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diff --git a/22784-h/22784-h.htm b/22784-h/22784-h.htm new file mode 100644 index 0000000..3c8e362 --- /dev/null +++ b/22784-h/22784-h.htm @@ -0,0 +1,11622 @@ +<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"> +<html> +<head> +<meta http-equiv="Content-Type" content="text/html; charset=UTF-8"> +<title>ON LABORATORY ARTS</title> +</head> +<body> + + +<pre> + +The Project Gutenberg EBook of On Laboratory Arts, by Richard Threlfall + +This eBook is for the use of anyone anywhere 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 www.gutenberg.org + + +Title: On Laboratory Arts + +Author: Richard Threlfall + +Release Date: September 27, 2007 [EBook #22784] + +Language: English + +Character set encoding: UTF-8 + +*** START OF THIS PROJECT GUTENBERG EBOOK ON LABORATORY ARTS *** + + + + +Produced by Jon Richfield + + + + + +</pre> + +<p align="center"><b><font face="Bookman Old Style" size="6">ON +LABORATORY ARTS</font></b></p> +<p align="center"> </p> +<p align="center"> </p> +<p align="center"> </p> +<p align="center"> </p> +<p align="center"> </p> +<p align="center"> </p> +<p align="center"><font face="Bookman Old Style" size= +"2">BY</font></p> +<p align="center"> </p> +<p align="center"><b><font face="Bookman Old Style" size= +"4">RICHARD THRELFALL, M.A.</font></b></p> +<p align="center"> </p> +<p align="center"><font face="Bookman Old Style" size= +"2">PROFESSOR OF PHYSICS IN THE UNIVERSITY OF SYDNEY;</font></p> +<p align="center">MEMBER OF THE INSTITUTE OF ELECTRICAL +ENGINEERS;</p> +<p align="center">ASSOCIATE-MEMBER OF THE INSTITUTE OF CIVIL +ENGINEERS;</p> +<p align="center">MEMBER OF THE PHYSICAL SOCIETY</p> +<p align="center"> </p> +<p align="center"> </p> +<p align="center"> </p> +<p align="center"> </p> +<p align="center"> </p> +<p align="center"> </p> +<p align="center">London</p> +<p align="center">MACMILLAN AND CO., LIMITED</p> +<p align="center"><font face="Bookman Old Style" size="2">NEW +YORK: THE MACMILLAN COMPANY</font></p> +<p align="center"><font face="Bookman Old Style">1898</font></p> +<p align="center"><font face="Bookman Old Style" size="1">All +rights reserved</font></p> +<p><b><font size="2">PREFACE</font></b> <a href= +"#Toc158108886">*</a></p> +<p><b><font size="2">CHAPTER I</font></b> <a href= +"#Toc158108887">*</a></p> +<div style="margin-left: 2em"> +<p><font size="2">HINTS ON THE MANIPULATION OF GLASS AND ON +GLASS-BLOWING FOR LABORATORY PURPOSES</font> <a href= +"#Toc158108888">*</a></p> +<p><font size="2">§ 4. Soft Soda Glass,</font> <a href= +"#Toc158108889">*</a></p> +<p><font size="2">§ 6. Flint Glass. —</font> <a href= +"#Toc158108890">*</a></p> +<p><font size="2">§ 9. Hard or Bohemian, Glass. +—</font> <a href="#Toc158108891">*</a></p> +<p><font size="2">§ 10. On the Choice of Sizes of Glass +Tube. —</font> <a href="#Toc158108892">*</a></p> +<p><font size="2">§ 11. Testing Glass. —</font> +<a href="#Toc158108893">*</a></p> +<p><font size="2">§ 13. Cleaning Glass Tubes. —</font> +<a href="#Toc158108894">*</a></p> +<p><font size="2">§ 14. The Blow-pipe. —</font> +<a href="#Toc158108895">*</a></p> +<p><font size="2">§ 18. <i>The Table</i>. —</font> +<a href="#Toc158108896">*</a></p> +<p><font size="2">§ 19. Special Operations. —</font> +<a href="#Toc158108897">*</a></p> +<p><font size="2">§ 20. Closing and blowing out the End of a +Tube. —</font> <a href="#Toc158108898">*</a></p> +<p><font size="2">§ 21. To make a Weld. —</font> +<a href="#Toc158108899">*</a></p> +<p><font size="2">§ 22. To weld two Tubes of different +Sizes. —</font> <a href="#Toc158108900">*</a></p> +<p><font size="2">§ 24. To weld Tubes of very small Bore. +—</font> <a href="#Toc158108901">*</a></p> +<p><font size="2">§ 30. To cut very thick Tubes.</font> +<a href="#Toc158108902">*</a></p> +<p><font size="2">§ 31. To blow a Bulb at the End of a Tube. +—</font> <a href="#Toc158108903">*</a></p> +<p><font size="2">§ 32. To blow a bulb in the middle of a +tube,</font> <a href="#Toc158108904">*</a></p> +<p><font size="2">§ 33. To make a side Weld. —</font> +<a href="#Toc158108905">*</a></p> +<p><font size="2">§ 34. Inserted Joints. —</font> +<a href="#Toc158108906">*</a></p> +<p><font size="2">§ 35. Bending Tubes. —</font> +<a href="#Toc158108907">*</a></p> +<p><font size="2">§ 36. Spiral Tubes. —</font> +<a href="#Toc158108908">*</a></p> +<p><font size="2">§ 37. On Auxiliary Operations on Glass:- +</font> <a href="#Toc158108909">*</a></p> +<p><font size="2">§ 38. Boring small Holes. —</font> +<a href="#Toc158108910">*</a></p> +<p><font size="2">§ 39. For boring large holes through thick +glass sheets,</font> <a href="#Toc158108911">*</a></p> +<p><font size="2">§ 41. Operations depending on Grinding: +Ground-in Joints. —</font> <a href= +"#Toc158108912">*</a></p> +<p><font size="2">§ 42. Use of the Lathe in Glass-working. +—</font> <a href="#Toc158108913">*</a></p> +<p><font size="2">§ 46. Making Ground Glass. —</font> +<a href="#Toc158108914">*</a></p> +<p><font size="2">§ 47. Glass-cutting. —</font> +<a href="#Toc158108915">*</a></p> +<p><font size="2">§ 48. <i>Cementing</i>. —</font> +<a href="#Toc158108916">*</a></p> +<p><font size="2">§ 49. Fusing Electrodes into Glass. +—</font> <a href="#Toc158108917">*</a></p> +<p><font size="2">§ 51. The Art of making Air-tight Joints. +—</font> <a href="#Toc158108918">*</a></p> +</div> +<p><b><font size="2">APPENDIX TO CHAPTER I</font></b> <a href= +"#Toc158108919">*</a></p> +<div style="margin-left: 2em"> +<p><font size="2">ON THE PREPARATION OF VACUUM TUBES FOR THE +PRODUCTION OF PROFESSOR ROENTGEN'S RADIATION</font> <a href= +"#Toc158108920">*</a></p> +</div> +<p><b><font size="2">CHAPTER II</font></b> <a href= +"#Toc158108921">*</a></p> +<div style="margin-left: 2em"> +<p><font size="2">GLASS-GRINDING AND OPTICIANS' WORK</font> +<a href="#Toc158108922">*</a></p> +<p><font size="2">§ 61. Details of the Process of Fine +Grinding. —</font> <a href="#Toc158108923">*</a></p> +<p><font size="2">§ 62. <i>Polishing</i>. —</font> +<a href="#Toc158108924">*</a></p> +<p><font size="2">§ 63. <i>Centering</i>. —</font> +<a href="#Toc158108925">*</a></p> +<p><font size="2">§ 65. Preparing Small Mirrors for +Galvanometers. —</font> <a href="#Toc158108926">*</a></p> +<p><font size="2">§ 66. Preparation of Large Mirrors or +Lenses for Telescopes. —</font> <a href= +"#Toc158108927">*</a></p> +<p><font size="2">§ 69. The Preparation of Flat Surfaces of +Rock Salt. —</font> <a href="#Toc158108928">*</a></p> +<p><font size="2">§ 70. Casting Specula for Mirrors. +—</font> <a href="#Toc158108929">*</a></p> +<p><font size="2">§ 71. Grinding and polishing Specula. +—</font> <a href="#Toc158108930">*</a></p> +<p><font size="2">§ 72. Preparation of Flat Surfaces. +—</font> <a href="#Toc158108931">*</a></p> +<p><font size="2">§ 73. Polishing Flat Surfaces on Glass or +on Speculum Metal. —</font> <a href= +"#Toc158108932">*</a></p> +</div> +<p><b><font size="2">CHAPTER III</font></b> <a href= +"#Toc158108933">*</a></p> +<div style="margin-left: 2em"> +<p><font size="2">MISCELLANEOUS PROCESSES</font> <a href= +"#Toc158108934">*</a></p> +<p><font size="2">§ 74. Coating Glass with Aluminium and +Soldering Aluminium. —</font> <a href= +"#Toc158108935">*</a></p> +<p><font size="2">§ 75. The Use of the Diamond-cutting +Wheel. —</font> <a href="#Toc158108936">*</a></p> +<p><font size="2">§ 76. Arming a Wheel. —</font> +<a href="#Toc158108937">*</a></p> +<p><font size="2">§ 77. Cutting a Section. —</font> +<a href="#Toc158108938">*</a></p> +<p><font size="2">§ 78. Grinding Rock Sections, or Thin +Slips of any Hard Material.—</font> <a href= +"#Toc158108939">*</a></p> +<p><font size="2">§ 79. Cutting Sections of Soft Substances. +—</font> <a href="#Toc158108940">*</a></p> +<p><font size="2">§ 80. On the Production of Quartz +Threads.' —</font> <a href="#Toc158108941">*</a></p> +<p><font size="2">§ 84. Drawing Quartz Threads. +—</font> <a href="#Toc158108942">*</a></p> +<p><font size="2">§ 86. Drawing Threads by the Catapult. +—</font> <a href="#Toc158108943">*</a></p> +<p><font size="2">§ 87. Drawing Threads by the Flame alone. +—</font> <a href="#Toc158108944">*</a></p> +<p><font size="2">§ 88. Properties of Threads. +—</font> <a href="#Toc158108945">*</a></p> +<p><font size="2">§ 90. On the Attachment of Quartz Fibres. +—</font> <a href="#Toc158108946">*</a></p> +<p><font size="2">§ 91. Other Modes of soldering Quartz. +—</font> <a href="#Toc158108947">*</a></p> +<p><font size="2">§ 92. Soldering. —</font> <a href= +"#Toc158108948">*</a></p> +<p><font size="2">§ 94. Preparing a Soldering Bit. +—</font> <a href="#Toc158108949">*</a></p> +<p><font size="2">§ 95. Soft Soldering. —</font> +<a href="#Toc158108950">*</a></p> +<p><font size="2">§ 97. Soldering Zinc. —</font> +<a href="#Toc158108951">*</a></p> +<p><font size="2">§ 98. Soldering other Metals +—</font> <a href="#Toc158108952">*</a></p> +<p><font size="2">§ 99. <i>Brazing</i>.</font> <a href= +"#Toc158108953">*</a></p> +<p><font size="2">§ 100. Silver Soldering. —</font> +<a href="#Toc158108954">*</a></p> +<p><font size="2">§ 101. On the Construction of Electrical +Apparatus - Insulators. —</font> <a href= +"#Toc158108955">*</a></p> +<p><font size="2">§ 102. Sulphur. —</font> <a href= +"#Toc158108956">*</a></p> +<p><font size="2">§ 103. <i>Fused Quartz</i>. —</font> +<a href="#Toc158108957">*</a></p> +<p><font size="2">§ 104. <i>Glass.</i> —</font> +<a href="#Toc158108958">*</a></p> +<p><font size="2">§ 105. Ebonite or Hard Rubber. +—</font> <a href="#Toc158108959">*</a></p> +<p><font size="2">§ 106. <i>Mica</i>. —</font> +<a href="#Toc158108960">*</a></p> +<p><font size="2">§ 107. Use of Mica in Condensers. +—</font> <a href="#Toc158108961">*</a></p> +<p><font size="2">§ 108. <i>Micanite</i>. —</font> +<a href="#Toc158108962">*</a></p> +<p><font size="2">§ 109. <i>Celluloid</i>. —</font> +<a href="#Toc158108963">*</a></p> +<p><font size="2">§ 110. <i>Paper</i>.</font> <a href= +"#Toc158108964">*</a></p> +<p><font size="2">§ 111. Paraffined Paper. —</font> +<a href="#Toc158108965">*</a></p> +<p><font size="2">§ 112. <i>Paraffin</i> —</font> +<a href="#Toc158108966">*</a></p> +<p><font size="2">§ 113. Vaseline, Vaseline Oil, and +Kerosene Oil. —</font> <a href="#Toc158108967">*</a></p> +<p><font size="2">§ 114. Imperfect Conductors. +—</font> <a href="#Toc158108968">*</a></p> +<p><font size="2">§ 116. <i>Conductors</i>. —</font> +<a href="#Toc158108969">*</a></p> +<p><font size="2">§ 117. <i>Platinoid</i>. —</font> +<a href="#Toc158108970">*</a></p> +<p><font size="2">§ 119. Platinum Silver. —</font> +<a href="#Toc158108971">*</a></p> +<p><font size="2">§ 120. Platinum Iridium. —</font> +<a href="#Toc158108972">*</a></p> +<p><font size="2">§ 121. <i>Manganin</i>. —</font> +<a href="#Toc158108973">*</a></p> +<p><font size="2">§ 122. <i>Other Alloys</i>. —</font> +<a href="#Toc158108974">*</a></p> +<p><font size="2">§ 123. <i>Nickelin</i>. —</font> +<a href="#Toc158108975">*</a></p> +<p><font size="2">§ 124. Patent Nickel. —</font> +<a href="#Toc158108976">*</a></p> +<p><font size="2">§ 125. <i>Constantin</i>. —</font> +<a href="#Toc158108977">*</a></p> +<p><font size="2">126. Nickel Manganese Copper. —</font> +<a href="#Toc158108978">*</a></p> +</div> +<p><b><font size="2">CHAPTER IV</font></b> <a href= +"#Toc158108979">*</a></p> +<div style="margin-left: 2em"> +<p><font size="2">ELECTROPLATING AND ALLIED ARTS</font> <a href= +"#Toc158108980">*</a></p> +<p><font size="2">§ 127. Electroplating. —</font> +<a href="#Toc158108981">*</a></p> +<p><font size="2">§ 128. The Dipping Bath. —</font> +<a href="#Toc158108982">*</a></p> +<p><font size="2">§ 130. Scratch-brushing. —</font> +<a href="#Toc158108983">*</a></p> +<p><font size="2">§ 131. Burnishing. —</font> <a href= +"#Toc158108984">*</a></p> +<p><font size="2">§ 132. Silver-plating. —</font> +<a href="#Toc158108985">*</a></p> +<p><font size="2">§ 133. Cold Silvering. —</font> +<a href="#Toc158108986">*</a></p> +<p><font size="2">§ 134. <i>Gilding</i>. —</font> +<a href="#Toc158108987">*</a></p> +<p><font size="2">§ 135. Preparing Surfaces for Gilding. +—</font> <a href="#Toc158108988">*</a></p> +<p><font size="2">§ 136. Gilding Solutions. —</font> +<a href="#Toc158108989">*</a></p> +<p><font size="2">§ 137. Plating with Copper. —</font> +<a href="#Toc158108990">*</a></p> +<p><font size="2">§ 138. Coppering Aluminium. —</font> +<a href="#Toc158108991">*</a></p> +<p><font size="2">§ 140. Alkaline Coppering Solution +—</font> <a href="#Toc158108992">*</a></p> +<p><font size="2">§ 141. Nickel-plating.—</font> +<a href="#Toc158108993">*</a></p> +<p><font size="2">142. Miscellaneous Notes on Electroplating. +</font> <a href="#Toc158108994">*</a></p> +<p><font size="2">§ 143. Blacking Brass Surfaces. +—</font> <a href="#Toc158108995">*</a></p> +<p><font size="2">§ 144. <i>Sieves</i>. —</font> +<a href="#Toc158108996">*</a></p> +<p><font size="2">§ 145. Pottery making in the Laboratory. +—</font> <a href="#Toc158108997">*</a></p> +</div> +<p><b><font size="2">APPENDIX</font></b> <a href= +"#Toc158108998">*</a></p> +<div style="margin-left: 2em"> +<p><font size="2">PLATINISING GLASS</font> <a href= +"#Toc158108999">*</a></p> +</div> +<p><b><a name="Toc158108886" id="Toc158108886"><font face= +"Bookman Old Style" size="4">PREFACE</font></a></b></p> +<p><font face="Bookman Old Style">EXPERIMENTAL work in physical +science rests ultimately upon the mechanical arts. It is true +that in a well-appointed laboratory, where apparatus is collected +together in greater or less profusion, the appeal is often very +indirect, and to a student carrying out a set experiment with +apparatus provided to his hand, the temptation to ignore the +mechanical basis of his work is often irresistible.</font></p> +<p>It often happens that young physicists are to be found whose +mathematical attainments are adequate, whose observational powers +are perfectly trained, and whose general capacity is +unquestioned, but who are quite unable to design or construct the +simplest apparatus with due regard to the facility with which it +ought to be constructed. That ultimate knowledge of materials and +of processes which by long experience becomes intuitive in the +mind of a great inventor of course cannot be acquired from books +or from any set course of instruction.</p> +<p>There are, however, many steps between absolute ignorance and +consummate knowledge of the mechanical arts, and it is the object +of the following pages to assist the young physicist in making +his first steps towards acquiring a working knowledge of +"laboratory arts." However humble the ambition may be, no one can +be more keenly alive than the writer to the inadequacy of his +attempt; and it is only from a profound sense of the necessity +which exists for some beginning to be made, that he has had the +courage to air his views on matters about which there are +probably hundreds or thousands of people whose knowledge is +superior to his own.</p> +<p>Moreover, nothing has been further from the writer's mind than +any idea of "instructing" any one; his desire is — if +happily it may so befall — to be of assistance, especially +to young physicists or inventors who wish to attain definite +mechanical ends with the minimum expenditure of time. Most people +will agree that one condition essential to success in such an +undertaking is brevity, and it is for this reason that +alternative methods as a rule have not been given, which, of +course, deprives the book of any pretence to being a "treatise." +The writer, therefore, is responsible for exercising a certain +amount of discretion in the selection he has made, and it is +hardly to be hoped that he has in all — or even in the +majority of cases — succeeded in recommending absolutely +the best method of procedure.</p> +<p>This brings another point into view. Before all things the +means indicated must be definite and reliable. It is for this +reason that the writer has practically confined himself to +matters lying within his own immediate experience, and has never +recommended any process (with one or two minor exceptions, which +he has noted) which he has not actually and personally carried +through to a successful issue. This, although it is a matter +which he considers of the highest importance, and which is his +only title to a hearing, has unfortunately led to a very personal +tone in the book.</p> +<p>With regard to the arts treated of in the following pages, +matters about which information is easily acquired — such +as carpentering, blacksmithing, turning, and the arts of the +watchmaker — have been left on one side. With regard to the +last, which is of immense use in the laboratory, there happen to +be at least two excellent and handy books, viz. Saunier's +Watchmakers' Handbook, Tripplin, London, 1892; and Britton's +Watchmakers' Dictionary and Guide.</p> +<p>With regard to carpentering, turning, and blacksmithing, +almost any one who so desires can obtain a little practical +experience in any village. A short chapter has been devoted to +GLASS-BLOWING, in spite of there being an excellent and handy +book by Mr. Shenstone (The Methods of GLASS-BLOWING, Rivington) +on the subject already in existence. The reason for this +exception lies in the fact that the writer's methods differ +considerably from those advocated by Mr. Shenstone.</p> +<p>The chapter on opticians' work has had to be compressed to an +extent which is undesirable in dealing with so complex and +delicate an art, but it is hoped that it will prove a sufficient +introduction for laboratory purposes. In this matter the writer +is under great obligations to his friend and assistant, Mr. James +Cook, F.R.A.S., who gave him his first lessons in lens-making +some twenty years ago. To Mr. John A. Brashear of Allegheny, Pa., +thanks are due for much miscellaneous information on optical +work, which is included verbatim in the text, some of it +contained originally in printed papers, and some most kindly +communicated to the writer for the purpose of this book. In +particular, the writer would thank Mr. Brashear for his +generously accorded information as to the production of those +"flat" surfaces for which he is so justly famous.</p> +<p>The writer is also indebted to Mr. A. E. Kennelly for some +information as to American practice in the use of insulating +material for electrical work, and to his friends Mr. J. A. +Pollock and Dr. C. J. Martin for many valuable suggestions. For +the illustrations thanks are due to Mrs. Threlfall and Mr. James +Cook. With regard to matters which have come to the writer's +knowledge by his being specifically instructed in them from time +to time, due acknowledgment is, it is hoped, made in the +text.</p> +<p>With regard to the question as to what matters might be +included and what omitted, the general rule has been to include +information which the author has obtained with difficulty, and to +leave on one side that which he has more easily attained. All the +"unities" have been consistently outraged by a deliberate use of +the English and metric systems side by side. So long as all the +materials for mechanical processes have to be purchased to +specifications in inches and feet, it is impossible to use the +centimetre consistently without introducing inconvenience. +However, everybody ought to, and probably does, use either system +with equal facility.</p> +<p>No attempt has been made at showing how work can be done +without tools. Though, no doubt, a great deal can be done with +inferior appliances where great economy of money and none of time +is an object, the writer has long felt very strongly that English +physical laboratory practice has gone too far in the direction of +starving the workshop, and he does not wish, even indirectly, 'to +give any countenance to such a mistaken policy. Physical research +is too difficult in itself, and students' time is too valuable, +for it to be remunerative to work with insufficient +appliances.</p> +<p>In conclusion, the writer would ask his readers to regard the +book to some extent as tentative, and as a means to the procuring +and organising of information bearing upon laboratory arts. Any +information which can be given will be always thankfully +received, and the author hereby requests any reader who may +happen to learn something of value from the book to communicate +any special information he may possess, so that it may be of use +to others should another edition ever be called for.</p> +<p><b><a name="Toc158108887" id="Toc158108887"><font face= +"Bookman Old Style" size="4">CHAPTER I</font></a></b></p> +<p><a name="Toc158108888" id="Toc158108888">HINTS ON THE +MANIPULATION OF GLASS AND ON GLASS-BLOWING FOR LABORATORY +PURPOSES</a></p> +<p>§ 1. THE art of GLASS-BLOWING has the conspicuous +advantage, from the point of view of literary presentation, of +being to a great extent incommunicable. As in the case of other +delightful arts — such as those treated of in the Badminton +Library, for instance — the most that can be done by +writing is to indicate suitable methods and to point out +precautions which experience has shown to be necessary, and which +are not always obvious when the art is first approached. It is +not the object of this work to deal with the art of GLASS-BLOWING +or any other art after the manner befitting a complete treatise, +in which every form of practice is rightly included. On the +contrary, it is my wish to avoid the presentation of alternative +methods.</p> +<p>I consider that the presentation of alternative methods would, +for my present purpose, be a positive disadvantage, for it would +swell this book to an outrageous size; and to beginners — I +speak from experience — too lavish a treatment acts rather +by way of obscuring the points to be aimed at than as a means of +enlightenment. The student often does not know which particular +bit of advice to follow, and obtains the erroneous idea that +great art has to be brought to bear to enable him to accomplish +what is, after all, most likely a perfectly simple and +straightforward operation.</p> +<p>This being understood, it might perhaps be expected that I +should describe nothing but the very best methods for obtaining +any proposed result. Such, of course, has been my aim, but it is +not likely that I have succeeded in every case, or even in the +majority of cases, for I have confined myself to giving such +directions as I know from my own personal experience will, if +properly carried out, lead to the result claimed. In the few +cases in which I have to refer to methods of which I have no +personal experience, I have endeavoured to give references +(usually taking the form of an acknowledgment), so that an idea +of their value may be formed. All methods not particularised may +be assumed by the reader to have come within my personal +experience.</p> +<p>§ 2. Returning to GLASS-BLOWING, we may note that two +forms of GLASS-BLOWING are known in the arts, "Pot" blowing and +"Table" blowing. In the former case large quantities of fluid +"metal" (technical term for melted glass) are assumed to be +available, and as this is seldom the case in the laboratory, and +as I have not yet felt the want of such a supply, I shall deal +only with "table" blowing. Fortunately there is a convenient book +on this subject, by Dr. Shenstone (Rivingtons), so that what I +have to say will be as brief as possible, consistent with +sufficiency for everyday work. As a matter of fact there is not +very much to say, for if ever there was an art in which manual +dexterity is of the first and last importance, that art is +glass-working.</p> +<p>I do not think that a man can become an accomplished +glass-blower from book instructions merely — at all events, +not without much unnecessary labour, — but he can learn to +do a number of simple things which will make an enormous +difference to him both as regards the progress of his work and +the state of his pocket.</p> +<p>§ 3. The first thing is to select the glass. In general, +it will suffice to purchase tubes and rods; in the case where +large pieces (such as the bulbs of Geissler pumps) have to be +specially prepared by pot-blowing, the student will have to +observe precautions to be mentioned later on. There are three +kinds of glass most generally employed in laboratories.</p> +<p><b><a name="Toc158108889" id="Toc158108889"><font face= +"Bookman Old Style" size="4">§ 4. Soft Soda +Glass,</font></a></b></p> +<p><font face="Bookman Old Style">obtained for the most part from +factories in Thuringia, and generally used in assembling chemical +apparatus. — This glass is cheap, and easily obtainable +from any large firm of apparatus dealers or chemists. It should +on no account be purchased from small druggists, for the +following reasons:-</font></p> +<p>(a) It is usually absurdly dear when obtained in this way.</p> +<p>(b) It is generally made up of selections of different age and +different composition, and pieces of different composition, even +if the difference is slight, will not fuse together and remain +together unless joined in a special manner.</p> +<p>(c) It is generally old, and this kind of glass often +devitrifies with age, and is then useless for blowpipe work, +though it may be bent sufficiently for assembling chemical +apparatus. Devitrified glass looks frosty, or, in the earlier +stages, appears to be covered by cobwebs, and is easily picked +out and rejected.</p> +<p>§ 5. It might be imagined that the devitrification would +disappear when the glass is heated to the fusing point; and so it +does to a great extent, but for many operations one only requires +to soften the glass, and the devitrification often persists up to +this temperature. My experience is that denitrified glass is also +more likely to crack in the flame than good new glass, though the +difference in this respect is not very strongly marked with +narrow tubes.</p> +<p><b><a name="Toc158108890" id="Toc158108890"><font face= +"Bookman Old Style" size="4">§ 6. Flint Glass. +—</font></a></b></p> +<p><font face="Bookman Old Style">Magnificent flint glass is made +both in England and France. The English experimenter will +probably prefer to use English glass, and, if he is wise, will +buy a good deal at a time, since it does not appear to devitrify +with age, and uniformity is thereby more likely to be secured. I +have obtained uniformly good results with glass made by Messrs. +Powell of Whitefriars, but I daresay equally good glass may be +obtained elsewhere.</font></p> +<p>For general purposes flint glass is vastly superior to the +soft soda mentioned above. In the first place, it is very much +stronger, and also less liable to crack when heated — not +alone when it is new, but also, and especially, after it has been +partly worked. Apparatus made of flint glass is less liable to +crack and break at places of unequal thickness than if made of +soda glass. This is not of much importance where small pieces of +apparatus only are concerned, because these can generally be +fairly annealed; and if the work is well done, the thickness will +not be uneven. It is a different matter where large pieces of +apparatus, such as connections to Geissler pumps, are concerned, +for the glass has often to be worked partly in situ, and can only +be imperfectly annealed.</p> +<p>Joints made between specimens of different composition are +much more likely to stand than when fashioned in soda glass. +Indeed, if it is necessary to join two bits of soda glass of +different kinds, it is better to separate them by a short length +of flint glass; they are more likely to remain joined to it than +to each other. A particular variety of flint glass, known as +white enamel, is particularly suitable for this purpose, and, +indeed, may be used practically as a cement.</p> +<p>§ 7, It is, however, when the necessity of altering or +repairing apparatus complicated by joints arises that the +advantage of flint glass is most apparent. A crack anywhere near +to a side, or inserted joint, can scarcely ever be repaired in +the case of soda glass apparatus, even when the glass is quite +thin and the dimensions small.</p> +<p>It should also be mentioned that flint glass has a much more +brilliant appearance than soda glass. Of course, there is a +considerable difference between different kinds of flint glass as +to the melting point, and this may account for the divergency of +the statements usually met with as to its fusibility compared +with that of soda glass. The kind of flint glass made by Messrs. +Powell becomes distinctly soft soon after it is hot enough to be +appreciably luminous in a darkened room, and at a white heat is +very fluid. This fluidity, though of advantage to the practised +worker, is likely to give a beginner some trouble.</p> +<p>§ 8. As against the advantages enumerated, there are some +drawbacks. The one which will first strike the student is the +tendency of the glass to become reduced in the flame of the +blow-pipe. This can be got over by proper adjustment of the +flame, as will be explained later on. A more serious drawback in +exact work is the following. In making a joint with lead glass it +is quite possible to neglect to fuse the glass completely +together at every point; in fact, the joint will stand perfectly +well even if it be left with a hole at one side, a thing which is +quite impossible with soft soda glass, or is at least exceedingly +unusual. An accident of this kind is particularly likely to +happen if the glass be at all reduced. Hence, if a joint does not +crack when cold, the presumption is, in the case of soda glass, +that the joint is perfectly made, and will not allow of any leak; +but this is not the case with flint glass, for which reason all +joints between flint glass tubes require the most minute +examination before they are passed. If there are any air bubbles +in the glass, especial care must be exercised.</p> +<p><b><a name="Toc158108891" id="Toc158108891"><font face= +"Bookman Old Style" size="4">§ 9. Hard or Bohemian, Glass. +—</font></a></b></p> +<p><font face="Bookman Old Style">This is, of course, used where +high temperatures are to be employed, and also in certain cases +where its comparative insolubility in water is of importance. It +is very unusual for the investigator to have to make complicated +apparatus from this glass. Fused joints may be made between hard +glass and flint glass without using enamel, and though they often +break in the course of time, still there is no reason against +their employment, provided the work be done properly, and they +are not required to last too long.</font></p> +<p><b><a name="Toc158108892" id="Toc158108892"><font face= +"Bookman Old Style" size="4">§ 10. On the Choice of Sizes of +Glass Tube. —</font></a></b></p> +<p><font face="Bookman Old Style">It will be found that for +general purposes tubes about one-quarter inch in inside diameter, +and from one-twentieth to one-fortieth of an inch thick, are most +in demand. Some very thin soda glass of these dimensions +(so-called "cylinder" tubes) will be found very handy for many +purposes. For physico-chemical work a good supply of tubing, from +one-half to three-quarters of an inch inside diameter, and from +one-twentieth to one-eighth inch thick, is very necessary. A few +tubes up to three inches diameter, and of various thicknesses, +will also be required for special purposes.</font></p> +<p>Thermometer and "barometer" tubing is occasionally required, +the latter, by the way, making particularly bad barometers. The +thermometer tubing should be of all sizes of bore, from the +finest obtainable up to that which has a bore of about +one-sixteenth of an inch. Glass rods varying from about +one-twentieth of an inch in diameter up to, say, half an inch +will be required, also two or three sticks of white enamel glass +for making joints.