and the angle of incidence of the rays.</p> - -<p class='c006'><span class='pageno' id='Page_81'>81</span>We shall see later (<a href='#chap04'>chapter <span class='fss'>IV.</span></a>) that similar effects are produced -when the rays from radio-active substances impinge upon -solid bodies.</p> - -<hr class='c008'> - -<p class='c006'>In this chapter an account of the ionization theory of gases has -been given to the extent that is necessary for the interpretation -of the measurements of radio-activity by the electric method. It -would be out of place here to discuss the development of that -theory in detail, to explain the passage of electricity through -flames and vapours, the discharge of electricity from hot bodies, -and the very complicated phenomena observed in the passage of -electricity through a vacuum tube.</p> - -<p class='c006'>For further information on this important subject, the reader -is referred to J. J. Thomson’s <i>Conduction of Electricity through -Gases</i>, in which the whole subject is treated in a full and complete -manner. A simple account of the effect of moving charges and -the electronic theory of matter was given by the same author in -the Silliman Lectures of Yale University and published under the -title <i>Electricity and Matter</i> (Scribner, New York, 1904).</p> - -<div class='chapter'> - <span class='pageno' id='Page_82'>82</span> - <h2 id='chap03' class='c004'>CHAPTER III. <br> METHODS OF MEASUREMENT.</h2> -</div> -<p class='c005'><b>53. Methods of Measurement.</b> Three general methods -have been employed for examination of the radiations from radio-active -bodies, depending on</p> - -<div class='lg-container-b c019'> - <div class='linegroup'> - <div class='group'> - <div class='line'>(1) The action of the rays on a photographic plate.</div> - </div> - <div class='group'> - <div class='line'>(2) The ionizing action of the rays on the surrounding gas.</div> - </div> - <div class='group'> - <div class='line'>(3) The fluorescence produced by the rays on a screen of</div> - <div class='line'>platinocyanide of barium, zinc sulphide, or similar substance.</div> - </div> - </div> -</div> - -<p class='c018'>The third method is very restricted in its application, and can -only be employed for intensely active substances like radium or -polonium.</p> - -<p class='c006'>The photographic method has been used very widely, especially -in the earlier development of the subject, but has gradually been -displaced by the electrical method, as a quantitative determination -of the radiations became more and more necessary. In certain -directions, however, it possesses distinct advantages over the electrical -method. For example, it has proved a very valuable means -of investigating the curvature of the path of the rays, when -deflected by a magnetic or electric field, and has allowed us to -determine the constants of these rays with considerable accuracy.</p> - -<p class='c006'>On the other hand, as a general method of study of the -radiations, it is open to many objections. A day’s exposure is -generally required to produce an appreciable darkening of the -sensitive film when exposed to a weak source of radiation like -uranium and thorium. It cannot, in consequence, be employed -to investigate the radiations of those active products which -<span class='pageno' id='Page_83'>83</span>rapidly lose their activity. Moreover, W. J. Russell has shown -that the darkening of a photographic plate can be produced by -many agents which do not give out rays like those of the radio-active -bodies. This darkening of the plate is produced under the -most varied conditions, and very special precautions are necessary -when long exposures to a weak source of radiation are required.</p> - -<p class='c006'>The main objection to the photographic method, however, lies -in the fact that the radiations which produce the strongest electrical -effect are very weak photographically. For example, Soddy<a id='r99' href='#f99' class='c012'><sup>[99]</sup></a> has -shown that the photographic action of uranium is due almost -entirely to the more penetrating rays, and that the easily absorbed -rays produce in comparison very little effect. Speaking generally, -the penetrating rays are the most active photographically, and, -under ordinary conditions, the action on the plate is almost -entirely due to them.</p> - -<p class='c006'>Most of the energy radiated from active bodies is in the form -of easily absorbed rays which are comparatively inactive photographically. -These rays are difficult to study by the photographic -method, as the layer of black paper which, in many cases, is required -in order to absorb the phosphorescent light from active substances, -cuts off at the same time most of the rays under examination. -These easily absorbed rays will be shown to play a far more important -part in the processes occurring in radio-active bodies than -the penetrating rays which are more active photographically.</p> - -<p class='c006'>The electrical method, on the other hand, offers a rapid and -accurate method of quantitatively examining the radiations. It can -be used as a means of measurement of all the types of radiation -emitted, excluding light waves, and is capable of accurate measurement -over an extremely wide range. With proper precautions it -can be used to measure effects produced by radiations of extremely -small intensity.</p> -<p class='c005'><b>54. Electrical Methods.</b> The electrical methods employed -in studying radio-activity are all based on the property of the -radiation in question of ionizing the gas, <i>i.e.</i> of producing positively -and negatively charged carriers throughout the volume of the gas. -The discussion of the application of the ionization theory of gases to -<span class='pageno' id='Page_84'>84</span>measurements of radio-activity has been given in the last chapter. -It has been shown there that the essential condition to be fulfilled -for comparative measurements of the intensity of the radiations -is that the electrical field shall in all cases be strong enough to -obtain the maximum or saturation current through the gas.</p> - -<p class='c006'>The electric field required to produce practical saturation -varies with the intensity of the ionization and consequently with -the activity of the preparations to be examined. For preparations -which have an activity not more than 500 times that of uranium, -under ordinary conditions, a field of 100 volts per cm. is sufficient to -produce a practical saturation current. For very active samples -of radium, it is often impossible to obtain conveniently a high -enough electromotive force to give even approximate saturation. -Under such conditions comparative measurement can be made -by measuring the current under diminished pressure of the gas, -when saturation is more readily obtained.</p> - -<p class='c006'>The method to be employed in the measurement of this ionization -current depends largely on the intensity of the current to be -measured. If some very active radium is spread on the lower of -two insulated plates as in <a href='#fig001'>Fig. 1</a>, and a saturating electric field -applied, the current may readily be measured by a sensitive galvanometer -of high resistance. For example, a weight of ·45 gr. -of radium chloride of activity 1000 times that of uranium oxide, -spread over a plate of area 33 sq. cms., gave a maximum current of -1·1 × 10<sup>-8</sup> -amperes when the plates were 4·5 cms. apart. In this -case the difference of potential to be applied to produce practical -saturation was about 600 volts. Since most of the ionization is -due to rays which are absorbed in passing through a few centimetres -of air, the current is not much increased by widening the -distance between the two plates. In cases where the current is -not quite large enough for direct deflection, the current may be -determined by connecting the upper insulated plate with a well -insulated condenser. After charging for a definite time, say one or -more minutes, the condenser is discharged through the galvanometer, -and the current can readily be deduced.</p> -<p class='c005'><b>55.</b> In most cases, however, when dealing with less active -substances like uranium or thorium, or with small amounts of active -<span class='pageno' id='Page_85'>85</span>material, it is necessary to employ methods for measuring much -smaller currents than can be detected conveniently by an ordinary -galvanometer. The most convenient apparatus to employ for this -purpose is one of the numerous types of quadrant electrometer or -an electroscope of special design. For many observations, especially -where the activity of the two substances is to be compared under -constant conditions, an electroscope offers a very certain and easy -method of measurement. As an example of a simple apparatus -of this kind, a brief description will be given of the electroscope -used by M. and Mme Curie in many of their earlier observations.</p> - -<div id='fig011' class='figcenter id004'> -<img src='images/fig-011.png' alt='Fig. 11.' class='ig001'> -<div class='ic002'> -<p>Fig. 11.</p> -</div> -</div> - -<p class='c006'>The connections are clearly seen from <a href='#fig011'>Fig. 11</a>. The active -material is placed on a plate laid on top of the fixed circular plate -<i>P</i>, connected with the case of the instrument and with earth. The -upper insulated plate <i>P´</i> is connected with the insulated gold-leaf -system <i>LL´</i>. <i>S</i> is an insulating support and <i>L</i> the gold-leaf.</p> - -<p class='c006'>The system is first charged to a suitable potential by means of -the rod <i>C</i>. The rate of movement of the gold-leaf is observed by -means of a microscope. In comparisons of the activity of two -specimens, the time taken by the gold leaf to pass over a certain -number of divisions of the micrometer scale in the eye-piece is -observed. Since the capacity of the charged system is constant, the -average rate of movement of the gold-leaf is directly proportional -to the ionization current between <i>P</i> and <i>P´</i>, <i>i.e.</i> to the intensity of -the radiation emitted by the active substance. Unless very active -<span class='pageno' id='Page_86'>86</span>material is being examined, the difference of potential between <i>P</i> -and <i>P´</i> can easily be made sufficient to produce saturation.</p> - -<p class='c006'>When necessary, a correction can be made for the rate of leak -when no active material is present. In order to avoid external -disturbances, the plates <i>PP´</i> and the rod <i>C</i> are surrounded by -metal cylinders, <i>E</i> and <i>F</i>, connected with earth.</p> -<p class='c005'><a id='section056'></a> -<b>56.</b> A modified form of the gold-leaf electroscope can be used -to determine extraordinarily minute currents -with accuracy, and can be employed -in cases where a sensitive electrometer is -unable to detect the current. A special -type of electroscope has been used by -Elster and Geitel, in their experiments on -the natural ionization of the atmosphere. -A very convenient type of electroscope to -measure the current due to minute ionization -of the gas is shown in <a href='#fig012'>Fig. 12</a>.</p> - -<div id='fig012' class='figcenter id005'> -<img src='images/fig-012.png' alt='Fig. 12.' class='ig001'> -<div class='ic002'> -<p>Fig. 12.</p> -</div> -</div> - -<p class='c006'>This type of instrument was first used -by C. T. R. Wilson<a id='r100' href='#f100' class='c012'><sup>[100]</sup></a> in his experiments -of the natural ionization of air in closed -vessels. A brass cylindrical vessel is taken -of about 1 litre capacity. The gold-leaf -system, consisting of a narrow strip of gold-leaf <i>L</i> attached to a flat -rod <i>R</i>, is insulated inside the vessel by the small sulphur bead or -piece of amber <i>S</i>, supported from the rod <i>P</i>. In a dry atmosphere -a clean sulphur bead or piece of amber is almost a perfect insulator. -The system is charged by a light bent rod <i>CC´</i> passing through -an ebonite cork<a id='r101' href='#f101' class='c012'><sup>[101]</sup></a>. The rod <i>C</i> is connected to one terminal of -a battery of small accumulators of 200 to 300 volts. If these are -absent, the system can be charged by means of a rod of sealing-wax. -The charging rod <i>CC´</i> is then removed from contact with -the gold-leaf system. The rods <i>P</i> and <i>C</i> and the cylinder are -then connected with earth.</p> - -<p class='c006'>The rate of movement of the gold-leaf is observed by a reading -<span class='pageno' id='Page_87'>87</span>microscope through two holes in the cylinder, covered with thin -mica. In cases where the natural ionization due to the enclosed -air in the cylinder is to be measured accurately, it is advisable to -enclose the supporting and charging rod and sulphur bead inside -a small metal cylinder <i>M</i> connected to earth, so that only the -charged gold-leaf system is exposed in the main volume of the air.</p> - -<p class='c006'>In an apparatus of this kind the small leakage over the sulphur -bead can be eliminated almost completely by keeping the rod <i>P</i> -charged to the average potential of the gold-leaf system during -the observation. This method has been used with great success by -C. T. R. Wilson (<i>loc. cit.</i>). Such refinements, however, are generally -unnecessary, except in investigations of the natural ionization of -gases at low pressures, when the conduction leak over the sulphur -bead is comparable with the discharge due to the ionized gas.</p> -<p class='c005'><b>57.</b> The electric capacity <i>C</i> of a gold-leaf system about 4 cms. -long is usually about 1 electrostatic unit. If <i>V</i> is the decrease of -potential of the gold-leaf system in t seconds, the current i through -the gas is given by</p> - -<div class='calc'> - -<div class='lg-container-b c017'> - <div class='linegroup'> - <div class='group'> - <div class='line in6'><i>CV</i></div> - <div class='line'><i>i</i> = ----</div> - <div class='line in6'><i>t</i>.</div> - </div> - </div> -</div> - -</div> - -<p class='c018'>With a well cleaned brass electroscope of volume 1 litre, the -fall of potential due to the natural ionization of the air was found -to be about 6 volts per hour. Since the capacity of the gold-leaf -system was about 1 electrostatic unit</p> - -<div class='calc'> - -<div class='lg-container-b c017'> - <div class='linegroup'> - <div class='group'> - <div class='line in14'>6</div> - <div class='line'><i>i</i> = 1 × ------------ = 5·6 × 10<sup>-6</sup> <span class='fss'>E.S.</span> units = 1·9 × 10<sup>-15</sup> amperes.</div> - <div class='line in10'>3600 × 300</div> - </div> - </div> -</div> - -</div> - -<p class='c018'>With special precautions a rate of discharge of ⅒ or even -¹⁄₁₀₀ of this amount can be measured accurately.</p> - -<p class='c006'>The number of ions produced in the gas can be calculated if -the charge on an ion is known. J. J. Thomson has shown that the -charge <i>e</i> on an ion is equal to -3·4 × 10<sup>-10</sup> -electrostatic units or -1·13 × 10<sup>-19</sup> -coulombs.</p> - -<div class='calc'> - -<div class='lg-container-b c017'> - <div class='linegroup'> - <div class='group'> - <div class='line'>Let <i>q</i> = number of ions produced per second per cubic centimetre</div> - <div class='line in10'>throughout the volume of the electroscope,</div> - </div> - <div class='group'> - <div class='line in4'><i>S</i> = volume of electroscope in cubic centimetres.</div> - </div> - </div> -</div> - -</div> - -<p class='c018'>If the ionization be uniform, the saturation current <i>i</i> is given -by <i>i</i> = <i>qSe</i>.</p> - -<p class='c006'><span class='pageno' id='Page_88'>88</span>Now for an electroscope with a volume of 1000 c.c., <i>i</i> was equal -to about -1·9 × 10<sup>-15</sup> -amperes. Substituting the values given above</p> - -<div class='calc'> - -<div class='lg-container-b c017'> - <div class='linegroup'> - <div class='group'> - <div class='line'><i>q</i> = 17 ions per cubic centimetre per second.</div> - </div> - </div> -</div> - -</div> - -<p class='c018'>With suitable precautions an electroscope can thus readily -measure an ionization current corresponding to the production of -1 ion per cubic centimetre per second.</p> - -<p class='c006'>The great advantage of an apparatus of this kind lies in the -fact that the current measured is due to the ionization inside the -vessel and is not influenced by the ionization of the external air -or by electrostatic disturbances<a id='r102' href='#f102' class='c012'><sup>[102]</sup></a>. Such an apparatus is very -convenient for investigating the very penetrating radiations from -the radio-elements, since these rays pass readily through the walls -of the electroscope. When the electroscope is placed on a lead -plate 3 or 4 mms. thick, the ionization in the electroscope, due to -a radio-active body placed under the lead, is due entirely to the -very penetrating rays, since the other two types of rays are -completely absorbed in the lead plate. If a circular opening is -cut in the base of the electroscope and covered with thin aluminium -of sufficient thickness to absorb the α rays, measurements of the -intensity of the β rays from an active substance placed under it, -can be made with ease and certainty.</p> -<p class='c005'><b>58.</b> A modified form of electroscope, which promises to be of -great utility for measuring currents even more minute than those -to be observed with the type of instrument already described, has -recently been devised by C. T. R. Wilson<a id='r103' href='#f103' class='c012'><sup>[103]</sup></a>. The construction of -the apparatus is shown in <a href='#fig013'>Fig. 13</a>.</p> - -<p class='c006'>The case consists of a rectangular brass box 4 cms. × 4 cms. -× 3 cms. A narrow gold-leaf <i>L</i> is attached to a rod <i>R</i> passing -through a clean sulphur cork. Opposite the gold-leaf is fixed an -insulated brass plate <i>P</i>, placed about 1 mm. from the wall of the -box. The movement of the gold-leaf is observed through two -small windows by means of a microscope provided with a micrometer -scale. The plate <i>P</i> is maintained at a constant potential (generally -<span class='pageno' id='Page_89'>89</span>about 200 volts). The electrometer case is placed in an inclined -position as shown in the figure, the angle of inclination and the -potential of the plate being adjusted to give the desired sensitiveness. -The gold-leaf is initially connected to the case, and the -microscope adjusted so that the gold-leaf is seen in the centre of -the scale. For a given potential of the plate, the sensitiveness -depends on the angle of tilt of the case. There is a certain critical -inclination below which the gold-leaf is unstable. The most -sensitive position lies just above the critical angle. In a particular -experiment Wilson found that with an angle of tilt of 30° and with -the plate at a constant potential of 207 volts, the gold-leaf, when -raised to a potential of one volt above the case, moved over 200 -scale divisions of the eye-piece, 54 divisions corresponding to one -millimetre.</p> - -<div id='fig013' class='figcenter id007'> -<img src='images/fig-013.png' alt='Fig. 13.' class='ig001'> -<div class='ic002'> -<p>Fig. 13.</p> -</div> -</div> - -<p class='c006'>In use, the rod <i>R</i> is connected with the external insulated -system whose rise or fall of potential is to be measured. On -account of the small capacity of the system and the large movement -of the gold-leaf for a small difference of potential, the electroscope -is able to measure extraordinarily minute currents. The apparatus -is portable. If the plate <i>P</i> be connected to one pole of a dry pile -the gold-leaf is stretched out towards the plate, and in this position -can be carried without risk of injury.</p> -<p class='c005'><span class='pageno' id='Page_90'>90</span><b>59. Electrometers.</b> Although the electroscope can be used -with advantage in special cases, it is limited in its application. -The most generally convenient apparatus for measurement of -ionization currents through gases is one of the numerous types of -quadrant electrometer. With the help of auxiliary capacities, the -electrometer can be used to measure currents with accuracy over -a wide range, and can be employed for practically every kind of -measurement required in radio-activity.</p> - -<p class='c006'>The elementary theory of the symmetrical quadrant electrometer -as given in the text-books is very imperfect. It is deduced that -the sensibility of the electrometer—measured by the deflection of -the needle for 1 volt <span class='fss'>P.D.</span> between the quadrants—varies directly -as the potential of the charged needle, provided that this potential -is high compared with the <span class='fss'>P.D.</span> between the quadrants. In most -electrometers however, the sensibility rises to a maximum, and then -decreases with increase of potential of the needle. For electrometers -in which the needle lies close to the quadrants, this maximum -sensibility is obtained for a comparatively low potential of the -needle. A theory of the quadrant electrometer, accounting for this -action, has been recently given by G. W. Walker<a id='r104' href='#f104' class='c012'><sup>[104]</sup></a>. The effect -appears to be due to the presence of the air space that necessarily -exists between adjoining quadrants.</p> - -<div id='fig014' class='figcenter id002'> -<img src='images/fig-014.png' alt='Fig. 14.' class='ig001'> -<div class='ic002'> -<p>Fig. 14.</p> -</div> -</div> - -<p class='c006'>Suppose that it is required to measure with an electrometer -the ionization current between two -horizontal metal plates <i>A</i> and <i>B</i> -(<a href='#fig014'>Fig. 14</a>) on the lower of which some -active material has been spread. If -the saturation current is required, -the insulated plate <i>A</i> is connected -with one pole of a battery of sufficient -<span class='fss'>E.M.F.</span> to produce saturation, the -other pole being connected to earth. -The insulated plate <i>B</i> is connected -with one pair of quadrants of the -electrometer, the other pair being -earthed. By means of a suitable key -<i>K</i>, the plate <i>B</i> and the pair of quadrants connected with it may be -<span class='pageno' id='Page_91'>91</span>either insulated or connected with earth. When a measurement -is to be taken, the earth connection is broken. If the positive pole -of the battery is connected with <i>A</i>, the plate <i>B</i> and the electrometer -connections immediately begin to be charged positively, and -the potential, if allowed, will steadily rise until it is very nearly -equal to the potential of <i>A</i>. As soon as the potential of the -electrometer system begins to rise, the electrometer needle commences -to move at a uniform rate. Observations of the angular -movement of the needle are made either by the telescope and scale -or by the movement of the spot of light on a scale in the usual -way. If the needle is damped so as to give a uniform motion -over the scale, the rate of movement of the needle, <i>i.e.</i> the number -of divisions of the scale passed over per second, may be taken as -a measure of the current through the gas. The rate of movement -is most simply obtained by observing with a stop-watch the time -taken for the spot of light, after the motion has become steady, to -pass over 100 divisions of the scale. As soon as the observation is -made, the plate <i>B</i> is again connected with earth, and the electrometer -needle returns to its original position.</p> - -<p class='c006'>In most experiments on radio-activity, only comparative measurements -of saturation currents are required. If these measurements -are to extend over weeks or months, as is sometimes the case, it is -necessary to adopt some method of standardizing the electrometer -from day to day, so as to correct for variation in its sensibility. -This is done most simply by comparing the current to be measured -with that due to a standard sample of uranium oxide, which is -placed in a definite position in a small testing vessel, always kept -in connection with the electrometer. Uranium oxide is a very -constant source of radiation, and the saturation current due to it -is the same from day to day. By this method of comparison -accurate observations may be made on the variation of activity of -a substance over long intervals of time, although the sensibility -of the electrometer may vary widely between successive measurements.</p> -<p class='c005'><b>60. Construction of electrometers.</b> As the quadrant -electrometer has gained the reputation of being a difficult and -uncertain instrument for accurate measurements of current, it may -<span class='pageno' id='Page_92'>92</span>be of value to give some particular details in regard to the best -method of construction and insulation. In most of the older types -of quadrant electrometers the needle system was made unnecessarily -heavy. In consequence of this, if a sensibility of the order -of 100 mms. deflection for 1 volt was required, it was necessary to -charge the Leyden jar connected to the needle to a fairly high -potential. This at once introduced difficulties, for at a high -potential it is not easy to insulate the Leyden jar satisfactorily, or -to charge it to the same potential from day to day. This drawback -is to a large extent avoided in the White pattern of the Kelvin -electrometer, which is provided with a replenisher and attracted -disc for keeping the potential of the needle at a definite value. If -sufficient trouble is taken in insulating and setting up this type -of electrometer, it proves a very useful instrument of moderate -sensibility, and will continue in good working order for a year or -more without much attention.</p> - -<p class='c006'>Simpler types of electrometer of greater sensibility can however -be readily constructed to give accurate results. The old type of -quadrant electrometer, to be found in every laboratory, can readily -be modified to prove a useful and trustworthy instrument. A light -needle can be made of thin aluminium, of silvered paper or of -a thin plate of mica, covered with gold-leaf to make it conducting. -The aluminium wire and mirror attached should be made as light -as possible. The needle should be supported either by a fine -quartz fibre or a long bifilar suspension of silk. A very fine -phosphor bronze wire of some length is also very satisfactory. -A magnetic control is not very suitable, as it is disturbed by coils -or dynamos working in the neighbourhood. In addition, the zero -point of the needle is not as steady as with the quartz or bifilar -suspension.</p> - -<p class='c006'>When an electrometer is used to measure a current by noting -the rate of movement of the needle, it is essential that the needle -should be damped sufficiently to give a uniform motion of the spot -of light over the scale. The damping requires fairly accurate -adjustment. If it is too little, the needle has an oscillatory movement -superimposed on the steady motion; if it is too great, it -moves too sluggishly from rest and takes some time to attain -a state of uniform motion. With a light needle, very little, if -<span class='pageno' id='Page_93'>93</span>any, extra damping is required. A light platinum wire with a -single loop dipping in sulphuric acid is generally sufficient for the -purpose.</p> - -<p class='c006'>With light needle systems and delicate suspensions, it is only -necessary to charge the needle to a potential of a few hundred volts -to give a sensibility of several thousand divisions for a volt. With -such low potentials, the difficulty of insulation of the condenser, -with which the needle is in electrical connection, is much reduced. -It is convenient to use a condenser such that the potential of the -needle does not fall more than a few per cent. per day. The -ordinary short glass jar partly filled with sulphuric acid is, in most -cases, not easy to insulate to this extent. It is better to replace it -by an ebonite (or sulphur) condenser<a id='r105' href='#f105' class='c012'><sup>[105]</sup></a> such as is shown in <a href='#fig015'>Fig. 15</a>.</p> - -<div id='fig015' class='figcenter id005'> -<img src='images/fig-015.png' alt='Fig. 15.' class='ig001'> -<div class='ic002'> -<p>Fig. 15.</p> -</div> -</div> - -<p class='c006'>A circular plate of ebonite about 1 cm. thick is turned down -until it is not more than ½ mm. thick in the centre. Into this -circular recess a brass plate <i>B</i> fits -loosely. The ebonite plate rests -on another brass plate <i>C</i> connected -with earth. The condenser -thus formed has a considerable -capacity and retains a charge for -a long time. In order to make -connection with the needle, a -small glass vessel <i>D</i>, partly filled -with sulphuric acid, is placed on -the plate <i>B</i> and put in connection -with the needle by means -of a fine platinum wire. The -platinum wire from the needle -dips into the acid, and serves to damp the needle. In a dry atmosphere, -a condenser of this kind will not lose more than 20 per cent. -of its charge in a week. If the insulation deteriorates, it can -readily be made good by rubbing the edge of the ebonite <i>A</i> with -sand-paper, or removing its surface in a lathe.</p> - -<p class='c006'>If a sufficient and steady <span class='fss'>E.M.F.</span> is available, it is much better -to keep the battery constantly connected with the needle, and to -<span class='pageno' id='Page_94'>94</span>avoid the use of the condenser altogether. If a battery of small -accumulators is used, their potential can be kept at a constant -value, and the electrometer always has a constant sensibility.</p> -<p class='c005'><b>61.</b> A very useful electrometer of great sensibility has been -devised by Dolezalek<a id='r106' href='#f106' class='c012'><sup>[106]</sup></a>. It is of the ordinary quadrant type -with a very light needle of silvered paper, spindle shaped, which -lies fairly close to the quadrants. A very fine quartz suspension is -employed. In consequence of the lightness of the needle and its -nearness to the quadrants, it acts as its own damper. This is -a great advantage, for difficulties always arise when the wire dips -into sulphuric acid, on account of the thin film which collects after -some time on the surface of the acid. This film obstructs the -motion of the platinum wire dipping into the acid, and has to be -removed at regular intervals. These instruments can readily be -made to give a sensibility of several thousand divisions for a volt -when the needle is charged to about one hundred volts. The -sensibility of the electrometer passes through a maximum as the -potential of the needle is increased. It is always advisable to -charge the needle to about the value of this critical potential. The -capacity of the electrometer is in general high (about 50 electrostatic -units) but the increased sensibility more than compensates -for this. The needle may either be charged by lightly touching -it with one terminal of a battery, or it may be kept charged to -a constant potential through the quartz suspension.</p> - -<p class='c006'>Dolezalek states that the fibre can be made sufficiently conducting -for the purpose by dipping it into a dilute solution of -calcium chloride or phosphoric acid. I have not found this method -satisfactory in dry climates as in many cases the fibre practically -loses its conductivity after a few days exposure to dry air.</p> - -<p class='c006'>In addition to its great sensibility, the advantage of this -instrument is in the steadiness of the zero and in the self-damping.</p> - -<p class='c006'>A sensibility of 10,000 millimetre divisions per volt can be -readily obtained with this electrometer, if a very fine fibre be used. -The use of such high sensibilities cannot, however, be recommended -except for very special experiments. The period of swing of the -needle under these conditions is several minutes and the natural -<span class='pageno' id='Page_95'>95</span>leak of the testing vessels employed, as well as electrostatic and -other disturbances, make themselves only too manifest. If measurements -of minute currents are required, an electroscope of the -type described in <a href='#section056'>Section 56</a> is much to be preferred to a very -sensitive electrometer. The electroscope readings in such a case are -more accurate than similar measurements made by an electrometer.</p> - -<p class='c006'>For most measurements in radio-activity, an electrometer which -has a sensibility of 100 divisions per volt is very suitable, and no -advantage is gained by using an electrometer of greater sensibility. -If still smaller effects require to be measured, the sensibility may -be increased to several thousand divisions per volt.</p> -<p class='c005'><b>62. Adjustment and screening.</b> In adjusting an electrometer, -it is important to arrange that the needle shall lie symmetrically -with regard to the quadrants. This is best tested by -observing whether the needle is deflected on charging, the quadrants -all being earthed. In most electrometers there is an adjustable -quadrant, the position of which may be altered until the needle -is not displaced on charging. When this condition is fulfilled, -the zero reading of the electrometer remains unaltered as -the needle loses its charge, and the deflection on both sides of -the zero should be the same for equal and opposite quantities of -electricity.</p> - -<p class='c006'>The supports of the quadrants require to be well insulated. -Ebonite rods are as a rule more satisfactory for this purpose than -glass. In testing for the insulation of the quadrants and the -connections attached, the system is charged to give a deflection -of about 200 scale divisions. If the needle does not move more -than one or two divisions after standing for one minute, the -insulation may be considered quite satisfactory. When a suitable -desiccator is placed inside the tight-fitting electrometer case, the -insulation of the quadrants should remain good for months. If the -insulation of the ebonite deteriorates, it can easily be made good -by removing the surface of the ebonite in a lathe.</p> - -<p class='c006'>In working with a sensitive instrument like the Dolezalek -electrometer, it is essential that the electrometer and the testing -apparatus should be completely enclosed in a screen of wire-gauze -connected with earth, in order to avoid electrostatic disturbances. -<span class='pageno' id='Page_96'>96</span>If an apparatus is to be tested at some distance from the electrometer, -the wires leading to it should be insulated in metal cylinders -connected with earth. The size of the insulators used at various -points should be made as small as possible, in order to avoid -disturbances due to their electrification. In damp climates, paraffin, -amber, or sulphur insulates better than ebonite. The objection -to paraffin as an insulator for sensitive electrometers lies in the -difficulty of getting entirely rid of any electrification on its surface. -When paraffin has been once charged, the residual charge, after -diselectrifying it with a flame, continues to leak out for a long -interval. All insulators should be diselectrified by means of a -spirit-lamp or still better by leaving some uranium near them. -Care should be taken not to touch the insulation when once -diselectrified.</p> - -<p class='c006'>In accurate work it is advisable to avoid the use of gas jets or -Bunsen flames in the neighbourhood of the electrometer, as the -flame gases are strongly ionized and take some time to lose their -conductivity. If radio-active substances are present in the room, -it is necessary to enclose the wires leading to the electrometer in -fairly narrow tubes, connected with earth. If this is not done, it -will be found that the needle does not move at a constant rate, -but rapidly approaches a steady deflection where the rate of loss -of charge of the electrometer and connections, due to the ionization -of the air around them, is balanced by the current to be measured. -This precaution must always be taken when observations are made -on the very penetrating rays from active substances. These rays -readily pass through ordinary screens, and ionize the air around -the electrometer and connecting wires. For this reason it is -impossible to make accurate measurements of small currents in -a room which is used for the preparation of radio-active material. -In course of time the walls of the room become radio-active owing -to the dissemination of dust and the action of the radio-active -emanations<a id='r107' href='#f107' class='c012'><sup>[107]</sup></a>.</p> -<p class='c005'><span class='pageno' id='Page_97'>97</span><b>63. Electrometer key.</b> For work with electrometers of -high sensibility, a special key is -necessary to make and break from -a distance the connection of the -quadrants with earth in order to -avoid electrostatic disturbances at -the moment the current is to be -measured. The simple key shown -in <a href='#fig016'>Fig. 16</a> has been found very -satisfactory for this purpose. A -small brass rod <i>BM</i>, to which a -string is attached, can be moved -vertically up and down in a brass -tube <i>A</i>, which is rigidly attached -to a bent metal support connected -with earth. When the string is released, this rod makes contact with -the mercury <i>M</i>, which is placed in a small metal vessel resting on -a block of ebonite <i>P</i>. The electrometer and testing vessel are -connected with the mercury. When the string is pulled, the rod -<i>BM</i> is removed from the mercury and the earth connection of the -electrometer system is broken. On release of the string, the rod -<i>BM</i> falls and the electrometer is again earthed. By means of this -key, which may be operated at any distance from the electrometer, -the earth connection may be made and broken at definite intervals -without any appreciable disturbance of the needle.</p> - -<div id='fig016' class='figcenter id002'> -<img src='images/fig-016.png' alt='Fig. 16.' class='ig001'> -<div class='ic002'> -<p>Fig. 16.</p> -</div> -</div> -<p class='c005'><b>64. Testing apparatus.</b> The arrangement shown in <a href='#fig017'>Fig. 17</a> -is very convenient for many measurements in radio-activity. Two -parallel insulated metal plates <i>A</i> and <i>B</i> are placed inside a metal -vessel <i>V</i>, provided with a side door. The plate <i>A</i> is connected with -one terminal of a battery of small storage cells, the other pole of -which is earthed; the plate <i>B</i> with the electrometer, and the vessel -<i>V</i> with earth. The shaded areas in the figure indicate the position -of ebonite insulators. The active material to be tested is spread -uniformly in a shallow groove (about 5 cms. square and 2 mms. -deep) in the brass plate <i>A</i>. In order to avoid breaking the -battery connection every time the plate <i>A</i> is removed, the wire -from the battery is permanently connected with the metal block <i>N</i> -<span class='pageno' id='Page_98'>98</span>resting on the ebonite support. In this arrangement there is no -possibility of a conduction leak from the plate <i>A</i> to <i>B</i>, since the -earth-connected vessel <i>V</i> intervenes.</p> - -<div id='fig017' class='figcenter id007'> -<img src='images/fig-017.png' alt='Fig. 17.' class='ig001'> -<div class='ic002'> -<p>Fig. 17.</p> -</div> -</div> - -<p class='c006'>An apparatus of this kind is very convenient for testing the -absorption of the radiations by solid screens, as well as for making -comparative studies of the activity of different bodies. Unless -very active preparations of radium are employed, a battery of -300 volts is sufficient to ensure saturation when the plates are not -more than 5 centimetres apart. If substances which give off a radio-active -emanation are being tested, the effect of the emanation can -be eliminated by passing a steady current of air from a gas bag -between the plates. This removes the emanation as fast as it is -produced.</p> - -<p class='c006'>If a clean plate is put in the place of <i>A</i>, a small movement of -the electrometer needle is always observed. If there is no radio-active -substance in the neighbourhood, this effect is due to the -small natural ionization of the air. We can correct for this natural -leak when necessary.</p> -<p class='c005'><b>65.</b> We have often to measure the activity due to the -emanations of thorium or radium, or the excited activity produced -by those emanations on rods or wires. A convenient apparatus for -this purpose is shown in <a href='#fig018'>Fig. 18</a>. The cylinder <i>B</i> is connected with -<span class='pageno' id='Page_99'>99</span>the battery in the usual way, and the central conductor <i>A</i> with the -electrometer. This central rod is insulated from the external -cylinder by an ebonite cork, which is divided into two parts by a -metal ring <i>CC´</i> connected to earth. This ring acts the part of a -guard-ring, and prevents any conduction leak between <i>B</i> and <i>A</i>. -The ebonite is thus only required to insulate satisfactorily for the -small rise of potential produced on <i>A</i> during the experiment. In all -accurate measurements of current in radio-activity the guard-ring -principle should always be used to ensure good insulation. This -is easily secured when the ebonite is only required to insulate -for a fraction of a volt, instead of for several hundred volts, as is -the case when the guard-ring is absent.</p> - -<div id='fig018' class='figcenter id004'> -<img src='images/fig-018.png' alt='Fig. 18.' class='ig001'> -<div class='ic002'> -<p>Fig. 18.</p> -</div> -</div> -<p class='c005'><b>66.</b> For measurements of radio-activity with an electrometer, -a steady source of <span class='fss'>E.M.F.</span> of at least 300 volts is necessary. This -is best obtained by a battery of small cells simply made by -immersing strips of lead in dilute sulphuric acid, or by a battery -of small accumulators of the usual construction. Small accumulators -of capacity about one-half ampere-hour can now be obtained -at a moderate price, and are more constant and require less -attention than simple lead cells.</p> - -<p class='c006'>In order to measure currents over a wide range, a graduated -series of capacities is required. The capacity of an electrometer and -testing apparatus is usually about 50 electrostatic units or ·000056 -microfarads. Subdivided condensers of mica are constructed in -which capacities varying from ·001 to ·2 microfarads are provided. -With such a condenser, another extra capacity is required to -bridge over the gap between the capacity of the electrometer and -<span class='pageno' id='Page_100'>100</span>the lowest capacity of the condenser. This capacity of value about -200 electrostatic units can readily be made by using parallel plates -or still better concentric cylinders. With this series of capacities, -currents may be measured between -3 × 10<sup>-14</sup> -and -3 × 10<sup>-8</sup> -amperes—a -range of over one million. Still larger currents can be -measured if the sensibility of the electrometer is reduced, or if -larger capacities are available.</p> - -<p class='c006'>In a room devoted to electrometer measurements of radio-activity, -it is desirable to have no radio-active matter present -except that to be tested. The room should also be as free from -dust as possible. The presence of a large quantity of dust in the -air (see <a href='#section031'>section 31</a>) is a very disturbing factor in all radio-active -measurements. A larger <span class='fss'>E.M.F.</span> is required to produce saturation -on account of the diffusion of the ions to the dust particles. The -presence of dust in the air also leads to uncertainty in the distribution -of excited activity in an electric field (see <a href='#section181'>section 181</a>).</p> -<p class='c005'><b>67. Measurement of Current.</b> In order to determine -the current in the electrometer circuit by measuring the rate of -movement of the needle, it is necessary to know both the capacity -of the circuit and the sensibility of the electrometer.</p> - -<div class='calc'> - -<div class='lg-container-b c017'> - <div class='linegroup'> - <div class='group'> - <div class='line'>Let <i>C</i> = capacity of electrometer and its connections in <span class='fss'>E.S.</span> units,</div> - <div class='line in4'><i>d</i> = number of divisions of the scale passed over per second,</div> - <div class='line in4'><i>D</i> = sensibility of the electrometer measured in scale divisions</div> - <div class='line in16'>for 1 volt <span class='fss'>P.D.</span> between the quadrants.</div> - </div> - </div> -</div> - -</div> - -<p class='c018'>The current <i>i</i> is given by the product of the capacity of the -system and the rate of rise of potential.</p> - -<div class='calc'> - -<div class='lg-container-b c017'> - <div class='linegroup'> - <div class='group'> - <div class='line'>Thus</div> - <div class='line in11'><i>Cd</i></div> - <div class='line in5'><i>i</i> = ----- <span class='fss'>E.S.</span> units,</div> - <div class='line in10'>300<i>D</i></div> - </div> - <div class='group'> - <div class='line in10'><i>Cd</i></div> - <div class='line in6'>= ----------- amperes.</div> - <div class='line in9'>9 × 10<sup>11</sup> <i>D</i></div> - </div> - </div> -</div> - -</div> - -<p class='c018'>Suppose, for example,</p> - -<div class='calc'> - -<div class='lg-container-b c017'> - <div class='linegroup'> - <div class='group'> - <div class='line'><i>C</i> = 50, <i>d</i> = 5, <i>D</i> = 1000;</div> - <div class='line'>then <i>i</i> = 2·8 × 10<sup>-13</sup> amperes.</div> - </div> - </div> -</div> - -</div> - -<p class='c018'>Since the electrometer can readily measure a current corresponding -to a movement of half a scale division per second, -we see that an electrometer can measure a current of -3 × 10<sup>-14</sup> -<span class='pageno' id='Page_101'>101</span>amperes, which is considerably below the range of the most -sensitive galvanometer.</p> - -<p class='c006'>The capacity of the electrometer itself must not be considered -as equal to that of the pair of quadrants and the needle when in a -position of rest. The actual capacity is very much larger than this, -on account of the motion of the charged needle. Suppose, for -example, that the needle is charged to a high negative potential, and -kept at the zero position by an external constraint. If a quantity <i>Q</i> -of positive electricity is given to the electrometer and its connections, -the whole system is raised to a potential <i>V</i>, such that <i>Q</i> = <i>CV</i>, -where <i>C</i> is the capacity of the system. When however the needle -is allowed to move, it is attracted into the charged pair of quadrants. -This corresponds to the introduction of a negatively charged -body between the quadrants, and in consequence the potential of -the system is lowered to <i>V´</i>. The actual capacity <i>C´</i> of the system -when the needle moves is thus greater than <i>C</i>, and is given by</p> - -<div class='calc'> - -<div class='lg-container-b c017'> - <div class='linegroup'> - <div class='group'> - <div class='line'><i>C´V´</i> = <i>CV</i>.</div> - </div> - </div> -</div> - -</div> - -<p class='c018'>Thus the capacity of the electrometer is not a constant, but -depends on the potential of the needle, <i>i.e.</i> on the sensibility of the -electrometer.</p> - -<p class='c006'>An interesting result of practical importance follows from the -variation of the capacity of the electrometer with the potential of -the needle. If the external capacity attached to the electrometer -is small compared with that of the electrometer itself, the rate of -movement of the needle for a constant current is, in some cases, -independent of the sensibility. An electrometer may be used for -several days or even weeks to give nearly equal deflections for -a constant current, without recharging the needle, although its -potential has been steadily falling during the interval. In such -a case the decrease in sensibility is nearly proportional to the -decrease in capacity of the electrometer, so that the deflection for -a given current is only slightly altered. The theory of this action -has been given by J. J. Thomson<a id='r108' href='#f108' class='c012'><sup>[108]</sup></a>.</p> -<p class='c005'><b>68.</b> The capacity of the electrometer and its connections -cannot be measured by any of the commutator methods used for -the determination of small capacities, for in such cases the needle -<span class='pageno' id='Page_102'>102</span>does not move, and the capacity measured is not that of the -electrometer system when in actual use. The value of the capacity -may, however, be determined by the method of mixtures.</p> - -<div class='calc'> - -<div class='lg-container-b c017'> - <div class='linegroup'> - <div class='group'> - <div class='line'>Let <i>C</i> = capacity of electrometer and connections,</div> - <div class='line in4'><i>C</i><sub>1</sub> = capacity of a standard condenser.</div> - </div> - </div> -</div> - -</div> - -<p class='c018'>The electrometer and its connections are charged to a potential -<i>V</i><sub>1</sub> -by a battery, and the deflection -<i>d</i><sub>1</sub> -of the needle is noted. By -means of an insulated key, the capacity of the standard condenser -is added in parallel with the electrometer system. Let -<i>V</i><sub>2</sub> -be the -potential of the system, and -<i>d</i><sub>2</sub> -the new deflection.</p> - -<p>Then</p> - -<div class='calc'> - -<div class='lg-container-b c017'> - <div class='linegroup'> - <div class='group'> - <div class='line'><i>CV<sub>1</sub></i> = (<i>C</i> + <i>C<sub>1</sub></i>) <i>V<sub>2</sub></i>,</div> - <div class='line in4'><i>C</i> + <i>C<sub>1</sub></i> <i>V<sub>1</sub></i> <i>d<sub>1</sub></i></div> - <div class='line in3'>-------- = ----- = -----</div> - <div class='line in6'><i>C</i> <i>V<sub>2</sub></i> <i>d<sub>2</sub></i></div> - </div> - <div class='group'> - <div class='line in15'><i>d<sub>2</sub></i></div> - <div class='line'>and <i>C</i> = <i>C<sub>1</sub></i> --------</div> - <div class='line in13'><i>d<sub>1</sub></i> – <i>d<sub>2</sub></i></div> - </div> - </div> -</div> - -</div> - -<div id='fig019' class='figcenter id004'> -<img src='images/fig-019.png' alt='Fig. 19.' class='ig001'> -<div class='ic002'> -<p>Fig. 19.</p> -</div> -</div> - -<p class='c018'>A simple standard capacity for this purpose can be constructed -of two concentric brass tubes the diameters of which can be -accurately measured. The external cylinder <i>D</i> (<a href='#fig019'>Fig. 19</a>) is mounted -on a wooden base, which is covered with a sheet of metal or tinfoil -connected to earth. The tube <i>C</i> is supported centrally on ebonite -rods at each end. The capacity is given approximately by the -formula</p> - -<div class='figcenter id010'> -<img src='images/form-027.png' alt='Formula.' class='ig001'> -</div> - -<p class='c006'><span class='pageno' id='Page_103'>103</span>where <i>b</i> is the internal diameter of <i>D</i>, <i>a</i> the external diameter of <i>C</i>, -and <i>l</i> the length of the tubes.</p> - -<p class='c006'>The following method can be used in some cases with advantage. -While a testing vessel is in connection with the electrometer, a -sample of uranium is placed on the lower plate <i>A</i>. Let -<i>d</i><sub>2</sub> and <i>d</i><sub>1</sub> -be the number of divisions passed over per second by the needle -with and without the standard capacity in connection.</p> - -<div class='calc'> - -<div class='lg-container-b c017'> - <div class='linegroup'> - <div class='group'> - <div class='line in6'><i>C</i> + <i>C<sub>1</sub></i> <i>d<sub>1</sub></i></div> - <div class='line'>Then ------ = ------ ,</div> - <div class='line in7'><i>C</i> <i>d<sub>2</sub></i></div> - </div> - <div class='group'> - <div class='line in15'><i>d<sub>2</sub></i></div> - <div class='line'>and <i>C</i> = <i>C<sub>1</sub></i> --------</div> - <div class='line in13'><i>d<sub>1</sub></i> – <i>d<sub>2</sub></i></div> - </div> - </div> -</div> - -</div> - -<p class='c018'>This method has the advantage that the relative capacities are -expressed in terms of the motion of the needle under the actual -conditions of measurement.</p> -<p class='c005'><a id='section069'></a> -<b>69. Steady deflection method.</b> The methods of measurement -previously described depend upon the rate of angular -movement of a suspended gold-leaf or of an electrometer needle. -The galvanometer can only be employed for measurements with -intensely active matter. A need, however, has long been felt for a -method in which ordinary ionization currents can be measured by -means of a steady deflection of an electrometer needle. This is -especially the case, where measurements have to be made with -active substances whose activity alters rapidly in the course of a -few minutes.</p> - -<p class='c006'>This can obviously be secured if the electrometer system (one -pair of quadrants being earthed) is connected to earth through a -suitable high resistance. A steady deflection of the electrometer -needle will be obtained when the rate of supply of electricity to -the electrometer system is balanced by the loss due to conduction -through the resistance. If the high resistance obeys Ohm’s law, -the deflection should be proportional to the ionization current to -be measured.</p> - -<p class='c006'>A simple calculation shows that the resistance required is very -great. Suppose, for example, that a current is to be measured -corresponding to a rate of movement of the needle of 5 divisions -per second, with a sensibility of 1000 divisions per volt, and where -<span class='pageno' id='Page_104'>104</span>the capacity of the electrometer system is 50 electrostatic units. -This current is equal to -2·8 × 10<sup>-13</sup> -amperes. If a steady deflection -of 10 divisions is required, which corresponds to a rise of potential -of the system of ¹⁄₁₀₀ of a volt, the resistance should be 36,000 -megohms. For a deflection of 100 divisions, the resistance should -be 10 times as large. Dr Bronson<a id='r109' href='#f109' class='c012'><sup>[109]</sup></a>, working in the laboratory of -the writer, has recently made some experiments in order to devise a -practical method for measurements of this character. It is difficult -to obtain sufficiently high and constant resistances to answer the -purpose. Tubes of xylol had too great a resistance, while special -carbon resistances were not sufficiently constant. The difficulty -was finally got over by the use of what may be called an “air -resistance.” The arrangement of the experiment is shown in -<a href='#fig020'>Fig. 20</a>.</p> - -<div id='fig020' class='figcenter id006'> -<img src='images/fig-020.png' alt='Fig. 20.' class='ig001'> -<div class='ic002'> -<p>Fig. 20.</p> -</div> -</div> - -<p class='c006'>The electrometer system was connected with the upper of two -insulated parallel plates <i>AB</i>, on the lower of which was spread a -layer of a very active substance. An active bismuth plate, coated -with radio-tellurium, which had been obtained from Sthamer of -Hamburg, proved very convenient for this purpose.</p> - -<p class='c006'>The lower plate <i>B</i> was connected to earth. The charge -communicated to the upper plate of the testing vessel <i>CD</i> and -the electrometer system leaked away in consequence of the strong -<span class='pageno' id='Page_105'>105</span>ionization between the plates <i>AB</i>, and a steady deflection was -obtained when the rate of supply was equal to the rate of discharge.</p> - -<p class='c006'>This air resistance obeyed Ohm’s law over a considerable range, -<i>i.e.</i> the steady deflection was proportional to the current. It is -advisable, in such an arrangement, to test whether the deflection is -proportional to the ionization current over the range required for -measurement. This can readily be done by the use of a number -of metal vessels filled with a constant radio-active substance like -uranium oxide. The effect of these, when placed in the testing -vessel, can be tested separately and in groups, and in this way the -scale can be calibrated accurately.</p> - -<p class='c006'>The plates <i>AB</i> were placed inside a closed vessel to avoid air -currents. The contact difference of potential between the plates -<i>AB</i>, which shows itself by a steady deflection when no radio-active -matter is present in <i>CD</i>, was for the most part eliminated by covering -the surface of the plates <i>A</i> and <i>B</i> with very thin aluminium foil.</p> - -<p class='c006'>This method proved very accurate and convenient for measurement -of rapid changes in activity, and possesses many advantages -over the ordinary rate-method of use of an electrometer. A thin -layer of radium of moderate activity would probably serve in place -of the radio-tellurium, but the emanation and the β and γ rays -emitted from it would be a possible source of disturbance to the -measurements. The deflection of the electrometer needle in this -arrangement is independent of the capacity of the electrometer -system, and thus comparative measurements of current can be made -without the necessity of determining the capacity in each case.</p> -<p class='c005'><b>70.</b> <b>Quartz piezo-electrique.</b> In measurements of the -strength of currents by electrometers, it is always necessary to -determine the sensibility of the instrument and the capacity of the -electrometer and the apparatus attached thereto. By means of the -quartz piezo-electrique devised by the brothers MM. J. and P. Curie<a id='r110' href='#f110' class='c012'><sup>[110]</sup></a>, -measurements of the current can be made with rapidity and -accuracy over a wide range. These measurements are quite independent -of the capacity of the electrometer and external circuit.</p> - -<p class='c006'><span class='pageno' id='Page_106'>106</span>The essential part of this instrument consists of a plate of -quartz which is cut in a special manner. When this plate is -placed under tension, there is a liberation of electricity equal in -amount but opposite in sign on the two sides of the plate. The -plate of quartz <i>AB</i> (<a href='#fig021'>Fig. 21</a>) is hung vertically and weights are -added to the lower end. The plate is cut so that the optic axis of -the crystal is horizontal and at right angles to the plane of the -paper.</p> - -<div id='fig021' class='figcenter id002'> -<img src='images/fig-021.png' alt='Fig. 21.' class='ig001'> -<div class='ic002'> -<p>Fig. 21.</p> -</div> -</div> - -<p class='c006'>The two faces <i>A</i> and <i>B</i> are normal to one of the binary axes -(or electrical axes) of the crystal. The tension must be applied in -a direction normal to the optic and electric axes. The two faces -<i>A</i> and <i>B</i> are silvered, but the main portion of the plate is electrically -insulated by removing a narrow strip of the silvering near the upper -and lower ends of the plate. One side of the plate is connected with -the electrometer and with the conductor, the rate of leak of which -is to be measured. The quantity of electricity set free on one face -of the plate is accurately given by</p> - -<div class='calc'> - -<div class='lg-container-b c017'> - <div class='linegroup'> - <div class='group'> - <div class='line in12'><i>L</i></div> - <div class='line'><i>Q</i> = ·063 ---- <i>F</i></div> - <div class='line in12'><i>b</i></div> - </div> - <div class='group'> - </div> - </div> -</div> - -</div> - -<p class='c018'><span class='pageno' id='Page_107'>107</span>where <i>L</i> is the length of the insulated portion of the plate, <i>b</i> the -thickness <i>AB</i>, and <i>F</i> the weight attached in kilogrammes. <i>Q</i> is -then given in electrostatic units.</p> - -<p class='c006'>Suppose, for example, that it is required to measure the current -between the plates <i>CD</i> (<a href='#fig021'>Fig. 21</a>) due to some radio-active material -on the plate <i>C</i>, for a given difference of potential between <i>C</i> and <i>D</i>. -At a given instant the connection of the quadrants of the electrometer -with the earth is broken. The weight is attached to the -quartz plate, and is held in the hand so as to apply the tension -gradually. This causes a release of electricity opposite in sign to -that given to the plate <i>D</i>. The electrometer needle is kept at the -position of rest as nearly as possible by adjusting the tension by -hand. The tension being fully applied, the moment the needle -commences to move steadily from zero is noted. The current -between the plates <i>CD</i> is then given by <i>Q</i>/<i>t</i> where <i>t</i> is the time of -the observation. The value of <i>Q</i> is known from the weight attached.</p> - -<p class='c006'>In this method the electrometer is only used as a detector to -show that the system is kept at zero potential. No knowledge of -the capacity of the insulated system is required. With practice, -measurements of the current can be made in this way with rapidity -and certainty.</p> - -<div class='chapter'> - <span class='pageno' id='Page_108'>108</span> - <h2 id='chap04' class='c004'>CHAPTER IV. <br> NATURE OF THE RADIATIONS.</h2> -</div> -<h3 class='c020'>PART I. <br> Comparison of the Radiations.</h3> -<p class='c005'><b>71. The Three Types of Radiation.</b> All the radio-active -substances possess in common the power of acting on a photographic -plate and of ionizing the gas in their immediate neighbourhood. -The intensity of the radiations may be compared by means of their -photographic or electrical action; and, in the case of the strongly -radio-active substances, by the power they possess of lighting up -a phosphorescent screen. Such comparisons, however, do not throw -any light on the question whether the radiations are of the same -or of different kinds, for it is well known that such different types -of radiations as the short waves of ultra-violet light, Röntgen and -cathode rays, all possess the property of producing ions throughout -the volume of a gas, lighting up a fluorescent screen, and acting -on a photographic plate. Neither can the ordinary optical methods -be employed to examine the radiations under consideration, as -they show no trace of regular reflection, refraction, or polarization.</p> - -<p class='c006'>Two general methods can be used to distinguish the types of -the radiations given out by the same body, and also to compare -the radiations from the different active substances. These methods -are as follows:</p> - -<p class='c021'>(1) By observing whether the rays are appreciably deflected -in a magnetic field.</p> - -<p class='c011'>(2) By comparing the relative absorption of the rays by solids -and gases.</p> - -<p class='c018'>Examined in these ways, it has been found that there are three -different types of radiation emitted from radio-active bodies, which -<span class='pageno' id='Page_109'>109</span>for brevity and convenience have been termed by the writer the -α, β, and γ rays.</p> - -<p class='c006'>(i) The α rays are very readily absorbed by thin metal foil -and by a few centimetres of air. They have been shown to consist -of positively charged bodies projected with a velocity of about -⅒ the velocity of light. They are deflected by intense magnetic -and electric fields, but the amount of deviation is minute -in comparison with the deviation, under the same conditions, of -the cathode rays produced in a vacuum tube.</p> - -<p class='c006'>(ii) The β rays are far more penetrating in character than the -α rays, and consist of negatively charged bodies projected with -velocities of the same order as the velocity of light. They are far -more readily deflected than the α rays, and are in fact identical -with the cathode rays produced in a vacuum tube.</p> - -<p class='c006'>(iii) The γ rays are extremely penetrating, and non-deviable -by a magnetic field. Their true nature is not definitely settled, but -they are analogous in most respects to very penetrating Röntgen rays.</p> - -<p class='c006'>The three best known radio-active substances, uranium, thorium, -and radium, all give out these three types of rays, each in an amount -approximately proportional to its relative activity measured by the -α rays. Polonium stands alone in giving only the α or easily -absorbed rays<a id='r111' href='#f111' class='c012'><sup>[111]</sup></a>.</p> -<p class='c005'><b>72. Deflection of the rays.</b> The rays emitted from the -active bodies thus present a very close analogy with the rays which -are produced in a highly exhausted vacuum tube when an electric -<span class='pageno' id='Page_110'>110</span>discharge passes through it. The α rays correspond to the canal -rays, discovered by Goldstein, which have been shown by Wien to -consist of positively charged bodies projected with great velocity -(see <a href='#section051'>section 51</a>). The β rays are the same as the cathode rays, -while the γ rays resemble the Röntgen rays. In a vacuum -tube, a large amount of electric energy is expended in producing -the rays, but, in the radio-active bodies, the rays are emitted -spontaneously, and at a rate uninfluenced by any chemical or -physical agency. The α and β rays from the active bodies are -projected with much greater velocity than the corresponding rays -in a vacuum tube, while the γ rays are of much greater penetrating -power than Röntgen rays.</p> - -<p class='c006'>The effect of a magnetic field on a pencil of rays from a -radio-active substance giving out the three kinds of rays is very -well illustrated in <a href='#fig022'>Fig. 22</a><a id='r112' href='#f112' class='c012'><sup>[112]</sup></a>.</p> - -<div id='fig022' class='figcenter id002'> -<img src='images/fig-022.png' alt='Fig. 22.' class='ig001'> -<div class='ic002'> -<p>Fig. 22.</p> -</div> -</div> - -<p class='c006'>Some radium is placed in the bottom of a narrow cylindrical -lead vessel <i>R</i>. A narrow pencil -of rays consisting of α, β, and -γ rays escapes from the opening. -If a strong uniform -magnetic field is applied at -right angles to the plane of -the paper, and directed towards -the paper, the three types of -rays are separated from one -another. The γ rays continue -in a straight line without any -deviation. The β rays are -deflected to the right, describing -circular orbits the radii of which vary within wide limits. -If the photographic plate <i>AC</i> is placed under the radium vessel, -the β rays produce a diffuse photographic impression on the right -of the vessel <i>R</i>. The α rays are bent in the direction opposite to -that of the β rays, and describe a portion of the arc of a circle of -large radius, but they are rapidly absorbed after traversing a -distance of a few centimetres from the vessel <i>R</i>. The amount -<span class='pageno' id='Page_111'>111</span>of the deviation of the α rays compared with that of the β rays is -much exaggerated in the figure.</p> -<p class='c005'><b>73. Ionizing and penetrating power of the rays.</b> Of -the three kinds of rays, the α rays produce most of the ionization -in the gas and the γ rays the least. With a thin layer of unscreened -active material spread on the lower of two parallel plates -5 cms. apart, the amount of ionization due to the α, β, and γ rays -is of the relative order 10,000, 100, and 1. These numbers are only -rough approximations, and the differences become less marked -as the thickness of the radio-active layer increases.</p> - -<p class='c006'>The average penetrating power of the rays is shown below. In -the first column is given the thickness of the aluminium, which -cuts each radiation down to half its value, and in the second the -relative power of penetration of the rays.</p> - -<table class='table9' > -<colgroup> -<col class='colwidth27'> -<col class='colwidth46'> -<col class='colwidth25'> -</colgroup> - <tr> - <th class='c013'>Radiation</th> - <th class='c013'>Thickness of Aluminium in cms. which cuts off half the radiation</th> - <th class='c016'>Relative power of penetration</th> - </tr> - <tr> - <td class='c013'> </td> - <td class='c013'> </td> - <td class='c016'> </td> - </tr> - <tr> - <td class='c013'>α rays</td> - <td class='c013'>0·0005 cms.</td> - <td class='c016'>1</td> - </tr> - <tr> - <td class='c013'>β „</td> - <td class='c013'>0·05 cms.</td> - <td class='c016'>100</td> - </tr> - <tr> - <td class='c013'>γ „</td> - <td class='c013'>8 cms.</td> - <td class='c016'>10000</td> - </tr> -</table> - -<p class='c006'>The relative power of penetration is thus approximately inversely -proportional to the relative ionization. These numbers, however, -only indicate the order of relative penetrating power. This power -varies considerably for the different active bodies.</p> - -<p class='c006'>The α rays from uranium and polonium are the least penetrating, -and those from thorium the most. The β radiations from -thorium and radium are very complex, and consist of rays widely -different in penetrating power. Some of the β rays from these -substances are much less and others much more penetrating than -those from uranium, which gives out fairly homogeneous rays.</p> -<p class='c005'><b>74. Difficulties of comparative measurements.</b> It is -difficult to make quantitative or even qualitative measurements of -the relative intensity of the three types of rays from active substances. -The three general methods employed depend upon the -action of the rays in ionizing the gas, in acting on a photographic -<span class='pageno' id='Page_112'>112</span>plate, and in causing phosphorescent or fluorescent effects in certain -substances. In each of these methods the fraction of the rays which -is absorbed and transformed into another form of energy is different -for each type of ray. Even when one specific kind of ray is under -observation, comparative measurements are rendered difficult by -the complexity of that type of rays. For example, the β rays from -radium consist of negatively charged particles projected with a -wide range of velocity, and, in consequence, they are absorbed -in different amounts in passing through a definite thickness of -matter. In each case, only a fraction of the energy absorbed -is transformed into the particular type of energy, whether ionic, -chemical, or luminous, which serves as a means of measurement.</p> - -<p class='c006'>The rays which are the most active electrically are the least -active photographically. Under ordinary conditions, most of the -photographic action of uranium, thorium, and radium, is due to the -β or cathodic rays. The α rays from uranium and thorium, on -account of their weak action, have not yet been detected photographically. -With active substances like radium and polonium, -the α rays readily produce a photographic impression. So far the -γ rays have been detected photographically from radium only. -That no photographic action of these rays has yet been established -for uranium and thorium is probably due merely to the fact that -the effect sought for is very small, and during exposures for long -intervals it is very difficult to avoid fogging of the plates owing to -other causes. Considering the similarity of the radiations in other -respects, there can be little doubt that the γ rays do produce some -photographic action, though it is too small to observe with certainty.</p> - -<p class='c006'>These differences in the photographic and ionizing properties -of the radiations must always be taken into account in comparing -results obtained by the two methods. The apparent contradiction -of results obtained by different observers using these two methods -is found to be due to their differences in relative photographic -and ionizing action. For example, with the unscreened active -material, the ionization observed by the electrical method is due -almost entirely to α rays, while the photographic action under the -same condition is due almost entirely to the β rays.</p> - -<p class='c006'>It is often convenient to know what thickness of matter is -sufficient to absorb a specific type of radiation. A thickness of -<span class='pageno' id='Page_113'>113</span>aluminium or mica of ·01 cms. or a sheet of ordinary writing-paper -is sufficient to absorb completely all the α rays. With such a -screen over the active material, the effects are due only to the -β and γ rays, which pass through with a very slight absorption. -Most of the β rays are absorbed in 5 mms. of aluminium or 2 mms. -of lead. The radiation passing through such screens consists very -largely of the γ rays. As a rough working rule, it may be taken -that a thickness of matter required to absorb any type of rays is -inversely proportional to the density of the substance, <i>i.e.</i> the -absorption is proportional to the density. This rule holds approximately -for light substances, but, in heavy substances like -mercury and lead, the radiations are about twice as readily absorbed -as the density rule would lead us to expect.</p> -<h3 class='c020'>PART II.</h3> -<h4 class='c022'>The β or Cathodic Rays.</h4> -<p class='c005'><b>75. Discovery of the β rays.</b> A discovery which gave -a great impetus to the study of the radiations from active bodies -was made in 1899, almost simultaneously in Germany, France, and -Austria. It was observed that preparations of radium gave out -some rays which were deviable by a magnetic field, and very -similar in character to the cathode rays produced in a vacuum tube. -The observation of Elster and Geitel that a magnetic field altered -the conductivity produced in air by radium rays, led Giesel<a id='r113' href='#f113' class='c012'><sup>[113]</sup></a> to -examine the effect of a magnetic field on the radiations. In his -experiments, the radio-active preparation was placed in a small -vessel between the poles of an electromagnet. The vessel was -arranged to give a pencil of rays which was approximately perpendicular -to the field. The rays caused a small fluorescent patch -on the screen. On exciting the electromagnet, the fluorescent -zone was observed to broaden out on one side. On reversing the -field, the extension of the zone was in the opposite direction. The -deviation of the rays thus indicated was in the same direction and -of the same order of magnitude as that for cathode rays.</p> - -<p class='c006'>S. Meyer and Schweidler<a id='r114' href='#f114' class='c012'><sup>[114]</sup></a> also obtained similar results. They -<span class='pageno' id='Page_114'>114</span>showed, in addition, the deviation of the rays by the alteration -of the conductivity of the air when a magnetic field was -applied. Becquerel<a id='r115' href='#f115' class='c012'><sup>[115]</sup></a>, a little later, showed the magnetic deflection -of the radium rays by using the photographic method. -P. Curie<a id='r116' href='#f116' class='c012'><sup>[116]</sup></a>, by the electrical method, showed furthermore that the -rays from radium consisted of two kinds, one apparently non-deviable -and easily absorbed (now known as the α rays), and the -other penetrating and deviable by a magnetic field (now known -as the β rays). The ionization effect due to the β rays was -only a small fraction of that due to the α rays. At a later date -Becquerel, by the photographic method, showed that uranium gave -out some deflectable rays. It had been shown previously<a id='r117' href='#f117' class='c012'><sup>[117]</sup></a> that the -rays from uranium consisted of α and β rays. The deflected rays -in Becquerel’s experiment consisted entirely of β rays, as the -α rays from uranium produce no appreciable photographic action. -Rutherford and Grier<a id='r118' href='#f118' class='c012'><sup>[118]</sup></a>, using the electric method, showed that -compounds of thorium, like those of uranium, gave out, besides -α rays, some penetrating β rays, deviable in a magnetic field. As -in the case of radium, the ionization due to the α rays of uranium -and thorium is large compared with that due to the β rays.</p> -<p class='c005'><b>76. Examination of the magnetic deviation by the -photographic method.</b> Becquerel has made a very complete -study, by the photographic method, of the β rays from radium, -and has shown that they behave in all respects like cathode rays, -which are known to be negatively charged particles moving with -a high velocity. The motion of a charged ion acted on by a -magnetic field has been discussed in section 49. It has been -shown that if a particle of mass <i>m</i> and charge <i>e</i> is projected -with a velocity <i>u</i>, at an angle α with the direction of a uniform -field of strength <i>H</i>, it will describe a helix round the magnetic -lines of force. This helix is wound on a cylinder of radius <i>R</i>, with -the axis parallel to the field, where <i>R</i> is given by</p> - -<div class='calc'> - -<div class='lg-container-b c017'> - <div class='linegroup'> - <div class='group'> - <div class='line in6'><i>mu</i></div> - <div class='line'><i>R</i> = ---- sin α.</div> - <div class='line in6'><i>He</i></div> - </div> - </div> -</div> - -</div> - -<p class='c018'><span class='pageno' id='Page_115'>115</span>When α = π/2, <i>i.e.</i> when the rays are projected normally to the -field, the particles describe circles of radius</p> - -<div class='calc'> - -<div class='lg-container-b c017'> - <div class='linegroup'> - <div class='group'> - <div class='line in6'><i>mu</i></div> - <div class='line'><i>R</i> = ----</div> - <div class='line in6'><i>He</i></div> - </div> - </div> -</div> - -</div> - -<p class='c018'>The planes of these circles are normal to the field. Thus, for -a particular velocity <i>u</i>, the value of <i>R</i> varies inversely as the -strength of the field. In a uniform field the rays projected normally -to the field describe circles, and their directions of projection -are the tangents at the origin.</p> - -<p class='c006'>This conclusion has been verified experimentally by Becquerel -for the β rays of radium, by an arrangement similar to that shown -in <a href='#fig023'>Fig. 23</a>.</p> - -<div id='fig023' class='figcenter id007'> -<img src='images/fig-023.png' alt='Fig. 23.' class='ig001'> -<div class='ic002'> -<p>Fig. 23.</p> -</div> -</div> - -<p class='c006'>A photographic plate <i>P</i>, with the film downwards, is enveloped -in black paper and placed horizontally in the uniform horizontal -magnetic field of an electromagnet. The magnetic field is supposed -to be uniform, and, in the figure, is at right angles to the -plane of the paper. The plate was covered with a sheet of lead, -and on the edge of the plate, in the centre of the magnetic field, -is placed a small lead vessel <i>R</i> containing the radio-active matter.</p> - -<p class='c006'>On exciting the magnet, so that the rays are bent to the left -of the figure, it is observed that a photographic impression is produced -directly below the source of the rays, which have been bent -round by the magnetic field. The active matter sends out rays -equally in all directions. The rays perpendicular to the field -describe circles, which strike the plate immediately under the -source. A few of these rays, -<i>A</i><sub>1</sub>, <i>A</i><sub>2</sub>, <i>A</i><sub>3</sub>, -are shown in the figure. -The rays, normal to the plate, strike the plate almost normally, -<span class='pageno' id='Page_116'>116</span>while the rays nearly parallel to the plate strike the plate at -grazing incidence. The rays, inclined to the direction of the -field, describe spirals and produce effects on an axis parallel -to the field passing through the source. In consequence of this, -any opaque screen placed in the path of the rays has its shadow -thrown near the edge of the photographic plate.</p> -<p class='c005'><a id='section077'></a> -<b>77. Complexity of the rays.</b> The deviable rays from -radium are complex, <i>i.e.</i> they are composed of a flight of particles -projected with a wide range of velocity. In a magnetic field every -ray describes a path, of which the radius of curvature is directly -proportional to the velocity of projection. The complexity of -the radiation has been shown very clearly by Becquerel<a id='r119' href='#f119' class='c012'><sup>[119]</sup></a> in the -following way.</p> - -<p class='c006'>An uncovered photographic plate, with the film upwards, was -placed horizontally in the horizontal uniform magnetic field of -an electromagnet. A small, open, lead box, containing the -radio-active matter, was placed in the centre of the field, on -the photographic plate. The light, due to the phosphorescence -of the radio-active matter, therefore, could not reach the plate. -The whole apparatus was placed in a dark room. The impression -on the plate took the form of a large, diffuse, but continuous -band, elliptic in shape, produced on one side of the plate.</p> - -<p class='c006'>Such an impression is to be expected if the rays are sent out -in all directions, even if their velocities of projection are the same, -for it can readily be shown theoretically, that the path of the rays -is confined within an ellipse whose minor axis, which is at right -angles to the field, is equal to 2<i>R</i>, and whose major axis is equal -to π<i>R</i>. If, however, the active matter is placed in the bottom of -a deep lead cylinder of small diameter, the rays have practically -all the same direction of projection, and in that case each part of -the plate is acted on by rays of a definite curvature.</p> - -<p class='c006'>In this case also, a diffuse impression is observed on the plate, -giving, so to speak, a continuous spectrum of the rays and showing -that the radiation is composed of rays of widely different curvatures. -<a href='#fig024'>Fig. 24</a> shows a photograph of this kind obtained by Becquerel, -with strips of paper, aluminium, and platinum placed on the plate.</p> - -<div id='fig024' class='figcenter id007'> -<span class='pageno' id='Page_117'>117</span> -<img src='images/fig-024.png' alt='Fig. 24.' class='ig001'> -<div class='ic002'> -<p>Fig. 24.</p> -</div> -</div> - -<p class='c006'>If screens of various thickness are placed on the plate, it is -observed that the plate is not appreciably affected within a certain -distance from the active matter, and that this distance increases -with the thickness of the screen. This distance is obviously equal -to twice the radius of curvature of the path of the rays, which are -just able to produce an impression through the screen.</p> - -<p class='c006'>These experiments show very clearly that the most deviable -rays are those most readily absorbed by matter. By observations -of this kind Becquerel has determined approximately the inferior -limit of the value of <i>HR</i> for rays which are transmitted through -different thicknesses of matter.</p> - -<p class='c006'>The results are given in the table below:</p> - -<table class='table5' > -<colgroup> -<col class='colwidth38'> -<col class='colwidth23'> -<col class='colwidth38'> -</colgroup> - <tr> - <th class='c013'>Substance</th> - <th class='c013'>Thickness in mms.</th> - <th class='c016'>Inferior limit of <i>HR</i> for transmitted rays</th> - </tr> - <tr> - <td class='c013'> </td> - <td class='c013'> </td> - <td class='c016'> </td> - </tr> - <tr> - <td class='c013'>Black paper</td> - <td class='c013'>0·065</td> - <td class='c016'>650</td> - </tr> - <tr> - <td class='c013'>Aluminium</td> - <td class='c013'>0·010</td> - <td class='c016'>350</td> - </tr> - <tr> - <td class='c013'>“</td> - <td class='c013'>0·100</td> - <td class='c016'>1000</td> - </tr> - <tr> - <td class='c013'>”</td> - <td class='c013'>0·200</td> - <td class='c016'>1480</td> - </tr> - <tr> - <td class='c013'>Mica</td> - <td class='c013'>0·025</td> - <td class='c016'>520</td> - </tr> - <tr> - <td class='c013'>Glass</td> - <td class='c013'>0·155</td> - <td class='c016'>1130</td> - </tr> - <tr> - <td class='c013'>Platinum</td> - <td class='c013'>0·030</td> - <td class='c016'>1310</td> - </tr> - <tr> - <td class='c013'>Copper</td> - <td class='c013'>0·085</td> - <td class='c016'>1740</td> - </tr> - <tr> - <td class='c013'>Lead</td> - <td class='c013'>0·130</td> - <td class='c016'>2610</td> - </tr> -</table> - -<p class='c006'>If <i>e</i>/<i>m</i> is a constant for all the rays, the value of <i>HR</i> is proportional -to the velocity of the rays, and it follows from the table that -the velocity of the rays which just produce an effect on the plate -through ·13 mms. of lead is about 7 times that of the rays which -<span class='pageno' id='Page_118'>118</span>just produce an impression through ·01 mm. of aluminium. It -will be shown, however, in <a href='#section082'>section 82</a>, that <i>e</i>/<i>m</i> is not a constant for -all speeds, but decreases with increase of velocity of the rays. The -difference in velocity between the rays is in consequence not as -great as this calculation would indicate. On examination of the -rays from uranium, Becquerel found that the radiation is not as -complex as that from radium, but consists wholly of rays for -which the value of <i>HR</i> is about 2000.</p> -<p class='c005'><b>78. Examination of the β rays by the electric method.</b> -The presence of easily deviable rays given off from an active -substance can most readily be shown by the photographic method, -but it is necessary, in addition, to show that the penetrating rays -which produce the ionization in the gas are the same as those -which cause the photographic action. This can be conveniently -tested in an arrangement similar to that shown in <a href='#fig025'>Fig. 25</a>.</p> - -<div id='fig025' class='figcenter id005'> -<img src='images/fig-025.png' alt='Fig. 25.' class='ig001'> -<div class='ic002'> -<p>Fig. 25.</p> -</div> -</div> - -<p class='c006'>The radio-active matter <i>A</i> is placed on a lead block <i>B´´</i> between -the two parallel lead plates <i>BB´</i>. The -rays pass between the parallel plates and -ionize the gas between the plates <i>PP´</i> of -the testing vessel. The magnetic field is -applied at right angles to the plane of -the paper. The dotted rectangle <i>EEEE</i> -represents the position of the pole piece. -If a compound of radium or thorium is -under investigation, a stream of air is -required to prevent the diffusion of the -radio-active emanations into the testing -vessel. When a layer of uranium, thorium -or radium compound is placed at <i>A</i>, the -ionization in the testing vessel is due -mainly to the action of the α and β rays. The α rays are cut -off by adding a layer of aluminium ·01 cm. thick over the active -material. When the layer of active matter is not more than a few -millimetres thick, the ionization due to the γ rays is small compared -with that produced by the β rays, and may be neglected. -On the application of a magnetic field at right angles to the mean -<span class='pageno' id='Page_119'>119</span>direction of the rays, the ionization in the testing vessel due to -the rays steadily decreases as the strength of the field increases, -and in a strong field it is reduced to a very small fraction of its -original value. In this case the rays are bent so that none of -them enter the testing vessel.</p> - -<p class='c006'>Examined in this way, it has been found that the β rays of -uranium, thorium, and radium consist entirely of rays readily -deflected by a magnetic field. The rays from polonium consist -entirely of α rays, the deviation of which can be detected only in -very intense magnetic fields.</p> - -<p class='c006'>When the screen covering the active material is removed, in -a strong magnetic field, the ionization in the vessel is mainly due -to the α rays. On account of the slight deviation of the α rays -under ordinary experimental conditions, a still greater increase of -the magnetic field does not appreciably alter the current due to -them in the testing vessel.</p> - -<p class='c006'>The action of a magnetic field on a very active substance like -radium is easily shown by the electrical method, as the ionization -current due to the deviable rays is large. With substances of -small activity like uranium and thorium, the ionization current -due to the deviable rays is very small, and a sensitive electrometer -or an electroscope is required to determine the variation, in a -magnetic field, of the very small current involved. This is -especially the case for thorium oxide, which gives out only about -⅕ of the amount of deviable rays given out by the same weight -of uranium oxide.</p> -<p class='c005'><b>79. Experiments with a fluorescent screen.</b> The β -rays from a few milligrams of pure radium bromide produce -intense fluorescence in barium platinocyanide and other substances -which can be made luminous under the influence of the cathode -rays. Using a centigram of radium bromide, the luminosity on -a screen, placed upon it, is bright enough to be observed in -daylight. With the aid of such a screen in a dark room many -of the properties of the β rays may be simply illustrated and their -complex nature clearly shown. A small quantity of radium is -placed in the bottom of a short, narrow, lead tube open at one end. -This is placed between the pole pieces of an electromagnet, and -<span class='pageno' id='Page_120'>120</span>the screen placed below it. With no magnetic field, a faint -luminosity of the screen is observed due to the very penetrating -γ rays which readily pass through the lead. When the magnetic -field is put on, the screen is brightly lighted up on one side over -an area elliptical in shape (<a href='#section077'>section 77</a>). The direction of deviation -is reversed by reversal of the field. The broad extent of the -illumination shows the complex nature of the β rays. On placing -a metallic object at various points above the screen, the trajectory -of the rays can readily be traced by noticing the position of the -shadow cast upon the screen. By observing the density of the -shadow, it can be seen that the rays most easily deviated are the -least penetrating.</p> -<h4 class='c022'>Comparison of the β rays with cathode rays.</h4> -<p class='c005'><a id='section080'></a> -<b>80. Means of comparison.</b> In order to prove the identity -of the β rays from active bodies with the cathode rays produced -in a vacuum tube, it is necessary to show</p> - -<p class='c021'>(1) That the rays carry with them a negative charge;</p> - -<p class='c011'>(2) That they are deviated by an electric as well as by a -magnetic field;</p> - -<p class='c011'>(3) That the ratio <i>e</i>/<i>m</i> is the same as for the cathode rays.</p> -<p class='c005'><b>Electric charge carried by the β rays.</b> The experiments -of Perrin and J. J. Thomson have shown that the cathode rays -carry with them a negative charge. In addition, Lenard has -shown that the rays still carry a charge after traversing thin -layers of matter. When the rays are absorbed, they give up their -charge to the body which absorbs them. The total amount of -charge carried by the β rays from even a very active preparation -of radium is, in general, small compared with that carried by the -whole of the cathode rays in a vacuum tube, and can be detected -only by delicate methods.</p> - -<hr class='c008'> - -<p class='c006'>Suppose that a layer of very active radium is spread on a metal -plate connected to earth, and that the β rays are absorbed by -a parallel plate connected with an electrometer. If the rays are -negatively charged, the top plate should receive a negative charge -increasing with the time. On account, however, of the great -<span class='pageno' id='Page_121'>121</span>ionization produced by the rays between the plates, any charge -given to one of them is almost instantly dissipated. In many -cases, the plate does become charged to a definite positive or -negative potential depending on the metal, but this is due to the -contact difference of potential between the plates, and would be -produced whether the rays were charged or not. The ionization of -the gas is greatly diminished by placing over the active material -a metal screen which absorbs the α rays, but allows the β rays to -pass through with little absorption.</p> - -<p class='c006'>The rapid loss of any charge communicated to the top plate -can be very much reduced, either by diminishing the pressure -of the gas surrounding it or by enclosing the plate with suitable -insulators. In their experiments to determine the amount of -charge carried by the radium rays, M. and Mme Curie<a id='r120' href='#f120' class='c012'><sup>[120]</sup></a> used -the second method.</p> - -<p class='c006'>A metal disc <i>MM</i> (<a href='#fig026'>Fig. 26</a>) is connected with an electrometer -by the wire <i>T</i>. The disc and wire are completely surrounded by -insulating matter <i>ii</i>. The whole is surrounded by a metal envelope -<i>EEEE</i> connected with earth. On the lower side of the disc, the -insulator and the metallic covering are very thin. This side is -exposed to the rays of the radium <i>R</i> placed in a depression in -a lead plate <i>AA</i>.</p> - -<div id='fig026' class='figcenter id006'> -<img src='images/fig-026.png' alt='Fig. 26.' class='ig001'> -<div class='ic002'> -<p>Fig. 26.</p> -</div> -</div> - -<p class='c006'>The rays of the radium pass through the metal cover and -insulator with little absorption, but they are completely absorbed -by the disc <i>MM</i>. It was observed that the disc received a negative -charge which increased uniformly with the time, showing that the -rays carry with them a negative charge. The current observed -was very small. With an active preparation of radium<a id='r121' href='#f121' class='c012'><sup>[121]</sup></a>, forming -<span class='pageno' id='Page_122'>122</span>a layer 2·5 sq. cms. in area and 2 mms. thick, a current of the order -of -10<sup>-11</sup> -amperes was observed after the rays had traversed a layer -of aluminium ·01 mm. thick and a layer of ebonite ·3 mm. thick. -The current was the same with discs of lead, copper, and zinc, and -also when the ebonite was replaced by paraffin.</p> - -<p class='c006'>Curie also observed in another experiment of a similar character -that the radium itself acquired a positive charge. This necessarily -follows if the rays carry with them a negative charge. If the -β rays alone carried with them a charge, a pellet of radium, if -perfectly insulated, and surrounded by a non-conducting medium, -would in the course of time be raised to a high positive potential. -Since, however, the α rays carry with them a charge opposite in -sign to the β rays, the ratio of the charge carried off by the two -types of rays must be determined, before it can be settled whether -the radium would acquire a positive or a negative charge. If, -however, the radium is placed in an insulated metal vessel of a -thickness sufficient to absorb all the α rays, but not too thick to -allow most of the β rays to escape, the vessel will acquire a -positive charge in a vacuum.</p> - -<p class='c006'>An interesting experimental result bearing upon this point -has been described by Dorn<a id='r122' href='#f122' class='c012'><sup>[122]</sup></a>. A small quantity of radium was -placed in a sealed glass tube and left for several months. On -opening the tube with a file, a bright electric spark was observed -at the moment of fracture, showing that there was a large difference -of potential between the inside of the tube and the earth.</p> - -<p class='c006'>In this case the α rays were absorbed in the walls of the tube, -but a large proportion of the β rays escaped. The inside of the -tube thus became charged, in the course of time, to a high positive -potential; a steady state would be reached when the rate of escape -of negative electricity was balanced by the leakage of positive -electricity through the walls of the tube. The external surface of -the glass would be always practically at zero potential, on account -of the ionization of the air around it.</p> - -<p class='c006'>Strutt<a id='r123' href='#f123' class='c012'><sup>[123]</sup></a> has recently described a simple and striking experiment -to illustrate still more clearly that a radium preparation acquires -a positive charge, if it is enclosed in an envelope thick enough to -<span class='pageno' id='Page_123'>123</span>absorb all the α particles, but thin enough to allow most of the -β particles to escape. The experimental arrangement is clearly -seen in <a href='#fig027'>Fig. 27</a>. A sealed tube <i>AA</i> containing -the radium, was attached at one end -to a pair of thin gold leaves in metallic -connection with the radium, and was insulated -inside a larger tube by means of a -quartz rod <i>B</i>. The inner surface of the tube -was coated with tinfoil <i>EE</i> connected to -earth. The glass surface of <i>AA</i> was made -conducting by a thin coating of phosphoric -acid. The air in the outer tube was exhausted -as completely as possible by means -of a mercury pump, in order to reduce the -ionization in the gas, and consequently the -loss of any charge gained by the gold leaves. -After an interval of 20 hours, the gold leaves -were observed to diverge to their full extent, -indicating that they had acquired a large -positive charge. In this experiment Strutt -used ½ gram of radiferous barium of activity -only 100 times that of uranium.</p> - -<div id='fig027' class='figcenter id009'> -<img src='images/fig-027.png' alt='Fig. 27.' class='ig001'> -<div class='ic002'> -<p>Fig. 27.</p> -</div> -</div> - -<p class='c006'>If the tube is filled with 30 mgrs. of pure -radium bromide, the leaves diverge to their -full extent in the course of about a minute. -If it is arranged that the gold leaf, at a -certain angle of divergence, comes in contact -with a piece of metal connected with earth, the -apparatus can be made to work automatically. The leaf diverges, -touches the metal, and at once collapses, and this periodic movement -of the leaf will continue, if not indefinitely, at any rate as -long as the radium lasts. This “radium clock” should work at -a sensibly uniform rate for many years, but, from evidence considered -later (<a href='#section261'>Section 261</a>), there is reason to believe that the -number of β particles emitted would decrease exponentially with -the time, falling to half value in about 1200 years. The period of -movement of the leaf should thus gradually increase with the time, -and ultimately the effect would become too small to observe.</p> - -<p class='c006'><span class='pageno' id='Page_124'>124</span>The action of this radium clock is the nearest approach to an -apparent perpetual motion that has so far been observed.</p> - -<p class='c006'>A determination of the amount of the charge carried off -by the β rays of radium has been made by Wien<a id='r124' href='#f124' class='c012'><sup>[124]</sup></a>. A small -quantity of radium, placed in a sealed platinum vessel, was hung -by an insulating thread inside a glass cylinder, which was exhausted -to a low pressure. A connection between the platinum vessel and -an electrode sealed on to the external glass cylinder could be made, -when required, by tilting the tube. Wien found that in a good -vacuum the platinum vessel became charged to about 100 volts. -The rate of escape of negative electricity from the platinum vessel -containing 4 milligrams of radium bromide corresponded to -2·91 × 10<sup>-12</sup> -amperes. If the charge on each particle is taken as -1·1 × 10<sup>-20</sup> -electromagnetic units, this corresponds to an escape of -2·66 × 10<sup>7</sup> -particles per second. From 1 gram of radium bromide -the corresponding number would be 6·6 × 10<sup>9</sup> per second. Since -some of the β rays are absorbed in their passage through the walls -of the containing vessel and through the radium itself, the actual -number projected per second from 1 gram of radium bromide must -be greater than the above value. This has been found by the -writer to be the case. The method employed reduced the -absorption of the β rays to a minimum, and the total number -emitted per second by 1 gram of radium bromide in radio-active -equilibrium was found to be -4·1 × 10<sup>10</sup>, -or about six times the -number found by Wien. A detailed account of the method -employed cannot be given with advantage at this stage, but will -be found later in <a href='#section253'>Section 253</a>.</p> -<p class='c005'><b>81. Determination of</b> <i>e</i>/<i>m</i>. We have seen (<a href='#section050'>Section 50</a>) that, -in their passage between the plates of a condenser, the cathode -rays are deflected towards the positive plate. Shortly after the -discovery of the magnetic deviation of the β rays from radium, -Dorn<a id='r125' href='#f125' class='c012'><sup>[125]</sup></a> and Becquerel<a id='r126' href='#f126' class='c012'><sup>[126]</sup></a> showed that they also were deflected by an -electric field.</p> - -<p class='c006'>By observing separately the amount of the electric and magnetic -deviation, Becquerel was able to determine the ratio of <i>e</i>/<i>m</i> and -the velocity of the projected particles. Two rectangular copper -<span class='pageno' id='Page_125'>125</span>plates, 3·45 cms. high and 1 cm. apart, were placed in a vertical -plane and insulated on paraffin blocks. One plate was charged to -a high potential by means of an influence machine, and the other -was connected with earth. The active matter was placed in a narrow -groove cut in a lead plate parallel to the copper plates and placed -midway between them. The photographic plate, enveloped in -black paper, was placed horizontally above the plate containing -the active substance. The large and diffuse pencil of rays thus -obtained was deflected by the electric field, but the deviation -amounted to only a few millimetres and was difficult to measure. -The method finally adopted was to place vertically above the -active matter a thin screen of mica, which cut the field into two -equal parts. Thus, in the absence of an electric field, a narrow -rectangular shadow was produced on the plate.</p> - -<p class='c006'>When the electric field was applied, the rays were deflected -and a part of the pencil of rays was stopped by the mica screen. -A shadow was thus cast on the plate which showed the direction -of deviation and corresponded to the least deviable rays which -gave an impression through the black paper.</p> - -<p class='c006'>If a particle of mass <i>m</i>, charge <i>e</i>, and velocity <i>u</i>, is projected -normally to an electric field of strength <i>X</i>, the acceleration α is in -the direction of the field, and is given by</p> - -<div class='calc'> - -<div class='lg-container-b c017'> - <div class='linegroup'> - <div class='group'> - <div class='line in6'><i>Xe</i></div> - <div class='line'>α = ----- .</div> - <div class='line in6'><i>m</i></div> - </div> - </div> -</div> - -</div> - -<p class='c018'>Since the particle moves with a constant acceleration parallel to -the field, the path of the particle is the same as that of a body -projected horizontally from a height with a constant velocity and -acted on by gravity. The path of the particle is thus a parabola, -whose axis is parallel to the field and whose apex is at the point -where the particle enters the electric field. The linear deviation -<i>d</i><sub>1</sub> -of the ray parallel to the field after traversing a distance <i>l</i> is -given by</p> - -<div class='calc'> - -<div class='lg-container-b c017'> - <div class='linegroup'> - <div class='group'> - <div class='line in6'>1 <i>Xe</i> <i>l<sup>2</sup></i></div> - <div class='line'><i>d<sub>1</sub></i> = -- ----- -- .</div> - <div class='line in6'>2 <i>m</i> <i>u<sup>2</sup></i></div> - </div> - </div> -</div> - -</div> - -<p class='c018'>On leaving the electric field, the particle travels in the direction of -the tangent to the path at that point. If θ is the angular deviation -of the path at that point</p> - -<div class='calc'> - -<div class='lg-container-b c017'> - <div class='linegroup'> - <div class='group'> - <div class='line in10'><i>eXl</i></div> - <div class='line'>tan θ = ----- .</div> - <div class='line in10'><i>mu<sup>2</sup></i></div> - </div> - </div> -</div> - -</div> - -<p class='c018'><span class='pageno' id='Page_126'>126</span>The photographic plate was at a distance <i>h</i> above the extremity of -the field. Thus the particles struck the plate at a distance -<i>d</i><sub>2</sub> -from -the original path given by</p> - -<div class='figcenter id009'> -<img src='images/form-028.png' alt='Formula.' class='ig001'> -</div> - -<p class='c006'>In the experimental arrangement the values were</p> - -<div class='calc'> - -<div class='lg-container-b c017'> - <div class='linegroup'> - <div class='group'> - <div class='line'><i>d</i><sub>2</sub> = ·4 cms.;</div> - <div class='line'><i>X</i> = 1·02 × 10<sup>12</sup>;</div> - <div class='line'><i>l</i> = 3·45 cms.;</div> - <div class='line'><i>h</i> = 1·2 cms.</div> - </div> - </div> -</div> - -</div> - -<p class='c018'>If the radius <i>R</i> of curvature of the path of the same rays is observed -in a magnetic field of strength <i>H</i> perpendicular to the rays,</p> - -<div class='calc'> - -<div class='lg-container-b c017'> - <div class='linegroup'> - <div class='group'> - <div class='line in1'><i>e</i> <i>V</i></div> - <div class='line'>--- = ----</div> - <div class='line in1'><i>m</i> <i>HR</i></div> - </div> - </div> -</div> - -</div> - -<p class='c018'>Combining these two equations we get</p> - -<div class='figcenter id009'> -<img src='images/form-029.png' alt='Formula.' class='ig001'> -</div> - -<p class='c006'>A difficulty arose in identifying the part of the complex pencil of -rays for which the electric and magnetic deviations were determined. -Becquerel estimated that the value of <i>HR</i> for the rays deflected -by the electric field was about 1600 <span class='fss'>C.G.S.</span> units. Thus</p> - -<div class='calc'> - -<div class='lg-container-b c017'> - <div class='linegroup'> - <div class='group'> - <div class='line'><i>u</i> = 1·6 × 10<sup>10</sup> cms. per second,</div> - </div> - <div class='group'> - <div class='line'>and</div> - <div class='line in1'><i>e</i></div> - <div class='line'>--- = 10<sup>7</sup>.</div> - <div class='line in1'><i>m</i></div> - </div> - </div> -</div> - -</div> - -<p class='c018'>Thus these rays had a velocity more than half the velocity of light, -and an apparent mass about the same as the cathode ray particles, -<i>i.e.</i> about ¹⁄₁₀₀₀ of the mass of the hydrogen atom. The β ray is -therefore analogous in all respects to the cathode ray, except that -it differs in velocity. In a vacuum tube the cathode rays generally -have a velocity of about -2 × 10<sup>9</sup> -cms. per sec. In special tubes -with strong fields the velocity may be increased to about -10<sup>10</sup> -cms. -per sec. These β particles, then, behave like isolated units of -negative electricity, identical with the electrons set free by an -electric discharge in a vacuum tube. The electrons projected -<span class='pageno' id='Page_127'>127</span>from radium have velocities varying from about 0·2<i>V</i> to at least -0·96<i>V</i>, where <i>V</i> is the velocity of light, and thus have an average -speed considerably greater than that of the electrons produced in -a vacuum tube. These moving electrons are able to pass through -much greater thicknesses of matter before they are absorbed than -the slower electrons produced in a vacuum tube, but the difference -is one merely of degree and not of kind. Since electrons are -continuously and spontaneously expelled from radium with -enormous velocities, they must acquire their energy of motion from -the matter itself. It is difficult to avoid the conclusion, that this -velocity has not been suddenly impressed on the electron. Such -a sudden gain of velocity would mean an immense and sudden -concentration of energy on a small particle, and it is more probable -that the electron before its expulsion has been in rapid orbital or -oscillatory motion in the atom, and, by some means, suddenly -escapes from its orbit. According to this view, the energy of the -electron is not suddenly created but is only made obvious by its -escape from the system to which it belongs.</p> -<p class='c005'><a id='section082'></a> -<b>82. Variation of</b> <i>e</i>/<i>m</i> <b>with the velocity of the electron</b>. -The fact that radium throws off electrons with rates of speed -varying from ⅕ to ⁹⁄₁₀ the velocity of light has been utilised by -Kaufmann<a id='r127' href='#f127' class='c012'><sup>[127]</sup></a> to examine whether the ratio <i>e</i>/<i>m</i> of the electrons -varies with the speed. We have seen (<a href='#section048'>Section 48</a>) that, according -to the electromagnetic theory, a charge of electricity in motion -behaves as if it had apparent mass. For small speeds, this -additional electrical mass is equal to</p> - -<div class='calc'> - -<div class='lg-container-b c017'> - <div class='linegroup'> - <div class='group'> - <div class='line'>2 <i>e</i><sup>2</sup></div> - <div class='line'>- --- ,</div> - <div class='line'>3 <i>a</i></div> - </div> - </div> -</div> - -</div> - -<p class='c018'>where <i>a</i> is the radius of -the body, but it increases rapidly as the speed of light is approached. -It is very important to settle whether the mass of the electron is -due partly to mechanical and partly to electrical mass, or whether -it can be explained by virtue of electricity in motion independently -of the usual conception of mass.</p> - -<p class='c006'>Slightly different formulae expressing the variation of mass -with speed have been developed by J. J. Thomson, Heaviside, -and Searle. To interpret his results Kaufmann used a formula -developed by M.</p> - -<p class='c006'><span class='pageno' id='Page_128'>128</span>Abraham<a id='r128' href='#f128' class='c012'><sup>[128]</sup></a>.</p> - -<div class='calc'> - -<div class='lg-container-b c017'> - <div class='linegroup'> - <div class='group'> - <div class='line'>Let <i>m</i>₀ = mass of electron for slow speeds;</div> - <div class='line in4'><i>m</i> = apparent mass of electron at any speed;</div> - <div class='line in4'><i>u</i> = velocity of electron;</div> - <div class='line in4'><i>V</i> = velocity of light.</div> - </div> - <div class='group'> - <div class='line'>Let β = <i>u</i>/<i>V</i>; then it can be shown that</div> - </div> - </div> -</div> - -</div> - -<div class='figcenter id002'> -<img src='images/form-030.png' alt='Formula.' class='ig001'> -</div> - -<p class='c018'>where</p> - -<div class='figcenter id007'> -<img src='images/form-031.png' alt='Formula.' class='ig001'> -</div> - -<p class='c006'>The experimental method employed to determine <i>e</i>/<i>m</i> and <i>u</i> is -similar to the method of crossed spectra. Some strongly active -radium was placed at the bottom of a brass box. The rays from -this passed between two brass plates insulated and about 1·2 mm. -apart. These rays fell on a platinum diaphragm, containing a -small tube about 0·2 mm. in diameter, which allowed a narrow -bundle of rays to pass. The rays then struck a photographic -plate enveloped in a thin layer of aluminium.</p> - -<p class='c006'>In the experiments the diaphragm was about 2 cms. from the -active material and at the same distance from the photographic -plate. When the whole apparatus was placed in a vacuum, a <span class='fss'>P.D.</span> -of from 2000 to 5000 volts could be applied between the plates -without a spark. The rays were deflected in their passage through -the electric field, and produced what may be termed an electric -spectrum on the plate.</p> - -<div id='fig028' class='figcenter id005'> -<img src='images/fig-028.png' alt='Fig. 28.' class='ig001'> -<div class='ic002'> -<p>Fig. 28.</p> -</div> -</div> - -<p class='c006'>If a magnetic field is superimposed parallel to the electric field -by means of an electromagnet, a magnetic spectrum is obtained -perpendicular to the electric spectrum. The combination -of the two spectra gives rise to a curved -line on the plate. The double trace obtained on -the photographic plate with reversal of the magnetic -field is shown in <a href='#fig028'>Fig. 28</a>. Disregarding -some small corrections, it can readily be shown -that if <i>y</i> and <i>z</i> are the electric and magnetic deviations respectively,</p> - -<div class='calc'> - -<div class='lg-container-b c017'> - <div class='linegroup'> - <div class='group'> - <div class='line in10'><i>z</i></div> - <div class='line'>β = κ<sub>1</sub> ----- (3),</div> - <div class='line in10'><i>y</i></div> - </div> - <div class='group'> - <div class='line'>and</div> - </div> - <div class='group'> - <div class='line'><i>e</i> <i>z</i><sup>2</sup></div> - <div class='line'>-- = κ --- (4).</div> - <div class='line'><i>m</i> <i>y</i></div> - </div> - </div> -</div> - -</div> - -<p class='c018'><span class='pageno' id='Page_129'>129</span>From these two equations, combined with (1), we obtain</p> - -<div class='figcenter id002'> -<img src='images/form-032.png' alt='Formula.' class='ig001'> -</div> - -<p class='c006'>where κ, κ<sub>1</sub>, κ<sub>2</sub> are constants.</p> - -<p class='c006'>Equation (5) gives the curve that should be obtained on the -plate according to the electromagnetic theory. This is compared -by trial with the actual curve obtained on the plate.</p> - -<p class='c006'>In this way Kaufmann<a id='r129' href='#f129' class='c012'><sup>[129]</sup></a> found that the value of <i>e</i>/<i>m</i> decreased -with the speed, showing that, assuming the charge constant, the -mass of the electron increased with the speed.</p> - -<p class='c006'>The following numbers give some of the preliminary results -obtained by this method.</p> - -<table class='table10' > -<colgroup> -<col class='colwidth57'> -<col class='colwidth42'> -</colgroup> - <tr> - <th class='c013'>Velocity of electron</th> - <th class='c014'><i>e</i>/<i>m</i></th> - </tr> - <tr> - <td class='c013'> </td> - <td class='c014'> </td> - </tr> - <tr> - <td class='c013'>2·36 × 10<sup>10</sup> cms. per sec.</td> - <td class='c014'>1·31 × 10<sup>7</sup></td> - </tr> - <tr> - <td class='c013'>2·48 „ „</td> - <td class='c014'>1·17 × 10<sup>7</sup></td> - </tr> - <tr> - <td class='c013'>2·59 „ „</td> - <td class='c014'>0·97 × 10<sup>7</sup></td> - </tr> - <tr> - <td class='c013'>2·72 „ „</td> - <td class='c014'>0·77 × 10<sup>7</sup></td> - </tr> - <tr> - <td class='c013'>2·85 „ „</td> - <td class='c014'>0·63 × 10<sup>7</sup></td> - </tr> -</table> - -<p class='c006'>For the cathode rays S. Simon<a id='r130' href='#f130' class='c012'><sup>[130]</sup></a> obtained a value for <i>e</i>/<i>m</i> of -1·86 × 10<sup>7</sup> -for an average speed of about 7 × 10<sup>9</sup> cms. per second.</p> - -<p class='c006'>In a later paper<a id='r131' href='#f131' class='c012'><sup>[131]</sup></a> with some very active radium, more satisfactory -photographs were obtained, which allowed of accurate -measurement. The given equation of the curve was found to -agree satisfactorily with experiment.</p> - -<p class='c006'>The table given below, deduced from the results given by -Kaufmann, shows the agreement between the theoretical and -experimental values, <i>u</i> being the velocity of the electron and <i>V</i> -that of light.</p> - -<p class='c006'>The average percentage error between the observed and calculated -value is thus not much more than one per cent. It is -<span class='pageno' id='Page_130'>130</span>remarkable how nearly the velocity of the electron has to approach -the velocity of light before the value of -<i>m</i>/<i>m</i>₀ -becomes large. This -is shown in the following table which gives the calculated values -of -<i>m</i>/<i>m</i>₀ -for different velocities of the electron.</p> - -<table class='table11' > -<colgroup> -<col class='colwidth27'> -<col class='colwidth41'> -<col class='colwidth30'> -</colgroup> - <tr> - <th class='c013'>Value of <i>u</i>/<i>V</i></th> - <th class='c013'>Observed value of <i>m</i>/<i>m</i>₀</th> - <th class='c014'>Percentage difference from theoretical values</th> - </tr> - <tr> - <td class='c013'> </td> - <td class='c013'> </td> - <td class='c014'> </td> - </tr> - <tr> - <td class='c013'>Small</td> - <td class='c013'>1</td> - <td class='c014'> </td> - </tr> - <tr> - <td class='c013'>·732</td> - <td class='c013'>1·34</td> - <td class='c014'>-1·5 %</td> - </tr> - <tr> - <td class='c013'>·752</td> - <td class='c013'>1·37</td> - <td class='c014'>-0·9 „</td> - </tr> - <tr> - <td class='c013'>·777</td> - <td class='c013'>1·42</td> - <td class='c014'>-0·6 „</td> - </tr> - <tr> - <td class='c013'>·801</td> - <td class='c013'>1·47</td> - <td class='c014'>+0·5 „</td> - </tr> - <tr> - <td class='c013'>·830</td> - <td class='c013'>1·545</td> - <td class='c014'>+0·5 „</td> - </tr> - <tr> - <td class='c013'>·860</td> - <td class='c013'>1·65</td> - <td class='c014'>0 „</td> - </tr> - <tr> - <td class='c013'>·883</td> - <td class='c013'>1·73</td> - <td class='c014'>+2·8 „</td> - </tr> - <tr> - <td class='c013'>·933</td> - <td class='c013'>2·05</td> - <td class='c014'>-7·8 „ ?</td> - </tr> - <tr> - <td class='c013'>·949</td> - <td class='c013'>2·145</td> - <td class='c014'>-1·2 „</td> - </tr> - <tr> - <td class='c013'>·963</td> - <td class='c013'>2·42</td> - <td class='c014'>+0·4 „</td> - </tr> -</table> - -<table class='table12' > -<colgroup> -<col class='colwidth19'> -<col class='colwidth15'> -<col class='colwidth9'> -<col class='colwidth9'> -<col class='colwidth7'> -<col class='colwidth7'> -<col class='colwidth7'> -<col class='colwidth9'> -<col class='colwidth13'> -</colgroup> - <tr> - <td class='c013'>Value of <i>u</i>/<i>V</i></td> - <td class='c013'>small</td> - <td class='c013'>·1</td> - <td class='c013'>·5</td> - <td class='c013'>·9</td> - <td class='c013'>·99</td> - <td class='c013'>·999</td> - <td class='c013'>·9999</td> - <td class='c014'>·999999</td> - </tr> - <tr> - <td class='c013'>Calculated value m/m₀</td> - <td class='c013'>1·00</td> - <td class='c013'>1·015</td> - <td class='c013'>1·12</td> - <td class='c013'>1·81</td> - <td class='c013'>3·28</td> - <td class='c013'>4·96</td> - <td class='c013'>6·68</td> - <td class='c014'>10·1</td> - </tr> -</table> - -<p class='c006'>Thus for velocities varying from 0 to ⅒ the velocity of light, -the mass of the electron is practically constant. The increase of -mass becomes appreciable at about half the velocity of light, and -increases steadily as the velocity of light is approached. Theoretically -the mass becomes infinite at the velocity of light, but -even when the velocity of the electron only differs from that of -light by one part in a million, its mass is only 10 times the value -for slow speeds.</p> - -<p class='c006'>The above results are therefore in agreement with the view -that the mass of the electron is altogether electrical in origin and -can be explained purely by electricity in motion. The value of -<i>e</i>/<i>m</i>₀, -for slow speeds, deduced from the results was -1·84 × 10<sup>7</sup>, -which is in very close agreement with the value obtained by -Simon for the cathode rays, viz. -1·86 × 10<sup>7</sup>.</p> - -<p class='c006'><span class='pageno' id='Page_131'>131</span>If the electricity carried by the electron is supposed to be -distributed uniformly over a sphere of radius <i>a</i>, for speeds slow -compared with the velocity of light, the apparent mass</p> - -<div class='calc'> - -<div class='lg-container-b c017'> - <div class='linegroup'> - <div class='group'> - <div class='line in6'>2 <i>e</i><sup>2</sup></div> - <div class='line'><i>m</i>₀ = --- ----</div> - <div class='line in6'>3 <i>a</i></div> - </div> - </div> -</div> - -</div> - -<p class='c018'>Therefore</p> - -<div class='calc'> - -<div class='lg-container-b c017'> - <div class='linegroup'> - <div class='group'> - <div class='line in5'>2 <i>e</i></div> - <div class='line'><i>a</i> = --- ---- . <i>e</i></div> - <div class='line in5'>3 <i>m</i>₀</div> - </div> - </div> -</div> - -</div> - -<p class='c018'>Taking the value of <i>e</i> as -1·13 × 10<sup>-20</sup>, <i>a</i> is 1·4 × 10<sup>-13</sup> cms.</p> - -<p class='c006'>Thus the diameter of an electron is minute compared with the -diameter of an atom.</p> -<p class='c005'><b>83. Distribution of velocity amongst the β particles</b>. -Some interesting experiments have been recently made by Paschen<a id='r132' href='#f132' class='c012'><sup>[132]</sup></a> -to determine the relative number of β particles which are expelled -from radium at the different speeds. The experimental arrangement -is shown in <a href='#fig029'>Fig. 29</a>.</p> - -<div id='fig029' class='figcenter id002'> -<img src='images/fig-029.png' alt='Fig. 29.' class='ig001'> -<div class='ic002'> -<p>Fig. 29.</p> -</div> -</div> - -<p class='c006'>A small thin silvered glass tube <i>b</i>, containing 15 mgrs. of -radium bromide, was placed in the axis of a number of lead vanes -arranged round a cylinder of diameter 2 cms. and length 2·2 cms. -<span class='pageno' id='Page_132'>132</span>When no magnetic field was acting, the β particles from the radium -passed through the openings and were absorbed in an outer concentric -cylinder <i>aa</i> of lead of inner diameter 3·7 cms. and of -thickness 5·5 mms. This outer cylinder was rigidly connected to -the inner cylinder <i>cc</i> by quartz rods <i>ii</i>, which also served to insulate -it. The cylinder <i>c</i> and the radium were connected with earth. -A gold-leaf electroscope <i>E</i> was attached to <i>a</i>, and the whole -apparatus was enclosed in a glass vessel which was exhausted to -a low vacuum by means of a mercury pump. The glass vessel was -placed in the uniform field of a large electromagnet, so that the -axis of the lead cylinder was parallel to the lines of force.</p> - -<p class='c006'>The outer cylinder gains a negative charge on account of the -particles which are absorbed in it. This negative charge, which -is indicated by the movement of the gold-leaf, tends to be dissipated -by the small ionization produced in the residual gas by the passage -of the β rays. This action of the gas can be eliminated by -observing the rate of movement of the gold leaf when charged -alternately to an initial positive and negative potential. The -mean of the two rates is proportional to the number of β particles -which give up their charge to the lead cylinder. This is evidently -the case, since, when the charge is positive, the ionization of -the gas assists the rate of movement of the gold-leaf, and, when -negative, diminishes it to an equal extent.</p> - -<p class='c006'>When a magnetic field is applied, each of the particles describes -a curved path, whose radius of curvature depends on the velocity -of the particle. For weak fields, only the particles of smallest -velocity will be deflected sufficiently not to strike the outer -cylinder, but, as the field is raised, the number will increase until -finally all the β particles fail to reach the outer cylinder. The -decrease of the charge communicated to the outer cylinder with -the increase of the strength of the magnetic field is shown graphically -in <a href='#fig030'>Fig. 30</a>, Curve I.</p> - -<p class='c006'>The ordinates represent in arbitrary units the charge communicated -to the lead cylinder per second, and thus serve as -a measure of the number of β particles which reach the cylinder. -Knowing the dimensions of the apparatus, and assuming the value -<i>e</i>/<i>m</i> found by Kaufmann, the velocity of the particles which just -fail to reach the lead cylinder can be deduced from any strength -<span class='pageno' id='Page_133'>133</span>of the magnetic field. Curve II, <a href='#fig030'>Fig. 30</a> is the first differential of -Curve I, and the ordinates represent the relative number of β -particles which are projected at each velocity.</p> - -<div id='fig030' class='figcenter id004'> -<img src='images/fig-030.png' alt='Fig. 30.' class='ig001'> -<div class='ic002'> -<p>Fig. 30.</p> -</div> -</div> - -<p class='c006'>From the data given by Kaufmann (see <a href='#section082'>section 82</a>) Paschen -deduced that the group of rays examined by the former, which -had velocities lying between -2·12 × 10<sup>10</sup> and 2·90 × 10<sup>10</sup> cms. -per second, corresponded to the group of rays between the points -<i>A</i> and <i>B</i>, that is, to the group of rays which were completely -deflected from the lead cylinder between the magnetic fields of -strengths of 1875 and 4931 <span class='fss'>C.G.S.</span> units. Since radium gives off -β particles which require a field of strength over 7000 units to -deflect them, Paschen concluded that β particles are expelled from -radium with still greater velocities than the highest recorded by -Kaufmann.</p> - -<p class='c006'>Paschen considered that the small charge observed in still -higher fields was mainly due to the γ rays. The effect is small -and is probably not due to an actual charge carried by the γ rays -but to a secondary effect produced by them. This question will -be discussed in more detail in <a href='#section112'>section 112</a>.</p> - -<p class='c006'>There is a group of low velocity β particles emitted by radium -(see <a href='#fig030'>Fig. 30</a>) which have about the same speed as the electrons -<span class='pageno' id='Page_134'>134</span>set free in a vacuum tube. In consequence of their small velocity, -these probably produce a large proportion of the ionization due to -the β rays at short distances from the radium, for it will be shown -(section 103) that the ionization produced by an electron per unit -length of path steadily decreases with increase of its velocity above -a small limiting value. This observation is confirmed by experiments -on the absorption of the β rays in passing through matter.</p> - -<p class='c006'>In Paschen’s experiments, the glass tube containing the radium -was ·5 mms. thick, so that a considerable proportion of the low -velocity β particles must have been stopped by it. This is borne -out by some later experiments of Seitz which will be described in -<a href='#section085'>section 85</a>.</p> -<p class='c005'><b>84. Absorption of the β rays by matter</b>. The β particles -produce ions in their passage through the gas and their energy -of motion is consequently diminished. A similar action takes -place also when the β rays pass through solid and liquid media, -and the mechanism of absorption is probably similar in all cases. -Some of the particles in their passage through matter are completely -stopped, while others have their velocity reduced. In -addition, there is a considerable scattering or diffuse reflection of -the rays in traversing matter. The amount of this scattering -depends upon the density of the substance and also upon the -angle of incidence of the rays. This scattering of the rays will be -discussed later in <a href='#section111'>section 111</a>.</p> - -<p class='c006'>There are two general methods of determining the absorption -of the β rays. In the first method, the variation of the ionization -current is observed in a testing vessel when the active matter is -covered by screens differing in material and thickness. This -ionization in the vessel depends upon two quantities, viz. the -number of β particles which pass through the matter and also -upon the number of ions produced by them per unit path. In the -absence of any definite information in regard to the variation of -ionization by the electron with its velocity, no very definite conclusions -can be drawn from such experiments.</p> - -<p class='c006'>The advent of pure radium-bromide has made it possible to -determine the actual number of electrons which are absorbed in -their passage through a definite thickness of matter, by measuring -<span class='pageno' id='Page_135'>135</span>the negative charge carried by the issuing rays. Experiments of -this character have been made by Seitz and will be considered later.</p> - -<p class='c006'>These two methods of determining the absorption of β rays -are quite distinct in principle, and it is not to be expected that the -values of the coefficients of absorption obtained in the two cases -should be the same. The whole question of the absorption of -electrons by matter is very complicated, and the difficulty is still -further increased by the complexity of the β rays emitted by the -radio-active substances. Many of the results obtained by different -methods, while pointing to the same general conclusion, are -quantitatively in wide disagreement. Before any definite advance -can be made to a better understanding of the mechanism of -absorption, it will be necessary to determine the variation of the -ionization with the speed of the electron over a very wide range. -Some work has already been done in this direction but not between -sufficiently wide limits.</p> -<h3 class='c020'>Ionization method.</h3> -<p class='c005'>We shall first consider the results obtained on the absorption of -β rays by measuring the variation of the ionization current, when -screens of different thickness are placed over the active substance. -When the active matter is covered with aluminium foil of thickness -·1 mm., the current in a testing vessel such as is shown in Fig. 17, -is due almost entirely to the β rays. If a uranium compound is -used, it is found that the saturation current decreases with the -thickness of matter traversed nearly according to an exponential -law. Taking the saturation current as a measure of the intensity -of the rays, the intensity <i>I</i> after passing through a thickness <i>d</i> of -matter is given by</p> - -<div class='figcenter id010'> -<img src='images/form-033.png' alt='Formula.' class='ig001'> -</div> - -<p class='c006'>where λ is the constant of absorption of the rays and -<i>I</i>₀ -is the -initial intensity. For uranium rays, the current is reduced to half -its value after passing through about ·5 mm. of aluminium.</p> - -<p class='c006'>If a compound of thorium or radium is examined in the same -way, it is found that the current does not decrease regularly -<span class='pageno' id='Page_136'>136</span>according to the above equation. Results of this kind for radium -rays have been given by Meyer and Schweidler<a id='r133' href='#f133' class='c012'><sup>[133]</sup></a>. The amount of -absorption of the rays by a certain thickness of matter decreases -with the thickness traversed. This is exactly opposite to what is -observed for the α rays. This variation in the absorption is due to -the fact that the β rays are made up of rays which vary greatly in -penetrating power. The rays from uranium are fairly homogeneous -in character, <i>i.e.</i> they consist of rays projected with about the same -velocity. The rays from radium and thorium are complex, <i>i.e.</i> they -consist of rays projected with a wide range of velocity and consequently -with a wide range of penetrating power. The electrical -examination of the deviable rays thus leads to the same results as -their examination by the photographic method.</p> - -<p class='c006'>Results on the absorption of cathode rays have been given by -Lenard<a id='r134' href='#f134' class='c012'><sup>[134]</sup></a>, who has shown that the absorption of cathode rays is -nearly proportional to the density of the absorbing matter, and is -independent of its chemical state. If the deviable rays from active -bodies are similar to cathode rays, a similar law of absorption is to -be expected. Strutt<a id='r135' href='#f135' class='c012'><sup>[135]</sup></a>, working with radium rays, has determined -the law of absorption, and has found it roughly proportional to the -density of matter over a range of densities varying from 0·041 for -sulphur dioxide to 21·5 for platinum. In the case of mica and -cardboard, the values of λ divided by the density were 3·94 and -3·84 respectively, while the value for platinum was 7·34. In order -to deduce the absorption coefficient, he assumed that the radiation -fell off according to an exponential law with the distance traversed. -As the rays from radium are complex, we have seen that this is -only approximately the case.</p> - -<p class='c006'>Since the β rays from uranium are fairly homogeneous, and are -at the same time penetrating in character, they are more suitable -for such a determination than the complex rays of radium. I -have in consequence made some experiments with uranium rays -to determine the dependence of absorption on the density. The -results obtained are given in the following table, where λ is the -coefficient of absorption.</p> - -<table class='table13' > -<colgroup> -<col class='colwidth26'> -<col class='colwidth26'> -<col class='colwidth21'> -<col class='colwidth26'> -</colgroup> - <tr><td class='c023' colspan='4'><span class='pageno' id='Page_137'>137</span></td></tr> - <tr> - <th class='c013'>Substance</th> - <th class='c013'>λ</th> - <th class='c013'>Density</th> - <th class='c014'>λ/Density</th> - </tr> - <tr> - <td class='c013'> </td> - <td class='c013'> </td> - <td class='c013'> </td> - <td class='c014'> </td> - </tr> - <tr> - <td class='c013'>Glass</td> - <td class='c013'>14·0</td> - <td class='c013'>2·45</td> - <td class='c014'>5·7</td> - </tr> - <tr> - <td class='c013'>Mica</td> - <td class='c013'>14·2</td> - <td class='c013'>2·78</td> - <td class='c014'>5·1</td> - </tr> - <tr> - <td class='c013'>Ebonite</td> - <td class='c013'>6·5</td> - <td class='c013'>1·14</td> - <td class='c014'>5·7</td> - </tr> - <tr> - <td class='c013'>Wood</td> - <td class='c013'>2·16</td> - <td class='c013'>·40</td> - <td class='c014'>5·4</td> - </tr> - <tr> - <td class='c013'>Cardboard</td> - <td class='c013'>3·7</td> - <td class='c013'>·70</td> - <td class='c014'>5·3</td> - </tr> - <tr> - <td class='c013'>Iron</td> - <td class='c013'>44</td> - <td class='c013'>7·8</td> - <td class='c014'>5·6</td> - </tr> - <tr> - <td class='c013'>Aluminium</td> - <td class='c013'>14·0</td> - <td class='c013'>2·60</td> - <td class='c014'>5·4</td> - </tr> - <tr> - <td class='c013'>Copper</td> - <td class='c013'>60</td> - <td class='c013'>8·6</td> - <td class='c014'>7·0</td> - </tr> - <tr> - <td class='c013'>Silver</td> - <td class='c013'>75</td> - <td class='c013'>10·5</td> - <td class='c014'>7·1</td> - </tr> - <tr> - <td class='c013'>Lead</td> - <td class='c013'>122</td> - <td class='c013'>11·5</td> - <td class='c014'>10·8</td> - </tr> - <tr> - <td class='c013'>Tin</td> - <td class='c013'>96</td> - <td class='c013'>7·3</td> - <td class='c014'>13·2</td> - </tr> -</table> - -<p class='c006'>It will be observed that the value of the absorption constant -divided by the density is very nearly the same for such different -substances as glass, mica, ebonite, wood, iron and aluminium. The -divergences from the law are great, however, for the other metals -examined, viz. copper, silver, lead and tin. In tin the value of λ -divided by the density is 2·5 times its value for iron and aluminium. -These differences show that a law for the absorption of the β rays -depending only on the density does not hold for all substances. -With an exception in the case of tin, the value of λ divided by the -density for the metals increases in the same order as their atomic -weights.</p> - -<p class='c006'>The absorption of the β rays by matter decreases very rapidly -with increase of speed. For example, the absorption of cathode -rays in Lenard’s experiment (<i>loc. cit.</i>) is about 500 times as great -as for the uranium β rays. The velocity of the β rays of uranium -was found by Becquerel to be about -1·6 × 10<sup>10</sup> -cms. per sec. The -velocity of the cathode rays used in Lenard’s experiment was -certainly not less than ⅒ of this, so that, for a decrease of -speed of less than 10 times, the absorption has increased over -500 times.</p> -<p class='c005'><a id='section085'></a> -<b>85. Number of electrons stopped by matter.</b> An account -will now be given of the experiments made by Seitz<a id='r136' href='#f136' class='c012'><sup>[136]</sup></a>, to determine -<span class='pageno' id='Page_138'>138</span>the relative number of electrons which are stopped in their passage -through different thicknesses of matter. The experimental -arrangement is shown in <a href='#fig031'>Fig. 31</a>.</p> - -<div id='fig031' class='figcenter id005'> -<img src='images/fig-031.png' alt='Fig. 31.' class='ig001'> -<div class='ic002'> -<p>Fig. 31.</p> -</div> -</div> - -<p class='c006'>The radium was placed outside a glass vessel containing an -insulated brass plate <i>P</i>, the connection -of which with a wire leading to -the electrometer could be made or -broken by a simple electromagnetic -device. The β rays from the radium -<i>R</i>, after passing through openings in -a brass plate <i>A</i>, covered with thin -aluminium foil, were absorbed in the -plate <i>P</i>. The glass vessel was exhausted, -and the charge communicated -to <i>P</i> by the β rays was measured by -an electrometer.</p> - -<p class='c006'>In a good vacuum, the magnitude -of the current observed is a measure -of the number of β particles absorbed -by the upper plate<a id='r137' href='#f137' class='c012'><sup>[137]</sup></a>. The following -table shows the results obtained when -different thicknesses of tin foil were -placed over the radium. The second -table gives the ratio -<i>I</i>/<i>I</i>₀ -where -<i>I</i>₀ -is the -rate of discharge observed before the -absorbing screen is introduced. The -mean value of the absorption constant -λ was deduced from the equation</p> - -<div class='figcenter id010'> -<img src='images/form-034.png' alt='Formula.' class='ig001'> -</div> - -<p class='c006'>where <i>d</i> is the thickness -of matter traversed.</p> - -<p class='c006'>The values included in the brackets have not the same accuracy -as the others. There is thus a wide difference in penetrating -power of the β particles emitted from radium, and some of them -are very readily absorbed.</p> - -<p class='c006'><span class='pageno' id='Page_139'>139</span>When a lead screen 3 mms. thick was placed over the radium—a -thickness sufficient to absorb all the readily deflectable β rays—a -small negative charge was still given to the plate, corresponding -to ·29 per cent. of the maximum. This is a very much -smaller value than was observed by Paschen (see <a href='#fig030'>Fig. 30</a>).</p> - -<table class='table8' > -<colgroup> -<col class='colwidth37'> -<col class='colwidth37'> -<col class='colwidth25'> -</colgroup> - <tr> - <th class='c013'>Thickness of Tin in mms.</th> - <th class='c013'><i>I</i>/<i>I</i>₀</th> - <th class='c014'>λ</th> - </tr> - <tr> - <td class='c013'> </td> - <td class='c013'> </td> - <td class='c014'> </td> - </tr> - <tr> - <td class='c013'>0·00834</td> - <td class='c013'>·869</td> - <td class='c014'>175</td> - </tr> - <tr> - <td class='c013'>0·0166</td> - <td class='c013'>·802</td> - <td class='c014'>132·5</td> - </tr> - <tr> - <td class='c013'>0·0421</td> - <td class='c013'>·653</td> - <td class='c014'>101·5</td> - </tr> - <tr> - <td class='c013'>0·0818</td> - <td class='c013'>·466</td> - <td class='c014'>93·5</td> - </tr> - <tr> - <td class='c013'>0·124</td> - <td class='c013'>·359</td> - <td class='c014'>82·5</td> - </tr> - <tr> - <td class='c013'>0·166</td> - <td class='c013'>·289</td> - <td class='c014'>74·9</td> - </tr> - <tr> - <td class='c013'>0·205</td> - <td class='c013'>·230</td> - <td class='c014'>71·5</td> - </tr> - <tr> - <td class='c013'>0·270</td> - <td class='c013'>·170</td> - <td class='c014'>65·4</td> - </tr> - <tr> - <td class='c013'>0·518</td> - <td class='c013'>·065 }</td> - <td class='c014'>53}</td> - </tr> - <tr> - <td class='c013'>0·789</td> - <td class='c013'>·031 }</td> - <td class='c014'>44}</td> - </tr> - <tr> - <td class='c013'>1·585</td> - <td class='c013'>·0059}</td> - <td class='c014'>32}</td> - </tr> - <tr> - <td class='c013'>2·16</td> - <td class='c013'>·0043}</td> - <td class='c014'>25}</td> - </tr> -</table> - -<p class='c006'>This difference may, in part, be due to the fact that, in Paschen’s -experiments, a large proportion of the slow velocity electrons were -absorbed in the glass tube of ·5 mm. thickness containing the -radium.</p> - -<p class='c006'>Seitz also determined the relative thickness, compared with -tin, of different substances which reduced the negative charge -communicated to <i>P</i> by a definite amount. A few of the numbers -are given below, and expressed in terms of tin as unity.</p> - -<table class='table14' > -<colgroup> -<col class='colwidth50'> -<col class='colwidth50'> -</colgroup> - <tr> - <th class='c013'>Substance</th> - <th class='c016'>Thickness Tin = 1</th> - </tr> - <tr> - <td class='c013'> </td> - <td class='c016'> </td> - </tr> - <tr> - <td class='c013'>Lead</td> - <td class='c016'>·745</td> - </tr> - <tr> - <td class='c013'>Gold</td> - <td class='c016'>·83</td> - </tr> - <tr> - <td class='c013'>Platinum</td> - <td class='c016'>·84</td> - </tr> - <tr> - <td class='c013'>Silver</td> - <td class='c016'>1</td> - </tr> - <tr> - <td class='c013'>Steel</td> - <td class='c016'>1·29</td> - </tr> - <tr> - <td class='c013'>Aluminium</td> - <td class='c016'>1·56</td> - </tr> - <tr> - <td class='c013'>Water</td> - <td class='c016'>1·66</td> - </tr> - <tr> - <td class='c013'>Paraffin</td> - <td class='c016'>1·69</td> - </tr> -</table> - -<p class='c006'>The thickness required to stop a given proportion of the β rays -thus decreases with the density, but not nearly so fast as the -<span class='pageno' id='Page_140'>140</span>density increases. These results are difficult to reconcile with -the density-law of absorption found by Lenard from the cathode -rays, or with the results of the ionization method already considered. -A further experimental examination of the whole -question is very much to be desired.</p> -<p class='c005'><a id='section086'></a> -<b>86. Variation of the amount of radiation with the -thickness of the layer of radiating material.</b> The radiations -are sent out equally from all portions of the active mass, but the -ionization of the gas which is measured is due only to the radiations -which escape into the air. The depth from which the radiations -can reach the surface depends on the absorption of the radiation -by the active matter itself.</p> - -<p class='c006'>Let λ be the absorption constant of the homogeneous radiation -by the active material. It can readily be shown that the intensity -<i>I</i> of the rays issuing from a layer of active matter, of thickness <i>d</i>, -is given by</p> - -<div class='figcenter id010'> -<img src='images/form-035.png' alt='Formula.' class='ig001'> -</div> - -<p class='c006'>where -<i>I</i>₀ -is the intensity at the surface due to a very thick layer.</p> - -<p class='c006'>This equation has been confirmed experimentally by observing -the current due to the β rays for different thicknesses of uranium -oxide. In this case -<i>I</i> = (½)<i>I</i>₀ -for a thickness of oxide corresponding -to ·11 gr. per sq. cm. This gives a value of λ divided by density of -6·3. This is a value slightly greater than that observed for the -absorption of the same rays in aluminium. Such a result shows -clearly that the substance which gives rise to the β rays does not -absorb them to a much greater extent than does ordinary matter -of the same density.</p> - -<p class='c006'>The value of λ will vary, not only for the different active -substances, but also for the different compounds of the same -substance.</p> -<div> - <span class='pageno' id='Page_141'>141</span> - <h3 class='c001'>PART III.</h3> -</div> -<h4 class='c022'>The α Rays.</h4> -<p class='c005'><b>87. The α rays</b>. The magnetic deviation of the β rays was -discovered towards the end of 1899, at a comparatively early stage -in the history of radio-activity, but three years elapsed before -the true character of the α rays was disclosed. It was natural -that great prominence should have been given in the early stages -of the subject to the β rays, on account of their great penetrating -power and marked action in causing phosphorescence in many -substances. The α rays were, in comparison, very little studied, -and their importance was not generally recognized. It will, however, -be shown that the α rays play a far more important part -in radio-active processes than the β rays, and that the greater -portion of the energy emitted in the form of ionizing radiations -is due to them.</p> -<p class='c005'><b>88. The nature of the α rays</b>. The nature of the α rays -was difficult to determine, for a magnetic field sufficient to cause -considerable deviation of the β rays produced no appreciable effect -on the α rays. It was suggested by several observers that they -were, in reality, secondary rays set up by the β or cathode rays in -the active matter from which they were produced. Such a view, -however, failed to explain the radio-activity of polonium, which -gave out α rays only. Later work also showed that the matter, -which gave rise to the β rays from uranium, could be chemically -separated from the uranium, while the intensity of the α rays was -unaffected. These and other results show that the α and β rays -are produced quite independently of one another. The view that -they are an easily absorbed type of Röntgen rays fails to explain -a characteristic property of the α rays, viz. that the absorption of -the rays in a given thickness of matter, determined by the electrical -method, increases with the thickness of matter previously -traversed. It does not seem probable that such an effect could -be produced by a radiation like X rays, but the result is to be -expected if the rays consist of projected bodies, which fail to -<span class='pageno' id='Page_142'>142</span>ionize the gas when their velocity is reduced below a certain -value. From observations of the relative ionization produced in -gases by the α and β rays, Strutt<a id='r138' href='#f138' class='c012'><sup>[138]</sup></a> suggested in 1901 that the α -rays might consist of positively charged bodies projected with -great velocity. Sir William Crookes<a id='r139' href='#f139' class='c012'><sup>[139]</sup></a>, in 1902, advanced the same -hypothesis. From a study of the α rays of polonium Mme. Curie<a id='r140' href='#f140' class='c012'><sup>[140]</sup></a> -in 1900 suggested the probability that these rays consisted of -bodies, projected with great velocity, which lost their energy by -passing through matter.</p> - -<p class='c006'>The writer was led independently to the same view by a mass -of indirect evidence which received an explanation only on the -hypothesis that the rays consisted of matter projected with great -velocity. Preliminary experiments with radium of activity 1000 -showed that it was very difficult to determine the magnetic deviation -of the α rays. When the rays were passed through slits -sufficiently narrow to enable a minute deviation of the rays to be -detected, the ionizing effect of the issuing rays was too small to be -measured with certainty. It was not until radium of activity 19,000 -was obtained that it was possible to detect the deviation of these -rays in an intense magnetic field. How small the magnetic deviation -is may be judged from the fact that the α rays, projected at -right angles to a magnetic field of 10,000 <span class='fss'>C.G.S.</span> units, describe the -arc of a circle of about 39 cms. radius, while under the same conditions -the cathode rays produced in a vacuum tube would describe -a circle of about ·01 cm. radius. It is therefore not surprising -that the α rays were for some time thought to be non-deviable in -a magnetic field.</p> -<p class='c005'><a id='section089'></a> -<b>89. Magnetic deviation of the α rays</b>. The general -method employed<a id='r141' href='#f141' class='c012'><sup>[141]</sup></a> to detect the magnetic deviation of the α rays -was to allow the rays to pass through narrow slits and to observe -whether the rate of discharge of an electroscope, due to the issuing -rays, was altered by the application of a strong magnetic field. -<a href='#fig032'>Fig. 32</a> shows the general arrangement of the experiment. The -<span class='pageno' id='Page_143'>143</span>rays from a thin layer of radium of activity 19,000 passed upwards -through a number of narrow slits <i>G</i>, in parallel, and then through -a thin layer of aluminium foil, ·00034 cm. thick, into the testing -vessel <i>V</i>. The ionization produced by the rays in the testing -vessel was measured by the rate of movement of the leaves of a -gold-leaf electroscope <i>B</i>. The gold-leaf system was insulated inside -the vessel by a sulphur bead <i>C</i>, and could be charged by means of -a movable wire <i>D</i>, which was afterwards earthed. The rate of -movement of the gold-leaf was observed through small mica -windows in the testing vessel by means of a microscope provided -with a micrometer eye-piece.</p> - -<div id='fig032' class='figcenter id007'> -<img src='images/fig-032.png' alt='Fig. 32.' class='ig001'> -<div class='ic002'> -<p>Fig. 32.</p> -</div> -</div> - -<p class='c006'>In order to increase the ionization in the testing vessel, the -rays passed through 20 to 25 slits of equal width, placed side by -side. This was arranged by cutting grooves at regular intervals in -side-plates into which brass plates were slipped. The width of the -slit varied in different experiments between ·042 cm. and ·1 cm. -The magnetic field was applied perpendicular to the plane of the -paper, and parallel to the plane of the slits. The rays are thus -deflected in a direction perpendicular to the plane of the slits and -a very small amount of deviation is sufficient to cause the rays to -impinge on the sides of the plate where they are absorbed.</p> - -<p class='c006'>The testing vessel and system of plates were waxed to a lead -plate <i>P</i> so that the rays entered the vessel <i>V</i> only through the -<span class='pageno' id='Page_144'>144</span>aluminium foil. It is necessary in these experiments to have a -steady stream of gas passing downwards between the plates in -order to prevent the diffusion of the emanation from the radium -upwards into the testing vessel. The presence in the testing -vessel of a small amount of this emanation, which is always given -out by radium, would produce great ionization and completely mask -the effect to be observed. For this purpose, a steady current -of dry electrolytic hydrogen of about 2 c.c. per second was passed -into the testing vessel; it then streamed through the porous aluminium -foil, and passed between the plates carrying the emanation -with it away from the apparatus. The use of a stream of hydrogen -instead of air greatly simplifies the experiment, for it <i>increases</i> the -ionization current due to the α rays in the testing vessel, and at -the same time greatly <i>diminishes</i> that due to the β and γ rays. -This is caused by the fact that the α rays are much more readily -absorbed in air than in hydrogen, while the rate of production of -ions due to the β and γ rays is much less in hydrogen than in air. -The intensity of the α rays after passing between the plates is -consequently greater when hydrogen is used; and since the rays -pass through a sufficient distance of hydrogen in the testing vessel -to be largely absorbed, the total amount of ionization produced by -them is greater with hydrogen than with air.</p> - -<p class='c006'>The following is an example of an observation on the magnetic -deviation:—</p> - -<div class='calc'> - -<div class='lg-container-b c017'> - <div class='linegroup'> - <div class='group'> - <div class='line'>Pole-pieces 1·90 × 2·50 cms.</div> - </div> - <div class='group'> - <div class='line'>Strength of field between pole-pieces 8370 units.</div> - </div> - <div class='group'> - <div class='line'>Apparatus of 25 parallel plates of length 3·70 cms., width ·70 cm., with an average air-space between plates of ·042 cm.</div> - </div> - <div class='group'> - <div class='line'>Distance of radium below plates 1·4 cm.</div> - </div> - </div> -</div> - -</div> - -<table class='table15' > -<colgroup> -<col class='colwidth66'> -<col class='colwidth33'> -</colgroup> - <tr> - <th class='c013'></th> - <th class='c014'>Rate of discharge of electroscope in volts per minute</th> - </tr> - <tr> - <td class='c013'>(1) Without magnetic field</td> - <td class='c014'>8·33</td> - </tr> - <tr> - <td class='c013'>(2) With magnetic field</td> - <td class='c014'>1·72</td> - </tr> - <tr> - <td class='c013'>(3) Radium covered with thin layer of mica to absorb all α rays</td> - <td class='c014'>0·93</td> - </tr> - <tr> - <td class='c013'>(4) Radium covered with mica and magnetic field applied</td> - <td class='c014'>0·92</td> - </tr> -</table> - -<p class='c006'><span class='pageno' id='Page_145'>145</span>The mica plate, ·01 cm. thick, was of sufficient thickness to -absorb completely all the α rays, while it allowed the β rays -and γ rays to pass through without appreciable absorption. The -difference between (1) and (3), 7·40 volts per minute, gives the rate -of discharge due to the α rays alone; the difference between (2) -and (3), 0·79 volts per minute, that due to the α rays not deviated -by the magnetic field employed.</p> - -<p class='c006'>The amount of α rays not deviated by the field is thus about -11% of the total. The small difference between (3) and (4) -measures the small ionization due to the β rays, for they would -be completely deviated by the magnetic field; (4) comprises the -effect of the γ rays together with the natural leak of the electroscope -in hydrogen.</p> - -<p class='c006'>In this experiment there was a good deal of stray magnetic -field acting on the rays before they reached the pole-pieces. The -diminution of the rate of discharge due to the α rays was found to -be proportional to the strength of field between the pole-pieces. -With a more powerful magnetic field, the whole of the α rays were -deviated, showing that they consisted <i>entirely</i> of projected charged -particles.</p> - -<p class='c006'>In order to determine the <i>direction</i> of deviation of the rays, -the rays were passed through slits one mm. in width, each of which -was half covered with a brass strip. The diminution of the rate of -discharge in the testing vessel for a given magnetic field in such a -case depends upon the <i>direction</i> of the field. In this way it was -found that the rays were deviated in the <i>opposite sense</i> to the cathode -rays. Since the latter consist of negatively charged particles, the -α rays must consist of <i>positively</i> charged particles.</p> - -<p class='c006'>These results were soon after confirmed by Becquerel<a id='r142' href='#f142' class='c012'><sup>[142]</sup></a>, by the -photographic method, which is very well adapted to determine the -character of the path of the rays acted on by a magnetic field. -The radium was placed in a linear groove cut in a small block of -lead. Above this source, at a distance of about 1 centimetre, was -placed a metallic screen, formed of two plates, leaving between them -a narrow opening parallel to the groove. Above this was placed -the photographic plate. The whole apparatus was placed in a -strong magnetic field parallel to the groove. The strength of the -<span class='pageno' id='Page_146'>146</span>magnetic field was sufficient to deflect the β rays completely away -from the plate. When the plate was parallel to the opening, -there was produced on it an impression, due to the α rays alone, -which became more and more diffuse as the distance from the -opening increased. This distance should not exceed 1 or 2 centimetres -on account of the absorption of the rays in air. If, during -the exposure, the magnetic field is reversed for equal lengths of -time, on developing the plate two images of the α rays are -observed which are deflected in opposite directions. This deviation, -even in a strong field, is small though quite appreciable and -is opposite in sense to the deviation observed for the β or cathodic -rays from the same material.</p> - -<p class='c006'>M. Becquerel<a id='r143' href='#f143' class='c012'><sup>[143]</sup></a>, by the same method, found that the α rays from -polonium were deviated in the same direction as the α rays from -radium; and thus that they also consist of projected positive bodies. -In both cases, the photographic impressions were sharply marked -and did not show the same diffusion which always appears in -photographs of the β rays.</p> -<p class='c005'><a id='section090'></a> -<b>90. Electrostatic deviation of the α rays</b>. If the rays -are charged bodies, they should be deflected in passing through a -strong electric field. This was found by the writer to be the case, -but the electric deviation is still more difficult to detect than the -magnetic deviation, as the intensity of the electric field must of -necessity be less than that required to produce a spark in the -presence of radium. The apparatus was similar to that employed -for the magnetic deviation (<a href='#fig032'>Fig. 32</a>) with this exception, that the -brass sides which held the plates in position, were replaced by -ebonite. Alternate plates were connected together and charged -to a high potential by means of a battery of small accumulators. -The discharge in the electroscope, due to the α rays, was found to -be diminished by application of the electric field. With plates -·055 cm. apart and 4·5 cms. high, the diminution was only 7% -with a <span class='fss'>P.D.</span> of 600 volts between the slits. With a special arrangement -of plates, with slits only ·01 cm. apart, the discharge was -diminished about 45% with an electric field corresponding to -10,000 volts per cm.</p> -<p class='c005'><span class='pageno' id='Page_147'>147</span><a id='section091'></a> -<b>91. Determination of the constants of the rays.</b> If the -deviation of the rays in both an electric and magnetic field is -known, the values of the velocity of the rays, and the ratio <i>e</i>/<i>m</i> of -the charge of the particle to its mass can be determined by the -method, first used by J. J. Thomson for the cathode rays, which is -described in section 50. From the equations of a moving charged -body, the radius of curvature ρ of the path of the rays in a -magnetic field of strength <i>H</i> perpendicular to the path of the rays -is given by</p> - -<div class='calc'> - -<div class='lg-container-b c017'> - <div class='linegroup'> - <div class='group'> - <div class='line in7'><i>m</i></div> - <div class='line'><i>H</i>ρ = ---- <i>V</i> .</div> - <div class='line in7'><i>e</i></div> - </div> - </div> -</div> - -</div> - -<p class='c018'>If the particle, after passing through a uniform magnetic field for -a distance -<i>l</i><sub>1</sub>, -is deviated through a small distance -<i>d</i><sub>1</sub> -from its -original direction,</p> - -<div class='calc'> - -<div class='lg-container-b c017'> - <div class='linegroup'> - <div class='group'> - <div class='line in8'>2ρ<i>d</i><sub>1</sub> = <i>l</i><sub>1</sub><sup>2</sup></div> - </div> - <div class='group'> - <div class='line'>or</div> - <div class='line in15'><i>l</i><sub>1</sub><sup>2</sup> <i>e</i> <i>H</i></div> - <div class='line in8'><i>d</i><sub>1</sub> = ----- --- --- (1).</div> - <div class='line in15'>2 <i>m</i> <i>V</i></div> - </div> - <div class='group'> - </div> - </div> -</div> - -</div> - -<p class='c018'>If the rays pass through a uniform electric field of strength <i>X</i> and -length -<i>l</i><sub>2</sub> with a deviation <i>d</i><sub>2</sub>,</p> - -<div class='calc'> - -<div class='lg-container-b c017'> - <div class='linegroup'> - <div class='group'> - <div class='line in7'>1 <i>Xel<sub>2</sub><sup>2</sup></i></div> - <div class='line'><i>d</i><sub>2</sub> = --- ----- (2),</div> - <div class='line in7'>2 <i>mV</i><sup>2</sup></div> - </div> - <div class='group'> - </div> - </div> -</div> - -</div> - -<p class='c018'>since <i>Xe</i>/<i>m</i> is the acceleration of the particle, at right angles to its -direction, and -<i>l</i><sub>2</sub>/<i>V</i> -is the time required to travel through the electric -field.</p> - -<p class='c006'>From equations (1) and (2)</p> - -<div class='calc'> - -<div class='lg-container-b c017'> - <div class='linegroup'> - <div class='group'> - <div class='line in12'><i>d</i><sub>1</sub> <i>l</i><sub>2</sub><sup>2</sup> <i>X</i></div> - <div class='line in6'><i>V</i> = ----- ----- --- ,</div> - <div class='line in12'><i>d</i><sub>2</sub> <i>l</i><sub>1</sub><sup>2</sup> <i>H</i></div> - </div> - <div class='group'> - <div class='line'>and</div> - </div> - <div class='group'> - <div class='line in7'><i>e</i> 2<i>d</i><sub>1</sub> <i>V</i></div> - <div class='line in6'>---- = ------ --- .</div> - <div class='line in7'><i>m</i> <i>l</i><sub>1</sub><sup>2</sup> <i>H</i></div> - </div> - </div> -</div> - -</div> - -<p class='c018'>The values of <i>V</i> and <i>e</i>/<i>m</i> are thus completely determined from the -combined results of the electric and magnetic deviation. It was -found that</p> - -<div class='calc'> - -<div class='lg-container-b c017'> - <div class='linegroup'> - <div class='group'> - <div class='line'><i>V</i> = 2·5 × 10<sup>9</sup> cms. per sec.</div> - <div class='line'><i>e</i>/<i>m</i> = 6 × 10<sup>3</sup>.</div> - </div> - </div> -</div> - -</div> - -<p class='c018'>On account of the difficulty of obtaining a large electrostatic -deviation, these values are only approximate in character.</p> - -<p class='c006'><span class='pageno' id='Page_148'>148</span>The results on the magnetic and electric deviation of the -α rays of radium have been confirmed by Des Coudres<a id='r144' href='#f144' class='c012'><sup>[144]</sup></a>, by the -photographic method. Some pure radium bromide was used as a -source of radiation. The whole apparatus was enclosed in a vessel -which was exhausted to a low vacuum. In this way, not only -was he able to determine the photographic action of the rays at -a much greater distance from the source, but he was also able -to apply a stronger electric field without the passage of a spark. -He found values of the constants given by</p> - -<div class='calc'> - -<div class='lg-container-b c017'> - <div class='linegroup'> - <div class='group'> - <div class='line'><i>V</i> = 1·65 × 10<sup>9</sup> cms. per sec.</div> - <div class='line'><i>e</i>/<i>m</i> = 6·4 × 10<sup>3</sup>.</div> - </div> - </div> -</div> - -</div> - -<p class='c018'>These values are in very good agreement with the numbers found -by the electric method. The α rays from radium are complex, and -probably consist of a stream of positively charged bodies projected -at velocities lying between certain limits. The amount of deviation -of the particles in a magnetic field will thus differ according -to the velocity of the particle. The photographic results of -Becquerel seem to indicate that the velocity of the rays of radium -can vary only within fairly narrow limits, since the trajectory of -the rays in a magnetic field is sharply marked and not nearly as -diffuse as in similar experiments with the β rays. The evidence, -however, discussed in the following section, shows that the velocities -of the α particles from a thick layer of radium vary over a -considerable range.</p> -<p class='c005'><a id='section092'></a> -<b>92.</b> Becquerel<a id='r145' href='#f145' class='c012'><sup>[145]</sup></a> has examined the amount of magnetic deviation -of the α rays at different distances from the source of the rays -in a very simple way. A narrow vertical pencil of the rays, after -its passage through a narrow slit, fell on a photographic plate, -which was inclined at a small angle to the vertical and had its -lower edge perpendicular to the slit. The trajectory of the rays -is shown by a fine line traced on the plate. If a strong magnetic -field is applied parallel to the slit, the trajectory of the rays is -displaced to the right or left according to the direction of the -field. If equal times of exposure are given for the magnetic field -<span class='pageno' id='Page_149'>149</span>in the two directions, on developing the plate two fine diverging -lines are found traced on the plate. The distance between these -lines at any point is a measure of twice the average deviation -at that point, corresponding to the value of the magnetic field. -By measuring the distance between the trajectories at various -points, Becquerel found that the radius of curvature of the path of -the rays <i>increased</i> with the distance from the slit. The product -<i>H</i>ρ of the strength of the field and the radius of curvature of the -path of the rays is shown in the following table.</p> - -<table class='table8' > -<colgroup> -<col class='colwidth50'> -<col class='colwidth50'> -</colgroup> - <tr> - <th class='c013'>Distance in mms. from the slit</th> - <th class='c014'><i>H</i>ρ</th> - </tr> - <tr> - <td class='c013'>1</td> - <td class='c014'>2·91 × 10<sup>5</sup></td> - </tr> - <tr> - <td class='c013'>3</td> - <td class='c014'>2·99 „</td> - </tr> - <tr> - <td class='c013'>5</td> - <td class='c014'>3·06 „</td> - </tr> - <tr> - <td class='c013'>7</td> - <td class='c014'>3·15 „</td> - </tr> - <tr> - <td class='c013'>8</td> - <td class='c014'>3·27 „</td> - </tr> - <tr> - <td class='c013'>9</td> - <td class='c014'>3·41 „</td> - </tr> -</table> - -<p class='c006'>The writer (<i>loc. cit.</i>) showed that the <i>maximum</i> value of <i>H</i>ρ -for complete deviation of the α rays was 390,000. The results are -thus in good agreement. Since</p> - -<div class='calc'> - -<div class='lg-container-b c017'> - <div class='linegroup'> - <div class='group'> - <div class='line in8'><i>m</i></div> - <div class='line'><i>H</i>ρ = ----- <i>V</i></div> - <div class='line in8'><i>e</i></div> - </div> - </div> -</div> - -</div> - -<p class='c018'>these results show -that the values either of <i>V</i> or of <i>e</i>/<i>m</i> for the projected particles vary -at different distances from the source. Becquerel considered that -the rays were homogeneous, and, in order to explain the results, -has suggested that the charge on the projected particles may -gradually decrease with the distance traversed, so that the radius -of curvature of the path steadily increases with the distance from -the source. It, however, seems more probable that the rays consist -of particles projected with different velocities, and that the -slower particles are more quickly absorbed in the gas. In consequence -of this, only the swifter particles are present some distance -from the source.</p> - -<p class='c006'>This conclusion is borne out by some recent experiments of -Bragg and Kleeman<a id='r146' href='#f146' class='c012'><sup>[146]</sup></a> on the nature of the absorption of α particles -by matter, which are discussed in more detail in sections 103 and -104. They found that the α particles from a thick layer of radium -are complex, and have a wide range of penetrating power and -presumably of velocity. This is due to the fact that the α particles -<span class='pageno' id='Page_150'>150</span>emitted from the radium come from different depths. Since their -velocity is reduced in their transit through matter, a pencil of -α rays will consist of particles which differ considerably in speed. -Those which are just able to emerge from the radium will be -absorbed in a very short depth of air, while those that come from -the surface will be able to pass through several centimetres of air -before they lose their power of ionizing the gas. Since the α -particles have different velocities, they will be unequally deflected -by the magnetic field, the slower moving particles describing a -more curved path than the swifter ones. Consequently, the -outer edge of the trace of the pencil of rays on the photographic -plate, as obtained by Becquerel, will be the locus of the points -where the photographic action of the α particles end. It was -found that the α particles are most efficient as ionizers of the gas -just before their power of ionizing ends. The loss of ionizing -power of the α particles seems to be fairly abrupt, and, for particles -of the same velocity, to occur always after traversing a definite -distance in air. On the assumption that the photographic as well -as the ionizing action is most intense just before the particles are -stopped, and ceases fairly abruptly, Bragg has been able to -account numerically for the measurements (see above table) -recorded by Becquerel. Quite apart from the special assumptions -required for such a quantitative comparison of theory with -experiment, there can be little doubt that the increase of value -of <i>H</i>ρ with distance can be satisfactorily explained as a consequence -of the complex character of the pencil of rays<a id='r147' href='#f147' class='c012'><sup>[147]</sup></a>.</p> - -<p class='c006'>Becquerel states that the amount of deviation, in a given -magnetic field, was the same for the α rays of polonium and of -radium. This shows that the value of</p> - -<div class='calc'> - -<div class='lg-container-b c017'> - <div class='linegroup'> - <div class='group'> - <div class='line in2'><i>m</i></div> - <div class='line'>--- <i>V</i></div> - <div class='line in2'><i>e</i></div> - </div> - </div> -</div> - -</div> - -<p class='c018'>is the same for the -α rays from the two substances. Since the α rays from polonium -are far more readily absorbed than the α rays from radium, this -result would indicate that the value of <i>m</i>/<i>e</i> is greater for the α particles -of polonium than of radium. Further experimental evidence -is required on this important point.</p> -<p class='c005'><span class='pageno' id='Page_151'>151</span><a id='section093'></a> -<b>93. Charge carried by the α rays</b>. We have seen that -the negative charge carried by the β particles has been readily -measured. Since there is reason to believe (<a href='#section229'>section 229</a>) that -four α particles are expelled from radium for each β particle, it is -to be expected that the positive charge carried by the α particles -should be determined still more readily. All the initial experiments, -however, made to detect this charge, gave negative results; -and, before successful results were obtained, it was found necessary -to eliminate some secondary actions, which at first completely -masked the effects to be looked for.</p> - -<p class='c006'>In consequence of the importance of this question, a brief -account will be given of the methods of measurement adopted and -the special experimental difficulties which have arisen.</p> - -<p class='c006'>In the first place, it must be remembered that only a small -fraction of the α rays, emitted from a layer of powdered radium -bromide, escape into the surrounding gas. On account of the -ease with which the α rays are stopped in their passage through -matter, only those escape which are expelled from a superficial -layer, and the rest are absorbed by the radium itself. On the -other hand, a much larger proportion of the β rays escape, on -account of their greater power of penetration. In the second -place, the α particle is a far more efficient ionizer of the gas -than the β particle, and, in consequence, if the charge carried by -the α rays is to be determined by methods similar to those -employed for the β rays (see section 80), the pressure of the gas -surrounding the conductor to be charged must be very small -in order to eliminate, as far as possible, the loss of charge resulting -from the ionization of the residual gas by the α rays<a id='r148' href='#f148' class='c012'><sup>[148]</sup></a>.</p> - -<p class='c006'>The experimental arrangement used by the writer is shown in -<a href='#fig033'>Fig. 33</a>.</p> - -<p class='c006'>A thin film of radium was obtained on a plate <i>A</i> by evaporation -of a radium solution containing a known weight of radium -bromide. Some hours after evaporation, the activity of the -radium, measured by the α rays, is about 25 per cent. of its -maximum value, and the β rays are almost completely absent. -The activity measured by the α and β rays is then slowly regained, -and recovers its original value after about a month’s interval (see -<span class='pageno' id='Page_152'>152</span><a href='#chap11'>chapter <span class='fss'>XI.</span></a>). The experiments were made on the active plate when -its activity was a minimum, in order to avoid complications due to -the presence of β rays. The film of radium was so thin that only -a very small fraction of the α rays was absorbed.</p> - -<div id='fig033' class='figcenter id006'> -<img src='images/fig-033.png' alt='Fig. 33.' class='ig001'> -<div class='ic002'> -<p>Fig. 33.