</p> +<p>To facilitate choice, there is appended a diagram of sizes +from the catalogue of a reliable German firm, Messrs. Desaga of +Heidelberg, and the experimenter will be able to see at a glance +what sizes of glass to order. It is a good plan to stock the +largest and smallest size of each material as well as the most +useful working sizes. </p> +<p><img src="images/Image27.gif" alt="images/Image27.gif" width= +"505" height="818">Fig. 1.</p> +<p><b><a name="Toc158108893" id="Toc158108893"><font face= +"Bookman Old Style" size="4">§ 11. Testing Glass. +—</font></a></b></p> +<p><font face="Bookman Old Style">"Reject glass which has lumps +or knots, is obviously conical, or has long drawn-out bubbles +running through the substance." If a scratch be made on the +surface of a glass tube, and one end of the scratch be touched by +a very fine point of fused glass, say not more than one-sixteenth +inch in diameter, the tube, however large it is (within reason), +ought to crack in the direction of the scratch. If a big crack +forms and does not run straight, but tends to turn +longitudinally, it is a sign that the glass is ill annealed, and +nothing can be done with it. If such glass be hit upon in the +course of blow-pipe work, it is inadvisable to waste time upon +it; the best plan is to reject it at once, and save it for some +experiment where it will not have to be heated.</font></p> +<p>The shortest way of selecting glass is to go to a good firm, +and let it be understood that if the glass proves to be badly +annealed it will be returned. Though it was stated above that the +glass should not be distinctly conical, of course allowance must +be made for the length of the pieces, and, on the other hand, a +few highly conical tubes will be of immense service in special +cases, and a small supply of such should be included.</p> +<p>The glass, as it is obtained, should be placed in a rack, and +covered by a cloth to reduce the quantity of dust finding its way +into the tubes. It has been stated by Professor Ostwald that +tubes when reared up on end tend to bend permanently. I have not +noticed this with lead glass well supported. Each different +supply should be kept by itself and carefully described on a +label pasted on to the rack, and tubes from different lots should +not be used for critical welds. This remark is more important in +the case of soda than of lead glass.</p> +<p>In the case of very fine thermometer tubes it will be +advisable to cover the ends with a little melted shellac, or, in +special cases, to obtain the tubes sealed from the works. Soda +glass can generally be got in rather longer lengths than lead +glass; the longer the lengths are the better, for the waste is +less.</p> +<p>It is useful to be able to distinguish the different kinds of +glass by the colour. This is best observed by looking towards a +bright surface along the whole length of the tube and through the +glass. Lead glass is yellow, soda glass is green, and hard glass +purple in the samples in my laboratory, and I expect this is +practically true of most samples. <i>[Footnote:</i> Some new lead +glass I have is also almost purple in hue. If any doubt exists as +to the kind of glass, it may be tested at once in the blow-pipe +flame, or by a mixture of oils of different refractive indices, +as will be explained later.<i>]</i></p> +<p>§ 12. The question of the solubility of glass in reagents +is one of great importance in accurate work, though it does not +always meet with the attention it deserves. It is impossible here +to go into the matter with sufficient detail, and the reader is +therefore referred to the <i>Abstracts</i> of the Chemical +Society, particularly for the years 1889 and 1892. The memoir by +F. Kohlrausch, <i>Wied. Ann. xliv</i>., should be consulted in +the original. The following points may be noted. A method of +testing the quality of glass is given by Mylius (<i>C. S. J. +Abstracts</i>, 1889, p. 549), and it is stated that the +resistance of glass to the action of water can generally be much +increased by leaving it in contact with cold water for several +days, and then heating it to 300° to 400° C. This +improvement seems to be due to the formation of a layer of moist +silica on the surface, and its subsequent condensation into a +resisting layer by the heating. Mylius (<i>C. S. J. +Abstracts,</i> 1892, p. 411), and Weber, and Sauer (<i>C. S. J. +Abstracts</i>, 1892, p. 410) have also shown that the best glass +for general chemical purposes consists of</p> +<div style="margin-left: 4em"> +<p>Silica, 7 to 8 parts</p> +<p>Lime, 1 part</p> +<p>Alkali, 1.5 to 1.1 parts.</p> +</div> +<p>This is practically "Bohemian" tube glass.</p> +<p>The exact results are given in the <i>Berichte</i> of the +German Chemical Society, vol. xxv. An excellent account of the +properties of glass will be found in Grove's edition of Miller's +<i>Elements of Chemistry</i>.</p> +<p><b><a name="Toc158108894" id="Toc158108894"><font face= +"Bookman Old Style" size="4">§ 13. Cleaning Glass Tubes. +—</font></a></b></p> +<p><font face="Bookman Old Style">This is one of the most +important arts in chemistry. If the tubes are new, they are +generally only soiled by dust, and can be cleaned fairly easily +— first by pushing a bit of cotton waste through with a +cane, or pulling a rag through with string — and then +washing with sand and commercial hydrochloric acid. I have heard +of glass becoming scratched by this process, and breaking in +consequence when heated, but have never myself experienced this +inconvenience. In German laboratories little bits of bibulous +paper are sometimes used instead of sand; they soon break into a +pulp, and this pulp has a slightly scouring action.</font></p> +<p>As soon as the visible impurities are removed and the tube +when washed looks bright and clean, it may be wiped on the +outside and held perpendicularly so as to allow the water film to +drain down. If the tube be greasy (and perhaps in other cases) it +will be observed that as the film gets thinner the water begins +to break away and leave dry spots. For accurate work this grease, +or whatever it is, must be removed; and after trying many plans +for many years, I have come back to the method I first employed, +viz. boiling out with aqua regia.</p> +<p>For this purpose, close one end of the tube by a cork (better +than a rubber bung, because cheaper), and half fill the tube with +aqua regia; then, having noted the greasy places, proceed to boil +the liquid in contact with the glass at these points, and in the +case of very obstinate dirt — such as lingers round a fused +joint which has been made between undusted tubes — leave +the whole affair for twelve hours. If the greasiness is only +slight, then simply shaking with hot aqua regia will often remove +it, and the aqua regia is conveniently heated in this case by the +addition of a little strong sulphuric acid.</p> +<p>The spent aqua regia may be put into a bottle. It is generally +quite good enough for the purpose of washing glass vessels with +sand, as above explained.</p> +<p>However carefully a tube is cleaned before being subjected to +blowpipe operations, it will be fouled wherever there is an +opening during the process of heating, unless the extreme tip +only of an oxidising flame be employed. Even this should not be +trusted too implicitly unless an oxygas or hydrogen flame is +employed.</p> +<p>When a tube or piece of apparatus has been cleaned by acid, so +that on clamping it vertically, dry spaces do not appear, it may +be rinsed with platinum distilled water and left to drain, the +dust being, of course, kept out by placing a bit of paper round +the top. For accurate work water thus prepared is to be preferred +to anything else. When the glass is very clean interference +colours will be noticed as the water dries away.</p> +<p>Carefully-purified alcohol may in some cases be employed where +it is desired to dry the tube or apparatus quickly. In this case +an alcohol wash bottle should be used, and a little alcohol +squirted into the top of the tube all round the circumference. +The water film drags the alcohol after it, and by waiting a few +minutes and then adding a few more drops of alcohol, the water +may be practically entirely removed, especially if a bit of +filter paper be held against the lower end of the tube. It is +customary in some laboratories to use ether for a final rinse, +but unless the ether is freshly distilled and very pure, it +leaves a distinct organic residue.</p> +<p>When no more liquid can be caused to drain away, the tube may +be dried by heating it along its length, beginning at the top (to +get the advantage of the reduction of surface tension), and so on +all down. It will then be possible to mop up a little more of the +rinsing liquid. When the tube is nearly dry a loose plug of +cotton wool may be inserted at the bottom. The wool must be put +in so that the fibres lie on an even surface inside the tube, and +the wool must be blown free from dust. Ordinary cotton wool is +useless, from being dusty and the fibres short, and the same +remark applies to wadding. Use nothing but what is known as +"medicated" cotton wool with a good long fibre.</p> +<p>The tube will usually soon dry of itself when the cover is +lifted an inch or so. If water has been used, the air-current may +be assisted by means of the water-pump, the air being sucked from +the top, so that the wool has an opportunity of acting as a dust +filter; a very slow stream of air only must be employed. For +connecting the tube to the pump, a bit of India-rubber tube about +an inch in diameter, with a bore of about one-eighth of an inch, +may be employed. The end of the rubber tube is merely pressed +against the edge of the glass.</p> +<p>These remarks apply, with suitable modification, to all kinds +of finished apparatus having two openings. For flasks and so on, +it is convenient to employ a blowing apparatus, dust being +avoided by inserting a permanent plug of cotton wool in one of +the leading tubes. The efficiency of this method is greatly +increased by using about one foot of thin copper tube, bent into +a helix, and heated by means of a Bunsen burner; the hot air +(previously filtered) is passed directly into the flask, bottle, +or whatever the apparatus may be. This has proved so convenient +that a copper coil is now permanently fastened to the wall in one +of the rooms of my laboratory.</p> +<p>The above instructions indicate greater refinement than is in +general necessary or proper for tubes that have to be afterwards +worked by the blow-pipe. In the majority of cases all that is +necessary is to remove the dust, and this is preferably done by a +wad of cotton waste (which does not leave shreds like cotton +wool), followed by a bit of bibulous filter paper. I would +especially warn a beginner against neglecting this precaution, +for in the process of blowing, the dust undergoes some change at +the heated parts of the apparatus, and forms a particularly +obstinate kind of dirt.</p> +<p>In special cases the methods I have advocated for removing +dirt and drying without covering the damp surfaces with dust are +inadequate, but an experimenter who has got to that stage will +have nothing to learn from such a work as this.</p> +<p><b><a name="Toc158108895" id="Toc158108895"><font face= +"Bookman Old Style" size="4">§ 14. The Blow-pipe. +—</font></a></b></p> +<p><font face="Bookman Old Style">I suppose a small book might +easily be written on this subject but what I have to say — +in accordance with the limitation imposed — will be brief. +For working lead glass I never use anything but an oxygas +blow-pipe, except for very large work, and should never dream of +using anything else. Of course, to a student who requires +practice in order to attain dexterity this plan would be a good +deal too dear. My advice to such a one is — procure good +soda glass, and work it by means of a modification of a gas +blow-pipe, to be described directly. The Fletcher's blow-pipes on +long stems are generally very inconvenient. The flame should not +be more than 5 or 6 inches from the working table at most, +especially for a beginner, who needs to rest his arms on the edge +of the table to secure steadiness.</font></p> +<p>The kind of oxygas blow-pipe I find most convenient is +indicated in the sketch. (Fig. 2) I like to have two nozzles, +which will slip on and off, one with a jet of about 0.035 inch in +diameter, the other of about double this dimension. The oxygen is +led into the main tube of the blow-pipe by another tube of much +smaller diameter, concentric with the main tube (Fig. 3, at A). +The oxygen is mixed with the gas during its escape from the inner +tube, which is pierced by a number of fine holes for the purpose, +the extreme end being closed up. The inner tube may run up to +within half an inch of the point where the cap carrying the +nozzle joins the larger tube.</p> +<p><img src="images/Image28.gif" alt="images/Image28.gif" width= +"358" height="243"><img src="images/Image29.gif" alt= +"images/Image29.gif" width="470" height="372">Fig. 2.</p> +<p>Fig. 3.</p> +<p>If it is desired to use the blow-pipe for working glass which +is already fixed in position to a support, it will be found very +advantageous to use a hooked nozzle. The nozzle shown in the +sketch is not hooked enough for this work, which requires that +the flame be directed 'backwards towards the worker. With a +little practice such a flame may be used perfectly well for +blowing operations on the table, as well as for getting at the +back of fixed tubes.</p> +<p>To warm up the glass, the gas supply is turned full on, and +enough oxygen is allowed to pass in to clear the flame. The work +is held in front of, but not touching, the flame, until it is +sufficiently hot to bear moving into the flame itself. The, work +is exposed to this flame until, in the case of lead glass, traces +of reduction begin to appear. When this point is reached the +oxygen tap is thrown wide open. I generally regulate the pressure +on the bags, so that under these circumstances the flame is +rather overfed with oxygen. This condition is easily recognised, +as follows. The flame shrinks down to a very small compass, and +the inner blue cone almost disappears; also flashes of yellow +light begin to show themselves — a thing which does not +occur when the proportions of the gases are adjusted for maximum +heating effect.</p> +<p>For many purposes the small dimensions of the flame render it +very convenient, and the high temperature which can be attained +at exact spots enables glass to be fused together after a certain +amount of mixing, which is an enormous advantage in fusing lead +glass on to hard glass. The lead glass should not be heated hot +enough to burn, but, short of this, the more fluid it is the +better for joints between dissimilar samples.</p> +<p>It will be noticed that the blow-pipe can be rotated about a +vertical axis so as to throw the flame in various directions. +This is often indispensable.</p> +<p>§ 15. In general the oxygen flame does not require to be +delivered under so high a pressure as for the production of a +lime light. In England, I presume, most experimenters will obtain +their oxygen ready prepared in bottles, and will not have to +undergo the annoyance of filling a bag. If, however, a bag is +used, and it has some advantages (the valves of bottles being +generally stiff), I find that a pressure produced by placing +about two hundredweight (conveniently divided into four fifty-six +pound weights) on bags measuring 3' x 2'6" x 2' (at the thicker +end) does very well. To fill such a bag with oxygen, about 700 +grms of potassium chlorate is required.</p> +<p>If the experimenter desires to keep his bag in good order, he +must purify his oxygen by washing it with a solution of caustic +soda, and then passing it through a "tower" of potash or soda in +sticks, and, finally, through a calcium chloride tower. This +purifying apparatus should be permanently set up on a board, so +that it may be carried about by the attendant to wherever it is +required. Oxygen thus purified does not seem to injure a good bag +— at least during the first six or seven years:</p> +<p>In order to reduce the annoyance of preparing oxygen, the use +of the usual thin copper conical bottle should be avoided. The +makers of steel gas bottles provide retorts of wrought iron or +steel for oxygen-making, and these do very well. They have the +incidental advantage of being strong enough to resist the attacks +of a servant when a spent charge is being removed.</p> +<p>The form of retort referred to is merely a large tube, closed +at one end, and with a screw coupling at the other; the +dimensions may be conveniently about 5 inches by 10. The screw +threads should be filled with fireclay (as recommended by +Faraday) before the joint is screwed up. Before purchasing a +bottle the experimenter will do well to remember that unless it +is of sufficiently small diameter to go into his largest vice, he +will be inconvenienced in screwing the top on and off. Why these +affairs are not made with union joints, as they should be, is a +question which will perhaps be answered when we learn why cork +borers are still generally made of brass, though steel tube has +long been available.</p> +<p><img src="images/Image30.gif" alt="images/Image30.gif" width= +"526" height="318">Fig. 4.</p> +<p>These little matters may appear very trivial — and so +they are — but the purchaser of apparatus will generally +find that unless he looks after details himself, they will not be +attended to for him. Whether a union joint is provided or not, +let it be seen that the end of the delivery tube is either small +enough to fit a large rubber tube connection going to the +wash-bottle, or large enough to allow of a cork carrying a bit of +glass tube for the same purpose to be inserted. This tube should +not be less than half an inch in inside diameter. Never use a new +bottle before it has been heated sufficiently to get rid of +grease and carbonaceous dirt. A convenient oxygen-making +apparatus is shown in Fig. 4, which is drawn from "life."</p> +<p>§ 16. For large blow-pipe work with lead glass I +recommend a system of four simple blow-pipes, in accordance with +the sketch annexed. I first saw this system in operation in the +lamp factory of the Westinghouse Electric Company at Pittsburg in +1889, and since then I have seen it used by an exceedingly clever +"trick" glass-worker at a show. After trying both this +arrangement and the "brush flame" recommended by Mr. Shenstone, I +consider the former the more convenient; however, I daresay that +either can be made to work in competent hands, but I shall here +describe only my own choice. <i>[Footnote:</i> A brush flame is +one which issues from the blow-pipe nozzle shaped like a brush, +i.e. it expands on leaving the jet. It is produced by using a +cylindrical air jet or a conical jet with a large aperture, say +one-eighth of an inch. See Fig. 25.<i>]</i></p> +<p>As will be seen, the blow-pipe really consists of four simple +brass tube blow-pipes about three-eighths of an inch internal +diameter and 3 inches long, each with its gas and air tap and +appropriate nozzle. Each blowpipe can turn about its support (the +gas-entry pipe) to some extent, and this possibility of +adjustment is of importance, The air jets are merely bits of very +even three-sixteenths inch glass tubing, drawn down to conical +points, the jets themselves being about 0.035 inch diameter.</p> +<p>Fig. 5 <img src="images/Image31.gif" alt="images/Image31.gif" +width="458" height="318">.</p> +<p>The flames produced are the long narrow blow-pipe flames used +in blow-pipe analysis, and arranged so as to consist mostly of +oxidising flame. The air-supply does not require to be large, nor +the pressure high — 5 to 10 inches of water will do — +but it must be very regular. The "trick" glass-blower I referred +to employed a foot bellows in connection with a small weighted +gasometer, the Westinghouse Company used their ordinary +air-blast, and I have generally used a large gas-holder with +which I am provided, which is supplied by a Roots blower worked +by an engine.</p> +<p>I have also used a "velocity pump" blower, which may be +purchased amongst others from Gerhardt of Bonn. The arrangement +acts both as a sucking and blowing apparatus, and is furnished +with two manometers and proper taps, etc. As I have reason to +know that arrangements of this kind work very ill unless really +well made, I venture to add that the Gerhardt arrangement to +which I refer is No. 239 in his catalogue, and costs about three +pounds. It hardly gives enough air, however, to work four +blow-pipes, and the blast requires to be steadied by passing the +air through a vessel covered with a rubber sheet.</p> +<p>In default of any of these means being available, one of +Fletcher's foot-blowers may be employed, but it must be worked +very regularly. A table mounted with one blow-pipe made on this +plan, and worked by a double-acting bellows, is recommended for +students' use. For working flint glass, the air jet may be +one-eighth of an inch in diameter and the pressure higher — +this will give a brush flame. See Fig. 25.</p> +<p>It will be seen, on looking at the sketch of the blowpipe +system, that the pair of blow-pipes farther from the observer can +be caused to approach or recede at will by means of a handle +working a block on a slide. It often happens that after using all +four blow-pipes at once it is necessary to have recourse to one +blow-pipe only, and to do this conveniently and quickly is rather +an object. Now, in my arrangement I have to turn off both the gas +and air from the farther system, and then put in a bit of +asbestos board to prevent the nozzles being damaged by the flame +or flames kept alight. As I said before, when some experience is +gained, glassblowing, becomes a very simple art, and work can be +done under circumstances so disadvantageous that they would +entirely frustrate the efforts of a beginner. This is not any +excuse, however, for recommending inferior arrangements.</p> +<p>Consequently, I say that the pipes leading in gas and air +should be all branches of one gas and one air pipe, in so far as +the two remote and one proximate blow-pipe are concerned, and +these pipes should come up to the table to the right hand of the +operator, and should have main taps at that point, each with a +handle at least 2 inches long. By this arrangement the operator +can instantly turn down all the blow-pipes but one, while, if the +inverse operation is required, all the three pipes can be started +at once. <i>[Footnote:</i> I find, since writing the above, that +I have been anticipated in this recommendation by Mr. G. S. Ram, +The Incandescent Lamp and its Manufacture, p. 114.<i>]</i></p> +<p>The separate air and gas taps must be left for permanent +regulation, and must not be used to turn the supply on or cut it +off. In some respects this blow-pipe will be found more easy to +manage than an oxygas blow-pipe, for the glass is not so readily +brought to the very fluid state, and this will often enable a +beginner who proceeds cautiously to do more than he could with +the more powerful instrument.</p> +<p>Though I have mentioned glass nozzles for the air supply, +there is no difficulty in making nozzles of brass. For this +purpose let the end of a brass tube of about one-eighth of an +inch diameter be closed by a bit of brass wire previously turned +to a section as shown (Fig. 6), and then bored by a drill of the +required diameter, say .035 inch. It is most convenient to use +too small a drill, and to gradually open the hole by means of +that beautiful tool, the watchmaker's "broach." The edges of the +jet should be freed from burr by means of a watchmaker's +chamfering tool (see Saunier's Watchmaker's Hand-book, Tripplin, +1882, p. 232, § 342), or by the alternate use of a slip of +Kansas stone and the broach.</p> +<p>Fig. 6 <img src="images/Image32.gif" alt="images/Image32.gif" +width="325" height="60">.</p> +<p>The construction of this blow-pipe is so simple, that in case +any one wishes to use a brush flame, he can easily produce one +simply by changing his air jets to bits of the same size (say +one-eighth to one-sixteenth of an inch) tubing, cut off clean. To +insure success, the ends of the tubes must be absolutely plane +and regular; the slightest inequality makes all the difference in +the action of the instrument. If a jet is found to be defective, +cut it down a little and try again; a clean-cut end is better +than one which has been ground flat on a stone. The end of a tube +may, however, be turned in a manner hereafter to be described so +as to make an efficient jet. Several trials by cutting will +probably have to be made before success is attained. For this +kind of jet the air-pressure must be greatly increased, and a +large Fletcher's foot-blower or, better still, a small +double-action bellows worked with vigour will be found very +suitable. A fitting for this auxiliary blow-pipe is shown in Fig. +5 at B.</p> +<p>Professor Roentgen's discovery has recently made it necessary +to give more particular attention to the working of soft soda +glass, and I have been obliged to supplement the arrangements +described by a table especially intended for work with glass of +this character. The arrangement has proved so convenient for +general work that I give the following particulars. The table +measures 5 feet long, 2 feet 11 inches wide, and is 2 feet 9 +inches high.</p> +<p><img src="images/Image33.gif" alt="images/Image33.gif" width= +"474" height="364">Fig. 7.</p> +<p>It is provided with a single gas socket, into which either a +large or small gas tube may be screwed. The larger tube is 5.5 +inches long and 0.75 of an inch in diameter. The smaller tube is +the same length, and half an inch in diameter. The axis of the +larger tube is 3.5 inches above the table at the point of +support, and is inclined to the horizontal at an angle of +12°. The axis of the smaller tube is 2.5 inches above the +surface of the table, and is inclined to the horizontal at the +same angle as the larger one.</p> +<p>The air jets are simply pieces of glass tube held in position +by corks. The gas supply is regulated by a well-bored tap. The +air supply is regulated by treading the bellows — no tap is +requisite. The bellows employed are ordinary smiths' bellows, +measuring 22 inches long by 13 inches wide in the widest part. +They are weighted by lead weights, weighing 26 lbs. The treadle +is connected to the bellows by a small steel chain, for the +length requires to be invariable. As the treadle only acts in +forcing air from the lower into the upper chamber of the bellows, +a weight of 13 lbs. is hung on to the lower cover, so as to open +the bellows automatically.</p> +<p>The air jets which have hitherto been found convenient +are:</p> +<p>for the small gas tube</p> +<div style="margin-left: 2em"> +<ol> +<li>a tube 0.12 inch diameter drawn down to a jet of 0.032 inch +diameter for small work;</li> +<li>plain tubes not drawn down of 0.14 inch, 0.127 inch, and +–0.245 inch diameter, and for the large gas tube, plain +tubes up to 0.3 inch in diameter.</li> +</ol> +</div> +<p>The table is placed in such a position that the operator sits +with his back to a window and has the black calico screen in +front of him and to his right. The object of the screen is to +protect the workman against draughts. The table is purposely left +unscreened to the left of the workman, so that long tubes may be +treated.</p> +<p>§ 17. Other appliances which will be required for +GLASS-BLOWING are of the simplest character.</p> +<p>(1) Small corks for closing the ends of tubes.</p> +<p>(2) Soft wax — a mixture of bees' wax and resin softened +by linseed oil to the proper consistency, easily found by trial, +also used for temporarily closing tubes.</p> +<p>(3) A bottle of vaseline for lubricating.</p> +<p>(4) An old biscuit tin filled with asbestos in shreds, and an +asbestos towel or cloth for annealing glass after removal from +the flame. As asbestos absorbs moisture, which would defeat its +use as an annealing material, it must be dried if necessary.</p> +<p>(5) <i>A Glass-Cutter's Knife</i>. — This is best made +out of a fine three-cornered file, with the file teeth almost +ground out, but not quite; it should be about 2 inches long. +After the surface has been ground several times, it may be +necessary to reharden the steel. This is best done by heating to +a full red and quenching in mercury. The grindstone employed for +sharpening the knife should be "quick," so as to leave a rough +edge. I have tried many so-called glass knives "made in Germany," +but, with one exception, they were nothing like so good as a +small French or Sheffield file. In this matter I have the support +of Mr. Shenstone's experience.</p> +<p>(6) A wire nail, about 2 inches long, mounted very accurately +in a thin cylindrical wooden handle about 5 inches long by +one-quarter of an inch diameter, or, better still, a bit of +pinion wire 6 inches long, of which 1.5 inches are turned down as +far as the cylindrical core., An old dentists' chisel or filling +tool is also a very good form of instrument.</p> +<p>(7) A bit of charcoal about 3.5 inches long and 2 wide, and of +any thickness, will be found very useful in helping to heat a +very large tube. The charcoal block is provided with a stout wire +handle, bent in such a manner that the block can be held close +above a large glass tube on which the flames impinge. In some +cases it is conveniently held by a clip stand. By the use of such +a slab of charcoal the temperature obtainable over a large +surface can be considerably increased.</p> +<p>I have seen a wine-glass (Venetian sherry-glass) worked on a +table with four blow-pipes, such as is here described, with the +help of a block of hard wood held over the heated glass, and +helping the attainment of a high temperature by its own +combustion.</p> +<p>(8) Several retort stands with screw clips.</p> +<p>(9) Some blocks of wood about 5" X 2" X 2" with V-shaped +notches cut in from the top.</p> +<p>(10) A strong pair of pliers.</p> +<p>(11) An apparatus for cleaning and drying the breath, when +blowing directly by the mouth is not allowable. The apparatus +consists of a solid and heavy block of wood supporting a +calcium-chloride tube permanently connected with a tube of +phosphorus pentoxide divided into compartments by plugs of glass +wool. Care should be taken to arrange these tubes so as to occupy +the smallest space, and to have the stand particularly stable. +The exit tube from the phosphorus pentoxide should be drawn down +to form a nozzle, from, say, half an inch to one-eighth of an +inch in diameter, so as to easily fit almost any bit of rubber +tube. The entry to the calcium chloride should be permanently +fitted to about a yard of fine soft rubber tubing, as light as +possible. The ends of this tube should terminate in a glass +mouthpiece, which should not be too delicate.