</p> -</div> -</div> - -<p class='c006'>The active plate <i>A</i> was insulated in a metal vessel <i>D</i>, and was -connected to one pole of the battery, the other pole being earthed. -The upper electrode, which was insulated and connected with a -Dolezalek electrometer, consisted of a rectangular copper vessel -<i>BC</i>, the lower part of which was covered with a thin sheet of -aluminium foil. The α rays passed through the foil, but were -stopped by the copper sides of the vessel. This arrangement was -found to reduce the secondary ionization produced at the surface -of the upper plate. The outside vessel <i>D</i> could be connected with -either <i>A</i> or <i>B</i> or with earth. By means of a mercury pump, the -vessel was exhausted to a very low pressure. If the rays carry a -positive charge, the current between the two plates measured by -the electrometer should be greater when <i>A</i> is charged positively. -No certain difference, however, between the currents in the two -directions was observed, even when a very good vacuum was -obtained. In some arrangements, it was found that the current -was even greater when the lower plate was negative than when -it was positive. An unexpected experimental result was also -noticed. The current between the parallel plates at first -diminished with the pressure, but soon reached a limiting -value which was not altered however good a vacuum was produced. -For example, in one experiment, the current between -<span class='pageno' id='Page_153'>153</span>the two parallel plates, placed about 3 mms. apart, was initially -6·5 × 10<sup>-9</sup> -amperes and fell off directly as the pressure. The -current reached a limiting value of about -6 × 10<sup>-12</sup> -amperes, -or about ¹⁄₁₀₀₀ of the value at atmospheric pressure. The -magnitude of this limiting current was not much altered if the -air was replaced by hydrogen.</p> - -<p class='c006'>Experiments of a similar character have been made by Strutt<a id='r149' href='#f149' class='c012'><sup>[149]</sup></a> -and J. J. Thomson<a id='r150' href='#f150' class='c012'><sup>[150]</sup></a>; using an active bismuth plate coated with -radio-tellurium (polonium) after Marckwald’s method. This substance -emits only α rays, and is thus especially suitable for -experiments of this kind. Strutt employed the method used by -him to show the charge carried by the β rays (<a href='#fig027'>Fig. 27</a>). He -found, however, that, even in the lowest possible vacuum, the -electroscope rapidly lost its charge and at the same rate whether -it was charged positively or negatively. This is in agreement -with the results found by the writer with radium.</p> - -<p class='c006'>In the experiments of J. J. Thomson, the electroscope was -attached to a metal disc placed 3 cms. from the plate of radio-tellurium. -A very low vacuum was produced by Dewar’s method -by absorbing the residual gas in cocoanut charcoal immersed in -liquid air. When the electroscope was charged negatively, an -extremely slow rate of leak was observed, but when charged -positively the leak was about 100 times greater. This showed -that the polonium gave out large quantities of negative electricity, -but not enough positive to be detected. By placing the -apparatus in a strong magnetic field, the negative particles were -prevented from reaching the electroscope and the positive leak -was stopped.</p> - -<p class='c006'>These results indicate that these negative particles are not -projected with sufficient velocity to move against the repulsion -exerted by the electrified body, and are bent by a magnetic field. -There thus seems little doubt that a stream of negative particles -(electrons) is projected from the active surface at a very slow -speed. Such low velocity electrons are also projected from -uranium and radium. It is probable that these electrons are -<span class='pageno' id='Page_154'>154</span>a type of secondary radiation, set up at the surfaces on which the -α rays fall. The particles would be extremely readily absorbed -in the gas, and their presence would be difficult to detect except -in low vacua. J. J. Thomson at first obtained no evidence -that the α particles of polonium were charged; but in later -experiments, where the plates were closer together, the electroscope -indicated that the α rays did carry a positive charge.</p> - -<p class='c006'>In order to see whether the positive charge due to the α rays -from radium could be detected when the slow moving ions were -prevented from escaping by a magnetic field, I placed the -apparatus of <a href='#fig033'>Fig. 33</a> between the pole-pieces of a large electromagnet, -so that the magnetic field was parallel to the plane of the -plates<a id='r151' href='#f151' class='c012'><sup>[151]</sup></a>. A very marked alteration was observed both on the -magnitude of the positive and negative currents. In a good -vacuum, the upper plate received a positive charge, independently -of whether the lower plate was charged positively or negatively or -was connected with earth. After the magnetic field had reached a -certain value, a great increase in its strength had no appreciable -effect on the magnitude of the current.</p> - -<p class='c006'>The following table illustrates the results obtained when the -two plates were 3 mms. apart, and were both coated with thin -aluminium foil.</p> - -<table class='table16' > -<colgroup> -<col class='colwidth18'> -<col class='colwidth37'> -<col class='colwidth37'> -<col class='colwidth7'> -</colgroup> - <tr> - <th class='c013'>Potential of lower plate</th> - <th class='c013'>Current in</th> - <th class='c013'>arbitrary units</th> - <th class='c014'> </th> - </tr> - <tr> - <td class='c013'> </td> - <td class='c013'>Without magnetic field</td> - <td class='c013'>With magnetic field</td> - <td class='c014'> </td> - </tr> - <tr> - <td class='c013'>0</td> - <td class='c013'>—</td> - <td class='c013'>+·36</td> - <td class='c014'> </td> - </tr> - <tr> - <td class='c013'>+2 volts</td> - <td class='c013'>2·0</td> - <td class='c013'>+·46}</td> - <td class='c014'> </td> - </tr> - <tr> - <td class='c013'> </td> - <td class='c013'> </td> - <td class='c013'>}</td> - <td class='c014'>·39</td> - </tr> - <tr> - <td class='c013'>-2 „</td> - <td class='c013'>2·5</td> - <td class='c013'>+·33}</td> - <td class='c014'> </td> - </tr> - <tr> - <td class='c013'> </td> - <td class='c013'> </td> - <td class='c013'> </td> - <td class='c014'> </td> - </tr> - <tr> - <td class='c013'>+4 „</td> - <td class='c013'>2·8</td> - <td class='c013'>+·47}</td> - <td class='c014'> </td> - </tr> - <tr> - <td class='c013'> </td> - <td class='c013'> </td> - <td class='c013'>}</td> - <td class='c014'>·41</td> - </tr> - <tr> - <td class='c013'>-4 „</td> - <td class='c013'>3·5</td> - <td class='c013'>+·35}</td> - <td class='c014'> </td> - </tr> - <tr> - <td class='c013'> </td> - <td class='c013'> </td> - <td class='c013'> </td> - <td class='c014'> </td> - </tr> - <tr> - <td class='c013'>+8 „</td> - <td class='c013'>3·1</td> - <td class='c013'>+·56}</td> - <td class='c014'> </td> - </tr> - <tr> - <td class='c013'> </td> - <td class='c013'> </td> - <td class='c013'>}</td> - <td class='c014'>·43</td> - </tr> - <tr> - <td class='c013'>-8 „</td> - <td class='c013'>4·0</td> - <td class='c013'>+·31}</td> - <td class='c014'> </td> - </tr> - <tr> - <td class='c013'> </td> - <td class='c013'> </td> - <td class='c013'> </td> - <td class='c014'> </td> - </tr> - <tr> - <td class='c013'>+84 „</td> - <td class='c013'>3·5</td> - <td class='c013'>+·77}</td> - <td class='c014'> </td> - </tr> - <tr> - <td class='c013'> </td> - <td class='c013'> </td> - <td class='c013'>}</td> - <td class='c014'>·50</td> - </tr> - <tr> - <td class='c013'>-84 „</td> - <td class='c013'>5·2</td> - <td class='c013'>+·24}</td> - <td class='c014'> </td> - </tr> -</table> - -<p class='c006'>Let <i>n</i> be the number of α particles, carrying a charge <i>e</i>, which -are absorbed in the upper plate. Let -ι₀ -be the current due to the -slight ionization of the residual gas.</p> - -<p class='c006'>If only a small potential is applied to the lower plate, this -current should be equal in magnitude but opposite in sign when -<span class='pageno' id='Page_155'>155</span>the potential is reversed. Let -ι<sub>1</sub> -be the charge per sec. communicated -to the upper electrode when the lower plate is charged -positively and -ι<sub>2</sub> -the value when charged negatively. Then</p> - -<div class='calc'> - -<div class='lg-container-b c017'> - <div class='linegroup'> - <div class='group'> - <div class='line in16'>ι<sub>1</sub> = ι₀ + <i>ne</i>,</div> - <div class='line in16'>ι<sub>2</sub> = ι₀ + <i>ne</i>;</div> - </div> - <div class='group'> - <div class='line'>adding we get</div> - </div> - <div class='group'> - <div class='line in23'>ι<sub>1</sub> + ι<sub>2</sub></div> - <div class='line in16'><i>ne</i> = ------ .</div> - <div class='line in23'>2</div> - </div> - </div> -</div> - -</div> - -<p class='c018'>Now in the third column of the above table it is seen that -(ι<sub>1</sub> + ι<sub>2</sub>)/2 -has the values ·39, ·41, ·43 for 2, 4, and 8 volts respectively. The -numbers are thus in fairly good agreement. Similar results were -obtained when a brass plate was substituted for the upper electrode -shown in the figure. Taking into consideration that the -magnitude of <i>ne</i> is independent of the strength of the magnetic -field above a certain small value, and the good agreement of -the numbers obtained with variation of voltage, I think that there -can be no doubt that the positive charge communicated to the -upper electrode was carried by the α particles. This positive -charge was not small, for using a weight of ·48 mgrs. radium -bromide spread in a thin foil over an area of about 20 sq. cms., the -charge communicated by the particles corresponded to a current -8·8 × 10<sup>-13</sup> -amperes, and, with the Dolezalek electrometer employed, -it was necessary to add a capacity of ·0024 microfarads to the -electrometer system.</p> - -<p class='c006'>In these experiments, the film of radium bromide was so thin, -that only a very small percentage of the α particles was stopped -by the radium itself. Assuming that each α particle carries the -same charge as an ion, viz. -1·1 × 10<sup>-19</sup> -coulombs, and remembering -that half of the α particles are absorbed in the lower plate, the -total number <i>N</i> of α particles expelled per second from one gram -of radium bromide (at its minimum activity) can be deduced. -In two separate experiments where the amount of radium used -was ·194 and ·484 mgrs. respectively, the values of <i>N</i> were in close -agreement and equal to -3·6 × 10<sup>10</sup>. -Now it will be shown later -that in radium there are three other products in radio-active -equilibrium, each of which probably gives out the same number of -α particles as radium itself. If this is the case, the total number -of α particles expelled per second from 1 gram of radium bromide -<i>in radio-active equilibrium</i> is 4<i>N</i> or -1·44 × 10<sup>11</sup>. -Assuming the -<span class='pageno' id='Page_156'>156</span>composition of radium bromide as -RaBr<sub>2</sub>, -the number per second -per gram of radium is -2·5 × 10<sup>10</sup>. -This number will be found to -be in very good agreement with that deduced from indirect data -(<a href='#chap13'>chapter <span class='fss'>XIII</span></a>.). The value of <i>N</i> is of great importance in -determining the magnitude of various quantities in radio-active -calculations.</p> -<p class='c005'><b>94. Mass and energy of the α particle.</b> It has been -pointed out that the α rays from radium and polonium are -analogous to the Canal rays of Goldstein, for both carry a positive -charge and are difficult to deflect by a magnetic field. The experiments -of Wien have shown that the velocity of projection of the -canal rays varies with the gas in the tube and the intensity of the -electric field applied, but it is generally about ⅒ of the velocity -of the α particle from radium. The value of <i>e</i>/<i>m</i> is also variable, -depending upon the gas in the tube.</p> - -<p class='c006'>It has been shown that for the α rays of radium</p> - -<div class='calc'> - -<div class='lg-container-b c017'> - <div class='linegroup'> - <div class='group'> - <div class='line in23'><i>e</i></div> - <div class='line'>V = 2·5 × 10<sup>9</sup> and ------- = 6 × 10<sup>3</sup>.</div> - <div class='line in23'><i>m</i></div> - </div> - </div> -</div> - -</div> - -<p class='c018'>Now the value of <i>e</i>/<i>m</i> for the hydrogen atom, liberated in the -electrolysis of water, is -10<sup>4</sup>. -Assuming the charge carried by the -α particle to be the same as that carried by the hydrogen atom, the -mass of the α particle is about twice that of the hydrogen atom. -Taking into consideration the uncertainty attaching to the experimental -value of <i>e</i>/<i>m</i> for the α particle, if the α particle consists of -any known kind of matter, this result indicates that it consists -either of projected helium or hydrogen. Further evidence on this -important question is given in section 260.</p> - -<p class='c006'>The α rays from all the radio-active substances and their -products, such as the radio-active emanations and the matter -causing excited activity, possess the same general properties and -do not vary very much in penetrating power. It is thus probable -that in all cases the α rays from the different radio-active substances -consist of positively charged bodies projected with great -velocity. Since the rays from radium are made up in part of α -rays from the emanation stored in the radium, and from the -excited activity which it produces, the α rays from each of these -products must consist of positively charged bodies; for it has been -shown that <i>all</i> the α rays from radium are deviated in a strong -magnetic field.</p> - -<p class='c006'><span class='pageno' id='Page_157'>157</span>The kinetic energy of each projected particle is enormous, compared -with its mass. The kinetic energy of each α particle is</p> - -<div class='calc'> - -<div class='lg-container-b c017'> - <div class='linegroup'> - <div class='group'> - <div class='line in1'>1 1 <i>m</i></div> - <div class='line'>--- <i>mV</i><sup>2</sup> = --- --- <i>V<sup>2</sup>e</i> = 5·9 × 10<sup>-6</sup> ergs.</div> - <div class='line in1'>2 2 <i>e</i></div> - </div> - </div> -</div> - -</div> - -<p class='c018'>Taking the velocity of a rifle bullet as -10<sup>5</sup> -cms. per second, it is -seen that, mass for mass, the energy of motion of the α rays is -6 × 10<sup>8</sup> -times as great as that of the rifle bullet. In this projection -of bodies atomic in size with great velocity probably lies the -principal cause of the heating effects produced by radium -(<a href='#chap12'>chapter <span class='fss'>XII</span></a>).</p> -<p class='c005'><b>95. Atomic disintegration.</b> The radio-activity of the radio-elements -is an atomic and not a molecular property. The rate of -emission of the radiations depends only on the amount of the -element present and is independent of its combination with inactive -substances. In addition, it will be shown later that the rate of -emission is not affected by wide variations of temperature, or by -the application of any known chemical or physical forces. Since -the power of radiating is a property of the radio-atoms, and the -radiations consist for the most part of positively and negatively -charged masses projected with great velocity, it is necessary to -suppose that the atoms of the radio-elements are undergoing disintegration, -in the course of which parts of the atom escape from -the atomic system. It seems very improbable that the α and β -particles can suddenly acquire their enormous velocity of projection -by the action of forces existing inside or outside the atom. For -example, the α particle would have to travel from rest between two -points differing in potential by 5·2 million volts in order to acquire -the kinetic energy with which it escapes. Thus it seems probable -that these particles are not set suddenly in motion, but that they -escape from an atomic system in which they were already in -rapid oscillatory or orbital motion. On this view, the energy is -not communicated to the projected particles, but exists beforehand -in the atoms from which they escape. The idea that the atom is -a complicated structure consisting of charged parts in rapid oscillatory -or orbital motion has been developed by J. J. Thomson, -Larmor and Lorentz. Since the α particle is atomic in size, it is -<span class='pageno' id='Page_158'>158</span>natural to suppose that the atoms of the radio-active elements -consist not only of the electrons in motion, but also of positively -charged particles whose mass is about the same as that of the -hydrogen or helium atom.</p> - -<p class='c006'>It will be shown later that only a minute fraction of the atoms -of the radio-element need break up per second in order to account -for the radiations even of an enormously active element like -radium. The question of the possible causes which lead to this -atomic disintegration and the consequences which follow from it -will be discussed later in <a href='#chap13'>chapter <span class='fss'>XIII</span></a>.</p> -<p class='c005'><b>96. Experiments with a zinc sulphide screen.</b> A screen -of Sidot’s hexagonal blend (phosphorescent crystalline zinc -sulphide) lights up brightly under the action of the α rays of -radium and polonium. If the surface of the screen is examined -with a magnifying glass, the light from the screen is found not to -be uniformly distributed but to consist of a number of scintillating -points of light. No two flashes succeed one another at the same -point, but they are scattered over the surface, coming and going -rapidly without any movement of translation. This remarkable -action of the radium and polonium rays on a zinc sulphide screen -was discovered by Sir William Crookes<a id='r152' href='#f152' class='c012'><sup>[152]</sup></a>, and independently by -Elster and Geitel<a id='r153' href='#f153' class='c012'><sup>[153]</sup></a>, who observed it with the rays given out from -a wire which has been charged negatively either in the open air -or in a vessel containing the emanation of thorium.</p> - -<p class='c006'>In order to show the scintillations of radium on the screen, -Sir William Crookes has devised a simple apparatus which he has -called the “Spinthariscope.” A small piece of metal, which has -been dipped in a radium solution, is placed several millimetres away -from a small zinc sulphide screen. This screen is fixed at one -end of a short brass tube and is looked at through a lens fixed at -the other end of the tube. Viewed in this way, the surface of the -screen is seen as a dark background, dotted with brilliant points -of light which come and go with great rapidity. The number of -points of light per unit area to be seen at one time falls off rapidly -as the distance from the radium increases, and, at several centimetres -<span class='pageno' id='Page_159'>159</span>distance, only an occasional one is seen. The experiment -is extremely beautiful, and brings vividly before the observer the -idea that the radium is shooting out a stream of projectiles, the -impact of each of which on the screen is marked by a flash of light.</p> - -<p class='c006'>The scintillating points of light on the screen are the result -of the impact of the α particles on its surface. If the radium is -covered with a layer of foil of sufficient thickness to absorb all the -α rays the scintillations cease. There is still a phosphorescence to -be observed on the screen due to the β and γ rays, but this -luminosity is not marked by scintillations to any appreciable -extent. Sir William Crookes showed that the number of -scintillations was about the same in vacuo as in air at atmospheric -pressure. If the screen was kept at a constant temperature, -but the radium cooled down to the temperature of liquid air, no -appreciable difference in the number of scintillations was observed. -If, however, the screen was gradually cooled to the temperature of -liquid air, the scintillations diminished in number and finally -ceased altogether. This is due to the fact that the screen loses -to a large extent its power of phosphorescence at such a low -temperature.</p> - -<p class='c006'>Not only are scintillations produced by radium, actinium, -and polonium, but also by the emanations and other radio-active -products which emit α rays. In addition, F. H. Glew<a id='r154' href='#f154' class='c012'><sup>[154]</sup></a> has found -that they can be observed from the metal uranium, thorium -compounds and various varieties of pitchblende. In order to -show the scintillations produced by pitchblende, a flat surface -was ground, and a transparent screen, whose lower surface was -coated with zinc sulphide, placed upon it. Glew has designed -a modified and very simple form of spinthariscope. A transparent -screen, coated on one side with a thin layer of zinc sulphide, -is placed in contact with the active material, and the scintillations -observed by a lens in the usual way.</p> - -<p class='c006'>Since there is no absorption in the air, the luminosity is a -maximum. The relative transparency of different substances -placed between the active material and the screen may, in this -way, be directly studied.</p> - -<p class='c006'>The production of scintillations appears to be a general -property of the α rays from all radio-active substances. The -<span class='pageno' id='Page_160'>160</span>scintillations are best shown with a zinc sulphide screen; but -are also observed with willemite (zinc silicate), powdered diamond, -and potassium platinocyanide (Glew, <i>loc. cit.</i>). If a screen of -barium platinocyanide is exposed to the α rays from radium, the -scintillations are difficult to observe, and the luminosity is far -more persistent than for a zinc sulphide screen exposed under -the same conditions. The duration of the phosphorescence in -this case probably accounts for the absence of visible scintillations.</p> - -<p class='c006'>There can be no doubt that the scintillations result from the -continuous bombardment of the sensitive screen by the α particles. -Each of these particles moves with enormous velocity, and has a -considerable energy of motion. On account of the ease with -which these particles are stopped, most of this energy is given up -at the surface of the screen, and a portion of the energy is in -some way transformed into light. Zinc sulphide is very sensitive to -mechanical shocks. Luminosity is observed if a penknife is drawn -across the screen, or if a current of air is directed on to the screen. -The disturbance effected by the impact of the α particle extends -over a distance very large compared with the size of the impinging -particle, so that the spots of light produced have an appreciable -area. Recently Becquerel<a id='r155' href='#f155' class='c012'><sup>[155]</sup></a> has made an examination of the -scintillations produced by different substances, and has concluded -that the scintillations are due to irregular cleavages in the crystals -composing the screen, produced by the action of the α rays. -Scintillations can be mechanically produced by crushing a crystal. -Tommasina<a id='r156' href='#f156' class='c012'><sup>[156]</sup></a> found that a zinc sulphide screen removed from the -action of the radium rays for several days, showed the scintillations -again when an electrified rod was brought near it.</p> - -<p class='c006'>The number of scintillations produced in zinc sulphide depends -upon the presence of a slight amount of impurity and on its crystalline -state. It can be shown that even with the most sensitive -zinc sulphide screens, the number of scintillations is probably only -a small fraction of the total number of α particles which fall upon -it. It would appear that the crystals are in some way altered by -the bombardment of the α particles, and that some of the crystals -occasionally break up with emission of light<a id='r157' href='#f157' class='c012'><sup>[157]</sup></a>.</p> - -<p class='c006'><span class='pageno' id='Page_161'>161</span>Although the scintillations from a particle of pure radium -bromide are very numerous, they are not too numerous to be -counted. Close to the radium, the luminosity is very bright, but -by using a high power microscope the luminosity can still be -shown to consist of scintillations. Since the number of scintillations -probably bears no close relation to the number of α -particles emitted, a determination of the number of scintillations -would have no special physical significance. The relation between -the number of α particles and the number of scintillations would -probably be variable, depending greatly on the exact chemical -composition of the sensitive substance and also upon its crystalline -state.</p> -<p class='c005'><b>97. Absorption of the α rays by matter</b>. The α rays from -the different radio-active substances can be distinguished from -one another by the relative amounts of their absorption by gases -or by thin screens of solid substances. When examined under -the same conditions, the α rays from the active substances can be -arranged in a definite order with reference to the amount of -absorption in a given thickness of matter.</p> - -<p class='c006'>In order to test the amount of absorption of the α rays for -different thicknesses of matter, an apparatus similar to that shown -in <a href='#fig017'>Fig. 17</a>, p. 98, was employed<a id='r158' href='#f158' class='c012'><sup>[158]</sup></a>. A thin layer of the active -material was spread uniformly over an area of about 30 sq. cms., -and the saturation current observed between two plates 3·5 cms. -apart. With a thin layer<a id='r159' href='#f159' class='c012'><sup>[159]</sup></a> of active material, the ionization between -the plates is due almost entirely to the α rays. The ionization due -to the β and γ rays is generally less than 1% of the total.</p> - -<p class='c006'>The following table shows the variation of the saturation current -between the plates due to the α rays from radium and polonium, -with successive layers of aluminium foil interposed, each ·00034 cm. -in thickness. In order to get rid of the ionization due to the β -rays from radium, the radium chloride employed was dissolved in -water and evaporated. This renders the active compound, for the -time, nearly free from β rays.</p> - -<p class='c006'><span class='pageno' id='Page_162'>162</span>The initial current with 1 layer of aluminium over the active -material is taken as 100. It will be observed that the current due</p> - -<table class='table1' > -<colgroup> -<col class='colwidth15'> -<col class='colwidth18'> -<col class='colwidth18'> -<col class='colwidth15'> -<col class='colwidth15'> -<col class='colwidth18'> -</colgroup> - <tr> - <th class='c013'></th> - <th class='c013'><i>Polonium.</i></th> - <th class='c013'> </th> - <th class='c013'> </th> - <th class='c013'><i>Radium.</i></th> - <th class='c014'> </th> - </tr> - <tr> - <th class='c013'>Layers of aluminium</th> - <th class='c013'>Current</th> - <th class='c013'>Ratio of decrease for each layer</th> - <th class='c013'>Layers of aluminium</th> - <th class='c013'>Current</th> - <th class='c014'>Ratio of decrease for each layer</th> - </tr> - <tr> - <td class='c013'> </td> - <td class='c013'> </td> - <td class='c013'> </td> - <td class='c013'> </td> - <td class='c013'> </td> - <td class='c014'> </td> - </tr> - <tr> - <td class='c013'>0</td> - <td class='c013'>100</td> - <td class='c013'> </td> - <td class='c013'>0</td> - <td class='c013'>100</td> - <td class='c014'> </td> - </tr> - <tr> - <td class='c013'> </td> - <td class='c013'> </td> - <td class='c013'>·41</td> - <td class='c013'> </td> - <td class='c013'> </td> - <td class='c014'>·48</td> - </tr> - <tr> - <td class='c013'>1</td> - <td class='c013'>41</td> - <td class='c013'> </td> - <td class='c013'>1</td> - <td class='c013'>48</td> - <td class='c014'> </td> - </tr> - <tr> - <td class='c013'> </td> - <td class='c013'> </td> - <td class='c013'>·31</td> - <td class='c013'> </td> - <td class='c013'> </td> - <td class='c014'>·48</td> - </tr> - <tr> - <td class='c013'>2</td> - <td class='c013'>12·6</td> - <td class='c013'> </td> - <td class='c013'>2</td> - <td class='c013'>23</td> - <td class='c014'> </td> - </tr> - <tr> - <td class='c013'> </td> - <td class='c013'> </td> - <td class='c013'>·17</td> - <td class='c013'> </td> - <td class='c013'> </td> - <td class='c014'>·60</td> - </tr> - <tr> - <td class='c013'>3</td> - <td class='c013'>2·1</td> - <td class='c013'> </td> - <td class='c013'>3</td> - <td class='c013'>13·6</td> - <td class='c014'> </td> - </tr> - <tr> - <td class='c013'> </td> - <td class='c013'> </td> - <td class='c013'>·067</td> - <td class='c013'> </td> - <td class='c013'> </td> - <td class='c014'>·47</td> - </tr> - <tr> - <td class='c013'>4</td> - <td class='c013'>·14</td> - <td class='c013'> </td> - <td class='c013'>4</td> - <td class='c013'>6·4</td> - <td class='c014'> </td> - </tr> - <tr> - <td class='c013'> </td> - <td class='c013'> </td> - <td class='c013'> </td> - <td class='c013'> </td> - <td class='c013'> </td> - <td class='c014'>·39</td> - </tr> - <tr> - <td class='c013'>5</td> - <td class='c013'>0</td> - <td class='c013'> </td> - <td class='c013'>5</td> - <td class='c013'>2·5</td> - <td class='c014'> </td> - </tr> - <tr> - <td class='c013'> </td> - <td class='c013'> </td> - <td class='c013'> </td> - <td class='c013'> </td> - <td class='c013'> </td> - <td class='c014'>·36</td> - </tr> - <tr> - <td class='c013'> </td> - <td class='c013'> </td> - <td class='c013'> </td> - <td class='c013'>6</td> - <td class='c013'>·9</td> - <td class='c014'> </td> - </tr> - <tr> - <td class='c013'> </td> - <td class='c013'> </td> - <td class='c013'> </td> - <td class='c013'> </td> - <td class='c013'> </td> - <td class='c014'> </td> - </tr> - <tr> - <td class='c013'> </td> - <td class='c013'> </td> - <td class='c013'> </td> - <td class='c013'>7</td> - <td class='c013'>0</td> - <td class='c014'> </td> - </tr> -</table> - -<p class='c006'>to the radium rays decreases very nearly by half its value for each -additional thickness until the current is reduced to about 6% of -the maximum. It then decays more rapidly to zero. Thus, for -radium, over a wide range, the current decreases approximately -according to an exponential law with the thickness of the screen, -or</p> - -<div class='figcenter id010'> -<img src='images/form-036.png' alt='Formula.' class='ig001'> -</div> - -<p class='c006'>where <i>i</i> is the current for a thickness <i>d</i>, and -<i>i</i>₀ -the initial current. -In the case of polonium, the decrease is far more rapid than would -be indicated by the exponential law. By the first layer, the -current is reduced to the ratio ·41. The addition of the third -layer cuts the current down to a ratio of ·17. For most of the -active bodies, the current diminishes slightly faster than the -exponential law would lead one to expect, especially when the -radiation is nearly all absorbed.</p> -<p class='c005'><a id='section098'></a> -<b>98.</b> The increase of absorption of the α rays of polonium with -the thickness of matter traversed has been very clearly shown -in some experiments made by Mme Curie. The apparatus -employed is shown in <a href='#fig034'>Fig. 34</a>.</p> - -<div id='fig034' class='figcenter id002'> -<span class='pageno' id='Page_163'>163</span> -<img src='images/fig-034.png' alt='Fig. 34.' class='ig001'> -<div class='ic002'> -<p>Fig. 34.</p> -</div> -</div> - -<p class='c006'>The saturation current was measured between two parallel -plates <i>PP´</i> 3 cms. apart. The polonium <i>A</i> was placed in the -metal box <i>CC</i>, and the rays -from it, after passing through -an opening in the lower plate -<i>P´</i>, covered with a layer of -thin foil <i>T</i>, ionized the gas -between the plates. For a -certain distance <i>AT</i>, of 4 cms. -or more, no appreciable current -was observed between <i>P</i> -and <i>P´</i>. As the distance <i>AT</i> -was diminished, the current increased in a very sudden manner, so -that for a small variation of the distance <i>AT</i> there was a large -increase of current. With still further decrease of distance the -current increases in a more regular manner. The results are -shown in the following table, where the screen <i>T</i> consisted of one -and two layers of aluminium foil respectively. The current due -to the rays, without the aluminium screen, is in each case taken -as 100.</p> - -<table class='table11' > -<colgroup> -<col class='colwidth55'> -<col class='colwidth8'> -<col class='colwidth8'> -<col class='colwidth8'> -<col class='colwidth11'> -<col class='colwidth8'> -</colgroup> - <tr> - <th class='c013'>Distance AT in cms.</th> - <th class='c013'>3·5</th> - <th class='c013'>2·5</th> - <th class='c013'>1·9</th> - <th class='c013'>1·45</th> - <th class='c014'>0·5</th> - </tr> - <tr> - <td class='c013'> </td> - <td class='c013'> </td> - <td class='c013'> </td> - <td class='c013'> </td> - <td class='c013'> </td> - <td class='c014'> </td> - </tr> - <tr> - <td class='c013'>For 100 rays transmitted by one layer</td> - <td class='c013'>0</td> - <td class='c013'>0</td> - <td class='c013'>5</td> - <td class='c013'>10</td> - <td class='c014'>25</td> - </tr> - <tr> - <td class='c013'>For 100 rays transmitted by two layers</td> - <td class='c013'>0</td> - <td class='c013'>0</td> - <td class='c013'>0</td> - <td class='c013'>0</td> - <td class='c014'>0·7</td> - </tr> -</table> - -<p class='c006'>The metallic screen thus cuts off a greater proportion of the -rays the greater the distance of air which the radiations traverse. -The effects are still more marked if the plates <i>PP´</i> are close -together. Results similar but not so marked are found if radium -is substituted for the polonium.</p> - -<p class='c006'>It follows from these experiments that the ionization per unit -volume, due to a large plate uniformly covered with the radio-active -matter, falls off rapidly with the distance from the plate. -At a distance of 10 cms. the α rays from uranium, thorium, or -radium have been completely absorbed in the gas, and the small -ionization then observed in the gas is due to the more penetrating -β and γ rays. The relative amount of the ionization observed at -<span class='pageno' id='Page_164'>164</span>a distance from the source will increase with the thickness of the -layer of active matter, but will reach a maximum for a layer of a -certain thickness. The greater proportion of the ionization, due -to unscreened active matter, is thus entirely confined to a shell of -air surrounding it not more than 10 cms. in depth.</p> - -<div id='fig035' class='figcenter id004'> -<img src='images/fig-035.png' alt='Fig. 35.' class='ig001'> -<div class='ic002'> -<p>Fig. 35.</p> -</div> -</div> -<p class='c005'><a id='section099'></a> -<b>99.</b> The α rays from different compounds of the same active -element, although differing in amount, have about the same average -penetrating power. Experiments on this point have been made by -the writer<a id='r160' href='#f160' class='c012'><sup>[160]</sup></a> and by Owens<a id='r161' href='#f161' class='c012'><sup>[161]</sup></a>. Thus in comparing the relative -power of penetration of the α rays from the different radio-elements, -it is only necessary to determine the penetrating power -for one compound of each of the radio-elements. Rutherford and -Miss Brooks<a id='r162' href='#f162' class='c012'><sup>[162]</sup></a> have determined the amount of absorption of the -α rays from the different active substances in their passage -through successive layers of aluminium foil ·00034 cm. thick. The -<span class='pageno' id='Page_165'>165</span>curves of absorption are given in <a href='#fig035'>Fig. 35</a>. For the purpose of -comparison in each case, the initial current with the bare active -compound was taken as 100. A very thin layer of the active -substance was used, and, in the case of thorium and radium, the -emanations given off were removed by a slow current of air through -the testing vessel. A potential difference of 300 volts was applied -between the plates, which was sufficient to give the maximum -current in each case.</p> - -<p class='c006'>Curves for the minerals organite and thorite were very nearly -the same as for thoria.</p> - -<p class='c006'>For comparison, the absorption curves of the excited radiations -of thorium and radium are given, as well as the curve for the -radio-elements uranium, thorium, radium, and polonium. The α -radiations may be arranged in the following order, as regards their -power of penetration, beginning with the most penetrating.</p> - -<div class='calc'> - -<div class='lg-container-b c017'> - <div class='linegroup'> - <div class='group'> - <div class='line'>Thorium}</div> - <div class='line'>Radium } excited radiation.</div> - <div class='line'>Thorium.</div> - <div class='line'>Radium.</div> - <div class='line'>Polonium.</div> - <div class='line'>Uranium.</div> - </div> - </div> -</div> - -</div> - -<p class='c018'>The same order is observed for all the absorbing substances -examined, viz., aluminium, Dutch metal, tinfoil, paper, and air and -other gases. The differences in the absorption of the α rays from -the active bodies are thus considerable, and must be ascribed either -to a difference of mass or of velocity of the α particles or to a -variation in both these quantities.</p> - -<p class='c006'>Since the α rays differ either in mass or velocity, it follows -that they cannot be ascribed to any single radio-active impurity -common to all radio-active bodies.</p> -<p class='c005'><a id='section100'></a> -<b>100. Absorption of the α rays by gases</b>. The α rays from -the different radio-active substances are quickly absorbed in their -passage through a few centimetres of air at atmospheric pressure -and temperature. In consequence of this, the ionization of the air, -due to the α rays, is greatest near the surface of the radiating body -and falls off very rapidly with the distance (see <a href='#section098'>section 98</a>).</p> - -<div id='fig036' class='figcenter id002'> -<span class='pageno' id='Page_166'>166</span> -<img src='images/fig-036.png' alt='Fig. 36.' class='ig001'> -<div class='ic002'> -<p>Fig. 36.</p> -</div> -</div> - -<p class='c006'>A simple method of determining the absorption in gases is -shown in <a href='#fig036'>Fig. 36</a>. The maximum -current is measured between two -parallel plates <i>A</i> and <i>B</i> kept at a -<i>fixed</i> distance of 2 cms. apart, and -then moved by means of a screw to -different distances from the radio-active -surface. The radiation from -this active surface passed through a -circular opening in the plate <i>A</i>, -covered with thin aluminium foil, -and was stopped by the upper plate. -For observations on other gases besides -air, and for examining the -effect at different pressures, the apparatus is enclosed in an air-tight -cylinder.</p> - -<p class='c006'>If the radius of the active surface is large compared with the -distance of the plate <i>A</i> from it, the intensity of the radiation is -approximately uniform over the opening in the plate <i>A</i>, and falls -off with the distance <i>x</i> traversed according to an exponential law. -Thus</p> - -<div class='figcenter id010'> -<img src='images/form-037.png' alt='Formula.' class='ig001'> -</div> - -<p class='c006'>where λ is the “absorption constant” of the radiation for the gas -under consideration<a id='r163' href='#f163' class='c012'><sup>[163]</sup></a>. Let</p> - -<div class='calc'> - -<div class='lg-container-b c017'> - <div class='linegroup'> - <div class='group'> - <div class='line'><i>x</i> = distance of lower plate from active material,</div> - <div class='line'><i>l</i> = distance between the two fixed plates.</div> - </div> - </div> -</div> - -</div> - -<p class='c018'>The energy of the radiation at the lower plate is then</p> - -<div class='figcenter id010'> -<img src='images/form-038.png' alt='Formula.' class='ig001'> -</div> - -<p class='c006'>and at the upper plate</p> - -<div class='figcenter id010'> -<img src='images/form-039.png' alt='Formula.' class='ig001'> -</div> - -<p class='c006'>The total number of ions produced -between the parallel plates <i>A</i> and <i>B</i> is therefore proportional -to</p> - -<div class='figcenter id002'> -<img src='images/form-040.png' alt='Formula.' class='ig001'> -</div> - -<p class='c006'>Since the factor</p> - -<div class='figcenter id010'> -<img src='images/form-041.png' alt='Formula.' class='ig001'> -</div> - -<p class='c006'>is a constant, the saturation current -<span class='pageno' id='Page_167'>167</span>between <i>A</i> and <i>B</i> varies as</p> - -<div class='figcenter id010'> -<img src='images/form-042.png' alt='Formula.' class='ig001'> -</div> - -<p class='c006'><i>i.e.</i> it decreases according to an -exponential law with the distance traversed.</p> - -<div id='fig037' class='figcenter id004'> -<img src='images/fig-037.png' alt='Fig. 37.' class='ig001'> -<div class='ic002'> -<p>Fig. 37.</p> -</div> -</div> - -<p class='c006'>The variation of the current between <i>A</i> and <i>B</i> with the distance -from a thin layer of uranium oxide is shown in <a href='#fig037'>Fig. 37</a> for different -gases. The initial measurements were taken at a distance of about -3·5 mms. from the active surface. The actual values of this initial -current were different for the different gases, but, for the purposes -of comparison, the value is in each case taken as unity.</p> - -<p class='c006'>It will be seen that the current falls off with the distance -approximately in a geometrical progression, a result which is in -agreement with the simple theory given above. The distance -through which the rays pass before they are absorbed is given -below for different gases.</p> - -<table class='table6' > -<colgroup> -<col class='colwidth50'> -<col class='colwidth50'> -</colgroup> - <tr> - <th class='c013'>Gas</th> - <th class='c014'>Distance in mms. to absorb half of radiation</th> - </tr> - <tr> - <td class='c013'>Carbonic acid</td> - <td class='c014'>3</td> - </tr> - <tr> - <td class='c013'>Air</td> - <td class='c014'>4·3</td> - </tr> - <tr> - <td class='c013'>Coal-gas</td> - <td class='c014'>7·5</td> - </tr> - <tr> - <td class='c013'>Hydrogen</td> - <td class='c014'>16</td> - </tr> -</table> - -<p class='c006'>The results for hydrogen are only approximate, as the absorption -is small over the distance examined.</p> - -<p class='c006'><span class='pageno' id='Page_168'>168</span>The absorption is least in hydrogen and greatest in carbonic -acid, and follows the same order as the densities of the gases. -In the case of air and carbonic acid, the absorption is proportional -to the density, but this rule is widely departed from in the case -of hydrogen. Results for the relative absorption by air of the α rays -from the different active bodies are shown in <a href='#fig038'>Fig. 38</a>.</p> - -<div id='fig038' class='figcenter id004'> -<img src='images/fig-038.png' alt='Fig. 38.' class='ig001'> -<div class='ic002'> -<p>Fig. 38.</p> -</div> -</div> - -<p class='c006'>The initial observation was made about 2 mms. from the active -surface, and the initial current is in each case taken as 100. The -current, as in the case of uranium, falls off at first approximately -in geometrical progression with the distance. The thickness of -air, through which the radiation passes before the intensity is -reduced to half value, is given below.</p> - -<table class='table8' > -<colgroup> -<col class='colwidth50'> -<col class='colwidth50'> -</colgroup> - <tr> - <th class='c013'></th> - <th class='c014'>Distance in mms.</th> - </tr> - <tr> - <td class='c013'>Uranium</td> - <td class='c014'>4·3</td> - </tr> - <tr> - <td class='c013'>Radium</td> - <td class='c014'>7·5</td> - </tr> - <tr> - <td class='c013'>Thorium</td> - <td class='c014'>10</td> - </tr> - <tr> - <td class='c013'>Excited radiation from Thorium and Radium</td> - <td class='c014'>16·5</td> - </tr> -</table> - -<p class='c006'>The order of absorption by air of the radiations from the active -substances is the same as the order of absorption by the metals -and solid substances examined.</p> -<p class='c005'><span class='pageno' id='Page_169'>169</span><b>101. Connection between absorption and density.</b> Since -in all cases the radiations first diminish approximately according -to an exponential law with the distance traversed, the intensity <i>I</i> -after passing through a thickness <i>x</i> is given by</p> - -<div class='figcenter id010'> -<img src='images/form-043.png' alt='Formula.' class='ig001'> -</div> - -<p class='c006'>where λ -is the absorption constant and -<i>I</i>₀ -the initial intensity.</p> - -<p class='c006'>The following table shows the value of λ with different radiations -for air and aluminium.</p> - -<table class='table8' > -<colgroup> -<col class='colwidth50'> -<col class='colwidth25'> -<col class='colwidth25'> -</colgroup> - <tr> - <th class='c013'>Radiation</th> - <th class='c015'>λ for aluminium</th> - <th class='c016'>λ for air</th> - </tr> - <tr> - <td class='c013'>Excited radiation</td> - <td class='c015'>830</td> - <td class='c016'>·42</td> - </tr> - <tr> - <td class='c013'>Thorium</td> - <td class='c015'>1250</td> - <td class='c016'>·69</td> - </tr> - <tr> - <td class='c013'>Radium</td> - <td class='c015'>1600</td> - <td class='c016'>·90</td> - </tr> - <tr> - <td class='c013'>Uranium</td> - <td class='c015'>2750</td> - <td class='c016'>1·6</td> - </tr> -</table> - -<p class='c006'>Taking the density of air at 20° C. and 760 mms. as 0·00120 -compared with water as unity, the following table shows the value -of λ divided by density for the different radiations.</p> - -<table class='table8' > -<colgroup> -<col class='colwidth50'> -<col class='colwidth25'> -<col class='colwidth25'> -</colgroup> - <tr> - <th class='c013'>Radiation</th> - <th class='c015'>Aluminium</th> - <th class='c016'>Air</th> - </tr> - <tr> - <td class='c013'>Excited radiation</td> - <td class='c015'>320</td> - <td class='c016'>350</td> - </tr> - <tr> - <td class='c013'>Thorium</td> - <td class='c015'>480</td> - <td class='c016'>550</td> - </tr> - <tr> - <td class='c013'>Radium</td> - <td class='c015'>620</td> - <td class='c016'>740</td> - </tr> - <tr> - <td class='c013'>Uranium</td> - <td class='c015'>1060</td> - <td class='c016'>1300</td> - </tr> -</table> - -<p class='c006'>Comparing aluminium and air, the absorption is thus roughly -proportional to the density for all the radiations. The divergence, -however, between the absorption-density numbers is large when -two metals like tin and aluminium are compared. The value of λ -for tin is not much greater than for aluminium, although the -density is nearly three times as great.</p> - -<p class='c006'>If the absorption is proportional to the density, the absorption -in a gas should vary directly as the pressure, and this is found to -be the case. Some results on this subject have been given by the -writer (<i>loc. cit.</i>) for uranium rays between pressures of ¼ and 1 -atmosphere. Owens (<i>loc. cit.</i>) examined the absorption of the α -radiation in air from thoria between the pressures of 0·5 to 3 -atmospheres and found that the absorption varied directly as the -pressure.</p> - -<p class='c006'>The variation of absorption with density for the projected -positive particles is thus very similar to the law for the projected -negative particles and for cathode rays. The absorption, in both -cases, depends mainly on the density, but is not in all cases directly -<span class='pageno' id='Page_170'>170</span>proportional to it. Since the absorption of the α rays in gases is -probably mainly due to the exhaustion of the energy of the rays -by the production of ions in the gas, it seems probable that the -absorption in metals is due to a similar cause.</p> -<p class='c005'><b>102. Relation between ionization and absorption in -gases.</b> It has been shown (<a href='#section045'>section 45</a>) that if the α rays are -completely absorbed in a gas, the <i>total</i> ionization produced is about -the same for all the gases examined. Since the rays are unequally -absorbed in different gases, there should be a direct connection -between the relative ionization and the relative absorption. This -is seen to be the case if the results of Strutt (<a href='#section045'>section 45</a>) are compared -with the relative absorption constants (<a href='#section100'>section 100</a>).</p> - -<table class='table17' > -<colgroup> -<col class='colwidth40'> -<col class='colwidth30'> -<col class='colwidth30'> -</colgroup> - <tr> - <th class='c013'>Gas</th> - <th class='c015'>Relative absorption</th> - <th class='c016'>Relative ionization</th> - </tr> - <tr> - <td class='c013'>Air</td> - <td class='c015'>1</td> - <td class='c016'>1</td> - </tr> - <tr> - <td class='c013'>Hydrogen</td> - <td class='c015'>·27</td> - <td class='c016'>·226</td> - </tr> - <tr> - <td class='c013'>Carbon dioxide</td> - <td class='c015'>1·43</td> - <td class='c016'>1·53</td> - </tr> -</table> - -<p class='c006'>Considering the difficulty of obtaining accurate determinations -of the absorption, the relative ionization in a gas is seen to be -directly proportional to the relative absorption within the limits of -experimental error. This result shows that the energy absorbed -in producing an ion is about the same in air, hydrogen, and carbon -dioxide.</p> -<p class='c005'><a id='section103'></a> -<b>103. Mechanism of the absorption of α rays by -matter</b>. The experiments, already described, show that the -ionization of the gas, due to the α rays from a large plane surface -of radio-active matter, falls off in most cases approximately -according to an exponential law, until most of the rays are -absorbed, whereupon the ionization decreases at a much faster -rate. In the case of polonium, the ionization falls off more rapidly -than is to be expected on the simple exponential law.</p> - -<p class='c006'>The ionization produced in the gas is due to the collision -of the rapidly moving α particles with the molecules of the gas in -their path. On account of its large mass, the α particle is a far -more efficient ionizer than the β particle moving at the same -speed. It can be deduced from the results of experiment that -<span class='pageno' id='Page_171'>171</span>each projected α particle is able to produce about 100,000 ions -in passing through a few centimetres of the gas before its velocity -is reduced to the limiting value, below which it no longer ionizes -the gas in its path.</p> - -<p class='c006'>Energy is required to ionize the gas, and this energy can only -be obtained at the expense of the kinetic energy of the projected -α particle. Thus it is to be expected that the α particle should -gradually lose its velocity and energy of motion in its passage -through the gas.</p> - -<p class='c006'>Since the rate of absorption of the α rays in gases is deduced -from measurements of the ionization of the gas at different distances -from the source of radiation, a knowledge of the law of variation -of the ionizing power of the projected α particle with its speed is -required in order to interpret the results. The experimental data -on this question are, however, too incomplete to be applied -directly to a solution of this question. Townsend<a id='r164' href='#f164' class='c012'><sup>[164]</sup></a> has shown that -a moving electron produces ions in the gas after a certain limiting -velocity is reached. The number of ions produced per centimetre -of its path through the gas then rises to a maximum, and for still -higher speeds continuously decreases. For example, Townsend -found that the number of ions produced by an electron moving in -an electric field was small at first for weak fields, but increased -with the strength of the electric field to a maximum corresponding -to the production of 20 ions per cm. of path in air at a pressure of -1 mm. of mercury. Durack<a id='r165' href='#f165' class='c012'><sup>[165]</sup></a> found that the electrons, generated -in a vacuum tube, moving with a velocity of about 5 × 10<sup>9</sup> cms. -per second produced a pair of ions every 5 cms. of path at 1 mm. -pressure. In a later paper, Durack showed that for the electrons -from radium, which are projected with a velocity greater than half -the velocity of light, a pair of ions was produced every 10 cms. of -path. The high speed electron from radium is thus a very -inefficient ionizer and produces only about ¹⁄₁₀₀ of the ionization -per unit path observed by Townsend for the slow moving electron.</p> -<p class='c005'><a id='section104'></a> -<b>104.</b> In the case of the α particle, no direct measurements -have been made upon the variation of the ionization with the -<span class='pageno' id='Page_172'>172</span>velocity of the particle, so that the law of absorption of the rays -cannot be deduced directly. An indirect attack upon the question -has, however, been made recently by Bragg and Kleeman<a id='r166' href='#f166' class='c012'><sup>[166]</sup></a> who -have formulated a simple theory to account for the experimental -results which they have obtained upon the absorption of the -α rays. The α particles from each simple type of radio-active -matter are supposed to be projected with the same velocity, and -to pass through a definite distance a in air at atmospheric pressure -and temperature before they are all absorbed. As a first approximation -the ionization per unit path is supposed to be the same -over the whole length traversed before absorption, and to cease -fairly suddenly at a definite distance from the source of radiation. -This is in agreement with the observed fact that the ionization -between parallel plates increases very rapidly when it approaches -nearer than a certain distance to the radiant source. The range -<i>a</i> depends upon the initial energy of motion of the α particle and -will thus be different for different kinds of radio-active matter. If -a thick layer of radio-active matter is employed, only the α -particles from the surface have a range <i>a</i>. Those which reach the -surface from a depth <i>d</i> have their range diminished by an amount ρ<i>d</i>, -where ρ is the density of the radio-active matter compared with -air. This is merely an expression of the fact that the absorption -of the α rays is proportional to the thickness and density of matter -traversed. The rays from a thick layer of active matter will thus -be complex, and will consist of particles of different velocity whose -ranges have all values between 0 and <i>a</i>.</p> - -<p class='c006'>Suppose that a narrow pencil of -α rays is emitted from a thick layer -of radio-active material, and confined -by metal stops as in <a href='#fig039'>Fig. 39</a>.</p> - -<div id='fig039' class='figcenter id002'> -<img src='images/fig-039.png' alt='Fig. 39.' class='ig001'> -<div class='ic002'> -<p>Fig. 39.</p> -</div> -</div> - -<p class='c006'>The pencil of rays passes into -an ionization vessel <i>AB</i> through a -fine wire gauze <i>A</i>. The amount of -ionization is to be determined between -<i>A</i> and <i>B</i> for different distances -<i>h</i> from the source of the -rays <i>R</i> to the plate <i>A</i>.</p> - -<p class='c006'><span class='pageno' id='Page_173'>173</span>All the particles coming from a depth <i>x</i> of the material given -by <i>h</i> = <i>a</i> – ρ<i>x</i> will enter the ionization vessel. The number of -ions produced in a depth <i>dh</i> of the ionization vessel is equal to -<i>nxdh</i>, <i>i.e.</i> to</p> - -<div class='calc'> - -<div class='lg-container-b c017'> - <div class='linegroup'> - <div class='group'> - <div class='line in3'><i>a</i> – <i>h</i></div> - <div class='line'><i>n</i> ------ <i>dh</i> ,</div> - <div class='line in5'>ρ</div> - </div> - </div> -</div> - -</div> - -<p class='c018'>where <i>n</i> is a constant.</p> - -<p class='c006'>If the depth of the ionization vessel be <i>b</i>, the total number of -ions produced in the vessel is</p> - -<div class='figcenter id005'> -<img src='images/form-044.png' alt='Formula.' class='ig001'> -</div> - -<p class='c006'>This supposes that the stream of particles passes completely -across the vessel. If not, the expression becomes</p> - -<div class='figcenter id005'> -<img src='images/form-045.png' alt='Formula.' class='ig001'> -</div> - -<p class='c006'>If the ionization in the vessel <i>AB</i> is measured, and a curve -plotted showing its relation to <i>h</i>, the curve in the former case -should be a straight line whose slope is <i>nb</i>/ρ and in the latter a -parabola.</p> - -<p class='c006'>Thus if a thin layer of radio-active material is employed and a -shallow ionization vessel, the ionization -would be represented by a curve -such as <i>APM</i> (<a href='#fig040'>Fig. 40</a>), where the -ordinates represent distances from -the source of radiation, and the -abscissae the ionization current between -the plates <i>AB</i>.</p> - -<div id='fig040' class='figcenter id002'> -<img src='images/fig-040.png' alt='Fig. 40.' class='ig001'> -<div class='ic002'> -<p>Fig. 40.</p> -</div> -</div> - -<p class='c006'>In this case, <i>PM</i> is the range of -the α particles from the lowest layer -of the radio-active matter. The -current should be constant for all -distances less than <i>PM</i>.</p> - -<p class='c006'>For a thick layer of radio-active -matter, the curve should be a straight line such as <i>APB</i>.</p> - -<p class='c006'>Curves of the above character should only be obtained when -definite cones of rays are employed, and where the ionization -vessel is shallow and includes the whole cone of rays. In such a -case the inverse square law need not be taken into account.</p> - -<p class='c006'><span class='pageno' id='Page_174'>174</span>In the experiments previously recorded (sections <a href='#section099'>99</a> and <a href='#section100'>100</a>), -the ionization was measured between parallel plates several centimetres -apart for a large area of radio-active material. Such an -arrangement was necessary at the time at which the experiments -were made, as only weak radio-active material was available. -Measurable electrical effects could not then be obtained with -narrow cones of rays and shallow ionization vessels, but this -disadvantage is removed by the advent of pure radium bromide -as a source of radiation.</p> - -<p class='c006'>The interesting experiments described by Bragg and Kleeman -show that the theoretical curves are approximately realized in -practice. The chief difficulty experienced in the analysis of the -experimental results was due to the fact that radium is a complex -radio-active substance and contains four radio-active products each -of which gives rise to α rays which have different ranges. The -general character of the results obtained from radium are shown -graphically in <a href='#fig041'>Fig. 41</a>, curves <i>A</i>, <i>B</i>, <i>C</i>, <i>D</i>.</p> - -<div id='fig041' class='figcenter id004'> -<img src='images/fig-041.png' alt='Fig. 41.' class='ig001'> -<div class='ic002'> -<p>Fig. 41.</p> -</div> -</div> - -<p class='c006'>The ordinates represent the distance between the radium and -the gauze of the testing vessel; the abscissae the current in the -ionization vessel in arbitrary units. Five milligrams of radium -bromide were used, and the depth of the ionization vessel was -<span class='pageno' id='Page_175'>175</span>about 5 mms. Curve <i>A</i> is for a cone of rays of angle 20°. The -initial current at a distance of 7 cms. is due to the β and γ rays -and natural leak. This curve is initially parabolic, and then is -made up of two straight lines. Curve <i>B</i> is for a smaller cone, and -shows the straight line character of the curve to within a short -distance of the radium. Curve <i>C</i> was obtained under the same -condition as curve <i>A</i>, but with a layer of gold beater’s skin placed -over the radium. The effect of this is to reduce all the ordinates -of curve <i>A</i> by the same quantity. This is to be expected on the -simple theory already considered. Curve <i>D</i> was obtained when -the radium was heated so as to get rid of the emanation and its -products. The α particles of greatest range are quite absent and -the curve is simpler in character.</p> - -<div id='fig042' class='figcenter id007'> -<img src='images/fig-042.png' alt='Fig. 42.' class='ig001'> -<div class='ic002'> -<p>Fig. 42.</p> -</div> -</div> - -<p class='c006'>The complex character of the radium curves are more clearly -brought out by a careful examination of a portion of the curve at -distances between 2 and 5 cms. from the radium, using an -ionization vessel of depth only 2 mms. The results are shown -in <a href='#fig042'>Fig. 42</a>, where the curve is seen to consist approximately of -<span class='pageno' id='Page_176'>176</span>four straight lines of different slopes represented by <i>PQ</i>, <i>QR</i>, -<i>RS</i>, <i>ST</i>.</p> - -<p class='c006'>Such a result is to be expected, for it will be shown later that -four distinct α ray products exist in radium when in radio-active -equilibrium. Each of these products of radium emits an equal -number of α particles per second, but the range of each is -different. If -<i>a</i><sub>1</sub> -is the range of one stream, -<i>a</i><sub>2</sub> -of another, the -ionization in the vessel <i>AB</i>, when two streams enter the vessel, -should be</p> - -<div class='calc'> - -<div class='lg-container-b c017'> - <div class='linegroup'> - <div class='group'> - <div class='line'> <i>nb</i> <i>nb</i></div> - <div class='line'>---- (<i>a</i><sub>1</sub>-<i>h</i>-<i>b</i>/2) + ----- (<i>a</i><sub>2</sub> – <i>h</i> – <i>b</i>/2),</div> - <div class='line in1'> ρ ρ</div> - </div> - </div> -</div> - -</div> - -<p class='c018'><i>i.e.</i></p> - -<div class='calc'> - -<div class='lg-container-b c017'> - <div class='linegroup'> - <div class='group'> - <div class='line'> <i>nb</i></div> - <div class='line'>---- (<i>a</i><sub>1</sub> + <i>a</i><sub>2</sub> – 2<i>h</i> – <i>b</i>) .</div> - <div class='line in1'>ρ</div> - </div> - </div> -</div> - -</div> - -<p class='c018'>Thus the slope of the curve should in this case be 2<i>nb</i>/ρ, while if -only one stream enters, it should be <i>nb</i>/ρ. When three reach it, the -slope should be 3<i>nb</i>/ρ and for four 4<i>nb</i>/ρ. These results are realized -fairly closely in practice. The curve (<a href='#fig042'>Fig. 42</a>) consists of four -parts, whose slopes are in the proportion 16, 34, 45, 65, <i>i.e.</i> very -nearly in the ratio 1, 2, 3, 4.</p> - -<p class='c006'>Experiments were also made with very thin layers of radium -bromide, when, as we have seen (<a href='#fig040'>Fig. 40</a>) a very different shape of -curve is to be expected. An example of the results is shown in -<a href='#fig043'>Fig. 43</a>, curves I., II. and III. Curve I. is obtained from radium -bromide which has been heated to drive off the emanation, and -curves II. and III. from the same substance several days later, -when the emanation was again accumulating. The portion <i>PQ</i>, -which is absent in the first curve, is probably due to the “excited” -activity produced by the emanation. By careful examination of -the successive changes in the curves after the radium has been -heated to drive off the emanation, it is possible to tell the range -of the α rays from each of the different products, and this has been -done to some extent by Bragg and Kleeman.</p> - -<p class='c006'>It will be seen later that the results here obtained support in -a novel way the theory of radio-active changes which has been -advanced from data of quite a different character.</p> - -<p class='c006'>The inward slope of the curve in <a href='#fig043'>Fig. 43</a> due to the radium -indicates that the α particles become more efficient ionizers as -<span class='pageno' id='Page_177'>177</span>their velocity decreases. This is in agreement with observations -on the β rays. In some cases Bragg also observed that the α -particles are the most efficient ionizers just before they lose their -power of ionizing the gas.</p> - -<div id='fig043' class='figcenter id006'> -<img src='images/fig-043.png' alt='Fig. 43.' class='ig001'> -<div class='ic002'> -<p>Fig. 43.</p> -</div> -</div> - -<p class='c006'>Thus we may conclude from these experiments that the α -particles from a simple radio-active substance traverse a definite -distance in air, at a definite pressure and temperature, and that -the ionization ends fairly abruptly. If the rays traverse a sheet of -metal, the effective range of ionization is diminished by a distance -corresponding to ρ<i>d</i>, where ρ is the density of the material -compared with air and <i>d</i> its thickness. The α rays from a thick -layer of a simple radio-active substance consist of α particles of -<span class='pageno' id='Page_178'>178</span>different velocities, which have ranges in air lying between 0 and -the maximum range. The ionization of the particles per unit -path is greatest near the end of its range, and decreases somewhat -as we approach the radiant source. A complex source of rays like -radium gives out four types of rays, each of which has a different -but distinct range.</p> - -<p class='c006'>From this theory it is possible to calculate approximately the -decrease of current to be observed when sheets of metal foil are -placed over a large area of radio-active substance. This is the method -that has been employed to obtain the curves of Figs. <a href='#fig035'>35</a> and <a href='#fig038'>38</a>.</p> - -<p class='c006'>Suppose a very thin layer of simple radio-active matter is -employed (for example a bismuth plate covered with radio-tellurium -or a metal plate made active by exposure to the presence -of the thorium or radium emanations) and that the ionization -vessel is of sufficient depth to absorb the α rays completely.</p> - -<p class='c006'>Let <i>d</i> be the thickness of the metal plate, ρ its density -compared with air. Consider a point <i>P</i> close to the upper side of -the plate. The range of the particles moving from a point, when -the path makes an angle θ with the normal at <i>P</i>, is <i>a</i> – ρ<i>d</i> sec θ, -where <i>a</i> is the range in air. The rays coming from points such -that the paths make an angle with the normal greater than</p> - -<div class='figcenter id010'> -<img src='images/form-046.png' alt='Formula.' class='ig001'> -</div> - -<p class='c006'>will thus be absorbed in the plate. By integrating over the circular -area under the point <i>P</i>, it is easy to show that the total ionization -in the vessel is proportional to</p> - -<div class='figcenter id006'> -<img src='images/form-047.png' alt='Formula.' class='ig001'> -</div> - -<p class='c006'>The curves showing the relation between current and distance -of metal traversed should thus be parabolic with respect to <i>d</i>. -This is approximately the case for a simple substance like radio-tellurium. -The curve for a thick layer of radium would be -difficult to calculate on account of the complexity of the rays, but -we know from experiment that it is approximately exponential. -An account of some recent investigations made to determine the -range of velocity over which the α particle is able to ionize the gas -is given in <a href='#appa'>Appendix A</a>. The results there given strongly support -the theory of absorption of the α rays discussed above.</p> -<div> - <span class='pageno' id='Page_179'>179</span> - <h3 class='c001'>PART IV.</h3> -</div> -<h4 class='c022'>The γ or very penetrating Rays.</h4> -<p class='c005'><b>105.</b> In addition to the α and β rays, the three active substances, -uranium, thorium, and radium, all give out a radiation of -an extraordinarily penetrating character. These γ rays are considerably -more penetrating than the X rays produced in a “hard” -vacuum tube. Their presence can readily be observed for an active -substance like radium, but is difficult to detect for uranium and -thorium unless a large quantity of active material is used. -Villard<a id='r167' href='#f167' class='c012'><sup>[167]</sup></a>, using the photographic method, first drew attention -to the fact that radium gave out these very penetrating rays, and -found that they were non-deviable by a magnetic field. This result -was confirmed by Becquerel<a id='r168' href='#f168' class='c012'><sup>[168]</sup></a>.</p> - -<p class='c006'>Using a few milligrams of radium bromide, the γ rays can -be detected in a dark room by the luminosity they excite in -the mineral willemite or a screen of platinocyanide of barium. -The α and β rays are completely absorbed by placing a thickness -of 1 centimetre of lead over the radium, and the rays which then -pass through the lead consist entirely of γ rays. The very great -penetrating power of these rays is easily observed by noting the -slight diminution of the luminosity of the screen when plates of -metal several centimetres thick are placed between the radium and -the screen. These rays also produce ionization in gases and are -best investigated by the electrical method. The presence of the -γ rays from 30 mgrs. of radium bromide can be observed in an -electroscope after passing through 30 cms. of iron.</p> -<p class='c005'><b>106. Absorption of the γ rays</b>. In an examination of the -active substances by the electrical method, the writer<a id='r169' href='#f169' class='c012'><sup>[169]</sup></a> found that -both uranium and thorium gave out γ rays in amount roughly -proportional to their activity. An electroscope of the type shown -in <a href='#fig012'>Fig. 12</a> was employed. This was placed on a large lead plate -·65 cm. thick, the active substance being placed in a closed vessel -beneath.</p> - -<p class='c006'><span class='pageno' id='Page_180'>180</span>The discharge due to the natural ionization of the air in the -electroscope was first observed. The additional ionization due to -the active substance must be that produced by rays which have -passed through the lead plate and the walls of the electroscope. -The following table shows that the discharge due to these rays -decreases approximately according to an exponential law with the -thickness of lead traversed.</p> - -<table class='table8' > -<colgroup> -<col class='colwidth50'> -<col class='colwidth50'> -</colgroup> - <tr> - <th class='c013'>Thickness of lead</th> - <th class='c016'>Rate of discharge</th> - </tr> - <tr> - <td class='c013'>·62 cms.</td> - <td class='c016'>100</td> - </tr> - <tr> - <td class='c013'>„ + ·64 cms.</td> - <td class='c016'>67</td> - </tr> - <tr> - <td class='c013'>„ + 2·86 „</td> - <td class='c016'>23</td> - </tr> - <tr> - <td class='c013'>„ + 5·08 „</td> - <td class='c016'>8</td> - </tr> -</table> - -<p class='c006'>Using 100 grs. of uranium and thorium, the discharge due to -the rays through 1 cm. of lead was quite appreciable, and readily -measured. The results showed that the amount of γ rays was -about the same for equal weights of thorium and uranium oxides. -The penetrating power was also about the same as for the radium -rays.</p> - -<div id='fig044' class='figcenter id004'> -<img src='images/fig-044.png' alt='Fig. 44.' class='ig001'> -<div class='ic002'> -<p>Fig. 44.</p> -</div> -</div> - -<p class='c006'><span class='pageno' id='Page_181'>181</span>The writer showed that the absorption of the γ rays from -radium was approximately proportional to the density of the -substance traversed. A more detailed examination of the absorption -of these rays in various substances has been recently made -by McClelland<a id='r170' href='#f170' class='c012'><sup>[170]</sup></a>. The curve (<a href='#fig044'>Fig. 44</a>) shows the decrease of the -ionization current in a testing vessel due to the β and γ rays -with successive layers of lead. It is seen that the β rays are -almost completely stopped by 4 mms. of lead; the ionization is -then due entirely to the γ rays.</p> - -<p class='c006'>In order to leave no doubt that all the β rays were absorbed, -the radium was covered with a thickness of 8 mms. of lead, and -measurements of the coefficient of absorption λ were made for -additional thicknesses. The average value of λ was calculated -from the usual equation</p> - -<div class='figcenter id010'> -<img src='images/form-033.png' alt='Formula.' class='ig001'> -</div> - -<p class='c006'>where <i>d</i> is the thickness of -matter traversed. The following table shows the value of λ, (I) -for the first 2·5 mms. of matter traversed (after initially passing -through 8 mms. of lead), (II) for the thickness 2·5 to 5 mms., -(III) for 5 to 10 mms., (IV) 10 to 15 mms.</p> - -<p class='c006'>TABLE A.</p> - -<table class='table13' > -<colgroup> -<col class='colwidth39'> -<col class='colwidth21'> -<col class='colwidth13'> -<col class='colwidth13'> -<col class='colwidth13'> -</colgroup> - <tr> - <th class='c013'>Substance</th> - <th class='c015'>I</th> - <th class='c015'>II</th> - <th class='c015'>III</th> - <th class='c016'>IV</th> - </tr> - <tr> - <td class='c013'> </td> - <td class='c015'> </td> - <td class='c015'> </td> - <td class='c015'> </td> - <td class='c016'> </td> - </tr> - <tr> - <td class='c013'>Platinum</td> - <td class='c015'>1·167</td> - <td class='c015'> </td> - <td class='c015'> </td> - <td class='c016'> </td> - </tr> - <tr> - <td class='c013'>Mercury</td> - <td class='c015'>·726</td> - <td class='c015'>·661</td> - <td class='c015'>·538</td> - <td class='c016'>·493</td> - </tr> - <tr> - <td class='c013'>Lead</td> - <td class='c015'>·641</td> - <td class='c015'>·563</td> - <td class='c015'>·480</td> - <td class='c016'>·440</td> - </tr> - <tr> - <td class='c013'>Zinc</td> - <td class='c015'>·282</td> - <td class='c015'>·266</td> - <td class='c015'>·248</td> - <td class='c016'>·266</td> - </tr> - <tr> - <td class='c013'>Aluminium</td> - <td class='c015'>·104</td> - <td class='c015'>·104</td> - <td class='c015'>·104</td> - <td class='c016'>·104</td> - </tr> - <tr> - <td class='c013'>Glass</td> - <td class='c015'>·087</td> - <td class='c015'>·087</td> - <td class='c015'>·087</td> - <td class='c016'>·087</td> - </tr> - <tr> - <td class='c013'>Water</td> - <td class='c015'>·034</td> - <td class='c015'>·034</td> - <td class='c015'>·034</td> - <td class='c016'>·034</td> - </tr> -</table> - -<p class='c006'>In the above table, the absorption in aluminium, glass and -water was too small to determine with accuracy the variation of λ -with distance traversed. It will be observed that, for the denser -substances, the coefficient of absorption decreases with the -distance through which the rays have passed. This indicates -that the rays are heterogeneous. The variation of λ is more -marked in heavy substances.</p> - -<p class='c006'><span class='pageno' id='Page_182'>182</span>Table B gives the values of λ divided by density for the -above numbers. If the absorption were directly proportional to -the density, the quotient would be the same in all cases.</p> - -<p class='c006'>TABLE B.</p> - -<p class='c006'>λ <i>divided by density</i>.</p> - -<table class='table13' > -<colgroup> -<col class='colwidth39'> -<col class='colwidth21'> -<col class='colwidth13'> -<col class='colwidth13'> -<col class='colwidth13'> -</colgroup> - <tr> - <th class='c013'>Substance</th> - <th class='c015'>I</th> - <th class='c015'>II</th> - <th class='c015'>III</th> - <th class='c016'>IV</th> - </tr> - <tr> - <td class='c013'> </td> - <td class='c015'> </td> - <td class='c015'> </td> - <td class='c015'> </td> - <td class='c016'> </td> - </tr> - <tr> - <td class='c013'>Platinum</td> - <td class='c015'>·054</td> - <td class='c015'> </td> - <td class='c015'> </td> - <td class='c016'> </td> - </tr> - <tr> - <td class='c013'>Mercury</td> - <td class='c015'>·053</td> - <td class='c015'>·048</td> - <td class='c015'>·039</td> - <td class='c016'>·036</td> - </tr> - <tr> - <td class='c013'>Lead</td> - <td class='c015'>·056</td> - <td class='c015'>·049</td> - <td class='c015'>·042</td> - <td class='c016'>·037</td> - </tr> - <tr> - <td class='c013'>Zinc</td> - <td class='c015'>·039</td> - <td class='c015'>·037</td> - <td class='c015'>·034</td> - <td class='c016'>·033</td> - </tr> - <tr> - <td class='c013'>Aluminium</td> - <td class='c015'>·038</td> - <td class='c015'>·038</td> - <td class='c015'>·038</td> - <td class='c016'>·038</td> - </tr> - <tr> - <td class='c013'>Glass</td> - <td class='c015'>·034</td> - <td class='c015'>·034</td> - <td class='c015'>·034</td> - <td class='c016'>·034</td> - </tr> - <tr> - <td class='c013'>Water</td> - <td class='c015'>·034</td> - <td class='c015'>·034</td> - <td class='c015'>·034</td> - <td class='c016'>·034</td> - </tr> -</table> - -<p class='c006'>The numbers in column I vary considerably, but the agreement -becomes closer in the succeeding columns, until in column IV the -absorption is very nearly proportional to the density.</p> - -<p class='c006'>It is seen that the absorption of all three types of rays from -radio-active substances is approximately proportional to the density -of the substance traversed—a relation first observed by Lenard for -the cathode rays. This law of absorption thus holds for both -positively and negatively electrified particles projected from the -radio-active substances, and also for the electromagnetic pulses -which are believed to constitute the γ rays; although the absorption -of the α rays, for example, is 10,000 times greater than for -the γ rays. We have seen in section 84 that the value of the -absorption constant λ for lead is 122 for the β rays from uranium. -The value for the γ rays from radium varies between ·64 and ·44, -showing that the γ rays are more than 200 times as penetrating -as the β rays.</p> - -<p class='c006'><a id='section107'></a> -<b>107. Nature of the rays.</b> In addition to their great -penetrating power, the γ rays differ from the α and β rays in not -being deflected to an appreciable degree by a magnetic or -electric field. In a strong magnetic field, it can be shown, using -the photographic method, that there is an abrupt discontinuity -between the β and γ rays, for the former are bent completely away -<span class='pageno' id='Page_183'>183</span>from the latter. This indicates that, as regards the action of a -magnetic field, there is no gradual transition of magnetic properties -between the β and γ rays. Paschen<a id='r171' href='#f171' class='c012'><sup>[171]</sup></a> has examined the γ rays in -a very intense magnetic field, and, from the absence of deflection -of these rays, has calculated that, if they consist of electrified -particles carrying an ionic charge, and projected with a velocity -approaching that of light, their apparent mass must be at least 45 -times greater than that of the hydrogen atom.</p> - -<p class='c006'>It now remains for us to consider whether the γ rays are -corpuscular in character, or whether they are a type of electromagnetic -pulse in the ether similar to Röntgen rays. They resemble -Röntgen rays in their great penetrating power and in their absence -of deflection in a magnetic field. Earlier experiments seemed to -indicate an important difference between the action of γ and X -rays. It is well known that ordinary X rays produce much greater -ionization in gases such as sulphuretted hydrogen and hydrochloric -acid gas, than in air, although the differences in density -are not large. For example, exposed to X rays, sulphuretted -hydrogen has six times the conductivity of air, while with γ rays -the conductivity only slightly exceeds that of air. The results -obtained by Strutt, in this connection, have already been given -in section 45. It is there shown that the relative conductivity of -gases exposed to γ rays (and also to α and β rays) is, in most cases, -nearly proportional to their relative densities; but, under X rays, -the relative conductivity for some gases and vapours is very much -greater than for the γ rays. It must be remembered, however, -that the results obtained by Strutt were for “soft X rays,” whose -penetrating power was very much less than that of the γ rays. -In order to see if the relative conductivity of gases produced by -X rays depended upon their penetrating power, A. S. Eve<a id='r172' href='#f172' class='c012'><sup>[172]</sup></a> made -some experiments with a very “hard” X ray bulb, which gave an -unusually penetrating type of rays.</p> - -<p class='c006'>The results of the measurements are shown in the table -below, where the conductivity for each type of rays is expressed -relative to air as unity. The results obtained for “soft” X rays -by Strutt and by Eve for γ rays are added for comparison.</p> - -<p class='c006'><span class='pageno' id='Page_184'>184</span>It is seen that the hard rays show a much closer agreement -than the soft rays with the density law found for the γ rays. The -high values previously obtained for the vapours of chloroform and -carbon tetrachloride are greatly reduced, and are very nearly the same -as for the γ rays. On the other hand, the vapour of methyl iodide -is an exception, and still shows a high conductivity. The γ rays -were, however, forty times as penetrating as the hard X rays, and -it is probable that the value of methyl iodide would be reduced -with still more penetrating X rays.</p> - -<p class='c006'><i>Relative conductivities of gases.</i></p> - -<table class='table18' > -<colgroup> -<col class='colwidth36'> -<col class='colwidth19'> -<col class='colwidth14'> -<col class='colwidth14'> -<col class='colwidth14'> -</colgroup> - <tr> - <th class='c013'>Gas</th> - <th class='c015'>Relative Density</th> - <th class='c015'>“Soft” X rays</th> - <th class='c015'>“Hard” X rays</th> - <th class='c016'>γ rays</th> - </tr> - <tr> - <td class='c013'> </td> - <td class='c015'> </td> - <td class='c015'> </td> - <td class='c015'> </td> - <td class='c016'> </td> - </tr> - <tr> - <td class='c013'>Hydrogen</td> - <td class='c015'>·07</td> - <td class='c015'>·11</td> - <td class='c015'>·42</td> - <td class='c016'>·19</td> - </tr> - <tr> - <td class='c013'>Air</td> - <td class='c015'>1·0</td> - <td class='c015'>1·0</td> - <td class='c015'>1·0</td> - <td class='c016'>1·0</td> - </tr> - <tr> - <td class='c013'>Sulphuretted Hydrogen</td> - <td class='c015'>1·2</td> - <td class='c015'>6</td> - <td class='c015'>·9</td> - <td class='c016'>1·23</td> - </tr> - <tr> - <td class='c013'>Chloroform</td> - <td class='c015'>4·3</td> - <td class='c015'>32</td> - <td class='c015'>4·6</td> - <td class='c016'>4·8</td> - </tr> - <tr> - <td class='c013'>Methyl Iodide</td> - <td class='c015'>5·0</td> - <td class='c015'>72</td> - <td class='c015'>13·5</td> - <td class='c016'>5·6</td> - </tr> - <tr> - <td class='c013'>Carbon Tetrachloride</td> - <td class='c015'>5·3</td> - <td class='c015'>45</td> - <td class='c015'>4·9</td> - <td class='c016'>5·2</td> - </tr> -</table> - -<p class='c006'>The hard X rays were found to give far more secondary -radiation than the γ rays, but this effect is probably also a -function of the penetrating power of the primary rays. It will be -seen later (<a href='#section112'>section 112</a>) that γ rays give rise to a secondary -radiation of the β ray type. This has also been observed for -the X rays.</p> - -<p class='c006'>Considering the experimental evidence as a whole, there is -undoubtedly a very marked similarity between the properties -of γ and X rays. The view that the γ rays are a type of very -penetrating X rays, also receives support from theoretical considerations. -We have seen (<a href='#section052'>section 52</a>) that the X rays are -believed to be electromagnetic pulses, akin in some respects to -short light waves, which are set up by the sudden stoppage of the -cathode ray particles. Conversely, it is also to be expected that -X rays will be produced at the sudden starting, as well as -at the sudden stopping, of electrons. Since most of the β -particles from radium are ejected from the radium atom with -velocities much greater than the cathode particles in a vacuum -tube, X rays of a very penetrating character will arise. But -<span class='pageno' id='Page_185'>185</span>the strongest argument in support of this view is derived from -an examination of the origin and connection of the β and γ rays -from radio-active substances. It will be shown later that the -α ray activity observed in radium arises from several disintegration -products, stored up in the radium, while the β and γ rays arise -only from one of these products named radium <i>C</i>. It is found, -too, that the activity measured by the γ rays is always proportional -to the activity measured by the β rays, although by separation of -the products the activity of the latter may be made to undergo -great variations in value.</p> - -<p class='c006'>Thus the intensity of the γ rays is always proportional to the -rate of expulsion of β particles, and this result indicates that there -is a close connection between the β and γ rays. Such a result is -to be expected if the β particle is the parent of the γ ray, for the -expulsion of each electron from radium will give rise to a narrow -spherical pulse travelling from the point of disturbance with the -velocity of light.</p> -<p class='c005'><b>108.</b> There is another possible hypothesis in regard to the -nature of these rays. It has been shown (sections 48 and 82) that -the apparent mass of an electron increases as the speed of light -is approached; theoretically it should be very great when the -velocity of the electron is exceedingly close to the velocity of -light. In such a case, a moving electron would be difficult to -deflect by a magnetic or electric field.</p> - -<p class='c006'>The view that the γ rays are electrons carrying a negative -charge and moving with a velocity nearly equal to that of light -has recently been advocated by Paschen<a id='r173' href='#f173' class='c012'><sup>[173]</sup></a>. He concluded from -experiment that the γ rays like the β rays carried a negative -charge. We have seen (<a href='#section085'>section 85</a>) that Seitz also observed that -a small negative charge was communicated to bodies on which the -γ rays impinged, but the magnitude of this charge was much -smaller than that observed by Paschen. I do not think that -much weight can be attached to observations that a small positive -or negative charge is communicated to bodies on which the γ rays -fall, for it will be shown later that a strong secondary radiation, -<span class='pageno' id='Page_186'>186</span>consisting in part of electrons, is set up during the passage of the -γ rays through matter. It is not improbable that the small -charge observed is not a direct result of the charge carried by -the γ rays, but is an indirect effect due to the secondary radiations -emitted from the surface of bodies. There is no doubt that a -thick lead vessel, completely enclosing a quantity of radium, -acquires a small positive charge, but this result would follow -whether the γ rays carry a charge or not, since the secondary -radiations from the lead surface consist of projected particles -which carry with them a negative charge.</p> - -<p class='c006'>On this corpuscular theory of the nature of the γ rays, each -electron must have a large apparent mass, or otherwise it would be -appreciably deflected by an intense magnetic field. The energy of -motion of the electron must, in consequence, be very great, and, if -the number of the electrons constituting the γ rays is of the same -order of magnitude as the number of the β particles, a large -heating effect is to be expected when the γ rays are stopped in -matter. Paschen<a id='r174' href='#f174' class='c012'><sup>[174]</sup></a> made some experiments on the heat emission -of radium due to the γ rays; he concluded that the γ rays were -responsible for more than half of the total heat emission of radium -and carried away energy at the rate of over 100 gram calories per -hour per gram of radium. This result was not confirmed by later -experiments of Rutherford and Barnes<a id='r175' href='#f175' class='c012'><sup>[175]</sup></a>, who found that the heating -effect of the γ rays could not be more than a few per cent. of the -total heat emission of radium. These results will be considered -later in <a href='#chap12'>chapter XII</a>.</p> - -<p class='c006'>The weight of evidence, both experimental and theoretical, at -present supports the view that the γ rays are of the same nature -as the X rays but of a more penetrating type. The theory that -the X rays consist of non-periodic pulses in the ether, set up when -the motion of electrons is arrested, has found most favour, although -it is difficult to provide experimental tests to decide definitely the -question. The strongest evidence in support of the wave nature -of the X rays is derived from the experiments of Barkla<a id='r176' href='#f176' class='c012'><sup>[176]</sup></a>, who -found that the amount of secondary radiation set up by the X rays -<span class='pageno' id='Page_187'>187</span>on striking a metallic surface depended on the orientation of the -X ray bulb. The rays thus showed evidence of a one-sidedness or -polarization which is only to be expected if the rays consist of -a wave motion in the ether.</p> -<h3 class='c001'>PART V.</h3> -<h4 class='c022'>Secondary Rays.</h4> -<p class='c005'><b>109. Production of secondary rays.</b> It has long been -known that Röntgen rays, when they impinge on solid obstacles, -produce secondary rays of much less penetrating power than the -incident rays. This was first shown by Perrin and has been -investigated in detail by Sagnac, Langevin, Townsend and others. -Thus it is not surprising that similar phenomena should be -observed for the radiation from radio-active substances. By -means of the photographic method, Becquerel<a id='r177' href='#f177' class='c012'><sup>[177]</sup></a> has made a close -study of the secondary radiations produced by radio-active substances. -In his earliest observations, he noticed that radiographs -of metallic objects were always surrounded by a diffuse border. -This effect is due to the secondary rays set up by the incident -rays at the surface of the screen.</p> - -<p class='c006'>The secondary rays produced by the α rays are very feeble. -They are best shown by polonium, which gives out only α rays, -so that the results are not complicated by the action of the -β rays. Strong secondary rays are set up at the point of -impact of the β or cathodic rays. Becquerel found that the -magnitude of this action depended greatly on the velocity of -the rays. The rays of lowest velocity gave the most intense -secondary action, while the penetrating rays gave, in comparison, -scarcely any secondary effect. In consequence of the presence of -this secondary radiation, the photographic impression of a screen -pierced with holes is not clear and distinct. In each case there is -a double photographic impression, due to the primary rays and the -secondary rays set up by them.</p> - -<p class='c006'>These secondary rays are deviable by a magnetic field, and in -turn produce tertiary rays and so on. The secondary rays are in all -cases more readily deviated and absorbed than the primary rays, -<span class='pageno' id='Page_188'>188</span>from which they arise. The very penetrating γ rays give rise to -secondary rays, which cause intense action on the photographic -plate. When some radium was placed in a cavity inside a deep -lead block, rectangular in shape, besides the impression due to the -direct rays through the lead, Becquerel observed that there was -also a strong impression due to the secondary rays emitted from -the surface of the lead. The action of these secondary rays on -the plate is so strong that the effect on the plate is, in many cases, -increased by adding a metal screen between the active material -and the plate.</p> - -<p class='c006'>The comparative photographic action of the primary and -secondary rays cannot be taken as a relative measure of the -intensity of their radiations. For example, only a small portion -of the energy of the β rays is in general absorbed in the sensitive -film. Since the secondary rays are far more easily absorbed than -the primary rays, a far greater proportion of their energy is expended -in producing photographic action than in the case of the -primary rays. It is thus not surprising that the secondary rays -set up by the β and γ rays may in some cases produce a photographic -impression comparable with, if not greater than, the effect -of the incident rays.</p> - -<p class='c006'>On account of these secondary rays, radiographs produced by -the β rays of radium in general show a diffuse border round the -shadow of the object. For this reason radiographs of this kind -lack the sharpness of outline of X ray photographs.</p> -<p class='c005'><b>110. Secondary radiation produced by α rays</b>. -Mme Curie<a id='r178' href='#f178' class='c012'><sup>[178]</sup></a> has shown by the electric method that the α rays -of polonium produce secondary rays. The method adopted was to -compare the ionization current between two parallel plates, when -two screens of different material, placed over the polonium, were -interchanged.</p> - -<p class='c006'>These results show that the α rays of polonium are modified in -passing through matter, and that the amount of secondary rays set -up varies with screens of different material. Mme Curie, using the -same method, was unable to observe any such effect for the β rays -of radium. The production of secondary rays by the β rays of -<span class='pageno' id='Page_189'>189</span>radium is, however, readily shown by the photographic method. -We have already seen (<a href='#section093'>section 93</a>) that very low velocity electrons -accompany the α rays from radium or radio-tellurium spread on a -metal plate. These electrons are probably liberated when the -α rays escape from or impinge upon matter, and the number -emitted depends upon the kind of matter used as a screen. The -differences shown in the above table when the screens were interchanged -are explained simply in this way.</p> - -<table class='table8' > -<colgroup> -<col class='colwidth50'> -<col class='colwidth25'> -<col class='colwidth25'> -</colgroup> - <tr> - <th class='c013'>Screens employed</th> - <th class='c013'>Thickness in mms.</th> - <th class='c014'>Current observed</th> - </tr> - <tr> - <td class='c013'> </td> - <td class='c013'> </td> - <td class='c014'> </td> - </tr> - <tr> - <td class='c013'>Aluminium</td> - <td class='c013'>0·01</td> - <td class='c014'> </td> - </tr> - <tr> - <td class='c013'>Cardboard</td> - <td class='c013'>0·005</td> - <td class='c014'>17·9</td> - </tr> - <tr> - <td class='c013'> </td> - <td class='c013'> </td> - <td class='c014'> </td> - </tr> - <tr> - <td class='c013'>Cardboard</td> - <td class='c013'>0·005</td> - <td class='c014'> </td> - </tr> - <tr> - <td class='c013'>Aluminium</td> - <td class='c013'>0·01</td> - <td class='c014'>6·7</td> - </tr> - <tr> - <td class='c013'> </td> - <td class='c013'> </td> - <td class='c014'> </td> - </tr> - <tr> - <td class='c013'>Aluminium</td> - <td class='c013'>0·01</td> - <td class='c014'> </td> - </tr> - <tr> - <td class='c013'>Tin</td> - <td class='c013'>0·005</td> - <td class='c014'>150</td> - </tr> - <tr> - <td class='c013'> </td> - <td class='c013'> </td> - <td class='c014'> </td> - </tr> - <tr> - <td class='c013'>Tin</td> - <td class='c013'>0·005</td> - <td class='c014'> </td> - </tr> - <tr> - <td class='c013'>Aluminium</td> - <td class='c013'>0·01</td> - <td class='c014'>126</td> - </tr> - <tr> - <td class='c013'> </td> - <td class='c013'> </td> - <td class='c014'> </td> - </tr> - <tr> - <td class='c013'>Tin</td> - <td class='c013'>0·005</td> - <td class='c014'> </td> - </tr> - <tr> - <td class='c013'>Cardboard</td> - <td class='c013'>0·005</td> - <td class='c014'>13·9</td> - </tr> - <tr> - <td class='c013'> </td> - <td class='c013'> </td> - <td class='c014'> </td> - </tr> - <tr> - <td class='c013'>Cardboard</td> - <td class='c013'>0·005</td> - <td class='c014'> </td> - </tr> - <tr> - <td class='c013'>Tin</td> - <td class='c013'>0·005</td> - <td class='c014'>4·4</td> - </tr> -</table> - -<div id='fig045' class='figcenter id006'> -<img src='images/fig-045.png' alt='Fig. 45.' class='ig001'> -<div class='ic002'> -<p>Fig. 45.</p> -</div> -</div> -<p class='c005'><a id='section111'></a> -<b>111. Secondary rays produced by</b> β <b>and</b> γ <b>rays</b>. An -examination of the amount and character of the secondary -radiation emitted by various substances, when exposed to the -<span class='pageno' id='Page_190'>190</span>β and γ rays of radium, has recently been made by A. S. Eve<a id='r179' href='#f179' class='c012'><sup>[179]</sup></a>. -The general experimental method employed is shown in <a href='#fig045'>Fig. 45</a>.</p> - -<p class='c006'>The electroscope (<a href='#fig045'>Fig. 45</a>) was placed behind a lead screen -4·5 cms. thick, which stopped all the β rays and absorbed the -greater proportion of the γ rays from the radium tube placed at <i>R</i>. -On bringing near a plate of matter <i>M</i>, the primary rays fell upon -it and some of the secondary rays, emitted in all directions, passed -into the side of the electroscope, which was covered with aluminium -foil of thickness ·05 mm. Before the plate <i>M</i> was placed in position -the rate of discharge of the electroscope was due to the natural leak -and the γ rays from <i>R</i>, and the secondary radiation from the air. -On bringing the radiator <i>M</i> into position, the rate of discharge -was much increased, and the difference between the rate of -movement of the gold-leaf in the two cases was taken as a -measure of the amount of secondary rays from <i>M</i>. The absorption -of the secondary rays was tested by placing an aluminium plate -·85 mm. thick before the face of the electroscope.</p> - -<p class='c006'>The secondary rays were found to be fairly homogeneous, for -the intensity fell off according to an exponential law with the -distance traversed. The value of the absorption constant λ was -determined from the usual equation</p> - -<div class='figcenter id010'> -<img src='images/form-033.png' alt='Formula.' class='ig001'> -</div> - -<p class='c006'>where <i>d</i> is the -thickness of the screen. The table given below shows the results -obtained when thick plates of different substances of the same -dimensions were placed in a definite position at <i>M</i>. The secondary -radiation from fluids was obtained by a slight alteration of the -experimental arrangements.</p> - -<p class='c006'>Thirty milligrammes of radium bromide were used, and the -results are expressed in terms of the number of scale divisions -passed over per second by the gold-leaf.</p> - -<p class='c006'>It will be noticed that the amount of secondary radiation -follows in most cases the same order as the densities, and is -greatest for mercury. The value of (secondary radiation)/density is not -a constant, but varies considerably, being greatest for light -substances. The absorption constant of the secondary rays -from different radiators is not very different, with the exception -<span class='pageno' id='Page_191'>191</span>of substances such as granite, brick, and cement, which give out -secondary rays of nearly twice the penetrating power of other -substances.</p> - -<p class='c006'>β <i>and</i> γ <i>rays</i>.</p> - -<table class='table2' > -<colgroup> -<col class='colwidth26'> -<col class='colwidth17'> -<col class='colwidth19'> -<col class='colwidth17'> -<col class='colwidth19'> -</colgroup> - <tr> - <th class='c013'>Radiator</th> - <th class='c015'>Density</th> - <th class='c015'>Secondary Radiation</th> - <th class='c015'>Sec. Rad. / Density</th> - <th class='c014'>Aluminium ·085 cm. λ</th> - </tr> - <tr> - <td class='c013'> </td> - <td class='c015'> </td> - <td class='c015'> </td> - <td class='c015'> </td> - <td class='c014'> </td> - </tr> - <tr> - <td class='c013'>Mercury</td> - <td class='c015'>13·6</td> - <td class='c015'>147</td> - <td class='c015'>10·8</td> - <td class='c014'> </td> - </tr> - <tr> - <td class='c013'>Lead</td> - <td class='c015'>11·4</td> - <td class='c015'>141</td> - <td class='c015'>12·4</td> - <td class='c014'>18·5</td> - </tr> - <tr> - <td class='c013'>Copper</td> - <td class='c015'>8·8</td> - <td class='c015'>79</td> - <td class='c015'>9·0</td> - <td class='c014'>20</td> - </tr> - <tr> - <td class='c013'>Brass</td> - <td class='c015'>8·4</td> - <td class='c015'>81</td> - <td class='c015'>9·6</td> - <td class='c014'>21</td> - </tr> - <tr> - <td class='c013'>Iron (wrought)</td> - <td class='c015'>7·8</td> - <td class='c015'>75</td> - <td class='c015'>9·6</td> - <td class='c014'>20</td> - </tr> - <tr> - <td class='c013'>Tin</td> - <td class='c015'>7·4</td> - <td class='c015'>73</td> - <td class='c015'>9·9</td> - <td class='c014'>20·3</td> - </tr> - <tr> - <td class='c013'>Zinc</td> - <td class='c015'>7·0</td> - <td class='c015'>79</td> - <td class='c015'>11·3</td> - <td class='c014'> </td> - </tr> - <tr> - <td class='c013'>Granite</td> - <td class='c015'>2·7</td> - <td class='c015'>54</td> - <td class='c015'>20·0</td> - <td class='c014'>12·4</td> - </tr> - <tr> - <td class='c013'>Slate</td> - <td class='c015'>2·6</td> - <td class='c015'>53</td> - <td class='c015'>20·4</td> - <td class='c014'>12·1</td> - </tr> - <tr> - <td class='c013'>Aluminium</td> - <td class='c015'>2·6</td> - <td class='c015'>42</td> - <td class='c015'>16·1</td> - <td class='c014'>24</td> - </tr> - <tr> - <td class='c013'>Glass</td> - <td class='c015'>2·5</td> - <td class='c015'>44</td> - <td class='c015'>17·6</td> - <td class='c014'>24</td> - </tr> - <tr> - <td class='c013'>Cement</td> - <td class='c015'>2·4</td> - <td class='c015'>47</td> - <td class='c015'>19·6</td> - <td class='c014'>13·5</td> - </tr> - <tr> - <td class='c013'>Brick</td> - <td class='c015'>2·2</td> - <td class='c015'>49</td> - <td class='c015'>22·3</td> - <td class='c014'>13·0</td> - </tr> - <tr> - <td class='c013'>Ebonite</td> - <td class='c015'>1·1</td> - <td class='c015'>32</td> - <td class='c015'>29·1</td> - <td class='c014'>26</td> - </tr> - <tr> - <td class='c013'>Water</td> - <td class='c015'>1·0</td> - <td class='c015'>24</td> - <td class='c015'>24·0</td> - <td class='c014'>21</td> - </tr> - <tr> - <td class='c013'>Ice</td> - <td class='c015'>·92</td> - <td class='c015'>26</td> - <td class='c015'>28·2</td> - <td class='c014'> </td> - </tr> - <tr> - <td class='c013'>Paraffin solid</td> - <td class='c015'>·9</td> - <td class='c015'>17</td> - <td class='c015'>18·8</td> - <td class='c014'>21</td> - </tr> - <tr> - <td class='c013'>„ liquid</td> - <td class='c015'>·85</td> - <td class='c015'>16</td> - <td class='c015'>18·8</td> - <td class='c014'> </td> - </tr> - <tr> - <td class='c013'>Mahogany</td> - <td class='c015'>·56</td> - <td class='c015'>21·4</td> - <td class='c015'>38·2</td> - <td class='c014'>23</td> - </tr> - <tr> - <td class='c013'>Paper</td> - <td class='c015'>·4?</td> - <td class='c015'>21·0</td> - <td class='c015'>52</td> - <td class='c014'>22</td> - </tr> - <tr> - <td class='c013'>Millboard</td> - <td class='c015'>·4?</td> - <td class='c015'>19·4</td> - <td class='c015'>48</td> - <td class='c014'>20·5</td> - </tr> - <tr> - <td class='c013'>Papier-mâché</td> - <td class='c015'>...</td> - <td class='c015'>21·9</td> - <td class='c015'> </td> - <td class='c014'> </td> - </tr> - <tr> - <td class='c013'>Basswood</td> - <td class='c015'>·36</td> - <td class='c015'>20·7</td> - <td class='c015'>57</td> - <td class='c014'>22</td> - </tr> - <tr> - <td class='c013'>Pine</td> - <td class='c015'>·35</td> - <td class='c015'>21·8</td> - <td class='c015'>62</td> - <td class='c014'>21</td> - </tr> - <tr> - <td class='c013'> </td> - <td class='c015'> </td> - <td class='c015'> </td> - <td class='c015'> </td> - <td class='c014'> </td> - </tr> - <tr> - <td class='c013'>X ray screen</td> - <td class='c015'> </td> - <td class='c015'>75·2</td> - <td class='c015'> </td> - <td class='c014'>23·6</td> - </tr> -</table> - -<p class='c006'>The secondary radiation not only comes from the surface of -the radiator but from a considerable depth. The amount of -secondary rays increases with the thickness of the radiator, -and, in the case of glass and aluminium, reaches a practical -maximum for a plate about 3 mms. thick.</p> - -<p class='c006'>In the above table, the secondary radiation arises from both -the β rays and γ rays together. When the β rays were cut off by -a layer of lead 6·3 mms. thick, placed between the radium and the -radiator, the effect on the electroscope was reduced to less than -20 per cent. of its former value, showing that the β rays supplied -<span class='pageno' id='Page_192'>192</span>more than 80 per cent. of the secondary radiation. The following -table shows the relative amount of secondary rays from different -substances when exposed to β and γ rays together and to γ rays -alone. The amount from lead in each case is taken as a standard -and equal to 100. The amount of secondary radiation found by -Townsend from soft X rays is added for comparison.</p> - -<p class='c006'><i>Secondary Radiations.</i></p> - -<table class='table19' > -<colgroup> -<col class='colwidth34'> -<col class='colwidth19'> -<col class='colwidth19'> -<col class='colwidth26'> -</colgroup> - <tr> - <th class='c013'>Radiator</th> - <th class='c015'>β and γ rays</th> - <th class='c015'>γ rays</th> - <th class='c016'>Röntgen</th> - </tr> - <tr> - <td class='c013'> </td> - <td class='c015'> </td> - <td class='c015'> </td> - <td class='c016'> </td> - </tr> - <tr> - <td class='c013'>Lead</td> - <td class='c015'>100</td> - <td class='c015'>100</td> - <td class='c016'>100</td> - </tr> - <tr> - <td class='c013'>Copper</td> - <td class='c015'>57</td> - <td class='c015'>61</td> - <td class='c016'>291</td> - </tr> - <tr> - <td class='c013'>Brass</td> - <td class='c015'>58</td> - <td class='c015'>59</td> - <td class='c016'>263</td> - </tr> - <tr> - <td class='c013'>Zinc</td> - <td class='c015'>57</td> - <td class='c015'>...</td> - <td class='c016'>282</td> - </tr> - <tr> - <td class='c013'>Aluminium</td> - <td class='c015'>30</td> - <td class='c015'>30</td> - <td class='c016'>25</td> - </tr> - <tr> - <td class='c013'>Glass</td> - <td class='c015'>31</td> - <td class='c015'>35</td> - <td class='c016'>31</td> - </tr> - <tr> - <td class='c013'>Paraffin</td> - <td class='c015'>12</td> - <td class='c015'>20</td> - <td class='c016'>125</td> - </tr> -</table> - -<p class='c006'>It will be observed that the relative amounts are about the -same for the γ rays alone as for the β and γ rays together. On -the other hand, the amount of secondary radiation set up by -X rays is very different, lead for example giving much less than -brass or copper. The secondary rays from the γ rays alone are -slightly less penetrating than for the β and γ rays together, but -are far more penetrating than the secondary radiation from the -X rays examined by Townsend.</p> - -<p class='c006'>The amount of secondary radiation set up by the β and γ rays -is mainly independent of the state of the surface of the radiator. -About the same amount is obtained from iron as from iron filings; -from liquid as from solid paraffin; and from ice as from water<a id='r180' href='#f180' class='c012'><sup>[180]</sup></a>.</p> - -<p class='c006'>Becquerel has shown that the secondary rays set up by -the β rays are deflected by a magnet and consist of negatively -<span class='pageno' id='Page_193'>193</span>charged particles (electrons). It has been pointed out in -<a href='#section052'>section 52</a> that the cathode rays are diffusely reflected from the metal -on which they fall. These secondary rays consist in part of -electrons moving with about the same velocity as the primary, and -in part of some electrons with a much slower speed. The secondary -rays set up by the β rays of radium have on an average less -penetrating power than the primary rays, and consequently less -velocity than the primary rays. It must be remembered that the -β rays from radium are very complex, and consist of electrons -projected with a considerable range of velocities. The secondary -rays are, on an average, certainly more penetrating than the most -easily absorbed β rays emitted from radium, and probably move -with a velocity of about half that of light.</p> - -<p class='c006'>It is still uncertain whether the secondary rays are produced -by the action of the primary rays on matter, or whether -they consist of a portion of the primary rays whose direction -of motion has been deflected in their passage through matter, so -that they emerge again with diminished velocity from the surface.</p> -<p class='c005'><a id='section112'></a> -<b>112. Magnetic deflection of secondary rays from γ rays</b>. -It has been seen that the secondary rays set up by the γ rays -alone are very similar in character to those caused by the β rays. -This result was still further confirmed by Eve, who showed that -the secondary rays produced by the γ rays are readily deflected -by a magnetic field. The experimental arrangement is shown in -<a href='#fig046'>Fig. 46</a>.</p> - -<div id='fig046' class='figcenter id007'> -<img src='images/fig-046.png' alt='Fig. 46.' class='ig001'> -<div class='ic002'> -<p>Fig. 46.</p> -</div> -</div> - -<p class='c006'>A small electroscope was mounted on one side of a lead -platform 1·2 cms. thick, which rested on a lead cylinder 10 cms. -high and 10 cms. in diameter. The radium was placed at the -bottom of a hole reaching to the centre of the cylinder.</p> - -<p class='c006'>On applying a strong magnetic field, at right angles to the -plane of the paper, so as to bend the secondary rays from the -platform towards the electroscope, the rate of discharge was much -increased. On reversing the field, the effect was much diminished. -Since the γ rays are not themselves deflected by a magnetic field, -this result shows that the secondary radiation is quite different in -character from the primary rays, and consists of electrons projected -with a velocity (deduced from the penetrating power) of about half -<span class='pageno' id='Page_194'>194</span>the velocity of light. We have already pointed out that the -emission of electrons from a substance traversed by the rays will -account sufficiently well for the charge observed by Paschen, -without the necessity of assuming that the γ rays carry a negative -charge of electricity.</p> - -<p class='c006'>The secondary radiation set up by Röntgen rays, like that due -to the β and γ rays, consists in part of electrons projected with -considerable velocity. These three types of rays seem about equally -efficient in causing the expulsion of electrons from the substance -through which they pass. We have seen that the X and γ rays -are, in all probability, electromagnetic pulses set up by the sudden -starting or stopping of electrons, and, since these rays in turn cause -the removal of electrons, the process appears to be reversible. Since -the β rays pass through some thickness of matter before their energy -of motion is arrested, theory would lead us to expect that a type of -soft X rays should be generated in the absorbing matter.</p> -<div> - <span class='pageno' id='Page_195'>195</span> - <h3 class='c001'>PART VI.</h3> -</div> -<p class='c005'><b>113. Comparison of the ionization produced by the α -and β rays</b>. With unscreened active material the ionization -produced between two parallel plates, placed as in <a href='#fig017'>Fig. 17</a>, is mainly -due to the α rays. On account of the slight penetrating power of -the α rays, the current due to them practically reaches a maximum -with a small thickness of radio-active material. The following -saturation currents were observed<a id='r181' href='#f181' class='c012'><sup>[181]</sup></a> for different thicknesses of -uranium oxide between parallel plates sufficiently far apart for all -the α rays to be absorbed in the gas between them.</p> - -<p class='c006'><i>Surface of uranium oxide 38 sq. cms.</i></p> - -<table class='table8' > -<colgroup> -<col class='colwidth50'> -<col class='colwidth50'> -</colgroup> - <tr> - <th class='c013'>Weight of uranium oxide in grammes per sq. cm. of surface</th> - <th class='c014'>Saturation current in amperes per sq. cm. of surface</th> - </tr> - <tr> - <td class='c013'>.</td> - <td class='c014'> </td> - </tr> - <tr> - <td class='c013'>·0036</td> - <td class='c014'>1·7 × 10<sup>-13</sup></td> - </tr> - <tr> - <td class='c013'>·0096</td> - <td class='c014'>3·2 × 10<sup>-13</sup></td> - </tr> - <tr> - <td class='c013'>·0189</td> - <td class='c014'>4·0 × 10<sup>-13</sup></td> - </tr> - <tr> - <td class='c013'>·0350</td> - <td class='c014'>4·4 × 10<sup>-13</sup></td> - </tr> - <tr> - <td class='c013'>·0955</td> - <td class='c014'>4·7 × 10<sup>-13</sup></td> - </tr> -</table> - -<p class='c006'>The current reached about half its maximum value for a -weight of oxide ·0055 gr. per sq. cm. If the α rays are cut off -by a metallic screen, the ionization is then mainly due to the -β rays, since the ionization produced by the γ rays is small in -comparison. For the β rays from uranium oxide it has been -shown (<a href='#section086'>section 86</a>) that the current reaches half its maximum -value for a thickness of 0·11 gr. per sq. cm.</p> - -<p class='c006'>Meyer and Schweidler<a id='r182' href='#f182' class='c012'><sup>[182]</sup></a> have found that the radiation from -a water solution of uranium nitrate is very nearly proportional to -the amount of uranium present in the solution.</p> - -<p class='c006'>On account of the difference in the penetrating power of the α -and β rays, the ratio of the ionization currents produced by them -<span class='pageno' id='Page_196'>196</span>depends on the thickness of the radio-active layer under examination. -The following comparative values of the current due to the -α and β rays were obtained for very thin layers of active matter<a id='r183' href='#f183' class='c012'><sup>[183]</sup></a>. A -weight of ⅒ gramme of fine powder, consisting of uranium oxide, -thorium oxide, or radium chloride of activity 2000, was spread as -uniformly as possible over an area of 80 sq. cms. The saturation -current was observed between parallel plates 5·7 cms. apart. This -distance was sufficient to absorb most of the α rays from the active -substances. A layer of aluminium ·009 cm. thick absorbed all -the α rays.</p> - -<table class='table20' > -<colgroup> -<col class='colwidth18'> -<col class='colwidth27'> -<col class='colwidth27'> -<col class='colwidth27'> -</colgroup> - <tr> - <th class='c013'></th> - <th class='c015'>Current due to α rays</th> - <th class='c015'>Current due to β rays</th> - <th class='c016'>Ratio of currents β/α</th> - </tr> - <tr> - <td class='c013'> </td> - <td class='c015'> </td> - <td class='c015'> </td> - <td class='c016'> </td> - </tr> - <tr> - <td class='c013'>Uranium</td> - <td class='c015'>1</td> - <td class='c015'>1</td> - <td class='c016'>·0074</td> - </tr> - <tr> - <td class='c013'>Thorium</td> - <td class='c015'>1</td> - <td class='c015'>·27</td> - <td class='c016'>·0020</td> - </tr> - <tr> - <td class='c013'>Radium</td> - <td class='c015'>2000</td> - <td class='c015'>1350</td> - <td class='c016'>·0033</td> - </tr> -</table> - -<p class='c006'>In the above table the saturation current due to the α and -β rays of uranium is, in each case, taken as unity. The third -column gives the ratio of the currents observed for equal weights -of substance. The results are only approximate in character, for -the ionization due to a given weight of substance depends on its -fineness of division. In all cases, the current due to the β rays is -small compared with that due to the α rays, being greatest for -uranium and least for thorium. As the thickness of layer increases, -the ratio of currents β/α steadily increases to a constant value.</p> -<p class='c005'><a id='section114'></a> -<b>114. Comparison of the energy radiated by the α and -β rays</b>. It has not yet been found possible to measure directly -the energy of the α and β rays. A comparison of the energy -radiated in the two forms of rays can, however, be made indirectly -by two distinct methods.</p> - -<p class='c006'>If it be assumed that the same amount of energy is required to -produce an ion by either the α or the β ray, and that the same -proportion of the total energy is used up in producing ions, an -approximate estimate can be made of the ratio of the energy -<span class='pageno' id='Page_197'>197</span>radiated by the α and β rays by measuring the ratio of the total -number of ions produced by them. If λ is the coefficient of -absorption of the β rays in air, the rate of production of ions -per unit volume at a distance x from the source is</p> - -<div class='figcenter id010'> -<img src='images/form-048.png' alt='Formula.' class='ig001'> -</div> - -<p class='c006'>where -<i>q</i>₀ -is the rate of ionization at the source.</p> - -<p class='c006'>The total number of ions produced by complete absorption of -the rays is</p> - -<div class='figcenter id005'> -<img src='images/form-049.png' alt='Formula.' class='ig001'> -</div> - -<p class='c006'>Now λ is difficult to measure experimentally for air, but an -approximate estimate can be made of its value from the known -fact that the absorption of β rays is approximately proportional to -the density of any given substance.</p> - -<p class='c006'>For β rays from uranium the value of λ for aluminium is about -14, and λ divided by the density is 5·4. Taking the density of air -as ·0012, we find that for air</p> - -<p class='c006'>λ = ·0065.</p> - -<p class='c006'>The total number of ions produced in air is thus -154<i>q</i>₀ -when -the rays are completely absorbed.</p> - -<p class='c006'>Now from the above table the ionization due to the β rays -is ·0074 of that produced by α rays, when the β rays passed -through a distance of 5·7 cms. of air.</p> - -<p class='c006'>Thus we have approximately</p> - -<div class='calc'> - -<div class='lg-container-b c017'> - <div class='linegroup'> - <div class='group'> - <div class='line'>Total number of ions produced by β rays ·0074</div> - <div class='line'>--------------------------------------- = ----- × 154 = 0·20.</div> - <div class='line'>Total number of ions produced by α rays 5·7</div> - </div> - </div> -</div> - -</div> - -<p class='c018'>Therefore about ⅙ of the total energy radiated into air by a -thin layer of uranium is carried by the β rays or electrons. The -ratio for thorium is about ¹⁄₂₂ and for radium about ¹⁄₁₄, assuming -the rays to have about the same average value of λ.</p> - -<p class='c006'>This calculation takes into account only the energy which is -radiated out into the surrounding gas; but on account of the ease -with which the α rays are absorbed, even with a thin layer, the -greater proportion of the radiation is <i>absorbed by the radio-active -substance itself</i>. This is seen to be the case when it is recalled -that the α radiation of thorium or radium is reduced to half -value after passing through a thickness of about 0·0005 cm. of -<span class='pageno' id='Page_198'>198</span>aluminium. Taking into consideration the great density of the -radio-active substances, it is probable that most of the radiation -which escapes into the air is due to a thin skin of the powder not -much more than ·0001 cm. in thickness.</p> - -<hr class='c008'> - -<p class='c006'>An estimate, however, of the relative rate of emission of -energy by the α and β rays from a thick layer of material can be -made in the following way:—For simplicity suppose a thick layer -of radio-active substance spread uniformly over a large plane area. -There seems to be no doubt that the radiations are emitted -uniformly from each portion of the mass; consequently, the -radiation, which produces the ionizing action in the gas above -the radio-active layer, is the sum total of all the radiation which -reaches the surface of the layer.</p> - -<hr class='c008'> - -<p class='c006'>Let -λ<sub>1</sub> -be the average coefficient of absorption of the α rays in -the radio-active <i>substance itself</i> and σ the specific gravity of the -substance. Let -<i>E</i><sub>1</sub> -be the <i>total</i> energy radiated per sec. per unit -mass of the substance when the absorption of the rays in the -substance itself is disregarded. The energy per sec. radiated to -the upper surface by a thickness <i>dx</i> of a layer of unit area at a -distance <i>x</i> from the surface is given by</p> - -<div class='figcenter id009'> -<img src='images/form-050.png' alt='Formula.' class='ig001'> -</div> - -<p class='c006'>The total energy -<i>W</i><sub>1</sub> -per unit area radiated to the surface per -sec. by a thickness <i>d</i> is given by</p> - -<div class='figcenter id005'> -<img src='images/form-051.png' alt='Formula.' class='ig001'> -</div> - -<p class='c006'>if -λ<sub>1</sub><i>d</i> -is large.</p> - -<hr class='c008'> - -<p class='c006'>In a similar way it may be shown that the energy -<i>W</i><sub>2</sub> -of the -β rays reaching the surface is given by</p> - -<div class='figcenter id010'> -<img src='images/form-052.png' alt='Formula.' class='ig001'> -</div> - -<p class='c006'>where -<i>E</i><sub>2</sub> and λ<sub>2</sub> -are the values for the β rays corresponding to -<i>E</i><sub>1</sub> and λ<sub>1</sub> -for the -α rays. Thus it follows that</p> - -<div class='calc'> - -<div class='lg-container-b c017'> - <div class='linegroup'> - <div class='group'> - <div class='line'> <i>E</i><sub>1</sub> λ<sub>1</sub><i>W</i><sub>1</sub></div> - <div class='line'>---- = ------</div> - <div class='line'> <i>E</i><sub>2</sub> λ<sub>2</sub><i>W</i><sub>2</sub></div> - </div> - </div> -</div> - -</div> - -<p class='c018'><span class='pageno' id='Page_199'>199</span>λ<sub>1</sub> and λ<sub>2</sub> -are difficult to determine directly for the radio-active -substance itself, but it is probable that the ratio -λ<sub>1</sub>/λ<sub>2</sub> -is not very -different from the ratio for the absorption coefficients for another -substance like aluminium. This follows from the general result -that the absorption of both α and β rays is proportional to the -density of the substance; for it has already been shown in the -case of the β rays from uranium that the absorption of the rays in -the radio-active material is about the same as for non-radio-active -matter of the same density.</p> - -<p class='c006'>With a thick layer of uranium oxide spread over an area of -22 sq. cms., it was found that the saturation current between -parallel plates 6·1 cms. apart, due to the α rays, was 12·7 times -as great as the current due to the β rays. Since the α rays were -entirely absorbed between the plates and the total ionization -produced by the β rays is 154 times the value at the surface of the -plates,</p> - -<div class='calc'> - -<div class='lg-container-b c017'> - <div class='linegroup'> - <div class='group'> - <div class='line'> <i>W</i><sub>1</sub> total number of ions due to α rays</div> - <div class='line'>---- = ------------------------------------</div> - <div class='line'> <i>W</i><sub>2</sub> total number of ions due to β rays</div> - </div> - <div class='group'> - <div class='line in5'> 12·7 × 6·1</div> - <div class='line in4'>= ------------- = 0·5 approximately.</div> - <div class='line in7'> 154</div> - </div> - </div> -</div> - -</div> - -<p class='c018'>Now the value of -λ<sub>1</sub> -for aluminium is 2740 and of -λ<sub>2</sub> -for the -same metal 14, thus</p> - -<div class='calc'> - -<div class='lg-container-b c017'> - <div class='linegroup'> - <div class='group'> - <div class='line'> <i>E</i><sub>1</sub> λ<sub>1</sub><i>W</i><sub>1</sub></div> - <div class='line'>---- = ------- = 100 approximately</div> - <div class='line'> <i>E</i><sub>2</sub> λ<sub>2</sub><i>W</i><sub>2</sub></div> - </div> - </div> -</div> - -</div> - -<p class='c018'>This shows that the energy radiated from a thick layer of -material by the β rays is only about 1 per cent. of the energy -radiated in the form of α rays.</p> - -<p class='c006'>This estimate is confirmed by calculations based on independent -data. Let -<i>m</i><sub>1</sub>, <i>m</i><sub>2</sub> -be the masses of the α and β particles -respectively and -<i>v</i><sub>1</sub>, <i>v</i><sub>2</sub> -their velocities.</p> - -<div class='figcenter id007'> -<img src='images/form-053.png' alt='Formula.' class='ig001'> -</div> - -<p class='c006'>Now it has been shown that for the α rays of radium</p> - -<div class='calc'> - -<div class='lg-container-b c017'> - <div class='linegroup'> - <div class='group'> - <div class='line'><i>v</i><sub>1</sub> = 2·5 × 10<sup>9</sup>,</div> - <div class='line'> <i>e</i></div> - <div class='line in1'>--- = 6 × 10<sup>3</sup>.</div> - <div class='line'> <i>m</i><sub>1</sub></div> - </div> - </div> -</div> - -</div> - -<p class='c018'><span class='pageno' id='Page_200'>200</span>The velocity of the β rays of radium varies between wide -limits. Taking for an average value</p> - -<div class='calc'> - -<div class='lg-container-b c017'> - <div class='linegroup'> - <div class='group'> - <div class='line'><i>v</i><sub>2</sub> = 1·5 × 10<sup>10</sup>,</div> - <div class='line'> <i>e</i></div> - <div class='line in1'>---- = 1·8 × 10<sup>7</sup>,</div> - <div class='line'> <i>m</i><sub>1</sub></div> - </div> - </div> -</div> - -</div> - -<p class='c018'>it follows that the energy of the α particle from radium is almost -83 times the energy of the β particle. If equal numbers of α and -β particles are projected per second, the total energy radiated in -the form of α rays is about 83 times the amount in the form of -β rays.</p> - -<p class='c006'>Evidence will be given later (<a href='#section253'>section 253</a>) to show that -the number of α particles projected is probably four times the -number of β particles; so that a still greater proportion of the -energy is emitted in the form of α rays. These results thus lead -to the conclusion that, from the point of view of the energy -emitted, the α rays are far more important than the β rays. -This conclusion is supported by other evidence which is discussed in -chapters <a href='#chap12'><span class='fss'>XII</span></a> and <a href='#chap13'><span class='fss'>XIII</span></a>, where it will be shown that the α rays play by -far the most important part in the changes occurring in radio-active -bodies, and that the β rays only appear in the latter stages of the -radio-active processes. From data based on the relative absorption -and ionization of the β and γ rays in air, it can be shown that the -γ rays carry off about the same amount of energy as the β rays. -These conclusions are confirmed by direct measurement of the -heating effect of radium, which is discussed in detail in <a href='#chap12'>chapter <span class='fss'>XII</span></a>.</p> - -<div class='chapter'> - <span class='pageno' id='Page_201'>201</span> - <h2 id='chap05' class='c004'>CHAPTER V. <br> PROPERTIES OF THE RADIATIONS.</h2> -</div> -<p class='c005'><b>115.</b> Besides their power of acting on a photographic plate, -and of ionizing gases, the radiations from active bodies are able -to produce marked chemical and physical actions in various substances. -Most of these effects are due either to the α or β rays. -The γ rays produce little effect in comparison. Since the β rays -are similar in all respects to high velocity cathode rays, it is to be -expected that they will produce effects similar in character to -those produced by the cathode rays in a vacuum tube.</p> -<h3 class='c020'>Phosphorescent action.</h3> -<p class='c005'>Becquerel<a id='r184' href='#f184' class='c012'><sup>[184]</sup></a> has studied the action of radium rays in producing -phosphorescence in various bodies. The substance to be tested -was placed above the radium in the form of powder on a very thin -mica plate. Examination was made of the sulphides of calcium -and strontium, ruby, diamond, varieties of spar, phosphorus and -hexagonal blende. Substances like the ruby and spar, which phosphoresce -under luminous rays, did not phosphoresce under the -radium rays. On the other hand, those which were made luminous -by ultra-violet light were also luminous under the action of radium -rays. The radium rays show distinct differences from X rays. For -example, a diamond which was very luminous with radium rays -was unaffected by X rays. The double sulphate of uranium and -potassium is more luminous than hexagonal blende under X rays, -but the reverse is true for radium rays; under the influence of -these rays, sulphide of calcium gave a blue luminosity but was -hardly affected by X rays.</p> - -<p class='c006'><span class='pageno' id='Page_202'>202</span>The following table shows the relative phosphorescence excited -in various bodies.</p> - -<table class='table17' > -<colgroup> -<col class='colwidth40'> -<col class='colwidth30'> -<col class='colwidth30'> -</colgroup> - <tr> - <th class='c013'>Substance</th> - <th class='c015'>Without screen. Intensity</th> - <th class='c016'>Across screen of black paper</th> - </tr> - <tr> - <td class='c013'> </td> - <td class='c015'> </td> - <td class='c016'> </td> - </tr> - <tr> - <td class='c013'>Hexagonal blende</td> - <td class='c015'>13·36</td> - <td class='c016'>·04</td> - </tr> - <tr> - <td class='c013'>Platino-cyanide of barium</td> - <td class='c015'>1·99</td> - <td class='c016'>·05</td> - </tr> - <tr> - <td class='c013'>Diamond</td> - <td class='c015'>1·14</td> - <td class='c016'>·01</td> - </tr> - <tr> - <td class='c013'>Double sulphate of Uranium and Potassium</td> - <td class='c015'>1·00</td> - <td class='c016'>·31</td> - </tr> - <tr> - <td class='c013'>Calcium Fluoride</td> - <td class='c015'>·30</td> - <td class='c016'>·02</td> - </tr> -</table> - -<p class='c006'>In the last column the intensity without the screen is in each -case taken as unity. The great diminution of intensity after the -rays have passed through black paper shows that most of the phosphorescence -developed without the screen is, in the majority of -cases, due to the α rays.</p> - -<p class='c006'>Bary<a id='r185' href='#f185' class='c012'><sup>[185]</sup></a> has made a very complete examination of the class of -substances which become luminous under radium rays. He found -that the great majority of substances belong to the alkali metals -and alkaline earths. All these substances were also phosphorescent -under the action of X rays.</p> - -<p class='c006'>Crystalline zinc sulphide (Sidot’s blende) phosphoresces very -brightly under the influence of the rays from radium and other -very active substances. This was observed by Curie and Debierne -in their study of the radium emanation and the excited activity -produced by it. It has also been largely used by Giesel as an -optical means of detecting the presence of emanations from very -active substances. It is an especially sensitive means of detecting -the presence of α rays, when it exhibits the “scintillating” property -already discussed in section 96. In order to show the luminosity -due to the α rays, the screen should be held close to the active -substance, as the rays are absorbed in their passage through a few -centimetres of air. Zinc sulphide is also luminous under the action -of the β rays, but the phosphorescence is far more persistent than -when produced by the α rays.</p> - -<p class='c006'>Very beautiful luminous effects are produced by large crystals -of the platinocyanides exposed to the radium rays. Those -<span class='pageno' id='Page_203'>203</span>containing lithium give a brilliant pink colour. The calcium and -barium salts fluoresce with a deep green light, and the sodium compound -with a lemon yellow. The mineral willemite (zinc silicate) -was recently found by Kunz to be an even more sensitive means -of detecting the presence of the radiations than platinocyanide of -barium. It fluoresces showing a beautiful greenish colour, and a -piece of mineral exposed to the action of the rays appears quite -translucent. The crystals of the platinocyanides of barium and -lithium are especially suited for showing the action of the γ rays, -and, in this respect, are superior to willemite.</p> - -<p class='c006'>A very striking effect is shown by the mineral kunzite—a -new variety of spodumene discovered by Kunz<a id='r186' href='#f186' class='c012'><sup>[186]</sup></a>. This is a -transparent gem like crystal, often of very large size, which -glows with a beautiful reddish colour under the action of the β or -γ rays, but does not appear to be sensitive to the α rays. The -luminosity extends throughout the crystal, but is not so marked as -in the platinocyanides or willemite. The mineral sparteite<a id='r187' href='#f187' class='c012'><sup>[187]</sup></a>, a form -of calcite containing a few per cent. of manganese, has been found -by Ambrecht to fluoresce with a very deep orange light under the β -and γ rays. The colour appears to depend on the intensity of the -rays, and is deeper close to the radium than at some distance away.</p> - -<p class='c006'>If kunzite and sparteite are exposed to the action of the -cathode rays in a vacuum tube, the colour is different from that -produced by the radium rays. The former appears a deep yellow, -instead of the deep red observed with the radium rays.</p> - -<p class='c006'>The different actions of the radium rays on these fluorescent -substances can be illustrated very simply and beautifully by the -following experiment. A small U tube is filled with fragments of -the fluorescent substance arranged in layers. The U tube is -immersed in liquid air and the emanation from about 30 mgrs. -of radium bromide is condensed in the tube. On closing the tube -and removing it from the liquid air, the emanation distributes -itself uniformly in the tube. The shades of colour produced in the -different substances are clearly seen.</p> - -<p class='c006'>It is observed that all the crystals increase in luminosity -for several hours, on account of the excited activity produced -<span class='pageno' id='Page_204'>204</span>by the emanation. This effect is especially observed in kunzite, -which at first hardly responds to the rays, since the β and γ rays, -which causes it to fluoresce, are not given out by the emanation -itself but by one of its later products. The intensity of the -β and γ rays is, in consequence, small at first but rises to a -maximum after several hours; the luminosity observed varies in -a corresponding manner.</p> - -<p class='c006'>Sir William Crookes<a id='r188' href='#f188' class='c012'><sup>[188]</sup></a> has made an examination of the effect -of continued exposure of a diamond to the radium rays. An -“off-colour” diamond, of a pale yellow colour, was placed inside a -tube with radium bromide. After 78 days’ exposure, the diamond -had darkened and become bluish green in tint; when heated at -50° in a mixture of potassium chlorate for ten days, the diamond -lost its dull surface colour and was bright and transparent, and its -tint had changed to a pale bluish green. The rays have thus -a double action on the diamond; the less penetrating β rays -produce a superficial darkening due to the change of the surface -into graphite, while the more penetrating β rays and the γ rays -produce a change of colour throughout its mass. The diamond -phosphoresced brightly during the whole course of its exposure to -the rays. Crookes also observed that the diamond still retained -enough activity to affect a photographic plate 35 days after -removal, although, during the period of 10 days, it was heated -in a mixture sufficiently powerful to remove the outer skin of -graphite. This residual activity may possibly be due to a slow -transformation product of the emanation which is deposited on -the surface of bodies (see <a href='#chap11'>chapter <span class='fss'>XI</span></a>).</p> - -<p class='c006'>Marckwald observed that the α rays from radio-tellurium -produced marked phosphorescence on some kinds of diamonds. -An account of the various luminous effects produced on different -gems by exposure to the radium and actinium rays has been given -by Kunz and Baskerville<a id='r189' href='#f189' class='c012'><sup>[189]</sup></a>.</p> - -<p class='c006'>Both zinc sulphide and platinocyanide of barium diminish in -luminosity after exposure for some time to the action of the rays. -To regenerate a screen of the latter, exposure to solar light is -necessary. A similar phenomenon has been observed by Villard -<span class='pageno' id='Page_205'>205</span>for a screen exposed to Röntgen rays. Giesel made a screen of -platinocyanide of radio-active barium. The screen, very luminous -at first, gradually turned brown in colour, and at the same time -the crystals became dichroic. In this condition the luminosity -was much less, although the active substance had increased in -activity after preparation. Many of the substances which are -luminous under the rays from active substances lose this property -to a large extent at low temperatures<a id='r190' href='#f190' class='c012'><sup>[190]</sup></a>.</p> -<p class='c005'><b>116. Luminosity of radium compounds.</b> All radium -compounds are spontaneously luminous. This luminosity is especially -brilliant in the dry haloid salts, and persists for long -intervals of time. In damp air the salts lose a large amount of -their luminosity, but they recover it on drying. With very active -radium chloride, the Curies have observed that the light changes -in colour and intensity with time. The original luminosity is -recovered if the salt is dissolved and dried. Many inactive preparations -of radiferous barium are strongly luminous. The writer -has seen a preparation of impure radium bromide which gave out -a light sufficient to read by in a dark room. The luminosity of -radium persists over a wide range of temperature and is as bright -at the temperature of liquid air as at ordinary temperatures. A -slight luminosity is observed in a solution of radium, and if crystals -are being formed in the solution, they can be clearly distinguished -in the liquid by their greater luminosity.</p> -<p class='c005'><b>117. Spectrum of the phosphorescent light of radium -and actinium.</b> Compounds of radium, with a large admixture -of barium, are usually strongly self-luminous. This luminosity -decreases with increasing purity, and pure radium bromide is only -very feebly self-luminous. A spectroscopic examination of the -slight phosphorescent light of pure radium bromide has been -made by Sir William and Lady Huggins<a id='r191' href='#f191' class='c012'><sup>[191]</sup></a>. On viewing the light -with a direct vision spectroscope, there were faint indications of a -variation of luminosity at different points along the spectrum. In -<span class='pageno' id='Page_206'>206</span>order to get a photograph of the spectrum within a reasonable -time, they made use of a quartz spectroscope of special design -which had been previously employed in a spectroscopic examination -of faint celestial objects. After three days’ exposure with a -slit of ¹⁄₄₅₀ of an inch in width, a negative was obtained which -showed a number of bright lines. The magnified spectrum is -shown in <a href='#fig046a'>Fig. 46 <span class='fss'>A</span></a>. The lines of this spectrum were found to agree -not only in position but also in relative intensity with the band -spectrum of nitrogen. The band spectrum of nitrogen and also -the spark spectrum<a id='r192' href='#f192' class='c012'><sup>[192]</sup></a> of radium are shown in the same figure.</p> - -<p class='c006'>Some time afterwards Sir William Crookes and Prof. Dewar -showed that this spectrum of nitrogen was not obtained if the -radium was contained in a highly exhausted tube. Thus it -appears that the spectrum is due to the action of the radium rays -either on occluded nitrogen or the nitrogen in the atmosphere -surrounding the radium.</p> - -<p class='c006'>It is very remarkable that a phosphorescent light, like that of -radium bromide, should show a bright line spectrum of nitrogen. -It shows that radium at ordinary temperatures is able to set up -radiations which are produced only by the electric discharge under -special conditions.</p> - -<p class='c006'>Sir William and Lady Huggins were led to examine the -spectrum of the natural phosphorescent light of radium with the -hope that some indications might be obtained thereby of the -processes occurring in the radium atom. Since the main radiation -from radium consists of positively charged atoms projected with -great velocity, radiations must be set up both in the expelled body -and in the system from which it escapes.</p> - -<div id='fig046a' class='figcenter id004'> -<img src='images/fig-046a.png' alt='Fig. 46a.' class='ig001'> -<div class='ic002'> -<p>Fig. 46a.</p> -</div> -</div> - -<p class='c006'>Giesel<a id='r193' href='#f193' class='c012'><sup>[193]</sup></a> observed that the spectrum of the phosphorescent light -of actinium consists of three bright lines. Measurements of the -wave length were made by Hartmann<a id='r194' href='#f194' class='c012'><sup>[194]</sup></a>. The luminosity was very -slight and a long exposure was required. The lines observed were -<span class='pageno' id='Page_207'>207</span>in the red, blue and green. The wave length λ and velocity are -shown below.</p> - -<table class='table10' > -<colgroup> -<col class='colwidth14'> -<col class='colwidth28'> -<col class='colwidth57'> -</colgroup> - <tr> - <td class='c013'>Line</td> - <td class='c015'>Intensity</td> - <td class='c014'>λ</td> - </tr> - <tr> - <td class='c013'>1</td> - <td class='c015'>10</td> - <td class='c014'>4885·4 ± 0·1 Ångström units</td> - </tr> - <tr> - <td class='c013'>2</td> - <td class='c015'>6</td> - <td class='c014'>5300 ± 6 „</td> - </tr> - <tr> - <td class='c013'>3</td> - <td class='c015'>1</td> - <td class='c014'>5909 ± 10 „</td> - </tr> -</table> - -<p class='c006'>The line 4885 was very broad; the other two lines were so -feeble that it was difficult to determine their wave length with -accuracy. Hartmann suggests that these lines may be found in -the spectrum of the new stars. The lines observed have no -connection with radium or its emanation<a id='r195' href='#f195' class='c012'><sup>[195]</sup></a>.</p> -<p class='c005'><b>118. Thermo-luminescence.</b> E. Wiedemann and Schmidt<a id='r196' href='#f196' class='c012'><sup>[196]</sup></a> -have shown that certain bodies after exposure to the cathode rays -or the electric spark become luminous when they are heated to -a temperature much below that required to cause incandescence. -This property of thermo-luminescence is most strikingly exhibited -in certain cases where two salts, one of which is much in excess -of the other, are precipitated together. It is to be expected that -such bodies would also acquire the property when exposed to the -β or cathodic rays of radium. This has been found to be the case -by Wiedemann<a id='r197' href='#f197' class='c012'><sup>[197]</sup></a>. Becquerel showed that fluor-spar, exposed to the -radium rays, was luminous when heated. The glass tubes in which -radium is kept are rapidly blackened. On heating the tube, a -strong luminosity is observed, and the coloration to a large extent -disappears. The peculiarity of many of these bodies lies in the -fact that the property of becoming luminous when heated is retained -for a long interval of time after the body is removed from the -influence of the exciting cause. It appears probable that the rays -cause chemical changes in these bodies, which are permanent until -heat is applied. A portion of the chemical energy is then released -in the form of visible light.</p> -<h3 class='c020'>Physical actions.</h3> -<p class='c005'><b>119. Some electric effects.</b> Radium rays have the same -effect as ultra-violet light and Röntgen rays in increasing the -<span class='pageno' id='Page_208'>208</span>facility with which a spark passes between electrodes. Elster and -Geitel<a id='r198' href='#f198' class='c012'><sup>[198]</sup></a> showed that if two electrodes were separated by a distance -such that the spark just refused to pass, on bringing near a specimen -of radium the spark at once passes. This effect is best shown with -short sparks from a small induction coil. The Curies have observed -that radium completely enveloped by a lead screen 1 cm. -thick produces a similar action. The effect in that case is due to -the γ rays alone. This action of the rays can be very simply -illustrated by connecting two spark-gaps with the induction coil -in parallel. The spark-gap of one circuit is adjusted so that the -discharge just refuses to pass across it, but passes by the other. -When some radium is brought near the silent spark-gap, the spark -at once passes and ceases in the other<a id='r199' href='#f199' class='c012'><sup>[199]</sup></a>.</p> - -<p class='c006'>Hemptinne<a id='r200' href='#f200' class='c012'><sup>[200]</sup></a> found that the electrodeless discharge in a vacuum -tube began at a higher pressure when a strong preparation of -radium was brought near the tube. In one experiment the discharge -without the rays began at 51 mms. but with the radium -rays at 68 mms. The colour of the discharge was also altered.</p> - -<p class='c006'>Himstedt<a id='r201' href='#f201' class='c012'><sup>[201]</sup></a> found that the resistance of selenium was diminished -by the action of radium rays in the same way as by ordinary light.</p> - -<p class='c006'>F. Henning<a id='r202' href='#f202' class='c012'><sup>[202]</sup></a> examined the electrical resistance of a barium -chloride solution containing radium of activity 1000, but could -observe no appreciable difference between it and a similar pure -solution of barium chloride. This experiment shows that the -action of the rays from the radium does not produce any appreciable -change in the conductivity of the barium solution.</p> - -<p class='c006'>Kohlrausch and Henning<a id='r203' href='#f203' class='c012'><sup>[203]</sup></a> have recently made a detailed -examination of the conductivity of pure radium bromide solutions, -and have obtained results very similar to those for the corresponding -barium solutions. Kohlrausch<a id='r204' href='#f204' class='c012'><sup>[204]</sup></a> found that the conductivity -of water exposed to the radiations from radium increased -more rapidly than water which had not been exposed. -<span class='pageno' id='Page_209'>209</span>This increase of conductivity may have been due to an increase of -the conductivity of the water itself, or to an increased rate of -solution of the glass of the containing vessel.</p> - -<p class='c006'>Specimens of strongly active material have been employed to -obtain the potential at any point of the atmosphere. The ionization -due to the active substance is so intense that the body to which it -is attached rapidly takes up the potential of the air surrounding -the active substance. In this respect it is more convenient and -rapid in its action than the ordinary taper or water dropper, but -on account of the disturbance of the electric field by the strong -ionization produced, it is probably not so accurate a method as -that of the water dropper.</p> -<p class='c005'><b>120. Effect on liquid and solid dielectrics.</b> P. Curie<a id='r205' href='#f205' class='c012'><sup>[205]</sup></a> -made the very important observation that liquid dielectrics became -partial conductors under the influence of radium rays. In these -experiments the radium, contained in a glass tube, was placed in -an inner thin cylinder of copper. This was surrounded by a concentric -copper cylinder, and the liquid to be examined filled the -space between. A strong electric field was applied, and the current -through the liquid measured by means of an electrometer.</p> - -<p class='c006'>The following numbers illustrate the results obtained:</p> - -<table class='table8' > -<colgroup> -<col class='colwidth50'> -<col class='colwidth50'> -</colgroup> - <tr> - <th class='c013'>Substance</th> - <th class='c016'>Conductivity in megohms per 1 cm.<sup>3</sup></th> - </tr> - <tr> - <td class='c013'> </td> - <td class='c016'> </td> - </tr> - <tr> - <td class='c013'>Carbon bisulphide</td> - <td class='c016'>20 × 10<sup>-14</sup></td> - </tr> - <tr> - <td class='c013'>Petroleum ether</td> - <td class='c016'>15 „</td> - </tr> - <tr> - <td class='c013'>Amyline</td> - <td class='c016'>14 „</td> - </tr> - <tr> - <td class='c013'>Carbon chloride</td> - <td class='c016'>8 „</td> - </tr> - <tr> - <td class='c013'>Benzene</td> - <td class='c016'>4 „</td> - </tr> - <tr> - <td class='c013'>Liquid air</td> - <td class='c016'>1·3 „</td> - </tr> - <tr> - <td class='c013'>Vaseline oil</td> - <td class='c016'>1·6 „</td> - </tr> -</table> - -<p class='c006'>Liquid air, vaseline oil, petroleum ether, amyline, are normally -nearly perfect insulators. The conductivity of amyline and petroleum -ether due to the rays at -17° C. was only ⅒ of its -value at 0° C. There is thus a marked action of temperature -on the conductivity. For very active material the current was -<span class='pageno' id='Page_210'>210</span>proportional to the voltage. With material of only ¹⁄₅₀₀ of the -activity, it was found that Ohm’s law was not obeyed.</p> - -<p class='c006'>The following numbers were obtained:</p> - -<table class='table14' > -<colgroup> -<col class='colwidth50'> -<col class='colwidth50'> -</colgroup> - <tr> - <th class='c015'>Volts</th> - <th class='c016'>Current</th> - </tr> - <tr> - <td class='c015'>50</td> - <td class='c016'>109</td> - </tr> - <tr> - <td class='c015'>100</td> - <td class='c016'>185</td> - </tr> - <tr> - <td class='c015'>200</td> - <td class='c016'>255</td> - </tr> - <tr> - <td class='c015'>400</td> - <td class='c016'>335</td> - </tr> -</table> - -<p class='c006'>For an increase of voltage of 8 times, the current only increases -about 3 times. The current in the liquid thus tends to become -“saturated” as does the ordinary ionization current through a gas. -These results have an important bearing on the ionization theory, -and show that the radiation probably produces ions in the liquid as -well as in the gas. It was also found that X rays increased the -conductivity to about the same extent as the radium rays.</p> - -<p class='c006'>Becquerel<a id='r206' href='#f206' class='c012'><sup>[206]</sup></a> has recently shown that solid paraffin exposed to -the β and γ rays of radium acquires the property of conducting -electricity to a slight extent. After removal of the radium the -conductivity diminishes with time according to the same law as for -an ionized gas. These results show that a solid as well as a liquid -and gaseous dielectric is ionized under the influence of radium rays.</p> -<p class='c005'><b>121. Effect of temperature on the radiations.</b> Becquerel<a id='r207' href='#f207' class='c012'><sup>[207]</sup></a>, -by the electric method, determined the activity of uranium at the -temperature of liquid air, and found that it did not differ more -than 1 per cent. from the activity at ordinary temperatures. In -his experiments, the α rays from the uranium were absorbed before -reaching the testing vessel, and the electric current measured was -due to the β rays alone. P. Curie<a id='r208' href='#f208' class='c012'><sup>[208]</sup></a> found that the luminosity of -radium and its power of exciting fluorescence in bodies were -retained at the temperature of liquid air. Observations by the -electric method showed that the activity of radium was unaltered -at the temperature of liquid air. If a radium compound is heated -in an open vessel, it is found that the activity, measured by the -α rays, falls to about 25 per cent. of its original value. This is -however not due to a change in the radio-activity, but to the -release of the radio-active emanation, which is stored in the -<span class='pageno' id='Page_211'>211</span>radium. No alteration is observed if the radium is heated in -a closed vessel from which none of the radio-active products are -able to escape.</p> -<p class='c005'><b>122. Motion of radium in an electric field.</b> Joly<a id='r209' href='#f209' class='c012'><sup>[209]</sup></a> found -that a disc, one side of which is coated with a few milligrams of -radium bromide, exhibits, when an electrified body is brought -near it, motions very different to those observed in the case of -an inactive substance. The electrified body, whether positive or -negative, repels the suspended body if brought up to it on the -side coated with radium, but attracts it if presented to the naked -side.</p> - -<p class='c006'>This effect is very simply shown by constructing a small -apparatus like a radiometer. Two covered glasses are attached -to the end of a glass fibre about 6 cms. long, the surfaces lying in -the same plane. The apparatus is free to rotate on a pivot. The -two vanes are coated on alternate faces with radium bromide, and -the whole apparatus contained within a glass receiver. If an -electrified rod of ebonite or sealing wax is brought up close to -the receiver, a rotation is communicated to the vane which -increases as the pressure of the air is lowered to 5 or 6 cms. -of mercury. By placing the apparatus between parallel plates -connected with the terminals of a Wimshurst machine, a steady -rotation is communicated to the vanes. The rotation is always in -such a direction that the radium coated surface is repelled from -the electrified body.</p> - -<p class='c006'>This action was examined still further by attaching the vanes -to the glass beam of a Coulomb’s balance. A metal sphere, which -could be charged from without, was fixed facing the side coated -with radium. A repulsion was always observed except when the -charge was very strong and the vane near the sphere. If, however, -the two vanes were connected by a light wire and a similar -sphere placed exactly opposite the other, an attraction was -observed if one sphere was charged, but a repulsion if both -were charged. These effects were observed whether the vanes -were of aluminium or glass.</p> - -<p class='c006'>Joly found that the effect could not be explained by any direct -<span class='pageno' id='Page_212'>212</span>action due to the movement of the ions in an electric field. The -recoil, due to the expulsion of α particles from one side of the vane, -is far too small to account for the movement observed.</p> - -<p class='c006'>This effect can, I think, be simply accounted for by taking into -consideration the difference in conductivity of the gas on the two -sides of the radium coated vane. If a small vane, coated uniformly -with radium on both sides, and mounted on an insulating support, -be brought near a charged body kept at a constant potential, it acts -like a water dropper and rapidly acquires very nearly the average -potential which existed at that point before the vane was brought -up. The mechanical force acting on the vane will, in consequence, -be small. If, however, the vane is only coated with radium on the -side near the charged body, the ionization and consequently the -conductivity of the gas is much greater between the vane and the -charged body than on the opposite side. Suppose, for simplicity, -the body is charged to a positive potential. On account of the -greater conductivity of the gas on the side facing the charged -body, it will rapidly acquire a positive charge, and the potential of -the vane will reach a higher value than existed at that place -before the vane was introduced. This will result in a repulsion -of the vane. This also accounts for the attraction observed in the -experiment with the Coulomb’s balance already referred to. -Suppose that one sphere is positively charged and the other -earthed, and the two vanes metallically connected together. The -vane next to the charged body will become charged positively, but -this charge will be dissipated rapidly on account of the ionization -of the gas close to the opposite vane, and, in most conditions, this -loss of charge will be so rapid that the potential of the vane -is unable to reach the value which would exist at that place -in the field, if the vane were removed. There will, in consequence, -be an attracting force acting on the vane towards the sphere.</p> - -<p class='c006'>The repulsion observed by Joly is thus only an indirect result -of the ionization in the gas produced by the radium, and should -be shown under conditions where similar unequal distribution of -ionization is produced by any other sources.</p> - -<p class='c006'>Since radium gives out heat at a fairly rapid rate, a radiometer -in which the vanes were coated on one side with radium instead of -lampblack, should rotate at low pressure of the gas, even if no -<span class='pageno' id='Page_213'>213</span>source of light is brought near it. This should evidently be the -case, since the face coated with radium should reach a slightly -higher temperature than the other. This experiment has been -tried, but the effect seems too small to produce rotation of the -vanes.</p> -<h3 class='c020'>Chemical actions.</h3> -<p class='c005'><b>123.</b> Rays from active radium preparations change oxygen -into ozone<a id='r210' href='#f210' class='c012'><sup>[210]</sup></a>. Its presence can be detected by the smell or by -the action on iodide of potassium paper. This effect is due to the -α and β rays from the radium, and not to the luminous rays from -it. Since energy is required to produce ozone from oxygen, this -must be derived from the energy of the radiations.</p> - -<p class='c006'>The Curies found that radium compounds rapidly produced -coloration in glass. For moderately active material the colour -is violet, for more active material it is yellow. Long continued -action blackens the glass, although the glass may have no lead in -its composition. This coloration gradually extends through the -glass, and is dependent to some extent on the kind of glass used.</p> - -<p class='c006'>Giesel<a id='r211' href='#f211' class='c012'><sup>[211]</sup></a> found that he could obtain as much coloration in rock-salt -and fluor-spar by radium rays, as by exposure to the action of -cathode rays in a vacuum tube. The coloration, however, extended -much deeper than that produced by the cathode rays. This is to -be expected, since the radium rays have a higher velocity, and -consequently greater penetrating power, than the cathode rays -produced in an ordinary vacuum tube. Goldstein observed that -the coloration is far more intense and rapid when the salts are -melted or heated to a red heat. Melted potassium sulphate, -under the action of a very active preparation of radium, was -rapidly coloured a strong greenish blue which gradually changed -into a dark green. Salomonsen and Dreyer<a id='r212' href='#f212' class='c012'><sup>[212]</sup></a> found that plates of -quartz were coloured by exposure to radium rays. When examined -minutely, plates cut perpendicular to the optic axis showed the -presence of lines and striae, parallel to the binary axes. Adjacent -portions of the striated system differed considerably in intensity of -<span class='pageno' id='Page_214'>214</span>coloration and clearly revealed the heterogeneity of structures of -the crystal.</p> - -<p class='c006'>The cause of these colorations by cathode and radium rays -has been the subject of much discussion. Elster and Geitel<a id='r213' href='#f213' class='c012'><sup>[213]</sup></a> -observed that a specimen of potassium sulphate, coloured green by -radium rays, showed a strong photo-electric action, <i>i.e.</i> it rapidly -lost a negative charge of electricity when exposed to the action of -ultra-violet light. All substances coloured by cathode rays show -a strong photo-electric action, and, since the metals sodium and -potassium themselves show photo-electric action to a very remarkable -degree, Elster and Geitel have suggested that the colorations -are caused by a solid solution of the metal in the salt.</p> - -<p class='c006'>Although the coloration due to radium rays extends deeper -than that due to the cathode rays, when exposed to light the -colour fades away at about the same rate in the two cases.</p> - -<p class='c006'>Becquerel<a id='r214' href='#f214' class='c012'><sup>[214]</sup></a> found that white phosphorus is changed into the -red variety by the action of radium rays. This action was shown -to be due mainly to the β rays. The secondary radiation set up -by the primary rays also produced a marked effect. Radium rays, -like ordinary light rays, also caused a precipitate of calomel in the -presence of oxalic acid.</p> - -<p class='c006'>Hardy and Miss Wilcock<a id='r215' href='#f215' class='c012'><sup>[215]</sup></a> found that a solution of iodoform in -chloroform turned purple after exposure for 5 minutes to the rays -from 5 milligrams of radium bromide. This action is due to the -liberation of iodine. By testing the effect of screens of different -thicknesses, over the radium, this action was found to be mainly -due to the β rays from the radium. Röntgen rays produce a -similar coloration.</p> - -<p class='c006'>Hardy<a id='r216' href='#f216' class='c012'><sup>[216]</sup></a> also observed an action of the radium rays on the -coagulation of globulin. Two solutions of globulin from ox serum -were used, one made electro-positive by adding acetic acid, and the -other electro-negative by adding ammonia. When the globulin -was exposed close to the radium in naked drops, the opalescence of -the electro-positive solution rapidly diminished, showing that the -<span class='pageno' id='Page_215'>215</span>solution became more complete. The electro-negative solution was -rapidly turned to a jelly and became opaque. These actions were -found to be due to the α rays of radium alone.</p> - -<p class='c006'>This is further evidence in favour of the view that the α rays -consist of projected positively charged bodies of atomic dimensions, -for a similar coagulation effect is produced by the metallic ions of -liquid electrolytes, and has been shown by W. C. D. Whetham<a id='r217' href='#f217' class='c012'><sup>[217]</sup></a> to -be due to the electric charges carried by the ions.</p> -<p class='c005'><a id='section124'></a> -<b>124. Gases evolved from radium.</b> Curie and Debierne<a id='r218' href='#f218' class='c012'><sup>[218]</sup></a> -observed that radium preparations placed in a vacuum tube continually -lowered the vacuum. The gas evolved was always accompanied -by the emanation, but no new lines were observed in its -spectrum. Giesel<a id='r219' href='#f219' class='c012'><sup>[219]</sup></a> has observed a similar evolution of gas from -solutions of radium bromide. Giesel forwarded some active material -to Runge and Bödlander, in order that they might test the gas -spectroscopically. From 1 gram of a 5 per cent. radium preparation -they obtained 3·5 c.c. of gas in 16 days. This gas was found, -however, to be mainly hydrogen, with 12 per cent. of oxygen. In -later experiments Ramsay and Soddy<a id='r220' href='#f220' class='c012'><sup>[220]</sup></a> found that 50 milligrams of -radium bromide evolved gases at the rate of about 0·5 c.c. per day. -This is a rate of evolution about twice that observed by Runge -and Bödlander. On analysing the gases about 28·9 per cent. -consisted of oxygen, and the rest hydrogen. The slight excess -of hydrogen over that attained in the decomposition of water, they -consider to be due to the action of oxygen on the grease of the -stop-cocks. The radio-active emanation from radium has a strong -oxidizing action and rapidly produces carbon dioxide, if carbonaceous -matter is present. The production of gas is probably due to the -action of the radiations in decomposing water. The amount of -energy required to produce the rate of decomposition observed by -Ramsay and Soddy—about 10 c.c. per day for 1 gram of radium -bromide—corresponds to about 30 gram-calories per day. This -amount of energy is about two per cent. of the total energy emitted -in the form of heat.</p> - -<p class='c006'><span class='pageno' id='Page_216'>216</span>Ramsay and Soddy (<i>loc. cit.</i>) have also observed the presence of -helium in the gases evolved by solution of radium bromide. This -important result is considered in detail in <a href='#section267'>section 267</a>.</p> -<h3 class='c020'>Physiological actions.</h3> -<p class='c005'><b>125.</b> Walkhoff first observed that radium rays produce burns -of much the same character as those caused by Röntgen rays. -Experiments in this direction have been made by Giesel, Curie and -Becquerel, and others, with very similar results. There is at first -a painful irritation, then inflammation sets in, which lasts from 10 -to 20 days. This effect is produced by all preparations of radium, -and appears to be due mainly to the α and β rays.</p> - -<p class='c006'>Care has to be taken in handling radium on account of the -painful inflammation set up by the rays. If a finger is held for -some minutes at the base of a capsule containing a radium preparation, -the skin becomes inflamed for about 15 days and then peels -off. The painful feeling does not disappear for two months.</p> - -<p class='c006'>Danysz<a id='r221' href='#f221' class='c012'><sup>[221]</sup></a> found that this action is mainly confined to the skin, -and does not extend to the underlying tissue. Caterpillars subjected -to the action of the rays lost their power of motion in -several days and finally died.</p> - -<p class='c006'>Radium rays have been found beneficial in certain cases of -cancer. The effect is apparently similar to that produced by -Röntgen rays, but the use of radium possesses the great advantage -that the radiating source can be enclosed in a fine tube and introduced -at the particular point at which the action of the rays is -required. The rays have also been found to hinder or stop the -development of microbes<a id='r222' href='#f222' class='c012'><sup>[222]</sup></a>.</p> - -<p class='c006'>It would be out of place here to give an account of the -numerous experiments that have been made by physicists and -physiologists on the action of the rays of radium and of other -radio-active substances on different organisms, such as caterpillars, -mice and guinea-pigs. In some cases, the experiments have been -carried out by placing the organisms in an atmosphere impregnated -<span class='pageno' id='Page_217'>217</span>with the radium emanation. The effect of an exposure under such -conditions for several days or weeks has been found generally -harmful and in many cases fatal. The literature in this new -department of study is already large and is increasing rapidly.</p> - -<p class='c006'>Another interesting action of the radium rays has been observed -by Giesel. On bringing up a radium preparation to the -closed eye, in a dark room, a sensation of diffuse light is observed. -This effect has been examined by Himstedt and Nagel<a id='r223' href='#f223' class='c012'><sup>[223]</sup></a> who have -shown that it is due to a fluorescence produced by the rays in the -eye itself. The blind are able to perceive this luminosity if the -retina is intact, but not if the retina is diseased. Hardy and -Anderson<a id='r224' href='#f224' class='c012'><sup>[224]</sup></a> have examined this effect in some detail. The -sensation of light is produced both by the β and γ rays. The -eyelid practically absorbs all the β rays, so that the luminosity -observed with a closed eye is due to the γ rays alone. The lens -and retina of the eye are strongly phosphorescent under the action -of the β and γ rays. Hardy and Anderson consider that the -luminosity observed in a dark room with the open eye (the phosphorescent -light of the radium itself being stopped by black paper) -is to a large extent due to the phosphorescence set up in the -eyeball. The γ rays, for the most part, produce the sensation of -light when they strike the retina.</p> - -<p class='c006'>Tommasina stated that the air exhaled by man contained a -larger proportion of ions than ordinary air, and, in consequence, -caused an increased rate of discharge of an electroscope. The -experiment was repeated by Elster and Geitel but with negative -results. On the other hand, they found that the breath of -Dr Giesel, of Braunschweig, who had been engaged continuously -in the chemical separation of the radio-active bodies, caused a -rapid loss of charge of an electroscope. This increased rate of -discharge was probably mainly due to the radium emanation, with -which his system had become impregnated by inhaling the -emanation-laden air of the laboratory.</p> - -<div class='chapter'> - <span class='pageno' id='Page_218'>218</span> - <h2 id='chap06' class='c004'>CHAPTER VI. <br> CONTINUOUS PRODUCTION OF RADIO-ACTIVE MATTER.</h2> -</div> -<p class='c005'><b>126.</b> An account will now be given of some experiments -which have thrown much light, not only on the nature of the -processes which serve to maintain the radio-activity of the -radio-active bodies, but also on the source of the energy continuously -emitted by those bodies. In this chapter, for simplicity, -the radio-activity of uranium and thorium will alone be considered, -for it will be seen later that the changes taking place -in these two substances are typical of those which occur in all -radio-active substances.</p> - -<p class='c006'>We have seen (<a href='#section023'>section 23</a>) that there is some doubt whether -the radio-activity of thorium is due to that element itself, or to an -unknown radio-active constituent associated with it. This uncertainty, -however, will present no serious difficulty when we are -discussing the radio-activity of thorium, for the general conclusions -are, for the most part, independent of whether thorium is the -primary radio-active constituent or not. For simplicity, however, -it will be assumed for the present that the radio-activity is due to -thorium itself. If future research should definitely show that the -radio-activity, ordinarily observed in thorium, is due to a new -radio-active element mixed with it, the radio-active processes -considered will refer to this new element.</p> -<p class='c005'><a id='section127'></a> -<b>127. Uranium X.</b> The experiments of Mme Curie show -that the radio-activity of uranium and radium is an atomic phenomenon. -The activity of any uranium compound depends only -on the amount of that element present, and is unaffected by its -chemical combination with other substances, and is not appreciably -affected by wide variations of temperature. It would thus seem -<span class='pageno' id='Page_219'>219</span>probable, since the activity of uranium is a specific property of -the element, that the activity could not be separated from it by -chemical agencies.</p> - -<p class='c006'>In 1900, however, Sir William Crookes<a id='r225' href='#f225' class='c012'><sup>[225]</sup></a> showed that, by a single -chemical operation, uranium could be obtained photographically -inactive while the whole of the activity could be concentrated -in a small residue free from uranium. This residue, to which -he gave the name of Ur X, was many hundred times more active -photographically, weight for weight, than the uranium from which -it had been separated. The method employed for this separation -was to precipitate a solution of the uranium with ammonium carbonate. -On dissolving the precipitate in an excess of the reagent, a -light precipitate remained behind. This was filtered, and constituted -the Ur X. The active substance Ur X was probably present in -very small quantity, mixed with impurities derived from the -uranium. No new lines were observed in its spectrum. A partial -separation of the activity of uranium was also effected by -another method. Crystallized uranium nitrate was dissolved in -ether, when it was found that the uranium divided itself between -the ether and water present in two unequal fractions. The small -part dissolved in the water layer was found to contain practically -all the activity when examined by the photographic method, while -the other fraction was almost inactive. These results, taken by -themselves, pointed very strongly to the conclusion that the -activity of uranium was not due to the element itself, but to -some other substance, associated with it, which had distinct -chemical properties.</p> - -<p class='c006'>Results of a similar character were observed by Becquerel<a id='r226' href='#f226' class='c012'><sup>[226]</sup></a>. -It was found that barium could be made photographically very -active by adding barium chloride to the uranium solution and -precipitating the barium as sulphate. By a succession of precipitations -the uranium was rendered photographically almost inactive, -while the barium was strongly active.</p> - -<p class='c006'>The inactive uranium and the active barium were laid aside; -but, on examining them a year later, it was found <i>that the uranium -had completely regained its activity, while that of the barium had -<span class='pageno' id='Page_220'>220</span>completely disappeared</i>. The loss of activity of uranium was thus -only temporary in character.</p> - -<p class='c006'>In the above experiments, the activity of uranium was examined -by the photographic method. The photographic action produced -by uranium is due almost entirely to the β rays. The α rays, in -comparison, have little if any effect. Now the radiation from Ur X -consists entirely of β rays, and is consequently photographically -very active. If the activity of uranium had been measured -electrically without any screen over it, the current observed would -have been due very largely to the α rays, and little change would -have been observed after the removal of Ur X, since only the constituent -responsible for the β rays was removed. This important -point is discussed in more detail in <a href='#section205'>section 205</a>.</p> -<p class='c005'><b>128. Thorium X.</b> Rutherford and Soddy<a id='r227' href='#f227' class='c012'><sup>[227]</sup></a>, working with -thorium compounds, found that an intensely active constituent -could be separated from thorium by a single chemical operation. -If ammonia is added to a thorium solution, the thorium is precipitated, -but a large amount of the activity is left behind in the -filtrate, which is chemically free from thorium. This filtrate was -evaporated to dryness, and the ammonium salts driven off by -ignition. A small residue was obtained which, weight for weight, -was in some cases several thousand times more active than the -thorium from which it was obtained, while the activity of the -precipitated thorium was reduced to less than one half of its -original value. This active constituent was named Th X from -analogy to Crookes’ Ur X.</p> - -<p class='c006'>The active residue was found to consist mainly of impurities -from the thorium; the Th X could not be examined chemically, -and probably was present only in minute quantity. It was also -found that an active constituent could be partly separated from -thorium oxide by shaking it with water for some time. On -filtering the water, and evaporating down, a very active residue -was obtained which was analogous in all respects to Th X.</p> - -<p class='c006'>On examining the products a month later, it was found that -the <i>Th X was no longer active, while the thorium had completely -<span class='pageno' id='Page_221'>221</span>regained its activity</i>. A long series of measurements was then -undertaken to examine the time-rate of these processes of decay -and recovery of activity.</p> - -<div id='fig047' class='figcenter id006'> -<img src='images/fig-047.png' alt='Fig. 47.' class='ig001'> -<div class='ic002'> -<p>Fig. 47.</p> -</div> -</div> - -<p class='c006'>The results are shown graphically in <a href='#fig047'>Fig. 47</a>, where the final -activity of the thorium and the initial activity of the Th X are in -each case taken as 100. The ordinates represent the activities -determined by means of the ionization current, and the abscissae -represent the time in days. It will be observed that both curves -are irregular for the first two days. The activity of the Th X -increased at first, while the activity of the thorium diminished. -Disregarding these initial irregularities of the curves, which will be -<span class='pageno' id='Page_222'>222</span>explained in detail in <a href='#section208'>section 208</a>, it will be seen that, after the -first two days, the time taken for the thorium to recover half its -lost activity is about equal to the time taken by the Th X to lose -half its activity. This time in each case is about four days. The -percentage proportion of the activity regained by the thorium, over -any given interval, is approximately equal to the percentage proportion -of the activity lost by the Th X during the same interval.</p> - -<div id='fig048' class='figcenter id004'> -<img src='images/fig-048.png' alt='Fig. 48.' class='ig001'> -<div class='ic002'> -<p>Fig. 48.</p> -</div> -</div> - -<p class='c006'>If the recovery curve is produced backwards to meet the -vertical axis, it does so at a minimum of 25 per cent., and -the above conclusions hold more accurately, if the recovery is -assumed to start from this minimum. This is clearly shown by -<a href='#fig048'>Fig. 48</a>, where the percentages of activity recovered, reckoned -from the 25 per cent. minimum, are plotted as ordinates. In -the same figure the decay curve, after the second day, is shown -on the same scale. The activity of the Th X decays with the time -according to an exponential law, falling to half value in about -four days. If -<i>I</i>₀ -is the initial activity and <i>I<sub>t</sub></i> is the activity after -a time <i>t</i>, then</p> - -<div class='figcenter id010'> -<img src='images/form-054.png' alt='Formula.' class='ig001'> -</div> -<p class='c006'><span class='pageno' id='Page_223'>223</span>where λ is a constant and <i>e</i> the natural base of logarithms. The -experimental curve of the rise of activity from a minimum to a -maximum value is therefore expressed by the equation</p> - -<div class='figcenter id009'> -<img src='images/form-055.png' alt='Formula.' class='ig001'> -</div> - -<p class='c006'>where -<i>I</i>₀ -is the amount of activity recovered when the state of -constant activity is reached, <i>I<sub>t</sub></i> the activity recovered after a -time <i>t</i>, and λ is the <i>same constant</i> as before.</p> -<p class='c005'><a id='section129'></a> -<b>129. Uranium X.</b> Similar results were obtained when -uranium was examined. The Ur X was separated by Becquerel’s -method of successive precipitations with barium. The decay of -the separated activity and the recovery of the lost activity are -shown graphically in <a href='#fig049'>Fig. 49</a>. A more detailed discussion of this -experiment is given in <a href='#section205'>section 205</a>.</p> - -<div id='fig049' class='figcenter id006'> -<img src='images/fig-049.png' alt='Fig. 49.' class='ig001'> -<div class='ic002'> -<p>Fig. 49.</p> -</div> -</div> - -<p class='c006'>The curves of decay and recovery exhibit the same peculiarities -and can be expressed by the same equations as in the case of -thorium. The time-rate of decay and recovery is, however, much -slower than for thorium, the activity of the Ur X falling to half its -value in about 22 days.</p> - -<p class='c006'><span class='pageno' id='Page_224'>224</span>A large number of results of a similar character have been -obtained from other radio-active products, separated from the -radio-elements, but the cases of thorium and uranium will suffice -for the present to form a basis for the discussion of the processes -that are taking place in radio-active bodies.</p> -<p class='c005'><a id='section130'></a> -<b>130. Theory of the phenomena.</b> These processes of decay -and recovery go on at exactly the same rate if the substances are -removed from the neighbourhood of one another, or enclosed in -lead, or placed in a vacuum tube. It is at first sight a remarkable -phenomenon that the processes of decay and recovery should -be so intimately connected, although there is no possibility of -mutual interaction between them. These results, however, receive -a complete explanation on the following hypotheses:</p> - -<p class='c021'>(1) That there is a constant rate of production of fresh -radio-active matter by the radio-active body;</p> - -<p class='c011'>(2) That the activity of the matter so formed decreases -according to an exponential law with the time from -the moment of its formation.</p> - -<p class='c018'>Suppose that -<i>q</i>₀ -particles of new matter are produced per second -from a given mass of matter. The rate of emission of energy due -to the particles produced in the time <i>dt</i>, is, at the moment of their -formation, equal to -<i>Kq</i>₀<i>dt</i>, -where <i>K</i> is a constant.</p> - -<p class='c006'>It is required to find the activity due to the whole matter -produced after the process has continued for a time <i>T</i>.</p> - -<p class='c006'>The activity <i>dI</i>, due to the matter produced during the time <i>dt</i> -at the time <i>t</i>, decays according to an exponential law during the -time <i>T</i> – <i>t</i> that elapses before its activity is estimated, and in -consequence is given by</p> - -<div class='figcenter id005'> -<img src='images/form-056.png' alt='Formula.' class='ig001'> -</div> - -<p class='c006'>where λ is the constant of decay of activity of the active matter. -The activity <i>I<sub>T</sub></i> due to the whole matter produced in the time <i>T</i> is -thus given by</p> - -<div class='figcenter id002'> -<img src='images/form-057.png' alt='Formula.' class='ig001'> -</div> - -<p class='c006'><span class='pageno' id='Page_225'>225</span>The activity reaches a maximum value -<i>I</i>₀ -when <i>T</i> is very great, -and is then given by</p> - -<div class='calc'> - -<div class='lg-container-b c017'> - <div class='linegroup'> - <div class='group'> - <div class='line in6'><i>Kq</i>₀</div> - <div class='line'><i>I</i>₀ = ----</div> - <div class='line in7'>λ</div> - </div> - </div> -</div> - -</div> - -<p class='c018'>thus</p> - -<div class='figcenter id009'> -<img src='images/form-058.png' alt='Formula.' class='ig001'> -</div> - -<p class='c006'>This equation agrees with the experimental results for the -recovery of lost activity. Another method for obtaining this -equation is given later in <a href='#section133'>section 133</a>.</p> - -<p class='c006'>A state of equilibrium is reached when the rate of loss of -activity of the matter already produced is balanced by the activity -supplied by the production of new active matter. According to -this view, the radio-active bodies are undergoing change, but the -activity remains constant owing to the action of two opposing -processes. Now, if this active matter can at any time be separated -from the substance in which it is produced, the decay of -its activity, as a whole, should follow an exponential law with -the time, since each portion of the matter decreases in activity -according to an exponential law with the time, whatever its age -may be. If -<i>I</i>₀ -is the initial activity of the separated product, the -activity <i>I<sub>t</sub></i> after an interval <i>t</i> is given by</p> - -<div class='figcenter id009'> -<img src='images/form-059.png' alt='Formula.' class='ig001'> -</div> - -<p class='c006'>Thus, the two assumptions—of uniform production of active -matter and of the decay of its activity in an exponential law from -the moment of its formation—satisfactorily explain the relation -between the curves of decay and recovery of activity.</p> -<p class='c005'><b>131. Experimental evidence.</b> It now remains to consider -further experimental evidence in support of these hypotheses. -The primary conception is that the radio-active bodies are able to -produce from themselves matter of chemical properties different -from those of the parent substance, and that this process goes -on at a constant rate. This new matter initially possesses -the property of activity, and loses it according to a definite law. -The fact that a proportion of the activity of radium and thorium -can be concentrated in small amounts of active matter like Th X -<span class='pageno' id='Page_226'>226</span>or Ur X does not, of itself, prove directly that a material constituent -responsible for the activity has been chemically separated. -For example, in the case of the separation of Th X from thorium, -it might be supposed that the non-thorium part of the solution is -rendered temporarily active by its association with thorium, and -that this property is retained through the processes of precipitation, -evaporation, and ignition, and finally manifests itself in the -residue remaining. According to this view it is to be expected -that any precipitate capable of removing the thorium completely -from its solution should yield active residues similar to those obtained -from ammonia. No such case has, however, been observed. -For example, when thorium nitrate is precipitated by sodium or -ammonium carbonate, the residue from the filtrate after evaporation -and ignition is free from activity and the thorium carbonate -obtained has the normal amount of activity. In fact, ammonia is -the only reagent yet found capable of completely separating Th X -from thorium. A partial separation of the Th X can be made by -shaking thorium oxide with water owing to the greater solubility -of Th X in water.</p> - -<p class='c006'>Thorium and uranium behave quite differently with regard to -the action of ammonia and ammonium carbonate. Ur X is completely -precipitated with the uranium in an ammonia solution -and the filtrate is inactive. Ur X is separated by ammonium -carbonate, while Th X under the same conditions is completely -precipitated with the thorium. The Ur X and the Th X thus -behave like distinct types of matter with well-marked chemical -properties quite distinct from those of the substances in which -they are produced. The removal of Ur X by the precipitation -of barium is probably not directly connected with the chemical -properties of Ur X. The separation is probably due to the -dragging down of the Ur X with the dense barium precipitate. -Sir William Crookes found that the Ur X was dragged down by -precipitates when no question of insolubility was involved, and -such a result is to be expected if the Ur X exists in extremely -minute quantity. It must be borne in mind that the actual -amount of the active constituents Th X and Ur X, separated from -thorium and uranium, is probably infinitesimal, and that the -greater proportion of the residues is due to impurities present -<span class='pageno' id='Page_227'>227</span>in the salt and the reagents, a very small amount of active matter -being mixed with them.</p> -<p class='c005'><b>132. Rate of production of Th X.</b> If the recovery of -the activity of uranium or thorium is due to the continuous -production of new active matter, it should be possible to obtain -experimental evidence of the process. As the case of thorium -has been most fully investigated, a brief account will be given of -some experiments made by Rutherford and Soddy<a id='r228' href='#f228' class='c012'><sup>[228]</sup></a> to show that -Th X is produced continuously at a constant rate. Preliminary -experiments showed that three successive precipitations were -sufficient to remove the Th X almost completely from the thorium. -The general method employed was to precipitate a solution of -5 grams of thorium-nitrate with ammonia. The precipitate was -then redissolved in nitric acid and the thorium again precipitated -as before, as rapidly as possible, so that the Th X produced in the -time between successive precipitations should not appreciably -affect the results. The removal of the Th X was followed by -measurements of the activity of the residues obtained from successive -filtrates. In three successive precipitations the activities of -the residues were proportional to 100, 8, 1·6 respectively. Thus -two precipitations are nearly sufficient to free the thorium -from Th X.</p> - -<p class='c006'>The thorium freed from Th X was then allowed to stand for -a definite time, and the amount of Th X formed during that -time found by precipitating it, and measuring its radio-activity. -According to the theory, the activity <i>I<sub>t</sub></i> of the thorium formed in -the time <i>t</i> is given by</p> - -<div class='figcenter id009'> -<img src='images/form-055.png' alt='Formula.' class='ig001'> -</div> - -<p class='c006'>where -<i>I</i>₀ -is the total activity of Th X, when there is radio-active -equilibrium.</p> - -<p class='c006'>If λ<i>t</i> is small,</p> - -<div class='calc'> - -<div class='lg-container-b c017'> - <div class='linegroup'> - <div class='group'> - <div class='line'> <i>I<sub>t</sub></i></div> - <div class='line'>---- = λ<i>t</i>.</div> - <div class='line'> <i>I</i>₀</div> - </div> - </div> -</div> - -</div> - -<p class='c018'>Since the activity of Th X falls to half value in 4 days, the -<span class='pageno' id='Page_228'>228</span>value of λ expressed in hours = ·0072. After standing a period -of 1 hour about ¹⁄₁₄₀, after 1 day ⅙, after 4 days ½ of the -maximum should be obtained. The experimental results obtained -showed an agreement, as good as could be expected, with the -equation expressing the result that the Th X was being produced -at a constant rate.</p> - -<p class='c006'>The thorium-nitrate which had been freed from Th X was -allowed to stand for one month, and then it was again subjected -to the same process. The activity of the Th X was found to be -the same as that obtained from an equal amount of the original -thorium-nitrate. In one month, therefore, the Th X had been -regenerated, and had reached a maximum value. By leaving the -thorium time to recover fully its activity, this process can be repeated -indefinitely, and equal amounts of Th X are obtained at -each precipitation. Ordinary commercial thorium-nitrate and the -purest nitrate obtainable showed exactly the same action, and -equal amounts of Th X could be obtained from equal weights. -These processes thus appear to be independent of the chemical -purity of the substance<a id='r229' href='#f229' class='c012'><sup>[229]</sup></a>.</p> - -<p class='c006'>The process of the production of Th X is continuous, and no -alteration has been observed in the amount produced in the given -time after repeated separations. After 23 precipitations extending -over 9 days, the amount produced in a given interval was about -the same as at the beginning of the process.</p> - -<p class='c006'>These results are all in agreement with the view that the -Th X is being continuously produced from the thorium compound -at a constant rate. The amount of active matter produced from -1 gram of thorium is probably extremely minute, but the electrical -effects due to its activity are so large that the process of -production can be followed after extremely short intervals. With -a sensitive electrometer the amount of Th X produced per minute -in 10 grams of thorium-nitrate gives a rapid movement to the -electrometer needle. For larger intervals it is necessary to add -additional capacity to the system to bring the effects within range -of the instrument.</p> -<p class='c005'><span class='pageno' id='Page_229'>229</span><a id='section133'></a> -<b>133. Rate of decay of activity.</b> It has been shown that -the activity of Ur X and Th X decays according to an exponential -law with the time. This, we shall see later, is the general law of -decay of activity in any type of active matter, obtained by itself, -and freed from any secondary active products which it may, itself, -produce. In any case, when this law is not fulfilled, it can be -shown that the activity is due to the superposition of two or -more effects, each of which decays in an exponential law with -the time. The physical interpretation of this law still remains -to be discussed.</p> - -<p class='c006'>It has been shown that in uranium and thorium compounds -there is a continuous production of active matter which keeps the -compound in radio-active equilibrium. The changes by which -the active matter is produced must be chemical in nature, since -the products of the action are different in chemical properties -from the matter in which the changes take place. The activity -of the products has afforded the means of following the changes -occurring in them. It now remains to consider the connection -between the activity at any time, and the amount of chemical -change taking place at that time.</p> - -<p class='c006'>In the first place, it is found experimentally that the saturation -ionization current <i>i<sub>t</sub></i>, after the active product has been allowed to -decay for a time <i>t</i>, is given by</p> - -<div class='figcenter id009'> -<img src='images/form-060.png' alt='Formula.' class='ig001'> -</div> - -<p class='c006'>where -<i>i</i>₀ -is the initial saturation current and λ the constant of -decay.</p> - -<p class='c006'>Now the saturation current is a measure of the total number -of ions produced per second in the testing vessel. It has already -been shown that the α rays, which produce the greater proportion -of ionization in the gas, consist of positively charged particles -projected with great velocity. Suppose for simplicity that each -atom of active matter, in the course of its change, gives rise to -one projected α particle. Each α particle will produce a certain -average number of ions in its path before it strikes the boundaries -or is absorbed in the gas. Since the number of projected particles -per second is equal to the number of atoms changing per second, -<span class='pageno' id='Page_230'>230</span>the number of atoms <i>n<sub>t</sub></i> which change per second at the time <i>t</i> is -given by</p> - -<div class='figcenter id009'> -<img src='images/form-061.png' alt='Formula.' class='ig001'> -</div> - -<p class='c006'>where -<i>n</i>₀ -is the initial number which change per second. On this -view, then, the law of decay expresses the result that the number -of atoms changing in unit time, diminishes according to an exponential -law with the time. The number of atoms <i>N<sub>t</sub></i> which -remain <i>unchanged</i> after an interval <i>t</i> is given by</p> - -<div class='figcenter id009'> -<img src='images/form-062.png' alt='Formula.' class='ig001'> -</div> - -<p class='c006'>If -<i>N</i>₀ -is the number of atoms at the beginning,</p> - -<div class='figcenter id010'> -<img src='images/form-063.png' alt='Formula.' class='ig001'> -</div> - -<p class='c006'>Thus</p> - -<div class='figcenter id007'> -<img src='images/form-064.png' alt='Formula.' class='ig001'> -</div> - -<p class='c006'>or the law of decay expresses the fact that the <i>activity of a product -at any time is proportional to the number of atoms which -remain unchanged at that time</i>.</p> - -<p class='c006'>This is the same as the law of monomolecular change in -chemistry, and expresses the fact that there is only one changing -system. If the change depended on the mutual action of two -systems, the law of decay would be different, since the rate of -decay in that case would depend on the relative concentration -of the two reacting substances. This is not so, for not a single -case has yet been observed in which the law of decay was affected -by the amount of active matter present.</p> - -<p class='c006'>From the above equation (1)</p> - -<div class='calc'> - -<div class='lg-container-b c017'> - <div class='linegroup'> - <div class='group'> - <div class='line in1'><i>dN<sub>t</sub></i></div> - <div class='line'>---- = -λ<i>N<sub>t</sub></i>,</div> - <div class='line in1'><i>dt</i></div> - </div> - </div> -</div> - -</div> - -<p class='c018'>or the number of systems changing in unit time is proportional to -the number unchanged at that time.</p> - -<p class='c006'>In the case of recovery of activity, after an active product has -been removed, the number of systems changing in unit time, when -<span class='pageno' id='Page_231'>231</span>radio-active equilibrium is produced, is equal to -λ<i>N</i>₀. -This must -be equal to the number -<i>q</i>₀ -of new systems applied in unit time, or</p> - -<div class='calc'> - -<div class='lg-container-b c017'> - <div class='linegroup'> - <div class='group'> - <div class='line'><i>q</i>₀ = λ<i>N</i>₀,</div> - </div> - <div class='group'> - <div class='line in10'><i>q</i>₀</div> - <div class='line'>and λ = ----- ;</div> - <div class='line in10'><i>N</i>₀</div> - </div> - </div> -</div> - -</div> - -<p class='c018'>λ has thus a distinct physical meaning, and may be defined as -the proportion of the total number of systems present which -change per second. It has different values for different types of -active matter, but is invariable for any particular type of matter. -For this reason, λ will be termed the “<i>radio-active constant</i>„ of -the product.</p> - -<p class='c006'>We are now in a position to discuss with more physical -definiteness the gradual growth of Th X in thorium, after the -Th X has been completely removed from it. Let -<i>q</i>₀ -particles of -Th X be produced per second by the thorium, and let <i>N</i> be the -number of particles of Th X present at any time <i>t</i> after the -original Th X was removed. The number of particles of Th X -which change every second is λ<i>N</i>, where λ is the radio-active -constant of Th X. Now, at any time during the process of recovery, -the rate of increase of the number of particles of Th X = the rate -of production – the rate of change; that is</p> - -<div class='calc'> - -<div class='lg-container-b c017'> - <div class='linegroup'> - <div class='group'> - <div class='line in1'><i>dN</i></div> - <div class='line'>---- = <i>q</i>₀ – λ<i>N</i>.</div> - <div class='line in1'><i>dt</i></div> - </div> - </div> -</div> - -</div> - -<p class='c018'>The solution of this equation is of the form</p> - -<div class='figcenter id010'> -<img src='images/form-065.png' alt='Formula.' class='ig001'> -</div> - -<p class='c006'>where <i>a</i> and <i>b</i> are constants.</p> - -<p class='c006'>Now when <i>t</i> is very great, the number of particles of Th X -present reach a maximum value -<i>N</i>₀.</p> - -<div class='calc'> - -<div class='lg-container-b c017'> - <div class='linegroup'> - <div class='group'> - <div class='line'>Thus, since <i>N</i> = <i>N</i>₀ when <i>t</i> = infinity,</div> - <div class='line in31'><i>b</i> = <i>N</i>₀;</div> - </div> - <div class='group'> - <div class='line'>since <i>N</i> = 0 when <i>t</i> = 0,</div> - </div> - <div class='group'> - <div class='line in10'><i>a</i> + <i>b</i> = 0;</div> - <div class='line'>hence <i>b</i> = -<i>a</i> = <i>N</i>₀,</div> - </div> - </div> -</div> - -</div> - -<p class='c018'>and the equation becomes</p> - -<div class='figcenter id010'> -<img src='images/form-066.png' alt='Formula.' class='ig001'> -</div> - -<p class='c006'>This is equivalent to the equation already obtained in <a href='#section130'>section 130</a>, -<span class='pageno' id='Page_232'>232</span>since the intensity of the radiation is always proportional to the -number of particles present.</p> -<p class='c005'><b>134. Influence of conditions on the rate of decay.</b> -Since the activity of any product, at any time, may be taken as -a measure of the rate at which chemical change takes place, it -may be used as a means of determining the effect of conditions -on the changes occurring in radio-active matter. If the rate of -change should be accelerated or retarded, it is to be expected -that the value of the radio-active constant λ will be increased or -decreased, <i>i.e.</i> that the decay curve will be different under different -conditions.</p> - -<p class='c006'>No such effect, however, has yet been observed in any case of -radio-active change, where none of the active products produced -are allowed to escape from the system. The rate of decay is -unaltered by any chemical or physical agency, and in this respect -the changes in radio-active matter are sharply distinguished from -ordinary chemical changes. For example, the rate of decay of -activity from any product takes place at the same rate when the -substance is exposed to light as when it is kept in the dark, and -at the same rate in a vacuum as in air or any other gas at atmospheric -pressure. Its rate of decay is unaltered by surrounding -the active matter by a thick layer of lead under conditions where -no ordinary radiation from outside can affect it. The activity of -the matter is unaffected by ignition or chemical treatment. The -material giving rise to the activity can be dissolved in acid and -re-obtained by evaporation of the solution without altering the -activity. The rate of decay is the same whether the active -matter is retained in the solid state or kept in solution. When -a product has lost its activity, resolution or heat does not regenerate -it, and as we shall see later, the rate of decay of the -active products, so far examined, is the same at a red heat as at -the temperature of liquid air. In fact, no variation of physical or -chemical conditions has led to any observable difference in the -decay of activity of any of the numerous types of active matter -which have been examined.</p> -<p class='c005'><b>135. Effect of conditions on the rate of recovery of -activity.</b> The recovery of the activity of a radio-element with -<span class='pageno' id='Page_233'>233</span>time, when an active product is separated from it, is governed by -the rate of production of fresh active matter and by the decay of -activity of that already produced. Since the rate of decay of the -activity of the separated product is independent of conditions, the -rate of recovery of activity can be modified only by a change of -the rate of production of fresh active matter. As far as experiments -have gone, the rate of production, like the rate of decay, is -independent of chemical or physical conditions. There are indeed -certain cases which are apparent exceptions to this rule. For -example, the escape of the radio-active emanations from thorium -and radium is readily affected by heat, moisture and solution. -A more thorough investigation, however, shows that the exception -is only apparent and not real. These cases will be discussed -more in detail in <a href='#chap07'>chapter <span class='fss'>VII</span></a>, but it may be stated here that -the differences observed are due to differences in the rate of escape -of the emanations into the surrounding gas, and not to differences -in the rate of production. For this reason it is difficult to test the -question at issue in the case of the thorium compounds, which -in most cases readily allow the emanation produced by them to -escape into the air.</p> - -<p class='c006'>In order to show that the rate of production is independent -of molecular state, temperature, etc., it is necessary in such a -case to undertake a long series of measurements extending -over the whole time of recovery. It is impossible to make accurate -relative comparisons to see if the activity is altered by the -conversion of one compound into another. The relative activity -in such a case, when measured by spreading a definite weight of -material uniformly on a metal plate, varies greatly with the physical -conditions of the precipitate, although the total activity of two -compounds may be the same.</p> - -<p class='c006'>The following method<a id='r230' href='#f230' class='c012'><sup>[230]</sup></a> offers an accurate and simple means -of studying whether the rate of production of active matter is -influenced by molecular state. The substance is chemically converted -into any compound required, care being taken that active -products are recovered during the process. The new compound is -then spread on a metal plate and compared with a standard sample -of uranium for several days or weeks as required. If the rate of -<span class='pageno' id='Page_234'>234</span>production of active matter is altered by the conversion, there -should be an increase or decrease of activity to a new steady value, -where the production of active matter is again balanced by the -rate of decay. This method has the great advantage of being independent -of the physical condition of the precipitate. It can be -applied satisfactorily to a compound of thorium like the nitrate -and the oxide which has been heated to a white heat, after which -treatment only a slight amount of emanation escapes. The nitrate -was converted into the oxide in a platinum crucible by treatment -with sulphuric acid and ignition to a white heat. The oxide so -obtained was spread on a plate, but no change of its activity was -observed with time, showing that in this case the rate of production -was independent of molecular state. This method, which is limited -in the case of thorium, may be applied generally to the uranium -compounds where the results are not complicated by the presence -of an emanation.</p> - -<p class='c006'>No differences have yet been observed in the recovery curves -of different thorium compounds after the removal of Th X. For -example, the rate of recovery is the same whether the precipitated -hydroxide is converted into the oxide or into the sulphate.</p> -<p class='c005'><a id='section136'></a> -<b>136. Disintegration hypothesis.</b> In the discussion of the -changes in radio-active bodies, only the active products Ur X -and Th X have been considered. It will, however, be shown later -that these two products are only examples of many other types of -active matter which are produced by the radio-elements, and that -each of these types of active matter has definite chemical as well -as radio-active properties, which distinguish it, not only from the -other active products, but also from the substance from which it is -produced.</p> - -<p class='c006'>The full investigation of these changes will be shown to -verify in every particular the hypothesis that radio-activity is the -accompaniment of chemical changes of a special kind occurring in -matter, and that the constant activity of the radio-elements is -due to an equilibrium process, in which the rate of production of -fresh active matter balances the rate of change of that already -formed.</p> - -<p class='c006'>The nature of the process taking place in the radio-elements, -<span class='pageno' id='Page_235'>235</span>in order to give rise to the production at a constant rate of new -kinds of active matter, will now be considered. Since in thorium -or uranium compounds there is a continuous production of radio-active -matter, which differs in chemical properties from the parent -substance, some kind of change must be taking place in the radio-element. -This change, by which new matter is produced, is very -different in character from the molecular changes dealt with in -chemistry, for no chemical change is known which proceeds at the -same rate at the temperatures corresponding to a red heat and -to liquid air, and is independent of all physical and chemical -actions. If, however, the production of active matter is supposed -to be the result of changes, not in the molecule, but in the <i>atom -itself</i>, it is not to be expected that the temperature would exert -much influence. The general experience of chemistry in failing -to transform the elements by the action of temperature is itself -strong evidence that wide ranges of temperature have not much -effect in altering the stability of the chemical atom.</p> - -<p class='c006'>The view that the atoms of the radio-elements are undergoing -spontaneous disintegration was put forward by Rutherford and -Soddy as a result of evidence of this character. The discovery of -the <i>material</i> nature of the α rays added strong confirmation to -the hypothesis; for it has been pointed out (section 95) that the -expulsion of α particles must be the result of a disintegration -of the atoms of the radio-element. Taking the case of thorium -as an example, the processes occurring in the atom may be -pictured in the following way. It must be supposed that the -thorium atoms are not permanently stable systems, but, on an -average, a constant small proportion of them—about one atom in -every -10<sup>16</sup> -will suffice—break up per second. The disintegration -consists in the expulsion from the atom of one or more α particles -with great velocity. For simplicity, it will be supposed that each -atom expels <i>one</i> α particle. It has been shown that the α particle -of radium has a mass about twice that of the hydrogen atom. -From the similarity of the α rays from thorium and radium, it is -probable that the α particle of thorium does not differ much in -mass from that of radium, and may be equal to it. The α particles -expelled from the thorium atoms as they break up constitute what -is known as the “non-separable activity” of thorium. This activity, -<span class='pageno' id='Page_236'>236</span>measured by the α rays, is about 25 per cent. of the maximum. -After the escape of an α particle, the part of the atom left behind, -which has a mass slightly less than that of the thorium atom, tends -to rearrange its components to form a temporarily stable system. -It is to be expected that it will differ in chemical properties from -the thorium atom from which it was derived. The atom of the -substance Th X is, on this view, the thorium atom minus one α -particle. The atoms of Th X are far more unstable than the atoms -of thorium, and one after the other they break up, each atom -expelling one α particle as before. These projected α particles give -rise to the <i>radiation</i> from the Th X. Since the activity of Th X -falls to half its original value in about four days, on an average -half of the atoms of Th X break up in four days, the number -breaking up per second being always proportional to the number -present. After an atom of Th X has expelled an α particle, the -mass of the system is again reduced, and its chemical properties -are changed. It will be shown (<a href='#section154'>section 154</a>) that the Th X produces -the thorium emanation, which exists as a radio-active gas, and -that this in turn is transformed into matter which is deposited on -solid bodies and gives rise to the phenomena of excited activity. -The first few successive changes occurring in thorium are shown -diagrammatically below (<a href='#fig050'>Fig. 50</a>).</p> - -<div id='fig050' class='figcenter id001'> -<img src='images/fig-050.png' alt='Fig. 50.' class='ig001'> -<div class='ic002'> -<p>Fig. 50.</p> -</div> -</div> - -<p class='c006'>Thus as a result of the disintegration of the thorium atom, -a series of chemical substances is produced, each of which has -distinctive chemical properties. Each of these products is radio-active, -and loses its activity according to a definite law. Since -thorium has an atomic weight of 237, and the weight of the -α particle is about 2, it is evident that, if only <i>one</i> α particle -is expelled at each change, the process of disintegration could -pass through a number of successive stages and yet leave behind, -<span class='pageno' id='Page_237'>237</span>at the end of the process, a mass comparable with that of the -parent atom.</p> - -<p class='c006'>It will be shown later that a process of disintegration, very -similar to that already described for thorium, must be supposed -to take place also in uranium, actinium and radium. The full -discussion of this subject cannot be given with advantage until two -of the most important products of the three substances thorium, -radium and actinium, viz. the radio-active emanations and the -matter which causes excited activity, have been considered in detail.</p> -<p class='c005'><b>137. Magnitude of the changes.</b> It can be calculated by -several independent methods (see <a href='#section246'>section 246</a>) that, in order -to account for the radio-activity observed in thorium, about -3 × 10<sup>4</sup> -atoms in each gram of thorium suffer disintegration -per second. It is well known (<a href='#section039'>section 39</a>) that 1 cubic centimetre -of hydrogen at atmospheric pressure and temperature -contains about -3·6 × 10<sup>19</sup> -molecules. From this it follows that -one gram of thorium contains -3·6 × 10<sup>21</sup> -atoms. The fraction -which breaks up per second is thus about -10<sup>-17</sup>. -This is an -extremely small ratio, and it is evident that the process could -continue for long intervals of time, before the amount of matter -changed would be capable of detection by the spectroscope or -by the balance. With the electroscope it is possible to detect -the radiation from -10<sup>-5</sup> -gram of thorium, <i>i.e.</i> the electroscope -is capable of detecting the ionization which accompanies the -disintegration of a single thorium atom per second. The electroscope -is thus an extraordinarily delicate means for detection of -minute changes in matter, which are accompanied, as in the case of -the radio-elements, by the expulsion of charged particles with great -velocity. It is possible to detect by its radiation the amount of -Th X produced in a second from 1 gram of thorium, although -the process would probably need to continue thousands of years -before it could be detected by the balance or the spectroscope. It -is thus evident that the changes occurring in thorium are of an -order of magnitude quite different from that of ordinary chemical -changes, and it is not surprising that they have never been -observed by direct chemical methods.</p> - -<div class='chapter'> - <span class='pageno' id='Page_238'>238</span> - <h2 id='chap07' class='c004'>CHAPTER VII. <br> RADIO-ACTIVE EMANATIONS.</h2> -</div> -<p class='c005'><b>138. Introduction.</b> A most important and striking property -possessed by radium, thorium, and actinium, but not by uranium or -polonium, is the power of continuously emitting into the surrounding -space a material emanation, which has all the properties of a -radio-active gas. This emanation is able to diffuse rapidly through -gases and through porous substances, and may be separated from -the gas with which it is mixed by condensation by the action of -extreme cold. This emanation forms a connecting link between -the activity of the radio-elements themselves and their power of -exciting activity on surrounding objects, and has been studied more -closely than the other active products on account of its existence in -the gaseous state. The emanations from the three active bodies all -possess similar radio-active properties, but the effects are more -marked in the case of the emanation from radium, on account of -the very great activity of that element.</p> -<h3 class='c020'>Thorium Emanation.</h3> -<p class='c005'><b>139. Discovery of the emanation.</b> In the course of -examination of the radiations of thorium, several observers had -noted that some of the thorium compounds, and especially the -oxide, were very inconstant sources of radiation, when examined in -open vessels by the electrical method. Owens<a id='r231' href='#f231' class='c012'><sup>[231]</sup></a> found that this -inconstancy was due to the presence of air currents. When a -closed vessel was used, the current, immediately after the introduction -of the active matter, increased with the time, and finally -<span class='pageno' id='Page_239'>239</span>reached a constant value. By drawing a steady stream of air -through the vessel the value of the current was much reduced. It -was also observed that the radiations could apparently pass through -large thicknesses of paper, which completely absorbed the ordinary -α radiation.</p> - -<p class='c006'>In an investigation of these peculiar properties of thorium -compounds, the writer<a id='r232' href='#f232' class='c012'><sup>[232]</sup></a> found that the effects were due to an -emission of radio-active particles of some kind from the thorium -compounds. This “emanation,” as it was termed for convenience, -possesses the properties of ionizing the gas and acting on a photographic -plate, and is able to diffuse rapidly through porous -substances like paper and thin metal foil.</p> - -<p class='c006'>The emanation, like a gas, is completely prevented from escaping -by covering the active matter with a thin plate of mica. The -emanation can be carried away by a current of air; it passes -through a plug of cotton-wool and can be bubbled through solutions -without any loss of activity. In these respects, it behaves very -differently from the ions produced in the gas by the rays from -active substances, for these give up their charges completely under -the same conditions.</p> - -<p class='c006'>Since the emanation passes readily through large thicknesses -of cardboard, and through filters of tightly packed cotton-wool, it -does not seem likely that the emanation consists of particles of -dust given off by the active matter. This point was tested still -further by the method used by Aitken and Wilson, for detecting -the presence of dust particles in the air. The oxide, enclosed in -a paper cylinder, was placed in a glass vessel, and the dust was -removed by repeated small expansions of the air over a water -surface. The dust particles act as nuclei for the formation of -small drops and are then removed from the air by the action of -gravity. After repeated expansions, no cloud was formed, and the -dust was considered to be removed. After waiting for some time -to allow the thorium emanation to collect, further expansions were -made but no cloud resulted, showing that for the small expansions -used, the particles were too small to become centres of condensation. -The emanation then could not be regarded as dust emitted -from thorium.</p> - -<p class='c006'><span class='pageno' id='Page_240'>240</span>Since the power of diffusing rapidly through porous substances, -and acting on a photographic plate, is also possessed by a chemical -substance like hydrogen peroxide, some experiments were made -to see if the emanation could be an agent of that character. It -was found, however, that hydrogen peroxide is not radio-active, -and that its action on the plate is a purely chemical one, while -it is the <i>radiation</i> from the emanation and not the <i>emanation</i> itself -that produces ionizing and photographic effects.</p> -<p class='c005'><b>140. Experimental arrangements.</b> The emanation from -thorium is given off in minute quantity. No appreciable lowering -of the vacuum is observed when an emanating compound is placed -in a vacuum tube and no new spectrum lines are observed.</p> - -<p class='c006'>For an examination of the emanation, an apparatus similar in -principle to that shown in <a href='#fig051'>Fig. 51</a> is convenient.</p> - -<div id='fig051' class='figcenter id001'> -<img src='images/fig-051.png' alt='Fig. 51.' class='ig001'> -<div class='ic002'> -<p>Fig. 51.</p> -</div> -</div> - -<p class='c006'>The thorium compound, either bare or enclosed in a paper -envelope, was placed in a glass tube <i>C</i>. A current of air from a -gasometer, after passing through a tube containing cotton-wool to -remove dust particles, bubbled through sulphuric acid in the vessel -<i>A</i>. It then passed through a bulb containing tightly packed cotton-wool -to prevent any spray being carried over. The emanation, -mixed with air, was carried from the vessel <i>C</i> through a plug of -cotton-wool <i>D</i>, which removed completely all the ions carried with -the emanation. The latter then passed into a long brass cylinder, -75 cm. in length and 6 cm. in diameter. The insulated cylinder -was connected with a battery in the usual way. Three insulated -electrodes, <i>E</i>, <i>F</i>, <i>H</i>, of equal lengths, were placed along the axis of -the cylinder, supported by brass rods passing through ebonite -corks in the side of the cylinder. The current through the gas, -due to the presence of the emanation, was measured by means of -<span class='pageno' id='Page_241'>241</span>an electrometer. An insulating key was arranged so that any one -of the electrodes <i>E</i>, <i>F</i>, <i>H</i> could be rapidly connected with one pair -of quadrants of the electrometer, the other two being always connected -with earth. The current observed in the testing cylinder -vessel was due entirely to the ions produced by the emanation -carried into the vessel by the current of air. On substituting a -uranium compound for the thorium, not the slightest current was -observed. After a constant flow has passed for about 10 minutes, -the current due to the emanation reaches a constant value.</p> - -<p class='c006'>The variation of the ionization current with the voltage is -similar to that observed for the gas ionized by the radiations from -the active bodies. The current at first increases with the voltage, -but finally reaches a saturation value.</p> -<p class='c005'><b>141. Duration of the activity of the emanation.</b> The -emanation rapidly loses its activity with time. This is very readily -shown with the apparatus of Fig. 51. The current is found to -diminish progressively along the cylinder, and the variation from -electrode to electrode depends on the velocity of the flow of air.</p> - -<p class='c006'>If the velocity of the air current is known, the decay of activity -of the emanation with time can be deduced. If the flow of air is -stopped, and the openings of the cylinder closed, the current -steadily diminishes with time. The following numbers illustrate -the variation with time of the saturation current, due to the -emanation in a closed vessel. The observations were taken successively, -and as rapidly as possible after the current of air was -stopped.</p> - -<table class='table21' > -<colgroup> -<col class='colwidth60'> -<col class='colwidth40'> -</colgroup> - <tr> - <td class='c015'>Time in seconds</td> - <td class='c016'>Current</td> - </tr> - <tr> - <td class='c015'>0</td> - <td class='c016'>100</td> - </tr> - <tr> - <td class='c015'>28</td> - <td class='c016'>69</td> - </tr> - <tr> - <td class='c015'>62</td> - <td class='c016'>51</td> - </tr> - <tr> - <td class='c015'>118</td> - <td class='c016'>25</td> - </tr> - <tr> - <td class='c015'>155</td> - <td class='c016'>14</td> - </tr> - <tr> - <td class='c015'>210</td> - <td class='c016'>6·7</td> - </tr> - <tr> - <td class='c015'>272</td> - <td class='c016'>4·1</td> - </tr> - <tr> - <td class='c015'>360</td> - <td class='c016'>1·8</td> - </tr> -</table> - -<p class='c006'>Curve <i>A</i>, <a href='#fig052'>Fig. 52</a>, shows the relation existing between the -current through the gas and the time. The current just before -the flow of air was stopped is taken as unity. The current through -<span class='pageno' id='Page_242'>242</span>the gas, which is a measure of the activity of the emanation, -diminishes according to an exponential law with the time like the -activity of the products Ur X and Th X. The rate of decay is, -however, much more rapid, the activity of the emanation decreasing -to half value in about one minute. According to the view -developed in <a href='#section136'>section 136</a>, this implies that half of the emanation -particles have undergone change in one minute. After an interval -of 10 minutes the current due to the emanation is very small, -showing that practically all the emanation particles present have -undergone change.</p> - -<div id='fig052' class='figcenter id001'> -<img src='images/fig-052.png' alt='Fig. 52.' class='ig001'> -<div class='ic002'> -<p>Fig. 52.</p> -</div> -</div> - -<p class='c006'>The rate of decay has been more accurately determined by -Rossignol and Gimingham<a id='r233' href='#f233' class='c012'><sup>[233]</sup></a> who found that the activity fell to half -value in about 51 seconds. Bronson<a id='r234' href='#f234' class='c012'><sup>[234]</sup></a>, using the steady deflection -method described in section 69, found the corresponding time -54 seconds.</p> - -<p class='c006'>The decrease of the current with the time is an actual measure -of the decrease of the activity of the emanation, and is not in any -<span class='pageno' id='Page_243'>243</span>way influenced by the time that the ions produced take to reach -the electrodes. If the ions had been produced from a uranium -compound the duration of the conductivity for a saturation voltage -would only have been a fraction of a second.</p> - -<p class='c006'>The rate of decay of the activity of the emanation is independent -of the electromotive force acting on the gas. This shows that the -radio-active particles are not destroyed by the electric field. The -current through the gas at any particular instant, after stoppage of -the flow of air, was found to be the same whether the electromotive -force had been acting the whole time or had been just applied for -the time of the test.</p> - -<p class='c006'>The emanation itself is unaffected by a strong electric field and -so cannot be charged. By testing its activity after passing it -through long concentric cylinders, charged to a high potential, it -was found that the emanation certainly did not move with a -velocity greater than ·00001 cm. per second, for a gradient of -1 volt per cm., and there was no evidence to show that it moved at -all. This conclusion has been confirmed by the experiments of -McClelland<a id='r235' href='#f235' class='c012'><sup>[235]</sup></a>.</p> - -<p class='c006'>The rate at which the emanation is produced is independent -of the gas surrounding the active matter. If in the apparatus of -<a href='#fig051'>Fig. 51</a> air is replaced by hydrogen, oxygen, or carbonic acid, -similar results are obtained, though the current observed in the -testing vessel varies for the different gases on account of the -unequal absorption by them of the radiation from the emanation.</p> - -<p class='c006'>If a thorium compound, enclosed in paper to absorb the α -radiation, is placed in a closed vessel, the saturation current due to -the emanation is found to vary directly as the pressure. Since -the rate of ionization is proportional to the pressure for a constant -source of radiation, this experiment shows that the rate of emission -of the emanation is independent of the pressure of the gas. The -effect of pressure on the rate of production of the emanation is -discussed in more detail later in <a href='#section157'>section 157</a>.</p> -<p class='c005'><b>142. Effect of thickness of layer.</b> The amount of emanation -emitted by a given area of thorium compound depends on -the thickness of the layer. With a very thin layer, the current -between two parallel plates, placed in a closed vessel as in <a href='#fig017'>Fig. 17</a>, -is due very largely to the α rays. Since the α radiation is very -<span class='pageno' id='Page_244'>244</span>readily absorbed, the current due to it practically reaches a maximum -when the surface of the plate is completely covered by a thin layer -of the active material. On the other hand the current produced -by the emanation increases until the layer is several millimetres in -thickness, and then is not much altered by adding fresh active -matter. This falling off of the current after a certain thickness -has been reached is to be expected, since the emanation, which -takes several minutes to diffuse through the layer above it, has -already lost a large proportion of its activity.</p> - -<p class='c006'>With a thick layer of thorium oxide in a closed vessel, the -current between the plates is largely due to the radiation from the -emanation lying between the plates. The following tables illustrate -the way in which the current varies with the thickness of -paper for both a thin and a thick layer.</p> - -<p class='c005'><span class='sc'>Table I.</span> <i>Thin Layer.</i></p> - -<p class='c006'>Thickness of sheets of paper ·0027.</p> - -<table class='table14' > -<colgroup> -<col class='colwidth50'> -<col class='colwidth50'> -</colgroup> - <tr> - <th class='c015'>No. of layers of paper</th> - <th class='c016'>Current</th> - </tr> - <tr> - <td class='c015'> </td> - <td class='c016'> </td> - </tr> - <tr> - <td class='c015'>0</td> - <td class='c016'>1</td> - </tr> - <tr> - <td class='c015'>1</td> - <td class='c016'>·37</td> - </tr> - <tr> - <td class='c015'>2</td> - <td class='c016'>·16</td> - </tr> - <tr> - <td class='c015'>3</td> - <td class='c016'>·08</td> - </tr> -</table> -<p class='c005'><span class='sc'>Table II.</span> <i>Thick Layer.</i></p> - -<p class='c006'>Thickness of paper ·008 cm.</p> - -<table class='table14' > -<colgroup> -<col class='colwidth50'> -<col class='colwidth50'> -</colgroup> - <tr> - <th class='c015'>No. of layers of paper</th> - <th class='c016'>Current</th> - </tr> - <tr> - <td class='c015'> </td> - <td class='c016'> </td> - </tr> - <tr> - <td class='c015'>0</td> - <td class='c016'>1</td> - </tr> - <tr> - <td class='c015'>1</td> - <td class='c016'>·74</td> - </tr> - <tr> - <td class='c015'>2</td> - <td class='c016'>·74</td> - </tr> - <tr> - <td class='c015'>5</td> - <td class='c016'>·72</td> - </tr> - <tr> - <td class='c015'>10</td> - <td class='c016'>·67</td> - </tr> - <tr> - <td class='c015'>20</td> - <td class='c016'>·55</td> - </tr> -</table> - -<p class='c006'>The initial current with the unscreened compound is taken as -unity. In Table I, for a thin layer of thorium oxide, the current -diminished rapidly with additional layers of thin paper. In this -case the current is due almost entirely to the α rays. In Table II -the current falls to ·74 for the first layer. In this case about 26% -of the current is due to the α rays, which are practically absorbed -by the layer ·008 cm. in thickness. The slow decrease with -additional layers shows that the emanation diffuses so rapidly -through a few layers of paper that there is little loss of activity -during the passage. The time taken to diffuse through 20 layers -is however appreciable, and the current consequently has decreased. -After passing through a layer of cardboard 1·6 mms. in thickness -the current is reduced to about one-fifth of its original value. In -<span class='pageno' id='Page_245'>245</span>closed vessels the proportion of the total current, due to the emanation, -varies with the distance between the plates as well as with the -thickness of the layer of active material. It also varies greatly -with the compound examined. In the nitrate, which gives off only -a small amount of emanation, the proportion is very much smaller -than in the hydroxide, which gives off a large amount of emanation.</p> -<p class='c005'><b>143. Increase of current with time.</b> The current due to -the emanation does not reach its final value for some time after -the active matter has been introduced into the closed vessel. The -variation with time is shown in the following table. The saturation -current due to thorium oxide, covered with paper, was observed -between concentric cylinders of 5·5 cms. and ·8 cm. diameter.</p> - -<p class='c006'>Immediately before observations on the current were made, -a rapid stream of air was blown through the apparatus. This -removed most of the emanation. However, the current due to the -ionization of the gas by the emanation, as it was carried along by -the current of air, was still appreciable. The current consequently -does not start from zero.</p> - -<table class='table14' > -<colgroup> -<col class='colwidth50'> -<col class='colwidth50'> -</colgroup> - <tr> - <td class='c015'>Time in seconds</td> - <td class='c016'>Current</td> - </tr> - <tr> - <td class='c015'>0</td> - <td class='c016'>9</td> - </tr> - <tr> - <td class='c015'>23</td> - <td class='c016'>25</td> - </tr> - <tr> - <td class='c015'>53</td> - <td class='c016'>49</td> - </tr> - <tr> - <td class='c015'>96</td> - <td class='c016'>67</td> - </tr> - <tr> - <td class='c015'>125</td> - <td class='c016'>76</td> - </tr> - <tr> - <td class='c015'>194</td> - <td class='c016'>88</td> - </tr> - <tr> - <td class='c015'>244</td> - <td class='c016'>98</td> - </tr> - <tr> - <td class='c015'>304</td> - <td class='c016'>99</td> - </tr> - <tr> - <td class='c015'>484</td> - <td class='c016'>100</td> - </tr> -</table> -<p class='c006'>The results are shown graphically in <a href='#fig052'>Fig. 52</a>, curve <i>B</i>. The -decay of the activity of the emanation with time, and the rate of -increase of the activity due to the emanation in a closed space, -are connected in the same way as the decay and recovery curves of -Th X and Ur X.</p> - -<p class='c006'>With the previous notation, the decay curve is given by</p> - -<div class='figcenter id010'> -<img src='images/form-059.png' alt='Formula.' class='ig001'> -</div> - -<p class='c006'>and the recovery curve by</p> - -<div class='figcenter id010'> -<img src='images/form-055.png' alt='Formula.' class='ig001'> -</div> - -<p class='c006'>where λ is the radio-active constant of the emanation.</p> - -<p class='c006'>This relation is to be expected, since the decay and recovery -<span class='pageno' id='Page_246'>246</span>curves of the emanation are determined by exactly the same conditions -as the decay and recovery curves of Ur X and Th X. In -both cases there is:</p> - -<p class='c021'>(1) A supply of fresh radio-active particles produced at a -constant rate.</p> - -<p class='c011'>(2) A loss of activity of the particles following an exponential -law with the time.</p> - -<p class='c018'>In the case of Ur X and Th X, the active matter produced -manifests its activity in the position in which it is formed; in this -new phenomenon, a proportion of the active matter in the form of -the emanation escapes into the surrounding gas. The activity of -the emanation, due to a thorium compound kept in a closed vessel, -thus reaches a maximum when the rate of supply of fresh emanation -particles from the compound is balanced by the rate of change -of those already present. The time for recovery of half the final -activity is about 1 minute, the same as the time taken for the -emanation, when left to itself, to lose half its activity.</p> - -<p class='c006'>If -<i>q</i>₀ -is the number of emanation particles escaping into the -gas per second, and -<i>N</i>₀ -the final number when radio-active equilibrium -is reached, then (<a href='#section133'>section 133</a>),</p> - -<div class='calc'> - -<div class='lg-container-b c017'> - <div class='linegroup'> - <div class='group'> - <div class='line'><i>q</i>₀ = λ<i>N</i>₀.</div> - </div> - </div> -</div> - -</div> - -<p class='c018'>Since the activity of the emanation falls to half value in -1 minute</p> - -<div class='calc'> - -<div class='lg-container-b c017'> - <div class='linegroup'> - <div class='group'> - <div class='line'>λ = ¹⁄₈₇,</div> - </div> - </div> -</div> - -</div> - -<p class='c018'>and -<i>N</i>₀ = 87<i>q</i>₀, -or the number of emanation particles present when -a steady state is reached is 87 times the number produced per -second.</p> -<h3 class='c020'>Radium Emanation.</h3> -<p class='c005'><b>144. Discovery of the emanation.</b> Shortly after the -discovery of the thorium emanation, Dorn<a id='r236' href='#f236' class='c012'><sup>[236]</sup></a> repeated the results, -and, in addition, showed that radium compounds also gave off -radio-active emanations, and that the amount given off was much -increased by heating the compound. The radium emanation differs -from the thorium emanation in the rate at which it loses its -activity. It decays far more slowly, but in other respects the -emanations of thorium and radium have much the same properties. -Both emanations ionize the gas with which they are mixed, and -<span class='pageno' id='Page_247'>247</span>affect a photographic plate. Both diffuse readily through porous -substances but are unable to pass through a thin plate of mica; -both behave like a temporarily radio-active gas, mixed in minute -quantity with the air or other gas in which they are conveyed.</p> -<p class='c005'><a id='section145'></a> -<b>145. Decay of activity of the emanation.</b> Very little -emanation escapes from radium chloride in the solid state, but -the amount is largely increased by heating, or by dissolving the -compound in water. By bubbling air through a radium chloride -solution, or passing air over a heated radium compound, a large -amount of emanation may be obtained which can be collected, -mixed with air, in a suitable vessel.</p> - -<p class='c006'>Experiments to determine accurately the rate of decay of -activity of the emanation have been made by P. Curie<a id='r237' href='#f237' class='c012'><sup>[237]</sup></a>, and -Rutherford and Soddy<a id='r238' href='#f238' class='c012'><sup>[238]</sup></a>. In the experiments of the latter, the -emanation mixed with air was stored over mercury in an ordinary -gas-holder. From time to time, equal quantities of air mixed with -the emanation were measured off by a gas pipette and delivered -into a testing vessel. The latter consisted of an air-tight brass -cylinder carrying a central insulated electrode. A saturation -voltage was applied to the cylinder, and the inner electrode was -connected to the electrometer with a suitable capacity in parallel. -The saturation current was observed <i>immediately</i> after the introduction -of the active gas into the testing vessel, and was taken as -a measure of the activity of the emanation present. The current -increased rapidly with the time owing to the production of excited -activity on the walls of the containing vessel. This effect is -described in detail in <a href='#chap08'>chapter <span class='fss'>VIII</span></a>.</p> - -<p class='c006'>The measurements were made at suitable intervals over a -period of 33 days. The following table expresses the results, the -initial activity being taken as 100.</p> - -<table class='table14' > -<colgroup> -<col class='colwidth50'> -<col class='colwidth50'> -</colgroup> - <tr> - <th class='c015'>Time in hours</th> - <th class='c016'>Relative Activity</th> - </tr> - <tr> - <td class='c015'>0</td> - <td class='c016'>100</td> - </tr> - <tr> - <td class='c015'>20·8</td> - <td class='c016'>85·7</td> - </tr> - <tr> - <td class='c015'>187·6</td> - <td class='c016'>24·0</td> - </tr> - <tr> - <td class='c015'>354·9</td> - <td class='c016'>6·9</td> - </tr> - <tr> - <td class='c015'>521·9</td> - <td class='c016'>1·5</td> - </tr> - <tr> - <td class='c015'>786·9</td> - <td class='c016'>0·19</td> - </tr> -</table> - -<p class='c006'><span class='pageno' id='Page_248'>248</span>The activity falls off according to an exponential law with the -time, and decays to half value in 3·71 days. With the usual -notation</p> - -<div class='figcenter id010'> -<img src='images/form-059.png' alt='Formula.' class='ig001'> -</div> - -<p class='c006'>the mean value of λ deduced from the results is given by</p> - -<div class='calc'> - -<div class='lg-container-b c017'> - <div class='linegroup'> - <div class='group'> - <div class='line'>λ = 2·16 × 10<sup>-6</sup> = ¹⁄₄₆₃₀₀₀.</div> - </div> - </div> -</div> - -</div> - -<p class='c018'>P. Curie determined the rate of decay of activity of the emanation -by another method. The active matter was placed at one end -of a sealed tube. After sufficient time had elapsed the portion of -the tube containing the radium compound was removed. The loss -of activity of the emanation, stored in the other part, was tested at -regular intervals by observing the ionization current due to the -rays which passed through the -walls of the glass vessel. The -testing apparatus and the connections -are shown clearly in -<a href='#fig053'>Fig. 53</a>. The ionization current -is observed between the vessels -<i>BB</i> and <i>CC</i>. The glass tube -<i>A</i> contains the emanation.</p> - -<div id='fig053' class='figcenter id005'> -<img src='images/fig-053.png' alt='Fig. 53.' class='ig001'> -<div class='ic002'> -<p>Fig. 53.</p> -</div> -</div> - -<p class='c006'>Now it will be shown later -that the emanation itself gives -off only α rays, and these rays -are completely absorbed by the -glass envelope, unless it is made -extremely thin. The rays producing -ionization in the testing -vessel were thus not due to the -α rays from the emanation at -all, but to the β and γ rays due to the excited activity produced -on the walls of the glass tube by the emanation inside it. What -was actually measured was thus the decay of the excited activity -derived from the emanation, and not the decay of activity of the -emanation itself. Since, however, when a steady state is reached, -the amount of excited activity is nearly proportional at any time -to the activity of the emanation, the rate of decay of the excited -<span class='pageno' id='Page_249'>249</span>activity on the walls of the vessel indirectly furnishes a measure -of the rate of decay of the emanation itself. This is only true if -the emanation is placed for four or five hours in the tube before -observations begin, in order to allow the excited activity time to -reach a maximum value.</p> - -<p class='c006'>Using this method P. Curie obtained results similar to those -obtained by Rutherford and Soddy by the direct method. The -activity decayed according to an exponential law with the time, -falling to half value in 3·99 days.</p> - -<p class='c006'>The experiments were performed under the most varied conditions -but the rate of decay was found to remain unaltered. The -rate of decay did not depend on the material of the vessel containing -the emanation or on the nature or pressure of the gas with -which the emanation was mixed. It was unaffected by the amount -of emanation present, or by the time of exposure to the radium, -provided sufficient time had elapsed to allow the excited activity -to reach a maximum value before the observations were begun. -P. Curie<a id='r239' href='#f239' class='c012'><sup>[239]</sup></a> found that the rate of decay of activity was not altered -by exposing the vessel containing the emanation to different -temperatures, ranging from +450° to -180° C.</p> - -<p class='c006'>In this respect the emanations of thorium and radium are -quite analogous. The rate of decay seems to be unaffected by -any physical or chemical agency, and the emanations behave in -exactly the same way as the radio-active products Th X and Ur X, -already referred to. The radio-active constant λ is thus a fixed -and unalterable quantity for both emanations, although in one case -its value is about 5000 times greater than in the other.</p> -<h3 class='c020'>Emanations from Actinium.</h3> -<p class='c005'><b>146.</b> Debierne<a id='r240' href='#f240' class='c012'><sup>[240]</sup></a> found that actinium gives out an emanation -similar to the emanation of thorium and radium. The loss -of activity of the emanation is even more rapid than for the -thorium emanation, for its activity falls to half value in 3·9 -seconds. In consequence of the rapid decay of activity, the -emanation is able to diffuse through the air only a short distance -from the active matter before it loses the greater proportion of its -<span class='pageno' id='Page_250'>250</span>activity. Giesel early observed that the radio-active substance -separated by him, which we have seen (<a href='#section018'>section 18</a>) is identical -in radio-active properties with actinium, gave off a large amount -of emanation. It was in consequence of this property, that he -gave it the name of the “emanating substance” and later -“emanium.” The impure preparations of this substance emit -the emanation very freely and in this respect differ from most -of the thorium compounds. The emanation from actinium like -those from thorium and radium possesses the property of exciting -activity on inactive bodies, but it has not yet been studied so -completely as the better known emanations of thorium and radium.</p> -<h3 class='c020'>Experiments with large amounts of Radium Emanation.</h3> -<p class='c005'><b>147.</b> With very active specimens of radium a large amount -of emanation can be obtained, and the electrical, photographic, and -fluorescent effects are correspondingly intense. On account of -the small activity of thorium and the rapid decay of its emanation -the effects due to it are weak, and can be studied only for a few -minutes after its production. The emanation from radium, on the -other hand, in consequence of the slow decay of its activity, may -be stored mixed with air in an ordinary gas-holder, and its photographic -and electrical actions may be examined several days or -even weeks after, quite apart from those of the radium from which -it was obtained.</p> - -<p class='c006'>It is, in general, difficult to study the radiation due to the -emanation alone, on account of the fact that the emanation is -continually producing a secondary type of activity on the surface -of the vessel in which the emanation is enclosed. This excited -activity reaches a maximum value several hours after the introduction -of the emanation, and, as long as it is kept in the vessel, -this excited activity on the walls decays at the same rate as the -emanation itself, <i>i.e.</i> it falls to half its initial value in about 4 days. -If, however, the emanation is blown out, the excited activity -remains behind on the surface, but rapidly loses its activity in the -course of a few hours. After several hours the intensity of the -residual radiation is very small.</p> - -<p class='c006'>These effects and their connection with the emanation are -discussed more fully in <a href='#chap08'>chapter <span class='fss'>VIII</span></a>. -<span class='pageno' id='Page_251'>251</span>Giesel<a id='r241' href='#f241' class='c012'><sup>[241]</sup></a> has recorded some interesting observations of the effect -of the radium emanation on a screen of phosphorescent zinc sulphide. -When a few centigrams of moist radium bromide were placed on a -screen any slight motion of the air caused the luminosity to move -to and fro on the screen. The direction of phosphorescence could -be altered at will by a slow current of air. The effect was still -further increased by placing the active material in a tube and -blowing the air through it towards the screen. A screen of barium -platinocyanide or of Balmain’s paint failed to give any visible -light under the same conditions. The luminosity was not altered -by a magnetic field, but it was affected by an electric field. If the -screen were charged the luminosity was more marked when it was -negative than when it was positive.</p> - -<p class='c006'>Giesel states that the luminosity was not equally distributed, -but was concentrated in a peculiar ring-shaped manner over the -surface of the screen. The concentration of luminosity on the -negative, rather than on the positive, electrode is probably due to -the excited activity, caused by the emanation, and not to the -emanation itself, for this excited activity is concentrated chiefly on -the negative electrode in an electric field (see <a href='#chap08'>chapter <span class='fss'>VIII</span></a>).</p> - -<p class='c006'>An experiment to illustrate the phosphorescence produced in -some substances by the rays from a large amount of emanation is -described in <a href='#section165'>section 165</a>.</p> -<p class='c005'><b>148.</b> Curie and Debierne<a id='r242' href='#f242' class='c012'><sup>[242]</sup></a> have investigated the emanation -from radium, and the excited activity produced by it. Some -experiments were made on the amount of emanation given off -from radium under very low pressures. The tube containing the -emanation was exhausted to a good vacuum by a mercury pump. -It was observed that a gas was given off from the radium which -produced excited activity on the glass walls. This gas was -extremely active, and rapidly affected a photographic plate through -the glass. It caused fluorescence on the surface of the glass and -rapidly blackened it, and was still active after standing ten days. -When spectroscopically examined, this gas did not show any new -lines, but generally those of the spectra of carbonic acid, hydrogen, -<span class='pageno' id='Page_252'>252</span>and mercury. In the light of the results described in <a href='#section124'>section 124</a> -the gas, given off by the radium, was probably the non-active -gases hydrogen and oxygen, in which the active emanation was -mixed in minute quantity. It will be shown later (<a href='#section242'>section 242</a>) -that the energy radiated from the emanation is enormous compared -with the amount of matter involved, and that the effects observed, -in most cases, are produced by an almost infinitesimal amount of -the emanation.</p> - -<p class='c006'>In further experiments, Curie and Debierne<a id='r243' href='#f243' class='c012'><sup>[243]</sup></a> found that many -substances were phosphorescent under the action of the emanation -and the excited activity produced by it. In their experiments, two -glass bulbs <i>A</i> and <i>B</i> (<a href='#fig054'>Fig. 54</a>) were connected with a glass tube. -The active material was placed in the bulb <i>A</i> and the substance -to be examined in the other.</p> - -<div id='fig054' class='figcenter id004'> -<img src='images/fig-054.png' alt='Fig. 54.' class='ig001'> -<div class='ic002'> -<p>Fig. 54.</p> -</div> -</div> - -<p class='c006'>They found that, in general, substances that were phosphorescent -in ordinary light became luminous. The sulphide of zinc was -especially brilliant and became as luminous as if exposed to a -strong light. After sufficient time had elapsed the luminosity -reached a constant value. The phosphorescence is partly due to -the excited activity produced by the emanation on its surface, and -partly to the direct radiation from the emanation.</p> - -<p class='c006'>Phosphorescence was also produced in glass. Thuringian glass -showed the most marked effects. The luminosity of the glass was -found to be about the same in the two bulbs, but was more marked -in the connecting tube. The effect in the two bulbs was the same -even if connected by a very narrow tube.</p> - -<p class='c006'>Some experiments were also made with a series of phosphorescent -plates placed in the vessel at varying distances apart. With -the plates 1 mm. apart the effect was very feeble, but increased -directly as the distance and was large for a distance of 3 cms.</p> - -<p class='c006'><span class='pageno' id='Page_253'>253</span>These effects receive a general explanation on the views already -put forward. When the radium is placed in the closed vessel, the -emanation is given off at a constant rate and gradually diffuses -throughout the enclosure. Since the time taken for diffusion of -the emanation through tubes of ordinary size is small compared -with the time required for the activity to be appreciably reduced, -the emanation, and also the excited activity due to it, will be -nearly equally distributed throughout the vessel.</p> - -<p class='c006'>The luminosity due to it should thus be equal at each end of -the tube. Even with a capillary tube connecting the two bulbs, the -gas continuously given off by the radium will always carry the -emanation with it and cause a practically uniform distribution.</p> - -<p class='c006'>The gradual increase of the amount of emanation throughout -the tube will be given by the equation</p> - -<div class='figcenter id009'> -<img src='images/form-067.png' alt='Formula.' class='ig001'> -</div> - -<p class='c006'>where <i>N<sub>t</sub></i> is the number of emanation particles present at the -time <i>t</i>, -<i>N</i>₀ -the number present when radio-active equilibrium is -reached, and λ is the radio-active constant of the emanation. The -phosphorescent action, which is due partly to the radiations from -the emanation and partly to the excited activity on the walls, -should thus reach half the maximum value in four days and should -practically reach its limit after three weeks’ interval.</p> - -<p class='c006'>The variation of luminosity with different distances between -the screens is to be expected. The amount of excited activity -deposited on the boundaries is proportional to the amount of -emanation present. Since the emanation is equally distributed, -the amount of excited activity deposited on the screens, due to the -emanation between them, varies directly as the distance, provided -the distance between the screens is small compared with their -dimensions. Such a result would also follow if the phosphorescence -were due to the radiation from the emanation itself, provided that -the pressure of the gas was low enough to prevent absorption of -the radiation from the emanation in the gas itself between the -screens.</p> -<div> - <span class='pageno' id='Page_254'>254</span> - <h3 class='c020'>Measurements of Emanating Power.</h3> -</div> -<p class='c005'><b>149. Emanating power.</b> The compounds of thorium in the -solid state vary very widely in the amount of emanation they emit -under ordinary conditions. It is convenient to use the term -<i>emanating power</i> to express the amount of emanation given off per -second by one gram of the compound. Since, however, we have -no means of determining absolutely the amount of emanation -present, all measurements of emanating power are of necessity -comparative. In most cases, it is convenient to take a given weight -of a thorium compound, kept under conditions as nearly as possible -constant, and to compare the amount of emanation of the compound -to be examined with this standard.</p> - -<p class='c006'>In this way comparisons of the emanating power of thorium -compounds have been made by Rutherford and Soddy<a id='r244' href='#f244' class='c012'><sup>[244]</sup></a>, using an -apparatus similar to that shown in <a href='#fig051'>Fig. 51</a> on page <a href='#Page_240'>240</a>.</p> - -<p class='c006'>A known weight of the substance to be tested was spread on a -shallow dish, placed in the glass tube <i>C</i>. A stream of dry dust-free -air, kept constant during all the experiments, was passed over the -compound and carried the emanation into the testing vessel. After -ten minutes interval, the current due to the emanation in the -testing vessel reached a constant value. The compound was then -removed, and the standard comparison sample of equal weight -substituted; the saturation current was observed when a steady -state was again reached. The ratio of these two currents gives -the ratio of the emanating power of the two samples.</p> - -<p class='c006'>It was found experimentally that, for the velocities of air -current employed, the saturation current in the testing vessel was -directly proportional to the weight of thorium, for weights up to -20 grams. This is explained by the supposition that the emanation -is removed by the current of air from the mass of the compound, -as fast as it is formed.</p> - -<div class='calc'> - -<div class='lg-container-b c017'> - <div class='linegroup'> - <div class='group'> - <div class='line'>Let <i>i</i><sub>1</sub> = saturation current due to a weight ω<sub>1</sub> of the standard,</div> - <div class='line in4'><i>i</i><sub>2</sub> = „ „ „ „ ω<sub>2</sub> of the sample to be tested.</div> - </div> - <div class='group'> - <div class='line in7'>(emanating power of specimen) <i>i</i><sub>2</sub> ω<sub>1</sub></div> - <div class='line'>Then ------------------------------- = --- ---</div> - <div class='line in7'>(emanating power of standard) <i>i</i><sub>1</sub> ω<sub>2</sub></div> - </div> - </div> -</div> - -</div> - -<p class='c018'><span class='pageno' id='Page_255'>255</span>By means of this relation the emanating power of compounds -which are not of equal weight can be compared.</p> - -<p class='c006'>It was found that thorium compounds varied enormously in -emanating power, although the percentage proportion of thorium -present in the compound was not very different. For example, -the emanating power of thorium hydroxide was generally 3 to 4 -times greater than that of ordinary thoria, obtained from the manufacturer. -Thorium nitrate, in the solid state, had only ¹⁄₂₀₀ of the -emanating power of ordinary thoria, while preparations of the -carbonate were found to vary widely among themselves in emanating -power, which depended upon slight variations in the method -of preparation.</p> -<p class='c005'><b>150. Effect of conditions on emanating power.</b> The -emanating power of different compounds of thorium and radium is -much affected by the alteration of chemical and physical conditions. -In this respect the emanating power, which is a measure of the -rate of escape of the emanation into the surrounding gas, must not -be confused with the rate of decay of the activity of the emanations -themselves, which has already been shown to be unaffected by -external conditions.</p> - -<p class='c006'>Dorn (<i>loc. cit.</i>) first observed that the emanating power of -thorium and radium compounds was much affected by moisture. -In a fuller investigation of this point by Rutherford and Soddy, it -was found that the emanating power of thoria is from two to three -times greater in a moist than in a dry gas. Continued desiccation -of the thoria in a glass tube, containing phosphorus pentoxide, did -not reduce the emanating power much below that observed in -ordinary dry air. In the same way radium chloride in the solid -state gives off very little emanation when in a dry gas, but the -amount is much increased in a moist gas.</p> - -<p class='c006'>The rate of escape of emanation is much increased by solution -of the compound. For example, thorium nitrate, which has an -emanating power of only ¹⁄₂₀₀ that of thoria in the solid state, -has in solution an emanating power of 3 to 4 times that of thoria. -P. Curie and Debierne observed that the emanating power of -radium was also much increased by solution.</p> - -<p class='c006'>Temperature has a very marked effect on the emanating power. -<span class='pageno' id='Page_256'>256</span>The writer<a id='r245' href='#f245' class='c012'><sup>[245]</sup></a> showed that the emanating power of ordinary thoria -was increased three to four times by heating the substance to a dull -red heat in a platinum tube. If the temperature was kept constant -the emanation continued to escape at the increased rate, -but returned to its original value on cooling. If, however, the -compound was heated to a white heat, the emanating power was -greatly reduced, and it returned on cooling to about 10% of the -original value. Such a compound is said to be <i>de-emanated</i>. -The emanating power of radium compounds varies in a still more -striking manner with rise of temperature. The rate of escape -of the emanation is momentarily increased even 10,000 times by -heating to a dull red heat. This effect does not continue, for the -large escape of the emanation by heating is in reality due to the -release of the emanation stored up in the radium compound. Like -thoria, when the compound has once been heated to a very high -temperature, it loses its emanating power and does not regain it. -It regains its power of emanating, however, after solution and -re-separation.</p> - -<p class='c006'>A further examination of the effect of temperature was made -by Rutherford and Soddy<a id='r246' href='#f246' class='c012'><sup>[246]</sup></a>. The emanating power of thoria -decreases very rapidly with lowering of temperature, and at the -temperature of solid carbonic acid it is only about 10% of its -ordinary value. It rapidly returns to its original value when the -cooling agent is removed.</p> - -<p class='c006'>Increase of temperature from 80° C. to a dull red heat of platinum -thus increases the emanating power about 40 times, and the -effects can be repeated again and again, with the same compound, -provided the temperature is not raised to the temperature at which -de-emanation begins. De-emanation sets in above a red heat, and -the emanating power is then permanently diminished, but even -long-continued heating at a white heat never entirely destroys the -emanating power.</p> -<p class='c005'><b>151. Regeneration of emanating power.</b> An interesting -question arises whether the de-emanation of thorium and radium is -due to a removal or alteration of the substance which produces the -<span class='pageno' id='Page_257'>257</span>emanation, or whether intense ignition merely changes the rate -of escape of the emanation from the solid into the surrounding -atmosphere.</p> - -<p class='c006'>It is evident that the physical properties of the thoria are -much altered by intense ignition. The compound changes in -colour from white to pink; it becomes denser and also far less readily -soluble in acids. In order to test if the emanating power could be -regenerated by a cyclic chemical process, the de-emanated thoria -was dissolved, precipitated as hydroxide and again converted into -oxide. At the same time a specimen of the ordinary oxide was -subjected to an exactly parallel process. The emanating power of -both these compounds was the same, and was from two to three -times greater than that of ordinary thoria.</p> - -<p class='c006'>Thus de-emanation does not permanently destroy the power -of thorium of giving out an emanation, but merely produces an -alteration of the amount of the emanation which escapes from the -compound.</p> -<p class='c005'><a id='section152'></a> -<b>152. Rate of production of the emanation.</b> The emanating -power of thorium compounds, then, is a very variable quantity, -much affected by moisture, heat, and solution. Speaking generally, -increased temperatures and solution greatly increase the emanating -power of both thorium and radium.</p> - -<p class='c006'>The wide differences between the emanating powers of these -substances in the solid state and in solution pointed to the conclusion -that the differences were probably due to the rate of escape of -the emanation into the surrounding gas, and not to a variation of -the rate of reaction which gave rise to the emanation. It is -obvious that a very slight retardation in the rate of escape of the -thorium emanation from the compound into the gas, will, on account -of the rapid decay of activity of the emanation, produce great -changes in emanating power. The regeneration of the emanating -power of de-emanated thoria and radium by solution and chemical -treatment made it evident that the original power of thorium and -radium of producing the emanation still persisted in an unaltered -degree.</p> - -<p class='c006'>The question whether the emanation was produced at the same -rate in emanating as in non-emanating compounds can be put to a -<span class='pageno' id='Page_258'>258</span>sharp quantitative test. If the rate of production of emanation -goes on at the same rate in the solid compound where very -little escapes, as in the solution where probably all escapes, the -emanation must be <i>occluded</i> in the compound, and consequently -there must be a sudden release of this emanation on solution of -the compound. On account of the very slow decay of the activity -of the emanation of radium, the effects should be far more marked -in that compound than in thorium.</p> - -<p class='c006'>From the point of view developed in <a href='#section133'>section 133</a>, the exponential -law of decay of the emanation expresses the result that -<i>N<sub>t</sub></i> the number of particles remaining unchanged at the time <i>t</i> -is given by</p> - -<div class='figcenter id002'> -<img src='images/form-064.png' alt='Formula.' class='ig001'> -</div> - -<p class='c006'>where -<i>N</i>₀ -is the initial number of particles present. When a -steady state is reached, the rate of production -<i>q</i>₀ -of fresh emanation -particles is exactly balanced by the rate of change of the particles -<i>N</i>₀ -already present, <i>i.e.</i></p> - -<div class='calc'> - -<div class='lg-container-b c017'> - <div class='linegroup'> - <div class='group'> - <div class='line'><i>q</i>₀ = λ<i>N</i>₀,</div> - </div> - </div> -</div> - -</div> - -<p class='c018'><i>N</i>₀ -in this case represents the amount of emanation “occluded” in -the compound. Substituting the value of λ found for the radium -emanation in <a href='#section145'>section 145</a>,</p> - -<div class='calc'> - -<div class='lg-container-b c017'> - <div class='linegroup'> - <div class='group'> - <div class='line in2'><i>N</i>₀</div> - <div class='line in1'>---- = 1/λ = 463,000.</div> - <div class='line in2'><i>q</i>₀</div> - </div> - </div> -</div> - -</div> - -<p class='c018'>The amount of emanation stored in a non-emanating radium -compound should therefore be nearly 500,000 times the amount -produced per second by the compound. This result was tested in -the following way<a id='r247' href='#f247' class='c012'><sup>[247]</sup></a>.</p> - -<p class='c006'>A weight of ·03 gr. of radium chloride of activity 1000 times that -of uranium was placed in a Drechsel bottle and a sufficient amount of -water drawn in to dissolve it. The released emanation was swept -out by a current of air into a small gas holder and then into a testing -cylinder. The initial saturation current was proportional to -<i>N</i>₀. -A rapid current of air was then passed through the radium solution -for some time in order to remove any slight amount of emanation -which had not been removed initially. The Drechsel bottle was -<span class='pageno' id='Page_259'>259</span>closed air-tight, and allowed to stand undisturbed for a definite -time <i>t</i>. The accumulated emanation was then swept out as before -into the testing vessel. The new ionization current represents -the value of <i>N<sub>t</sub></i> the amount of emanation formed in the compound -during the interval <i>t</i>.</p> - -<div class='calc'> - -<div class='lg-container-b c017'> - <div class='linegroup'> - <div class='group'> - <div class='line'>In the experiment <i>t</i> = 105 minutes,</div> - <div class='line'>and the observed value</div> - <div class='line in2'><i>N</i><sub>t</sub></div> - <div class='line in1'>---- = ·0131.</div> - <div class='line in2'><i>N</i>₀</div> - </div> - </div> -</div> - -</div> - -<p class='c018'>Assuming that there is no decay during the interval,</p> - -<div class='calc'> - -<div class='lg-container-b c017'> - <div class='linegroup'> - <div class='group'> - <div class='line'><i>N<sub>t</sub></i> = 105 × 60 × <i>q</i>₀.</div> - </div> - <div class='group'> - <div class='line in9'><i>N</i>₀</div> - <div class='line'>Thus -------- = 480,000.</div> - <div class='line in9'><i>q</i>₀</div> - </div> - </div> -</div> - -</div> - -<p class='c018'>Making the small correction for the decay of activity during -the interval,</p> - -<div class='calc'> - -<div class='lg-container-b c017'> - <div class='linegroup'> - <div class='group'> - <div class='line in1'><i>N</i>₀</div> - <div class='line'>---- = 477,000.</div> - <div class='line in1'><i>q</i>₀</div> - </div> - </div> -</div> - -</div> - -<p class='c018'>We have previously shown that from the theory</p> - -<div class='calc'> - -<div class='lg-container-b c017'> - <div class='linegroup'> - <div class='group'> - <div class='line in1'><i>N</i>₀ 1</div> - <div class='line'>----- = --- = 463,000.</div> - <div class='line in1'><i>q</i>₀ λ</div> - </div> - </div> -</div> - -</div> - -<p class='c018'>The agreement between theory and experiment is thus as close -as could be expected from the nature of the experiments. This -experiment proves conclusively that the rate of production of -emanation in the solid compound is the same as in the solution. -In the former case it is occluded, in the latter it escapes as fast as -it is produced.</p> - -<p class='c006'>It is remarkable how little emanation, compared with the -amount stored up in the compound, escapes from solid radium -chloride in a dry atmosphere. One experiment showed that the -emanating power in the dry solid state was less than ½% of the -emanating power of the solution. Since nearly 500,000 times as -much emanation is stored up in the solid compound as is produced -per second, this result showed that the amount of emanation -which escaped per second was less than -10<sup>-8</sup> -of that occluded in -the compound.</p> - -<p class='c006'><span class='pageno' id='Page_260'>260</span>If a solid radium chloride compound is kept in a moist atmosphere, -the emanating power becomes comparable with the amount -produced per second in the solution. In such a case, since the rate -of escape is continuous, the amount occluded will be much less than -the amount for the non-emanating material.</p> - -<p class='c006'>The phenomenon of occlusion of the radium emanation is -probably not connected in any way with its radio-activity, although -this property has here served to measure it. The occlusion of -helium by minerals presents almost a complete analogy to the -occlusion of the radium emanation. Part of the helium is given -off by fergusonite, for example, when it is heated and all of it -when the mineral is dissolved.</p> -<p class='c005'><b>153.</b> Similar results hold for thorium, but, on account of the -rapid loss of activity of the emanation, the amount of emanation -occluded in a non-emanating compound is very small compared -with that observed for radium. If the production of the thorium -emanation proceeds at the same rate under all conditions, the -solution of a solid non-emanating compound should be accompanied -by a rush of emanation greater than that subsequently produced. -With the same notation as before we have for the thorium -emanation,</p> - -<div class='calc'> - -<div class='lg-container-b c017'> - <div class='linegroup'> - <div class='group'> - <div class='line in1'><i>N</i>₀ 1</div> - <div class='line'>----- = --- = 87.</div> - <div class='line in1'><i>q</i>₀ λ</div> - </div> - </div> -</div> - -</div> - -<p class='c018'>This result was tested as follows: a quantity of finely powdered -thorium nitrate, of emanating power ¹⁄₂₀₀ of ordinary thoria, -was dropped into a Drechsel bottle containing hot water and the -emanation rapidly swept out into the testing vessel by a current of -air. The ionization current rose quickly to a maximum, but soon -fell again to a steady value; showing that the amount of emanation -released when the nitrate dissolves, is greater than the subsequent -amount produced from the solution.</p> - -<p class='c006'>The rapid loss of the activity of the thorium emanation makes -a quantitative comparison like that for radium very difficult. -By slightly altering the conditions of the experiment, however, a -definite proof was obtained that the rate of production of emanation -is the same in the solid compound as in the solution. After -dropping in the nitrate, a rapid air stream was blown through the -<span class='pageno' id='Page_261'>261</span>solution for 25 seconds into the testing vessel. The air stream was -stopped and the ionization current immediately measured. The -solution was then allowed to stand undisturbed for 10 minutes. -In that time the accumulation of the emanation again attained a -practical maximum and again represented a steady state. The -stream of air was blown through, as before, for 25 seconds, stopped -and the current again measured. In both cases, the electrometer -recorded a movement of 14·6 divisions per second. By blowing -the same stream of air continuously through the solution the final -current corresponded to 7·9 divisions per second or about one-half -of that observed after the first rush.</p> - -<p class='c006'>Thus the rate of production of emanation is the same in the -solid nitrate as in the solution, although the emanating power, <i>i.e.</i> -the rate of escape of the emanation, is over 600 times greater in -the solution than in the solid.</p> - -<p class='c006'>It seems probable that the rate of production of emanation -by thorium, like the rate of production of Ur X and Th X, is independent -of conditions. The changes of emanating power of the -various compounds by moisture, heat, and solution must therefore -be ascribed solely to an alteration in the rate of escape of the -emanation into the surrounding gas and not to an alteration in -the rate of its production in the compound.</p> - -<p class='c006'>On this view, it is easy to see that slight changes in the mode -of preparation of a thorium compound may produce large changes -in emanating power. Such effects have been often observed, and -must be ascribed to slight physical changes in the precipitate. -The fact that the rate of production of the emanation is independent -of the physical or chemical conditions of the thorium, in which -it is produced, is thus in harmony with what had previously been -observed for the radio-active products Ur X and Th X.</p> -<h3 class='c020'>Source of the Thorium Emanation.</h3> -<p class='c005'><a id='section154'></a> -<b>154.</b> Some experiments of Rutherford and Soddy<a id='r248' href='#f248' class='c012'><sup>[248]</sup></a> will now -be considered, which show that the thorium emanation is produced, -not directly by the thorium itself, but by the active -product Th X.</p> - -<p class='c006'><span class='pageno' id='Page_262'>262</span>When the Th X, by precipitation with ammonia, is removed from -a quantity of thorium nitrate, the precipitated thorium hydroxide -does not at first possess appreciable emanating power. This loss -of emanating power is not due, as in the case of the de-emanated -oxide, to a retardation in the rate of escape of the emanation -produced; for the hydroxide, when dissolved in acid, still gives -off no emanation. On the other hand, the solution, containing -the Th X, possesses emanating power to a marked degree. -When the precipitated hydroxide and the Th X is left for some -time, it is found that the Th X decreases in emanating power, -while the hydroxide gradually regains its emanating power. After -about a month’s interval, the emanating power of the hydroxide -has nearly reached a maximum, while the emanating power of -the Th X has almost disappeared.</p> - -<p class='c006'>The curves of decay and recovery of emanating power with -time are found to be exactly the same as the curves of decay -and recovery of activity of Th X and the precipitated hydroxide -respectively, shown in <a href='#fig047'>Fig. 47</a>. The emanating power of Th X, -as well as its activity, falls to half value in four days, while the -hydroxide regains half its final emanating power as well as half its -lost activity in the same interval.</p> - -<p class='c006'>It follows from these results that the emanating power of Th X -is directly proportional to its activity, <i>i.e.</i> that the rate of production -of emanating particles is always proportional to the number -of α particles, projected from the Th X per second. <i>The radiation -from Th X thus accompanies the change of the Th X into the -emanation.</i> Since the emanation has chemical properties distinct -from those of the Th X, and also a distinctive rate of decay, it -cannot be regarded as a vapour of Th X, but it is a distinct -chemical substance, produced by the changes occurring in Th X. -On the view advanced in <a href='#section136'>section 136</a>, the atom of the emanation -consists of the part of the atom of Th X left behind after the -expulsion of one or more α particles. The atoms of the emanation -are unstable, and in turn expel α particles. This projection -of α particles constitutes the radiation from the emanation, which -serves as a measure of the amount of emanation present. Since -the activity of the emanation falls to half value in <i>one</i> minute -while that of Th X falls to half value in four days, the emanation -<span class='pageno' id='Page_263'>263</span>consists of atoms which disintegrate at intervals nearly 6000 times -shorter than those of the atoms of Th X.</p> -<h3 class='c020'>Source of the Radium and Actinium Emanation.</h3> -<p class='c005'><b>155.</b> No intermediate stage—Radium X—between radium -and its emanation, corresponding to the Th X for thorium, has -so far been observed. The emanation from radium is probably -produced directly from that element. In this respect, the radium -emanation holds the same position in regard to radium as Th X -does to thorium, and its production from radium can be explained -on exactly similar lines. It will be shown later in chapter X, that -the emanation of actinium, like that of thorium, does not arise -directly from the parent element but from an intermediate product -actinium X, which is very analogous in physical and chemical -properties to Th X.</p> -<h3 class='c020'>Radiations from the Emanations.</h3> -<p class='c005'><a id='section156'></a> -<b>156.</b> Special methods are necessary to examine the nature of -the radiation from the emanations, for the radiations arise from -the volume of the gas in which the emanations are distributed. -Some experiments to examine the radiations from the thorium -emanation were made by the writer in the following way.</p> - -<div id='fig055' class='figcenter id002'> -<img src='images/fig-055.png' alt='Fig. 55.' class='ig001'> -<div class='ic002'> -<p>Fig. 55.</p> -</div> -</div> - -<p class='c006'>A highly emanating thorium compound wrapped in paper was -placed inside a lead box <i>B</i> about 1 cm. deep, shown in Fig. 55. -An opening was cut in the -top of the box, over which a -very thin sheet of mica was -waxed. The emanation rapidly -diffused through the paper into -the vessel, and after ten minutes -reached a state of radio-active -equilibrium. The penetrating power of the radiation from the -emanation which passed through the thin mica window was -examined by the electrical method in the usual way by adding -<span class='pageno' id='Page_264'>264</span>screens of thin aluminium foil. The results are expressed in the -following table:</p> - -<div class='lg-container-b c019'> - <div class='linegroup'> - <div class='group'> - <div class='line'>Thickness of mica window ·0015 cm.</div> - <div class='line'>Thickness of aluminium foil ·00034 cm.</div> - </div> - </div> -</div> - -<table class='table22' > -<colgroup> -<col class='colwidth50'> -<col class='colwidth50'> -</colgroup> - <tr> - <td class='c015'>Layers of foil</td> - <td class='c016'>Current</td> - </tr> - <tr> - <td class='c015'>0</td> - <td class='c016'>100</td> - </tr> - <tr> - <td class='c015'>1</td> - <td class='c016'>59</td> - </tr> - <tr> - <td class='c015'>2</td> - <td class='c016'>30</td> - </tr> - <tr> - <td class='c015'>3</td> - <td class='c016'>10</td> - </tr> - <tr> - <td class='c015'>4</td> - <td class='c016'>3·2</td> - </tr> -</table> - -<p class='c006'>The greater proportion of the conductivity is thus due to -α rays, as in the case of the radio-active elements. The amount -of absorption of these α rays by aluminium foil is about the same -as that of the rays from the active bodies. No direct comparison -can be made, for the α rays from the emanation show the characteristic -property of increased rate of absorption with thickness -of matter traversed. Before testing, the rays have been largely -absorbed by the mica window, and the penetrating power has -consequently decreased.</p> - -<p class='c006'>No alteration in the radiation from the emanation was observed -on placing an insulated wire inside the emanation vessel, -and charging it to a high positive or negative potential. When -a stream of air through the vessel carried away the emanation as -fast as it was produced, the intensity of the radiation fell to a small -fraction of its former value.</p> - -<p class='c006'>No evidence of any β rays in the radiations was found in -these experiments, although a very small effect would have been -detected. After standing some hours, however, β rays began to -appear. These were due to the excited activity deposited on the -walls of the vessel from the emanation, and not directly to the -emanation itself.</p> - -<p class='c006'>The radium emanation, like that of thorium, only gives rise to -α rays. This was tested in the following way<a id='r249' href='#f249' class='c012'><sup>[249]</sup></a>:</p> - -<p class='c006'>A large amount of emanation was introduced into a cylinder -made of sheet copper ·005 cm. thick, which absorbed all the -α rays but allowed the β and γ rays, if present, to pass through -with but little loss. The external radiation from the cylinder -<span class='pageno' id='Page_265'>265</span>was determined at intervals, commencing about two minutes after -the introduction of the emanation. The amount observed at first -was extremely small, but increased rapidly and practically reached -a maximum in three or four hours. Thus the radium emanation -only gives out α rays, the β rays appearing as the excited activity is -produced on the walls of the vessel. On sweeping out the emanation -by a current of air, there was no immediately appreciable -decrease of the radiation. This is another proof that the emanation -does not emit any β rays. In a similar way it can be shown that -the emanation does not give out γ rays; these rays always make -their appearance at the same time as the β rays.</p> - -<p class='c006'>The method of examination of the radiations from the -emanations has been given in some detail, as the results are of -considerable importance in the discussion, which will be given -later in chapters <a href='#chap10'><span class='fss'>X</span></a> and <a href='#chap11'><span class='fss'>XI</span></a>, of the connection between the changes -occurring in radio-active products and the radiations they emit. -There is no doubt that the emanations, apart from the excited -activity to which they give rise, only give out α rays, consisting -most probably of positively charged bodies projected with great -velocity.</p> -<h3 class='c020'>Effect of pressure on the rate of production of the Emanation.</h3> -<p class='c005'><a id='section157'></a> -<b>157.</b> It has already been mentioned that the conductivity -due to the thorium emanation is proportional to the pressure of -the gas, pointing to the conclusion that the rate of production -of the emanation is independent of the pressure, as well as of the -nature of the surrounding gas. This result was directly confirmed -with the apparatus of <a href='#fig055'>Fig. 55</a>. When the pressure of the gas -under the vessel was slowly reduced, the radiation, tested outside -the window, increased to a limit, and then remained constant -over a wide range of pressure. This increase, which was far more -marked in air than in hydrogen, is due to the fact that the α rays -from the emanation were partially absorbed in the gas inside the -vessel when at atmospheric pressure. At pressures of the order -of 1 millimetre of mercury the external radiation decreased, but -experiment showed that this must be ascribed to a removal of -the emanation by the pump, and not to a change in the rate of -<span class='pageno' id='Page_266'>266</span>production. The thorium compounds very readily absorb water-vapour, -which is slowly given off at low pressures, and in consequence -some of the emanation is carried out of the vessel with -the water-vapour.</p> - -<p class='c006'>Curie and Debierne<a id='r250' href='#f250' class='c012'><sup>[250]</sup></a> found that both the amount of excited -activity produced in a closed vessel containing active samples of -radium, and also the time taken to reach a maximum value, were -independent of the pressure and nature of the gas. This was true -in the case of a solution down to the pressure of the saturated -vapour, and in the case of solid salts to very low pressures. When -the pump was kept going at pressures of the order of ·001 mm. of -mercury, the amount of excited activity was much diminished. -This was probably not due to any alteration of the rate of escape -of the emanation, but to the removal of the emanation by the -action of the pump as fast as it was formed.</p> - -<p class='c006'>Since the amount of excited activity, when in a state of -radio-active equilibrium, is a measure of the amount of emanation -producing it, these results show that the amount of emanation -present when the rate of production balances the rate of decay is -independent of the pressure and nature of the gas. It was also -found that the time taken to reach the point of radio-active -equilibrium was independent of the size of the vessel or the -amount of active matter present. This proves that the state of -equilibrium cannot in any way be ascribed to the possession by the -emanation of any appreciable vapour pressure; for if such were the -case, the time taken to reach the equilibrium value should depend -on the size of the vessel and the amount of active matter present. -The results are, however, in agreement with the view that the -emanation is present in minute quantity in the tube, and that the -equilibrium is governed purely by the radio-active constant λ, the -constant of decay of activity of the emanation. This has been seen -to be the same under all conditions of concentration, pressure and -temperature, and, provided the rate of supply of the emanation -from the active compound is not changed, the time-rate of increase -of activity to the equilibrium value will always be the same, -whatever the size of the vessel or the nature and pressure of the -surrounding gas.</p> -<div> - <span class='pageno' id='Page_267'>267</span> - <h3 class='c020'>Chemical Nature of the Emanations.</h3> -</div> -<p class='c005'><b>158.</b> We shall now consider some experiments on the physical -and chemical properties of the emanations themselves, without -reference to the material producing them, in order to see if they -possess any properties which connect them with any known kind -of matter.</p> - -<p class='c006'>It was soon observed that the thorium emanation passed -unchanged through acid solutions, and later the same result was -shown to hold true in the case of both emanations for every -reagent that was tried. Preliminary observations<a id='r251' href='#f251' class='c012'><sup>[251]</sup></a> showed that the -thorium emanation, obtained in the usual way by passing air over -thoria, passed unchanged in amount through a platinum tube -heated electrically to the highest temperature obtainable. The -tube was then filled with platinum-black, and the emanation passed -through it in the cold, and with gradually increasing temperatures, -until the limit was reached. In another experiment, the emanation -was passed through a layer of red-hot lead-chromate in a -glass tube. The current of air was replaced by a current of -hydrogen, and the emanation was sent through red-hot magnesium-powder -and red-hot palladium-black, and, by using a current of -carbon dioxide, through red-hot zinc-dust. In every case the -emanation passed through without sensible change in the amount. -If anything, a slight increase occurred, owing to the time taken for -the gas-current to pass through the tubes when hot being slightly -less than when cold, the decay <i>en route</i> being consequently less. -The only known gases capable of passing in unchanged amount -through all the reagents employed are the recently discovered -members of the argon family.</p> - -<p class='c006'>But another possible interpretation might be put upon the -results. If the emanation were the manifestation of a type of -excited radio-activity on the surrounding atmosphere, then, since -from the nature of the experiments it was necessary to employ in -each case as the atmosphere, a gas not acted on by the reagent -employed, the result obtained might be expected. Red-hot magnesium -would not retain an emanation consisting of radio-active -hydrogen, nor red-hot zinc-dust an emanation consisting of radio-active -<span class='pageno' id='Page_268'>268</span>carbon dioxide. The incorrectness of this explanation was -shown in the following way. Carbon dioxide was passed over -thoria, then through a <b>T</b>-tube, where a current of air met and -mixed with it, both passing on to the testing-cylinder. But -between this and the <b>T</b>-tube a large soda-lime tube was introduced, -and the current of gas was thus freed from its admixed -carbon dioxide, before being tested in the cylinder for the emanation. -The amount of emanation found was quite unchanged, -whether carbon dioxide was sent over thoria in the manner described, -or whether, keeping the other arrangements as before, -an equally rapid current of air was substituted for it. The theory -that the emanation is an effect of the excited activity on the -surrounding medium is thus excluded.</p> - -<p class='c006'>Experiments of a similar kind on the radium emanation were -made later. A steady stream of gas was passed through a radium -chloride solution and then through the reagent to be employed, -into a testing-vessel of small volume, so that any change in the -amount of emanation passing through could readily be detected. -The radium emanation, like that of thorium, passed unchanged in -amount through every reagent used.</p> - -<p class='c006'>In later experiments by Sir William Ramsay and Mr Soddy<a id='r252' href='#f252' class='c012'><sup>[252]</sup></a>, -the emanation from radium was exposed to still more drastic -treatment. The emanation in a glass tube was sparked for -several hours with oxygen over alkali. The oxygen was then -removed by ignited phosphorus and no visible residue was left. -When, however, another gas was introduced, mixed with the -minute amount of emanation in the tube and withdrawn, the -activity of emanation was found to be unaltered. In another -experiment, the emanation was introduced into a magnesium lime -tube, which was heated for three hours at a red heat. The -emanation was then removed and tested, but no diminution in its -discharging power was observed.</p> - -<p class='c006'>The emanations of thorium and radium thus withstand chemical -treatment in a manner hitherto unobserved except in gases of the -argon family.</p> -<p class='c005'><b>159.</b> Ramsay and Soddy (<i>loc. cit.</i>) record an interesting -experiment to illustrate the gaseous nature of the emanation. -<span class='pageno' id='Page_269'>269</span>A large amount of the radium emanation was collected in a -small glass tube. This tube phosphoresced brightly under the -influence of the rays from the emanation. The passage of the -emanation from point to point was observed in a darkened -room by the luminosity excited in the glass. On opening the -stop-cock connecting with the Töpler pump, the slow flow through -the capillary tube was noticed, the rapid passage along the wider -tubes, the delay in passing through a plug of phosphorous pentoxide, -and the rapid expansion into the reservoir of the pump. -When compressed, the luminosity of the emanation increased, and -became very bright as the small bubble containing the emanation -was expelled through the fine capillary tube.</p> -<h3 class='c020'>Diffusion of the Emanations.</h3> -<p class='c005'><b>160.</b> It has been shown that the emanations of thorium and -radium behave like radio-active gases, distributed in minute amount -in the air or other gas in which they are tested. With the small -quantities of active material so far investigated, the emanations -have not yet been collected in sufficient amount to determine -their density. Although the molecular weight of the emanations -cannot yet be obtained by direct chemical methods, an indirect -estimate of it can be made by determining the rate of their inter-diffusion -into air or other gases. The coefficients of inter-diffusion -of various gases have long been known, and the results show that -the coefficient of diffusion of one gas into another is, for the -simpler gases, approximately inversely proportional to the square -root of the product of their molecular weights. If, therefore, the -coefficient of diffusion of the emanation into air is found to have -a value, lying between that of two known gases <i>A</i> and <i>B</i>, it is -probable that the molecular weight of the emanation lies between -that of <i>A</i> and <i>B</i>.</p> - -<p class='c006'>Although the volume of the emanation given off from radium -is very small, the electrical conductivity produced by the emanation -in the gas, with which it is mixed, is often very large, and offers -a ready means of measuring the emanation present.</p> - -<p class='c006'>Some experiments have been made by Miss Brooks and the -writer<a id='r253' href='#f253' class='c012'><sup>[253]</sup></a> to determine the rate of the diffusion of the radium emanation -<span class='pageno' id='Page_270'>270</span>into air, by a method similar to that employed by Loschmidt<a id='r254' href='#f254' class='c012'><sup>[254]</sup></a> -in 1871, in his investigations of the coefficient of inter-diffusion -of gases.</p> - -<div id='fig056' class='figcenter id001'> -<img src='images/fig-056.png' alt='Fig. 56.' class='ig001'> -<div class='ic002'> -<p>Fig. 56.</p> -</div> -</div> - -<p class='c006'><a href='#fig056'>Fig. 56</a> shows the general arrangement. A long brass cylinder -<i>AB</i>, of length 73 cms., and diameter 6 cms., was divided into two -equal parts by a moveable metal slide <i>S</i>. The ends of the cylinder -were closed with ebonite stoppers. Two insulated brass rods, <i>a</i> -and <i>b</i>, each half the length of the tube, passed through the ebonite -stoppers and were supported centrally in the tube. The cylinder -was insulated and connected with one pole of a battery of 300 -volts, the other pole of which was earthed. The central rods could -be connected with a sensitive quadrant electrometer. The cylinder -was covered with a thick layer of felt, and placed inside a metal -box filled with cotton wool in order to keep temperature conditions -as steady as possible.</p> - -<p class='c006'>In order to convey a sufficient quantity of emanation into -the half-cylinder <i>A</i>, it was necessary to heat the radium slightly. -The slide <i>S</i> was closed and the side tubes opened. A slow -current of dry air from a gasometer was passed through a platinum -tube, in which a small quantity of radium compound was placed. -The emanation was carried with the air into the cylinder <i>A</i>. When -a sufficient quantity had been introduced, the stream of air was -stopped. The side tubes were closed by fine capillary tubes. -These prevented any appreciable loss of gas due to the diffusion, -but served to keep the pressure of the gas inside <i>A</i> at the pressure -of the outside air. The three entrance tubes into the cylinder, -shown in the figure, were for the purpose of initially mixing the -emanation and gas as uniformly as possible.</p> - -<p class='c006'><span class='pageno' id='Page_271'>271</span>After standing several hours to make temperature conditions -steady, the slide was opened, and the emanation began to diffuse -into the tube <i>B</i>. The current through the tubes <i>A</i> and <i>B</i> was -measured at regular intervals by an electrometer, with a suitable -capacity in parallel. Initially there is no current in <i>B</i>, but after -the opening of the slide, the amount in <i>A</i> decreased and the -amount in <i>B</i> steadily increased. After several hours the amount -in each half is nearly the same, showing that the emanation is -nearly uniformly diffused throughout the cylinder.</p> - -<p class='c006'>It can readily be shown<a id='r255' href='#f255' class='c012'><sup>[255]</sup></a> that if</p> - -<div class='calc'> - -<div class='lg-container-b c017'> - <div class='linegroup'> - <div class='group'> - <div class='line in4'><i>K</i> = coefficient of diffusion of the emanation into air,</div> - <div class='line in4'><i>t</i> = duration of diffusion experiments in secs.,</div> - <div class='line in4'><i>a</i> = total length of cylinder,</div> - <div class='line'><i>S</i><sub>1</sub> = partial pressure of emanation in tube <i>A</i> at end of diffusion,</div> - <div class='line'><i>S</i><sub>2</sub> = partial pressure of emanation in tube <i>B</i> at end of diffusion,</div> - </div> - </div> -</div> - -</div> - -<p class='c018'>then</p> - -<div class='figcenter id007'> -<img src='images/form-068.png' alt='Formula.' class='ig001'> -</div> - -<p class='c006'>Now the values of -<i>S</i><sub>1</sub> and <i>S</i><sub>2</sub> -are proportional to the saturation -ionization currents due to the emanations in the two halves of the -cylinder. From this equation <i>K</i> can be determined, if the relative -values of -<i>S</i><sub>1</sub> and <i>S</i><sub>2</sub> -are observed after diffusion has been in progress -for a definite interval <i>t</i>.</p> - -<p class='c006'>The determination of -<i>S</i><sub>1</sub> and <i>S</i><sub>2</sub> -is complicated by the excited -activity produced on the walls of the vessel. The ionization due -to this must be subtracted from the total ionization observed in -each half of the cylinder, for the excited activity is produced from -the material composing the emanation, and is removed to the -electrodes in an electric field. The ratio of the current due to -excited activity to the current due to the emanation depends on -the time of exposure to the emanation, and is only proportional to -it for exposures of several hours.</p> - -<p class='c006'>The method generally adopted in the experiments was to open -the slide for a definite interval, ranging in the experiments from -15 to 120 minutes. The slide was then closed and the currents -in each half determined at once. The central rods, which had -<span class='pageno' id='Page_272'>272</span>been kept negatively charged during the experiments, had most -of the excited activity concentrated on their surfaces. These -were removed, new rods substituted and the current immediately -determined. The ratio of the currents in the half cylinders under -these conditions was proportional to -<i>S</i><sub>1</sub> and <i>S</i><sub>2</sub>, -the amounts of -emanation present in the two halves of the cylinder.</p> - -<p class='c006'>The values of <i>K</i>, deduced from different values of <i>t</i>, were found -to be in good agreement. In the earlier experiments the values -of <i>K</i> were found to vary between ·08 and ·12. In some later -experiments, where great care was taken to ensure that temperature -conditions were very constant, the values of <i>K</i> were found to -vary between ·07 and ·09. The lower value ·07 is most likely -nearer the true value, as temperature disturbances tend to give -too large a value of <i>K</i>. No certain differences were observed in -the value of <i>K</i> whether the air was dry or damp, or whether an -electric field was acting or not.</p> -<p class='c005'><a id='section161'></a> -<b>161.</b> Some experiments on the rate of diffusion of the radium -emanation into air were made at a later date by P. Curie and Danne<a id='r256' href='#f256' class='c012'><sup>[256]</sup></a>. -If the emanation is contained in a closed reservoir, it has been shown -that its activity, which is a measure of the amount of emanation -present, decreases according to an exponential law with the time. -If the reservoir is put in communication with the outside air -through a capillary tube, the emanation slowly diffuses out, and -the amount of emanation in the reservoir is found to decrease -according to the same law as before, but at a faster rate. Using -tubes of different lengths and diameters, the rate of diffusion was -found to obey the same laws as a gas. The value of <i>K</i> was found -to be 0·100. This is a slightly greater value of <i>K</i> than the lowest -value 0·07 found by Rutherford and Miss Brooks. No mention -is made by Curie and Danne of having taken any special precautions -against temperature disturbances, and this may account for -the higher value of <i>K</i> obtained by them.</p> - -<p class='c006'>They also found that the emanation, like a gas, always divided -itself between two reservoirs, put in connection with one another, -in the proportion of their volumes. In one experiment one reservoir -was kept at a temperature of 10° C. and the other at 350° C. -<span class='pageno' id='Page_273'>273</span>The emanation divided itself between the two reservoirs in the -same proportion as would a gas under the same conditions.</p> -<p class='c005'><a id='section162'></a> -<b>162.</b> For the purpose of comparison, a few of the coefficients -of inter-diffusion of gases, compiled from Landolt and Bernstein’s -tables, are given below.</p> - -<table class='table8' > -<colgroup> -<col class='colwidth37'> -<col class='colwidth37'> -<col class='colwidth25'> -</colgroup> - <tr> - <th class='c013'>Gas or vapour</th> - <th class='c013'>Coefficient of diffusion into air</th> - <th class='c016'>Molecular weight</th> - </tr> - <tr> - <td class='c013'> </td> - <td class='c013'> </td> - <td class='c016'> </td> - </tr> - <tr> - <td class='c013'>Water vapour</td> - <td class='c013'>0·198</td> - <td class='c016'>18</td> - </tr> - <tr> - <td class='c013'>Carbonic acid gas</td> - <td class='c013'>0·142</td> - <td class='c016'>44</td> - </tr> - <tr> - <td class='c013'>Alcohol vapour</td> - <td class='c013'>0·101</td> - <td class='c016'>46</td> - </tr> - <tr> - <td class='c013'>Ether vapour</td> - <td class='c013'>0·077</td> - <td class='c016'>74</td> - </tr> - <tr> - <td class='c013'>Radium emanation</td> - <td class='c013'>0·07</td> - <td class='c016'>?</td> - </tr> -</table> - -<p class='c006'>The tables, although not very satisfactory for the purpose of -comparison, show that the coefficient of inter-diffusion follows the -inverse order of the molecular weights. The value of <i>K</i> for the -radium emanation is slightly less than for ether vapour, of which -the molecular weight is 74. We may thus conclude that the -emanation is of greater molecular weight than 74. It seems -likely that the emanation has a molecular weight somewhere in -the neighbourhood of 100, and is probably greater than this, -for the vapours of ether and alcohol have higher diffusion -coefficients compared with carbonic acid than the theory would -lead us to anticipate. Comparing the diffusion coefficients of the -emanation and carbonic acid into air, the value of the molecular -weight of the emanation should be about 176 if the result -observed for the simple gases, viz. that the coefficient of diffusion -is inversely proportional to the square root of the molecular -weights, holds true in the present case. Bumstead and Wheeler<a id='r257' href='#f257' class='c012'><sup>[257]</sup></a> -compared the rates of diffusion of the radium emanation and of -carbon dioxide through a porous plate, and concluded that the -molecular weight of the emanation was about 180. On the disintegration -theory, the atom of the emanation is derived from the -radium atom by the expulsion of one α particle. Thus, it is to be -expected that its molecular weight would be over 200.</p> - -<p class='c006'>It is of interest to compare the value of <i>K</i> = ·07 with the value -of <i>K</i> determined by Townsend (<a href='#section037'>section 37</a>) for the gaseous ions -<span class='pageno' id='Page_274'>274</span>produced in air at ordinary pressure and temperature, by Röntgen -rays or by the radiations from active substances. Townsend found -that the value of <i>K</i> in dry air was ·028 for the positive ions -and ·043 for the negative ions. The radium emanation thus -diffuses more rapidly than the ions produced by its radiation in -the gas, and behaves as if its mass were smaller than that of -the ions produced in air, but considerably greater than that of -the air molecules with which it is mixed.</p> - -<p class='c006'>It is not possible to regard the emanation as a temporarily -modified condition of the gas originally in contact with the active -body. Under such conditions a much larger value of <i>K</i> would be -expected. The evidence derived from the experiments on diffusion -strongly supports the view that the emanation is a gas of heavy -molecular weight.</p> - -<p class='c006'>Makower<a id='r258' href='#f258' class='c012'><sup>[258]</sup></a> has recently attacked the question of the molecular -weight of the radium emanation by another method. The rate of -diffusion of the emanation through a porous plug of plaster-of-Paris -was compared with that of the gases oxygen, carbon dioxide, -and sulphur dioxide. It was found that Graham’s law, viz. that -the coefficient of diffusion <i>K</i> is inversely proportional to the -square root of its molecular weight <i>M</i>, was not strictly applicable. -The value of <i>K</i> √<i>M</i> was not found to be constant for these gases, -but decreased with increase of molecular weight of the gas. If, -however, a curve was plotted with <i>K</i> √<i>M</i> as ordinate and <i>K</i> as -abscissa, the points corresponding to the values of O, CO<sub>2</sub> and SO<sub>2</sub> -were found to lie on a straight line. By linear extrapolation, the -molecular weight of the emanation was estimated. The value -obtained from experiments on three different porous plugs was -85·5, 97, and 99 respectively. This method indicates that the -molecular weight of the radium emanation is about 100; but in -all the experiments on diffusion, it must be remembered that the -emanation, whose rate of inter-diffusion is being examined, exists -in minute quantity mixed with the gas, and is compared with the -rate of inter-diffusion of gases which are present in large quantity. -For this reason, deductions of the molecular weight of the -emanation may be subject to comparatively large errors, for which -it is difficult to make correction.</p> -<div> - <span class='pageno' id='Page_275'>275</span> - <h3 class='c020'>Diffusion of the Thorium Emanation.</h3> -</div> -<p class='c005'><b>163.</b> On account of the rapid decay of the activity of the -thorium emanation, it is not possible to determine the value of <i>K</i> -its coefficient of diffusion into air by the methods employed for the -radium emanation. The value of <i>K</i> has been determined by the -writer in the following way. A plate <i>C</i>, -<a href='#fig057'>Fig. 57</a>, covered with thorium hydroxide, was -placed horizontally near the base of a long -vertical brass cylinder <i>P</i>. The emanation -released from the thorium compound diffuses -upwards in the cylinder.</p> - -<div id='fig057' class='figcenter id005'> -<img src='images/fig-057.png' alt='Fig. 57.' class='ig001'> -<div class='ic002'> -<p>Fig. 57.</p> -</div> -</div> - -<p class='c006'>Let <i>p</i> be the partial pressure of the emanation -at a distance <i>x</i> from the source <i>C</i>. This -will be approximately uniform over the cross -section of the cylinder. From the general -principles of diffusion we get the equation</p> - -<div class='calc'> - -<div class='lg-container-b c017'> - <div class='linegroup'> - <div class='group'> - <div class='line in3'><i>d<sup>2</sup>p</i> <i>dp</i></div> - <div class='line'><i>K</i> ---- = – ---- .</div> - <div class='line in3'><i>dx<sup>2</sup></i> <i>dt</i></div> - </div> - </div> -</div> - -</div> - -<p class='c018'>The emanation is continuously breaking -up and expelling α particles. The emanation-residue gains a positive -charge, and, in an electric field, is removed at once from the -gas to the negative electrode.</p> - -<p class='c006'>Since the activity of the emanation at any time is always -proportional to the number of particles which have not broken up, -and since the activity decays with the time according to an -exponential law,</p> - -<div class='figcenter id010'> -<img src='images/form-069.png' alt='Formula.' class='ig001'> -</div> - -<p class='c006'>where -<i>p</i><sub>1</sub> -is the value of <i>p</i> when <i>t</i> = 0 -and λ is the <i>radio-active constant</i> of the emanation.</p> - -<div class='calc'> - -<div class='lg-container-b c017'> - <div class='linegroup'> - <div class='group'> - <div class='line'>Then</div> - </div> - <div class='group'> - <div class='line in1'><i>dp</i></div> - <div class='line'>---- = -λ<i>p</i>,</div> - <div class='line in1'><i>dt</i></div> - </div> - <div class='group'> - <div class='line'>and</div> - </div> - <div class='group'> - <div class='line in3'><i>d<sup>2</sup>p</i></div> - <div class='line'>K ---- = λ<i>p</i>.</div> - <div class='line in3'><i>dx<sup>2</sup></i></div> - </div> - </div> -</div> - -</div> - -<p class='c018'>Thus</p> - -<div class='figcenter id002'> -<img src='images/form-070.png' alt='Formula.' class='ig001'> -</div> - -<p class='c006'>Since <i>p</i> = 0 when <i>x</i> = -∞. -<i>B</i> = 0. -If <i>p</i> = <i>p</i>₀ when <i>x</i> = 0, <i>A</i> = <i>p</i>₀.</p> - -<p class='c006'>Thus</p> - -<div class='figcenter id002'> -<img src='images/form-071.png' alt='Formula.' class='ig001'> -</div> - -<p class='c006'><span class='pageno' id='Page_276'>276</span>It was not found convenient in the experiments to determine -the activity of the emanation along the cylinder, but an equivalent -method was used which depends upon measuring the distribution -of “excited activity,” produced along a central rod <i>AB</i>, which is -charged negatively.</p> - -<p class='c006'>It will be shown later (<a href='#section177'>section 177</a>) that the amount of excited -activity at any point is always proportional to the amount of -emanation at that point. The distribution of “excited activity” -along the central rod from the plate <i>C</i> upwards thus gives the -variation of <i>p</i> for the emanation along the tube.</p> - -<p class='c006'>In the experiments, the cylinder was filled with dry air at -atmospheric pressure and was kept at a constant temperature. -The central rod was charged negatively and exposed from one to -two days in the presence of the emanation. The rod was then -removed, and the distribution of the excited activity along it -determined by the electric method. It was found that the amount -of excited activity fell off with the distance <i>x</i> according to an -exponential law, falling to half value in about 1·9 cms. This is in -agreement with the above theory.</p> - -<p class='c006'>Since the activity of the emanation falls to half value in -1 minute, λ = ·0115. The value <i>K</i> = ·09 was deduced from the -average of a number of experiments. This is a slightly greater -value than <i>K</i> = ·07, obtained for the radium emanation, but the -results show that the two emanations do not differ much from -one another in molecular weight.</p> - -<p class='c006'>Makower (<i>loc. cit.</i>) compared the rates of diffusion of the -thorium and radium emanation through a porous plate, and -concluded that the two emanations were of about the same -molecular weight, thus confirming the results obtained by the -above method.</p> -<h3 class='c020'>Diffusion of the Emanation into Liquids.</h3> -<p class='c005'><b>164.</b> Experiments have been made by Wallstabe<a id='r259' href='#f259' class='c012'><sup>[259]</sup></a> on the -coefficient of diffusion of the radium emanation into various liquids. -The radium emanation was allowed to diffuse into a closed reservoir, -containing a cylinder of the liquid under observation. The cylinder -<span class='pageno' id='Page_277'>277</span>was provided with a tube and a stop-cock extending beyond the -closed vessel, so that different layers of the liquid could be removed. -The liquid was then placed in a closed testing vessel, where the -ionization current due to the escape of the emanation from the -liquid was observed to rise to a maximum after several hours, and -then to decay. This maximum value of the current was taken as -a measure of the amount of emanation absorbed in the liquid.</p> - -<p class='c006'>The coefficient of diffusion <i>K</i> of the emanation into the liquid -can be obtained from the same equation used to determine the -diffusion of the thorium emanation into air,</p> - -<div class='figcenter id005'> -<img src='images/form-071.png' alt='Formula.' class='ig001'> -</div> - -<p class='c006'>where λ is the constant of decay of activity of the radium emanation -and <i>x</i> the depth of the layer of water from the surface.</p> - -<p class='c006'>Putting</p> - -<div class='figcenter id009'> -<img src='images/form-072.png' alt='Formula.' class='ig001'> -</div> - -<p class='c006'>it was found that</p> - -<div class='calc'> - -<div class='lg-container-b c017'> - <div class='linegroup'> - <div class='group'> - <div class='line'>for water α = 1·6,</div> - <div class='line'>for toluol α = ·75.</div> - </div> - </div> -</div> - -</div> - -<p class='c018'>The value of λ expressed in terms of a day as the unit of time -is about ·17.</p> - -<p class='c006'>Thus the value of <i>K</i> for the diffusion of the radium emanation -into water = ·066 cm.<sup>2</sup> / day.</p> - -<p class='c006'>The value of <i>K</i> found by Stefan<a id='r260' href='#f260' class='c012'><sup>[260]</sup></a> for the diffusion of carbon -dioxide into water was -1·36 cm.<sup>2</sup>/day. -These results are thus in harmony -with the conclusion drawn from the diffusion of the radium -emanation into air, and show that the radium emanation behaves -as a gas of high molecular weight.</p> -<h3 class='c020'>Condensation of the Emanations.</h3> -<p class='c005'><a id='section165'></a> -<b>165. Condensation of the emanations.</b> During an investigation -of the effect of physical and chemical agencies on -the thorium emanation, Rutherford and Soddy<a id='r261' href='#f261' class='c012'><sup>[261]</sup></a> found that the -<span class='pageno' id='Page_278'>278</span>emanation passed unchanged in amount through a white-hot -platinum tube and through a tube cooled to the temperature -of solid carbon dioxide. In later experiments the effects of still -lower temperatures were examined, and it was then found that at -the temperature of liquid air both emanations were condensed<a id='r262' href='#f262' class='c012'><sup>[262]</sup></a>.</p> - -<p class='c006'>If either emanation is conveyed by a slow stream of hydrogen, -oxygen, or air through a metal spiral immersed in liquid air, and -placed in connection with a testing vessel as in Fig. 51, no trace -of emanation escapes in the issuing gas. When the liquid air is -removed and the spiral plunged into cotton-wool, several minutes -elapse before any deflection of the electrometer needle is observed, -and then the condensed emanation volatilizes rapidly, and the -movement of the electrometer needle is very sudden, especially -in the case of radium. With a fairly large amount of radium -emanation, under the conditions mentioned, a very few seconds -elapse after the first sign of movement before the electrometer -needle indicates a deflection of several hundred divisions per -second. It is not necessary in either case that the emanating -compound should be retained in the gas stream. After the -emanation is condensed in the spiral, the thorium or radium -compound may be removed and the gas stream sent directly -into the spiral. But in the case of thorium, under these conditions, -the effects observed are naturally small owing to the rapid -loss of the activity of the emanation with time, which proceeds at -the same rate at the temperature of liquid air as at ordinary -temperatures.</p> - -<p class='c006'>If a large amount of radium emanation is condensed in a glass -<b>U</b> tube, the progress of the condensation can be followed by the -eye, by means of the phosphorescence which the radiations excite -in the glass. If the ends of the tube are sealed and the temperature -allowed to rise, the glow diffuses uniformly throughout the -tube, and can be concentrated at any point to some extent by -local cooling of the tube with liquid air.</p> -<p class='c005'><b>166. Experimental arrangements.</b> A simple experimental -arrangement to illustrate the condensation and volatilization of the -emanation and some of its characteristic properties is shown in -<span class='pageno' id='Page_279'>279</span><a href='#fig058'>Fig. 58</a>. The emanation obtained from a few milligrams of radium -bromide by solution or heating is condensed in the glass <b>U</b> -tube <i>T</i> immersed in liquid air. -This <b>U</b> tube is then put into connection -with a larger glass tube <i>V</i>, -in the upper part of which is placed -a piece of zinc sulphide screen <i>Z</i>, -and in the lower part of the tube -a piece of the mineral willemite. -The stop-cock <i>A</i> is closed and the -<b>U</b> tube and the vessel <i>V</i> are partially -exhausted by a pump through -the stop-cock <i>B</i>. This lowering of -the pressure causes a more rapid -diffusion of the emanation when -released. The emanation does not -escape if the tube <i>T</i> is kept immersed -in liquid air. The stop-cock <i>B</i> is then closed, and the -liquid air removed. No luminosity of the screen or the willemite -in the tube <i>V</i> is observed for several minutes, until the temperature -of <i>T</i> rises above the point of volatilization of the emanation. -The emanation is then rapidly carried into the vessel <i>V</i>, -partly by expansion of the gas in the tube <i>T</i> with rising temperature, -and partly by the process of diffusion. The screen <i>Z</i> and -the willemite <i>W</i> are caused to phosphoresce brilliantly under the -influence of the rays from the emanation surrounding them.</p> - -<div id='fig058' class='figcenter id005'> -<img src='images/fig-058.png' alt='Fig. 58.' class='ig001'> -<div class='ic002'> -<p>Fig. 58.</p> -</div> -</div> - -<p class='c006'>If the end of the vessel <i>V</i> is then plunged into liquid air, the -emanation is again condensed in the lower end of the tube, and the -willemite phosphoresces much more brightly than before. This is -not due to an increase of the phosphorescence of willemite at the -temperature of the liquid air, but to the effect of the rays from -the emanation condensed around it. At the same time the luminosity -of the zinc sulphide gradually diminishes, and practically -disappears after several hours if the end of the tube is kept in -the liquid air. If the tube is removed from the liquid air, -the emanation again volatilizes and lights up the screen <i>Z</i>. The -luminosity of the willemite returns to its original value after the -lapse of several hours. This slow change of the luminosity of -the zinc sulphide screen and of the willemite is due to the gradual -<span class='pageno' id='Page_280'>280</span>decay of the “excited activity” produced by the emanation on -the surface of all bodies exposed to its action (<a href='#chap08'>chapter <span class='fss'>VIII</span></a>). -The luminosity of the screen is thus due partly to the radiation -from the emanation and partly to the excited radiation caused -by it. As soon as the emanation is removed from the upper -to the lower part of the tube, the “excited” radiation gradually -diminishes in the upper and increases in the lower part of the -tube.</p> - -<p class='c006'>The luminosity of the screen gradually diminishes with the -time as the enclosed emanation loses its activity, but is still -appreciable after an interval of several weeks.</p> - -<p class='c006'>An apparatus of a similar character to illustrate the condensation -of the radium emanation has been described by P. Curie<a id='r263' href='#f263' class='c012'><sup>[263]</sup></a>.</p> - -<div id='fig059' class='figcenter id006'> -<img src='images/fig-059.png' alt='Fig. 59.' class='ig001'> -<div class='ic002'> -<p>Fig. 59.</p> -</div> -</div> -<p class='c005'><b>167. Determination of the temperature of condensation.</b> -A detailed investigation was made by Rutherford and -Soddy (<i>loc. cit.</i>) of the temperatures at which condensation and -volatilization commenced for the two emanations. The experimental -arrangement of the first method is shown clearly in <a href='#fig059'>Fig. 59</a>. -A slow constant stream of gas, entering at <i>A</i>, was passed through -a copper spiral <i>S</i>, over 3 metres in length, immersed in a bath -of liquid ethylene. The copper spiral was made to act as its -own thermometer by determining its electrical resistance. The -<span class='pageno' id='Page_281'>281</span>resistance temperature curve was obtained by observation of the -resistances at 0°, the boiling point of liquid ethylene -103·5°, -the solidification point of ethylene -169° and in liquid air. The -temperature of the liquid air was deduced from the tables given -by Baly for the boiling point of liquid air for different percentages -of oxygen. The resistance-temperature curve, for the particular -spiral employed, was found to be nearly a straight line between -0° and -192°C., cutting the temperature axis if produced nearly -at the absolute zero. The resistance of the spiral, deduced from -readings on an accurately calibrated Weston millivoltmeter, with -a constant current through the spiral, was thus very approximately -proportional to the absolute temperature. The liquid ethylene was -kept vigorously stirred by an electric motor, and was cooled to any -desired temperature by surrounding the vessel with liquid air.</p> - -<p class='c006'>The general method employed for the radium emanation was -to pass a suitable amount of emanation, mixed with the gas to be -used, from the gas holder <i>B</i> into the spiral, cooled below the -temperature of condensation. After the emanation was condensed -in the spiral, a current of electrolytic hydrogen or oxygen was -passed through the spiral. The temperature was allowed to -rise gradually, and was noted at the instant when a deflection of -the electrometer, due to the presence of emanation in the testing -vessel <i>T</i>, was observed. The resistance, subject to a slight correction -due to the time taken for the emanation to be carried into -the testing vessel, gave the temperature at which some of the -emanation commenced to volatilize. The ionization current in -the testing vessel rose rapidly to a maximum value, showing that, -for a small increase of temperature, the whole of the radium -emanation was volatilized. The following table gives an illustration -of the results obtained for a current of hydrogen of 1·38 cubic -centimetres per second.</p> - -<table class='table6' > -<colgroup> -<col class='colwidth50'> -<col class='colwidth50'> -</colgroup> - <tr> - <th class='c015'>Temperature</th> - <th class='c016'>Divisions per second of the electrometer</th> - </tr> - <tr> - <td class='c015'> </td> - <td class='c016'> </td> - </tr> - <tr> - <td class='c015'>-160°</td> - <td class='c016'>0</td> - </tr> - <tr> - <td class='c015'>-156°</td> - <td class='c016'>0</td> - </tr> - <tr> - <td class='c015'>-154°·3</td> - <td class='c016'>1</td> - </tr> - <tr> - <td class='c015'>-153°·8</td> - <td class='c016'>21</td> - </tr> - <tr> - <td class='c015'>-152°·5</td> - <td class='c016'>24</td> - </tr> -</table> - -<p class='c006'><span class='pageno' id='Page_282'>282</span>The following table shows the results obtained for different -currents of hydrogen and oxygen.</p> - -<table class='table23' > -<colgroup> -<col class='colwidth16'> -<col class='colwidth41'> -<col class='colwidth20'> -<col class='colwidth20'> -</colgroup> - <tr> - <th class='c013'></th> - <th class='c015'>Current of Gas</th> - <th class='c013'><i>T</i><sub>1</sub></th> - <th class='c014'><i>T</i><sub>2</sub></th> - </tr> - <tr> - <td class='c013'> </td> - <td class='c015'> </td> - <td class='c013'> </td> - <td class='c014'> </td> - </tr> - <tr> - <td class='c013'>Hydrogen</td> - <td class='c015'>·25 c.c. per sec.</td> - <td class='c013'>-151·3</td> - <td class='c014'>-150</td> - </tr> - <tr> - <td class='c013'>“</td> - <td class='c015'>·32 „ „</td> - <td class='c013'>-153·7</td> - <td class='c014'>-151</td> - </tr> - <tr> - <td class='c013'>”</td> - <td class='c015'>·92 „ „</td> - <td class='c013'>-152</td> - <td class='c014'>-151</td> - </tr> - <tr> - <td class='c013'>“</td> - <td class='c015'>1·38 „ „</td> - <td class='c013'>-154</td> - <td class='c014'>-153</td> - </tr> - <tr> - <td class='c013'>”</td> - <td class='c015'>2·3 „ „</td> - <td class='c013'>-162·5</td> - <td class='c014'>-162</td> - </tr> - <tr> - <td class='c013'>Oxygen</td> - <td class='c015'>·34 „ „</td> - <td class='c013'>-152·5</td> - <td class='c014'>-151·5</td> - </tr> - <tr> - <td class='c013'>“</td> - <td class='c015'>·58 „ „</td> - <td class='c013'>-155</td> - <td class='c014'>-153</td> - </tr> -</table> - -<p class='c006'>The temperature -<i>T</i><sub>1</sub> -in the above table gives the temperature -of initial volatilization, -<i>T</i><sub>2</sub> -the temperature for which half of the -condensed emanation had been released. For slow currents of -hydrogen and oxygen, the values of -<i>T</i><sub>1</sub> -and -<i>T</i><sub>2</sub> -are in good agreement. -For a stream of gas as rapid as 2·3 cubic centimetres per -second the value of -<i>T</i><sub>1</sub> -is much lower. Such a result is to be -expected; for, in too rapid a stream, the gas is not cooled to the -temperature of the spiral, and, in consequence, the inside surface -of the spiral is above the mean temperature, and some of the -emanation escapes at a temperature apparently much lower. In -the case of oxygen, this effect appears for a gas stream of 0·58 cubic -centimetres per second.</p> - -<p class='c006'>In the experiments on the thorium emanation, on account of -the rapid loss of activity, a slightly different method was necessary. -The steady stream of gas was passed over the thorium -compound, and the temperature was observed at the instant when -an appreciable movement of the electrometer appeared. This gave -the temperature at which a small fraction of the thorium emanation -escaped condensation, and not the value -<i>T</i><sub>1</sub> -observed for the radium -emanation, which gave the temperature for which a small fraction -of the previously condensed emanation was volatilized.</p> - -<p class='c006'>The following table illustrates the results obtained.</p> - -<table class='table20' > -<colgroup> -<col class='colwidth27'> -<col class='colwidth40'> -<col class='colwidth32'> -</colgroup> - <tr> - <th class='c013'></th> - <th class='c013'>Current of Gas</th> - <th class='c014'>Temperature</th> - </tr> - <tr> - <td class='c013'> </td> - <td class='c013'> </td> - <td class='c014'> </td> - </tr> - <tr> - <td class='c013'>Hydrogen</td> - <td class='c013'>·71 c.c. per sec.</td> - <td class='c014'>-155° C.</td> - </tr> - <tr> - <td class='c013'>“</td> - <td class='c013'>1·38 „ „</td> - <td class='c014'>-159° C.</td> - </tr> - <tr> - <td class='c013'>Oxygen</td> - <td class='c013'>·58 „ „</td> - <td class='c014'>-155° C.</td> - </tr> -</table> - -<p class='c006'><span class='pageno' id='Page_283'>283</span>On comparing these results with the values obtained for the -radium emanation, it will be observed that with equal gas streams -the temperatures are nearly the same.</p> - -<p class='c006'>A closer examination of the thorium emanation showed, however, -that this apparent agreement was only accidental, and that -there was, in reality, a very marked difference in the effect of temperature -on the two emanations. It was found experimentally that -the radium emanation was condensed very near the temperature -at which volatilization commenced, and that the points of condensation -and volatilization were defined fairly sharply.</p> - -<div id='fig060' class='figcenter id004'> -<img src='images/fig-060.png' alt='Fig. 60.' class='ig001'> -<div class='ic002'> -<p>Fig. 60.</p> -</div> -</div> - -<p class='c006'>On the other hand, the thorium emanation required a range -of over 30° C. after condensation had started in order to ensure -complete condensation. <a href='#fig060'>Fig. 60</a> is an example of the results -obtained with a steady gas stream of 1·38 c.c. per sec. of oxygen. -The ordinates represent the percentage proportion of the emanation -uncondensed at different temperatures. It will be observed -that condensation commences about -120°, and that very little of -the emanation escapes condensation at -155° C.</p> - -<p class='c006'>To investigate this difference of behaviour in the two emanations, -a static method was employed, which allowed an examination -<span class='pageno' id='Page_284'>284</span>of the two emanations to be made under comparable conditions. -The emanation, mixed with a small amount of the gas to be used, -was introduced into the cool spiral, which had been exhausted -previously by means of a mercury pump. The amount of emanation -remaining uncondensed after definite intervals was rapidly -removed by means of the pump, and was carried with a constant -auxiliary stream of gas into the testing vessel.</p> - -<p class='c006'>Tested in this way, it was found that the volatilization point -of the radium emanation was very nearly the same as that obtained -by the blowing method, viz. -150° C. With thorium, on -the other hand, the condensation started at about -120° C., and, -as in the blowing method, continued over a range of about 30° C. -The proportion of the emanation condensed at any temperature -was found to depend on a variety of conditions, although the point -at which condensation commenced, viz. -120° C., was about the -same in each case. It depended on the pressure and nature of the -gas, on the concentration of the emanation, and on the time for -which it was left in the spiral. For a given temperature a greater -proportion of the emanation was condensed, the lower the pressure -and the longer the time it was left in the spiral. Under the -same conditions, the emanation was condensed more rapidly in -hydrogen than in oxygen.</p> -<p class='c005'><b>168.</b> Thus there is no doubt that the thorium emanation -begins to condense at a temperature higher than that at which -the radium emanation condenses. The explanation of the peculiar -behaviour of the thorium emanation is clear when the small -number of emanation particles present in the gas are taken into -consideration. It has been shown that both emanations give -out only α rays. It is probable that the α particles from the -two emanations are similar in character and produce about the -same number of ions in their passage through the gas. The -number of ions produced by each α particle before its energy -is dissipated is probably about 70,000. (See <a href='#section252'>section 252</a>.)</p> - -<p class='c006'>Now, in the experiment, the electrometer readily measured -a current of -10<sup>-3</sup> -electrostatic units. Taking the charge on an ion -as -3·4 × 10<sup>-10</sup> -electrostatic units, this corresponds to a production in -the testing vessel of about -3 × 10<sup>6</sup> -ions per sec., which would be -<span class='pageno' id='Page_285'>285</span>produced by about 40 expelled α particles per second. Each -radiating particle cannot expel less than one α particle and may -expel more, but it is likely that the number expelled by an atom -of the thorium emanation is not greatly different from that -expelled by an atom of the radium emanation.</p> - -<p class='c006'>In <a href='#section133'>section 133</a> it has been shown that, according to the law of -decay, λ<i>N</i> particles change per second when <i>N</i> are present. Thus, -to produce 40 α particles, λ<i>N</i> cannot be greater than 40. Since for -the thorium emanation λ is ¹⁄₈₇, it follows that <i>N</i> cannot be greater -than 3500. The electrometer thus detected the presence of 3500 -particles of the thorium emanation, and since in the static method -the volume of the condensing spiral was about 15 c.c., this corresponded -to a concentration of about 230 particles per c.c. An -ordinary gas at atmospheric pressure and temperature probably -contains about -3·6 × 10<sup>19</sup> -molecules per c.c. Thus the emanation -would have been detected on the spiral if it had possessed a partial -pressure of less than -10<sup>-17</sup> -of an atmosphere.</p> - -<p class='c006'>It is not surprising then that the condensation point of the -thorium emanation is not sharply defined. It is rather a matter -of remark that condensation should occur so readily with so sparse -a distribution of emanation particles in the gas; for, in order -that condensation may take place, it is probable that the particles -must approach within one another’s sphere of influence.</p> - -<p class='c006'>Now in the case of the radium emanation, the rate of decay -is about 5000 times slower than that of the thorium emanation, -and consequently the actual number of particles that must be -present to produce the same ionization per second in the two -cases must be about 5000 times greater in the case of radium -than in the case of thorium. This conclusion involves only the -assumption that the same number of rays is produced by a -particle of emanation in each case, and that the expelled particles -produce in their passage through the gas the same number of -ions. The number of particles present, in order to be detected -by the electrometer, in this experiment, must therefore have -been about 5000 × 3500, <i>i.e.</i> about -2 × 10<sup>7</sup>. -The difference of -behaviour in the two cases is well explained by the view -that, <i>for equal electrical effects</i>, the number of radium emanation -particles must be far larger than the number of thorium -<span class='pageno' id='Page_286'>286</span>emanation particles. The probability of the particles coming into -each other’s sphere of influence will increase very rapidly as the -concentration of the particles increases, and, in the case of the -radium emanation, once the temperature of condensation is attained, -all but a small proportion of the total number of particles -present will condense in a very short time. In the case of the -thorium emanation, however, the temperature might be far below -that of condensation, and yet a considerable portion remain -uncondensed for comparatively long intervals. On this view the -experimental results obtained might reasonably be expected. A -greater proportion of emanation condenses the longer the time -allowed for condensation under the same conditions. The condensation -occurs more rapidly in hydrogen than in oxygen, as the -diffusion is greater in the former gas. For the same reason the -condensation occurs faster the lower the pressure of the gas -present. Finally, when the emanation is carried by a steady -stream of gas, a smaller proportion condenses than in the other -cases, because the concentration of emanation particles per unit -volume of gas is less under these conditions.</p> - -<p class='c006'>It is possible that the condensation of the emanations may not -occur in the gas itself but at the surface of the containing vessel. -Accurate observations of the temperature of condensation have so -far only been made in a copper spiral, but condensation certainly -occurs in tubes of lead or glass at about the same temperature as -in tubes of copper.</p> -<p class='c005'><b>169.</b> In experiments that were made by the static method -with a very large quantity of radium emanation, a slight amount -of escape of the condensed emanation was observed several degrees -below the temperature at which most of the emanation was released. -This is to be expected, since, under such conditions, the electrometer -is able to detect a very minute proportion of the whole quantity of -the emanation condensed.</p> - -<p class='c006'>Special experiments, with a large quantity of emanation, that -were made with the spiral immersed in a bath of rapidly boiling -nitric oxide, showed this effect very clearly. For example, the condensed -emanation began to volatilize at -155° C. In 4 minutes -the temperature had risen to -153·5°, and the amount volatilized -<span class='pageno' id='Page_287'>287</span>was four times as great as at -155°. In the next 5-½ minutes the -temperature had increased to -152·3° and practically the whole -quantity, which was at least fifty times the amount at the -temperature of -153·5°, had volatilized.</p> - -<p class='c006'>It thus seems probable that, if the temperature were kept -steady at the point at which volatilization was first observed, -and the released emanation removed at intervals, the whole of -the emanation would in course of time be liberated at that temperature. -Curie and Dewar and Ramsay have observed that the -emanation condensed in a <b>U</b> tube, immersed in liquid air, slowly -escapes if the pump is kept steadily working. These results point -to the probability that the condensed emanation possesses a true -vapour pressure, but great refinements in experimental methods -would be necessary before such a conclusion could be definitely -established.</p> - -<p class='c006'>The true temperature of condensation of the thorium emanation -is probably about -120° C., and that of radium about --150° C. Thus there is no doubt that the two emanations are -quite distinct from each other in this respect, and also with regard -to their radio-activity, although they both possess the property -of chemical inertness. These results on the temperatures of -condensation do not allow us to make any comparison of the -condensation points of the emanations with those of known gases, -since the lowering of the condensation points of gases with diminution -of pressure has not been studied at such extremely minute -pressures.</p> -<p class='c005'><b>170.</b> It has been found<a id='r264' href='#f264' class='c012'><sup>[264]</sup></a> that the activity of the thorium -emanation, when condensed in the spiral at the temperature of -liquid air, decayed at the same rate as at ordinary temperatures. -This is in accord with results of a similar kind obtained by -P. Curie for the radium emanation (section 145), and shows that -the value of the radio-active constant is unaffected by wide -variations of temperature.</p> -<div> - <span class='pageno' id='Page_288'>288</span> - <h3 class='c020'>Amount of Emanation from Radium and Thorium.</h3> -</div> -<p class='c005'><b>171.</b> It has been shown in section 93 from experimental data -that 1 gram of radium bromide at its minimum activity emits -about -3·6 × 10<sup>10</sup> -α particles per second. Since the activity due to -the emanation stored up in radium, when in a state of radio-active -equilibrium, is about one quarter of the whole and about equal to -the minimum activity, the number of α particles projected per -second by the emanation from 1 gram of radium bromide is about -3·6 × 10<sup>10</sup>. -It has been shown in <a href='#section152'>section 152</a> that 463,000 times -the amount of emanation produced per second is stored up in the -radium. But, in a state of radio-active equilibrium, the number of -emanation particles breaking up per second is equal to the number -produced per second. Assuming that each emanation particle in -breaking up expels one α particle, it follows that the number of -emanation particles present in 1 gram of radium bromide in radio-active -equilibrium is -463,000 × 3·6 × 10<sup>10</sup>, <i>i.e.</i> 1·7 × 10<sup>16</sup>. -Taking -the number of hydrogen molecules in 1 c.c. of gas at atmospheric -pressure and temperature as -3·6 × 10<sup>19</sup> -(<a href='#section039'>section 39</a>), the volume of -the emanation from 1 gram of radium bromide is -4·6 × 10<sup>-4</sup> cubic -centimetres at atmospheric pressure and temperature. Assuming -the composition of radium bromide as -RaBr<sub>2</sub>, -the amount from -1 gram of radium in radio-active equilibrium is 0·82 cubic -millimetres. Quite independently of any method of calculation -it was early evident that the volume of the emanation was very -small, for all the earlier attempts made to detect its presence -by its volume were unsuccessful. It will be seen, however, that, -when larger quantities of radium were available for experiment, -the emanation has been collected in volume sufficiently large -to measure.</p> - -<p class='c006'>In the case of thorium, the maximum quantity of emanation to -be obtained from 1 gram of the solid is very minute, both on account -of the small activity of thorium and of the rapid break up of the -emanation after its production. Since the amount of emanation, -stored in a non-emanating thorium compound, is only 87 times -the rate of production, while in radium it is 463,000 times, and the -rate of production of the emanation by radium is about 1 million -<span class='pageno' id='Page_289'>289</span>times faster than by thorium, it follows that the amount of -emanation to be obtained from 1 gram of thorium is not greater -than -10<sup>-10</sup> -of the amount from an equal weight of radium, <i>i.e.</i> -its volume is not greater than -10<sup>-13</sup> -c.c. at the ordinary pressure -and temperature. Even with large quantities of thorium, -the amount of emanation is too small ever to be detected by its -volume.</p> -<p class='c005'><a id='section172'></a> -<b>172. Volume of the emanation from radium.</b> The -evidence already considered points very strongly to the conclusion -that the emanation possesses all the properties of a chemically -inert gas of high molecular weight.</p> - -<p class='c006'>Since the emanation continuously breaks up, and is transformed -into a solid type of matter, which is deposited on the surface -of bodies, the volume of the emanation, when separated from -radium, should contract at the same rate as it loses its activity, -<i>i.e.</i> it should decrease to half value in about four days. The -amount of emanation to be obtained from a given quantity of -radium is a maximum when the rate of production of new emanation -balances its rate of change. This condition is practically -attained when the emanation has been allowed to collect for an -interval of one month. The probable volume of the emanation to -be obtained from 1 gram of radium was early calculated on certain -assumptions, and from data then available the writer<a id='r265' href='#f265' class='c012'><sup>[265]</sup></a> deduced -that the volume of the emanation from 1 gram of radium lay -between ·06 and ·6 cubic millimetre at atmospheric pressure and -temperature, and was probably nearer the latter value. The -volume to be expected on the latest data has been discussed in -the preceding section and shown to be about ·82 cubic mm. The -volume of the emanation is thus very small, but not too small to -be detected if several centigrams of radium are available. This -has been proved to be the case by Ramsay and Soddy<a id='r266' href='#f266' class='c012'><sup>[266]</sup></a> who, -by very careful experiment, finally succeeded in isolating a -small quantity of the emanation and in determining its volume. -The experimental method employed by them will now be briefly -described.</p> - -<div id='fig061' class='figcenter id002'> -<span class='pageno' id='Page_290'>290</span> -<img src='images/fig-061.png' alt='Fig. 61.' class='ig001'> -<div class='ic002'> -<p>Fig. 61.</p> -</div> -</div> - -<p class='c006'>The emanation from 60 milligrams of radium bromide in -solution was allowed to collect for 8 days -and then drawn off through the inverted -siphon <i>E</i> (<a href='#fig061'>Fig. 61</a>) into the explosion -burette <i>F</i>. This gas consisted for the most -part of hydrogen and oxygen, produced by -the action of the radiations on the water of -the solution. After explosion, the excess of -hydrogen mixed with emanation was left -some time in contact with caustic soda, -placed in the upper part of the burette, in -order to remove all trace of carbon dioxide. -In the meantime the upper part of the -apparatus had been completely evacuated. -The connection <i>C</i> to the pump was closed, -and the hydrogen and emanation were -allowed to enter the apparatus, passing -over a phosphorous pentoxide tube <i>D</i>. The emanation was -condensed in the lower part of the capillary tube <i>A</i>, by -surrounding it with the tube <i>B</i> filled with liquid air. The -process of condensation was rendered manifest by the brilliant -luminosity of the lower part of the tube. The mercury from -the burette was then allowed to run to <i>G</i>, and the apparatus -again completely evacuated. The connection of the pump was -again closed, the liquid air was removed and the volatilized -emanation forced into the fine capillary tube <i>A</i>. Observations -were then made, from day to day, of the volume of the emanation. -The results are given in the table below.</p> - -<table class='table2' > -<colgroup> -<col class='colwidth15'> -<col class='colwidth28'> -<col class='colwidth21'> -<col class='colwidth34'> -</colgroup> - <tr> - <th class='c013'>Time</th> - <th class='c013'>Volume</th> - <th class='c013'>Time</th> - <th class='c014'>Volume</th> - </tr> - <tr> - <td class='c013'>Start</td> - <td class='c013'>0·124 cub. mm.</td> - <td class='c013'>7 days</td> - <td class='c014'>0·0050 cub. mm.</td> - </tr> - <tr> - <td class='c013'>1 day</td> - <td class='c013'>0·027 „</td> - <td class='c013'>9 „</td> - <td class='c014'>0·0041 „</td> - </tr> - <tr> - <td class='c013'>3 „</td> - <td class='c013'>0·011 „</td> - <td class='c013'>11 „</td> - <td class='c014'>0·0020 „</td> - </tr> - <tr> - <td class='c013'>4 „</td> - <td class='c013'>0·0095 „</td> - <td class='c013'>12 „</td> - <td class='c014'>0·0011 „</td> - </tr> - <tr> - <td class='c013'>6 „</td> - <td class='c013'>0·0063 „</td> - <td class='c013'>28 „</td> - <td class='c014'>0·0004 „</td> - </tr> -</table> - -<p class='c006'>The volume contracted with the time, and was very small -after a month’s interval, but the minute bubble of the emanation -still retained its luminosity to the last. The tube became deep -purple in colour, which rendered readings difficult except with -a strong light. There was a sudden decrease in the first day, -<span class='pageno' id='Page_291'>291</span>which may have been due to the mercury sticking in the capillary -tube.</p> - -<p class='c006'>The experiments were repeated with another capillary tube -and the volume of gas observed at normal pressure was -0·0254 c. mm. The gas obtained was found to obey Boyle’s -law within the limit of experimental error over a considerable -range of pressure. But, unlike in the first experiment, the gas -did not contract but expanded rapidly during the first few hours, -and then more slowly, finally reaching a volume after 23 days -of 0·262 c. mm. or about 10 times the initial volume. The -measurements were complicated by the appearance of bubbles -of gas in the top of the mercury column. The differences -observed in these two experiments are difficult to account for. -We shall see, later, that the emanation always produces helium, -and, in the first experiment, the decrease of the volume to zero -indicates that the helium was buried or absorbed in the walls -of the tube. In the second case, probably owing to some difference -in the glass of the capillary tube, the helium may have been -released. This suggestion is confirmed by the observation that -the volume of gas, after the experiment ended, gave a brilliant -spectrum of helium.