</p> +<p>As an additional precaution against dust, I sometimes add a +tube containing a long plug of glass wool, between the phosphorus +pentoxide and the delivery tube, and also a tube containing stick +potash on the entry side of the calcium chloride tube, but it may +safely be left to individual judgment to determine when these +additions require to be made. In practice I always keep the +affair set up with these additions. The communication between all +the parts should be perfectly free, and the tubes should be +nearly filled with reagents, so as to avoid having a large volume +of air to compress before a pressure can be got up.</p> +<p>The arrangement will be clear by a reference to Fig. 8, which +illustrates the apparatus in use for joining two long tubes. I +have tried blowing-bags, etc., but, on the whole, prefer the +above arrangement, for, after a time, the skill one acquires in +regulating the pressure by blowing by the mouth and lips is such +an advantage that it is not to be lightly foregone.</p> +<p><img src="images/Image34.gif" alt="images/Image34.gif" width= +"446" height="277">Fig. 8.</p> +<p><b><a name="Toc158108896" id="Toc158108896"><font face= +"Bookman Old Style" size="4">§ 18. <i>The Table</i>. +—</font></a></b></p> +<p><font face="Bookman Old Style">The system of four blow-pipes +is, of course, a fixture. In this case the table may be about a +yard square, and may be covered with asbestos mill-board neatly +laid down, but this is not essential. The table should have a rim +running round it about a quarter of an inch high. The tools +should be laid to the right of the worker, and for this purpose +the blow-pipes are conveniently fixed rather to the left of the +centre of the table, but not so far as to make the leg of the +table come so close to the operator as to make him uncomfortable, +for a cheerful and contented spirit ought to be part of the +glass-worker's outfit.</font></p> +<p>The most convenient height for a blow-pipe table — with +the blow-pipes about 2 inches above the table top — is 3 +feet 2 inches. Nothing is so convenient to sit upon as a rough +music-stool with a good range of adjustment. The advantage of an +adjustable seat lies in the fact that for some operations one +wants to be well over the work, while in others the advantage of +resting the arms against the table is more important.</p> +<p><b><a name="Toc158108897" id="Toc158108897"><font face= +"Bookman Old Style" size="4">§ 19. Special Operations. +—</font></a></b></p> +<p><font face="Bookman Old Style">The preliminary to most +operations before the blow-pipe, is to draw down a tube and pull +it out to a fine point. This is also the operation on which a +beginner should exercise himself in the first instance. I will +suppose that it is desired to draw out a tube about one-quarter +of an inch in diameter, with the object of closing it, either +permanently or temporarily, and leaving a handle for future +operations in the shape of the point, thin enough to cool quickly +and so not delay further work.</font></p> +<p>For this simple operation most of the glass-blower's skill is +required. The tube must be grasped between the first finger and +thumb of both hands, and held so that the part to be operated on +lies evenly between the two hands. The distance between the +operator's thumbs may conveniently vary from 2.5 to 4 inches. +Releasing the grip of the left hand, let the operator assure +himself of his ability to easily rotate the tube about its axis +— by the right thumb and finger — he will +incidentally observe by the "feel" whether the tube is straight +or not.</p> +<p>A good deal of progress can be made from this point before the +tube is heated at all. The operator can acquire a habit of +instinctively rotating the tube by both hands, however the tube +itself be moved about in space, or however it be pushed or +pulled. The habit of constant and instinctive rotation is +literally about one-third of the whole art of glassblowing. It is +unlikely, however, that the beginner will discover that he has +not got this habit, until a few failures draw his attention to +it.</p> +<p>The glass tube being held in position lightly yet firmly, and +the operator being sure that he feels comfortable and at his +ease, and that the blow-pipe flame (a single flame in this +instance) is well under control, the preliminary heating may be +commenced. With a tube of the dimensions given this is a very +simple affair. Turn the air partly off, or blow gently, to get a +partly luminous gas flame; hold the tube about an inch from the +end of this flame, and turn it round and round till it commences +to soften.</p> +<p>In the case of soda glass it is usual to employ the gas flame +only, but I find that it is better in most cases to use the hot +air of a gently-blown flame, rather than have the disadvantage of +the soot deposited in the alternative operation. When the glass +begins to soften, or even before, it may be moved right into the +blow-pipe flame, and the latter may be properly urged.</p> +<p>It is not possible to give quite explicit and definite +instructions, applicable to every case, as to when the time is +ripe for passing the work into the flame, but the following notes +will indicate the general rules to be observed:—</p> +<p>(1) A thick tube must be warmed more slowly and raised to a +higher temperature than a thin tube.</p> +<p>(2) The same remark applies to a tube of large diameter, as +compared with one of small diameter, whatever the thickness.</p> +<p>(3) In the case of very large or thick tubes the hot air is +advantageously employed at first, and to complete the preliminary +heating, the luminous flame alone may be used. The object of this +is to enable the operator to judge, by the presence of soot, its +inability to deposit — or its burning off if deposited +— of the temperature of the glass, and of the equality of +this temperature all over the surface, for a large and thick tube +might be heated quite enough to enable it to be safely exposed to +the full heat before it is appreciably yielding to the fingers. +In general, when the soot burns off freely, or lead glass begins +to show the faintest sign of reduction, or soda glass begins to +colour the flame, it is more than safe to proceed.</p> +<p>In order to turn on the full flame the operator will form a +habit of holding the work in the left hand only, and he will also +take care not to let anything his right hand may be doing cause +him to stop rotating the tube with his left thumb and finger.</p> +<p>The preliminary adjustment of air or oxygen supply will enable +the change to a flame of maximum power to be made very quickly. +The tube having been introduced with constant rotation, it will +soon soften sufficiently to be worked. The beginner will find it +best to decide the convenient degree of softness by trial.</p> +<p>With soda glass it does not much matter how soft the glass +becomes, for it remains viscous, but with lead glass the +viscosity persists for a longer time and then suddenly gives +place to a much greater degree of fluidity. <i>[Footnote:</i> +This is only drawn from my impressions acquired in glass-working. +I have never explicitly tested the matter +experimentally.<i>]</i></p> +<p>It is just at this point that a beginner will probably meet +with his first difficulty. As soon as the glass gets soft he will +find that he no longer rotates the glass at the same speed by the +right and left hand, and, moreover, he will probably +unconsciously bend the tube, and even deform it, by pushing or +pulling.</p> +<p>The second third of the art of the glass-blower consists in +being able to move both hands about, rotating a tube with each +thumb and finger, and keeping the distance between the hands, and +also the speed of rotation, constant. Nothing but long practice +can give this facility, but it is essential that it be acquired +to some extent, or no progress can be made. Some people acquire a +moderate proficiency very quickly, others, of whom the writer is +one, only become reasonably proficient by months, or even years, +of practice.</p> +<p>Supposing that the tube is now ready to be drawn down, the +operator will remove it from the flame, and will gently pull the +ends apart, interrupting his turning as little as possible. If +the tube be pulled too hard, or if the area heated be too small +(about three-eighths of an inch in length in the case given would +be proper), it will be found that the ends of the two portions of +the tube will be nearly closed at a very sharp angle (nearly a +right angle to the length of the tube), that the ends will be +thin, and that a long length of very fine tube will be produced. +To heat a short length of tube and pull hard and suddenly is the +proper way to make a very fine capillary tube, but, in general, +this is what we want to avoid.</p> +<p>If the operation be successfully performed, the drawn-down +tube will have the appearance exhibited, which is suitable either +for subsequently closing or handling by means of the drawn-down +portion. The straightness of the point can be obtained by a +little practice in "feeling" the glass when the tube is rotated +as it cools just before it loses its viscous condition.</p> +<p>When the operation is carried out properly the shoulder of the +"draw" should be perfectly symmetrical and of even thickness, and +its axis regarded as that of a cone should lie in the axis of the +tube produced. The operation should be repeated till the student +finds that he can produce this result with certainty, and he +should not be discouraged if this takes several days, or even +weeks. Of course, it is probable that within the first hour he +will succeed in making a tolerable job, but it is his business to +learn never to make anything else.</p> +<p><img src="images/Image35.gif" alt="images/Image35.gif" width= +"486" height="32">Fig. 9.</p> +<p>Fig. <img src="images/Image36.gif" alt="images/Image36.gif" +width="523" height="63">10. — Diagram of a folded end.</p> +<p><b><a name="Toc158108898" id="Toc158108898"><font face= +"Bookman Old Style" size="4">§ 20. Closing and blowing out +the End of a Tube. —</font></a></b></p> +<p><font face="Bookman Old Style">When it is desired to close the +end of a particular bit of tube, this is easily done by heating +the end, and at the same time heating the end of a waste bit of +tube or rod; the ends, when placed in contact, stick together, +and a point can be drawn down as before. <i>[Footnote:</i> +"Point" is here used in the technical sense, i.e. it is a thin +tail of glass produced by drawing down a tube.<i>]</i> Having got +a point, it will be found that the thin glass cools enough to +allow of the point being handled after a few moments.</font></p> +<p>The most convenient way of reducing the point to a suitable +length (say 1.5 inch) is to fuse it off in the flame, but this +must be done neatly; if a tail is left it may cause inconvenience +by catching, or even piercing the finger and breaking off. The +blow-pipe flame being turned down to a suitable size, and the +shoulder of the "draw" having been kept warm meanwhile, let the +tip of the flame impinge on a point where the diameter is about +half that of the undrawn tube, and let the temperature be very +high (Fig. 11). The tube is to be inclined to the flame so that +the latter strikes the shoulder normally, or nearly so. Then, +according to circumstances, little or much of the glass can be +removed at will by drawing off the tail (Fig. 12), till, finally, +a small drop of melted glass only, adheres to the end of the now +closed tube (Fig. 13).</p> +<p>Fig. <img src="images/Image37.gif" alt="images/Image37.gif" +width="510" height="113">11.</p> +<p><img src="images/Image38.gif" alt="images/Image38.gif" width= +"493" height="51">Fig. 12.</p> +<p><img src="images/Image39.gif" alt="images/Image39.gif" width= +"365" height="54">Fig. 13.</p> +<p>Fig. <img src="images/Image40.gif" alt="images/Image40.gif" +width="314" height="50">14.</p> +<p>When this is satisfactorily accomplished, heat the extreme end +of the tube most carefully and equally, holding it in such a +position that the glass will tend to flow from the bead back on +to the tube, i.e. hold the closed end up to the flame, the tube +being, say, at 45 degrees to the horizontal. Then when the +temperature is such as to indicate complete softness lift the +tube to the mouth, still holding the tube pointing with its +closed end a little above the horizontal, and blow gently. A +beginner almost always blows too hard.</p> +<p>What is wanted, of course, is a continued pressure, to give +the viscous glass time to yield gradually, if it is uniform; or +else intermittent puffs to enable the thinner parts, if there are +any, to cool more, and hence become more resisting than the +thicker ones. In any case a little practice will enable the +operator to blow out a round and even end — neither thicker +nor thinner than the rest of the tube.</p> +<p><b><a name="Toc158108899" id="Toc158108899"><font face= +"Bookman Old Style" size="4">§ 21. To make a Weld. +—</font></a></b></p> +<p><font face="Bookman Old Style">To begin with, try on two bits +of glass of the same size, i.e. cut a seven-inch length of glass +in half by scratching it with the knife, and pulling the ends +apart with a slight inclination away from the scratch. In other +words, combine a small bending moment with a considerable +tensional stress. It is important to learn to do this properly. +If the proportions are not well observed, the tube will break +with difficulty, and the section will not be perpendicular to the +main length. If the knife is in good order it will make a fine +deep scratch — the feel of the glass under the knife will +enable the operator to decide when the scratch is made. The +operation of cutting large tubes will be treated further on. The +two halves of the tube being held one in each hand, and one tube +closed at one end, the extremities to be united will be warmed, +and then put in the flame as before.</font></p> +<p>Fig. <img src="images/Image41.gif" alt="images/Image41.gif" +width="487" height="113">16.</p> +<p>There are many ways of proceeding - perhaps the easiest is as +follows. As soon as the glass shows signs of melting at the ends +— and care should be taken that much more is not heated +— take both bits out of the flame. Stop rotating for a +moment, and resting the arms carefully on the edge of the table, +raise the tubes above the flame and bring the ends swiftly and +accurately together. This is a case of "sudden death no second +attempt at making the ends meet can be allowed; if the tubes join +in any other than a perfectly exact manner a kink more or less +objectionable will result. In practice the operator will learn to +bring the ends together, commencing at one point; i.e. the axes +of the tubes will be inclined at first, so as to cause adherence +at one spot only. If this is not quite "fair," then less damage +is done in moving one tube slightly up or down to get the contact +exact. The tubes will then be closed upon one another as if they +were hinged at the joint. This must be done lightly, yet +sufficiently, to ensure that the glass is actually in contact all +round.</p> +<p>Having gone so far, replace the tubes — now one — +in the flame, and carefully rotating the glass, raise the +temperature higher than in the operation just described, in fact +the higher the temperature, short of burning the glass, the +better. Take the tube out of the flame and blow into the open +end, turning constantly as before. One puff is enough. Then turn +and pull the glass apart till it is of the same diameter and +thickness throughout, and feel that it is straight as before.</p> +<p>Though it is in general of high importance that the joint +should be well heated, the beginner will probably find that he +"ties up" his glass as soon as it gets really soft.</p> +<p>If his object is to make one joint — at any cost — +then let him be careful to use two bits of exactly the same kind +of glass, and only get the temperature up to the viscous stage. +If the joint be then pulled out till it is comparatively thin, it +will probably stand (if of soda glass); certainly, if of lead +glass, though in this case it may not be sound. In any case the +joint should be annealed in the asbestos box if practicable, +otherwise (unless between narrow tubes) with the asbestos rag. +Care must be taken that the asbestos is dry.</p> +<p><b><a name="Toc158108900" id="Toc158108900"><font face= +"Bookman Old Style" size="4">§ 22. To weld two Tubes of +different Sizes. —</font></a></b></p> +<p><font face="Bookman Old Style">To do this, the diameter of the +larger tube must be reduced to that of the smaller. The general +procedure described in drawing down must be followed, with the +following modification. In general, a greater length of the tube +must be heated, and it must be made hotter. The tube is to be +gradually drawn in the flame with constant turning till the +proper diameter and thickness of glass are attained.</font></p> +<p>Fig. <img src="images/Image42.gif" alt="images/Image42.gif" +width="455" height="79">16.</p> +<p>For this operation time must be allowed if the operator's +hands are steady enough to permit of it; the shoulder should form +partly by the glass sinking in and partly by the process of +drawing the hot glass out. A shoulder properly prepared is shown +in the sketch. Beginners generally make the neck too thin on +large tubes, and too thick on smaller ones. There ought to be no +great difference in thickness of glass between the neck on the +larger tube, and the smaller tube. The diameters should be as +nearly as possible alike.</p> +<p>Having drawn down the larger tube to a neck, take it out of +the flame, and as it cools pull and turn till the neck is of the +right thickness and is perfectly straight, i.e. make the final +adjustment outside the flame, and to that end have the neck +rather too thick (as to glass) before it is taken out. It is not +necessary to wait till the neck gets cold before the end can be +cut off. Make a scratch as before — this will probably +slightly damage the temper of the file knife, but that must be +put up with. Hold the tube against the edge of the table, so that +the scratch is just above the level of the rim, and strike the +upper part a smart blow with the handle of the glass knife rather +in the direction of its length. <i>[Footnote:</i> A bit of hoop +iron nailed against the side of the table is a very convenient +arrangement, and it need not project appreciably above the +general level of the rim.<i>]</i></p> +<p>Of course this applies to a tube where economy has been +exercised and the end is short. If the tail is long enough to +form a handle, the tube may be pulled apart as before. As a rule +a temporary joint between a tube and a rod is not strong enough +to enable the shoulder to be broken at the scratch by mere +pulling. The ends to be welded must be broken off very clean and +true. Subsequent operations are to be carried out as already +described.</p> +<p>§ 23. The above operations will be easily performed on +tubes up to half an inch in diameter, if they are not too long. +It is the length of tube, and consequent difficulty in giving +identity of motion with the two hands, which make the jointing of +long tubes difficult. There are also difficulties if the tubes +are very thin, have a very fine bore or a very large +diameter.</p> +<p>All these difficulties merely amuse a good glass-blower, but +to an experimenter who wants to get on to other things before +sufficient skill is acquired (in the movement of the hands and +arms) the following method is recommended. First, use flint +glass. Then, assuming that any drawing down has to be done, do it +as well as possible, for on this the success of the method to be +described especially depends. Be sure that the tubes to be welded +are cut off clean and are as nearly as may be of the same size at +the point of junction.</p> +<p>To fix the description, suppose it is desired to join two +tubes (see Fig. 8), each about one inch in diameter and a yard +long. Get four clip stands and place them on a level table. Be +sure that the stands are firm and have not warped so as to rock. +In each pair of clips place a tube, so that the two tubes are at +the same height from the table, and, in fact, exactly abut, with +axes in the same straight line. Close one tube by a cork and then +fix the blowing apparatus as shown to the other.</p> +<p>In such an operation as this the drying apparatus may be +dispensed with, and a rubber tube simply connected to one end of +the system and brought to the mouth. Take the oxygen blow-pipe +and turn the nozzle till the flame issues towards you, and see +that the flame is in order. Then turn down the oxygen till it +only suffices to clear the smoky flame, and commence to heat the +proposed joint by a current of hot air, moving the flame round +the joint. Finally, bring to bear the most powerful flame you can +get out of the blow-pipe, and carry it round the joint so quickly +that you have the latter all hot at once. Put down the blow-pipe, +and, using both hands, press the tubes together (which wooden +clips will readily allow), and after seeing that the glass has +touched everywhere, pull the tubes a trifle apart. Apply the +blow-pipe again, passing lightly over the thin parts, if any, and +heating thicker ones; having the end of the rubber tube in his +mouth, the operator will be able to blow out thick places. When +all is hot, blow out slightly, and having taken the flame away, +pull the tubes a little apart, and see that they are +straight.</p> +<p>Throw an asbestos rag over the joint, loosen one pair of the +clamps slightly, and leave the joint to anneal. It is important +that the least possible amount of glass should be heated, hence +the necessity of having the ends well prepared, and it is also +important that the work should be done quickly; otherwise glass +will flow from the upper side downwards and no strong joint will +be obtained.</p> +<p>Fig. <img src="images/Image43.gif" alt="images/Image43.gif" +width="382" height="146">17. — Tube being opened at one +end.</p> +<p><b><a name="Toc158108901" id="Toc158108901"><font face= +"Bookman Old Style" size="4">§ 24. To weld Tubes of very +small Bore. —</font></a></b></p> +<p><font face="Bookman Old Style">If the bore is not so small as +to prevent the entrance of the point of the iron nail, get the +ends of the tubes hot, and open the bore by inserting the end of +the nail previously smeared over with a trace of vaseline. Work +the nail round by holding the handle between the thumb and first +finger of the right hand, the tube being similarly placed in the +left. The tube and nail should be inclined as shown in the +sketch.</font></p> +<p>Never try to force the operation; the nail soon cools the +glass, so that only a very short time is available after each +heat; during this the tube should be rotated against the nail +rather than the nail against the tube. Be careful not to heat a +greater length of tube than is necessary, or the nail will, by +its component of pressure along the tube, cause the latter to +"jump up" or thicken and bulge. Both ends being prepared, and if +possible, kept hot, the weld may be made as before, and the +heating continued till the glass falls in to about its previous +thickness, leaving a bore only slightly greater than before.</p> +<p>It is in operations such as this that the asbestos box will be +found of great use. As soon as one end of the weld is ready cool +it in the flame till soot deposits, and then plunge it into the +asbestos. This will cause it to cool very slowly, and renders it +less likely to crack when again brought into the flame. +Turned-out ends, if the glass is at all thick, are very liable to +crack off on reheating, so that they must be reintroduced (into +the flame) with especial care. This liability to breakage is +reduced, but not eliminated, by the asbestos annealing.</p> +<p><img src="images/Image44.gif" alt="images/Image44.gif" width= +"515" height="121">Figs. 18 and 19.</p> +<p>§ 25. When the bore is very fine, it is best to seal off +the tubes, and blow an incipient bulb near one end of each tube. +These bulbs may be cooled in asbestos, and cut across when cold +by means of a scratch touched at one end (Figs. 18 and 19) by a +fine point of highly incandescent glass. For details of this +method see p. 46, Fig. 21. Time is occasionally saved by blowing +off the ends of the bulbs. The details of this process will be +described when the operation of making thistle-headed tubes is +dealt with.</p> +<p>§ 26. When the tubes are both of large diameter, long, +and very thin (cylinder tubes), a considerable amount of +difficulty will be experienced. On the whole, it is best to heat +each end separately till the glass thickens a little, anneal in +the flame and in asbestos, and then proceed as in § 22. If +the ends are not quite true, it will be found that quite a +thickness of glass may be "jumped" together at one side of the +tubes, while the edges are still apart at the other. When this +looks likely to happen, incline the tubes as if the joint were a +hinge, and bend back quickly; do not simply continue to push the +tubes together in a straight line, or an unmanageable lump of +glass will be formed on one side.</p> +<p>If in spite of these precautions such a lump does form, +proceed as follows. Take a rod of glass, at least one-eighth of +an inch thick, and warm it in the flame at one end. Heat the +imperfect joint till it softens all round, and then bring the +flame right up to the thick part, and heat that as rapidly and +locally as possible. The oxygas flame does this magnificently. +Press the heated end of the glass rod against the thick part, and +pull off as much of the lump as it is desired to remove, +afterwards blowing the dint out by a judicious puff. Finish off +as before.</p> +<p>§ 27. Occasionally, when it is seen that in order to +produce a joint closed all round, one side of the tube would be +too much thickened, it is better to patch the open side. For this +purpose take a glass rod about one-sixteenth inch in diameter, +and turn the flame to give its greatest effect, still keeping +rather an excess of air or oxygen. See that the side of the joint +already made is kept fairly hot — it need not be soft; +interrupt any other work often enough to ensure this. Then, +directing the flame chiefly on the thin rod, begin to melt and +pull the glass over the edges of the gap. When the gap is closed +get the lump very hot, so that all the glass is well melted +together, and then, if necessary, pull the excess of glass off, +as before described.</p> +<p>It must be remembered that this and the method of the previous +section are emergency methods, and never give such nice joints as +a manipulation which avoids them, i.e. when the ends of the tubes +are perfectly straight and true to begin with. Also note that, as +the tubes cannot be kept in rotation while being patched, it is +as well to work at as low a temperature as possible, consistently +with the other conditions, or the glass will tend to run down and +form a drop, leaving a correspondingly thin place behind.</p> +<p><img src="images/Image45.gif" alt="images/Image45.gif" width= +"381" height="51">Fig. 20.</p> +<p>§ 28. A very common fault in cutting a tube of about an +inch in diameter is to leave it with a projecting point, as +shown. This can be slowly chipped off by the pliers, using the +jaws to crush and grind away the edge of the projection; it is +fatal to attempt to break off large pieces of glass all at +once.</p> +<p>§ 29. It will be convenient here to mention some methods +of cutting large tubes. With tubes up to an inch and a half in +diameter, and even over this — provided that the glass is +not very thick — we may proceed as follows: Make a good +scratch about half an inch long, and pretty deep, i.e. pass the +knife backwards and forwards two or three times. Press a point of +melted glass exactly on one end of the scratch; the glass point +even when pressed out of shape should not be as large as a button +one-twelfth of an inch in diameter. If this fails at first, +repeat the operation two or three times.</p> +<p><img src="images/Image46.gif" alt="images/Image46.gif" width= +"476" height="195">Fig. 21.</p> +<p>If a crack does not form, touch the hot place with the cold +end of the nail. If no success is obtained, try the other end of +the scratch. If failure still pursues the operator, let him make +another cut on the opposite side of the tube and try again. In +general, the tube will yield the first or second time the hot +drop of glass is applied. Never apply the drop at the centre of +the scratch, or a ragged crack, which may run in any direction, +will result. Very often, with a large tube, the crack formed by a +successful operation will only extend a short distance. In this +case it is desirable to entice the crack round the tube, and not +trust to its running straight when the tube is pulled apart.</p> +<p>On the whole, the best method in this case is to employ a +flame pencil, which should be kept ready for use. This merely +consists of a bit of glass tube of about the same dimensions as +an ordinary lead pencil, drawn down to a very fine jet at one +end. The jet must not be very long or thin, or the glass will +soon fuse up. A few trials will enable the operator to get the +proper proportions, which are such that the tube has the general +appearance of a pencil normally sharpened (say with a cone of +60'). This tube is best made of hard glass. Connect it to a gas +supply by light flexible tubing, and turn down the gas till the +flame from the end of the jet is not more than one-tenth of an +inch long. Then apply the jet, beginning from the end of the +crack, and gradually draw it (the crack) round the tube. The +operation will be assisted if a rubber ring is slipped on the +tube to begin with, so that the eye has some guide as to whether +the flame is being drawn round properly or not. The ring must, of +course, be far enough away to escape the effect of the flame. The +crack will be found to follow the flame in the most docile +manner, unless the tube is thick or badly annealed. Some +operators recommend a pencil of glowing charcoal, but the flame +is undoubtedly better.</p> +<p><b><a name="Toc158108902" id="Toc158108902"><font face= +"Bookman Old Style" size="4">§ 30. To cut very thick Tubes. +-</font></a></b></p> +<p><font face="Bookman Old Style">A large number of methods have +been proposed, and nearly everybody has his favourite. The +following has always succeeded with me. First mark on the tube, +by means of a little dead black spirit paint, exactly where the +cut is to be. Then sharpen the glass knife and scratch a quite +deep cut all round: there is no difficulty in making the cut +one-twentieth of an inch deep. It will be proper to lubricate the +knife with kerosene after the first mark is made. +<i>[Footnote:</i> The edge of the knife may be advantageously +saved by using an old file moistened with kerosene for the +purpose. I find kerosene is not worse, but, if anything, better +than the solution of camphor in turpentine recommended by Mr. +Shenstone.<i>]</i></font></p> +<p>If the glass is about one-eighth of an inch thick, the scratch +maybe conveniently about one-twentieth of an inch deep, but if +the glass is anything like one-quarter of an inch thick, the +scratch must be much deeper, in fact, the glass may be half cut +through. To make a very deep scratch, a wheel armed with diamond +dust, which will be described later on, may be used. However, it +is not essential to use a diamond wheel, though it saves +time.