</p> - -<p class='c006'>We shall see later that there is considerable evidence that the -α particles expelled from radio-active substances consist of helium -atoms. Since the particles are projected with great velocity, they -will first be buried in the walls of the tube, and then may -gradually diffuse out into the gas again under conditions -probably depending on the kind of glass employed. Since α -particles are projected from the emanation and also from two -of the rapidly changing products which arise from it, the volume -of helium should, on this view, be three times the initial volume -of the emanation. If the helium produced escaped from the walls -of the tube into the gas, the apparent volume of the gas in the -capillary should increase to three times the initial volume in -a month’s interval, for during that time the emanation itself -has been transformed into a solid type of matter deposited on -the walls of the tube.</p> - -<p class='c006'>Ramsay and Soddy concluded from their experiments that the -maximum volume of emanation to be obtained from 1 gram -of radium was about 1 cubic millimetre at standard pressure -<span class='pageno' id='Page_292'>292</span>and temperature, and that the emanation was produced from -1 gram of radium at the rate of -3 × 10<sup>-6</sup> -c. mm. per second. This -amount is in very good agreement with the calculated value, -and is a strong indication of the general correctness of the theory -on which the calculations are based.</p> -<p class='c005'><b>173. Spectrum of the emanation.</b> After the separation -of the emanation and the determination of its volume, Ramsay -and Soddy made numerous attempts to obtain its spectrum. In -some of the earlier experiments several bright lines were seen for a -short time, but these lines were soon masked by the appearance -of the hydrogen lines. In later experiments Ramsay and Collie<a id='r267' href='#f267' class='c012'><sup>[267]</sup></a> -succeeded in obtaining a spectrum of the emanation, which persisted -for a short time, during which a rapid determination of the -wave-lengths was made. They state that the spectrum was very -brilliant, consisting of very bright lines, the spaces between being -perfectly dark. The spectrum bore a striking resemblance in -general character to the spectrum of the gases of the argon -family.</p> - -<p class='c006'>The spectrum soon faded, and the spectrum of hydrogen began -to appear. The following table shows the wave-length of the -lines observed in the spectrum. The degree of coincidence of the -lines of known wave-lengths shows that the error is probably less -than five Ångström units.</p> - -<table class='table1' > -<colgroup> -<col class='colwidth18'> -<col class='colwidth81'> -</colgroup> - <tr> - <th class='c013'>Wave-length</th> - <th class='c014'>Remarks</th> - </tr> - <tr> - <td class='c013'>6567</td> - <td class='c014'>Hydrogen C; true wave-length, 6563; observed each time.</td> - </tr> - <tr> - <td class='c013'>6307</td> - <td class='c014'>Observed only at first; evanescent.</td> - </tr> - <tr> - <td class='c013'>5975</td> - <td class='c014'>„ „ „</td> - </tr> - <tr> - <td class='c013'>5955</td> - <td class='c014'>„ „ „</td> - </tr> - <tr> - <td class='c013'>5805</td> - <td class='c014'>Observed each time; persistent.</td> - </tr> - <tr> - <td class='c013'>5790</td> - <td class='c014'>Mercury; true wave-length, 5790.</td> - </tr> - <tr> - <td class='c013'>5768</td> - <td class='c014'>„ „ 5769.</td> - </tr> - <tr> - <td class='c013'>5725</td> - <td class='c014'>Observed only at first; evanescent.</td> - </tr> - <tr> - <td class='c013'>5595</td> - <td class='c014'>Observed each time; persistent and strong.</td> - </tr> - <tr> - <td class='c013'>5465</td> - <td class='c014'>Mercury; true wave-length, 5461.</td> - </tr> - <tr> - <td class='c013'>5105</td> - <td class='c014'>Not observed at first; appeared after some seconds; persisted and was visible during the second examination.</td> - </tr> - <tr> - <td class='c013'>4985</td> - <td class='c014'>Observed each time; persistent and strong.</td> - </tr> - <tr> - <td class='c013'>4865</td> - <td class='c014'>Hydrogen F; true wave-length, 4861.</td> - </tr> - <tr> - <td class='c013'>4690</td> - <td class='c014'>Observed only at first.</td> - </tr> - <tr> - <td class='c013'>4650</td> - <td class='c014'>Not observed when the emanation was examined again.</td> - </tr> - <tr> - <td class='c013'>4630</td> - <td class='c014'>„ „ „</td> - </tr> - <tr> - <td class='c013'>4360</td> - <td class='c014'>Mercury: true wave-length, 4359.</td> - </tr> -</table> - -<p class='c006'><span class='pageno' id='Page_293'>293</span>The experiments were repeated with a new supply of emanation, -and some of the stronger lines were observed again, while -some new lines made their appearance. Ramsay and Collie -suggest that the strong line 5595 may be identical with a line -which was observed by Pickering<a id='r268' href='#f268' class='c012'><sup>[268]</sup></a> in the spectrum of lightning, -and was not identified with the spectrum of any known gas.</p> - -<p class='c006'>Until large quantities of radium are available for the experimenter -it would appear difficult to make sure how many of these -lines must be ascribed to the spectrum of the emanation or to -measure the wave-lengths with accuracy.</p> - -<p class='c006'>The results are of great interest, as showing that the emanation -has a definite and new spectrum of the same general -character as the argon group of gases to which, as we have seen, -it is chemically allied.</p> -<h3 class='c020'>Summary of Results.</h3> -<p class='c005'><b>174.</b> The investigations into the nature of the radio-active -emanations have thus led to the following conclusions:—The -radio-elements thorium, radium and actinium continuously produce -from themselves radio-active emanations at a rate which is -constant under all conditions. In some cases, the emanations -continuously diffuse from the radio-active compounds into the -surrounding gas; in other cases, the emanations are unable to -escape from the material in which they are produced, but are -occluded, and can only be released by solution or by the action of -heat.</p> - -<p class='c006'>The emanations possess all the properties of radio-active gases. -They diffuse through gases, liquids, and porous substances, and can -be occluded in some solids. Under varying conditions of pressure, -volume, and temperature, the emanations distribute themselves in -the same way and according to the same laws as does a gas.</p> - -<p class='c006'>The emanations possess the important property of condensation -under the influence of extreme cold, and by that means can be -separated from the gases with which they are mixed. The radiation -from the emanation is material in nature, and consists of a -stream of positively charged particles projected with great velocity.</p> - -<p class='c006'><span class='pageno' id='Page_294'>294</span>The emanations possess the property of chemical inertness, -and in this respect resemble the gases of the argon family. The -emanations are produced in minute amount; but a sufficient -quantity of the radium emanation has been obtained to determine -its volume and its spectrum. With regard to their rates of -diffusion, the emanations of both thorium and radium behave -like gases of high molecular weight.</p> - -<p class='c006'>These emanations have been detected and their properties -investigated by the property they possess of emitting radiations of -a special character. These radiations consist entirely of α rays, -<i>i.e.</i> particles, projected with great velocity, which carry a positive -charge and have a mass about twice that of the hydrogen atom. -The emanations do not possess the property of permanently -radiating, but the intensity of the radiations diminishes according -to an exponential law with the time, falling to half value, from -actinium in 4 seconds, from thorium in one minute, and from -radium in about four days. The law of decay of activity does not -seem to be influenced by any physical or chemical agency.</p> - -<p class='c006'>The emanation particles gradually break up, each particle as it -breaks up expelling a charged body. The emanation after it has -radiated ceases to exist as such, but is transformed into a new -kind of matter, which is deposited on the surface of bodies and -gives rise to the phenomena of excited activity. This last property, -and the connection of the emanation with it, are discussed in detail -in the next chapter.</p> - -<div class='chapter'> - <span class='pageno' id='Page_295'>295</span> - <h2 id='chap08' class='c004'>CHAPTER VIII. <br> EXCITED RADIO-ACTIVITY.</h2> -</div> -<p class='c005'><b>175. Excited radio-activity.</b> One of the most interesting -and remarkable properties of thorium, radium, and actinium, is -their power of “exciting” or “inducing” temporary activity on all -bodies in their neighbourhood. A substance which has been -exposed for some time in the presence of radium or thorium -behaves as if its surface were covered with an invisible deposit of -intensely radio-active material. The “excited” body emits radiations -capable of affecting a photographic plate and of ionizing a -gas. Unlike the radio-elements themselves, however, the activity -of the body does not remain constant after it has been removed -from the influence of the exciting active material, but decays with -the time. The activity lasts for several hours when due to radium -and several days when due to thorium.</p> - -<p class='c006'>This property was first observed by M. and Mme. Curie<a id='r269' href='#f269' class='c012'><sup>[269]</sup></a> for -radium, and independently by the writer<a id='r270' href='#f270' class='c012'><sup>[270]</sup></a> for thorium<a id='r271' href='#f271' class='c012'><sup>[271]</sup></a>.</p> - -<p class='c006'><span class='pageno' id='Page_296'>296</span>If any solid body is placed inside a closed vessel containing an -emanating compound of thorium or radium, its surface becomes -radio-active. For thorium compounds the amount of excited -activity on a body is in general greater the nearer it is to -the active material. In the case of radium, however, provided -the body has been exposed for several hours, the amount of excited -activity is to a large extent independent of the position of the -body in the vessel containing the active material. Bodies are -made active whether exposed directly to the action of the radio-active -substance or screened from the action of the direct rays. -This has been clearly shown in some experiments of P. Curie. A -small open vessel <i>a</i> (<a href='#fig062'>Fig. 62</a>) containing a solution of radium -is placed inside a larger closed vessel <i>V</i>.</p> - -<div id='fig062' class='figcenter id006'> -<img src='images/fig-062.png' alt='Fig. 62.' class='ig001'> -<div class='ic002'> -<p>Fig. 62.</p> -</div> -</div> - -<p class='c006'>Plates <i>A</i>, <i>B</i>, <i>C</i>, <i>D</i>, <i>E</i> are placed in various positions in the -enclosure. After exposure for a day, the plates after removal are -found to be radio-active even in positions completely shielded from -the action of the direct rays. For example, the plate <i>D</i> shielded -from the direct radiation by the lead plate <i>P</i> is as active as the -plate <i>E</i>, exposed to the direct radiation. The amount of activity -produced in a given time on a plate of given area in a definite -position is independent of the material of the plate. Plates of -mica, copper, cardboard, ebonite, all show equal amounts of activity. -The amount of activity depends on the area of the plate and on -<span class='pageno' id='Page_297'>297</span>the amount of free space in its neighbourhood. Excited radio-activity -is also produced in water if exposed to the action of an -emanating compound.</p> -<p class='c005'><b>176. Concentration of excited radio-activity on the -negative electrode.</b> When thorium or radium is placed in a -closed vessel, the whole interior surface becomes strongly active. -In a strong electric field, on the other hand, the writer found that -the activity was confined entirely to the negative electrode. By -suitable arrangements, the whole of the excited activity, which -was previously distributed over the surface of the vessel, can be -concentrated on a small negative electrode placed inside the vessel. -An experimental arrangement for this purpose is shown in <a href='#fig063'>Fig. 63</a>.</p> - -<div id='fig063' class='figcenter id006'> -<img src='images/fig-063.png' alt='Fig. 63.' class='ig001'> -<div class='ic002'> -<p>Fig. 63.</p> -</div> -</div> - -<p class='c006'>The metal vessel <i>V</i> containing a large amount of thoria is connected -with the positive pole of a battery of about 300 volts. The -wire <i>AB</i> to be made active is fastened to a stouter rod <i>BC</i>, passing -through an ebonite cork inside a short cylinder <i>D</i>, fixed in the side -of the vessel. This rod is connected with the negative pole of the -battery. In this way the wire <i>AB</i> is the only conductor exposed -in the field with a negative charge, and it is found that the whole -of the excited activity is concentrated upon it.</p> - -<p class='c006'>In this way it is possible to make a short thin metal wire over -10,000 times as active per unit surface as the thoria from which -the excited activity is derived. In the same way, the excited -activity due to radium can be concentrated mainly on the negative -<span class='pageno' id='Page_298'>298</span>electrode. In the case of thorium, if the central wire be charged -positively, it shows no appreciable activity. With radium, however, -a positively charged body becomes slightly active. In most cases, -the amount of activity produced on the positive electrode is not -more than 5% of the corresponding amount when the body is -negatively charged. For both thorium and radium, the amount of -excited activity on electrodes of the same size is independent of -their material.</p> - -<p class='c006'>All metals are made active to equal extents for equal times of -exposure. When no electric field is acting, the same amount -of activity is produced on insulators like mica and glass as on -conductors of equal dimensions.</p> -<p class='c005'><a id='section177'></a> -<b>177. Connection between the emanations and excited -activity.</b> An examination of the conditions under which excited -activity is produced shows that there is a very close connection -between the emanation and the excited activity. If a thorium -compound is covered with several sheets of paper, which cut off the -α rays but allow the emanation to pass through, excited activity is -still produced in the space above it. If a thin sheet of mica is -waxed down over the active material, thus preventing the escape of -the emanation, no excited activity is produced outside it. Uranium -and polonium which do not give off an emanation are not able to -produce excited activity on bodies. Not only is the presence of -the emanation necessary to cause excited activity, but the amount -of excited activity is always proportional to the amount of emanation -present. For example, de-emanated thoria produces very -little excited activity compared with ordinary thoria. In all cases -the amount of excited activity produced is proportional to the -emanating power. When passing through an electric field the -emanation loses its property of exciting activity at the same -rate as the radiating power diminishes. This was shown by the -following experiment.</p> - -<p class='c006'>A slow constant current of air from a gasometer, freed from -dust by its passage through cotton-wool, passed through a rectangular -wooden tube 70 cms. long. Four equal insulated metal plates -<i>A</i>, <i>B</i>, <i>C</i>, <i>D</i>, were placed at regular intervals along the tube. The -positive pole of a battery of 300 volts was connected with a metal -<span class='pageno' id='Page_299'>299</span>plate placed in the bottom of the tube, while the negative pole -was connected with the four plates. A mass of thoria was placed -in the bottom of the tube under the plate <i>A</i>, and the current due -to the emanation determined at each of the four plates. After -passing a current of air of 0·2 cm. per second for 7 hours along the -tube, the plates were removed and the amount of excited activity -produced on them was tested by the electric method. The following -results were obtained.</p> - -<table class='table5' > -<colgroup> -<col class='colwidth38'> -<col class='colwidth30'> -<col class='colwidth30'> -</colgroup> - <tr> - <th class='c013'></th> - <th class='c013'>Relative current due to emanation</th> - <th class='c014'>Relative excited activity</th> - </tr> - <tr> - <td class='c013'> </td> - <td class='c013'> </td> - <td class='c014'> </td> - </tr> - <tr> - <td class='c013'>Plate <i>A</i></td> - <td class='c013'>1</td> - <td class='c014'>1</td> - </tr> - <tr> - <td class='c013'>„ <i>B</i></td> - <td class='c013'>·55</td> - <td class='c014'>·43</td> - </tr> - <tr> - <td class='c013'>„ <i>C</i></td> - <td class='c013'>·18</td> - <td class='c014'>·16</td> - </tr> - <tr> - <td class='c013'>„ <i>D</i></td> - <td class='c013'>·072</td> - <td class='c014'>·061</td> - </tr> -</table> - -<p class='c006'>Within the errors of measurement, the amount of excited -activity is thus proportional to the radiation from the emanation, -<i>i.e.</i> to the amount of emanation present. The same considerations -hold for the radium emanation. The emanation in this case, on -account of the slow loss of its activity, can be stored mixed with -air for long periods in a gasometer, and its effects tested quite -independently of the active matter from which it is produced. -The ionization current due to the excited activity produced by the -emanation is always proportional to the current due to the emanation -for the period of one month or more that its activity is large -enough to be measured conveniently by an electrometer.</p> - -<p class='c006'>If, at any time during the interval, some of the emanation is -removed and introduced into a new testing vessel, the ionization -current will immediately commence to increase, rising in the course -of four or five hours to about twice its original value. This increase -of the current is due to the excited activity produced on the walls -of the containing vessel. On blowing out the emanation, the -excited activity is left behind, and at once begins to decay. -Whatever its age, the emanation still possesses the property of -causing excited activity, and in amount always proportional to its -activity, <i>i.e.</i> to the amount of emanation present.</p> - -<p class='c006'>These results show that the power of exciting activity on -<span class='pageno' id='Page_300'>300</span>inactive substances is a property of the radio-active emanations, -and is proportional to the amount of emanation present.</p> - -<p class='c006'>The phenomenon of excited activity cannot be ascribed to a -type of phosphorescence produced by the rays from the emanation -on bodies; for it has been shown that the activity can be concentrated -on the negative electrode in a strong electric field, even if -the electrode is shielded from the direct radiation from the active -substance which gives off the emanation. The amount of excited -activity does not seem in any way connected with the ionization -produced by the emanation in the gas with which it is mixed. -For example, if a closed vessel is constructed with two large -parallel insulated metal plates on the lower of which a layer of -thoria is spread, the amount of the excited activity on the upper -plate when charged negatively, is independent of the distance -between the plates when that distance is varied from 1 millimetre -to 2 centimetres. This experiment shows that the amount of -excited activity depends only on the amount of emanation emitted -from the thoria; for the ionization produced with a distance of -2 centimetres between the plates is about ten times as great as -with a distance of 1 millimetre.</p> -<p class='c005'><b>178.</b> If a platinum wire be made active by exposure to the -emanation of thoria, its activity can be removed by treating the -wire with certain acids<a id='r272' href='#f272' class='c012'><sup>[272]</sup></a>. For example, the activity is not much -altered by immersing the wire in hot or cold water or nitric acid, -but more than 80% of it is removed by dilute or concentrated -solutions of sulphuric or hydrochloric acid. The activity has not -been destroyed by this treatment but is manifested in the solution. -If the solution be evaporated, the activity remains behind on the -dish.</p> - -<p class='c006'>These results show that the excited activity is due to a deposit -on the surface of bodies of <i>radio-active matter</i> which has definite -properties as regards solution in acids. This active matter is -dissolved in some acids, but, when the solvent is evaporated, the -active matter is left behind. This active matter is deposited on -the surface of bodies, for it can be partly removed by rubbing the -body with a cloth, and almost completely by scouring the plate -<span class='pageno' id='Page_301'>301</span>with sand or emery paper. If a negatively charged wire is placed -in the presence of a large quantity of radium emanation, it -becomes intensely active. If the wire, after removal, is drawn -across a screen of zinc sulphide, or willemite, a portion of the -active matter is rubbed off, and a luminous trail is left behind on -the screen. The amount of active matter deposited is extremely -small, for no difference of weight has been detected in a platinum -wire when made extremely active. On examining the wire under -a microscope, no trace of foreign matter is observed. It follows -from these results that the matter which causes excited activity is -many thousand times more active, weight for weight, than radium -itself.</p> - -<p class='c006'>It is convenient to have a definite name for this radio-active -matter, for the term “excited activity” only refers to the radiation -from the active matter and not to the matter itself. The term -“active deposit” will be generally applied to this matter. The -active deposit from the three substances thorium, radium, and -actinium is, in each case, derived from its respective emanation, -and possesses the same general property of concentration on the -negative electrode in an electric field and of acting as a non-volatile -type of matter which is deposited from the gas on to the surface -of bodies. These active deposits, while all soluble in strong acids, -are chemically distinct from each other.</p> - -<p class='c006'>The term “active deposit” can, however, only be used when -the matter is spoken of as a whole; for it will be shown later that -the matter, under ordinary conditions, is complex and contains -several constituents which have distinctive physical and chemical -properties and also a distinctive rate of change. According to the -theory advanced in <a href='#section136'>section 136</a>, we may suppose that the emanation -of thorium, radium, and actinium is unstable and breaks up with -the expulsion of an α particle. The residue of the atom of the -emanation diffuses to the sides of the vessel or is removed to the -negative electrode in an electric field. This active deposit is in -turn unstable and breaks up in several successive stages.</p> - -<p class='c006'>The “excited activity” proper is the radiation set up by the -active deposit in consequence of the changes occurring in it. On -this view, the emanation is the parent of the active deposit in -the same way that Th X is the parent of the emanation. The -<span class='pageno' id='Page_302'>302</span>proportionality which always exists between the activity of the -emanation and the excited activity to which it gives rise, is at -once explained, if one substance be the parent of the other.</p> -<p class='c005'><b>179. Decay of the excited activity produced by thorium.</b> -The excited activity produced in a body after a <i>long</i> exposure to -the emanations of thorium, decays in an exponential law with the -time, falling to half value in about 11 hours. The following table -shows the rate of decay of the excited activity produced on a brass -rod.</p> - -<table class='table14' > -<colgroup> -<col class='colwidth50'> -<col class='colwidth50'> -</colgroup> - <tr> - <th class='c015'>Time in hours</th> - <th class='c014'>Current</th> - </tr> - <tr> - <td class='c015'>0</td> - <td class='c014'>100</td> - </tr> - <tr> - <td class='c015'>7·9</td> - <td class='c014'>64</td> - </tr> - <tr> - <td class='c015'>11·8</td> - <td class='c014'>47·4</td> - </tr> - <tr> - <td class='c015'>23·4</td> - <td class='c014'>19·6</td> - </tr> - <tr> - <td class='c015'>29·2</td> - <td class='c014'>13·8</td> - </tr> - <tr> - <td class='c015'>32·6</td> - <td class='c014'>10·3</td> - </tr> - <tr> - <td class='c015'>49·2</td> - <td class='c014'>3·7</td> - </tr> - <tr> - <td class='c015'>62·1</td> - <td class='c014'>1·86</td> - </tr> - <tr> - <td class='c015'>71·4</td> - <td class='c014'>0·86</td> - </tr> -</table> - -<p class='c006'>The results are shown graphically in <a href='#fig064'>Fig. 64</a>, Curve <i>A</i>.</p> - -<div id='fig064' class='figcenter id001'> -<img src='images/fig-064.png' alt='Fig. 64.' class='ig001'> -<div class='ic002'> -<p>Fig. 64.</p> -</div> -</div> - -<p class='c006'><span class='pageno' id='Page_303'>303</span>The intensity of the radiation <i>I</i> after any time <i>t</i> is given by</p> - -<div class='figcenter id010'> -<img src='images/form-073.png' alt='Formula.' class='ig001'> -</div> - -<p class='c006'>where λ is the radio-active constant.</p> - -<p class='c006'>The rate of decay of excited activity, like that of the activity of -other radio-active products, is not appreciably affected by change of -conditions. The rate of decay is independent of the concentration of -the excited activity, and of the material of the body on which it is -produced. It is independent also of the nature and pressure of the -gas in which it decays. The rate of decay is unchanged whether -the excited activity is produced on the body with or without an -electric field.</p> - -<p class='c006'>The amount of excited activity produced on a body increases -at first with the time, but reaches a maximum after an exposure -of several days. An example of the results is given in the following -table. In this experiment a rod was made the cathode in a closed -vessel containing thoria. It was removed at intervals for the short -time necessary to test its activity and then replaced.</p> - -<table class='table14' > -<colgroup> -<col class='colwidth50'> -<col class='colwidth50'> -</colgroup> - <tr> - <th class='c015'>Time in hours</th> - <th class='c016'>Current</th> - </tr> - <tr> - <td class='c015'>1·58</td> - <td class='c016'>6·3</td> - </tr> - <tr> - <td class='c015'>3·25</td> - <td class='c016'>10·5</td> - </tr> - <tr> - <td class='c015'>5·83</td> - <td class='c016'>29</td> - </tr> - <tr> - <td class='c015'>9·83</td> - <td class='c016'>40</td> - </tr> - <tr> - <td class='c015'>14·00</td> - <td class='c016'>59</td> - </tr> - <tr> - <td class='c015'>23·41</td> - <td class='c016'>77</td> - </tr> - <tr> - <td class='c015'>29·83</td> - <td class='c016'>83</td> - </tr> - <tr> - <td class='c015'>47·00</td> - <td class='c016'>90</td> - </tr> - <tr> - <td class='c015'>72·50</td> - <td class='c016'>95</td> - </tr> - <tr> - <td class='c015'>96·00</td> - <td class='c016'>100</td> - </tr> -</table> - -<p class='c006'>These results are shown graphically in Curve <i>B</i>, <a href='#fig064'>Fig. 64</a>. It is -seen that the decay and recovery curves may be represented -approximately by the following equations.</p> - -<p class='c006'>For the decay curve <i>A</i>,</p> - -<div class='figcenter id010'> -<img src='images/form-073.png' alt='Formula.' class='ig001'> -</div> - -<p class='c006'>For the recovery curve <i>B</i>,</p> - -<div class='figcenter id009'> -<img src='images/form-074.png' alt='Formula.' class='ig001'> -</div> - -<p class='c006'>The two curves are thus complementary to one another; they -are connected in the same way as the decay and recovery curves of -Ur X, and are susceptible of a similar explanation.</p> - -<p class='c006'><span class='pageno' id='Page_304'>304</span>The amount of excited radio-activity reaches a maximum value -when the rate of supply of fresh radio-active particles balances the -rate of change of those already deposited.</p> -<p class='c005'><a id='section180'></a> -<b>180. Excited radio-activity produced by a short exposure.</b> -The initial portion of the recovery curve <i>B</i>, <a href='#fig064'>Fig. 64</a>, is -not accurately represented by the above equation. The activity -for the first few hours increases more slowly than would be -expected from the equation. This result, however, is completely -explained in the light of later results. The writer<a id='r273' href='#f273' class='c012'><sup>[273]</sup></a> found that, for -a <i>short exposure</i> of a body to the thorium emanation, the excited -activity upon it after removal, instead of at once decaying at the -normal rate, <i>increased</i> for several hours. In some cases the activity -of the body increased to three or four times its original value in -the course of a few hours and then decayed with the time at -the normal rate.</p> - -<p class='c006'>For an exposure of 41 minutes to the emanation the excited -activity after removal rose to three times its initial value in about -3 hours and then fell again at about the normal rate to half value -in 11 hours.</p> - -<p class='c006'>With a longer time of exposure to the emanation, the ratio of -the increase after removal is much less marked. For a day’s exposure, -the activity after removal begins at once to diminish. In -this case, the increase of activity of the matter deposited in the -last few hours does not compensate for the decrease of activity of -the active matter as a whole, and consequently the activity at once -commences to decay. This increase of activity with time explains -the initial irregularity in the recovery curve, for the active matter -deposited during the first few hours takes some time to reach its -maximum activity, and the initial activity is, in consequence, -smaller than would be expected from the equation.</p> - -<p class='c006'>The increase of activity on a rod exposed for a short interval in -the presence of the thorium emanation has been further investigated -by Miss Brooks. The curve <i>C</i> in <a href='#fig065'>Fig. 65</a> shows the variation with -time of the activity of a brass rod exposed for 10 minutes in the -emanation vessel filled with dust-free air. The excited activity -after removal increased in the course of 3·7 hours to five times its -<span class='pageno' id='Page_305'>305</span>initial value, and afterwards decayed at the normal rate. The -dotted line curve <i>D</i> represents the variation of activity to be expected -if the activity decayed exponentially with the time. The -explanation of this remarkable action is considered in detail in -<a href='#section207'>section 207</a>.</p> - -<div id='fig065' class='figcenter id004'> -<img src='images/fig-065.png' alt='Fig. 65.' class='ig001'> -<div class='ic002'> -<p>Fig. 65.</p> -</div> -</div> -<p class='c005'><a id='section181'></a> -<b>181. Effect of dust on the distribution of excited activity.</b> -Miss Brooks<a id='r274' href='#f274' class='c012'><sup>[274]</sup></a>, working in the Cavendish Laboratory, observed that -the excited activity due to the thorium emanation appeared in -some cases on the anode in an electric field, and that the distribution -of excited activity varied in an apparently capricious manner. -This effect was finally traced to the presence of dust in the air of -the emanation vessel. For example, with an exposure of 5 minutes -the amount of excited activity to be observed on a rod depended -on the time that the air had been allowed to remain undisturbed -in the emanation vessel beforehand. The effect increased with the -time of standing, and was a maximum after about 18 hours. The -amount of excited activity obtained on the rod was then about -20 times as great as the amount observed for air freshly introduced. -<span class='pageno' id='Page_306'>306</span>The activity of this rod did not increase after removal, but with -fresh air, the excited activity, for an exposure of 5 minutes, increased -to five or six times its initial value.</p> - -<p class='c006'>This anomalous behaviour was found to be due to the presence -of dust particles in the air of the vessel, in which the bodies were -made radio-active. These particles of dust, when shut up in the -presence of the emanation, become radio-active. When a negatively -charged rod is introduced into the vessel, a part of the -radio-active dust is concentrated on the rod and its activity is -added to the normal activity produced on the wire. After the air -in the vessel has been left undisturbed for an interval sufficiently -long to allow each of the particles of dust to reach a state of radio-active -equilibrium, on the application of an electric field, all the -positively charged dust particles will at once be carried to the -negative electrode. The activity of the electrode at once commences -to decay, since the decay of the activity of the dust particles -on the wire quite masks the initial rise of the normal activity -produced on the wire.</p> - -<p class='c006'>Part of the radio-active dust is also carried to the anode, and -the proportion increases with the length of time during which the -air has been undisturbed. The greatest amount obtained on the -anode was about 60% of that on the cathode.</p> - -<p class='c006'>These anomalous effects were found to disappear if the air was -made dust-free by passing through a plug of glass wool, or by -application for some time of a strong electric field.</p> -<p class='c005'><a id='section182'></a> -<b>182. Decay of excited activity from radium.</b> The excited -activity produced on bodies by exposure to the radium emanation -decays much more rapidly than the thorium excited activity. For -short times of exposure<a id='r275' href='#f275' class='c012'><sup>[275]</sup></a> to the emanation the decay curve is very -irregular. This is shown in <a href='#fig066'>Fig. 66</a>.</p> - -<p class='c006'>It was found that the intensity of the radiation measured by -the α rays decreased rapidly for the first 10 minutes after removal, -but about 15 minutes after removal reached a value which -remained nearly constant for an interval of about 20 minutes. -It then decayed to zero, finally following an exponential law, the -intensity falling to half value in about 28 minutes. With longer -<span class='pageno' id='Page_307'>307</span>times of exposure, the irregularities in the curve are not so -marked.</p> - -<div id='fig066' class='figcenter id001'> -<img src='images/fig-066.png' alt='Fig. 66.' class='ig001'> -<div class='ic002'> -<p>Fig. 66.</p> -</div> -</div> - -<p class='c006'>Miss Brooks has recently determined the decay curves of the -excited activity of radium for different times of exposure, measured -by the α rays. The results are shown in <a href='#fig067'>Fig. 67</a>, where the initial -ordinates represent the activity communicated to the body from -different times of exposure to a constant supply of emanation. It -will be observed that in all cases there is a sudden initial drop -of activity, which becomes less marked with increasing time of -exposure. The activity, several hours after removal, decreases exponentially -in all cases, falling to half value in about 28 minutes.</p> - -<p class='c006'>Not only do the curves of variation of the excited activity after -removal depend upon the time of exposure to the emanation, but -they also depend upon whether the α or β and γ rays are used as -a means of measurement. The curves obtained for the γ rays are -identical with those from the β rays, showing that these two types -<span class='pageno' id='Page_308'>308</span>of rays always occur together and in the same proportion. The -curves measured by the β rays are very different, especially for the -case of a short exposure to the emanation. This is clearly shown -in Fig. 68, which gives the β and γ ray curves for exposures of 10 -minutes, 40 minutes, and 1 hour, and also the limiting case of an -exposure of 24 hours.</p> - -<div id='fig067' class='figcenter id004'> -<img src='images/fig-067.png' alt='Fig. 67.' class='ig001'> -<div class='ic002'> -<p>Fig. 67.</p> -</div> -</div> - -<div id='fig068' class='figcenter id004'> -<img src='images/fig-068.png' alt='Fig. 68.' class='ig001'> -<div class='ic002'> -<p>Fig. 68.</p> -</div> -</div> - -<p class='c006'>About 25 minutes after removal, the activity decays approximately -at the same rate in each case. For convenience of representation, -<span class='pageno' id='Page_309'>309</span>the ordinates of the curves were adjusted so that they -all passed through a common point. We shall see later (<a href='#chap11'>chapter <span class='fss'>XI</span></a>) -that the rates of decay are not identically the same until several -hours after removal; but, in the above figure, it is difficult to -represent the slight variations. It will be observed that for the -short exposure of 10 minutes the activity measured by the β rays -is small at first but rises to a maximum in about 22 minutes, and -then dies away with the time. The curve of decay of activity, -measured by the β rays for a long exposure, does not show the -rapid initial drop which occurs in all the α ray curves. Curie and -Danne<a id='r276' href='#f276' class='c012'><sup>[276]</sup></a> made an investigation of the curves of decay of excited -activity for different times of exposure to the radium emanation, -<span class='pageno' id='Page_310'>310</span>but apparently did not take into account the fact that measurements -made by the α and β rays give quite different curves of -decay. Some of the family of curves, given in their paper, refer to -the α rays and others to the β rays. They showed, however, the -important fact that the curve of decay obtained by them for a -long exposure (which is identical with the β ray curve) could -be empirically expressed by an equation of the form</p> - -<div class='figcenter id005'> -<img src='images/form-075.png' alt='Formula.' class='ig001'> -</div> - -<p class='c006'>where -<i>I</i>₀ -is the initial intensity and <i>I</i><sub><i>t</i></sub> the intensity after any -time <i>t</i>; -λ<sub>1</sub> = ¹⁄₂₄₂₀, λ<sub>2</sub> = ¹⁄₁₈₆₀. -The numerical constant <i>a</i> = 4·20. -After an interval of 2·5 hours, the logarithmic decay curve is nearly -a straight line, that is, the activity falls off according to an exponential -law with the time, decreasing to half value in about 28 -minutes.</p> - -<p class='c006'>The full explanation of this equation, and of the peculiarities of -the various decay curves of the excited activity of radium, will be -discussed in detail in <a href='#chap11'>chapter <span class='fss'>XI</span></a>.</p> - -<p class='c006'>As in the case of the excited activity from thorium, the rate of -decay of the excited activity from radium is for the most part -independent of the nature of the body made active. Curie and -Danne (<i>loc. cit.</i>) observed that the active bodies gave off an emanation -itself capable of exciting activity in neighbouring bodies. -This property rapidly disappeared, and was inappreciable 2 hours -after removal. In certain substances like celluloid and caoutchouc, -the decay of activity is very much slower than for the metals. -This effect becomes more marked with increase of time of exposure -to the emanation. A similar effect is exhibited by lead, but to a -less marked degree. During the time the activity lasts, these -substances continue to give off an emanation.</p> - -<p class='c006'>It is probable that these divergencies from the general law are -not due to an actual change in the rate of decay of the true excited -activity but to an occlusion of the emanation by these substances -during the interval of exposure. After exposure the emanation -gradually diffuses out, and thus the activity due to this occluded -emanation and the excited activity produced by it decays very -slowly with the time.</p> -<p class='c005'><span class='pageno' id='Page_311'>311</span><a id='section183'></a> -<b>183. Active deposit of very slow decay.</b> M. and Mme -Curie<a id='r277' href='#f277' class='c012'><sup>[277]</sup></a> have observed that bodies which have been exposed for a -long interval in the presence of the radium emanation do not lose -all their activity. The excited activity at first decays rapidly at -the normal rate, falling to half value in about 28 minutes, but a -residual activity, which they state is of the order of ½0,000 of the -initial activity, always remains. A similar effect was observed by -Giesel. The writer has examined the variation of this residual -activity, and has found that it increases for several years. The -results are discussed in detail in <a href='#chap11'>chapter <span class='fss'>XI</span></a>. It will there be -shown that this active deposit of slow transformation contains the -radio-active constituents present in polonium, radio-tellurium and -radio-lead.</p> - -<div id='fig069' class='figcenter id004'> -<img src='images/fig-069.png' alt='Fig. 69.' class='ig001'> -<div class='ic002'> -<p>Fig. 69.</p> -</div> -</div> -<p class='c005'><b>184. The excited activity from actinium.</b> The emanation -of actinium, like that of thorium and radium, produces excited -activity on bodies, which is concentrated on the negative electrode -in an electric field. Debierne<a id='r278' href='#f278' class='c012'><sup>[278]</sup></a> found that the excited activity -<span class='pageno' id='Page_312'>312</span>decays approximately according to an exponential law, falling to -half value in 41 minutes. Giesel<a id='r279' href='#f279' class='c012'><sup>[279]</sup></a> examined the rate of decay of -the excited activity of “emanium”—which, we have seen, probably -contains the same radio-active constituents as actinium—and found -that it decayed to half value in 34 minutes. Miss Brooks<a id='r280' href='#f280' class='c012'><sup>[280]</sup></a> found -that the curves of decay of the excited activity from Giesel’s -emanium varied with the time of exposure to the emanation. The -results are shown graphically in <a href='#fig069'>Fig. 69</a>, for time exposures of -1, 2, 5, 10 and 30 minutes, and also for a long exposure of 21 hours. -After 10 minutes the curves have approximately the same rate of -decay. For convenience, the ordinates of the curves are adjusted -to pass through a common point. For a very short exposure, the -activity is small at first, but reaches a maximum about 9 minutes -later and finally decays exponentially to zero.</p> - -<p class='c006'>The curve of variation of activity for a very short exposure has -been determined accurately by Bronson; it is shown later in -<a href='#fig083'>Fig. 83</a>. He found that the decay of activity is finally exponential, -falling to half value in 36 minutes.</p> - -<p class='c006'>The explanation of these curves is discussed in detail in -<a href='#chap10'>chapter <span class='fss'>X</span></a>, <a href='#section212'>section 212</a>.</p> -<p class='c005'><a id='section185'></a> -<b>185. Physical and chemical properties of the active -deposit.</b> On account of the slow decay of the activity of the -active deposit from the thorium emanation, its physical and -chemical properties have been more closely examined than the -corresponding deposit from radium. It has already been mentioned -that the active deposit of thorium is soluble in some acids. -The writer<a id='r281' href='#f281' class='c012'><sup>[281]</sup></a> found that the active matter was dissolved off the -wire by strong or dilute solutions of sulphuric, hydrochloric and -hydrofluoric acids, but was only slightly soluble in water or nitric -acid. The active matter was left behind when the solvent was -evaporated. The rate of decay of activity was unaltered by -dissolving the active matter in sulphuric acid, and allowing it to -decay in the solution. In the experiment, the active matter was -dissolved off an active platinum wire; then equal portions of -the solutions were taken at definite intervals, evaporated down in -<span class='pageno' id='Page_313'>313</span>a platinum dish, and the activity of the residue tested by the -electric method. The rate of decay was found to be exactly the -same as if the active matter had been left on the wire. In another -experiment, an active platinum wire was made the cathode in a -copper sulphate solution, and a thin film of copper deposited on it. -The rate of decay of the activity was unchanged by the process.</p> - -<p class='c006'>A detailed examination of the physical and chemical properties -of the active deposit of thorium has been made by F. von -Lerch<a id='r282' href='#f282' class='c012'><sup>[282]</sup></a> and some important and interesting results have been -obtained. A solution of the active deposit was prepared by -dissolving the metal which had been exposed for some time in the -presence of the thorium emanation. In most cases the active -matter was precipitated with the metal. For example, an active -copper wire was dissolved in nitric acid and then precipitated by -caustic potash. The precipitate was strongly active. An active -magnesium wire, dissolved in hydrochloric acid and then precipitated -as phosphate, also gave an active precipitate. The activity -of the precipitates decayed at the normal rate, <i>i.e.</i> the activity fell -to half value in about 11 hours.</p> - -<p class='c006'>Experiments were also made on the solubility of the active -deposit in different substances. A platinum plate was made active -and then placed in different solutions, and the decrease of the -activity observed. In addition to the acids already mentioned, a -large number of substances were found to dissolve the active -deposit to some extent. The active matter was however not -dissolved to an appreciable extent in ether or alcohol. Many -substances became active if added to the active solution and then -precipitated. For example, an active solution of hydrochloric acid -was obtained by dissolving the deposit on an active platinum wire. -Barium chloride was then added and precipitated as sulphate. -The precipitate was strongly active, thus suggesting that the -active matter was carried down by the barium.</p> -<p class='c005'><a id='section186'></a> -<b>186. Electrolysis of solutions.</b> Dorn showed that, if solutions -of radiferous barium chloride were electrolysed, both electrodes -became temporarily active, but the anode to a greater degree than -the cathode. F. von Lerch has made a detailed examination of -the action of electrolysis on a solution of the active deposit of -<span class='pageno' id='Page_314'>314</span>thorium. The matter was dissolved off an active platinum plate -by hydrochloric acid, and then electrolysed between platinum -electrodes. The cathode was very active, but there was no trace -of activity on the anode. The cathode lost its activity at a -rate much <i>faster</i> than the normal. With an amalgamated zinc -cathode on the other hand, the rate of decay was normal. When -an active solution of hydrochloric acid was electrolysed with an -electromotive force smaller than that required to decompose water, -the platinum became active. The activity decayed to half value -in 4·75 hours while the normal fall is to half value in 11 hours. -These results point to the conclusion that the active matter is -complex and consists of two parts which have different rates of -decay of activity, and can be separated by electrolysis.</p> - -<p class='c006'>Under special conditions it was found possible to make the -anode active. This was the case if the anion attached itself to -the anode. For example, if an active hydrochloric solution was -electrolysed with a silver anode, the chloride of silver formed was -strongly active and its activity decayed at a normal rate. The -amount of activity obtained by placing different metals in active -solutions for equal times varied greatly with the metal. For -example, it was found that if a zinc plate and an amalgamated -zinc plate, which show equal potential differences with regard to -hydrochloric acid, were dipped for equal times in two solutions -of equal activity, the zinc plate was seven times as active as -the other. The activity was almost removed from the solution -in a few minutes by dipping a zinc plate into it. Some metals -became active when dipped into an active solution while others -did not. Platinum, palladium, and silver remained inactive, -while copper, tin, lead, nickel, iron, zinc, cadmium, magnesium, -and aluminium became active. These results strongly confirm the -view that excited activity is due to a deposit of active matter -which has distinctive chemical behaviour.</p> - -<p class='c006'>G. B. Pegram<a id='r283' href='#f283' class='c012'><sup>[283]</sup></a> has made a detailed study of the active deposits -obtained by electrolysis of pure and commercial thorium salts. -The commercial thorium nitrate obtained from P. de Haen gave, -when electrolysed, a deposit of lead peroxide on the anode. This -deposit was radio-active, and its activity decayed at the normal -rate of the excited activity due to thorium. From solutions of -<span class='pageno' id='Page_315'>315</span>pure thorium nitrate, no visible deposit was obtained on the anode, -but it was, however, found to be radio-active. The activity -decayed rapidly, falling to half value in about one hour. Some -experiments were also made on the effect of adding metallic salts -to thorium solutions and then electrolysing them. Anode and -cathode deposits of the oxides or metals obtained in this way were -found to be radio-active, but the activity fell to half value in a few -minutes. The gases produced by electrolysis were radio-active, -but this was due to the presence of the thorium emanation. The -explanation of the results obtained by Pegram and von Lerch will -be considered later in <a href='#section207'>section 207</a>. It will be shown that the -active deposit of thorium contains two distinct substances which -have different rates of transformation.</p> -<p class='c005'><a id='section187'></a> -<b>187. Effect of temperature.</b> The activity of a platinum -wire which has been exposed in the presence of the thorium -emanation is almost completely lost by heating the wire to a white -heat. Miss F. Gates<a id='r284' href='#f284' class='c012'><sup>[284]</sup></a> found that the activity was not destroyed -by the intense heat, but manifested itself on neighbouring bodies. -When the active wire was heated electrically in a closed cylinder, -the activity was transferred from the wire to the interior surface -of the cylinder in unaltered amount. The rate of decay of the -activity was not altered by the process. By blowing a current of -air through the cylinder during the heating, a part of the active -matter was removed from the cylinder. Similar results were found -for the excited activity due to radium.</p> - -<p class='c006'>F. von Lerch (<i>loc. cit.</i>) determined the amount of activity -removed at different temperatures. The results are shown in -the following table for a platinum wire excited by the thorium -emanation<a id='r285' href='#f285' class='c012'><sup>[285]</sup></a>.</p> - -<table class='table24' > -<colgroup> -<col class='colwidth42'> -<col class='colwidth25'> -<col class='colwidth31'> -</colgroup> - <tr><th class='c023' colspan='3'></th></tr> - <tr> - <td class='c013'> </td> - <td class='c013'>Temperature</td> - <td class='c014'>Percentage of activity removed</td> - </tr> - <tr> - <td class='c013'> </td> - <td class='c013'> </td> - <td class='c014'> </td> - </tr> - <tr> - <td class='c013'>Heated 2 minutes</td> - <td class='c013'>800° C.</td> - <td class='c014'>0</td> - </tr> - <tr> - <td class='c013'>then „ ½ minute more</td> - <td class='c013'>1020° C.</td> - <td class='c014'>16</td> - </tr> - <tr> - <td class='c013'>„ „ ½ „ „</td> - <td class='c013'>1260° C.</td> - <td class='c014'>52</td> - </tr> - <tr> - <td class='c013'>„ „ ½ „ „</td> - <td class='c013'>1460° C.</td> - <td class='c014'>99</td> - </tr> -</table> - -<p class='c006'><span class='pageno' id='Page_316'>316</span>The effect of heat on the volatilization of the active deposit of -radium has been examined in detail by Curie and Danne. The -interesting and important results obtained by them will be -discussed in <a href='#chap11'>chapter <span class='fss'>XI</span></a>, <a href='#section226'>section 226</a>.</p> -<p class='c005'><b>188. Effect of variation of E.M.F. on amount of -excited activity from thorium.</b> It has been shown that the -excited activity is confined to the cathode in a strong electric field. -In weaker fields the activity is divided between the cathode and -the walls of the vessel. This was tested in an apparatus<a id='r286' href='#f286' class='c012'><sup>[286]</sup></a> shown -in <a href='#fig070'>Fig. 70</a>.</p> - -<div id='fig070' class='figcenter id001'> -<img src='images/fig-070.png' alt='Fig. 70.' class='ig001'> -<div class='ic002'> -<p>Fig. 70.</p> -</div> -</div> - -<p class='c006'><i>A</i> is a cylindrical vessel of 5·5 cms. diameter, <i>B</i> the negative -electrode passing through insulating ends <i>C</i>, <i>D</i>. For a potential -difference of 50 volts, most of the excited activity was deposited -on the electrode <i>B</i>. For about 3 volts, half of the total excited -activity was produced on the rod <i>B</i>, and half on the walls of the -vessel. Whatever the voltage applied, the sum of the activities -on the central rod and the walls of the cylinder was found to -be a constant when a steady state was reached.</p> - -<p class='c006'>When no voltage was applied, diffusion alone was operative, -and in that case about 13 per cent. of the total activity was on the -rod <i>B</i>. The application of an electric field has thus no influence -on the sum total of excited activity, but merely controls the proportion -concentrated on the negative electrode.</p> - -<p class='c006'>A more detailed examination of the variation with strength of -field of the amount on the negative electrode was made in a similar -manner by F. Henning<a id='r287' href='#f287' class='c012'><sup>[287]</sup></a>. He found that in a strong electric field -the amount of excited activity was practically independent of the -diameter of the rod <i>B</i>, although the diameter varied between -<span class='pageno' id='Page_317'>317</span>·59 mm. and 6·0 mms. With a small voltage, the amount on the -negative electrode varied with its diameter. The curves showing -the relation between the amount of excited activity and voltage -are very similar in character to those obtained for the variation of -the current through an ionized gas with the voltage applied.</p> - -<p class='c006'>The amount of excited activity reaches a maximum when all -the active matter is removed from the gas as rapidly as it is -formed. With weaker fields, a portion diffuses to the sides of the -vessel, and produces excited activity on the positive electrode.</p> -<p class='c005'><b>189. Effect of pressure on distribution of excited -activity.</b> In a strong electric field, the amount of excited activity -produced on the cathode is independent of the pressure down to a -pressure of about 10 mms. of mercury. In some experiments made -by the writer<a id='r288' href='#f288' class='c012'><sup>[288]</sup></a>, the emanating thorium compound was placed -inside a closed cylinder about 4 cms. in diameter, through which -passed an insulated central rod. The central rod was connected to -the negative pole of a battery of 50 volts. When the pressure was -reduced below 10 mms. of mercury, the amount of excited activity -produced on the negative electrode diminished, and was a very -small fraction of its original value at a pressure of ⅒ mm. Some -excited activity was in this case found to be distributed over the -interior surface of the cylinder. It may thus be concluded that at -low pressures the excited activity appears on both anode and -cathode, even in a strong electric field. The probable explanation -of this effect is given in the next section.</p> - -<p class='c006'>Curie and Debierne<a id='r289' href='#f289' class='c012'><sup>[289]</sup></a> observed that when a vessel containing -an emanating radium compound was kept pumped down to a low -pressure, the amount of excited activity produced on the vessel -was much reduced. In this case the emanation given off by the -radium was removed by the pump with the other gases continuously -evolved from the radium compound. On account of the -very slow decay of activity of the emanation, the amount of excited -activity produced on the walls of the vessel, in the passage of the -emanation through it, was only a minute fraction of the amount -produced when none of the emanation given off was allowed to -escape.</p> -<p class='c005'><span class='pageno' id='Page_318'>318</span><b>190. Transmission of excited activity.</b> The characteristic -property of excited radio-activity is that it can be confined to the -cathode in a strong electric field. Since the activity is due to a -deposit of radio-active matter on the electrified surface, the matter -must be transported by positively charged carriers. The experiments -of Fehrle<a id='r290' href='#f290' class='c012'><sup>[290]</sup></a> showed that the carriers of excited activity travel -along the lines of force in an electric field. For example, when a -small negatively charged metal plate was placed in the centre of -a metal vessel containing an emanating thorium compound, more -excited activity was produced on the sides and corners of the plate -than at the central part.</p> - -<p class='c006'>A difficulty however arises in connection with the positive -charge of the carrier. According to the view developed in -<a href='#section136'>section 136</a> and later in chapters <a href='#chap10'><span class='fss'>X</span></a> and <a href='#chap11'><span class='fss'>XI</span></a>, the active matter which -is deposited on bodies and gives rise to excited activity, is itself -derived from the emanation. The emanations of thorium and -radium emit only α rays, <i>i.e.</i> positively charged particles. After -the expulsion of an α particle, the residue, which is supposed to -constitute the primary matter of the active deposit, should retain -a negative charge, and be carried to the anode in an electric field. -The exact opposite however is observed to be the case. The -experimental evidence does not support the view that the positively -charged α particles, expelled from the emanation, are directly -responsible for the phenomena of excited activity; for no excited -activity is produced in a body exposed to the α rays of the -emanation, provided the emanation itself does not come in contact -with it.</p> - -<p class='c006'>There has been a tendency to attach undue importance to this -apparent discrepancy between theory and experiment. The difficulty -is not so much to offer a probable explanation of the results -as to select from a number of possible causes. While there can be -little doubt that the main factor in the disintegration of the atom -consists in the expulsion of an α particle carrying a positive -charge, a complicated series of processes probably occurs before the -residue of the atom is carried to the negative electrode. The -experimental evidence suggests that one or more negative electrons -of slow velocity escape from the atom at the same time as the -<span class='pageno' id='Page_319'>319</span>particle. This is borne out by the recent discovery that the -particle expelled from radium, freed from the ordinary β rays, and -also from polonium, is accompanied by a number of slowly moving -and consequently easily absorbed electrons. If two negative -electrons escaped at the same time as the α particle, the residue -would be left with a positive charge and would be carried to the -negative electrode. There is also another experimental point -which is of importance in this connection. In the absence of -an electric field, the carriers remain in the gas for a considerable -time and undergo their transformation <i>in situ</i>. There is also some -evidence (<a href='#section227'>section 227</a>) that, even in an electric field, the carriers -of the active deposit are not swept to the electrode immediately -after the break up of the emanation, but remain some time in the -gas before they gain a positive charge. It must be remembered -that the atoms of the active deposit do not exist as a gas and by -the process of diffusion would tend to collect together to form -aggregates. These aggregates would act as small metallic particles, -and, if they were electro-positive in regard to the gas, -would gain a positive charge from the gas.</p> - -<p class='c006'>There can be little doubt that the processes occurring between -the break up of the emanation and the deposit of the residue in -the cathode in an electric field are complicated, and further careful -experiment is required to elucidate the sequence of the phenomena.</p> - -<p class='c006'>Whatever view is taken of the process by which these carriers -obtain a positive charge, there can be little doubt that the expulsion -of an α particle with great velocity from the atom of the -emanation must set the residue in motion. On account of the -comparatively large mass of this residue, the velocity acquired -will be small compared with that of the expelled α particle, and -the moving mass will rapidly be brought to rest at atmospheric -pressure by collision with the gas molecules in its path. At low -pressures, however, the collisions will be so few that it will not be -brought to rest until it strikes the boundaries of the vessel. -A strong electric field would have very little effect in controlling -the motion of such a heavy mass, unless it has been initially -brought to rest by collision with the gas molecules. This would -explain why the active matter is not deposited on the cathode at -low pressures in an electric field. Some direct evidence of a -<span class='pageno' id='Page_320'>320</span>process of this character, obtained by Debierne on examination -of the excited activity produced by actinium, is discussed in -<a href='#section192'>section 192</a>.</p> -<p class='c005'><a id='section191'></a> -<b>191.</b> The following method has been employed by the writer<a id='r291' href='#f291' class='c012'><sup>[291]</sup></a> to -determine the velocity of the positive carriers of excited activity of -radium and thorium in an electric field. Suppose <i>A</i> and <i>B</i> (<a href='#fig071'>Fig. 71</a>) -are two parallel plates exposed to the influence of the emanation, -which is uniformly distributed between them. If an alternating -<span class='fss'>E.M.F.</span> -<i>E</i>₀ -is applied between the plates, the same amount of -excited activity is produced on each electrode. If, in series with -the source of the alternating <span class='fss'>E.M.F.</span>, a battery of <span class='fss'>E.M.F.</span> -<i>E</i><sub>1</sub> less than <i>E</i>₀ -is placed, the positive carrier moves in a stronger electric -field in one half alternation than in the other. A carrier consequently -moves over unequal distances during the two half -alternations, since the velocity of the carrier is proportional to -the strength of the electric field in which it moves. The excited -activity will in consequence be unequally distributed over the two -electrodes. If the frequency of alternation is sufficiently great, -only the positive carriers within a certain small distance of one -plate can be conveyed to it, and the rest, in the course of several -succeeding alternations, are carried to the other plate.</p> - -<div id='fig071' class='figcenter id001'> -<img src='images/fig-071.png' alt='Fig. 71.' class='ig001'> -<div class='ic002'> -<p>Fig. 71.</p> -</div> -</div> - -<p class='c006'>When the plate <i>B</i> is negatively charged, the <span class='fss'>E.M.F.</span> between -the plates is -<i>E</i>₀ – <i>E</i><sub>1</sub>, when -<i>B</i> is positive the <span class='fss'>E.M.F.</span> is <i>E</i>₀ + <i>E</i><sub>1</sub>.</p> - -<p class='c006'><span class='pageno' id='Page_321'>321</span>Let</p> - -<div class='calc'> - -<div class='lg-container-b c017'> - <div class='linegroup'> - <div class='group'> - <div class='line in4'><i>d</i> = distance between the plates,</div> - <div class='line in4'><i>T</i> = time of a half alternation,</div> - <div class='line in4'>ρ = ratio of the excited radio-activity on the plate <i>B</i> to the</div> - <div class='line in17'>sum of the radio-activities on the plates <i>A</i> and <i>B</i>,</div> - <div class='line in4'><i>K</i> = velocity of the positive carriers for a potential-gradient</div> - <div class='line in17'>of 1 volt per centimetre.</div> - </div> - </div> -</div> - -</div> - -<p class='c018'>On the assumption that the electric field between the plates is -uniform, and that the velocity of the carrier is proportional to the -electric field, the velocity of the positive carrier towards <i>B</i> is</p> - -<div class='calc'> - -<div class='lg-container-b c017'> - <div class='linegroup'> - <div class='group'> - <div class='line in5'><i>E</i>₀ – <i>E</i><sub>1</sub></div> - <div class='line in4'>-------- <i>K</i></div> - <div class='line in8'><i>d</i></div> - </div> - </div> -</div> - -</div> - -<p class='c018'>and, in the course of the next half alternation,</p> - -<div class='calc'> - -<div class='lg-container-b c017'> - <div class='linegroup'> - <div class='group'> - <div class='line in5'><i>E</i>₀ + <i>E</i><sub>1</sub></div> - <div class='line in4'>-------- <i>K</i></div> - <div class='line in7'><i>d</i></div> - </div> - </div> -</div> - -</div> - -<p class='c018'>towards the plate <i>A</i>.</p> - -<p class='c006'>If <i>x</i><sub>1</sub> is less than <i>d</i>, the greatest distances -<i>x</i><sub>1</sub>, <i>x</i><sub>2</sub> passed over by -the positive carrier during two succeeding half alternations is thus -given by</p> - -<div class='calc'> - -<div class='lg-container-b c017'> - <div class='linegroup'> - <div class='group'> - <div class='line in7'><i>E</i>₀ – <i>E</i><sub>1</sub></div> - <div class='line'><i>x</i><sub>1</sub> = --------- <i>KT</i></div> - <div class='line in9'><i>d</i></div> - </div> - <div class='group'> - <div class='line'>and</div> - </div> - <div class='group'> - <div class='line in8'><i>E</i>₀ + <i>E</i><sub>1</sub></div> - <div class='line'><i>x</i><sub>2</sub> = ---------- <i>KT</i></div> - <div class='line in10'><i>d</i></div> - </div> - </div> -</div> - -</div> - -<p class='c018'>Suppose that the positive carriers are produced at a uniform -rate of <i>q</i> per second for unit distance between the plates. The -number of positive carriers which reach <i>B</i> during a half alternation -consists of two parts:</p> - -<p class='c006'>(1) One half of those carriers which are produced within the -distance -<i>x</i><sub>1</sub> -of the plate <i>B</i>. This number is equal to</p> - -<div class='calc'> - -<div class='lg-container-b c017'> - <div class='linegroup'> - <div class='group'> - <div class='line in1'>1</div> - <div class='line'>--- <i>x</i><sub>1</sub> <i>qT</i></div> - <div class='line in1'>2</div> - </div> - </div> -</div> - -</div> - -<p class='c018'>(2) All the carriers which are left within the distance -<i>x</i><sub>1</sub> from -<i>B</i> at the end of the previous half alternation. The number of -these can readily be shown to be</p> - -<div class='calc'> - -<div class='lg-container-b c017'> - <div class='linegroup'> - <div class='group'> - <div class='line in1'>1 <i>x</i><sub>1</sub></div> - <div class='line'>--- <i>x</i><sub>1</sub> ---- <i>qT</i></div> - <div class='line in1'>2 <i>x</i><sub>2</sub></div> - </div> - </div> -</div> - -</div> - -<p class='c018'>The remainder of the carriers, produced between <i>A</i> and <i>B</i> -during a complete alternation, will reach the other plate <i>A</i> in the -course of succeeding alternations, provided no appreciable recombination -<span class='pageno' id='Page_322'>322</span>takes place. This must obviously be the case, since the -positive carriers travel further in a half alternation towards <i>A</i> than -they return towards <i>B</i> during the next half alternation. The -carriers thus move backwards and forwards in the changing electric -field, but on the whole move towards the plate <i>A</i>.</p> - -<p class='c006'>The total number of positive carriers produced between the -plates during a complete alternation is 2<i>dqT</i>. The ratio ρ of the -number which reach <i>B</i> to the total number produced is thus -given by</p> - -<div class='figcenter id006'> -<img src='images/form-076.png' alt='Formula.' class='ig001'> -</div> - -<p class='c006'>Substituting the values of <i>x</i><sub>1</sub> and <i>x</i><sub>2</sub>, we find that</p> - -<div class='figcenter id005'> -<img src='images/form-077.png' alt='Formula.' class='ig001'> -</div> - -<p class='c006'>In the experiments, the values of <i>E</i>₀, <i>E</i><sub>1</sub>, <i>d</i>, and <i>T</i> were varied, -and the results obtained were in general agreement with the above -equation.</p> - -<p class='c006'>The following were the results for thorium:</p> - -<p class='c006'><i>Plates 1·30 cms. apart.</i></p> - -<table class='table2' > -<colgroup> -<col class='colwidth21'> -<col class='colwidth21'> -<col class='colwidth30'> -<col class='colwidth13'> -<col class='colwidth13'> -</colgroup> - <tr> - <th class='c015'><i>E</i>₀ + <i>E</i><sub>1</sub></th> - <th class='c015'><i>E</i>₀ – <i>E</i><sub>1</sub></th> - <th class='c015'>Alternations per second</th> - <th class='c015'>ρ</th> - <th class='c016'><i>K</i></th> - </tr> - <tr> - <td class='c015'> </td> - <td class='c015'> </td> - <td class='c015'> </td> - <td class='c015'> </td> - <td class='c016'> </td> - </tr> - <tr> - <td class='c015'>152</td> - <td class='c015'>101</td> - <td class='c015'>57</td> - <td class='c015'>·27</td> - <td class='c016'>1·25</td> - </tr> - <tr> - <td class='c015'>225</td> - <td class='c015'>150</td> - <td class='c015'>57</td> - <td class='c015'>·38</td> - <td class='c016'>1·17</td> - </tr> - <tr> - <td class='c015'>300</td> - <td class='c015'>200</td> - <td class='c015'>57</td> - <td class='c015'>·44</td> - <td class='c016'>1·24</td> - </tr> -</table> - -<p class='c006'><i>Plates 2 cms. apart.</i></p> - -<table class='table2' > -<colgroup> -<col class='colwidth21'> -<col class='colwidth21'> -<col class='colwidth30'> -<col class='colwidth13'> -<col class='colwidth13'> -</colgroup> - <tr> - <th class='c015'><i>E</i>₀ + <i>E</i><sub>1</sub></th> - <th class='c015'><i>E</i>₀ – <i>E</i><sub>1</sub></th> - <th class='c015'>Alternations per second</th> - <th class='c015'>ρ</th> - <th class='c016'><i>K</i></th> - </tr> - <tr> - <td class='c015'> </td> - <td class='c015'> </td> - <td class='c015'> </td> - <td class='c015'> </td> - <td class='c016'> </td> - </tr> - <tr> - <td class='c015'>273</td> - <td class='c015'>207</td> - <td class='c015'>44</td> - <td class='c015'>·37</td> - <td class='c016'>1·47</td> - </tr> - <tr> - <td class='c015'>300</td> - <td class='c015'>200</td> - <td class='c015'>53</td> - <td class='c015'>·286</td> - <td class='c016'>1·45</td> - </tr> -</table> - -<p class='c006'>The average mobility <i>K</i> deduced from a large number of -experiments was 1·3 cms. per sec. per volt per cm. for atmospheric -<span class='pageno' id='Page_323'>323</span>pressure and temperature. This velocity is about the same as -the velocity of the positive ion produced by Röntgen rays in air, -viz. 1·37 cms. per sec. The results obtained with the radium -emanation were more uncertain than those for thorium on account -of the distribution of some excited activity on the positive electrode. -The values of the velocities of the carriers were however -found to be roughly the same for radium as for thorium.</p> - -<p class='c006'>These results show that the carriers of the active deposit -travel in the gas with about the same velocity as the positive or -negative ions produced by the radiations in the gas. This -indicates either that the active matter becomes attached to positive -ions, or that the active matter itself, acquiring in some way a -positive charge, collects a cluster of neutral molecules which travel -with it.</p> -<p class='c005'><a id='section192'></a> -<b>192. Carriers of the excited activity from actinium -and “emanium.”</b> Giesel<a id='r292' href='#f292' class='c012'><sup>[292]</sup></a> observed that “emanium” gave off -a large quantity of emanation, and that this emanation gave rise to -a type of radiation which he termed the <i>E</i> rays. A narrow metal -cylinder containing the active substance was placed with the open -end downwards, about 5 cms. above the surface of a zinc sulphide -screen. The screen was charged negatively to a high potential by -an electric machine, and the cylinder connected with earth. A -luminous spot of light was observed on the screen, which was -brighter at the edge than at the centre. A conductor, connected -with earth, brought near the luminous spot apparently repelled it. -An insulator did not show such a marked effect. On removal of -the active substance, the luminosity of the screen persisted for -some time. This was probably due to the excited activity produced -on the screen.</p> - -<p class='c006'>The results obtained by Giesel support the view that the -carriers of excited activity of “emanium” have a positive charge. -In a strong electric field the carriers travel along the lines of force -to the cathode, and there cause excited activity on the screen. -The movement of the luminous zone on the approach of a conductor -is due to the disturbance of the electric field. -<span class='pageno' id='Page_324'>324</span>Debierne<a id='r293' href='#f293' class='c012'><sup>[293]</sup></a> found that actinium also gave off a large amount of -emanation, the activity of which decayed very rapidly with the -time, falling to half value in 3·9 seconds.</p> - -<p class='c006'>This emanation produces excited activity on surrounding objects, -and at diminished pressure the emanation produces a uniform -distribution of excited activity in the enclosure containing the -emanation. The excited activity falls to half value in 41 minutes.</p> - -<p class='c006'>Debierne observed that the distribution of excited activity was -altered by a strong magnetic field. The experimental -arrangement is shown in <a href='#fig071a'>Fig. 71<span class='fss'>A</span></a>. The -active matter was placed at <i>M</i>, and two plates -<i>A</i> and <i>B</i> were placed symmetrically with regard -to the source. On the application of a strong -magnetic field normal to the plane of the paper, -the excited activity was unequally distributed -between the plates <i>A</i> and <i>B</i>. The results showed -that the carriers of excited activity were deviated -by a magnetic field in the opposite sense to the -cathode rays, <i>i.e.</i> the carriers were positively -charged. In some cases, however, the opposite -effect was obtained. Debierne considers that the excited activity -of actinium is due to “ions activants,” the motion of which is -altered by a magnetic field. Other experiments showed that the -magnetic field acted on the “ions activants” and not on the -emanation.</p> - -<div id='fig071a' class='figcenter id002'> -<img src='images/fig-071a.png' alt='Fig. 71A.' class='ig001'> -<div class='ic002'> -<p>Fig. 71A.</p> -</div> -</div> - -<p class='c006'>The results of Debierne thus lead to the conclusion that the -carriers of excited activity are derived from the emanation and are -projected with considerable velocity. This result supports the -view, advanced in section 190, that the expulsion of α particles -from the emanation must set the part of the system left behind in -rapid motion. A close examination of the mode of transference of -the excited activity by actinium and the emanation substance is -likely to throw further light on the processes which give rise -to the deposit of active matter on the electrodes.</p> - -<div class='chapter'> - <span class='pageno' id='Page_325'>325</span> - <h2 id='chap09' class='c004'>CHAPTER IX. <br> THEORY OF SUCCESSIVE CHANGES.</h2> -</div> -<p class='c005'><b>193. Introduction.</b> We have seen in previous chapters -that the radio-activity of the radio-elements is always accompanied -by the production of a series of new substances with some distinctive -physical and chemical properties. For example, thorium -produces from itself an intensely radio-active substance, Th X, -which can be separated from the thorium in consequence of its -solubility in ammonia. In addition, thorium gives rise to a gaseous -product, the thorium emanation, and also to another substance -which is deposited on the surface of bodies in the neighbourhood -of the thorium, where its presence is indicated by the phenomenon -known as “excited activity.”</p> - -<p class='c006'>A close examination of the origin of these products shows that -they are not produced simultaneously, but arise in consequence -of a succession of changes originating in the radio-element. -Thorium first of all gives rise to the product Th X. The Th X -produces from itself the thorium emanation, and this in turn is -transformed into a non-volatile substance. A similar series of -changes is observed in radium, with the exception that there is -no product in radium corresponding to the Th X in the case of -thorium. Radium first of all produces an emanation, which, like -thorium, is transformed into a non-volatile substance. In uranium -only one product, Ur X, has been observed, for uranium does not -give off an emanation and in consequence does not produce excited -activity on bodies.</p> - -<p class='c006'>As a typical example of the evidence, from which it is deduced -that one substance is the parent of another, we will consider the -connection of the two products Th X and the thorium emanation. -It has been shown (<a href='#section154'>section 154</a>) that after the separation of Th X -<span class='pageno' id='Page_326'>326</span>from a thorium solution, by precipitation with ammonia, the -precipitated thorium hydroxide has lost to a large extent its -power of emanating. This cannot be ascribed to a prevention of -escape of the emanation produced in it, for very little emanation -is observed when a current of air is drawn through the hydroxide -in a state of solution, when most of the emanation present would -be carried off. On the other hand, the solution containing the -Th X gives off a large quantity of emanation, showing that the -power of giving off an emanation belongs to the product Th X. -Now it is found that the quantity of emanation given off by the -separated Th X decreases according to an exponential law with -the time, falling to half value in four days. The rate of production -of emanation thus falls off according to the same law and at the -same rate as the activity of the Th X measured in the ordinary -manner by the α rays. Now this is exactly the result to be -expected if the Th X is the parent of the emanation, for the -activity of Th X at any time is proportional to its rate of change, -<i>i.e.</i>, to the rate of production of the secondary type of matter by -the emanation in consequence of a change in it. Since the rate -of change of the emanation (half transformed in 1 minute) is very -rapid compared with the rate of change of Th X, the amount of -emanation present will be practically proportional to the activity -of the Th X at any instant, <i>i.e.</i>, to the amount of unchanged Th X -present. The observed fact that the hydroxide regains its power -of emanating in the course of time is due to the production of -fresh Th X by the thorium, which in turn produces the emanation.</p> - -<p class='c006'>In a similar way, excited activity is produced on bodies over -which the emanation is passed, and in amount proportional to the -activity of the emanation, <i>i.e.</i>, to the amount of the emanation -present. This shows that the active deposit, which gives rise to -the phenomenon of excited activity, is itself a product of the -emanation. The evidence thus seems to be conclusive that Th X -is the parent of the emanation and that the emanation is the -parent of the deposited matter.</p> -<p class='c005'><b>194. Chemical and Physical properties of the active -products.</b> Each of these radio-active products is marked by some -distinctive chemical and physical properties which differentiate -<span class='pageno' id='Page_327'>327</span>it from the preceding and succeeding products. For example, -Th X behaves as a solid. It is soluble in ammonia, while thorium -is not. The thorium emanation behaves as a chemically inert gas -and condenses at a temperature of -120° C. The active deposit -from the emanation behaves as a solid and is readily soluble in -sulphuric and hydrochloric acids and is only slightly soluble in -ammonia.</p> - -<p class='c006'>The striking dissimilarity which exists in many cases between -the chemical and the physical properties of the parent matter and -the product to which it gives rise is very well illustrated by the -case of radium and the radium emanation. Radium is an element -so closely allied in chemical properties to barium that, apart from -a slight difference in the solubility of the chlorides and bromides, -it is difficult to distinguish chemically between them. It has a -definite spectrum of bright lines similar in many respects to the -spectra of the alkaline earths. Like barium, it is non-volatile at -ordinary temperature. On the other hand, the emanation which -is continually produced from radium is a radio-active and chemically -inert gas, which is condensed at a temperature of -150° C. -Both in its spectrum and in the absence of definite chemical -properties, it resembles the argon-helium group of inert gases, -but differs from these gases in certain marked features.</p> - -<p class='c006'>The emanation must be considered to be an unstable gas -which breaks down into a non-volatile type of matter, the disintegration -being accompanied by the expulsion of heavy atoms -of matter (α particles) projected with great velocity. This rate of -breaking up is not affected by temperature over the considerable -range which has been examined. After a month’s interval, the -volume of the emanation has shrunk to a small portion of its -initial value. But the most striking property of the emanation, -which, as we shall see later (<a href='#chap12'>chapter <span class='fss'>XII</span></a>), is a direct consequence -of its radio-activity, is the enormous amount of energy emitted -from it. The emanation in breaking up through its successive -stages emits about 3 million times as much energy as is given -out by the explosion of an equal volume of hydrogen and oxygen, -mixed in the proper proportions to form water; and yet, in this -latter chemical reaction more heat is emitted than in any other -known chemical change.</p> - -<p class='c006'><span class='pageno' id='Page_328'>328</span>We have seen that the two emanations and the products Ur X, -Th X lose their activity with the time according to a simple -exponential law, and at a rate that is independent—as far as -observation has gone—of the chemical and physical agents at our -disposal. The time taken for each of these products to fall to -half its value is thus a definite physical constant which serves to -distinguish it from all other products.</p> - -<p class='c006'>On the other hand, the variation of the excited activity -produced by these emanations does not even approximately obey -such a law. The rate of decay depends not only on the time of -exposure to the respective emanations, but also, in the case of -radium, on the type of radiation which is used as a means of -comparative measurement. It will be shown, in succeeding -chapters, that the complexity of the decay is due to the fact that -the matter in the active deposits undergoes several successive -transformations, and that the peculiarities of the curves of decay, -obtained under different conditions, can be explained completely -on the assumption that two changes occur in the active deposit -from both thorium and actinium and six in the active deposit -from radium.</p> -<p class='c005'><b>195. Nomenclature.</b> The nomenclature to be applied to -the numerous radio-active products is a question of great importance -and also one of considerable difficulty. Since there are at -least seven distinct substances produced from radium, and probably -five from thorium and actinium, it is neither advisable nor convenient -to give each a special name such as is applied to the -parent elements. At the same time, it is becoming more and -more necessary that each product should be labelled in such a -way as to indicate its place in the succession of changes. This -difficulty is especially felt in discussing the numerous changes in -the active deposits from the different emanations. Many of the -names attached to the products were given at the time of their -discovery, before their position in the scheme of changes was -understood. In this way the names Ur X, Th X were applied to -the active residues obtained by chemical treatment of uranium -and thorium. Since, in all probability, these substances are the -first products of the two elements, it may be advisable to retain -<span class='pageno' id='Page_329'>329</span>these names, which certainly have the advantage of brevity. The -name “emanation” was originally given to the radio-active gas -from thorium, and has since been applied to the similar gaseous -products of radium and actinium.</p> - -<p class='c006'>Finding the name “radium emanation” somewhat long and -clumsy, Sir William Ramsay<a id='r294' href='#f294' class='c012'><sup>[294]</sup></a> has recently suggested “ex-radio” -as an equivalent. This name is certainly brief and is also suggestive -of its origin; but at least six other ex-radios, whose -parentage is as certain as that of the emanation, remain unnamed. -A difficulty arises in applying the corresponding names ex-thorio, -ex-actinio to the other gaseous products, for, unlike radium, the -emanations of thorium and actinium are probably the second, -not the first, disintegration product of the radio-elements in -question. Another name thus has to be applied to the first -product in these cases. It may be advisable to give a special -name to the emanation, since it has been the product most investigated -and was the first to be isolated chemically; but, on the -other hand, the name “radium emanation” is historically interesting, -and suggests a type of volatile or gaseous matter. Since -the term “excited” or “induced” activity refers only to the -radiations from the active body, a name is required for the -radiating matter itself. The writer in the first edition of this -book suggested the name “emanation X.”<a id='r295' href='#f295' class='c012'><sup>[295]</sup></a> This title was given -from analogy to the names Ur X and Th X, to indicate that the -active matter was product of the emanation. The name, however, -is not very suitable, and, in addition, can only be applied to the -initial product deposited, and not to the further products of its -decomposition. It is very convenient in discussing mathematically -the theory of successive changes to suppose that the deposited -matter called <i>A</i> is changed into <i>B</i>, <i>B</i> into <i>C</i>, <i>C</i> into <i>D</i>, and so on. -I have therefore discarded the name emanation <i>X</i>, and have used -the terms radium <i>A</i>, radium <i>B</i>, and so on, to signify the successive -products of the decomposition of the emanation of radium. A -similar nomenclature is applied to thorium and actinium. This -system of notation is elastic and simple, and I have found it of -great convenience in the discussion of successive products. In -<span class='pageno' id='Page_330'>330</span>speaking generally of the active matter, which causes excited -activity, without regard to its constituents, I have used the term -“active deposit.” The scheme of nomenclature employed in this -book is clearly shown below:—</p> - -<table class='table8' > -<colgroup> -<col class='colwidth25'> -<col class='colwidth25'> -<col class='colwidth25'> -<col class='colwidth25'> -</colgroup> - <tr> - <td class='c013'>Radium</td> - <td class='c013'>Thorium</td> - <td class='c013'>Uranium</td> - <td class='c014'>Actinium</td> - </tr> - <tr> - <td class='c013'>Radium emanation</td> - <td class='c013'>Th X</td> - <td class='c013'>Ur X</td> - <td class='c014'>Actinium X</td> - </tr> - <tr> - <td class='c013'>Radium <i>A</i> (Active)</td> - <td class='c013'>Thorium emanation</td> - <td class='c013'>Final product</td> - <td class='c014'>Actinium emanation</td> - </tr> - <tr> - <td class='c013'>Radium <i>B</i> (Active)</td> - <td class='c013'>Thorium <i>A</i> (Active)</td> - <td class='c013'> </td> - <td class='c014'>Actinium <i>A</i> (Active)</td> - </tr> - <tr> - <td class='c013'>Radium <i>C</i> (Active)</td> - <td class='c013'>Thorium <i>B</i> (Active)</td> - <td class='c013'> </td> - <td class='c014'>Actinium <i>B</i> (Active)</td> - </tr> - <tr> - <td class='c013'>Radium <i>D</i> (Active)</td> - <td class='c013'>Thorium <i>C</i> (final)</td> - <td class='c013'> </td> - <td class='c014'>Actinium <i>C</i> (final)</td> - </tr> - <tr> - <td class='c013'>&c.</td> - <td class='c013'> </td> - <td class='c013'> </td> - <td class='c014'> </td> - </tr> -</table> - -<p class='c006'>Each product on this scheme is the parent of the product -below it. Since only two products have been observed in the -active deposit of thorium and actinium, thorium <i>C</i> and actinium <i>C</i> -respectively refer to their final inactive products. It will be -shown in the next chapter that, as in the case of thorium, an -intermediate product exists between actinium and its emanation. -From analogy to the products Th X and Ur X, this substance is -termed “actinium X.”</p> -<p class='c005'><b>196. Theory of Successive Changes.</b> Before considering -the evidence from which these changes are deduced, the general -theory of successive changes of radio-active matter will be considered. -It is supposed that the matter <i>A</i> changes into <i>B</i>, -<i>B</i> into <i>C</i>, <i>C</i> into <i>D</i>, and so on.</p> - -<p class='c006'>Each of these changes is supposed to take place according to -the same law as a monomolecular change in chemistry, <i>i.e.</i>, the -number <i>N</i> of particles unchanged after a time <i>t</i> is given by</p> - -<div class='figcenter id010'> -<img src='images/form-078.png' alt='Formula.' class='ig001'> -</div> - -<p class='c006'>where <i>N</i>₀ is the initial number and λ the constant of -the change.</p> - -<p class='c006'>Since <i>dN</i>/<i>dt</i> = -λ<i>N</i>, the rate of change at any time is always -proportional to the amount of matter unchanged. It has previously -been pointed out that this law of decay of the activity of the -radio-active products is an expression of the fact that the change -is of the same type as a monomolecular chemical change.</p> - -<p class='c006'><span class='pageno' id='Page_331'>331</span>Suppose that <i>P</i>, <i>Q</i>, <i>R</i> represent the number of particles of the -matter <i>A</i>, <i>B</i>, and <i>C</i> respectively at any time <i>t</i>. -Let λ<sub>1</sub>, λ<sub>2</sub>, λ<sub>3</sub> be -the constants of change of the matter <i>A</i>, <i>B</i>, and <i>C</i> respectively.</p> - -<p class='c006'>Each atom of the matter <i>A</i> is supposed to give rise to one -atom of the matter <i>B</i>, one atom of <i>B</i> to one of <i>C</i>, and so on.</p> - -<p class='c006'>The expelled “rays” or particles are non-radio-active, and so do -not enter into the theory.</p> - -<p class='c006'>It is not difficult to deduce mathematically the number of -atoms of <i>P</i>, <i>Q</i>, <i>R</i>, ... of the matter <i>A</i>, <i>B</i>, <i>C</i>, ... existing at any time <i>t</i> -after this matter is set aside, if the initial values of <i>P</i>, <i>Q</i>, <i>R</i>, ... -are given. In practice, however, it is generally only necessary to -employ three special cases of the theory which correspond, for -example, to the changes in the active deposit, produced on a wire -exposed to a constant amount of radium emanation and then -removed, (1) when the time of exposure is extremely short -compared with the period of the changes, (2) when the time of -exposure is so long that the amount of each of the products has -reached a steady limiting value, and (3) for any time of exposure.</p> - -<p class='c006'>There is also another case of importance which is practically -a converse of Case 3, viz. when the matter <i>A</i> is supplied at a -constant rate from a primary source and the amounts of <i>A</i>, <i>B</i>, <i>C</i> -are required at any subsequent time. The solution of this can, -however, be deduced immediately from Case 3 without analysis.</p> -<p class='c005'><a id='section197'></a> -<b>197.</b> <span class='sc'>Case 1.</span> <i>Suppose that the matter initially considered -is all of one kind A. It is required to find the number of -particles P, Q, R of the matter A, B, C respectively present after -any time t.</i></p> - -<p class='c006'>Then</p> - -<div class='figcenter id009'> -<img src='images/form-079.png' alt='Formula.' class='ig001'> -</div> - -<p class='c006'>if <i>n</i> is the number of particles of <i>A</i> initially -present. Now <i>dQ</i>, the increase of the number of particles of the -matter <i>B</i> per unit time, is the number supplied by the change in -the matter <i>A</i>, less the number due to the change of <i>B</i> into <i>C</i>, -thus</p> - -<div class='calc'> - -<div class='lg-container-b c017'> - <div class='linegroup'> - <div class='group'> - <div class='line in15'><i>dQ</i>/<i>dt</i> = λ<sub>1</sub><i>P</i> – λ<sub>2</sub><i>Q</i> (1).</div> - <div class='line'>Similarly <i>dR</i>/<i>dt</i> = λ<sub>2</sub><i>Q</i> – λ<sub>3</sub><i>R</i> (2).</div> - </div> - </div> -</div> - -</div> - -<p class='c018'>Substituting in (1) the value of <i>P</i> in terms of <i>n</i>,</p> - -<div class='figcenter id005'> -<img src='images/form-080.png' alt='Formula.' class='ig001'> -</div> - -<p class='c006'><span class='pageno' id='Page_332'>332</span>The solution of this equation is of the form</p> - -<div class='figcenter id006'> -<img src='images/form-081.png' alt='Formula.' class='ig001'> -</div> - -<p class='c006'>By substitution it is found that <i>a</i> = λ<sub>1</sub>/(λ<sub>2</sub> – λ<sub>1</sub>).</p> - -<p class='c006'>Since <i>Q</i> = 0 when <i>t</i> = 0, <i>b</i> = -λ<sub>1</sub>(λ<sub>2</sub> – λ<sub>1</sub>).</p> - -<p class='c006'>Thus</p> - -<div class='figcenter id006'> -<img src='images/form-082.png' alt='Formula.' class='ig001'> -</div> - -<p class='c006'>Substituting this value of <i>Q</i> in (2), it can readily be shown that</p> - -<div class='figcenter id006'> -<img src='images/form-083.png' alt='Formula.' class='ig001'> -</div> - -<p class='c006'>where</p> - -<div class='figcenter id005'> -<img src='images/form-084.png' alt='Formula.' class='ig001'> -</div> - -<div class='figcenter id005'> -<img src='images/form-085.png' alt='Formula.' class='ig001'> -</div> - -<div class='figcenter id005'> -<img src='images/form-086.png' alt='Formula.' class='ig001'> -</div> - -<div id='fig072' class='figcenter id004'> -<img src='images/fig-072.png' alt='Fig. 72.' class='ig001'> -<div class='ic002'> -<p>Fig. 72.</p> -</div> -</div> - -<p class='c006'>The variation of the values of <i>P</i>, <i>Q</i>, <i>R</i> with the time <i>t</i>, after -removal of the source, is shown graphically in <a href='#fig072'>Fig. 72</a>, curves <i>A</i>, <i>B</i>, -and <i>C</i> respectively. In order to draw the curves for the practical -case which will be considered later corresponding to the first three -<span class='pageno' id='Page_333'>333</span>changes in radium <i>A</i>, the -values of λ<sub>1</sub>, λ<sub>2</sub>, λ<sub>3</sub> were taken as -3·85 × 10<sup>-3</sup>, 5·38 × 10<sup>-4</sup>, 4·13 × 10<sup>-4</sup> -respectively, <i>i.e.</i>, the times -required for each successive type of matter to be half transformed -are about 3, 21, and 28 minutes respectively.</p> - -<p class='c006'>The ordinates of the curves represent the relative number of -atoms of the matter <i>A</i>, <i>B</i>, and <i>C</i> existing at any time, and the -value of <i>n</i>, the original number of atoms of the matter <i>A</i> -deposited, is taken as 100. The amount of matter <i>B</i> is initially -zero, and in this particular case, passes through a maximum about -10 minutes later, and then diminishes with the time. In a -similar way, the amount of <i>C</i> passes through a maximum about -37 minutes after removal. After an interval of several hours the -amount of both <i>B</i> and <i>C</i> diminishes very approximately according -to an exponential law with the time, falling to half value after -intervals of 21 and 28 minutes respectively.</p> -<p class='c005'><a id='section198'></a> -<b>198.</b> <span class='sc'>Case 2.</span> <i>A primary source supplies the matter A at a -constant rate and the process has continued so long that the amount -of the products A, B, C, ... has reached a steady limiting value. -The primary source is then suddenly removed. It is required to -find the amounts of A, B, C, ... remaining at any subsequent time t.</i></p> - -<p class='c006'>In this case, the number <i>n</i>₀ of particles of <i>A</i>, deposited -per second from the source, is equal to the number of particles -of <i>A</i> which change into <i>B</i> per second, and of <i>B</i> into <i>C</i>, and so -on. This requires the relation</p> - -<div class='calc'> - -<div class='lg-container-b c017'> - <div class='linegroup'> - <div class='group'> - <div class='line'><i>n</i>₀ = λ<sub>1</sub><i>P</i>₀ = λ<sub>2</sub><i>Q</i>₀ = λ<sub>3</sub><i>R</i>₀ (6),</div> - </div> - </div> -</div> - -</div> - -<p class='c018'>where <i>P</i>₀, <i>Q</i>₀, <i>R</i>₀ -are the maximum numbers of particles of the -matter <i>A</i>, <i>B</i>, and <i>C</i> when a steady state is reached.</p> - -<p class='c006'>The values of <i>P</i>, <i>Q</i>, <i>R</i> at any time <i>t</i> after removal of the -source are given by equations of the same form as (3) and (5) -for a short exposure. Remembering the condition that initially</p> - -<div class='calc'> - -<div class='lg-container-b c017'> - <div class='linegroup'> - <div class='group'> - <div class='line'><i>P</i> = <i>P</i>₀ = <i>n</i>₀/λ<sub>1</sub>,</div> - <div class='line'><i>Q</i> = <i>Q</i>₀ = <i>n</i>₀/λ<sub>2</sub>,</div> - <div class='line'><i>R</i> = <i>R</i>₀ = <i>n</i>₀/λ<sub>3</sub>,</div> - </div> - </div> -</div> - -</div> - -<p class='c018'><span class='pageno' id='Page_334'>334</span>it can readily be shown that</p> - -<div class='figcenter id006'> -<img src='images/form-087.png' alt='Formula.' class='ig001'> -</div> - -<p class='c006'>where</p> - -<div class='figcenter id002'> -<img src='images/form-088.png' alt='Formula.' class='ig001'> -</div> - -<div class='figcenter id002'> -<img src='images/form-089.png' alt='Formula.' class='ig001'> -</div> - -<div class='figcenter id002'> -<img src='images/form-090.png' alt='Formula.' class='ig001'> -</div> - -<div id='fig073' class='figcenter id004'> -<img src='images/fig-073.png' alt='Fig. 73.' class='ig001'> -<div class='ic002'> -<p>Fig. 73.</p> -</div> -</div> - -<p class='c006'>The relative numbers of atoms of <i>P</i>, <i>Q</i>, <i>R</i> existing at any -time are shown graphically in <a href='#fig073'>Fig. 73</a>, curves <i>A</i>, <i>B</i>, <i>C</i> respectively. -The number of atoms <i>R</i>₀ is taken as 100 for comparison, and the -values of λ<sub>1</sub>, λ<sub>2</sub>, λ<sub>3</sub> are taken corresponding to the 3, 21, and 28-minute -changes in the active deposit of radium. A comparison -with <a href='#fig072'>Fig. 72</a> for a short exposure brings out very clearly the -variation in the relative amounts of <i>P</i>, <i>Q</i>, <i>R</i> in the two cases. -Initially the amount of <i>R</i> decreases very slowly. This is a result -of the fact that the supply of <i>C</i> due to the breaking up of <i>B</i> at -<span class='pageno' id='Page_335'>335</span>first, nearly compensates for the breaking up of <i>C</i>. The values -of <i>Q</i> and <i>R</i> after several hours decrease exponentially, falling to -half value in 28 minutes.</p> -<p class='c005'><a id='section199'></a> -<b>199.</b> <span class='sc'>Case 3.</span> <i>Suppose that a primary source has supplied -the matter A at a constant rate for any time T and is then -suddenly removed. Required the amounts of A, B, C at any -subsequent time.</i></p> - -<p class='c006'>Suppose that -<i>n</i>₀ -particles of the matter <i>A</i> are deposited each -second. After a time of exposure <i>T</i>, the number of particles <i>P<sub>T</sub></i> -of the matter <i>A</i> present is given by</p> - -<div class='figcenter id007'> -<img src='images/form-091.png' alt='Formula.' class='ig001'> -</div> - -<p class='c006'>At any time <i>t</i>, after removal of the source, the number of -particles <i>P</i> of the matter <i>A</i> is given by</p> - -<div class='figcenter id007'> -<img src='images/form-092.png' alt='Formula.' class='ig001'> -</div> - -<p class='c006'>Consider the number of particles -<i>n</i>₀<i>dt</i> -of the matter <i>A</i> produced -during the interval <i>dt</i>. At any later time <i>t</i>, the number of -particles <i>dQ</i> of the matter <i>B</i>, which result from the change in <i>A</i>, -is given (see equation 4) by</p> - -<div class='figcenter id006'> -<img src='images/form-093.png' alt='Formula.' class='ig001'> -</div> - -<p class='c006'>After a time of exposure <i>T</i>, the number of particles <i>Q<sub>T</sub></i> of the -matter <i>B</i> present is readily seen to be given by</p> - -<div class='figcenter id007'> -<img src='images/form-094.png' alt='Formula.' class='ig001'> -</div> - -<p class='c006'>If the body is removed from the emanation after an exposure -<i>T</i>, at any later time <i>t</i> the number of particles of <i>B</i> is in the same -way given by</p> - -<div class='figcenter id009'> -<img src='images/form-095.png' alt='Formula.' class='ig001'> -</div> - -<p class='c006'>It will be noted that the method of deduction of <i>Q<sub>T</sub></i> and <i>Q</i> is -independent of the particular form of the function <i>f</i>(<i>t</i>).</p> - -<p class='c006'><span class='pageno' id='Page_336'>336</span>Substituting the particular value of <i>f</i>(<i>t</i>) given in equation (10) -and integrating, it can readily be deduced that</p> - -<div class='figcenter id007'> -<img src='images/form-096.png' alt='Formula.' class='ig001'> -</div> - -<p class='c006'>where</p> - -<div class='figcenter id007'> -<img src='images/form-097.png' alt='Formula.' class='ig001'> -</div> - -<p class='c006'>In a similar way, the number of particles <i>R</i> of the matter <i>C</i> -present at any time can be deduced by substitution of the value -of <i>f</i>(<i>t</i>) in equation (5). These equations are, however, too complicated -in form for simple application to experiment, and will not -be considered here.</p> -<p class='c005'><a id='section200'></a> -<b>200.</b> <span class='sc'>Case 4.</span> <i>The matter A is supplied at a constant rate -from a primary source. Required to find the number of particles -of A, B, C at any subsequent time t, when initially A, B, C are -absent.</i></p> - -<p class='c006'>The solution can be simply obtained in the following way. -Suppose that the conditions of Case 2 are fulfilled. The products -<i>A</i>, <i>B</i>, <i>C</i> are in radio-active equilibrium and -let <i>P</i>₀, <i>Q</i>₀, <i>R</i>₀ be the -number of particles of each present. Suppose the source is -removed. The values of <i>P</i>, <i>Q</i>, <i>R</i> at any subsequent time are given -by equations (7), (8) and (9) respectively. Now suppose the -source, which has been removed, still continues to supply <i>A</i> at -the same constant rate and let -<i>P</i><sub>1</sub>, <i>Q</i><sub>1</sub>, <i>R</i><sub>1</sub> be the number of -particles of <i>A</i>, <i>B</i>, <i>C</i> again present with the source at any -subsequent time. Now we have seen, that the rate of change of -any individual product, considered by itself, is independent of -conditions and is the same whether the matter is mixed with the -parent substance or removed from it. Since the values of -<i>P</i>₀, <i>Q</i>₀, <i>R</i>₀ -represent a steady state where the rate of supply of each kind -of matter is equal to its rate of change, the sum of the number -of particles <i>A</i>, <i>B</i>, <i>C</i> present at any time with the source, and in -the matter from which it was removed, must at all times be equal -to <i>P</i>₀, <i>Q</i>₀, <i>R</i>₀, ..., that is</p> - -<div class='calc'> - -<div class='lg-container-b c017'> - <div class='linegroup'> - <div class='group'> - <div class='line'><i>P</i><sub>1</sub> + <i>P</i> = <i>P</i>₀,</div> - <div class='line'><i>Q</i><sub>1</sub> + <i>Q</i> = <i>Q</i>₀,</div> - <div class='line'><i>R</i><sub>1</sub> + <i>R</i> = <i>R</i>₀.</div> - </div> - </div> -</div> - -</div> - -<p class='c018'><span class='pageno' id='Page_337'>337</span>This must obviously be the case, for otherwise there would be a -destruction or creation of matter by the mere process of separation -of the source from its products; but, by hypothesis, neither the -rate of supply from the source, nor the law of change of the -products, has been in any way altered by removal.</p> - -<p class='c006'>Substituting the values of <i>P</i>, <i>Q</i>, <i>R</i> from equations (7), (8), and -(9), we obtain</p> - -<div class='figcenter id002'> -<img src='images/form-098.png' alt='Formula.' class='ig001'> -</div> - -<p class='c006'>where <i>a</i>, <i>b</i>, and <i>c</i> have the values given after equation (9). The -curves representing the increase of <i>P</i>, <i>Q</i>, <i>R</i>, are thus, in all cases, -complementary to the curves shown in <a href='#fig073'>Fig. 73</a>. The sum of -the ordinates of the two curves of rise and decay at any time is -equal to 100. We have already seen examples of this in the case -of the decay and recovery curves of Ur X and Th X.</p> -<p class='c005'><b>201. Activity of a mixture of products.</b> In the previous -calculations we have seen how the number of particles of each -of the successive products varies with the time under different -conditions. It is now necessary to consider how this number is -connected with the activity of the mixture of products.</p> - -<p class='c006'>If <i>N</i> is the number of particles of a product, the number of -particles breaking up per second is λ<i>N</i>, where λ is the constant -of change. If each particle of each product, in breaking up, emits -one α particle, we see that the number of α particles expelled per -second from the mixture of products at any time is equal to -λ<sub>1</sub><i>P</i> + λ<sub>2</sub><i>Q</i> + λ<sub>3</sub><i>R</i> -+ ..., where <i>P</i>, <i>Q</i>, <i>R</i>, ... are the numbers of particles -of the successive products <i>A</i>, <i>B</i>, <i>C</i>, .... Substituting the values of -<i>P</i>, <i>Q</i>, <i>R</i> already found from any one of the four cases previously -considered, the variation of the number of α particles expelled per -second with the time can be determined.</p> - -<p class='c006'>The ideal method of measuring the activity of any mixture -of radio-active products would be to determine the number of α -<span class='pageno' id='Page_338'>338</span>or β particles expelled from it per second. In practice, however, -this is inconvenient and also very difficult experimentally.</p> - -<p class='c006'>Certain practical difficulties arise in endeavouring to compare -the activity of one product with another. We shall see later that, -in many cases, all of the successive products do not emit α rays. -Some give out β and γ rays alone, while there are several “rayless” -products, that is, products which do not emit either α, β, or γ rays. -In the case of radium, for example, radium <i>A</i> gives out only α rays, -radium <i>B</i> no rays at all, while radium <i>C</i> gives out α, β, and γ rays.</p> - -<p class='c006'>In practice, the relative activity of any individual product at -any time is usually determined by relative measurements of the -saturation ionization current produced between the electrodes of a -suitable testing vessel.</p> - -<p class='c006'>Let us consider, for example, the case of a product which gives -out only α rays. The passage of the α particles through the gas -produces a large number of ions in its path. Since the α particles -from any individual product are projected with the same average -velocity under all conditions, the relative amount of the ionization -produced per second in the testing vessel serves as an accurate -means of determining the variation of its activity. No two -products, however, emit α particles with the same average velocity. -We have seen that the rays from some products are more readily -stopped in the gas than others. Thus the relative saturation -current, due to two different products in a testing vessel, does not -serve as an accurate method of comparing the relative number -of α particles expelled per second. The ratio of the currents will -in general depend upon the distance between the plates of the -testing vessel, and, unless the relative ionization due to the -average α particle from the two products is known from other data, -the comparison of the currents can, at best, be only an approximate -guide to the relative number of α particles escaping into the gas.</p> -<p class='c005'><b>202.</b> Some examples will now be considered to show how the -factors, above considered, influence the character of the curves -of activity obtained under different experimental conditions. For -the purpose of illustration, we shall consider the variation after -removal of the excited activity on a body exposed for different -times to a constant supply of the radium emanation. The active -<span class='pageno' id='Page_339'>339</span>deposit on removal consists in general of a mixture of the products -radium <i>A</i>, <i>B</i>, and <i>C</i>. The nature of the rays from each product, -the time for each product to be transformed, and the value of λ -are tabulated below for convenience:—</p> - -<table class='table8' > -<colgroup> -<col class='colwidth25'> -<col class='colwidth25'> -<col class='colwidth25'> -<col class='colwidth25'> -</colgroup> - <tr> - <th class='c013'>Product</th> - <th class='c013'>Rays</th> - <th class='c013'>T.</th> - <th class='c014'>λ (sec<sup>-1</sup>)</th> - </tr> - <tr> - <td class='c013'>Radium <i>A</i></td> - <td class='c013'>α rays</td> - <td class='c013'>3 min.</td> - <td class='c014'>3·85 × 10<sup>-3</sup></td> - </tr> - <tr> - <td class='c013'>Radium <i>B</i></td> - <td class='c013'>no rays</td> - <td class='c013'>21 min.</td> - <td class='c014'>5·38 × 10<sup>-4</sup></td> - </tr> - <tr> - <td class='c013'>Radium <i>C</i></td> - <td class='c013'>α, β, γ rays</td> - <td class='c013'>28 min.</td> - <td class='c014'>4·13 × 10<sup>-4</sup></td> - </tr> -</table> - -<p class='c006'>Since only the product <i>C</i> gives rise to β and γ rays, the -activity measured by either of these types of rays will be proportional -to the amount of <i>C</i> present at any time, <i>i.e.</i> to the value -of <i>R</i> at any time. For a long exposure, the variation of activity -with time measured by the β and γ rays will thus be represented -by the upper curve <i>CC</i> of <a href='#fig073'>Fig. 73</a>, where the ordinates represent -activity. This curve will be seen to be very similar in shape to -the experimental curve for a long exposure which is given in -<a href='#fig068'>Fig. 68</a>.</p> - -<p class='c006'>Since radium <i>B</i> does not give out rays, the number of -α particles expelled from the active deposit per second is proportional -to λ<sub>1</sub><i>P</i> + λ<sub>3</sub><i>R</i>. -The activity measured by the α rays, -using the electrical method, is thus proportional at any time to -λ<sub>1</sub><i>P</i> + <i>K</i>λ<sub>3</sub><i>R</i>, -where <i>K</i> is a constant which represents the ratio of -the number of ions, produced in the testing vessel, by an α particle -from <i>C</i> compared with that from an α particle emitted by <i>A</i>.</p> - -<p class='c006'>It will be seen later that, for this particular case, <i>K</i> is nearly -unity. Taking <i>K</i> = 1, the activity at any time after removal is -proportional to λ<sub>1</sub><i>P</i> + λ<sub>3</sub><i>R</i>.</p> - -<p class='c006'><span class='sc'>Case 1.</span> We shall first consider the activity curve for a short -exposure to the radium emanation. The relative values of <i>P</i>, <i>Q</i>, -and <i>R</i> at any time corresponding to this case are graphically -shown in <a href='#fig074'>Fig. 74</a>. The activity measured by the α rays at any -time will be the sum of the activities due to <i>A</i> and <i>C</i> separately.</p> - -<p class='c006'>Let curve <i>AA</i> (<a href='#fig074'>Fig. 74</a>) represent the activity due to <i>A</i>. This -decreases exponentially, falling to half value in 3 minutes. In -order to show the small activity due to <i>C</i> clearly in the Figure, -the activity due to <i>A</i> is plotted after an interval of 6 minutes, -when the activity has been reduced to 25 per cent. of its maximum -<span class='pageno' id='Page_340'>340</span>value. The activity due to <i>C</i> is proportional to λ<sub>3</sub><i>R</i>, and in order -to represent the activity due to <i>C</i> to the same scale as <i>A</i>, it is -necessary to reduce the scale of the ordinates of curve <i>CC</i> in -<a href='#fig072'>Fig. 72</a> in the ratio -λ<sub>3</sub>/λ<sub>1</sub>.</p> - -<div id='fig074' class='figcenter id004'> -<img src='images/fig-074.png' alt='Fig. 74.' class='ig001'> -<div class='ic002'> -<p>Fig. 74.</p> -</div> -</div> - -<p class='c006'>The activity due to <i>C</i> is thus represented by the curve <i>CCC</i>, -<a href='#fig074'>Fig. 74</a>. The total activity is thus represented by a curve <i>A</i> + <i>C</i> -whose ordinates are the sum of the ordinates of <i>A</i> and <i>C</i>.</p> - -<p class='c006'>This theoretical activity curve is seen to be very similar in -its general features to the experimental curve shown in <a href='#fig066'>Fig. 66</a>, -where the activity from a very short exposure is measured by the -α rays.</p> - -<p class='c006'><span class='sc'>Case 2.</span> The activity curve for a long exposure to the emanation -will now be considered. The activity after removal of <i>A</i> and -<i>C</i> is proportional to -λ<sub>1</sub><i>P</i> + λ<sub>3</sub><i>R</i>, -where the values of <i>P</i> and <i>R</i> are -graphically shown in <a href='#fig075'>Fig. 75</a> by the curves <i>AA</i>, <i>CC</i>. Initially after -removal, -λ<sub>1</sub><i>P</i>₀ = λ<sub>3</sub><i>R</i>₀, -since <i>A</i> and <i>C</i> are in radio-active equilibrium, -and the same number of particles of each product break -up per second. The activity due to <i>A</i> alone is shown in curve -<i>AA</i>, <a href='#fig075'>Fig. 75</a>. The activity decreases exponentially, falling to half -value in 3 minutes. The activity due to <i>C</i> at any time is proportional -<span class='pageno' id='Page_341'>341</span>to <i>R</i>, and is initially equal to that of <i>A</i>. The activity -curve due to <i>C</i> is thus represented by the curve <i>CC</i>, which is the -same curve as the upper curve <i>CC</i> of <a href='#fig073'>Fig. 73</a>. The activity of -<i>A</i> and <i>C</i> together is represented by the upper curve <i>A</i> + <i>C</i> (<a href='#fig075'>Fig. 75</a>), -where the ordinates are equal to the sum of the ordinates of the -curves <i>A</i> and <i>C</i>. This theoretical curve is seen to be very similar -in shape to the experimental curve (<a href='#fig067'>Fig. 67</a>) showing the decay -of activity of the active deposit from a long exposure measured by -the α rays.</p> - -<div id='fig075' class='figcenter id004'> -<img src='images/fig-075.png' alt='Fig. 75.' class='ig001'> -<div class='ic002'> -<p>Fig. 75.</p> -</div> -</div> -<p class='c005'><a id='section203'></a> -<b>203. Effect of a rayless change on the activity curves.</b> -Certain important cases occur in the analysis of radio-active -changes, when one of the products does not give rise to rays and -so cannot be detected directly. The presence of this rayless -change can, however, be readily observed by the variations which -occur in the activity of the succeeding product.</p> - -<p class='c006'>Let us consider, for example, the case where the inactive -matter <i>A</i>, initially all of one kind, changes into the matter <i>B</i> -which gives out rays. The inactive matter <i>A</i> is supposed to be -transformed according to the same law as the radio-active products. -Let λ<sub>1</sub>, λ<sub>2</sub> -be the constants of the change of <i>A</i> and <i>B</i> respectively. -If <i>n</i> is the number of particles of <i>A</i>, initially present, we see from -<span class='pageno' id='Page_342'>342</span>the equation (4), <a href='#section197'>section 197</a>, that the number of particles of the -matter <i>B</i> present at any time is given by</p> - -<div class='figcenter id002'> -<img src='images/form-099.png' alt='Formula.' class='ig001'> -</div> - -<p class='c006'>Differentiating and equating to zero, it is seen that the value -of <i>Q</i> passes through a maximum at a time <i>T</i> given by the equation</p> - -<div class='figcenter id005'> -<img src='images/form-100.png' alt='Formula.' class='ig001'> -</div> - -<p class='c006'>For the sake of illustration, we shall consider the variation of -the activity of the active deposit of thorium, due to a very short -exposure to the emanation. Thorium <i>A</i> gives out no rays, and -thorium <i>B</i> gives out α, β, and γ rays, while thorium <i>C</i> is inactive.</p> - -<p class='c006'>The matter <i>A</i> is half transformed in 11 hours, and <i>B</i> is half -transformed in 55 minutes. The value of -λ<sub>1</sub> = 1·75 x 10<sup>-5</sup>(sec.)<sup>-1</sup> -and λ<sub>2</sub> = 2·08 x 10<sup>-4</sup>(sec.)<sup>-1</sup>.</p> - -<p class='c006'>The activity of the mixture of products <i>A</i> + <i>B</i> is due to <i>B</i> -alone, and will, in consequence, be always proportional to the -amount of <i>B</i> present, that is, to the value of <i>Q</i>.</p> - -<div id='fig076' class='figcenter id004'> -<img src='images/fig-076.png' alt='Fig. 76.' class='ig001'> -<div class='ic002'> -<p>Fig. 76.</p> -</div> -</div> - -<p class='c006'>The variation of activity with time is shown graphically in -<a href='#fig076'>Fig. 76</a>. The activity rises from zero to a maximum in 220 -minutes and then decays, finally decreasing, according to an -exponential law, with the time, falling to half value in 11 hours.</p> - -<p class='c006'><span class='pageno' id='Page_343'>343</span>This theoretical curve is seen to agree closely in shape with -the experimental curve (<a href='#fig065'>Fig. 65</a>), which shows the variation of the -activity of the active deposit of thorium, produced by a short -exposure in presence of the emanation.</p> - -<p class='c006'>There are several points of interest in connection with an -activity curve of this character. The activity, some hours after -removal, decays according to an exponential law, not at the rate -of the product <i>B</i>, from which the activity rises, but at the same -rate as the first rayless transformation. This will also be the case -if the rayless product has a slower rate of change than the -succeeding active product. Given an activity curve of the -character of <a href='#fig076'>Fig. 76</a>, we can deduce from it that the first change -is not accompanied by rays and also the period of the two changes -in question. We are, however, unable to determine from the curve -which of the periods of change refers to the rayless product. It -is seen that the activity curve is unaltered if the values of λ<sub>1</sub>, λ<sub>2</sub>, -that is, if the periods of the products are interchanged, for the -equation is symmetrical in λ<sub>1</sub>, λ<sub>2</sub>. For example, in the case of -the active deposit of thorium, without further data it is impossible -to decide whether the period of the first change has a value of -55 minutes or 11 hours. In such cases the question can only be -settled by using some physical or chemical means in order to -separate the product <i>A</i> from <i>B</i>, and then testing the rate of decay -of their activity separately. In practice, this can often be effected -by electrolysis or by utilizing the difference in volatility of the -two products. If now a product is separated from the mixture of -<i>A</i> and <i>B</i> which loses its activity according to an exponential law, -falling to half value in 55 minutes (and such is experimentally -observed), we can at once conclude that the active product <i>B</i> has -the period of 55 minutes.</p> - -<p class='c006'>The characteristic features of the activity curve shown in -<a href='#fig076'>Fig. 76</a> becomes less marked with increase of the time of exposure -of a body to the emanation, that is, when more and more of <i>B</i> is -mixed with <i>A</i> at the time of removal. For a long time of -exposure, when the products <i>A</i> and <i>B</i> are in radio-active equilibrium, -the activity after removal is proportional to <i>Q</i>, where</p> - -<div class='figcenter id002'> -<img src='images/form-101.png' alt='Formula.' class='ig001'> -</div> - -<p class='c006'><span class='pageno' id='Page_344'>344</span>(see equation 8, <a href='#section198'>section 198</a>). The value of <i>Q</i>, in this case, does -not increase after removal, but at once commences to diminish. -The activity, in consequence, decreases from the moment of -removal, but more slowly than would be given by an exponential -law. The activity finally decays exponentially, as in the previous -case, falling to half value in 11 hours.</p> - -<p class='c006'>In the previous case we have discussed the activity curve -obtained when both the active and inactive product have comparatively -rapid rates of transformation. In certain cases which arise -in the analysis of the changes in actinium and radium, the rayless -product has a rate of change extremely slow compared with -that of the active product. This corresponds to the case where -the active matter <i>B</i> is supplied from <i>A</i> at a constant rate. The -activity curve will thus be identical in form with the recovery -curves of Th X and Ur X, that is, the activity <i>I</i> at any time <i>t</i> -will be represented by the equation</p> - -<div class='figcenter id009'> -<img src='images/form-102.png' alt='Formula.' class='ig001'> -</div> - -<p class='c006'>where <i>I</i>₀ is the maximum value of the activity and λ<sub>2</sub> the constant of change -of <i>B</i>.</p> -<p class='c005'><b>204.</b> In this chapter we have considered the variation with -time, under different conditions, of the number of atoms of the -successive products, when the period and number of the changes -are given. It has been seen that the activity curves to be expected -under various conditions can be readily deduced from the simple -theory. In practice, however, the investigator has been faced -with the much more difficult inverse problem of deducing the -period, number, and character of the products, by analysis of the -activity curves obtained under various conditions.</p> - -<p class='c006'>In the case of radium, where at least seven distinct changes -occur, the problem has been one of considerable difficulty, and a -solution has only been possible by devising special physical and -chemical methods of isolation of some of the products.</p> - -<p class='c006'>We shall see later that two rayless changes occur in radium -and actinium and one in thorium. It is at first sight a very -striking fact that the presence of a substance which does not emit -rays can be detected, and its properties investigated. This is only -possible when the rayless product is transformed into another -<span class='pageno' id='Page_345'>345</span>substance which emits rays; for the variation of the activity of the -latter may be such as to determine not only the period but also -the physical and chemical properties of the parent product. In -the two following chapters the application of the theory of -successive changes will be shown to account satisfactorily for the -complicated processes occurring in the radio-elements.</p> - -<div class='chapter'> - <span class='pageno' id='Page_346'>346</span> - <h2 id='chap10' class='c004'>CHAPTER X. <br> TRANSFORMATION PRODUCTS OF URANIUM, THORIUM, AND ACTINIUM.</h2> -</div> -<p class='c005'><a id='section205'></a> -<b>205.</b> In the last chapter the mathematical theory of successive -changes has been considered. The results there obtained will now -be applied to explain the radio-active phenomena observed with -uranium, thorium, actinium, radium, and their products.</p> -<h3 class='c020'>Transformation products of Uranium.</h3> -<p class='c005'>It has been shown in sections <a href='#section127'>127</a> and <a href='#section129'>129</a> that a radio-active -constituent Ur X can be separated from uranium by several -different processes. The activity of the separated Ur X decays -with the time, falling to half value in about 22 days. At the same -time the uranium, from which the Ur X has been separated, -gradually regains its lost activity. The laws of decay of Ur X and -of the recovery of the lost activity of the uranium are expressed -by the equations</p> - -<div class='figcenter id010'> -<img src='images/form-073.png' alt='Formula.' class='ig001'> -</div> - -<p class='c006'>and</p> - -<div class='figcenter id010'> -<img src='images/form-055.png' alt='Formula.' class='ig001'> -</div> - -<p class='c006'>where λ is the radio-active constant of Ur X. The substance Ur X -is produced from uranium at a constant rate, and the constant -radio-activity observed in uranium represents a state of equilibrium, -where the rate of production of new active matter is balanced by -the rate of change of the Ur X already produced.</p> - -<p class='c006'>The radio-active processes occurring in uranium present several -points of difference from the processes occurring in thorium and -radium. In the first place, uranium does not give off an emanation, -and in consequence does not produce any excited activity on bodies. -So far only one active product Ur X has been observed in uranium. -This active product Ur X differs from Th X and the emanations, -<span class='pageno' id='Page_347'>347</span>inasmuch as the radiation from it consists almost entirely of β rays. -This peculiarity of the radiations from Ur X initially led to some -confusion in the interpretation of observations on Ur X and the -uranium from which it had been separated. When examined by -the photographic method, the uranium freed from Ur X showed -no activity, while the Ur X possessed it to an intense degree. -With the electric method, on the other hand, the results obtained -were exactly the reverse. The uranium freed from Ur X -showed very little loss of activity, while the activity of the Ur X -was very small. The explanation of these results was given by -Soddy<a id='r296' href='#f296' class='c012'><sup>[296]</sup></a> and by Rutherford and Grier<a id='r297' href='#f297' class='c012'><sup>[297]</sup></a>. The α rays of uranium are -photographically almost inactive, but produce most of the ionization -in the gas. The β rays, on the other hand, produce a strong -photographic action, but very little ionization compared with the α -rays. When the Ur X is separated from the uranium, the uranium -does not at first give out any β rays. In the course of time fresh -Ur X is produced from the uranium, and β rays begin to appear, -gradually increasing in intensity until they reach the original value -shown before the separation of the Ur X.</p> - -<p class='c006'>In order to determine the recovery curves of uranium after the -separation of Ur X, it was thus necessary to measure the rate of -increase of the β rays. This was done by covering the uranium -with a layer of aluminium of sufficient thickness to absorb all the -α rays, and then measuring the ionization due to the rays in an -apparatus similar to <a href='#fig017'>Fig. 17</a>.</p> - -<p class='c006'>Uranium has not yet been obtained inactive when tested by -the electric method. Becquerel<a id='r298' href='#f298' class='c012'><sup>[298]</sup></a> has stated that he was able to -obtain inactive uranium, but in his experiments the uranium was -covered with a layer of black paper, which would entirely absorb -the α rays. There is no evidence that the α radiation of uranium -has been altered either in character or amount by any chemical -treatment. The α rays appear to be inseparable from the uranium, -and it will be shown later that thorium and radium as well as -uranium also possess a non-separable activity consisting entirely -of α rays. The changes occurring in uranium must then be -<span class='pageno' id='Page_348'>348</span>considered to be of two kinds, (1) the change which gives rise to -the α rays and the product Ur X, (2) the change which gives rise -to the β rays from Ur X.</p> - -<p class='c006'>The possibility of separating the Ur X, which gives rise to the -β rays of uranium, shows that the α and β rays are produced quite -independently of one another, and by matter of different chemical -properties.</p> - -<p class='c006'>Following the general considerations discussed in section 136 -we may suppose that every second some of the atoms of uranium—a -very minute fraction of the total number present will suffice—become -unstable and break up, expelling an α particle with great -velocity. The uranium atom, minus one α particle, becomes the -atom of the new substance, Ur X. This in turn is unstable and -breaks up with the expulsion of the β particle and the appearance -of a γ ray.</p> - -<p class='c006'>The changes occurring in uranium are graphically shown in -<a href='#fig077'>Fig. 77</a>.</p> - -<div id='fig077' class='figcenter id004'> -<img src='images/fig-077.png' alt='Fig. 77.' class='ig001'> -<div class='ic002'> -<p>Fig. 77.</p> -</div> -</div> - -<p class='c006'>On this view the α ray activity of uranium should be an -inherent property of the uranium, and should be non-separable -from it by physical or chemical means. The β and γ ray -activity of uranium is a property of Ur X, which differs in chemical -properties from the parent substance and can at any time be completely -removed from it. The final product, after the decay of -Ur X, is so slightly active that its activity has not yet been -observed. We shall see later (<a href='#chap13'>chapter <span class='fss'>XIII.</span></a>) that there is some -reason to believe that the changes in uranium do not end at this -point but continue through one or more stages, finally giving rise -to radium, or in other words that radium is a product of the disintegration -of the uranium atom. -Meyer and Schweidler<a id='r299' href='#f299' class='c012'><sup>[299]</sup></a>, in a recent paper, state that the -activity due to uranium preparations increases somewhat in a -<span class='pageno' id='Page_349'>349</span>closed vessel. On removing the uranium no residual activity, -however, was observed. They consider that this effect may be due -to a very short-lived emanation emitted by uranium.</p> -<p class='c005'><b>206. Effect of crystallization on the activity of uranium.</b> -Meyer and Schweidler<a id='r300' href='#f300' class='c012'><sup>[300]</sup></a> recently observed that uranium nitrate, -after certain methods of treatment, showed remarkable variations -of its activity, measured by the β rays. The α ray activity, on the -other hand, was unaltered. Some uranium nitrate was dissolved -in water and then shaken up with ether, and the ether fraction -drawn off. The early experiments of Crookes showed that, by this -method, the uranium in the ether portion was photographically inactive. -This is simply explained by supposing that the uranium X is -insoluble in ether, and consequently remained behind in the water -fraction. The ether fraction gradually regained its β ray activity -at the normal rate to be expected if Ur X was produced by the -uranium at a constant rate, for it recovered half its final activity in -about 22 days. Some of the uranium in the water fraction was -crystallized and placed under an electroscope. The β ray activity -fell rapidly at first to half its value in the course of four days. The -activity then remained constant, and no further change was -observed over an interval of one month. Other experiments were -made with crystals of uranium nitrate, which had not been treated -with ether. The nitrate was dissolved in water and a layer of -crystals separated. The β ray activity of these crystals fell rapidly -at first, the rate varying somewhat in different experiments, but -reached a minimum value after about five days. The β ray -activity then rose again at a slow rate for several months.</p> - -<p class='c006'>The rapid drop of activity of the crystals seemed, at first sight, -to indicate that crystallization was able in some way to alter the -activity of uranium.</p> - -<p class='c006'>Dr Godlewski, working in the laboratory of the writer, repeated -the work of Meyer and Schweidler, and obtained results of a -similar character, but the initial drop of activity was found to vary -both in rate and amount in different experiments. These results -were at first very puzzling and difficult to explain, for the mother -liquor, left behind after removal of the crystals, did not show the -<span class='pageno' id='Page_350'>350</span>corresponding initial rise, which would be expected if the variation -of activity were due to the partial separation of some new product -of uranium.</p> - -<p class='c006'>The cause of this effect was, however, rendered very evident -by a few well-considered experiments made by Godlewski. The -uranium nitrate was dissolved in hot water in a flat dish, and -allowed to crystallize under the electroscope. Up to the moment -of crystallization the β ray activity remained constant, but as soon -as the crystals commenced to form at the bottom of the solution the -β ray activity rapidly rose in the course of a few minutes to five -times the initial value. After reaching a maximum, the activity -very gradually decreased again to the normal value. If, however, -the plate of crystals was reversed, the β ray activity was found at -first to be much smaller than the normal, but increased as fast as -that of the other side diminished.</p> - -<p class='c006'>The explanation of this effect is simple. Ur X is very soluble -in water and, at first, does not crystallize with the uranium, but -remains in the solution, and, consequently, when the crystallization -commences at the bottom of the vessel the upper layer of liquid -becomes richer in uranium X. Since the β rays arise only from -the product Ur X and not from the uranium itself, and the Ur X is -mostly confined to the upper layer, a much greater proportion of -the β rays escape than if the Ur X were uniformly distributed -throughout the thick layer of uranium. When the amount of -water added is just sufficient to supply the water of crystallization, -the Ur X in the upper layer of crystals gradually diffuses back -through the mass and, in consequence, the activity of the upper -surface diminishes and of the lower surface rises. A similar explanation -applies to the effects observed by Meyer and Schweidler. -The water fraction, left behind after treatment with ether, contained -all the Ur X. The first layer of crystals formed in it contained -some Ur X, and this was for the most part confined to the top -layer of crystals. The amount of β rays at first diminished owing -to the gradual diffusion of the Ur X from the surface. In the first -experiment, the amount of Ur X present was in radio-active -equilibrium with the uranium, and, after the initial drop, the β ray -activity remained constant. In the second experiment, the gradual -rise is due to the fact that the crystals of uranium first formed -<span class='pageno' id='Page_351'>351</span>contained less than the equilibrium amount of Ur X. After falling -to a minimum, the β ray activity, in consequence, slowly rose again -to the equilibrium value.</p> - -<p class='c006'>These effects exhibited by uranium are of great interest, and -illustrate in a striking manner the difference in properties of Ur X -and the uranium. The gradual diffusion of the Ur X throughout -the mass of crystals is noteworthy. By measurements of the -variation with time of the β ray activity, it should be possible to -deduce its rate of diffusion into the crystallized mass.</p> -<h3 class='c020'>Transformation products of Thorium.</h3> -<p class='c005'><a id='section207'></a> -<b>207. Analysis of the active deposit.</b> The radio-active -processes occurring in thorium are far more complicated than those -in uranium. It has already been shown in chapter vi that a radio-active -product Th X is continuously produced from the thorium. -This Th X breaks up, giving rise to the radio-active emanation. -The emanation produces from itself a type of active matter which -is deposited on the surface of bodies, where it gives rise to the -phenomena of excited or induced activity. This active deposit -possesses some distinctive chemical and physical properties which -distinguish it from the emanation and the Th X. We have seen -(<a href='#section180'>section 180</a>) that the rate at which the active deposit loses its -activity depends upon the time of exposure of the body made active -to the emanation. The explanation of the activity curves for -different time of exposure will now be considered.</p> - -<p class='c006'>The curve of variation of activity for a short exposure of 10 -minutes has already been given in <a href='#fig065'>Fig. 65</a>. The activity is small -at first but increases rapidly with the time; it passes through a -maximum about 4 hours later, and finally decays exponentially -with the time, falling to half value in 11 hours. -This remarkable effect can be explained completely<a id='r301' href='#f301' class='c012'><sup>[301]</sup></a> if it be -supposed that the active deposit consists of two distinct substances. -The matter initially deposited from the emanation, which will be -called thorium <i>A</i>, is supposed to be changed into thorium <i>B</i>. -Thorium <i>A</i> is transformed according to the ordinary exponential -<span class='pageno' id='Page_352'>352</span>law, but the change is not accompanied by any ionizing rays. In -other words, the change from <i>A</i> to <i>B</i> is a “rayless” change. On -the other hand, <i>B</i> breaks up into <i>C</i> with the accompaniment of all -three kinds of rays. On this view the activity of the active -deposit at any time represents the amount of the substance <i>B</i> -present, since <i>C</i> is inactive or active to a very minute extent.</p> - -<p class='c006'>If the variation of the activity imparted to a body exposed for -a short interval in the presence of the thorium emanation, is due -to the fact that there are two successive changes in the deposited -matter <i>A</i>, the first of which is a “rayless” change, the activity <i>I<sub>t</sub></i> -at any time <i>t</i> after removal should be proportional to the number -<i>Q<sub>t</sub></i> of particles of the matter <i>B</i> present at that time. Now, from -equation (4) <a href='#section197'>section 197</a>, it has been shown that</p> - -<div class='figcenter id007'> -<img src='images/form-082.png' alt='Formula.' class='ig001'> -</div> - -<p class='c006'>The value of <i>Q<sub>t</sub></i> passes through a maximum <i>Q<sub>T</sub></i> at the time <i>T</i> -when</p> - -<div class='figcenter id005'> -<img src='images/form-103.png' alt='Formula.' class='ig001'> -</div> - -<p class='c006'>The maximum activity <i>I<sub>T</sub></i> is proportional to <i>Q<sub>T</sub></i> and</p> - -<div class='figcenter id005'> -<img src='images/form-104.png' alt='Formula.' class='ig001'> -</div> - -<p class='c006'>It will be shown later that the variation with time of the -activity, imparted to a body by a short exposure, is expressed by -an equation of the above form. It thus remains to fix the values -of λ<sub>1</sub>, λ<sub>2</sub>. Since the above equation is symmetrical with regard to λ<sub>1</sub>, λ<sub>2</sub>, -it is not possible to settle from the agreement of the -theoretical and experimental curve which value of λ refers to the -first change. The curve of variation of activity with time is -unaltered if the values of λ<sub>1</sub> and λ<sub>2</sub> are interchanged.</p> - -<p class='c006'>It is found experimentally that the activity 5 or 6 hours after -removal decays very approximately according to an exponential -law with the time, falling to half value in 11 hours. This is the -normal rate of decay of thorium for all times of exposure, provided -measurements are not begun until several hours after the removal -of the active body from the emanation.</p> - -<p class='c006'><span class='pageno' id='Page_353'>353</span>This fixes the value of the constants of one of the changes. -Let us assume for the moment that this gives the value of λ<sub>1</sub>.</p> - -<div class='calc'> - -<div class='lg-container-b c017'> - <div class='linegroup'> - <div class='group'> - <div class='line'>Then λ<sub>1</sub> = 1·75 × 10<sup>-5</sup> (sec)<sup>-1</sup>.</div> - </div> - </div> -</div> - -</div> - -<p class='c018'>Since the maximum activity is reached after an interval <i>T</i> = 220 -minutes (see <a href='#fig065'>Fig. 65</a>), substituting the values of -λ<sub>1</sub> and <i>T</i> in the equation, the value of λ<sub>2</sub> comes out to be</p> - -<div class='calc'> - -<div class='lg-container-b c017'> - <div class='linegroup'> - <div class='group'> - <div class='line'>λ<sub>2</sub> = 2·08 × 10<sup>-4</sup> (sec)<sup>-1</sup>.</div> - </div> - </div> -</div> - -</div> - -<p class='c018'>This value of λ<sub>2</sub> corresponds to a change in which half the -matter is transformed in 55 minutes.</p> - -<p class='c006'>Substituting now the values of λ<sub>1</sub>, λ<sub>2</sub>, <i>T</i>, the equation reduces to</p> - -<div class='figcenter id005'> -<img src='images/form-105.png' alt='Formula.' class='ig001'> -</div> - -<p class='c006'>The agreement between the results of the theoretical equation -and the observed values is shown in the following table:</p> - -<table class='table8' > -<colgroup> -<col class='colwidth25'> -<col class='colwidth37'> -<col class='colwidth37'> -</colgroup> - <tr> - <th class='c015'>Time in minutes</th> - <th class='c015'>Theoretical value of <i>I<sub>t</sub></i>/<i>I<sub>T</sub></i></th> - <th class='c016'>Observed value of <i>I<sub>t</sub></i>/<i>I<sub>T</sub></i></th> - </tr> - <tr> - <td class='c015'> </td> - <td class='c015'> </td> - <td class='c016'> </td> - </tr> - <tr> - <td class='c015'>15</td> - <td class='c015'>·22</td> - <td class='c016'>·23</td> - </tr> - <tr> - <td class='c015'>30</td> - <td class='c015'>·38</td> - <td class='c016'>·37</td> - </tr> - <tr> - <td class='c015'>60</td> - <td class='c015'>·64</td> - <td class='c016'>·63</td> - </tr> - <tr> - <td class='c015'>120</td> - <td class='c015'>·90</td> - <td class='c016'>·91</td> - </tr> - <tr> - <td class='c015'>220</td> - <td class='c015'>1·00</td> - <td class='c016'>1·00</td> - </tr> - <tr> - <td class='c015'>305</td> - <td class='c015'>·97</td> - <td class='c016'>·96</td> - </tr> -</table> - -<p class='c006'>After 5 hours the activity decreased nearly exponentially with -the time, falling to half value in 11 hours.</p> - -<p class='c006'>It is thus seen that the curve of rise of activity for a short -exposure is explained very satisfactorily on the supposition that -two changes occur in the deposited matter, of which the first is a -rayless change.</p> - -<p class='c006'>Further data are required in order to fix which of the time -constants of the changes refers to the first change. In order to -settle this point, it is necessary to isolate one of the products of the -changes and to examine the variation of its activity with time. If, -for example, a product can be separated whose activity decays to -half value in 55 minutes, it would show that the second change is -the more rapid of the two. Now Pegram<a id='r302' href='#f302' class='c012'><sup>[302]</sup></a> has examined the -radio-active products obtained by electrolysis of thorium solutions. -<span class='pageno' id='Page_354'>354</span>The rates of decay of the active products depended upon conditions, -but he found that, in several cases, rapidly decaying products were -obtained whose activity fell to half value in about 1 hour. Allowing -for the probability that the product examined was not completely -isolated by the electrolysis, but contained also a trace of -the other product, this result would indicate that the last change -which gives rise to rays is the more rapid of the two.</p> - -<p class='c006'>This point is very clearly brought out by some recent experiments -of Miss Slater<a id='r303' href='#f303' class='c012'><sup>[303]</sup></a>, who has made a detailed examination of the -effect of temperature on the active deposit of thorium.</p> - -<p class='c006'>A platinum wire was made active by exposure for a long -interval to the thorium emanation, and then heated for a few -minutes to any desired temperature by means of the electric -current. The wire, while being heated, was surrounded by a lead -cylinder in order that any matter driven off from it should be -collected on its surface. The decay of activity both of the wire -and of the lead cylinder was then tested separately. After heating -to a dull red heat, no sensible diminution of the activity was -observed at first, but the rate of decay of the activity on the wire -was found to be more rapid than the normal. The activity of the -lead cylinder was small at first but increased to a maximum after -about 4 hours and then decayed at the normal rate with the time.</p> - -<p class='c006'>These results are to be expected if some thorium A is volatilized -from the wire; for the rise of activity on the lead cylinder is -very similar to that observed on a wire exposed for a short time in -the presence of the thorium emanation, <i>i.e.</i>, under the condition -that only thorium A is initially present.</p> - -<p class='c006'>On heating the wire above 700° C. the activity was found to be -reduced, showing that some thorium B had also been removed. By -heating for a few minutes at about 1000° C. nearly all the thorium -A was driven off. The activity on the wire then decayed exponentially -with the time, falling to half value in about 1 hour. -After heating for a minute at about 1200° C. all the activity was -removed. These results show that thorium A is more volatile -than B, and that the product which gives out rays, viz. thorium B, -has a period of about 55 minutes.</p> - -<p class='c006'>Another series of experiments was made, in which an active -<span class='pageno' id='Page_355'>355</span>aluminium disc was placed in an exhausted tube, and exposed to -the cathode ray discharge. Under these conditions, a part of the -activity of the disc was removed. When the disc was made the -anode, the loss of activity was usually 20 to 60 per cent. for half-an-hour’s -exposure. If the disc was made the cathode, the loss -was much greater, amounting to about 90 per cent. in 10 minutes. -Part of the active matter removed from the disc was collected on -a second disc placed near it. This second disc on removal lost its -activity at a far more rapid rate than the normal. The rate of -decay on the first disc was also altered, the activity sometimes -even increasing after removal. These results indicate that, in this -case, the apparent volatility of the products is reversed. Thorium B -is driven off from the disc more readily than thorium A. The -rates of decay obtained under different conditions were satisfactorily -explained by supposing that the surfaces of the discs after -exposure to the discharge were coated with different proportions of -thorium A and B.</p> - -<p class='c006'>The escape of thorium B from the disc under the influence of -the discharge seems rather to be the result of an action similar -to the well-known “sputtering” of electrodes than to a direct -influence of temperature.</p> - -<p class='c006'>The results obtained by von Lerch<a id='r304' href='#f304' class='c012'><sup>[304]</sup></a> on the electrolysis of a -solution of the active deposit also admit of a similar interpretation. -Products were obtained on the electrodes of different rates of -decay, losing half their activity in times varying from about -1 hour to 5 hours. This variation is due to the admixture -of the two products in different proportions. The evidence, as a -whole, thus strongly supports the conclusion that the active deposit -from thorium undergoes two successive transformations as follows:</p> - -<p class='c006'>(1) A “rayless” change for which λ<sub>1</sub> = 1·75 × 10<sup>-5</sup>, <i>i.e.</i>, in -which half the matter is transformed in 11 hours;</p> - -<p class='c006'>(2) A second change giving rise to α, β and γ rays, for which -λ<sub>2</sub> = 2·08 × 10<sup>-4</sup>, <i>i.e.</i>, in which half the matter is transformed in 55 -minutes<a id='r305' href='#f305' class='c012'><sup>[305]</sup></a>.</p> - -<p class='c006'><span class='pageno' id='Page_356'>356</span>It is, at first sight, a somewhat unexpected result that the final -rate of decay of the active deposit from thorium gives the rate of -change not of the last product itself, but of the preceding product, -which does not give rise to rays at all.</p> - -<p class='c006'>A similar peculiarity is observed in the decay of the excited -activity of actinium, which is discussed in <a href='#section212'>section 212</a>.</p> - -<p class='c006'>For a long exposure in the presence of a constant supply of -thorium emanation, the equation expressing the variation of -activity with time is found from equation (8), <a href='#section198'>section 198</a>,</p> - -<div class='figcenter id002'> -<img src='images/form-106.png' alt='Formula.' class='ig001'> -</div> - -<p class='c006'>About 5 hours after removal the second term in the brackets -becomes very small, and the activity after that time will decay -nearly according to an exponential law with the time, falling to -half value in 11 hours. For any time of exposure <i>T</i>, the activity -at time <i>t</i> after the removal (see equation 11, <a href='#section199'>section 199</a>) is -given by</p> - -<div class='figcenter id002'> -<img src='images/form-107.png' alt='Formula.' class='ig001'> -</div> - -<p class='c006'>where <i>I</i>₀ is the initial value of the activity, immediately after -removal, and</p> - -<div class='figcenter id007'> -<img src='images/form-108.png' alt='Formula.' class='ig001'> -</div> - -<p class='c006'>By variation of <i>T</i> the curves of variation of activity for any time -of exposure can be accurately deduced from the equation, when the -values of the two constants -λ<sub>1</sub>, λ<sub>2</sub> -are substituted. Miss Brooks<a id='r306' href='#f306' class='c012'><sup>[306]</sup></a> -has examined the decay curves of excited activity for thorium for -different times of exposure and has observed a substantial agreement -between experiment and theory.</p> - -<div id='fig078' class='figcenter id004'> -<img src='images/fig-078.png' alt='Fig. 78.' class='ig001'> -<div class='ic002'> -<p>Fig. 78.</p> -</div> -</div> - -<p class='c006'>The results are shown graphically in <a href='#fig078'>Fig. 78</a>. The maximum -<span class='pageno' id='Page_357'>357</span>value of the activity is, for each time of exposure, taken as 100. -The theoretical and observed values are shown in the Figure.</p> -<p class='c005'><span class='pageno' id='Page_358'>358</span><a id='section208'></a> -<b>208. Analysis of the decay and recovery curves of -Th X.</b> The peculiarities of the initial portions of the decay and -recovery curves of Th X and thorium respectively (Curves <i>A</i> and <i>B</i>, -<a href='#fig047'>Fig. 47</a>, p. 221), will now be considered. It was shown that when -the Th X was removed from the thorium by precipitation with -ammonia, the radiation increased about 15 per cent. during the -first day, passed through a maximum, and then fell off according -to an exponential law, decreasing to half value in four days. At -the same time the activity of the separated hydroxide decreased for -the first day, passed through a minimum, and then slowly increased -again, rising to its original value after the lapse of about one month.</p> - -<p class='c006'>When a thorium compound is in a state of radio-active equilibrium, -the series of changes in which Th X, the emanation, and -thorium A and B are produced, go on simultaneously. Since a -state of equilibrium has been reached for each of these products, -the amount of each product changing in unit time is equal to the -amount of that product supplied from the preceding change in -unit time. Now the matter Th X is soluble in ammonia, while -thorium A and B are not. The Th X is thus removed from the -thorium by precipitation with ammonia, but A and B are left -behind with the thorium. Since the active deposit is produced -from the emanation, which in turn arises from Th X, on the -removal of the parent matter Th X, the radiation due to this -active deposit will decay, since the rate of production of fresh -matter no longer balances its own rate of change. Disregarding -the initial irregularity in the decay curve of the active deposit, -its activity will have decayed to half value in about 11 hours, and -to one quarter value at the end of 22 hours. As soon, however, -as the Th X has been separated, new Th X is produced in the -thorium compound. The activity of this new Th X is not, however, -sufficient to compensate at first for the loss of activity due -to the change in the active deposit, so that, as a whole, the -activity will at first <i>decrease</i>, then pass through a minimum, then -increase again.</p> - -<p class='c006'>The correctness of this point of view has been tested by Rutherford -and Soddy<a id='r307' href='#f307' class='c012'><sup>[307]</sup></a> as follows: If the precipitated thorium hydroxide -<span class='pageno' id='Page_359'>359</span>after the removal of Th X is put through a series of precipitations -with ammonia at short intervals, the Th X is removed almost as -fast as it is formed, and, at the same time, the activity of thorium B -in the thorium decays.</p> - -<p class='c006'>The following table indicates the results obtained. A portion -of the precipitated hydroxide was removed after each series of -precipitations and its activity tested in the usual way.</p> - -<table class='table8' > -<colgroup> -<col class='colwidth75'> -<col class='colwidth25'> -</colgroup> - <tr> - <th class='c013'></th> - <th class='c016'>Activity of hydroxide per cent.</th> - </tr> - <tr> - <td class='c013'>After 1 precipitation</td> - <td class='c016'>46</td> - </tr> - <tr> - <td class='c013'>After 3 precipitations at intervals of 24 hours</td> - <td class='c016'>39</td> - </tr> - <tr> - <td class='c013'>After 3 more precipitations at intervals of 24 hours and 3 at intervals of 8 hours</td> - <td class='c016'>22</td> - </tr> - <tr> - <td class='c013'>After 3 more each of 8 hours</td> - <td class='c016'>24</td> - </tr> - <tr> - <td class='c013'>After 6 more each of 4 hours</td> - <td class='c016'>25</td> - </tr> -</table> - -<div id='fig079' class='figcenter id004'> -<img src='images/fig-079.png' alt='Fig. 79.' class='ig001'> -<div class='ic002'> -<p>Fig. 79.</p> -</div> -</div> - -<p class='c006'>The differences in the last three numbers are not significant, -for it is difficult to make accurate comparisons of the activity of -thorium compounds which have been precipitated under slightly -different conditions. It is thus seen that as a result of successive -precipitations, the activity is reduced to a minimum of about 25 per -cent. The recovery curve of the activity of this 23 times precipitated -<span class='pageno' id='Page_360'>360</span>hydroxide is shown in <a href='#fig079'>Fig. 79</a>. The initial drop in the curve is -quite absent, and the curve, starting from the minimum, is practically -identical with the curve shown in <a href='#fig048'>Fig. 48</a>, which gives the -recovery curve of thorium hydroxide after the first two days. This -residual activity—about 25 per cent. of the maximum—is non-separable -from the thorium by any chemical process that has been -tried.</p> - -<p class='c006'>The initial rise of activity of Th X, after it has been separated, -will now be considered. In all cases it was found that the activity -of the separated Th X had increased about 15 per cent. at the -end of 24 hours, and then steadily decayed, falling to half value in -about four days.</p> - -<p class='c006'>This peculiarity of the Th X curve follows, of necessity, from the -considerations already advanced to explain the drop in the recovery -curve. As soon as the Th X is separated, it at once produces from -itself the emanation, and this in turn produces thorium A and B. -The activity due to B at first more than compensates for the decay -of activity of the Th X itself. The total activity thus increases to -a maximum, and then slowly decays to zero according to an -exponential law with the time. The curve expressing the variation -of the activity of the separated Th X with time can be deduced -from the theory of successive changes already considered in -<a href='#chap09'>chapter <span class='fss'>IX</span></a>. In the present case there are four successive changes -occurring at the same time, viz. the change of Th X into the -emanation, of the emanation into thorium A, of A into B, and of -B into an inactive product. Since, however, the change of the -emanation into thorium A (about half changed in one minute) is -far more rapid than the changes occurring in Th X or thorium A -and B, for the purposes of calculation it may be assumed without -serious error that the Th X changes at once into the active deposit. -The 55 minute change will also be disregarded for the same -reason.</p> - -<p class='c006'>Let λ<sub>1</sub> and λ<sub>2</sub> be the constants of decay of activity of Th X -and of thorium A respectively. Since the activity of Th X and of -thorium A falls to half value in 4 days and 11 hours respectively, -the value of λ<sub>1</sub> = ·0072 and of λ<sub>2</sub> = ·063, where 1 hour is taken as -the unit of time.</p> - -<p class='c006'>The problem reduces to the following: <i>Given the matter A -<span class='pageno' id='Page_361'>361</span>(thorium X) all of one kind, which changes into B (thorium B), -find the activity of A and B together at any subsequent time.</i> -This corresponds to Case I. (<a href='#section197'>section 197</a>). The amount <i>Q</i> of B at -any time <i>T</i> is given by</p> - -<div class='figcenter id002'> -<img src='images/form-109.png' alt='Formula.' class='ig001'> -</div> - -<p class='c006'>and the activity <i>I</i> at any time of the two together is proportional -to λ<sub>1</sub><i>P</i> + <i>K</i>λ<sub>2</sub><i>Q</i>, -where <i>K</i> is the ratio of the ionization of B compared -with that of A.</p> - -<p class='c006'>Then</p> - -<div class='figcenter id004'> -<img src='images/form-110.png' alt='Formula.' class='ig001'> - -</div> - -<p class='c006'>where <i>I</i>₀ is the initial activity due to <i>n</i>₀ particles of Th X.</p> - -<p class='c006'>By comparison of this equation with the curve of variation of -the activity of Th X with time, shown in <a href='#fig047'>Fig. 47</a>, it is found that -<i>K</i> is almost ·44. It must be remembered that the activity of the -emanation and Th X are included together, so that the activity -of thorium B is about half of the activity of the two preceding -products.</p> - -<p class='c006'>The calculated values of -<i>I<sub>t</sub></i>/<i>I</i>₀ -for different values of <i>t</i> are shown -in the second column of the following table, and the observed values -in the third column.</p> - -<table class='table25' > -<colgroup> -<col class='colwidth32'> -<col class='colwidth35'> -<col class='colwidth32'> -</colgroup> - <tr> - <th class='c015'>Time</th> - <th class='c013'>Theoretical value</th> - <th class='c014'>Observed value</th> - </tr> - <tr> - <td class='c015'> </td> - <td class='c013'> </td> - <td class='c014'> </td> - </tr> - <tr> - <td class='c015'>0</td> - <td class='c013'>1·00</td> - <td class='c014'>1·00</td> - </tr> - <tr> - <td class='c015'>·25 days</td> - <td class='c013'>1·09</td> - <td class='c014'>—</td> - </tr> - <tr> - <td class='c015'>·5 „</td> - <td class='c013'>1·16</td> - <td class='c014'>—</td> - </tr> - <tr> - <td class='c015'>1 „</td> - <td class='c013'>1·15</td> - <td class='c014'>1·17</td> - </tr> - <tr> - <td class='c015'>1·5 „</td> - <td class='c013'>1·11</td> - <td class='c014'>—</td> - </tr> - <tr> - <td class='c015'>2 „</td> - <td class='c013'>1·04</td> - <td class='c014'>—</td> - </tr> - <tr> - <td class='c015'>3 „</td> - <td class='c013'>·875</td> - <td class='c014'>·88</td> - </tr> - <tr> - <td class='c015'>4 „</td> - <td class='c013'>·75</td> - <td class='c014'>·72</td> - </tr> - <tr> - <td class='c015'>6 „</td> - <td class='c013'>·53</td> - <td class='c014'>·53</td> - </tr> - <tr> - <td class='c015'>9 „</td> - <td class='c013'>·315</td> - <td class='c014'>·295</td> - </tr> - <tr> - <td class='c015'>13 „</td> - <td class='c013'>·157</td> - <td class='c014'>·152</td> - </tr> -</table> - -<div id='fig080' class='figcenter id004'> -<span class='pageno' id='Page_362'>362</span> -<img src='images/fig-080.png' alt='Fig. 80.' class='ig001'> -<div class='ic002'> -<p>Fig. 80.</p> -</div> -</div> - -<p class='c006'>The theoretical and observed values thus agree within the -limit of error in the measurements. The theoretical curve is -shown in Curve <i>A</i>, <a href='#fig080'>Fig. 80</a> (with the observed points marked, for -comparison). The curve <i>B</i> shows the theoretical curve of the decay -of the activity of Th X and the emanation, supposing there is -no further change into the active deposit. Curve <i>C</i> shows the -difference curve between the curves <i>A</i> and <i>B</i>, <i>i.e.</i> the proportion -of the activity at different times due to the active deposit. -The activity due to the latter thus rises to a maximum about -two days after removal of the Th X, and then decays with the -time at the same rate as the Th X itself, <i>i.e.</i> the activity falls -to half value every four days. When <i>t</i> exceeds four days, the -term</p> - -<div class='figcenter id009'> -<img src='images/form-111.png' alt='Formula.' class='ig001'> -</div> - -<p class='c006'>in the theoretical equation is very small.</p> - -<p class='c006'><span class='pageno' id='Page_363'>363</span>The equation of decay after this time is therefore expressed by</p> - -<div class='figcenter id005'> -<img src='images/form-112.png' alt='Formula.' class='ig001'> -</div> - -<p class='c006'><i>i.e.</i> the activity decays according to an exponential law with the -time.</p> -<p class='c005'><b>209. Radiations from Thorium products.</b> It has been -shown in the last section that the activity of thorium, by successive -precipitations with ammonia, is reduced to a limiting value of -almost 25 per cent. of the initial activity. This “non-separable -activity” consists of α rays, the β and γ rays being altogether -absent. According to the disintegration theory, this is an expression -of the fact that the initial break-up of the thorium atom is -accompanied only by the expulsion of α particles. We have seen -in <a href='#section156'>section 156</a> that the thorium emanation also gives out only -α rays. In the active deposit, thorium A gives out no rays, while -thorium B emits all three types of rays.</p> - -<p class='c006'>Some hours after separation, Th X gives out α, β, and γ rays, -but the appearance of β and γ rays is probably due to the thorium -B associated with it. The β and γ ray activity of Th X is much -reduced if a current of air is continuously aspirated through a -solution of Th X to remove the emanation. It seems likely that -if the emanation could be removed as fast as it was formed, so as -to prevent the formation of thorium B in its mass, Th X itself -would give out only α rays: but, on account of the rapid rate of -change of the thorium emanation, it is difficult to realize this -experimentally.</p> -<p class='c005'><b>210. Transformation products of Thorium.</b> The transformation -products of thorium and the rays emitted by them are -graphically shown below (<a href='#fig081'>Fig. 81</a>).</p> - -<div id='fig081' class='figcenter id001'> -<img src='images/fig-081.png' alt='Fig. 81.' class='ig001'> -<div class='ic002'> -<p>Fig. 81.</p> -</div> -</div> - -<p class='c006'><span class='pageno' id='Page_364'>364</span>A table of the transformation products of thorium is shown -below, with some of their physical and chemical properties.</p> - -<table class='table1' > -<colgroup> -<col class='colwidth20'> -<col class='colwidth20'> -<col class='colwidth20'> -<col class='colwidth20'> -<col class='colwidth20'> -</colgroup> - <tr> - <th class='c013'>Product</th> - <th class='c015'>Time to be half transformed</th> - <th class='c015'>λ (sec)<sup>-1</sup></th> - <th class='c015'>Radiations</th> - <th class='c016'>Physical and chemical properties</th> - </tr> - <tr> - <td class='c013'> </td> - <td class='c015'> </td> - <td class='c015'> </td> - <td class='c015'> </td> - <td class='c016'> </td> - </tr> - <tr> - <td class='c013'>Thorium</td> - <td class='c015'> </td> - <td class='c015'> </td> - <td class='c015'>α rays</td> - <td class='c016'>Insoluble in ammonia</td> - </tr> - <tr> - <td class='c013'>Th. X</td> - <td class='c015'>4 days</td> - <td class='c015'>2·00 × 10<sup>-6</sup></td> - <td class='c015'>α rays</td> - <td class='c016'>Soluble in ammonia</td> - </tr> - <tr> - <td class='c013'>Emanation</td> - <td class='c015'>54 secs.</td> - <td class='c015'>1·28 × 10<sup>-2</sup></td> - <td class='c015'>α rays</td> - <td class='c016'>Inert gas, condenses -120° C.</td> - </tr> - <tr> - <td class='c013'>Thorium A</td> - <td class='c015'>11 hours</td> - <td class='c015'>1·75 × 10<sup>-5</sup></td> - <td class='c015'>no rays</td> - <td class='c016'>Soluble in strong acids. Volatile at a white heat. B can be separated from A by electrolysis and by difference of volatility.</td> - </tr> - <tr> - <td class='c013'>Thorium B</td> - <td class='c015'>55 mins.</td> - <td class='c015'>2·1 × 10<sup>-4</sup></td> - <td class='c015'>α, β, γ rays</td> - <td class='c016'>Same</td> - </tr> - <tr> - <td class='c013'>?</td> - <td class='c015'>—</td> - <td class='c015'>—</td> - <td class='c015'>—</td> - <td class='c016'>-</td> - </tr> -</table> -<p class='c005'><b>211. Transformation products of Actinium.</b> It has -previously been pointed out (sections <a href='#section017'>17</a> and <a href='#section018'>18</a>) that the -actinium of Debierne and the emanium of Giesel contain the same -radio-active constituent. Both give out a short-lived emanation -which imparts activity to the surface of bodies. Recently, thanks -to Dr Giesel of Braunschweig, preparations of “emanium” have -been placed on the market, and most of the investigations that are -described later have been made with this substance.</p> - -<p class='c006'><i>Actinium X.</i> Actinium and thorium are very closely allied in -radio-active properties. Both emit an emanation which is rapidly -transformed, but the rate of change of the actinium emanation is -still more rapid than that of thorium, the activity decreasing to -half value in 3·7 seconds. Miss Brooks<a id='r308' href='#f308' class='c012'><sup>[308]</sup></a> has analysed the active -deposit from the emanation of actinium, and has shown that two -successive changes occur in it, very similar in character to those -observed in the active deposit of thorium. It thus seemed -<span class='pageno' id='Page_365'>365</span>probable, from analogy, that an intermediate product, corresponding -to Th X in thorium, would be found in actinium<a id='r309' href='#f309' class='c012'><sup>[309]</sup></a>. -Recent work has verified this supposition. Giesel<a id='r310' href='#f310' class='c012'><sup>[310]</sup></a> and Godlewski<a id='r311' href='#f311' class='c012'><sup>[311]</sup></a> -independently observed that a very active substance could be -separated from “emanium,” very similar in chemical and physical -properties to Th X in thorium. This product will, from analogy, -be called “actinium X.” The same method, which was used by -Rutherford and Soddy to separate Th X from thorium, is also -effective in separating actinium X from actinium. After precipitation -of the active solution with ammonia, actinium X is left -behind in the filtrate. After evaporation and ignition, a very -active residue remains. At the same time, the precipitated -actinium loses a large proportion of its activity.</p> - -<p class='c006'>Giesel observed the separation of an active product, using a -fluorescent screen to detect the radiations. A very complete -examination of the product actinium X has been made by -Godlewski in the laboratory of the writer.</p> - -<p class='c006'>After separation of actinium X, the activity, whether measured -by the α or β rays, increases about 15 per cent. during the first -day, and afterwards decays exponentially with the time, falling to -half value in 10·2 days. The activity of the separated actinium -was small at first but steadily increased with the time, reaching a -practical maximum after an interval of sixty days. After the first -day, the decay and recovery curves of activity are complementary -to one another. The curves of rise and decay are shown graphically -in <a href='#fig082'>Fig. 82</a>, curves I and II respectively.</p> - -<p class='c006'>Godlewski observed that a solution of actinium, freed from -actinium X, gave out very little emanation, while a solution of -actinium X gave off the emanation in large quantity. The -amount of emanation from the solution was measured by observing -the activity produced in a testing vessel, similar to that shown in -Fig. 51, when a constant current of air was passed through the -solution. The emanating power of actinium X decreased exponentially -with the time at the same rate as that at which the actinium X -lost its activity. At the same time the actinium solution increased -<span class='pageno' id='Page_366'>366</span>in emanating power, reaching its original value after about 60 days. -The behaviour of actinium and thorium is thus quite analogous, -and the explanation advanced to explain the decay and recovery -curves of thorium applies equally well to the corresponding curves -of actinium.</p> - -<div id='fig082' class='figcenter id004'> -<img src='images/fig-082.png' alt='Fig. 82.' class='ig001'> -<div class='ic002'> -<p>Fig. 82.</p> -</div> -</div> - -<p class='c006'>The actinium X is produced at a constant rate from the parent -matter actinium, and is transformed according to an exponential -law with the time. The constant of change λ = ·068 (day)<sup>-1</sup>, and -this value is characteristic of the product actinium X. As in the -case of thorium, the above experiments show that the emanation -does not arise from actinium itself but from actinium X. The -emanation in turn breaks up and gives rise to an active deposit on -the surface of bodies.</p> -<p class='c005'><a id='section212'></a> -<b>212. Analysis of the active deposit from the emanation.</b> -Debierne<a id='r312' href='#f312' class='c012'><sup>[312]</sup></a> observed that the excited activity produced by actinium -decayed to half value in about 41 minutes. Miss Brooks<a id='r313' href='#f313' class='c012'><sup>[313]</sup></a> showed -<span class='pageno' id='Page_367'>367</span>that the curves of decay of the excited activity after removal -depended upon the duration of exposure to the emanation. The -curves for different times of exposure have already been shown in -<a href='#fig069'>Fig. 69</a>.</p> - -<p class='c006'>Bronson, using the direct deflection method described in -<a href='#section069'>section 69</a>, accurately determined the activity curve corresponding -to a short exposure to the actinium emanation. The curve -obtained is shown in <a href='#fig083'>Fig. 83</a>.</p> - -<div id='fig083' class='figcenter id004'> -<img src='images/fig-083.png' alt='Fig. 83.' class='ig001'> -<div class='ic002'> -<p>Fig. 83.</p> -</div> -</div> - -<p class='c006'>This curve is similar in shape to the corresponding curve -obtained for the active deposit from thorium, and is explained -in a similar way. The activity <i>I<sub>t</sub></i> at any time <i>t</i> is given by</p> - -<div class='figcenter id005'> -<img src='images/form-113.png' alt='Formula.' class='ig001'> -</div> - -<p class='c006'>where λ<sub>1</sub> and λ<sub>2</sub> are two constants, and <i>I</i><sub><i>T</i></sub> the maximum activity -reached after an interval <i>T</i>. After 20 minutes the activity -decreased exponentially with the time, falling to half value in -35·7 minutes. This gives the value λ<sub>1</sub> = ·0194 (min.)<sup>-1</sup>. By comparison -with the curve, the value of λ<sub>2</sub> was found to be ·317 (min.)<sup>-1</sup>. -This corresponds to a change in which half the matter is transformed -in 2·15 minutes. Exactly as in the analogous curve for -thorium, it can be shown that the matter initially deposited -undergoes two changes, the first of which is a rayless one. The -same difficulty arises in fixing which of the values of λ refers to -<span class='pageno' id='Page_368'>368</span>the first change. An experiment made by Miss Brooks (<i>loc. cit.</i>) -shows that the rayless product has the slower period of transformation. -The active deposit of actinium was dissolved off a -platinum wire and then electrolysed. The anode was found to be -active, and the activity fell off exponentially with the time, decreasing -to half value in about 1·5 minutes. Allowing for the -difficulty of accurately measuring such a rapid rate of decay, this -result indicates that the product which gives out rays has the -rapid period of 2·15 minutes. The analysis of the active deposit of -actinium thus leads to the following conclusions:</p> - -<p class='c006'>(1) The matter initially deposited from the emanation, called -actinium A, does not give out rays, and is half transformed in -35·7 minutes.</p> - -<p class='c006'>(2) A changes into B, which is half transformed in 2·15 -minutes, and gives out both α and β (and probably γ) rays.</p> - -<p class='c006'>Godlewski found that the active deposit of actinium was very -easily volatilized. Heating for several minutes at a temperature -of 100° C. was sufficient to drive off most of the active matter. -The active deposit is readily soluble in ammonia and in strong -acids.</p> -<p class='c005'><b>213. Radiations from actinium and its products.</b> -Actinium in radio-active equilibrium gives out α, β, and γ rays. -Godlewski found several points of distinction between the β and γ -rays of actinium and of radium. The β rays of actinium appear -to be homogeneous, for the activity measured by an electroscope -was found to fall off accurately according to an exponential law -with the thickness of matter traversed. The β rays were half -absorbed in a thickness of 0·21 mm. of aluminium. This indicates -that the β particles are all projected from actinium with the same -velocity. In this respect actinium behaves very differently from -radium, for the latter gives out β particles whose velocities vary -over a wide range.</p> - -<p class='c006'>After the β rays were absorbed, another type of more penetrating -rays was observed, which probably corresponds to the -γ rays from the other radio-elements. The γ rays of actinium -were, however, far less penetrating than those from radium. The -activity due to these rays was reduced to one-half after passing -<span class='pageno' id='Page_369'>369</span>through 1·9 mms. of lead, while the thickness of lead required -in order to absorb half the γ rays of radium is about 9 mms.</p> - -<p class='c006'>The active deposit gave out α and β (and probably γ) rays. It -was difficult to decide definitely whether actinium X gave out β -as well as α rays. When the actinium X was heated to a red heat, -the β activity was temporarily reduced to about half its initial -value. This decrease was probably due to the removal of the -active deposit, which, we have seen, is readily volatilized by heat. -If the β ray activity cannot be further reduced, this would point -to the conclusion that actinium X, as well as actinium B, gives out -β rays, but the evidence so far obtained is not conclusive.</p> - -<p class='c006'>The ease with which the active deposit is volatilized by heat -offers a very simple explanation of the initial peculiarities of the -decay and recovery curves (<a href='#fig082'>Fig. 82</a>) of actinium X. The activity -of actinium X rises at first, but there is no corresponding decrease -in the activity of the actinium left behind. It has been shown -that the active deposit is soluble in ammonia, and, in consequence, -is removed with the actinium X. The products actinium A and B -and actinium X, immediately after separation, are in radio-active -equilibrium and we should not therefore expect to find any increase -of activity after removal, such as is observed in the case of thorium, -where thorium A and B are not removed with thorium X. However, -in heating the actinium X to drive off the ammonium salts, -some of the active deposit is volatilized. After cooling, the amount -of the active deposit increases to nearly its old value and there is a -corresponding increase of the activity.</p> - -<div id='fig084' class='figcenter id008'> -<img src='images/fig-084.png' alt='Fig. 84.' class='ig001'> -<div class='ic002'> -<p>Fig. 84.</p> -</div> -</div> -<p class='c005'><b>214. Products of Actinium.</b> There is one very interesting -point of distinction between the radio-active behaviour of thorium -and actinium. The latter after removal of actinium X, shows only -about 5 per cent. of the original activity, while thorium, after -<span class='pageno' id='Page_370'>370</span>removal of Th X, always shows a residual activity of about 25 per -cent. of the maximum value. This very small residual activity -indicates that actinium, if completely freed from all its products, -would not give out rays at all, in other words, the first change in -actinium is a rayless one.</p> - -<p class='c006'>The radio-active products of actinium are shown graphically in -<a href='#fig084'>Fig. 84</a>. Some of their chemical and physical properties are -tabulated below.</p> - -<table class='table16' > -<colgroup> -<col class='colwidth20'> -<col class='colwidth20'> -<col class='colwidth22'> -<col class='colwidth37'> -</colgroup> - <tr> - <th class='c013'>Products</th> - <th class='c013'>Time to be half transformed</th> - <th class='c013'>Rays</th> - <th class='c014'>Some Physical and Chemical properties</th> - </tr> - <tr> - <td class='c013'> </td> - <td class='c013'> </td> - <td class='c013'> </td> - <td class='c014'> </td> - </tr> - <tr> - <td class='c013'>Actinium</td> - <td class='c013'>?</td> - <td class='c013'>No rays</td> - <td class='c014'>Insoluble in ammonia</td> - </tr> - <tr> - <td class='c013'>Actinium X</td> - <td class='c013'>10·2 days</td> - <td class='c013'>α, (β and γ)</td> - <td class='c014'>Soluble in ammonia</td> - </tr> - <tr> - <td class='c013'>Emanation</td> - <td class='c013'>3·9 secs.</td> - <td class='c013'>α rays</td> - <td class='c014'>Behaves as a gas</td> - </tr> - <tr> - <td class='c013'>Actinium A</td> - <td class='c013'>35·7 mins.</td> - <td class='c013'>No rays</td> - <td class='c014'>Soluble in ammonia and strong acids.</td> - </tr> - <tr> - <td class='c013'>Actinium B</td> - <td class='c013'>2·15 mins.</td> - <td class='c013'>α, β and γ</td> - <td class='c014'>Volatilized at 100°C. B can be separated from A by electrolysis</td> - </tr> -</table> - -<div class='chapter'> - <span class='pageno' id='Page_371'>371</span> - <h2 id='chap11' class='c004'>CHAPTER XI. <br> TRANSFORMATION PRODUCTS OF RADIUM.</h2> -</div> -<p class='c005'><a id='section215'></a> -<b>215. Radio-activity of radium.</b> Notwithstanding the -enormous difference in their relative activities, the radio-activity -of radium presents many close analogies to that of thorium and -actinium. Both substances give rise to emanations which in turn -produce “excited activity” on bodies in their neighbourhood. -Radium, however, does not give rise to any intermediate product -between the element itself and the emanation it produces, or in -other words there is no product in radium corresponding to Th X -in thorium.</p> - -<p class='c006'>Giesel first drew attention to the fact that a radium compound -gradually increased in activity after preparation, and only reached -a constant value after a month’s interval. If a radium compound -is dissolved in water and boiled for some time, or a current of air -drawn through the solution, on evaporation it is found that the -activity has been diminished. The same result is observed if -a solid radium compound is heated in the open air. This loss -of activity is due to the removal of the emanation by the process -of solution or heating. Consider the case of a radium compound -which has been kept for some time in solution in a shallow vessel, -exposed to the open air, and then evaporated to dryness. The -emanation which, in the state of solution, was removed as fast as -it was formed, is now occluded, and, together with the active -deposit which it produces, adds its radiations to that of the original -radium. The activity will increase to a maximum value when the -rate of production of fresh emanation balances the rate of change -of that already produced.</p> - -<p class='c006'><span class='pageno' id='Page_372'>372</span>If now the compound is again dissolved or heated, the emanation -escapes. Since the active deposit is not volatile and is -insoluble in water, it is not removed by the process of solution or -heating. Since, however, the parent matter is removed, the activity -due to the active deposit will immediately begin to decay, and -in the course of a few hours will have almost disappeared. The -activity of the radium measured by the α rays is then found to be -about 25 per cent. of its original value. This residual activity of -radium, consisting entirely of α rays, is non-separable, and has not -been further diminished by chemical or physical means. Rutherford -and Soddy<a id='r314' href='#f314' class='c012'><sup>[314]</sup></a> examined the effect of aspiration for long intervals -through a radium chloride solution. After the first few hours the -activity was found to be reduced to 25 per cent., and further -aspiration for three weeks did not produce any further diminution. -The radium was then evaporated to dryness, and the rise of its -activity with time determined. The results are shown in the -following table. The final activity in the second column is taken -as one hundred. In column 3 is given the percentage proportion -of the activity recovered.</p> - -<table class='table6' > -<colgroup> -<col class='colwidth26'> -<col class='colwidth30'> -<col class='colwidth43'> -</colgroup> - <tr> - <th class='c015'>Time in days</th> - <th class='c015'>Activity</th> - <th class='c016'>Percentage Activity recovered</th> - </tr> - <tr> - <td class='c015'> </td> - <td class='c015'> </td> - <td class='c016'> </td> - </tr> - <tr> - <td class='c015'>0</td> - <td class='c015'>25·0</td> - <td class='c016'>0</td> - </tr> - <tr> - <td class='c015'>0·70</td> - <td class='c015'>33·7</td> - <td class='c016'>11·7</td> - </tr> - <tr> - <td class='c015'>1·77</td> - <td class='c015'>42·7</td> - <td class='c016'>23·7</td> - </tr> - <tr> - <td class='c015'>4·75</td> - <td class='c015'>68·5</td> - <td class='c016'>58·0</td> - </tr> - <tr> - <td class='c015'>7·83</td> - <td class='c015'>83·5</td> - <td class='c016'>78·0</td> - </tr> - <tr> - <td class='c015'>16·0</td> - <td class='c015'>96·0</td> - <td class='c016'>95·0</td> - </tr> - <tr> - <td class='c015'>21·0</td> - <td class='c015'>100·0</td> - <td class='c016'>100·0</td> - </tr> -</table> - -<p class='c006'>The results are shown graphically in <a href='#fig085'>Fig. 85</a>.</p> - -<p class='c006'>The decay curve of the radium emanation is shown in the -same figure. The curve of recovery of the lost activity of radium -is thus analogous to the curves of recovery of uranium and thorium -which have been freed from the active products Ur X and Th X -respectively. The intensity <i>I<sub>t</sub></i> of the recovered activity at any -time is given by</p> - -<div class='figcenter id010'> -<img src='images/form-055.png' alt='Formula.' class='ig001'> -</div> - -<p class='c006'>where <i>I</i>₀ is the final value, and λ is -<span class='pageno' id='Page_373'>373</span>the radio-active constant of the emanation. The decay and recovery -curves are complementary to one another.</p> - -<div id='fig085' class='figcenter id004'> -<img src='images/fig-085.png' alt='Fig. 85.' class='ig001'> -<div class='ic002'> -<p>Fig. 85.</p> -</div> -</div> - -<p class='c006'>Knowing the rate of decay of activity of the radium emanation, -the recovery curve of the activity of radium can thus at once be -deduced, provided all of the emanation formed is occluded in the -radium compound.</p> - -<p class='c006'>When the emanation is removed from a radium compound by -solution or heating, the activity <i>measured by the</i> β <i>rays</i> falls -almost to zero, but increases in the course of a month to its -original value. The curve showing the rise of β and γ rays with -time is practically identical with the curve, <a href='#fig085'>Fig. 85</a>, showing the -recovery of the lost activity of radium measured by the α rays. -The explanation of this result lies in the fact that the β and γ rays -from radium only arise from the active deposit, and that the non-separable -activity of radium gives out only α rays. On removal of -the emanation, the activity of the active deposit decays nearly to -zero, and in consequence the β and γ rays almost disappear. -When the radium is allowed to stand, the emanation begins to -accumulate, and produces in turn the active deposit, which gives -<span class='pageno' id='Page_374'>374</span>rise to β and γ rays. The amount of β and γ rays (allowing for a -period of retardation of a few hours) will then increase at the same -rate as the activity of the emanation, which is continuously produced -from the radium.</p> -<p class='c005'><b>216. Effect of escape of emanation.</b> If the radium -allows some of the emanation produced to escape into the air, the -curve of recovery will be different from that shown in Fig. 85. -For example, suppose that the radium compound allows a constant -fraction α of the amount of emanation, present in the compound at -any time, to escape per second. If <i>n</i> is the number of emanation -particles present in the compound at the time <i>t</i>, the number of -emanation particles changing in the time <i>dt</i> is λ<i>ndt</i>, where λ is the -constant of decay of activity of the emanation. If <i>q</i> is the rate of -production of emanation particles per second, the increase of the -number <i>dn</i> in the time <i>dt</i> is given by</p> - -<div class='calc'> - -<div class='lg-container-b c017'> - <div class='linegroup'> - <div class='group'> - <div class='line in8'><i>dn</i> = <i>qdt</i> – λ<i>ndt</i> – α<i>ndt</i>,</div> - </div> - <div class='group'> - <div class='line'>or <i>dn</i></div> - <div class='line in4'>----- = <i>q</i> – (λ + α)<i>n</i>.</div> - <div class='line in4'> <i>dt</i></div> - </div> - </div> -</div> - -</div> - -<p class='c018'>The same equation is obtained when no emanation escapes, -with the difference that the constant λ + α is replaced by λ. -When a steady state is reached, <i>dn</i>/<i>dt</i> is zero, and the maximum value -of <i>n</i> is equal to <i>q</i>/(λ + α).</p> - -<p class='c006'>If no escape takes place, the maximum value of <i>n</i> is equal to <i>q</i>/λ. -The escape of emanation will thus lower the amount of activity -recovered in the proportion λ/(λ + α). If -<i>n</i>₀ -is the final number of -emanation particles stored up in the compound, the integration of -the above equation gives</p> - -<div class='figcenter id005'> -<img src='images/form-114.png' alt='Formula.' class='ig001'> -</div> - -<p class='c006'>The curve of recovery of activity is thus of the same general -form as the curve when no emanation escapes, but the constant -λ is replaced by λ + α.</p> - -<p class='c006'><span class='pageno' id='Page_375'>375</span>For example, if α = λ = ¹⁄₄₆₃₀₀₀, the equation of rise of activity -is given by</p> - -<div class='figcenter id009'> -<img src='images/form-115.png' alt='Formula.' class='ig001'> -</div> - -<p class='c006'>and, in consequence, the increase of -activity to the maximum will be far more rapid than in the -case of no escape of emanation.</p> - -<p class='c006'>A very slight escape of emanation will thus produce large alterations -both in the final maximum and in the curve of recovery of -activity.</p> - -<p class='c006'>A number of experiments have been described by Mme Curie -in her <i>Thèse présentée à la Faculté des Sciences de Paris</i> on the -effect of solution and of heat in diminishing the activity of radium. -The results obtained are in general agreement with the above view, -that 75 per cent. of the activity of radium is due to the emanation -and the excited activity it produces. If the emanation is -wholly or partly removed by solution or heating, the activity of -the radium is correspondingly diminished, but the activity of the -radium compound is spontaneously recovered owing to the production -of fresh emanation. A state of radio-active equilibrium is -reached, when the rate of production of fresh emanation balances -the rate of change in the emanation stored up in the compound. -The differences observed in the rate of recovery of radium under -different conditions were probably due to variations in the rate of -escape of the emanation.</p> -<p class='c005'><b>217.</b> It has been shown in section 152 that the emanation is -produced at the same rate in the solid as in the solution, and all -the results obtained point to the conclusion that the emanation is -produced from radium at a constant rate, which is independent -of physical conditions. Radium, like thorium, shows a non-separable -activity of 25 per cent. of the maximum activity, and -consisting entirely of α rays. The β and γ rays arise only from -the active deposit. The emanation itself (<a href='#section156'>section 156</a>) gives out -only α rays. These results thus admit of the explanation given in -the case of thorium (<a href='#section136'>section 136</a>). The radium atoms break up at -a constant rate with the emission of α particles. The residue of -the radium atom becomes the atom of the emanation. This -in turn is unstable and breaks up with the expulsion of an α -particle. The emanation is half transformed in four days. We -<span class='pageno' id='Page_376'>376</span>have seen that this emanation gives rise to an active deposit. -The results obtained up to this stage are shown diagrammatically -below.</p> - -<div class='calc'> - -<div class='lg-container-b c017'> - <div class='linegroup'> - <div class='group'> - <div class='line in13'>α <i>particle</i> α <i>particle</i></div> - <div class='line in12'>/ /</div> - <div class='line in11'>/ /</div> - <div class='line'><span class='sc'>Radium atom</span> ——> <span class='sc'>atom of Emanation</span> ——> <span class='fss'>ATOM OF ACTIVE DEPOSIT</span></div> - </div> - </div> -</div> - -</div> -<p class='c005'><b>218. Analysis of the active deposit from radium.</b> We -have seen in <a href='#chap08'>chapter <span class='fss'>VIII</span></a> that the excited activity produced -on bodies, by the action of the radium emanation, is due to a thin -film of active matter deposited on the surface of bodies. This -active deposit is a product of the decomposition of the radium -emanation, and is not due to any action of the radiations on the -surface of the matter.</p> - -<p class='c006'>The curves showing the variation of the excited activity with -time are very complicated, depending not only upon the time -of exposure in the presence of the emanation, but also upon -the type of radiation used for measurement. The greater portion -of the activity of this deposit dies away in the course of 24 hours, -but a very small fraction still remains, which then changes -very slowly.</p> - -<p class='c006'>It will be shown in this chapter that at least six successive -transformations occur in the active deposit. The matter initially -produced from the emanation is called radium A, and the succeeding -products B, C, D, E, F. The equations expressing the -quantity of A, B, C,...... present at any time are very complicated, -but the comparison of theory with experiment may be much -simplified by temporarily disregarding some unimportant terms: -for example, the products A, B, C are transformed at a very rapid -rate compared with D. The activity due to D + E + F is, in most -cases, negligible compared with that of A or C, being usually -less than ¹⁄₁₀₀₀₀₀ of the initial activity observed for A or C. -The analysis of the active deposit of radium may thus be conveniently -divided into two stages:</p> - -<p class='c021'>(1) Analysis of the deposit of rapid change, which is -mainly composed of radium A, B, and C;</p> - -<p class='c011'>(2) Analysis of the deposit of slow change, which is -composed of radium D, E, and F.</p> -<p class='c005'><span class='pageno' id='Page_377'>377</span><b>219. Analysis of the deposit of rapid change.</b> In the -experiments described below, a radium solution was placed in -a closed glass vessel. The emanation then collected in the air -space above the solution. The rod, to be made active, was introduced -through an opening in the stopper and exposed in the -presence of the emanation for a definite interval. If the decay -was to be measured by the α rays, the rod was made the central -electrode in a cylindrical vessel such as is shown in Fig. 18. -A saturating voltage was applied, and the current between the -cylinders measured by an electrometer. If a very active rod is -to be tested, a sensitive galvanometer can be employed, but, in -such a case, a large voltage is required to produce saturation. A -slow current of dust-free air was continuously circulated through -the cylinder, in order to remove any emanation that may have -adhered to the rod. For experiments on the β and γ rays, it was -found advisable to use an electroscope, such as is shown in <a href='#fig012'>Fig. 12</a>, -instead of an electrometer. For measurements with the γ rays, -the active rod was placed under the electroscope, and before -entering the vessel the rays passed through a sheet of metal -of sufficient thickness to absorb all the α rays. For measurements -with the γ rays, the electroscope was placed on a lead plate -0·6 cms. thick, and the active rod placed under the lead plate. -The α and β rays were completely stopped by the lead, and the -discharge in the electroscope was then due to the γ rays alone. -The electroscope is very advantageous for measurements of this -character, and accurate observations can be made simply and -readily.</p> - -<p class='c006'>The curve of decay of activity, measured by the α rays, for an -exposure of 1 minute in the presence of the radium emanation is -shown in <a href='#fig086'>Fig. 86</a>, curve <i>BB</i>.</p> - -<p class='c006'>The curve exhibits three stages:—</p> - -<p class='c021'>(1) A rapid decay in the course of 15 minutes to less than -10 per cent. of the value immediately after removal;</p> - -<p class='c011'>(2) A period of 30 minutes in which the activity varies -very little;</p> - -<p class='c011'>(3) A gradual decrease almost to zero.</p> - -<p class='c018'>The initial drop decays very approximately according to an -<span class='pageno' id='Page_378'>378</span>exponential law with the time, falling to half value in about -3 minutes. Three or four hours after removal the activity again -decays according to an exponential law with the time, falling to -half value in about 28 minutes. The family of curves obtained -for different times of exposure have already been shown in <a href='#fig067'>Fig. 67</a>. -These results thus indicate:—</p> - -<p class='c021'>(1) An initial change in which half the matter is transformed -in 3 minutes;</p> - -<p class='c011'>(2) A final change in which half the matter is transformed -in 28 minutes.</p> - -<div id='fig086' class='figcenter id004'> -<img src='images/fig-086.png' alt='Fig. 86.' class='ig001'> -<div class='ic002'> -<p>Fig. 86.</p> -</div> -</div> - -<p class='c018'>Before considering the explanation of the intermediate portion -of the curve further experimental results will be considered.</p> - -<p class='c006'>The curve of decay of the excited activity for a long exposure -(24 hours) is shown graphically in <a href='#fig086'>Fig. 86</a>, curve <i>AA</i>. There is at -first a rapid decrease for the first 15 minutes to about 50 per cent. -of the initial value, then a slower decay, and, after an interval -of about 4 hours, a gradual decay nearly to zero, according to -an exponential law with the time, falling to half value in 28 -minutes.</p> - -<p class='c006'><span class='pageno' id='Page_379'>379</span>The curves of variation with time of the excited activity when -measured by the β <i>rays</i> are shown graphically in Figs. <a href='#fig087'>87</a> and <a href='#fig088'>88</a>.</p> - -<p class='c006'><a href='#fig087'>Fig. 87</a> is for a short exposure of 1 minute. <a href='#fig088'>Fig. 88</a> shows the -decay for a long exposure of about 24 hours.</p> - -<div id='fig087' class='figcenter id004'> -<img src='images/fig-087.png' alt='Fig. 87.' class='ig001'> -<div class='ic002'> -<p>Fig. 87.</p> -</div> -</div> - -<p class='c006'>The curves obtained for the β rays are quite different from -those obtained for the α rays. For a short exposure, the activity -measured by the β rays is at first small, then passes through -a maximum about 36 minutes after removal. There is then -a gradual decrease, and after several hours the activity decays -according to an exponential law, falling, as in the other cases, -to half value in 28 minutes.</p> - -<p class='c006'>The curve shown in <a href='#fig088'>Fig. 88</a> for the β rays is very similar -in shape to the corresponding curve, <a href='#fig086'>Fig. 86</a>, curve <i>AA</i>, for the α -rays, with the exception that the rapid initial drop observed for -the α-ray curve is quite absent. The later portions of the curve -are similar in shape, and, disregarding the first 15 minutes after -removal, the activity decays at exactly the same rate in both -cases.</p> - -<p class='c006'>The curves obtained by means of the γ rays are identical with -those obtained for the β rays. This shows that the β and γ rays -always occur together and in the same proportion.</p> - -<p class='c006'><span class='pageno' id='Page_380'>380</span>For increase of the time of exposure from 1 minute to 24 hours -the curves obtained are intermediate in shape between the two -representative limiting curves, Figs. <a href='#fig087'>87</a> and <a href='#fig088'>88</a>. Some of these -curves have already been shown in <a href='#fig068'>Fig. 68</a>.</p> - -<div id='fig088' class='figcenter id004'> -<img src='images/fig-088.png' alt='Fig. 88.' class='ig001'> -<div class='ic002'> -<p>Fig. 88.</p> -</div> -</div> -<p class='c005'><b>220. Explanation of the curves.</b> It has been pointed -out that the rapid initial drop for curves <i>A</i> and <i>B</i>, <a href='#fig086'>Fig. 86</a>, is due -to a change giving rise to α rays, in which half of the matter -is transformed in about 3 minutes. The absence of the drop -in the corresponding curves, when measured by the β rays, shows -that the first 3-minute change does not give rise to β rays; for if -it gave rise to β rays, the activity should fall off at the same rate -as the corresponding α-ray curve.</p> - -<p class='c006'>It has been shown that the activity several hours after removal -decays in all cases according to an exponential law with the time, -falling to half value in about 28 minutes. This is the case -whether for a short or long exposure, or whether the activity -is measured by the α, β, or γ rays. This indicates that the final -28-minute change gives rise to all three types of rays.</p> - -<p class='c006'><span class='pageno' id='Page_381'>381</span>It will be shown that these results can be completely explained -on the supposition that three successive changes occur in the -deposited matter of the following character<a id='r315' href='#f315' class='c012'><sup>[315]</sup></a>:—</p> - -<p class='c021'>(1) A change of the matter A initially deposited in which -half is transformed in about 3 minutes. This gives -rise only to α rays.</p> - -<p class='c011'>(2) A second “rayless” change in which half the matter B -is transformed in 21 minutes.</p> - -<p class='c011'>(3) A third change in which half the matter C is transformed -in 28 minutes. This gives rise to α, β, -and γ rays.</p> -<p class='c005'><b>221. Analysis of the β-ray curves</b>. The analysis of the -changes is much simplified by temporarily disregarding the first -3-minute change. In the course of 6 minutes after removal, three -quarters of the matter A has been transformed into B and 20 -minutes after removal all but 1 per cent. has been transformed. -The variation of the amount of matter B or C present at any time -agrees more closely with the theory, if the first change is disregarded -altogether. A discussion of this important point is -given later (<a href='#section228'>section 228</a>).</p> - -<p class='c006'>The explanation of the β-ray curves (see Figs. <a href='#fig087'>87</a> and <a href='#fig088'>88</a>), -obtained for different times of exposure, will be first considered. -For a very short exposure, the activity measured by the β rays is -small at first, passes through a maximum about 36 minutes later, -and then decays steadily with the time.</p> - -<p class='c006'>The curve shown in <a href='#fig087'>Fig. 87</a> is very similar in general shape -to the corresponding thorium and actinium curves. It is thus -necessary to suppose that the change of the matter B into C does -not give rise to β rays, while the change of C into D does. In -such a case the activity (measured by the β rays) is proportional -to the amount of C present. Disregarding the first rapid change, -the activity <i>I<sub>t</sub></i> at any time <i>t</i> should be given by an equation of the -same form (<a href='#section207'>section 207</a>) as for thorium and actinium, viz.,</p> - -<div class='figcenter id005'> -<img src='images/form-116.png' alt='Formula.' class='ig001'> -</div> - -<p class='c006'><span class='pageno' id='Page_382'>382</span>where <i>I<sub>T</sub></i> is the maximum activity observed, which is reached after -an interval <i>T</i>. Since the activity finally decays according to an -exponential law (half value in 28 minutes), one of the values of λ -is equal to -4·13 × 10<sup>-4</sup>. -As in the case of thorium and actinium, -the experimental curves do not allow us to settle whether this -value of λ is to be given to λ<sub>2</sub> or λ<sub>3</sub>. From other data (see -<a href='#section226'>section 226</a>) it will be shown later that it must refer to -λ<sub>3</sub>. Thus λ<sub>3</sub> = 4·13 × 10<sup>-4</sup> (sec)<sup>-1</sup>.</p> - -<p class='c006'>The experimental curve agrees very closely with theory if -λ<sub>2</sub> = 5·38 × 10<sup>-4</sup> (sec)<sup>-1</sup>.</p> - -<p class='c006'>The agreement between theory and experiment is shown by -the table given below. The maximum value <i>I<sub>T</sub></i> (which is taken as -100) is reached at a time <i>T</i> = 36 minutes.</p> - -<p class='c006'>In order to obtain the β-ray curve, the following procedure -was adopted. A layer of thin aluminium was placed inside a -glass tube, which was then exhausted. A large quantity of -radium emanation was then suddenly introduced by opening a -stop-cock communicating with the emanation vessel, which was at -atmospheric pressure. The emanation was left in the tube for -1·5 minutes and then was rapidly swept out by a current of -air. The aluminium was then removed and was placed under -an electroscope, such as is shown in <a href='#fig012'>Fig. 12</a>. The α rays from the -aluminium were cut off by an interposed screen of aluminium -·1 mm. thick. The time was reckoned from a period of 45 -seconds after the introduction of the emanation.</p> - -<table class='table26' > -<colgroup> -<col class='colwidth25'> -<col class='colwidth37'> -<col class='colwidth37'> -</colgroup> - <tr> - <th class='c015'>Time in minutes</th> - <th class='c015'>Theoretical value of activity</th> - <th class='c016'>Observed value of activity</th> - </tr> - <tr> - <td class='c015'> </td> - <td class='c015'> </td> - <td class='c016'> </td> - </tr> - <tr> - <td class='c015'>0</td> - <td class='c015'>0</td> - <td class='c016'>0</td> - </tr> - <tr> - <td class='c015'>10</td> - <td class='c015'>58·1</td> - <td class='c016'>55</td> - </tr> - <tr> - <td class='c015'>20</td> - <td class='c015'>88·6</td> - <td class='c016'>86</td> - </tr> - <tr> - <td class='c015'>30</td> - <td class='c015'>97·3</td> - <td class='c016'>97</td> - </tr> - <tr> - <td class='c015'>36</td> - <td class='c015'>100</td> - <td class='c016'>100</td> - </tr> - <tr> - <td class='c015'>40</td> - <td class='c015'>99·8</td> - <td class='c016'>99·5</td> - </tr> - <tr> - <td class='c015'>50</td> - <td class='c015'>93·4</td> - <td class='c016'>92</td> - </tr> - <tr> - <td class='c015'>60</td> - <td class='c015'>83·4</td> - <td class='c016'>82</td> - </tr> - <tr> - <td class='c015'>80</td> - <td class='c015'>63·7</td> - <td class='c016'>61·5</td> - </tr> - <tr> - <td class='c015'>100</td> - <td class='c015'>44·8</td> - <td class='c016'>42·5</td> - </tr> - <tr> - <td class='c015'>120</td> - <td class='c015'>30·8</td> - <td class='c016'>29</td> - </tr> -</table> - -<p class='c006'><span class='pageno' id='Page_383'>383</span>There is thus a good agreement between the calculated and -observed values of the activity measured by the β rays.</p> - -<p class='c006'>The results are satisfactorily explained if it is supposed:—</p> - -<p class='c021'>(1) That the change B into C (half transformed in 21 -minutes) does not give rise to β rays;</p> - -<p class='c011'>(2) That the change C into D (half transformed in 28 -minutes) gives rise to β rays.</p> -<p class='c005'><a id='section222'></a> -<b>222.</b> These conclusions are very strongly supported by observations -of the decay measured by the β rays for a long exposure. -The curve of decay is shown in <a href='#fig088'>Fig. 88</a> and <a href='#fig089'>Fig. 89</a>, curve I.</p> - -<div id='fig089' class='figcenter id004'> -<img src='images/fig-089.png' alt='Fig. 89.' class='ig001'> -<div class='ic002'> -<p>Fig. 89.</p> -</div> -</div> - -<p class='c006'>P. Curie and Danne made the important observation that the -curve of decay <i>C</i>, corresponding to that shown in <a href='#fig088'>Fig. 88</a>, for -a long exposure, could be accurately expressed by an empirical -equation of the form</p> - -<div class='figcenter id005'> -<img src='images/form-117.png' alt='Formula.' class='ig001'> -</div> - -<p class='c006'>where λ<sub>2</sub> = 5·38 × 10<sup>-4</sup> (sec)<sup>-1</sup> and λ<sub>3</sub> = 4·13 × 10<sup>-4</sup> (sec)<sup>-1</sup>, and -α = 4·20 is a numerical constant.</p> - -<p class='c006'>I have found that within the limit of experimental error this -equation represents the decay of excited activity of radium for a -<span class='pageno' id='Page_384'>384</span>long exposure, measured by the β rays. The equation expressing -the decay of activity, measured by the α rays, differs considerably -from this, especially in the early part of the curve. Several hours -after removal the activity decays according to an exponential law -with the time, decreasing to half value in 28 minutes. This fixes -the value of λ<sub>3</sub>. The constant α and the value of λ<sub>2</sub> are deduced -from the experimental curve by trial. Now we have already -shown (<a href='#section207'>section 207</a>) that in the case of the active deposit from -thorium, where there are two changes of constants λ<sub>2</sub> and λ<sub>3</sub>, -in which only the second change gives rise to a radiation, the -intensity of the radiation is given by</p> - -<div class='figcenter id002'> -<img src='images/form-118.png' alt='Formula.' class='ig001'> -</div> - -<p class='c006'>for a long time of exposure (see equation 8, <a href='#section198'>section 198</a>). This is -an equation of the same form as that found experimentally by -Curie and Danne. On substituting the values λ<sub>2</sub>, λ<sub>3</sub> found by -them,</p> - -<div class='figcenter id002'> -<img src='images/form-119.png' alt='Formula.' class='ig001'> -</div> - -<p class='c006'>Thus the theoretical equation agrees in form with that deduced -from observation, and the values of the numerical constants are -also closely concordant. If the first as well as the second change -gave rise to a radiation, the equation would be of the same general -form, but the value of the numerical constants would be different, -the values depending upon the ratio of the ionization in the first -and second changes. If, for example, it is supposed that both -changes give out β rays in equal amounts, it can readily be -calculated that the equation of decay would be</p> - -<div class='figcenter id007'> -<img src='images/form-120.png' alt='Formula.' class='ig001'> -</div> - -<p class='c006'>Taking the values of -λ<sub>2</sub> and λ<sub>3</sub> -found by Curie, the numerical -factor</p> - -<div class='figcenter id009'> -<img src='images/form-121.png' alt='Formula.' class='ig001'> -</div> - -<p class='c006'>becomes 2·15 instead of 4·3 and 1·15 instead of 3·3. -The theoretical curve of decay in this case would be readily -distinguishable from the observed curve of decay. The fact that -the equation of decay found by Curie and Danne involves the -necessity of an initial rayless change can be shown as follows:—</p> - -<p class='c006'><span class='pageno' id='Page_385'>385</span>Curve I (<a href='#fig089'>Fig. 89</a>) shows the experimental curve. At the -moment of removal of the body from the emanation (disregarding -the initial rapid change), the matter must consist of both B and -C. Consider the matter which existed in the form C at the -moment of removal. It will be transformed according to an -exponential law, the activity falling by one-half in 28 minutes. -This is shown in curve II. Curve III represents the difference -between the ordinates of curves I and II. It will be seen that it -is identical in shape with the curve (<a href='#fig087'>Fig. 87</a>) showing the variation -of the activity for a short exposure, measured by the β rays. It -passes through a maximum at the same time (about 36 minutes). -The explanation of such a curve is only possible on the assumption -that the first change is a rayless one. The ordinates of curve III -express the activity added in consequence of the change of the -matter B, present after removal, into the matter C. The matter -B present gradually changes into C, and this, in its change to D, -gives rise to the radiation observed. Since the matter B alone is -considered, the variation of activity with time due to its further -changes, shown by curve III, should agree with the curve obtained -for a short exposure (see <a href='#fig087'>Fig. 87</a>), and this, as we have seen, is the -case.</p> - -<p class='c006'>The agreement between theory and experiment is shown in -the following table. The first column gives the theoretical curve -of decay for a long exposure deduced from the equation</p> - -<div class='figcenter id007'> -<img src='images/form-118.png' alt='Formula.' class='ig001'> -</div> - -<p class='c006'>taking the value of λ<sub>2</sub> = 5·38 × 10<sup>-4</sup> and λ<sub>3</sub> = 4·13 × 10<sup>-4</sup>.</p> - -<table class='table26' > -<colgroup> -<col class='colwidth25'> -<col class='colwidth37'> -<col class='colwidth37'> -</colgroup> - <tr> - <th class='c015'>Time in minutes</th> - <th class='c015'>Calculated values</th> - <th class='c016'>Observed values</th> - </tr> - <tr> - <td class='c015'> </td> - <td class='c015'> </td> - <td class='c016'> </td> - </tr> - <tr> - <td class='c015'>0</td> - <td class='c015'>100</td> - <td class='c016'>100</td> - </tr> - <tr> - <td class='c015'>10</td> - <td class='c015'>96·8</td> - <td class='c016'>97·0</td> - </tr> - <tr> - <td class='c015'>20</td> - <td class='c015'>89·4</td> - <td class='c016'>88·5</td> - </tr> - <tr> - <td class='c015'>30</td> - <td class='c015'>78·6</td> - <td class='c016'>77·5</td> - </tr> - <tr> - <td class='c015'>40</td> - <td class='c015'>69·2</td> - <td class='c016'>67·5</td> - </tr> - <tr> - <td class='c015'>50</td> - <td class='c015'>59·9</td> - <td class='c016'>57·0</td> - </tr> - <tr> - <td class='c015'>60</td> - <td class='c015'>49·2</td> - <td class='c016'>48·2</td> - </tr> - <tr> - <td class='c015'>80</td> - <td class='c015'>34·2</td> - <td class='c016'>33·5</td> - </tr> - <tr> - <td class='c015'>100</td> - <td class='c015'>22·7</td> - <td class='c016'>22·5</td> - </tr> - <tr> - <td class='c015'>120</td> - <td class='c015'>14·9</td> - <td class='c016'>14·5</td> - </tr> -</table> - -<p class='c006'><span class='pageno' id='Page_386'>386</span>The second column gives the observed activity (measured by -means of an electroscope) for a long exposure of 24 hours in the -presence of the emanation.</p> - -<p class='c006'>In cases where a steady current of air is drawn over the active -body, the observed values are slightly lower than the theoretical. -This is probably due to a slight volatility of the product radium B -at ordinary temperatures.</p> - -<div id='fig090' class='figcenter id004'> -<img src='images/fig-090.png' alt='Fig. 90.' class='ig001'> -<div class='ic002'> -<p>Fig. 90.</p> -</div> -</div> -<p class='c005'><b>223. Analysis of the α-ray curves</b>. The analysis of the -decay curves of the excited activity of radium, measured by the α -rays, will now be discussed. The following table shows the variation -of the intensity of the radiation after a long exposure in the -presence of the radium emanation. A platinum plate was made -active by exposure for several days in a glass tube containing -a large quantity of emanation. The active platinum after removal -was placed on the lower of two parallel insulated lead plates, and -a saturating electromotive force of 600 volts was applied. The -ionization current was sufficiently large to be measured by means -of a sensitive high-resistance galvanometer, and readings were -taken as quickly as possible after removal of the platinum from -the emanation vessel. The initial value of the current (taken -<span class='pageno' id='Page_387'>387</span>as 100) was deduced by continuing the curves backwards to meet -the vertical axis (see <a href='#fig090'>Fig. 90</a>), and was found to be -3 × 10<sup>-8</sup> ampere.</p> - -<table class='table27' > -<colgroup> -<col class='colwidth44'> -<col class='colwidth55'> -</colgroup> - <tr> - <th class='c015'>Time in minutes</th> - <th class='c016'>Current</th> - </tr> - <tr> - <td class='c015'> </td> - <td class='c016'> </td> - </tr> - <tr> - <td class='c015'>0</td> - <td class='c016'>100</td> - </tr> - <tr> - <td class='c015'>2</td> - <td class='c016'>80</td> - </tr> - <tr> - <td class='c015'>4</td> - <td class='c016'>69·5</td> - </tr> - <tr> - <td class='c015'>6</td> - <td class='c016'>62·4</td> - </tr> - <tr> - <td class='c015'>8</td> - <td class='c016'>57·6</td> - </tr> - <tr> - <td class='c015'>10</td> - <td class='c016'>52·0</td> - </tr> - <tr> - <td class='c015'>15</td> - <td class='c016'>48·4</td> - </tr> - <tr> - <td class='c015'>20</td> - <td class='c016'>45·4</td> - </tr> - <tr> - <td class='c015'>30</td> - <td class='c016'>40·4</td> - </tr> - <tr> - <td class='c015'>40</td> - <td class='c016'>35·6</td> - </tr> - <tr> - <td class='c015'>50</td> - <td class='c016'>30·4</td> - </tr> - <tr> - <td class='c015'>60</td> - <td class='c016'>25·4</td> - </tr> - <tr> - <td class='c015'>80</td> - <td class='c016'>17·4</td> - </tr> - <tr> - <td class='c015'>100</td> - <td class='c016'>11·6</td> - </tr> - <tr> - <td class='c015'>120</td> - <td class='c016'>7·6</td> - </tr> -</table> - -<p class='c006'>These results are shown graphically in the upper curve of -<a href='#fig090'>Fig. 90</a>. The initial rapid decrease is due to the decay of the -activity of the matter A. If the slope of the curve is produced -backwards from a time 20 minutes after removal, it cuts the -vertical axis at about 50. The difference between the ordinates of -the curves <i>A</i> + <i>B</i> + <i>C</i> and <i>LL</i> at any time is shown in the curve -<i>AA</i>. The curve <i>AA</i> represents the activity at any time supplied -by the change in radium <i>A</i>. The curve <i>LL</i> starting from the -vertical axis is identical with the curve already considered, representing -the decay of activity measured by the β rays for a long -exposure (see <a href='#fig088'>Fig. 88</a>).</p> - -<table class='table26' > -<colgroup> -<col class='colwidth25'> -<col class='colwidth37'> -<col class='colwidth37'> -</colgroup> - <tr> - <th class='c015'>Time in minutes</th> - <th class='c015'>Calculated value of activity</th> - <th class='c016'>Observed value of activity</th> - </tr> - <tr> - <td class='c015'> </td> - <td class='c015'> </td> - <td class='c016'> </td> - </tr> - <tr> - <td class='c015'>0</td> - <td class='c015'>100</td> - <td class='c016'>100</td> - </tr> - <tr> - <td class='c015'>10</td> - <td class='c015'>96·8</td> - <td class='c016'>97·0</td> - </tr> - <tr> - <td class='c015'>20</td> - <td class='c015'>89·4</td> - <td class='c016'>89·2</td> - </tr> - <tr> - <td class='c015'>30</td> - <td class='c015'>78·6</td> - <td class='c016'>80·8</td> - </tr> - <tr> - <td class='c015'>40</td> - <td class='c015'>69·2</td> - <td class='c016'>71·2</td> - </tr> - <tr> - <td class='c015'>50</td> - <td class='c015'>59·9</td> - <td class='c016'>60·8</td> - </tr> - <tr> - <td class='c015'>60</td> - <td class='c015'>49·2</td> - <td class='c016'>50·1</td> - </tr> - <tr> - <td class='c015'>80</td> - <td class='c015'>34·2</td> - <td class='c016'>34·8</td> - </tr> - <tr> - <td class='c015'>100</td> - <td class='c015'>22·7</td> - <td class='c016'>23·2</td> - </tr> - <tr> - <td class='c015'>120</td> - <td class='c015'>14·9</td> - <td class='c016'>15·2</td> - </tr> -</table> - -<p class='c006'>This is shown by the agreement of the -numbers in the above table. The first column in the table -<span class='pageno' id='Page_388'>388</span>above gives the theoretical values of the activity deduced from the -equation</p> - -<div class='figcenter id002'> -<img src='images/form-118.png' alt='Formula.' class='ig001'> -</div> - -<p class='c006'>for the values of λ<sub>2</sub>, λ<sub>3</sub> previously employed. The second column -gives the observed values of the activity deduced from the decay -curve <i>LL</i>.</p> - -<p class='c006'>The close agreement of the curve <i>LL</i> with the theoretical -curve deduced on the assumption that there are two changes, the -first of which does not emit rays, shows that the change of radium -B into C does not emit α rays. In a similar way, as in the curve I, -<a href='#fig089'>Fig. 89</a>, the curve <i>LL</i> may be analysed into its two components -represented by the two curves <i>CC</i> and <i>BB</i>. The curve <i>CC</i> represents -the activity supplied by the matter C present at the moment -of removal. The curve <i>BB</i> represents the activity resulting from -the change of B into C and is identical with the corresponding curve -in <a href='#fig089'>Fig. 89</a>. Using the same line of reasoning as before, we may -thus conclude that the change of B into C is not accompanied by -α rays. It has already been shown that it does not give rise to -β rays, and the identity of the β and γ-ray curves shows that -it does not give rise to γ rays. The change of B into C is thus -a “rayless” change, while the change of C into D gives rise to -all three kinds of rays.</p> - -<p class='c006'>An analysis of the decay of the excited activity of radium thus -shows that three distinct rapid changes occur in the matter -deposited, viz.:—</p> - -<p class='c021'>(1) The matter A, derived from the change in the emanation, -is half transformed in 3 minutes and is accompanied by -α rays alone;</p> - -<p class='c011'>(2) The matter B is half transformed in 21 minutes and gives -rise to no ionizing rays;</p> - -<p class='c011'>(3) The matter C is half transformed in 28 minutes and is -accompanied by α, β, and γ rays;</p> - -<p class='c011'>(4) A fourth very slow change will be discussed later.</p> -<p class='c005'><b>224. Equations representing the activity curves.</b> The -equations representing the variation of activity with time are for -<span class='pageno' id='Page_389'>389</span>convenience collected below, where -λ<sub>1</sub> = 3·8 × 10<sup>-3</sup>, λ<sub>2</sub> = 5·38 × 10<sup>-4</sup>, λ<sub>3</sub> = 4·13 × 10<sup>-4</sup>:—</p> - -<p class='c006'>(1) Short exposure: activity measured by β rays,</p> - -<div class='figcenter id005'> -<img src='images/form-122.png' alt='Formula.' class='ig001'> -</div> - -<p class='c006'>where <i>I<sub>T</sub></i> is the maximum value of the activity;</p> - -<p class='c006'>(2) Long exposure: activity measured by β rays,</p> - -<div class='figcenter id005'> -<img src='images/form-123.png' alt='Formula.' class='ig001'> -</div> - -<p class='c006'>where <i>I</i>₀ is the initial value;</p> - -<p class='c006'>(3) Any time of exposure <i>T</i>: activity measured by the β rays,</p> - -<div class='figcenter id005'> -<img src='images/form-124.png' alt='Formula.' class='ig001'> -</div> - -<p class='c006'>where</p> - -<div class='figcenter id002'> -<img src='images/form-125.png' alt='Formula.' class='ig001'> -</div> - -<p class='c006'>(4) Activity measured by α rays: long time of exposure,</p> - -<div class='figcenter id007'> -<img src='images/form-126.png' alt='Formula.' class='ig001'> -</div> - -<p class='c006'>The equations for the α rays for any time of exposure can be -readily deduced, but the expressions are somewhat complicated.</p> - -<div id='fig091' class='figcenter id004'> -<img src='images/fig-091.png' alt='Fig. 91.' class='ig001'> -<div class='ic002'> -<p>Fig. 91.</p> -</div> -</div> -<p class='c005'><b>225. Equations of rise of excited activity.</b> The curves -expressing the gradual increase to a maximum of the excited -<span class='pageno' id='Page_390'>390</span>activity produced on a body exposed in the presence of a constant -amount of emanation are complementary to the curves of decay for -a long exposure. The sum of the ordinates of the rise and decay -curves is at any time a constant. This follows necessarily from the -theory and can also be deduced simply from <i>à priori</i> considerations. -(See <a href='#section200'>section 200</a>.)</p> - -<p class='c006'>The curves of rise and decay of the excited activity for both -the α and β rays are shown graphically in <a href='#fig091'>Fig. 91</a>. The thick line -curves are for the α rays. The difference between the shapes -of the decay curves when measured by the α or β rays is clearly -brought out in the figure. The equations representing the rise of -activity to a maximum are given below.</p> - -<p class='c006'>For the β and γ rays,</p> - -<div class='figcenter id002'> -<img src='images/form-127.png' alt='Formula.' class='ig001'> -</div> - -<p class='c006'>For the α rays,</p> - -<div class='figcenter id007'> -<img src='images/form-128.png' alt='Formula.' class='ig001'> -</div> -<p class='c005'><a id='section226'></a> -<b>226. Effect of temperature.</b> We have so far not considered -the evidence on which the 28-minute rather than the -21-minute change is supposed to take place in the matter C. -This evidence has been supplied by some recent important -experiments of P. Curie and Danne<a id='r316' href='#f316' class='c012'><sup>[316]</sup></a> on the volatilization of -the active matter deposited by the emanation. Miss Gates<a id='r317' href='#f317' class='c012'><sup>[317]</sup></a> -showed that this active matter was volatilized from a platinum -wire above a red heat and deposited on the surface of a cold -cylinder surrounding the wire. Curie and Danne extended these -results by subjecting an active platinum wire <i>for a short time</i> -to the action of temperatures varying between 15° C. and 1350° C., -and then examining at room temperatures the decay curves not -only for the active matter remaining on the wire, but also for -the volatilized part. They found that the activity of the distilled -part always increased after removal, passed through a maximum, -and finally decayed according to an exponential law to half value in -28 minutes. At a temperature of about 630° C. the active matter -left behind on the wire decayed at once according to an exponential -<span class='pageno' id='Page_391'>391</span>law, falling to half value in 28 minutes. P. Curie and Danne -showed that the matter B is much more volatile than C. The -former is completely volatilized at about 600° C., while the latter -is not completely volatilized even at a temperature of 1300° C. -The fact that the matter C, left behind when B is completely -volatilized, decays at once to half value in 28 minutes shows that -the matter C itself and not B is half transformed in 28 minutes.</p> - -<p class='c006'>Curie and Danne also found that the rate of decay of the active -matter varied with the temperature to which the platinum wire -had been subjected. At 630° C. the rate of decay was normal, at -1100° C. the activity fell to half value in about 20 minutes, while -at 1300° C. it fell to about half value in about 25 minutes.</p> - -<p class='c006'>I have repeated the experiments of Curie and Danne and -obtained very similar results. It was thought possible that the -measured rate of decay observed after heating might be due -to a permanent increase in the rate of volatilization of C at -ordinary temperatures. This explanation, however, is not tenable, -for it was found that the activity decreased at the same rate -whether the activity of the wire was tested in a closed tube or in -the open with a current of air passed over it.</p> - -<p class='c006'>These results are of great importance, for they indicate that -the rate of change of the product C is not a constant, but is -affected by differences of temperature. This is the first case -where temperature has been shown to exert an appreciable -influence on the rate of change of any radio-active product.</p> -<p class='c005'><a id='section227'></a> -<b>227. Volatility of radium B at ordinary temperature.</b> -Miss Brooks<a id='r318' href='#f318' class='c012'><sup>[318]</sup></a> has observed that a body, made active by exposure -to the radium emanation, possesses the power of exciting secondary -activity on the walls of a vessel in which it is placed. This -activity was usually about ¹⁄₁₀₀₀ of the whole, but the amount -was increased to about ¹⁄₂₀₀ if the active wire was washed in -water and dried over a gas flame—the method often adopted to -free the wire of any trace of the radium emanation. This effect of -producing activity was most marked immediately after removal of -the wire from the emanation, and was almost inappreciable ten -minutes afterwards.</p> - -<p class='c006'><span class='pageno' id='Page_392'>392</span>The effect was particularly noticeable in some experiments -with a copper plate, which was made active by leaving it a short -time in a solution of the active deposit from radium. This active -solution was obtained by placing an active platinum wire in -dilute hydrochloric acid. On placing the copper plate in a testing -vessel for a few minutes, and then removing it, activity was -observed on the walls of the vessel amounting to about one per -cent. of the activity of the copper plate.</p> - -<p class='c006'>It was found that this effect was not due to the emission of an -emanation from the active body, but must be ascribed to a slight -volatility of radium B at ordinary temperatures. This was proved -by observations on the variation of the activity of the matter -deposited on the walls of the vessel. The activity was small at -first, but rose to a maximum after about 30 minutes, and then -decayed with the time. The curve of rise was very similar to that -shown in Fig. 87, and shows that the inactive matter radium B -was carried to the walls and there changed into C, which gave rise -to the radiation observed.</p> - -<p class='c006'>The product B only escapes from the body for a short time -after removal. This is a strong indication that its apparent -volatility is connected with the presence of the rapidly changing -product radium A. Since A breaks up with an expulsion of an α -particle, some of the residual atoms constituting radium B may -acquire sufficient velocity to escape into the gas, and are then -transferred by diffusion to the walls of the vessel.</p> - -<p class='c006'>Miss Brooks observed that the activity was not concentrated -on the negative electrode in an electric field but was diffused -uniformly over the walls of the vessel. This observation is of -importance in considering the explanation of the anomalous effects -exhibited by the active deposit of radium, which will be discussed -in the following section.</p> -<p class='c005'><a id='section228'></a> -<b>228. Effect of the first rapid change.</b> We have seen that -the law of decay of activity, measured by the β or γ rays, can be -explained very satisfactorily if the first 3-minute change is disregarded. -The full theoretical examination of the question given -in sections <a href='#section197'>197</a> and <a href='#section198'>198</a> and the curves of Figs. <a href='#fig072'>72</a> and <a href='#fig073'>73</a> show, -however, that the presence of the first change should exercise an -<span class='pageno' id='Page_393'>393</span>effect of sufficient magnitude to be detected in measurements of -the activity due to the succeeding changes. The question is of -great interest, for it involves the important theoretical point -whether the substances A and B are produced independently of -one another, or whether A is the parent of B. In the latter case, -the matter A which is present changes into B, and, in consequence, -the amount of B present after A is transformed should be somewhat -greater than if B were produced independently. Since the -change of A is fairly rapid, the effect should be most marked in -the early part of the curve.</p> - -<p class='c006'>In order to examine this point experimentally, the curve -of rise of activity, measured by the β rays, was determined -immediately after the introduction of a large quantity of the -radium emanation into a closed vessel. The curve of decay of -activity on a body for a long exposure after removal of the -emanation, and the rise of activity after the introduction of the -emanation, are in all cases complementary to one another. While, -however, it is difficult to measure with certainty whether the -activity has fallen in a given time, for example, from 100 to 99 or -98·5, it is easy to be sure whether the corresponding rise of -activity in the converse experiment is 1 or 1·5 per cent. of the -final amount. <a href='#fig092'>Fig. 92</a>, curve I, shows the rise of activity -(measured by the β rays) obtained for an interval of 20 minutes -after the introduction of the emanation. The ordinates represent -the percentage amount of the final activity regained at any time.</p> - -<p class='c006'>Curve III shows the theoretical curve obtained on the -assumption that A is a parent of B. This curve is calculated -from equation (9) discussed in <a href='#section198'>section 198</a>, and -λ<sub>1</sub>, λ<sub>2</sub>, λ<sub>3</sub> are the -values previously found.</p> - -<p class='c006'>Curve II gives the theoretical activity at any time on the -assumption that the substances A and B arise independently. -This is calculated from an equation of the same form as (8), -<a href='#section198'>section 198</a>.</p> - -<div id='fig092' class='figcenter id004'> -<img src='images/fig-092.png' alt='Fig. 92.' class='ig001'> -<div class='ic002'> -<p>Fig. 92.</p> -</div> -</div> - -<p class='c006'>It is seen that the experimental results agree best with the -view that A and B arise independently. Such a conclusion, -however, is of too great importance to be accepted before -examining closely whether the theoretical conditions are fulfilled -in the experiments. In the first place, it is assumed that the -<span class='pageno' id='Page_394'>394</span>carriers which give rise to excited activity are deposited on -the surface of the body, to be made active immediately after their -formation. There is some evidence, however, that some of these -carriers exist for a considerable interval in the gas before their -deposit on the body. For example, it is found that if a body is -introduced for a short interval, about 1 minute, into a vessel -containing the radium emanation, which has remained undisturbed -for several hours, the activity after the first rapid decay -(see <a href='#fig086'>Fig. 86</a>, curve <i>B</i>) is in much greater proportion than if an -electric field had been acting for some time previously. This -result indicates that the carriers of B and C both collect in the -gas and are swept to the electrode when an electric field is -applied. I have also observed that if radium emanation, which -has stood undisturbed for some time, is swept into a testing -vessel, the rise curve is not complementary to the decay curve, -but indicates that a large amount of radium B and C was present -with the emanation. The experiments of Miss Brooks, previously -<span class='pageno' id='Page_395'>395</span>referred to, indicate that radium B does not obtain a charge and -so will remain in the gas. Dr. Bronson, working in the laboratory -of the writer, has obtained evidence that a large amount of -radium D remains in the gas even in a strong electric field. -If the matter B exists to some extent in the gas, the difference -between the theoretical curves for three successive changes would -be explained; for, in transferring the emanation to another vessel, -the matter B mixed with it would commence at once to change -into C and give rise to a part of the radiation observed.</p> - -<p class='c006'>The equal division of the activity between the products A and -C (see <a href='#fig090'>Fig. 90</a>) supports the view that C is a product of A, for -when radio-active equilibrium is reached, the number of particles -of A changing per second is equal to the number of B or C -changing per second. If each atom of A and C expels an α -particle of the same mass and with the same average velocity, the -activity due to the matter A should be equal to that due to the -matter C; and this, as we have seen, is the case.</p> - -<p class='c006'>While it is a matter of great difficulty to give a definite -experimental proof that radium A and B are consecutive products, -I think there is little doubt of its correctness. Accurate determinations -of the curves of rise and decay may throw further light -on the complicated processes which undoubtedly occur between -the breaking up of the atoms of the emanation and the appearance -of the active deposit on the electrodes.</p> -<p class='c005'><a id='section229'></a> -<b>229. Relative activity supplied by the α-ray products -of radium.</b> There are four products in radium which give out -α rays, viz. radium itself, the emanation, radium A and C. If -these products are in radio-active equilibrium, the same number -of particles of each product are transformed per second and, if -each atom breaks up with the emission of one α particle, the -number of α particles expelled per second should be the same -for each product.</p> - -<p class='c006'>Since, however, the α particles from the different products -are not projected with the same velocity, the activity, measured -by the ionization current in the usual manner, will not be the -same for all products. The activity, when measured by the -saturation current between parallel plates at sufficient distance -<span class='pageno' id='Page_396'>396</span>apart to absorb all the α rays in the gas, is proportional to the -energy of the α particles escaping into the gas.</p> - -<p class='c006'>It has been shown that the minimum activity of radium after -removal of the emanation, measured by the α rays, is 25 per cent. -of the maximum value. The remaining 75 per cent. is due to the -α particles from the other products. Now the activity supplied by -radium A and C is nearly the same (<a href='#section228'>section 228</a>). If the emanation -is introduced into a cylindrical vessel about 5 cms. in -diameter, the activity increases to about twice its initial value -owing to the deposit of radium A and C on the surface of the -vessel. This shows that the activity of the emanation is of about -the same magnitude as that supplied by radium A or C, but an -accurate comparison is beset with difficulty, for the emanation -is distributed throughout the gas, while radium A and C are -deposited on the walls of the vessel. In addition, the relative -absorption of the emanation compared with that of radium A and -C is not known.</p> - -<p class='c006'>The writer has made some experiments on the decrease of -activity of radium immediately after heating to a sufficient -temperature to drive off the emanation. The results obtained by -this method are complicated by the alteration of the radiating -surface in consequence of the heating, but indicate that the -emanation supplies about 70 per cent. of the activity of radium -A or C.</p> - -<p class='c006'>This points to the conclusion that the α particles from the -emanation are projected with less velocity than those from -radium C.</p> - -<p class='c006'>The following table shows approximately the activity supplied -by the different products of radium in radio-active equilibrium.</p> - -<table class='table14' > -<colgroup> -<col class='colwidth50'> -<col class='colwidth50'> -</colgroup> - <tr> - <th class='c013'>Product</th> - <th class='c014'>Percentage proportion of total activity</th> - </tr> - <tr> - <td class='c013'>Radium</td> - <td class='c014'>25 per cent.</td> - </tr> - <tr> - <td class='c013'>Emanation</td> - <td class='c014'>17 „</td> - </tr> - <tr> - <td class='c013'>Radium A</td> - <td class='c014'>29 „</td> - </tr> - <tr> - <td class='c013'>Radium B</td> - <td class='c014'>0 „</td> - </tr> - <tr> - <td class='c013'>Radium C</td> - <td class='c014'>29 „</td> - </tr> -</table> - -<p class='c006'>The products of radium and their radiation are graphically -shown later in <a href='#fig095'>Fig. 95</a>.</p> -<p class='c005'><span class='pageno' id='Page_397'>397</span><b>230. Active deposit of radium of slow transformation.</b> -It has been pointed out (<a href='#section183'>section 183</a>) that a body, exposed in the -presence of the radium emanation, does not lose all its activity for -a long time after removal; a small residual activity is always -observed. The magnitude of this residual activity is dependent -not only upon the amount of emanation employed, but also upon -the time of exposure of the body in the presence of the emanation. -For an exposure of several hours in the presence of the emanation, -the residual activity is less than one-millionth of the activity -immediately after removal.</p> - -<p class='c006'>An account will now be given of some investigations made by -the writer<a id='r319' href='#f319' class='c012'><sup>[319]</sup></a> on the nature of this residual activity and the chemical -properties of the active matter itself. It is first of all necessary to -show that the residual activity arises in consequence of a deposit -of radio-active matter, and is not due to some action of the intense -radiations to which the body made active has been subjected.</p> - -<p class='c006'>The inside of a long glass tube was covered with equal areas -of thin metal, including aluminium, iron, copper, silver, lead, and -platinum. A large amount of radium emanation was introduced -into the tube, and the tube closed. After seven days the metal -plates were removed, and, after allowing two days to elapse for the -ordinary excited activity to disappear, the residual activity of the -plates was tested by an electrometer. The activity of the plates -was found to be unequal, being greatest for copper and silver, and -least for aluminium. The activity of copper was twice as great as -that of aluminium. After standing for another week the activity -of the plates was again tested. The activity of each had diminished -in the interval to some extent, but the initial differences observed -had to a large extent disappeared. After reaching a minimum -value the activity of each plate slowly but steadily increased at -the same rate. After a month’s interval the activity of each of -the plates was nearly the same, and more than three times the -minimum value. The initial irregularities in the decay curves of the -different metals are, in all probability, due to slight but different -degrees of absorption of the radium emanation by the metal plates, -the absorption being greatest for copper and silver and least for -<span class='pageno' id='Page_398'>398</span>aluminium. As the occluded emanation was slowly released or -lost its activity, the activity of the metal fell to a limiting value. -The absorption of the radium emanation by lead, paraffin, and -caoutchouc has been noticed by Curie and Danne (<a href='#section182'>section 182</a>).</p> - -<p class='c006'>The residual activity on the plates comprised both α and β -rays, the latter being present, in all cases, in a very unusual -proportion. The equality of the activity and the identity of the -radiation emitted from each plate show that the residual activity -is due to changes of some form of matter deposited on the plates, -and that it cannot be ascribed to an action of the intense radiations; -for if such were the case, it would be expected that the -activity produced on the different plates would vary not only in -quantity, but also in quality. This result is confirmed by the -observation that the active matter can be removed from a platinum -plate by solution in sulphuric acid, and has other distinctive -chemical and physical properties.</p> - -<p class='c006'>The variation with time of the residual activity measured by -the α rays will first be considered. A platinum plate was exposed -in the presence of the radium emanation for seven days. The -amount of emanation initially present was equal to that obtained -from about 3 milligrams of pure radium bromide. The plate -immediately after removal gave a saturation-current, measured -between parallel plates by a galvanometer, of 1·5 × 10<sup>-7</sup> ampere. -Some hours after removal, the activity decayed according to an -exponential law with the time, falling to half value in 28 minutes. -Three days after removal the active plate gave a saturation-current, -measured by an electrometer, of 5 × 10<sup>-13</sup> ampere; <i>i.e.</i> ¹⁄₃₀0,000 -of the initial activity. The activity was observed to increase -steadily with the time. The results are shown in <a href='#fig093'>Fig. 93</a>, where -the time is reckoned from the middle of the time of exposure to -the emanation.</p> - -<p class='c006'>The curve is initially nearly a straight line passing through -the origin. The activity increases with the time for the interval -of eight months over which the observations have extended. The -latter portions of the curve, however, fall below the tangent to the -curve drawn through the origin, showing that the activity is not -increasing proportionately with the time.</p> - -<p class='c006'>The active deposit, obtained in a different manner, has been -<span class='pageno' id='Page_399'>399</span>examined for a still longer period. The emanation from 30 milligrams -of radium bromide was condensed in a glass tube and then -sealed. After a month’s interval, the tube was opened and dilute -sulphuric acid introduced. The acid dissolved off the active deposit -in the tube and on driving off the acid by heat, a radio-active -residue was obtained. The activity of this residue, measured by -the α rays, steadily increased for a period of 18 months, but the -curve of variation of activity with time plotted as in <a href='#fig093'>Fig. 93</a> tends -to become more flattened, and is obviously approaching a maximum -value.</p> - -<div id='fig093' class='figcenter id004'> -<img src='images/fig-093.png' alt='Fig. 93.' class='ig001'> -<div class='ic002'> -<p>Fig. 93.</p> -</div> -</div> - -<p class='c006'>The explanation of this curve will be considered later in -<a href='#section236'>section 236</a>.</p> -<p class='c005'><b>231. Variation of the β ray activity.</b> The residual -activity consists of both α and β rays, the latter being present -initially in an unusually large proportion. The proportion of α to -β rays from the platinum plate, one month after removal, was at -the most one-fiftieth of that from a thin film of radium bromide -in radio-active equilibrium. Unlike the α ray activity, the activity -measured by the β rays remains constant after the active deposit -is about one month old, and, in consequence, the proportion of -α to β rays steadily increases with the time. The experiments -<span class='pageno' id='Page_400'>400</span>showed that the intensity of the β rays did not vary much, if -at all, over a further period of eighteen months. The want of -proportionality between the α and β rays shows that the two -types of rays arise from different products. This conclusion is -confirmed by experiments, to be described later, which show that -the products giving rise to α and β rays can be temporarily -separated from one another by physical and chemical means.</p> - -<div id='fig094' class='figcenter id004'> -<img src='images/fig-094.png' alt='Fig. 94.' class='ig001'> -<div class='ic002'> -<p>Fig. 94.</p> -</div> -</div> - -<p class='c006'>If observations of the active deposit are begun shortly after its -formation, it is found that the activity, measured by the β rays, is -small at first, but increases with the time, reaching a practical -maximum about 40 days later. Experiments were made on a -platinum plate, which was exposed for 3·75 days in a vessel -containing the radium emanation. The observations of the β ray -activity began 24 hours after removal. The results are shown in -<a href='#fig094'>Fig. 94</a>, where the time was measured from the middle of the time -of exposure to the emanation. Similar results were obtained for -a negatively charged wire exposed to the emanation. The curve, -if produced back to the origin, is seen to be very similar to the -recovery curves of Ur X, and other active products, and can be -expressed by the equation</p> - -<div class='figcenter id010'> -<img src='images/form-055.png' alt='Formula.' class='ig001'> -</div> - -<p class='c006'>where <i>I</i>₀ is the maximum -<span class='pageno' id='Page_401'>401</span>activity. The activity reaches half its final value in about six -days, and the value of λ is equal to ·115 (day)<sup>-1</sup>. We have shown -in <a href='#section203'>section 203</a> that a rising curve of this character indicates that -the β ray activity arises from a product which is supplied at a -constant rate from a primary source. Before discussing in detail -the explanation of these curves, showing the rise with time of -the α and β ray activity, further experimental results will be -considered.</p> -<p class='c005'><a id='section232'></a> -<b>232. Effect of temperature on the activity.</b> A platinum -plate, made active in the manner described, was exposed to varying -temperatures in an electric furnace, and the activity tested at -atmospheric temperature after exposure. Four minutes’ exposure -in the furnace, at first at 430° C., and afterwards at 800° C., had -little, if any, effect on the activity. After four minutes at about -1000° C. the activity decreased about 20 per cent., and a further -exposure of eight minutes at a temperature of about 1050° C. -almost completely removed the α ray activity. On the other hand, -the β ray activity, when measured immediately after removal, was -not altered by the heating, but exposure to a still higher temperature -caused it to decrease. These results show that the active -matter consists of two kinds. The part which emits β rays is not -volatile at 1000° C., but the other part, which emits α rays, is almost -completely volatilized at that temperature.</p> - -<p class='c006'>It was found, however, that the β ray activity after heating -to about 1000° was not permanent, but decayed according to an -exponential law with the time, the activity decreasing to half -value in about 4·5 days. From the recovery curve of the β ray -activity already considered, it was to be expected that the activity -would decay to half value in six days. This difference in the -periods is possibly due to an effect of the high temperature in -altering the rate of decay of radium E. The period of six days is -more probably correct. The results obtained on the rise and decay -of the β rays, taken together, show:—</p> - -<p class='c021'>(1) That the product giving β rays is supplied at a constant -rate from some parent matter of very slow rate of change.</p> - -<p class='c011'>(2) That this parent matter is volatilized at or below 1000° C., -and the β ray product is left behind. Since the parent -<span class='pageno' id='Page_402'>402</span>matter is removed, the product immediately begins to -lose its activity at its characteristic rate, viz. the activity -falls to half value in about six days.</p> -<p class='c005'><b>233. Separation of the constituents by means of a -bismuth plate.</b> The active matter of slow decay was obtained -in solution by introducing dilute sulphuric acid into a glass tube -in which the emanation from 30 milligrams of radium bromide -had been stored for a month. The solution showed strong activity -and gave out both α and β rays, the latter, as in other cases, being -present in an unusually large proportion.</p> - -<p class='c006'>When a polished bismuth disk was kept for some hours in the -solution, it became strongly active. The active matter deposited -on the bismuth gave out α rays, but no trace of β rays. After -several bismuth disks had been successively left in the solution, -the active matter, which emits α rays, was almost completely -removed. This was shown by evaporating down the solution after -treatment. The β ray activity remained unchanged, but that of -the α rays had been reduced to about 10 per cent. of its original -value. Three bismuth disks, made active in this way, were set -aside and their activity measured at regular intervals. The -activity fell off according to an exponential law with the time -during the 200 days since their removal, while that of each fell to -half value on an average in about 143 days.</p> - -<p class='c006'>At the same time it was observed that the solution, from -which the α ray activity was removed, gradually regained its -activity, showing that the active substance which gave out α rays -was continuously produced from the matter left behind in the -solution.</p> -<p class='c005'><b>234. Explanation of the results.</b> We have seen that a -close examination of the active deposit of slow change has disclosed,</p> - -<p class='c021'>(1) the presence of a β ray product which loses half of its -activity in about six days;</p> - -<p class='c011'>(2) the presence of an α ray product, which is deposited on -bismuth and is volatilized at 1000° C. This product -loses half of its activity in 143 days;</p> - -<p class='c011'>(3) the presence of a parent substance, which produces the -β ray product at a constant rate.</p> - -<p class='c018'><span class='pageno' id='Page_403'>403</span>This parent product must be transformed very slowly since the -β ray product, which arises from it, soon reaches an equilibrium -value, which does not change appreciably over a period of more -than one year. The experimental evidence points to the conclusion -that the parent product does not give rise to β rays, but that the -β rays arise entirely from the next product. This parent product -cannot give rise to α rays, for we have seen that the initial α ray -activity is at first extremely small, but increases steadily with the -time for a period of at least eighteen months. Thus the parent -product does not give rise to either α or β rays, and must be a -“rayless” product.</p> - -<p class='c006'>The first three transition products of the radium emanation, -viz. radium A, B and C, have already been analysed, and shown to -be consecutive. It thus seems probable that the active deposit of -slow change must arise from the successive transformations of the -last product radium C. The results already obtained can be completely -explained if it is supposed that three transition products, -viz. radium D, E and F, are present in the active deposit of slow -rate of change. The properties of these products are summarized -below.</p> - -<p class='c021'><i>Radium D</i> is a rayless product of very slow rate of change. -It will be shown later that it is half transformed in -about 40 years. It is volatile below 1000° C. and is -soluble in strong acids.</p> - -<p class='c011'><i>Radium E</i> is produced from radium D. In breaking up, it -emits β (and probably γ) rays but no α rays. It is half -transformed in about 6 days and is not so volatile as -radium D and F.</p> - -<p class='c011'><i>Radium F</i> is produced from radium E. It emits only α rays -and is half transformed in 143 days. This substance in -solution attaches itself to bismuth. It is volatile at -about 1000° C.</p> - -<p class='c018'>Apart from their value and interest in showing the stages of -transformation of the radium atom, the results of this analysis -have an important bearing upon the origin of some of the well-known -radio-active substances separated from pitchblende; for it -will be shown later that the product radium F is the radio-active -substance present in radio-tellurium and probably also in polonium. -<span class='pageno' id='Page_404'>404</span>In addition, there is very strong evidence that the radio-active lead -obtained by Hofmann contains the three products radium D, E -and F together.</p> - -<p class='c006'>The changes of radium as far as they are at present known, are -shown diagrammatically in <a href='#fig095'>Fig. 95</a>. It is possible that further -investigation will show that the transformation does not end with -radium F.</p> - -<div id='fig095' class='figcenter id008'> -<img src='images/fig-095.png' alt='Fig. 95.' class='ig001'> -<div class='ic002'> -<p>Fig. 95.</p> -</div> -</div> - -<p class='c006'>While we have shown that radium D is the parent of E, we -have not given any conclusive evidence that E is the parent of F. -This evidence is, however, supplied by the following experiment. -A platinum plate, made active in the manner already described, -was placed in an electric furnace and heated for four minutes at -about 1000° C. Most of the products D and F were volatilized, -but E was left behind. Since the parent matter D was removed, -E at once commenced to lose its β ray activity. At the same time -it was observed that the small α ray activity, left behind on the -platinum plate, increased rapidly at first and then more slowly, as -the activity of E became smaller and smaller. This experiment -shows conclusively that E was the parent of F, the α ray product.</p> -<p class='c005'><b>235. Rate of transformation of radium D.</b> It has been -observed experimentally that each of the products of radium, -which emit α rays, supplies about an equal proportion of the -activity of radium when in radio-active equilibrium. Since, when -equilibrium is reached, the same number of particles of each of -the successive products must break up per second, this is an -expression of the fact that every atom of each product breaks up -with the expulsion of an equal number (probably one) of α particles. -Now radium D is directly derived from radium C, and, since the -rate of change of D is very slow compared with that of C, the -number of particles of D initially present must be very nearly -equal to the number of particles of radium C which break up -<span class='pageno' id='Page_405'>405</span>during the time that radium D is being formed. Now D does -not itself give out rays, but the succeeding product E does. The -products D and E are practically in radio-active equilibrium one -month after D is set aside, and the variation of the β ray activity -of E then serves as a measure of the variation of the parent -product D. Suppose that a vessel is filled with a large quantity -of radium emanation. After several hours, the product radium C, -which emits β rays, reaches a maximum value, and then decreases -at the same rate as the emanation loses its activity, <i>i.e.</i> it falls -to half value in 3·8 days. If <i>N</i><sub>1</sub> is the number of β particles -expelled from radium C at its maximum value, the total number -<i>Q</i><sub>1</sub> of β particles expelled during the life of the emanation is given -approximately by</p> - -<div class='figcenter id002'> -<img src='images/form-129.png' alt='Formula.' class='ig001'> -</div> - -<p class='c006'>where λ<sub>1</sub> is the constant of change of the emanation.</p> - -<p class='c006'>After the emanation has disappeared, and the final products -D + E are in radio-active equilibrium, suppose that the number of -β particles <i>N</i><sub>2</sub> expelled per second by radium E is determined. -If <i>Q</i><sub>2</sub> is the total number of particles expelled during the life of -D + E, then <i>Q</i><sub>2</sub> as before is approximately given by <i>Q</i><sub>2</sub> = <i>N</i><sub>2</sub>/λ<sub>2</sub> where -λ<sub>2</sub> is the constant of change of radium D. Now we have seen that -if each particle of C and of E gives rise to one β particle, it is to -be expected that</p> - -<div class='calc'> - -<div class='lg-container-b c017'> - <div class='linegroup'> - <div class='group'> - <div class='line'><i>Q</i><sub>1</sub> = <i>Q</i><sub>2</sub>,</div> - </div> - <div class='group'> - <div class='line'>or</div> - <div class='line in3'>λ<sub>2</sub> <i>N</i><sub>2</sub></div> - <div class='line in2'>---- = ---- .</div> - <div class='line in3'>λ<sub>1</sub> <i>N</i><sub>1</sub></div> - </div> - </div> -</div> - -</div> - -<p class='c018'>The ratio -<i>N</i><sub>2</sub>/<i>N</i><sub>1</sub> -was determined by measuring the activity due to -the β rays from C and E in the same testing-vessel. Then, since -<i>N</i><sub>2</sub>/<i>N</i><sub>1</sub> is known, and also the value of λ<sub>1</sub>, -the value of the constant of -change, λ<sub>2</sub>, of radium D is obtained. In this way it was calculated -that D is half transformed in about 40 years.</p> - -<p class='c006'>In the above calculations it is assumed, as a first approximation, -that the β rays from C and E have the same average velocity. -This is probably not accurately the case, but the above number -certainly serves to fix the order of magnitude of the period of the -<span class='pageno' id='Page_406'>406</span>product D. This calculation is confirmed by observations to be -given later on the amount of D and E in old radium.</p> - -<p class='c006'>It may be of interest to mention that the writer calculated the -period of radium F by a similar method, before its value was -experimentally determined, and found that F should be half -transformed in about one year. This is not very different from -the experimental value of 143 days found later. In addition, it -was assumed in the calculation that the α particles from C and F -were projected with the same velocity, and in consequence produced -the same amount of ionization. In practice, however, it is -found that the α particle of F is absorbed in about half the -distance of the α particles of C, and in consequence produces -only about half of the ionization of the latter. If this correction -were made, the calculated period for half transformation would be -six months instead of one year.</p> - -<p class='c006'>A table of the transformation products of radium, together -with some of their physical and chemical properties, is given -below.</p> - -<table class='table28' > -<colgroup> -<col class='colwidth26'> -<col class='colwidth19'> -<col class='colwidth26'> -<col class='colwidth26'> -</colgroup> - <tr> - <th class='c013'>Transformation Products</th> - <th class='c013'>Time to be half transformed</th> - <th class='c013'>Rays</th> - <th class='c014'>Chemical and Physical Properties</th> - </tr> - <tr> - <td class='c013'> </td> - <td class='c013'> </td> - <td class='c013'> </td> - <td class='c014'> </td> - </tr> - <tr> - <td class='c013'>Radium</td> - <td class='c013'>1200 years</td> - <td class='c013'>α rays</td> - <td class='c014'>—</td> - </tr> - <tr> - <td class='c013'>Emanation</td> - <td class='c013'>3·8 days</td> - <td class='c013'>α rays</td> - <td class='c014'>Chemically inert gas; condenses at -150° C.</td> - </tr> - <tr> - <td class='c013'>Radium A (active deposit of rapid change)</td> - <td class='c013'>3 mins.</td> - <td class='c013'>α rays</td> - <td class='c014'>Behaves as solid; deposited on the surface of bodies; concentrated on cathode in electric field. Soluble in strong acids; volatile at a white heat. B is more volatile than A or C.</td> - </tr> - <tr> - <td class='c013'>:: B (same)</td> - <td class='c013'>21 mins.</td> - <td class='c013'>no rays</td> - <td class='c014'>Same</td> - </tr> - <tr> - <td class='c013'>:: C (same)</td> - <td class='c013'>28 mins.</td> - <td class='c013'>α, β, γ rays</td> - <td class='c014'>Same</td> - </tr> - <tr> - <td class='c013'>:: D (active deposit of slow change)</td> - <td class='c013'>about 40 years</td> - <td class='c013'>no rays</td> - <td class='c014'>Soluble in strong acids and volatized below 1000° C.</td> - </tr> - <tr> - <td class='c013'>:: E (same)</td> - <td class='c013'>6 days</td> - <td class='c013'>β (and γ)</td> - <td class='c014'>Non-volatile at 1000°C.</td> - </tr> - <tr> - <td class='c013'>:: F (same)</td> - <td class='c013'>143 days</td> - <td class='c013'>α rays</td> - <td class='c014'>Volatile at 1000° C; deposited from solution on to bismuth plate.</td> - </tr> - <tr> - <td class='c013'>?</td> - <td class='c013'>—</td> - <td class='c013'>—</td> - <td class='c014'>—</td> - </tr> -</table> -<p class='c005'><span class='pageno' id='Page_407'>407</span><a id='section236'></a> -<b>236. Variation of the activity over long periods of -time.</b> We are now in a position to calculate the variation of the -α and β ray activity of the active deposit over long periods of -time. If it is supposed that the matter initially deposited consists -only of D, the amounts <i>P</i>, <i>Q</i> and <i>R</i> of radium D, E -and F existing at any later time are given by the equations -3, 4, 5, <a href='#section197'>section 197</a>.</p> - -<p class='c006'>Since, however, the intermediate product E has a much more -rapid rate of change than D or F, the equations can be simplified, -without much loss of accuracy, by disregarding the change E, and -by supposing that D gives out β rays and changes directly into -the α ray product F.</p> - -<p class='c006'>Let λ<sub>1</sub>, λ<sub>2</sub> be the constants of change D and F respectively. Let <i>n</i>₀ -be the number of particles of D present initially. Then -using the notation of <a href='#section197'>section 197</a>, the amount <i>P</i> of radium D at -any time <i>t</i> is given by</p> - -<div class='figcenter id009'> -<img src='images/form-130.png' alt='Formula.' class='ig001'> -</div> - -<p class='c006'>The amount <i>Q</i> of radium F is -given by</p> - -<div class='figcenter id005'> -<img src='images/form-131.png' alt='Formula.' class='ig001'> -</div> - -<div id='fig096' class='figcenter id004'> -<img src='images/fig-096.png' alt='Fig. 96.' class='ig001'> -<div class='ic002'> -<p>Fig. 96.</p> -</div> -</div> - -<p class='c006'>The number of β particles emitted by D + E per second, some -<span class='pageno' id='Page_408'>408</span>months afterwards, is</p> - -<div class='figcenter id009'> -<img src='images/form-132.png' alt='Formula.' class='ig001'> -</div> - -<p class='c006'>and the number of α particles -emitted by radium F is</p> - -<div class='figcenter id005'> -<img src='images/form-133.png' alt='Formula.' class='ig001'> -</div> - -<p class='c006'>The results are shown graphically in <a href='#fig096'>Fig. 96</a>, by the curves <i>EE</i> -and <i>FF</i>, in which the ordinates represent the number of β and α -particles expelled per second by the products D and F respectively. -The complete calculation for three changes shows that the number -of β particles soon reaches a practical maximum, and then decays -nearly exponentially with the time, falling to half value in 40 years. -The number of α particles expelled per second increases for several -years, but reaches a maximum after 2·6 years and then diminishes, -finally falling off exponentially with the time to half value in -40 years.</p> - -<p class='c006'>The experimental curve of the rise of α ray activity, shown in -<a href='#fig093'>Fig. 93</a>, as far as it has been determined, lies accurately on this -curve, if the maximum is calculated from the above theory. The -observed activity after a period of 250 days is marked by the -point <i>X</i> on the curve.</p> -<p class='c005'><b>237. Experiments with old radium.</b> Since the substance -radium D is produced from radium at a constant rate, the amount -present mixed with the radium will increase with its age. The -writer had in his possession a small quantity of impure radium -chloride, kindly presented by Professors Elster and Geitel four -years before. The amount of radium D present in it was tested -in the following way:—The substance was dissolved in water and -kept continuously boiling for a period of about six hours. Under -these conditions the emanation is removed as rapidly as it is -formed, and the β rays from the radium, due to the product -radium C, practically disappear. A newly prepared specimen of -radium bromide under these conditions retains only a fraction of -1 per cent. of its original β radiation. The old radium, however, -showed (immediately after this treatment) an activity measured -by the β rays of about 8 per cent. of its original amount. The -activity could not be reduced any lower by further boiling or -aspiration of air through the solution. This residual β ray activity -was due to the product radium E stored up in the radium. The -<span class='pageno' id='Page_409'>409</span>β ray activity due to radium E was thus about 9 per cent. of that -due to radium C. Disregarding the differences in the absorption -of the β rays, when the activity of the product E in radium -reaches a maximum value, the β ray activity due to it should be -the same as that due to C. Since the parent product D is half -transformed in forty years, the amount present in the radium -after four years should be about 7 per cent. of the maximum -amount; <i>i.e.</i> it should show a β ray activity of about 7 per cent. -of that due to radium C. The observed and calculated values -(7 and 9 per cent. respectively) are thus of the same order of -magnitude. The amount of β rays from radium E present in -pure radium bromide about one year old was about 2 per cent. of -the total.</p> - -<p class='c006'>The amount of radium F present in old radium was measured -by observations of the activity imparted to a bismuth disk left for -several days in the solution, and was found to be of the same order -as the theoretical value. Radium F is not deposited to an -appreciable extent on the bismuth from a water solution of radium -bromide. If, however, a trace of sulphuric acid is added to the -solution, the radium F is readily deposited on the bismuth. The -addition of sulphuric acid to the radium solution practically effected -a separation of radium D, E and F from the radium proper; for -the latter was precipitated as sulphate and the products D, E and F -remained in solution. After filtering, the solution contained the -greater proportion of the products D, E, and F and very little radium.</p> -<p class='c005'><a id='section238'></a> -<b>238. Variation of the activity of radium with time.</b> -It has been shown that the activity of freshly prepared radium -increases at first with the time and practically reaches a maximum -value after an interval of about one month. The results already -considered show that there is a still further slow increase of -activity with the time. This is the case whether the activity is -measured by the α or β rays. It will be shown later that radium -is probably half transformed in about 1000 years. From this it -can readily be calculated that after a lapse of about 200 years the -amount of the products radium D, E and F will have reached a -maximum value. The same number of atoms of each of the -products C and E will then break up per second. If each atom -of these products in disintegrating throws off an equal number -<span class='pageno' id='Page_410'>410</span>(probably one) of β particles, the number of β particles thrown -off per second will be twice as great as from radium a few months -old. The number will increase at first at the rate of about 2 per -cent. a year.</p> - -<p class='c006'>Similar considerations apply to the α ray activity. Since, however, -there are four other products of radium besides radium itself -which expel α particles, the number of α particles emitted per -second from old radium will not be more than 25 per cent. greater -than the number from radium a few months old. The activity -measured by the α rays will thus not increase more than 25 per -cent., and probably still less, as the α particles from radium F -produce less ionization than the α particles expelled from the other -radium products. The activity of radium will consequently rise to -a maximum after 200 years and then slowly die away with the time.</p> -<p class='c005'><a id='section239'></a> -<b>239. Presence of these products in pitchblende.</b> The -products radium D, E and F must be present in pitchblende in -amounts proportional to the quantity of radium present, and -should be capable of separation from the mineral by suitable -chemical methods. The radio-active properties of these substances, -if obtained in the pure state, are summarized below.</p> - -<p class='c006'><i>Radium D</i> when first separated, should give out very little -α or β radiation. The β ray activity will rapidly increase, reaching -half its maximum value in 6 days. The α ray activity will at first -increase nearly proportionately with the time, and will reach a -maximum value after an interval of about 3 years. The α and β -ray activity, after reaching a maximum, will finally decay, the -activity falling to half value in about 40 years. Since radium D -is half transformed in 40 years, and radium in 1200 years, the -maximum β ray activity of radium D, weight for weight, will be -about 300 times that of radium.</p> - -<p class='c006'>The α ray activity, at any time, will be removed by placing a -bismuth disk in the solution.</p> - -<p class='c006'><i>Radium F</i>, after separation, will give out only α rays. Its -activity, after separation, will decrease according to an exponential -law, falling to half value in 143 days. Since radium in radio-active -equilibrium contains four products which emit α rays, the number -of α particles expelled per second from radium F will, weight for -weight, be about 800 times as numerous as from new radium in -<span class='pageno' id='Page_411'>411</span>radio-active equilibrium. Since the α particles from radium F -produce only about half as much ionization as the α particles from -the other radium products, the activity of radium F, measured by -the electric method, will be about 400 times that of radium.</p> -<p class='c005'><b>240. Origin of radio-tellurium and polonium.</b> It is now -necessary to consider whether these products of radium have been -previously separated from pitchblende, and known by other names.</p> - -<p class='c006'>We shall first consider the α ray product, radium F. The -radio-tellurium of Marckwald and the polonium of Mme Curie both -resemble radium F in giving out only α rays, and in being deposited -on a bismuth disk from a solution. If the active constituent -present in radio-tellurium is the same as radium F, its activity -should decay at the same rate as the latter. The writer<a id='r320' href='#f320' class='c012'><sup>[320]</sup></a> has -carefully compared the rates of decay of the activity of radium F -and of the radio-tellurium of Marckwald and found them to be the -same within the limits of experimental error. Both lose half of their -activity in about 143 days<a id='r321' href='#f321' class='c012'><sup>[321]</sup></a>. A similar value of the rate of decay -of radio-tellurium has been obtained by Meyer and Schweidler<a id='r322' href='#f322' class='c012'><sup>[322]</sup></a>.</p> - -<p class='c006'>The experiments on radio-tellurium were made upon the active -bismuth plates supplied by Dr Sthamer of Hamburg, which were -prepared under Marckwald’s directions.</p> - -<p class='c006'>An additional proof<a id='r323' href='#f323' class='c012'><sup>[323]</sup></a> of the identity of these two products was -obtained by comparing the absorption of the α rays by aluminium -foil. The α rays from different products are projected with different -velocities, and, in consequence, are unequally absorbed by matter. -The absorption of the rays from the two products by aluminium -foil agreed very closely, indicating the probable identity of the -substances from which they were emitted.</p> - -<p class='c006'>There can thus be no doubt that the active constituent present -in the radio-tellurium of Marckwald is identical with the product -radium F. This is a very interesting result, and shows how the -close examination of the successive transformations of the radio-active -bodies may throw light on the origin of the various -substances found in pitchblende.</p> - -<p class='c006'><span class='pageno' id='Page_412'>412</span>We have already seen (<a href='#section021'>section 21</a>) that Marckwald, by special -chemical methods, was able to obtain a few milligrams of very -active substance by working over 2 tons of pitchblende. We have -already seen (<a href='#section239'>section 239</a>) that this substance, if obtained in the -pure state, should be about 400 times as active as radium. Comparative -measurements of the activity of this substance with -radium will thus indicate the amount of impurity that is present -with the former. This method should be of value in purifying -radium F for the purpose of determining its spectrum, which has -not yet been observed.</p> -<p class='c005'><b>241. Polonium.</b> Since the separation of the active substance -by Marckwald, called by him radio-tellurium, there has been some -discussion as to whether the active constituent is the same as that -present in the polonium of Mme Curie. Both of these substances -have similar radio-active and chemical properties, but the main -objection to the view that the active constituents were identical -has rested on an early statement of Marckwald that the -activity of one of his very active preparations did not decay -appreciably in the course of six months. This objection is now -removed, for we have seen that the activity of radio-tellurium does -decay fairly rapidly. It was early recognised that the activity of -the polonium, separated from pitchblende by the methods of -Mme Curie, was not permanent, but decayed with the time. -Observations on the rate of decay have not been very precise, but -Mme Curie states that some of her preparations lost half of their -activity in about six months but in others the rate of decay was somewhat -smaller. It is possible that the initial differences observed in -the rates of decay of different specimens of polonium may be due -to the presence of some radium D with the polonium. The -polonium in my possession lost its activity fairly rapidly, and was -reduced to a small portion of its value in the course of about four -years. Rough observations of its activity, made from time to time, -showed that its activity diminished to half value in about six -months. If it is identical with radio-tellurium, the activity should -decay to half value in 143 days, and I think there is little doubt -that more accurate measurement will prove this to be the case.</p> - -<p class='c006'>While the proof of the identity of the active constituent in -polonium is not so definite as for radio-tellurium, I think there can -<span class='pageno' id='Page_413'>413</span>be no reasonable doubt that these substances both contain the -same active substance, which is the seventh transformation product -of radium. Marckwald has noticed some chemical differences in -the behaviour of polonium and radio-tellurium, but little weight -can be attached to such observations, for it must be remembered -that the active constituent in both cases is present in minute -quantity in the material under examination, and that the apparent -chemical properties of the active substance are much influenced by -the presence of impurities. The most important and trustworthy test -rests upon the identity of the radiations and the period of decay.</p> -<p class='c005'><b>241 A. Origin of radio-active lead.</b> Some experiments -will now be discussed which show that the radio-lead first separated -from pitchblende by Hofmann (<a href='#section022'>section 22</a>) contains the products -radium D, E and F. Hofmann has observed that the activity of this -substance did not appreciably decay in the course of several years. -In some recent experiments, Hofmann, Gonder and Wölfl<a id='r324' href='#f324' class='c012'><sup>[324]</sup></a> have -made a close chemical examination of the radio-active lead, and have -shown the presence of two radio-active constituents, which are -probably identical with the products radium E and F. The radio-active -measurements were unfortunately not very precise, and the -periods of change of the separated products have not been examined -very closely.</p> - -<p class='c006'>Experiments were made on the effect of adding substances to -a solution of radio-lead, and then removing them by precipitation. -Small quantities of iridium, rhodium, palladium, and platinum, in -the form of chlorides, were left in the solution for three weeks, and -then precipitated by formalin or hydroxylamine. All of these -substances were found to give out both α and β rays, the activity -being greatest for rhodium and least for platinum. A large proportion -of the β ray activity disappeared in the course of six weeks, -and of the α ray activity in one year. It is probable that the two -products radium E and F were in part removed with the metals -from the radio-lead. We have seen that radium E gives out β rays -and loses half of its activity in about six days, while radium F -gives out only α rays and its activity falls to half value in 143 -days. This conclusion is further confirmed by experiments on the -effect of heat on the activity of these substances. By heating to a -full red heat, the α ray activity was lost in a few seconds. This is -<span class='pageno' id='Page_414'>414</span>in agreement with the results (<a href='#section232'>section 232</a>) where we have seen -that radium F is volatilized at about 1000° C. and radium E is left -behind.</p> - -<p class='c006'>Salts of gold, silver and mercury added to the radio-lead were -found to show only α ray activity on removal. This is in accordance -with the view that radium F alone is removed with these substances. -Bismuth salts on the other hand showed initially α and β -ray activity, but the latter rapidly died away. The presence of β -rays in freshly prepared polonium was early observed by Mme Curie. -The α and β ray activity of the radio-lead is much reduced by the -precipitation of bismuth added to the solution. The α and β ray -activity of the radio-lead, however, recovers itself again. This -result is exactly what is to be expected if radio-lead contains -radium D, E and F. Radium E and F are removed with the -bismuth, but the parent substance, radium D, is left behind, and, -in consequence, a fresh supply of radium E and F is produced.</p> - -<p class='c006'>While further experiments are required to settle definitely -whether the products separated from radio-lead are identical with -radium E and F, there can be little doubt that such is the case. -This conclusion is strengthened by some experiments which I have -made on a specimen of radio-lead, which was kindly forwarded to -me by Mr Boltwood of New Haven. This active lead gave out α -and β rays, the latter being in unusually large proportion. The -active lead was four months old when first tested. The β ray -activity in the following six months has remained sensibly constant, -but the α ray activity has steadily increased. These results are to -be expected if the radio-lead contains radium D. Radium E will -reach a practical maximum about 40 days after separation of the -product radium D with the lead. The α ray activity due to -radium F should increase to a maximum in about 2·6 years (see -section 236).</p> - -<p class='c006'>Further experiments are required to settle whether the lead -immediately after separation from pitchblende contains only -radium D, or whether radium E also appears with it. It seems -likely, however, that the bismuth, which is initially present in -solution at the time of separation of the lead, will retain both -radium E and F, and that the presence of these products in radio-lead -is due to their production, after separation, by the parent -substance, radium D.</p> - -<p class='c006'><span class='pageno' id='Page_415'>415</span>It would be of scientific value to separate radium D from -pitchblende and obtain it in the pure state, for, a month after -removal, the β ray activity from it would be about 300 times as -great as from an equal weight of radium. By placing a bismuth -plate in a solution of this substance, radium F (polonium) should -be separated, and, provided a sufficient interval is allowed to -elapse, a fresh supply of radium F can at any time be obtained.</p> - -<p class='c006'>The rate of transformation of radium D (half transformed in -40 years) is sufficiently slow not to interfere seriously with its -utility in most experiments.</p> - -<p class='c006'>The results of the comparison of the products of radium with -those contained in polonium, radio-tellurium and radio-lead are -summarized below.</p> - -<p class='c021'>Radium D = product in <i>new radio-lead</i>, no rays. Half transformed in 40 years.</p> - -<p class='c011'>Radium E gives out β rays, separated with bismuth, iridium and platinum. Half transformed in 6 days.</p> - -<p class='c011'>Radium F = product in <i>polonium</i> and <i>radio-tellurium</i>. Gives out only α rays. Half transformed in 143 days.</p> -<p class='c005'><a id='section242'></a> -<b>242. Temporary activity of inactive matter separated -from radio-active substances.</b> We have seen in the last -section that the platinum metals and bismuth acquire temporary -activity by their admixture with a solution of radio-lead, and that -these effects are very satisfactorily explained on the view that -some of the products of change of radio-lead are removed with the -inactive substances. Very similar effects have been observed by -Pegram and von Lerch (<a href='#section186'>section 186</a>), when inactive substances -were added to solutions of thorium and of the active deposit of -thorium. These results, too, are almost certainly due to the -removal of one or more of the products of thorium with the -inactive matter. Examples of this character may readily be -multiplied, and some of the more interesting and important of -these will be briefly discussed later.</p> - -<p class='c006'>There have been two general points of view regarding the -character of this activity which is temporarily acquired by inactive -matter. Some people have supposed that the inactive molecules of -the substance, mixed with the solution, acquire by “radio-active induction” -temporary activity, the underlying idea being that the close -admixture of an inactive and an active substance has communicated -<span class='pageno' id='Page_416'>416</span>the property of radiating to some of the molecules of the former. -According to the disintegration theory of radio-activity, on the -other hand, the temporary activity of originally inactive matter is -not due to any alteration of the inactive substance itself, but to -an admixture with it of one or more of the numerous radio-active -products. The idea of “radio-active induction” has no definite -experimental evidence in support of it, while there is much indirect -evidence against it.</p> - -<p class='c006'>We shall now consider how these facts are interpreted according -to the disintegration theory. In a specimen of old radium, for -example, there are present, besides radium itself, the seven -successive products which arise from it. Each of these differs in -chemical and physical properties from the others. If now, for -example, a bismuth rod is introduced into the solution, one or more -of these products are deposited on the bismuth. This action is -most probably electrolytic in nature, and will depend upon the -electro-chemical behaviour of the bismuth compared with that of -the products in solution. An electro-negative substance will tend -to remove the product or products which are strongly electro-positive. -This point of view serves to explain why different -metals are made active to different degrees, depending upon their -position in the electro-chemical series.</p> - -<p class='c006'>It seems probable that the activity communicated to inactive -matter by precipitation from an active solution occurs only during -the precipitation. The correctness of this view could readily be -tested by observing whether the time that the inactive substance -is present in solution has any effect on the magnitude of the -activity imparted to it.</p> - -<p class='c006'>When it is remembered that in pitchblende there are present -the radio-elements uranium, thorium, radium and actinium and -their numerous family of products, it is not surprising that many -of the inactive substances separated from it may show very considerable -activity due to the mixture of products which may be -removed with them. In carrying out experiments on the separation -of radium from pitchblende, M. and Mme Curie observed that -the separation of the active substance is fairly complete if the -stage of purification is not far advanced. Copper, antimony and -arsenic can be separated only slightly active, but other substances -like lead and iron always show activity. When the stage of -<span class='pageno' id='Page_417'>417</span>precipitation is more advanced, every substance separated from the -active solution shows activity.</p> - -<p class='c006'>One of the earliest observations in this direction was made by -Debierne, who found that barium could be made active by solution -with actinium. The active barium removed from the actinium -still preserved its activity after chemical treatment, and, in this -way, barium chloride was obtained whose activity was 6000 times -that of uranium. Although the activity of the barium chloride -could be concentrated in the same way as the activity of radiferous -barium chloride, it did not show any of the spectroscopic lines of -radium, and could not have been due to the admixture of that -element with the barium. The activity of the barium was not -permanent, and Debierne states that the activity fell to about one-third -of its value in three months. It seems probable that the -precipitated barium carried down with it the product actinium X, -and also some of the actinium itself, and that the decay observed -was due to the transformation of actinium X. It is interesting to -note that barium is capable of removing a large number of products -of the different radio-elements. This effect is probably connected -with its position in the electro-chemical series, for barium is highly -electro-positive.</p> - -<p class='c006'>Giesel showed in 1900 that bismuth could be made active by -placing it in a radium solution, and considered that polonium was -in reality bismuth made active by the process of induction. In -later experiments, he found that the bismuth plate gave out only -α rays, and that the activity of the bismuth could not be ascribed -to radium, since no β rays were present. We have seen that this -activity of the bismuth is due to the product radium F deposited -on its surface.</p> - -<p class='c006'>Mme Curie also found that bismuth was made active by -solution with a radium compound, and succeeded in fractionating -the above bismuth in the same way as polonium. In this way -bismuth was obtained 2000 times as active as uranium, but the -activity, like that of polonium separated from pitchblende, decreased -with the time. In the light of the experiments on the -transformation products of radium, it is seen that these early -experiments of Mme Curie add additional confirmation to the view -that the product (radium F) separated from radium itself is -identical with the polonium obtained directly from pitchblende.</p> - -<div class='chapter'> - <span class='pageno' id='Page_418'>418</span> - <h2 id='chap12' class='c004'>CHAPTER XII. <br> RATE OF EMISSION OF ENERGY.</h2> -</div> -<p class='c005'><b>243.</b> It was early recognised that a considerable amount -of energy is emitted by the radio-active bodies in the form of -their characteristic radiations. Most of the early estimates of the -amount of this energy were based on the number and energy of -the expelled particles, and were much too small. It has been -pointed out (<a href='#section114'>section 114</a>) that the greater part of the energy -emitted from the radio-active bodies in the form of ionizing -radiations is due to the α rays, and that the β rays in comparison -supply only a very small fraction.</p> - -<p class='c006'>Rutherford and McClung<a id='r325' href='#f325' class='c012'><sup>[325]</sup></a> made an estimate of the energy -of the rays, emitted by a thin layer of active matter, by determining -the total number of ions produced by the complete -absorption of the α rays. The energy required to produce an ion -was determined experimentally by observations of the heating -effect of X rays, and of the total number of ions produced when -the rays were completely absorbed in air. The energy required -to produce an ion in air was found to be 1·90 × 10<sup>-10</sup> ergs. This, -as will be shown in <a href='#appa'>Appendix A</a>, is probably an over-estimate, -but was of the right order of magnitude. From this it was -calculated that one gram of uranium oxide spread over a plate -in the form of a thin powdered layer emitted energy into the -air at the rate of 0·032 gram calories per year. This is a very -small emission of energy, but in the case of an intensely radio-active -substance like radium, whose activity is about two million -times that of uranium, the corresponding emission of energy is -69000 gram calories per year. This is obviously an under-estimate, -<span class='pageno' id='Page_419'>419</span>for it includes only the energy radiated into the air. -The actual amount of energy released in the form of α rays -is evidently much greater than this on account of the absorption -of the α rays by the active matter itself.</p> - -<p class='c006'>It will be shown later that the heating effect of radium and of -its products is a measure of the energy of the expelled α particles.</p> -<p class='c005'><b>244. Heat emission of radium.</b> P. Curie and Laborde<a id='r326' href='#f326' class='c012'><sup>[326]</sup></a> -first drew attention to the striking result that a radium compound -kept itself continuously at a temperature several degrees higher -than that of the surrounding atmosphere. Thus the energy -emitted from radium can be demonstrated by its direct heating -effect, as well as by photographic and electric means. Curie -and Laborde determined the rate of the emission of heat in -two different ways. In one method the difference of temperature -was observed by means of an iron-constantine thermo-couple -between a tube containing one gram of radiferous chloride -of barium, of activity about ⅙ of pure radium, and an exactly -similar tube containing one gram of pure barium chloride. -The difference of temperature observed was 1·5° C. In order to -measure the rate of emission of heat, a coil of wire of known -resistance was placed in the pure barium chloride, and the -strength of the electric current required to raise the barium to -the same temperature as the radiferous barium was observed. In -the other method, the active barium, enclosed in a glass tube, was -placed inside a Bunsen calorimeter. Before the radium was introduced, -it was observed that the level of the mercury in the stem -remained steady. As soon as the radium, which had previously -been cooled in melting ice, was placed in the calorimeter, the -mercury column began to move at a regular rate. If the radium -tube was removed, the movement of the mercury ceased. It was -found from these experiments that the heat emission from the -1 gram of radiferous barium, containing about ⅙ of its weight of -pure radium chloride, was 14 gram-calories per hour. Measurements -were also made with 0·08 gram of pure radium chloride. -Curie and Laborde deduced from these results that 1 gram of pure -radium emits a quantity of heat equal to about 100 gram-calories -per hour. This result was confirmed by the experiments of Runge -<span class='pageno' id='Page_420'>420</span>and Precht<a id='r327' href='#f327' class='c012'><sup>[327]</sup></a> and others. As far as observation has gone at present, -this rate of emission of heat is continuous and unchanged with -lapse of time. Therefore, 1 gram of radium emits in the course of -a day 2400, and in the course of a year 876,000 gram-calories. -The amount of heat evolved in the union of hydrogen and oxygen -to form 1 gram of water is 3900 gram-calories. It is thus seen -that 1 gram of radium emits <i>per day</i> nearly as much energy as is -required to dissociate 1 gram of water.</p> - -<p class='c006'>In some later experiments using 0·7 gram of pure radium -bromide, P. Curie<a id='r328' href='#f328' class='c012'><sup>[328]</sup></a> found that the temperature of the radium -indicated by a mercury thermometer was 3° C. above that of the -surrounding air. This result was confirmed by Giesel, who obtained -a difference of temperature of 5° C. with 1 gram of radium bromide. -The actual rise of temperature observed will obviously depend upon -the size and nature of the vessel containing -the radium.</p> - -<p class='c006'>During their visit to England in -1903 to lecture at the Royal Institution, -M. and Mme Curie performed -some experiments with Professor -Dewar, to test by another method the -rate of emission of heat from radium -at very low temperatures. This method -depended on the measurement of the -amount of gas volatilized when a -radium preparation was placed inside -a tube immersed in a liquefied gas -at its boiling point. The arrangement -of the calorimeter is shown in -<a href='#fig097'>Fig. 97</a>.</p> - -<div id='fig097' class='figcenter id002'> -<img src='images/fig-097.png' alt='Fig. 97.' class='ig001'> -<div class='ic002'> -<p>Fig. 97.</p> -</div> -</div> - -<p class='c006'>The small closed Dewar flask <i>A</i> contains the radium in a glass -tube <i>R</i>, immersed in the liquid to be employed. The flask <i>A</i> is -surrounded by another Dewar bulb <i>B</i>, containing the same liquid, -so that no heat is communicated to <i>A</i> from the outside. The gas -liberated in the tube <i>A</i> is collected in the usual way over water or -mercury, and its volume determined. By this method, the rate -of heat emission of the radium was found to be about the same in -<span class='pageno' id='Page_421'>421</span>boiling carbon dioxide and oxygen, and also in liquid hydrogen. -Especial interest attaches to the result obtained with liquid -hydrogen, for at such a low temperature ordinary chemical activity -is suspended. The fact that the heat emission of radium -is unaltered over such a wide range of temperature indirectly -shows that the rate of expulsion of α particles from radium is -independent of temperature, for it will be shown later that the -heating effect observed is due to the bombardment of the radium -by the α particles.</p> - -<p class='c006'>The use of liquid hydrogen is very convenient for demonstrating -the rate of heat emission from a small amount of radium. -From 0·7 gram of radium bromide (which had been prepared only -10 days previously) 73 c.c. of gas were given off per minute.</p> - -<p class='c006'>In later experiments P. Curie (<i>loc. cit.</i>) found that the rate of -emission of heat from a given quantity of radium depended upon -the time which had elapsed since its preparation. The emission -of heat was at first small, but after a month’s interval practically -attained a maximum. If a radium compound is dissolved and -placed in a sealed tube, the rate of heat emission rises to the same -maximum as that of an equal quantity of radium in the solid -state.</p> -<p class='c005'><b>245. Connection of the heat emission with the radiations.</b> -The observation of Curie that the rate of heat emission -depended upon the age of the radium preparation pointed to the -conclusion that the phenomenon of heat emission of radium was -connected with the radio-activity of that element. It had long -been known that radium compounds increased in activity for about -a month after their preparation, when they reached a steady state. -It has been shown (<a href='#section215'>section 215</a>), that this increase of activity is -due to the continuous production by the radium of the radio-active -emanation, which is occluded in the radium compound and -adds its radiation to that of the radium proper. It thus seemed -probable that the heating effect was in some way connected with -the presence of the emanation. Some experiments upon this point -were made by Rutherford and Barnes<a id='r329' href='#f329' class='c012'><sup>[329]</sup></a>. In order to measure the -small amounts of heat emitted, a form of differential air calorimeter -shown in Fig. 98 was employed. Two equal glass flasks -<span class='pageno' id='Page_422'>422</span>of about 500 c.c. were filled with dry air at atmospheric pressure. -These flasks were connected through a glass <b>U</b>-tube filled with -xylene, which served as a manometer to determine any variation -of pressure of the air in the flasks. A small glass tube, closed -at the lower end, was introduced into the middle of each of the -flasks. When a continuous source of heat was introduced into the -glass tube, the air surrounding it was heated and the pressure was -increased. The difference of pressure, when a steady state was -reached, was observed on the manometer by means of a microscope -with a micrometer scale in the eye-piece. On placing the source -of heat in the similar tube in the other flask, the difference in -pressure was reversed. In order to keep the apparatus at a -constant temperature, the two flasks were immersed in a water-bath, -which was kept well stirred.</p> - -<div id='fig098' class='figcenter id006'> -<img src='images/fig-098.png' alt='Fig. 98.' class='ig001'> -<div class='ic002'> -<p>Fig. 98.</p> -</div> -</div> - -<p class='c006'>Observations were first made on the heat emission from 30 -milligrams of radium bromide. The difference in pressure observed -on the manometer was standardized by placing a small coil of wire -of known resistance in the place of the radium. The strength of -the current through the wire was adjusted to give the same difference -of pressure on the manometer. In this way it was found that -the heat emission per gram of radium bromide corresponded to -65 gram-calories per hour. Taking the atomic weight of radium -as 225, this is equivalent to a rate of emission of heat from one -gram of metallic radium of 110 gram-calories per hour.</p> - -<p class='c006'>The emanation from the 30 milligrams of radium bromide was -then removed by heating the radium (<a href='#section215'>section 215</a>). By passing -<span class='pageno' id='Page_423'>423</span>the emanation through a small glass tube immersed in liquid air, -the emanation was condensed. The tube was sealed off while the -emanation was still condensed in the tube. In this way the -emanation was concentrated in a small glass tube about 4 cms. -long. The heating effects of the “de-emanated” radium and of the -emanation tube were then determined at intervals. It was found -that, after removal of the emanation, the heating effect of the -radium decayed in the course of a few hours to a minimum, -corresponding to about 25 per cent. of the original heat emission, -and then gradually increased again, reaching its original value after -about a month’s interval. The heating effect of the emanation -tube was found to increase for the first few hours after separation -to a maximum, and then to decay regularly with the time according -to an exponential law, falling to half its maximum value in about -four days. The actual heat emission of the emanation tube was -determined by sending a current through a coil of wire occupying -the same length and position as the emanation tube.</p> - -<p class='c006'>The variation with time of the heating effect from 30 milligrams -of radium and the emanation from it is shown in <a href='#fig099'>Fig. 99</a>.</p> - -<div id='fig099' class='figcenter id004'> -<img src='images/fig-099.png' alt='Fig. 99.' class='ig001'> -<div class='ic002'> -<p>Fig. 99.</p> -</div> -</div> - -<p class='c006'>Curve <i>A</i> shows the variation with time of the heat emission -of the radium and curve <i>B</i> of the emanation. The sum total of -<span class='pageno' id='Page_424'>424</span>the rate of heat emission of the radium and the emanation -together, was at any time found to be equal to that of the -original radium. The maximum heating effect of the tube containing -the emanation from 30 milligrams of radium bromide was -1·26 gram-calories per hour. The emanation together with the -secondary products which arise from it, obtained from one gram -of radium, would thus give out 42 gram-calories per hour. The -emanation stored up in the radium is thus responsible for more -than two-thirds of the total heat emission from radium. It will -be seen later that the decrease to a minimum of the heating effect -of radium, after removal of the emanation, is connected with -the decay of the excited activity. In a similar way, the increase -of the heating effect of the emanation to a maximum some hours -after removal is also a result of the excited activity produced by -the emanation on the walls of the containing vessel. Disregarding -for the moment these rapid initial changes in heat emission, it is -seen that the heating effect of the emanation and its further -products, after reaching a maximum, decreases at the same rate as -that at which the emanation loses its activity, that is, it falls to half -value in four days. If <i>Q</i><sub>max.</sub> is the maximum heating effect and <i>Q<sub>t</sub></i> the -heating effect at any time <i>t</i> later, then</p> - -<div class='figcenter id010'> -<img src='images/form-134.png' alt='Formula.' class='ig001'> -</div> - -<p class='c006'>where λ is the -constant of change of the emanation.</p> - -<p class='c006'>The curve of recovery of the heating effect of radium from -its minimum value is identical with the curve of recovery of its -activity measured by the α rays. Since the minimum heating -effect is 25 per cent. of the total, the heat emission <i>Q<sub>t</sub></i> at any time <i>t</i> -after reaching a minimum is given by</p> - -<div class='figcenter id005'> -<img src='images/form-135.png' alt='Formula.' class='ig001'> -</div> - -<p class='c006'>where <i>Q</i><sub>max.</sub> is the maximum rate of heat emission and λ, as before, -is the constant of change of the emanation.</p> - -<p class='c006'>The identity of the curves of recovery and fall of the heating -effect of radium and its emanation respectively with the corresponding -curves for the rise and fall of radio-activity shows that -the heat emission of radium and its products is directly connected -with their radio-activity. The variation in the heat emission of -both radium and its emanation is approximately proportional to -their activity measured by the α rays. It is not proportional to -<span class='pageno' id='Page_425'>425</span>the activity measured by the β or γ rays, for the intensity of these -rays falls nearly to zero some hours after removal of the emanation, -while the α ray activity, like the heating effect, is 25 per cent. -of the maximum value. These results are thus in accordance with -the view that the heat emission of radium accompanies the -expulsion of α particles, and is approximately proportional to the -number expelled. Before such a conclusion can be considered -established, it is necessary to show that the heating effect of the -active deposit from the emanation varies in the same way as its -α ray activity. Experiments made to test this point will now be -considered.</p> -<p class='c005'><a id='section246'></a> -<b>246. Heat emission of the active deposit from the -emanation.</b> New radium in radio-active equilibrium contains -four successive products which break up with the emission of -α particles, viz. radium itself, the emanation, radium A and C. -Radium B does not emit rays at all. The effect of the later -products radium D, E and F may be neglected, if the radium has -not been prepared for more than a year.</p> - -<p class='c006'>It is not easy to settle definitely the relative activity supplied -by each of these products when in radio-active equilibrium, but -it has been shown in <a href='#section229'>section 229</a> that the activity is not very -different for the four α ray products. The α particles from radium -A and C are more penetrating than those from radium itself and -the emanation. The evidence at present obtained points to the -conclusion that the activity supplied by the emanation is less -than that supplied by the other products. This indicates that -the α particles from the emanation are projected with less velocity -than in the other cases.</p> - -<p class='c006'>When the emanation is suddenly released from radium by -heat or solution, the products radium A, B and C are left behind. -Since the parent matter is removed, the amount of the products -A, B, C at once commences to diminish, and at the end of about -three hours reaches a very small value. If the heating effect -depends upon the α ray activity, it is thus to be expected that the -heat emission of the radium should rapidly diminish to a minimum -after the removal of the emanation.</p> - -<p class='c006'>When the emanation is introduced into a vessel, the products -radium A, B and C at once appear and increase in quantity, -<span class='pageno' id='Page_426'>426</span>reaching a practical maximum about 3 hours later. The heating -effect of the emanation tube should thus increase for several hours -after the introduction of the emanation.</p> - -<p class='c006'>In order to follow the rapid changes in the heating effect of -radium, after removal of the emanation, Rutherford and Barnes -(<i>loc. cit.</i>) used a pair of differential platinum thermometers. Each -thermometer consisted of 35 cms. of fine platinum wire, wound -carefully on the inside of a thin glass tube 5 mms. in diameter, -forming a coil 3 cms. long. The glass tube containing the radium -and also the tube containing the emanation were selected to slide -easily into the interior of the coils, the wire thus being in direct -contact with the glass envelope containing the source of heat. -The change in resistance of the platinum thermometers, when the -radium or emanation tube was transferred from one coil to the -other, was readily measured.</p> - -<div id='fig100' class='figcenter id004'> -<img src='images/fig-100.png' alt='Fig. 100.' class='ig001'> -<div class='ic002'> -<p>Fig. 100.</p> -</div> -</div> - -<p class='c006'>The heating effect of the radium in radio-active equilibrium -was first accurately determined. The radium tube was heated to -drive off the emanation, which was rapidly condensed in a small -glass tube 3 cms. long and 3 mms. internal diameter. After -allowing a short time for temperature conditions to become steady, -the heating effect of the radium tube was measured. The results -are shown in <a href='#fig100'>Fig. 100</a>. An observation could not be taken until -<span class='pageno' id='Page_427'>427</span>about 12 minutes after the removal of the emanation, and the -heating effect was then found to have fallen to about 55 per cent. -of the maximum value. It steadily diminished with the time, -finally reaching a minimum value of 25 per cent. several hours later.</p> - -<p class='c006'>It is not possible in experiments of this character to separate -the heating effect of the emanation from that supplied by radium A. -Since A is half transformed in three minutes, its heating effect -will have largely disappeared after 10 minutes, and the decrease -is then mainly due to changes in radium B and C.</p> - -<p class='c006'>The variation with time of the heating effect of the active -deposit is still more clearly brought out by an examination of the -rise of the heating effect when the emanation is introduced into a -small tube, and of the decrease of the heating effect after the -emanation is removed. The curve of rise is shown in the upper -curve of <a href='#fig101'>Fig. 101</a>. 40 minutes after the introduction of the emanation, -the heating effect had risen to 75 per cent. of the maximum -value which was reached after an interval of about 3 hours.</p> - -<div id='fig101' class='figcenter id004'> -<img src='images/fig-101.png' alt='Fig. 101.' class='ig001'> -<div class='ic002'> -<p>Fig. 101.</p> -</div> -</div> - -<p class='c006'>After the heating effect of the emanation tube had attained a -maximum, the emanation was removed, and the decay with time -observed as soon as possible afterwards. The results are shown in -the lower curve of <a href='#fig101'>Fig. 101</a>. It is seen that the two curves of -<span class='pageno' id='Page_428'>428</span>rise and decay are complementary to one another. The first -observation was made 10 minutes after removal, and the heating -effect had then dropped to 47 per cent. of the original value. -This sudden drop is due partly to the removal of the emanation, -and partly to the rapid transformation of radium A. The lower -curve is almost identical in shape with the corresponding α ray -curve for the decay of the excited activity after a long exposure -(see <a href='#fig086'>Fig. 86</a>) and clearly shows that the heating effect is directly -proportional to the activity measured by the α rays over the whole -range examined. The heating effect decreases according to the -same law and at the same rate as the activity measured by the α -rays.</p> - -<p class='c006'>Twenty minutes after the removal of the emanation, radium A -has been almost completely transformed, and the activity is then -proportional to the amount of radium C present, since the intermediate -product B does not give out rays. The close agreement -of the activity and heat emission curves shows that the heating -effect is proportional also to the amount of radium C. We may -thus conclude that the rayless product B supplies little if any -of the heat emission observed. If radium B supplied the same -amount as radium C, the curve of decrease of heating effect with -time would differ considerably from the activity curve.</p> - -<p class='c006'>The conclusion that the transformation of radium B is not -accompanied by the release of as much heat as the other changes -is to be expected if the heating effect is mainly due to the energy -of motion of the expelled α particles.</p> - -<p class='c006'>The relative heating effect due to the radium products is -shown in the following table. The initial heating effect of C is -deduced by comparison with the corresponding activity curve.</p> - -<table class='table17' > -<colgroup> -<col class='colwidth20'> -<col class='colwidth20'> -<col class='colwidth60'> -</colgroup> - <tr> - <th class='c013'>Products</th> - <th class='c013'>Radiation</th> - <th class='c014'>Initial rate of heat emission</th> - </tr> - <tr> - <td class='c013'>Radium</td> - <td class='c013'>α rays</td> - <td class='c014'>25 per cent. of total</td> - </tr> - <tr> - <td class='c013'>Emanation</td> - <td class='c013'>α „</td> - <td class='c014'> </td> - </tr> - <tr> - <td class='c013'>Radium A</td> - <td class='c013'>α „</td> - <td class='c014'>44 „ „</td> - </tr> - <tr> - <td class='c013'>Radium B</td> - <td class='c013'>no rays</td> - <td class='c014'>0 „ „</td> - </tr> - <tr> - <td class='c013'>Radium C</td> - <td class='c013'>α, β, γ rays</td> - <td class='c014'>31 „ „</td> - </tr> -</table> - -<p class='c006'>Since radium A and C supply almost an equal proportion of -activity, it is probable that they have equal initial heating effects. -If this is the case, the heating effect of the emanation alone is -13 per cent. of the total.</p> -<p class='c005'><span class='pageno' id='Page_429'>429</span><b>247. Heating effects of the β and</b> γ <b>rays</b>. It has been -shown in <a href='#section114'>section 114</a> that the kinetic energy of the β particles -emitted from radium is probably not greater than one per cent. -of that due to the α particles. If the heat emission is a result -of bombardment by the particles expelled from its mass, it is to -be expected that the heating effect of the β rays will be very -small compared with that due to the α rays. This anticipation is -borne out by experiment. Curie measured the heating effect of -radium (1) when enclosed in a thin envelope, and (2) when surrounded -by one millimetre of lead. In the former case a large -proportion of the β rays escaped, and, in the latter, nearly all were -absorbed. The increase of heating effect in case (2) was not more -than five per cent., and this is probably an over-estimate.</p> - -<p class='c006'>In a similar way, since the total ionization due to the β rays -is about equal to that produced by the γ rays, we should expect -that the heating effect of the γ rays will be very small compared -with that arising from the α rays.</p> - -<p class='c006'>Paschen made some experiments on the heating effect of -radium in a Bunsen ice calorimeter where the radium was surrounded -by a thickness of 1·92 cms. of lead—a depth sufficient to -absorb a large proportion of the γ rays. In his first publication<a id='r330' href='#f330' class='c012'><sup>[330]</sup></a>, -results were given which indicated that the heating effect of the -γ rays was even greater than that of the α rays. This was not -confirmed by later observations by the same method. He concluded -that the ice calorimeter could not be relied on to measure -such very small quantities of heat.</p> - -<p class='c006'>After the publication of Paschen’s first paper Rutherford and -Barnes<a id='r331' href='#f331' class='c012'><sup>[331]</sup></a> examined the question by a different method. An air -calorimeter of the form shown in <a href='#fig098'>Fig. 98</a> was employed which -was found to give very satisfactory results. The heat emission -of radium was measured (1) when the radium was surrounded by -a cylinder of aluminium and (2) when surrounded by a cylinder -of lead of the same dimensions. The aluminium absorbed only -a small fraction of the γ rays while the lead stopped more than -half. No certain difference between the heating effect in the two -cases was observed, although from the earlier experiments of -Paschen a difference of at least 50 per cent. was to be expected.</p> - -<p class='c006'><span class='pageno' id='Page_430'>430</span>We must therefore conclude that the β and γ rays together do -not supply more than a small percentage of the total heat emission -of radium—a result which is in accordance with the calculations -based on the total ionization produced by the different types of -rays.</p> -<p class='c005'><b>248. Source of the energy.</b> It has been shown that the -heating effect of radium is closely proportional to the activity -measured by the α rays. Since the activity is generally measured -between parallel plates such a distance apart that most of the α -particles are absorbed in the gas, this result shows that the heating -effect is proportional to the energy of the emitted α particles. -The rapid heat emission of radium follows naturally from the disintegration -theory of radio-activity. The heat is supposed to be -derived not from external sources, but from the internal energy of -the radium atom. The atom is supposed to be a complex system -consisting of charged parts in very rapid motion, and in consequence -contains a large store of latent energy, which can only be manifested -when the atom breaks up. For some reason, the atomic -system becomes unstable, and an α particle, of mass about twice -that of the hydrogen atom, escapes, carrying with it its energy of -motion. Since the α particles would be practically absorbed in a -thickness of radium of less than ·001 cm., the greater proportion -of the α particles, expelled from a mass of radium, would be stopped -in the radium itself and their energy of motion would be manifested -in the form of heat. The radium would thus be heated by its own -bombardment above the temperature of the surrounding air. The -energy of the expelled α particles probably does not account for -the whole emission of heat by radium. It is evident that the -violent expulsion of a part of the atom must result in intense -electrical disturbances in the atom. At the same time, the residual -parts of the disintegrated atom rearrange themselves to form a -permanently or temporarily stable system. During this process -also some energy is probably emitted, which is manifested in the -form of heat in the radium itself.</p> - -<p class='c006'>The view that the heat emission of radium is due very largely -to the kinetic energy possessed by the expelled α particles is -strongly confirmed by calculations of the magnitude of the heating -effect to be expected on such an hypothesis. It has been shown -<span class='pageno' id='Page_431'>431</span>in <a href='#section093'>section 93</a> that one gram of radium bromide emits about -1·44 × 10<sup>11</sup> α particles per second. The corresponding number for -1 gram of radium (Ra = 225) is 2·5 × 10<sup>11</sup>. Now it has been -calculated from experimental data in section 94, that the average -kinetic energy of the α particles expelled from radium is 5·9 × 10<sup>-6</sup> -ergs. Since all of the α particles are absorbed either in the radium -itself or the envelope surrounding it, the total energy of the α -particles emitted per second is 1·5 × 10<sup>6</sup> ergs. This corresponds -to an emission of energy of about 130 gram calories per hour. -Now the observed heating effect of radium is about 100 gram -calories per hour. Considering the nature of the calculation, the -agreement between the observed and experimental values is as -close as would be expected, and directly supports the view that -the heat emission of radium is due very largely to the bombardment -of the radium and containing vessel by the α particles -expelled from its mass.</p> -<p class='c005'><a id='section249'></a> -<b>249. Heating effect of the radium emanation.</b> The -enormous amount of heat liberated in radio-active transformations -which are accompanied by the expulsion of α particles is very well -illustrated by the case of the radium emanation.</p> - -<p class='c006'>The heat emission of the emanation released from 1 gram of -radium is 75 gram calories per hour at its maximum value. This -heat emission is not due to the emanation alone, but also to its -further products which are included with it. Since the rate of -heat emission decays exponentially with the time to about half -value in four days, the total amount of heat liberated during the -life of the emanation from 1 gram of radium is equal to</p> - -<div class='figcenter id007'> -<img src='images/form-136.png' alt='Formula.' class='ig001'> -</div> - -<p class='c006'>since λ = ·0072(hour)<sup>-1</sup>. Now the volume of the emanation from -1 gram of radium is about 1 cubic millimetre at standard pressure -and temperature (<a href='#section172'>section 172</a>). Thus 1 cubic centimetre of the -emanation would during its transformation emit 10<sup>7</sup> gram calories. -The heat emitted during the combination of 1 c.c. of hydrogen and -oxygen to form water is about 2 gram calories. The emanation -thus gives out during its changes 5 × 10<sup>6</sup> times as much energy -as the combination of an equal volume of hydrogen and oxygen -<span class='pageno' id='Page_432'>432</span>to form water, although this latter reaction is accompanied by a -larger release of energy than any other known to chemistry.</p> - -<p class='c006'>The production of heat from 1 c.c. of the radium emanation is -about 21 gram calories per second. This generation of heat would -be sufficient to heat to redness, if not to melt down, the walls of -the glass tube containing the emanation.</p> - -<p class='c006'>The probable rate of heat emission from 1 gram weight of the -emanation can readily be deduced, assuming that the emanation -has about 100 times the molecular weight of hydrogen. Since -100 c.c. of the emanation would weigh about 1 gram, the total -heat emission from 1 gram of the emanation is about 10<sup>9</sup> gram -calories.</p> - -<p class='c006'>It can readily be calculated that one pound weight of the -emanation would, at its maximum, radiate energy at the rate of about -10,000 horse-power. This radiation of energy would fall off with -the time, but the total emission of energy during the life of the -emanation would correspond to 60,000 horse-power days.</p> -<p class='c005'><b>250. Heating effects of uranium, thorium, and actinium.</b> -Since the heat emission of radium is a direct consequence of its -bombardment by the α particles expelled from its mass, it is to be -expected that all the radio-elements which emit α rays should -also emit heat at a rate proportional to their α ray activity.</p> - -<p class='c006'>Since the activity of pure radium is probably about two -million times that of uranium or thorium, the heat emission from -1 gram of thorium or uranium should be about 5 × 10<sup>-5</sup> gram -calories per hour, or about 0·44 gram calories per year. This is a -very small rate of generation of heat, but it should be detectable if -a large quantity of uranium or thorium is employed. Experiments -to determine the heating effect of thorium have been made by -Pegram<a id='r332' href='#f332' class='c012'><sup>[332]</sup></a>. Three kilograms of thorium oxide, enclosed in a Dewar -bulb, were kept in an ice-bath, and the difference of temperature -between the thorium and ice-bath determined by a set of iron-constantan -thermo-electric couples. The maximum difference of -temperature observed was 0·04° C., and, from the rate of change -of temperature, it was calculated that one gram of thorium oxide -liberated 8 × 10<sup>-5</sup> gram calories per hour. A more accurate -determination of the heat emission is in progress, but the results -obtained are of the order of magnitude to be expected.</p> -<p class='c005'><span class='pageno' id='Page_433'>433</span><b>251. Energy emitted by a radio-active product.</b> An -important consequence follows from the fact that the heat -emission is a measure of the energy of the expelled α particles. -If each atom of each product emits α particles, the total emission -of energy from 1 gram of the product can at once be determined. -The α particles from the different products are projected with -about the same velocity, and consequently carry off about the -same amount of energy. Now it has been shown that the energy -of each α particle expelled from radium is about 5·9 × 10<sup>-6</sup> ergs. -Most of the products probably have an atomic weight in the -neighbourhood of 200. Since there are 3·6 × 10<sup>19</sup> molecules in -one cubic centimetre of hydrogen, it can easily be calculated -that there are about 3·6 × 10<sup>21</sup> atoms in one gram of the -product.</p> - -<p class='c006'>If each atom of the product expels one α particle, the total -energy emitted from 1 gram of the matter is about 2 × 10<sup>16</sup> ergs or -8 × 10<sup>8</sup> gram calories. The total emission of energy from a product -which emits only β rays is probably about one-hundredth of the -above amount.</p> - -<p class='c006'>In this case we have only considered the energy emitted from -a single product independently of the successive products which -may arise from it. Radium, for example, may be considered a -radio-active product which slowly breaks up and gives rise to four -subsequent α ray products. The total heat emission from one -gram of radium and products is thus about five times the above -amount, or 4 × 10<sup>9</sup> gram calories.</p> - -<p class='c006'>The total emission of energy from radium is discussed later in -<a href='#section266'>section 266</a> from a slightly different point of view.</p> -<p class='c005'><a id='section252'></a> -<b>252. Number of ions produced by an α particle.</b> In -the first edition of this book it was calculated by several independent -methods that 1 gram of radium emitted about 10<sup>11</sup> α -particles per second. Since the actual number has later been -determined by measuring the charge carried by the α rays -(<a href='#section093'>section 93</a>) we can, conversely, use this number to determine with -more certainty some of the constants whose values were assumed -in the original calculation.</p> - -<p class='c006'>For example, the total number of ions produced by an α -<span class='pageno' id='Page_434'>434</span>particle in the gas can readily be determined. The method -employed is as follows. 0·484 mgr. of radium bromide was -dissolved in water and then spread uniformly over an aluminium -plate. After evaporation, the saturation ionization current, due -to the radium at its minimum activity, was found to be 8·4 × 10<sup>-8</sup> -ampere. The plates of the testing vessel were sufficiently far apart -to absorb all the α rays in the gas. The number of α particles -expelled per second into the gas was found experimentally to be -8·7 × 10<sup>6</sup>. Taking the charge on an ion as 1·13 × 10<sup>-19</sup> coulombs -(<a href='#section036'>section 36</a>), the total number of ions produced per second in the -gas was 7·5 × 10<sup>11</sup>. Thus each α particle on an average produced -86,000 ions in the gas before it was absorbed.</p> - -<p class='c006'>Now Bragg (<a href='#section104'>section 104</a>) has shown that the α particles from -radium at its minimum activity are stopped in about 3 cms. of -air. The results obtained by him indicate that the ionization of -the particles per cm. of path is less near the radium than some -distance away. Assuming, however, as a first approximation that -the ionization is uniform along the path, the number of ions -produced per cm. of path by the α particle is 29,000. Since the -ionization varies directly as the pressure, at a pressure of 1 mm. -of mercury the number of ions per unit path would be about 38. -Now Townsend (<a href='#section103'>section 103</a>) found that the maximum number -of ions produced per unit path of air at 1 mm. pressure by an -electron in motion was 20, and in this case a fresh pair of ions -was produced at each encounter of the electron with the molecules -in its path. In the present case the α particle, which has a very -large mass compared with the electron, appears to have a larger -sphere of influence than the electron and to ionize twice as many -molecules.</p> - -<p class='c006'>In addition, the α particle produces many more ions per unit -path than an electron moving with the same velocity, for it has -been shown (<a href='#section103'>section 103</a>) that the electron becomes a less -efficient ionizer after a certain velocity is reached. As Bragg -(<i>loc. cit.</i>) has pointed out, this is to be expected, since the α -particle consists of a large number of electrons and consequently -would be a far more efficient ionizer than an isolated electron. A -calculation of the energy required to produce an ion by an α -particle is given in <a href='#appa'>Appendix A</a>.</p> -<p class='c005'><span class='pageno' id='Page_435'>435</span><a id='section253'></a> -<b>253. Number of β particles expelled from one gram of -radium.</b> It is of importance to compare the total number of -β particles expelled from one gram of radium in radio-active -equilibrium, as, theoretically, this number should bear a definite -relation to the total number of α particles emitted. We have seen -that new radium in radio-active equilibrium contains four products -which emit α rays, viz. radium itself, the emanation, radium A -and radium C. On the other hand, β rays are expelled from only -one product, radium C. The same number of atoms of each of -these successive products in equilibrium break up per second. If -the disintegration of each atom is accompanied by the expulsion -of one α particle and, in the case of radium C, also of one β particle, -the number of α particles emitted from radium in radio-active -equilibrium will be four times the number of β particles.</p> - -<p class='c006'>The method employed by Wien to determine the number of -β particles emitted from a known quantity of radium has already -been discussed in <a href='#section080'>section 80</a>. On account of the absorption of -some of the β particles in the radium envelope and in the radium -itself, the number found by him is far too small. It has been -shown in <a href='#section085'>section 85</a> that a number of easily absorbed β rays are -projected from radium, many of which would be stopped in the -radium itself or in the envelope containing it.</p> - -<p class='c006'>In order to eliminate as far as possible the error due to this -absorption, in some experiments made by the writer, the active -deposit obtained from the radium emanation rather than radium -itself was used as a source of β rays. A lead rod, 4 cms. long and -4 mms. in diameter, was exposed as the negative electrode in a -large quantity of the radium emanation for three hours. The rod -was then removed and the γ ray effect from it immediately -measured by an electroscope and compared with the corresponding -γ ray effect from a known weight of radium bromide in radio-active -equilibrium. Since the active deposit contains the product -radium C which alone emits β rays, and, since the intensities -of the β and γ rays are always proportional to each other, the -number of β particles expelled from the lead rod per second is -equal to the corresponding number from the weight of radium -bromide which gives the same γ ray effect as the lead rod.</p> - -<p class='c006'>The rod was then enveloped in a thickness of aluminium foil -<span class='pageno' id='Page_436'>436</span>of ·0053 cms.—a thickness just sufficient to absorb the α rays—and -made the insulated electrode in a cylindrical metal vessel -which was rapidly exhausted to a low pressure. The current in -the two directions was measured at intervals by an electrometer, -and, as we have seen in <a href='#section093'>section 93</a>, the algebraic sum of these -currents is proportional to <i>ne</i>, where <i>n</i> is the number of β particles -expelled per second from the lead rod, and <i>e</i> the charge on each -particle. The activity of the radium C decayed with the time, -but, from the known curve of decay, the results could be corrected -in terms of the initial value immediately after the rod was removed -from the emanation.</p> - -<p class='c006'>Taking into account that half of the β particles emitted by -the active deposit were absorbed in the radium itself, and reckoning -the charge on the β particle as 1·13 × 10<sup>-19</sup> coulombs, two separate -experiments gave 7·6 × 10<sup>10</sup> and 7·0 × 10<sup>10</sup> as the total number of -β particles expelled per second from one gram of radium. Taking -the mean value, we may conclude that the total number of -β particles expelled per second from one gram of radium in radio-active -equilibrium is about 7·3 × 10<sup>10</sup>.</p> - -<p class='c006'>The total number of α particles expelled from one gram of -radium at its minimum activity has been shown to be 6·2 × 10<sup>10</sup> -(<a href='#section093'>section 93</a>). The approximate agreement between these numbers -is a strong indication of the correctness of the theoretical views -previously discussed. It is to be expected that the number of -β particles, deduced in this way, will be somewhat greater than -the true value, since the β particles give rise to a secondary -radiation consisting also of negatively charged particles moving -at a high speed. These secondary β particles, arising from the -impact of the β particles on the lead, will pass through the -aluminium screen and add their effect to the primary β rays.</p> - -<p class='c006'>The results, however, indicate that four α particles are expelled -from radium in radio-active equilibrium for each β particle and -thus confirm the theory of successive changes.</p> - -<div class='chapter'> - <span class='pageno' id='Page_437'>437</span> - <h2 id='chap13' class='c004'>CHAPTER XIII. <br> RADIO-ACTIVE PROCESSES.</h2> -</div> -<p class='c005'><b>254. Theories of radio-activity.</b> In previous chapters, a -detailed account has been given of the nature and properties of -the radiations, and of the complex processes taking place in the -radio-active substances. The numerous products arising from the -radio-elements have been closely examined, and have been shown -to result from a transformation of the parent element through a -number of well-marked stages. In this chapter, the application of -the disintegration theory to the explanation of radio-active phenomena -will be considered still further, and the logical deductions -to be drawn from the theory will be discussed briefly.</p> - -<p class='c006'>A review will first be given of the working hypotheses which -have served as a guide to the investigators in the field of radio-activity. -These working theories have in many cases been modified -or extended with the growth of experimental knowledge.</p> - -<p class='c006'>The early experiments of Mme Curie had indicated that radio-activity -was an atomic and not a molecular phenomenon. This -was still further substantiated by later work, and the detection and -isolation of radium from pitchblende was a brilliant verification of -the truth of this hypothesis.</p> - -<p class='c006'>The discovery that the β rays of the radio-elements were -similar to the cathode rays produced in a vacuum tube was an -important advance, and has formed the basis of several subsequent -theories. J. Perrin<a id='r333' href='#f333' class='c012'><sup>[333]</sup></a>, in 1901, following the views of J. J. Thomson -and others, suggested that the atoms of bodies consisted of parts -and might be likened to a miniature planetary system. In the -<span class='pageno' id='Page_438'>438</span>atoms of the radio-elements, the parts composing the atoms more -distant from the centre might be able to escape from the central -attraction and thus give rise to the radiation of energy observed. -In December 1901, Becquerel<a id='r334' href='#f334' class='c012'><sup>[334]</sup></a> put forward the following hypothesis, -which, he stated, had served him as a guide in his investigations. -According to the view of J. J. Thomson, radio-active -matter consists of negatively and positively charged particles. The -former have a mass about ¹⁄₁₀₀₀ of the mass of the hydrogen -atom, while the latter have a mass about one thousand times -greater than that of the negative particle. The negatively charged -particles (the β rays) would be projected with great velocity, but -the larger positive particles with a much lower velocity forming a -sort of gas (the emanation) which deposits itself on the surface of -bodies. This in turn would subdivide, giving rise to rays (excited -activity).</p> - -<p class='c006'>In a paper communicated to the Royal Society in June 1900, -Rutherford and McClung<a id='r335' href='#f335' class='c012'><sup>[335]</sup></a> estimated that the energy, radiated in -the form of ionizing rays into the gas, was 3000 gram-calories per -year for radium of activity 100,000 times that of uranium. Taking -the latest estimate of the activity of a pure radium compound as -2,000,000, this would correspond to an emission of energy into the -gas in the form of α rays of about 66,000 gram-calories per gram -per year. The suggestion was made that this energy might be -derived from a re-grouping of the constituents of the atom of the -radio-elements, and it was pointed out that the possible energy -to be derived from a greater concentration of the components of -the atom was large compared with that given out in molecular -reactions.</p> - -<p class='c006'>In the original papers<a id='r336' href='#f336' class='c012'><sup>[336]</sup></a> giving an account of the discovery of the -emanation of thorium and the excited radio-activity produced by -it, the view was taken that both of these manifestations were -due to radio-active material. The emanation behaved like a gas, -while the matter which caused excited activity attached itself to -solids and could be dissolved in some acids but not in others. -Rutherford and Miss Brooks showed that the radium emanation -<span class='pageno' id='Page_439'>439</span>diffused through air like a gas of heavy molecular weight. At -a later date Rutherford and Soddy showed that the radium and -thorium emanations behaved like chemically inert gases, since -they were unaffected by the most drastic physical and chemical -treatment.</p> - -<p class='c006'>On the other hand, P. Curie, who, in conjunction with Debierne, -had made a series of researches on the radium emanation, expressed -dissent from this view. P. Curie<a id='r337' href='#f337' class='c012'><sup>[337]</sup></a> did not consider that there was -sufficient evidence that the emanation was material in nature, and -pointed out that no spectroscopic evidence of its presence had yet -been obtained, and also that the emanation disappeared when -contained in a sealed vessel. It was pointed out by the writer<a id='r338' href='#f338' class='c012'><sup>[338]</sup></a> -that the failure to detect spectroscopic lines was probably a consequence -of the minute quantity of the emanation present, under -ordinary conditions, although the electrical and phosphorescent -actions produced by this small quantity are very marked. This -contention is borne out by later work. P. Curie at first took the -view that the emanation was not material, but consisted of centres -of condensation of energy attached to the gas molecules and moving -with them.</p> - -<p class='c006'>M. and Mme Curie have throughout taken a very general view -of the phenomena of radio-activity, and have not put forward any -definite theory. In Jan. 1902, they gave an account of the general -working theory<a id='r339' href='#f339' class='c012'><sup>[339]</sup></a> which had guided them in their researches. -Radio-activity is an atomic property, and the recognition of this -fact had created their methods of research. Each atom acts as a -constant source of emission of energy. This energy may either -be derived from the potential energy of the atom itself, or each -atom may act as a mechanism which instantly regains the energy -which is lost. They suggested that this energy may be borrowed -from the surrounding air in some way not accounted for by the -principle of Carnot.</p> - -<p class='c006'>In the course of a detailed study of the radio-activity of thorium, -Rutherford and Soddy<a id='r340' href='#f340' class='c012'><sup>[340]</sup></a> found that it was necessary to suppose -<span class='pageno' id='Page_440'>440</span>that thorium was continuously producing from itself new kinds of -active matter, which possess temporary activity and differ in chemical -properties from the thorium itself. The constant radio-activity -of thorium was shown to be the result of equilibrium between the -processes of production of active matter and the change of that -already produced. At the same time, the theory was advanced -that the production of active matter was a consequence of the disintegration -of the atom. The work of the following year was -devoted to an examination of the radio-activity of uranium and -radium on similar lines, and it was found that the conclusions -already advanced for thorium held equally for uranium and radium<a id='r341' href='#f341' class='c012'><sup>[341]</sup></a>. -The discovery of a condensation of the radio-active emanations<a id='r342' href='#f342' class='c012'><sup>[342]</sup></a> -gave additional support to the view that the emanations were -gaseous in character. In the meantime, the writer<a id='r343' href='#f343' class='c012'><sup>[343]</sup></a> had found that -the rays consisted of positively charged bodies atomic in size, -projected with great velocity. The discovery of the material -nature of these rays served to strengthen the theory of atomic -disintegration, and at the same time to offer an explanation of -the connection between the α rays and the changes occurring in -the radio-elements. In a paper entitled “Radio-active Change,” -Rutherford and Soddy<a id='r344' href='#f344' class='c012'><sup>[344]</sup></a> put forward in some detail the theory -of atomic disintegration as an explanation of the phenomena of -radio-activity, and at the same time some of the more important -consequences which follow from the theory were discussed.</p> - -<p class='c006'>In a paper announcing the discovery of the heat emission of -radium, P. Curie and Laborde<a id='r345' href='#f345' class='c012'><sup>[345]</sup></a> state that the heat energy may be -equally well supposed to be derived from a breaking up of the -radium atom or from energy absorbed by the radium from some -external source.</p> - -<p class='c006'>J. J. Thomson in an article on “Radium,” communicated to -<i>Nature</i><a id='r346' href='#f346' class='c012'><sup>[346]</sup></a>, put forward the view that the emission of energy from -radium is probably due to some change within the atom, and -<span class='pageno' id='Page_441'>441</span>pointed out that a large store of energy would be released by a -contraction of the atom.</p> - -<p class='c006'>Sir William Crookes<a id='r347' href='#f347' class='c012'><sup>[347]</sup></a>, in 1899, proposed the theory that the -radio-active elements possess the property of abstracting energy -from the gas. If the moving molecules, impinging more swiftly -on the substance, were released from the active substance at a -much lower velocity, the energy released from the radio-elements -might be derived from the atmosphere. This theory was advanced -again later on to account for the large heat emission of radium, -discovered by P. Curie and Laborde.</p> - -<p class='c006'>F. Re<a id='r348' href='#f348' class='c012'><sup>[348]</sup></a> recently advanced a very general theory of matter -with a special application to radio-active bodies. He supposes -that the parts of the atom were originally free, constituting a -nebula of extreme tenuity. These parts have gradually become -united round centres of condensation, and have thus formed the -atoms of the elements. On this view an atom may be likened -to an extinct sun. The radio-active atoms occupy a transitional -stage between the original nebula and the more stable chemical -atoms, and in the course of their contraction give rise to the -heat emission observed.