</p> +<p>When the cut is made to a sufficient depth proceed thus: +Obtain two strips of bibulous paper or bits of tape and twist +them round the tube on each side of the scratch, allowing not +more than one-eighth of an inch between them. Then add a few +drops of water to each, till it is thoroughly soaked, but not +allowing water to run away. Dry out the scratch by a shred of +blotting paper.</p> +<p>Turn down the oxygas flame to the smallest dimensions, and +then boldly apply it with its hottest part playing right into the +nick and at a single point. Probably in about two seconds, or +less, the tube will break. If it does not, rotate the tube, but +still so that the flame plays in the nick. After making the tube +very hot all round — if it has not broken — apply the +flame again steadily at one point for a few seconds and then +apply a bit of cold iron. If the tube does not break at once +during these processes, let it cool, and cut the groove deeper; +then try again. <i>[Footnote:</i> This method is continually +being reinvented and published in the various journals. It is of +unknown antiquity.<i>]</i></p> +<p><img src="images/Image47.gif" alt="images/Image47.gif" width= +"416" height="187">Fig. 22.</p> +<p>If the tube breaks after great heating and long efforts, it +will probably leave incipient cracks running away from the break, +or may even break irregularly. A good break is nearly always one +that was easily made. If a number of rings have to be cut, or a +number of cuts made on glass tubes of about the same size, it +will be found economical in the end to mount a glazier's diamond +for the purpose. A simple but suitable apparatus is figured (Fig. +23).</p> +<p><img src="images/Image48.gif" alt="images/Image48.gif" width= +"355" height="316">Fig. 23.</p> +<p>The only difficulty is to regulate the position of the diamond +so that it cuts. In order to do this, carefully note its cutting +angle by preliminary trials on sheet glass, and then adjust the +diamond by clamps, or by wriggling it in a fork, as shown. Weight +the board very slightly, so as to give the small necessary +pressure, and produce the cut by rotating the tube by hand. When +a cut is nearly completed take great care that the two ends join, +or irregularity will result. This is not always easy to do unless +the tube happens to be straight. Having got a cut, start a crack +by means of a fine light watchmaker's hammer, or even a bit of +fused glass, and entice the crack round the cut by tapping with +the hammer or by means of the flame pencil.</p> +<p>If the cut is a true "cut" the tube will break at once. As a +supply of electrical current for lighting will, in the near +future, be as much a matter of course for laboratory purposes as +a gas supply, I add the following note. To heat a tube round a +scratch, nothing — not even the oxygas blow-pipe — is +so good as a bit of platinum or iron wire electrically heated. If +the crack does not start by considerable heating of the glass, +stop the current, unwind the wire, and touch the glass on the +crack either with a bit of cold copper wire or a wet match stem. +I prefer the copper wire, for in my experience the water will +occasionally produce an explosion of cracks. On the other hand, +the cold wire frequently fails to start a crack.</p> +<p>Judging from the appearance of thick tubes as supplied by the +dealers, the factory method of cutting off appears to be to grind +a nick almost through the tube, and right round; and for really +thick glass this is the safest but slowest way; a thin emery +wheel kept wet will do this perfectly. Suitable wheels may be +purchased from the "Norton" Emery Wheel Co. of Bedford, Mass., +U.S.A. — in England through Messrs. Churchill and Co. of +London, importers.</p> +<p><b><a name="Toc158108903" id="Toc158108903"><font face= +"Bookman Old Style" size="4">§ 31. To blow a Bulb at the End +of a Tube. —</font></a></b></p> +<p><font face="Bookman Old Style">I must admit at once that this +is a difficult operation — at all events, if a large bulb +is required. However, all there is to be said can be said in few +words. In general, when a bulb is required at the end of a tube +it will be necessary to thicken up the glass. A professional +glass-worker will generally accomplish this by "jumping up" the +tube, i.e. by heating it where the bulb is required, and +compressing it little by little until a sufficient amount of +glass is collected. The amateur will probably find that he gets a +very irregular mass in this way, and will be tempted to begin by +welding on a short bit of wide and thick tubing preparatory to +blowing out the bulb.</font></p> +<p>However, supposing that enough glass is assembled by-either of +these methods, and that it is quite uniform in thickness, let the +thickened part be heated along a circle till it becomes +moderately soft, and let it then be expanded about one-fifth, say +by gently blowing. It is perhaps more important to keep turning +the glass during bulb-blowing than in any other operation, and +this both when the glass is in the flame and while the bulb is +being blown. It is also very important to avoid draughts. In +general, a bulb is best blown with the tube in a nearly +horizontal position, but sloping slightly upwards from the mouth. +If it be noticed that a bulb tends to blow out more at one side +than another, let the side of greatest protuberance be turned +down, so that it is at the lowest point, reduce the pressure for +an instant, and then blow again. It will be observed that the +bulb will now expand at the top.</p> +<p>The reason of this is chiefly that the under side cools most +rapidly (according to Faraday, <i>Chemical Manipulation</i>, +§ 1194), and consequently can expand no further; but also it +is not unlikely that the glass tends to flow somewhat from the +upper side, which remains hot, and consequently the bulb, when +the next puff reaches it, will tend to yield at this point. By +heating several zones the tube will become gradually +expanded.</p> +<p>Fig. <img src="images/Image49.gif" alt="images/Image49.gif" +width="433" height="31">24.</p> +<p><img src="images/Image50.gif" alt="images/Image50.gif" width= +"405" height="72">Fig. 25.</p> +<p>Fig. <img src="images/Image51.gif" alt="images/Image51.gif" +width="399" height="92">26.</p> +<p>When the length of the thickened part of the tube only +slightly exceeds its diameter (Fig. 25), let the whole be brought +to a temperature which, with flint glass, should be just short of +that of perfect fluidity; and then, holding the tube horizontally +and constantly turning it, let the bulb be blown out to its full +size, noting the appearances and correcting too great +protuberance on any side by the means above mentioned. If the +bulb appears pear-shaped turn the tube so that the melted mass is +directed upwards; if the bulb have the contrary fault, correct in +the corresponding manner.</p> +<p>The bulb when finished may be lightly tapped on the table, +when, if there is any weak place owing to inequality of +thickness, the bulb will break, and the operation may be started +afresh. "A good bulb is round, set truly on the tube, and free +from lumps of thick glass or places of excessive thinness." When +the amateur has succeeded in blowing a bulb two inches in +diameter on the end of a strong bit of thermometer tube — +say for an air thermometer — he may well seek the +congratulations of his friends.</p> +<p>In case the bulb is not satisfactory on a first attempt, it +may be melted down again, if the following precautions are taken. +Directly creases begin to appear in the bulb let it be withdrawn +from the flame, and gently blown till the creases come out. By +alternate heating and blowing the glass can be got back to its +original form, or nearly so, but unless the operator shows great +skill and judgment, the probability is that the glass will be +uneven. By heating and keeping the thicker parts in the higher +position, and blowing a little now and again, the glass may be +got even, and a new attempt may be made. It must not be supposed +that this process can be carried on indefinitely, for the glass +tends to lose its viscous properties after a time, or, at all +events, it "perishes" in some way, especially if it has been +allowed to get very thin; consequently too frequent attempts on +the same glass are unprofitable. Two or three trials are as many +as it generally pays to make. As a rule the largest possible +flame may be used with advantage in this operation.</p> +<p><b><a name="Toc158108904" id="Toc158108904"><font face= +"Bookman Old Style" size="4">§ 32. To blow a bulb in the +middle of a tube,</font></a></b></p> +<p><font face="Bookman Old Style">the procedure is much like that +already treated, but the manipulation is, if anything, more +difficult, for the further end of the tube must be carried and +turned as well as the end which is held to the lips.</font></p> +<p><b><a name="Toc158108905" id="Toc158108905"><font face= +"Bookman Old Style" size="4">§ 33. To make a side Weld. +—</font></a></b></p> +<p><font face="Bookman Old Style">This is by no means difficult, +but is easier with lead glass than with soda glass. The tube to +which it is desired to make a side connection having been +selected, it is closed at one end by rubber tube stops, or in any +other suitable manner. The zone of the proposed connection is +noted, and the tube is brought to near softness round that circle +(if the tube is made actually soft, inconvenience will arise from +the bending, which is sure to occur). Two courses are then open +to the operator, one suitable to a thick tube, the other to a +tube of moderate thickness.</font></p> +<p>Taking the former first. Provide a piece of glass rod and warm +its end. Direct a small flame against the spot on the thick tube +where the proposed joint is to be. When the glass becomes almost +incandescent at this spot, put the end of the rod against it and +draw out a thread of glass till sufficient "metal" has been +removed. Then fuse off the thread close to the tube.</p> +<p><img src="images/Image52.gif" alt="images/Image52.gif" width= +"467" height="118">Fig. 27.</p> +<p>The subsequent procedure is the same as for thin tubes. In +this case heat the spot by the smallest flame available, and get +the spot very hot. Blow it out gently into a bubble, perhaps +extending to a height equal to its diameter. Then heat the top of +the bubble till it is incandescent and blow violently. This will +produce an opening fringed by glass so thin as to exhibit +interference colours. Remove the filmy part, and heat the frayed +edges till they cohere and form an incipient tube. If the flame +has been of a correct size, the tube will now be of the same +diameter as the tube to be welded on, and will project perhaps +one-sixteenth of an inch from the surface of the main tube (Fig. +28).</p> +<p><img src="images/Image53.gif" alt="images/Image53.gif" width= +"423" height="68">Fig. 28.</p> +<p>Fig. <img src="images/Image54.gif" alt="images/Image54.gif" +width="453" height="160"> 29.</p> +<p>When this stage is reached, again heat the tube all round till +it nearly softens, and by means of the other hand heat the end of +the other tube which it is proposed to weld. Just before the main +tube actually softens, turn it so as to heat the edges of the +aperture, and at the same time get the end of the side tube very +hot. Take both out of the flame for an instant, and press the +parts together, instantly slightly withdrawing the side tube. If +the operation is well performed, it will be found that the point +of maximum thickness of glass is now clear of the main tube. The +joint is then to be heated all round and blown out — a +rather awkward operation, and one requiring some practice, but it +can be done.</p> +<p>Fig. <img src="images/Image55.gif" alt="images/Image55.gif" +width="362" height="139"> 30.</p> +<p>If great strength is wanted, heat the main tube all round the +joint bit by bit, and blow each section slightly outwards. If the +operator is confident in his skill, he should then heat the whole +joint to the softening point, blow it out slightly, and then +adjust by pulling and pushing. Cool first in the gas flame, and +then plunge the joint into the asbestos and cover it up — +or if too large, throw the asbestos cloth round it.</p> +<p>In the case of soda glass this final "general heat" is almost +essential, but it is not so with flint glass, and as the general +heat is the most difficult part of the job, it will be found +easier to use lead glass and omit the general heating. With soda +glass a very small irregularity will cause the joint to break +when cold, but flint glass is much more long-suffering. It is +easy to perform the above operation on small tubes. For large +ones it will be found best to employ flint glass and use the clip +stands as in the case of direct welds, treated above, but, of +course, with suitable modifications. Never let the main tube cool +after the hole is made until the work is done.</p> +<p><b><a name="Toc158108906" id="Toc158108906"><font face= +"Bookman Old Style" size="4">§ 34. Inserted Joints. +—</font></a></b></p> +<p><font face="Bookman Old Style">In many instances the +performance of apparatus is much improved by joints of this kind, +even when their use is not absolutely essential.</font></p> +<p>There are two ways in which inserted joints may be made. The +first method is the easier, and works well with flint glass; but +when one comes to apply it to soda glass there is a danger of the +glass becoming too thick near the joint, and this often leads to +a cracking of the joint as the glass cools.</p> +<p>Fig. 31. <img src="images/Image56.gif" alt= +"images/Image56.gif" width="407" height="202"></p> +<p>Suppose it is desired to insert the tube B into the tube A +(Fig. 31). Begin by reducing the size of the end of tube A till B +will just slip in quite easily. With B about one-quarter inch in +diameter, a clearance of about one-twentieth of an inch, or less, +in all (i.e. one-fortieth of an inch on each side) will be +proper.</p> +<p>Heat B by itself at the proposed zone of junction, and blow +out a very narrow ring; then compress this slightly so that it +forms an almost closed ring of glass. The figure refers to the +close of this operation (Fig. 31, B). It does not matter much +whether the ring remains a mere flattened bulb, or whether it is +a solid ring, but it must be one or the other. Some judgment must +be exercised in preparing the ring. In general, the beginner will +collect too much glass in the ring, and consequently the joint, +when made, will either be thick and liable to crack easily, or it +will be blown out into an erratic shape in endeavours to reduce +this thickness. Accordingly, the operator will, if necessary, +thin the tube B by drawing slightly, if he considers it +desirable, before the little enlargement is blown out. In +general, two heats must be used for this operation.</p> +<p>Fig. <img src="images/Image57.gif" alt="images/Image57.gif" +width="498" height="63"> 32.</p> +<p>Get the approximating parts of both A and B up to a +temperature just below that at which they will adhere, and having +closed the other end of A, place B carefully within it up to the +ring, and if it can be arranged, have a mica wad in A, with a +central hole through which the end of B can project. This will +very much facilitate the operation, especially if B is long, but +may be dispensed with by the exercise of care and skill.</p> +<p>The operation is now simple. Fuse the junction and press the +tubes lightly together, being careful not to collect more glass +than can be helped; finally, blow out the joint and reduce the +thickness by mild drawing (Fig. 33). In order to make a really +good joint, two points must be particularly attended to — +the rim must be thin and its plane perfectly perpendicular to the +axis of tube B; the end of tube A must be cut off quite clean and +perpendicular to its axis before B is inserted. So important are +these conditions — especially the latter that the writer +has even occasionally used the grindstone to get the end of A +into a proper condition, an admission which will probably earn +the contempt of the expert glass-worker.</p> +<p><img src="images/Image58.gif" alt="images/Image58.gif" width= +"467" height="139">Fig. 33.</p> +<p>Now for the second method, which is often practised in +Germany, where soda glass is chiefly used. With this glass the +chief point is to get a very even and not too thick ring at the +junction, and consequently the extra thickening produced by +making a rim on B is rather a drawback. The method consists in +cutting off from B the length which it is desired to insert, +slipping this into A (which may be an otherwise closed bulb, for +instance), and then gradually melting up the open end of A till +the piece of B inside will no longer fall out. By holding the +joint downwards so that the inserted portion of B rests on the +edges of the opening, a joint may be made with the minimum +thickening.</p> +<p>The external part of B, previously heated, is then applied, +and the joint subjected to a "general" heat and blown out. Very +nice joints may be made by this method, and it is perhaps the +better one where the external part of B is to be less in diameter +than the inserted part. It was in this manner that the writer was +taught to make glass velocity pumps, one of which, of a good +design, is figured as an example.</p> +<p>In all cases good annealing should follow this operation. If +the inserted part of the inner tube (B) is anything like an inch +in diameter, and especially if it is of any length, as in some +forms of ozone apparatus, or in a large Bunsen's ice calorimeter, +the arrangements for supporting the inner part must be very good. +A convenient way of proceeding when the inner tube is well +supported is to make the mouth of A only very little larger than +the diameter of B, so that B will only just slip in. Then the +mouth of A and the zone of B may be heated together, and B blown +out upon A. This, of course, must be arranged for, if necessary, +by temporarily stopping the inner end of B.</p> +<p><img src="images/Image59.gif" alt="images/Image59.gif" width= +"497" height="125">The inner support of B should be removed as +soon as practicable after the joint is made, or, at all events, +should not be perfectly rigid; a tightly-fitting cork, for +instance, is too rigid. The reason is, of course, that in cooling +there may be a tendency to set B a little to one side or the +other, and if it is not free to take such a set, the joint most +probably will give way. Good annealing both with flame and +asbestos is a <i>sine qua non</i> in all inserted work.</p> +<p>Fig. 34.</p> +<p><b><a name="Toc158108907" id="Toc158108907"><font face= +"Bookman Old Style" size="4">§ 35. Bending Tubes. +—</font></a></b></p> +<p><font face="Bookman Old Style">I have hitherto said nothing +about bending tubes, for to bend a tube of a quarter of an inch +in diameter, and of ordinary thickness, is about the first thing +one learns in any laboratory, while to bend large tubes nicely is +as difficult an operation as the practice of GLASS-BLOWING +affords. However, even in bending a narrow tube it is possible to +proceed in the wrong way. The wrong way is to heat a short length +of the tube and then bend it rapidly, holding the plane of the +bend horizontal. The right way, <i>per contra</i>, is to use a +batswing burner to heat, say, two inches of the tube with +constant turning till it is very soft, and then, holding the +glass so that the bend will be in a vertical plane passing +through one eye (the other being shut), to make the bend rather +slowly.</font></p> +<p>If an exact angle is required, it is as well to have it drawn +out on a sheet of asbestos board. In this case bend the glass as +described till it is approximately right, and finish by laying it +on the asbestos board and bringing it up to the marks. A suitable +bit of wood may be substituted for the asbestos on occasion.</p> +<p>N.B. — The laboratory table is not a suitable piece of +wood. A right-angled bend is often wanted. In this case the +corner of a table will serve as a good guide to the eye, the +glass being finished by being held just above it. If great +accuracy is wanted, make a wooden template and suspend it by a +screw from the side of the table, so that the vertex of the gauge +for the interior angle projects downwards, then finish by bending +the tube round it. The wood may be about half an inch thick.</p> +<p>If a sharp bend is required, heat the tube in the blow-pipe, +and bend it rapidly, blowing out the glass meanwhile. The reason +why a long bend should be held in a vertical plane is that the +hot part tends to droop out of the plane of the bend if the +latter be made in a horizontal position. To bend a tube above +half an inch in diameter is a more or less difficult operation, +and one which increases in difficulty as the diameter of the tube +increases.</p> +<p>A U-tube, for instance, may be made as follows: Use the four +blow-pipe arrangement so as to heat a fair length of tube, and +get, say, two inches of tube very hot--almost fluid, in fact +— by means of the carbon block supported from a stand. +Remove the tube rapidly from the flame and draw the hot part out +to, say, three inches. Then, holding the tube so as to make the +bend in a vertical plane, bend it and blow it out together to its +proper size.</p> +<p>This operation seems to present no difficulties to experienced +glass-workers, even with tubes of about one inch in diameter, but +to the amateur it is very difficult. I always look on a large +U-tube with feelings of envy and admiration, which the complex +trick work of an elaborate vacuum tube does not excite in the +least. It will be noted that this method may, and often does, +involve a preliminary thickening of the glass.</p> +<p>With tubes over an inch in diameter I have no idea as to what +is the best mode of procedure — whether, for instance, a +quantity of sand or gas coke might not be used to stuff out the +tube during bending, but in this case there would be the +difficulty of removing the fragments, which would be sure to +stick to the glass.</p> +<p>Of course, if the bend need not be short, the tube could be +softened in a tube furnace and bent in a kind of way. I must +admit that with tubes of even less than one inch in diameter I +have generally managed best by proceeding little by little. I +heat as much of the glass as I can by means of a gigantic +blow-pipe, having a nozzle of about an inch in diameter, and +driven by a machine-blower.</p> +<p>When I find that, in spite of blowing, the tube begins to +collapse, I suspend operations, reheat the tube a little farther +on, and so proceed. If by any chance any reader knows a good +laboratory method of performing this operation, I hope he will +communicate it to me. After all, the difficulty chiefly arises +from laboratory heating appliances being as a rule too limited in +scope for such work.</p> +<p>The bending of very thin tubes also is a difficulty. I have +only succeeded here by making very wide bends, but of course the +blowing method is quite applicable to this case, and the effect +may be obtained by welding in a rather thicker bit of tube, and +drawing and blowing it till it is of the necessary thinness. This +is, however, a mere evasion of the difficulty.</p> +<p><b><a name="Toc158108908" id="Toc158108908"><font face= +"Bookman Old Style" size="4">§ 36. Spiral Tubes. +—</font></a></b></p> +<p><font face="Bookman Old Style">These are easily made where +good heating apparatus is available. As, however, one constantly +requires to bend tubes of about one-eighth inch in diameter into +spirals in order to make spring connections for continuous glass +apparatus, I will describe a method by which this is easily done. +Provide a bit of iron pipe about an inch and a quarter in outside +diameter. Cover this with a thick sheath of asbestos cloth, and +sew the edges with iron wire. Hammer the wire down so that a good +cylindrical surface is obtained. Make two wooden plugs for the +ends of the iron pipe. Bore one to fit a nail, which may be held +in a small retort clip, and fasten a stout wire crank handle into +the other one. Support the neck of the handle by means of a +second clip. In this way we easily get a sort of windlass quite +strong enough for our purpose.</font></p> +<p>Fig. <img src="images/Image60.gif" alt="images/Image60.gif" +width="419" height="215"> 35.</p> +<p>Provide a large blow-pipe, such as the blow-pipe of a Fletcher +crucible furnace, Select a length of tubing and clean it. Lash +one end to the cylinder by means of a bit of wire, and hold the +other end out nearly horizontally. Then start the blow-pipe to +play on the tube just where it runs on to the asbestos cylinder, +and at first right up to the lashing. Get an attendant to assist +in turning the handle of the windlass, always keeping his eye on +the tube, and never turning so fast as to tilt the tube upwards. +By means of the blow-pipe, which may be moved round the tubing, +heat the latter continuously as it is drawn through the flame, +and lay it on the cylinder in even spirals.</p> +<p>If the tubing is thin, a good deal of care will have to be +exercised in order to prevent a collapse. A better arrangement, +which, however, I have not yet tried, would, I think, be to +replace the blow-pipe by two bats-wing burners, permanently +fastened to a stand, one of them playing its flame downwards on +to the top of the flame of the other. The angle between the +directions of the jets might be, say, 130°, or whatever is +found convenient. In this way the glass would not be so likely to +get overheated in spots, and better work would doubtless result. +However, I have made numbers of perfectly satisfactory spirals as +described. Three or four turns only make a sufficiently springy +connection for nearly all purposes.</p> +<p><b><a name="Toc158108909" id="Toc158108909"><font face= +"Bookman Old Style" size="4">§ 37. On Auxiliary Operations +on Glass:-</font></a></b></p> +<p><font face="Bookman Old Style">Boring Holes through Glass. +— This is much more easily done than is generally supposed. +The best mode of procedure depends on the circumstances. The +following three cases will be considered:-</font></p> +<p>1. Boring holes up to one-quarter inch diameter through thick +glass (say over one-eighth inch), or rather larger holes through +thin glass.</p> +<p>2. Boring holes of any size through thick glass.</p> +<p>3. Boring round holes through ordinary window glass.</p> +<p><b><a name="Toc158108910" id="Toc158108910"><font face= +"Bookman Old Style" size="4">§ 38. Boring small Holes. +—</font></a></b></p> +<p><font face="Bookman Old Style">Take a three-cornered file of +appropriate dimensions, and snip the point off by means of a +hammer; grind out most of the file marks to get sharp corners. +Dip the file in kerosene, and have plenty of kerosene at hand in +a small pot. Place the broken end of the file against the glass, +and with considerable pressure begin to rotate it (the file) +backwards and forwards with the fingers, very much as one would +operate a bradawl against a hard piece of wood. The surface of +the glass will shortly be ground away, and then the file bradawl +will make much quicker progress than might be expected. Two or +three minutes should suffice to bore a bit of sheet +window-glass.</font></p> +<p>The following points require attention:</p> +<p>(1) Use any quantity of oil.</p> +<p>(2) After getting through the skin reduce the pressure on the +file.</p> +<p>(3) Be sure to turn the file backwards and forwards through a +complete revolution at least.</p> +<p>(4) When the hole is nearly through reduce the pressure.</p> +<p>(5) When the hole is through the glass be exceedingly careful +not to force the file through too rapidly, otherwise it will +simply act as a wedge and cause a complete fracture.</p> +<p>(6) In many cases it is better to harden the file in mercury +before commencing operations; both files and glass differ so much +in hardness that this point can only be decided by a trial. If it +is found necessary to harden the file, use either a large +blow-pipe and a coke or charcoal bed, or else a small forge. A +small blowpipe, such as is generally found in laboratories, does +more harm than good, either by burning the end of the file or +raising it to an insufficient temperature.</p> +<p>(7) To sharpen the file, which is often necessary after +passing through the "skin" of the glass, put it in a vice so that +the point just protrudes clear of the jaws. Then, using a bit of +waste iron as an intermediary anvil or punch, knock off the least +bit from the point, so as to expose a fresh natural surface. The +same result may be brought about by the use of a pair of +pliers.</p> +<p>If several holes have to be bored, it is convenient to mount +the file in the lathe and use a bit of flat hard wood to press up +the glass by means of the back rest. A drilling machine, if not +too heavy, does very well, and has the advantage of allowing the +glass to remain horizontal so that plenty of oil can be kept in +the hole.</p> +<p>Use a very slow speed in either case — much slower than +would be used for drilling wrought iron. It is essential that the +lubricant should flow on to the end of the file very freely, +either from a pipette or from the regular oil-feed. If a little +chipping where the file pierces the back surface is inadmissible, +it is better, on the whole, to finish the bore by hand, using a +very taper file. It is not necessary to use a special file for +the lathe, for a well-handled file can be chucked very +conveniently in a three-jaw chuck by means of the handle.</p> +<p>Mr. Shenstone recommends a lubricant composed of camphor +dissolved in turpentine for general purposes. With the object of +obtaining some decisive information as to the use of this +lubricant, and to settle other points, I made the following +experiments. Using an old three-cornered French file, I chipped +off the point and adjusted the handle carefully. I also ground +out the file marks near the point, without hardening the file in +mercury. Using kerosene and turpentine and camphor, I began to +bore holes in a hard bit of 3/32 inch window glass.</p> +<p>Each hole was bored to about one-eighth inch in diameter in +four minutes with either lubricant. After hardening the file in +mercury and using kerosene, I also required four minutes per +hole. After mounting the file in a lathe which had been speeded +to turn up brass rods of about one-half-inch diameter, and +therefore ran too fast, I required one and a half minutes per +hole, and bored them right through, using kerosene. On the whole, +I think kerosene does as well as anything, and for filing is, I +think, better than the camphor solution. However, I ought to say +that the camphor-turpentine compound has probably a good deal to +recommend it, for it has survived from long ago. My assistant +tells me he has seen his grandfather use it when filing +glass.</p> +<p>I beg to acknowledge my indebtedness to Mr. Pye, of the +Cambridge Scientific Instrument Company, for showing me in 1886 +(by the courtesy of the Company) the file method of glass-boring; +it is also described by Faraday in <i>Chemical Manipulation</i>, +1228.