</p> - -<p class='c006'>Lord Kelvin in a paper to the British Association meeting, -1903, has suggested that radium may obtain its energy from -external sources. If a piece of white paper is put into one vessel -and a piece of black paper into an exactly similar vessel, on exposure -of both vessels to the light the vessel containing the black -paper is found to be at a higher temperature. He suggests that -radium in a similar manner may keep its temperature above the -surrounding air by its power of absorption of unknown radiations.</p> - -<p class='c006'>Richarz and Schenck<a id='r349' href='#f349' class='c012'><sup>[349]</sup></a> have suggested that radio-activity may -be due to the production and breaking up of ozone which is known -to be produced by radium salts.</p> -<p class='c005'><b>255. Discussion of Theories.</b> From the survey of the -general hypotheses advanced as possible explanations of radio-activity, -<span class='pageno' id='Page_442'>442</span>it is seen that they may be divided broadly into two -classes, one of which assumes that the energy emitted from the -radio-elements is obtained at the expense of the internal energy of -the atom, and the other that the energy is derived from external -sources, but that the radio-elements act as mechanisms capable of -transforming this borrowed energy into the special forms manifested -in the phenomena of radio-activity. Of these two sets of hypotheses -the first appears to be the more probable, and to be best -supported by the experimental evidence. Up to the present not -the slightest experimental evidence has been adduced to show -that the energy of radium is derived from external sources.</p> - -<p class='c006'>J. J. Thomson (<i>loc. cit.</i>) has discussed the question in the -following way:—</p> - -<p class='c006'>“It has been suggested that the radium derives its energy from -the air surrounding it, that the atoms of radium possess the faculty -of abstracting the kinetic energy from the more rapidly moving air -molecules while they are able to retain their own energy when in -collision with the slowly moving molecules of air. I cannot see, -however, that even the possession of this property would explain -the behaviour of radium; for imagine a portion of radium placed -in a cavity in a block of ice; the ice around the radium gets -melted; where does the energy for this come from? By the hypothesis -there is no change in the air-radium system in the cavity, -for the energy gained by the radium is lost by the air, while heat -cannot flow into the cavity from the outside, for the melted ice -round the cavity is hotter than the ice surrounding it.”</p> - -<p class='c006'>The writer has recently found that the activity of radium is -not altered by surrounding it with a large mass of lead. A cylinder -of lead was cast 10 cms. in diameter and 10 cms. high. A hole -was bored in one end of the cylinder to the centre, and the radium, -enclosed in a small glass tube, was placed in the cavity. The -opening was then hermetically closed. The activity was measured -by the rate of discharge of an electroscope by the γ rays transmitted -through the lead, but no appreciable change was observed -during a period of one month.</p> - -<p class='c006'>M. and Mme Curie early made the suggestion that the radiation -of energy from the radio-active bodies might be accounted for by -supposing that space is traversed by a type of Röntgen rays, and -<span class='pageno' id='Page_443'>443</span>that the radio-elements possess the property of absorbing them. -Recent experiments (<a href='#section279'>section 279</a>) have shown that there is present -at the surface of the earth a very penetrating type of rays, similar -to the γ rays of radium. Even if it were supposed that the radio-elements -possessed the power of absorbing this radiation, the -energy of the rays is far too minute to account even for the energy -radiated from an element of small activity like uranium. In -addition, all the evidence so far obtained points to the conclusion -that the radio-active bodies do not absorb the type of rays they -emit to any greater extent than would be expected from their -density. It has been shown (<a href='#section086'>section 86</a>) that this is true in the -case of uranium. Even if it were supposed that the radio-elements -possess the property of absorbing the energy of some unknown -type of radiation, which is able to pass through ordinary matter -with little absorption, there still remains the fundamental difficulty -of accounting for the peculiar radiations from the radio-elements, -and the series of changes that occur in them. It is not sufficient -for us to account for the heat emission only, for it has been shown -(<a href='#chap12'>chapter <span class='fss'>XII</span></a>) that the emission of heat is directly connected with -the radio-activity.</p> - -<p class='c006'>In addition, the distribution of the heat emission of radium -amongst the radio-active products which arise from it is extremely -difficult to explain on the hypothesis that the energy emitted -is borrowed from external sources. It has been shown that more -than two-thirds of the heat emitted by radium is due to the -emanation together with the active deposit which is produced -by the emanation. When the emanation is separated from the -radium, its power of emitting heat, after reaching a maximum, -decreases with the time according to an exponential law. It -would thus be necessary on the absorption hypothesis to postulate -that most of the heat emission of radium, observed under ordinary -conditions, is not due to the radium itself but to something produced -by the radium, whose power of absorbing energy from -external sources diminishes with time.</p> - -<p class='c006'>A similar argument also applies to the variation with time of -the heating effect of the active deposit produced from the emanation. -It has been shown in the last chapter that most of the -heating effect observed in radium and its products must be ascribed -<span class='pageno' id='Page_444'>444</span>to the bombardment of the α particles expelled from these substances. -It has already been pointed out (<a href='#section136'>section 136</a>) that it is -difficult to imagine any mechanism, either internal or external, -whereby such enormous velocity can suddenly be impressed upon -the α particles. We are forced to the conclusion that the α particle -did not suddenly acquire this energy of motion, but was initially -in rapid motion in the atom, and for some reason, was suddenly -released with the velocity which it previously possessed in its -orbit.</p> - -<p class='c006'>The strongest evidence against the hypothesis of absorption of -external energy is that such a theory ignores the fact, that, whenever -radio-activity is observed, it is always accompanied by some -change which can be detected by the appearance of new products -having chemical properties distinct from those of the original -substances. This leads to some form of “chemical” theory, and -other results show that the change is atomic and not molecular.</p> -<p class='c005'><b>256. Theory of radio-active change.</b> The processes occurring -in the radio-elements are of a character quite distinct from any -previously observed in chemistry. Although it has been shown -that the radio-activity is due to the spontaneous and continuous -production of new types of active matter, the laws which control -this production are different from the laws of ordinary chemical -reactions. It has not been found possible in any way to alter -either the rate at which the matter is produced or its rate of -change when produced. Temperature, which is such an important -factor in altering the rate of chemical reactions, is, in these cases, -almost entirely without influence. In addition, no ordinary -chemical change is known which is accompanied by the expulsion -of charged atoms with great velocity. It has been suggested -by Armstrong and Lowry<a id='r350' href='#f350' class='c012'><sup>[350]</sup></a> that radio-activity may be an -exaggerated form of fluorescence or phosphorescence with a very -slow rate of decay. But no form of phosphorescence has yet been -shown to be accompanied by radiations of the character of those -emitted by the radio-elements. Whatever hypothesis is put -forward to explain radio-activity must account not only for the -production of a series of active products, which differ in chemical -<span class='pageno' id='Page_445'>445</span>and physical properties from each other and from the parent -element, but also for the emission of rays of a special character. -Besides this, it is necessary to account for the large amount of -energy continuously radiated from the radio-elements.</p> - -<p class='c006'>The radio-elements, besides their high atomic weights, do not -possess in common any special chemical characteristics which differentiate -them from the other elements, which do not possess the -property of radio-activity to an appreciable degree. Of all the -known elements, uranium, thorium, and radium possess the -greatest atomic weights, viz.: radium 225, thorium 232·5, and -uranium 240.</p> - -<p class='c006'>If a high atomic weight is taken as evidence of a complicated -structure of the atom, it might be expected that disintegration -would occur more readily in heavy than in light atoms. At the -same time, there is no reason to suppose that the elements of the -highest atomic weight must be the most radio-active; in fact, -radium is far more active than uranium, although its atomic -weight is less. This is seen to be the case also in the radio-active -products; for example, the radium emanation is enormously more -active weight for weight than the radium itself, and there is every -reason to believe that the emanation has an atom lighter than -that of radium.</p> - -<p class='c006'>In order to explain the phenomena of radio-activity, Rutherford -and Soddy have advanced the theory that the atoms of the radio-elements -suffer spontaneous disintegration, and that each disintegrated -atom passes through a succession of well-marked changes, -accompanied in most cases by the emission of α rays.</p> - -<p class='c006'>A preliminary account of this hypothesis has already been -given in <a href='#section136'>section 136</a>, while the mathematical theory of successive -changes, which is based upon it, has been discussed in <a href='#chap09'>chapter <span class='fss'>IX</span></a>. -The general theory has been utilized in chapters <a href='#chap10'><span class='fss'>X</span></a> and <a href='#chap11'><span class='fss'>XI</span></a> to -account for the numerous active substances found in uranium, -thorium, actinium and radium.</p> - -<p class='c006'>The theory supposes that, on an average, a definite small -proportion of the atoms of each radio-active substance becomes -unstable at a given time. As a result of this instability, the -atoms break up. In most cases, the disintegration is explosive in -violence and is accompanied by the ejection of an α particle with -<span class='pageno' id='Page_446'>446</span>great velocity; in a few cases, α and β particles are expelled -together, while in others a β particle alone escapes. In a few -cases, the change in the atom appears to be less violent in -character, and is not accompanied by the expulsion of either an -α or β particle. The explanation of these rayless changes is -considered in <a href='#section259'>section 259</a>. The expulsion of an α particle, of mass -about twice that of the hydrogen atom, leaves behind it a new -system lighter than the original one, and possessing chemical -and physical properties quite different from those of the original -element. This new system again becomes unstable, and expels -another α particle. The process of disintegration, once started, -proceeds from stage to stage at a definite measurable rate in -each case.</p> - -<p class='c006'>At any time after the disintegration has commenced, there -exists a proportion of the original matter, which is unchanged, -mixed with the part which has undergone change. This is in -accordance with the observed fact that the spectrum of radium, -for example, does not change progressively with time. The -radium breaks up so slowly that only a small fraction has been -transformed in the course of a few years. The unchanged part -still shows its characteristic spectrum, and will continue to do so as -long as any radium exists. At the same time it is to be expected -that, in old radium, the spectrum of those products which exist in -any quantity should also appear.</p> - -<p class='c006'>The term metabolon has been suggested as a convenient -expression for each of these changing atoms, derived from the -successive disintegration of the atoms of the radio-elements. -Each metabolon, on an average, exists only for a limited time. -In a collection of metabolons of the same kind the number <i>N</i>, -which are unchanged at a time <i>t</i> after production, is given by</p> - -<div class='figcenter id010'> -<img src='images/form-137.png' alt='Formula.' class='ig001'> -</div> - -<p class='c006'>where <i>N</i>₀ is the original number. Now <i>dN</i>/<i>dt</i> = -λ<i>N</i>, -or the fraction of the metabolons present, which change in unit -time, is equal to λ. The value 1/λ may be taken as the <i>average -life</i> of each metabolon.</p> - -<p class='c006'>This may be simply shown as follows:—At any time <i>t</i> -after <i>N</i>₀ metabolons have been set aside, the number which -change in the time <i>dt</i> is equal to λ<i>Ndt</i> or</p> - -<div class='figcenter id010'> -<img src='images/form-138.png' alt='Formula.' class='ig001'> -</div> - -<p class='c006'>Each -<span class='pageno' id='Page_447'>447</span>metabolon has a life <i>t</i>, so that the average life of the whole -number is given by</p> - -<div class='figcenter id005'> -<img src='images/form-139.png' alt='Formula.' class='ig001'> -</div> - -<p class='c006'>The various metabolons from the radio-elements are distinguished -from ordinary matter by their great instability and consequent -rapid rate of change. Since a body which is radio-active -must <i>ipso facto</i> be undergoing change, it follows that none of the -active products, for example, the emanations and Th X, can consist -of any known kind of matter; for there is no evidence to show that -inactive matter can be made radio-active, or that two forms of the -same element can exist, one radio-active and the other not. For -example, half of the matter constituting the radium emanation -has undergone change after an interval of four days. After the -lapse of about one month the emanation as such has nearly -disappeared, having been transformed through several stages into -other and more stable types of matter, which are in consequence -difficult to detect by their radio-activity.</p> - -<p class='c006'>The striking difference in chemical and physical properties -which exists in many cases between the various products themselves, -and also between the primary active substance and its -products, has already been drawn attention to in <a href='#chap09'>chapter <span class='fss'>IX</span></a>. -Some of the products show distinctive electro-chemical behaviour -and can be removed from a solution by electrolysis. Others show -differences in volatility which have been utilized to effect a partial -separation. There can be no doubt that each of these products is -a definite new chemical substance, and if it could be collected in -sufficient quantity to be examined by ordinary chemical means, -would be found to behave like a distinct chemical element. It -would differ, however, from the ordinary chemical element in the -shortness of its life, and the fact that it is continuously changing -into another substance. We shall see later (<a href='#section261'>section 261</a>) that there -is every reason to believe that radium itself is a metabolon in the -true sense of the term, since it is continuously changing, and is -itself produced from another substance. The main point of -difference between it and the other products lies in the comparative -slowness of its rate of change.</p> - -<p class='c006'><span class='pageno' id='Page_448'>448</span>It is for this reason that radium exists in pitchblende in -greater quantity than the other more rapidly changing products. -By working up a large amount of the mineral, we have seen -that a sufficient quantity of the pure product has been obtained -for chemical examination.</p> - -<p class='c006'>On account of the short life of the emanation, it exists in -pitchblende in much less quantity than radium, but it, too, has -been isolated chemically and its volume measured. The extraordinary -properties of this emanation, or gas, have already been -discussed, and there can be no doubt that, while it exists, it must -be considered a new element allied in chemical properties to the -argon-helium group of gases.</p> - -<p class='c006'>There can be no doubt that in the radio-elements we are -witnessing the spontaneous transformation of matter, and that the -different products which arise mark the stages or halting-places in -the process of transformation, where the atoms are able to exist for -a short time before again breaking up into new systems.</p> -<p class='c005'><b>257. Radio-active products.</b> The following table gives -the list of the active products or metabolons known to result from -the disintegration of the three radio-elements. In the second -column is given the value of the radio-active constant λ for each -active product, <i>i.e.</i> the proportion of the active matter undergoing -change per second; in the third column the time <i>T</i> required for -the activity to fall to one-half, <i>i.e.</i> the time taken for half the active -product to undergo change; in the fourth column, the nature of the -rays from each active product, not including the rays from the -products which result from it; in the fifth column, a few of the -more marked physical and chemical properties of each metabolon.</p> - -<table class='table1' > -<colgroup> -<col class='colwidth20'> -<col class='colwidth20'> -<col class='colwidth16'> -<col class='colwidth16'> -<col class='colwidth26'> -</colgroup> - <tr> - <th class='bbm c013'>Products</th> - <th class='bbm c013'>λ(sec)<sup>-1</sup></th> - <th class='bbm c013'>T</th> - <th class='bbm c013'>Nature of the rays</th> - <th class='bbm c014'>Chemical and Physical properties of the product</th> - </tr> - <tr> - <td class='c013'>Uranium</td> - <td class='c013'>—</td> - <td class='c013'>—</td> - <td class='c013'>α</td> - <td class='c014'>Soluble in excess of ammonium carbonate, soluble in ether.</td> - </tr> - <tr> - <td class='bbm c013'>Uranium X</td> - <td class='bbm c013'>3·6 × 10<sup>-7</sup></td> - <td class='bbm c013'>22 days</td> - <td class='bbm c013'>β and γ</td> - <td class='bbm c014'>Insoluble in excess of ammonium carbonate, soluble in ether and water.</td> - </tr> - <tr> - <td class='c013'>Thorium</td> - <td class='c013'>—</td> - <td class='c013'>—</td> - <td class='c013'>α</td> - <td class='c014'>Insoluble in ammonia.</td> - </tr> - <tr> - <td class='c013'>Thorium X</td> - <td class='c013'>2·0 × 10<sup>-6</sup></td> - <td class='c013'>4 days</td> - <td class='c013'>α</td> - <td class='c014'>Soluble in ammonia and water.</td> - </tr> - <tr> - <td class='c013'>Emanation</td> - <td class='c013'>1·3 × 10<sup>-2</sup></td> - <td class='c013'>53 secs.</td> - <td class='c013'>α</td> - <td class='c014'>Chemically inert gas of heavy molecular weight. Condenses at -120° C.</td> - </tr> - <tr> - <td class='c013'>Thorium A</td> - <td class='c013'>1·74 × 10<sup>-5</sup></td> - <td class='c013'>11 hours</td> - <td class='c013'>no rays</td> - <td class='c014'>Deposited on bodies; concentrated on the cathode in an electric field. Soluble in some acids; Th A more volatile than Th B; shows definite electro-chemical behaviour.</td> - </tr> - <tr> - <td class='c013'>Thorium B</td> - <td class='c013'>2·2 × 10<sup>-4</sup></td> - <td class='c013'>55 mins.</td> - <td class='c013'>α, β, γ</td> - <td class='c014'>Same</td> - </tr> - <tr> - <td class='bbm c013'>?</td> - <td class='bbm c013'>—</td> - <td class='bbm c013'>—</td> - <td class='bbm c013'>—</td> - <td class='bbm c014'> </td> - </tr> - <tr> - <td class='c013'>Actinium</td> - <td class='c013'>—</td> - <td class='c013'>—</td> - <td class='c013'>no rays</td> - <td class='c014'>Insoluble in ammonia.</td> - </tr> - <tr> - <td class='c013'>Actinium X</td> - <td class='c013'>7·8 × 10<sup>-7</sup></td> - <td class='c013'>10·2 days</td> - <td class='c013'>α (and β?)</td> - <td class='c014'>Soluble in ammonia.</td> - </tr> - <tr> - <td class='c013'>Emanation</td> - <td class='c013'>·17</td> - <td class='c013'>3·9 secs.</td> - <td class='c013'>α</td> - <td class='c014'>Behaves like a gas.</td> - </tr> - <tr> - <td class='c013'>Actinium A</td> - <td class='c013'>3·2 × 10<sup>-4</sup></td> - <td class='c013'>36 mins.</td> - <td class='c013'>no rays</td> - <td class='c014'>Deposited on bodies; concentrated on the cathode in an electric field, soluble in ammonia and strong acids; volatilized at a temperature of 100° C., A and B can be separated by electrolysis.</td> - </tr> - <tr> - <td class='c013'>Actinium B</td> - <td class='c013'>5·4 × 10<sup>-3</sup></td> - <td class='c013'>2·15 mins.</td> - <td class='c013'>α, β, γ</td> - <td class='c014'>Same</td> - </tr> - <tr> - <td class='bbm c013'>?</td> - <td class='bbm c013'>—</td> - <td class='bbm c013'>—</td> - <td class='bbm c013'>—</td> - <td class='bbm c014'> </td> - </tr> - <tr> - <td class='c013'>Radium</td> - <td class='c013'>—</td> - <td class='c013'>1300 years</td> - <td class='c013'>α</td> - <td class='c014'>Allied chemically to barium.</td> - </tr> - <tr> - <td class='c013'>Emanation</td> - <td class='c013'>2·1 × 10<sup>-6</sup></td> - <td class='c013'>3·8 days</td> - <td class='c013'>α</td> - <td class='c014'>Chemically inert gas of heavy molecular weight; condenses at -150° C.</td> - </tr> - <tr> - <td class='c013'>Radium A (active deposit of rapid change)</td> - <td class='c013'>3·85 × 10<sup>-3</sup></td> - <td class='c013'>3 mins.</td> - <td class='c013'>α</td> - <td class='c014'>} Deposited on surface of bodies; concentrated on cathode in electric field; soluble in strong acids; B volatized at about 700° C., A and C at about 1000° C.</td> - </tr> - <tr> - <td class='c013'>Radium B (same)</td> - <td class='c013'>5·38 × 10<sup>-4</sup></td> - <td class='c013'>21 mins.</td> - <td class='c013'>no rays</td> - <td class='c014'>Same</td> - </tr> - <tr> - <td class='c013'>Radium C (same)</td> - <td class='c013'>4·13 × 10<sup>-4</sup></td> - <td class='c013'>28 mins.</td> - <td class='c013'>α, β, γ</td> - <td class='c014'>Same</td> - </tr> - <tr> - <td class='c013'>Radium D (active deposit of slow change)</td> - <td class='c013'>—</td> - <td class='c013'>about 40</td> - <td class='c013'>no rays</td> - <td class='c014'>Soluble in acids; volatile below 1000° C.</td> - </tr> - <tr> - <td class='c013'>Radium E (same)</td> - <td class='c013'>1·3 × 10<sup>-6</sup></td> - <td class='c013'>6 days</td> - <td class='c013'>β and γ</td> - <td class='c014'>Non-volatile at 1000° C.</td> - </tr> - <tr> - <td class='c013'>Radium F (same)</td> - <td class='c013'>5·6 × 10<sup>-8</sup></td> - <td class='c013'>143 days</td> - <td class='c013'>α</td> - <td class='c014'>Deposited on bismuth from solution; volatile at about 1000° C., same properties as radio-tellurium and polonium.</td> - </tr> -</table> - -<p class='c006'>The products and their radiations are indicated graphically in -<a href='#fig102'>Fig. 102</a> on page <a href='#Page_448'>448</a>.</p> - -<div id='fig102' class='figcenter id008'> -<img src='images/fig-102.png' alt='Fig. 102.' class='ig001'> -<div class='ic002'> -<p>Fig. 102.</p> -</div> -</div> - -<p class='c006'>One product has been observed in uranium, four in thorium, -four in actinium and seven in radium. It is not improbable that -a closer examination of the radio-elements may reveal still further -changes. If any very rapid transformations exist, they would be -very difficult to detect. The change of thorium X into the -emanation, for example, would probably not have been discovered -if the product of the change had not been gaseous in character. -<span class='pageno' id='Page_451'>451</span>The electrolysis of solutions is, in many cases, a very powerful -method of separating active products from one another, and its -possibilities have not yet been exhausted. The main family of -changes of the radio-elements, as far as they are known, have been -investigated closely, and it is not likely that any product of -comparatively slow rate of change has been overlooked. There is -a possibility, however, that two radio-active products may in some -cases arise from the disintegration of a single substance. This -point is discussed further in section 260.</p> - -<p class='c006'>The remarkable way in which the disintegration theory can -be applied to unravel the intricacies of the succession of radio-active -changes is very well illustrated in the case of radium. -Without its aid, it would not have been possible to disentangle -the complicated processes which occur. We have already seen -that this analysis has been instrumental in showing that the -substances polonium, radio-tellurium and radio-lead are in reality -products of radium.</p> - -<p class='c006'>After the radio-active substances have undergone the succession -of changes traced above, a final stage is reached where the atoms -are either permanently stable, or change so slowly that it is -difficult to detect their presence by means of their radio-activity. -It is probable, however, that the process of transformation still -continues through further slow stages.</p> - -<p class='c006'>There is now considerable evidence that the elements uranium, -radium and actinium are intimately connected together. The two -latter probably result from the breaking up of uranium. The -evidence in support of this idea is given in <a href='#section262'>section 262</a>, but there -still remains much work to be done to bridge over the gaps which -at present appear to separate these elements from one another.</p> - -<p class='c006'>After the series of transformations have come to an end, there -will probably remain a product or products which will be inactive, -or active only to a minute extent. In addition, since the α -particles, expelled during the transformation, are material in -nature, and are non-radio-active, they must collect in some quantity -in radio-active matter. The probability that the α particles consist -of helium is considered later in <a href='#section268'>section 268</a>.</p> - -<p class='c006'>The value of <i>T</i>, the time for a product to be half-transformed, -may be taken as a comparative measure of the stability of the -<span class='pageno' id='Page_452'>452</span>different metabolons. The stability of the products varies over -a very wide range. For example, the value of <i>T</i> for radium D is -40 years, and for the actinium emanation 3·9 secs. This corresponds -to a range of stability measured by 3·8 × 10<sup>8</sup>. The range of -stability is still further extended, when it is remembered that the -atoms of the radio-elements themselves are very slowly changing.</p> - -<p class='c006'>The only two metabolons of about the same stability are -thorium X and the radium emanation. In each case, the transformation -is half completed in about four days. I consider that -the approximate agreement of the numbers is a mere coincidence, -and that the two types of matter are quite distinct from one -another; for, if the metabolons were identical, it would be -expected that the changes which follow would take place in the -same way and at the same rate, but such is not the case. Moreover, -Th X and the radium emanation have chemical and physical -properties quite distinct from one another.</p> - -<p class='c006'>It is very remarkable that the three radio-active substances, -radium, thorium and actinium, should exhibit such a close similarity -in the succession of changes which occur in them. Each -of them at one stage of its disintegration emits a radio-active gas, -and in each case this gas is transformed into a solid which is -deposited upon the surface of bodies. It would appear that, after -disintegration of an atom of any of these has once begun, there is -a similar succession of changes, in which the resulting systems -have allied chemical and physical properties. Such a connection -is of interest as indicating a possible origin of the recurrence of -properties in the atoms of the elements, as exemplified by the -periodic law. The connection between thorium and actinium is -especially close both as regards the number and nature of the -products. The period of transformation of the successive products, -though differing in magnitude, rises and falls in a very analogous -manner. This indicates that the atoms of these two elements are -very similarly constituted.</p> -<p class='c005'><b>258. Amount of the products.</b> By application of the -theory of successive changes, the probable amount of each of the -products present in radium and the other radio-elements can -readily be estimated.</p> - -<p class='c006'><span class='pageno' id='Page_453'>453</span>Since each radio-atom expels one α particle of atomic weight -about that of hydrogen or helium, the atoms of the intermediate -products will not differ much in weight from the parent atom.</p> - -<p class='c006'>The approximate weight of each product present in a gram -of radium can be readily deduced. Let <i>N<sub>A</sub></i>, <i>N<sub>B</sub></i>, <i>N<sub>C</sub></i> be the -number of atoms of the products A, B, C present per gram in -radio-active equilibrium. Let λ<sub><i>A</i></sub>, λ<sub><i>B</i></sub>, λ<sub><i>C</i></sub> be the corresponding -constants of change. Then if <i>q</i> is the number of the parent atoms -breaking up per second, per gram,</p> - -<div class='calc'> - -<div class='lg-container-b c017'> - <div class='linegroup'> - <div class='group'> - <div class='line'><i>q</i> = λ<sub><i>A</i></sub><i>N<sub>A</sub></i> = λ<sub><i>B</i></sub><i>N<sub>B</sub></i> = λ<sub><i>C</i></sub><i>N<sub>C</sub></i>.</div> - </div> - </div> -</div> - -</div> - -<p class='c018'>Consider the case of the radium products, where the value of <i>q</i> -is 6·2 × 10<sup>10</sup> (<a href='#section093'>section 93</a>). Knowing the value of λ and <i>q</i>, the -value of <i>N</i> can at once be calculated. The corresponding weight -can be deduced, since in one gram of matter of atomic weight -about 200, there are about 4 × 10<sup>21</sup> atoms (section 39). The -results are shown in the following table:—</p> - -<table class='table23' > -<colgroup> -<col class='colwidth25'> -<col class='colwidth25'> -<col class='colwidth25'> -<col class='colwidth25'> -</colgroup> - <tr> - <th class='c013'>Product</th> - <th class='c013'>Value of λ (sec)<sup>-1</sup></th> - <th class='c013'>Number of atoms, <i>N</i>, present per gram</th> - <th class='c014'>Weight of product gram of radium</th> - </tr> - <tr> - <td class='c013'> </td> - <td class='c013'> </td> - <td class='c013'> </td> - <td class='c014'> </td> - </tr> - <tr> - <td class='c013'>Radium emanation</td> - <td class='c013'>2·0 × 10<sup>-6</sup></td> - <td class='c013'>3·2 × 10<sup>16</sup></td> - <td class='c014'>8 × 10<sup>-3</sup></td> - </tr> - <tr> - <td class='c013'>Radium A</td> - <td class='c013'>3·8 × 10<sup>-3</sup></td> - <td class='c013'>1·7 × 10<sup>13</sup></td> - <td class='c014'>4 × 10<sup>-6</sup></td> - </tr> - <tr> - <td class='c013'>Radium B</td> - <td class='c013'>5·4 × 10<sup>-4</sup></td> - <td class='c013'>1·3 × 10<sup>14</sup></td> - <td class='c014'>3 × 10<sup>-5</sup></td> - </tr> - <tr> - <td class='c013'>Radium C</td> - <td class='c013'>4·1 × 10<sup>-4</sup></td> - <td class='c013'>1·6 × 10<sup>14</sup></td> - <td class='c014'>4 × 10<sup>-5</sup></td> - </tr> -</table> - -<p class='c006'>With the small quantities of radium available, the amounts -of the products radium A, B and C are too small to weigh. It -may be possible, however, to detect their presence by means of -the spectroscope.</p> - -<p class='c006'>In the case of thorium, the weight of the product Th X, which -is present in greatest quantity, is far too small to be detected. -Since the value of λ for Th X is about the same as for the radium -emanation, the maximum weight present per gram is about -4 × 10<sup>-12</sup> of a gram, remembering that <i>q</i> for radium is about -2 × 10<sup>6</sup> times the value for thorium. Even with a kilogram of -<span class='pageno' id='Page_454'>454</span>thorium, the amount of Th X is far too small to be detected by its -weight.</p> - -<p class='c006'>This method can be used generally to calculate the relative -amounts of any successive products in radio-active equilibrium, -provided the value of λ for each product is known. For example, -it will be shown later that uranium is the parent of radium and -is half transformed in about 6 × 10<sup>8</sup> years, while radium and -radium D are half transformed in 1300 and 40 years respectively. -The weight of radium present in one gram of uranium, when -equilibrium is established, is thus -2 × 10<sup>-6</sup> grams, and the weight of radium D is 7 × 10<sup>-8</sup> grams. -In a mineral containing a ton of -uranium there should be about 1·8 grams of radium and ·063 grams -of radium D. Some recent experiments described in <a href='#section262'>section 262</a> -show that these theoretical estimates are about twice too great.</p> -<p class='c005'><a id='section259'></a> -<b>259. Rayless Changes.</b> The existence of well-marked -changes in radium, thorium, and actinium, which are not accompanied -by the expulsion of α or β particles, is of great interest -and importance.</p> - -<p class='c006'>Since the rayless changes are not accompanied by any -appreciable ionization of the gas, their presence cannot be detected -by direct means. The rate of change of the substance can, -however, be determined indirectly, as we have seen, by measurement -of the variation with time of the activity of the succeeding -product. The law of change has been found to be the same as -for the changes which give rise to α rays. The rayless changes -are thus analogous, in some respects, to the monomolecular changes -observed in chemistry, with the difference that the changes are -in the atom itself, and are not due to the decomposition of a -molecule into simpler molecules or into its constituent atoms.</p> - -<p class='c006'>It must be supposed that a rayless change is not of so violent -a character as one which gives rise to the expulsion of α or β -particles. The change may be accounted for either by supposing -that there is a rearrangement of the components of the atom, -or that the atom breaks up without the expulsion of its parts -with sufficient velocity to produce ionization by collision with the -gas. The latter point of view, if correct, at once indicates the -possibility that undetected changes of a similar character may be -<span class='pageno' id='Page_455'>455</span>taking place slowly in the non-radio-active elements; or, in other -words, that all matter may be undergoing a slow process of change. -The changes taking place in the radio-elements have been observed -only in consequence of the expulsion with great velocity of the -parts of the disintegrated atom. Some recent experiments described -in <a href='#appa'>Appendix A</a> show that the α particle from radium -ceases to ionize the gas when its velocity falls below about 10<sup>9</sup> cms. -per second. It is thus seen that α particles may be projected -with a great velocity, and yet fail to produce ionization in the gas. -In such a case, it would be difficult to follow the changes by the -electrical method, as the electrical effects would be very small in -comparison with those produced by the known radio-active bodies.</p> -<p class='c005'><b>260. Radiations from the products.</b> We have seen that -the great majority of the radio-active products break up with the -expulsion of α particles, and that the β particle with its accompaniment -of the γ ray appears in most cases only in the last rapid -change. In the case of radium, for example, which has been most -closely investigated on account of its great activity, radium itself, -the emanation and radium A emit only α particles; radium B -emits no rays at all; while radium C emits all three kinds of rays. -It is difficult to settle with certainty whether the products thorium -X and actinium X emit β particles or not, but the β and γ rays -certainly appear in each case in the last rapid change in the -active deposit, and, in this respect, behave in a similar manner -to radium.</p> - -<p class='c006'>The very slow moving electrons which accompany the particles -emitted from radium (<a href='#section093'>section 93</a>) are not taken into account, for -they appear to be liberated as a result of the impact of α particles -on matter, and are expelled with a speed insignificant compared -with that of the β particles emitted from radium C.</p> - -<p class='c006'>The appearance of β and γ rays only in the last rapid changes -of the radio-elements is very remarkable, and cannot be regarded -as a mere coincidence. The final expulsion of a β particle results -in the appearance of a product of great stability, or, in the case -of radium, of a product (radium D) which has far more stability -than the preceding one. It would appear that the initial changes -are accompanied by the expulsion of an α particle, and that once -<span class='pageno' id='Page_456'>456</span>the β particle is expelled, the components of the residual atom -fall into an arrangement of fairly stable equilibrium, where the -rate of transformation is very slow. It thus appears probable that -the β particle, which is finally expelled, may be regarded as the -active agent in promoting the disintegration of the radio-atom -through the successive stages. A discussion of this question will -be given with more advantage later (<a href='#section270'>section 270</a>), when the -general question of the stability of the atom is under consideration.</p> - -<p class='c006'>It is significant that the change in which the three types of -rays appear is far more violent in character than the preceding -changes. Not only are the α particles expelled with greater -velocity than in any other change, but the β particles are -projected with a velocity very closely approaching that of light.</p> - -<p class='c006'>There is always a possibility that, in such a violent explosion -in the atom, not only may the α and β particles be expelled, but -the atom itself may be disrupted into several fragments. If the -greater proportion of the matter resulting from the disintegration -is of one kind, it would be difficult to detect the presence of a -small quantity of rapidly changing matter from observations of -the rate of decay; but, if the products have distinctive electro-chemical -behaviour, a partial separation should, in some cases, -be effected by electrolysis. It has already been pointed out that -the results of Pegram and von Lerch (<a href='#section207'>section 207</a>) on the electrolysis -of thorium solutions may be explained on the supposition -that thorium A and B have distinctive electro-chemical behaviour. -Pegram, however, in addition observed the presence of a product -which decayed to half value in six minutes. This active product -was obtained by electrolysing a solution of pure thorium salt, to -which a small quantity of copper nitrate had been added. The -copper deposit was slightly active and lost half of its activity in -about six minutes.</p> - -<p class='c006'>The presence of such radio-active products, which do not come -under the main scheme of changes, indicates that, at some stage -of the disintegration, more than one substance results. In the -violent disintegration which occurs in radium C and thorium B, -such a result is to be expected, for it is not improbable that there -are several arrangements whereby the constituents of the atom -<span class='pageno' id='Page_457'>457</span>form a system of some slight stability. The two products resulting -from the disintegration would probably be present in unequal -proportion, and, unless they gave out different kinds of rays, -would be difficult to separate from each other.</p> -<p class='c005'><a id='section261'></a> -<b>261. Life of radium.</b> Since the atoms of the radio-elements -are continuously breaking up, they must also be considered to be -metabolons, the only difference between them and metabolons such -as the emanations Th X and others being their comparatively great -stability and consequent very slow rate of change. There is no -evidence that the process of change, traced above, is reversible -under present conditions, and in the course of time a quantity of -radium, uranium, or thorium left to itself must gradually be transformed -into other types of matter.</p> - -<p class='c006'>There seems to be no escape from this conclusion. Let us -consider, for example, the case of radium. The radium is continuously -producing from itself the radium emanation, the rate of -production being always proportional to the amount of radium -present. All the radium must ultimately be changed into emanation, -which in turn is transformed through a succession of stages -into other kinds of matter. There is no doubt that the emanation -is chemically quite different from radium itself. The quantity of -radium must diminish, to compensate for the emanation which is -formed; otherwise it is necessary to assume that matter in the -form of emanation is created from some unknown source.</p> - -<p class='c006'>An approximate estimate of the rate of change of radium can -easily be made by two different methods depending upon (1) the -number of atoms of radium breaking up per second, and (2) the -amount of emanation produced per second.</p> - -<p class='c006'>It has been shown experimentally (<a href='#section093'>section 93</a>) that 1 gram of -radium at its minimum activity expels 6·2 × 10<sup>10</sup> α particles per -second. The heating effect of radium and also its volume agree -closely with calculation, if it is supposed that each atom of each -product in breaking up emits one α particle. On this supposition -it is seen that 6·2 × 10<sup>10</sup> atoms of radium break up per second.</p> - -<p class='c006'>Now it has been shown experimentally (section 39) that one -cubic centimetre of hydrogen at standard pressure and temperature -contains 3·6 × 10<sup>19</sup> molecules. Taking the atomic weight of radium -<span class='pageno' id='Page_458'>458</span>as 225, the number of atoms in 1 gram of radium is equal to -3·6 × 10<sup>21</sup>. The fraction λ of radium which breaks up is thus 1·95 × 10<sup>-11</sup> -per second, or 5·4 × 10<sup>-4</sup> per year. It follows that -in each gram of radium about half a milligram breaks up per year. -The average life of radium is about 1800 years, and half of the -radium is transformed in about 1300 years.</p> - -<p class='c006'>We shall now consider the calculation, based on the observed -result of Ramsay and Soddy, that the volume of emanation to be -obtained from one gram of radium is about 1 cubic millimetre. -The experimental evidence based on diffusion results indicates that -the molecular weight of the emanation is about 100. If the disintegration -theory is correct, the emanation is an atom of radium -minus one particle, and therefore must have a molecular weight of -at least 200. This high value is more likely to be correct than the -experimental number, which is based on evidence that must -necessarily be somewhat uncertain. Now the rate of production -of emanation per second is equal to λ<i>N</i>₀, where <i>N</i>₀ is the equilibrium -amount. Taking the molecular weight as 200, the weight -of emanation produced per second from 1 gram of radium -= 8·96 × 10<sup>-6</sup>λ = 1·9 × 10<sup>-11</sup> gram.</p> - -<p class='c006'>Now the weight of emanation produced per second is very -nearly equal to the weight of radium breaking up per second. -Thus the fraction of radium breaking up per second is about -1·9 × 10<sup>-11</sup>, which is in agreement with the number previously -calculated by the first method.</p> - -<p class='c006'>We may thus conclude that <i>radium is half transformed in -about 1300 years</i>.</p> - -<p class='c006'>Taking the activity of pure radium as about two million times -that of uranium, and remembering that only one change, which -gives rise to α rays, occurs in uranium and four in radium, it can -readily be calculated that the fraction of uranium changing per -year is about 10<sup>-9</sup>. From this it follows that uranium should be half transformed in about 6 × 10<sup>8</sup> years.</p> - -<p class='c006'>If thorium is a true radio-active element, the time for half -transformation is about 2·4 × 10<sup>9</sup> years, since thorium has about -the same activity as uranium but contains four products which -emit α rays. If the activity of thorium is due to some radio-active -impurity, no estimate of the length of its life can be made until -<span class='pageno' id='Page_459'>459</span>the primary active substance has been isolated and its activity -measured.</p> -<p class='c005'><a id='section262'></a> -<b>262. Origin of radium.</b> The changes in radium are thus -fairly rapid, and a mass of radium if left to itself should in the -course of a few thousand years have lost a large proportion of its -radio-activity. Taking the above estimate of the life of radium, -the value of λ is 5·4 × 10<sup>-4</sup>, with a year as the unit of time. A -mass of radium left to itself should be half transformed in 1300 -years and only one-millionth part would remain after 26,000 years. -Thus supposing, for illustration, that the earth was originally composed -of pure radium, its activity per gram 26,000 years later -would not be greater than the activity observed to-day in a good -specimen of pitchblende. Even supposing this estimate of the life -of radium is too small, the time required for the radium practically -to disappear is short compared with the probable age of the -earth. We are thus forced to the conclusion that radium is being -continuously produced in the earth, unless the very improbable -assumption is made, that radium was in some way suddenly formed -at a date recent in comparison with the age of the earth. It was -early suggested by Rutherford and Soddy<a id='r351' href='#f351' class='c012'><sup>[351]</sup></a> that radium might be -a disintegration product of one of the radio-elements found in -pitchblende. Both uranium and thorium fulfil the conditions -required in a possible source of production of radium. Both are -present in pitchblende, have atomic weights greater than that of -radium, and have rates of change which are slow compared with -that of radium. In some respects, uranium fulfils the conditions -required better than thorium; for it has not been observed that -minerals rich in thorium contain much radium, while on the other -hand, the pitchblendes containing the most radium contain a large -proportion of uranium.</p> - -<p class='c006'>If radium is not produced from uranium, it is certainly a -remarkable coincidence that the greatest activity of pitchblende -yet observed is about five or six times that of uranium. Since -radium has a life short compared with that of uranium, the -amount of radium produced should reach a maximum value after -a few thousand years, when the rate of production of fresh radium—which -<span class='pageno' id='Page_460'>460</span>is also a measure of the rate of change of uranium—balances -the rate of change of that product. In this respect the -process would be exactly analogous to the production of the -emanation by radium, with the difference that the radium changes -much more slowly than the emanation. But since radium itself -in its disintegration gives rise to at least five changes with the -corresponding production of α rays, the activity due to the radium -(measured by the α rays), when in a state of radio-active equilibrium -with uranium, should be about five times that of the -uranium that produces it; for it has been shown that only one -change has so far been observed in uranium in which α rays are -expelled. Taking into account the presence of actinium in pitchblende, -the activity observed in the best pitchblende is about the -same as would be expected if the radium were a disintegration -product of uranium. If this hypothesis is correct, the amount of -radium in any pitchblende should be proportional to the amount -of uranium present, provided the radium is not removed from the -mineral by percolating water.</p> - -<p class='c006'>This question has been experimentally attacked by Boltwood<a id='r352' href='#f352' class='c012'><sup>[352]</sup></a>, -McCoy<a id='r353' href='#f353' class='c012'><sup>[353]</sup></a> and Strutt<a id='r354' href='#f354' class='c012'><sup>[354]</sup></a>. McCoy measured the relative activities of -different minerals in the form of powder by means of an electroscope, -and determined the amount of uranium present by chemical -analysis. His results indicated that the activity observed in the -minerals was very approximately proportional to their content of -uranium. Since actinium is present as well as uranium and its -products, this would indicate that the amount of radium and -actinium taken together is proportional to the amount of uranium. -This problem has been attacked more directly by Boltwood and -Strutt by measuring the relative amount of the radium emanation -evolved by different minerals. By dissolving the mineral and -then setting it aside in a closed vessel, the amount of emanation -present reaches a maximum value after about a month’s interval. -The emanation is then introduced into a closed vessel containing -a gold-leaf electroscope similar to that shown in <a href='#fig012'>Fig. 12</a>. The -rate of movement of the gold-leaf is proportional to the amount -<span class='pageno' id='Page_461'>461</span>of emanation from the solution, and this in turn is proportional -to the amount of radium. Boltwood has made in this way a very -complete and accurate comparison of the radium content of different -varieties of pitchblende and other ores containing radium. It -was found that many of the minerals in the solid state allowed -a considerable fraction of the emanation to escape into the air. -The percentage fraction of the total amount of emanation lost in -this way is shown in Column II of the following table. Column I -gives the maximum amount of emanation present in 1 gram of -the mineral in arbitrary units when none of the emanation escapes; -Column III the weight in grams of uranium contained in 1 gram; -and Column IV the ratio obtained by dividing the quantity of -emanation by the quantity of uranium. The numbers in Column IV -should be constant, if the amount of radium is proportional to the -amount of uranium.</p> - -<table class='table9' > -<colgroup> -<col class='colwidth27'> -<col class='colwidth27'> -<col class='colwidth11'> -<col class='colwidth9'> -<col class='colwidth13'> -<col class='colwidth9'> -</colgroup> - <tr> - <th class='c013'>Substance</th> - <th class='c013'>Locality</th> - <th class='c015'>I</th> - <th class='c015'>II</th> - <th class='c015'>III</th> - <th class='c016'>IV</th> - </tr> - <tr> - <td class='c013'> </td> - <td class='c013'> </td> - <td class='c015'> </td> - <td class='c015'> </td> - <td class='c015'> </td> - <td class='c016'> </td> - </tr> - <tr> - <td class='c013'>Uraninite</td> - <td class='c013'>North Carolina</td> - <td class='c015'>170·0</td> - <td class='c015'>11·3</td> - <td class='c015'>0·7465</td> - <td class='c016'>228</td> - </tr> - <tr> - <td class='c013'>Uraninite</td> - <td class='c013'>Colorado</td> - <td class='c015'>155·1</td> - <td class='c015'>5·2</td> - <td class='c015'>0·6961</td> - <td class='c016'>223</td> - </tr> - <tr> - <td class='c013'>Gummite</td> - <td class='c013'>North Carolina</td> - <td class='c015'>147·0</td> - <td class='c015'>13·7</td> - <td class='c015'>0·6538</td> - <td class='c016'>225</td> - </tr> - <tr> - <td class='c013'>Uraninite</td> - <td class='c013'>Joachimsthal</td> - <td class='c015'>139·6</td> - <td class='c015'>5·6</td> - <td class='c015'>0·6174</td> - <td class='c016'>226</td> - </tr> - <tr> - <td class='c013'>Uranophane</td> - <td class='c013'>North Carolina</td> - <td class='c015'>117·7</td> - <td class='c015'>8·2</td> - <td class='c015'>0·5168</td> - <td class='c016'>228</td> - </tr> - <tr> - <td class='c013'>Uraninite</td> - <td class='c013'>Saxony</td> - <td class='c015'>115·6</td> - <td class='c015'>2·7</td> - <td class='c015'>0·5064</td> - <td class='c016'>228</td> - </tr> - <tr> - <td class='c013'>Uranophane</td> - <td class='c013'>North Carolina</td> - <td class='c015'>113·5</td> - <td class='c015'>22·8</td> - <td class='c015'>0·4984</td> - <td class='c016'>228</td> - </tr> - <tr> - <td class='c013'>Thorogummite</td> - <td class='c013'>North Carolina</td> - <td class='c015'>72·9</td> - <td class='c015'>16·2</td> - <td class='c015'>0·3317</td> - <td class='c016'>220</td> - </tr> - <tr> - <td class='c013'>Carnotite</td> - <td class='c013'>Colorado</td> - <td class='c015'>49·7</td> - <td class='c015'>16·3</td> - <td class='c015'>0·2261</td> - <td class='c016'>220</td> - </tr> - <tr> - <td class='c013'>Uranothorite</td> - <td class='c013'>Norway</td> - <td class='c015'>25·2</td> - <td class='c015'>1·3</td> - <td class='c015'>0·1138</td> - <td class='c016'>221</td> - </tr> - <tr> - <td class='c013'>Samarskite</td> - <td class='c013'>North Carolina</td> - <td class='c015'>23·4</td> - <td class='c015'>0·7</td> - <td class='c015'>0·1044</td> - <td class='c016'>224</td> - </tr> - <tr> - <td class='c013'>Orangite</td> - <td class='c013'>Norway</td> - <td class='c015'>23·1</td> - <td class='c015'>1·1</td> - <td class='c015'>0·1034</td> - <td class='c016'>223</td> - </tr> - <tr> - <td class='c013'>Euxinite</td> - <td class='c013'>Norway</td> - <td class='c015'>19·9</td> - <td class='c015'>0·5</td> - <td class='c015'>0·0871</td> - <td class='c016'>228</td> - </tr> - <tr> - <td class='c013'>Thorite</td> - <td class='c013'>Norway</td> - <td class='c015'>16·6</td> - <td class='c015'>6·2</td> - <td class='c015'>0·0754</td> - <td class='c016'>220</td> - </tr> - <tr> - <td class='c013'>Fergusonite</td> - <td class='c013'>Norway</td> - <td class='c015'>12·0</td> - <td class='c015'>0·5</td> - <td class='c015'>0·0557</td> - <td class='c016'>215</td> - </tr> - <tr> - <td class='c013'>Aeschynite</td> - <td class='c013'>Norway</td> - <td class='c015'>10·0</td> - <td class='c015'>0·2</td> - <td class='c015'>0·0452</td> - <td class='c016'>221</td> - </tr> - <tr> - <td class='c013'>Xenotine</td> - <td class='c013'>Norway</td> - <td class='c015'>1·54</td> - <td class='c015'>26·0</td> - <td class='c015'>0·0070</td> - <td class='c016'>220</td> - </tr> - <tr> - <td class='c013'>Monazite (sand)</td> - <td class='c013'>North Carolina</td> - <td class='c015'>0·88</td> - <td class='c015'> </td> - <td class='c015'>0·0043</td> - <td class='c016'>205</td> - </tr> - <tr> - <td class='c013'>Monazite (crys.)</td> - <td class='c013'>Norway</td> - <td class='c015'>0·84</td> - <td class='c015'>1·2</td> - <td class='c015'>0·0041</td> - <td class='c016'>207</td> - </tr> - <tr> - <td class='c013'>Monazite (sand)</td> - <td class='c013'>Brazil</td> - <td class='c015'>0·76</td> - <td class='c015'> </td> - <td class='c015'>0·0031</td> - <td class='c016'>245</td> - </tr> - <tr> - <td class='c013'>Monazite (massive)</td> - <td class='c013'>Conn.</td> - <td class='c015'>0·63</td> - <td class='c015'> </td> - <td class='c015'>0·0030</td> - <td class='c016'>210</td> - </tr> -</table> - -<p class='c006'>With the exception of some of the monazites, the numbers -show a surprisingly good agreement, and, taking into consideration -the great variation of the content of uranium in the different -<span class='pageno' id='Page_462'>462</span>minerals, and the wide range of locality from which they were -obtained, the results afford a direct and satisfactory proof that the -amount of radium in the minerals is directly proportional to the -amount of uranium.</p> - -<p class='c006'>In this connection, it is of interest to note that Boltwood -found that a considerable quantity of radium existed in various -varieties of monazite, although most of the previous analyses -agreed in stating that no uranium was present. A careful examination -was in consequence made to test this point, and it was -found by special methods that uranium was present, and in about -the amount to be expected from the theory. The ordinary -methods of analysis failed to give correct results on account of -the presence of phosphates. -Results of a similar character have recently been given by -Strutt<a id='r355' href='#f355' class='c012'><sup>[355]</sup></a>.</p> - -<p class='c006'>The weight of radium in a mineral per gram of uranium is -thus a definite constant of considerable practical importance. Its -value was recently determined by Boltwood by comparison of the -emanation, liberated from a known weight of uraninite, with that -liberated from a known quantity of pure radium bromide, supplied -for the purpose by the writer. A measured weight of radium -bromide was taken from a stock which gave out heat at a rate of -slightly over 100 gram calories per hour per gram, and was thus -probably pure. This was dissolved in water, and, by successive -dilutions, a standard solution was made up containing 10⁻⁷ gram -of radium bromide per c.c. Taking the constitution of radium -bromide as RaBr<sub>2</sub>, it was deduced that the weight of radium per -gram of uranium in any mineral was 8·0 × 10⁻⁷ gram. The -amount of radium in a mineral per ton of uranium is thus 0·72 -gram.</p> - -<p class='c006'>Strutt (<i>loc. cit.</i>) obtained a value nearly twice as great, -but he had no means of ascertaining the purity of his radium -bromide.</p> - -<p class='c006'>This amount of radium per gram of uranium is of the right -order of magnitude to be expected on the disintegration theory, if -uranium is the parent of radium. The activity of pure radium, -compared with uranium, is not known with sufficient accuracy to -<span class='pageno' id='Page_463'>463</span>determine with accuracy the theoretical proportion of radium to -uranium.</p> - -<p class='c006'>The production of radium from uranium, while very strongly -supported by these experiments, cannot be considered definitely -established until direct experimental evidence is obtained of the -growth of radium in uranium. The rate of production of radium -to be expected on the disintegration theory can readily be estimated. -The fraction of uranium breaking up per year has been -calculated (<a href='#section261'>section 261</a>) and shown to be about 10<sup>-9</sup> per year. -This number represents the weight of radium produced per year -from 1 gram of uranium. The emanation, released from the -amount of radium produced in one year from 1 gram of uranium, -would cause an ordinary gold-leaf electroscope to be discharged -in about half-an-hour. If a kilogram of uranium is used, the -amount of radium produced in a single day should be easily -detectable.</p> - -<p class='c006'>Experiments to detect the growth of radium in uranium have -been made by several observers. Soddy<a id='r356' href='#f356' class='c012'><sup>[356]</sup></a> examined the amount -of emanation given off at different times from one kilogram of -uranium nitrate in solution, which was originally freed from the -small trace of radium present by a suitable chemical process. The -solution was kept stored in a closed vessel, and the amount of -emanation which collected in the solution was measured at regular -intervals.</p> - -<p class='c006'>Preliminary experiments showed that the actual rate of production -of radium was far less than the amount to be expected -theoretically, and at first very little indication was obtained that -radium was produced at all. After allowing the uranium to stand -for eighteen months, Soddy states that the amount of emanation -was distinctly greater than at first. The solution after this interval -contained about 1·5 × 10<sup>-9</sup> gram of radium. This gives the value -of about 2 × 10<sup>-12</sup> for the fraction of uranium changing per year, -while the theoretical value is about 10<sup>-9</sup>.</p> - -<p class='c006'>Whetham<a id='r357' href='#f357' class='c012'><sup>[357]</sup></a> also found that a quantity of uranium nitrate which -had been set aside for a year showed an appreciable increase in -the content of radium, and considers that the rate of production is -<span class='pageno' id='Page_464'>464</span>faster than that found by Soddy. In his case, the uranium was -not originally completely freed from radium.</p> - -<p class='c006'>Observations extending over years will be required before the -question can be considered settled, for the accurate estimation of -small quantities of radium by the amount of emanation is beset -with difficulties. This is especially the case where observations -are made over wide intervals of time.</p> - -<p class='c006'>The writer has made an examination to see if radium is produced -from actinium or thorium. It was thought possible that -actinium might prove to be an intermediate product between -uranium and radium. The solutions, freed from radium, have -been set aside for a year, but no certain increase in the content of -radium has been observed.</p> - -<p class='c006'>There is little doubt that the production of radium by uranium -first proceeds at only a small fraction of the rate to be expected -from theory. This is not surprising when we consider that probably -several changes intervene between the product Ur X and the -radium. In the case of radium, for example, it has been shown -that a number of slow changes follow the rapid changes ordinarily -observed. On account of the feeble activity of uranium, it would -not be easy to detect directly the occurrence of such changes. If, -for example, one or more rayless products occurred between Ur X -and radium, which were removed from the uranium by the same -chemical process used to free it from radium, the rate of production -of radium would be very small at first, but would be expected to -increase with time as more of the intermediary products were -stored up in the uranium. The fact that the contents of uranium -and radium in radio-active minerals are always proportional to -each other, coupled with definite experimental evidence that -radium is produced from uranium, affords an almost conclusive -proof that uranium is in some way the parent of radium.</p> - -<p class='c006'>The general evidence which has been advanced to show that -radium must be continuously produced from some other substance -applies also to actinium, which has an activity of the same order of -magnitude as that of radium. The presence of actinium with -radium in pitchblende would indicate that this substance also is -in some way derived from uranium. It is possible that actinium -may prove to be produced either from radium or to be the intermediary -<span class='pageno' id='Page_465'>465</span>substance between uranium and radium. If it could be -shown that the amount of actinium in radio-active minerals is, like -radium, proportional to the amount of uranium, this would afford -indirect proof of such a connection. It is not so simple to settle -this point for actinium as for radium, since actinium gives out a -very short-lived emanation, and the methods adopted to determine -the content of radium in minerals cannot be applied without -considerable modifications to determine the amount of actinium -present.</p> - -<p class='c006'>The experimental data, so far obtained, do not throw much -light upon the origin of the primary active matter in thorium. -Hofmann and others (<a href='#section023'>section 23</a>) have shown that thorium separated -from minerals containing uranium is always more active the -greater the quantity of uranium present. This would indicate -that the active substance in thorium also may be derived from -uranium.</p> - -<p class='c006'>While much work still remains to be done, a promising beginning -has already been made in determining the origin and relation -of the radio-elements. We have seen that the connection between -polonium, radio-tellurium, and radio-lead with radium has already -been established. Radium itself is now added to the list, and it is -probable that actinium will soon follow.</p> - -<p class='c006'>While the experiments undoubtedly show that there is a -definite relation between the amount of uranium and radium -present in the ordinary radio-active minerals, Danne<a id='r358' href='#f358' class='c012'><sup>[358]</sup></a> has recently -called attention to a very interesting apparent exception. Considerable -quantities of radium were found in certain deposits -in the neighbourhood of Issy-l’Evêque in the Saône-Loire district, -although no trace of uranium was present. The active matter -is found in pyromorphite (phosphate of lead), in clays containing -lead, and in pegmatite, but the radium is usually present in -greater quantities in the former. The pyromorphite is found in -veins of the quartz and felspar rocks. The veins are always wet -owing to the presence of a number of springs in the neighbourhood. -The content of uranium in the pyromorphite varies considerably, -but Danne considers that about a centigram of radium is present -per ton. It seems probable that the radium found in this locality -<span class='pageno' id='Page_466'>466</span>has been deposited from water flowing through it, possibly in past -times. The presence of radium is not surprising, since crystals of -autunite have been found about 40 miles distant, and probably -there are deposits containing uranium in that region. This result -is of interest, as suggesting that radium may be removed with water -and deposited by physical or chemical action some distance away.</p> - -<p class='c006'>It will be shown in the next chapter that radium has been -found very widely distributed over the surface of the earth, but -generally in very small quantities.</p> -<p class='c005'><b>263. Does the radio-activity of radium depend upon its -concentration?</b> We have seen that the radio-active constant λ -of any product is independent of the concentration of the product. -This result has been established over a very wide range for some -substances, and especially for the radium emanation. No certain -difference in the rate of decay of the emanation has been observed, -although the amount present in unit volume of the air has been -varied a millionfold.</p> - -<p class='c006'>It has been suggested by J. J. Thomson<a id='r359' href='#f359' class='c012'><sup>[359]</sup></a> that the rate of disintegration -of radium may be influenced by its own radiations. -This, at first sight, appears very probable, for a small mass of a pure -radium compound is subjected to an intense bombardment by the -radiations arising from it, and the radiations are of such a character -that they might be expected to produce a breaking up of the -atoms of matter which they traverse. If this be the case, the -radio-activity of a given quantity of radium should be a function -of its concentration, and should be greater in the solid state than -when disseminated through a large mass of matter.</p> - -<p class='c006'>The writer has made an experiment to examine this question. -Two glass tubes were taken, in one of which was placed a few -milligrams of pure radium bromide in a state of radio-active -equilibrium, and in the other a solution of barium chloride. The -two tubes were connected near the top by a short cross tube, and -the open ends sealed off. The activity of the radium in the solid -state was tested immediately after its introduction by placing it -in a definite position near an electroscope made of thin metal of -the type shown in <a href='#fig012'>Fig. 12</a>. The increased rate of discharge of the -<span class='pageno' id='Page_467'>467</span>electroscope due to the β and γ rays from the radium was -observed. When a lead plate 6 mms. in thickness was placed -between the radium and the electroscope, the rate of discharge -observed was due to the γ rays alone. By slightly tilting the -apparatus, the barium solution flowed into the radium tube and -dissolved the radium. The tube was well shaken, so as to distribute -the radium uniformly throughout the solution. No appreciable -change of the activity measured by the γ rays was observed over -the period of one month. The activity measured by the β and γ -rays was somewhat reduced, but this was not due to a decrease of -the radio-activity, but to an increased absorption of the β rays in -their passage through the solution. The volume of the solution -was at least 1000 times greater than that of the solid radium -bromide, and, in consequence, the radium was subjected to the -action of a much weaker radiation. I think we may conclude -from this experiment that the radiations emitted by radium have -little if any influence in causing the disintegration of the radium -atoms.</p> - -<p class='c006'>Voller<a id='r360' href='#f360' class='c012'><sup>[360]</sup></a> recently published some experiments which appeared -to show that the life of radium varied enormously with its concentration. -In his experiments, solutions of radium bromide of -known strengths were evaporated down in a platinum vessel -1·2 sq. cms. in area, and their activity tested from time to time. -The activity of the radium, so deposited, at first showed the -normal rise to be expected on account of the production of the -emanation, but after reaching a maximum, it rapidly decayed. -For a weight of 10<sup>-6</sup> mgrs. of radium bromide, the activity for -example, practically disappeared in 26 days after reaching its -maximum. The time taken for the activity to disappear increased -rapidly with the amount of radium present. In another set of -experiments, he states that the activity observed on the vessel -was not proportional to the amount of radium present. For example, -the activity only increased 24 times for a millionfold increase of -the radium present, from 10<sup>-9</sup> mgrs. to 10<sup>-3</sup> mgrs.</p> - -<p class='c006'>These results, however, have not been confirmed by later -experiments made by Eve. He found that, over the range -examined, the activity was directly proportional to the amount -<span class='pageno' id='Page_468'>468</span>of radium present, within the limits of experimental error. The -following table illustrates the results obtained. The radium was -evaporated down in platinum vessels 4·9 sq. cms. in area.</p> - -<table class='table6' > -<colgroup> -<col class='colwidth50'> -<col class='colwidth50'> -</colgroup> - <tr> - <th class='c013'>Weight of radium in milligrams</th> - <th class='c014'>Activity in arbitrary units</th> - </tr> - <tr> - <td class='c013'>10<sup>-4</sup></td> - <td class='c014'>1000</td> - </tr> - <tr> - <td class='c013'>10<sup>-5</sup></td> - <td class='c014'>106</td> - </tr> - <tr> - <td class='c013'>10<sup>-6</sup></td> - <td class='c014'>11·8</td> - </tr> - <tr> - <td class='c013'>10<sup>-7</sup></td> - <td class='c014'>1·25</td> - </tr> -</table> - -<p class='c006'>For an increase of one-thousandfold of the quantity of radium, -the activity increased 800 times, while Voller states that the -activity, in his experiments, only increased 3 to 4 times.</p> - -<p class='c006'>In the experiments of Eve, the activity was measured by -observing the increased rate of discharge of a gold-leaf electroscope -when the platinum vessel containing the active deposit was -placed inside the electroscope. The activity of 10<sup>-8</sup> mgrs. was -too small to be measured with accuracy in the electroscope employed, -while 10<sup>-3</sup> mgrs. gave too rapid a rate of discharge. On the other -hand, the method of measurement employed by Voller was unsuitable -for the measurement of very weak radio-activity.</p> - -<p class='c006'>Eve also found that a small quantity of radium <i>kept in a closed -vessel</i> did not lose its activity with time. A silvered glass vessel -contained a gold-leaf system, such as is shown in <a href='#fig012'>Fig. 12</a>. A -solution containing 10<sup>-6</sup> mgrs. of radium bromide was evaporated -over the bottom of the vessel of area 76 sq. cms. The activity, -after reaching a maximum, has remained constant over the -100 days during which observations have so far been made.</p> - -<p class='c006'>These experiments of Eve, as far as they go, show that the -activity of radium is proportional to the amount of radium present, -and that radium, kept in a closed vessel, shows no signs of -decreasing in activity. On the other hand, I think there is no -doubt that a very small quantity of radium deposited on a plate -and <i>left in the open air</i> does lose its activity fairly rapidly. This -loss of activity has nothing whatever to do with the shortness of -life of the radium itself, but is due to the escape of the radium from -the plate into the surrounding gas. Suppose, for example, that a -solution containing 10<sup>-9</sup> mgrs. of radium bromide is evaporated -in a vessel of one sq. cm. in area. This amount of radium is far -<span class='pageno' id='Page_469'>469</span>too small to form even a layer of molecular thickness. It seems -likely that, during the process of evaporation, the radium would -tend to collect in small masses and be deposited on the surface -of the vessel. These would very readily be removed by slow -currents of air and so escape from the plate. The disappearance -of such minute amounts of radium is to be expected, and would -probably occur with all kinds of matter present in such minute -amount. Such an effect has nothing to do with an alteration of -the life of radium and must not be confused with it.</p> - -<p class='c006'>The result that the total radiation from a given quantity of -radium depends only on the quantity of radium and not on the -degree of its concentration is of great importance, for it allows us -to determine with accuracy the content of radium in minerals and -in soils in which the radium exists in a very diffused state.</p> -<p class='c005'><a id='section264'></a> -<b>264. Constancy of the radiations.</b> The early observations -on uranium and thorium had shown that their radio-activity -remained constant over the period of several years during which -they were examined. The possibility of separating from uranium -and thorium the active products Ur X and Th X respectively, -the activity of which decayed with the time, seemed at first sight -to contradict this. Further observation, however, showed that -the total radio-activity of these bodies was not altered by the -chemical processes, for it was found that the uranium and -thorium from which the active products were removed, spontaneously -regained their radio-activity. At any time after removal -of the active product, the sum total of the radio-activity -of the separated product together with that of the substance -from which it has been separated is always equal to that of -the original compound before separation. In cases where active -products, like Ur X and the radium emanation, decay with time -according to an exponential law, this follows at once from the -experimental results. If <i>I</i><sub>1</sub> is the activity of the product at any -time t after separation, and <i>I</i>₀ the initial value, we know that</p> - -<div class='figcenter id010'> -<img src='images/form-140.png' alt='Formula.' class='ig001'> -</div> - -<p class='c006'>At the same time the activity <i>I</i><sub>2</sub> recovered during the -same interval <i>t</i> is given by</p> - -<div class='figcenter id010'> -<img src='images/form-141.png' alt='Formula.' class='ig001'> -</div> - -<p class='c006'>where λ is the same -<span class='pageno' id='Page_470'>470</span>constant as before. It thus follows that <i>I</i><sub>1</sub> + <i>I</i><sub>2</sub> = <i>I</i>₀, which is an -expression of the above result. The same is also true whatever -the law of decay of activity of the separated product (see -<a href='#section200'>section 200</a>). For example, the activity of Th X after separation from -thorium at first increases with the time. At the same time, the -activity of the residual thorium compound at first decreases, and -at such a rate that the sum of the activities of the thorium and -its separated product is always equal to that of the original -thorium.</p> - -<p class='c006'>This apparent constancy of the total radiation follows from the -general result that the radio-active processes cannot in any way be -changed by the action of known forces. It may be recalled that -the constant of decay of the activity of a radio-active product has a -definite fixed value under all conditions. It is independent of the -concentration of the active matter, of the pressure, and of the -nature of the gas in which the substance is placed, and it is not -affected by wide ranges of temperature. The only observed exception -is the product radium C. Its value of λ increases with -temperature to some extent at about 1000° C., but at 1200° C. -returns nearly to the normal value. In the same way, it has not -been found possible to alter the rate of production of active matter -from the radio-elements. In addition, there is not a single well-authenticated -case where radio-activity has been altered or destroyed -in any active body or created in an inactive element.</p> - -<p class='c006'>Certain cases have been observed, which at first sight seem to -indicate a destruction of radio-activity. For example, the excited -radio-activity is removed from a platinum wire when heated above -a red heat. It has been shown, however, by Miss Gates -(<a href='#section187'>section 187</a>) that the radio-activity is not destroyed, but is deposited -in unaltered amount on the colder bodies surrounding it. Thorium -oxide has been shown to lose to a large extent its power of emanating -by ignition to a white heat. But a close examination shows -that the emanation is still being produced at the same rate, but is -occluded in the compound.</p> - -<p class='c006'>The total radio-activity of a given mass of a radio-element, -measured by the peculiar radiations emitted, is a quantity which -can neither be increased nor diminished, although it may be manifested -in a series of products which are capable of separation from -<span class='pageno' id='Page_471'>471</span>the radio-element. The term “conservation of radio-activity” is -thus a convenient expression of the facts known at the present -time. It is quite possible, however, that further experiments at -very high or very low temperatures may show that the radio-activity -does vary.</p> - -<p class='c006'>Although no difference has been observed in the radio-activity -of uranium over an interval of five years, it has been shown -(<a href='#section261'>section 261</a>) that on theoretical grounds the radio-activity of a <i>given -quantity</i> of a radio-element should decrease with the time. The -change will, however, be so slow in uranium, that probably -millions of years must elapse before a measurable change can -take place, while the total radio-activity of a given quantity of -matter left to itself should thus decrease, but it ought to be -constant for a <i>constant mass</i> of the radio-element. It has already -been pointed out (<a href='#section238'>section 238</a>) that the activity of radium, -measured by the α and β rays, will probably increase for several -hundred years after its separation. This is due to the appearance -of fresh products in the radium. Ultimately, however, the activity -must decrease according to an exponential law with the time, -falling to half value in about 1300 years.</p> - -<p class='c006'>The conservation of radio-activity applies not only to the -radiations taken as a whole, but also to each specific type of -radiation. If the emanation is removed from a radium compound, -the amount of β radiation of the radium at once commences to -decrease, but this is compensated by the appearance of β rays -in the radiations from the vessel in which the separated emanation -is stored. At any time the sum total of the β radiations from the -radium and the emanation vessel is always the same as that from -the radium compound before the emanation was removed.</p> - -<p class='c006'>Similar results have also been found to hold for the γ rays. -This was tested by the writer in the following way. The emanation -from some solid radium bromide was released by heat, -and condensed in a small glass tube which was then sealed off. -The radium so treated, and the emanation tube, were placed -together under an electroscope, with a screen of lead 1 cm. thick -interposed in order to let through only the γ rays. The experiments -were continued over three weeks, but the sum total of the -γ rays from the radium and the emanation tube was, over the -<span class='pageno' id='Page_472'>472</span>whole interval, equal to that of the original radium. During this -period the amount of γ rays from the radium at first decreased to -only a few per cent. of the original value, and then slowly increased -again, until at the end of the three weeks it had nearly regained -its original value, before the emanation was removed. At the same -time the amount of γ rays from the emanation tube rose from zero -to a maximum and then slowly decreased again at the same rate -as the decay of the activity of the emanation in the tube. This -result shows that the amount of γ rays from radium was a constant -quantity over the interval of observation, although the amount of -γ rays from the radium and emanation tube had passed through a -cycle of changes.</p> - -<p class='c006'>There is one interesting possibility in this connection that -should be borne in mind. The rays from the active substances -carry off energy in a very concentrated form, and this energy -is dissipated by the absorption of the rays in matter. The rays -might be expected to cause a disintegration of the atoms of -inactive matter on which they fall and thus give rise to a kind -of radio-activity. This effect has been looked for by several -observers. Ramsay and W. T. Cooke<a id='r361' href='#f361' class='c012'><sup>[361]</sup></a> state that they have -noticed such an action, using about a decigram of radium as -a source of radiation. The radium, sealed in a glass vessel, was -surrounded by an external glass tube and exposed to the action of -the β and γ rays of radium for several weeks. The inside and -outside of the glass tube were found to be active, and the active -matter was removed by solution in water. The radio-activity -observed was very minute, corresponding to only about 1 milligram -of uranium. The writer has, at various times, tried experiments of -this character but with negative results. The greatest care is -necessary in such experiments to ensure that the radio-activity is -not due to other causes besides the rays from the radium. This -care is especially necessary in laboratories where considerable -quantities of the radium emanation have been allowed to escape -into the air. The surface of every substance becomes coated with -the slow transformation products of radium, viz. radium D, E, and -F. The activity communicated in this way to originally inactive -matter is often considerable. This infection by the radium emanation -<span class='pageno' id='Page_473'>473</span>extends throughout the whole laboratory, on account of the -distribution of the emanation by convection and diffusion. For -example, Eve<a id='r362' href='#f362' class='c012'><sup>[362]</sup></a> found that every substance which he examined -in the laboratory of the writer showed much greater activity than -the normal. In this case the radium had been in use in the -building for about two years.</p> -<p class='c005'><b>265. Loss of weight of the radio-elements.</b> Since the -radio-elements are continually throwing off α particles atomic in -size, an active substance, enclosed in a vessel sufficiently thin to -allow the α particles to escape, must gradually lose in weight. -This loss of weight will be small under ordinary conditions, since -the greater proportion of the α rays produced are absorbed in the -mass of the substance. If a very thin layer of a radium compound -were spread on a very thin sheet of substance, which did not -appreciably absorb the α particles, a loss of weight due to the -expulsion of α particles might be detectable. Since <i>e</i>/<i>m</i> = -6 × 10<sup>3</sup> for the α particle and <i>e</i> = 1·1 × 10<sup>-20</sup> electromagnetic units and -2·5 × 10<sup>11</sup> α particles are expelled per second per gram of radium, -the proportion of the mass expelled is 4·8 × 10<sup>-13</sup> per second and -10<sup>-5</sup> per year. There is one condition, however, under which -the radium should lose in weight fairly rapidly. If a current of -air is slowly passed over a radium solution, the emanation produced -would be removed as fast as it was formed. Since the atom of -the emanation has a mass probably not much smaller than the -radium atom, the fraction of the mass removed per year should -be nearly equal to the fraction of the radium which changes per -year, <i>i.e.</i> one gram of radium should diminish in weight about -half a milligram (<a href='#section261'>section 261</a>) per year.</p> - -<p class='c006'>If it is supposed that the β particles have weight, the loss of -weight due to their expulsion is very small compared with that -due to the emission of α particles. The writer has shown -(<a href='#section253'>section 253</a>) -that about 7 × 10<sup>10</sup> β particles are projected per second from -1 gram of radium. The consequent loss of weight would only be -about 10<sup>-9</sup> grams per year.</p> - -<p class='c006'>Except under very special experimental conditions, it would -thus be difficult to detect the loss of weight of radium due to -<span class='pageno' id='Page_474'>474</span>the expulsion of β particles from its mass. There is, however, a -possibility that radium might change in weight even though none -of the radio-active products were allowed to escape. For example, -if the view is taken that gravitation is the result of forces having -their origin in the atom, it is possible that, if the atom were -disintegrated, the weight of the parts might not be equal to that -of the original atom.</p> - -<p class='c006'>A large number of experiments have been made to see if -radium preparations, kept in a sealed tube, alter in weight. With -the small quantities of radium available to the experimenter, no -difference of weight of radium preparations with time has yet -been established with certainty. Heydweiller stated that he had -observed a loss of weight of radium and Dorn also obtained a -slight indication of change in weight. These results have not, -however, been confirmed. Forch, later, was unable to observe any -appreciable change.</p> - -<p class='c006'>J. J. Thomson<a id='r363' href='#f363' class='c012'><sup>[363]</sup></a> has made experiments to see if the ratio of -weight to mass for radium is the same as for inactive matter. We -have seen in <a href='#section048'>section 48</a> that a charge in motion possesses an -apparent mass which is constant for slow speeds but increases as -the speed of light is approached. Now radium emits some electrons -at a velocity comparable with the velocity of light, and -presumably these electrons were in rapid motion in the atom -before their expulsion. It might thus be possible that the ratio for -radium would differ from that for ordinary matter. The pendulum -method was used, and the radium was enclosed in a small light -tube suspended by a silk fibre. Within the limit of experimental -error the ratio of weight to mass was found to be the same as for -ordinary matter, so that we may conclude that the number of -electrons moving with a velocity approaching that of light is small -compared with the total number present.</p> -<p class='c005'><a id='section266'></a> -<b>266. Total emission of energy from the radio-element.</b> -It has been shown that 1 gram of radium emits energy at the -rate of 100 gram-calories per hour or 876,000 gram-calories per -year. If 1 gram of radium in radio-active equilibrium be set -apart, its radio-activity and consequent heat emission is given at a -<span class='pageno' id='Page_475'>475</span>time <i>t</i> by</p> - -<div class='figcenter id010'> -<img src='images/form-142.png' alt='Formula.' class='ig001'> -</div> - -<p class='c006'>where λ is the constant of decay of activity of -radium and of the initial heating effect; the total heat emission -from 1 gram of radium is given by</p> - -<div class='figcenter id009'> -<img src='images/form-143.png' alt='Formula.' class='ig001'> -</div> - -<p class='c006'>Now on the estimate of the life of radium given in section 261 -the value of λ is ¹⁄₁₈₅₀ when 1 year is taken as the unit of time. -The total heat emission from 1 gram of radium during its life is -thus 1·6 × 10<sup>9</sup> gram-calories. The heat emitted in the union of -hydrogen and oxygen to form 1 gram of water is about 4 × 10<sup>3</sup> -gram-calories, and in this reaction more heat is given out for -equal weights than in any other chemical reaction known. It is -thus seen that the total energy emitted from 1 gram of radium -during its changes is about one million times greater than in any -known molecular change. That matter is able, under special conditions, -to emit an enormous amount of energy, is well exemplified -by the case of the radium emanation. Calculations of the amount -of this energy have already been given in <a href='#section249'>section 249</a>.</p> - -<p class='c006'>Since the other radio-elements only differ from radium in the -slowness of their change, the total heat emission from uranium -and thorium must be of a similar high order of magnitude. There -is thus reason to believe that there is an enormous store of latent -energy resident in the atoms of the radio-elements. This store of -energy could not have been recognized if the atoms had not been -undergoing a slow process of disintegration. The energy emitted -in radio-active changes is derived from the internal energy of the -atoms. The emission of this energy does not disobey the law of -the conservation of energy, for it is only necessary to suppose that, -when the radio-active changes have ceased, the energy stored up -in the atoms of the final products is less than that of the original -atoms of the radio-elements. The difference between the energy -originally possessed by the matter which has undergone the -change, and the final inactive products which arise, is a measure -of the total amount of energy released.</p> - -<p class='c006'>There seems to be every reason to suppose that the atomic -energy of all the elements is of a similar high order of magnitude. -With the exception of their high atomic weights, the radio-elements -do not possess any special chemical characteristics which -differentiate them from the inactive elements. The existence of -<span class='pageno' id='Page_476'>476</span>a latent store of energy in the atoms is a necessary consequence -of the modern view developed by J. J. Thomson, Larmor, and -Lorentz, of regarding the atom as a complicated structure consisting -of charged parts in rapid oscillatory or orbital motion in regard to -one another. The energy may be partly kinetic and partly potential, -but the mere concentration of the charged particles, which probably -constitute the atom, in itself implies a large store of energy in -the atom, in comparison with which the energy emitted during -the changes of radium is insignificant.</p> - -<p class='c006'>The existence of this store of latent energy does not ordinarily -manifest itself, since the atoms cannot be broken up into -simpler forms by the physical or chemical agencies at our disposal. -Its existence at once explains the failure of chemistry to transform -the atoms, and also accounts for the rate of change of the radio-active -processes being independent of all external agencies. It -has not so far been found possible to alter in any way the rate -of emission of energy from the radio-elements. If it should ever -be found possible to control at will the rate of disintegration of -the radio-elements, an enormous amount of energy could be obtained -from a small quantity of matter.</p> -<p class='c005'><a id='section267'></a> -<b>267. Production of helium from radium and the radium -emanation.</b> Since the final products, resulting from a disintegration -of the radio-elements, are not radio-active, they should in -the course of geologic ages collect in some quantity, and should -always be found associated with the radio-elements. Now the -inactive products resulting from the radio-active changes are the α -particles expelled at each stage, and the final inactive product or -products which remain, when the process of disintegration can no -longer be traced by the property of radio-activity.</p> - -<p class='c006'>Pitchblende, in which the radio-elements are mostly found, -contains in small quantity a large proportion of all the known -elements. In searching for a possible disintegration product -common to all the radio-elements, the presence of helium in the -radio-active minerals is noteworthy; for helium is only found in -the radio-active minerals, and is an invariable companion of the -radio-elements. Moreover, the presence in minerals of a light, -inert gas like helium had always been a matter of surprise. The -<span class='pageno' id='Page_477'>477</span>production by radium and thorium of the radio-active emanations, -which behave like chemically inert gases of the helium-argon -family, suggested the possibility that one of the final inactive -products of the disintegration of the radio-elements might prove -to be a chemically inert gas. The later discovery of the material -nature of the α rays added weight to the suggestion; for the -measurement of the ratio <i>e</i>/<i>m</i> of the α particle indicated that if -the α particle consisted of any known kind of matter, it must either -be hydrogen or helium. For these reasons, it was suggested in -1902 by Rutherford and Soddy<a id='r364' href='#f364' class='c012'><sup>[364]</sup></a> that helium might be a product -of the disintegration of the radio-elements.</p> - -<p class='c006'>Sir William Ramsay and Mr Soddy in 1903 undertook an investigation -of the radium emanation, with the purpose of seeing if -it were possible to obtain any spectroscopic evidence of the presence -of a new substance. First of all, they exposed the emanation to -very drastic treatment (section 158), and confirmed and extended -the results previously noted by Rutherford and Soddy that the -emanation behaved like a chemically inert gas, and in this respect -possessed properties analogous to the gases of the helium-argon -group.</p> - -<p class='c006'>On obtaining 30 milligrams of pure radium bromide (prepared -about three months previously) Ramsay and Soddy<a id='r365' href='#f365' class='c012'><sup>[365]</sup></a> examined -the gases, liberated by solution of the radium bromide in -water, for the presence of helium. A considerable quantity of -hydrogen and oxygen was released by the solution (see -<a href='#section124'>section 124</a>). The hydrogen and oxygen were removed by passing the -liberated gases over a red-hot spiral of partially oxidized copper-wire -and the resulting water vapour was absorbed in a phosphorus -pentoxide tube.</p> - -<p class='c006'>The gas was then passed into a small vacuum tube which was -in connection with a small <b>U</b> tube. By placing the <b>U</b> tube in -liquid air, most of the emanation present was condensed, and also -most of the CO<sub>2</sub> present in the gas. On examining the spectrum -of the gas in the vacuum tube, the characteristic line <i>D</i><sub>3</sub> of helium -was observed.</p> - -<p class='c006'><span class='pageno' id='Page_478'>478</span>This experiment was repeated with 30 milligrams of radium -bromide about four months old, lent for the purpose by the writer. -The emanation and CO<sub>2</sub> were removed by passing them through a -<b>U</b> tube immersed in liquid air. A practically complete spectrum -of helium was observed, including the lines of wave-lengths 6677, -5876, 5016, 4972, 4713 and 4472. There were also present three -other lines of wave-lengths about 6180, 5695, 5455 which have not -yet been identified.</p> - -<p class='c006'>In later experiments, the emanation from 50 milligrams of the -radium bromide was conveyed with oxygen into a small <b>U</b> tube, -cooled in liquid air, in which the emanation was condensed. Fresh -oxygen was added, and the <b>U</b> tube again pumped out. The small -vacuum tube, connected with the <b>U</b> tube, showed at first no -helium lines when the liquid air was removed. The spectrum -obtained was a new one, and Ramsay and Soddy considered it -to be probably that of the emanation itself. After allowing the -emanation tube to stand for four days, the helium spectrum appeared -with all the characteristic lines, and in addition, three new lines -present in the helium obtained by solution of the radium. These -results have since been confirmed. The experiments, which have -led to such striking and important results, were by no means easy -of performance, for the quantity of helium and of emanation released -from 50 mgrs. of radium bromide is extremely small. It was -necessary, in all cases, to remove almost completely the other gases, -which were present in sufficient quantity to mask the spectrum of -the substance under examination. The success of the experiments -has been largely due to the application, to this investigation, -of the refined methods of gas analysis, previously employed -by Sir William Ramsay with so much skill in the separation of -the rare gases xenon and krypton, which exist in minute proportions -in the atmosphere. The fact that the helium spectrum -was not present at first, but appeared <i>after</i> the emanation had -remained in the tube for some days, shows that the helium must -have been produced from the emanation. The emanation cannot -be helium itself, for, in the first place, helium is not radio-active, -and in the second place, the helium spectrum was not present -at first, when the quantity of emanation in the tube was at -its maximum. Moreover, the diffusion experiments, already discussed, -<span class='pageno' id='Page_479'>479</span>point to the conclusion that the emanation is of high -molecular weight. There can thus be no doubt that the helium is -derived from the emanation of radium in consequence of changes -of some kind occurring in it.</p> - -<p class='c006'>These results were confirmed later by other observers. Curie -and Dewar<a id='r366' href='#f366' class='c012'><sup>[366]</sup></a> performed the following experiment: A weight of -about ·42 gr. of radium bromide was placed in a quartz tube, and the -tube exhausted until no further gas came off. The radium was then -heated to fusion, about 2·6 c.c. of gas being liberated in the process. -The tube was then sealed, and some weeks afterwards the spectrum -of the gas liberated in the tube by the radium was examined by -Deslandres and found to give the entire spectrum of helium. The -gas, liberated during the initial heating of the radium, was collected -and found to contain a large amount of emanation, although the -gas had been passed through two tubes immersed in liquid air. -The tube containing these gases was very luminous and rapidly -turned violet, while more than half of the gases was absorbed. The -spectrum of the phosphorescent light was found to be discontinuous, -consisting of three nitrogen bands. No sign of the helium spectrum -was observed, although helium must have been present.</p> - -<p class='c006'>Himstedt and Meyer<a id='r367' href='#f367' class='c012'><sup>[367]</sup></a> placed 50 mgrs. of radium bromide in -a <b>U</b> tube connected with a small vacuum tube. The tube was -carefully exhausted and then sealed off. The spectrum of hydrogen -and carbon dioxide alone was observed for three months, but after -four months the red, yellow, green and blue lines of the helium -spectrum were visible. The slow appearance of the helium spectrum -was probably due to the presence in the tube of a considerable -quantity of hydrogen. In another experiment, some radium -sulphate which had been heated to a bright red heat in a quartz -tube was connected with a small vacuum tube. After three weeks, -some of the lines of helium were clearly seen, and increased in -brightness with time.</p> -<p class='c005'><a id='section268'></a> -<b>268. Connection between helium and the α particles</b>. -The appearance of helium in a tube containing the radium emanation -may indicate either that the helium is one of the final -<span class='pageno' id='Page_480'>480</span>products, which appear at the end of the series of radio-active -changes, or that the helium is in reality the expelled α particle. -The evidence at present points to the latter as being the more -probable explanation. In the first place, the emanation diffuses -like a gas of heavy molecular weight, and it appears probable that -after the expulsion of a few α particles, the atomic weight of the -final product is comparable with that of the emanation. On the -other hand, the value of <i>e</i>/<i>m</i> determined for the projected α particle -points to the conclusion that, if it consists of any known kind of -matter, it is either hydrogen or helium.</p> - -<p class='c006'>There has been a tendency to assume that the helium produced -from the radium emanation is the last transformation product of -that substance. The evidence, however, does not support this -view. We have seen that the emanation, after the initial rapid -changes, is transformed very slowly. If the helium were the final -product, the amount present in the emanation tube after a few -days or weeks would be insignificant, since the product radium -D intervenes, which takes 40 years to be half transformed. Since -the helium cannot be the final product of the series of changes, -and since all the other products are radio-active, and almost -certainly of high atomic weight, it is difficult to see what position -the helium atom occupies in the scheme of transformation, unless -it be the α particle expelled during the successive changes.</p> - -<p class='c006'>It is a matter of great difficulty to settle definitely whether the -α particle is a projected helium atom or not. On account of the -very small deflection of the α rays in an electric field, and the -complex nature of the α radiation from radium, an accurate determination -of the value <i>e</i>/<i>m</i> for the α particle is beset with -difficulties.</p> - -<p class='c006'>It may be possible to settle the question by accurate measurements -of the volume of gas in a tube, filled originally with the -radium emanation. Since the emanation itself, and two of the -rapidly changing products which result from it, emit α particles, -the final volume of the α particles, if they can exist in the -gaseous state, would be three times the volume of the emanation. -Ramsay and Soddy (<a href='#section172'>section 172</a>) have made experiments of this -kind, but the results obtained were very contradictory, depending -upon the kind of glass employed. In one case, the volume of the -<span class='pageno' id='Page_481'>481</span>residual gases shrank almost to zero, in another the initial volume -increased to about ten times its initial value. In the latter experiment -a brilliant spectrum of helium was observed in the -residual gas. This difference of behaviour is probably due to -different degrees of absorption of helium by the glass tubes.</p> - -<p class='c006'>If the α particles are helium atoms, we may expect that a -large proportion of the helium, which is produced in a tube containing -the radium emanation, will be buried in the wall of the -glass tube; for the α particles are projected with sufficient velocity -to penetrate some distance into the glass. This helium may either -remain in the glass, or in some cases rapidly diffuse out again. -In any case, a fraction of the helium would be liberated when an -intense electric discharge is passed through the tube. Ramsay -and Soddy have in some instances observed that a slight amount -of helium is liberated on heating the walls of the tube in which -the emanation had been stored for some time.</p> - -<p class='c006'>The volume of helium produced per year by 1 gram of radium -can easily be calculated on the assumption that the α particle is -in reality a helium atom.</p> - -<p class='c006'>It has been shown that 2·5 × 10<sup>11</sup> α particles are projected per -second from 1 gram of radium. Since there are 3·6 × 10<sup>19</sup> molecules -in one cubic centimetre of any gas at standard pressure and -temperature, the volume of the α particles released per second is -7 × 10<sup>-9</sup> c.c. and per year 0·24 c.c. It has already been pointed out -that, on this hypothesis, the volume of helium released by the -emanation is three times the volume of the latter. The amount -of helium to be obtained from the emanation released from -1 gram of radium in radio-active equilibrium is thus about -3 cubic mms.</p> - -<p class='c006'>Ramsay and Soddy have tried to estimate experimentally the -probable volume of helium produced per second by one gram of -radium. The helium, obtained from 50 mgrs. of radium bromide, -which had been kept in solution in a closed vessel for 60 days, -was introduced into a vacuum tube. Another similar tube was -placed in series with it, and the amount of the helium in the -latter adjusted until on passing a discharge through the two tubes -in series the helium lines in each tube were of about the same -brightness. In this way they calculated that the amount of helium -<span class='pageno' id='Page_482'>482</span>present was 0·1 cubic mm. On this estimate, the amount of helium -produced per year per gram of radium is about 20 cubic mms. -We have seen that the calculated amount is about 240 cubic mms., -on the assumption that the α particle is a helium atom. Ramsay -and Soddy consider that the presence of argon in one of the tubes -may have seriously interfered with the correctness of the estimation. -On account of the great uncertainty attaching to estimates -of the above character, the value deduced by Ramsay and Soddy -does not exclude the probability that the calculated volume may -be of the right order of magnitude.</p> - -<p class='c006'>In order to explain the presence of helium in radium on ordinary -chemical lines, it has been suggested that radium is not -a true element, but a molecular compound of helium with some -substance known or unknown. The helium compound gradually -breaks down, giving rise to the helium observed. It is at once -obvious that this postulated helium compound is of a character -entirely different from that of any other compound previously -observed in chemistry. Weight for weight, it emits during its -change an amount of energy at least one million times greater than -any molecular compound known (see section 249). In addition, it -must be supposed that the rate of breaking up of the helium compound -is independent of great ranges of temperature—a result never -before observed in any molecular change. The helium compound -in its breaking up must give rise to the peculiar radiations and -also pass through the successive radio-active changes observed in -radium.</p> - -<p class='c006'>Thus in order to explain the production of helium and radio-activity -on this view, a unique kind of molecule must be postulated—a -molecule, in fact, which is endowed with every single property -which on the disintegration theory is ascribed to the atom of the -radio-elements. On the other hand, radium as far as it has been -examined, has fulfilled every test required for an element. It has -a well-marked and characteristic spectrum, and there is no reason -to suppose that it is not an element in the ordinarily accepted -sense of the term.</p> - -<p class='c006'>On the theory that the radio-elements are undergoing atomic -disintegration, the helium must be considered to be a constituent -of the radium atom, or, in other words, the radium atom is -<span class='pageno' id='Page_483'>483</span>built up of parts, one of which, at least, is the atom of helium. -The theory that the heavy atoms are all built up of some simple -fundamental unit of matter or protyle has been advanced at various -times by many prominent chemists and physicists. Prout’s hypothesis -that all elements are built up out of hydrogen is an example -of this point of view of regarding the subject.</p> - -<p class='c006'>On the disintegration theory, the changes occurring in the -radio-atoms involve an actual transformation of the atoms through -successive changes. This change is so slow in uranium and thorium -that at least a million years would be required before the amount -of change could be measured by the balance. In radium it is a -million times faster, but even in this case it is doubtful whether -any appreciable change would have been observed by ordinary -chemical methods for many years had not the possibility of such a -change been suggested from other lines of evidence.</p> - -<p class='c006'>The similarity of the α particles from the different radio-elements -indicates that they consist of expelled particles of the -same kind. On this view, helium should be produced by each of -the radio-elements. Its presence in minerals containing thorium, -for example in monazite sand and the Ceylon mineral described -by Ramsay, indicates that helium may be a product of thorium -as well as of radium. Strutt<a id='r368' href='#f368' class='c012'><sup>[368]</sup></a> has recently suggested that most -of the helium observed in radio-active minerals may be a decomposition -product of thorium rather than of uranium and radium; -for he finds that minerals rich in helium always contain thorium, -while many uranium minerals nearly free from thorium contain -little helium. The evidence in support of this view is, however, -not altogether satisfactory, for some of the uranium minerals in -question are secondary uranium minerals (see <a href='#appb'>Appendix B</a>), deposited -by the action of water or other agencies at a comparatively -late date, and are also, in many cases, highly emanating, and consequently -could not be expected to retain more than a fraction of -the helium produced in them.</p> - -<p class='c006'>Taking the view that the α particles are projected helium atoms, -we must regard the atoms of the radio-elements as compounds of -some known or unknown substance with helium. These compounds -break up spontaneously, and at a very slow rate even in the -<span class='pageno' id='Page_484'>484</span>case of radium. The disintegration takes place in successive stages, -and at most of the stages a helium atom is projected with great -velocity. This disintegration is accompanied by an enormous -emission of energy. The liberation of such a large amount of -energy in the radio-active changes at once explains the constancy -of the rate of change under the action of any of the physical and -chemical agencies at our command. On this view, uranium, -thorium and radium are in reality compounds of helium. The -helium, however, is held in such strong combination that the -compound cannot be broken up by chemical or physical forces, -and, in consequence, these bodies behave as chemical elements in -the ordinary accepted chemical sense.</p> - -<p class='c006'>It appears not unlikely that many of the so-called chemical -elements may prove to be compounds of helium, or, in other words, -that the helium atom is one of the secondary units with which the -heavier atoms are built up. In this connection it is of interest to -note that many of the elements differ in their atomic weight by -four—the atomic weight of helium.</p> - -<p class='c006'>If the α particle is a helium atom, at least three α particles -must be expelled from uranium (238·5) to reduce its atomic weight -to that of radium (225). It is known that five α particles are -expelled from radium during its successive transformations. This -would make the atomic weight of the final residue 225 – 20 = 205. -This is very nearly the atomic weight of lead, 206·5. I have, for -some time, considered it probable that lead is the end or final -product of radium. The same suggestion has recently been made -by Boltwood<a id='r369' href='#f369' class='c012'><sup>[369]</sup></a>. This point of view is supported by the fact that -lead is always found in small quantity in all uranium minerals, -and that the relative proportions of lead and helium in the radio-active -minerals are about the same as would be expected if lead -and helium were both decomposition products of radium. Dr -Boltwood has drawn my attention to the fact that the proportion -of lead in many radio-active minerals varies with the content of -helium. A mineral rich in helium in nearly all cases contains -more lead than a mineral poor in helium. This cannot be considered, -at present, more than a speculation, but the facts as they -stand are very suggestive.</p> -<p class='c005'><span class='pageno' id='Page_485'>485</span><b>269. Age of radio-active minerals.</b> Helium is only found -in the radio-active minerals, and this fact, taken in conjunction -with the liberation of helium by radium, indicates that the helium -must have been produced as a result of the transformation of -radium and the other radio-active substances contained in the -minerals. Now in a mineral about half the helium is, in many -cases, released by heat and the residue by solution. It seems -probable that the helium produced throughout the mass of the -mineral is mechanically imprisoned in it. Moss<a id='r370' href='#f370' class='c012'><sup>[370]</sup></a> found that, by -grinding pitchblende in vacuo, helium is evolved, apparently showing -that the helium exists in cavities of the mineral. Travers<a id='r371' href='#f371' class='c012'><sup>[371]</sup></a> -has suggested that, since helium is liberated on heating, the -effect may be due to the heat generated by grinding. The -escape of the helium from the heated mineral is probably connected -with the fact observed by Jaquerod<a id='r372' href='#f372' class='c012'><sup>[372]</sup></a> that helium passes -through the walls of a quartz tube, heated above 500° C. The -substance of the mineral probably possesses a similar property. -Travers considers that helium is present in the mineral in a state -of supersaturated solid solution, but the facts are equally well -explained by assuming that the helium is mechanically imprisoned -in the mass of the mineral.</p> - -<p class='c006'>The sudden rise of temperature observed in the mineral fergusonite, -at the time the helium is released, has been found to have -nothing to do with the presence of helium, for it also takes place -in minerals not containing helium. The old view that helium was -in a state of chemical combination with the mineral must be -abandoned in the light of these more recent experiments.</p> - -<p class='c006'>Since the helium is only released from some minerals by the -action of high temperatures and solution, it appears probable that -a large proportion of the helium found in the minerals is unable -to escape under normal conditions. Thus if the rate of production -of helium by the radio-active substance were definitely known, it -should be possible to calculate the age of the mineral by observing -the volume of helium liberated from it by solution.</p> - -<p class='c006'>In the absence of such definite information, an approximate -<span class='pageno' id='Page_486'>486</span>calculation will be made to indicate the order of magnitude of the -time that has elapsed since the mineral was formed or was at a -temperature low enough to prevent the escape of the helium.</p> - -<p class='c006'>Let us take, for example, the mineral fergusonite, which was -found by Ramsay and Travers<a id='r373' href='#f373' class='c012'><sup>[373]</sup></a> to evolve 1·81 c.c. of helium. The -fergusonite contained about 7 per cent. of uranium. Now uranium -in old minerals probably contains about 8 × 10<sup>-7</sup> of its weight of -radium (see <a href='#section262'>section 262</a>). One gram of the mineral thus contained -about 5·6 × 10<sup>-8</sup> grams of radium. Now if the α particle is helium, -it has been shown that 1 gram of radium produces 0·24 c.c. of -helium per year. The volume of helium produced per year in -1 gram of fergusonite is thus 1·3 × 10<sup>-8</sup> c.c. Assuming that the -rate of production of helium has been uniform, the time required -to produce 1·81 c.c. per gram is about 140 million years. If the -calculated rate of production of helium by radium is an over-estimate, -the time is correspondingly lengthened.</p> - -<p class='c006'>I think that, when the constants required for these calculations -are more definitely fixed, this method will probably give fairly -trustworthy information as to the probable age of some of the -radio-active minerals of the earth’s crust, and indirectly as to the -age of the strata in which they are found.</p> - -<p class='c006'>In this connection it is of interest to note that Ramsay<a id='r374' href='#f374' class='c012'><sup>[374]</sup></a> found -that a Ceylon mineral, thorianite, contained as much as 9·5 c.c. of -helium per gram. According to the analysis by Dunstan, this -mineral contains about 76 per cent. of thorium and 12 per cent. -of uranium. The unusually large amount of helium evolved from -this mineral would indicate that it was formed at an earlier date -than the fergusonite previously considered.</p> -<p class='c005'><a id='section270'></a> -<b>270. Possible causes of disintegration.</b> In order to explain -the phenomena of radio-activity, it has been supposed that a -certain small fraction of the radio-atoms undergoes disintegration -per second, but no assumptions have been made as to the cause -which produces the instability and consequent disintegration. -The instability of the atoms may be supposed to be brought about -either by the action of external forces or by that of forces inherent -<span class='pageno' id='Page_487'>487</span>in the atoms themselves. It is conceivable, for example, that the -application of some slight external force might cause instability and -consequent disintegration, accompanied by the liberation of a large -amount of energy, on the same principle that a detonator is -necessary to start some explosives. It has been shown that the -number of atoms of any radio-active product which break up per -second is always proportional to the number present. This law -of change does not throw any light on the question, for it would -be expected equally on either hypothesis. It has not been found -possible to alter the rate of change of any product by the -application of any known physical or chemical forces, unless -possibly it is assumed that the force of gravitation which is not -under our control may influence in some way the stability of the -radio-atoms.</p> - -<p class='c006'>It seems likely therefore that the cause of the disruption of -the atoms of the radio-elements and their products resides in the -atoms themselves. According to the modern views of the constitution -of the atom, it is not so much a matter of surprise that -some atoms disintegrate as that the atoms of the elements are so -permanent as they appear to be. In accordance with the hypothesis -of J. J. Thomson, it may be supposed that the atoms consist of a -number of small positively and negatively charged particles in -rapid internal movement, and held in equilibrium by their mutual -forces. In a complex atom, where the possible variations in the -relative motion of the parts are very great, the atom may arrive -at such a phase that one part acquires sufficient kinetic energy -to escape from the system, or that the constraining forces are -momentarily neutralised, so that the part escapes from the system -with the velocity possessed by it at the instant of its release.</p> - -<p class='c006'>Sir Oliver Lodge<a id='r375' href='#f375' class='c012'><sup>[375]</sup></a> has advanced the view that the instability of -the atom may be a result of radiation of energy by the atom. Larmor -has shown that an electron, subject to acceleration, radiates energy -at a rate proportional to the square of its acceleration. An electron -moving uniformly in a straight line does not radiate energy, but -an electron, constrained to move in a circular orbit with constant -velocity, is a powerful radiator, for in such a case the electron is -continuously accelerated towards the centre. Lodge considered -<span class='pageno' id='Page_488'>488</span>the simple case of a negatively charged electron revolving round -an atom of mass relatively large but having an equal positive -charge and held in equilibrium by electrical forces. This system -will radiate energy, and, since the radiation of energy is equivalent -to motion in a resisting medium, the particle tends to move -towards the centre, and its speed consequently increases. The -rate of radiation of energy will increase rapidly with the speed -of the electron. When the speed of the electron becomes very -nearly equal to the velocity of light, according to Lodge, another -effect supervenes. It has been shown (<a href='#section082'>section 82</a>) that the -apparent mass of an electron increases very rapidly as the speed -of light is approached, and is theoretically infinite at the speed -of light. There will be at this stage a sudden increase of the -mass of the revolving atom, and, on the supposition that this stage -can be reached, a consequent disturbance of the balance of forces -holding the system together. Lodge considers it probable that, -under these conditions, the parts of the system will break asunder -and escape from the sphere of one another’s influence.</p> - -<p class='c006'>It seems probable that the primary cause of the disintegration -of the atom must be looked for in the loss of energy of the atomic -system due to electromagnetic radiation (<a href='#section052'>section 52</a>). Larmor<a id='r376' href='#f376' class='c012'><sup>[376]</sup></a> -has shown that the condition to be fulfilled in order that a system -of rapidly moving electrons may persist without loss of energy is -that the vector sum of the accelerations towards the centre should -be permanently zero. While a single electron moving in a circular -orbit is a powerful radiator of energy, it is remarkable how rapidly -the radiation of energy diminishes if several electrons are revolving -in a ring. This has recently been shown by J. J. Thomson<a id='r377' href='#f377' class='c012'><sup>[377]</sup></a>, -who examined mathematically the case of a system of negatively -electrified corpuscles, situated at equal intervals round the circumference -of a circle, and rotating in one plane with uniform velocity -round its centre. For example, he found that the radiation from -a group of six particles moving with a velocity of ⅒ of the velocity -of light is less than one-millionth part of the radiation from a -single particle describing the same orbit with the same velocity. -When the velocity is ¹⁄₁₀₀ of that of light the amount of radiation -<span class='pageno' id='Page_489'>489</span>is only 10<sup>-16</sup> that of a single particle moving with the same -velocity in the same orbit.</p> - -<p class='c006'>Results of this kind indicate that an atom consisting of a large -number of revolving electrons may radiate energy extremely slowly, -and yet, finally, this minute but continuous drain of energy from -the atom must result either in a rearrangement of its component -parts into a new system, or of an expulsion of electrons or groups -of electrons from the atom.</p> - -<p class='c006'>Simple models of atoms to imitate the behaviour of polonium -in shooting out α particles, and of radium in shooting out β -particles have been discussed by Lord Kelvin<a id='r378' href='#f378' class='c012'><sup>[378]</sup></a>. It is possible to -devise certain stable arrangements of the positively and negatively -electrified particles, supposed to constitute an atom, which, on the -application of some disturbing force, break up with the expulsion -of a part of the system with great velocity.</p> - -<p class='c006'>J. J. Thomson<a id='r379' href='#f379' class='c012'><sup>[379]</sup></a> has mathematically investigated the possible -stable arrangements of a number of electrons moving about in a -sphere of uniform positive electrification. The properties of such -a model atom are very striking, and indirectly suggest a possible -explanation of the periodic law in chemistry. He has shown -that the electrons, if in one plane, arrange themselves in a -number of concentric rings; and generally, if they are not constrained -to move in one plane, in a number of concentric shells -like the coats of an onion.</p> - -<p class='c006'>The mathematical problem is much simplified if the electrons -are supposed to rotate in rings in one plane, the electrons in each -ring being arranged at equal angular intervals. The ways in -which the number of electrons group themselves, for numbers -ranging from 60 to 5 at intervals of 5, are shown in the following -table:—</p> - -<table class='table29' > -<colgroup> -<col class='colwidth35'> -<col class='colwidth10'> -<col class='colwidth10'> -<col class='colwidth10'> -<col class='colwidth10'> -<col class='colwidth10'> -<col class='colwidth10'> -</colgroup> - <tr> - <th class='c013'>Number of electrons</th> - <th class='c015'>60</th> - <th class='c015'>55</th> - <th class='c015'>50</th> - <th class='c015'>45</th> - <th class='c015'>40</th> - <th class='c016'>35</th> - </tr> - <tr> - <td class='c013'> </td> - <td class='c015'> </td> - <td class='c015'> </td> - <td class='c015'> </td> - <td class='c015'> </td> - <td class='c015'> </td> - <td class='c016'> </td> - </tr> - <tr> - <td class='c013'>Number in successive rings</td> - <td class='c015'>20</td> - <td class='c015'>19</td> - <td class='c015'>18</td> - <td class='c015'>17</td> - <td class='c015'>16</td> - <td class='c016'>16</td> - </tr> - <tr> - <td class='c013'> </td> - <td class='c015'>16</td> - <td class='c015'>16</td> - <td class='c015'>15</td> - <td class='c015'>14</td> - <td class='c015'>13</td> - <td class='c016'>12</td> - </tr> - <tr> - <td class='c013'> </td> - <td class='c015'>13</td> - <td class='c015'>12</td> - <td class='c015'>11</td> - <td class='c015'>10</td> - <td class='c015'>8</td> - <td class='c016'>6</td> - </tr> - <tr> - <td class='c013'> </td> - <td class='c015'>8</td> - <td class='c015'>7</td> - <td class='c015'>5</td> - <td class='c015'>4</td> - <td class='c015'>3</td> - <td class='c016'>1</td> - </tr> - <tr> - <td class='c013'> </td> - <td class='c015'>3</td> - <td class='c015'>1</td> - <td class='c015'>1</td> - <td class='c015'> </td> - <td class='c015'> </td> - <td class='c016'> </td> - </tr> -</table> - -<table class='table29' > -<colgroup> -<col class='colwidth35'> -<col class='colwidth10'> -<col class='colwidth10'> -<col class='colwidth10'> -<col class='colwidth10'> -<col class='colwidth10'> -<col class='colwidth10'> -</colgroup> - <tr><td class='c023' colspan='7'><span class='pageno' id='Page_490'>490</span></td></tr> - <tr> - <th class='c013'>Number of electrons</th> - <th class='c015'>30</th> - <th class='c015'>25</th> - <th class='c015'>20</th> - <th class='c015'>15</th> - <th class='c015'>10</th> - <th class='c016'>5</th> - </tr> - <tr> - <td class='c013'> </td> - <td class='c015'> </td> - <td class='c015'> </td> - <td class='c015'> </td> - <td class='c015'> </td> - <td class='c015'> </td> - <td class='c016'> </td> - </tr> - <tr> - <td class='c013'>Number in successive rings</td> - <td class='c015'>15</td> - <td class='c015'>13</td> - <td class='c015'>12</td> - <td class='c015'>10</td> - <td class='c015'>8</td> - <td class='c016'>5</td> - </tr> - <tr> - <td class='c013'> </td> - <td class='c015'>10</td> - <td class='c015'>9</td> - <td class='c015'>7</td> - <td class='c015'>5</td> - <td class='c015'>2</td> - <td class='c016'> </td> - </tr> - <tr> - <td class='c013'> </td> - <td class='c015'>5</td> - <td class='c015'>3</td> - <td class='c015'>1</td> - <td class='c015'> </td> - <td class='c015'> </td> - <td class='c016'> </td> - </tr> -</table> - -<p class='c006'>In the next table is given the possible series of arrangements -of electrons which can have an outer ring of 20:—</p> - -<table class='table20' > -<colgroup> -<col class='colwidth27'> -<col class='colwidth8'> -<col class='colwidth8'> -<col class='colwidth8'> -<col class='colwidth8'> -<col class='colwidth8'> -<col class='colwidth8'> -<col class='colwidth8'> -<col class='colwidth8'> -<col class='colwidth8'> -</colgroup> - <tr> - <th class='c013'>Number of electrons</th> - <th class='c015'>59</th> - <th class='c015'>60</th> - <th class='c015'>61</th> - <th class='c015'>62</th> - <th class='c015'>63</th> - <th class='c015'>64</th> - <th class='c015'>65</th> - <th class='c015'>66</th> - <th class='c016'>67</th> - </tr> - <tr> - <td class='c013'> </td> - <td class='c015'> </td> - <td class='c015'> </td> - <td class='c015'> </td> - <td class='c015'> </td> - <td class='c015'> </td> - <td class='c015'> </td> - <td class='c015'> </td> - <td class='c015'> </td> - <td class='c016'> </td> - </tr> - <tr> - <td class='c013'>Number in successive rings</td> - <td class='c015'>20</td> - <td class='c015'>20</td> - <td class='c015'>20</td> - <td class='c015'>20</td> - <td class='c015'>20</td> - <td class='c015'>20</td> - <td class='c015'>20</td> - <td class='c015'>20</td> - <td class='c016'>20</td> - </tr> - <tr> - <td class='c013'> </td> - <td class='c015'>16</td> - <td class='c015'>16</td> - <td class='c015'>16</td> - <td class='c015'>17</td> - <td class='c015'>17</td> - <td class='c015'>17</td> - <td class='c015'>17</td> - <td class='c015'>17</td> - <td class='c016'>17</td> - </tr> - <tr> - <td class='c013'> </td> - <td class='c015'>13</td> - <td class='c015'>13</td> - <td class='c015'>13</td> - <td class='c015'>13</td> - <td class='c015'>13</td> - <td class='c015'>13</td> - <td class='c015'>14</td> - <td class='c015'>14</td> - <td class='c016'>15</td> - </tr> - <tr> - <td class='c013'> </td> - <td class='c015'>8</td> - <td class='c015'>8</td> - <td class='c015'>9</td> - <td class='c015'>9</td> - <td class='c015'>10</td> - <td class='c015'>10</td> - <td class='c015'>10</td> - <td class='c015'>10</td> - <td class='c016'>10</td> - </tr> - <tr> - <td class='c013'> </td> - <td class='c015'>2</td> - <td class='c015'>3</td> - <td class='c015'>3</td> - <td class='c015'>3</td> - <td class='c015'>3</td> - <td class='c015'>4</td> - <td class='c015'>4</td> - <td class='c015'>5</td> - <td class='c016'>5</td> - </tr> -</table> - -<p class='c006'>The smallest number of electrons which can have an outer -ring of 20 is 59, while 67 is the greatest.</p> - -<p class='c006'>The various arrangements of electrons can be classified into -families, in which the groupings of the electrons have certain -features in common. Thus the group of 60 electrons consists of -the same arrangement of electrons as the group of 40 with the -addition of an outer ring of 20 electrons; the group of 40 is the -same as the group of 24 with an additional ring outside; and the -group of 24 in turn is the same as the group of 11 with an extra -ring. A series of model atoms may be formed in this way, in -which each atom is derived from the preceding member by an -additional ring of electrons. Such atoms would be expected to -possess many properties in common, and would correspond to the -elements in the same vertical column of the periodic table of -Mendeléef.</p> - -<p class='c006'>Different arrangements of electrons vary widely in stability. -Some may acquire an extra electron or two and yet remain stable, -others readily lose an electron without disturbing their stability. -The former would correspond to an electro-negative atom, the -latter to an electro-positive.</p> - -<p class='c006'>Certain arrangements of electrons are stable if the electrons -move with an angular velocity greater than a certain value, but -<span class='pageno' id='Page_491'>491</span>become unstable when the velocity falls below this value. Four -electrons in motion, for example, are stable in one plane, but -when the velocity falls below a certain critical value, the system -is unstable, and the electrons tend to arrange themselves at the -corners of a regular tetrahedron. J. J. Thomson (<i>loc. cit.</i>) applies -this property to explain why an atom of radio-active matter breaks -up, as follows:—</p> - -<p class='c006'>“Consider now the properties of an atom containing a system -of corpuscles (electrons) of this kind. Suppose the corpuscles -were originally moving with velocities far exceeding the critical -velocity; in consequence of the radiation from the moving corpuscles, -their velocity will slowly—very slowly—diminish; when, -after a long interval, the velocity reaches the critical velocity, -there will be what is equivalent to an explosion of the corpuscles, -the corpuscles will move far away from their original position, -their potential energy will decrease, while their kinetic energy -will increase. The kinetic energy gained in this way might be -sufficient to carry the system out of the atom, and we should -have, as in the case of radium, a part of the atom shot off. In -consequence of the very slow dissipation of energy by radiation -the life of the atom would be very long. We have taken the -case of the four corpuscles as the type of a system which, like -a top, requires for its stability a certain amount of rotation. Any -system possessing this property would, in consequence of the -gradual dissipation of energy by radiation, give to the atom containing -it radio-active properties similar to those conferred by the -four corpuscles.”</p> - -<p class='c005'><b>271. Heat of the sun and earth.</b> It was pointed out by -Rutherford and Soddy<a id='r380' href='#f380' class='c012'><sup>[380]</sup></a> that the maintenance of the sun’s heat -for long intervals of time did not present any fundamental difficulty -if a process of disintegration, such as occurs in the radio-elements, -were supposed to be taking place in the sun. In a letter -to <i>Nature</i> (July 9, 1903) W. E. Wilson showed that the presence -of 3·6 grams of radium in each cubic metre of the sun’s mass -was sufficient to account for the present rate of emission of energy -by the sun. This calculation was based on the estimate of Curie -<span class='pageno' id='Page_492'>492</span>and Laborde that 1 gram of radium emits 100 gram-calories per -hour, and on the observation of Langley that each square centimetre -of the sun’s surface emits 8·28 × 10<sup>6</sup> gram-calories per hour. -Since the average density of the sun is 1·44, the presence of radium -in the sun, to the extent of 2·5 parts by weight in a million, -would account for its present rate of emission of energy.</p> - -<p class='c006'>An examination of the spectrum of the sun has not so far -revealed any of the radium lines. It is known, however, from -spectroscopic evidence that helium is present, and this indirectly -suggests the existence of radio-active matter also. It can readily -be shown<a id='r381' href='#f381' class='c012'><sup>[381]</sup></a> that the absence of penetrating rays from the sun at -the surface of the earth does not imply that the radio-elements -are not present in the sun. Even if the sun were composed of -pure radium, it would hardly be expected that the γ rays emitted -would be appreciable at the surface of the earth, since the rays -would be almost completely absorbed in passing through the -atmosphere, which corresponds to a thickness of 76 centimetres of -mercury.</p> - -<p class='c006'>In the Appendix E of Thomson and Tait’s <i>Natural Philosophy</i>, -Lord Kelvin has calculated the energy lost in the concentration of -the sun from a condition of infinite dispersion, and concludes that -it seems “on the whole probable that the sun has not illuminated -the earth for 100,000,000 years and almost certain that he has not -done so for 500,000,000 years. As for the future we may say, with -equal certainty, that inhabitants of the earth cannot continue to -enjoy the light and heat essential to their life for many million -years longer, unless sources now unknown to us are prepared in -the great storehouses of creation.”</p> - -<p class='c006'>The discovery that a small mass of a substance like radium -can emit spontaneously an enormous quantity of heat renders it -possible that this estimate of the age of the sun’s heat may be -much increased. In a letter to <i>Nature</i> (Sept. 24, 1903) G. H. Darwin -drew attention to this probability, and at the same time pointed -out that, on Kelvin’s hypotheses, his estimate of the duration of -the sun’s heat was probably much too high, and stated that, “The -lost energy of the sun, supposed to be a homogeneous sphere -of mass <i>M</i> and radius <i>a</i>, -is (⅗)μ<i>M</i><sup>2</sup>/<i>a</i> where μ is the constant of -<span class='pageno' id='Page_493'>493</span>gravitation. On introducing numerical values for the symbols in -this formula, I find the lost energy to be 2·7 × 10<sup>7</sup> <i>M</i> calories where -<i>M</i> is expressed in grams. If we adopt Langley’s value of the solar -constant, this heat suffices to give a supply for 12 million years. -Lord Kelvin used Pouillet’s value for that constant, but if he had -been able to use Langley’s, his 100 million would have been -reduced to 60 million. The discrepancy between my results of -12 million and his of 60 million is explained by a conjectural -augmentation of the lost energy to allow for the concentration -of the solar mass towards its central parts.” Now it has been -shown (<a href='#section266'>section 266</a>) that one gram of radium emits during its -life an amount of heat corresponding to 1·6 × 10<sup>9</sup> gram-calories. -It has also been pointed out that there is every reason to suppose -that a similar amount of energy is resident in the chemical atoms -of the inactive elements. It is not improbable that, at the -enormous temperature of the sun, the breaking up of the elements -into simpler forms may be taking place at a more rapid -rate than on the earth. If the energy resident in the atoms -of the elements is thus available, the time during which the sun -may continue to emit heat at the present rate may be at least 50 -times longer than the value computed from dynamical data.</p> - -<p class='c006'>Similar considerations apply to the question of the age of -the earth. A full discussion of the probable age of the earth, -computed from its secular cooling from a molten mass, is given -by Lord Kelvin in Appendix D of Thomson and Tait’s <i>Natural -Philosophy</i>. He has there shown that about 100 million years -after the earth was a molten mass, the gradual cooling due to -radiation from its surface would account for the average temperature -gradient of ¹⁄₅₀° F. per foot, observed to-day near the earth’s -surface.</p> - -<p class='c006'>Some considerations will now be discussed which point to the -probability that the present temperature gradient observed in the -earth cannot be used as a guide to estimate the length of time -that has elapsed since the earth has been at a temperature capable -of supporting animal and vegetable life; for it will be shown that -probably there is sufficient radio-active matter on the earth to -supply as much heat to the earth as is lost by radiation from its -surface. Taking the average conductivity <i>K</i> of the materials of -<span class='pageno' id='Page_494'>494</span>the earth as ·004 (<span class='fss'>C.G.S.</span> units) and the temperature gradient <i>T</i> near -the surface as ·00037° C. per cm., the heat <i>Q</i> in gram-calories -conducted to the surface of the earth per second is given by</p> - -<div class='calc'> - -<div class='lg-container-b c017'> - <div class='linegroup'> - <div class='group'> - <div class='line'><i>Q</i> = 4π<i>R</i><sup>2</sup><i>KT</i>,</div> - </div> - </div> -</div> - -</div> - -<p class='c018'>where <i>R</i> is the radius of the earth.</p> - -<p class='c006'>Let <i>X</i> be the average amount of heat liberated per second per -cubic centimetre of the earth’s volume owing to the presence of -radio-active matter. If the heat <i>Q</i> radiated from the earth is -equal to the heat supplied by the radio-active matter in the -earth,</p> - -<div class='calc'> - -<div class='lg-container-b c017'> - <div class='linegroup'> - <div class='group'> - <div class='line in7'><i>X</i> . (⁴⁄₃)π<i>R</i><sup>3</sup> = 4π<i>R</i><sup>2</sup><i>KT</i>,</div> - </div> - <div class='group'> - <div class='line'>or</div> - <div class='line in6'>3<i>KT</i></div> - <div class='line'><i>X</i> = ------ .</div> - <div class='line in6'><i>R</i></div> - </div> - </div> -</div> - -</div> - -<p class='c018'>Substituting the values of these constants,</p> - -<div class='calc'> - -<div class='lg-container-b c017'> - <div class='linegroup'> - <div class='group'> - <div class='line'><i>X</i> = 7 × 10<sup>-15</sup> gram-calories per second</div> - <div class='line in4'>= 2·2 × 10<sup>-7</sup> gram-calories per year.</div> - </div> - </div> -</div> - -</div> - -<p class='c018'>Since 1 gram of radium emits 876,000 gram-calories per year, -the presence of 2·6 × 10<sup>-13</sup> grams of radium per unit volume, or -4·6 × 10<sup>-14</sup> grams per unit mass, would compensate for the heat lost -from the earth by conduction.</p> - -<p class='c006'>Now it will be shown in the following chapter that radio-active -matter seems to be distributed fairly uniformly through the earth -and atmosphere. In addition, it has been found that all substances -are radio-active to a feeble degree, although it is not yet settled -whether this radio-activity may not be due mainly to the presence -of a radio-element as an impurity. For example, Strutt<a id='r382' href='#f382' class='c012'><sup>[382]</sup></a> observed -that a platinum plate was about ¹⁄₃₀₀₀ as active as a crystal of -uranium nitrate, or about 2 × 10<sup>-10</sup> as active as radium. This corresponds -to a far greater activity than is necessary to compensate -for the loss of heat of the earth. A more accurate deduction, -however, can be made from data of the radio-activity exhibited by -matter dug out of the earth. Elster and Geitel<a id='r383' href='#f383' class='c012'><sup>[383]</sup></a> filled a dish of -<span class='pageno' id='Page_495'>495</span>volume 3·3 × 10<sup>3</sup> c.c. with clay dug up from the garden, and placed -it in a vessel of 30 litres capacity in which was placed an electroscope -to determine the conductivity of the enclosed gas. After -standing for several days, they found that the conductivity of the -air reached a constant maximum value, corresponding to three times -the normal. It will be shown later (<a href='#section284'>section 284</a>) that the normal -conductivity observed in sealed vessels corresponds to the production -of about 30 ions per c.c. per second. The number of ions -produced per second in the vessel by the radio-active earth was -thus about 2 × 10<sup>6</sup>. This would give a saturation current through -the gas of 2·2 × 10<sup>-14</sup> electromagnetic units. Now the emanation -from 1 gram of radium stored in a metal cylinder gives a saturation -current of about 3·2 × 10<sup>-5</sup> electromagnetic units. Elster and -Geitel considered that most of the conductivity observed in the -gas was due to a radio-active emanation, which gradually diffused -from the clay into the air in the vessel. The increased conductivity -in the gas observed by Elster and Geitel would thus be -produced by the emanation from 7 × 10<sup>-10</sup> gram of radium. -Taking the density of clay as 2, this corresponds to about 10<sup>-13</sup> -gram of radium per gram of clay. But it has been shown that if -4·6 × 10<sup>-14</sup> gram of radium were present in each gram of earth, the -heat emitted would compensate for the loss of heat of the earth by -conduction and radiation. The amount of activity observed in the -earth is thus about the right order of magnitude to account for the -heat emission required. In the above estimate, the presence of -uranium and thorium minerals in the earth has not been considered. -Moreover, it is probable that the total amount of radio-activity -in the clay was considerably greater than that calculated, -for it is likely that other radio-active matter was present which -did not give off an emanation.</p> - -<p class='c006'>If the earth is supposed to be in a state of thermal equilibrium -in which the heat lost by radiation is supplied from radio-active -matter, there must be an amount of radio-active matter in the -earth corresponding to about 270 million tons of radium. If there -were more radium than this in the earth, the temperature gradient -would be greater than that observed to-day. This may appear to -be a very large quantity of radium, but recent determinations -(<a href='#section281'>section 281</a>) of the amount of radium emanation in the atmosphere -<span class='pageno' id='Page_496'>496</span>strongly support the view that a large quantity of radium must -exist in the surface soil of the earth. Eve found, on a minimum -estimate, that the amount of emanation always present in the -atmosphere is equivalent to the equilibrium amount derived from -100 tons of radium. There is every reason to believe that the -emanation found in the atmosphere is supplied both by the diffusion -of the emanation from the soil and by the action of springs. -Since the emanation loses half its activity in four days, it cannot -diffuse from any great depth. Assuming that the radium is -uniformly distributed throughout the earth, the quantity of the -radium emanation produced in a thin shell of earth about thirteen -metres in depth, is sufficient to account for the amount ordinarily -observed in the atmosphere.</p> - -<p class='c006'>I think we may conclude that the present rate of loss of heat -of the earth might have continued unchanged for long periods of -time in consequence of the supply of heat from radio-active matter -in the earth. It thus seems probable that the earth may have -remained for very long intervals of time at a temperature not very -different from that observed to-day, and that, in consequence, the -time during which the earth has been at a temperature capable of -supporting the presence of animal and vegetable life may be very -much longer than the estimate made by Lord Kelvin from other -data.</p> -<p class='c005'><b>272. Evolution of matter.</b> Although the hypothesis that -all matter is composed of some elementary unit of matter or -protyle has been advanced as a speculation at various times -by many prominent physicists and chemists, the first definite -experimental evidence showing that the chemical atom was -not the smallest unit of matter was obtained in 1897 by -J. J. Thomson in his classic research on the nature of the -cathode rays produced by an electric discharge in a vacuum -tube. We have seen that Sir William Crookes, who was the first -to demonstrate the remarkable properties of these rays, had -suggested that they consisted of streams of projected charged -matter and represented—as he termed it—a new or “fourth state -of matter.”</p> - -<p class='c006'>J. J. Thomson showed by two distinct methods (<a href='#section050'>section 50</a>), -<span class='pageno' id='Page_497'>497</span>that the cathode rays consisted of a stream of negatively charged -particles projected with great velocity. The particles behaved as -if their mass was only about ¹⁄₁₀₀₀ of the mass of the atom of -hydrogen, which is the lightest atom known. These corpuscles, -as they were termed by Thomson, were found at a later date to be -produced from a glowing carbon filament and from a zinc plate -exposed to the action of ultra-violet light. They acted as isolated -units of negative electricity, and, as we have seen, may be identified -with the electrons studied mathematically by Larmor and Lorentz. -Not only were these electrons produced by the action of light, -heat, and the electric discharge, but similar bodies were also -found to be emitted spontaneously from the radio-elements with -a velocity far greater than that observed for the electrons in a -vacuum tube.</p> - -<p class='c006'>The electrons produced in these various ways were all found to -carry a negative charge, and to be apparently identical; for the -ratio <i>e</i>/<i>m</i> of the charge of the electron to its mass was in all cases -the same within the limits of experimental error. Since electrons, -produced from different kinds of matter and under different -conditions, were in all cases identical, it seemed probable that they -were a constituent part of all matter. J. J. Thomson suggested -that the atom is built up of a number of these negatively charged -electrons combined in some way with corresponding positively -charged bodies.</p> - -<p class='c006'>On this view the atoms of the chemical elements differ from -one another only in the number and arrangement of the component -electrons.</p> - -<p class='c006'>The removal of an electron from the atom in the case of -ionization does not appear to affect permanently the stability of -the system, for no evidence has so far been obtained to show that -the passage of an intense electric discharge through a gas results -in a permanent alteration of the structure of the atom. On the -other hand, in the case of the radio-active bodies, a positively -charged particle of mass about twice that of the hydrogen atom -escapes from the heavy radio-atom. This loss appears to result at -once in a permanent alteration of the atom, and causes a marked -change in its physical and chemical properties. In addition there -is no evidence that the process is reversible.</p> - -<p class='c006'><span class='pageno' id='Page_498'>498</span>The expulsion of a β particle with great velocity from an atom -of radio-active matter also results in a transformation of the atom. -For example radium E emits a β particle, and, in consequence, -gives rise to a distinct substance radium F (polonium). A case -of this kind, where the expulsion of a β particle with great -velocity causes a complete rearrangement of the parts of an -atom, is probably quite distinct from the process which occurs -during ionization, where a slow speed electron escapes from the -atom without apparently affecting the stability of the atom left -behind.</p> - -<p class='c006'>The only direct experimental evidence of the transformation -of matter has been derived from a study of the radio-active -bodies. If the disintegration theory, advanced to account for the -phenomena of radio-activity, is correct in the main essentials, then -the radio-elements are undergoing a spontaneous and continuous -process of transformation into other and different kinds of matter. -The rate of transformation is slow in uranium and thorium, but -is fairly rapid in radium. It has been shown that the fraction -of a mass of radium which is transformed per year is about -¹⁄₂₀₀₀ of the total amount present. In the case of uranium -and thorium probably a million years would be required to -produce a similar amount of change. Thus the process of -transformation in uranium and thorium is far too slow to be -detected within a reasonable time by the use of the balance or -spectroscope, but the radiations which accompany the transformation -can easily be detected. Although the process of change is -slow it is continuous, and in the course of ages the uranium and -thorium present in the earth must be transformed into other -types of matter.</p> - -<p class='c006'>Those who have considered the possibility of atoms undergoing -a process of transformation have generally thought that the -matter as a whole would undergo a progressive change, with a -gradual alteration of physical and chemical properties of the whole -mass of substance. On the theory of disintegration this is not the -case. Only a minute fraction of the matter present breaks up in -unit time, and in each of the successive stages through which -the disintegrated atoms pass, there is in most cases a marked -alteration in the chemical and physical properties of the matter. -<span class='pageno' id='Page_499'>499</span>The transformation of the radio-elements is thus a transformation -of a part <i>per saltum</i>, and not a progressive change of the whole. -At any time after the process of transformation has been in -progress there will thus remain a part of the matter which is -unchanged, and, mixed with it, the products which have resulted -from the transformation of the remainder.</p> - -<p class='c006'>The question naturally arises whether the process of degradation -of matter is confined to the radio-elements or is a universal -property of matter. It will be shown in <a href='#chap14'>chapter <span class='fss'>XIV</span></a> that all -matter, so far examined, exhibits the property of radio-activity to -a slight degree. It is very difficult, however, to make certain -that the observed radio-activity is not due to the presence in the -matter of a slight trace of a radio-element. If ordinary matter is -radio-active, it is certain that its activity is much less than that of -uranium, and consequently that its rate of transformation must -be excessively slow. There is, however, another possibility to be -considered. The changes occurring in the radio-elements would -probably never have been detected if the change had not been -accompanied by the expulsion of charged particles with great -velocity. It does not seem unlikely that an atom may undergo -disintegration without projecting a part of its system with sufficient -velocity to ionize the gas. In fact, we have seen that, even -in the radio-elements, several of the series of changes in both -thorium, radium, and actinium are unaccompanied by ionizing -rays. The experimental results given in <a href='#appa'>Appendix A</a> strongly -support this point of view. It may thus be possible that all -matter is undergoing a slow process of transformation, which has -so far only been detected in the radio-elements on account of the -expulsion of charged particles with great velocity during the -change. This process of degradation of matter continuing for ages -must reduce the constituents of the earth to the simpler and -more stable forms of matter.</p> - -<p class='c006'>The idea that helium is a transformation product of radium -suggests the probability that helium is one of the more elementary -substances of which the heavier atoms are composed. Sir Norman -Lockyer, in his interesting book on “Inorganic Evolution,” has -pointed out that the spectra of helium and of hydrogen predominate -in the hottest stars. In the cooler stars the more -<span class='pageno' id='Page_500'>500</span>complex types of matter appear. Sir Norman Lockyer has based -his theory of evolution of matter on evidence of a spectroscopic -examination of the stars, and considers that temperature is the -main factor in breaking up matter into its simpler forms. The -transformation of matter occurring in the radio-elements is on the -other hand spontaneous, and independent of temperature over the -range examined.</p> - -<div class='chapter'> - <span class='pageno' id='Page_501'>501</span> - <h2 id='chap14' class='c004'>CHAPTER XIV. <br> RADIO-ACTIVITY OF THE ATMOSPHERE AND OF ORDINARY MATERIALS.</h2> -</div> -<p class='c005'><b>273. Radio-activity of the atmosphere.</b> The experiments -of Geitel<a id='r384' href='#f384' class='c012'><sup>[384]</sup></a> and C. T. R. Wilson<a id='r385' href='#f385' class='c012'><sup>[385]</sup></a> in 1900 showed that a positively -or negatively charged conductor placed inside a closed vessel gradually -lost its charge. This loss of charge was shown to be due to a -small ionization of the air inside the vessel. Elster and Geitel -also found that a charged body exposed in the open air lost its -charge rapidly, and that the rate of discharge was dependent -on the locality and on atmospheric conditions. A more detailed -description and discussion of these results will be given later in -section 284.</p> - -<p class='c006'>In the course of these experiments, Geitel observed that -the rate of discharge increased slightly for some time after the -vessel had been closed. He considered that this might possibly -be due to the existence of some radio-active substances in the air, -which produced excited activity on the walls of the vessel and so -increased the rate of dissipation of the charge. In 1901 Elster -and Geitel<a id='r386' href='#f386' class='c012'><sup>[386]</sup></a> tried the bold experiment of seeing whether it were -possible to extract a radio-active substance from the air. The -experiments of the writer had shown that the excited radio-activity -from the thorium emanation could be concentrated on the -negative electrode in a strong electric field. This result indicated -that the carriers of the radio-activity had a positive charge of -<span class='pageno' id='Page_502'>502</span>electricity. Elster and Geitel therefore tried an experiment to see -whether positively charged carriers, possessing a similar property, -were present in the atmosphere. For this purpose a cylinder of -wire-netting, charged negatively to 600 volts, was exposed for several -hours in the open air. The cylinder was then removed, and quickly -placed in a large bell-jar, inside which was placed an electroscope -to detect the rate of discharge. It was found that the rate of -discharge was increased to a slight extent. In order to multiply -the effect a wire about 20 metres in length was exposed at some -height from the ground, and was kept charged to a high potential -by connecting it to the negative terminal of an influence machine. -After exposure for some hours, this wire was removed and placed -inside the dissipation vessel. The rate of discharge was found to -be increased many times by the presence of the wire. No increase -was observed when the wire was charged positively instead of -negatively. The results also showed that the radio-active matter -could be removed from the wire in the same way as from a wire -made active by exposure in the presence of the thorium emanation. -A piece of leather moistened with ammonia was rubbed over the -active wire. On testing the leather, it was found to be strongly -radio-active. When a long wire was used, the amount of activity -obtained on the leather was comparable with that possessed by a -gram of uranium oxide.</p> - -<p class='c006'>The activity produced on the wire was not permanent, but -disappeared to a large extent in the course of a few hours. The -amount of activity produced on a wire of given size, exposed under -similar conditions, was independent of the material of the wire. -Lead, iron and copper wires gave about equal effects.</p> - -<p class='c006'>The amount of activity obtained was greatly increased by exposing -a negatively charged wire in a mass of air which had been -undisturbed for a long time. Experiments were made in the great -cave of Wolfenbüttel, and a very large amount of activity was -observed. By transferring the activity to a piece of leather it -was found that the rays could appreciably light up a screen of -barium platinocyanide in the dark<a id='r387' href='#f387' class='c012'><sup>[387]</sup></a>. The rays also darkened a -photographic plate through a piece of aluminium 0·1 mm. in -thickness.</p> - -<p class='c006'><span class='pageno' id='Page_503'>503</span>These remarkable experiments show that the excited radio-activity -obtained from the atmosphere is very similar in character -to the excited activity produced by the emanations of radium and -thorium. No investigators have contributed more to our knowledge -of the radio-activity and ionization of the atmosphere than -Elster and Geitel. The experiments here described have been the -starting-point of a series of researches by them and others on the -radio-active properties of the atmosphere, which have led to a -great extension of our knowledge of that important subject.</p> - -<p class='c006'>Rutherford and Allan<a id='r388' href='#f388' class='c012'><sup>[388]</sup></a> determined the rate of decay of the -excited activity produced on a negatively charged wire exposed in -the open air. A wire about 15 metres long was exposed in the -open air, and kept charged by an influence machine to a potential -of about -10,000 volts. An hour’s exposure was sufficient to obtain -a large amount of excited activity on the wire. The wire was -then rapidly removed and wound on a framework which formed -the central electrode in a large cylindrical metal vessel. The -ionization current for a saturation voltage was measured by -means of a sensitive Dolezalek electrometer. The current, which -is a measure of the activity of the wire, was found to diminish -according to an exponential law with the time, falling to half value -in about 45 minutes. The rate of decay was independent of the -material of the wire, of the time of exposure, and of the potential -of the wire.</p> - -<p class='c006'>An examination was also made of the nature of the rays emitted -by the radio-active wire. For this purpose a lead wire was made -radio-active in the manner described, and then rapidly wound into -the form of a flat spiral. The penetrating power of the rays was -tested in a vessel similar to that shown in <a href='#fig017'>Fig. 17</a>. Most of the -ionization was found to be due to some very easily absorbed rays, -which were of a slightly more penetrating character than the α -rays emitted from a wire made active by the radium or thorium -emanations. The intensity of the rays was cut down to half value -by about 0·001 cm. of aluminium. The photographic action observed -by Elster and Geitel through 0·1 mm. of aluminium showed -that some penetrating rays were also present. This was afterwards -confirmed by Allan, who used the electric method. These penetrating -<span class='pageno' id='Page_504'>504</span>rays are probably similar in character to the β rays from the -radio-elements.</p> -<p class='c005'><b>274.</b> The excited activity produced on the negatively charged -wire cannot be due to an action of the strong electric field on the -surface of the wire; for very little excited activity is produced if -the wire is charged to the same potential inside a closed cylinder.</p> - -<p class='c006'>We have seen that the excited activity produced on the wire -can be partially removed by rubbing and by solution in acids, and, -in this respect, it is similar to the excited activity produced in -bodies by the emanations of radium and thorium. The very close -similarity of the excited activity obtained from the atmosphere -to that obtained from the radium and thorium emanations suggests -the probability that a radio-active emanation exists in the -atmosphere. This view is confirmed by a large amount of indirect -evidence discussed in sections <a href='#section276'>276</a>, <a href='#section277'>277</a> and <a href='#section280'>280</a>.</p> - -<p class='c006'>Assuming the presence of a radio-active emanation in the -atmosphere, the radio-active effects observed receive a simple -explanation. The emanation in the air gradually breaks up, -giving rise in some way to positively charged radio-active carriers. -These are driven to the negative electrode in the electric field, -and there undergo a further change, giving rise to the radiations -observed at the surface of the wire. The matter which causes -excited activity will thus be analogous to the active deposit of -radium and thorium.</p> - -<p class='c006'>Since the earth is negatively electrified with regard to the -upper atmosphere, these positive radio-active carriers produced in -the air are continuously deposited on the surface of the earth. -Everything on the surface of the earth, including the external -surface of buildings, the grass, and leaves of trees, must be covered -with an invisible deposit of radio-active material. A hill, or -mountain peak, or any high mass of rock or land, concentrates the -earth’s electric field at that point and consequently will receive -more excited radio-activity per unit area than the plain. Elster -and Geitel have pointed out that the greater ionization of the air -observed in the neighbourhood of projecting peaks receives a -satisfactory explanation on this view.</p> - -<p class='c006'>If the radio-active carriers are produced at a uniform rate in -<span class='pageno' id='Page_505'>505</span>the atmosphere, the amount of excited activity <i>I<sub>t</sub></i>, produced on -a wire exposed under given conditions, will, after exposure for a -time <i>t</i>, be given by</p> - -<div class='figcenter id009'> -<img src='images/form-144.png' alt='Formula.' class='ig001'> -</div> - -<p class='c006'>where <i>I</i>₀ is the maximum -activity on the wire and λ is the constant of decay of the excited -activity. Since the activity of a wire after removal falls to half -value in about 45 minutes, the value of λ is 0·92 (hour)<sup>-1</sup>. Some -experiments made by Allan<a id='r389' href='#f389' class='c012'><sup>[389]</sup></a> are in rough agreement with the -above equation. Accurate comparative results are difficult to -obtain on account of the inconstancy of the radio-activity of the -open air. After an exposure of a wire for several hours, the -activity reached a practical maximum, and was not much increased -by continued exposure.</p> - -<p class='c006'>We have seen (<a href='#section191'>section 191</a>) that the carriers of the active -deposit of radium and thorium move in an electric field with about -the same velocity as the ions. We should expect therefore that a -long wire charged to a high negative potential would abstract the -active carriers from the atmosphere for a considerable distance. -This does not appear to be the case, for Eve (see <a href='#section281'>section 281</a>) has -found that the carriers are only abstracted from the air for a -radius of less than one metre, for a potential of the wire of -10,000 -volts. It seems probable that the carriers of the active matter -are deposited on the numerous fine dust particles present in the -air and thus move very slowly even in a strong electric field.</p> - -<p class='c006'>The amount of excited activity produced on a wire, supported -some distance from the surface of the earth, should increase steadily -with the voltage, for the greater the potential, the greater the -volume of air from which the radio-active carriers are abstracted.</p> - -<p class='c006'>The presence of radio-active matter in the atmosphere will -account for a considerable portion of the ionization of the air -observed near the earth. This important question is discussed in -more detail in <a href='#section281'>section 281</a>.</p> -<p class='c005'><b>275. Radio-activity of freshly fallen rain and snow.</b> -C. T. R. Wilson<a id='r390' href='#f390' class='c012'><sup>[390]</sup></a> tried experiments to see if any of the radio-active -material from the air was carried down by rain. For this -purpose a quantity of freshly fallen rain was collected, rapidly -<span class='pageno' id='Page_506'>506</span>evaporated to dryness in a platinum vessel, and the activity of the -residue tested by placing the vessel in an electroscope. In all -cases, the rate of discharge of the electroscope was considerably -increased. From about 50 c.c. of rain water, an amount of activity -was obtained sufficient to increase the rate of discharge of the -electroscope four or five times, after the rays had traversed a thin -layer of aluminium or gold-leaf. The activity disappeared in the -course of a few hours, falling to half value in about 30 minutes. -Rain water, which had stood for some hours, showed no trace of -activity. Tap water, when evaporated, left no active residue.</p> - -<p class='c006'>The amounts of activity obtained from a given quantity of rain -water were all of the same order of magnitude, whether the rain -was precipitated in fine or in large drops, by night or by day, or -whether the rain was tested at the beginning or at the end of a -heavy rainfall lasting several hours.</p> - -<p class='c006'>The activity obtained from rain is not destroyed by heating -the platinum vessel to a red heat. In this and other respects it -resembles the excited activity obtained on negatively charged -wires exposed in the open air.</p> - -<p class='c006'>C. T. R. Wilson<a id='r391' href='#f391' class='c012'><sup>[391]</sup></a> obtained a radio-active precipitate from rain -water by adding a little barium chloride and precipitating the -barium with sulphuric acid. An active precipitate was also -obtained when alum was added to the water, and the aluminium -precipitated by ammonia. The precipitates obtained in this way -showed a large activity. The filtrate when boiled down was quite -inactive, showing that the active matter had been completely -removed by precipitation. This effect is quite analogous to the -production of active precipitates from a solution containing the -active deposit of thorium (see <a href='#section185'>section 185</a>).</p> - -<p class='c006'>The radio-activity of freshly fallen snow was independently observed -by C. T. R. Wilson<a id='r392' href='#f392' class='c012'><sup>[392]</sup></a> in England, and Allan<a id='r393' href='#f393' class='c012'><sup>[393]</sup></a> and McLennan<a id='r394' href='#f394' class='c012'><sup>[394]</sup></a> -in Canada. In order to obtain a large amount of activity, the -surface layer of snow was removed, and evaporated to dryness -in a metal vessel. An active residue was obtained with radio-active -<span class='pageno' id='Page_507'>507</span>properties similar to those observed for freshly fallen rain. -Both Wilson and Allan found that the activity of rain and snow -decayed at about the same rate, the activity falling to half value -in about 30 minutes. McLennan states that he found a smaller -amount of radio-activity in the air after a prolonged fall of snow.</p> - -<p class='c006'>Schmauss<a id='r395' href='#f395' class='c012'><sup>[395]</sup></a> has observed that drops of water falling through air -ionized by Röntgen rays acquire a negative charge. This effect is -ascribed to the fact that the negative ions in air diffuse faster -than the positive. On this view the drops of rain and flakes of -snow would acquire a negative charge in falling through the air. -They would in consequence act as collectors of the positive radio-active -carriers from the air. On evaporation of the water the -radio-active matter would be left behind.</p> -<p class='c005'><a id='section276'></a> -<b>276. Radio-active emanations from the earth.</b> Elster -and Geitel observed that the air in caves and cellars was, in most -cases, abnormally radio-active, and showed very strong ionization. -This action might possibly be due to an effect of stagnant air, by -which it produced a radio-active emanation from itself, or to a -diffusion of a radio-active emanation from the soil. To test -whether this emanation was produced by the air itself, Elster and -Geitel shut up the air for several weeks in a large boiler, but no -appreciable increase of the activity or ionization was observed. To -see whether the air imprisoned in the capillaries of the soil was -radio-active, Elster and Geitel<a id='r396' href='#f396' class='c012'><sup>[396]</sup></a> put a pipe into the earth and sucked -up the air into a testing vessel by means of a water pump.</p> - -<p class='c006'>The apparatus employed to test the ionization of the air is -shown in <a href='#fig103'>Fig. 103</a>. <i>C</i> is an electroscope connected with a wire net, -<i>Z</i>. The active air was introduced into a large bell-jar of 27 litres -capacity, the inside of which was covered with wire netting, <i>MM´</i>. -The bell-jar rested on an iron plate <i>AB</i>. The electroscope could -be charged by the rod <i>S</i>. The rate of discharge of the electroscope, -before the active air was introduced, was noted. On allowing -the active air to enter, the rate of discharge increased rapidly, -rising in the course of a few hours in one experiment to 30 times -the original value. They found that the emanation produced -<span class='pageno' id='Page_508'>508</span>excited activity on the walls of the containing vessel. The air -sucked up from the earth was even more active than that observed -in caves and cellars. There can thus be little doubt that the -abnormal activity observed in caves and cellars is due to a radio-active -emanation, present in the earth, which gradually diffuses to -the surface and collects in places where the air is not disturbed.</p> - -<p class='c006'>Results similar to those obtained by Elster and Geitel for the air -removed from the earth at Wolfenbüttel were also obtained later -by Ebert and Ewers<a id='r397' href='#f397' class='c012'><sup>[397]</sup></a> at Munich. They found a strongly active -emanation in the soil, and, in addition, examined the variation with -time of the activity due to the emanation in a sealed vessel. After -the introduction of the active air into the testing vessel, the activity -was observed to increase for several hours, and then to decay, -according to an exponential law, with the time, falling to half -value in about 3·2 days. This rate of decay is more rapid than -that observed for the radium emanation, which decays to half -value in a little less than four days. The increase of activity with -time is probably due to the production of excited activity on the -walls of the vessel by the emanation. In this respect it is analogous -to the increase of activity observed when the radium emanation -is introduced into a closed vessel. No definite experiments were -made by Ebert and Ewers on the rate of decay of this excited -activity. In one experiment the active emanation, after standing -in the vessel for 140 hours, was removed by sucking ordinary air -of small activity through the apparatus. The activity rapidly fell -to about half value, and this was followed by a very slow decrease -of the activity with time. This result indicates that about half -the rate of discharge observed was due to the radiation from the -emanation and the other half to the excited activity produced -by it.</p> - -<p class='c006'>The apparatus employed by Ebert and Ewers in these experiments -was very similar to that employed by Elster and Geitel, -shown in <a href='#fig103'>Fig. 103</a>. Ebert and Ewers observed that, when the wire -net attached to the electroscope was charged negatively, the rate -of discharge observed was always greater than when it was charged -positively. The differences observed between the two rates of -discharge varied between 10 and 20 per cent. A similar effect -<span class='pageno' id='Page_509'>509</span>has been observed by Sarasin, Tommasina and Micheli<a id='r398' href='#f398' class='c012'><sup>[398]</sup></a> for a wire -made active by exposure to the open air. This difference in -the rates of discharge for positive and negative electricity is -probably connected with the presence of particles of dust or small -water globules suspended in the gas. The experiments of Miss -Brooks (<a href='#section181'>section 181</a>) have shown that the particles of dust present -in the air containing the thorium emanation become radio-active. -A large proportion of these dust particles acquire a positive charge -and are carried to the negative electrode in an electric field. This -effect would increase the rate of discharge of the electroscope when -charged negatively. In later experiments, Ebert and Ewers -noticed that, in some cases, when the air had been kept in the -vessel for several days, the effect was reversed, and the electroscope -showed a great rate of discharge when charged positively.</p> - -<div id='fig103' class='figcenter id007'> -<img src='images/fig-103.png' alt='Fig. 103.' class='ig001'> -<div class='ic002'> -<p>Fig. 103.</p> -</div> -</div> - -<p class='c006'>J. J. Thomson<a id='r399' href='#f399' class='c012'><sup>[399]</sup></a> has observed that the magnitude of the ionization -current depends on the direction of the electric field, if fine -water globules are suspended in the ionized gas.</p> - -<p class='c006'><span class='pageno' id='Page_510'>510</span>In later experiments, Ebert<a id='r400' href='#f400' class='c012'><sup>[400]</sup></a> found that the radio-active emanation -could be removed from the air by condensation in liquid air. -This property of the emanation was independently discovered by -Ebert before he was aware of the results of Rutherford and Soddy -on the condensation of the emanations of radium and thorium. -To increase the amount of radio-active emanation in a given -volume of air, a quantity of the active air, obtained by sucking the -air from the soil, was condensed by a liquid air machine. The air -was then allowed partially to evaporate, but the process was stopped -before the point of volatilization of the emanation was reached. -This process was repeated with another quantity of air and the -residues added together. Proceeding in this way, he was able to -concentrate the emanation in a small volume of air. On allowing -the air to evaporate, the ionization of the air in the testing vessel -increased rapidly for a time and then slowly diminished. Ebert -states that the maximum for the emanation which had been liquefied -for some time was reached earlier than for fresh air. The rate -of decay of activity of the emanation was not altered by keeping -it at the temperature of liquid air for some time. In this respect -it behaves like the emanations of radium and thorium.</p> - -<p class='c006'>J. J. Thomson<a id='r401' href='#f401' class='c012'><sup>[401]</sup></a> found that air bubbled through Cambridge tap -water showed much greater conductivity than ordinary air. The -air was drawn through the water by means of a water pump into a -large gasometer, when the ionization current was tested with a -sensitive electrometer. When a rod charged negatively was introduced -into this conducting air it became active. After an exposure -for a period of 15 to 30 minutes in the conducting gas, the rod, -when introduced into a second testing vessel, increased the saturation -current in the vessel to about five times the normal amount. -Very little effect was produced when the rod was uncharged or -charged positively for the same time. The activity of the rod -decayed with the time, falling to half value in about 40 minutes. -The amount of activity produced on a wire under constant conditions -was independent of the material of the wire. The rays from -the rod were readily absorbed in a few centimetres of air.</p> - -<p class='c006'>These effects were, at first, thought to be due to the action of -<span class='pageno' id='Page_511'>511</span>the small water drops suspended in the gas, for it was well known -that air rapidly drawn through water causes a temporary increase -in its conductivity. Later results, however, showed that there -was a radio-active emanation present in Cambridge tap water. -This led to an examination of the waters from deep wells in -various parts of England, and J. J. Thomson found that, in some -cases, a large amount of emanation could be obtained from the -well water. The emanation was released either by bubbling air -through the water or by boiling the water. The gases obtained by -boiling the water were found to be strongly active. A sample of -air mixed with the radio-active emanation was condensed. The -liquefied gas was allowed to evaporate, and the earlier and later -portions of the gas were collected in separate vessels. The final -portion was found to be about 30 times as active as the first portion.</p> - -<p class='c006'>An examination of the radio-active properties of the active -gases so obtained has been made by Adams<a id='r402' href='#f402' class='c012'><sup>[402]</sup></a>. He found that the -activity of the emanation decayed, according to an exponential law, -with the time, falling to half value in about 3·4 days. This is not -very different from the rate of decay of the activity of the radium -emanation, which falls to half value in a little less than four days. -The excited activity produced by the emanation decayed to half -value in about 35 minutes. The decay of the excited activity from -radium is at first irregular, but after some time falls off, according -to an exponential law, diminishing to half value in 28 minutes. -Taking into account the uncertainty attaching to measurements of -the very small ionization observed in these experiments, the results -indicate that the emanation obtained from well water in England -is similar to, if not identical with, the radium emanation. Adams -observed that the emanation was slightly soluble in water. After -well water had been boiled for a while and then put aside, it -was found to recover its power of giving off an emanation. The -amount obtained after standing for some time was never more -than 10 per cent. of the amount first obtained. Thus it is probable -that the well water, in addition to the emanations mixed with it, -has also a slight amount of a permanent radio-active substance -dissolved in it. Ordinary rain water or distilled water does not -give off an emanation.</p> - -<p class='c006'><span class='pageno' id='Page_512'>512</span>Bumstead and Wheeler<a id='r403' href='#f403' class='c012'><sup>[403]</sup></a> have made a very careful examination -of the radio-activity of the emanation obtained from -the surface water and soil at New Haven, Connecticut. The -emanation, obtained from the water by boiling, was passed into -a large testing cylinder, and measurements of the current were -made by means of a sensitive electrometer. The current gradually -rose to a maximum, after the introduction of the emanation, in -exactly the same way as the current increases in a vessel after the -introduction of the radium emanation. The decay of activity of -the emanations obtained from the water and soil was carefully -measured, and, within the limits of experimental error, agreed with -the rate of decay of activity observed for the radium emanation. -The identity of the emanations from the water and soil with the -radium emanation was still further established by experiments -on the rate of diffusion of the emanation through a porous plate. -By comparative tests it was found that the coefficient of diffusion -of the emanations from the water and soil was the same as for -the radium emanation. Also, by comparison of the rate of -diffusion of carbonic acid, it was found that the density of the -emanation was about four times that of carbonic acid, a result -in good agreement with that found for the radium emanation -(sections <a href='#section161'>161</a> and <a href='#section162'>162</a>).</p> - -<p class='c006'>Bumstead<a id='r404' href='#f404' class='c012'><sup>[404]</sup></a> has found that a considerable amount of thorium -as well as radium emanation exists in the air of New Haven. For -a three hour exposure in the open air, 3 to 5 per cent. of the -excited activity on the wire is due to thorium. For a twelve hour -exposure, the thorium activity was sometimes 15 per cent. of the -whole. On account of the comparatively slow decay of the excited -activity of thorium, the activity on the wire after removal for -three or four hours was due almost entirely to thorium. The rate -of decay could then be measured accurately, and was found to be -the same as for a wire exposed in the presence of the thorium -emanation.</p> - -<p class='c006'>Dadourian<a id='r405' href='#f405' class='c012'><sup>[405]</sup></a> has made an examination of the underground air -in New Haven, and has found that this too contains a large -<span class='pageno' id='Page_513'>513</span>quantity of the thorium emanation. A circular hole about 50 cms. -in diameter and 2 metres deep was dug in the ground. A number -of wires were wound on an insulated frame and suspended in -the hole, the top of the hole then being covered over. The wire -was charged negatively by a Wimshurst machine. After a long -exposure the excited activity on the wire diminished at a rate -that showed it to be a mixture of the excited activities of thorium -and radium.</p> - -<p class='c006'>A very large amount of work has been done in examining -various hot and mineral springs for the presence of the radium -emanation, and it is not possible here to refer more than briefly to -a few of the very numerous papers that have been published -on this subject both in Europe and America. H. S. Allen and -Lord Blythswood<a id='r406' href='#f406' class='c012'><sup>[406]</sup></a> have observed that the hot springs at Bath and -Buxton gave off a radio-active emanation. This was confirmed by -Strutt<a id='r407' href='#f407' class='c012'><sup>[407]</sup></a>, who found that the escaping gases contained the radium -emanation, and also that the mud deposited from the springs -contained a trace of radium salts. These results are of considerable -interest, for Lord Rayleigh has observed that helium is -contained among the gases evolved by the springs. It appears -probable that the helium observed is produced from the radium -or radio-active deposits through which the water flows. Many -mineral and hot springs which are famous for their curative -properties have been found to contain traces of radium and also -considerable amounts of radium emanation. It has been suggested -that the curative properties may be due to some extent to -the presence of these minute quantities of radium.</p> - -<p class='c006'>Himstedt<a id='r408' href='#f408' class='c012'><sup>[408]</sup></a> found that the thermal springs at Baden Baden -contained the radium emanation, while Elster and Geitel<a id='r409' href='#f409' class='c012'><sup>[409]</sup></a> -examined the deposits formed by these springs and found them -to contain small quantities of radium salts. Results of a similar -character were obtained for a number of waters in Germany by -Dorn<a id='r410' href='#f410' class='c012'><sup>[410]</sup></a>, Schenck<a id='r411' href='#f411' class='c012'><sup>[411]</sup></a>, and H. Mache<a id='r412' href='#f412' class='c012'><sup>[412]</sup></a>.</p> - -<p class='c006'><span class='pageno' id='Page_514'>514</span>Curie and Laborde<a id='r413' href='#f413' class='c012'><sup>[413]</sup></a> have tested the waters of a large number -of mineral springs and found that the great majority contain the -radium emanation. In this connection, it is of interest to note -that Curie and Laborde found very little emanation in the waters -of Salins-Moutiers, while Blanc<a id='r414' href='#f414' class='c012'><sup>[414]</sup></a> observed, on the other hand, that -the sediment from the spring was very active. A closer examination -of this deposit by Blanc revealed the fact that it contained a -considerable quantity of thorium. This was proved by finding that -it gave out an emanation, which lost half of its activity in one -minute, and produced excited activity, which fell to half value in -about 11 hours. Boltwood<a id='r415' href='#f415' class='c012'><sup>[415]</sup></a> has tested a number of samples of -spring water from different sources in America and has found -that many of them contain the radium emanation.</p> - -<p class='c006'>Most of the results upon the amount of radium emanation from -different sources have been expressed in arbitrary units without, -in many cases, any comparative standard being given. Boltwood -(<i>loc. cit.</i>) has described a satisfactory method for collecting and -testing the emanation from different waters, and has suggested -that the rate of discharge observed by the electroscope or the -electrometer should be expressed in terms of the effect due to the -emanation liberated on solution of a definite weight of the mineral -uraninite. Since in every mineral so far examined, the amount -of radium present is proportional to the amount of uranium, such -a standard would be sufficiently definite for practical purposes. -The emanation liberated from a few centigrams of the mineral is -sufficient to give a convenient rate of discharge of an electroscope. -Such a method is preferable to using a known quantity of a -radium compound as a standard, since it is difficult to know with -certainty the activity of the preparations of radium which may be -in the possession of the different experimenters.</p> -<p class='c005'><a id='section277'></a> -<b>277. Radio-activity of constituents of the earth.</b> Elster -and Geitel<a id='r416' href='#f416' class='c012'><sup>[416]</sup></a> observed that, although in many cases the conductivity -of the air was abnormally high in underground enclosures, the -conductivity varied greatly in different places. In the Baumann -<span class='pageno' id='Page_515'>515</span>Cave, for example, the conductivity of the air was nine times the -normal, but in the Iberg Cave only three times the normal. In a -cellar at Clausthal the conductivity was only slightly greater than -the normal, but the excited radio-activity obtained on a negatively -charged wire exposed in it was only ¹⁄₁₁ of the excited radio-activity -obtained when the wire was exposed in the free air. They -concluded from these experiments that the amount of radio-activity -in the different places probably varied with the nature -of the soil. Observations were then made on the conductivity of -the air sucked up from the earth at different parts of the country. -The clayey and limestone soils at Wolfenbüttel were found to be -strongly active, the conductivity varying from four to sixteen times -the normal amount. A sample of air from the shell limestone of -Würzburg and from the basalt of Wilhelmshöhe showed very little -activity.</p> - -<p class='c006'>Experiments were made to see whether any radio-active substance -could be detected in the soil itself. For this purpose some -earth was placed on a dish and introduced under a bell-jar, similar -to that shown in <a href='#fig103'>Fig. 103</a>. The conductivity of the air in the bell-jar -increased with the time, rising to three times the normal value -after several days. Little difference was observed whether the -earth was dry or moist. The activity of the soil seemed to be -permanent, for no change in the activity was observed after the -earth had been laid aside for eight months.</p> - -<p class='c006'>Attempts were then made to separate the radio-active constituent -from the soil by chemical treatment. For this purpose -a sample of clay was tested. By extraction with hydrochloric -acid all the calcium carbonate was removed. On drying the -clay the activity was found to be reduced, but it spontaneously -regained its original activity in the course of a few days. It seems -probable, therefore, that an active product had been separated -from the soil by the acid. Elster and Geitel consider that an -active substance was present in the clay, which formed a product -more readily soluble in hydrochloric acid than the active material -itself. There seemed to be a process of separation analogous to -that of Th X from thorium by precipitation with ammonia.</p> - -<p class='c006'>Experiments were also made to see whether substances placed -in the earth acquired any radio-activity. For this purpose samples -<span class='pageno' id='Page_516'>516</span>of potter’s clay, whitening, and heavy spar, wrapped in linen, were -placed in the earth 50 cms. below the surface. After an interval -of a month, these were dug up and their activity examined. The -clay was the only substance which showed any activity. The -activity of the clay diminished with the time, showing that activity -had been excited in it by the emanations present in the soil.</p> - -<p class='c006'>Elster and Geitel<a id='r417' href='#f417' class='c012'><sup>[417]</sup></a> have found that a large quantity of the -radio-active emanation can be obtained by sucking air through -clay. In some cases, the conductivity of the air in the testing -vessel was increased over 100 times. They have also found that -the so-called “fango”—a fine mud obtained from hot springs in -Battaglia, Northern Italy—gives off three or four times as much -emanation as clay. By treating the fango with acid, the active -substance present was dissolved. On adding some barium chloride -to the solution, and precipitating the barium as sulphate, the active -substance was removed, and in this way a precipitate was obtained -over 100 times as active, weight for weight, as the original fango. -Comparisons were made of the rate of decay of the excited activity, -due to the emanation from fango, with that due to the radium -emanation, and within the limits of error, the decay curves obtained -were found to be identical. There can thus be no doubt that the -activity observed in fango is due to the presence of a small -quantity of radium. Elster and Geitel calculate that the amount -of radium, contained in it, is only about one-thousandth of the -amount to be obtained from an equal weight of pitchblende from -Joachimsthal.</p> - -<p class='c006'>Vincenti and Levi Da Zara<a id='r418' href='#f418' class='c012'><sup>[418]</sup></a> have found that the waters and -sediments of a number of hot springs in Northern Italy contain -the radium emanation. Elster and Geitel observed that natural -carbonic acid obtained from great depths of old volcanic soil was -radio-active, while Burton<a id='r419' href='#f419' class='c012'><sup>[419]</sup></a> found that the petroleum from a deep -well in Ontario, Canada, contained a large quantity of emanation, -probably of radium, since its activity fell to half value in 3·1 days, -while the excited activity produced by the emanation fell to half -<span class='pageno' id='Page_517'>517</span>value in about 35 minutes. A permanently active deposit was -left behind after volatilization of the oil, indicating that probably -one or more of the radio-elements were present in minute -quantity.</p> - -<p class='c006'>Elster and Geitel<a id='r420' href='#f420' class='c012'><sup>[420]</sup></a> have found that the active sediments -obtained from springs at Nauheim and Baden Baden showed -abnormal rates of decay of the excited activity. This was finally -traced to the presence in the deposit of both thorium and radium. -By suitable chemical methods, the two active substances were -separated from each other and were then tested separately.</p> -<p class='c005'><b>278. Effect of meteorological conditions upon the -radio-activity of the atmosphere.</b> The original experiments -of Elster and Geitel on the excited radio-activity derived from -the atmosphere were repeated by Rutherford and Allan<a id='r421' href='#f421' class='c012'><sup>[421]</sup></a> in -Canada. It was found that a large amount of excited radio-activity -could be derived from the air, and that the effects were -similar to those observed by Elster and Geitel in Germany. This -was the case even on the coldest day in winter, when the ground -was covered deeply with snow and wind was blowing from the -north over snow-covered lands. The results showed that the -radio-activity present in the air was not much affected by the -presence of moisture, for the air during a Canadian winter is -extremely dry. The greatest amount of excited activity on a -negatively charged wire was obtained in a strong wind. In some -cases the amount produced for a given time of exposure was ten -to twenty times the normal amount. A cold bright day of winter -usually gave more effect than a warm dull day in summer.</p> - -<p class='c006'>Elster and Geitel<a id='r422' href='#f422' class='c012'><sup>[422]</sup></a> have made a detailed examination of the -effect of meteorological conditions on the amount of excited radio-activity -to be derived from the atmosphere. For this purpose a -simple portable apparatus was devised by them and used for the -whole series of experiments. A large number of observations were -taken, extending over a period of twelve months. They found -that the amount of excited activity obtained was subject to great -<span class='pageno' id='Page_518'>518</span>variations. The extreme values obtained varied in the ratio of -16 to 1. No direct connection could be traced between the amount -of ionization in the atmosphere and the amount of excited activity -produced. They found that the greatest amount of excited activity -was obtained during a fog, when the amount of ionization in the -air was small. This result, however, is not necessarily contradictory -to the view that the ionization and activity of the air -are to a certain extent connected. From the experiments of -Miss Brooks on the effect of dust in acting as carriers of excited -activity, more excited activity should be obtained during a fog -than in clear air. The particles of water become centres for the -deposit of radio-active matter. The positive carriers are thus -anchored and are not removed from the air by the earth’s field. -In a strong electric field, these small drops will be carried to the -negative electrode and manifest their activity on the surface of -the wire. On the other hand, the distribution of water globules -throughout the air causes the ions in the air to disappear rapidly -in consequence of their diffusion to the surface of the drops (see -<a href='#section031'>section 31</a>). For this reason the denser the fog, the smaller will -be the conductivity observed in the air.</p> - -<p class='c006'>Lowering the temperature of the air had a decided influence. -The average activity observed below 0° C. was 1·44 times the -activity observed above 0° C. The height of the barometer was -found to exert a marked influence on the amount of excited activity -to be derived from the air. The lower the barometer the greater -was the amount of excited activity in the air. The effect of -variation of the height of the barometer is intelligible, when it is -considered that probably a large proportion of the radio-activity -observed in the air is due to the radio-active emanations which -are continuously diffusing from the earth into the atmosphere. -Elster and Geitel have suggested that a lowering of the pressure -of the air would cause the air from the ground to be drawn up -from the capillaries of the earth into the atmosphere. This, however, -need not necessarily be the case if the conditions of the escape -of the emanation into the atmosphere are altered by the variation -of the position of underground water or by a heavy fall of rain.</p> - -<p class='c006'>The amount of excited activity to be derived from the air on -the Baltic Coast was only one-third of that observed inland at -<span class='pageno' id='Page_519'>519</span>Wolfenbüttel. Experiments on the radio-activity of the air in -mid-ocean would be of great importance in order to settle whether -the radio-activity observed in the air is due to the emanations -from the soil alone. It is probable that the radio-activity of the -air at different points of the earth may vary widely, and may -largely depend on the nature of the soil.</p> - -<p class='c006'>Saake<a id='r423' href='#f423' class='c012'><sup>[423]</sup></a> has found that the amount of emanation present in the -air at high altitudes in the valley of Arosa in Switzerland is much -greater than the normal amount at lower levels. Elster and Geitel -have observed that there is also a larger number of ions in the air -at high altitudes, and suggest that the curative effect of thermal -springs and the physiological actions of the air at high levels may be -connected with the presence of an unusual amount of radio-active -matter in the atmosphere. Simpson<a id='r424' href='#f424' class='c012'><sup>[424]</sup></a> made experiments on the -amount of excited activity at Karasjoh, Norway, at a height of about -150 feet above sea level. The sun did not rise above the level of -the horizon during the time the observations were taken. The -average amount of excited activity obtained from the air was -considerably greater than the normal amount observed by Elster -and Geitel in Germany. This was the more surprising as the -ground was frozen hard and covered with deep snow. Allan, -working in Montreal, Canada, early observed that the amount -of activity to be obtained from the air was about the same in -summer as in winter, although, in the latter case, the whole earth -was deeply frozen and covered with snow, and the winds blew -from the north over snow-covered lands. Under such conditions, -a diminution of the amount of activity is to be expected since the -diffusion of the emanation must be retarded, if not altogether -stopped, by the freezing of the soil. On the other hand, it -appears difficult to escape from the conclusion of Elster and -Geitel that the emanation present in the atmosphere is evolved -from the earth itself.</p> - -<p class='c006'>Some interesting experiments have been made by McLennan<a id='r425' href='#f425' class='c012'><sup>[425]</sup></a> -on the amount of excited radio-activity to be derived from the air -when filled with fine spray. The experiments were made at the -<span class='pageno' id='Page_520'>520</span>foot of the American Fall at Niagara. An insulated wire was -suspended near the foot of the Fall, and the amount of excited -activity on the wire compared with the amount to be obtained on -the same wire for the same exposure in Toronto. The amount of -activity obtained from the air at Toronto was generally five or six -times that obtained from the air at the Falls. In these experiments -it was not necessary to use an electric machine to charge -the wire negatively, for the falling spray kept the insulated wire -permanently charged to a potential of about -7500 volts. These -results indicate that the falling spray had a negative charge and -electrified the wire. The small amount of the excited radio-activity -at the Falls was probably due to the fact that the -negatively charged drops abstracted the positively charged radio-active -carriers from the atmosphere, and in falling carried them -to the river below. On collecting the spray and evaporating it, -no active residue was obtained. Such a result is, however, to be -expected on account of the minute proportion of the spray tested -compared with that present in the air.</p> -<p class='c005'><a id='section279'></a> -<b>279. A very penetrating radiation from the earth’s -surface.</b> McLennan<a id='r426' href='#f426' class='c012'><sup>[426]</sup></a>, and Rutherford and Cooke<a id='r427' href='#f427' class='c012'><sup>[427]</sup></a> independently, -observed the presence of a very penetrating radiation inside buildings. -McLennan measured the natural conductivity of the air in -a large closed metal cylinder by means of a sensitive electrometer. -The cylinder was then placed inside another and the space between -filled with water. For a thickness of water between the cylinders -of 25 cms. the conductivity of the air in the inner cylinder fell to -about 63 per cent. of its initial value. This result shows that part -of the ionization in the inner cylinder was due to a penetrating -radiation from an external source, which radiation was partially or -wholly absorbed in water.</p> - -<p class='c006'>Rutherford and Cooke observed that the rate of discharge of a -sealed brass electroscope was diminished by placing a lead screen -around the electroscope. A detailed investigation of the decrease of -the rate of discharge in the electroscope, when surrounded by metal -screens, was made later by Cooke<a id='r428' href='#f428' class='c012'><sup>[428]</sup></a>. A thickness of 5 cms. of lead -<span class='pageno' id='Page_521'>521</span>round the electroscope decreased the rate of discharge about 30 per -cent. Further increase of the thickness of the screen had no effect. -When the apparatus was surrounded by 5 tons of pig-lead the rate -of discharge was about the same as when it was surrounded by a -plate about 3 cms. thick. An iron screen also diminished the rate -of discharge to about the same extent as the lead. By suitably -arranging lead screens it was found that the radiation came equally -from all directions. It was of the same intensity by night as by -day. In order to be sure that this penetrating radiation did not -arise from the presence of radio-active substances in the laboratory, -the experiments were repeated in buildings in which radio-active -substances had never been introduced, and also on the open ground -far removed from any building. In all cases a diminution of the rate -of discharge of the electroscope, when surrounded by lead screens, -was observed. These results show that a penetrating radiation is -present at the surface of the earth, arising partly from the earth -itself and partly from the atmosphere.</p> - -<p class='c006'>The result is not surprising when the radio-activity of the -earth and atmosphere is taken into account. The writer has -found that bodies made active by exposure to the emanations from -thorium and radium give out γ rays. We may expect then -that the very similar excited radio-activity which is present in -the earth and atmosphere should also give rise to γ rays of -a similar character. More recent work, however (<a href='#section286'>section 286</a>), -indicates that this explanation is not sufficient to explain all -the facts observed.</p> -<p class='c005'><a id='section280'></a> -<b>280. Comparison of the radio-activity of the atmosphere -with that produced by the radio-elements.</b> The -radio-active phenomena observed in the earth and atmosphere are -very similar in character to those produced by thorium and radium. -Radio-active emanations are present in the air of caves and cellars, -in natural carbonic acid, and in deep well water, and these emanations -produce excited radio-activity on all bodies in contact with -them. The question now arises whether these effects are due -entirely to known radio-elements present in the earth or to -unknown kinds of radio-active matter. The simplest method of -testing this point is to compare the rate of decay of the radio-active -<span class='pageno' id='Page_522'>522</span>product in the atmosphere with those of the known radio-active -products of thorium and radium. A cursory examination of -the facts at once shows that the radio-activity of the atmosphere -is much more closely allied to effects produced by radium than to -those due to thorium. The activity of the emanation released -from well water, and also that sucked up from the earth, decays to -half value in about 3·3 days, while the activity of the radium -emanation decays to half value in an interval of 3·7 to 4 days. -Considering the difficulty of making accurate determinations of -these quantities, the rates of decay of the activity of the emanations -from the earth and from radium agree within the limits of -experimental error. A large number of observers have found -that the radium emanation is present in the water of thermal -springs and in the sediment deposited by them. Bumstead and -Wheeler have shown that the emanation from the soil and surface -water of New Haven is identical with that from radium. If the -emanations from the earth and from radium are the same, the -excited activities produced should have the same rate of decay. -The emanation from well water in England approximately fulfils -this condition (<a href='#section276'>section 276</a>), but an observation recorded by Ebert -and Ewers (<a href='#section276'>section 276</a>) seems to show that the excited activity -due to the emanation sucked up from the earth decays at a very -slow rate compared with that due to radium.</p> - -<p class='c006'>Bumstead has given undoubted evidence that the thorium as -well as the radium emanation is also present in the atmosphere at -New Haven, while Dadourian has shown that it is emitted by -New Haven soil. Blanc, and Elster and Geitel, have also found -that thorium is present in the sediment from some thermal -springs.</p> - -<p class='c006'>If the active matter in the atmosphere consists mainly of the -radium emanation, the active deposit on a negatively charged wire, -exposed in the open air, should initially consist of radium A, B -and C. The curve of decay should be identical with the decay -curve of the excited activity of radium, measured by the α rays, -that is, there should be a rapid initial drop corresponding to the -initial 3 minute change, then a slow rate of variation, the activity -after several hours decaying to half value in about 28 minutes -(see <a href='#section222'>section 222</a>). The rapid initial drop has been observed by -<span class='pageno' id='Page_523'>523</span>Bumstead for the air at New Haven. Allan<a id='r429' href='#f429' class='c012'><sup>[429]</sup></a> did not observe this -initial drop in Montreal, but found the activity fell to half value in -about 45 minutes, reckoning from a time about 10 minutes after -the removal of the active wire. This is about the rate of decay to -be expected for the active deposit of radium over the same interval. -Allan obtained evidence that there were several kinds of active -matter deposited on the wire. For example, the activity transferred -from the active wire to a piece of leather, moistened with -ammonia, fell to half value in 38 minutes; for a piece of absorbent -felt treated similarly, the activity fell to half value in 60 minutes, -the normal time for the untreated wire being 45 minutes.</p> - -<p class='c006'>It is probable that this variation of the rate of decay is due to -the fact that unequal proportions of radium B and C were transferred -from the wire to the rubber. If a greater proportion of B -than of C were removed, the decay would be slower and <i>vice versa</i>.</p> - -<p class='c006'>The fact that the activity of rain and snow falls to half value -in about 30 minutes is a strong indication that the radium emanation -is present in the atmosphere. The active matter with the -rain and snow after standing some time would consist mainly of -radium C and this should decay exponentially with the time, -falling to half value in 28 minutes.</p> - -<p class='c006'>On account of the rapid decay of the thorium emanation—half -value in one minute—it is not likely that much of the activity of -the atmosphere can be ascribed to it. Its effect would be most -marked near the surface of the soil.</p> - -<p class='c006'>There can be little doubt, that a large part of the radio-activity -of the atmosphere is due to the radium emanation, which is continually -diffusing into the atmosphere from the pores of the earth. -Since radio-activity has been observed in the atmosphere at all -points at which observations have, so far, been made, radio-active -matter must be distributed in minute quantities throughout the -soil of the earth. The volatile emanations escape into the atmosphere -by diffusion, or are carried to the surface in spring water or -by the escape of underground gases, and cause the radio-active -phenomena observed in the atmosphere. The observation of Elster -and Geitel that the radio-activity of the air is much less near the -sea than inland is explained at once, if the radio-activity of the -<span class='pageno' id='Page_524'>524</span>atmosphere is due mainly to the diffusion of emanations from the -soil into the air above it.</p> - -<p class='c006'>The rare gases helium and xenon which exist in the atmosphere -have been tested and found to be non-radio-active. The radio-activity -of the air cannot be ascribed to a slight radio-activity -possessed by either of these gases.</p> -<p class='c005'><a id='section281'></a> -<b>281. Amount of the radium emanation in the atmosphere.</b> -It is a matter of great interest to form an estimate of -the amount of radium emanation present in the atmosphere, for -since it comes from the earth, it indirectly serves as a means of -estimating the amount of radium which is distributed over a thin -crust of the earth.</p> - -<p class='c006'>Some experiments in this direction have been made by Eve -in the laboratory of the writer. The experiments are not yet -completed but the results so far obtained allow us to calculate the -probable amount of emanation per cubic kilometre of the atmosphere -near the earth.</p> - -<p class='c006'>Experiments were first made with a large iron tank 154 cms. -square and 730 cms. deep, in a building in which no radium -or other radio-active material had ever been introduced. The -saturation ionization current for the air in the tank was first -measured by means of an electroscope, connected with an insulated -electrode passing up the centre of the closed tank. Assuming that -the ionization in the tank was uniform, the number of ions produced -per c.c. of the air in the tank was found to be 10. This is a -considerably lower value than has usually been observed in a small -closed vessel (see <a href='#section284'>section 284</a>). Cooke obtained the value 10 for a -well cleaned brass electroscope, surrounded by lead, while Schuster -obtained a value about 12 for the air in the laboratory of Owens -College, Manchester.</p> - -<p class='c006'>In order to measure the amount of the excited activity from -the tank, a central insulated wire was charged negatively to about -10,000 volts by a Wimshurst machine. After two hours, the wire -was removed and wound on an insulated frame connected with a -gold-leaf electroscope. The rate of decay of the activity on the wire -was found to be about the same as for the excited activity produced -by the radium emanation. In order to estimate the amount of -<span class='pageno' id='Page_525'>525</span>radium emanation present in the large tank, special experiments -were made with a smaller tank in which a known quantity of the -radium emanation was introduced by employing a solution of pure -radium bromide of known concentration. A central wire was made -the negative electrode as before, and, after removal, it was wound -on the frame and its activity tested. In this way it was found -that the amount of radium emanation present in the large tank, in -order to produce the excited activity observed, must have been -equal to the equilibrium or maximum amount to be obtained -from 9·5 × 10<sup>-9</sup> grams of pure radium bromide. The volume of -the large tank was 17 cubic metres, so that the amount of emanation -present per cubic metre was equivalent to that liberated from -5·6 × 10<sup>-10</sup> grams of radium bromide in radio-active equilibrium.</p> - -<p class='c006'>If the amount of the emanation in the tank is taken as the -average amount existing in the outside air, <i>the amount of radium -emanation present per cubic kilometre of the air is equivalent to -that supplied by 0·56 grams of radium bromide</i>.</p> - -<p class='c006'>For the purpose of calculation, suppose the emanation is -uniformly distributed over the land portion of the earth (¼ of the -total surface), and to extend to an average height of 5 kilometres. -The air over the sea is not taken into account as its radio-activity -has not been examined. The total amount of emanation present -in the atmosphere under these conditions corresponds to that -supplied by about 400 tons of radium bromide. In order to maintain -this amount of emanation in the atmosphere, it must be -supplied at a constant rate from the earth’s surface. Since the -greater amount of the emanation probably escapes into the air by -transpiration and diffusion through the soil, the emanation cannot -reach the surface except from a very thin layer of the earth. The -probable thickness of this layer can be estimated if it is assumed -that the present loss of heat from the earth is supplied from the -radio-active matter contained in it. We have seen (section 271) -that, on this hypothesis, there must be an amount of active matter -in the earth corresponding to about 300 million tons of radium. -If this is supposed to be uniformly distributed, a thickness of layer -of about 13 metres will suffice to maintain the calculated amount -of emanation in the atmosphere. This thickness of layer is about -the order of magnitude to be expected from general considerations.</p> - -<p class='c006'><span class='pageno' id='Page_526'>526</span>These results lead indirectly to the conclusion that a large -amount of emanation does undoubtedly exist in the surface crust -of the earth.</p> - -<p class='c006'>Experiments were also made by Eve with a large zinc cylinder -exposed in the open air. Volume for volume, the average amount -of excited activity derived from it was only about one-third of that -obtained from the large iron tank. This would reduce the amount -of emanation, previously deduced, to about one-third.</p> - -<p class='c006'>Before such calculations can be considered at all definite, it will -be necessary to make comparative measurements of the amount of -emanation in the atmosphere at various parts of the earth. The -air at Montreal is not abnormally active, so that the calculations -probably give the right order of magnitude of the quantities.</p> - -<p class='c006'>Eve also observed that the amount of activity to be obtained -per unit length of the wire in the zinc cylinder of about 70 cms. in -diameter was about the same as for a wire ·5 mms. in diameter -charged to 10,000 volts in the open air, supported 20 feet from the -ground. This shows that such a potential does not draw in the -carriers of excited activity which are more than half a metre away, -and probably the range is even less.</p> - -<p class='c006'>It is of great importance to find how large a proportion of the -number of ions produced in the atmosphere is due to the radio-active -matter distributed throughout it. The results of Eve with -the large iron tank, already referred to, indicate that a large proportion -of the ionization in the tank was due to the radio-active -matter contained in it, for the ratio of the excited activity on the -central electrode to the total ionization current in the tank was -about ⁷⁄₁₀ of the corresponding ratio for a smaller tank into which -a supply of the radium emanation had been introduced.</p> - -<p class='c006'>This result requires confirmation by experiments at other parts -of the earth, but the results point to the conclusion that a large -part, if not all, of the ionization at the earth’s surface is due to -radio-active matter distributed in the atmosphere. A constant -rate of production of 30 ions per second per c.c. of air, which has -been observed in the open air at the surface of the earth in various -localities, would be produced by the presence in each c.c. of the air -of the amount of emanation liberated from 2·4 × 10<sup>-15</sup> grams of -radium bromide in radio-active equilibrium. It is not likely, -<span class='pageno' id='Page_527'>527</span>however, that the ionization of the upper part of the atmosphere is -due to this cause alone. In order to explain the maintenance of -the large positive charge, which generally exists in the upper -atmosphere, there must be a strong ionization of the upper air, -which may possibly be due to ionizing radiations emitted by -the sun.</p> -<p class='c005'><b>282. Ionization of atmospheric air.</b> A large number of -measurements have been made during the last few years to -determine the relative amount of ionization in the atmosphere in -different localities and at different altitudes. Measurements of -this character were first undertaken by Elster and Geitel with a -special type of electroscope. A charged body exposed to the air -was attached to a portable electroscope, and the rate of loss of -charge was observed by the movement of the gold or aluminium -leaf. The rates of discharge of the electroscope for positive and -negative electricity were generally different, the ratio depending -on the locality and the altitude, and on the meteorological conditions. -This apparatus is not suitable for quantitative measurements -and the deductions to be drawn from the observations are -of necessity somewhat indefinite.</p> - -<p class='c006'>Ebert<a id='r430' href='#f430' class='c012'><sup>[430]</sup></a> has designed a portable apparatus in which the number -of ions per c.c. of the air can be determined easily. A constant -current of air is drawn between two concentric cylinders by means -of a fan actuated by a falling weight. The inner cylinder is insulated -and connected with an electroscope. Knowing the capacity -of the apparatus, and the velocity of the current of air, the rate of -movement of the gold-leaf affords a measure of the number of ions -present in unit volume of the air drawn between the cylinders.</p> - -<p class='c006'>In this way Ebert found that the number of ions in the air -was somewhat variable, but on an average corresponded to about -2600 per c.c. in the particular locality where the measurements -were made.</p> - -<p class='c006'>This is the equilibrium number of ions present per c.c. when -the rate of production balances the rate of recombination. If <i>q</i> is -the number of ions produced per second per unit volume of the air -<span class='pageno' id='Page_528'>528</span>and <i>n</i> is the equilibrium number, then <i>q</i> = α<i>n</i><sup>2</sup> where α is the constant -of recombination (<a href='#section030'>section 30</a>).</p> - -<p class='c006'>By a slight addition to the apparatus of Ebert, Schuster<a id='r431' href='#f431' class='c012'><sup>[431]</sup></a> has -shown that the constant of recombination for the particular sample -of air under investigation can be determined. The value so -obtained for air in the neighbourhood of Manchester was variable, -and two or three times as great as for dust-free air. The results of -some preliminary measurements showed that the number of ions -present per c.c. of the air in different localities varied from 2370 to -3660, while the value of <i>q</i>, the number of ions produced per c.c. per -second, varied between 12 and 38·5.</p> - -<p class='c006'>Rutherford and Allan and Eberts showed that the ions in the air -had about the same mobility as the ions produced in air by Röntgen -rays and radio-active substances. In some recent determinations -by Mache and Von Schweidler<a id='r432' href='#f432' class='c012'><sup>[432]</sup></a>, the velocity of the positive ion was -found to be about 1·02 cms. per second, and that of the negative -1·25 cms., for a potential gradient of one volt per cm.</p> - -<p class='c006'>Langevin<a id='r433' href='#f433' class='c012'><sup>[433]</sup></a> has recently shown that in addition to these swift -moving ions, there are also present in the atmosphere some ions -which travel extremely slowly in an electric field. The number of -these slowly moving ions in the air in Paris is about 40 times as -great as the number of the swifter ions. This result is of great -importance, for in the apparatus of Ebert these ions escape detection, -since the electric field is not strong enough to carry them -to the electrodes during the time of their passage between the -cylinders.</p> -<p class='c005'><b>283. Radio-activity of ordinary materials.</b> It has been -shown that radio-active matter seems to be distributed fairly -uniformly over the surface of the earth and in the atmosphere. -The very important question arises whether the small radio-activity -observed is due to known or unknown radio-elements present in -the earth and atmosphere, or to a feeble radio-activity of matter -in general, which is only readily detectable when large quantities -of matter are present. The experimental evidence is not yet -<span class='pageno' id='Page_529'>529</span>sufficient to answer this question, but undoubted proof has been -obtained that many of the metals show a very feeble radio-activity. -Whether this radio-activity is due to the presence of a slight trace -of the radio-elements or is an actual property of the metals themselves -will be discussed in more detail in <a href='#section286'>section 286</a>.</p> - -<p class='c006'>Schuster<a id='r434' href='#f434' class='c012'><sup>[434]</sup></a> has pointed out that every physical property hitherto -discovered for one element has been found to be shared by all -the others in varying degrees. For example, the property of -magnetism is most strongly marked in iron, nickel, and cobalt, but -all other substances are found to be either feebly magnetic or -diamagnetic. It might thus be expected on general principles -that all matter should exhibit the property of radio-activity in -varying degrees. On the view developed in <a href='#chap10'>chapter <span class='fss'>X.</span></a>, the -presence of this property is an indication that the matter is -undergoing change accompanied by the expulsion of charged -particles. It does not, however, by any means follow that because -the atom of one element in the course of time becomes unstable -and breaks up, that, therefore, the atoms of all the other elements -pass through similar phases of instability.</p> - -<p class='c006'>It has already been mentioned (<a href='#section008'>section 8</a>), that Mme Curie -made a very extensive examination of most of the elements and -their compounds for radio-activity. The electric method was -used, and any substance possessing an activity of ¹⁄₁₀₀ of that of -uranium would certainly have been detected. With the exception -of the known radio-elements and the minerals containing uranium -and thorium, no other substances were found to be radio-active -even to that degree.</p> - -<p class='c006'>Certain substances like phosphorus<a id='r435' href='#f435' class='c012'><sup>[435]</sup></a> possess the property of -ionizing a gas under special conditions. The air which is drawn -over the phosphorus is conducting, but it has not yet been settled -whether this conductivity is due merely to ions formed at the -surface of the phosphorus or to ions produced by the phosphorus -nuclei or emanations, as they have been termed, which are carried -along with the current of air. It does not however appear that -the ionization of the gas is in any way due to the presence of a -penetrating type of radiation such as is emitted by the radio-active -<span class='pageno' id='Page_530'>530</span>bodies. Le Bon (<a href='#section008'>section 8</a>) observed that quinine sulphate, -after being heated to a temperature below the melting point and -then allowed to cool, showed for a time strong phosphorescence -and was able rapidly to discharge an electroscope. The discharging -action of quinine sulphate under varying conditions has been very -carefully examined by Miss Gates<a id='r436' href='#f436' class='c012'><sup>[436]</sup></a>. The ionization could not be -observed through thin aluminium foil or gold-leaf, but appeared -to be confined to the surface of the sulphate. The current observed -by an electrometer was found to vary with the direction of the -electric field, indicating that the positive and negative ions had -very different mobilities. The discharging action appears to be -due either to an ionization of the gas very close to the surface by -some short ultra-violet light waves, accompanying the phosphorescence, -or to a chemical action taking place at the surface.</p> - -<p class='c006'>Thus, neither phosphorus nor quinine sulphate can be considered -to be radio-active, even under the special conditions when -they are able to discharge an electrified body. No evidence in -either case has been found that the ionization is due to the -emission of a penetrating radiation.</p> - -<p class='c006'>No certain evidence has yet been obtained that any body can -be made radio-active by exposure to Röntgen rays or cathode rays. -A metal exposed to the action of Röntgen rays gives rise to a -secondary radiation which is very readily absorbed in a few -centimetres of air. It is possible that this secondary radiation -may prove to be analogous in some respects to the α rays from -the radio-elements. The secondary radiation, however, ceases -immediately the Röntgen rays are cut off. Villard<a id='r437' href='#f437' class='c012'><sup>[437]</sup></a> stated that -a piece of bismuth produced a feeble photographic action after it -had been exposed for some time to the action of the cathode -rays in a vacuum. It has not however been shown that the -bismuth gives out rays of a character similar to those of the -radio-active bodies. The experiments of Ramsay and Cooke on -the production of apparent activity in inactive matter by the radiations -from radium have already been discussed in <a href='#section264'>section 264</a>.</p> - -<p class='c006'>The existence of a very feeble radio-activity of ordinary matter -has been deduced from the study of the conductivity of gases in -<span class='pageno' id='Page_531'>531</span>closed vessels. The conductivity is extremely minute, and special -methods are required to determine it with accuracy. A brief -account will now be given of the gradual growth of our knowledge -on this important question.</p> -<p class='c005'><a id='section284'></a> -<b>284. Conductivity of air in closed vessels.</b> Since the -time of Coulomb onwards several investigators have believed that -a charged conductor placed inside a closed vessel lost its charge -more rapidly than could be explained by the conduction leak -across the insulating support. Matteucci, as early as 1850, observed -that the rate of loss of charge was independent of the potential. -Boys, by using quartz insulators of different lengths and diameters, -arrived at the conclusion that the leakage must in part take place -through the air. This loss of charge in a closed vessel was believed -to be due in some way to the presence of dust particles in the air.</p> - -<p class='c006'>On the discovery that gases become temporary conductors of -electricity under the influence of Röntgen rays and the rays from -radio-active substances, attention was again drawn to this question. -Geitel<a id='r438' href='#f438' class='c012'><sup>[438]</sup></a> and C. T. R. Wilson<a id='r439' href='#f439' class='c012'><sup>[439]</sup></a> independently attacked the problem, -and both came to the conclusion that the loss of charge was due -to a constant ionization of the air in the closed vessel. Geitel -employed in his experiments an apparatus similar to that shown -in Fig. 103. The loss of charge of an Exner electroscope, with the -cylinder of wire netting <i>Z</i> attached, was observed in a closed vessel -containing about 30 litres of air. The electroscope system was -found to diminish in potential at the rate of about 40 volts per -hour, and this leakage was shown not to be due to a want of -insulation of the supports.</p> - -<p class='c006'>Wilson, on the other hand, used a vessel of very small volume, -in order to work with air which could be completely freed from -dust. In the first experiments a silvered glass vessel with a -volume of only 163 c.c. was employed. The experimental arrangement -is shown in <a href='#fig104'>Fig. 104</a>.</p> - -<div id='fig104' class='figcenter id004'> -<img src='images/fig-104.png' alt='Fig. 104.' class='ig001'> -<div class='ic002'> -<p>Fig. 104.</p> -</div> -</div> - -<p class='c006'>The conductor, of which the loss of charge was to be measured, -was placed near the centre of the vessel <i>A</i>. It consisted of a -<span class='pageno' id='Page_532'>532</span>narrow strip of metal with a gold-leaf attached. The strip of -metal was fixed to the upper rod by means of a small sulphur bead. -The upper rod was connected with a sulphur condenser with an -Exner electroscope <i>B</i> attached to indicate its potential. The -gold-leaf system was initially charged to the same potential as -the upper rod and condenser by means of a fine steel wire which -was caused to touch the gold-leaf system by the attraction of a -magnet brought near it. The rate of movement of the gold-leaf -was measured by means of a microscope provided with a micrometer -eye-piece. By keeping the upper rod at a slightly higher -potential than the gold-leaf system, it was ensured that the loss -of charge of the gold-leaf system should not be due in any way -to a conduction leakage across the sulphur bead.</p> - -<p class='c006'>The method employed by Wilson in these experiments is -very certain and convenient when an extremely small rate of -discharge is to be observed. In this respect the electroscope -measures with certainty a rate of loss of charge much smaller -than can be measured by a sensitive electrometer.</p> - -<p class='c006'><span class='pageno' id='Page_533'>533</span>Both Geitel and Wilson found that the leakage of the insulated -system in dust-free air was the same for a positive as for a negative -charge, and was independent of the potential over a considerable -range. The leakage was the same in the dark as in diffuse -daylight. The independence of leakage of the potential is strong -evidence that the loss of charge is due to a constant ionization of -the air. When the electric field acting on the gas exceeds a -certain value, all the ions are carried to the electrodes before recombination -occurs. A saturation current is reached, and it will -be independent of further increase of the electric field, provided, -of course, a potential sufficiently high to cause a spark to pass is -not applied.</p> - -<p class='c006'>C. T. R. Wilson has recently devised a striking experiment to -show the presence of ions in dust-free air which is not exposed to -any external ionizing agency. Two large metal plates are placed -in a glass vessel connected with an expansion apparatus similar to -that described in <a href='#section034'>section 34</a>. On expanding the air, the presence -of the ions is shown by the appearance of a slight cloud between -the plates. These condensation nuclei carry an electric charge, -and are apparently similar in all respects to the ions produced -in gases by X rays, or by the rays from active substances.</p> - -<p class='c006'>Wilson found that the loss of charge of the insulated system -was independent of the locality. The rate of discharge was unaltered -when the apparatus was placed in a deep tunnel, so that -it did not appear that the loss of charge was due to an external -radiation. From experiments already described, however -(<a href='#section279'>section 279</a>), it is probable that about 30 per cent. of the rate of discharge -observed was due to a very penetrating radiation. This experiment -of Wilson’s indicates that the intensity of the penetrating radiation -was the same in the tunnel as at the earth’s surface. Wilson -found that the ionization of the air was about the same in a brass -vessel as in one of glass, and came to the conclusion that the -air was spontaneously ionized.</p> - -<p class='c006'>Using a brass vessel of volume about 471 c.c., Wilson determined -the number of ions that must be produced in air -per unit volume per second, in order to account for the loss of -charge of the insulated system. The leakage system was found -to have a capacity of about 1·1 electrostatic units, and lost its -<span class='pageno' id='Page_534'>534</span>charge at the rate of 4·1 volts per hour for a potential of 210 volts, -and 4·0 volts per hour for a potential of 120 volts. Taking the -charge on an ion as 3·4 × 10<sup>-10</sup> electrostatic units, this corresponds -to a production of 26 ions per second.</p> - -<p class='c006'>Rutherford and Allan<a id='r440' href='#f440' class='c012'><sup>[440]</sup></a> repeated the results of Geitel and -Wilson, using an electrometer method. The saturation current -was observed between two concentric zinc cylinders of diameter -25·5 and 7·5 cms. respectively and length 154 cms. It was found -that the saturation current could practically be obtained with a -potential of a few volts. Saturation was however obtained with -a lower voltage after the air had remained undisturbed in the -cylinders for several days. This was probably due to the gradual -settling of the dust originally present in the air.</p> - -<p class='c006'>Later observations of the number of ions produced in air in -sealed vessels have been made by Patterson<a id='r441' href='#f441' class='c012'><sup>[441]</sup></a>, Harms<a id='r442' href='#f442' class='c012'><sup>[442]</sup></a>, and -Cooke<a id='r443' href='#f443' class='c012'><sup>[443]</sup></a>. The results obtained by different observers are shown -in the following table. The value of the charge on an ion is taken -as 3·4 × 10<sup>-10</sup> electrostatic units:</p> - -<table class='table4' > -<colgroup> -<col class='colwidth28'> -<col class='colwidth35'> -<col class='colwidth35'> -</colgroup> - <tr> - <th class='c013'>Material of vessel</th> - <th class='c015'>Number of ions produced per c.c. per second</th> - <th class='c014'>Observer</th> - </tr> - <tr> - <td class='c013'> </td> - <td class='c015'> </td> - <td class='c014'> </td> - </tr> - <tr> - <td class='c013'>Silvered glass</td> - <td class='c015'>36</td> - <td class='c014'>C. T. R. Wilson</td> - </tr> - <tr> - <td class='c013'>Brass</td> - <td class='c015'>26</td> - <td class='c014'>„ „</td> - </tr> - <tr> - <td class='c013'>Zinc</td> - <td class='c015'>27</td> - <td class='c014'>Rutherford and Allan</td> - </tr> - <tr> - <td class='c013'>Glass</td> - <td class='c015'>53 to 63</td> - <td class='c014'>Harms</td> - </tr> - <tr> - <td class='c013'>Iron</td> - <td class='c015'>61</td> - <td class='c014'>Patterson</td> - </tr> - <tr> - <td class='c013'>Cleaned brass</td> - <td class='c015'>10</td> - <td class='c014'>Cooke</td> - </tr> -</table> - -<p class='c006'>It will be shown later that the differences in these results are -probably due to differences in the radio-activity of the containing -vessel.</p> -<p class='c005'><b>285. Effect of pressure and nature of gas.</b> C. T. R. Wilson -(<i>loc. cit.</i>) found that the rate of leakage of a charged conductor -<span class='pageno' id='Page_535'>535</span>varied approximately as the pressure of the air between the pressures -examined, viz. 43 mms. and 743 mms. of mercury. These -results point to the conclusion that, in a good vacuum, a charged -body would lose its charge extremely slowly. This is in agreement -with an observation of Crookes, who found that a pair of gold-leaves -retained their charge for several months in a high vacuum.</p> - -<p class='c006'>Wilson<a id='r444' href='#f444' class='c012'><sup>[444]</sup></a> at a later date investigated the leakage for different -gases. The results are included in the following table, where the -ionization produced in air is taken as unity:</p> - -<table class='table13' > -<colgroup> -<col class='colwidth36'> -<col class='colwidth26'> -<col class='colwidth36'> -</colgroup> - <tr> - <th class='c013'>Gas</th> - <th class='c013'>Relative ionization</th> - <th class='c014'>(Relative ionization) / (density)</th> - </tr> - <tr> - <td class='c013'> </td> - <td class='c013'> </td> - <td class='c014'> </td> - </tr> - <tr> - <td class='c013'>Air</td> - <td class='c013'>1·00</td> - <td class='c014'>1·00</td> - </tr> - <tr> - <td class='c013'>Hydrogen</td> - <td class='c013'>0·184</td> - <td class='c014'>2·7</td> - </tr> - <tr> - <td class='c013'>Carbon dioxide</td> - <td class='c013'>1·69</td> - <td class='c014'>1·10</td> - </tr> - <tr> - <td class='c013'>Sulphur dioxide</td> - <td class='c013'>2·64</td> - <td class='c014'>1·21</td> - </tr> - <tr> - <td class='c013'>Chloroform</td> - <td class='c013'>4·7</td> - <td class='c014'>1·09</td> - </tr> -</table> - -<p class='c006'>With the exception of hydrogen, the ionization produced in -different gases is approximately proportional to their density. The -relative ionization is very similar to that observed by Strutt -(<a href='#section045'>section 45</a>) for gases exposed to the influence of the α and β rays -from radio-active substances, and points to the conclusion that the -ionization observed may be due either to a radiation from the -walls of the vessel or from external sources.</p> - -<p class='c006'>Jaffé<a id='r445' href='#f445' class='c012'><sup>[445]</sup></a> has made a careful examination of the natural ionization -in the very heavy gas nickel-carbonyl, Ni(CO)<sub>4</sub>, in a small silvered -glass vessel. The ionization of this gas was 5·1 times that of air -at normal pressure while its density is 5·9 times that of air. The -leak of the electroscope was nearly proportional to the pressures -except at low pressure, when the leak was somewhat greater than -would be expected if the pressure law held. The fact that a gas -of such high density and complicated structure behaves like the -simpler and lighter gases is a strong indication that the ionization -itself is due to a radiation from the walls of the vessel and not to -a spontaneous ionization of the gas.</p> - -<p class='c006'><span class='pageno' id='Page_536'>536</span>Patterson<a id='r446' href='#f446' class='c012'><sup>[446]</sup></a> examined the variation of the ionization of air -with pressure in a large iron vessel of diameter 30 cms. and length -20 cms. The current between a central electrode and the cylinder -was measured by means of a sensitive Dolezalek electrometer. -He found that the saturation current was practically independent -of the pressure for pressures greater than 300 mms. of mercury. -Below a pressure of 80 mms. the current varied directly as the -pressure. For air at atmospheric pressure, the current was independent -of the temperature up to 450° C. With further increase -of temperature, the current began to increase, and the increase -was more rapid when the central electrode was charged negatively -than when it was charged positively. This difference was ascribed -to the production of positive ions at the surface of the iron vessel. -The results obtained by Patterson render it very improbable that -the ionization observed in air is due to a spontaneous ionization -of the enclosed air: for we should expect the amount of -this ionization to depend on the temperature of the gas. On -the other hand, these results are to be expected if the ionization -of the enclosed air is mainly due to an easily absorbed radiation -from the walls of the vessel. If this radiation had a penetrating -power about equal to that observed for the α rays of the radio-elements, -the radiation would be absorbed in a few centimetres of -air. With diminution of pressure, the radiations would traverse -a greater distance of air before complete absorption, but the total -ionization produced by the rays would still remain about the same, -until the pressure was reduced sufficiently to allow the radiation -to traverse the air space in the vessel without complete absorption. -With still further diminution of pressure, the total ionization -produced by the radiation, and in consequence the current observed, -would vary directly as the pressure.</p> -<p class='c005'><a id='section286'></a> -<b>286. Examination of ordinary matter for radio-activity.</b> -Strutt<a id='r447' href='#f447' class='c012'><sup>[447]</sup></a>, McLennan and Burton<a id='r448' href='#f448' class='c012'><sup>[448]</sup></a>, and Cooke<a id='r449' href='#f449' class='c012'><sup>[449]</sup></a>, independently observed -<span class='pageno' id='Page_537'>537</span>about the same time that ordinary matter is radio-active -to a slight degree. Strutt, by means of an electroscope, observed -that the ionization produced in a closed vessel varied with the -material of the vessel. A glass vessel with a removable base -was employed and the vessel was lined with the material to be -examined. The following table shows the relative results obtained. -The amount of leakage observed is expressed in terms of the -number of scale divisions of the eye-piece passed over per hour -by the gold-leaf:</p> - -<table class='table6' > -<colgroup> -<col class='colwidth50'> -<col class='colwidth50'> -</colgroup> - <tr> - <th class='c013'>Material of lining of vessel</th> - <th class='c014'>Leakage in scale divisions per hour</th> - </tr> - <tr> - <td class='c013'> </td> - <td class='c014'> </td> - </tr> - <tr> - <td class='c013'>Tinfoil</td> - <td class='c014'>3·3</td> - </tr> - <tr> - <td class='c013'>„ another sample</td> - <td class='c014'>2·3</td> - </tr> - <tr> - <td class='c013'>Glass coated with phosphoric acid</td> - <td class='c014'>1·3</td> - </tr> - <tr> - <td class='c013'>Silver chemically deposited on glass</td> - <td class='c014'>1·6</td> - </tr> - <tr> - <td class='c013'>Zinc</td> - <td class='c014'>1·2</td> - </tr> - <tr> - <td class='c013'>Lead</td> - <td class='c014'>2·2</td> - </tr> - <tr> - <td class='c013'>Copper (clean)</td> - <td class='c014'>2·3</td> - </tr> - <tr> - <td class='c013'>„ (oxidized)</td> - <td class='c014'>1·7</td> - </tr> - <tr> - <td class='c013'>Platinum (various samples)</td> - <td class='c014'>2·0, 2·9, 3·9</td> - </tr> - <tr> - <td class='c013'>Aluminium</td> - <td class='c014'>1·4</td> - </tr> -</table> - -<p class='c006'>There are thus marked differences in the leakage observed for -different materials and also considerable differences in different -samples of the same metal. For example, one specimen of platinum -caused nearly twice the leakage of another sample from a different -stock.</p> - -<p class='c006'>McLennan and Burton, on the other hand, measured by means -of a sensitive electrometer the ionization current produced in the -air in a closed iron cylinder 25 cms. in diameter and 130 cms. in -length, in which an insulated central electrode was placed. The -open cylinder was first exposed for some time at the open window -of the laboratory. It was then removed, the top and bottom -closed, and the saturation current through the gas determined as -soon as possible. In all cases it was observed that the current -diminished for two or three hours to a minimum and then very -slowly increased again. In one experiment, for example, the initial -current observed corresponded to 30 on an arbitrary scale. In the -course of four hours the current fell to a minimum of 6·6, and -<span class='pageno' id='Page_538'>538</span>44 hours later had risen to a practical maximum of 24. The -initial decrease observed is probably due to a radio-activity of -the enclosed air or walls of the vessel, which decayed rapidly -with the time. The decay of the excited activity produced on -the interior surface of the cylinder when exposed to the air was -probably responsible for a part of the decrease observed. McLennan -ascribes the increase of current with time to a radio-active <i>emanation</i> -which is given off from the cylinder, and ionizes the enclosed -air. On placing linings of lead, tin, and zinc in the iron cylinder, -considerable differences were observed both for the minimum -current and also for the final maximum. Lead gave about twice -the current due to zinc, while tin gave an intermediate value. -These results are similar in character to those obtained by -Strutt.</p> - -<p class='c006'>McLennan and Burton also investigated the effect of diminution -of pressure on the current. The cylinder was filled with -air to a pressure of 7 atmospheres, and allowed to stand until -the current reached a constant value. The air was then allowed -to escape and the pressure reduced to 44 mms. of mercury. The -current was found to vary approximately as the pressure over the -whole range. These results are not in agreement with the results -of Patterson already described, nor with some later experiments -of Strutt. McLennan’s results however point to the conclusion -that the ionization was mainly due to an emanation emitted from -the metal. Since the air was rapidly removed, a proportionate -amount of the emanation would be removed also, and it might -thus be expected that the current would vary directly as the -pressure. If this is the case the current through the gas at low -pressures should increase again to a maximum if time is allowed -for a fresh emanation to form.</p> - -<p class='c006'>H. L. Cooke, using an electroscopic method, obtained results -very similar to those given by Strutt. Cooke observed that a penetrating -radiation was given out from brick. When a brass vessel -containing the gold-leaf system was surrounded by brick, the -discharge of the electroscope was increased by 40 to 50 per cent. -This radiation was of about the same penetrating power as the -rays from radio-active substances. The rays were completely -absorbed by surrounding the electroscope with a sheet of lead -<span class='pageno' id='Page_539'>539</span>2 mms. in thickness. This result is in agreement with the observation -of Elster and Geitel, already mentioned, that radio-active -matter was present in clay freshly dug up from the earth.</p> - -<p class='c006'>Cooke also observed that the ionization of the air in a brass -electroscope could be reduced to about one-third of its usual -value if the interior surface of the brass was carefully cleaned. -By removing the surface of the brass he was able to reduce -the ionization of the enclosed air from 30 to 10 ions per c.c. per -second. This is an important observation, and indicates that a -large proportion of the radio-activity observed in ordinary matter -is due to a deposit of radio-active matter on its surface. It has -already been shown that bodies which have been exposed in the -presence of the radium emanation retain a residual activity which -decays extremely slowly. There can be no doubt that the radium -emanation is present in the atmosphere, and the exposed surface -of matter, in consequence, will become coated with an invisible -film of radio-active matter, deposited from the atmosphere. On -account of the slow decay of this activity it is probable that the -activity of matter exposed in the open air would steadily increase -for a long interval. Metals, even if they are originally inactive, -would thus acquire a fairly permanent activity, but it should be -possible to get rid of this by removing the surface of the metal -or by chemical treatment. The rapid increase of activity of all -matter left in a laboratory in which a large quantity of emanation -has been released has been drawn attention to by Eve<a id='r450' href='#f450' class='c012'><sup>[450]</sup></a>. This -superficial activity, due to the products radium D, E, and F, was -mainly removed by placing the metal in strong acid.</p> - -<p class='c006'>A number of experiments have been made by J. J. Thomson, -N. R. Campbell, and A. Wood in the Cavendish laboratory to examine -whether the radio-activity observed in ordinary matter is a specific -property of such matter or is due to the presence of some radio-active -impurity. An account of these experiments was given by -Professor J. J. Thomson in a discussion on the Radio-activity of -Ordinary Matter at the British Association meeting at Cambridge, -1904. The results<a id='r451' href='#f451' class='c012'><sup>[451]</sup></a>, as a whole, support the view that each -substance gives out a characteristic type or types of radiation and -<span class='pageno' id='Page_540'>540</span>that the radiation is a specific property of the substance. J. J. -Thomson<a id='r452' href='#f452' class='c012'><sup>[452]</sup></a> has made experiments to observe the action of different -substances in cutting off the external very penetrating radiation -(<a href='#section279'>section 279</a>) observed by Cooke and McLennan. He found that -some substances cut off this external radiation, while others had -little if any effect. For example, the ionization in a closed vessel -was reduced 17 per cent. by surrounding it with a thick lead -envelope; but, on surrounding it with an equivalent absorbing thickness -of water, or water mixed with sand, no sensible diminution was -observed. In other experiments Wood<a id='r453' href='#f453' class='c012'><sup>[453]</sup></a> found that the diminution -of the ionization by a given screen depended upon the -material of the vessel. For example, the ionization in a lead -vessel, surrounded by a lead screen, was reduced 10 per cent., while -in an iron vessel it was reduced 24 per cent. He concludes from -his experiments that the ionization observed in a closed vessel has -a threefold origin. Part of it is due to an external penetrating -radiation, part to a secondary radiation set up by it, while the -remainder is due to an intrinsic radiation from the walls, altogether -independent of the external radiation.</p> - -<p class='c006'>In some experiments of Campbell<a id='r454' href='#f454' class='c012'><sup>[454]</sup></a>, the variation of the -ionization current between two parallel plates was observed for a -progressive increase of the distance between them. The effects -observed are shown in <a href='#fig105'>Fig. 105</a>. The curves at first rise rapidly, -then bend over and finally become a straight line. The knee of -the curve is at a different distance for the different substances. -The shape of these curves indicates that two types of radiation are -present, one of which is readily absorbed in the gas while the -other, a more penetrating type of radiation, extends over the whole -distance between the plates. In another series of experiments, -one side of the testing vessel was of thin aluminium, and the ionization -current was observed when an exterior screen was brought up -to it. Lead gave a considerable increase, but the radiation from -it was readily absorbed by an interposed screen. The radiation -emitted by carbon and zinc was more than twice as penetrating as -from lead.</p> - -<div id='fig105' class='figcenter id004'> -<span class='pageno' id='Page_541'>541</span> -<img src='images/fig-105.png' alt='Fig. 105.' class='ig001'> -<div class='ic002'> -<p>Fig. 105.</p> -</div> -</div> - -<p class='c006'>Attempts were made to see whether a radio-active emanation -was given off by dissolving solid substances and then keeping the -solutions in a closed vessel and afterwards testing the activity of -the air drawn from them. In some cases an emanation was -observed, but the amount varied with different specimens of the -same material; in others no effect was detected.</p> - -<p class='c006'>When linings of different substances were placed in a closed -testing vessel, the ionization current in most cases fell at first, -passed through a minimum, and then slowly increased to a -maximum. For lead the maximum was reached in 9 hours, for -tin in 14 and for zinc in 18 hours. These results indicate that an -emanation is given off from the metal, and that the amount reaches -a maximum value at different intervals in the various cases. This -was confirmed by an examination of a piece of lead which was left -<span class='pageno' id='Page_542'>542</span>in radium-free nitric acid. Twenty times the normal effect was -observed after this treatment. This is probably due to the -increase of porosity of the lead which allows a greater fraction of -the emanation produced in the metal to diffuse out with the gas.</p> - -<p class='c006'>The activity observed in ordinary matter is extremely small. -The lowest rate of production of ions yet observed is 10 per cubic -centimetre per second in a brass vessel. Suppose a spherical brass -vessel is taken of capacity 1 litre. The area of the interior surface -would be about 480 sq. cms. and the total number of ions produced -per second would be about 10<sup>4</sup>. Now it has been shown, in -<a href='#section252'>section 252</a>, that an α particle projected from radium itself gives rise to -8·6 × 10<sup>4</sup> ions before it is absorbed in the gas. An expulsion of -one α particle every 8 seconds from the whole vessel, or of one α -particle from each square centimetre of surface <i>per hour</i> would -thus account for the minute conductivity observed. Even if it -were supposed that this activity is the result of a breaking up of -the matter composing the vessel, the disintegration of one atom -per second per gram, provided it was accompanied by the expulsion -of an α particle, would fully account for the conductivity -observed.</p> - -<p class='c006'>While the experiments, already referred to, afford strong -evidence that ordinary matter does possess the property of radio-activity -to a feeble degree, it must not be forgotten that the -activity observed is excessively minute, compared even with a weak -radio-active substance like uranium or thorium. The interpretation -of the results is complicated, too, by the presence of the -radium emanation in the atmosphere, for we have seen that the -surface of every body exposed to the open air must become coated -with the slowly changing transformation products of the radium -emanation. The distribution of radio-active matter throughout -the constituents of the earth renders it difficult to be certain that -any substance, however carefully prepared, is freed from radio-active -impurities. If matter in general is radio-active, it must be -undergoing transformation at an excessively slow rate, unless it be -supposed (see <a href='#appa'>Appendix A</a>) that changes of a similar character -to those observed in the radio-elements may occur without the -appearance of their characteristic radiations.</p> - -<div class='chapter'> - <span class='pageno' id='Page_543'>543</span> - <h2 id='appa' class='c004'>APPENDIX A. <br> PROPERTIES OF THE α RAYS.</h2> -</div> -<p class='c005'>A brief account is given here of some investigations made by the -writer on the properties of the α rays from radium—investigations -which were not completed in time for the results to be incorporated -in the text.</p> - -<p class='c006'>The experiments were undertaken primarily with a view of determining -accurately the value of <i>e</i>/<i>m</i> of the α particle from radium, in -order to settle definitely whether or not it is an atom of helium. In -the previous experiments of the writer, Becquerel, and Des Coudres, on -this subject (sections <a href='#section089'>89</a>, <a href='#section090'>90</a>, and <a href='#section091'>91</a>), a thick layer of radium in radio-active -equilibrium has been used as a source of α rays. Bragg -(<a href='#section103'>section 103</a>) has shown that the rays emitted from radium under -such conditions are complex, and consist of particles projected over a -considerable range of velocity. In order to obtain a homogeneous -pencil of rays it is necessary to use a very thin layer of a simple -radio-active substance as a source of rays. In the experiments that -follow, this condition was fulfilled by using a fine wire which was -made active by exposure for several hours in the presence of a large -quantity of radium emanation. By charging the wire negatively the -active deposit was concentrated upon the wire, which was made intensely -active. The active deposit initially contains radium A, B, and C. -The activity of radium A practically disappears in about fifteen -minutes, and the α radiation is then due entirely to the single product -radium C, since radium B is a rayless product. The activity of radium -C decreases to about 15 per cent. of its initial value after two hours.</p> -<p class='c005'><b>Magnetic deflection of the α rays.</b> The photographic method -was employed to determine the deviation of the pencil of rays in a -magnetic field. The experimental arrangement is shown in <a href='#fig106'>Fig. 106</a>. -The rays from the active wire, which was placed in a slot, passed -through a narrow slit and fell normally on a photographic plate, placed -at a known distance above the slit. The apparatus was enclosed in a -<span class='pageno' id='Page_544'>544</span>brass tube which could be exhausted rapidly to a low pressure by -means of a Fleuss pump. The apparatus was placed in a strong -uniform magnetic field parallel to the plane -of the slit. The magnetic field was reversed -every ten minutes, so that on developing -the plate two narrow bands were -observed, the distance between which represented -twice the deviation from the normal -of the pencil of rays by the magnetic field. -The width of the band was found to be -the same whether the magnetic field was -applied or not, showing that the pencil of -rays was homogeneous and consisted of α -particles projected with the same velocity.</p> - -<div id='fig106' class='figcenter id007'> -<img src='images/fig-106.png' alt='Fig. 106.' class='ig001'> -<div class='ic002'> -<p>Fig. 106.</p> -</div> -</div> - -<p class='c006'>By placing the photographic plate at -different distances from the slit it was -found that the rays, after entering the magnetic field, described the -arc of a circle of radius ρ equal to 42·0 cms. The strength of field <i>H</i> -was 9470 <span class='fss'>C.G.S.</span> units, so that the value of <i>H</i>ρ for the α particles -expelled from radium C is 398,000. This is in good agreement with -the maximum values of <i>H</i>ρ, previously found for radium rays (see -<a href='#section092'>section 92</a>).</p> - -<p class='c006'>The electric deviation of the rays from radium C has not yet -been accurately measured, but an approximate determination of <i>e</i>/<i>m</i> -for the α particles can be obtained by assuming that the heating effect -of radium C is a measure of the kinetic energy of the α particles -expelled from it. We have seen in section 246 that the heating -effect of the radium C present in one gram of radium in radio-active -equilibrium is 31 gram calories per hour, which corresponds to an -emission of energy of 3·6 × 10<sup>5</sup> ergs per second. Now when radio-active -equilibrium is reached, the number of α particles expelled from -radium C per second is equal to the number of α particles expelled -per second from radium at its minimum activity. This number, <i>n</i>, is -6·2 × 10<sup>10</sup> (section 93).</p> - -<div class='calc'> - -<div class='lg-container-b c017'> - <div class='linegroup'> - <div class='group'> - <div class='line'>Then ½ <i>mnv</i><sup>2</sup> = 3·6 × 10<sup>5</sup>,</div> - </div> - <div class='group'> - <div class='line'>or (<i>m</i>/<i>e</i>)<i>v</i><sup>2</sup> = 1·03 × 10<sup>16</sup>,</div> - </div> - </div> -</div> - -</div> - -<p class='c018'>substituting the value of <i>n</i>, and the value of the ionic charge <i>e</i>. -The value of <i>e</i> in this case has not been assumed, since <i>n</i> = <i>i</i>/<i>e</i>, where -<span class='pageno' id='Page_545'>545</span><i>i</i> was the measured current due to the charge carried by the α -rays.</p> - -<p class='c006'>From the magnetic deflection, it is known that</p> - -<div class='calc'> - -<div class='lg-container-b c017'> - <div class='linegroup'> - <div class='group'> - <div class='line'>(<i>m</i>/<i>e</i>)<i>v</i> = 3·98 × 10<sup>5</sup>.</div> - </div> - </div> -</div> - -</div> - -<p class='c018'>From these two equations we obtain</p> - -<div class='calc'> - -<div class='lg-container-b c017'> - <div class='linegroup'> - <div class='group'> - <div class='line'><i>v</i> = 2·6 × 10<sup>9</sup> cms. per second.</div> - <div class='line'><i>e</i>/<i>m</i> = 6·5 × 10<sup>3</sup> electromagnetic units.</div> - </div> - </div> -</div> - -</div> - -<p class='c018'>These values are in surprisingly good agreement with the previous -values of the writer and Des Coudres (<a href='#section091'>section 91</a>). On account of -the uncertainty attaching to the value of <i>n</i>, not much weight can be -attached to the determination by this method of the constants of the -α particles.</p> -<p class='c005'><b>Decrease of velocity of the α particles in passing through -matter.</b> Some experiments were made to determine the velocity of -the α particles from radium C after passing through known thicknesses -of aluminium. The previous apparatus was employed, and the distance -between the photographic bands was observed for successive layers of -aluminium foil, each ·00031 cms. thick, placed over the active wire. -The photographic plate was placed 2 cms. above the slit, and the -magnetic field extended 1 cm. below the slit. The amount of deviation -of the rays is inversely proportional to their velocity after -traversing the aluminium screens. The impressions on the plate were -clear and distinct, and about the same in all cases, showing that the -rays were still homogeneous after passing through the aluminium.</p> - -<p class='c006'>A clear photographic impression was obtained for 12 layers of foil, -but it was not found possible to obtain any effect through 13 layers. -This result shows that the photographic action of the rays, like the -ionizing action, ceases very abruptly.</p> - -<p class='c006'>The results obtained are shown in the following table. Assuming -that the value of <i>e</i>/<i>m</i> is constant, the third column gives the velocity -of the α particles after traversing the aluminium. This is expressed -in terms of <i>V</i>₀, the velocity of the α particle when the screens are -removed.</p> - -<table class='table26' > -<colgroup> -<col class='colwidth31'> -<col class='colwidth37'> -<col class='colwidth31'> -</colgroup> - <tr> - <th class='c015'>Number of layers of aluminum foil</th> - <th class='c015'>Distance between bands on the plate</th> - <th class='c016'>Velocity of α particles</th> - </tr> - <tr> - <td class='c015'>0</td> - <td class='c015'>1·46 mms.</td> - <td class='c016'>1·00 <i>V</i>₀</td> - </tr> - <tr> - <td class='c015'>5</td> - <td class='c015'>1·71 „</td> - <td class='c016'>·85 „</td> - </tr> - <tr> - <td class='c015'>8</td> - <td class='c015'>1·91 „</td> - <td class='c016'>·76 „</td> - </tr> - <tr> - <td class='c015'>10</td> - <td class='c015'>2·01 „</td> - <td class='c016'>·73 „</td> - </tr> - <tr> - <td class='c015'>12</td> - <td class='c015'>2·29 „</td> - <td class='c016'>·64 „</td> - </tr> - <tr> - <td class='c015'>13</td> - <td class='c015'>No photographic effect</td> - <td class='c016'> </td> - </tr> -</table> - -<p class='c006'><span class='pageno' id='Page_546'>546</span>The velocity of the α particle is thus reduced only about 36 per -cent. of its initial value when it fails to produce any action on the -photographic plate.</p> - -<p class='c006'>Now Bragg has shown (<a href='#section104'>section 104</a>) that the α particle produces -approximately the same number of ions per cm. of path in air over its -whole range. Consequently, the simplest assumption to make is that -the energy of the α particle is diminished by a constant amount in -traversing each layer of foil. After passing through 12 layers the -kinetic energy is reduced to 41 per cent. of the maximum. Each -layer of foil thus absorbs 4·9 per cent. of the maximum energy. The -observed kinetic energy of the α particle after passing through successive -layers of foil, and the value calculated on the above assumptions, -are shown in the following table.</p> - -<table class='table6' > -<colgroup> -<col class='colwidth33'> -<col class='colwidth33'> -<col class='colwidth33'> -</colgroup> - <tr> - <td class='c015'>Number of layers of aluminum foil</td> - <td class='c015'>Observed energy</td> - <td class='c016'>Calculated energy</td> - </tr> - <tr> - <td class='c015'>0</td> - <td class='c015'>100</td> - <td class='c016'>100</td> - </tr> - <tr> - <td class='c015'>5</td> - <td class='c015'>73</td> - <td class='c016'>75</td> - </tr> - <tr> - <td class='c015'>8</td> - <td class='c015'>58</td> - <td class='c016'>61</td> - </tr> - <tr> - <td class='c015'>10</td> - <td class='c015'>53</td> - <td class='c016'>51</td> - </tr> - <tr> - <td class='c015'>12</td> - <td class='c015'>41</td> - <td class='c016'>41</td> - </tr> -</table> - -<p class='c006'>The experimental and theoretical values agree within the limits of -experimental error. We may thus conclude, as a first approximation, -that the same proportion of the total energy is abstracted from -the α particles in passing through equal distances of the absorbing -screen.</p> -<p class='c005'><b>Range of ionization and photographic action in air.</b> -The abrupt falling off of the photographic impression after the rays -had passed through 12 layers of foil suggested that it might be -directly connected with the corresponding abrupt falling off of the -ionization in air, so clearly brought out by Bragg. This was found to -be the case. It was found experimentally that the absorption in each -layer of aluminium foil was equivalent to that produced by a distance -of ·54 cms. of air. Twelve layers of foil thus corresponded to 6·5 cms. of -air. Now Bragg found that the α rays from radium C ionize the air -for a distance 6·7 cms., and that the ionization then falls off very -rapidly. We may thus conclude that the α rays cease to affect the -photographic plate at the same velocity as that at which they cease to -ionize the gas. This is a very important result, and, as we shall see -later, suggests that the action on the photographic plate is due to an -ionization of the photographic salts.</p> - -<p class='c006'><span class='pageno' id='Page_547'>547</span>The velocity of the α particles from the different radio-active products -can at once be calculated, knowing the maximum range in air -of the α rays from each product. The latter have been experimentally -determined by Bragg. The velocity is expressed in terms of <i>V</i>₀, the -initial velocity of the α particles from radium C. The rays from -radium C are projected with a greater velocity than the rays from the -other products of radium.</p> - -<table class='table6' > -<colgroup> -<col class='colwidth33'> -<col class='colwidth33'> -<col class='colwidth33'> -</colgroup> - <tr> - <th class='c013'>Product</th> - <th class='c013'>Maximum range of α particles in air</th> - <th class='c014'>Velocity of α particles</th> - </tr> - <tr> - <td class='c013'> </td> - <td class='c013'> </td> - <td class='c014'> </td> - </tr> - <tr> - <td class='c013'>Radium</td> - <td class='c013'>3 cms.</td> - <td class='c014'>·82 <i>V</i>₀</td> - </tr> - <tr> - <td class='c013'>Emanation</td> - <td class='c013'>3·8 or 4·4 cms.</td> - <td class='c014'>·87 or ·90 <i>V</i>₀</td> - </tr> - <tr> - <td class='c013'>Rad. A</td> - <td class='c013'>4·4 or 3·8 „</td> - <td class='c014'>·90 or ·87 <i>V</i>₀</td> - </tr> - <tr> - <td class='c013'>Rad. C</td> - <td class='c013'>6·7 „</td> - <td class='c014'>1·00 <i>V</i>₀</td> - </tr> -</table> - -<p class='c006'>It is difficult to determine from the experiments whether the range -3·8 cms. belongs to the rays from the emanation or from radium A. -The mean velocity of the α particles is thus ·90 <i>V</i>₀, and the maximum -variation for the individual products does not vary more than 10 per -cent. from the mean value.</p> - -<p class='c006'>The results of Becquerel, discussed in <a href='#section092'>section 92</a>, at once receive -an explanation on the above results. The α particles, expelled from -radium in radio-active equilibrium, have all ranges lying between -0 and 6·7 cms. of air. The velocity of the α particles which are able -to produce a photographic impression varies between ·64 <i>V</i>₀ and <i>V</i>₀. -The particles which have only a short range in air are projected with -a smaller velocity than those which have a greater range. The former -are in consequence more bent by a magnetic field. It is thus to be -expected that the apparent curvature of the path of rays in a uniform -magnetic field will be greater close to the radium than at some -distance away.</p> -<p class='c005'><b>Range of phosphorescent action in air.</b> Some experiments -were also made to see whether the action of the α rays in producing -luminosity in substances like zinc sulphide, barium platinocyanide, -and willemite, ceased at the same distance as the ionizing action.</p> - -<p class='c006'>A very active wire was placed on a moveable plate, the distance -of which from a fixed screen of phosphorescent substance could be -varied. The distance at which the phosphorescent action ceased -could be determined fairly accurately. Different thicknesses of -aluminium foil were then placed over the active wire, and the -corresponding distance at which the luminosity disappeared was -<span class='pageno' id='Page_548'>548</span>measured. The results are shown graphically in <a href='#fig107'>Fig. 107</a>, where the -ordinates represent the distance of the phosphorescent screen from -the active wire, and the abscissae the number of layers of aluminium -foil, each ·00031 cms. thick.</p> - -<div id='fig107' class='figcenter id006'> -<img src='images/fig-107.png' alt='Fig. 107.' class='ig001'> -<div class='ic002'> -<p>Fig. 107.</p> -</div> -</div> - -<p class='c006'>It is seen that the curve joining the points is a straight line. -12·5 thicknesses of foil absorbed the rays to the same extent as 6·8 cms. -of air, so that each thickness of aluminium corresponded in absorbing -power to ·54 cms. of air. For a screen of zinc sulphide, the phosphorescent -action ceased at a distance of air of 6·8 cms., showing that the -photographic and phosphorescent ranges of the α rays in air were -practically identical.</p> - -<p class='c006'>The experiments with barium platinocyanide and willemite were -more difficult, as the β and γ rays from the active wire produced -a luminosity comparable with that produced by the α rays. Fairly -concordant results, however, were obtained by introducing a thin -sheet of black paper between the active wire and the screen. If the -luminosity was sensibly changed, it was concluded that the α rays -still produced an effect, and in this way the point of cessation of -phosphorescent action could be approximately determined. For example, -with eight thicknesses of foil over the active wire the additional -thickness of air required to cut off the phosphorescent effect of the -a rays was 2·5 cms. for willemite, and 2·1 cms. for barium platinocyanide.</p> - -<p class='c006'><span class='pageno' id='Page_549'>549</span>The corresponding distance for zinc sulphide was 2·40 cms., a value -intermediate between the other two.</p> - -<p class='c006'>Since eight layers of foil are equivalent to 4·3 cms. of air, the -ranges in air of phosphorescent action for zinc sulphide, barium platinocyanide, -and willemite correspond to 6·7, 6·8, and 6·4 cms. respectively. -The differences observed are quite likely to be due to experimental -error.</p> -<p class='c005'><b>Discussion of results.</b> We have seen that the ionizing, -phosphorescent, and photographic actions of the α rays emitted from -radium C cease after traversing very nearly the same distance of air. -This is a surprising result when it is remembered that the α particle, -after passing through this depth of air, still possesses a velocity of at -least 60 per cent. of its initial value. Taking the probable value of -the initial velocity of the α particle from radium C as 2·5 × 10<sup>9</sup> cms. -per sec., the ionizing, phosphorescent, and photographic actions cease -when the velocity of the α particle falls below 1·5 × 10<sup>9</sup> cms. per second, -that is, a velocity of about ¹⁄₂₀ of that of light. The particle still -possesses nearly 40 per cent. of its initial energy of projection at this -stage.</p> - -<p class='c006'>These results show that the property of the α rays of producing -ionization in gases, of producing luminosity in some substances, and -of affecting a photographic plate, ceases when the velocity of the α -particle falls below a certain fixed value which is the same in each -case. It seems reasonable, therefore, to suppose that these three -properties of the α rays must be ascribed to a common cause. Now -the absorption of the α rays in gases is mainly a consequence of the -energy absorbed in the production of ions in the gas. When the α -particles are completely absorbed in the gas, the same total amount of -ionization is produced, showing that the energy required to produce an -ion is the same for all gases. On the other hand, for a constant -source of radiation, the ionization per unit volume of the gas is -approximately proportional to its density. Since the absorption of -the α rays in solid matter is approximately proportional to the density -of the absorbing medium compared with air, it is probable that this -absorption is also a result of the energy used up in producing ions in the -solid matter traversed, and that about the same amount of energy is -required to produce an ion in matter whether solid, liquid, or gaseous.</p> - -<p class='c006'>It is probable, therefore, that the production of ions in the phosphorescent -material and in the photographic film would cease at about -<span class='pageno' id='Page_550'>550</span>the same velocity for which the α particle is unable to ionize the gas. -On this view, then, the experimental results receive a simple explanation. -The action of the α rays in producing photographic and -phosphorescent actions is primarily a result of ionization. This -ionization may possibly give rise to secondary actions which influence -the effects observed.</p> - -<p class='c006'>This point of view is of interest in connection with the origin of -the “scintillations” observed in zinc sulphide and other substances -when exposed to the action of the α rays. This effect is ascribed by -Becquerel to the cleavage of the crystals under the bombardment of -the α particles. These results, however, show that we must look -deeper for the explanation of this phenomenon. The effect is primarily -due to the production of ions in the phosphorescent material and not -to direct bombardment, for we have seen that the α particle produces -no scintillations when it still possesses a large amount of kinetic -energy. It seems not unlikely that the scintillations produced by the -α rays must be ascribed to the recombination of the ions which are -produced by the α particle in the crystalline mass. It is difficult to -see how this ionization could result in a cleavage of the crystals.</p> - -<p class='c006'>This close connection of the photographic and phosphorescent -actions of the α rays with their property of producing ions, raises -the question whether photographic and phosphorescent actions in -general may not, in the first place, be due to a production of ions in -the substance.</p> -<p class='c005'><b>Ionization curve for the α rays from radium C.</b> Mr -McClung, working in the laboratory of the writer, has recently -determined the relative ionization per unit path of the α particles -projected from radium C, using the method first employed by Bragg -and discussed in <a href='#section104'>section 104</a>. An active wire, exposed for several -hours to the emanation from radium, was used as a source of rays. -The α particles were homogeneous, since the film of radio-active -matter was extremely thin.</p> - -<p class='c006'>The relation between the ionization observed over the cross section -of the narrow cone of rays and the distance from the source of rays -is shown in <a href='#fig108'>Fig. 108</a>.</p> - -<div id='fig108' class='figcenter id002'> -<img src='images/fig-108.png' alt='Fig. 108.' class='ig001'> -<div class='ic002'> -<p>Fig. 108.</p> -</div> -</div> - -<p class='c006'>The curve exhibits the same peculiarities as those given by Bragg -for a thin film of matter of one kind. The ionization of the α particle -per unit path increases slowly for about 4 cms. There is then a -more rapid increase just before the α particle ceases to ionize the -<span class='pageno' id='Page_551'>551</span>gas, and then a rapid falling off. The ionization does not appear to -end so abruptly as is really the case, since there is a correction to be -applied for the angle subtended by the cone of rays. The maximum -range of the α rays in air was 6·7 cms., a number in agreement with -that obtained by Bragg by measurements on the range of the rays -from radium.</p> - -<p class='c006'>These results show that the ionization per unit path of the α -particle increases at first slowly and then rapidly with decrease of -velocity until the rays cease to ionize the gas.</p> -<p class='c005'><b>Energy required to produce an ion.</b> From the above results -the energy required to produce an ion by collision of the α particle -with the gas molecules can readily be deduced. The α particles, -emitted from radium itself, are initially projected with a velocity ·88<i>V</i>₀ -<span class='pageno' id='Page_552'>552</span>where <i>V</i>₀ is the initial velocity of projection of the α particles from -radium C. The α particles cease to ionize the gas at a velocity ·64<i>V₀</i>. -From this it can at once be deduced that ·48 of the total energy of -the α particle, shot out by radium itself, is absorbed when it ceases to -ionize the gas. Assuming that the heating effect of radium at its -minimum activity—25 gram calories per hour per gram—is a measure -of the kinetic energy of the expelled α particles, it can be calculated -that the kinetic energy of each α particle is 4·7 × 10<sup>-6</sup> ergs. The -amount of energy absorbed when the α particle just ceases to ionize -the gas is 2·3 × 10<sup>-6</sup> ergs. Assuming that this energy is used up in -ionization, and remembering that the α particle from radium itself -produces 86000 ions in its path (<a href='#section252'>section 252</a>), the average energy -required to produce an ion is 2·7 × 10<sup>-11</sup> ergs. This is equivalent to -the energy acquired by an ion moving freely between two points -differing in potential by 24 volts.</p> - -<p class='c006'>Townsend found that fresh ions were produced by an electron for -a corresponding difference of potential of 10 volts. Stark, from -other data, obtained a value 45 volts, while Langevin considers that -60 volts is an average value. The value obtained by Rutherford and -McClung for ionization by X-rays was 175 volts, and is probably too -high.</p> -<p class='c005'><b>Rayless changes.</b> We have seen that the α particles from the -radio-active substances are projected with an average velocity not more -than 30 per cent. greater than the minimum velocity, below which -the α particles are unable to produce any ionizing, photographic, or -phosphorescent action. Such a conclusion suggests that the property -of the radio-active substances of emitting α particles has been detected -because the α particles were projected slightly above this minimum -velocity. A similar disintegration of matter may be taking place in -other substances at a rate much greater than in uranium without -producing much electrical effect, provided the α particles are projected -below the critical velocity.</p> - -<p class='c006'>The α particle, on an average, produces about 100,000 ions in the -gas before it is absorbed, so that the electrical effect observed is -about 100,000 times as great as that due to the charge carried by the -α particles alone.</p> - -<p class='c006'>It is not unlikely that the numerous rayless products which have -been observed may undergo disintegration of a similar character to -the products which obviously emit α rays. In the rayless product the -<span class='pageno' id='Page_553'>553</span>α particle may be expelled with a velocity less than 1·5 × 10<sup>9</sup> cms. per -second and so fail to produce much electrical effect.</p> - -<p class='c006'>These considerations have an important bearing on the question -whether matter in general is radio-active. The property of emitting -α particles above the critical velocity may well be a property only of a -special class of substances, and need not be exhibited by matter in -general. At the same time the results suggest that ordinary matter -may be undergoing transformation accompanied by the expulsion of -α particles at a rate much greater than that shown by uranium, -without producing appreciable electrical or photographic action.</p> -<div class='chapter'> - <span class='pageno' id='Page_554'>554</span> - <h2 id='appb' class='c004'>APPENDIX B. <br> RADIO-ACTIVE MINERALS.</h2> -</div> -<p class='c005'>Those natural mineral substances which possess marked radio-active -properties have been found to contain either uranium or thorium, -one of these elements being always present in sufficient proportion -readily to permit its chemical separation and identification by the -ordinary analytical methods<a id='r455' href='#f455' class='c012'><sup>[455]</sup></a>.</p> - -<p class='c006'>A large number of uranium and thorium minerals are known at -the present time, but they are for the most part found very sparingly, -and some of them have been observed to occur only in a single locality. -The chief commercial sources of uranium are uraninite, gummite, and -carnotite, while thorium is obtained almost exclusively from monazite.</p> - -<p class='c006'>Rutherford and Soddy (<i>Phil. Mag.</i> 65, 561 (1903)), were the first -to call attention to the important fact that the relations between the -various radio-active substances and the other elements could best be -determined from the study of the natural minerals in which these -bodies occur, since these minerals represent mixtures of extreme -antiquity, which have remained more or less undisturbed for almost -countless ages. In dealing with these matters, however, it is highly -important that we bring to our aid the data furnished by geology and -mineralogy, from which it is often possible to determine the relative -ages of the different substances with at least a rough degree of approximation. -Thus, for example, if a certain mineral occurs as a primary -constituent of a rock of remote geological period, it can safely be -assumed that its age is greater than that of a similar or different -mineral occurring in a later formation. It is, moreover, quite evident -that those minerals which are obviously produced by the decomposition -and alteration of the primary minerals, through the action of -percolating water and other agencies acting from the surface downward, -<span class='pageno' id='Page_555'>555</span>are of less antiquity than the primary minerals from which -they originated. Through the application of these considerations -it should, in general, be possible to arrange the various minerals -roughly in the order of their probable ages.</p> - -<p class='c006'>The most familiar and widely known uranium mineral is uraninite, -commonly called pitchblende, which consists essentially of uranium -dioxide (UO<sub>2</sub>), uranium trioxide (UO<sub>3</sub>), and lead oxide (PbO), present in -varying proportions. The uraninites can be distinguished as primary, -namely, those which occur as a primary constituent of pegmatitic -dikes and coarse granites, and secondary, when they occur in metalliferous -veins associated with the sulphides of silver, lead, copper, -nickel, iron, and zinc. The former varieties are quite frequently -crystalline in character, contain a larger proportion of the rare earths -and helium, and have a higher specific gravity than the latter, which -are always massive and botryoidal.</p> - -<p class='c006'>The following are the most prominent localities in which primary -uraninites occur:</p> - -<p class='c006'>1. North Carolina, U.S.A. (especially in Mitchell and Yancey -counties). The uraninite is found in a coarse pegmatitic dike which -is mined for the mica constituent. The associated feldspar of the -dike is considerably decomposed through the action of meteoric waters -and gases, and the uraninite itself is largely altered into the secondary -minerals gummite and uranophane through the same agencies. Among -the associated primary minerals are allanite, zircon, columbite, samarskite, -fergusonite and monazite, while the secondary minerals include -gummite, thorogummite, uranophane, autunite, phosphuranylite, -hatchettolite, and cyrtolite. The geological period of this formation -is difficult to establish with certainty, but is stated to be perhaps -Archean, or possibly to correspond with the close of the Ordovician or -with the Permian.</p> - -<p class='c006'>2. Connecticut, U.S.A. The best known localities are Glastonbury, -where the uraninite is found in the feldspar quarries, and -Branchville, where it occurs in an albitic granite. Both of these -localities have furnished fine crystals. The geological period probably -corresponds with the close of the Ordovician or Carboniferous eras, and -is stated to be certainly Post-Cambrian and Pre-Triassic. Among the -associated minerals are (primary) columbite, (secondary) torbernite -and autunite.</p> - -<p class='c006'>3. Southern Norway, particularly in the neighbourhood of Moss. -Here uraninite occurs in the augite-syenite and pegmatite. The -<span class='pageno' id='Page_556'>556</span>varieties found are known as cleveite and bröggerite, and among the -primary associated minerals are orthite, fergusonite, monazite, and -thorite. The period is stated to be Post-Devonian.</p> - -<p class='c006'>4. Llano County, Texas. The variety of uraninite known as -nivenite is found here in a quartzose pegmatite, associated with the -primary minerals gadolinite, allanite and fergusonite, and the secondary -minerals cyrtolite, yttrialite, gummite, and thorogummite.</p> - -<p class='c006'>Secondary uraninite is found at Johanngeorgenstadt, Marienberg -and Schneeberg in Saxony, at Joachimsthal and Pribam in Bohemia, -at Cornwall in England, and at Black Hawk, Colorado, and in the -Black Hills, South Dakota, in the United States. The exact geological -period of most of these secondary occurrences is somewhat uncertain, -but they are undoubtedly very much later than the primary occurrences -mentioned above.</p> - -<p class='c006'>As a matter of general interest the analysis of a typical primary -uraninite (No. 1) and of a typical secondary uraninite (No. 2) is -given below<a id='r456' href='#f455' class='c012'><sup>[456]</sup></a>:</p> - -<table class='table9' > -<colgroup> -<col class='colwidth27'> -<col class='colwidth27'> -<col class='colwidth44'> -</colgroup> - <tr> - <th class='c013'></th> - <th class='c015'>No. 1 Glastonbury, Conn.</th> - <th class='c016'>No. 2 Johanngeorgenstadt, Saxony</th> - </tr> - <tr> - <td class='c013'>Sp. Gr.</td> - <td class='c015'>9·59</td> - <td class='c016'>6·89</td> - </tr> - <tr> - <td class='c013'>UO<sub>3</sub></td> - <td class='c015'>26·48</td> - <td class='c016'>60·05</td> - </tr> - <tr> - <td class='c013'>UO<sub>2</sub></td> - <td class='c015'>57·43</td> - <td class='c016'>22·33</td> - </tr> - <tr> - <td class='c013'>ThO<sub>2</sub></td> - <td class='c015'>9·79</td> - <td class='c016'>...</td> - </tr> - <tr> - <td class='c013'>CeO<sub>2</sub></td> - <td class='c015'>0·25</td> - <td class='c016'>...</td> - </tr> - <tr> - <td class='c013'>La<sub>2</sub>O<sub>3</sub></td> - <td class='c015'>0·13</td> - <td class='c016'>...</td> - </tr> - <tr> - <td class='c013'>Y<sub>2</sub>O<sub>3</sub></td> - <td class='c015'>0·20</td> - <td class='c016'>...</td> - </tr> - <tr> - <td class='c013'>PbO</td> - <td class='c015'>3·26</td> - <td class='c016'>6·39</td> - </tr> - <tr> - <td class='c013'>CaO</td> - <td class='c015'>0·08</td> - <td class='c016'>1·00</td> - </tr> - <tr> - <td class='c013'>He</td> - <td class='c015'>und.</td> - <td class='c016'>und.</td> - </tr> - <tr> - <td class='c013'>H<sub>2</sub>O</td> - <td class='c015'>0·61</td> - <td class='c016'>3·17</td> - </tr> - <tr> - <td class='c013'>Fe<sub>2</sub>O<sub>3</sub></td> - <td class='c015'>0·40</td> - <td class='c016'>0·21</td> - </tr> - <tr> - <td class='c013'>SiO<sub>2</sub></td> - <td class='c015'>0·25</td> - <td class='c016'>0·50</td> - </tr> - <tr> - <td class='c013'>Al<sub>2</sub>O<sub>3</sub></td> - <td class='c015'>...</td> - <td class='c016'>0·20</td> - </tr> - <tr> - <td class='c013'>Bi<sub>2</sub>O<sub>3</sub></td> - <td class='c015'>...</td> - <td class='c016'>0·75</td> - </tr> - <tr> - <td class='c013'>CuO</td> - <td class='c015'>...</td> - <td class='c016'>0·17</td> - </tr> - <tr> - <td class='c013'>MnO</td> - <td class='c015'>...</td> - <td class='c016'>0·09</td> - </tr> - <tr> - <td class='c013'>MgO</td> - <td class='c015'>...</td> - <td class='c016'>0·17</td> - </tr> - <tr> - <td class='c013'>Na<sub>2</sub>O</td> - <td class='c015'>...</td> - <td class='c016'>0·31</td> - </tr> - <tr> - <td class='c013'>P<sub>2</sub>O<sub>5</sub></td> - <td class='c015'>...</td> - <td class='c016'>0·06</td> - </tr> - <tr> - <td class='c013'>SO<sub>3</sub></td> - <td class='c015'>...</td> - <td class='c016'>0·19</td> - </tr> - <tr> - <td class='c013'>As<sub>2</sub>O<sub>3</sub></td> - <td class='c015'>...</td> - <td class='c016'>2·34</td> - </tr> - <tr> - <td class='c013'>Insoluble</td> - <td class='c015'>0·70</td> - <td class='c016'>...</td> - </tr> -</table> - -<p class='c006'>The following list comprises the more important radio-active -minerals, with their approximate chemical composition and some notes -on their occurrence and probable origin.</p> - -<table class='table4'> -<colgroup> -<col class='colwidth35'> -<col class='colwidth30'> -<col class='colwidth33'> -</colgroup> - <tr><td class='c023' colspan='3'><span class='pageno' id='Page_557'>557</span></td></tr> - <tr> - <th class='c013'>Name</th> - <th class='c013'>Composition</th> - <th class='c014'>Remarks</th> - </tr> - <tr> - <td class='c013'> </td> - <td class='c013'> </td> - <td class='c014'> </td> - </tr> - <tr> - <td class='c013'>Uraninite, Cleveite, Bröggerite, Nivenite, Pitchblende</td> - <td class='c013'>Oxides of uranium and lead. Usually contains thorium, other rare earths and helium. Uranium 50-80%. Thorium 0–10%</td> - <td class='c014'>Occurs primary as a constituent of rocks and secondary in veins with metalliferous sulphides</td> - </tr> - <tr> - <td class='c013'>Gummite</td> - <td class='c013'>(Pb, Ca) U<sub>3</sub>SiO<sub>12</sub> . 6H<sub>2</sub>O? Uranium 50–65%</td> - <td class='c014'>An alteration product of uraninite. Formed by the action of percolating waters</td> - </tr> - <tr> - <td class='c013'>Uranophane, Uranotil</td> - <td class='c013'>CaO . 2UO<sub>3</sub> . 2SiO<sub>2</sub> . 6H<sub>2</sub>O Uranium 44–56%</td> - <td class='c014'>An alteration product of uraninite through gummite</td> - </tr> - <tr> - <td class='c013'>Carnotite</td> - <td class='c013'>A vanadate of uranium and potassium. Uranium 42–51%</td> - <td class='c014'>Occurs as a secondary mineral impregnating a porous, sedimentary sandstone. Found in Colorado and Utah</td> - </tr> - <tr><td> </td><td> </td><td> </td></tr> - <tr> - <td class='c013'>Uranosphaerite</td> - <td class='c013'>Bi<sub>2</sub>O<sub>3</sub> . 2UO<sub>3</sub> . 3H<sub>2</sub>O. Uranium 41%</td> - <td class='c014'>Alteration product of other uranium minerals</td> - </tr> - <tr> - <td class='c013'>Torbernite, Cuprouranite</td> - <td class='c013'>CuO . 2UO<sub>3</sub> . P<sub>2</sub>O<sub>5</sub> . 8H<sub>2</sub>O. Uranium 44–51%</td> - <td class='c014'>„ „</td> - </tr> - <tr> - <td class='c013'>Autunite, Calciouranite</td> - <td class='c013'>CaO . 2UO<sub>3</sub> . P<sub>2</sub>O<sub>5</sub> . 8H<sub>2</sub>O. Uranium 45–51%</td> - <td class='c014'>„ „</td> - </tr> - <tr> - <td class='c013'>Uranocircite</td> - <td class='c013'>BaO . 2UO<sub>3</sub> . P<sub>2</sub>O<sub>5</sub> . 8H<sub>2</sub>O. Uranium 46%</td> - <td class='c014'>„ „</td> - </tr> - <tr> - <td class='c013'>Phosphuranylite</td> - <td class='c013'>3UO<sub>3</sub> . P<sub>2</sub>O<sub>5</sub> . 6H<sub>2</sub>O. Uranium 58–64%</td> - <td class='c014'>„ „</td> - </tr> - <tr> - <td class='c013'>Zunerite</td> - <td class='c013'>CuO . 2UO<sub>3</sub> . As<sub>2</sub>O<sub>5</sub> . 8H<sub>2</sub>O. Uranium 46%</td> - <td class='c014'>„ „</td> - </tr> - <tr> - <td class='c013'>Uranospinite</td> - <td class='c013'>CaO . 2UO<sub>3</sub> . As<sub>2</sub>O<sub>5</sub> . 8H<sub>2</sub>O. Uranium 49%</td> - <td class='c014'>„ „</td> - </tr> - <tr> - <td class='c013'>Walpurgite</td> - <td class='c013'>5Bi<sub>2</sub>O<sub>3</sub> . 3UO<sub>3</sub> . As<sub>2</sub>O<sub>5</sub> . 12H<sub>2</sub>O. Uranium 16%</td> - <td class='c014'>„ „</td> - </tr> - <tr> - <td class='c013'>Thorogummite</td> - <td class='c013'>UO<sub>3</sub> . 3ThO<sub>2</sub> . 3SiO<sub>2</sub> . 6H<sub>2</sub>O? Uranium 41%</td> - <td class='c014'>A variety of gummite</td> - </tr> - <tr> - <td class='c013'>Thorite, Orangite, Uranothorite</td> - <td class='c013'>ThSiO<sub>4</sub>. Uranium 1–10%. Thorium oxide 48–71%</td> - <td class='c014'>A primary constituent of pegmatite dikes</td> - </tr> - <tr> - <td class='c013'>Thorianite</td> - <td class='c013'>Oxide of thorium, uranium, the rare earths and lead. Contains a relatively large proportion of helium. Uranium 9–10%. Thorium oxide 73–77%</td> - <td class='c014'>Occurs as a primary constituent of a pegmatite dike in Ceylon. Geological age probably Archean</td> - </tr> - <tr> - <td class='c013'><span class='pageno' id='Page_558'>558</span>Samarskite</td> - <td class='c013'>Niobate and tantalate of rare earths. Uranium 8–10%</td> - <td class='c014'>Primary constituent of pegmatite dikes</td> - </tr> - <tr> - <td class='c013'>Fergusonite</td> - <td class='c013'>Metaniobate and tantalate of rare earths. Uranium 1–6%</td> - <td class='c014'>„ „</td> - </tr> - <tr> - <td class='c013'>Euxenite</td> - <td class='c013'>Niobate and titanate of rare earths. Uranium 3–10%</td> - <td class='c014'>„ „</td> - </tr> - <tr> - <td class='c013'>Monazite</td> - <td class='c013'>Phosphate of the rare earths, chiefly cerium. Uranium 0·3–0·4%</td> - <td class='c014'>„ „</td> - </tr> -</table> - -<div class='chapter'> - <span class='pageno' id='Page_559'>559</span> - <h2 id='index' class='c004'>INDEX.</h2> -</div> -<p class='c005'><i>The numbers refer to the pages.</i></p> - -<ul class='index'> - <li class='c024'>α rays - <ul> - <li>discovery of, <a href='#Page_141'>141</a></li> - <li>nature of, <a href='#Page_141'>141</a></li> - <li>magnetic deviation of, <a href='#Page_142'>142</a> <i>et seq.</i></li> - <li>electrostatic deviation of, <a href='#Page_146'>146</a></li> - <li>velocity of, <a href='#Page_148'>148</a></li> - <li>value of <i>e</i>/<i>m</i> for, <a href='#Page_148'>148</a></li> - <li>charge carried by, <a href='#Page_151'>151</a> <i>et seq.</i></li> - <li>number of α particles expelled from one gram of radium, <a href='#Page_155'>155</a></li> - <li>mass and energy of, <a href='#Page_156'>156</a></li> - <li>origin of, in atomic disintegration, <a href='#Page_157'>157</a></li> - <li>scintillations produced by, <a href='#Page_158'>158</a> <i>et seq.</i></li> - <li>absorption of, by matter, <a href='#Page_161'>161</a> <i>et seq.</i></li> - <li>increase of absorption with thickness of matter traversed, <a href='#Page_163'>163</a></li> - <li>relative absorption of α rays from radio-elements, <a href='#Page_164'>164</a></li> - <li>absorption of, by gases, <a href='#Page_165'>165</a> <i>et seq.</i></li> - <li>connection between absorption and density, <a href='#Page_169'>169</a></li> - <li>relation between ionization and absorption, <a href='#Page_170'>170</a></li> - <li>theory of absorption of, <a href='#Page_170'>170</a> <i>et seq.</i></li> - <li>range of ionization of, <a href='#Page_172'>172</a> <i>et seq.</i></li> - <li>complexity of α rays from radium, <a href='#Page_174'>174</a> <i>et seq.</i></li> - <li>effect of thickness of layer of radiating matter on emission of, <a href='#Page_195'>195</a></li> - <li>relative ionization produced by α and β rays, <a href='#Page_196'>196</a> <i>et seq.</i></li> - <li>phosphorescence by α rays, <a href='#Page_202'>202</a> <i>et seq.</i></li> - <li>connection of, with radio-active changes, <a href='#Page_235'>235</a>, <a href='#Page_444'>444</a> <i>et seq.</i>, <a href='#Page_455'>455</a></li> - <li>from the emanations, <a href='#Page_263'>263</a></li> - <li>emission of energy from radio-elements in form of α rays, <a href='#Page_419'>419</a> <i>et seq.</i></li> - <li>connection of heat emission of radium with α rays, <a href='#Page_421'>421</a> <i>et seq.</i></li> - <li>number of ions produced by an α particle, <a href='#Page_433'>433</a></li> - <li>absence of, in rayless changes, <a href='#Page_454'>454</a></li> - <li>emission from active products, <a href='#Page_454'>454</a> <i>et seq.</i></li> - <li>loss of weight due to expulsion of, <a href='#Page_473'>473</a></li> - <li>α particles consist of helium, <a href='#Page_479'>479</a> <i>et seq.</i></li> - <li>magnetic deflection of, from radium C, <a href='#Page_543'>543</a></li> - <li>velocity and <i>e</i>/<i>m</i> for, from radium C, <a href='#Page_543'>543</a> <i>et seq.</i></li> - <li>diminution of velocity of, in passing through matter, <a href='#Page_545'>545</a></li> - <li>diminution in velocity of, in passing through aluminium, <a href='#Page_545'>545</a></li> - <li>velocity of, when ionization ceases, <a href='#Page_545'>545</a> <i>et seq.</i></li> - <li>connection of phosphorescent, photographic, and ionization effects produced by, <a href='#Page_546'>546</a> <i>et seq.</i></li> - <li>energy required to produce an ion by α rays, <a href='#Page_551'>551</a></li> - </ul> - </li> - <li class='c024'>Abraham - <ul> - <li>apparent mass of moving charged body, <a href='#Page_71'>71</a>, <a href='#Page_127'>127</a></li> - </ul> - </li> - <li class='c024'>Absorption - <ul> - <li>law of, in gases, <a href='#Page_64'>64</a> <i>et seq.</i></li> - <li>relative absorption of α, β and γ rays by matter, <a href='#Page_111'>111</a></li> - <li>connection between absorption and ionization, <a href='#Page_134'>134</a> <i>et seq.</i>, <a href='#Page_170'>170</a> <i>et seq.</i></li> - <li>of β rays by solids, <a href='#Page_134'>134</a> <i>et seq.</i></li> - <li>connection between absorption and density for β rays, <a href='#Page_137'>137</a></li> - <li>of β rays in radio-active matter, <a href='#Page_140'>140</a></li> - <li>of α rays by solids, <a href='#Page_161'>161</a> <i>et seq.</i></li> - <li>of α rays in gases, <a href='#Page_167'>167</a>, <a href='#Page_170'>170</a> <i>et seq.</i></li> - <li>connection between absorption and density for α rays, <a href='#Page_169'>169</a></li> - <li>theory of, <a href='#Page_170'>170</a> <i>et seq.</i></li> - <li>of γ rays by solids, <a href='#Page_179'>179</a> <i>et seq.</i></li> - <li>connection between absorption and density for γ rays, <a href='#Page_181'>181</a></li> - <li>of rays from the emanations, <a href='#Page_263'>263</a></li> - <li>of penetrating rays from the earth, <a href='#Page_520'>520</a>, <a href='#Page_540'>540</a></li> - </ul> - </li> - <li class='c024'><a id='index-actinium'></a></li> - <li class='c024'>Actinium - <ul> - <li>methods of separation of, <a href='#Page_20'>20</a> <i>et seq.</i></li> - <li>properties of, <a href='#Page_21'>21</a></li> - <li>similarity to “emanating substance” of Giesel, <a href='#Page_21'>21</a></li> - <li><span class='pageno' id='Page_560'>560</span>possible connection with radio-activity of thorium, <a href='#Page_28'>28</a></li> - <li>emanation from, <a href='#Page_249'>249</a></li> - <li>excited activity produced by, <a href='#Page_311'>311</a></li> - <li>effect of magnetic field on excited activity from, <a href='#Page_324'>324</a></li> - <li>separation of actinium X, <a href='#Page_365'>365</a></li> - <li>decay of actinium X, <a href='#Page_365'>365</a></li> - <li>source of actinium emanation, <a href='#Page_365'>365</a></li> - <li>analysis of active deposit of, <a href='#Page_366'>366</a></li> - <li>radiations from products of, <a href='#Page_368'>368</a></li> - <li>penetrating power of β and γ rays from, <a href='#Page_368'>368</a></li> - <li>products of, <a href='#Page_369'>369</a></li> - <li>table of products of, <a href='#Page_448'>448</a></li> - <li>possible origin of, <a href='#Page_464'>464</a></li> - </ul> - </li> - <li class='c024'>Actinium A - <ul> - <li>separation and period of, <a href='#Page_367'>367</a> <i>et seq.</i></li> - </ul> - </li> - <li class='c024'>Actinium B - <ul> - <li>period of, <a href='#Page_368'>368</a></li> - <li>properties of, <a href='#Page_368'>368</a></li> - </ul> - </li> - <li class='c024'>Actinium X - <ul> - <li>separation and decay of, <a href='#Page_364'>364</a> <i>et seq.</i></li> - <li>production of emanation by, <a href='#Page_365'>365</a></li> - </ul> - </li> - <li class='c024'>Adams - <ul> - <li>decay of activity of emanation from well water, <a href='#Page_511'>511</a></li> - <li>decay of excited activity from the emanation, <a href='#Page_511'>511</a></li> - </ul> - </li> - <li class='c024'>Age - <ul> - <li>of radium, <a href='#Page_457'>457</a></li> - <li>of sun and earth, <a href='#Page_492'>492</a> <i>et seq.</i></li> - </ul> - </li> - <li class='c024'>Allan, S. J. - <ul> - <li>increase with time of excited activity from atmosphere, <a href='#Page_505'>505</a></li> - <li>radio-activity of snow, <a href='#Page_506'>506</a></li> - <li>effect of conditions on decay of activity from air, <a href='#Page_519'>519</a>, <a href='#Page_523'>523</a></li> - </ul> - </li> - <li class='c024'>Allan and Rutherford - <ul> - <li>decay of excited activity from the atmosphere, <a href='#Page_503'>503</a></li> - <li>ionization of air in closed vessels, <a href='#Page_534'>534</a></li> - </ul> - </li> - <li class='c024'>Allen, H. S. and Lord Blythswood - <ul> - <li>radium emanation in Bath springs, <a href='#Page_513'>513</a></li> - </ul> - </li> - <li class='c024'>Anderson and Hardy - <ul> - <li>action of radium rays on the eye, <a href='#Page_217'>217</a></li> - </ul> - </li> - <li class='c024'>Armstrong and Lowry - <ul> - <li>radio-activity and phosphorescence, <a href='#Page_444'>444</a></li> - </ul> - </li> - <li class='c024'>Arnold - <ul> - <li>rays from phosphorescent substances, <a href='#Page_4'>4</a></li> - </ul> - </li> - <li class='c024'>Aschkinass and Caspari - <ul> - <li>action of radium rays on microbes, <a href='#Page_216'>216</a></li> - </ul> - </li> - <li class='c024'>Atmosphere - <ul> - <li>excited activity from, <a href='#Page_501'>501</a> <i>et seq.</i></li> - <li>radio-activity of, due to emanations, <a href='#Page_504'>504</a></li> - <li>diffusion of emanations into, from the earth, <a href='#Page_507'>507</a></li> - <li>effect of temperature, pressure, &c. on radio-activity of, <a href='#Page_517'>517</a> <i>et seq.</i></li> - <li>presence of very penetrating radiation in, <a href='#Page_520'>520</a></li> - <li>comparison of radio-activity of, with radio-elements, <a href='#Page_521'>521</a> <i>et seq.</i></li> - <li>amount of radium emanation in, <a href='#Page_524'>524</a> <i>et seq.</i></li> - <li>ionization of, due to radium emanation, <a href='#Page_526'>526</a></li> - </ul> - </li> - <li class='c024'>Atom - <ul> - <li>number of per c.c., <a href='#Page_54'>54</a></li> - <li>disintegration of, <a href='#Page_234'>234</a> <i>et seq.</i></li> - <li>complex nature of, <a href='#Page_235'>235</a></li> - <li>changing atoms, <a href='#Page_444'>444</a> <i>et seq.</i></li> - <li>possible causes of disintegration of, <a href='#Page_486'>486</a></li> - <li>evolution of, <a href='#Page_496'>496</a></li> - </ul> - </li> - <li class='c024'>Atomic weight - <ul> - <li>of radium by chemical methods, <a href='#Page_17'>17</a></li> - <li>from spectroscopic evidence, <a href='#Page_18'>18</a></li> - <li>emanations, <a href='#Page_273'>273</a></li> - <li>of radio-elements and connection with radio-activity, <a href='#Page_445'>445</a></li> - </ul> - </li> - <li class='c003'><a id='index-beta-rays'></a></li> - <li class='c024'>β rays - <ul> - <li>discovery of, <a href='#Page_113'>113</a></li> - <li>magnetic deflection of, <a href='#Page_114'>114</a></li> - <li>complexity of, <a href='#Page_116'>116</a></li> - <li>examination by the electrical method, <a href='#Page_118'>118</a></li> - <li>effect of, on a fluorescent screen, <a href='#Page_119'>119</a></li> - <li>charge carried by the, <a href='#Page_120'>120</a> <i>et seq.</i></li> - <li>electrostatic deviation of, <a href='#Page_124'>124</a></li> - <li>velocity of, and value of <i>e</i>/<i>m</i> for, <a href='#Page_126'>126</a></li> - <li>variation of <i>e</i>/<i>m</i> with velocity of, <a href='#Page_127'>127</a> <i>et seq.</i></li> - <li>distribution of velocity amongst β particles, <a href='#Page_131'>131</a></li> - <li>absorption of, <a href='#Page_134'>134</a> <i>et seq.</i></li> - <li>variation of absorption with density, <a href='#Page_136'>136</a> <i>et seq.</i></li> - <li>number of β particles stopped by matter, <a href='#Page_137'>137</a> <i>et seq.</i></li> - <li>variation of intensity of, with thickness of layer, <a href='#Page_140'>140</a></li> - <li>secondary β rays, <a href='#Page_189'>189</a> <i>et seq.</i></li> - <li>relative ionization produced by α and β rays, <a href='#Page_196'>196</a></li> - <li>relative energy emitted in form of α and β rays, <a href='#Page_196'>196</a> <i>et seq.</i></li> - <li>phosphorescent action of, <a href='#Page_201'>201</a> <i>et seq.</i></li> - <li>physical action produced by, <a href='#Page_207'>207</a> <i>et seq.</i></li> - <li>chemical action of, <a href='#Page_213'>213</a></li> - <li>physiological action of, <a href='#Page_216'>216</a></li> - <li>from Ur X, <a href='#Page_347'>347</a></li> - <li><span class='pageno' id='Page_561'>561</span>from active deposit of radium, <a href='#Page_377'>377</a> <i>et seq.</i></li> - <li>significance of appearance of, only in last radio-active changes, <a href='#Page_455'>455</a></li> - <li>change of weight due to expulsion of, <a href='#Page_473'>473</a></li> - </ul> - </li> - <li class='c024'>Barium platinocyanide - <ul> - <li>phosphorescence of, under radium rays, <a href='#Page_203'>203</a></li> - <li>change of colour due to radium rays, <a href='#Page_205'>205</a></li> - </ul> - </li> - <li class='c024'>Barkla - <ul> - <li>polarization of X rays, <a href='#Page_80'>80</a></li> - </ul> - </li> - <li class='c024'>Barnes and Rutherford - <ul> - <li>heating effect of radium emanation, <a href='#Page_421'>421</a>, <a href='#Page_429'>429</a></li> - <li>connection of heating effect with radio-activity, <a href='#Page_421'>421</a></li> - <li>heating effect of active deposit, <a href='#Page_425'>425</a></li> - <li>heating effect of γ rays, <a href='#Page_429'>429</a></li> - <li>heating effect of emanation, <a href='#Page_431'>431</a></li> - <li>division of heating effect among active products, <a href='#Page_433'>433</a></li> - </ul> - </li> - <li class='c024'>Bary - <ul> - <li>phosphorescence under radium rays, <a href='#Page_202'>202</a></li> - </ul> - </li> - <li class='c024'>Baskerville - <ul> - <li>activity of thorium, <a href='#Page_29'>29</a></li> - <li>phosphorescence of kunzite under radium rays, <a href='#Page_203'>203</a></li> - </ul> - </li> - <li class='c024'>Baskerville and Kunz - <ul> - <li>phosphorescence of substances under radium rays, <a href='#Page_204'>204</a></li> - </ul> - </li> - <li class='c024'>Beattie, Smolan and Kelvin - <ul> - <li>discharging power of uranium rays, <a href='#Page_7'>7</a></li> - </ul> - </li> - <li class='c024'>Becquerel - <ul> - <li>rays from calcium sulphide, <a href='#Page_4'>4</a></li> - <li>rays from uranium, <a href='#Page_5'>5</a> <i>et seq.</i></li> - <li>permanence of uranium rays, <a href='#Page_6'>6</a></li> - <li>discharging power of uranium rays, <a href='#Page_6'>6</a></li> - <li>magnetic deflection of radium rays by photographic method, <a href='#Page_114'>114</a> <i>et seq.</i></li> - <li>curvature of radium rays in a magnetic field, <a href='#Page_115'>115</a> <i>et seq.</i></li> - <li>complexity of radium rays, <a href='#Page_116'>116</a> <i>et seq.</i></li> - <li>electrostatic deflection of β rays of radium, <a href='#Page_124'>124</a> <i>et seq.</i></li> - <li>value of <i>e</i>/<i>m</i> for β rays of radium, <a href='#Page_126'>126</a> <i>et seq.</i></li> - <li>magnetic deviation of α rays of radium and polonium, <a href='#Page_145'>145</a></li> - <li>trajectory of rays of radium in magnetic field, <a href='#Page_148'>148</a></li> - <li>scintillations due to cleavage of crystals, <a href='#Page_160'>160</a></li> - <li>γ rays from radium, <a href='#Page_179'>179</a></li> - <li>secondary rays produced by active substances, <a href='#Page_187'>187</a></li> - <li>phosphorescence produced by radium rays, <a href='#Page_201'>201</a></li> - <li>conductivity of paraffin under radium radiation, <a href='#Page_210'>210</a></li> - <li>effect of temperature on uranium rays, <a href='#Page_210'>210</a></li> - <li>chemical action of radium rays, <a href='#Page_214'>214</a></li> - <li>removal of activity from uranium by precipitation with barium, <a href='#Page_219'>219</a></li> - <li>theory of radio-activity, <a href='#Page_438'>438</a></li> - </ul> - </li> - <li class='c024'>Bemont et M. et Mme Curie - <ul> - <li>discovery of radium, <a href='#Page_13'>13</a></li> - </ul> - </li> - <li class='c024'>Berndt - <ul> - <li>spectrum of polonium, <a href='#Page_23'>23</a></li> - </ul> - </li> - <li class='c024'>Blanc - <ul> - <li>thorium in sediments from hot springs, <a href='#Page_514'>514</a></li> - </ul> - </li> - <li class='c024'>Blythswood, Lord and Allen, H. S. - <ul> - <li>radium emanation in Bath springs, <a href='#Page_513'>513</a></li> - </ul> - </li> - <li class='c024'>Bödlander and Runge - <ul> - <li>evolution of gases from radium, <a href='#Page_215'>215</a></li> - </ul> - </li> - <li class='c024'>Boltwood - <ul> - <li>origin of radium, <a href='#Page_460'>460</a></li> - <li>amount of radium in minerals, <a href='#Page_460'>460</a></li> - <li>proportionality of uranium and radium in minerals, <a href='#Page_461'>461</a></li> - <li>production of lead by uranium, <a href='#Page_484'>484</a></li> - <li>radium emanation in spring water, <a href='#Page_514'>514</a></li> - <li>method of standardization of amount of emanation in waters, <a href='#Page_514'>514</a></li> - </ul> - </li> - <li class='c024'>Boys - <ul> - <li>rate of dissipation of charge, <a href='#Page_531'>531</a></li> - </ul> - </li> - <li class='c024'>Bragg and Kleeman - <ul> - <li>theory of absorption of α rays, <a href='#Page_172'>172</a> <i>et seq.</i></li> - <li>relation between ionization and absorption, <a href='#Page_174'>174</a> <i>et seq.</i></li> - <li>range of α rays in air, <a href='#Page_174'>174</a></li> - <li>four sets of α rays from radium, <a href='#Page_174'>174</a> <i>et seq.</i></li> - </ul> - </li> - <li class='c024'>Bronson - <ul> - <li>use of steady deflection method with an electrometer, <a href='#Page_104'>104</a></li> - <li>decay of thorium emanation, <a href='#Page_242'>242</a></li> - <li>decay of excited activity from actinium, <a href='#Page_312'>312</a></li> - </ul> - </li> - <li class='c024'>Brooks, Miss - <ul> - <li>variation of excited activity from thorium for short exposures, <a href='#Page_304'>304</a></li> - <li>effect of dust on distribution of excited activity, <a href='#Page_305'>305</a></li> - <li>decay curves of excited activity of radium measured by α and β rays, <a href='#Page_307'>307</a> <i>et seq.</i></li> - <li>decay curves of excited activity from actinium, <a href='#Page_312'>312</a></li> - </ul> - </li> - <li class='c024'>Brooks and Rutherford - <ul> - <li>absorption of α rays by matter, <a href='#Page_161'>161</a></li> - <li><span class='pageno' id='Page_562'>562</span>comparison of absorption of α rays from radio-elements, <a href='#Page_164'>164</a></li> - <li>diffusion of radium emanation, <a href='#Page_270'>270</a></li> - <li>decay of excited activity from radium, <a href='#Page_306'>306</a></li> - </ul> - </li> - <li class='c024'>Bumstead - <ul> - <li>presence of thorium emanation in atmosphere, <a href='#Page_512'>512</a></li> - </ul> - </li> - <li class='c024'>Bumstead and Wheeler - <ul> - <li>diffusion of radium emanation, <a href='#Page_273'>273</a></li> - <li>emanation from surface water and the soil, <a href='#Page_512'>512</a>, <a href='#Page_522'>522</a></li> - <li>identity of emanation from soil with radium emanation, <a href='#Page_512'>512</a>, <a href='#Page_522'>522</a></li> - </ul> - </li> - <li class='c024'>Burton - <ul> - <li>radium emanation in petroleum, <a href='#Page_516'>516</a></li> - </ul> - </li> - <li class='c024'>Burton and McLennan - <ul> - <li>penetrating radiation from the earth, <a href='#Page_520'>520</a></li> - <li>radio-activity of ordinary materials, <a href='#Page_537'>537</a></li> - <li>emanation from ordinary matter, <a href='#Page_538'>538</a></li> - </ul> - </li> - <li class='c003'>Campbell - <ul> - <li>radio-activity of ordinary materials, <a href='#Page_540'>540</a></li> - </ul> - </li> - <li class='c024'>Canal rays - <ul> - <li>discovery of, <a href='#Page_78'>78</a></li> - <li>magnetic and electric deflection of, <a href='#Page_78'>78</a></li> - <li>value of <i>e</i>/<i>m</i> for, <a href='#Page_78'>78</a></li> - <li>similarity of, to α rays, <a href='#Page_110'>110</a></li> - </ul> - </li> - <li class='c024'>Capacity - <ul> - <li>of electroscopes, <a href='#Page_87'>87</a></li> - <li>of electrometers, <a href='#Page_94'>94</a>, <a href='#Page_102'>102</a></li> - <li>standards of, <a href='#Page_102'>102</a></li> - </ul> - </li> - <li class='c024'>Carbonic acid - <ul> - <li>radio-activity of natural, <a href='#Page_516'>516</a></li> - </ul> - </li> - <li class='c024'>Caspari and Aschkinass - <ul> - <li>action of radium rays on microbes, <a href='#Page_216'>216</a></li> - </ul> - </li> - <li class='c024'>Cathode rays - <ul> - <li>discovery of, <a href='#Page_73'>73</a></li> - <li>magnetic and electric deflection of, <a href='#Page_74'>74</a></li> - <li>value of <i>e</i>/<i>m</i> for, <a href='#Page_75'>75</a></li> - <li>radiation of energy from, <a href='#Page_79'>79</a></li> - <li>comparison of, with β rays, <a href='#Page_120'>120</a></li> - <li>absorption of, by matter, <a href='#Page_136'>136</a>, <a href='#Page_137'>137</a></li> - <li><i>see also</i> <a href='#index-beta-rays'>β rays</a></li> - </ul> - </li> - <li class='c024'>Caves - <ul> - <li>radio-active matter present in air of, <a href='#Page_514'>514</a> <i>et seq.</i></li> - <li>radio-activity of air of, due to emanation from the soil, <a href='#Page_515'>515</a></li> - </ul> - </li> - <li class='c024'>Changes - <ul> - <li>(<i>see</i> <a href='#index-transformations'>Transformations</a>)</li> - </ul> - </li> - <li class='c024'>Charge - <ul> - <li>carried by the ions, <a href='#Page_50'>50</a> <i>et seq.</i></li> - <li>negative charge carried by β rays, <a href='#Page_120'>120</a></li> - <li>measurement of charge carried by β rays, <a href='#Page_121'>121</a> <i>et seq.</i></li> - <li>positive charge carried by α rays, <a href='#Page_145'>145</a></li> - <li>measurement of charge carried by α rays, <a href='#Page_151'>151</a> <i>et seq.</i></li> - </ul> - </li> - <li class='c024'>Chemical nature - <ul> - <li>of emanation, <a href='#Page_267'>267</a></li> - <li>of active deposit, <a href='#Page_312'>312</a></li> - </ul> - </li> - <li class='c024'>Chemical actions of radium rays - <ul> - <li>production of ozone, <a href='#Page_213'>213</a></li> - <li>coloration of glass and rock-salt, <a href='#Page_213'>213</a></li> - <li>on phosphorus, <a href='#Page_214'>214</a></li> - <li>on iodoform, <a href='#Page_214'>214</a></li> - <li>on globulin, <a href='#Page_214'>214</a></li> - <li>evolution of hydrogen and oxygen, <a href='#Page_215'>215</a></li> - </ul> - </li> - <li class='c024'>Child - <ul> - <li>potential gradient between electrodes, <a href='#Page_65'>65</a></li> - <li>variation of current with voltage for surface ionization, <a href='#Page_66'>66</a></li> - </ul> - </li> - <li class='c024'>Clouds - <ul> - <li>formation of, by condensation of water round ions, <a href='#Page_46'>46</a> <i>et seq.</i></li> - <li>difference between positive and negative ions in formation of, <a href='#Page_49'>49</a></li> - </ul> - </li> - <li class='c024'>Collie and Ramsay - <ul> - <li>spectrum of emanation, <a href='#Page_292'>292</a></li> - </ul> - </li> - <li class='c024'>Collision - <ul> - <li>ionization by, <a href='#Page_39'>39</a>, <a href='#Page_57'>57</a></li> - <li>number of ions produced by β rays per cm. of path, <a href='#Page_434'>434</a></li> - <li>total number of ions produced by collisions of α particles, <a href='#Page_434'>434</a></li> - </ul> - </li> - <li class='c024'>Coloration - <ul> - <li>of crystals of radiferous barium, <a href='#Page_15'>15</a></li> - <li>of bunsen flame by radium, <a href='#Page_15'>15</a></li> - <li>of glass by radium rays, <a href='#Page_213'>213</a></li> - <li>of rock-salt, fluor-spar and potassium sulphate by radium rays, <a href='#Page_213'>213</a></li> - </ul> - </li> - <li class='c024'>Concentration - <ul> - <li>of excited activity on negative electrode, <a href='#Page_297'>297</a></li> - <li>activity of radium independent of, <a href='#Page_466'>466</a></li> - </ul> - </li> - <li class='c024'>Condensation - <ul> - <li>of water round the ions, <a href='#Page_46'>46</a> <i>et seq.</i></li> - <li>of emanations, <a href='#Page_277'>277</a></li> - <li>experimental illustration of, <a href='#Page_279'>279</a></li> - <li>temperature of, <a href='#Page_280'>280</a></li> - <li>difference between point of, for emanations of thorium and radium, <a href='#Page_283'>283</a></li> - <li>from air sucked up from the earth, <a href='#Page_510'>510</a></li> - </ul> - </li> - <li class='c024'>Conductivity - <ul> - <li>of gases exposed to radiations, <a href='#Page_31'>31</a> <i>et seq.</i></li> - <li>variation of, with pressure, <a href='#Page_61'>61</a> <i>et seq.</i></li> - <li>variation of, with nature of gas, <a href='#Page_64'>64</a></li> - <li>comparison of, for gases exposed to α, β, and γ rays, <a href='#Page_64'>64</a></li> - <li><span class='pageno' id='Page_563'>563</span>comparison of, when exposed to γ rays and to hard X rays, <a href='#Page_184'>184</a></li> - <li>of insulators, <a href='#Page_209'>209</a></li> - <li>of air in caves and cellars, <a href='#Page_507'>507</a> <i>et seq.</i></li> - <li>of air in closed vessels, <a href='#Page_531'>531</a> <i>et seq.</i></li> - <li>variation of, in closed vessels with pressure and nature of gas, <a href='#Page_534'>534</a></li> - <li>variation of, with temperature for air in closed vessels, <a href='#Page_536'>536</a></li> - <li>increase of, with time, in a closed vessel, <a href='#Page_537'>537</a></li> - </ul> - </li> - <li class='c024'>Conservation of radio-activity - <ul> - <li>examples of, <a href='#Page_469'>469</a> <i>et seq.</i></li> - </ul> - </li> - <li class='c024'>Cooke, H. L. - <ul> - <li>penetrating rays from the earth, <a href='#Page_520'>520</a></li> - <li>number of ions per c.c. in closed vessels, <a href='#Page_534'>534</a></li> - <li>radio-activity from ordinary matter, <a href='#Page_536'>536</a></li> - </ul> - </li> - <li class='c024'>Cooke, W. T. and Ramsay - <ul> - <li>radio-activity produced by radiations of radium, <a href='#Page_472'>472</a></li> - </ul> - </li> - <li class='c024'>Corpuscle - <ul> - <li>(<i>see</i> <a href='#index-electron'>Electron</a>)</li> - </ul> - </li> - <li class='c024'>Crookes, Sir W. - <ul> - <li>spectrum of radium, <a href='#Page_17'>17</a></li> - <li>spectrum of polonium, <a href='#Page_23'>23</a></li> - <li>nature of cathode rays, <a href='#Page_73'>73</a></li> - <li>nature of α rays, <a href='#Page_142'>142</a></li> - <li>scintillations produced by radium, <a href='#Page_158'>158</a></li> - <li>spinthariscope, <a href='#Page_158'>158</a></li> - <li>number of scintillations independent of pressure and temperature, <a href='#Page_159'>159</a></li> - <li>phosphorescence of diamond, <a href='#Page_204'>204</a></li> - <li>separation of Ur X, <a href='#Page_219'>219</a></li> - <li>theory of radio-activity, <a href='#Page_441'>441</a></li> - </ul> - </li> - <li class='c024'>Crookes and Dewar - <ul> - <li>absence of nitrogen spectrum in phosphorescent light of radium at low pressures, <a href='#Page_206'>206</a></li> - </ul> - </li> - <li class='c024'>Crystallization - <ul> - <li>effect of, on activity of uranium, <a href='#Page_349'>349</a></li> - </ul> - </li> - <li class='c024'>Curie, Mme - <ul> - <li>permanence of uranium rays, <a href='#Page_6'>6</a></li> - <li>discovery of radio-activity of thorium, <a href='#Page_10'>10</a></li> - <li>radio-activity of uranium and thorium minerals, <a href='#Page_11'>11</a></li> - <li>relative activity of compounds of uranium, <a href='#Page_12'>12</a></li> - <li>coloration of radium crystals, <a href='#Page_15'>15</a></li> - <li>spectrum of radium, <a href='#Page_16'>16</a></li> - <li>discovery of polonium, <a href='#Page_22'>22</a></li> - <li>nature of a rays, <a href='#Page_142'>142</a></li> - <li>absorption of α rays from polonium, <a href='#Page_163'>163</a></li> - <li>secondary radiation tested by electric method, <a href='#Page_188'>188</a></li> - <li>slowly decaying excited activity from radium, <a href='#Page_311'>311</a></li> - <li>recovery of activity of radium, <a href='#Page_375'>375</a></li> - <li>bismuth made active by solution of barium, <a href='#Page_417'>417</a></li> - </ul> - </li> - <li class='c024'>Curie, P. - <ul> - <li>magnetic deviation of radium rays by electric method, <a href='#Page_114'>114</a></li> - <li>conductivity of dielectrics under radium rays, <a href='#Page_209'>209</a></li> - <li>radio-activity of radium unaffected by temperature, <a href='#Page_210'>210</a></li> - <li>decay of activity of radium emanation, <a href='#Page_247'>247</a></li> - <li>discovery of excited radio-activity from radium, <a href='#Page_295'>295</a></li> - <li>heat emission of radium at low temperature and variation of heat emission with age of radium, <a href='#Page_419'>419</a></li> - <li>nature of the emanation, <a href='#Page_439'>439</a></li> - </ul> - </li> - <li class='c024'>Curie, M. et Mme - <ul> - <li>discovery of radium, <a href='#Page_13'>13</a></li> - <li>charge carried by β rays, <a href='#Page_121'>121</a></li> - <li>luminosity of radium compounds, <a href='#Page_205'>205</a></li> - <li>production of ozone by radium rays, <a href='#Page_213'>213</a></li> - <li>coloration of glass by radium rays, <a href='#Page_213'>213</a></li> - <li>theory of radio-activity, <a href='#Page_439'>439</a></li> - <li>possible absorption by radio-elements of unknown radiations, <a href='#Page_442'>442</a></li> - </ul> - </li> - <li class='c024'>Curie, J. et P. - <ul> - <li>quartz piezo-électrique, <a href='#Page_105'>105</a> <i>et seq.</i></li> - </ul> - </li> - <li class='c024'>Curie, P. et Danne - <ul> - <li>diffusion of radio-active emanation, <a href='#Page_272'>272</a></li> - <li>decay of excited activity from radium, <a href='#Page_309'>309</a></li> - <li>decay curves of radium and equation, <a href='#Page_309'>309</a></li> - <li>occlusion of radium emanation in solids, <a href='#Page_310'>310</a></li> - <li>changes in radium, <a href='#Page_381'>381</a></li> - <li>effect of temperature on active deposit, <a href='#Page_390'>390</a></li> - </ul> - </li> - <li class='c024'>Curie, P. and Debierne - <ul> - <li>evolution of gas from radium, <a href='#Page_215'>215</a></li> - <li>active gases evolved from radium, <a href='#Page_251'>251</a></li> - <li>phosphorescence produced by radium emanation, <a href='#Page_252'>252</a></li> - <li>distribution of luminosity, <a href='#Page_252'>252</a></li> - <li>rate of production of emanation independent of pressure, <a href='#Page_266'>266</a></li> - <li>effect of pressure on amount of excited activity, <a href='#Page_266'>266</a>, <a href='#Page_317'>317</a></li> - </ul> - </li> - <li class='c024'>Curie and Dewar - <ul> - <li>production of helium by radium, <a href='#Page_479'>479</a></li> - </ul> - </li> - <li class='c024'>Curie, P. and Laborde - <ul> - <li>heat emission of radium, <a href='#Page_419'>419</a></li> - <li><span class='pageno' id='Page_564'>564</span>origin of heat from radium, <a href='#Page_440'>440</a></li> - <li>radium emanation in waters of hot springs, <a href='#Page_514'>514</a></li> - </ul> - </li> - <li class='c024'>Current - <ul> - <li>through gases, <a href='#Page_31'>31</a> <i>et seq.</i></li> - <li>variation of, with distance between the plates, <a href='#Page_59'>59</a> <i>et seq.</i></li> - <li>variation of, with pressure of gas, <a href='#Page_61'>61</a> <i>et seq.</i></li> - <li>variation of, with nature of gas, <a href='#Page_64'>64</a></li> - <li>measurement of, by galvanometer, <a href='#Page_84'>84</a></li> - <li>measurement of, by electroscope, <a href='#Page_85'>85</a> <i>et seq.</i></li> - <li>measurement of, by electrometer, <a href='#Page_90'>90</a> <i>et seq.</i></li> - <li>measurement of, by quartz piezo-électrique, <a href='#Page_105'>105</a></li> - </ul> - </li> - <li class='c003'>Dadourian - <ul> - <li>presence of thorium emanation in the earth, <a href='#Page_512'>512</a></li> - </ul> - </li> - <li class='c024'>Danne - <ul> - <li>on deposit of radium not containing uranium, <a href='#Page_465'>465</a></li> - </ul> - </li> - <li class='c024'>Danne et Curie - <ul> - <li>diffusion of radio-active emanation, <a href='#Page_272'>272</a></li> - <li>decay of excited activity from radium, <a href='#Page_309'>309</a></li> - <li>decay curves of radium and equation, <a href='#Page_309'>309</a></li> - <li>occlusion of radium emanation in solids, <a href='#Page_310'>310</a></li> - <li>changes in radium, <a href='#Page_381'>381</a></li> - <li>effect of temperature on active deposit, <a href='#Page_390'>390</a></li> - </ul> - </li> - <li class='c024'>Danysz - <ul> - <li>action of radium rays on skin, <a href='#Page_216'>216</a></li> - </ul> - </li> - <li class='c024'>Darwin, G. H. - <ul> - <li>age of sun, <a href='#Page_492'>492</a></li> - </ul> - </li> - <li class='c024'>Debierne - <ul> - <li>actinium, <a href='#Page_21'>21</a></li> - <li>emanation from actinium, <a href='#Page_249'>249</a></li> - <li>decay of excited activity from actinium, <a href='#Page_311'>311</a></li> - <li>effect of magnetic field on activity excited from actinium, <a href='#Page_324'>324</a></li> - <li>barium made active by actinium, <a href='#Page_417'>417</a></li> - </ul> - </li> - <li class='c024'>Debierne and Curie - <ul> - <li>evolution of gas from radium, <a href='#Page_215'>215</a></li> - <li>active gases evolved from radium, <a href='#Page_251'>251</a></li> - <li>phosphorescence produced by radium emanation, <a href='#Page_252'>252</a></li> - <li>distribution of luminosity, <a href='#Page_252'>252</a></li> - <li>rate of production of emanation independent of pressure, <a href='#Page_266'>266</a></li> - <li>effect of pressure on amount of excited activity, <a href='#Page_266'>266</a>, <a href='#Page_317'>317</a></li> - </ul> - </li> - <li class='c024'>Decay - <ul> - <li>of activity of Th X, <a href='#Page_221'>221</a></li> - <li>of activity of Ur X, <a href='#Page_223'>223</a></li> - <li>significance of law of, <a href='#Page_229'>229</a></li> - <li>effect of conditions on the rate of, <a href='#Page_232'>232</a></li> - <li>of activity of thorium emanation, <a href='#Page_241'>241</a></li> - <li>of activity of radium emanation, <a href='#Page_247'>247</a></li> - <li>of activity of actinium emanation, <a href='#Page_249'>249</a></li> - <li>of excited activity due to thorium for long exposure, <a href='#Page_302'>302</a></li> - <li>of excited activity due to thorium for short exposure, <a href='#Page_304'>304</a></li> - <li>of excited activity due to radium, <a href='#Page_306'>306</a> <i>et seq.</i></li> - <li>excited activity of slow decay due to radium, <a href='#Page_311'>311</a></li> - <li>of excited activity from actinium, <a href='#Page_311'>311</a></li> - <li>of radium A, B and C, <a href='#Page_377'>377</a> <i>et seq.</i></li> - <li>of radium D, E and F, <a href='#Page_397'>397</a> <i>et seq.</i></li> - <li>of heating effect of emanation, <a href='#Page_423'>423</a></li> - <li>of excited activity from atmosphere, <a href='#Page_502'>502</a></li> - <li>of activity of rain and snow, <a href='#Page_506'>506</a></li> - <li>of emanation from the earth, <a href='#Page_508'>508</a></li> - <li>differences in, of excited activity from atmosphere, <a href='#Page_521'>521</a> <i>et seq.</i></li> - </ul> - </li> - <li class='c024'>Demarçay - <ul> - <li>spectrum of radium, <a href='#Page_16'>16</a></li> - </ul> - </li> - <li class='c024'>Deposit, active - <ul> - <li>connection of, with excited activity, <a href='#Page_301'>301</a></li> - <li>physical and chemical properties of, <a href='#Page_312'>312</a></li> - <li>electrolysis of, <a href='#Page_313'>313</a></li> - <li>effect of temperature on, <a href='#Page_315'>315</a></li> - <li>effect of pressure on distribution of, <a href='#Page_317'>317</a></li> - <li>transmission of, by positive carriers, <a href='#Page_318'>318</a> <i>et seq.</i></li> - <li>nomenclature of, <a href='#Page_328'>328</a></li> - <li>theory of changes in, <a href='#Page_331'>331</a> <i>et seq.</i></li> - <li>theory of activity due to, <a href='#Page_337'>337</a> <i>et seq.</i></li> - <li>theory of rayless change in, <a href='#Page_341'>341</a> <i>et seq.</i></li> - <li>of thorium, <a href='#Page_302'>302</a> <i>et seq.</i>, <a href='#Page_351'>351</a> <i>et seq.</i> - <ul> - <li>analysis of, <a href='#Page_351'>351</a></li> - <li>rayless change in, <a href='#Page_352'>352</a></li> - <li>effect of temperature on, <a href='#Page_354'>354</a></li> - <li>period of products of, <a href='#Page_355'>355</a></li> - </ul> - </li> - <li>of actinium, <a href='#Page_311'>311</a> <i>et seq.</i> - <ul> - <li>decay curves of, <a href='#Page_311'>311</a></li> - <li>analysis of, <a href='#Page_367'>367</a></li> - <li>rayless change in, <a href='#Page_367'>367</a></li> - <li>period of products of, <a href='#Page_368'>368</a></li> - <li>radiations from, <a href='#Page_368'>368</a></li> - </ul> - </li> - <li>of radium, <a href='#Page_376'>376</a> <i>et seq.</i> - <ul> - <li>connection of excited activity with, <a href='#Page_306'>306</a></li> - <li>general analysis of, <a href='#Page_376'>376</a> <i>et seq.</i></li> - <li>analysis of, of rapid change, <a href='#Page_377'>377</a> <i>et seq.</i></li> - <li><span class='pageno' id='Page_565'>565</span>analysis of α ray curves, <a href='#Page_377'>377</a></li> - <li>α ray curves of, <a href='#Page_378'>378</a></li> - <li>β ray curves of, <a href='#Page_379'>379</a></li> - <li>analysis of β ray curves, <a href='#Page_381'>381</a></li> - <li>equations of activity curves, <a href='#Page_389'>389</a></li> - <li>effect of temperature on, <a href='#Page_390'>390</a></li> - <li>volatility of, <a href='#Page_391'>391</a></li> - <li>of slow transformation, <a href='#Page_311'>311</a>, <a href='#Page_397'>397</a></li> - <li>variation of α ray activity of, <a href='#Page_398'>398</a></li> - <li>variation of β ray activity of, <a href='#Page_399'>399</a></li> - <li>separation of constituents of, <a href='#Page_401'>401</a> <i>et seq.</i></li> - <li>successive products in, <a href='#Page_402'>402</a></li> - <li>variation of activity of, for long periods, <a href='#Page_407'>407</a></li> - <li>presence in old radium, <a href='#Page_408'>408</a></li> - <li>effect of, on variation of activity of radium with time, <a href='#Page_409'>409</a></li> - <li>presence in pitchblende, <a href='#Page_410'>410</a></li> - <li>connection with radio-tellurium, <a href='#Page_411'>411</a></li> - <li>connection with polonium, <a href='#Page_411'>411</a>, <a href='#Page_412'>412</a></li> - <li>connection with radio-lead, <a href='#Page_413'>413</a></li> - <li>connection of, with radio-active induction, <a href='#Page_415'>415</a> <i>et seq.</i></li> - <li>heat emission of, <a href='#Page_425'>425</a> <i>et seq.</i></li> - <li>use of, to determine number of β particles from radium, <a href='#Page_435'>435</a></li> - <li>use of, as source of α rays, <a href='#Page_543'>543</a></li> - </ul> - </li> - </ul> - </li> - <li class='c024'>Des Coudres - <ul> - <li>magnetic and electric deviation of α rays, <a href='#Page_148'>148</a></li> - <li>determination of <i>e</i>/<i>m</i> for α rays, <a href='#Page_148'>148</a></li> - </ul> - </li> - <li class='c024'>Dewar - <ul> - <li>emission of heat from radium in liquid hydrogen, <a href='#Page_420'>420</a></li> - </ul> - </li> - <li class='c024'>Dewar and Crookes - <ul> - <li>absence of nitrogen spectrum in phosphorescent light of radium at low pressures, <a href='#Page_206'>206</a></li> - </ul> - </li> - <li class='c024'>Dewar and Curie - <ul> - <li>production of helium by radium, <a href='#Page_479'>479</a></li> - </ul> - </li> - <li class='c024'>Dielectrics - <ul> - <li>condition of, under radium rays, <a href='#Page_209'>209</a></li> - </ul> - </li> - <li class='c024'>Diffusion - <ul> - <li>of ions, <a href='#Page_51'>51</a> <i>et seq.</i></li> - <li>of radium emanation into gases, <a href='#Page_270'>270</a></li> - <li>of thorium emanation into gases, <a href='#Page_275'>275</a></li> - <li>of radium emanation into liquids, <a href='#Page_276'>276</a></li> - </ul> - </li> - <li class='c024'>Discharge - <ul> - <li>action of rays on spark and electrodeless, <a href='#Page_208'>208</a></li> - </ul> - </li> - <li class='c024'>Disintegration - <ul> - <li>account of theory of, <a href='#Page_234'>234</a>, <a href='#Page_325'>325</a>, <a href='#Page_445'>445</a></li> - <li>list of products of, <a href='#Page_448'>448</a></li> - <li>rate of, in radio-elements, <a href='#Page_457'>457</a></li> - <li>emission of energy in consequence of, <a href='#Page_474'>474</a> <i>et seq.</i></li> - <li>helium a product of, <a href='#Page_476'>476</a> <i>et seq.</i></li> - <li>possible causes of, <a href='#Page_486'>486</a> <i>et seq.</i></li> - <li>of matter in general, <a href='#Page_496'>496</a> <i>et seq.</i></li> - </ul> - </li> - <li class='c024'>Dissipation of charge - <ul> - <li>in caves and cellars, <a href='#Page_514'>514</a> <i>et seq.</i></li> - <li>in closed vessels, <a href='#Page_531'>531</a></li> - <li>effect of pressure and nature of gas on, <a href='#Page_534'>534</a> <i>et seq.</i></li> - <li>effect of material of vessel on, <a href='#Page_536'>536</a> <i>et seq.</i></li> - </ul> - </li> - <li class='c024'>Dolezalek - <ul> - <li>electrometer, construction of, <a href='#Page_94'>94</a> <i>et seq.</i></li> - </ul> - </li> - <li class='c024'>Dorn - <ul> - <li>charge carried by β rays, <a href='#Page_122'>122</a></li> - <li>electrostatic deflection of β rays from radium, <a href='#Page_124'>124</a></li> - <li>discovery of radium emanation, <a href='#Page_246'>246</a></li> - <li>effect of moisture on emanating power of thorium, <a href='#Page_255'>255</a></li> - <li>electrolysis of radium solution, <a href='#Page_313'>313</a></li> - <li>loss of weight of radium, <a href='#Page_474'>474</a></li> - <li>radium emanation in springs, <a href='#Page_513'>513</a></li> - </ul> - </li> - <li class='c024'>Dreyer and Salomonsen - <ul> - <li>coloration of quartz by radium rays, <a href='#Page_213'>213</a></li> - </ul> - </li> - <li class='c024'>Dunston - <ul> - <li>analysis of thorianite, <a href='#Page_486'>486</a></li> - </ul> - </li> - <li class='c024'>Durack - <ul> - <li>ionization by collision of electrons of great velocity, <a href='#Page_171'>171</a></li> - </ul> - </li> - <li class='c024'>Dust - <ul> - <li>effect of, on recombination of ions, <a href='#Page_42'>42</a></li> - <li>effect of, on distribution of excited activity, <a href='#Page_305'>305</a></li> - </ul> - </li> - <li class='c003'>Earth - <ul> - <li>amount of radium in, <a href='#Page_493'>493</a> <i>et seq.</i></li> - <li>age of, <a href='#Page_496'>496</a></li> - <li>excited activity deposited on, <a href='#Page_504'>504</a></li> - <li>activity concentrated on peaks of, <a href='#Page_504'>504</a></li> - <li>emanation from, <a href='#Page_507'>507</a></li> - <li>very penetrating radiation from, <a href='#Page_520'>520</a></li> - </ul> - </li> - <li class='c024'>Ebert - <ul> - <li>condensation of emanation from the earth, <a href='#Page_510'>510</a></li> - <li>apparatus for determining number of ions per c.c. in air, <a href='#Page_527'>527</a></li> - <li>velocity of ions in air, <a href='#Page_528'>528</a></li> - </ul> - </li> - <li class='c024'>Ebert and Ewers - <ul> - <li>emanation from the earth, <a href='#Page_508'>508</a></li> - </ul> - </li> - <li class='c024'>Electrolysis - <ul> - <li>separation of radio-tellurium by, <a href='#Page_25'>25</a></li> - <li>of solutions of active deposit, <a href='#Page_313'>313</a></li> - <li>of radium solutions, <a href='#Page_313'>313</a></li> - <li>of thorium solutions, <a href='#Page_314'>314</a></li> - </ul> - </li> - <li class='c024'>Electrometer - <ul> - <li>description of, <a href='#Page_90'>90</a> <i>et seq.</i></li> - <li>use of, in measurements, <a href='#Page_90'>90</a></li> - <li>construction of, <a href='#Page_91'>91</a> <i>et seq.</i></li> - <li>Dolezalek, <a href='#Page_94'>94</a></li> - <li><span class='pageno' id='Page_566'>566</span>adjustment and screening of, <a href='#Page_95'>95</a></li> - <li>special key for, <a href='#Page_97'>97</a></li> - <li>application of, to measurements of radio-activity, <a href='#Page_97'>97</a> <i>et seq.</i></li> - <li>measurement of current by, <a href='#Page_100'>100</a></li> - <li>capacity of, <a href='#Page_101'>101</a></li> - <li>use with steady deflection, <a href='#Page_103'>103</a></li> - <li>use with quartz piezo-électrique, <a href='#Page_105'>105</a></li> - </ul> - </li> - <li class='c024'><a id='index-electron'></a></li> - <li class='c024'>Electron - <ul> - <li>definition of, <a href='#Page_56'>56</a></li> - <li>production of, under different conditions, <a href='#Page_76'>76</a> <i>et seq.</i></li> - <li>identity of β rays with electrons, <a href='#Page_120'>120</a> <i>et seq.</i></li> - <li>variation of apparent mass of electron with velocity, <a href='#Page_127'>127</a> <i>et seq.</i></li> - <li>evidence that mass of electron is electromagnetic, <a href='#Page_129'>129</a> <i>et seq.</i></li> - <li>diameter of, <a href='#Page_131'>131</a></li> - </ul> - </li> - <li class='c024'>Electroscope - <ul> - <li>description of, used by Curie, <a href='#Page_85'>85</a></li> - <li>construction of, for accurate measurements, <a href='#Page_86'>86</a></li> - <li>use of, in measurements of minute currents, <a href='#Page_86'>86</a></li> - <li>of C. T. R. Wilson, <a href='#Page_89'>89</a></li> - <li>use of, in measuring conductivity of air in closed vessels, <a href='#Page_531'>531</a> <i>et seq.</i></li> - <li>use of, for determining radio-activity of ordinary matter, <a href='#Page_537'>537</a></li> - </ul> - </li> - <li class='c024'>Elster and Geitel - <ul> - <li>radio-active lead, <a href='#Page_27'>27</a></li> - <li>effect of magnetic field on conductivity produced in air by β rays, <a href='#Page_113'>113</a></li> - <li>scintillations produced by active substances, <a href='#Page_158'>158</a></li> - <li>action of radium rays on spark, <a href='#Page_208'>208</a></li> - <li>photo-electric action of body, coloured by radium rays, <a href='#Page_214'>214</a></li> - <li>radio-active matter in earth, <a href='#Page_494'>494</a></li> - <li>discovery of excited activity in atmosphere, <a href='#Page_501'>501</a></li> - <li>emanations from the earth, <a href='#Page_507'>507</a></li> - <li>radio-activity of air in caves, <a href='#Page_507'>507</a></li> - <li>radio-activity of the soil, <a href='#Page_515'>515</a></li> - <li>radio-activity of fango, <a href='#Page_516'>516</a></li> - <li>variation of radio-activity in atmosphere with meteorological conditions, <a href='#Page_517'>517</a></li> - <li>effect of temperature and pressure on atmospheric radio-activity, <a href='#Page_518'>518</a></li> - </ul> - </li> - <li class='c024'>Emanation - <ul> - <li>of thorium, discovery and properties of, <a href='#Page_238'>238</a></li> - <li>methods of measurement of, <a href='#Page_240'>240</a></li> - <li>decay of activity of, <a href='#Page_241'>241</a></li> - <li>effect of thickness of layer on amount of, <a href='#Page_243'>243</a></li> - <li>increase of, with time to a maximum, <a href='#Page_245'>245</a></li> - <li>of radium, <a href='#Page_246'>246</a></li> - <li>decay of activity of, <a href='#Page_247'>247</a></li> - <li>of actinium, properties of, <a href='#Page_249'>249</a></li> - <li>of radium, phosphorescence produced by, <a href='#Page_251'>251</a></li> - <li>rate of emission of, <a href='#Page_254'>254</a></li> - <li>effect of conditions on rate of emission of, <a href='#Page_255'>255</a></li> - <li>regeneration of emanating power, <a href='#Page_256'>256</a></li> - <li>continuous rate of production of, <a href='#Page_257'>257</a></li> - <li>source of thorium emanation, <a href='#Page_261'>261</a></li> - <li>source of radium and actinium emanation, <a href='#Page_263'>263</a></li> - <li>radiations from, <a href='#Page_263'>263</a></li> - <li>effect of pressure on production of, <a href='#Page_265'>265</a></li> - <li>chemical nature of, <a href='#Page_267'>267</a></li> - <li>experiments to illustrate gaseous nature of, <a href='#Page_268'>268</a></li> - <li>rate of diffusion of radium emanation, <a href='#Page_269'>269</a></li> - <li>rate of diffusion of thorium emanation, <a href='#Page_275'>275</a></li> - <li>diffusion of, into liquids, <a href='#Page_276'>276</a></li> - <li>condensation of, <a href='#Page_277'>277</a></li> - <li>temperature of condensation of, <a href='#Page_280'>280</a></li> - <li>volume of, from one gram of radium and thorium, <a href='#Page_288'>288</a></li> - <li>measurement of volume of, from radium, <a href='#Page_289'>289</a></li> - <li>diminution of volume of, <a href='#Page_290'>290</a></li> - <li>spectrum of emanation, <a href='#Page_292'>292</a></li> - <li>connection between emanation and excited activity, <a href='#Page_298'>298</a></li> - <li>effect of removal of, on activity of radium, <a href='#Page_371'>371</a> <i>et seq.