</p> +<p>It is not necessary, however, to use a file at all, for the +twist drills made by the Morse Drill Company are quite hard +enough in their natural state to bore glass. The circumferential +speed of the drill should not much exceed 10 feet per minute. In +this way the author has bored holes through glass an inch thick +without any trouble except that of keeping the lubricant +sufficiently supplied. For boring very small holes watchmaker's +drills may be used perfectly well, especially those tempered for +boring hardened steel. The only difficulty is in obtaining a +sufficient supply of the lubricant, and to secure this the drill +must be frequently withdrawn.</p> +<p>My reason for describing the file method at such length is to +be found in the fact that a Morse drill requires to be sharpened +after drilling glass before it can be used in the ordinary way, +and this is often a difficulty.</p> +<p>I ought to say that I have never succeeded in boring the +<i>barrel of a glass tap</i> by either of these methods. +<i>[Footnote:</i> I have been lately informed that it is usual to +employ a splinter of diamond set in a steel wire holder both for +tap boring and for drilling earthenware for riveting. The diamond +must, of course, be set so as to give sufficient clearance for +the wire holder.</p> +<p>For methods of using and setting diamond tools see § 55. +It will suffice to say here that a steel wire is softened and +filed at one end so as to form a fork; into this the diamond is +set by squeezing with pliers. The diamond is arranged so as to +present a point in the axis of the wire, and must not project on +one side of the wire more than on the other. It is not always +easy to get a fragment satisfying these conditions, and at the +same time suitable for mounting. A drop of solder occasionally +assists the process of setting the diamond.</p> +<p>In drilling, the diamond must be held against the work by a +constant force, applied either by means of weight or a spring. I +made many trials by this method, using a watchmaker's lathe and +pressing up the work by a weight and string, which passed over a +pulley. I used about 40 ounces, and drilled a hole 3/32 in +diameter in flint glass at a speed of 900 revolutions per minute +to a depth of one-eighth of an inch in eight minutes. I used soap +and water as a lubricant, and the work was satisfactory.</p> +<p>Since this was set up, I have been informed by Mr. Hicks of +Hatton Garden that it is necessary to anneal glass rod by heating +it up to the softening point and allowing it to cool very slowly +under red-hot sand or asbestos before boring. If this be done, no +trouble will be experienced. The annealing must be +perfect.<i>]</i></p> +<p><b><a name="Toc158108911" id="Toc158108911"><font face= +"Bookman Old Style" size="4">§ 39. For boring large holes +through thick glass sheets,</font></a></b></p> +<p><font face="Bookman Old Style">or, indeed, through anything +where it is necessary to make sure that no accident can happen, +or where great precision of position and form of hole is +required, I find a boring tube mounted as shown in the picture +(Fig. 36) is of great service. Brass or iron tube borers do +perfectly well, and the end of the spindle may be provided once +for all with a small tube chuck, or the tubes may be separately +mounted as shown. A fairly high speed is desirable, and may be +obtained either by foot, or, if power is available, is readily +got by connecting to the speed cone of a lathe, which is +presumably permanently belted to the motor.</font></p> +<p>Fig. 36. <img src="images/Image61.gif" alt= +"images/Image61.gif" width="352" height="285"></p> +<p>After trying tubes armed with diamond dust, as will be +presently explained, I find that emery and thin oil or +turpentine, if liberally supplied below the glass, will do very +nearly as well. The tube should be allowed to rise from the work +every few seconds, so as to allow of fresh emery and oil being +carried into the circular grooves. This is done by lifting the +hinged upper bearing, the drill being lifted by a spiral spring +between the pulley and the lower bearing shown at B. The glass +may be conveniently supported on a few sheets of paper if flat, +or held firm in position by wooden clamps if of any other shape. +In any case it should be firmly held down and should be well +supported. Any desired pressure upon the drill is obtained by +weighting the hinged board A.</p> +<p>§ 40. The following method was shown to me by Mr. +Wimshurst, but I have not had occasion to employ it myself. It is +suitable for boring large holes through such glass as the plates +of Mr. Wimshurst's Influence machines are usually made of. A +diamond is mounted as the "pencil" of a compass, and with this a +circle is drawn on the glass in the desired position. The other +leg of the compass of course rests on a suitable washer.</p> +<p>To the best of my recollection the further procedure was as +follows. A piece of steel rod about one-eighth inch in diameter +was ground off flat and mounted in a vice vertically, so as to +cause its plane end to form a small horizontal anvil. The centre +(approximately) of the diamond-cut circle of the glass was laid +on this anvil so as to rest evenly upon it, and the upper surface +(i.e. that containing the cut) was then struck smartly with a +hammer, completely pulverising the glass above the anvil. The +hole was gradually extended in a similar manner right up to the +diamond cut, from which, of course, the glass broke away.</p> +<p>A similar method has long been known to glaziers, differing +from the preceding in that a series of diamond cuts are run +across the circle parallel to two mutually perpendicular +diameters. A smart tap on the back of the scored disc will +generally cause the fragments to tumble out. I have never tried +this myself, but I have seen it done.</p> +<p>Large discs may easily be cut from sheet glass by drawing a +circular diamond cut, and gradually breaking away the outer parts +by the aid of additional cuts and a pair of pliers or "shanks" +(see Fig. 44).</p> +<p><b><a name="Toc158108912" id="Toc158108912"><font face= +"Bookman Old Style" size="4">§ 41. Operations depending on +Grinding: Ground-in Joints. —</font></a></b></p> +<p><font face="Bookman Old Style">The process will be perfectly +understood by reference to a simple case. Suppose it is desired +to grind the end of a tube into the neck of a bottle. If a +stoppered bottle is available, the stopper must be taken out and +measured as to its diameter at the top and bottoM. Select a bit +of tube as nearly as possible of the same diameter as the stopper +at its thickest part. Draw down the glass in the blow-pipe flame +rather by allowing it to sink than by pulling it out. After a few +trials no difficulty will be experienced in making its taper +nearly equal to that of the stopper, though there will in all +probability be several ridges and inequalities. When this stage +is reached anneal the work carefully and see that the glass is +not too thin. Afterwards use emery and water, and grind the +stopper into the bottle.</font></p> +<p>There are six special directions to be note</p> +<p>(1 )Turn the stopper through at least one revolution in each +direction.</p> +<p>(2) Lift it out often so as to give the fresh emery a chance +of getting into the joint.</p> +<p>(3) Rotate the bottle as well as the stopper in case there is +any irregularity in the force brought to bear, which might cause +one side of the neck to be more ground than another, or would +cause the tube to set rather to one side or the other.</p> +<p>(4) Use emery passing a 50 sieve, i.e. a sieve with fifty +threads to the inch run (see § 144) to begin with, and when +the stopper nearly fits, wash this thoroughly away, and finish +with flour emery, previously washed to get rid of particles of +excessive size; the process of washing will be fully discussed in +the chapter on glass-grinding, which see.</p> +<p>(5) Any degree of fineness of surface may be obtained by using +graded emery, as will be explained, but, in general, it is +unnecessary to attempt a finer surface than can be got with +washed flour emery. A superficial and imperfect polish may be +given by grinding for a short time with powdered pumice +stone.</p> +<p>(6) If the proper taper is not attained by blowing, or if +ridges are left on the tapered part, the process may be both +hastened and improved by giving the taper a preliminary filing +with a three-cornered file and kerosene, just as one would +proceed with iron or brass. A little filing will often save a +good deal of grinding and make a better job.</p> +<p>If a bottle without a tapered neck is to be employed, it is as +well to do the preliminary grinding by means of a cone turned up +from a bit of cast iron. This is put in the lathe and pushed into +the mouth of the bottle, the latter being supported by the hands. +Use about the same surface speed as would be employed for turning +cast iron. In this case the emery is better used with +kerosene.</p> +<p>If a cylindrical bit of cast iron about an inch in diameter is +turned down conically nearly to a point, it will save a good deal +of trouble in making separate cones. If it gets ground into +rings, and it becomes necessary to turn it up, use a diamond tool +until the skin is thoroughly removed; the embedded emery merely +grinds the edge off any ordinary steel tool.</p> +<p>For diamond tools see § 55.</p> +<p><b><a name="Toc158108913" id="Toc158108913"><font face= +"Bookman Old Style" size="4">§ 42. Use of the Lathe in +Glass-working. —</font></a></b></p> +<p><font face="Bookman Old Style">If it is necessary to remove a +good deal of glass, time may be saved by actually turning the +glass in a lathe. According to the direction given above for +grinding a tube into the neck of a bottle, very little glass need +be removed if the drawing down is well done, so that for this +purpose turning is often unnecessary.</font></p> +<p>If the taper of the stopper be small and it is permissible to +use a thick tube, or if a solid stopper only has to be provided, +or an old stopper quickly altered to a new form, turning is very +useful. The glass may be "chucked" in any suitable manner, and +run at a speed not exceeding 10 feet per minute. Prepare a +three-cornered file by mercury-hardening and by grinding the end +flat so as to form a cutting angle of about 80°, and use a +moderate amount of kerosene lubrication, i.e. enough to keep the +glass damp, but even this is not essential. Use the file as an +ordinary brass turning tool, and press much more lightly than for +metal turning. The glass will be found to scrape off quite +pleasantly.</p> +<p>By chucking glass tubes on wooden mandrells the ends may be +nicely turned in this manner ready for accurate closing by glass +plates.</p> +<p>The process of grinding also is made much more rapid — +at all events in the earlier stages — by chucking either +the stopper or the bottle and holding the other member in the +fingers, or in a wooden vice held in the hands. The finishing +touches are best given by hand.</p> +<p>I ought to say that I think a good deal of glass-grinding, as +practised in laboratories, might be advantageously replaced by +glass turning or filing and certainly will be by any one who will +give these methods a trial.</p> +<p>If one tube is to be ground into another, as in grinding a +retort into a receiver, the latter must be drawn down from a +larger piece, few beginners being able to widen a tube by the +method explained with sufficient ease and certainty. The other +operations are similar to the operations above described.</p> +<p>§ 43. Funnels often require to be ground to an angle of +60°. For this purpose it is well to keep a cast-iron cone, +tapering from nothing up to four inches in diameter. This may be +mounted on a lathe, and will be found of great use for grinding +out the inside of funnels. Care must be taken to work the funnel +backwards and forwards, or it will tend to grind so as to form +rings, which interfere with filtering. A rough polish may be +given on the lines explained in the next section.</p> +<p>§ 44. A rough polish may be easily given to a surface +which has been finished by washed flour emery, in the following +manner. Turn up a disc of soft wood on the lathe, and run it at +the highest wood-turning speed. Rub into the periphery a paste of +sifted powdered pumice stone and water.</p> +<p>Any fairly smooth ground glass surface may be more or less +polished by holding it for a moment against the revolving disc. +Exact means of polishing will be described later on. Meanwhile +this simple method will be found both quick and convenient, and +is often quite sufficient where transparency, rather than figure, +is required. I daresay a fine polish may be got on the same +lines, using putty powder or washed rouge (not jewellers' rouge, +which is too soft, but glass-polishers' rouge) to follow the +pumice powder, but I have not required to try this.</p> +<p>§ 45. It is sometimes required to give to ground glass +surfaces a temporary transparency. This is to be done by using a +film of oil of the same refractive index as the glass. Cornu has +employed a varnish consisting of a mixture of turpentine and oil +of cloves, but the yellow-brown colour of the latter is often a +disadvantage. It will be found that a mixture of nut oil and oil +of bitter almonds, or of bromo-napthalene and acetone, can be +made of only a faint yellow colour; and by exact adjustment of +the proportions will have the same refractive index for any ray +as crown glass (ordinary window glass).</p> +<p>Procure a sample of the glass and smash it up to small +fragments in an iron mortar. Sift out the fine dust and the +larger pieces; bits about as large as small beads — say +one-sixteenth inch every way — do very well. Boil the +sifted glass with strong commercial hydrochloric acid to remove +iron, wash with distilled water and a few drops of alcohol, dry +on blotting paper in the sun or otherwise. Put the dry glass into +a bottle or beaker, and begin by adding almond oil (or +bromo-napthalene), then add nut oil (or acetone) till the glass +practically disappears when examined by sodium light, or light of +any other wave-length, as may be required.</p> +<p>The adjustment of the mixture is a matter of great delicacy, +one drop too much of either constituent, in, say, 50 cubic +centimetres, makes all the difference. The final adjustment is +best accomplished by having two mixtures of the oils, one just +too rich in almond, the other in nut oil; by adding one or other +of these, the required mixture is soon obtained.</p> +<p><i>It is to be noted</i></p> +<p>(1) That adjustment is only perfect for light of one +wave-length.</p> +<p>(2) That adjustment is only perfect at one temperature.</p> +<p>On examining a bottle of rather larger fragments of glass +immersed in an adjusted mixture by ordinary daylight, a +peculiarly beautiful play of colours is seen.</p> +<p>Of course, if it is only desired to make ground glass fairly +transparent, these precautions are unnecessary, but it seemed +better to dispose of the matter once for all in this +connection.</p> +<p>M. Cornu's object was to make a varnish which would prevent +reflection from the back of a photographic plate on to the film. +I have had occasion to require to do the same when using a scale +made by cutting lines through a film of black varnish on a slip +of glass. This succeeded perfectly by making the varnish out of +Canada balsam stained with a black aniline dye.</p> +<p>Mr. Russell, Government Astronomer of New South Wales, finds +that the "halation" of star photographs can be prevented by +pouring over the back of the plate a film of collodion suitably +stained.</p> +<p><b><a name="Toc158108914" id="Toc158108914"><font face= +"Bookman Old Style" size="4">§ 46. Making Ground Glass. +—</font></a></b></p> +<p><font face="Bookman Old Style">This is easily done by rubbing +the surface of polished glass with a bit of cast iron and washed +"flour of emery." Of course, if the fineness of grain of the +surface is of importance, appropriate sizes of emery must be +employed. The iron may be replaced by a bit of glass cut with +transverse grooves to allow the emery to distribute itself, or +even by a bit of glass without such grooves, provided it does not +measure more than one or two inches each way. If great speed is +an object rather than the fineness of the surface, use a bit of +lead and coarse emery, say any that will pass a sieve with fifty +threads to the inch.</font></p> +<p>It may perhaps be mentioned here that it is a pity to throw +away emery which has been used between glass and glass. In the +chapter dealing with fine optical work the use of emery of +various grades of fineness will be treated, and the finer grades +can only be obtained (to my knowledge) from emery which has been +crushed in the process of glass or metal grinding, especially the +former. A large jam-pot covered with a cardboard lid does well as +a receptacle of washings.</p> +<p><b><a name="Toc158108915" id="Toc158108915"><font face= +"Bookman Old Style" size="4">§ 47. Glass-cutting. +—</font></a></b></p> +<p><font face="Bookman Old Style">This is an art about which more +can be learned in five minutes by watching it well practised than +by pages of written description. My advice to any one about to +commence the practice of the art would be to make friends with a +glazier and see it done. What follows is therefore on the +supposition that this advice has been followed.</font></p> +<p>After some experience of cutters made of especially hardened +steel, I believe better work can generally be got out of a +diamond, provided the cost is not an objection. It is economy to +pay a good price for a good diamond. As is well known, the +natural angle of the crystal makes the best point, and a person +buying a diamond should examine the stone by the help of a lens, +so as to see that this condition is fulfilled. The natural angle +is generally, if not always, bounded by curved edges, which have +a totally different appearance from the sharp edges of a +"splinter."</p> +<p>When a purchase is to be made, it is as well for the student +to take a bit of glass and a foot-rule with him, and to test the +diamond before it is taken away. When a good diamond has been +procured, begin by taking cuts on bits of clean window glass +until the proper angle at which to hold the tool is ascertained. +Never try to cut over a scratch, if you value your diamond, and +never press hard on the glass; a good cut is accompanied by an +unmistakable ringing sound quite different from the sound made +when the diamond is only scratching.</p> +<p>Perhaps the most important advice that can be given is, +<i>Never lend the diamond to anybody — under any +circumstances</i>.</p> +<p>The free use of a diamond is an art which the physicist will +do well to acquire, for quite a variety of apparatus may be made +out of glass strips, and the accuracy with which the glass breaks +along a good cut reduces such an operation as glass-box-making to +a question of accurate drawing.</p> +<p><b><a name="Toc158108916" id="Toc158108916"><font face= +"Bookman Old Style" size="4">§ 48. <i>Cementing</i>. +—</font></a></b></p> +<p><font face="Bookman Old Style">One of the matters which is +generally confused by too great a profusion of treatment is the +art of cementing glass to other substances.</font></p> +<p>The following methods will be found to work, subject to two +conditions:</p> +<p>(1) The glass must be clean;</p> +<p>(2) it must be hot enough to melt the cement.</p> +<p>For ordinary mending purposes when the glass does not require +to be placed in water (especially if hot) nothing is better than +that kind of glue which is generally called "diamond cement." +This may be easily made by dissolving the best procurable +isinglass in a mixture of 20 per cent water and 80 per cent +glacial acetic acid — the exact proportions are not of +consequence.</p> +<p>First, the isinglass is to be tightly packed into a bottle +with a wide neck, then add the water, and let the isinglass soak +it up. Afterwards pour in the acetic acid, and keep the mixture +near 100°C. for an hour or two on the water bath — or +rather in it. The total volume of acetic acid and water should +not be more than about half of the volume of isinglass when the +latter is pressed into the bottle as tightly as possible.</p> +<p>The proper consistency of the cement may be ascertained by +lifting a drop out of the bottle and allowing it to cool on a +sheet of glass. In ten minutes it ought not to be more than +slightly sticky, and the mass in the bottle, after standing a few +hours cold, should not be sticky at all, and should yield, +jelly-like, to the pressure of the finger to only a slight +degree. If the glue is too weak, more isinglass may be added +(without any preliminary soaking).</p> +<p>A person making the mixture for the first time almost always +gets it too weak. It is difficult to give exact proportions by +weight, as isinglass and gelatine (which may replace it) differ +greatly in quality. This cement is applied like glue, and will +cement nearly anything as well as glass. Of course, as much +cement as possible must be squeezed out of any joint where it is +employed. The addition of gums, as recommended in some books, is +unnecessary.</p> +<p>Ordinary glue will serve perfectly for cementing glass to +wood.</p> +<p>"Chipped glass" ware is, I understand, made by painting clean +glass with glue. As the glue dries and breaks by contraction, it +chips off the surface of the glass. I have never seen this done. +In nearly all cases where alcohol is not to be employed very +strong joints may be made by shellac. Orange shellac is stronger +than the "bleached" variety.</p> +<p>A <i>sine qua non</i> is that the glass be hot enough to melt +the shellac. The best way is to heat the glass surfaces and rub +on the shellac from a bit of flake; the glass should not be so +hot as to discolour the shellac appreciably, or its valuable +properties will be partly destroyed. Both glass surfaces being +thus prepared, and the shellac being quite fluid on both, they +may be brought together and clamped tightly together till cool. +Shellac that has been overheated, or dissolved in alcohol, or +bleached, is of little use as compared with the pale orange flaky +product. Dark flakes have probably been overheated during the +preliminary refining.</p> +<p>For many purposes a cement is required capable of resisting +carbon bisulphide. This is easily made by adding a little treacle +(say 20 per cent) to ordinary glue. Since the mixture of glue and +treacle does not keep, i.e. it cannot be satisfactorily melted up +again after once it has set, no more should be made up than will +be wanted at the time. If the glue be thick, glass boxes for +carbon disulphide may be easily put together, even though the +edges of the glass strips are not quite smooth, for, unlike most +cements, this mixture remains tough, and is fairly strong in +itself.</p> +<p>I have found by experiment that most fixed and, to a less +degree, essential oils have little or no solvent action on +shellac, and I suspect that the same remark applies to the +treacle-glue mixture, but I have not tried. Turpenes act on +shellac slightly, but mineral oils apparently not at all. The +tests on which these statements are based were continued for +about two years, during which time kerosene and mineral oils had +no observable effect on shellac-fastened galvanometer +mirrors.</p> +<p><b><a name="Toc158108917" id="Toc158108917"><font face= +"Bookman Old Style" size="4">§ 49. Fusing Electrodes into +Glass. —</font></a></b></p> +<p><font face="Bookman Old Style">This art has greatly improved +since the introduction of the incandescent lamp; however, up to +the present, platinum seems to remain the only substance capable +of giving a certainly air-tight result. I have not tried the +aluminium-alumina method.</font></p> +<p>Many years ago it was the fashion to surround the platinum +wire with a drop of white enamel glass in order to cause better +adhesion between it and the ordinary glass. <i>[Footnote:</i> +Hittorf and Geissler (Pogg. Ann. 1864, § 35; English +translation, Phys. Soc. London, p. 138) found that it was +impossible to make air-tight joints between platinum and hard +potash glass, but that soft lead glass could be used with success +as a cement.<i>]</i> However, in the case of flint glass, if one +may judge from incandescent lamps, this is not essential — +a fact which entirely coincides with my own experience.</p> +<p>On the other hand, when sealing electrodes into German glass I +have often used a drop of enamel with perfect results, though +this is not always done in Germany. In all cases, however, in +which electrodes have to be sealed in — especially when +they are liable to heat — I recommend flint glass, and in +this have the support of Mr. Rain (<i>The Incandescent Lamp and +its Manufacture</i>, p. 131). The exact details for the +preparation of eudiometer tubes are given by Faraday (<i>Chemical +Manipulation</i>, § 1200).</p> +<p>In view of what has preceded, however, I will content myself +with the following notes. Make the hole through which the wire is +to protrude only slightly larger than the wire itself, and be +sure that the latter is clean. Allow the glass to cool +sufficiently not to stick to the wire when the latter is pushed +in. Be sure that, on heating, the glass does not get reduced, and +that it flows up to the wire all round; pull and push the wire a +little with a pair of pincers, to ensure this.</p> +<p>It is not a bad plan to get the glass exceedingly fluid round +the wire — even if the lump has to be blown out a little +afterwards--as it cools. The seal should finally be well annealed +in asbestos, but first by gradually moving it into the hot air in +front of the flame.</p> +<p>It was observed by Professor J. J. Thomson and the author some +years ago (<i>Proc</i>. Roy. Soc. 40. 331. 1886) that when very +violent discharges are taken through lightly sealed-in electrodes +in lead-glass tubes — say from a large battery of Leyden +jars — gas appears to be carried into the tube over and +above that naturally given off by the platinum, and this without +there being any apparent want of perfection in the seal. This +observation has since been confirmed by others. Consequently in +experiments on violent discharges in vacuo where certainty is +required as to the exclusion of air, the seals should be +protected by a guard tube or cap containing mercury; this must, +of course, be put in hot and clean, on hot and clean glass, and +in special cases should be boiled in situ.</p> +<p>A well-known German physicist (Warburg, I think) recommends +putting the seals under water, but I cannot think that this is a +good plan, for if air can get in, why not water? which has its +surface tension in its favour. The same reasoning prevents my +recommending a layer of sulphuric acid above the mercury-a method +used for securing air-tightness in "mercury joints" by Mr. +Gimingham, Proc. R. S. 1874.</p> +<p>Further protection may be attained for many purposes by +coating the platinum wire with a sheath of glass, say half an +inch long, fused to the platinum wire to a depth of one-twentieth +of an inch all round.</p> +<p>In some cases the electrodes must be expected to get very hot, +for instance, when it is desired to platinise mirrors by the +device of Professor Wright of Yale. In this and similar cases I +have met with great success by using "barometer" tubes of about +one-twelfth of an inch bore, and with walls, say, one-tenth of an +inch thick. <i>[Footnote:</i> "Barometer" tube is merely very +thick-walled glass tubing, and makes particularly bad barometers, +which are sold as weather glasses.<i>]</i></p> +<p>This tube is drawn down to a long point — say an inch +long by one-eighth of an inch external diameter, and the wire is +fused in for a length, say, of three-quarters of an inch, but +only in the narrow drawn-down part of the tube. At different +times I have tried four such seals, and though the electrodes +were red hot for hours, I have never had an accident — of +course they were well annealed.</p> +<p>Fig. <img src="images/Image62.gif" alt="images/Image62.gif" +width="439" height="52"> 37.</p> +<p>For directions as to the making of high vacuum tubes, see the +section dealing with that matter.</p> +<p>§ 50. As economy of platinum is often of importance, the +following little art will save money and trouble. Platinum is +easily caused to join most firmly to copper — with which, I +presume, it alloys — by the following method. Hold the +platinum wire against the copper wire, end to end, at the tip of +the reducing flame of a typical blowpipe — or anywhere +— preferably in the "reducing" part of the oxygas flame; in +a moment the metals will fuse together at the point of contact, +when they may be withdrawn.</p> +<p>Such a joint is very strong and wholly satisfactory, much +better than a soldered joint. If the work is not carried out +successfully so that a considerable drop of copper-platinum alloy +accumulates, cut it off and start again. The essence of success +is speed, so that the copper does not get "burned." If any +considerable quantity of alloy is formed it dissolves the copper, +and weakens it, so that we have first the platinum wire, then a +bead of alloy, and then a copper wire fused into the bead, but so +thin just outside the latter that the joint has no mechanical +strength.</p> +<p><b><a name="Toc158108918" id="Toc158108918"><font face= +"Bookman Old Style" size="4">§ 51. The Art of making +Air-light Joints. —</font></a></b></p> +<p><font face="Bookman Old Style">Lamp-manufacturers and others +have long since learned that when glass is in question not only +are fused joints made as easily as others, but that they afford +the only reliable form of joint. An experimenter who uses flint +glass, has a little experience, an oxygas blow-pipe and a blowing +apparatus, will prefer to make his joints in this way, simply +from the ease with which it may be done. When it comes to making +a tight joint between glass and other substances the problem is +by no means so easy. Thus Mr. Griffiths (Phil. Trans. 1893, p. +380) failed to make air-tight joints by cementing glass into +steel tubes, using hard shellac, and the tubes fitting closely. +These joints were satisfactory at first, but did not last; the +length of the joint is not stated. The difficulty was finally got +over by soldering very narrow platinum tubes into the steel, and +fusing the former into the glass.</font></p> +<p>Mr. Griffiths has since used an alloy with success as a +cement, but I cannot discover what it is made from. Many years +ago Professor Hittorf prepared good high vacuum tubes by plugging +the ends of glass tubes with sealing wax merely, though in all +cases the spaces to be filled with wax were long and narrow +(Hittorf, Pogg. Ann. 1869, § 5, English translation, Phys. +Soc. p. 113). Again, Regnault habitually used brass ferules, and +cemented glass into them by means of his mastic, which can still +be procured at a low rate from his instrument-makers (Golan, +Paris). Lenard also, in his investigations on Cathode Rays (Wied. +Ann., vol. li. p. 224), made use of sealing wax covered with +marine glue.</p> +<p>Surely in face of these facts we must admit that cement joints +can be made with fair success. I do not know the composition of +M. Regnault's mastic, but Faraday (Manipulations, § 1123) +gives the following receipt for a cement for joining ferules to +retorts, etc. —</p> +<div style="margin-left: 4em"> +<p>Resin 5 parts</p> +<p>Beeswax 1 part</p> +<p>Red ochre or Venetian red,</p> +<p>finely powdered and sifted 1 part</p> +</div> +<p>I believe this to be substantially the same as Regnault's +mastic, though I have never analysed the latter.