</i></li> - <li>fraction of activity of radium due to, <a href='#Page_374'>374</a></li> - <li>effect of rate of escape of, on activity of radium, <a href='#Page_374'>374</a></li> - <li>heat emission of, <a href='#Page_420'>420</a>, <a href='#Page_431'>431</a></li> - <li>variation of heat emission with time, <a href='#Page_421'>421</a> <i>et seq.</i></li> - <li>enormous emission of energy from emanation, <a href='#Page_431'>431</a></li> - <li>radio-activity of atmosphere due to emanations, <a href='#Page_504'>504</a></li> - <li>sucked up from the earth, <a href='#Page_507'>507</a></li> - <li>in caves, <a href='#Page_507'>507</a> <i>et seq.</i></li> - <li>rate of decay of activity of, from the earth, <a href='#Page_508'>508</a></li> - <li>condensation of, from the atmosphere, <a href='#Page_510'>510</a></li> - <li>in well water and springs, <a href='#Page_510'>510</a> <i>et seq.</i></li> - <li>from “fango,” <a href='#Page_516'>516</a></li> - <li>effect of meteorological conditions on amount of, in atmosphere, <a href='#Page_517'>517</a> <i>et seq.</i></li> - <li><span class='pageno' id='Page_567'>567</span>from metals, <a href='#Page_538'>538</a></li> - </ul> - </li> - <li class='c024'>Emanating power - <ul> - <li>measurement of, <a href='#Page_254'>254</a></li> - <li>effect of conditions on, <a href='#Page_255'>255</a></li> - <li>regeneration of, <a href='#Page_256'>256</a></li> - </ul> - </li> - <li class='c024'>Emanium or “emanating substance” of Giesel (<i>see</i> <a href='#index-actinium'>Actinium</a>) - <ul> - <li>discovery of, <a href='#Page_21'>21</a></li> - <li>separation and properties of, <a href='#Page_21'>21</a></li> - <li>similarity of, to actinium, <a href='#Page_21'>21</a></li> - <li>emanation from, <a href='#Page_249'>249</a></li> - <li>excited activity produced by, <a href='#Page_311'>311</a></li> - <li>action of an electric field on, <a href='#Page_323'>323</a></li> - </ul> - </li> - <li class='c024'>Energy - <ul> - <li>of α particle, <a href='#Page_156'>156</a></li> - <li>of β particle, <a href='#Page_196'>196</a></li> - <li>comparison of, for α and β particles, <a href='#Page_196'>196</a></li> - <li>emitted from radium in form of heat, <a href='#Page_419'>419</a> <i>et seq.</i></li> - <li>emission of, from the emanation, <a href='#Page_431'>431</a></li> - <li>emission of, from radio-active products of radium, <a href='#Page_433'>433</a></li> - <li>total emission of, from 1 gram of radio-elements, <a href='#Page_474'>474</a> <i>et seq.</i></li> - <li>latent store of, in matter, <a href='#Page_475'>475</a></li> - </ul> - </li> - <li class='c024'>Eve - <ul> - <li>conductivity of gases exposed to X rays, <a href='#Page_64'>64</a></li> - <li>conductivity of gases exposed to X rays and γ rays, <a href='#Page_183'>183</a>, <a href='#Page_184'>184</a></li> - <li>secondary rays produced by β and γ rays, <a href='#Page_189'>189</a> <i>et seq.</i></li> - <li>magnetic deflection of secondary rays from γ rays, <a href='#Page_193'>193</a></li> - <li>variation of activity of radium with concentration, <a href='#Page_467'>467</a></li> - <li>amount of radium emanation in the atmosphere, <a href='#Page_524'>524</a> <i>et seq.</i></li> - <li>ionization due to emanation in atmosphere, <a href='#Page_526'>526</a> <i>et seq.</i></li> - </ul> - </li> - <li class='c024'>Evolution of matter - <ul> - <li>evidence of, <a href='#Page_497'>497</a></li> - </ul> - </li> - <li class='c024'>Ewers and Ebert - <ul> - <li>emanation from the earth, <a href='#Page_508'>508</a></li> - </ul> - </li> - <li class='c024'><a id='index-excited-radio-activity'></a></li> - <li class='c024'>Excited radio-activity - <ul> - <li>discovery and properties of, <a href='#Page_295'>295</a> <i>et seq.</i></li> - <li>concentration of, on negative electrode, <a href='#Page_297'>297</a></li> - <li>connection of, with the emanations, <a href='#Page_298'>298</a></li> - <li>removal of, by acids, <a href='#Page_300'>300</a></li> - <li>decay of, due to thorium, <a href='#Page_302'>302</a></li> - <li>decay of, for short exposure to thorium, <a href='#Page_304'>304</a></li> - <li>effect of dust on distribution of, <a href='#Page_305'>305</a></li> - <li>decay curves for different times of exposure, <a href='#Page_306'>306</a> <i>et seq.</i></li> - <li>decay of, from radium, <a href='#Page_306'>306</a></li> - <li>decay curves of, measured by α rays, <a href='#Page_308'>308</a></li> - <li>decay curves of, measured by β rays, <a href='#Page_309'>309</a></li> - <li>decay curves of, from actinium, <a href='#Page_311'>311</a></li> - <li>of radium, of very slow decay, <a href='#Page_311'>311</a></li> - <li>effect of solution on, <a href='#Page_312'>312</a></li> - <li>electrolysis of active solutions, <a href='#Page_313'>313</a></li> - <li>effect of temperature on, <a href='#Page_315'>315</a></li> - <li>variation with electric field, of amount of, <a href='#Page_316'>316</a></li> - <li>effect of pressure on distribution of, <a href='#Page_317'>317</a></li> - <li>transmission of, <a href='#Page_318'>318</a></li> - <li>from actinium and emanium, <a href='#Page_323'>323</a></li> - <li>heat emission due to, <a href='#Page_425'>425</a> <i>et seq.</i></li> - <li>from the atmosphere, <a href='#Page_501'>501</a> <i>et seq.</i></li> - <li>decay of, <a href='#Page_502'>502</a></li> - <li>due to emanation in atmosphere, <a href='#Page_504'>504</a></li> - <li>distribution of, on surface of the earth, <a href='#Page_504'>504</a></li> - <li>concentration of, on prominences of the earth, <a href='#Page_504'>504</a></li> - <li>of rain and snow, <a href='#Page_506'>506</a></li> - <li>produced by emanation from tap water, <a href='#Page_510'>510</a></li> - <li>effect of meteorological conditions on amount of, <a href='#Page_517'>517</a> <i>et seq.</i></li> - <li>amount of, at Niagara Falls, <a href='#Page_520'>520</a></li> - <li>rate of decay of, dependent on conditions, <a href='#Page_522'>522</a> <i>et seq.</i></li> - </ul> - </li> - <li class='c024'>Exner and Haschek - <ul> - <li>spectrum of radium, <a href='#Page_17'>17</a></li> - </ul> - </li> - <li class='c024'>Eye - <ul> - <li>action of radium rays on, <a href='#Page_217'>217</a></li> - </ul> - </li> - <li class='c003'>Fehrle - <ul> - <li>distribution of excited activity on a plate in electric field, <a href='#Page_318'>318</a></li> - </ul> - </li> - <li class='c024'>Fluorescence - <ul> - <li>produced in substances by radium rays, <a href='#Page_18'>18</a></li> - <li>produced in substances by radium and polonium rays, <a href='#Page_201'>201</a> <i>et seq.</i></li> - </ul> - </li> - <li class='c024'>Fog - <ul> - <li>large amount of excited activity during, <a href='#Page_518'>518</a></li> - </ul> - </li> - <li class='c024'>Forch - <ul> - <li>loss of weight of radium, <a href='#Page_474'>474</a></li> - </ul> - </li> - <li class='c003'>γ rays - <ul> - <li>relative conductivity of gas exposed to γ and hard X rays, <a href='#Page_64'>64</a>, <a href='#Page_184'>184</a></li> - <li>discovery of, <a href='#Page_179'>179</a></li> - <li>absorption of, by matter, <a href='#Page_179'>179</a> <i>et seq.</i></li> - <li>connection between absorption of, and density, <a href='#Page_182'>182</a></li> - <li>discussion of nature of rays, <a href='#Page_182'>182</a> <i>et seq.</i></li> - <li><span class='pageno' id='Page_568'>568</span>secondary rays produced by γ rays, <a href='#Page_189'>189</a></li> - <li>measurement of radio-activity by means of, <a href='#Page_442'>442</a>, <a href='#Page_467'>467</a></li> - <li>conservation of radio-activity measured by, <a href='#Page_471'>471</a></li> - </ul> - </li> - <li class='c024'>Gases - <ul> - <li>evolved by radium, <a href='#Page_215'>215</a></li> - <li>presence of helium in gases from radium, <a href='#Page_216'>216</a></li> - </ul> - </li> - <li class='c024'>Gates, Miss F. - <ul> - <li>effect of temperature on excited activity, <a href='#Page_315'>315</a></li> - <li>discharge of quinine sulphate, <a href='#Page_530'>530</a></li> - </ul> - </li> - <li class='c024'>Geitel - <ul> - <li>natural conductivity of air in closed vessels, <a href='#Page_501'>501</a>, <a href='#Page_531'>531</a></li> - </ul> - </li> - <li class='c024'>Geitel and Elster - <ul> - <li>radio-active lead, <a href='#Page_27'>27</a></li> - <li>effect of magnetic field on conductivity produced by radium rays, <a href='#Page_113'>113</a></li> - <li>scintillations produced by active substances, <a href='#Page_158'>158</a></li> - <li>action of radium rays on spark, <a href='#Page_208'>208</a></li> - <li>photo-electric action of bodies coloured by radium rays, <a href='#Page_214'>214</a></li> - <li>radio-active matter in earth, <a href='#Page_494'>494</a></li> - <li>discovery of radio-active matter in atmosphere, <a href='#Page_501'>501</a></li> - <li>emanations from the earth, <a href='#Page_507'>507</a></li> - <li>radio-activity of air in caves, <a href='#Page_507'>507</a></li> - <li>radio-activity of the soil, <a href='#Page_515'>515</a></li> - <li>radio-activity of fango, <a href='#Page_516'>516</a></li> - <li>variation of radio-activity of air with meteorological conditions, <a href='#Page_517'>517</a></li> - <li>effect of temperature and pressure on radio-activity in atmosphere, <a href='#Page_518'>518</a></li> - </ul> - </li> - <li class='c024'>Giesel - <ul> - <li>coloration of bunsen flame by radium, <a href='#Page_15'>15</a></li> - <li>separation of radium by crystallization of bromide, <a href='#Page_15'>15</a></li> - <li>emanating substance, <a href='#Page_21'>21</a></li> - <li>radio-active lead, <a href='#Page_27'>27</a></li> - <li>magnetic deviation of β rays, <a href='#Page_113'>113</a></li> - <li>decrease with time of luminosity of radio-active screen, <a href='#Page_205'>205</a></li> - <li>spectrum of phosphorescent light of emanium due to didymium, <a href='#Page_206'>206</a>, <a href='#Page_207'>207</a></li> - <li>coloration of bodies by radium rays, <a href='#Page_213'>213</a></li> - <li>evolution of gases from radium, <a href='#Page_215'>215</a></li> - <li>action of radium rays on the eye, <a href='#Page_217'>217</a></li> - <li>emanation from the emanating substance, <a href='#Page_250'>250</a></li> - <li>luminosity produced by radium emanation, <a href='#Page_251'>251</a></li> - <li>decay of excited activity of emanium, <a href='#Page_312'>312</a></li> - <li>activity of radium dependent on age, <a href='#Page_371'>371</a></li> - <li>bismuth made active by radio-active solution, <a href='#Page_417'>417</a></li> - <li>temperature of radium bromide above air, <a href='#Page_420'>420</a></li> - </ul> - </li> - <li class='c024'>Gimingham and Rossignol - <ul> - <li>decay of thorium emanation, <a href='#Page_242'>242</a></li> - </ul> - </li> - <li class='c024'>Glass - <ul> - <li>coloration produced in, by radium rays, <a href='#Page_213'>213</a></li> - <li>phosphorescence produced in, by emanation, <a href='#Page_252'>252</a></li> - </ul> - </li> - <li class='c024'>Glew - <ul> - <li>simple form of spinthariscope, and scintillations, <a href='#Page_159'>159</a></li> - </ul> - </li> - <li class='c024'>Globulin - <ul> - <li>action of radium rays on, <a href='#Page_214'>214</a></li> - </ul> - </li> - <li class='c024'>Godlewski - <ul> - <li>effect of crystallization on activity of uranium, <a href='#Page_349'>349</a></li> - <li>diffusion of uranium X, <a href='#Page_350'>350</a></li> - <li>separation of actinium X, <a href='#Page_365'>365</a></li> - <li>source of actinium emanation, <a href='#Page_365'>365</a></li> - <li>recovery and decay curves of actinium, <a href='#Page_366'>366</a></li> - <li>penetrating power of β and γ rays from actinium, <a href='#Page_368'>368</a></li> - <li>radiations from active products, <a href='#Page_368'>368</a></li> - </ul> - </li> - <li class='c024'>Goldstein - <ul> - <li>canal rays, <a href='#Page_78'>78</a></li> - <li>coloration of bodies by radium rays, <a href='#Page_213'>213</a></li> - </ul> - </li> - <li class='c024'>Gonder, Hofmann, and Wölfl - <ul> - <li>properties of radio-active lead, <a href='#Page_27'>27</a>, <a href='#Page_413'>413</a></li> - </ul> - </li> - <li class='c024'>Grier and Rutherford - <ul> - <li>magnetic deviation of β rays of thorium, <a href='#Page_114'>114</a></li> - <li>relative current due to α and β rays, <a href='#Page_195'>195</a></li> - <li>nature of rays from Ur X, <a href='#Page_347'>347</a></li> - </ul> - </li> - <li class='c003'>Hardy - <ul> - <li>coagulation of globulin by radium rays, <a href='#Page_214'>214</a></li> - </ul> - </li> - <li class='c024'>Hardy and Miss Willcock - <ul> - <li>coloration of iodoform solutions by radium rays, <a href='#Page_214'>214</a></li> - </ul> - </li> - <li class='c024'>Hardy and Anderson - <ul> - <li>action of radium rays on the eye, <a href='#Page_217'>217</a></li> - </ul> - </li> - <li class='c024'>Harms - <ul> - <li>number of ions per c.c. in closed vessel, <a href='#Page_534'>534</a></li> - </ul> - </li> - <li class='c024'>Hartmann - <ul> - <li>spectrum of phosphorescent light of emanium, <a href='#Page_206'>206</a></li> - </ul> - </li> - <li class='c024'><span class='pageno' id='Page_569'>569</span>Haschek and Exner - <ul> - <li>spectrum of radium, <a href='#Page_17'>17</a></li> - </ul> - </li> - <li class='c024'>Heat - <ul> - <li>rate of emission of, from radium, <a href='#Page_419'>419</a> <i>et seq.</i></li> - <li>emission of, from radium at low temperatures, <a href='#Page_420'>420</a></li> - <li>connection of heat emission with the radio-activity, <a href='#Page_421'>421</a> <i>et seq.</i></li> - <li>source of heat energy, <a href='#Page_421'>421</a> <i>et seq.</i></li> - <li>rate of emission of, after removal of the emanation, <a href='#Page_422'>422</a> <i>et seq.</i></li> - <li>rate of emission of, by emanation, <a href='#Page_423'>423</a>, <a href='#Page_431'>431</a></li> - <li>variation with time of heat emission of radium, and of its emanation, <a href='#Page_423'>423</a></li> - <li>heating effect of the emanation, <a href='#Page_423'>423</a>, <a href='#Page_431'>431</a></li> - <li>heating effect of active deposit, <a href='#Page_425'>425</a></li> - <li>proportion of heating effect due to radio-active products, <a href='#Page_433'>433</a></li> - <li>origin of, in radium, <a href='#Page_442'>442</a> <i>et seq.</i></li> - <li>total heat emission during life of radio-elements, <a href='#Page_474'>474</a> <i>et seq.</i></li> - <li>heating of earth by radio-active matter, <a href='#Page_493'>493</a></li> - </ul> - </li> - <li class='c024'>Heaviside - <ul> - <li>apparent mass of moving charged body, <a href='#Page_71'>71</a>, <a href='#Page_127'>127</a></li> - </ul> - </li> - <li class='c024'>Helium - <ul> - <li>produced by radium and its emanation, <a href='#Page_476'>476</a> <i>et seq.</i></li> - <li>amount of, from radium, <a href='#Page_480'>480</a></li> - <li>origin of, <a href='#Page_480'>480</a></li> - </ul> - </li> - <li class='c024'>Helmholtz and Richarz - <ul> - <li>action of ions on steam jet, <a href='#Page_47'>47</a></li> - </ul> - </li> - <li class='c024'>Hemptinne - <ul> - <li>action of rays on spark and electrodeless discharge, <a href='#Page_208'>208</a></li> - </ul> - </li> - <li class='c024'>Henning - <ul> - <li>resistance of radium solutions, <a href='#Page_208'>208</a></li> - <li>effect of voltage on amount of excited activity, <a href='#Page_316'>316</a></li> - </ul> - </li> - <li class='c024'>Henning and Kohlrausch - <ul> - <li>conductivity of solutions of radium bromide, <a href='#Page_208'>208</a></li> - </ul> - </li> - <li class='c024'>Hertz - <ul> - <li>electric deviation of cathode rays, <a href='#Page_73'>73</a></li> - </ul> - </li> - <li class='c024'>Heydweiler - <ul> - <li>loss of weight of radium, <a href='#Page_474'>474</a></li> - </ul> - </li> - <li class='c024'>Himstedt - <ul> - <li>action of radium rays on selenium, <a href='#Page_208'>208</a></li> - <li>radium emanation in springs of Baden, <a href='#Page_513'>513</a></li> - </ul> - </li> - <li class='c024'>Himstedt and Meyer - <ul> - <li>production of helium by radium, <a href='#Page_479'>479</a></li> - </ul> - </li> - <li class='c024'>Himstedt and Nagel - <ul> - <li>action of radium rays on the eye, <a href='#Page_217'>217</a></li> - </ul> - </li> - <li class='c024'>Hofmann, Gonder, and Wölfl - <ul> - <li>properties of radio-active lead, <a href='#Page_27'>27</a>, <a href='#Page_413'>413</a></li> - </ul> - </li> - <li class='c024'>Hofmann and Strauss - <ul> - <li>radio-active lead, <a href='#Page_27'>27</a></li> - </ul> - </li> - <li class='c024'>Hofmann and Zerban - <ul> - <li>connection of activity of thorium with uranium, <a href='#Page_29'>29</a></li> - </ul> - </li> - <li class='c024'>Huggins, Sir W. and Lady - <ul> - <li>spectrum of phosphorescent light of radium bromide, <a href='#Page_205'>205</a></li> - </ul> - </li> - <li class='c024'>Hydrogen - <ul> - <li>production of, by radium rays, <a href='#Page_215'>215</a></li> - </ul> - </li> - <li class='c024'>Induced radio-activity (<i>see</i> <a href='#index-excited-radio-activity'>Excited radio-activity</a>)</li> - <li class='c024'>Induction - <ul> - <li>radio-active, <a href='#Page_24'>24</a></li> - <li>meaning and examples of, <a href='#Page_415'>415</a> <i>et seq.</i></li> - </ul> - </li> - <li class='c024'>Insulators - <ul> - <li>conduction of, under radium rays, <a href='#Page_209'>209</a></li> - </ul> - </li> - <li class='c024'>Iodoform - <ul> - <li>coloration produced in, by radium rays, <a href='#Page_214'>214</a></li> - </ul> - </li> - <li class='c024'>Ionization - <ul> - <li>theory of, to explain conductivity of gases, <a href='#Page_31'>31</a> <i>et seq.</i></li> - <li>by collision, <a href='#Page_39'>39</a>, <a href='#Page_57'>57</a></li> - <li>variation of, with pressure of gas, <a href='#Page_61'>61</a> <i>et seq.</i></li> - <li>variation of, with nature of gas, <a href='#Page_64'>64</a></li> - <li>comparison of, produced by rays, <a href='#Page_111'>111</a>, <a href='#Page_194'>194</a></li> - <li>production of, in insulators, <a href='#Page_209'>209</a></li> - <li>total, produced by 1 gram of radium, <a href='#Page_433'>433</a> <i>et seq.</i></li> - <li>natural ionization of gases, <a href='#Page_531'>531</a> <i>et seq.</i></li> - <li>connection of, with phosphorescent and photographic actions, <a href='#Page_549'>549</a></li> - </ul> - </li> - <li class='c024'>Ions - <ul> - <li>in explanation of conductivity of gases, <a href='#Page_31'>31</a> <i>et seq.</i></li> - <li>production of, by collision, <a href='#Page_39'>39</a>, <a href='#Page_57'>57</a></li> - <li>rate of recombination of, <a href='#Page_40'>40</a> <i>et seq.</i></li> - <li>mobility of, <a href='#Page_42'>42</a> <i>et seq.</i></li> - <li>difference between mobility of positive and negative, <a href='#Page_43'>43</a> <i>et seq.</i></li> - <li>condensation of water around, <a href='#Page_46'>46</a> <i>et seq.</i></li> - <li>difference between positive and negative, <a href='#Page_49'>49</a></li> - <li>charge carried by, <a href='#Page_50'>50</a></li> - <li>diffusion of, <a href='#Page_51'>51</a> <i>et seq.</i></li> - <li>charge on an ion same as on hydrogen atom, <a href='#Page_54'>54</a></li> - <li>number of, produced per c.c., <a href='#Page_54'>54</a></li> - <li>size and nature of, <a href='#Page_55'>55</a> <i>et seq.</i></li> - <li>definition of, <a href='#Page_56'>56</a> <i>et seq.</i></li> - <li><span class='pageno' id='Page_570'>570</span>velocity acquired by, between collisions, <a href='#Page_58'>58</a></li> - <li>energy required to produce, <a href='#Page_58'>58</a>, <a href='#Page_551'>551</a></li> - <li>comparative number of, produced in gases, <a href='#Page_65'>65</a></li> - <li>disturbance of potential gradient by movement of, <a href='#Page_65'>65</a></li> - <li>production of, in insulators, <a href='#Page_209'>209</a></li> - <li>number of, produced by α particle, <a href='#Page_433'>433</a></li> - <li>number produced per c.c. in closed vessels, <a href='#Page_533'>533</a> <i>et seq.</i></li> - </ul> - </li> - <li class='c003'>Joly - <ul> - <li>motion of radium in an electric field, <a href='#Page_211'>211</a></li> - <li>absorption of radium rays by atmosphere, <a href='#Page_492'>492</a> (see footnote)</li> - </ul> - </li> - <li class='c003'>Kaufmann - <ul> - <li>velocity of cathode rays, <a href='#Page_75'>75</a></li> - <li>variation of <i>e</i>/<i>m</i> with velocity of electron, <a href='#Page_127'>127</a> <i>et seq.</i></li> - </ul> - </li> - <li class='c024'>Kelvin - <ul> - <li>theory of radio-activity, <a href='#Page_441'>441</a></li> - <li>age of sun and earth, <a href='#Page_492'>492</a>, <a href='#Page_493'>493</a></li> - </ul> - </li> - <li class='c024'>Kelvin, Smolan and Beattie - <ul> - <li>discharging power of uranium rays, <a href='#Page_7'>7</a></li> - </ul> - </li> - <li class='c024'>Kleeman and Bragg - <ul> - <li>theory of absorption of α rays, <a href='#Page_172'>172</a> <i>et seq.</i></li> - <li>relation between ionization and absorption, <a href='#Page_174'>174</a> <i>et seq.</i></li> - <li>range of α rays in air, <a href='#Page_174'>174</a></li> - <li>four sets of α rays from radium, <a href='#Page_174'>174</a> <i>et seq.</i></li> - </ul> - </li> - <li class='c024'>Kohlrausch - <ul> - <li>conductivity of water altered by radium rays, <a href='#Page_208'>208</a></li> - </ul> - </li> - <li class='c024'>Kohlrausch and Henning - <ul> - <li>conductivity of solutions of radium bromide, <a href='#Page_208'>208</a></li> - </ul> - </li> - <li class='c024'>Kunz - <ul> - <li>phosphorescence of willemite and kunzite, <a href='#Page_203'>203</a></li> - </ul> - </li> - <li class='c024'>Kunz and Baskerville - <ul> - <li>phosphorescence of substance under radium rays, <a href='#Page_204'>204</a></li> - </ul> - </li> - <li class='c024'>Kunzite - <ul> - <li>phosphorescence of, under radium rays, <a href='#Page_203'>203</a></li> - </ul> - </li> - <li class='c003'>Laborde and Curie - <ul> - <li>heat emission of radium, <a href='#Page_419'>419</a></li> - <li>origin of heat from radium, <a href='#Page_440'>440</a></li> - <li>radium emanation in waters of hot springs, <a href='#Page_514'>514</a></li> - </ul> - </li> - <li class='c024'>Langevin - <ul> - <li>coefficient of recombination of ions, <a href='#Page_41'>41</a></li> - <li>velocity of ions, <a href='#Page_45'>45</a> <i>et seq.</i></li> - <li>energy required to produce an ion, <a href='#Page_58'>58</a></li> - <li>secondary radiation produced by X rays, <a href='#Page_187'>187</a></li> - <li>slow moving ions in air, <a href='#Page_528'>528</a></li> - </ul> - </li> - <li class='c024'>Larmor - <ul> - <li>radiation theory, <a href='#Page_77'>77</a></li> - <li>radiation of energy from moving electron, <a href='#Page_79'>79</a></li> - <li>structure of the atom, <a href='#Page_157'>157</a></li> - </ul> - </li> - <li class='c024'>Lead, radio-active - <ul> - <li>preparation of, <a href='#Page_26'>26</a></li> - <li>radiations from, <a href='#Page_26'>26</a></li> - </ul> - </li> - <li class='c024'>Le Bon - <ul> - <li>rays from bodies exposed to sunlight, <a href='#Page_5'>5</a></li> - <li>discharging power of quinine sulphate, <a href='#Page_9'>9</a>, <a href='#Page_530'>530</a></li> - </ul> - </li> - <li class='c024'>Lenard - <ul> - <li>ionization of gases by ultra-violet light, <a href='#Page_9'>9</a></li> - <li>action of ions on a steam jet, <a href='#Page_47'>47</a></li> - <li>penetrating power of cathode rays, <a href='#Page_73'>73</a></li> - <li>negative charge carried by Lenard rays, <a href='#Page_120'>120</a></li> - <li>absorption of cathode rays proportional to density, <a href='#Page_136'>136</a>, <a href='#Page_137'>137</a></li> - </ul> - </li> - <li class='c024'>Lerch, von - <ul> - <li>chemical properties of active deposit of thorium, <a href='#Page_313'>313</a></li> - <li>electrolysis of solution of active deposit, <a href='#Page_313'>313</a></li> - <li>effect of temperature on excited activity, <a href='#Page_315'>315</a></li> - <li>temporary activity of active deposit from thorium, <a href='#Page_415'>415</a></li> - </ul> - </li> - <li class='c024'>Lockyer - <ul> - <li>inorganic evolution, <a href='#Page_499'>499</a></li> - </ul> - </li> - <li class='c024'>Lodge, Sir Oliver - <ul> - <li>electronic theory, <a href='#Page_69'>69</a></li> - <li>instability of atoms, <a href='#Page_487'>487</a></li> - </ul> - </li> - <li class='c024'>Lorentz - <ul> - <li>structure of atoms, <a href='#Page_157'>157</a></li> - </ul> - </li> - <li class='c024'>Lowry and Armstrong - <ul> - <li>radio-activity and phosphorescence, <a href='#Page_444'>444</a></li> - </ul> - </li> - <li class='c024'>Luminosity - <ul> - <li>of radium compounds, <a href='#Page_205'>205</a></li> - <li>change of, in radium compounds with time, <a href='#Page_205'>205</a></li> - <li>spectrum of phosphorescent light from radium bromide, <a href='#Page_206'>206</a></li> - <li>of radium compounds unaffected by temperature, <a href='#Page_210'>210</a></li> - </ul> - </li> - <li class='c003'>Mache - <ul> - <li>radium emanation in hot springs, <a href='#Page_513'>513</a></li> - </ul> - </li> - <li class='c024'>Mache and von Schweidler - <ul> - <li>velocity of ions in air, <a href='#Page_528'>528</a></li> - </ul> - </li> - <li class='c024'><span class='pageno' id='Page_571'>571</span>Makower - <ul> - <li>diffusion of radium emanation, <a href='#Page_274'>274</a></li> - <li>diffusion of thorium emanation, <a href='#Page_276'>276</a></li> - </ul> - </li> - <li class='c024'>Marckwald - <ul> - <li>preparation of radio-tellurium, <a href='#Page_25'>25</a></li> - <li>rate of decay of radio-tellurium, <a href='#Page_411'>411</a></li> - </ul> - </li> - <li class='c024'>Mass - <ul> - <li>apparent mass of electron, <a href='#Page_71'>71</a>, <a href='#Page_127'>127</a></li> - <li>variation of mass of electron with speed, <a href='#Page_127'>127</a> <i>et seq.</i></li> - <li>of α particle, <a href='#Page_147'>147</a> <i>et seq.</i></li> - </ul> - </li> - <li class='c024'>Materials - <ul> - <li>radio-activity of ordinary, <a href='#Page_528'>528</a>, <a href='#Page_536'>536</a> <i>et seq.</i></li> - </ul> - </li> - <li class='c024'>Matteucci - <ul> - <li>rate of dissipation of charge in closed vessels, <a href='#Page_531'>531</a></li> - </ul> - </li> - <li class='c024'>McClelland - <ul> - <li>absorption of γ rays, <a href='#Page_181'>181</a></li> - <li>secondary rays from β and γ rays from radium, <a href='#Page_192'>192</a></li> - </ul> - </li> - <li class='c024'>McClung - <ul> - <li>coefficient of recombination of ions, <a href='#Page_41'>41</a></li> - <li>conductivity of gases exposed to X rays, <a href='#Page_64'>64</a></li> - <li>ionization by α rays from radium C, <a href='#Page_550'>550</a></li> - </ul> - </li> - <li class='c024'>McClung and Rutherford - <ul> - <li>energy required to produce an ion, <a href='#Page_58'>58</a></li> - <li>variation of current with thickness of layer of uranium, <a href='#Page_195'>195</a></li> - <li>estimate of energy radiated from radio-elements, <a href='#Page_418'>418</a></li> - <li>radiation of energy from radium, <a href='#Page_438'>438</a></li> - </ul> - </li> - <li class='c024'>McLennan - <ul> - <li>absorption of cathode rays, <a href='#Page_65'>65</a></li> - <li>radio-activity of snow, <a href='#Page_506'>506</a></li> - <li>excited radio-activity at Niagara Falls, <a href='#Page_519'>519</a></li> - </ul> - </li> - <li class='c024'>McLennan and Burton - <ul> - <li>penetrating radiation from the earth, <a href='#Page_520'>520</a></li> - <li>radio-activity of ordinary materials, <a href='#Page_537'>537</a></li> - <li>emanation from ordinary matter, <a href='#Page_538'>538</a></li> - </ul> - </li> - <li class='c024'>Metabolon - <ul> - <li>definition of, <a href='#Page_446'>446</a></li> - <li>table of metabolons, <a href='#Page_448'>448</a></li> - <li>radio-elements as metabolons, <a href='#Page_457'>457</a></li> - </ul> - </li> - <li class='c024'>Meteorological conditions - <ul> - <li>effect of, on radio-activity of atmosphere, <a href='#Page_517'>517</a></li> - </ul> - </li> - <li class='c024'>Methods of measurement - <ul> - <li>in radio-activity, <a href='#Page_82'>82</a> <i>et seq.</i></li> - <li>comparison of photographic and electrical, <a href='#Page_83'>83</a> <i>et seq.</i></li> - <li>description of electrical, <a href='#Page_84'>84</a> <i>et seq.</i></li> - </ul> - </li> - <li class='c024'>Meyer and Himstedt - <ul> - <li>production of helium by radium, <a href='#Page_479'>479</a></li> - </ul> - </li> - <li class='c024'>Meyer and Schweidler - <ul> - <li>magnetic deviation of β rays by electrical method, <a href='#Page_113'>113</a></li> - <li>absorption of β rays of radium by matter, <a href='#Page_136'>136</a></li> - <li>activity proportional to amount of uranium, <a href='#Page_195'>195</a></li> - <li>emanation from uranium, <a href='#Page_348'>348</a></li> - <li>effect of crystallization on activity of uranium, <a href='#Page_349'>349</a></li> - <li>rate of decay of radio-tellurium, <a href='#Page_411'>411</a></li> - </ul> - </li> - <li class='c024'>Minerals, radio-active - <ul> - <li>constant ratio of radium to uranium, <a href='#Page_459'>459</a> <i>et seq.</i></li> - <li>list of minerals, <a href='#Page_461'>461</a></li> - <li>age of, <a href='#Page_485'>485</a></li> - <li>composition of, <a href='#Page_554'>554</a> <i>et seq.</i></li> - </ul> - </li> - <li class='c024'>Mobility - <ul> - <li>of ions, <a href='#Page_43'>43</a> <i>et seq.</i></li> - </ul> - </li> - <li class='c024'>Moisture - <ul> - <li>effect of, on velocity of ions, <a href='#Page_43'>43</a>, <a href='#Page_45'>45</a></li> - <li>effect of, on emanating power, <a href='#Page_255'>255</a></li> - </ul> - </li> - <li class='c024'>Molecule - <ul> - <li>number of, in 1 c.c. of hydrogen, <a href='#Page_54'>54</a></li> - </ul> - </li> - <li class='c024'>Molecular weight - <ul> - <li>of radium emanation, <a href='#Page_273'>273</a></li> - <li>of thorium emanation, <a href='#Page_275'>275</a></li> - </ul> - </li> - <li class='c003'>Nagel and Himstedt - <ul> - <li>action of radium rays on the eye, <a href='#Page_217'>217</a></li> - </ul> - </li> - <li class='c024'>Niewenglowski - <ul> - <li>rays from sulphide of calcium, <a href='#Page_4'>4</a></li> - </ul> - </li> - <li class='c024'>Nomenclature - <ul> - <li>of successive products, <a href='#Page_328'>328</a> <i>et seq.</i></li> - </ul> - </li> - <li class='c024'>Number - <ul> - <li>of molecules per c.c. of hydrogen, <a href='#Page_54'>54</a></li> - <li>of ions produced in gas by active substances, <a href='#Page_55'>55</a></li> - <li>of β particles expelled from 1 gram of radium, <a href='#Page_124'>124</a></li> - <li>of α particles emitted per gram of radium, <a href='#Page_155'>155</a></li> - <li>of ions produced per c.c. in closed vessels, <a href='#Page_534'>534</a></li> - </ul> - </li> - <li class='c003'>Occlusion - <ul> - <li>of emanation in thorium and radium, <a href='#Page_258'>258</a></li> - <li>of radium emanation by solids, <a href='#Page_310'>310</a></li> - </ul> - </li> - <li class='c024'>Owens - <ul> - <li>saturation current affected by dust, <a href='#Page_42'>42</a></li> - <li>penetrating power of rays independent of compound, <a href='#Page_164'>164</a></li> - <li>absorption of α rays varies directly as the pressure of gas, <a href='#Page_169'>169</a></li> - <li>effect of air currents on conductivity produced by thorium, <a href='#Page_238'>238</a></li> - </ul> - </li> - <li class='c024'>Oxygen - <ul> - <li>change into ozone, by radium rays, <a href='#Page_213'>213</a></li> - <li><span class='pageno' id='Page_572'>572</span>production of, from radium solutions, <a href='#Page_215'>215</a></li> - </ul> - </li> - <li class='c024'>Ozone - <ul> - <li>production of, by radium rays, <a href='#Page_213'>213</a></li> - </ul> - </li> - <li class='c003'>Paraffin - <ul> - <li>objection to, as an insulator, <a href='#Page_96'>96</a></li> - <li>conductivity of, under radium rays, <a href='#Page_210'>210</a></li> - </ul> - </li> - <li class='c024'>Paschen - <ul> - <li>distribution of velocity amongst β particles, <a href='#Page_131'>131</a> <i>et seq.</i></li> - <li>absence of magnetic deflection of γ rays, <a href='#Page_183'>183</a></li> - <li>γ rays and electrons, <a href='#Page_185'>185</a></li> - <li>heating effect of γ rays, <a href='#Page_186'>186</a>, <a href='#Page_429'>429</a></li> - </ul> - </li> - <li class='c024'>Patterson - <ul> - <li>number of ions per c.c. in closed vessel, <a href='#Page_534'>534</a></li> - <li>natural conductivity of air due to an easily absorbed radiation, <a href='#Page_536'>536</a></li> - <li>effect of temperature on natural conductivity of air, <a href='#Page_536'>536</a></li> - </ul> - </li> - <li class='c024'>Peck and Willows - <ul> - <li>action of radium rays on spark, <a href='#Page_208'>208</a></li> - </ul> - </li> - <li class='c024'>Pegram - <ul> - <li>electrolysis of thorium solutions, <a href='#Page_314'>314</a></li> - <li>temporary activity of substances separated from thorium, <a href='#Page_415'>415</a></li> - </ul> - </li> - <li class='c024'>Penetrating power - <ul> - <li>comparison of, for α, β and γ rays, <a href='#Page_111'>111</a></li> - <li>variation in, of β rays, <a href='#Page_134'>134</a> <i>et seq.</i></li> - <li>variation of, with density for β rays, <a href='#Page_137'>137</a></li> - <li>comparison of, for α rays from radio-elements, <a href='#Page_164'>164</a></li> - <li>variation of, with density for α rays, <a href='#Page_169'>169</a></li> - <li>variation of, with density for γ rays, <a href='#Page_182'>182</a></li> - </ul> - </li> - <li class='c024'>Penetrating radiation - <ul> - <li>from the earth and atmosphere, <a href='#Page_520'>520</a></li> - </ul> - </li> - <li class='c024'>Perrin - <ul> - <li>charge carried by cathode rays, <a href='#Page_73'>73</a></li> - <li>theory of radio-activity, <a href='#Page_437'>437</a></li> - </ul> - </li> - <li class='c024'>Phosphorescence - <ul> - <li>production of, by radium, <a href='#Page_19'>19</a></li> - <li>production of, by radium and polonium rays, <a href='#Page_201'>201</a> <i>et seq.</i></li> - <li>comparison of, produced by α and β rays, <a href='#Page_202'>202</a></li> - <li>of zinc sulphide, <a href='#Page_202'>202</a></li> - <li>of barium platinocyanide, <a href='#Page_203'>203</a></li> - <li>of willemite and kunzite, <a href='#Page_203'>203</a></li> - <li>produced by radium emanation in substances, <a href='#Page_203'>203</a>, <a href='#Page_252'>252</a></li> - <li>diminution of, with time, <a href='#Page_205'>205</a></li> - <li>of radium compounds, <a href='#Page_205'>205</a></li> - <li>spectrum of phosphorescent light of radium bromide, <a href='#Page_205'>205</a></li> - <li>spectrum of phosphorescent light of “emanium,” <a href='#Page_206'>206</a></li> - <li>production of by heat (thermo-luminescence), <a href='#Page_207'>207</a></li> - <li>use of, to illustrate condensation of emanations, <a href='#Page_279'>279</a></li> - <li>connection of with ionization, <a href='#Page_547'>547</a> <i>et seq.</i></li> - </ul> - </li> - <li class='c024'>Phosphorus - <ul> - <li>action of radium rays on, <a href='#Page_214'>214</a></li> - <li>ionization produced by, <a href='#Page_529'>529</a></li> - </ul> - </li> - <li class='c024'>Photo-electric action - <ul> - <li>produced by radium rays in certain substances, <a href='#Page_214'>214</a></li> - </ul> - </li> - <li class='c024'>Photographic - <ul> - <li>method, advantages and disadvantages of, <a href='#Page_83'>83</a></li> - <li>relative photographic action of rays, <a href='#Page_83'>83</a></li> - <li>connection of photographic action with ionization, <a href='#Page_546'>546</a></li> - </ul> - </li> - <li class='c024'>Physical action of radium rays - <ul> - <li>on sparks, <a href='#Page_208'>208</a></li> - <li>on electrodeless discharge, <a href='#Page_208'>208</a></li> - <li>on selenium, <a href='#Page_208'>208</a></li> - <li>on conductivity of insulators, <a href='#Page_209'>209</a></li> - </ul> - </li> - <li class='c024'>Physiological action of radium rays - <ul> - <li>production of burns, <a href='#Page_216'>216</a></li> - <li>effect on bacteria, <a href='#Page_216'>216</a></li> - <li>effect on eye, <a href='#Page_217'>217</a></li> - </ul> - </li> - <li class='c024'>Piezo-électrique of quartz - <ul> - <li>description of, <a href='#Page_105'>105</a></li> - </ul> - </li> - <li class='c024'>Pitchblendes - <ul> - <li>comparison of radio-activity of, <a href='#Page_11'>11</a></li> - <li>radio-elements separated from, <a href='#Page_13'>13</a> <i>et seq.</i></li> - <li>radium continually produced from, <a href='#Page_459'>459</a></li> - <li>constitution of, <a href='#Page_557'>557</a></li> - </ul> - </li> - <li class='c024'>Polarization of uranium rays - <ul> - <li>absence of, <a href='#Page_7'>7</a></li> - </ul> - </li> - <li class='c024'>Polonium - <ul> - <li>methods of separation of, <a href='#Page_22'>22</a></li> - <li>rays from, <a href='#Page_23'>23</a></li> - <li>decay of activity of, <a href='#Page_23'>23</a></li> - <li>discussion of nature of, <a href='#Page_24'>24</a></li> - <li>similarity to radio-tellurium, <a href='#Page_26'>26</a></li> - <li>magnetic deviation of α rays from, <a href='#Page_146'>146</a>, <a href='#Page_150'>150</a></li> - <li>slow moving electrons, <a href='#Page_153'>153</a></li> - <li>increase of absorption with thickness of matter traversed, <a href='#Page_163'>163</a></li> - <li>connection of, with radium F, <a href='#Page_411'>411</a></li> - </ul> - </li> - <li class='c024'>Potential - <ul> - <li>required to produce saturation, <a href='#Page_32'>32</a> <i>et seq.</i></li> - <li>fall of potential needed to produce ions at each collision, <a href='#Page_58'>58</a></li> - <li><span class='pageno' id='Page_573'>573</span>gradient due to movement of ions, <a href='#Page_65'>65</a></li> - </ul> - </li> - <li class='c024'>Precht and Runge - <ul> - <li>spectrum of radium, <a href='#Page_17'>17</a></li> - <li>atomic weight of radium, <a href='#Page_18'>18</a></li> - <li>heating effect of radium, <a href='#Page_420'>420</a></li> - </ul> - </li> - <li class='c024'>Pressure - <ul> - <li>effect of, on velocity of ions, <a href='#Page_46'>46</a></li> - <li>effect of, on current through gases, <a href='#Page_61'>61</a> <i>et seq.</i></li> - <li>production of emanation independent of, <a href='#Page_265'>265</a></li> - <li>effect of, on distribution of excited activity, <a href='#Page_317'>317</a></li> - <li>effect of, on natural conductivity of air in closed vessels, <a href='#Page_534'>534</a></li> - </ul> - </li> - <li class='c024'>Products, radio-active - <ul> - <li>list of, from radio-elements, <a href='#Page_448'>448</a></li> - <li>properties of, <a href='#Page_448'>448</a></li> - <li>amount of in radium, <a href='#Page_452'>452</a> <i>et seq.</i></li> - <li>radiations from, <a href='#Page_455'>455</a></li> - </ul> - </li> - <li class='c003'>Quartz piezo-électrique - <ul> - <li>use of, in measurement of current, <a href='#Page_105'>105</a></li> - </ul> - </li> - <li class='c024'>Quinine sulphate - <ul> - <li>discharging power of, <a href='#Page_530'>530</a></li> - <li>phosphorescence of, <a href='#Page_530'>530</a></li> - </ul> - </li> - <li class='c003'>Radiations - <ul> - <li>emitted by uranium, <a href='#Page_8'>8</a></li> - <li>emitted by thorium, <a href='#Page_10'>10</a></li> - <li>emitted by radium, <a href='#Page_18'>18</a></li> - <li>emitted by actinium, <a href='#Page_21'>21</a></li> - <li>emitted by polonium, <a href='#Page_23'>23</a></li> - <li>method of measurement of, <a href='#Page_82'>82</a> <i>et seq.</i></li> - <li>methods of comparison of, <a href='#Page_108'>108</a></li> - <li>three kinds of, <a href='#Page_109'>109</a></li> - <li>analogy to rays from a Crookes tube, <a href='#Page_110'>110</a></li> - <li>relative ionizing and penetrating power of, <a href='#Page_111'>111</a></li> - <li>difficulties of comparative measurement of, <a href='#Page_112'>112</a></li> - <li>β rays, <a href='#Page_113'>113</a></li> - <li>α rays, <a href='#Page_141'>141</a></li> - <li>γ rays, <a href='#Page_179'>179</a></li> - <li>secondary rays, <a href='#Page_187'>187</a></li> - <li>comparison of ionization of α and β rays, <a href='#Page_194'>194</a></li> - <li>phosphorescent effect of, <a href='#Page_201'>201</a> <i>et seq.</i></li> - <li>physical actions of, <a href='#Page_207'>207</a> <i>et seq.</i></li> - <li>chemical actions of, <a href='#Page_213'>213</a> <i>et seq.</i></li> - <li>physiological actions of, <a href='#Page_216'>216</a></li> - <li>from the emanation, <a href='#Page_263'>263</a></li> - <li>from Ur X, <a href='#Page_347'>347</a></li> - <li>connection of, with heat emission, <a href='#Page_421'>421</a> <i>et seq.</i></li> - <li>from different active products, <a href='#Page_455'>455</a></li> - <li>conservation of energy of each specific type of, <a href='#Page_469'>469</a> <i>et seq.</i></li> - </ul> - </li> - <li class='c024'>Radio-lead - <ul> - <li>connection of, with polonium, <a href='#Page_411'>411</a> <i>et seq.</i></li> - <li>connection of, with radium D, <a href='#Page_413'>413</a></li> - </ul> - </li> - <li class='c024'>Radio-tellurium - <ul> - <li>rate of decay of, <a href='#Page_411'>411</a></li> - <li>connection of with radium F, <a href='#Page_411'>411</a></li> - </ul> - </li> - <li class='c024'>Radium - <ul> - <li>discovery of, <a href='#Page_13'>13</a></li> - <li>separation of, <a href='#Page_13'>13</a></li> - <li>spectrum of, <a href='#Page_16'>16</a></li> - <li>atomic weight of, <a href='#Page_17'>17</a></li> - <li>radiations from, <a href='#Page_18'>18</a></li> - <li>compounds of, <a href='#Page_19'>19</a></li> - <li>nature of radiations from, <a href='#Page_109'>109</a></li> - <li>β rays from, <a href='#Page_113'>113</a></li> - <li>α rays from, <a href='#Page_141'>141</a></li> - <li>γ rays from, <a href='#Page_179'>179</a></li> - <li>secondary rays from, <a href='#Page_187'>187</a></li> - <li>production of phosphorescence by, <a href='#Page_201'>201</a> <i>et seq.</i></li> - <li>spectrum of phosphorescent light of, <a href='#Page_206'>206</a></li> - <li>physical actions of, <a href='#Page_207'>207</a> <i>et seq.</i></li> - <li>chemical actions of, <a href='#Page_213'>213</a> <i>et seq.</i></li> - <li>physiological actions of, <a href='#Page_216'>216</a></li> - <li>emanation from, <a href='#Page_246'>246</a></li> - <li>properties of emanation from, <a href='#Page_247'>247</a> <i>et seq.</i></li> - <li>chemical nature of emanation from, <a href='#Page_267'>267</a></li> - <li>diffusion of emanation from, <a href='#Page_269'>269</a></li> - <li>condensation of emanation from, <a href='#Page_277'>277</a></li> - <li>amount of emanation from, <a href='#Page_288'>288</a></li> - <li>volume of emanation from, <a href='#Page_289'>289</a></li> - <li>spectrum of emanation from, <a href='#Page_292'>292</a></li> - <li>excited radio-activity from, <a href='#Page_295'>295</a> <i>et seq.</i></li> - <li>decay of excited activity from, <a href='#Page_306'>306</a> <i>et seq.</i></li> - <li>difference in properties of radium and the emanation, <a href='#Page_327'>327</a></li> - <li>nomenclature of products, <a href='#Page_328'>328</a></li> - <li>theory of successive changes in, <a href='#Page_330'>330</a></li> - <li>alteration of activity of, by removal of emanation, <a href='#Page_371'>371</a> <i>et seq.</i></li> - <li>recovery of activity of, after removal of emanation, <a href='#Page_372'>372</a></li> - <li>effect of escape of emanation on recovery of activity of, <a href='#Page_374'>374</a></li> - <li>non-separable activity of, <a href='#Page_375'>375</a></li> - <li>period and properties of radium A, B and C, <a href='#Page_376'>376</a> <i>et seq.</i></li> - <li>analysis of active deposit of rapid changes of radium, <a href='#Page_377'>377</a></li> - <li>analysis of β ray curves, <a href='#Page_381'>381</a> <i>et seq.</i></li> - <li>analysis of α ray curves, <a href='#Page_386'>386</a> <i>et seq.</i></li> - <li>equations of activity curves, <a href='#Page_389'>389</a></li> - <li><span class='pageno' id='Page_574'>574</span>effect of temperature on active deposit of, <a href='#Page_390'>390</a></li> - <li>relative activity due to products of, <a href='#Page_395'>395</a></li> - <li>active deposit of slow transformation, <a href='#Page_397'>397</a></li> - <li>physical and chemical properties of radium D, E and F, <a href='#Page_398'>398</a> <i>et seq.</i></li> - <li>effect of temperature on active deposit of slow change, <a href='#Page_401'>401</a></li> - <li>separation of radium F by bismuth, <a href='#Page_402'>402</a></li> - <li>products of, <a href='#Page_402'>402</a> <i>et seq.</i></li> - <li>rate of transformation of radium D, <a href='#Page_404'>404</a> <i>et seq.</i></li> - <li>variation of the activity of the active deposit over long periods of time, <a href='#Page_407'>407</a></li> - <li>amounts of radium D, E and F in old radium, <a href='#Page_408'>408</a></li> - <li>variation of activity of, with time, <a href='#Page_409'>409</a></li> - <li>products of in pitchblende, <a href='#Page_410'>410</a></li> - <li>origin of radio-tellurium, <a href='#Page_411'>411</a></li> - <li>origin of polonium, <a href='#Page_411'>411</a>, <a href='#Page_412'>412</a></li> - <li>origin of radio-lead, <a href='#Page_413'>413</a></li> - <li>temporary activity of inactive matter separated from pitchblende, <a href='#Page_415'>415</a> <i>et seq.</i></li> - <li>heat emission of, <a href='#Page_419'>419</a> <i>et seq.</i></li> - <li>heat emission of emanation from, <a href='#Page_420'>420</a>, <a href='#Page_431'>431</a></li> - <li>heating effects due to products of, <a href='#Page_433'>433</a></li> - <li>theories of radio-activity of, <a href='#Page_437'>437</a> <i>et seq.</i></li> - <li>discussion of theories of radio-activity of, <a href='#Page_441'>441</a> <i>et seq.</i></li> - <li>energy of radiations, not derived from external source, <a href='#Page_442'>442</a> <i>et seq.</i></li> - <li>theory of radio-active change, <a href='#Page_444'>444</a> <i>et seq.</i></li> - <li>list of active products of, <a href='#Page_448'>448</a></li> - <li>amount of products of, <a href='#Page_452'>452</a></li> - <li>rate of change of, <a href='#Page_457'>457</a></li> - <li>life of radium, <a href='#Page_457'>457</a></li> - <li>origin of, <a href='#Page_459'>459</a> <i>et seq.</i></li> - <li>production of, by uranium, <a href='#Page_459'>459</a> <i>et seq.</i></li> - <li>amount of in 1 gram of uranium, <a href='#Page_461'>461</a></li> - <li>amount of, in minerals, <a href='#Page_461'>461</a></li> - <li>radio-activity of, independent of concentration, <a href='#Page_466'>466</a> <i>et seq.</i></li> - <li>disappearance of, <a href='#Page_467'>467</a></li> - <li>life of, independent of concentration, <a href='#Page_468'>468</a></li> - <li>conservation of radio-activity of, <a href='#Page_469'>469</a> <i>et seq.</i></li> - <li>loss of weight of, <a href='#Page_473'>473</a></li> - <li>experiments to determine loss of weight of, <a href='#Page_474'>474</a></li> - <li>total emission of energy from 1 gram of, <a href='#Page_474'>474</a> <i>et seq.</i></li> - <li>production of helium from, <a href='#Page_476'>476</a></li> - <li>helium, disintegration product of, <a href='#Page_479'>479</a> <i>et seq.</i></li> - <li>amount of helium from, <a href='#Page_480'>480</a></li> - <li>possible causes of disintegration of, <a href='#Page_486'>486</a> <i>et seq.</i></li> - <li>amount of, to account for heat of sun, <a href='#Page_491'>491</a></li> - <li>possible connection of with heat of sun, <a href='#Page_491'>491</a></li> - <li>possible connection of with heat of earth, <a href='#Page_493'>493</a></li> - <li>probable amount of, in earth, <a href='#Page_495'>495</a></li> - <li>amount of, in atmosphere, <a href='#Page_495'>495</a>, <a href='#Page_524'>524</a></li> - <li>presence of, in atmosphere, <a href='#Page_521'>521</a> <i>et seq.</i></li> - </ul> - </li> - <li class='c024'>Radium A - <ul> - <li>decay curve of, <a href='#Page_378'>378</a></li> - <li>radiation from, <a href='#Page_381'>381</a></li> - <li>effect of, on activity curves, <a href='#Page_386'>386</a> <i>et seq.</i></li> - <li>connection with later changes, <a href='#Page_392'>392</a></li> - <li>activity supplied by, <a href='#Page_393'>393</a></li> - </ul> - </li> - <li class='c024'>Radium B - <ul> - <li>absence of rays in, <a href='#Page_381'>381</a></li> - <li>effect of, on activity curves, <a href='#Page_381'>381</a> <i>et seq.</i></li> - <li>effect of temperature on, <a href='#Page_390'>390</a></li> - <li>volatility of, <a href='#Page_390'>390</a></li> - <li>absence of heating effect of, <a href='#Page_433'>433</a></li> - <li>nature of rayless change in, <a href='#Page_454'>454</a>, <a href='#Page_552'>552</a></li> - </ul> - </li> - <li class='c024'>Radium C - <ul> - <li>radiations from, <a href='#Page_381'>381</a></li> - <li>analysis of β ray curves of, <a href='#Page_381'>381</a> <i>et seq.</i></li> - <li>analysis of α ray curves of, <a href='#Page_386'>386</a> <i>et seq.</i></li> - <li>effect of temperature on, <a href='#Page_390'>390</a></li> - <li>activity supplied by, <a href='#Page_394'>394</a> <i>et seq.</i></li> - <li>heating effect of, <a href='#Page_425'>425</a></li> - <li>use of, as a source of β rays, <a href='#Page_435'>435</a></li> - <li>explosive nature of change in, <a href='#Page_456'>456</a></li> - <li>magnetic deflection of rays from, <a href='#Page_543'>543</a></li> - <li>velocity and value of <i>e</i>/<i>m</i> for rays from, <a href='#Page_544'>544</a></li> - </ul> - </li> - <li class='c024'>Radium D - <ul> - <li>origin of name of, <a href='#Page_376'>376</a></li> - <li>connection of, with active deposit, <a href='#Page_403'>403</a></li> - <li>period of transformation of, <a href='#Page_406'>406</a></li> - <li>effect of, on variation of activity, <a href='#Page_407'>407</a></li> - <li>presence in old radium, <a href='#Page_408'>408</a></li> - <li>effect of, on activity of old radium, <a href='#Page_409'>409</a></li> - <li>presence in pitchblende, <a href='#Page_410'>410</a></li> - <li>connection with radio-lead, <a href='#Page_413'>413</a></li> - <li>amount of, in 1 ton of uranium, <a href='#Page_454'>454</a></li> - </ul> - </li> - <li class='c024'>Radium E - <ul> - <li>effect of temperature on, <a href='#Page_401'>401</a></li> - <li>connection of, with β ray activity active deposit, <a href='#Page_403'>403</a>, <a href='#Page_400'>400</a></li> - <li><span class='pageno' id='Page_575'>575</span>connection with radio-lead, <a href='#Page_413'>413</a></li> - </ul> - </li> - <li class='c024'>Radium F - <ul> - <li>variation of activity due to, <a href='#Page_398'>398</a></li> - <li>effect of temperature on, <a href='#Page_401'>401</a></li> - <li>separation of, on bismuth plate, <a href='#Page_402'>402</a></li> - <li>connection with active deposit, <a href='#Page_403'>403</a></li> - <li>variation of activity of, over long periods of time, <a href='#Page_407'>407</a></li> - <li>presence in old radium, <a href='#Page_409'>409</a></li> - <li>effect of, on activity of old radium, <a href='#Page_409'>409</a></li> - <li>presence in pitchblende, <a href='#Page_410'>410</a></li> - <li>connection with radio-tellurium, <a href='#Page_411'>411</a></li> - <li>connection with polonium, <a href='#Page_411'>411</a>, <a href='#Page_412'>412</a></li> - <li>connection with radio-lead, <a href='#Page_413'>413</a></li> - </ul> - </li> - <li class='c024'>Rain - <ul> - <li>radio-activity of, <a href='#Page_505'>505</a></li> - <li>decay of activity of, <a href='#Page_506'>506</a></li> - </ul> - </li> - <li class='c024'>Ramsay, Sir W. - <ul> - <li>amount of helium in thorianite, <a href='#Page_486'>486</a></li> - </ul> - </li> - <li class='c024'>Ramsay and Collie - <ul> - <li>spectrum of emanation, <a href='#Page_292'>292</a></li> - </ul> - </li> - <li class='c024'>Ramsay and Cooke - <ul> - <li>radio-activity produced by radiation from radium, <a href='#Page_472'>472</a></li> - </ul> - </li> - <li class='c024'>Ramsay and Soddy - <ul> - <li>evolution of gas from radium, <a href='#Page_215'>215</a></li> - <li>production of hydrogen and oxygen from radium, <a href='#Page_215'>215</a></li> - <li>chemical nature of the emanation, <a href='#Page_268'>268</a></li> - <li>gaseous nature of the emanation, <a href='#Page_268'>268</a></li> - <li>volume of emanation, and change with time, <a href='#Page_289'>289</a></li> - <li>helium from radium emanation, <a href='#Page_291'>291</a></li> - <li>amount of helium produced by radium, <a href='#Page_480'>480</a></li> - </ul> - </li> - <li class='c024'>Ramsay and Travers - <ul> - <li>amount of helium in fergusonite, <a href='#Page_486'>486</a></li> - </ul> - </li> - <li class='c024'>Rayless changes - <ul> - <li>discussion of, <a href='#Page_454'>454</a>, <a href='#Page_552'>552</a></li> - </ul> - </li> - <li class='c024'>Re, F. - <ul> - <li>theory of radio-activity, <a href='#Page_441'>441</a></li> - </ul> - </li> - <li class='c024'>Recombination - <ul> - <li>of ions, <a href='#Page_40'>40</a> <i>et seq.</i></li> - <li>constant of, <a href='#Page_42'>42</a></li> - </ul> - </li> - <li class='c024'>Recovery - <ul> - <li>of activity of thorium after removal of Th X, <a href='#Page_221'>221</a></li> - <li>of activity of uranium after removal of Ur X, <a href='#Page_223'>223</a></li> - <li>significance of law of, <a href='#Page_224'>224</a></li> - <li>effect of conditions on rate of, <a href='#Page_232'>232</a></li> - <li>of activity of radium after removal of emanation, <a href='#Page_372'>372</a></li> - <li>of heating effect of radium, <a href='#Page_423'>423</a></li> - </ul> - </li> - <li class='c024'>Reflection - <ul> - <li>no evidence of direct reflection for uranium rays, <a href='#Page_7'>7</a></li> - <li>diffuse reflection of rays, <a href='#Page_7'>7</a></li> - </ul> - </li> - <li class='c024'>Refraction - <ul> - <li>no evidence of, for uranium rays, <a href='#Page_7'>7</a></li> - </ul> - </li> - <li class='c024'>Regeneration - <ul> - <li>of emanating power, <a href='#Page_256'>256</a></li> - </ul> - </li> - <li class='c024'>Richarz and von Helmholtz - <ul> - <li>action of ions on steam jet, <a href='#Page_47'>47</a></li> - </ul> - </li> - <li class='c024'>Richarz and Schenck - <ul> - <li>theory of radio-activity, <a href='#Page_441'>441</a></li> - </ul> - </li> - <li class='c024'>Rossignol and Gimingham - <ul> - <li>decay of thorium emanation, <a href='#Page_242'>242</a></li> - </ul> - </li> - <li class='c024'>Runge - <ul> - <li>spectrum of radium, <a href='#Page_17'>17</a></li> - </ul> - </li> - <li class='c024'>Runge and Bödlander - <ul> - <li>evolution of gas from radium, <a href='#Page_215'>215</a></li> - </ul> - </li> - <li class='c024'>Runge and Precht - <ul> - <li>spectrum of radium, <a href='#Page_17'>17</a></li> - <li>atomic weight of radium, <a href='#Page_18'>18</a></li> - <li>heating effect of radium, <a href='#Page_420'>420</a></li> - </ul> - </li> - <li class='c024'>Russel - <ul> - <li>photographic action of substances, <a href='#Page_83'>83</a></li> - </ul> - </li> - <li class='c003'>Saake - <ul> - <li>amount of emanation in air at high altitudes, <a href='#Page_519'>519</a></li> - </ul> - </li> - <li class='c024'>Salomonsen and Dreyer - <ul> - <li>coloration of quartz by radium rays, <a href='#Page_213'>213</a></li> - </ul> - </li> - <li class='c024'>Saturation current - <ul> - <li>meaning of, <a href='#Page_33'>33</a> <i>et seq.</i></li> - <li>application of, to measurements of radio-activity, <a href='#Page_84'>84</a></li> - <li>measurement of, <a href='#Page_100'>100</a> <i>et seq.</i></li> - </ul> - </li> - <li class='c024'>Schenck - <ul> - <li>radium emanation in springs, <a href='#Page_513'>513</a></li> - </ul> - </li> - <li class='c024'>Schenck and Richarz - <ul> - <li>theory of radio-activity, <a href='#Page_441'>441</a></li> - </ul> - </li> - <li class='c024'>Schmidt - <ul> - <li>discovery of radio-activity of thorium, <a href='#Page_10'>10</a></li> - </ul> - </li> - <li class='c024'>Schmidt and Wiedemann - <ul> - <li>thermo-luminescence, <a href='#Page_207'>207</a></li> - </ul> - </li> - <li class='c024'>Schuster - <ul> - <li>number of ions per c.c. in air of Manchester, <a href='#Page_528'>528</a></li> - <li>radio-activity of matter, <a href='#Page_529'>529</a></li> - </ul> - </li> - <li class='c024'>Schweidler and Mache - <ul> - <li>velocity of ions in air, <a href='#Page_528'>528</a></li> - </ul> - </li> - <li class='c024'>Schweidler and Meyer - <ul> - <li>magnetic deviation of β rays by electrical method, <a href='#Page_113'>113</a></li> - <li>absorption of β rays of radium by matter, <a href='#Page_136'>136</a></li> - <li>activity proportional to amount of uranium, <a href='#Page_195'>195</a></li> - <li>emanation from uranium, <a href='#Page_348'>348</a></li> - <li>effect of crystallization on activity of uranium, <a href='#Page_349'>349</a></li> - <li>rate of decay of radio-tellurium, <a href='#Page_411'>411</a></li> - </ul> - </li> - <li class='c024'><span class='pageno' id='Page_576'>576</span>Scintillations - <ul> - <li>discovery of, in zinc sulphide screen, <a href='#Page_158'>158</a></li> - <li>connection of, with α rays, <a href='#Page_158'>158</a></li> - <li>illustration of, by spinthariscope, <a href='#Page_158'>158</a></li> - <li>cause of, <a href='#Page_160'>160</a></li> - <li>production of, by action of electric field, <a href='#Page_160'>160</a></li> - </ul> - </li> - <li class='c024'>Searle - <ul> - <li>apparent mass of moving charged body, <a href='#Page_71'>71</a>, <a href='#Page_127'>127</a></li> - </ul> - </li> - <li class='c024'>Secondary rays - <ul> - <li>examination of, by photographic method, <a href='#Page_187'>187</a></li> - <li>examination of, by electrical method, <a href='#Page_188'>188</a></li> - <li>production of, by β and γ rays, <a href='#Page_189'>189</a> <i>et seq.</i></li> - <li>from different materials, <a href='#Page_191'>191</a></li> - <li>amount of, depends upon atomic weight, <a href='#Page_192'>192</a></li> - <li>magnetic deflection of, <a href='#Page_193'>193</a></li> - </ul> - </li> - <li class='c024'>Seitz - <ul> - <li>absorption of electrons by matter, <a href='#Page_137'>137</a> <i>et seq.</i></li> - </ul> - </li> - <li class='c024'>Selenium - <ul> - <li>action of radium rays on, <a href='#Page_208'>208</a></li> - </ul> - </li> - <li class='c024'>Simon - <ul> - <li>value of <i>e</i>/<i>m</i> for cathode rays, <a href='#Page_75'>75</a>, <a href='#Page_129'>129</a></li> - </ul> - </li> - <li class='c024'>Simpson - <ul> - <li>amount of excited activity in north of Norway, <a href='#Page_519'>519</a></li> - </ul> - </li> - <li class='c024'>Slater, Miss - <ul> - <li>effect of temperature on active deposit of thorium, <a href='#Page_354'>354</a></li> - </ul> - </li> - <li class='c024'>Smolan, Beattie and Kelvin - <ul> - <li>discharging power of uranium rays, <a href='#Page_7'>7</a></li> - </ul> - </li> - <li class='c024'>Snow - <ul> - <li>radio-activity of, <a href='#Page_506'>506</a></li> - <li>decay of activity of, <a href='#Page_507'>507</a></li> - </ul> - </li> - <li class='c024'>Soddy - <ul> - <li>comparison of photographic and electrical action of uranium rays, <a href='#Page_83'>83</a></li> - <li>nature of rays from Ur X, <a href='#Page_347'>347</a></li> - <li>production of radium from uranium, <a href='#Page_463'>463</a></li> - </ul> - </li> - <li class='c024'>Soddy and Ramsay - <ul> - <li>evolution of gas from radium, <a href='#Page_215'>215</a></li> - <li>production of hydrogen and oxygen from radium, <a href='#Page_215'>215</a></li> - <li>chemical nature of the emanation, <a href='#Page_268'>268</a></li> - <li>gaseous nature of the emanation, <a href='#Page_268'>268</a></li> - <li>volume of the emanation, and change with time, <a href='#Page_289'>289</a></li> - <li>helium from radium emanation, <a href='#Page_291'>291</a></li> - <li>amount of helium produced by radium, <a href='#Page_480'>480</a></li> - </ul> - </li> - <li class='c024'>Soddy and Rutherford - <ul> - <li>separation of Th X, <a href='#Page_220'>220</a></li> - <li>decay of activity of Th X, <a href='#Page_221'>221</a></li> - <li>recovery of activity of thorium freed from Th X, <a href='#Page_221'>221</a></li> - <li>decay of activity of Ur X, <a href='#Page_223'>223</a></li> - <li>recovery of activity of uranium freed from Ur X, <a href='#Page_223'>223</a></li> - <li>explanation of decay and recovery curves, <a href='#Page_224'>224</a></li> - <li>rate of production of Th X, <a href='#Page_227'>227</a></li> - <li>theory of decay of activity, <a href='#Page_229'>229</a></li> - <li>influence of conditions on rate of decay and recovery of activity, <a href='#Page_233'>233</a></li> - <li>disintegration hypothesis, <a href='#Page_234'>234</a></li> - <li>decay of activity of radium emanation, <a href='#Page_247'>247</a></li> - <li>measurements of emanating power, <a href='#Page_254'>254</a></li> - <li>effect of temperature, moisture, and solution, on emanating power, <a href='#Page_255'>255</a></li> - <li>regeneration of emanating power, <a href='#Page_256'>256</a></li> - <li>constant rate of production of emanation of radium and thorium, <a href='#Page_257'>257</a></li> - <li>source of thorium emanation, <a href='#Page_261'>261</a></li> - <li>radiations from the emanation, <a href='#Page_264'>264</a></li> - <li>chemical nature of emanation, <a href='#Page_267'>267</a></li> - <li>condensation of emanations of radium and thorium, <a href='#Page_277'>277</a></li> - <li>temperature of condensation of emanation, <a href='#Page_278'>278</a></li> - <li>effect of successive precipitations on activity of thorium, <a href='#Page_358'>358</a></li> - <li>recovery of activity of radium, <a href='#Page_372'>372</a></li> - <li>theory of radio-activity, <a href='#Page_439'>439</a></li> - <li>theory of radio-active change, <a href='#Page_445'>445</a></li> - <li>conservation of radio-activity, <a href='#Page_469'>469</a></li> - </ul> - </li> - <li class='c024'>Soil - <ul> - <li>radio-activity of, <a href='#Page_507'>507</a> <i>et seq.</i></li> - <li>difference in activity of, <a href='#Page_508'>508</a> <i>et seq.</i></li> - </ul> - </li> - <li class='c024'>Solution - <ul> - <li>coloration of, by radium, <a href='#Page_15'>15</a></li> - <li>of active deposit in acids, <a href='#Page_312'>312</a></li> - <li>electrolysis of active, <a href='#Page_313'>313</a></li> - </ul> - </li> - <li class='c024'>Source - <ul> - <li>of thorium emanation, <a href='#Page_261'>261</a></li> - <li>of radium and actinium emanations, <a href='#Page_263'>263</a></li> - </ul> - </li> - <li class='c024'>Spark - <ul> - <li>action of radium rays on, <a href='#Page_208'>208</a></li> - </ul> - </li> - <li class='c024'>Spectrum - <ul> - <li>spark spectrum of radium, <a href='#Page_15'>15</a>, <a href='#Page_16'>16</a></li> - <li>flame spectrum of radium, <a href='#Page_17'>17</a></li> - <li>effect of a magnetic field on spectrum of radium, <a href='#Page_17'>17</a></li> - <li>of polonium, <a href='#Page_23'>23</a></li> - <li>of phosphorescent light of radium bromide, <a href='#Page_206'>206</a></li> - <li>of emanation, <a href='#Page_292'>292</a></li> - <li>of helium in radium gases and emanation, <a href='#Page_477'>477</a></li> - </ul> - </li> - <li class='c024'><span class='pageno' id='Page_577'>577</span>Spinthariscope - <ul> - <li>description of, <a href='#Page_158'>158</a></li> - </ul> - </li> - <li class='c024'>Springs - <ul> - <li>emanation from water of, <a href='#Page_513'>513</a></li> - </ul> - </li> - <li class='c024'>Stark - <ul> - <li>energy to produce an ion, <a href='#Page_58'>58</a></li> - </ul> - </li> - <li class='c024'>Stoney, Johnstone - <ul> - <li>use of term electron, <a href='#Page_76'>76</a></li> - </ul> - </li> - <li class='c024'>Strauss and Hofmann - <ul> - <li>radio-active lead, <a href='#Page_27'>27</a></li> - </ul> - </li> - <li class='c024'>Strutt - <ul> - <li>conductivity of gases for radiation, <a href='#Page_63'>63</a>, <a href='#Page_64'>64</a></li> - <li>conductivity of gases produced by γ rays, <a href='#Page_64'>64</a>, <a href='#Page_183'>183</a></li> - <li>negative charge carried by β rays, <a href='#Page_122'>122</a> <i>et seq.</i></li> - <li>absorption of β rays proportional to density, <a href='#Page_136'>136</a></li> - <li>nature of α rays, <a href='#Page_142'>142</a></li> - <li>attempt to measure charge of α rays, <a href='#Page_153'>153</a></li> - <li>constant ratio of uranium to radium in minerals, <a href='#Page_462'>462</a></li> - <li>connection of thorium with helium, <a href='#Page_483'>483</a></li> - <li>absorption of radium rays from sun by atmosphere, <a href='#Page_492'>492</a></li> - <li>presence of radium in Bath waters, <a href='#Page_513'>513</a></li> - <li>radio-activity of ordinary matter, <a href='#Page_536'>536</a></li> - </ul> - </li> - <li class='c024'>Sun - <ul> - <li>effect of radium in, <a href='#Page_491'>491</a></li> - <li>age of, <a href='#Page_492'>492</a></li> - </ul> - </li> - <li class='c003'>Temperature - <ul> - <li>effect of, on intensity of radiations from uranium and radium, <a href='#Page_210'>210</a></li> - <li>effect of, on luminosity, <a href='#Page_210'>210</a></li> - <li>rate of decay of radium emanation unaffected by, <a href='#Page_249'>249</a></li> - <li>of condensation of emanations, <a href='#Page_283'>283</a></li> - <li>rate of decay of thorium emanation unaffected by, <a href='#Page_287'>287</a></li> - <li>effect of, on excited activity, <a href='#Page_315'>315</a></li> - <li>effect of, on active deposit of thorium, <a href='#Page_354'>354</a></li> - <li>effect of, on active deposit of actinium, <a href='#Page_368'>368</a></li> - <li>effect of, on active deposit of rapid change of radium, <a href='#Page_390'>390</a></li> - <li>effect of, on active deposit of slow change, <a href='#Page_401'>401</a></li> - <li>of radium above surrounding space, <a href='#Page_419'>419</a></li> - <li>effect of, on amount of excited activity in atmosphere, <a href='#Page_518'>518</a></li> - <li>effect of, on natural ionization of air, <a href='#Page_536'>536</a></li> - </ul> - </li> - <li class='c024'>Theories - <ul> - <li>of radio-activity, review of, <a href='#Page_437'>437</a> <i>et seq.</i></li> - <li>discussion of, <a href='#Page_441'>441</a> <i>et seq.</i></li> - <li>disintegration theory, <a href='#Page_445'>445</a> <i>et seq.</i></li> - </ul> - </li> - <li class='c024'>Thermo-luminescence, <a href='#Page_207'>207</a></li> - <li class='c024'>Thomson, J. J. - <ul> - <li>relation between current and voltage for ionized gases, <a href='#Page_34'>34</a></li> - <li>difference between ions as condensation nuclei, <a href='#Page_49'>49</a></li> - <li>charge on ion, <a href='#Page_50'>50</a></li> - <li>magnetic field produced by an ion in motion, <a href='#Page_69'>69</a></li> - <li>apparent mass of electron, <a href='#Page_71'>71</a></li> - <li>action of magnetic field on moving ion, <a href='#Page_72'>72</a></li> - <li>determination of <i>e</i>/<i>m</i> for cathode stream, <a href='#Page_73'>73</a></li> - <li>origin of X rays, <a href='#Page_80'>80</a></li> - <li>slow velocity electrons from radio-tellurium, <a href='#Page_153'>153</a></li> - <li>charge carried by α rays, <a href='#Page_154'>154</a></li> - <li>theory of radio-activity, <a href='#Page_440'>440</a></li> - <li>cause of heat emission from radium, <a href='#Page_442'>442</a></li> - <li>structure of atom, <a href='#Page_487'>487</a></li> - <li>possible causes of disintegration of radium, <a href='#Page_487'>487</a></li> - <li>nature of electrons, <a href='#Page_496'>496</a></li> - <li>emanation from tap-water and deep wells, <a href='#Page_510'>510</a></li> - <li>radio-activity of ordinary materials, <a href='#Page_539'>539</a></li> - </ul> - </li> - <li class='c024'>Thomson, J. J. and Rutherford - <ul> - <li>ionization theory of gases, <a href='#Page_31'>31</a> <i>et seq.</i></li> - </ul> - </li> - <li class='c024'>Thorium - <ul> - <li>discovery of radio-activity of, <a href='#Page_10'>10</a></li> - <li>emanation from, <a href='#Page_11'>11</a></li> - <li>preparation of non-radio-active thorium, <a href='#Page_29'>29</a></li> - <li>nature of radiations from, <a href='#Page_109'>109</a></li> - <li>β rays from, <a href='#Page_114'>114</a></li> - <li>α rays from, <a href='#Page_141'>141</a></li> - <li>γ rays from, <a href='#Page_180'>180</a></li> - <li>separation of Th X from, <a href='#Page_220'>220</a></li> - <li>recovery of activity of, <a href='#Page_221'>221</a></li> - <li>disintegration of, <a href='#Page_234'>234</a></li> - <li>emanation from, <a href='#Page_238'>238</a></li> - <li>properties of emanation from, <a href='#Page_239'>239</a></li> - <li>diffusion of emanation from, <a href='#Page_275'>275</a></li> - <li>condensation of emanation from, <a href='#Page_277'>277</a></li> - <li>excited radio-activity from, <a href='#Page_295'>295</a> <i>et seq.</i></li> - <li>analysis of active deposit of, <a href='#Page_351'>351</a> <i>et seq.</i></li> - <li>rayless change in, <a href='#Page_352'>352</a></li> - <li>explanation of initial portion of decay curve, <a href='#Page_358'>358</a></li> - <li>explanation of initial portion of recovery curve, <a href='#Page_358'>358</a></li> - <li><span class='pageno' id='Page_578'>578</span>effect of successive precipitations on, <a href='#Page_358'>358</a></li> - <li>recovery curve after large number of precipitations, <a href='#Page_359'>359</a></li> - <li>products of, <a href='#Page_363'>363</a></li> - <li>non-separable activity of, <a href='#Page_363'>363</a></li> - <li>radiations from active products of, <a href='#Page_363'>363</a></li> - <li>division of activity amongst active products of, <a href='#Page_363'>363</a></li> - <li>rate of emission of energy by, <a href='#Page_432'>432</a></li> - <li>theories of radio-activity of, <a href='#Page_438'>438</a></li> - <li>discussion of theories of radio-activity, <a href='#Page_441'>441</a> <i>et seq.</i></li> - <li>source of energy of radiations, <a href='#Page_442'>442</a> <i>et seq.</i></li> - <li>theory of radio-active change, <a href='#Page_444'>444</a> <i>et seq.</i></li> - <li>table of radio-active products of, <a href='#Page_448'>448</a></li> - <li>rate of change of, <a href='#Page_458'>458</a></li> - <li>life of, <a href='#Page_458'>458</a></li> - <li>conservation of radio-activity of, <a href='#Page_469'>469</a></li> - <li>total emission of energy from 1 gram of, <a href='#Page_475'>475</a></li> - <li>possible causes of disintegration of, <a href='#Page_486'>486</a> <i>et seq.</i></li> - </ul> - </li> - <li class='c024'>Thorium A - <ul> - <li>period and properties of, <a href='#Page_352'>352</a> <i>et seq.</i></li> - <li>absence of rays in, <a href='#Page_352'>352</a></li> - <li>effect of temperature on, <a href='#Page_354'>354</a></li> - </ul> - </li> - <li class='c024'>Thorium B - <ul> - <li>period and properties of, <a href='#Page_352'>352</a> <i>et seq.</i></li> - <li>effect of temperature on, <a href='#Page_354'>354</a></li> - <li>radiations from, <a href='#Page_363'>363</a></li> - </ul> - </li> - <li class='c024'>Thorium X - <ul> - <li>methods of separation of, <a href='#Page_220'>220</a></li> - <li>law of decay of activity of, <a href='#Page_221'>221</a></li> - <li>law of recovery of activity of, <a href='#Page_221'>221</a></li> - <li>theory to explain production of, <a href='#Page_224'>224</a></li> - <li>material nature of, <a href='#Page_226'>226</a></li> - <li>continuous production of, <a href='#Page_227'>227</a></li> - <li>explanation of decay of activity of, <a href='#Page_229'>229</a></li> - <li>effect of conditions on the rate of change of, <a href='#Page_233'>233</a></li> - <li>disintegration hypothesis to explain production of, <a href='#Page_234'>234</a></li> - <li>minute amount of, produced, <a href='#Page_237'>237</a></li> - <li>effect of successive separations of, on activity of thorium, <a href='#Page_358'>358</a> <i>et seq.</i></li> - <li>analysis of decay and recovery curves of, <a href='#Page_358'>358</a></li> - <li>radiations from, <a href='#Page_363'>363</a></li> - </ul> - </li> - <li class='c024'>Tommasina - <ul> - <li>scintillations produced by electrification, <a href='#Page_160'>160</a></li> - </ul> - </li> - <li class='c024'>Townsend - <ul> - <li>ions by collision, <a href='#Page_39'>39</a>, <a href='#Page_57'>57</a></li> - <li>coefficient of recombination, <a href='#Page_41'>41</a></li> - <li>diffusion of ions, <a href='#Page_51'>51</a> <i>et seq.</i></li> - <li>comparison of charge on ion with that on hydrogen atom in electrolysis, <a href='#Page_53'>53</a></li> - <li>number of molecules per c.c. of gas, <a href='#Page_54'>54</a></li> - <li>ionization by collision for different speeds, <a href='#Page_171'>171</a></li> - </ul> - </li> - <li class='c024'><a id='index-transformations'></a></li> - <li class='c024'>Transformations, successive - <ul> - <li>theory of, <a href='#Page_325'>325</a> <i>et seq.</i></li> - <li>nomenclature of, <a href='#Page_328'>328</a></li> - <li>activity due to, <a href='#Page_337'>337</a></li> - <li>detection of a rayless change in, <a href='#Page_341'>341</a></li> - <li>in uranium, <a href='#Page_346'>346</a> <i>et seq.</i></li> - <li>in thorium, <a href='#Page_351'>351</a> <i>et seq.</i></li> - <li>in actinium, <a href='#Page_364'>364</a> <i>et seq.</i></li> - <li>in radium, <a href='#Page_371'>371</a> <i>et seq.</i></li> - <li>list of, <a href='#Page_448'>448</a></li> - <li>origin of radium in, <a href='#Page_459'>459</a></li> - <li>helium, a result of, <a href='#Page_476'>476</a> <i>et seq.</i></li> - <li>possible cause of, <a href='#Page_486'>486</a> <i>et seq.</i></li> - <li>application of, to evolution of matter, <a href='#Page_497'>497</a> <i>et seq.</i></li> - </ul> - </li> - <li class='c024'>Transmission - <ul> - <li>of excited radio-activity of radium and thorium, <a href='#Page_318'>318</a> <i>et seq.</i></li> - <li>of excited radio-activity of actinium, <a href='#Page_323'>323</a></li> - </ul> - </li> - <li class='c024'>Travers and Ramsay - <ul> - <li>amount of helium in fergusonite, <a href='#Page_486'>486</a></li> - </ul> - </li> - <li class='c024'>Troost - <ul> - <li>rays from hexagonal blende, <a href='#Page_4'>4</a></li> - </ul> - </li> - <li class='c003'>Uranium - <ul> - <li>discovery of radio-activity of, <a href='#Page_5'>5</a></li> - <li>persistence of radiations of, <a href='#Page_6'>6</a></li> - <li>discharging power of rays, <a href='#Page_7'>7</a></li> - <li>absence of reflection, refraction and polarization, <a href='#Page_7'>7</a></li> - <li>examination of uranium minerals, <a href='#Page_11'>11</a> <i>et seq.</i></li> - <li>relative activity of compounds of uranium, <a href='#Page_12'>12</a></li> - <li>nature of radiations from, <a href='#Page_109'>109</a></li> - <li>β rays from, <a href='#Page_114'>114</a></li> - <li>α rays from, <a href='#Page_141'>141</a></li> - <li>γ rays from, <a href='#Page_180'>180</a></li> - <li>separation of Ur X from, <a href='#Page_219'>219</a></li> - <li>recovery of activity of, <a href='#Page_219'>219</a></li> - <li>changes in, <a href='#Page_346'>346</a> <i>et seq.</i></li> - <li>non-separable activity of, <a href='#Page_347'>347</a></li> - <li>radiations from Ur X, <a href='#Page_347'>347</a> <i>et seq.</i></li> - <li>method of measurement of activity of Ur X, <a href='#Page_347'>347</a></li> - <li>emission of energy by, <a href='#Page_418'>418</a></li> - <li>theories of radio-activity of, <a href='#Page_437'>437</a> <i>et seq.</i></li> - <li>discussion of theories of radio-activity, <a href='#Page_441'>441</a> <i>et seq.</i></li> - <li><span class='pageno' id='Page_579'>579</span>source of energy of radiation, <a href='#Page_442'>442</a> <i>et seq.</i></li> - <li>theory of radio-active change, <a href='#Page_444'>444</a> <i>et seq.</i></li> - <li>table of active products, <a href='#Page_448'>448</a></li> - <li>rate of change of, <a href='#Page_458'>458</a></li> - <li>life of, <a href='#Page_458'>458</a></li> - <li>radium probable product of, <a href='#Page_459'>459</a> <i>et seq.</i></li> - <li>amount of radium in, <a href='#Page_460'>460</a> <i>et seq.</i></li> - <li>amount of, in radio-active minerals, <a href='#Page_461'>461</a></li> - <li>growth of radium in, <a href='#Page_463'>463</a></li> - <li>conservation of radio-activity of, <a href='#Page_469'>469</a></li> - <li>total emission of energy from 1 gram of, <a href='#Page_475'>475</a></li> - <li>possible causes of disintegration of, <a href='#Page_486'>486</a> <i>et seq.</i></li> - </ul> - </li> - <li class='c024'>Uranium X - <ul> - <li>separation of, by Crookes, <a href='#Page_219'>219</a></li> - <li>separation of, by Becquerel, <a href='#Page_219'>219</a></li> - <li>decay of activity of, <a href='#Page_223'>223</a></li> - <li>recovery of activity of, <a href='#Page_223'>223</a></li> - <li>theory to explain production of, <a href='#Page_224'>224</a> <i>et seq.</i></li> - <li>material nature of, <a href='#Page_226'>226</a></li> - <li>explanation of decay of activity of, <a href='#Page_229'>229</a></li> - <li>changes in, <a href='#Page_346'>346</a> <i>et seq.</i></li> - <li>radiations from, <a href='#Page_347'>347</a> <i>et seq.</i></li> - <li>method of measurement of radiations from, <a href='#Page_347'>347</a></li> - <li>effect of crystallization on activity of, <a href='#Page_349'>349</a></li> - <li>diffusion of, <a href='#Page_350'>350</a></li> - </ul> - </li> - <li class='c003'>Velocity - <ul> - <li>of ions in electric field, <a href='#Page_42'>42</a> <i>et seq.</i></li> - <li>difference between, of positive and negative ions, <a href='#Page_43'>43</a> <i>et seq.</i></li> - <li>of β particle or electron, <a href='#Page_126'>126</a> <i>et seq.</i></li> - <li>variation of mass of electron with, <a href='#Page_127'>127</a></li> - <li>of α particle, <a href='#Page_148'>148</a></li> - <li>of transmission of carriers of excited activity, <a href='#Page_320'>320</a> <i>et seq.</i></li> - <li>of ions in atmosphere, <a href='#Page_528'>528</a></li> - </ul> - </li> - <li class='c024'>Villard - <ul> - <li>discovery of γ rays from radium, <a href='#Page_179'>179</a></li> - <li>alteration of X ray screen with time, <a href='#Page_205'>205</a></li> - <li>activity produced by cathode rays, <a href='#Page_530'>530</a></li> - </ul> - </li> - <li class='c024'>Vincenti and Levi Da Zara - <ul> - <li>radium emanation in spring waters, <a href='#Page_516'>516</a></li> - </ul> - </li> - <li class='c024'>Voller - <ul> - <li>variation of activity of radium with concentration, <a href='#Page_467'>467</a></li> - </ul> - </li> - <li class='c024'>Volume - <ul> - <li>of radium emanation, calculation of, <a href='#Page_289'>289</a></li> - <li>decrease of, of radium emanation, <a href='#Page_290'>290</a></li> - </ul> - </li> - <li class='c003'>Walker, G. W. - <ul> - <li>theory of electrometer, <a href='#Page_90'>90</a></li> - </ul> - </li> - <li class='c024'>Walkhoff - <ul> - <li>action of radium rays on skin, <a href='#Page_216'>216</a></li> - </ul> - </li> - <li class='c024'>Wallstabe - <ul> - <li>diffusion of radium emanation into liquids, <a href='#Page_276'>276</a></li> - </ul> - </li> - <li class='c024'>Water - <ul> - <li>emanation from, <a href='#Page_510'>510</a> <i>et seq.</i></li> - <li>decay of activity of emanation from, <a href='#Page_511'>511</a> <i>et seq.</i></li> - </ul> - </li> - <li class='c024'>Water-falls - <ul> - <li>amount of excited activity produced at Niagara, <a href='#Page_520'>520</a></li> - <li>electrification produced near, <a href='#Page_520'>520</a></li> - </ul> - </li> - <li class='c024'>Watts, Marshall - <ul> - <li>atomic weight of radium, <a href='#Page_18'>18</a></li> - </ul> - </li> - <li class='c024'>Weichert - <ul> - <li>velocity of cathode rays, <a href='#Page_76'>76</a></li> - </ul> - </li> - <li class='c024'>Weight - <ul> - <li>loss of by radio-elements, <a href='#Page_473'>473</a></li> - <li>attempts to measure loss of in radium, <a href='#Page_474'>474</a></li> - </ul> - </li> - <li class='c024'>Wheeler and Bumstead - <ul> - <li>diffusion of radium emanation, <a href='#Page_273'>273</a></li> - <li>emanation from surface water and the soil, <a href='#Page_512'>512</a>, <a href='#Page_522'>522</a></li> - <li>identity of emanation from soil with radium emanation, <a href='#Page_512'>512</a>, <a href='#Page_522'>522</a></li> - </ul> - </li> - <li class='c024'>Whetham - <ul> - <li>effect of valency of ion on colloidal solutions, <a href='#Page_215'>215</a></li> - <li>production of radium from uranium, <a href='#Page_463'>463</a></li> - </ul> - </li> - <li class='c024'>Wiedemann - <ul> - <li>thermo-luminescence, <a href='#Page_207'>207</a></li> - </ul> - </li> - <li class='c024'>Wiedemann and Schmidt - <ul> - <li>thermo-luminescence, <a href='#Page_207'>207</a></li> - </ul> - </li> - <li class='c024'>Wien - <ul> - <li>value of <i>e</i>/<i>m</i> for canal rays, <a href='#Page_78'>78</a></li> - <li>positive charge of canal rays, <a href='#Page_78'>78</a></li> - <li>amount of charge carried by β rays, <a href='#Page_124'>124</a></li> - </ul> - </li> - <li class='c024'>Willcock, Miss and Hardy - <ul> - <li>coloration of iodoform solution by radium rays, <a href='#Page_214'>214</a></li> - </ul> - </li> - <li class='c024'>Willemite - <ul> - <li>phosphorescence of, under radium rays, <a href='#Page_203'>203</a></li> - <li>use of, to show condensation of emanation, <a href='#Page_279'>279</a></li> - </ul> - </li> - <li class='c024'>Willows and Peck - <ul> - <li>action of radium rays on spark, <a href='#Page_208'>208</a></li> - </ul> - </li> - <li class='c024'><span class='pageno' id='Page_580'>580</span>Wilson, C. T. R. - <ul> - <li>ions as nuclei of condensation, <a href='#Page_47'>47</a> <i>et seq.</i></li> - <li>difference between positive and negative ions as condensation nuclei, <a href='#Page_49'>49</a></li> - <li>equality of charges carried by positive and negative ions, <a href='#Page_50'>50</a></li> - <li>construction of electroscope, <a href='#Page_86'>86</a>, <a href='#Page_88'>88</a></li> - <li>natural ionization of air in vessels, <a href='#Page_501'>501</a></li> - <li>radio-activity of rain and snow, <a href='#Page_505'>505</a>, <a href='#Page_506'>506</a></li> - <li>loss of charge in closed vessels, <a href='#Page_531'>531</a> <i>et seq.</i>, <a href='#Page_534'>534</a></li> - <li>presence of ions in dust-free air shown by condensation, <a href='#Page_533'>533</a></li> - <li>number of ions produced per c.c., <a href='#Page_533'>533</a></li> - <li>effect of pressure and nature of gas on ionization in sealed vessels, <a href='#Page_534'>534</a></li> - </ul> - </li> - <li class='c024'>Wilson, H. A. - <ul> - <li>charge on ion, <a href='#Page_51'>51</a></li> - </ul> - </li> - <li class='c024'>Wilson, W. E. - <ul> - <li>radium in sun, <a href='#Page_491'>491</a></li> - </ul> - </li> - <li class='c024'>Wölfl, Hofmann and Gonder - <ul> - <li>properties of radio-active lead, <a href='#Page_27'>27</a>, <a href='#Page_413'>413</a></li> - </ul> - </li> - <li class='c024'>Wood, A. - <ul> - <li>radio-activity of ordinary materials, <a href='#Page_540'>540</a></li> - </ul> - </li> - <li class='c003'>Zara, Levi Da and Vincenti - <ul> - <li>radium emanation in spring waters, <a href='#Page_516'>516</a></li> - </ul> - </li> - <li class='c024'>Zeeman - <ul> - <li>action of magnetic field on light, <a href='#Page_77'>77</a></li> - </ul> - </li> - <li class='c024'>Zeleny - <ul> - <li>velocity of ions, <a href='#Page_42'>42</a> <i>et seq.</i></li> - <li>difference of velocity of ions, <a href='#Page_45'>45</a></li> - <li>potential gradient between electrodes, <a href='#Page_65'>65</a></li> - </ul> - </li> - <li class='c024'>Zerban and Hofmann - <ul> - <li>connection of activity of thorium with uranium, <a href='#Page_29'>29</a></li> - </ul> - </li> - <li class='c024'>Zinc Sulphide - <ul> - <li>scintillations produced in by α rays, <a href='#Page_158'>158</a></li> - <li>cause of luminosity of, <a href='#Page_160'>160</a>, <a href='#Page_549'>549</a></li> - </ul> - </li> -</ul> - -<p class='c006'>CAMBRIDGE: PRINTED BY JOHN CLAY, M.A. AT THE UNIVERSITY PRESS.</p> -<div class='chapter'> - <span class='pageno' id='Page_581'>581</span> - <h2 class='c004'>CAMBRIDGE PHYSICAL SERIES.</h2> -</div> - -<p class='c010'><b>Conduction of Electricity through Gases.</b> By <span class='sc'>J. J. -Thomson</span>, D.Sc., LL.D., Ph.D., F.R.S., Fellow of Trinity College -and Cavendish Professor of Experimental Physics. Demy 8vo. -viii + 568 pp. 16<i>s.</i></p> - -<p class='c018'>CONTENTS.</p> - -<p class='c021'>I. Electrical Conductivity of -Gases in a normal state.</p> - -<p class='c011'>II. Properties of a Gas when in -the conducting state.</p> - -<p class='c011'>III. Mathematical Theory of the -Conduction of Electricity -through a Gas containing -Ions.</p> - -<p class='c011'>IV. Effect produced by a Magnetic -Field on the Motion -of the Ions.</p> - -<p class='c011'>V. Determination of the Ratio -of the Charge to the Mass -of an Ion.</p> - -<p class='c011'>VI. Determination of the Charge -carried by the Negative -Ion.</p> - -<p class='c011'>VII. On some Physical Properties -of Gaseous Ions.</p> - -<p class='c011'>VIII. Ionisation by Incandescent -Solids.</p> - -<p class='c011'>IX. Ionisation in Gases from -Flames.</p> - -<p class='c011'>X. 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A considerable -portion of the present work has been in type for a long time; it was used -originally as a part of the practical work in Physics for Medical Students -at the Cavendish Laboratory in connexion with my lectures, and was -expanded by Mr Wilberforce and Mr Fitzpatrick in one of their Laboratory -Note-books of Practical Physics.</p> - -<p class='c006'>When I ceased to deliver the first year course I was asked to print my -lectures for the use, primarily, of the Students attending the practical -classes; the lectures on Mechanics, Heat and Light have been in type for -some years. 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Demy -8vo. pp. x + 400. 10s. 6<i>d.</i> Net.</p> - -<p class='c018'><i>Athenæum.</i>—“English students have had to wait till now for any -connected and detailed account of this new branch of physics from the -pen of one who has a first hand knowledge of it.”</p> - -<p class='c006'><i>Nature.</i>—“The arrangement of the matter and its treatment are -throughout admirable.”</p> -<p class='c010'><b>The Theory of Experimental Electricity.</b> By -<span class='sc'>W. C. D. Whetham</span>, M.A., F.R.S., Fellow of Trinity College. -Demy 8vo. 8<i>s.</i> Net.</p> - -<div class='lg-container-b c025'> - <div class='linegroup'> - <div class='group'> - <div class='line'>CAMBRIDGE UNIVERSITY PRESS WAREHOUSE,</div> - <div class='line'>C. F. CLAY, <span class='sc'>Manager</span>,</div> - <div class='line'>London: AVE MARIA LANE,</div> - <div class='line'>Glasgow: 50, WELLINGTON STREET.</div> - <div class='line'>ALSO</div> - <div class='line'>London: H. K. LEWIS, 136, GOWER STREET, W.C.</div> - </div> - </div> -</div> - -<div class='nf-center-c1'> -<div class='nf-center c002'> - <div><span class='large'>Footnotes</span></div> - </div> -</div> - -<div class='footnote' id='f1'> -<p class='c006'><span class='label'><a href='#r1'>1</a>. </span>Niewenglowski, <i>C. R.</i> 122, p. 385, 1896.</p> -</div> -<div class='footnote' id='f2'> -<p class='c006'><span class='label'><a href='#r2'>2</a>. </span>Becquerel, <i>C. R.</i> 122, p. 559, 1896.</p> -</div> -<div class='footnote' id='f3'> -<p class='c006'><span class='label'><a href='#r3'>3</a>. </span>Troost, <i>C. R.</i> 122, p. 564, 1896.</p> -</div> -<div class='footnote' id='f4'> -<p class='c006'><span class='label'><a href='#r4'>4</a>. </span>Arnold, <i>Annal. d. Phys.</i> 61, p. 316, 1897.</p> -</div> -<div class='footnote' id='f5'> -<p class='c006'><span class='label'><a href='#r5'>5</a>. </span>Le Bon, <i>C. R.</i> 122, pp. 188, 233, 386, 462, 1896.</p> -</div> -<div class='footnote' id='f6'> -<p class='c006'><span class='label'><a href='#r6'>6</a>. </span>Becquerel, <i>C. R.</i> 122, pp. 420, 501, 559, 689, 762, 1086, 1896.</p> -</div> -<div class='footnote' id='f7'> -<p class='c006'><span class='label'><a href='#r7'>7</a>. </span>Mme Curie, <i>Thèse présentée à la Faculté des Sciences de Paris</i>, 1903.</p> -</div> -<div class='footnote' id='f8'> -<p class='c006'><span class='label'><a href='#r8'>8</a>. </span><i>Nature</i>, 56, 1897; <i>Phil. Mag.</i> 43, p. 418, 1897; 45, p. 277, 1898.</p> -</div> -<div class='footnote' id='f9'> -<p class='c006'><span class='label'><a href='#r9'>9</a>. </span>Rutherford, <i>Phil. Mag.</i> Jan. 1899.</p> -</div> -<div class='footnote' id='f10'> -<p class='c006'><span class='label'><a href='#r10'>10</a>. </span><i>Ibid.</i></p> -</div> -<div class='footnote' id='f11'> -<p class='c006'><span class='label'><a href='#r11'>11</a>. </span>Le Bon, <i>C. R.</i> 130, p. 891, 1900.</p> -</div> -<div class='footnote' id='f12'> -<p class='c006'><span class='label'><a href='#r12'>12</a>. </span>Lenard, <i>Annal. d. Phys.</i> 1, p. 498; 3, p. 298, 1900.</p> -</div> -<div class='footnote' id='f13'> -<p class='c006'><span class='label'><a href='#r13'>13</a>. </span>Schmidt, <i>Annal. d. Phys.</i> 65, p. 141, 1898.</p> -</div> -<div class='footnote' id='f14'> -<p class='c006'><span class='label'><a href='#r14'>14</a>. </span>Mme Curie, <i>C. R.</i> 126, p. 1101, 1898.</p> -</div> -<div class='footnote' id='f15'> -<p class='c006'><span class='label'><a href='#r15'>15</a>. </span>Owens, <i>Phil. Mag.</i> Oct. 1899.</p> -</div> -<div class='footnote' id='f16'> -<p class='c006'><span class='label'><a href='#r16'>16</a>. </span>Rutherford, <i>Phil. Mag.</i> Jan. 1900.</p> -</div> -<div class='footnote' id='f17'> -<p class='c006'><span class='label'><a href='#r17'>17</a>. </span>M. and Mme Curie and G. Bemont, <i>C. R.</i> 127, p. 1215, 1898.</p> -</div> -<div class='footnote' id='f18'> -<p class='c006'><span class='label'><a href='#r18'>18</a>. </span>Giesel, <i>Phys. Zeit.</i> 3, No. 24, p. 578, 1902.</p> -</div> -<div class='footnote' id='f19'> -<p class='c006'><span class='label'><a href='#r19'>19</a>. </span>Giesel, <i>Annal. d. Phys.</i> 69, p. 91, 1890. <i>Ber. d. D. Chem. Ges.</i> p. 3608, 1902.</p> -</div> -<div class='footnote' id='f20'> -<p class='c006'><span class='label'><a href='#r20'>20</a>. </span>Demarçay, <i>C. R.</i> 127, p. 1218, 1898; 129, p. 716, 1899; 131, p. 258, 1900.</p> -</div> -<div class='footnote' id='f21'> -<p class='c006'><span class='label'><a href='#r21'>21</a>. </span>Runge, <i>Astrophys. Journal</i>, p. 1, 1900. <i>Annal. d. Phys.</i> No. 10, p. 407, 1903.</p> -</div> -<div class='footnote' id='f22'> -<p class='c006'><span class='label'><a href='#r22'>22</a>. </span>Exner and Haschek, <i>Wien. Ber.</i> July 4, 1901.</p> -</div> -<div class='footnote' id='f23'> -<p class='c006'><span class='label'><a href='#r23'>23</a>. </span>Crookes, <i>Proc. Roy. Soc.</i> 72, p. 295, 1904.</p> -</div> -<div class='footnote' id='f24'> -<p class='c006'><span class='label'><a href='#r24'>24</a>. </span>Runge and Precht, <i>Annal. d. Phys.</i> <span class='fss'>XIV</span>. 2, p. 418, 1904.</p> -</div> -<div class='footnote' id='f25'> -<p class='c006'><span class='label'><a href='#r25'>25</a>. </span>Runge and Precht, <i>Phil. Mag.</i> April, 1903.</p> -</div> -<div class='footnote' id='f26'> -<p class='c006'><span class='label'><a href='#r26'>26</a>. </span>Watts, <i>Phil. Mag.</i> July, 1903; August, 1904.</p> -</div> -<div class='footnote' id='f27'> -<p class='c006'><span class='label'><a href='#r27'>27</a>. </span>Runge, <i>Phil. Mag.</i> December, 1903.</p> -</div> -<div class='footnote' id='f28'> -<p class='c006'><span class='label'><a href='#r28'>28</a>. </span>Debierne, <i>C. R.</i> 129, p. 593, 1899; 130, p. 206, 1900.</p> -</div> -<div class='footnote' id='f29'> -<p class='c006'><span class='label'><a href='#r29'>29</a>. </span>Giesel, <i>Ber. d. D. Chem. Ges.</i> p. 3608, 1902; p. 342, 1903.</p> -</div> -<div class='footnote' id='f30'> -<p class='c006'><span class='label'><a href='#r30'>30</a>. </span>Debierne, <i>C. R.</i> 139, p. 538, 1904. Miss Brooks, <i>Phil. Mag.</i> Sept. 1904. -Giesel, <i>Phys. Zeit.</i> 5, p. 822, 1904. <i>Jahrbuch. d. Radioaktivität</i>, no. 4, p. 345, 1904.</p> -</div> -<div class='footnote' id='f31'> -<p class='c006'><span class='label'><a href='#r31'>31</a>. </span>Giesel, <i>Ber. d. D. Chem. Ges.</i> 37, p. 1696, 1904; Hartmann, <i>Phys. Zeit.</i> 5, -No. 18, p. 570, 1904.</p> -</div> -<div class='footnote' id='f32'> -<p class='c006'><span class='label'><a href='#r32'>32</a>. </span>Mme Curie, <i>C. R.</i> 127, p. 175, 1898.</p> -</div> -<div class='footnote' id='f33'> -<p class='c006'><span class='label'><a href='#r33'>33</a>. </span>Mme Curie, <i>Thèse</i>, Paris, 1903.</p> -</div> -<div class='footnote' id='f34'> -<p class='c006'><span class='label'><a href='#r34'>34</a>. </span>Crookes, <i>Proc. Roy. Soc.</i> May, 1900.</p> -</div> -<div class='footnote' id='f35'> -<p class='c006'><span class='label'><a href='#r35'>35</a>. </span>Berndt, <i>Phys. Zeit.</i> 2, p. 180, 1900.</p> -</div> -<div class='footnote' id='f36'> -<p class='c006'><span class='label'><a href='#r36'>36</a>. </span>Marckwald, <i>Phys. Zeit.</i> 4, No. 1 b, p. 51.</p> -</div> -<div class='footnote' id='f37'> -<p class='c006'><span class='label'><a href='#r37'>37</a>. </span>Marckwald, <i>Ber. d. D. Chem. Ges.</i> p. 2662, No. 12, 1903.</p> -</div> -<div class='footnote' id='f38'> -<p class='c006'><span class='label'><a href='#r38'>38</a>. </span>Elster and Geitel, <i>Annal. d. Phys.</i> 69, p. 83, 1899.</p> -</div> -<div class='footnote' id='f39'> -<p class='c006'><span class='label'><a href='#r39'>39</a>. </span>Giesel, <i>Ber. d. D. Chem. Ges.</i> p. 3775, 1901.</p> -</div> -<div class='footnote' id='f40'> -<p class='c006'><span class='label'><a href='#r40'>40</a>. </span>Hofmann and Strauss, <i>Ber. d. D. Chem. Ges.</i> p. 3035, 1901.</p> -</div> -<div class='footnote' id='f41'> -<p class='c006'><span class='label'><a href='#r41'>41</a>. </span>Hofmann, Gonder and Wölfl, <i>Annal. d. Phys.</i> No. 13, p. 615, 1904.</p> -</div> -<div class='footnote' id='f42'> -<p class='c006'><span class='label'><a href='#r42'>42</a>. </span>Hofmann and Zerban, <i>Ber. d. D. Chem. Ges.</i> No. 12, p. 3093, 1903.</p> -</div> -<div class='footnote' id='f43'> -<p class='c006'><span class='label'><a href='#r43'>43</a>. </span>Baskerville and Zerban, <i>Amer. Chem. Soc.</i> 26, p. 1642, 1904.</p> -</div> -<div class='footnote' id='f44'> -<p class='c006'><span class='label'><a href='#r44'>44</a>. </span>J. J. Thomson and Rutherford, <i>Phil. Mag.</i> Nov. 1896.</p> -</div> -<div class='footnote' id='f45'> -<p class='c006'><span class='label'><a href='#r45'>45</a>. </span>The word ion has now been generally adopted in the literature of the subject. -In using this word, it is not assumed that the ions in gases are the same as the -corresponding ions in the electrolysis of solutions.</p> -</div> -<div class='footnote' id='f46'> -<p class='c006'><span class='label'><a href='#r46'>46</a>. </span>A minute current is observed between the plates even if no radio-active matter -be present. This has been found to be due mainly to a slight natural radio-activity -of the matter composing them. (See <a href='#chap14'>chapter <span class='fss'>XIV.</span></a>)</p> -</div> -<div class='footnote' id='f47'> -<p class='c006'><span class='label'><a href='#r47'>47</a>. </span>This nomenclature has arisen from the similarity of the shape of the current-voltage -curves to the magnetization curves for iron. Since, on the ionization -theory, the maximum current is a result of the <i>removal</i> of all the ions from the gas, -before recombination occurs, the terms are not very suitable. They have however -now come into general use and will be retained throughout this work.</p> -</div> -<div class='footnote' id='f48'> -<p class='c006'><span class='label'><a href='#r48'>48</a>. </span>J. J. Thomson, <i>Phil. Mag.</i> 47, p. 253, 1899; <i>Conduction of Electricity through -Gases</i>, p. 73, 1903.</p> -</div> -<div class='footnote' id='f49'> -<p class='c006'><span class='label'><a href='#r49'>49</a>. </span>Rutherford, <i>Phil. Mag.</i> Jan. 1899.</p> -</div> -<div class='footnote' id='f50'> -<p class='c006'><span class='label'><a href='#r50'>50</a>. </span>Townsend, <i>Phil. Mag.</i> Feb. 1901.</p> -</div> -<div class='footnote' id='f51'> -<p class='c006'><span class='label'><a href='#r51'>51</a>. </span>Rutherford, <i>Phil. Mag.</i> Nov. 1897, p. 144, Jan. 1899.</p> -</div> -<div class='footnote' id='f52'> -<p class='c006'><span class='label'><a href='#r52'>52</a>. </span>Townsend, <i>Phil. Trans.</i> A, p. 157, 1899.</p> -</div> -<div class='footnote' id='f53'> -<p class='c006'><span class='label'><a href='#r53'>53</a>. </span>McClung, <i>Phil. Mag.</i> March, 1902.</p> -</div> -<div class='footnote' id='f54'> -<p class='c006'><span class='label'><a href='#r54'>54</a>. </span>Langevin, <i>Thèse présentée à la Faculté des Sciences</i>, p. 151, Paris, 1902.</p> -</div> -<div class='footnote' id='f55'> -<p class='c006'><span class='label'><a href='#r55'>55</a>. </span>Owens, <i>Phil. Mag.</i> Oct. 1899.</p> -</div> -<div class='footnote' id='f56'> -<p class='c006'><span class='label'><a href='#r56'>56</a>. </span>Rutherford, <i>Phil. Mag.</i> p. 429, Nov. 1897.</p> -</div> -<div class='footnote' id='f57'> -<p class='c006'><span class='label'><a href='#r57'>57</a>. </span>Zeleny, <i>Phil. Trans.</i> A, p. 193, 1901.</p> -</div> -<div class='footnote' id='f58'> -<p class='c006'><span class='label'><a href='#r58'>58</a>. </span>Langevin, <i>C. R.</i> 134, p. 646, 1902.</p> -</div> -<div class='footnote' id='f59'> -<p class='c006'><span class='label'><a href='#r59'>59</a>. </span>Zeleny, <i>Phil. Mag.</i> July, 1898.</p> -</div> -<div class='footnote' id='f60'> -<p class='c006'><span class='label'><a href='#r60'>60</a>. </span>Rutherford, <i>Phil. Mag.</i> Feb. 1899.</p> -</div> -<div class='footnote' id='f61'> -<p class='c006'><span class='label'><a href='#r61'>61</a>. </span>Zeleny, <i>Phil. Trans.</i> 195, p. 193, 1900.</p> -</div> -<div class='footnote' id='f62'> -<p class='c006'><span class='label'><a href='#r62'>62</a>. </span>Langevin, <i>C. R.</i> 134, p. 646, 1902, and Thesis, p. 191, 1902.</p> -</div> -<div class='footnote' id='f63'> -<p class='c006'><span class='label'><a href='#r63'>63</a>. </span>Rutherford, <i>Proc. Camb. Phil. Soc.</i> 9, p. 410, 1898.</p> -</div> -<div class='footnote' id='f64'> -<p class='c006'><span class='label'><a href='#r64'>64</a>. </span>Langevin, Thesis, p. 190, 1902.</p> -</div> -<div class='footnote' id='f65'> -<p class='c006'><span class='label'><a href='#r65'>65</a>. </span>Helmholtz and Richarz, <i>Annal. d. Phys.</i> 40, p. 161, 1890.</p> -</div> -<div class='footnote' id='f66'> -<p class='c006'><span class='label'><a href='#r66'>66</a>. </span>Wilson, <i>Phil. Trans.</i> p. 265, 1897; p. 403, 1899; p. 289, 1900.</p> -</div> -<div class='footnote' id='f67'> -<p class='c006'><span class='label'><a href='#r67'>67</a>. </span>Thomson, <i>Phil. Mag.</i> p. 528, Dec. 1898.</p> -</div> -<div class='footnote' id='f68'> -<p class='c006'><span class='label'><a href='#r68'>68</a>. </span>Wilson, <i>Phil. Trans.</i> A, 193, p. 289, 1899.</p> -</div> -<div class='footnote' id='f69'> -<p class='c006'><span class='label'><a href='#r69'>69</a>. </span>Thomson, <i>Phil. Mag.</i> p. 528, Dec. 1898, and March, 1903. <i>Conduction of -Electricity through Gases</i>, Camb. Univ. Press, 1903, p. 121.</p> -</div> -<div class='footnote' id='f70'> -<p class='c006'><span class='label'><a href='#r70'>70</a>. </span>Wilson, <i>Phil. Mag.</i> April, 1903.</p> -</div> -<div class='footnote' id='f71'> -<p class='c006'><span class='label'><a href='#r71'>71</a>. </span>Townsend, <i>Phil. Trans.</i> A, p. 129, 1899.</p> -</div> -<div class='footnote' id='f72'> -<p class='c006'><span class='label'><a href='#r72'>72</a>. </span>Townsend, <i>loc. cit.</i> p. 139.</p> -</div> -<div class='footnote' id='f73'> -<p class='c006'><span class='label'><a href='#r73'>73</a>. </span>Some difference of opinion has been expressed as to the value of <i>V</i> required -to produce ions at each collision. Townsend considers it to be about 20 volts; -Langevin 60 volts and Stark about 50 volts.</p> -</div> -<div class='footnote' id='f74'> -<p class='c006'><span class='label'><a href='#r74'>74</a>. </span>Rutherford, <i>Phil. Mag.</i> Jan. 1899.</p> -</div> -<div class='footnote' id='f75'> -<p class='c006'><span class='label'><a href='#r75'>75</a>. </span>Rutherford, <i>Phil. Mag.</i> Jan. 1899.</p> -</div> -<div class='footnote' id='f76'> -<p class='c006'><span class='label'><a href='#r76'>76</a>. </span>Strutt, <i>Phil. Trans.</i> A, p. 507, 1901 and <i>Proc. Roy. Soc.</i> p. 208, 1903.</p> -</div> -<div class='footnote' id='f77'> -<p class='c006'><span class='label'><a href='#r77'>77</a>. </span>McClung, <i>Phil. Mag.</i> Sept. 1904.</p> -</div> -<div class='footnote' id='f78'> -<p class='c006'><span class='label'><a href='#r78'>78</a>. </span>Eve, <i>Phil. Mag.</i> Dec. 1904.</p> -</div> -<div class='footnote' id='f79'> -<p class='c006'><span class='label'><a href='#r79'>79</a>. </span>Rutherford, <i>Phil. Mag.</i> p. 137, Jan. 1899.</p> -</div> -<div class='footnote' id='f80'> -<p class='c006'><span class='label'><a href='#r80'>80</a>. </span>Child, <i>Phys. Rev.</i> Vol. 12, 1901.</p> -</div> -<div class='footnote' id='f81'> -<p class='c006'><span class='label'><a href='#r81'>81</a>. </span>Rutherford, <i>Phil. Mag.</i> p. 210, August, 1901; <i>Phys. Rev.</i> Vol. 13, 1901.</p> -</div> -<div class='footnote' id='f82'> -<p class='c006'><span class='label'><a href='#r82'>82</a>. </span>Rutherford, <i>Phil. Mag.</i> Aug. 1901.</p> -</div> -<div class='footnote' id='f83'> -<p class='c006'><span class='label'><a href='#r83'>83</a>. </span>A simple and excellent account of the effects produced by the motion of a -charged ion and also of the electronic theory of matter was given by Sir Oliver -Lodge in 1903 in a paper entitled “Electrons” (<i>Proceedings of the Institution of -Electrical Engineers</i>, Part 159, Vol. 32, 1903). See also J. J. Thomson’s <i>Electricity -and Matter</i> (Scribner, New York, 1904).</p> -</div> -<div class='footnote' id='f84'> -<p class='c006'><span class='label'><a href='#r84'>84</a>. </span>J. J. Thomson, <i>Phil. Mag.</i> April, 1887.</p> -</div> -<div class='footnote' id='f85'> -<p class='c006'><span class='label'><a href='#r85'>85</a>. </span>Heaviside, <i>Collected Papers</i>, Vol. II. p. 514.</p> -</div> -<div class='footnote' id='f86'> -<p class='c006'><span class='label'><a href='#r86'>86</a>. </span>Searle, <i>Phil. Mag.</i> Oct. 1897.</p> -</div> -<div class='footnote' id='f87'> -<p class='c006'><span class='label'><a href='#r87'>87</a>. </span>Abraham, <i>Phys. Zeit.</i> 4, No. 1 b, p. 57, 1902.</p> -</div> -<div class='footnote' id='f88'> -<p class='c006'><span class='label'><a href='#r88'>88</a>. </span>A full account of the path described by a moving ion under various conditions -is given by J. J. Thomson, <i>Conduction of Electricity in Gases</i> (Camb. Univ. Press, -1903), pp. 79–90.</p> -</div> -<div class='footnote' id='f89'> -<p class='c006'><span class='label'><a href='#r89'>89</a>. </span>J. J. Thomson, <i>Phil. Mag.</i> p. 293, 1897.</p> -</div> -<div class='footnote' id='f90'> -<p class='c006'><span class='label'><a href='#r90'>90</a>. </span>Lenard, <i>Annal. d. Phys.</i> 64, p. 279, 1898.</p> -</div> -<div class='footnote' id='f91'> -<p class='c006'><span class='label'><a href='#r91'>91</a>. </span>Kaufmann, <i>Annal. d. Phys.</i> 61, p. 544; 62, p. 596, 1897; 65, p. 431, 1898.</p> -</div> -<div class='footnote' id='f92'> -<p class='c006'><span class='label'><a href='#r92'>92</a>. </span>Simon, <i>Annal. d. Phys.</i> 69, p. 589, 1899.</p> -</div> -<div class='footnote' id='f93'> -<p class='c006'><span class='label'><a href='#r93'>93</a>. </span>A complete discussion of the various methods employed to measure the -velocity and mass of electrons and also of the theory on which they are based will -be found in J. J. Thomson’s <i>Conduction of Electricity through Gases</i>.</p> -</div> -<div class='footnote' id='f94'> -<p class='c006'><span class='label'><a href='#r94'>94</a>. </span>Goldstein, <i>Berlin Sitzber.</i> 39, p. 691, 1896; <i>Annal. d. Phys.</i> 64, p. 45, 1898.</p> -</div> -<div class='footnote' id='f95'> -<p class='c006'><span class='label'><a href='#r95'>95</a>. </span>Wien, <i>Annal. d. Phys.</i> 65, p. 440, 1898.</p> -</div> -<div class='footnote' id='f96'> -<p class='c006'><span class='label'><a href='#r96'>96</a>. </span>Larmor, <i>Phil. Mag.</i> 44, p. 593, 1897.</p> -</div> -<div class='footnote' id='f97'> -<p class='c006'><span class='label'><a href='#r97'>97</a>. </span>J. J. Thomson, <i>Phil. Mag.</i> Feb. 1897.</p> -</div> -<div class='footnote' id='f98'> -<p class='c006'><span class='label'><a href='#r98'>98</a>. </span>Barkla, <i>Phil. Mag.</i> June, 1903.</p> -</div> -<div class='footnote' id='f99'> -<p class='c006'><span class='label'><a href='#r99'>99</a>. </span>Soddy, <i>Trans. Chem. Soc.</i> Vol. 81, p. 860, 1902.</p> -</div> -<div class='footnote' id='f100'> -<p class='c006'><span class='label'><a href='#r100'>100</a>. </span>Wilson, <i>Proc. Roy. Soc.</i> Vol. 68, p. 152, 1901.</p> -</div> -<div class='footnote' id='f101'> -<p class='c006'><span class='label'><a href='#r101'>101</a>. </span>If the apparatus is required to be air-tight, the gold-leaf system can be -charged by means of a piece of magnetized steel wire, which is made to touch the -rod <i>R</i> by the approach of a magnet.</p> -</div> -<div class='footnote' id='f102'> -<p class='c006'><span class='label'><a href='#r102'>102</a>. </span>It is sometimes observed that the motion of the gold-leaf, immediately after -charging, is irregular. In many cases, this can be traced to air currents set up in -the electroscope in consequence of unsymmetrical heating by the source of light used -for illumination.</p> -</div> -<div class='footnote' id='f103'> -<p class='c006'><span class='label'><a href='#r103'>103</a>. </span>Wilson, <i>Proc. Camb. Phil. Soc.</i> Vol. 12, Part <span class='fss'>II.</span> 1903.</p> -</div> -<div class='footnote' id='f104'> -<p class='c006'><span class='label'><a href='#r104'>104</a>. </span>Walker, <i>Phil. Mag.</i> Aug. 1903.</p> -</div> -<div class='footnote' id='f105'> -<p class='c006'><span class='label'><a href='#r105'>105</a>. </span>Strutt, <i>Phil. Trans.</i> A, p. 507, 1901.</p> -</div> -<div class='footnote' id='f106'> -<p class='c006'><span class='label'><a href='#r106'>106</a>. </span>Dolezalek, <i>Instrumentenkunde</i>, p. 345, Dec. 1901.</p> -</div> -<div class='footnote' id='f107'> -<p class='c006'><span class='label'><a href='#r107'>107</a>. </span>It is very desirable that care should be taken not to release large quantities -of the radium emanation inside a laboratory. This emanation has a slow rate of -decay and is carried by currents of air throughout the whole building and finally -leaves behind an active deposit of very slow rate of change (see <a href='#chap11'>chapter <span class='fss'>XI.</span></a>). Eve -(<i>Nature</i>, March 16, 1905) has drawn attention to the difficulty of making refined -radio-active measurements under such conditions.</p> -</div> -<div class='footnote' id='f108'> -<p class='c006'><span class='label'><a href='#r108'>108</a>. </span>J. J. Thomson, <i>Phil. Mag.</i> 46, p. 537, 1898.</p> -</div> -<div class='footnote' id='f109'> -<p class='c006'><span class='label'><a href='#r109'>109</a>. </span>Bronson, <i>Amer. Journ. Science</i>, Feb. 1905.</p> -</div> -<div class='footnote' id='f110'> -<p class='c006'><span class='label'><a href='#r110'>110</a>. </span>J. and P. Curie, <i>C. R.</i> 91, pp. 38 and 294, 1880. See also Friedel and -J. Curie, <i>C. R.</i> 96, pp. 1262 and 1389, 1883, and Lord Kelvin, <i>Phil. Mag.</i> 36, -pp. 331, 342, 384, 414, 453, 1893.</p> -</div> -<div class='footnote' id='f111'> -<p class='c006'><span class='label'><a href='#r111'>111</a>. </span>In an examination of uranium the writer (<i>Phil. Mag.</i> p. 116, Jan. 1899) found -that the rays from uranium consist of two kinds, differing greatly in penetrating -power, which were called the α and β rays. Later, it was found that similar types of -rays were emitted by thorium and radium. On the discovery that very penetrating -rays were given out by uranium and thorium as well as by radium, the term γ was -applied to them by the writer. The word “ray” has been retained in this work, -although it is now settled that the α and β rays consist of particles projected with -great velocity. The term is thus used in the same sense as by Newton, who applied -it in the <i>Principia</i> to the stream of corpuscles which he believed to be responsible for -the phenomenon of light. In some recent papers, the α and β rays have been called -the α and β “emanations.” This nomenclature cannot fail to lead to confusion, -since the term “radio-active emanation” has already been generally adopted in -radio-activity as applying to the material substance which gradually <i>diffuses</i> from -thorium and radium compounds, and itself emits rays.</p> -</div> -<div class='footnote' id='f112'> -<p class='c006'><span class='label'><a href='#r112'>112</a>. </span>This method of illustration is due to Mme Curie, <i>Thèse présentée à la Faculté -des Sciences de Paris</i>, 1903.</p> -</div> -<div class='footnote' id='f113'> -<p class='c006'><span class='label'><a href='#r113'>113</a>. </span>Giesel, <i>Annal. d. Phys.</i> 69, p. 834, 1899.</p> -</div> -<div class='footnote' id='f114'> -<p class='c006'><span class='label'><a href='#r114'>114</a>. </span>Meyer and Schweidler, <i>Phys. Zeit.</i> 1, pp. 90, 113, 1899.</p> -</div> -<div class='footnote' id='f115'> -<p class='c006'><span class='label'><a href='#r115'>115</a>. </span>Becquerel, <i>C. R.</i> 129, pp. 997, 1205, 1899.</p> -</div> -<div class='footnote' id='f116'> -<p class='c006'><span class='label'><a href='#r116'>116</a>. </span>Curie, <i>C. R.</i> 130, p. 73, 1900.</p> -</div> -<div class='footnote' id='f117'> -<p class='c006'><span class='label'><a href='#r117'>117</a>. </span>Rutherford, <i>Phil. Mag.</i> January, 1899.</p> -</div> -<div class='footnote' id='f118'> -<p class='c006'><span class='label'><a href='#r118'>118</a>. </span>Rutherford and Grier, <i>Phil. Mag.</i> September, 1902.</p> -</div> -<div class='footnote' id='f119'> -<p class='c006'><span class='label'><a href='#r119'>119</a>. </span>Becquerel, <i>C. R.</i> 130, pp. 206, 372, 810, 979. 1900.</p> -</div> -<div class='footnote' id='f120'> -<p class='c006'><span class='label'><a href='#r120'>120</a>. </span>M. and Mme Curie, <i>C. R.</i> 130, p. 647, 1900.</p> -</div> -<div class='footnote' id='f121'> -<p class='c006'><span class='label'><a href='#r121'>121</a>. </span>The activity of the radium preparation was not stated in the paper.</p> -</div> -<div class='footnote' id='f122'> -<p class='c006'><span class='label'><a href='#r122'>122</a>. </span>Dorn, <i>Phys. Zeit.</i> 4, No. 18, p. 507, 1903.</p> -</div> -<div class='footnote' id='f123'> -<p class='c006'><span class='label'><a href='#r123'>123</a>. </span>Strutt, <i>Phil. Mag.</i> Nov. 1903.</p> -</div> -<div class='footnote' id='f124'> -<p class='c006'><span class='label'><a href='#r124'>124</a>. </span>Wien, <i>Phys. Zeit.</i> 4, No. 23, p. 624, 1903.</p> -</div> -<div class='footnote' id='f125'> -<p class='c006'><span class='label'><a href='#r125'>125</a>. </span>Dorn, <i>C. R.</i> 130, p. 1129, 1900.</p> -</div> -<div class='footnote' id='f126'> -<p class='c006'><span class='label'><a href='#r126'>126</a>. </span>Becquerel, <i>C. R.</i> 130, p. 809, 1900.</p> -</div> -<div class='footnote' id='f127'> -<p class='c006'><span class='label'><a href='#r127'>127</a>. </span>Kaufmann, <i>Phys. Zeit.</i> 4, No. 1 b, p. 54, 1902.</p> -</div> -<div class='footnote' id='f128'> -<p class='c006'><span class='label'><a href='#r128'>128</a>. </span>Abraham, <i>Phys. Zeit.</i> 4, No. 1 b, p. 57, 1902.</p> -</div> -<div class='footnote' id='f129'> -<p class='c006'><span class='label'><a href='#r129'>129</a>. </span>Kaufmann, <i>Nachrichten d. Ges. d. Wiss. zu Gött.</i>, Nov. 8, 1901.</p> -</div> -<div class='footnote' id='f130'> -<p class='c006'><span class='label'><a href='#r130'>130</a>. </span>Simon, <i>Annal. d. Phys.</i> p. 589, 1899.</p> -</div> -<div class='footnote' id='f131'> -<p class='c006'><span class='label'><a href='#r131'>131</a>. </span>Kaufmann, <i>Phys. Zeit.</i> 4, No. 1 b, p. 54, 1902.</p> -</div> -<div class='footnote' id='f132'> -<p class='c006'><span class='label'><a href='#r132'>132</a>. </span>Paschen, <i>Annal. d. Phys.</i> 14, p. 389, 1904.</p> -</div> -<div class='footnote' id='f133'> -<p class='c006'><span class='label'><a href='#r133'>133</a>. </span>Meyer and Schweidler, <i>Phys. Zeit.</i> pp. 90, 113, 209, 1900.</p> -</div> -<div class='footnote' id='f134'> -<p class='c006'><span class='label'><a href='#r134'>134</a>. </span>Lenard, <i>Annal. d. Phys.</i> 56, p. 275, 1895.</p> -</div> -<div class='footnote' id='f135'> -<p class='c006'><span class='label'><a href='#r135'>135</a>. </span>Strutt, <i>Nature</i>, p. 539, 1900.</p> -</div> -<div class='footnote' id='f136'> -<p class='c006'><span class='label'><a href='#r136'>136</a>. </span>Seitz, <i>Phys. Zeit.</i> 5, No. 14, p. 395, 1904.</p> -</div> -<div class='footnote' id='f137'> -<p class='c006'><span class='label'><a href='#r137'>137</a>. </span>It is presumed that the results were corrected, if necessary, for the discharging -action due to the ionized gas, although no direct mention of this is made in the -paper by Seitz.</p> -</div> -<div class='footnote' id='f138'> -<p class='c006'><span class='label'><a href='#r138'>138</a>. </span>Strutt, <i>Phil. Trans.</i> A, p. 507, 1901.</p> -</div> -<div class='footnote' id='f139'> -<p class='c006'><span class='label'><a href='#r139'>139</a>. </span>Crookes, <i>Proc. Roy. Soc.</i> 1902. <i>Chem. News</i>, 85, p. 109, 1902.</p> -</div> -<div class='footnote' id='f140'> -<p class='c006'><span class='label'><a href='#r140'>140</a>. </span>Mme Curie, <i>C. R.</i> 130, p. 76, 1900.</p> -</div> -<div class='footnote' id='f141'> -<p class='c006'><span class='label'><a href='#r141'>141</a>. </span>Rutherford, <i>Phil. Mag.</i> Feb. 1903. <i>Phys. Zeit.</i> 4, p. 235, 1902.</p> -</div> -<div class='footnote' id='f142'> -<p class='c006'><span class='label'><a href='#r142'>142</a>. </span>Becquerel, <i>C. R.</i> 136, p. 199, 1903.</p> -</div> -<div class='footnote' id='f143'> -<p class='c006'><span class='label'><a href='#r143'>143</a>. </span>Becquerel, <i>C. R.</i> 136, p. 431, 1903.</p> -</div> -<div class='footnote' id='f144'> -<p class='c006'><span class='label'><a href='#r144'>144</a>. </span>Des Coudres, <i>Phys. Zeit.</i> 4, No. 17, p. 483, 1903.</p> -</div> -<div class='footnote' id='f145'> -<p class='c006'><span class='label'><a href='#r145'>145</a>. </span>Becquerel, <i>C. R.</i> 136, p. 1517, 1903.</p> -</div> -<div class='footnote' id='f146'> -<p class='c006'><span class='label'><a href='#r146'>146</a>. </span>Bragg, <i>Phil. Mag.</i> Dec. 1904; Bragg and Kleeman, <i>Phil. Mag.</i> Dec. 1904.</p> -</div> -<div class='footnote' id='f147'> -<p class='c006'><span class='label'><a href='#r147'>147</a>. </span>Further experimental results bearing on this important question are given in -an Appendix to this book.</p> -</div> -<div class='footnote' id='f148'> -<p class='c006'><span class='label'><a href='#r148'>148</a>. </span>Bakerian Lecture, <i>Phil. Trans.</i> A, p. 169, 1904.</p> -</div> -<div class='footnote' id='f149'> -<p class='c006'><span class='label'><a href='#r149'>149</a>. </span>Strutt, <i>Phil. Mag.</i> Aug. 1904.</p> -</div> -<div class='footnote' id='f150'> -<p class='c006'><span class='label'><a href='#r150'>150</a>. </span>J. J. Thomson, <i>Proc. Camb. Phil.</i> Soc. 13, Pt. <span class='fss'>I.</span> p. 39, 1905. <i>Nature</i>, -Dec. 15, 1904.</p> -</div> -<div class='footnote' id='f151'> -<p class='c006'><span class='label'><a href='#r151'>151</a>. </span>Rutherford, <i>Nature</i>, March 2, 1905. J. J. Thomson, <i>Nature</i>, March 9, 1905.</p> -</div> -<div class='footnote' id='f152'> -<p class='c006'><span class='label'><a href='#r152'>152</a>. </span>Crookes, <i>Proc. Roy. Soc.</i> 81, p. 405, 1903.</p> -</div> -<div class='footnote' id='f153'> -<p class='c006'><span class='label'><a href='#r153'>153</a>. </span>Elster and Geitel, <i>Phys. Zeit.</i> No. 15, p. 437, 1903.</p> -</div> -<div class='footnote' id='f154'> -<p class='c006'><span class='label'><a href='#r154'>154</a>. </span>Glew, <i>Arch. Röntgen Ray</i>, June 1904.</p> -</div> -<div class='footnote' id='f155'> -<p class='c006'><span class='label'><a href='#r155'>155</a>. </span>Becquerel, <i>C. R.</i> 137, Oct. 27, 1903.</p> -</div> -<div class='footnote' id='f156'> -<p class='c006'><span class='label'><a href='#r156'>156</a>. </span>Tommasina, <i>C. R.</i> 137, Nov. 9, 1903.</p> -</div> -<div class='footnote' id='f157'> -<p class='c006'><span class='label'><a href='#r157'>157</a>. </span>An interesting side-light is thrown on this question by the experiments -described in <a href='#appa'>Appendix A</a> of this book.</p> -</div> -<div class='footnote' id='f158'> -<p class='c006'><span class='label'><a href='#r158'>158</a>. </span>Rutherford and Miss Brooks, <i>Phil. Mag.</i> July 1902.</p> -</div> -<div class='footnote' id='f159'> -<p class='c006'><span class='label'><a href='#r159'>159</a>. </span>In order to obtain a thin layer, the compound to be tested is ground to a fine -powder and then sifted through a fine gauge uniformly over the area, so that the -plate is only partially covered.</p> -</div> -<div class='footnote' id='f160'> -<p class='c006'><span class='label'><a href='#r160'>160</a>. </span>Rutherford, <i>Phil. Mag.</i> Jan. 1899.</p> -</div> -<div class='footnote' id='f161'> -<p class='c006'><span class='label'><a href='#r161'>161</a>. </span>Owens, <i>Phil. Mag.</i> Oct. 1899.</p> -</div> -<div class='footnote' id='f162'> -<p class='c006'><span class='label'><a href='#r162'>162</a>. </span>Rutherford and Miss Brooks, <i>Phil. Mag.</i> July, 1900.</p> -</div> -<div class='footnote' id='f163'> -<p class='c006'><span class='label'><a href='#r163'>163</a>. </span>Since the ionization at any point above the plate is the resultant effect of the -α particles coming from all points of the large radio-active layer, λ is not the same -as the coefficient of absorption of the rays from a point source. It will however be -proportional to it. For this reason λ is called the “absorption constant.”</p> -</div> -<div class='footnote' id='f164'> -<p class='c006'><span class='label'><a href='#r164'>164</a>. </span>Townsend, <i>Phil. Mag.</i> Feb. 1901.</p> -</div> -<div class='footnote' id='f165'> -<p class='c006'><span class='label'><a href='#r165'>165</a>. </span>Durack, <i>Phil. Mag.</i> July 1902, May 1903.</p> -</div> -<div class='footnote' id='f166'> -<p class='c006'><span class='label'><a href='#r166'>166</a>. </span>Bragg and Bragg and Kleeman, <i>Phil. Mag.</i> Dec. 1904.</p> -</div> -<div class='footnote' id='f167'> -<p class='c006'><span class='label'><a href='#r167'>167</a>. </span>Villard, <i>C. R.</i> 130, pp. 1010, 1178, 1900.</p> -</div> -<div class='footnote' id='f168'> -<p class='c006'><span class='label'><a href='#r168'>168</a>. </span>Becquerel, <i>C. R.</i> 130, p. 1154, 1900.</p> -</div> -<div class='footnote' id='f169'> -<p class='c006'><span class='label'><a href='#r169'>169</a>. </span>Rutherford, <i>Phys. Zeit.</i> 3, p. 517, 1902.</p> -</div> -<div class='footnote' id='f170'> -<p class='c006'><span class='label'><a href='#r170'>170</a>. </span>McClelland, <i>Phil. Mag.</i> July 1904.</p> -</div> -<div class='footnote' id='f171'> -<p class='c006'><span class='label'><a href='#r171'>171</a>. </span>Paschen, <i>Phys. Zeit.</i> 5, No. 18, p. 563, 1904.</p> -</div> -<div class='footnote' id='f172'> -<p class='c006'><span class='label'><a href='#r172'>172</a>. </span>A. S. Eve, <i>Phil. Mag.</i> Nov. 1904.</p> -</div> -<div class='footnote' id='f173'> -<p class='c006'><span class='label'><a href='#r173'>173</a>. </span>Paschen, <i>Annal. d. Physik</i>, 14, p. 114, 1904; 14, 2, p. 389, 1904. <i>Phys. -Zeit.</i> 5, No. 18, p. 563, 1904.</p> -</div> -<div class='footnote' id='f174'> -<p class='c006'><span class='label'><a href='#r174'>174</a>. </span>Paschen, <i>Phys. Zeit.</i> 5, No. 18, p. 563, 1904.</p> -</div> -<div class='footnote' id='f175'> -<p class='c006'><span class='label'><a href='#r175'>175</a>. </span>Rutherford and Barnes, <i>Phil. Mag.</i> May 1905. <i>Nature</i>, p. 151, Dec. 15, 1904.</p> -</div> -<div class='footnote' id='f176'> -<p class='c006'><span class='label'><a href='#r176'>176</a>. </span>Barkla, <i>Nature</i>, March 17, 1904.</p> -</div> -<div class='footnote' id='f177'> -<p class='c006'><span class='label'><a href='#r177'>177</a>. </span>Becquerel, <i>C.R.</i> 132, pp. 371, 734, 1286. 1901.</p> -</div> -<div class='footnote' id='f178'> -<p class='c006'><span class='label'><a href='#r178'>178</a>. </span>Mme Curie, <i>Thèse présentée à la Faculté des Sciences</i>, Paris 1903, p. 85.</p> -</div> -<div class='footnote' id='f179'> -<p class='c006'><span class='label'><a href='#r179'>179</a>. </span>A. S. Eve, <i>Phil. Mag.</i> Dec. 1904.</p> -</div> -<div class='footnote' id='f180'> -<p class='c006'><span class='label'><a href='#r180'>180</a>. </span>In a recent paper (<i>Phil. Mag.</i> Feb. 1905), McClelland has, in the main, -confirmed the experimental results obtained by Eve. An electrometer was used -instead of an electroscope. He finds, in addition, that the amount of secondary -radiation depends on the angle of incidence of the primary rays, and is greatest for -an angle of 45°. In a letter to <i>Nature</i> (Feb. 23, p. 390, 1905), he states that more -recent experiments have shown that the amount of secondary radiation from -different substances is a function of their atomic weights rather than of their -densities. In every case examined, the amount of secondary radiation increases -with the atomic weight, but is not proportional to it.</p> -</div> -<div class='footnote' id='f181'> -<p class='c006'><span class='label'><a href='#r181'>181</a>. </span>Rutherford and McClung, <i>Phil. Trans.</i> A. p. 25, 1901.</p> -</div> -<div class='footnote' id='f182'> -<p class='c006'><span class='label'><a href='#r182'>182</a>. </span>Meyer and Schweidler, <i>Wien Ber.</i> 113, July, 1904.</p> -</div> -<div class='footnote' id='f183'> -<p class='c006'><span class='label'><a href='#r183'>183</a>. </span>Rutherford and Grier, <i>Phil. Mag.</i> Sept. 1902.</p> -</div> -<div class='footnote' id='f184'> -<p class='c006'><span class='label'><a href='#r184'>184</a>. </span>Becquerel, <i>C. R.</i> 129, p. 912, 1899.</p> -</div> -<div class='footnote' id='f185'> -<p class='c006'><span class='label'><a href='#r185'>185</a>. </span>Bary, <i>C. R.</i> 130, p. 776, 1900.</p> -</div> -<div class='footnote' id='f186'> -<p class='c006'><span class='label'><a href='#r186'>186</a>. </span>Kunz and Baskerville, <i>Amer. Journ. Science</i> <span class='fss'>XVI.</span> p. 335, 1903.</p> -</div> -<div class='footnote' id='f187'> -<p class='c006'><span class='label'><a href='#r187'>187</a>. </span>See <i>Nature</i>, p. 523, March 31, 1904.</p> -</div> -<div class='footnote' id='f188'> -<p class='c006'><span class='label'><a href='#r188'>188</a>. </span>Crookes, <i>Proc. Roy. Soc.</i> 74, p. 47, 1904.</p> -</div> -<div class='footnote' id='f189'> -<p class='c006'><span class='label'><a href='#r189'>189</a>. </span>Kunz and Baskerville, <i>Science</i> <span class='fss'>XVIII</span>, p. 769, Dec. 18, 1903.</p> -</div> -<div class='footnote' id='f190'> -<p class='c006'><span class='label'><a href='#r190'>190</a>. </span>Beilby in a recent communication to the Royal Society (Feb. 9 and 23, 1905) -has examined in some detail the production of phosphorescence by the β and γ rays -of radium and has put forward a theory to account for the different actions observed.</p> -</div> -<div class='footnote' id='f191'> -<p class='c006'><span class='label'><a href='#r191'>191</a>. </span>Huggins, <i>Proc. Roy. Soc.</i> 72, pp. 196 and 409, 1903.</p> -</div> -<div class='footnote' id='f192'> -<p class='c006'><span class='label'><a href='#r192'>192</a>. </span>The spark spectrum of the radium bromide showed the <i>H</i> and <i>K</i> lines of -calcium and also faintly some of the strong lines of barium. The characteristic -lines of radium of wave-lengths 3814·59, 3649·7, 4340·6 and 2708·6, as shown by -Demarçay and others are clearly shown in the figure. The strong line of wave-length -about 2814 is due to radium.</p> -</div> -<div class='footnote' id='f193'> -<p class='c006'><span class='label'><a href='#r193'>193</a>. </span>Giesel, <i>Ber. d. D. Chem. Ges.</i> 37, p. 1696, 1904.</p> -</div> -<div class='footnote' id='f194'> -<p class='c006'><span class='label'><a href='#r194'>194</a>. </span>Hartmann, <i>Phys. Zeit.</i> 5, No. 18, p. 570, 1904.</p> -</div> -<div class='footnote' id='f195'> -<p class='c006'><span class='label'><a href='#r195'>195</a>. </span>In a recent paper, Giesel (<i>Ber. d. D. Chem. Ges.</i> No. 3, p. 775, 1905) has -shown that the bright lines are due to didymium, which is present as an impurity. -Exposure of didymium to the radium rays also causes the appearance of the lines.</p> -</div> -<div class='footnote' id='f196'> -<p class='c006'><span class='label'><a href='#r196'>196</a>. </span>Wiedemann and Schmidt, <i>Wied. Annal.</i> 59, p. 604, 1895.</p> -</div> -<div class='footnote' id='f197'> -<p class='c006'><span class='label'><a href='#r197'>197</a>. </span>Wiedemann, <i>Phys. Zeit.</i> 2, p. 269, 1901.</p> -</div> -<div class='footnote' id='f198'> -<p class='c006'><span class='label'><a href='#r198'>198</a>. </span>Elster and Geitel, <i>Annal. d. Phys.</i> 69, p. 673, 1899.</p> -</div> -<div class='footnote' id='f199'> -<p class='c006'><span class='label'><a href='#r199'>199</a>. </span>Willons and Peck (<i>Phil. Mag.</i> March, 1905) found that under some conditions, -especially for long sparks, the rays of radium hindered the passage of the spark.</p> -</div> -<div class='footnote' id='f200'> -<p class='c006'><span class='label'><a href='#r200'>200</a>. </span>Hemptinne, <i>C. R.</i> 133, p. 934, 1901.</p> -</div> -<div class='footnote' id='f201'> -<p class='c006'><span class='label'><a href='#r201'>201</a>. </span>Himstedt, <i>Phys. Zeit.</i> p. 476, 1900.</p> -</div> -<div class='footnote' id='f202'> -<p class='c006'><span class='label'><a href='#r202'>202</a>. </span>Henning, <i>Annal. d. Phys.</i> p. 562, 1902.</p> -</div> -<div class='footnote' id='f203'> -<p class='c006'><span class='label'><a href='#r203'>203</a>. </span>Kohlrausch and Henning, <i>Verh. Deutsch. Phys. Ges.</i> 6, p. 144, 1904.</p> -</div> -<div class='footnote' id='f204'> -<p class='c006'><span class='label'><a href='#r204'>204</a>. </span>Kohlrausch, <i>Verh. Deutsch. Phys. Ges.</i> 5, p. 261, 1904.</p> -</div> -<div class='footnote' id='f205'> -<p class='c006'><span class='label'><a href='#r205'>205</a>. </span>P. Curie, <i>C. R.</i> 134, p. 420, 1902.</p> -</div> -<div class='footnote' id='f206'> -<p class='c006'><span class='label'><a href='#r206'>206</a>. </span>Becquerel, <i>C. R.</i> 136, p. 1173, 1903.</p> -</div> -<div class='footnote' id='f207'> -<p class='c006'><span class='label'><a href='#r207'>207</a>. </span>Becquerel, <i>C. R.</i> 133, p. 199, 1901.</p> -</div> -<div class='footnote' id='f208'> -<p class='c006'><span class='label'><a href='#r208'>208</a>. </span>P. Curie, Société de Physique, March 2, 1900.</p> -</div> -<div class='footnote' id='f209'> -<p class='c006'><span class='label'><a href='#r209'>209</a>. </span>Joly, <i>Phil. Mag.</i> March, 1904.</p> -</div> -<div class='footnote' id='f210'> -<p class='c006'><span class='label'><a href='#r210'>210</a>. </span>S. and P. Curie, <i>C. R.</i> 129, p. 823, 1899.</p> -</div> -<div class='footnote' id='f211'> -<p class='c006'><span class='label'><a href='#r211'>211</a>. </span>Giesel, <i>Verhandlg. d. D. Phys. Ges.</i> Jan. 5, 1900.</p> -</div> -<div class='footnote' id='f212'> -<p class='c006'><span class='label'><a href='#r212'>212</a>. </span>Salomonsen and Dreyer, <i>C. R.</i> 139, p. 533, 1904.</p> -</div> -<div class='footnote' id='f213'> -<p class='c006'><span class='label'><a href='#r213'>213</a>. </span>Elster and Geitel, <i>Phys. Zeit.</i> p. 113, No. 3, 1902.</p> -</div> -<div class='footnote' id='f214'> -<p class='c006'><span class='label'><a href='#r214'>214</a>. </span>Becquerel, <i>C. R.</i> 133, p. 709, 1901.</p> -</div> -<div class='footnote' id='f215'> -<p class='c006'><span class='label'><a href='#r215'>215</a>. </span>Hardy and Miss Wilcock, <i>Proc. Roy. Soc.</i> 72, p. 200, 1903.</p> -</div> -<div class='footnote' id='f216'> -<p class='c006'><span class='label'><a href='#r216'>216</a>. </span>Hardy, <i>Proc. Physiolog. Soc.</i> May 16, 1903.</p> -</div> -<div class='footnote' id='f217'> -<p class='c006'><span class='label'><a href='#r217'>217</a>. </span>Whetham, <i>Phil. Mag.</i> Nov. 1899; <i>Theory of Solution</i>, Camb. 1902, p. 396.</p> -</div> -<div class='footnote' id='f218'> -<p class='c006'><span class='label'><a href='#r218'>218</a>. </span>Curie and Debierne, <i>C. R.</i> 132, p. 768, 1901.</p> -</div> -<div class='footnote' id='f219'> -<p class='c006'><span class='label'><a href='#r219'>219</a>. </span>Giesel, <i>Ber. D. d. Chem. Ges.</i> 35, p. 3605, 1902.</p> -</div> -<div class='footnote' id='f220'> -<p class='c006'><span class='label'><a href='#r220'>220</a>. </span>Ramsay and Soddy, <i>Proc. Roy. Soc.</i> 72, p. 204, 1903.</p> -</div> -<div class='footnote' id='f221'> -<p class='c006'><span class='label'><a href='#r221'>221</a>. </span>Danysz, <i>C. R.</i> 136, p. 461, 1903.</p> -</div> -<div class='footnote' id='f222'> -<p class='c006'><span class='label'><a href='#r222'>222</a>. </span>Aschkinass and Caspari, <i>Arch. d. Ges. Physiologie</i>, 86, p. 603, 1901.</p> -</div> -<div class='footnote' id='f223'> -<p class='c006'><span class='label'><a href='#r223'>223</a>. </span>Himstedt and Nagel, <i>Drude’s Annal.</i> 4, p. 537, 1901.</p> -</div> -<div class='footnote' id='f224'> -<p class='c006'><span class='label'><a href='#r224'>224</a>. </span>Hardy and Anderson, <i>Proc. Roy. Soc.</i> 72, p. 393, 1903.</p> -</div> -<div class='footnote' id='f225'> -<p class='c006'><span class='label'><a href='#r225'>225</a>. </span>Crookes, <i>Proc. Roy. Soc.</i> 66, p. 409, 1900.</p> -</div> -<div class='footnote' id='f226'> -<p class='c006'><span class='label'><a href='#r226'>226</a>. </span>Becquerel, <i>C. R.</i> 131, p. 137, 1900; 133, p. 977, 1901.</p> -</div> -<div class='footnote' id='f227'> -<p class='c006'><span class='label'><a href='#r227'>227</a>. </span>Rutherford and Soddy, <i>Phil. Mag.</i> Sept. and Nov. 1902. <i>Trans. Chem. Soc.</i> -81, pp. 321 and 837, 1902.</p> -</div> -<div class='footnote' id='f228'> -<p class='c006'><span class='label'><a href='#r228'>228</a>. </span>Rutherford and Soddy, <i>Phil. Mag.</i> Sept. 1902.</p> -</div> -<div class='footnote' id='f229'> -<p class='c006'><span class='label'><a href='#r229'>229</a>. </span>The general method of regarding the subject would be unchanged, even if it -were proved that the radio-activity of thorium is not due to thorium at all but to a -small constant amount of a radio-active impurity mixed with it.</p> -</div> -<div class='footnote' id='f230'> -<p class='c006'><span class='label'><a href='#r230'>230</a>. </span>Rutherford and Soddy, <i>Phil. Mag.</i> Sept. 1902.</p> -</div> -<div class='footnote' id='f231'> -<p class='c006'><span class='label'><a href='#r231'>231</a>. </span>Owens, <i>Phil. Mag.</i> p. 360, Oct. 1899.</p> -</div> -<div class='footnote' id='f232'> -<p class='c006'><span class='label'><a href='#r232'>232</a>. </span>Rutherford, <i>Phil. Mag.</i> p. 1, Jan. 1900.</p> -</div> -<div class='footnote' id='f233'> -<p class='c006'><span class='label'><a href='#r233'>233</a>. </span>Rossignol and Gimingham, <i>Phil. Mag.</i> July, 1904.</p> -</div> -<div class='footnote' id='f234'> -<p class='c006'><span class='label'><a href='#r234'>234</a>. </span>Bronson, <i>Amer. Journ. Science</i>, Feb. 1905.</p> -</div> -<div class='footnote' id='f235'> -<p class='c006'><span class='label'><a href='#r235'>235</a>. </span><i>Phil. Mag.</i> April, 1904.</p> -</div> -<div class='footnote' id='f236'> -<p class='c006'><span class='label'><a href='#r236'>236</a>. </span>Dorn, <i>Abh. der. Naturforsch. Ges. für Halle-a-S.</i>, 1900.</p> -</div> -<div class='footnote' id='f237'> -<p class='c006'><span class='label'><a href='#r237'>237</a>. </span>P. Curie, <i>C. R.</i> 135, p. 857, 1902.</p> -</div> -<div class='footnote' id='f238'> -<p class='c006'><span class='label'><a href='#r238'>238</a>. </span>Rutherford and Soddy, <i>Phil. Mag.</i> April, 1903.</p> -</div> -<div class='footnote' id='f239'> -<p class='c006'><span class='label'><a href='#r239'>239</a>. </span>P. Curie, <i>C. R.</i> 136, p. 223, 1903.</p> -</div> -<div class='footnote' id='f240'> -<p class='c006'><span class='label'><a href='#r240'>240</a>. </span>Debierne, <i>C. R.</i> 136, p. 146, 1903.</p> -</div> -<div class='footnote' id='f241'> -<p class='c006'><span class='label'><a href='#r241'>241</a>. </span>Giesel, <i>Ber. D. deutsch. Chem. Ges.</i> p. 3608, 1902.</p> -</div> -<div class='footnote' id='f242'> -<p class='c006'><span class='label'><a href='#r242'>242</a>. </span>Curie and Debierne, <i>C. R.</i> 132, pp. 548 and 768, 1901.</p> -</div> -<div class='footnote' id='f243'> -<p class='c006'><span class='label'><a href='#r243'>243</a>. </span>Curie and Debierne, <i>C. R.</i> 133, p. 931, 1901.</p> -</div> -<div class='footnote' id='f244'> -<p class='c006'><span class='label'><a href='#r244'>244</a>. </span>Rutherford and Soddy, <i>Trans. Chem. Soc.</i> p. 321, 1902. <i>Phil. Mag.</i> Sept. 1902.</p> -</div> -<div class='footnote' id='f245'> -<p class='c006'><span class='label'><a href='#r245'>245</a>. </span>Rutherford, <i>Phys. Zeit.</i> 2, p. 429, 1901.</p> -</div> -<div class='footnote' id='f246'> -<p class='c006'><span class='label'><a href='#r246'>246</a>. </span>Rutherford and Soddy, <i>Phil. Mag.</i> Nov. 1902.</p> -</div> -<div class='footnote' id='f247'> -<p class='c006'><span class='label'><a href='#r247'>247</a>. </span>Rutherford and Soddy, <i>Phil. Mag.</i> April, 1903.</p> -</div> -<div class='footnote' id='f248'> -<p class='c006'><span class='label'><a href='#r248'>248</a>. </span>Rutherford and Soddy, <i>Phil. Mag.</i> Nov. 1902.</p> -</div> -<div class='footnote' id='f249'> -<p class='c006'><span class='label'><a href='#r249'>249</a>. </span>Rutherford and Soddy, <i>Phil. Mag.</i> April, 1903.</p> -</div> -<div class='footnote' id='f250'> -<p class='c006'><span class='label'><a href='#r250'>250</a>. </span>Curie and Debierne, <i>C. R.</i> 133, p. 931, 1901.</p> -</div> -<div class='footnote' id='f251'> -<p class='c006'><span class='label'><a href='#r251'>251</a>. </span>Rutherford and Soddy, <i>Phil. Mag.</i> Nov. 1902.</p> -</div> -<div class='footnote' id='f252'> -<p class='c006'><span class='label'><a href='#r252'>252</a>. </span>Ramsay and Soddy, <i>Proc. Roy. Soc.</i> 72, p. 204, 1903.</p> -</div> -<div class='footnote' id='f253'> -<p class='c006'><span class='label'><a href='#r253'>253</a>. </span>Rutherford and Miss Brooks, <i>Trans. Roy. Soc. Canada 1901</i>, <i>Chem. News 1902</i>.</p> -</div> -<div class='footnote' id='f254'> -<p class='c006'><span class='label'><a href='#r254'>254</a>. </span>Loschmidt, <i>Sitzungsber. d. Wien. Akad.</i> 61, <span class='fss'>II.</span> p. 367, 1871.</p> -</div> -<div class='footnote' id='f255'> -<p class='c006'><span class='label'><a href='#r255'>255</a>. </span>See Stefan, <i>Sitzungsber. d. Wien. Akad.</i> 63, <span class='fss'>II.</span> p. 82, 1871.</p> -</div> -<div class='footnote' id='f256'> -<p class='c006'><span class='label'><a href='#r256'>256</a>. </span>P. Curie and Danne, <i>C. R.</i> 136, p. 1314, 1903.</p> -</div> -<div class='footnote' id='f257'> -<p class='c006'><span class='label'><a href='#r257'>257</a>. </span>Bumstead and Wheeler, <i>Amer. Jour. Science</i>, Feb. 1904.</p> -</div> -<div class='footnote' id='f258'> -<p class='c006'><span class='label'><a href='#r258'>258</a>. </span>Makower, <i>Phil. Mag.</i> Jan. 1905.</p> -</div> -<div class='footnote' id='f259'> -<p class='c006'><span class='label'><a href='#r259'>259</a>. </span>Wallstabe, <i>Phys. Zeit.</i> 4, p. 721, 1903.</p> -</div> -<div class='footnote' id='f260'> -<p class='c006'><span class='label'><a href='#r260'>260</a>. </span>Stefan, <i>Wien. Ber.</i> 2, p. 371, 1878.</p> -</div> -<div class='footnote' id='f261'> -<p class='c006'><span class='label'><a href='#r261'>261</a>. </span>Rutherford and Soddy, <i>Phil. Mag.</i> Nov. 1902.</p> -</div> -<div class='footnote' id='f262'> -<p class='c006'><span class='label'><a href='#r262'>262</a>. </span><i>Phil. Mag.</i> May, 1903.</p> -</div> -<div class='footnote' id='f263'> -<p class='c006'><span class='label'><a href='#r263'>263</a>. </span>P. Curie, Société de Physique, 1903.</p> -</div> -<div class='footnote' id='f264'> -<p class='c006'><span class='label'><a href='#r264'>264</a>. </span>Rutherford and Soddy, <i>Phil. Mag.</i> May, 1903.</p> -</div> -<div class='footnote' id='f265'> -<p class='c006'><span class='label'><a href='#r265'>265</a>. </span><i>Nature</i>, Aug. 20, 1903.</p> -</div> -<div class='footnote' id='f266'> -<p class='c006'><span class='label'><a href='#r266'>266</a>. </span><i>Proc. Roy. Soc.</i> 73, No. 494, p. 346, 1904.</p> -</div> -<div class='footnote' id='f267'> -<p class='c006'><span class='label'><a href='#r267'>267</a>. </span><i>Proc. Roy. Soc.</i> 73, No. 495, p. 470, 1904.</p> -</div> -<div class='footnote' id='f268'> -<p class='c006'><span class='label'><a href='#r268'>268</a>. </span>Pickering, <i>Astrophys. Journ.</i> Vol. 14, p. 368, 1901.</p> -</div> -<div class='footnote' id='f269'> -<p class='c006'><span class='label'><a href='#r269'>269</a>. </span>M. and Mme. Curie, <i>C. R.</i> 129, p. 714, 1899.</p> -</div> -<div class='footnote' id='f270'> -<p class='c006'><span class='label'><a href='#r270'>270</a>. </span>Rutherford, <i>Phil. Mag.</i> Jan. and Feb. 1900.</p> -</div> -<div class='footnote' id='f271'> -<p class='c006'><span class='label'><a href='#r271'>271</a>. </span>As regards date of publication, the priority of the discovery of “excited -activity” belongs to M. and Mme. Curie. A short paper on this subject, entitled -“Sur la radioactivité provoquée par les rayons de Becquerel,” was communicated -by them to the <i>Comptes Rendus</i>, Nov. 6, 1899. A short note was added to the -paper by Becquerel in which the phenomena of excited activity were ascribed to a -type of phosphorescence. On my part, I had simultaneously discovered the -emission of an emanation from thorium compounds and the excited activity -produced by it, in July, 1899. I, however, delayed publication in order to work -out in some detail the properties of the emanation and of the excited activity and -the connection between them. The results were published in two papers in the -<i>Philosophical Magazine</i> (Jan. and Feb. 1900) entitled “A radio-active substance -emitted from thorium compounds,” and “Radio-activity produced in substances by -the action of thorium compounds.”</p> -</div> -<div class='footnote' id='f272'> -<p class='c006'><span class='label'><a href='#r272'>272</a>. </span>Rutherford, <i>Phil. Mag.</i> Feb. 1900.</p> -</div> -<div class='footnote' id='f273'> -<p class='c006'><span class='label'><a href='#r273'>273</a>. </span>Rutherford, <i>Phys. Zeit.</i> 3, No. 12, p. 254, 1902. <i>Phil. Mag.</i> Jan. 1903.</p> -</div> -<div class='footnote' id='f274'> -<p class='c006'><span class='label'><a href='#r274'>274</a>. </span>Miss Brooks, <i>Phil. Mag.</i> Sept. 1904.</p> -</div> -<div class='footnote' id='f275'> -<p class='c006'><span class='label'><a href='#r275'>275</a>. </span>Rutherford and Miss Brooks, <i>Phil. Mag.</i> July, 1902.</p> -</div> -<div class='footnote' id='f276'> -<p class='c006'><span class='label'><a href='#r276'>276</a>. </span>Curie and Danne, <i>C. R.</i> 136, p. 364, 1903.</p> -</div> -<div class='footnote' id='f277'> -<p class='c006'><span class='label'><a href='#r277'>277</a>. </span>Mme Curie, <i>Thèse</i>, Paris, 1903, p. 116.</p> -</div> -<div class='footnote' id='f278'> -<p class='c006'><span class='label'><a href='#r278'>278</a>. </span>Debierne, <i>C. R.</i> 138, p. 411, 1904.</p> -</div> -<div class='footnote' id='f279'> -<p class='c006'><span class='label'><a href='#r279'>279</a>. </span>Giesel, <i>Ber. d. D. Chem. Ges.</i> No. 3, p. 775, 1905.</p> -</div> -<div class='footnote' id='f280'> -<p class='c006'><span class='label'><a href='#r280'>280</a>. </span>Miss Brooks, <i>Phil. Mag.</i> Sept. 1904.</p> -</div> -<div class='footnote' id='f281'> -<p class='c006'><span class='label'><a href='#r281'>281</a>. </span>Rutherford, <i>Phys. Zeit.</i> 3, No. 12, p. 254, 1902.</p> -</div> -<div class='footnote' id='f282'> -<p class='c006'><span class='label'><a href='#r282'>282</a>. </span>F. von Lerch, <i>Annal. d. Phys.</i> 12, p. 745, 1903.</p> -</div> -<div class='footnote' id='f283'> -<p class='c006'><span class='label'><a href='#r283'>283</a>. </span>Pegram, <i>Phys. Review</i>, p. 424, Dec. 1903.</p> -</div> -<div class='footnote' id='f284'> -<p class='c006'><span class='label'><a href='#r284'>284</a>. </span>Miss Gates, <i>Phys. Review</i>, p. 300, 1903.</p> -</div> -<div class='footnote' id='f285'> -<p class='c006'><span class='label'><a href='#r285'>285</a>. </span>A more complete examination of the effect of temperature on the excited -activity of thorium has been made by Miss Slater (<a href='#section207'>section 207</a>).</p> -</div> -<div class='footnote' id='f286'> -<p class='c006'><span class='label'><a href='#r286'>286</a>. </span>Rutherford, <i>Phil. Mag.</i> Feb. 1900.</p> -</div> -<div class='footnote' id='f287'> -<p class='c006'><span class='label'><a href='#r287'>287</a>. </span>Henning, <i>Annal. d. Phys.</i> 7, p. 562, 1902.</p> -</div> -<div class='footnote' id='f288'> -<p class='c006'><span class='label'><a href='#r288'>288</a>. </span>Rutherford, <i>Phil. Mag.</i> Feb. 1900.</p> -</div> -<div class='footnote' id='f289'> -<p class='c006'><span class='label'><a href='#r289'>289</a>. </span>Curie and Debierne, <i>C. R.</i> 132, p. 768, 1901.</p> -</div> -<div class='footnote' id='f290'> -<p class='c006'><span class='label'><a href='#r290'>290</a>. </span>Fehrle, <i>Phys. Zeit.</i> 3, No. 7, p. 130, 1902.</p> -</div> -<div class='footnote' id='f291'> -<p class='c006'><span class='label'><a href='#r291'>291</a>. </span>Rutherford, <i>Phil. Mag.</i> Jan. 1903.</p> -</div> -<div class='footnote' id='f292'> -<p class='c006'><span class='label'><a href='#r292'>292</a>. </span>Giesel, <i>Ber. d. D. Chem. Ges.</i> 36, p. 342, 1903.</p> -</div> -<div class='footnote' id='f293'> -<p class='c006'><span class='label'><a href='#r293'>293</a>. </span>Debierne, <i>C. R.</i> 136, pp. 446 and 671, 1903; 138, p. 411, 1904.</p> -</div> -<div class='footnote' id='f294'> -<p class='c006'><span class='label'><a href='#r294'>294</a>. </span>Ramsay, <i>Proc. Roy. Soc.</i> p. 470, June, 1904; <i>C. R.</i> 138, June 6, 1904.</p> -</div> -<div class='footnote' id='f295'> -<p class='c006'><span class='label'><a href='#r295'>295</a>. </span><i>Phil. Mag.</i> February, 1904.</p> -</div> -<div class='footnote' id='f296'> -<p class='c006'><span class='label'><a href='#r296'>296</a>. </span>Soddy, <i>Trans. Chem. Soc.</i> 81, p. 460, 1902.</p> -</div> -<div class='footnote' id='f297'> -<p class='c006'><span class='label'><a href='#r297'>297</a>. </span>Rutherford and Grier, <i>Phil. Mag.</i> Sept. 1902.</p> -</div> -<div class='footnote' id='f298'> -<p class='c006'><span class='label'><a href='#r298'>298</a>. </span>Becquerel, <i>C. R.</i> 131, p. 137, 1900.</p> -</div> -<div class='footnote' id='f299'> -<p class='c006'><span class='label'><a href='#r299'>299</a>. </span>Meyer and Schweidler, <i>Wien Ber.</i> Dec. 1, 1904.</p> -</div> -<div class='footnote' id='f300'> -<p class='c006'><span class='label'><a href='#r300'>300</a>. </span>Meyer and Schweidler, <i>Wien Ber.</i> 113, July, 1904.</p> -</div> -<div class='footnote' id='f301'> -<p class='c006'><span class='label'><a href='#r301'>301</a>. </span>Rutherford, <i>Phil. Trans.</i> A. 204, pp. 169–219, 1904.</p> -</div> -<div class='footnote' id='f302'> -<p class='c006'><span class='label'><a href='#r302'>302</a>. </span>Pegram, <i>Phys. Rev.</i> p. 424, December, 1903.</p> -</div> -<div class='footnote' id='f303'> -<p class='c006'><span class='label'><a href='#r303'>303</a>. </span>Miss Slater, <i>Phil. Mag.</i> 1905.</p> -</div> -<div class='footnote' id='f304'> -<p class='c006'><span class='label'><a href='#r304'>304</a>. </span>von Lerch, <i>Ann. de d. Phys.</i> November, 1903.</p> -</div> -<div class='footnote' id='f305'> -<p class='c006'><span class='label'><a href='#r305'>305</a>. </span>The ‘rayless change’ certainly does not give out α rays, and special experiments -showed that no appreciable amount of β rays were present. On the other -hand, the second change gives out all three types of rays.</p> -</div> -<div class='footnote' id='f306'> -<p class='c006'><span class='label'><a href='#r306'>306</a>. </span>Miss Brooks, <i>Phil. Mag.</i> Sept. 1904.</p> -</div> -<div class='footnote' id='f307'> -<p class='c006'><span class='label'><a href='#r307'>307</a>. </span>Rutherford and Soddy, <i>Trans. Chem. Soc.</i> 81, p. 837, 1902. <i>Phil. Mag.</i> -Nov. 1902.</p> -</div> -<div class='footnote' id='f308'> -<p class='c006'><span class='label'><a href='#r308'>308</a>. </span>Miss Brooks, <i>Phil. Mag.</i> Sept. 1904.</p> -</div> -<div class='footnote' id='f309'> -<p class='c006'><span class='label'><a href='#r309'>309</a>. </span>Rutherford, <i>Phil. Trans.</i> A. p. 169, 1904.</p> -</div> -<div class='footnote' id='f310'> -<p class='c006'><span class='label'><a href='#r310'>310</a>. </span>Giesel, <i>Ber. d. D. Chem. Ges.</i> p. 775, 1905.</p> -</div> -<div class='footnote' id='f311'> -<p class='c006'><span class='label'><a href='#r311'>311</a>. </span>Godlewski, <i>Nature</i>, p. 294, Jan. 19, 1905.</p> -</div> -<div class='footnote' id='f312'> -<p class='c006'><span class='label'><a href='#r312'>312</a>. </span>Debierne, <i>C. R.</i> 138, p. 411, 1904.</p> -</div> -<div class='footnote' id='f313'> -<p class='c006'><span class='label'><a href='#r313'>313</a>. </span>Miss Brooks, <i>Phil. Mag.</i> Sept. 1904.</p> -</div> -<div class='footnote' id='f314'> -<p class='c006'><span class='label'><a href='#r314'>314</a>. </span>Rutherford and Soddy, <i>Phil. Mag.</i> April, 1903.</p> -</div> -<div class='footnote' id='f315'> -<p class='c006'><span class='label'><a href='#r315'>315</a>. </span>Rutherford, <i>Phil. Trans.</i> A. p. 169, 1904. Curie and Danne, <i>C. R.</i> p. 748, 1904.</p> -</div> -<div class='footnote' id='f316'> -<p class='c006'><span class='label'><a href='#r316'>316</a>. </span>P. Curie and Danne, <i>Comptes Rendus</i>, 138, p. 748, 1904.</p> -</div> -<div class='footnote' id='f317'> -<p class='c006'><span class='label'><a href='#r317'>317</a>. </span>Miss Gates, <i>Phys. Rev.</i> p. 300, 1903.</p> -</div> -<div class='footnote' id='f318'> -<p class='c006'><span class='label'><a href='#r318'>318</a>. </span>Miss Brooks, <i>Nature</i>, July 21, 1904.</p> -</div> -<div class='footnote' id='f319'> -<p class='c006'><span class='label'><a href='#r319'>319</a>. </span>Rutherford, <i>Phil. Mag.</i> Nov. 1904. <i>Nature</i>, p. 341, Feb. 9, 1905.</p> -</div> -<div class='footnote' id='f320'> -<p class='c006'><span class='label'><a href='#r320'>320</a>. </span>Rutherford, <i>Nature</i>, p. 341, Feb. 9, 1905.</p> -</div> -<div class='footnote' id='f321'> -<p class='c006'><span class='label'><a href='#r321'>321</a>. </span>Marckwald (<i>Ber. d. D. Chem. Ges.</i> p. 591, 1905) has recently found that the -activity of his radio-tellurium falls to half value in 139 days.</p> -</div> -<div class='footnote' id='f322'> -<p class='c006'><span class='label'><a href='#r322'>322</a>. </span>Meyer and Schweidler, <i>Wien Ber.</i> Dec. 1, 1904.</p> -</div> -<div class='footnote' id='f323'> -<p class='c006'><span class='label'><a href='#r323'>323</a>. </span>Rutherford, <i>Phil. Trans.</i> A. p. 169, 1904.</p> -</div> -<div class='footnote' id='f324'> -<p class='c006'><span class='label'><a href='#r324'>324</a>. </span>Hofmann, Gonder and Wölfl, <i>Annal. d. Phys.</i> 15, p. 615, 1904.</p> -</div> -<div class='footnote' id='f325'> -<p class='c006'><span class='label'><a href='#r325'>325</a>. </span><i>Phil. Trans.</i> A. p. 25, 1901.</p> -</div> -<div class='footnote' id='f326'> -<p class='c006'><span class='label'><a href='#r326'>326</a>. </span>P. Curie and Laborde, <i>C. R.</i> 136, p. 673, 1903.</p> -</div> -<div class='footnote' id='f327'> -<p class='c006'><span class='label'><a href='#r327'>327</a>. </span>Runge and Precht, <i>Sitz. Ak. Wiss. Berlin</i>, No. 38, 1903.</p> -</div> -<div class='footnote' id='f328'> -<p class='c006'><span class='label'><a href='#r328'>328</a>. </span>P. Curie, Société de Physique, 1903.</p> -</div> -<div class='footnote' id='f329'> -<p class='c006'><span class='label'><a href='#r329'>329</a>. </span>Rutherford and Barnes, <i>Nature</i>, Oct. 29, 1903. <i>Phil. Mag.</i> Feb. 1904.</p> -</div> -<div class='footnote' id='f330'> -<p class='c006'><span class='label'><a href='#r330'>330</a>. </span>Paschen, <i>Phys. Zeit.</i> Sept. 15, 1904.</p> -</div> -<div class='footnote' id='f331'> -<p class='c006'><span class='label'><a href='#r331'>331</a>. </span>Rutherford and Barnes, <i>Nature</i>, Dec. 18, 1904; <i>Phil. Mag.</i> May, 1905.</p> -</div> -<div class='footnote' id='f332'> -<p class='c006'><span class='label'><a href='#r332'>332</a>. </span>Pegram, <i>Science</i>, May 27, 1904.</p> -</div> -<div class='footnote' id='f333'> -<p class='c006'><span class='label'><a href='#r333'>333</a>. </span>Perrin, <i>Revue Scientifique</i>, April 13, 1901.</p> -</div> -<div class='footnote' id='f334'> -<p class='c006'><span class='label'><a href='#r334'>334</a>. </span>Becquerel, <i>C. R.</i> 133, p. 979, 1901.</p> -</div> -<div class='footnote' id='f335'> -<p class='c006'><span class='label'><a href='#r335'>335</a>. </span>Rutherford and McClung, <i>Phil. Trans.</i> A, p. 25, 1901.</p> -</div> -<div class='footnote' id='f336'> -<p class='c006'><span class='label'><a href='#r336'>336</a>. </span>Rutherford, <i>Phil. Mag.</i> Jan. and Feb. 1900.</p> -</div> -<div class='footnote' id='f337'> -<p class='c006'><span class='label'><a href='#r337'>337</a>. </span>P. Curie, <i>C. R.</i> 136, p. 223, 1903.</p> -</div> -<div class='footnote' id='f338'> -<p class='c006'><span class='label'><a href='#r338'>338</a>. </span>Rutherford, <i>Phil. Mag.</i> April, 1903.</p> -</div> -<div class='footnote' id='f339'> -<p class='c006'><span class='label'><a href='#r339'>339</a>. </span>M. and Mme Curie, <i>C. R.</i> 134, p. 85, 1902.</p> -</div> -<div class='footnote' id='f340'> -<p class='c006'><span class='label'><a href='#r340'>340</a>. </span>Rutherford and Soddy, <i>Trans. Chem. Soc.</i> 81, pp. 321, 837, 1902. <i>Phil. Mag.</i> -Sept. and Nov. 1902.</p> -</div> -<div class='footnote' id='f341'> -<p class='c006'><span class='label'><a href='#r341'>341</a>. </span>Rutherford and Soddy, <i>Phil. Mag.</i> April, 1903.</p> -</div> -<div class='footnote' id='f342'> -<p class='c006'><span class='label'><a href='#r342'>342</a>. </span>Rutherford and Soddy, <i>Phil. Mag.</i> May, 1903.</p> -</div> -<div class='footnote' id='f343'> -<p class='c006'><span class='label'><a href='#r343'>343</a>. </span>Rutherford, <i>Phys. Zeit.</i> 4, p. 235, 1902. <i>Phil. Mag.</i> Feb. 1903.</p> -</div> -<div class='footnote' id='f344'> -<p class='c006'><span class='label'><a href='#r344'>344</a>. </span>Rutherford, <i>Phil. Mag.</i> May, 1903.</p> -</div> -<div class='footnote' id='f345'> -<p class='c006'><span class='label'><a href='#r345'>345</a>. </span>Curie and Laborde, <i>C. R.</i> 136, p. 673, 1903.</p> -</div> -<div class='footnote' id='f346'> -<p class='c006'><span class='label'><a href='#r346'>346</a>. </span>J. J. Thomson, <i>Nature</i>, p. 601, 1903.</p> -</div> -<div class='footnote' id='f347'> -<p class='c006'><span class='label'><a href='#r347'>347</a>. </span>Crookes, <i>C. R.</i> 128, p. 176, 1899.</p> -</div> -<div class='footnote' id='f348'> -<p class='c006'><span class='label'><a href='#r348'>348</a>. </span>F. Re, <i>C. R.</i> p. 136, p. 1393, 1903.</p> -</div> -<div class='footnote' id='f349'> -<p class='c006'><span class='label'><a href='#r349'>349</a>. </span>Richarz and Schenck, <i>Berl. Ber.</i> p. 1102, 1903. Schenck, <i>Berl. Ber.</i> p. 37, -1904.</p> -</div> -<div class='footnote' id='f350'> -<p class='c006'><span class='label'><a href='#r350'>350</a>. </span>Armstrong and Lowry, <i>Proc. Roy. Soc.</i> 1903. <i>Chem. News</i>, 88, p. 89, 1903.</p> -</div> -<div class='footnote' id='f351'> -<p class='c006'><span class='label'><a href='#r351'>351</a>. </span>Rutherford and Soddy, <i>Phil. Mag.</i> May, 1903.</p> -</div> -<div class='footnote' id='f352'> -<p class='c006'><span class='label'><a href='#r352'>352</a>. </span>Boltwood, <i>Nature</i>, May 25, p. 80, 1904. <i>Phil. Mag.</i> April, 1905.</p> -</div> -<div class='footnote' id='f353'> -<p class='c006'><span class='label'><a href='#r353'>353</a>. </span>McCoy, <i>Ber. d. D. Chem. Ges.</i> No. 11, p. 2641, 1904.</p> -</div> -<div class='footnote' id='f354'> -<p class='c006'><span class='label'><a href='#r354'>354</a>. </span>Strutt, <i>Nature</i>, March 17 and July 7, 1904. <i>Proc. Roy. Soc.</i> March 2, 1905.</p> -</div> -<div class='footnote' id='f355'> -<p class='c006'><span class='label'><a href='#r355'>355</a>. </span>Strutt, <i>Proc. Roy. Soc.</i> March 2, 1905.</p> -</div> -<div class='footnote' id='f356'> -<p class='c006'><span class='label'><a href='#r356'>356</a>. </span>Soddy, <i>Nature</i>, May 12, 1904; Jan. 19, 1905.</p> -</div> -<div class='footnote' id='f357'> -<p class='c006'><span class='label'><a href='#r357'>357</a>. </span>Whetham, <i>Nature</i>, May 5, 1904; Jan. 26, 1905.</p> -</div> -<div class='footnote' id='f358'> -<p class='c006'><span class='label'><a href='#r358'>358</a>. </span>Danne, <i>C. R.</i> Jan. 23, 1905.</p> -</div> -<div class='footnote' id='f359'> -<p class='c006'><span class='label'><a href='#r359'>359</a>. </span>J. J. Thomson, <i>Nature</i>, April 30, p. 601, 1903.</p> -</div> -<div class='footnote' id='f360'> -<p class='c006'><span class='label'><a href='#r360'>360</a>. </span>Voller, <i>Phys. Zeit.</i> 5, No. 24, p. 781, 1904.</p> -</div> -<div class='footnote' id='f361'> -<p class='c006'><span class='label'><a href='#r361'>361</a>. </span>Ramsay and Cooke, <i>Nature</i>, Aug. 11, 1904.</p> -</div> -<div class='footnote' id='f362'> -<p class='c006'><span class='label'><a href='#r362'>362</a>. </span>Eve, <i>Nature</i>, March 16, 1905.</p> -</div> -<div class='footnote' id='f363'> -<p class='c006'><span class='label'><a href='#r363'>363</a>. </span>J. J. Thomson, International Electrical Congress, St Louis, Sept. 1904.</p> -</div> -<div class='footnote' id='f364'> -<p class='c006'><span class='label'><a href='#r364'>364</a>. </span>Rutherford and Soddy, <i>Phil. Mag.</i> p. 582, 1902; pp. 453 and 579, 1903.</p> -</div> -<div class='footnote' id='f365'> -<p class='c006'><span class='label'><a href='#r365'>365</a>. </span>Ramsay and Soddy, <i>Nature</i>, July 16, p. 246, 1903. <i>Proc. Roy. Soc.</i> 72, p. 204, -1903; 73, p. 346, 1904.</p> -</div> -<div class='footnote' id='f366'> -<p class='c006'><span class='label'><a href='#r366'>366</a>. </span>Curie and Dewar, <i>C. R.</i> 138, p. 190, 1904. <i>Chem. News</i>, 89, p. 85, 1904.</p> -</div> -<div class='footnote' id='f367'> -<p class='c006'><span class='label'><a href='#r367'>367</a>. </span>Himstedt and Meyer, <i>Ann. d. Phys.</i> 15, p. 184, 1904.</p> -</div> -<div class='footnote' id='f368'> -<p class='c006'><span class='label'><a href='#r368'>368</a>. </span>Strutt, <i>Proc. Roy. Soc.</i> March 2, 1905.</p> -</div> -<div class='footnote' id='f369'> -<p class='c006'><span class='label'><a href='#r369'>369</a>. </span>Boltwood, <i>Phil. Mag.</i> April, 1905.</p> -</div> -<div class='footnote' id='f370'> -<p class='c006'><span class='label'><a href='#r370'>370</a>. </span>Moss, <i>Trans. Roy. Soc. Dublin</i>, 1904.</p> -</div> -<div class='footnote' id='f371'> -<p class='c006'><span class='label'><a href='#r371'>371</a>. </span>Travers, <i>Nature</i>, p. 248, Jan. 12, 1905.</p> -</div> -<div class='footnote' id='f372'> -<p class='c006'><span class='label'><a href='#r372'>372</a>. </span>Jaquerod, <i>C. R.</i> p. 789, 1904.</p> -</div> -<div class='footnote' id='f373'> -<p class='c006'><span class='label'><a href='#r373'>373</a>. </span>Ramsay and Travers, <i>Zeitsch. Physik. Chem.</i> 25, p. 568, 1898.</p> -</div> -<div class='footnote' id='f374'> -<p class='c006'><span class='label'><a href='#r374'>374</a>. </span>Ramsay, <i>Nature</i>, April 7, 1904.</p> -</div> -<div class='footnote' id='f375'> -<p class='c006'><span class='label'><a href='#r375'>375</a>. </span>Lodge, <i>Nature</i>, June 11, p. 129, 1903.</p> -</div> -<div class='footnote' id='f376'> -<p class='c006'><span class='label'><a href='#r376'>376</a>. </span>Larmor, <i>Aether and Matter</i>, p. 233.</p> -</div> -<div class='footnote' id='f377'> -<p class='c006'><span class='label'><a href='#r377'>377</a>. </span>J. J. Thomson, <i>Phil. Mag.</i> p. 681, Dec. 1903.</p> -</div> -<div class='footnote' id='f378'> -<p class='c006'><span class='label'><a href='#r378'>378</a>. </span>Lord Kelvin, <i>Phil. Mag.</i> Oct. 1904.</p> -</div> -<div class='footnote' id='f379'> -<p class='c006'><span class='label'><a href='#r379'>379</a>. </span>Thomson, <i>Phil. Mag.</i> March, 1904.</p> -</div> -<div class='footnote' id='f380'> -<p class='c006'><span class='label'><a href='#r380'>380</a>. </span>Rutherford and Soddy, <i>Phil. Mag.</i> May, 1903.</p> -</div> -<div class='footnote' id='f381'> -<p class='c006'><span class='label'><a href='#r381'>381</a>. </span>See Strutt and Joly, <i>Nature</i>, Oct. 15, 1903.</p> -</div> -<div class='footnote' id='f382'> -<p class='c006'><span class='label'><a href='#r382'>382</a>. </span>Strutt, <i>Phil. Mag.</i> June, 1903.</p> -</div> -<div class='footnote' id='f383'> -<p class='c006'><span class='label'><a href='#r383'>383</a>. </span>Elster and Geitel, <i>Phys. Zeit.</i> 4, No. 19, p. 522, 1903. <i>Chem. News</i>, July 17, -p. 30, 1903.</p> -</div> -<div class='footnote' id='f384'> -<p class='c006'><span class='label'><a href='#r384'>384</a>. </span>Geitel, <i>Phys. Zeit.</i> 2, p. 116, 1900.</p> -</div> -<div class='footnote' id='f385'> -<p class='c006'><span class='label'><a href='#r385'>385</a>. </span>C. T. R. Wilson, <i>Proc. Camb. Phil. Soc.</i> 11, p. 32, 1900. <i>Proc. Roy. Soc.</i> 68, -p. 151, 1901.</p> -</div> -<div class='footnote' id='f386'> -<p class='c006'><span class='label'><a href='#r386'>386</a>. </span>Elster and Geitel, <i>Phys. Zeit.</i> 2, p. 590, 1901.</p> -</div> -<div class='footnote' id='f387'> -<p class='c006'><span class='label'><a href='#r387'>387</a>. </span>Elster and Geitel, <i>Phys. Zeit.</i> 3, p. 76, 1901.</p> -</div> -<div class='footnote' id='f388'> -<p class='c006'><span class='label'><a href='#r388'>388</a>. </span>Rutherford and Allan, <i>Phil. Mag.</i> Dec. 1902.</p> -</div> -<div class='footnote' id='f389'> -<p class='c006'><span class='label'><a href='#r389'>389</a>. </span>Allan, <i>Phil. Mag.</i> Feb. 1904.</p> -</div> -<div class='footnote' id='f390'> -<p class='c006'><span class='label'><a href='#r390'>390</a>. </span>C. T. R. Wilson, <i>Proc. Camb. Phil. Soc.</i> 11, p. 428, 1902.</p> -</div> -<div class='footnote' id='f391'> -<p class='c006'><span class='label'><a href='#r391'>391</a>. </span>C. T. R. Wilson, <i>Proc. Camb. Phil. Soc.</i> 11, p. 428, 1902; 12, p. 17, 1903.</p> -</div> -<div class='footnote' id='f392'> -<p class='c006'><span class='label'><a href='#r392'>392</a>. </span>C. T. R. Wilson, <i>Proc. Camb. Phil. Soc.</i> 12, p. 85, 1903.</p> -</div> -<div class='footnote' id='f393'> -<p class='c006'><span class='label'><a href='#r393'>393</a>. </span>Allan, <i>Phys. Rev.</i> 16, p. 106, 1903.</p> -</div> -<div class='footnote' id='f394'> -<p class='c006'><span class='label'><a href='#r394'>394</a>. </span>McLennan, <i>Phys. Rev.</i> 16, p. 184, 1903.</p> -</div> -<div class='footnote' id='f395'> -<p class='c006'><span class='label'><a href='#r395'>395</a>. </span>Schmauss, <i>Annal. d. Phys.</i> 9, p. 224, 1902.</p> -</div> -<div class='footnote' id='f396'> -<p class='c006'><span class='label'><a href='#r396'>396</a>. </span>Elster and Geitel, <i>Phys. Zeit</i>. 3, p. 574, 1902.</p> -</div> -<div class='footnote' id='f397'> -<p class='c006'><span class='label'><a href='#r397'>397</a>. </span>Ebert and Ewers, <i>Phys. Zeit.</i> 4, p. 162, 1902.</p> -</div> -<div class='footnote' id='f398'> -<p class='c006'><span class='label'><a href='#r398'>398</a>. </span>Sarasin, Tommasina and Micheli, <i>C. R.</i> 139, p. 917, 1905.</p> -</div> -<div class='footnote' id='f399'> -<p class='c006'><span class='label'><a href='#r399'>399</a>. </span>J. J. Thomson, <i>Phil. Mag.</i> Sept. 1902.</p> -</div> -<div class='footnote' id='f400'> -<p class='c006'><span class='label'><a href='#r400'>400</a>. </span>Ebert, <i>Sitz. Akad. d. Wiss. Munich</i>, 33, p. 133, 1903.</p> -</div> -<div class='footnote' id='f401'> -<p class='c006'><span class='label'><a href='#r401'>401</a>. </span>J. J. Thomson, <i>Phil. Mag.</i> Sept. 1902.</p> -</div> -<div class='footnote' id='f402'> -<p class='c006'><span class='label'><a href='#r402'>402</a>. </span>Adams, <i>Phil. Mag.</i> Nov. 1903.</p> -</div> -<div class='footnote' id='f403'> -<p class='c006'><span class='label'><a href='#r403'>403</a>. </span>Bumstead and Wheeler, <i>Amer. Journ. Science</i>, 17, p. 97, Feb. 1904.</p> -</div> -<div class='footnote' id='f404'> -<p class='c006'><span class='label'><a href='#r404'>404</a>. </span>Bumstead, <i>Amer. Journ. Science</i>, 18, July, 1904.</p> -</div> -<div class='footnote' id='f405'> -<p class='c006'><span class='label'><a href='#r405'>405</a>. </span>Dadourian, <i>Amer. Journ. Science</i>, 19, Jan. 1905.</p> -</div> -<div class='footnote' id='f406'> -<p class='c006'><span class='label'><a href='#r406'>406</a>. </span>H. S. Allen and Lord Blythswood, <i>Nature</i>, 68, p. 343, 1903; 69, p. 247, 1904.</p> -</div> -<div class='footnote' id='f407'> -<p class='c006'><span class='label'><a href='#r407'>407</a>. </span>Strutt, <i>Proc. Roy. Soc.</i> 73, p. 191, 1904.</p> -</div> -<div class='footnote' id='f408'> -<p class='c006'><span class='label'><a href='#r408'>408</a>. </span>Himstedt, <i>Ann. d. Phys.</i> 13, p. 573, 1904.</p> -</div> -<div class='footnote' id='f409'> -<p class='c006'><span class='label'><a href='#r409'>409</a>. </span>Elster and Geitel, <i>Phys. Zeit.</i> 5, No. 12, p. 321, 1904.</p> -</div> -<div class='footnote' id='f410'> -<p class='c006'><span class='label'><a href='#r410'>410</a>. </span>Dorn, <i>Abhandl. d. Natur. Ges. Halle</i>, 25, p. 107, 1904.</p> -</div> -<div class='footnote' id='f411'> -<p class='c006'><span class='label'><a href='#r411'>411</a>. </span>Schenck, Thesis Univ. Halle, 1904.</p> -</div> -<div class='footnote' id='f412'> -<p class='c006'><span class='label'><a href='#r412'>412</a>. </span>Mache, <i>Wien. Ber.</i> 113, p. 1329, 1904.</p> -</div> -<div class='footnote' id='f413'> -<p class='c006'><span class='label'><a href='#r413'>413</a>. </span>Curie and Laborde, <i>C. R.</i> 138, p. 1150, 1904.</p> -</div> -<div class='footnote' id='f414'> -<p class='c006'><span class='label'><a href='#r414'>414</a>. </span>Blanc, <i>Phil. Mag.</i> Jan. 1905.</p> -</div> -<div class='footnote' id='f415'> -<p class='c006'><span class='label'><a href='#r415'>415</a>. </span>Boltwood, <i>Amer. Journ. Science</i>, 18, Nov. 1904.</p> -</div> -<div class='footnote' id='f416'> -<p class='c006'><span class='label'><a href='#r416'>416</a>. </span>Elster and Geitel, <i>Phys. Zeit.</i> 4, p. 522, 1903.</p> -</div> -<div class='footnote' id='f417'> -<p class='c006'><span class='label'><a href='#r417'>417</a>. </span>Elster and Geitel, <i>Phys. Zeit.</i> 5, No. 1, p. 11, 1903.</p> -</div> -<div class='footnote' id='f418'> -<p class='c006'><span class='label'><a href='#r418'>418</a>. </span>Vincenti and Levi Da Zara, <i>Atti d. R. Instit. Veneto d. Scienze</i>, 54, p. 95, -1905.</p> -</div> -<div class='footnote' id='f419'> -<p class='c006'><span class='label'><a href='#r419'>419</a>. </span>Burton, <i>Phil. Mag.</i> Oct. 1904.</p> -</div> -<div class='footnote' id='f420'> -<p class='c006'><span class='label'><a href='#r420'>420</a>. </span>Elster and Geitel, <i>Phys. Zeit.</i> 6, No. 3, p. 67, 1905.</p> -</div> -<div class='footnote' id='f421'> -<p class='c006'><span class='label'><a href='#r421'>421</a>. </span>Rutherford and Allan, <i>Phil. Mag.</i> Dec. 1902.</p> -</div> -<div class='footnote' id='f422'> -<p class='c006'><span class='label'><a href='#r422'>422</a>. </span>Elster and Geitel, <i>Phys. Zeit.</i> 4, p. 138, 1902; 4, p. 522, 1903.</p> -</div> -<div class='footnote' id='f423'> -<p class='c006'><span class='label'><a href='#r423'>423</a>. </span>Saake, <i>Phys. Zeit.</i> 4, p. 626, 1903.</p> -</div> -<div class='footnote' id='f424'> -<p class='c006'><span class='label'><a href='#r424'>424</a>. </span>Simpson, <i>Proc. Roy. Soc.</i> 73, p. 209, 1904.</p> -</div> -<div class='footnote' id='f425'> -<p class='c006'><span class='label'><a href='#r425'>425</a>. </span>McLennan, <i>Phys. Rev.</i> 16, p. 184, 1903, and <i>Phil. Mag.</i> 5, p. 419, 1903.</p> -</div> -<div class='footnote' id='f426'> -<p class='c006'><span class='label'><a href='#r426'>426</a>. </span>McLennan, <i>Phys. Rev.</i> No. 4, 1903.</p> -</div> -<div class='footnote' id='f427'> -<p class='c006'><span class='label'><a href='#r427'>427</a>. </span>Rutherford and Cooke, <i>Americ. Phys. Soc.</i> Dec. 1902.</p> -</div> -<div class='footnote' id='f428'> -<p class='c006'><span class='label'><a href='#r428'>428</a>. </span>Cooke, <i>Phil. Mag.</i> Oct. 1903.</p> -</div> -<div class='footnote' id='f429'> -<p class='c006'><span class='label'><a href='#r429'>429</a>. </span>Allan, <i>Phil. Mag.</i> Feb. 1904.</p> -</div> -<div class='footnote' id='f430'> -<p class='c006'><span class='label'><a href='#r430'>430</a>. </span>Ebert, <i>Phys. Zeit.</i> 2, p. 622, 1901. <i>Zeitschr. f. Luftschiffahrt</i>, 4, Oct. 1902.</p> -</div> -<div class='footnote' id='f431'> -<p class='c006'><span class='label'><a href='#r431'>431</a>. </span>Schuster, <i>Proc. Manchester Phil. Soc.</i> p. 488, No. 12, 1904.</p> -</div> -<div class='footnote' id='f432'> -<p class='c006'><span class='label'><a href='#r432'>432</a>. </span>Mache and Von Schweidler, <i>Phys. Zeit.</i> 6, No. 3, p. 71, 1905.</p> -</div> -<div class='footnote' id='f433'> -<p class='c006'><span class='label'><a href='#r433'>433</a>. </span>Langevin, <i>C. R.</i> 140, p. 232, 1905.</p> -</div> -<div class='footnote' id='f434'> -<p class='c006'><span class='label'><a href='#r434'>434</a>. </span>Schuster, British Assoc. 1903.</p> -</div> -<div class='footnote' id='f435'> -<p class='c006'><span class='label'><a href='#r435'>435</a>. </span>J. J. Thomson, <i>Conduction of Electricity through Gases</i>, p. 324, 1903.</p> -</div> -<div class='footnote' id='f436'> -<p class='c006'><span class='label'><a href='#r436'>436</a>. </span>Miss Gates, <i>Phys. Rev.</i> 17, p. 499, 1903.</p> -</div> -<div class='footnote' id='f437'> -<p class='c006'><span class='label'><a href='#r437'>437</a>. </span>Villard, <i>Société de Physique</i>, July, 1900.</p> -</div> -<div class='footnote' id='f438'> -<p class='c006'><span class='label'><a href='#r438'>438</a>. </span>Geitel, <i>Phys. Zeit.</i> 2, p. 116, 1900.</p> -</div> -<div class='footnote' id='f439'> -<p class='c006'><span class='label'><a href='#r439'>439</a>. </span>C. T. R. Wilson, <i>Proc. Camb. Phil. Soc.</i> 11, p. 52, 1900. <i>Proc. Roy. Soc.</i> 68, -p. 152, 1901.</p> -</div> -<div class='footnote' id='f440'> -<p class='c006'><span class='label'><a href='#r440'>440</a>. </span>Rutherford and Allan, <i>Phil. Mag.</i> Dec. 1902.</p> -</div> -<div class='footnote' id='f441'> -<p class='c006'><span class='label'><a href='#r441'>441</a>. </span>Patterson, <i>Phil. Mag.</i> August, 1903.</p> -</div> -<div class='footnote' id='f442'> -<p class='c006'><span class='label'><a href='#r442'>442</a>. </span>Harms, <i>Phys. Zeit.</i> 4, No. 1, p. 11, 1902.</p> -</div> -<div class='footnote' id='f443'> -<p class='c006'><span class='label'><a href='#r443'>443</a>. </span>Cooke, <i>Phil. Mag.</i> Oct. 1903.</p> -</div> -<div class='footnote' id='f444'> -<p class='c006'><span class='label'><a href='#r444'>444</a>. </span>Wilson, <i>Proc. Roy. Soc.</i> 69, p. 277, 1901.</p> -</div> -<div class='footnote' id='f445'> -<p class='c006'><span class='label'><a href='#r445'>445</a>. </span>Jaffé, <i>Phil. Mag.</i> Oct. 1904.</p> -</div> -<div class='footnote' id='f446'> -<p class='c006'><span class='label'><a href='#r446'>446</a>. </span>Patterson, <i>Phil. Mag.</i> Aug. 1903.</p> -</div> -<div class='footnote' id='f447'> -<p class='c006'><span class='label'><a href='#r447'>447</a>. </span>Strutt, <i>Phil. Mag.</i> June, 1903. <i>Nature</i>, Feb. 19, 1903.</p> -</div> -<div class='footnote' id='f448'> -<p class='c006'><span class='label'><a href='#r448'>448</a>. </span>McLennan and Burton, <i>Phys. Rev.</i> No. 4, 1903. J. J. Thomson, <i>Nature</i>, -Feb. 26, 1903.</p> -</div> -<div class='footnote' id='f449'> -<p class='c006'><span class='label'><a href='#r449'>449</a>. </span>Cooke, <i>Phil. Mag.</i> Aug. 6, 1903. Rutherford, <i>Nature</i>, April 2, 1903.</p> -</div> -<div class='footnote' id='f450'> -<p class='c006'><span class='label'><a href='#r450'>450</a>. </span>Eve, <i>Nature</i>, March 16, 1905.</p> -</div> -<div class='footnote' id='f451'> -<p class='c006'><span class='label'><a href='#r451'>451</a>. </span>See article in <i>Le Radium</i>, No. 3, p. 81, Sept. 15, 1904.</p> -</div> -<div class='footnote' id='f452'> -<p class='c006'><span class='label'><a href='#r452'>452</a>. </span>J. J. Thomson, <i>Proc. Camb. Phil. Soc.</i> 12, p. 391, 1904.</p> -</div> -<div class='footnote' id='f453'> -<p class='c006'><span class='label'><a href='#r453'>453</a>. </span>Wood, <i>Phil. Mag.</i> April, 1905.</p> -</div> -<div class='footnote' id='f454'> -<p class='c006'><span class='label'><a href='#r454'>454</a>. </span>Campbell, <i>Nature</i>, p. 511, March 31, 1904. <i>Phil. Mag.</i> April, 1905.</p> -</div> -<div class='footnote' id='f455'> -<p class='c006'><span class='label'><a href='#r455'>455</a>. </span>An apparent exception has been observed by Danne in the case of certain -lead minerals which occur under peculiar conditions at d’Issy-l’Évêque, France. -See p. <a href='#Page_465'>465</a>.</p> -</div> -<div> - - <ul class='ul_1 c002'> - <li>Transcriber’s Notes: - <ul class='ul_2'> - <li>Inconsistent spelling and hyphenation were made consistent only when a predominant - form was found in this book. - </li> - <li>Footnotes have been collected at the end of the text, and are linked for ease of - reference. - </li> - </ul> - </li> - </ul> - -</div> - - - - -<div style='display:block;margin-top:4em'>*** END OF THE PROJECT GUTENBERG EBOOK RADIO-ACTIVITY ***</div> -<div style='display:block;margin:1em 0;'>This file should be named 12345-h.htm or 64693-h.zip</div> -<div style='display:block;margin:1em 0;'>This and all associated files of various formats will be found in https://www.gutenberg.org/6/4/6/9/64693/</div> - -<div style='text-align:left'> - -<div style='display:block; margin:1em 0'> -Updated editions will replace the previous one—the old editions will -be renamed. -</div> - -<div style='display:block; margin:1em 0'> -Creating the works from print editions not protected by U.S. copyright -law means that no one owns a United States copyright in these works, -so the Foundation (and you!) can copy and distribute it in the United -States without permission and without paying copyright -royalties. 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