</p> +<p>For chemical work the possibility of evolution of gas from +such a cement must be taken into account, and I should certainly +not trust it for this reason in vacuum tube work, where the +purity of the confined gas could come in question. Otherwise it +is an excellent cement, and does not in my experience tend to +crack away from glass to the same extent as paraffin or pure +shellac.</p> +<p>This cracking away from glass, by the way, is probably an +effect of difference in rate of expansion between the glass and +cement which probably always exists, and, if the cement be not +sufficiently viscous, must, beyond certain temperature limits, +either produce cracks or cause separation. Professor Wright of +Yale has used a hard mineral pitch as a cement in vacuum work +with success.</p> +<p>My attention has been directed to a fusible metal cement +containing mercury, and made according to the following receipt, +given by Mr. S. G. Rawson, Journal of the Society of Chemical +Industry, vol. ix. (1890), P. 150:—</p> +<div style="margin-left: 4em"> +<p>Bismuth 40 per cent</p> +<p>Lead 25 per cent</p> +<p>Tin 10 per cent</p> +<p>Cadmium 10 per cent</p> +<p>Mercury 15 per cent</p> +</div> +<p>This is practically one form of Rose's fusible metal with 15 +per cent mercury added. It takes nearly an hour to set +completely, and the apparatus must be clean and warm before it is +applied.</p> +<p>As the result of several trials by myself and friends, I am +afraid I must dissent from the claim of the author that such a +cement will make a really air-tight joint between glass tubes. +Indeed, the appearance of the surface as viewed through the glass +is not such as to give any confidence, no matter what care may +have been exercised in performing all the operations and cleaning +the glass; besides which the cement is rigid when cold, and the +expansion difficulty comes in.</p> +<p>On the other hand, if extreme air-tightness is not an object, +the cement is strong and easily applied, and has many uses. I +have an idea that if the joints were covered with a layer of soft +wax, the result would be satisfactory in so far as air-tightness +is concerned.</p> +<p>This anticipation has since been verified.</p> +<p>In many cases one can resort to the device already mentioned +of enclosing a rubber or tape-wrapped joint between two tubes in +a bath of mercury, but in this case the glass must be clean and +hot and the mercury also warm, dry, and pure when the joint is +put together, otherwise an appreciable air film is left against +the glass, and this may creep into the joint.</p> +<p>Perhaps the easiest way of making such a joint is to use an +outer tube of thin clean glass, and bore a narrow hole into it +from one side to admit the mercury; if the mercury is to be +heated in vacuo, it is better to seal on a side joint. It is +always better, if possible, to boil the mercury in situ, which +involves making the wrapping of asbestos, but, after all, we come +back to the position I began by taking up, viz. that the easiest +and most reliable method is by fusion of the glass — all +the rest are unsuitable for work of real precision.</p> +<p>I should be ungrateful, however, were I not to devote a few +lines to the great convenience and merit of so-called "centering +cement." This substance has two or three very valuable +properties. It is very tough and strong in itself, and it remains +plastic on cooling for some time before it really sets. If for +any reason a small tube has to be cemented into a larger one, +which is a good deal larger, so that an appreciable mass of +cement is necessary, and particularly if the joint requires to +have great mechanical strength, this cement is invaluable. I have +even used a plug of it instead of a cork for making the joint +between a gas delivery tube and a calcium chloride tower. (Why +are these affairs made with such abominable tubulures?)</p> +<p>The joint in question has never allowed the tube to sag though +it projects horizontally to a distance of 6 inches, and has had +to withstand nearly two years of Sydney temperature. The cement +consists of a mixture of shellac and 10 per cent of oil of +cassia.</p> +<p>The shellac is first melted in an iron ladle, and the oil of +cassia quickly added and stirred in, to an extent of about 10 per +cent, but the exact proportions are not of importance. Great care +must be taken not to overheat the shellac.</p> +<p><b><a name="Toc158108919" id="Toc158108919"><font face= +"Bookman Old Style" size="4">APPENDIX TO CHAPTER +I</font></a></b></p> +<p><a name="Toc158108920" id="Toc158108920">ON THE PREPARATION OF +VACUUM TUBES FOR THE PRODUCTION OF PROFESSOR ROENTGEN'S +RADIATION</a></p> +<p><i><font face="Bookman Old Style">[Footnote:</font> Written in +May 1896.]</i></p> +<p>WHEN Professor Roentgen's discovery was first announced at the +end of 1895 much difficulty was experienced in obtaining +radiation of the requisite intensity for the repetition of his +experiments. The following notes on the production of vacuum +tubes of the required quality may therefore be of use to those +who desire to prepare their own apparatus. It appears that flint +glass is much more opaque to Roentgen's radiation than soda +glass, and consequently the vacuum tubes require to be prepared +from the latter material.</p> +<p>Fig. <img src="images/Image63.gif" alt="images/Image63.gif" +width="412" height="262">39.</p> +<p>A form of vacuum tube which has proved very successful in the +author's hands is sketched in Fig. 38. It is most easily +constructed as follows. A bit of tubing about 2 centimetres +diameter, 15 centimetres long, and 1.5 millimetre wall thickness, +is drawn down to a point. The larger bulb, about 5 centimetres in +diameter, is blown at one end of this tube. The thinner the bulb +the better, provided that it does not collapse under atmospheric +pressure. A very good idea of a proper thickness may be obtained +from the statement that about 4 centimetres length of the tubing +should be blown out to form the bulb. This would give a bulb of +about the thickness of an ordinary fractionating bulb. Before +going any further it is as well to test the bulb by tapping on +the table and by exhausting it by means of an ordinary +water-velocity pump.</p> +<p>The side tube is next prepared out of narrower tubing, and is +provided with a smaller bulb, a blowing-out tube, and a terminal, +to be made as will be described. This side tube is next fused on +to the main tube, special care being taken about the annealing, +and the cathode terminal is then sealed into the main tube. After +using clean glass it is in general only necessary to rinse the +tube out with clean alcohol, after which it may be dried and +exhausted.</p> +<p>The success of the operation will depend primarily on the +attention given to the preparation and sealing-in of the +electrode facing the large bulb.</p> +<p><i>Preparation of Terminals</i>. — Some platinum wire of +about No. 26 B.W.G. — the exact size is unimportant — +must be provided, also some sheet aluminium about 1 millimetre +thick, some white enamel cement glass, and a "cane" of +flint-glass tube of a few millimetres bore.</p> +<p>The electrodes are prepared by cutting discs of aluminium of +from 1 to 1.5 centimetres diameter. The discs of aluminium are +bored in the centre, so as to admit the "stems" which are made +of aluminium wire of about 1 millimetre diameter. The stems are +then riveted into the discs. The "stems" are about I centimetre +long, and are drilled to a depth of about 3 millimetres, the +drill used being about double the diameter of the platinum wire +to be used for making the connections. The faces of the +electrodes — i.e. the free surfaces of the aluminium discs +— are then hammered flat and brought to a burnished surface +by being placed on a bit of highly polished steel and struck by a +"set" provided with a hole to allow of the "stem" escaping +damage. The operation will be obvious after a reference to Figs. +39 and 40; it is referred to again on page 96.</p> +<p>The platinum wires may be most conveniently attached by +melting one end of the piece of platinum wire in the oxygas +blow-pipe till it forms a bead just large enough to pass into the +hole drilled up the stem of the electrode. The junction between +the stein and the platinum wire is then made permanent by +squeezing the aluminium down upon the platinum wire with the help +of a pair of pliers. It is also possible to fuse the aluminium +round the platinum, but as I have had several breakages of such +joints, I prefer the mechanical connection described.</p> +<p><img src="images/Image64.gif" alt="images/Image64.gif" width= +"245" height="159">Fig. 39. — Sets for striking aluminium +electrodes</p> +<p>Fig. <img src="images/Image65.gif" alt="images/Image65.gif" +width="287" height="139"> 40.-</p> +<ol type="i"> +<li>Aluminium electrode.</li> +<li>Aluminium electrode connected to platinum wire.</li> +<li>Aluminium electrode connected to platinum wire and protected +by glass.</li> +<li>Detail of fastening platinum wire.</li> +</ol> +<p>The stem and platinum wire may now be protected by covering +them with a little flint glass. For this purpose the flint-glass +tube is pulled down till it will just slip over the stem and +wire, and is cut off so as to leave about half a centimetre of +platinum wire projecting. The flint-glass tube is then fused down +upon the platinum wire, care being taken to avoid the presence of +air bubbles. At the close of the operation a single drop of white +enamel glass is fused round the platinum wire at a high +temperature, so as to make a good joint with the protecting +flint-glass tube.</p> +<p>The negative electrode being nearly as large as the main tube, +it must be introduced before the latter is drawn down for +sealing. After drawing down the main tube in the usual manner, +taking care not to make it less than a millimetre in +wall-thickness, it is cut off so as to leave a hole not quite big +enough for the enamel drop to pass through. By heating and +opening, the aperture is got just large enough to allow the +enamel drop to pass into it, and when this is the case the joint +is sealed, pulled, and blown out until the electrode occupies the +right position — viz. in the centre of the tube and with +its face normal to the axis of the tube.</p> +<p>The glass walls near the negative electrode must not be less +than a millimetre thick, and may be rather more with advantage, +the glass must be even, and the joint between the flint glass and +the soda glass, or between the wire and the soda glass, must be +wholly through the enamel. The "seal" must be well annealed. It +will be found that the sealing-in process is much easier when the +stem of the electrode is short and when the glass coating is not +too heavy. Half a millimetre of glass thickness round the stein +is quite sufficient.</p> +<p>The diagram, of the tube shows that the main tube has been +expanded round the edges of the cathode. This is to reduce the +heating consequent on the projection of cathode rays from the +edges of the disc against the glass tube.</p> +<p>The anode is inserted into its bulb in a quite similar manner. +If desired it may be made considerably smaller, and does not need +the careful adjustment requisite in sealing-in the cathode, nor +does the glass near the entry wire require to be so thick.</p> +<p>More intense effects are often got by making the cathode +slightly concave, but in this case the risk of melting the thin +glass is considerably increased. No doubt, Bohemian glass might +be used throughout instead of soft soda glass, and this would not +melt so easily; the difficulties of manipulating the glass are, +however, more pronounced.</p> +<p>It will be shown directly that the best Roentgen effects are +got with a high vacuum, and it is for this reason that the glass +near the cathode seal requires to be strong. The potential right +up to the cathode is strongly positive inside the tube, and this +causes the glass to be exposed to a strong electric stress in the +neighbourhood of the seal.</p> +<p>Although the GLASS-BLOWING involved in the making of a +so-called focus tube is rather more difficult than in the case +just described, there is no reason why such a difficulty should +not be overcome; I will therefore explain how a focus tube may be +made.</p> +<p>Fig. <img src="images/Image66.gif" alt="images/Image66.gif" +width="403" height="386">41.</p> +<p>A bulb about 3 inches in diameter is blown from a bit of tube +of a little more than 1 inch diameter. Unless the walls of the +tube are about one-eighth of an inch in thickness, this will +involve a preliminary thickening up of the glass. This is not +difficult if care be taken to avoid making the glass too hot. The +larger gas jet described in connection with the +soda-GLASS-BLOWING table must be employed. In blowing a bulb of +this size it must not be forgotten that draughts exercise a very +injurious influence by causing the glass to cool unequally; this +leads to bulbs of irregular shape.</p> +<p>In the method of construction shown in Fig. 41, the anode is +put in first. This anode simply consists of a square bit of +platinum or platinum-iridium foil, measuring about 0.75 inch by 1 +inch, and riveted on to a bent aluminium wire stem.</p> +<p>As soon as the anode is fused in, and while the glass is still +hot, the side tube is put on. The whole of the anode end is then +carefully annealed. When the annealing is finished the side tube +is bent as shown to serve as a handle when the time comes to +mount the cathode. Before placing the cathode in position, and +while the main tube is still wide open, the anode is adjusted by +means of a tool thrust in through this open end. This is +necessary in view of the fact that the platinum foil is +occasionally bent during the operation of forcing the anode into +the bulb.</p> +<p>The cathode is a portion of a spherical surface of polished +aluminium, a mode of preparing which will be given directly. The +cathode having been placed inside the bulb, the wide glass tube +is carefully drawn down and cut off at such a point that when the +cathode is in position its centre of curvature will lie slightly +in front of the anode plate. For instance, if the radius of +curvature of the cathode be 1.5 inches, the centre of curvature +may lie something like an eighth of an inch or less in front of +the anode.</p> +<p>The cathode as shown in Fig. 41 is rather smaller than is +advantageous. To make it much larger than is shown, however, the +opening into the bulb would require to be considerably widened, +and though this is not really a difficult operation, still it +requires more practice than my readers are likely to have had. +The difficulty is not so much in widening out the entry as in +closing it down again neatly.</p> +<p>Now as to making the anode. A disc of aluminium is cut from a +sheet which must not be too thick — one twenty-fifth of an +inch is quite thick enough. This disc is bored at the centre to +allow of the stem being riveted in position. The disc is then +annealed in the Bunsen flame and the stem riveted on.</p> +<p>The curvature is best got by striking between steel dies (see +Figs. 39 and 40). Two bits of tool steel are softened and turned +on the lathe, one convex and the other concave. The concave die +has a small hole drilled up the centre to admit the stem. The +desired radius of curvature is easily attained by cutting out +templates from sheet zinc and using them to gauge the turning. +The two dies are slightly ground together on the lathe with emery +and oil and are then polished, or rather the convex die is +polished — the other one does not matter. The polishing is +most easily done by using graded emery and oil and polishing with +a rag. The method of grading emery will be described in the +chapter on glass-grinding.</p> +<p>The aluminium disc is now struck between the dies by means of +a hammer. If the radius of curvature is anything more than one +inch and the disc not more than one inch in diameter the cathode +can be struck at once from the flat as described. For very deep +curves no doubt it will be better to make an intermediate pair of +dies and to re-anneal the aluminium after the first striking.</p> +<p>When the tube is successfully prepared so far as the +glassblowing goes it may be rinsed with strong pure alcohol both +inside and out, and dried. The straight part of the side tube is +then constricted ready for fusing off and the whole affair is +placed on the vacuum pump.</p> +<p>In spite of the great improvements made during recent years in +the construction of so-called Geissler vacuum pumps — i.e. +pumps in which a Torricellian vacuum is continually reproduced +— I am of opinion that Sprengel pumps are, on the whole, +more convenient for exhausting Crooke's tubes. A full discussion +of the subject of vacuum pumps will be found in a work by Mr. G. +S. Ram (The Incandescent Lamp and its Manufacture), published by +the Electrician Publishing Company, and it is not my intention to +deal with the matter here; the simplest kind of Sprengel pump +will be found quite adequate for our purpose, provided that it is +well made.</p> +<p>Fig. 42 is intended to represent a modification of a pump +based on the model manufactured by Hicks of Hatton Garden, and +arranged to suit the amateur glass-blower. The only point of +importance is the construction of the head of the fall tube, of +which a separate and enlarged diagram is given. The fall tubes +may have an internal diameter up to 2 mm. (two millimetres) and +an effective length of 120 cm.</p> +<p>Free use is made of rubber tube connections in the part of the +pump exposed to the passage of mercury. The rubber employed +should be black and of the highest quality, having the walls +strengthened by a layer of canvas. If such tube cannot be easily +obtained, a very good substitute may be made by placing a bit of +ordinary black tube inside another and rather larger bit and +binding the outer tube with tape or ribbon. In any case the +tubing which comes in contact with the mercury should be boiled +in strong caustic potash or soda solution for at least ten +minutes to get rid of free sulphur, which fouls the mercury +directly it comes in contact with it. The tubing is well washed, +rinsed with alcohol, and carefully dried.</p> +<p><img src="images/Image67.gif" alt="images/Image67.gif" width= +"152" height="545">Fig. 42.</p> +<p>The diagram represents what is practically a system of three +Sprengel pumps, though they are all fed from the same mercury +reservoir and run down into the same mercury receiver. It is much +easier to make three pumps, each with separate pinch cocks to +regulate the mercury supply, than it is to make three jets, each +delivering exactly the proper stream of mercury to three fall +tubes.</p> +<p>Sprengel pumps only work at their highest efficiency when the +mercury supply is carefully regulated to suit the peculiarities +of each fall tube, and this is quite easily done in the model +figured. Since on starting the pump the rubber connections have +to stand a considerable pressure, the ends of the tubes must be +somewhat corrugated to enable the rubber to be firmly wired on to +them. The best binding wire is the purest Swedish iron wire, +previously annealed in a Bunsen gas flame.</p> +<p>The wire must never be twisted down on the bare rubber, but +must always be separated from it by a tape binding. By taking +this precaution the wire maybe twisted very much more tightly +than is otherwise possible without cutting the rubber.</p> +<p>The only difficulty in making such a pump as is described lies +in the bending of the heads of the fall tubes. This bending must +be done with perfect regularity and neatness, otherwise the drops +of mercury will not break regularly, or will break just inside +the top of the fall tube, and so obstruct its entrance that at +high vacua no air can get into the tube at all.</p> +<p>The connections at the head of the fall tubes must also be +well put on and the joints blown out so that the mercury in +dropping over the head is not interfered with by the upper +surface of the tube. However, a glance at the enlarged diagram +will show what is to be aimed at better than any amount of +description. In preparing the fall tubes it is generally +necessary to join at least two "canes" together. The joint must +be arranged to occur either in the tube leading the mercury to +the head of the fall, or in that part of the fall tube which +remains full of mercury when the highest vacuum is attained. On +no account must the joint be made at the fall itself (at least +not by an amateur), nor in that part of the fall tube where the +mercury falls freely, particularly at its lower end, where the +drops fall on the head of the column of mercury.</p> +<p>When a high vacuum is attained the efficacy of the pump +depends chiefly on the way in which the drops fall on the head of +the column. If the fall is too long the drops are apt to break up +and allow the small bubble of air to escape up the tube, also any +irregularity or dirt in the tube at this point makes it more easy +for the bubbles of air to escape to the surface of the +mercury.</p> +<p>Any pump in which the supply of mercury to the fall tube can +be regulated nicely will pump well until the lowest available +pressures are being attained; a good pump will then continue to +hold the air bubbles, while a bad one will allow them to slip +back <i>[Footnote:</i> For special methods of avoiding this +difficulty see Mr. Ram's book.<i>]</i> …</p> +<p>Though three fall tubes are recommended, it must not be +supposed that the pump will produce a Crooke's vacuum three times +more rapidly than one fall tube. Until the mercury commences to +hammer in the pump the three tubes will pump approximately three +times faster than one tube, but as soon as the major portion of +the air collected begins to come from the layer condensed on the +glass surface of the tube to be exhausted and from the +electrodes, the rate at which exhaustion will go on no longer +depends entirely on the pump.</p> +<p>In order that bubbles of air may not slip back up the fall +tube it is generally desirable to allow the mercury to fall +pretty briskly, and in this case the capacity of the pump to take +air is generally far in excess of the air supply. One advantage +of having more than one fall tube is that it often happens that a +fall tube gets soiled during the process of exhaustion and no +longer works up to its best performance. Out of three fall tubes, +however, one is pretty sure to be working well, and as soon as +the mercury begins to hammer in the tubes the supply may be shut +off from the two falls which are working least +satisfactorily.</p> +<p>Thus we are enabled to pump rapidly till a high degree of +exhaustion is attained, having practically three pumps instead of +one, whereas when the final stages are reached, and three pumps +are only a drawback in that they increase the mercury flow, the +apparatus is capable of instant modification to meet the new +conditions.</p> +<p>The thistle funnels at the head of the fall tubes are made +simply by blowing bulbs and then blowing the heads of the bulbs +into wider ones, and finally blowing the heads of the wider bulbs +off by vigorous blowing. The stoppers are ground in on the lathe +before the tubes are attached to the fall tubes. The stoppers +require to be at least half an inch long where they fit the +necks, and must be really well ground in. The stoppers must first +be turned up nicely and the necks ground out by a copper or iron +cone and emery. The stoppers are rotated on a lathe at quite a +slow speed, say 30 or 40 feet per minute, and the necks are held +against them, as described in the section dealing with this art. +The stoppers must in this case be finished with "two seconds" +emery, and lastly with pumice dust and water (see chapter on +glass-grinding).</p> +<p>Unless the stoppers fit exceedingly well trouble will arise +from the mercury (which is poured into the thistle heads to form +a seal) being forced downwards into the pump by atmospheric +pressure.</p> +<p>The joints between the three fall tubes and the single exhaust +main are easily made when the tubes are finally mounted, the +hooked nozzle of the oxygas blow-pipe being expressly made for +such work.</p> +<p>It is, on the whole, advisable to make the pump of flint +glass, or at all events the air-trap tube and the fall tubes. A +brush flame from the larger gas tube of the single blowpipe table +is most suitable for the work of bending the tubes. The jointing +of the long, narrow bore fall tubes is best accomplished by the +oxygas flame, for in this way the minimum of irregularity is +produced; the blowing tubes will of course be required for the +job, and the narrow tubes must be well cleaned to begin with.</p> +<p>The air trap is an important though simple part of the pump. +Its shoulder or fall should stand rather higher than the +shoulders of the fall tubes, so that the mercury may run in a +thin stream through a good Torricellian vacuum before it passes +down to the fall tubes. This is easily attained by regulating the +main mercury supply at the pinch cock situated between the tube +from the upper reservoir and the air-trap tube, the other cocks +being almost wide open.</p> +<p>It might be thought that the mercury would tend to pick up air +in passing through the rubber connections to the fall tubes, but +I have not found this to be the case in practice. There is, of +course, no difficulty in eliminating the rubber connections +between the fall tubes and the mercury supply from the air trap, +but it impresses a greater rigidity on the structure and, as I +say, is not in general necessary. It must not be forgotten that +the mercury always exercises considerable pressure on the rubber +joints, and so there is little tendency for gas to come out of +the rubber.</p> +<p>The thistle funnels at the head of the fall tubes provide a +simple and excellent means of cleaning the fall tubes. For this +purpose some "pure" sulphuric acid which has been boiled with +pure ammonium sulphate is placed in each thistle funnel, and when +the fall tube is dirty the connection to the mercury supply is +cut off at the pinch cock so as to leave the tube between this +entry and the head of the fall tube quite full of mercury, and +the sulphuric acid is allowed to run down the fall tube by +raising the stopper. The fall tube should be allowed to stand +full of acid for an hour or so, after which it will be found to +be fairly clean.</p> +<p>Of course the mercury reservoir thus obtains a layer of acid +above the mercury, and as it is better not to run the risk of any +acid getting into the pump except in the fall tubes, the +reservoir is best emptied from the bottom, by a syphon, if a +suitable vessel cannot be procured, so that clean mercury only is +withdrawn.</p> +<p>The phosphorus pentoxide tube is best made as shown simply +from a bit of wide tube, with two side connections fused to the +rest of the pump. It is no more trouble to cut the tube and fuse +it up again when the drying material is renewed than to adjust +the drying tube to two fixed stoppers, which is the alternative. +The practice here recommended is rendered possible only by the +oxygas blow-pipe with hooked nozzle. The connection between the +pump and tube to be exhausted is made simply by a short bit of +rubber tube immersed in mercury.</p> +<p>The phosphorus pentoxide should be pure, or rather free from +phosphorus and lower oxides; unless this be the case, the vapour +arising from it is apt to soil the mercury in the pump. The +phosphorus pentoxide is purified by distilling with oxygen over +red-hot platinum black; if this cannot be done, the pentoxide +should at least be strongly heated in a tube, in a current of dry +air or oxygen, before it is placed in the drying tube.</p> +<p>The mercury used for the pump must be scrupulously clean. It +does not, however, require to have been distilled in vacuo. It is +sufficient to purify it by allowing it to fall in a fine spray +into a large or rather tall jar of 25 per cent nitric acid and 75 +per cent water. The mercury is then to be washed and dried by +heating to, say, 110° C. in a porcelain dish.</p> +<p><i>Exhausting a Roentgen Tube.</i> —</p> +<p>With a pump such as has been described there is seldom any +advantage in fusing an extra connection to the vacuum tube so as +to allow of a preliminary exhaustion by means of a water pump. +About half an hour's pumping may possibly be saved by making use +of a water pump.</p> +<p>The tube to be exhausted is washed and dried by careful +heating over a Bunsen burner and by the passage of a current of +air. The exhausting tube is then drawn down preparatory to +sealing off, and the apparatus placed upon the pump. It is best +held in position by a wooden clamp supported by a long retort +stand.</p> +<p>Exhaustion may proceed till the mercury in the fall tubes +commences to hammer. At this point the tube must be carefully +heated by a Bunsen flame, the temperature being brought up to, +say, 400° C. The heating may be continued intermittently till +little or no effect due to the heating is discernible at the +pump. When this stage is reached, or even before, the electrodes +may be connected up to the coil and a discharge sent through the +tube.</p> +<p>Care must be taken to stop the discharge as soon as a purple +glow begins to appear, because when this happens, the resistance +of the tube is very low, the electrodes get very hot, and may +easily get damaged by a powerful discharge, and the platinum of +the anode (if a focus tube is in question) begins to be distilled +on to the glass. The heating and sparking are to be continued +till the resistance of the tube sharply increases. This is tested +by always having a spark gap, conveniently formed by the coil +terminals, in parallel with the tube. If the terminals are +points, it is convenient to set them at about one quarter of an +inch distance apart.</p> +<p>As soon as sparks begin to pass between the terminals of the +spark gap it becomes necessary to watch the process of exhaustion +very carefully. In the first place, stop the pump, but let the +coil run, and note whether the sparks continue to flow over the +terminals. If the glass and electrodes are getting gas free, the +discharge will continue to pass by the spark gap, but if gas is +still being freely given off, then in perhaps three minutes the +discharge will return to the tube, and pumping must be +recommenced. The Roentgen effect only begins to appear when the +tube has got to so high a state of exhaustion that the resistance +increases rapidly.</p> +<p>By pumping and sparking, the resistance of the tube may be +gradually raised till the spark would rather jump over 2 inches +of air than go through the tube. When this state is attained the +Roentgen effect as tested by a screen of calcium tungstate should +be very brilliant. No conclusion as to the equivalent resistance +of the tube can be arrived at so long as the discharge is kept +going continually. When the spark would rather go over an inch of +air in the spark gap than through the tube the pumping and +sparking may be interrupted and the tube allowed to rest for, +say, five minutes. It will generally be found that the equivalent +resistance of the tube will be largely increased by this period +of quiescence. It may even be found that the spark will now +prefer to pass an air gap 3 inches long.</p> +<p>In any case the sparking should now be continued, the pump +being at rest, and the variations of tube resistance watched by +adjusting the spark gap. If the resistance falls below an +equivalent of 2 inches of air in the gap the pump must be brought +into action again and continued until the resistance as thus +estimated remains fairly constant for, say, ten minutes. When +this occurs the narrow neck of the exhaust tube may be strongly +heated till the blow-pipe flame begins to show traces of sodium +light. The flame must then be withdrawn and the discharge again +tested. This is necessary because it occasionally happens that +gas is given off during the heating of the neck to the +neighbourhood of its fusion temperature.</p> +<p>If all is right the neck may now be fused entirely off and the +tube is finished. Tubes of the focus pattern with large platinum +anodes are in general (in my experience) much more difficult to +exhaust than tubes of the kind first described. This is possibly +to be attributed mainly to the gas given off by the platinum, but +is also, no doubt, due to the tubes being much larger and +exposing a larger glass surface. The type of tube described first +generally takes about two hours to exhaust by a pump made as +explained, while a "focus" tube has taken as long as nine hours, +eight of which have been consumed after the tube was exhausted to +the hammering point.</p> +<p>The pressure at which the maximum heating of the anode by the +cathode rays occurs is a good deal higher than that at which the +maximum Roentgen effect is produced. There is little doubt that +the Roentgen radiation changes in nature to some extent as the +vacuum improves either as a primary or secondary effect. It is +therefore of some importance to test the tube for the purpose for +which it is to be used during the actual exhaustion. It has been +stated, for instance, that the relative penetrability of bone and +flesh to Roentgen radiation attains a maximum difference at a +certain pressure; this is very likely the case. Whether this +effect is a direct function of the density of the gas in the +tube, or whether it is dependent on the voltage or time integral +of the current during the discharge, are questions which still +await a solution.</p> +<p>The preparation of calcium tungstate for fluorescent screens +is very simple.</p> +<p>Commercial sodium tungstate is fused with dried calcium +chloride in the proportion of three parts of the former to two +parts of the latter, both constituents being in fine powder and +well mixed together. The fusion is conducted in a Fletcher's +crucible furnace in a clay crucible. The temperature is raised as +rapidly as possible to the highest point which the furnace will +attain — i.e. a pure white heat. At this temperature the +mixture of salts becomes partly fluid, or at least pasty, and the +temperature may be kept at its highest point for, say, a quarter +of an hour. At the end of this time the mass is poured and scraped +on to a brick, and when cold is broken up and boiled with a +large excess of water to dissolve out all soluble matter. The +insoluble part, which consists of a gray shining powder, is +washed several times with hot water, and is finally dried on +filter paper in a water oven.</p> +<p>In order to prepare a screen the powder is ground slightly +with very dilute shellac varnish, and is then floated over a +glass plate so as to get an even covering. Unless the covering be +very even the screen is useless, and no pains should be spared to +secure evenness. It is not exactly easy to get a regular coat of +the fluorescent material, but it may be done with a little +care.</p> +<p><b><a name="Toc158108921" id="Toc158108921"><font face= +"Bookman Old Style" size="4">CHAPTER II</font></a></b></p> +<p><a name="Toc158108922" id="Toc158108922">GLASS-GRINDING AND +OPTICIANS' WORK</a></p> +<p><font face="Bookman Old Style">§ 52. As no instructions +of any practical value in this art have, so far as I know, +appeared in any book in English, though a great deal of valuable +information has been given in the <i>English Mechanic</i> and +elsewhere, I shall deal with the matter sufficiently fully for +all practical purposes. On the other hand, I do not propose to +treat of all the methods which have been proposed, but only those +requisite for the production of the results claimed. The student +is requested to read through the chapter before commencing any +particular operation.</font></p> +<p>§ 53. The simplest way will be to describe the process of +manufacture of some standard optical appliance, from which a +general idea of the nature of the operations will be obtained. +After this preliminary account special methods may be considered +in detail. I will begin with an account of the construction of an +achromatic object glass for a telescope, not because a student in +a physical laboratory will often require to make one, but because +it illustrates the usual processes very well; and requires to be +well and accurately made.</p> +<p>A knowledge of the ordinary principles of optics on the part +of the reader is assumed, for there are plenty of books on the +theory of lenses, and, in any case, it is my intention to treat +of the art rather than of the science of the subject. By far the +best short statement of the principles involved which I have seen +is Lord Rayleigh's article on Optics in the Encyclopaedia +Britannica, and this is amply sufficient.</p> +<p>The first question that crops up is, of course, the subject of +the choice of glass. It is obvious that the glass must be uniform +in refractive index throughout, and that it must be free from air +bubbles or bits of opaque matter. <i>[Footnote:</i> The complete +testing of glass for uniformity of refractive index can only be +arrived at by grinding and polishing a sufficient portion of the +surfaces to enable an examination to be made of every part. In +the case of a small disc it is sufficient to polish two or three +facets on the edge, and to examine the glass in a field of +uniform illumination through the windows thus formed. Very slight +irregularities will cause a "mirage" easily +recognised.<i>]</i></p> +<p>The simplest procedure is to obtain glass of the desired +quality from Messrs. Chance of Birmingham, according to the +following abbreviated list of names and refractive indices, which +may be relied upon:—</p> +<table border summary="" cellspacing="1" cellpadding="7" width= +"568"> +<tr> +<td width="17%" valign="top"></td> +<td width="17%" valign="top"> +<p><font face="Bookman Old Style">Density.</font></p> +</td> +<td width="67%" valign="top" colspan="4"> +<p><font face="Bookman Old Style">Refractive Index.</font></p> +</td> +</tr> +<tr> +<td width="17%" valign="top"></td> +<td width="17%" valign="top"></td> +<td width="17%" valign="top"> +<p><font face="Bookman Old Style">C</font></p> +</td> +<td width="17%" valign="top"> +<p><font face="Bookman Old Style">D</font></p> +</td> +<td width="17%" valign="top"> +<p><font face="Bookman Old Style">F</font></p> +</td> +<td width="17%" valign="top"> +<p><font face="Bookman Old Style">G</font></p> +</td> +</tr> +<tr> +<td width="17%" valign="top"> +<p><font face="Bookman Old Style">Hard crown</font></p> +</td> +<td width="17%" valign="top"> +<p><font face="Bookman Old Style">2.85</font></p> +</td> +<td width="17%" valign="top"> +<p><font face="Bookman Old Style">1.5146</font></p> +</td> +<td width="17%" valign="top"> +<p><font face="Bookman Old Style">1.5172</font></p> +</td> +<td width="17%" valign="top"> +<p><font face="Bookman Old Style">1.5232</font></p> +</td> +<td width="17%" valign="top"> +<p><font face="Bookman Old Style">1.5280</font></p> +</td> +</tr> +<tr> +<td width="17%" valign="top"> +<p><font face="Bookman Old Style">Soft crown</font></p> +</td> +<td width="17%" valign="top"> +<p><font face="Bookman Old Style">2.55</font></p> +</td> +<td width="17%" valign="top"> +<p><font face="Bookman Old Style">1.5119</font></p> +</td> +<td width="17%" valign="top"> +<p><font face="Bookman Old Style">1.5146</font></p> +</td> +<td width="17%" valign="top"> +<p><font face="Bookman Old Style">1.5210</font></p> +</td> +<td width="17%" valign="top"> +<p><font face="Bookman Old Style">1.5263</font></p> +</td> +</tr> +<tr> +<td width="17%" valign="top"> +<p><font face="Bookman Old Style">Light flint</font></p> +</td> +<td width="17%" valign="top"> +<p><font face="Bookman Old Style">3.21</font></p> +</td> +<td width="17%" valign="top"> +<p><font face="Bookman Old Style">1.5700</font></p> +</td> +<td width="17%" valign="top"> +<p><font face="Bookman Old Style">1.5740</font></p> +</td> +<td width="17%" valign="top"> +<p><font face="Bookman Old Style">1.5839</font></p> +</td> +<td width="17%" valign="top"> +<p><font face="Bookman Old Style">1.5922</font></p> +</td> +</tr> +<tr> +<td width="17%" valign="top"> +<p><font face="Bookman Old Style">Dense flint</font></p> +</td> +<td width="17%" valign="top"> +<p><font face="Bookman Old Style">3.66</font></p> +</td> +<td width="17%" valign="top"> +<p><font face="Bookman Old Style">1 6175</font></p> +</td> +<td width="17%" valign="top"> +<p><font face="Bookman Old Style">1.6224</font></p> +</td> +<td width="17%" valign="top"> +<p><font face="Bookman Old Style">1.6348</font></p> +</td> +<td width="17%" valign="top"> +<p><font face="Bookman Old Style">1.6453</font></p> +</td> +</tr> +<tr> +<td width="17%" valign="top"> +<p><font face="Bookman Old Style">Extra dense flint</font></p> +</td> +<td width="17%" valign="top"> +<p><font face="Bookman Old Style">3.85</font></p> +</td> +<td width="17%" valign="top"> +<p><font face="Bookman Old Style">1.6450</font></p> +</td> +<td width="17%" valign="top"> +<p><font face="Bookman Old Style">1.6504</font></p> +</td> +<td width="17%" valign="top"> +<p><font face="Bookman Old Style">1.6643</font></p> +</td> +<td width="17%" valign="top"> +<p><font face="Bookman Old Style">1.6761</font></p> +</td> +</tr> +<tr> +<td width="17%" valign="top"> +<p><font face="Bookman Old Style">Double extra dense +flint</font></p> +</td> +<td width="17%" valign="top"> +<p><font face="Bookman Old Style">4.45</font></p> +</td> +<td width="17%" valign="top"> +<p><font face="Bookman Old Style">1.7036</font></p> +</td> +<td width="17%" valign="top"> +<p><font face="Bookman Old Style">1.7103</font></p> +</td> +<td width="17%" valign="top"> +<p><font face="Bookman Old Style">1.7273</font></p> +</td> +<td width="17%" valign="top"> +<p><font face="Bookman Old Style">...</font></p> +</td> +</tr> +</table> +<p><font face="Bookman Old Style">The above glasses may be had in +sheets from 0.25 to 1 inch thick, and 6 to 12 inches square, at a +cost of, say, 7s. 6d. per pound.</font></p> +<p>Discs can also be obtained of any reasonable size. Discs 2 +inches in diameter cost about £1 per dozen, discs 3 inches +in diameter about 10s. each. The price of discs increases +enormously with the size. A 16-inch disc will cost about +£100.</p> +<p>For special purposes, where the desired quality of glass does +not appear on the list, an application may be made to the Jena +Factory of Herr Schott. In order to give a definite example, I +may mention that for ordinary telescopic objectives good results +may be obtained by combining the hard crown and dense flint of +Chance's list, using the crown to form a double convex, and the +flint to form a double concave lens. The convex lens is placed in +the more outward position in the telescope, i.e. the light passes +first through it.</p> +<p>The conditions to be fulfilled are:</p> +<ol> +<li>The glass must be achromatic;</li> +<li>it must have a small spherical aberration for rays converging +to the principal focus.</li> +</ol> +<p>It is impossible to discuss these matters without going into a +complete optical discussion. The radii of curvature of the +surfaces, beginning with the first, i.e. the external face of the +convex lens, are in the ratio of 1, 2, and 3; an allowance of 15 +inches focal length per inch of aperture is reasonable (see +Optics in Ency. Brit.), and the focal length is the same as the +greatest radius of curvature. Thus, for an object glass 2 inches +in diameter, the first surface of the convex lens would have a +radius of curvature of 10 inches, the surface common to the +convex and concave lens would have a radius of curvature of 20 +inches, and the last surface a radius of curvature of 30 inches. +This would also be about the focal length of the finished lens. +The surfaces in contact have, of course, a common curvature, and +need not be cemented together unless a slight loss of light is +inadmissible.</p> +<p>I will assume that a lens of about 2 inches diameter is to be +made by hand, i.e. without the help of a special grinding or +polishing machine; this can be accomplished perfectly well, so +long as the diameter of the glass is not above about 6 inches, +after which the labour is rather too severe. The two glass discs +having been obtained from the makers, it will be found that they +are slightly larger in diameter than the quoted size, something +having been left for the waste of working.</p> +<p>It is difficult to deal with the processes of lens manufacture +without entering at every stage into rather tedious details, and, +what is worse, without interrupting the main account for the +purpose of describing subsidiary instruments or processes. In +order that the reader may have some guide in threading the maze, +it is necessary that he should commence with a clear idea of the +broad principles of construction which are to be carried out. For +this purpose it seems desirable to begin by roughly indicating +the various steps which are to be taken.</p> +<p>(1) The glass is to be made circular in form and of a given +diameter.</p> +<p>(2) <i>Called Rough Grinding</i>. — The surfaces of the +glass are to be made roughly convex, plane, or concave, as may be +required; the glass is to be equally thick all round the edge. In +this process the glass is abraded by the use of sand or emery +rubbed over it by properly shaped pieces of iron or lead called +"tools."</p> +<p>(3) The glass is ground with emery to the correct spherical +figure as given by a spherometer.</p> +<p>(4) <i>Called Fine Grinding</i>. — The state of the +surface is gradually improved by grinding with finer and finer +grades of emery.</p> +<p>(5) The glass is polished by rouge.</p> +<p>(6) The glass is "figured." This means that it is gradually +altered in form by a polishing tool till it gives the best +results as found by trial.</p> +<p>In processes 2 to 5 counterpart tool surfaces are required +— as a rule two convex and two concave surfaces for each +lens surface. These subsidiary surfaces are worked (i.e. ground) +on discs of cast iron faced with glass, or on slate discs; and +discs thus prepared are called "tools."</p> +<p>Taking these processes in the order named, the mode of +manufacture is shortly as follows:—</p> +<p>(1) The disc of glass, obtained in a roughly circular form, is +mounted on an ordinary lathe, being conveniently cemented by +Regnault's mastic to a small face plate. The lathe is rotated +slowly, and the glass is gradually turned down to a circular +figure by means (1) of a tool with a diamond point; or (2) an +ordinary hand-file moistened with kerosene, as described in +§ 42; or (3) a mass of brass or iron served with a mixture +of emery — or sand — and water fed on to the disc, so +that the disc is gradually ground circular.</p> +<p>The operation of making a circular disc of given diameter does +not differ in any important particular from the similar operation +in the case of brass or iron, and is in fact merely a matter of +turning at a slow speed.</p> +<p>(2 and 3) Roughing or bringing the surfaces of the glass +roughly to the proper convex or concave shape. — This is +accomplished by grinding, generally with sand in large works, or +with emery in the laboratory, where the time saved is of more +importance than the value of the emery.</p> +<p>Discs of iron or brass are cast and turned so as to have a +diameter slightly less than that of the glass to be ground, and +are, say, half an inch thick. These discs are turned convex or +concave on one face according as they are to be employed in the +production of concave or convex glass surfaces. The proper degree +of convexity or concavity may be approximated to by turning with +ordinary turning tools, using a circular arc cut from zinc or +glass (as will be described) as a "template" or pattern. This +also is a mere matter of turning.</p> +<p>The first approximation to the desired convex or concave +surface of the glass is attained (in the case of small lenses, +say up to three inches diameter) by rotating the glass on the +lathe as described above (for the purpose of giving it a circular +edge) and holding the tool against the rotating glass, a +plentiful supply of coarse emery and water, or sand and water, +being supplied between the glass and metal surfaces. The tool is +held by hand against the surface of the revolving glass, and is +constantly moved about, both round its own axis of figure and to +and fro across the glass surface. In this way the glass gradually +gets convex or concave.</p> +<p>The curvature is tested from time to time by a spherometer, +and the tool is increased or decreased in curvature by turning it +on a lathe so as to cause it to grind the glass more at the edges +or in the middle according to the indications of the +spherometer.</p> +<p>This instrument, by the way — so important for lens +makers — consists essentially of a kind of three-legged +stool, with an additional leg placed at the centre of the circle +circumscribing the other three. This central leg is in reality a +fine screw with a very large head graduated on the edge, so that +it is easy to compute the fractions of a turn given to the screw. +The instrument is first placed on a flat plate, and the central +screw turned till its end just touches the plate, a state of +affairs which is very sharply discernible by the slight rocking +which it enables the instrument to undergo when pushed by the +hand. See the sketch.</p> +<p>On a convex or concave surface the screw has to be screwed in +or out, and from the amount of screwing necessary to bring all +four points into equal contact, the curvature may be +ascertained.</p> +<p>Let a be the distance between the equidistant feet, and d the +distance through which the screw is protruded or retracted from +its zero position on a flat surface. Then the radius of curvature +<font face="Bookman Old Style">ρ</font> <font face= +"Bookman Old Style">is given by the formula 2</font><font face= +"Microsoft Sans Serif">ρ</font><font face= +"Bookman Old Style">= a<sup>2</sup>/3d +d.</font></p> +<p>Fig. <img src="images/Image86.gif" alt="images/Image86.gif" +width="161" height="211"> 43.</p> +<p>The process of roughing is not always carried out exactly as +described, and will be referred to again.</p> +<p>(4) The glass being approximately of the proper radius of +curvature on one side, it is reversed on the chuck and the same +process gone through on the other side. After this the glass is +usually dismounted from the lathe and mounted by cement on a +pedestal, which is merely a wooden stand with a heavy foot, so +that the glass may be held conveniently for the workman. +Sometimes a pedestal about four feet high is fixed in the floor +of the room, so that the workman engaged in grinding the lens may +walk round and round it to secure uniformity. For ordinary +purposes, however, a short pedestal may be placed on a table and +rotated from time to time by hand, the operator sitting down to +his work.</p> +<p>Rough iron or brass tools do not succeed for fine grinding +— i.e. grinding with fine emery, because particles of emery +become embedded in the metal so tightly that they cannot be got +out by any ordinary cleaning. If we have been using emery passing +say a sieve with 60 threads to the inch, and then go on to some +passing say 100 threads to the inch, a few of the coarser +particles will adhere to the "tool", and go on cutting and +scratching all the time grinding by means of the finer emery is +in progress.</p> +<p>To get over this it is usual to use a rather different kind of +grinding tool. A very good kind is made by cementing small +squares of glass (say up to half an inch on the side), on to a +disc of slate slightly smaller than the lens surface to be formed +(Fig. 51). The glass-slate tool is then "roughed" just like the +lens surface, but, of course, if the lens has been roughed +"convex" the tool must be roughed "concave".</p> +<p>The "roughed" tool is then used to gradually improve the +fineness of grinding of the glass. For this purpose grinding by +hand is resorted to, the tool and lens being supplied continually +with finer and finer emery. Fig. 52 gives an idea of the way in +which the tool is moved across the glass surface. Very little +pressure is required. The tool is carried in small circular +sweeps round and round the lens, so that the centre of the tool +describes a many-looped curve on the lens surface. The tool must +be allowed to rotate about its own axis; and the lens and +pedestal must also be rotated from time to time.</p> +<p>Every few minutes the circular strokes are interrupted, and +simple, straight, transverse strokes taken. In no case (except to +correct a, defect, as will be explained) should the tool overhang +the lens surface by more than about one quarter the diameter of +the latter. After grinding say for an hour with one size of emery +fed in by means of a clean stick say every five minutes, the +emery is washed off, and everything carefully cleaned. The +process is then repeated with finer emery, and so on.</p> +<p>The different grades of emery are prepared by taking advantage +of the fact that the smaller the particles the longer do they +remain suspended in water. Some emery mud from a "roughing" +operation is stirred up with plenty of water and left a few +seconds to settle, the liquor is then decanted to a second jug +and left say for double the time, say ten seconds; it is decanted +again, and so on till four or five grades of emery have been +accumulated, each jug containing finer emery than its predecessor +in the process.</p> +<p>It is not much use using emery which takes more than half an +hour to settle in an ordinary bedroom jug. What remains in the +liquid to be decanted is mostly glass mud and not emery at all. +The process of fine grinding is continually checked by the +spherometer, and the art consists in knowing how to move the +grinding tool so as to make the lens surface more or less curved. +In general it may be said that if the tool is moved in small +sweeps, and not allowed to overhang much, the Centre of the lens +will be more abraded, while if bold free strokes are taken with +much overhanging, the edges of the lens will be more ground +away.</p> +<p>By the exercise of patience and perseverance any one will +succeed in gradually fine grinding the lens surface and keeping +it to the spherometer, but the skill comes in doing this rapidly +by varying the shape of the strokes before any appreciable +alteration of curvature has come about.</p> +<p><i>Polishing</i>. —</p> +<p>The most simple way of polishing is to coat the grinding tool +with paper, as will be described, and then to brush some rouge +into the paper. The polisher is moved over the work in much the +same way as the fine grinding tool, until the glass is polished. +Many operators prefer to use a tool made by squeezing a disc of +slate, armed with squares of warm pitch, against the lens surface +(finely ground), and then covering these squares with rouge and +water instead of emery and water as in the fine grinding +process.</p> +<p>The final process is called "figuring." It will in general be +unnecessary with a small lens. With large lenses or mirrors the +final touches have to be given after the optical behaviour of the +lens or mirror has been tested with the telescope itself, and +this process is called "figuring." A book might easily be written +on the optical indications of various imperfections in a mirror +or lens. Suffice it to say here that a sufficiently skilled +person will be able to decide from an observation of the +behaviour of a telescope whether a lens will be improved by +altering the curvature of one or all of the surfaces.</p> +<p>A very small alteration will make a large difference in the +optical properties, so that in general "figuring" is done merely +by using the rouge polishing tool as an abrading tool, and +causing it to alter the curves in the manner already suggested +for grinding. There are other methods based on knocking squares +out of the pitch-polisher so that some parts of the glass may be +more abraded than others.</p> +<p>The "figuring" and polishing may be done by hand just like the +grinding. There are machines, however, which can be made to +execute the proper motions, and a polisher is set in such a +machine, and the mechanical work done is by no means +inconsiderable. In fact for surfaces above six inches in diameter +few people are strong enough to work a polisher by hand owing to +the intense adhesion between it and the exactly fitting glass +surface.</p> +<p>Such is a general outline of the processes required to produce +a lens or mirror. These processes will now be dealt with in much +greater detail, and a certain amount of repetition of the above +will unfortunately be necessary: the reader is asked to pardon +this. It will also be advisable for the reader to begin by +reading the whole account before he commences any particular +operation. The reason for this is that it has been desirable to +keep to the main account as far as possible without inserting +special instructions for subsidiary operations, however important +they may be; consequently it may not always be quite clear how +the steps described are to be performed. It will be found, +however, that all necessary information is really given, though +perhaps not always exactly in the place the reader might at first +expect.</p> +<p>§ 54. All the discs that I have seen, come from the +makers already roughly ground on the edges to a circular +figure--but occasionally the figure is very rough indeed — +and in some cases, especially if small lenses have to be made, it +is convenient to begin by cutting the glass discs out of glass +sheet, which also may be purchased of suit-able glass. To do +this, the simplest way is to begin by cutting squares and then +cutting off the corners with the diamond, the approximate +circular figure being obtained by grinding the edges on an +ordinary grindstone.</p> +<p>If the pieces are larger, time and material may be saved by +using a diamond compass, i.e. an ordinary drawing compass armed +with a diamond to cut circles on the glass, and breaking the +superfluous glass away by means of a pair of spectacle-maker's +shanks (Fig. 44), or what does equally well, a pair of pliers +with soft iron jaws. With these instruments glass can be chipped +gradually up to any line, whether diamond-cut or not, the jaws of +the pincers being worked against the edge of the glass, so as to +gradually crush it away.</p> +<p>Fig. <img src="images/Image87.gif" alt="images/Image87.gif" +width="314" height="146"> 44.</p> +<p>Assuming that the glass has been bought or made roughly +circular, it must be finished on the lathe. For this purpose it +is necessary to chuck it on an iron or hardwood chuck, as shown +in Fig. 46. For a lens below say an inch in diameter, the +centering cement may be used; but for a lens of a diameter +greater than this, sufficient adhesion is easily obtained with +Regnault's mastic, and its low melting point gives it a decided +advantage over the shellac composition.</p> +<p>The glass may be heated gradually by placing it on the water +bath, or actually in the water, and gradually bringing the water +up to the boiling-point. The glass, being taken out, is rapidly +wiped, and rubbed with a bit of waste moistened, not wet, with a +little turpentine: its surface is then rubbed with a stick of +mastic previously warmed so as to melt easily. The surface of the +chuck being also warm, and covered with a layer of melted cement, +it is applied to the glass. The lathe is turned slowly by hand, +and the glass pushed gradually into the most central position; it +is then pressed tight against the chuck by the back rest, a bit +of wood being interposed for obvious reasons.</p> +<p>When all is cold the turning may be proceeded with. The +quickest way is to use the method already described (i.e. actual +turning by a file tool); but if the student prefers (time being +no object), he may accomplish the reduction to a circular form +very easily by grinding.</p> +<p><img src="images/Image88.gif" alt="images/Image88.gif" width= +"439" height="404">Fig. 45.</p> +<p><img src="images/Image89.gif" alt="images/Image89.gif" width= +"272" height="372">Fig. 46.</p> +<p>For this purpose he will require to make the following +arrangements (Fig. 45). If the lathe has a slide rest, a piece of +stout iron may be bent and cut so as to fit the tool rest, and +project beneath the glass. The iron must be fairly rigid, for if +it springs appreciably beneath the pressure of the glass, it will +not grind the latter really round. The lathe may run rather +faster than for turning cast iron of the same size. Coarse emery, +passing through a sieve of 80 threads to the inch (run), may be +fed in between the glass and iron, and the latter screwed up till +the disc just grinds slightly as it goes round.</p> +<p>A beginner will generally (in this as in all cases of grinding +processes) tend to feed too fast — no grinding process can +be hurried. If a slide rest is not available, a hinged board, +carrying a bit of iron, may (see Fig. 45) be arranged so as to +turn about its hinge at the back of the lathe; and it may be +screwed up readily enough by passing a long set-screw through the +front edge, so that the point of the screw bears upon the lathe +bed. I may add that emery behaves as if it were greasy, and it is +difficult to wet it with clean water. This is easily got over by +adding a little soap or alcohol to the water, or exercising a +little patience.</p> +<p>A good supply of emery and water should be kept between the +disc and the iron; a little putty may be arranged round the point +of contact on the iron to form a temporary trough. In any case +the resulting emery mud should on no account be thrown away, but +should be carefully kept for further use. The process is complete +when the glass is perfectly round and of the required diameter as +tested by callipers.</p> +<p>§ 55. The next step is to rough out the lens, and this +may easily be done by rotating it more slowly, i.e. with a +surface speed of ten feet per minute, and turning the glass with +a hard file, as explained in § 42. If it is desired to +employ the slide rest, it is quicker and better to use a diamond +tool — an instrument quite readily made, and of great +service for turning emery wheels and the like, — a thing, +in fact, which no workshop should be without. A bit of diamond +bort, or even a clear though off-colour stone, may be +employed.</p> +<p>An ordinary lathe tool is prepared by drawing down the tool +steel to a long cone, resembling the ordinary practice in +preparing a boring tool. The apex of the cone must be cut off +till it is only slightly larger than the greatest transverse +diameter of the diamond splinter. The latter may have almost any +shape — a triangular point, one side of a three-sided prism +is very convenient. A hole is drilled in the steel (which must +have been well softened), only just large enough to allow the +diamond to enter — if the splinter is thicker in the middle +than at either end, so much the better — the diamond is +fastened in position by squeezing the soft steel walls tightly +down upon it. Personally I prefer to use a tool holder, and in +this case generally mount the diamond in a bit of brass rod of +the proper diameter; and instead of pinching in the sides of the +cavity, I tin them, and set the diamond in position with a drop +of soft solder.</p> +<p><img src="images/Image90.gif" alt="images/Image90.gif" width= +"434" height="62">Fig. 47.</p> +<p>In purchasing diamond bort, a good plan is to buy fragments +that have been employed in diamond drilling, and have become too +small to reset; in this case some idea as to the hardness of the +bits may be obtained. Full details as to diamond tool-making are +given in books on watch-making, and in Holtzapffell's great work +on Mechanical Manipulation; but the above notes are all that are +really necessary — it is, in fact, a very simple matter. +The only advantage of using a diamond tool for glass turning is +that one does not need to be always taking it out of the rest to +sharpen it, which generally happens with hard steel, especially +if the work is turned a little too fast.</p> +<p>I recommend, therefore, that the student should boldly go to +work "free hand" with a hard file; but if he prefer the more +formal method, or distrust his skill (which he should not do), +then let him use a diamond point, even if he has the trouble of +making it. When using a diamond it is not necessary to employ a +lubricant, but there is some advantage in doing so.</p> +<p>The surface of the lens can be roughly shaped by turning to a +template or pattern made by cutting a circular arc (of the same +radius as the required surface) out of a bit of sheet zinc. +Another very handy way of making templates of great accuracy is +to use a beam compass (constructed from a light wooden bar) with +a glazier's diamond instead of a pencil. A bit of thin sheet +glass is cut across with this compass to the proper +curvature--which can be done with considerable accuracy and the +two halves of the plate, after breaking along the cut, are ground +together with a view to avoiding slight local irregularities, by +means of a little fine emery and water laid between the edges. In +this process the glass is conveniently supported on a clean board +or slate, and the bits are rubbed backwards and forwards against +each other.</p> +<p>§ 56. It is not very easy for a beginner to turn a bit of +anything — iron, wood, or glass - with great accuracy to +fit a template, and consequently time may be saved by the +following procedure, applied as soon as the figure of the +template is roughly obtained. A disc of lead or iron, of the same +diameter as the glass, and of approximately the proper curvature, +is prepared by turning, and is armed with a handle projecting +coaxially from the back of the disc. The glass revolving with +moderate speed on the lathe, the lead tool, supplied with coarse +emery and water, is held against it, care being taken to rotate +the tool by the handle, and also to move it backwards and +forwards across the disc, through a distance, say, up to half an +inch; if it is allowed to overhang too much the edges of the +glass disc will be overground. By the use of such a tool the +glass can readily be brought up to the template.</p> +<p>The only thing that remains, so far as the description of this +part of the process goes, is to give a note or two as to the best +way of making the lead tools, and for this purpose the main +narrative of processes must be interrupted. The easiest way is to +make a set of discs to begin with. For this purpose take the +mandrel out of the lathe, and place it nose downwards in the +centre of an iron ring of proper diameter on a flat and level +iron plate.</p> +<p>The discs are made by pouring lead round the screw-nose of the +mandrel. This method, of course, leaves them with a hole in the +centre; but this can be stopped up by placing the hot disc (from +which the mandrel has been unscrewed) on a hot plate, and pouring +in a sufficiency of very hot lead; or, better still, the mandrel +can be supported vertically at any desired distance above the +plate while the casting is being poured. Lead discs prepared in +this way are easily turned so as to form very convenient chucks +for brass work, and for use in the case now being treated, they +are easily turned to a template, using woodturners' tools, which +work better if oiled, and must be set to cut, not scrape.</p> +<p>If the operator does not mind the trouble of cutting a screw, +or if he has a jaw chuck, the lead may be replaced by iron with +some advantage.</p> +<p>The following is a neat way of making concave tools. It is an +application of the principle of having the cutting tool as long +as the radius of curvature, and allowing it to move about the +centre of curvature. Place the disc of iron or lead on the lathe +mandrel or in the chuck, and set the slide rest so that it is +free to slide up or down the lathe bed. Take a bar of tool steel +and cut it a little longer than the radius of curvature required. +Forge and finish one end of the bar into a pointed turning tool +of the ordinary kind. Measure the radius of curvature from the +point of the tool along the bar, and bore a hole, whose centre is +at this point, through the bar from the upper to the lower face. +I regard the upper face as the one whose horizontal plane +contains the cutting point when the tool is in use. Clamp a +temporary back centre to the lathe bed, and let it carry a pin in +the vertical plane through the lathe centres, and let this pin +exactly fit the hole in the bar.</p> +<p><img src="images/Image91.gif" alt="images/Image91.gif" width= +"128" height="383">Fig. 48.</p> +<p>Place the "radius" tool in position for cutting, and let it be +lightly held in the slide rest nearly at the cutting point, the +centre of rotation of the pedestal (or its equivalent) passing +through the central line of the bar. Then adjust the temporary +back rest, so that the tool will take a cut. In the sketch the +tool is shown swinging about the back centre instead of about a +pin — there is little to choose between the methods unless +economy of tool steel is an object. The tool must now be fed +across the work. The pedestal must of course be free to rotate, +and the slide rest to slip up and down the bed. In this way a +better concave grinding tool can be made than would be made by a +beginner by turning to a template — though an expert turner +would probably carry out the latter operation so as to obtain an' +accuracy of the same order, and would certainly do it in much +less time than would be required in setting up the special +arrangements here described.</p> +<p>On the other hand, if several surfaces have to be prepared, as +in the making of an achromatic lens, the quickest way would be by +the use of the radius tool, bored of course to work at the +several radii required. I have tried both methods, and my choice +would depend partly on the lathe at my disposal, and partly on +the number of grinding tools that had to be prepared.</p> +<p>Having obtained a concave tool of any given radius, it is +easily copied — negatively, so as to make a convex tool in +the following manner. Adjust the concave tool already made on the +back rest, so that if it rotated about the line of centres, it +would rotate about its axis of figure.</p> +<p>Arrangements for this can easily be made, but of course they +will depend on the detailed structure of the lathe. Use the slide +rest as before, i.e. let it grasp an ordinary turning tool +lightly, the pedestal being fixed, but the rest free to slide up +or down the lathe bed. Push the back rest up till the butt of the +turning tool (ground to a rounded point) rests against the +concave grinding tool. If the diameter of the convex tool +required be very small compared with the radius of curvature of +the surface (the most usual case), it is only necessary to feed +the cutting tool across to "copy" the concave surface +sufficiently nearly.</p> +<p>Fig. <img src="images/Image92.gif" alt="images/Image92.gif" +width="396" height="176">49.</p> +<p>There seems no reason, however, why these methods should not +be applied at once to the glass disc by means of a diamond point, +and the rough grinding thus entirely avoided. I am informed that +this has been done by Sir Henry Bessemer, but that the method was +found to present no great advantage in practice. A reader with a +taste for mechanical experimenting might try radius bar tools +with small carborundum wheels rapidly driven instead of a +diamond.</p> +<p>Enough has now been said to enable any one to prepare rough +convex or concave grinding tools of iron or lead, and of the same +diameter as the glass to be ground.</p> +<p>The general effect of the process of roughing the rotating +lens surface is to alter the radius of curvature of both tool and +glass; hence it is necessary to have for each grinding tool +another to fit it, and enable it to be kept (by working the two +together) at a constant figure. After a little practice it will +be found possible to bring the glass exactly up to the required +curvature as tested by template or spherometer. The art of the +process consists in altering the shape of the grinding tool so as +to take off the glass where required, as described in § 53, +and from this point of view lead has some advantages; (opinions +vary as to the relative advantages of lead and iron tools for +this purpose, however). The subsidiary grinding tool is not +actually needed for this preliminary operation, but it has to be +made some time with a view to further procedure, and occasionally +is of service here.</p> +<p>§ 57. 'The glass disc must be ground approximately to the +proper curvature on each side before any fine grinding is +commenced. It is precisely for this purpose that the previous +turning of the disc is recommended, for it is easy to unmount and +recentre a round object, but not so easy if the object have an +indefinite shape. Using a cement which is plastic before it sets, +the disc may be easily taken off the chuck and centred by a +little handicraft, i.e. by rotating the lathe slowly and pushing +the disc into such a position that it rotates about its axis. The +grinding of the second surface is accomplished exactly as in the +former case; of course on reversing the glass the chuck has to be +slightly turned up to fit the convex or concave surface.</p> +<p>§ 58. There is, however, one point of interest and +importance — attention to which will save a good deal of +useless labour afterwards. The glass must be ground in such a +manner that the thickness at the edge is the same all round. In +other words, the axes of figure of the two surfaces must +coincide. This will be the case if the recentering has been +accurately performed, and therefore no pains should be spared to +see that it is exactly carried out. Any simple form of vernier +gauge (such as Brown and Sharpe's vernier callipers) will serve +to allow of a sufficiently accurate measurement of the edge +thickness of the lens. If any difference of thickness is observed +as the gauge moves round the edge, one or other of the surfaces +must be reground. Of course the latitude of error which may be +permitted depends so much on the final arrangements for a special +finishing process called the "centering of the lens" — +which will be described — that it is difficult to fix a +limit, but perhaps one-thousandth of an inch may be mentioned as +a suitable amount for a 2-inch disc. For rough work, of course; +more margin may be admitted.</p> +<p>§ 59. In a large shop I imagine that lenses of only two +inches diameter would be ground in nests; or, in other words, a +number would be worked at a time, and centering, even of a rough +kind, would be left to the last; but this process will be treated +hereafter. At present I shall assume that only one lens will be +made at a time. Consequently we now enter on the stage of fine +grinding by hand. A leaden pedestal, for the sake of stability, +must be provided on which to mount the lens, so that the surface +to be operated on may be nearly horizontal (Fig. 50). Before this +can be done, however, fresh grinding tools (two for each surface) +must be properly prepared. After trying several plans I +unhesitatingly recommend that all fine-grinding surfaces should +be made of glass. This is easily done by taking two discs of +lead, or iron, or slate, cut to a one-tenth inch smaller radius +of curvature (in the case of a convex tool, and the opposite in +the other case) than the lens surface (Fig. 51, A). On these, +square bits of sheet glass, one-tenth of an inch thick, are to be +cemented, so as to leave channels of about one-eighth of an inch +between each bit of glass (Fig. 52, B). The "mastic" cement +formerly described may be employed for this purpose.</p> +<p><img src="images/Image93.gif" alt="images/Image93.gif" width= +"229" height="87">Fig. 50.</p> +<p>The bits of glass ought first to have their edges dressed +smooth on the grind-stone. A convex and concave glass surface +having been thus roughly prepared, they must be mounted in turn +in the lathe, and brought to the proper curvature by grinding +with the tools formerly employed and tested by the template or +spherometer. It is well to control this process by means of a +spherometer, so that the desired radius may be approximately +reached. The two glass-grinding tools are then ground together +by hand (see § 53 and § 61), the spherometer being +employed from time to time to check the progress of the work. In +general, if large circular sweeps are taken, greatly overhanging +the side of the glass surface to be figured, both the upper and +lower surfaces will be more ground at the edges, while in the +opposite event the centre will be chiefly affected.</p> +<p><img src="images/Image94.gif" alt="images/Image94.gif" width= +"381" height="235">Fig. 51.</p> +<p>A spherometer capable of measuring a 2-inch surface may be +procured, having a screw of, say, 50 threads to the inch, and a +micrometer surface divided into 200 parts, each part easily +capable of subdivision--into tenths or even twentieths. To get +the full advantage of the spherometer it must screw exceedingly +freely (i.e. must be well oiled with clock oil), and must not be +fingered except at the milled head. If one of the legs is held by +the fingers the expansion is sufficient to throw the instrument +quite out of adjustment. The glass-grinding tools being brought +to the proper figure, the next process is to transfer the same to +the lens, and this is done by similar means, the fellow tool +being used to correct the one employed in grinding the lens +surface. Before the grade of emery is changed all three surfaces +must agree, as nearly, at least, as the spherometer will +show.</p> +<p>In order to prevent confusion the following summary of the +steps already taken may be given. The discs of glass are first +ground or turned so as to be truly circular. Four "tools" are +made for each surface — a rough pair of iron or lead, and a +finishing pair of iron, lead, or slate faced by glass squares. +For a small lens the iron or lead backing may be used, for a +large one the slate. The rough tools are used to give an +approximate figure both to the lens and to the finishing +tools.</p> +<p>The final adjustment is attained by grinding one of the +glass-faced tools alternately upon the lens and upon the fellow +glass-faced tool. The spherometer is accepted at all stages of +the process as the final arbiter as to curvature. Some hints on +the form of strokes used in grinding will be given later on (see +§ 61). It suffices to state here that the object throughout +is to secure uniformity by allowing both the work and the tool to +rotate, and exercising no pressure by the fingers. The tool +backing may weigh from one to two pounds for a 2-inch lens.</p> +<p>§ 60. The tools and lens being all of the same curvature, +the state of the surface is gradually improved by grinding with +finer and finer emery. The best way of grading the emery is by +washing it with clean water, and allowing the emery (at first +stirred up with the water) to settle out. The longer the time +required for this part of the process the finer will be the emery +deposited. An ordinary bedroom jug is a very good utensil to +employ during this process; a large glass jug is even better. The +following grades will be found sufficient, though I daresay every +operative's practice differs a little on this point.</p> +<p>1st grade. — Flour emery, with the grit washed out, i.e. +allowed to stand for 2" (sec.) before being poured off.</p> +<p>2nd grade.--Stand 5" (secs.), settle in 1’ (min.)</p> +<p>3rd grade. — Stand 1', settle in 10'.</p> +<p>4th grade. — Stand 10', settle in 60'.</p> +<p>It is generally advisable to repeat the washing process with +each grade. Thus, selecting grade 2 for illustration, the liquor +for grade 3 must be poured off without allowing any of the +sediment to pass over with it. If any sediment at all passes, one +has no security against its containing perhaps the largest +particle in the jug. As soon as the liquor for No. 3 has been +decanted, jug No. 2 is filled up again with clean water (filtered +if necessary), and after standing 5" is decanted into jug No. 2b, +the sediment is returned to jug No. 1, and the liquor, after +standing 1', is transferred to jug No. 3.</p> +<p>The greatest care is necessary at each step of the operation +to prevent "sediment" passing over with liquor. There is a little +danger from the tendency which even comparatively large particles +of emery have to float, in consequence of their refusing to get +wet, and the emery worked up on the side of the jug is also a +source of danger, therefore wipe the jug round inside before +decanting.</p> +<p>In order to get a uniform grade stop the currents of water in +the jug, which may work up coarse particles, by holding a thin +bit of wood in the rotating liquid for a moment, and then gently +withdrawing it in its own plane. These precautions are +particularly necessary in the case of grades Nos. 2, 3, and 4, +especially No. 4, for if a single coarse particle gets on the +tool when the work has progressed up to this point it will +probably necessitate a return to grinding by means of No. 2, and +involve many hours' work.</p> +<p>The surface of the lens will require to be ground continuously +with each grade till it has the uniform state of roughness +corresponding to the grade in question. Two hours for each grade +is about the usual time required in working such a lens as is +here contemplated.</p> +<p>The coarser grades of emery may be obtained by washing +ordinary flour of emery, but the finer ones have to be got from +emery which has been used in the previous processes. It is not a +good plan to wash the finer grades of emery out of the proceeds +of very rough grinding say with anything coarser than flour of +emery — as there is a danger of thereby contaminating the +finer grades with comparatively coarse glass particles (owing to +their lightness) and this may lead to scratching. If the finer +grades are very light in colour, it may be inferred that a +considerable portion of the dust is composed of glass, and this +does no good. Consequently time may be saved by stirring up the +light-coloured mass with a little hydrofluoric acid in a platinum +capsule; this dissolves the finely divided glass almost +instantaneously. The emery and excess of hydrofluoric acid may +then be thrown into a large beaker of clean water and washed +several times. Fine emery thus treated has much the same dark +chocolate colour as the coarser varieties.</p> +<p>The operator should not wear a coat, and should have his arms +bare while working with fine emery, for a workshop coat is sure +to have gathered a good deal of dust, and increases the chances +of coarse particles getting between the surfaces.</p> +<p><b><a name="Toc158108923" id="Toc158108923"><font face= +"Bookman Old Style" size="4">§ 61. Details of the Process of +Fine Grinding. —</font></a></b></p> +<p><font face="Bookman Old Style">A lens of the size selected for +description is mounted as before mentioned on a leaden pedestal, +and the operator places the latter on a table of convenient +height in a room as free from dust as possible. Everything should +be as clean as a pin, and no splashes of emery mud should be +allowed to lie about. I have found it convenient to spread clean +newspapers on the table and floor, and to wear clean linen +clothes, which do not pick up dust. I have an idea that in large +work-shops some simpler means of avoiding scratches must have +been discovered, but I can only give the results of my own +experience. I never successfully avoided scratches till I adopted +the precautions mentioned.</font></p> +<p>Fig. 52. <img src="images/Image95.gif" alt= +"images/Image95.gif" width="155" height="162"></p> +<p>The left hand should be employed in rotating the pedestal +either continuously (though slowly) or at intervals of, say, one +minute. This point is rather important. Some operators require +two hands to work the grinding tool, and in any case this is the +safer practice. Under these circumstances the pedestal may be +rotated through one-eighth or tenth of a revolution every three +minutes, or thereabouts. The general motion given to the grinding +tool should be a series of circular sweeps of about one-fourth +the diameter of the glass disc, and gradually carried round an +imaginary circle drawn on the surface of the lens and concentric +with it (Fig. 52).</p> +<p>The tool may overhang the lens by a quarter of the diameter of +the latter as a maximum. The circuit may be completed in from +twelve to thirty sweeps. The grinding tool should be lightly held +by the fingers and the necessary force applied parallel to the +surface. The tool itself must be slowly rotated about its axis of +figure. If the tool be lightly held, it will be found that it +tends to rotate by itself. I say "tends to rotate," for if the +tool be touching evenly all over the surface it will rotate in a +direction opposite to the direction of the circular sweep. For +instance, if the tool be carried round its looped path clockwise, +it will tend to rotate about its own axis of figure +counter-clockwise. If it touch more in the middle, this rotation +will be increased, while if it touches more along the edge, the +rotation will be diminished, or even reversed in an extreme +case.</p> +<p>Every fifty sweeps or so the tool should be simply ground +backwards and forwards along a diameter of the lens surface. This +grinding should consist of three or four journeys to and fro +along, say, eight different diameters. About one-quarter of the +whole grinding should be accomplished by short straight strokes, +during which the tool should only overhang about one-quarter of +an inch. The object of the straight strokes is to counteract the +tendency to a gradual accumulation of the emery in the centre, +which results from the circular grinding.</p> +<p>A great deal of the art of the process consists in knowing how +to work the tool to produce any given effect. For instance, if +the lens requires to be ground down near the centre, the +epicycloidal strokes must be nearly central; the tool must never +overhang very much. If, on the other hand, it is the edges which +require attention, these must be dealt with by wider overhanging +strokes. The tool must be frequently tested on its fellow, and, +indeed, ground upon it if any marked unevenness of action (such +as that just described) is required for the lens. A check by +spherometer will be applied at intervals according to the +judgment of the operator, but, in any case, the fellow tool and +lens should be kept at very nearly the same figure.</p> +<p>The emery should never be allowed to become anything like dry +between the tool and the lens, for in some way (probably by +capillary action increasing the pressure of the tool) this seems +to lead to scratching and "rolling" of the emery. The channels in +the glass tool between the squares are of the greatest importance +in enabling the emery to distribute itself. Perhaps the best +guide in enabling one to judge as to when it is time to wash off +the emery and apply fresh is the "feel" of the tool; also when +the mud gets light in colour we know that it is full of glass +dust, and proportionately inoperative.</p> +<p>New emery may be put on, say, every five minutes, but no +absolute rule can be given, for much depends on the pressure of +the tool upon the lens. In the case considered a brass or lead, +or even slate tool, of an inch, or even less, in thickness, will +press quite heavily enough. In washing the lens and tool before +new emery is introduced, a large enamelled iron bucket is very +handy; the whole of the tool should be immersed and scrubbed with +a nail-brush. The lens surface may be wiped with a bit of clean +sponge, free from grit, or even a clean damp cloth.</p> +<p>When the time comes to alter the grade of emery, a fresh lot +of newspapers should be put down, and tools, lens, and pedestal +well washed and brushed by the nail-brush. The surfaces should be +wiped dry by a fresh piece of rag, and examined for scratches and +also for uniformity of appearance; a good opinion can be formed +as to the fit of the surfaces by noting whether — and if +so, to what degree — they differ in appearance from point +to point when held so that the light falls on them obliquely.</p> +<p>It is necessary to exercise the greatest care in the washing +between the application of successive grades of emery, and this +will be facilitated if the edges of the glass squares were +dressed on a grindstone before they were mounted. An additional +precaution which may be of immense advantage is to allow the tool +to dry between the application of successive grades of emery (of +course, after it has been scrubbed), and then to brush it +vigorously with a hat-brush. It sometimes happens that particles +of mud which have resisted the wet scrubbing with the nail-brush +may be removed by this method.</p> +<p>As my friend Mr. Cook informs me that his present practice +differs slightly from the above, I will depart from the rule I +laid down, and add a note on an alternative method.</p> +<p>Consider a single lens surface. This is roughed out as before +by an iron tool, a rough fellow tool being made at the same time. +The squares of glass are cemented to the roughing tool, and this +is ground to the spherometer by means of the counterpart tool. +The glass-coated tool is then applied to the lens surface and +grinding with the first grade of emery commenced. The curvature +is checked by the spherometer. Two auxiliary tools of, say, half +the diameter of the lens, are prepared from slate, or glass +backed with iron, and applied to grind down either the central +part of the lens surface or tool surface, according to the +indications of the spherometer. Any changes that may occur during +grinding are corrected by these tools. The spherometer is +accepted as the sole guide in obtaining the proper curvature. A +slate backing is preferred for tools of any diameter over, say, 2 +inches.</p> +<p><b><a name="Toc158108924" id="Toc158108924"><font face= +"Bookman Old Style" size="4">§ 62. <i>Polishing</i>. +—</font></a></b></p> +<p><font face="Bookman Old Style">After the surface has been +ground with the last grade of emery, and commences to become +translucent even when dry, the grinding may be considered to be +accomplished, and the next step is the polishing. There are many +ways of carrying out this process, and the relative suitability +of these methods depends on a good many, so to speak, accidental +circumstances. For instance, if the intention is to finish the +polishing at a sitting, the polishing tool may be faced with +squares of archangel — not mineral or coal-tar — +pitch and brought to shape simply by pressing while warm against +the face of the lens. A tool thus made is very convenient, +accurate, and good, but it is difficult to keep it in shape for +any length of time; if left on the lens it is apt to stick, and +if it overhangs ever so little will, of course, droop at the +edges.</font></p> +<p>On the whole, the following will be found a good and +sufficient plan. The glass-grinding tool is converted into a +polishing tool by pasting a bit of thin paper over its surface; a +bit of woven letter paper of medium thickness with a smooth but +not glazed surface does very well. We have found that what is +called Smith's "21 lbs. Vellum Wove" is excellent. This is +steeped in water till quite pliable and almost free from size. +The glass tool is brushed over with a little thin arrowroot or +starch paste, and the paper is laid upon it and squeezed down on +the glass squares as well as possible; if the paper is wet enough +and of the proper quality it will expand sufficiently to envelop +the tool without creases, unless the curvature is quite out of +the common.</p> +<p>This being accomplished, and the excess of water and paste +removed, the face of the paper is (for security) washed with a +little clean water and a bit of sponge, and, finally, the tool is +slightly pressed on the lens so as to get the paper to take up +the proper figure as nearly as possible. After the polishing tool +has been thus brought to the proper figure, it is lifted off and +allowed to dry slowly. When the paper is dry it may be trimmed +round the edges so as not to project sensibly beyond the glass +squares. The next step is to brush the surface over very +carefully with polishing rouge (prepared as is described at the +end of this section) by means of a hat-brush. When the surface of +the paper is filled with rouge all excess must be removed by +vigorous brushing.</p> +<p>Fig. <img src="images/Image96.gif" alt="images/Image96.gif" +width="444" height="52"> 53.</p> +<p>The tool being placed on the lens, two or three strokes +similar to those used in grinding may be taken, and the tool is +then lifted off and examined. It will be found to be dotted with +a few bright points, produced by the adhesion of glass at the +places of contact. These points are then to be removed in the +following manner. An old three-cornered file is ground on each +side till the file marks disappear, and sharp edges are produced +(Fig. 53). This tool is used as an ink eraser, and it will be +found to scrape the paper of the polishing tool very cleanly and +well.</p> +<p>The bright spots are the objects of attention, and they must +be erased by the old file, and the polisher reapplied to the +glass. A few strokes will develop other points, more numerous +than before, and these in turn must be erased. The process is +continued till the whole surface of the polishing tool is evenly +covered with bright specks, and then the polishing may be +proceeded with. The specks should not be more than about +one-eighth of an inch apart, or the polishing will be +irregular.</p> +<p>The operation of polishing is similar to that of grinding. A +reasonable time for polishing a glass surface is twenty hours; if +more time is required it is a sign that the fine grinding has not +been carried far enough. The progress of the operation may be +best watched by looking at the surface — not through it. +For this purpose a good light is requisite. When the lens is +dismounted it may be examined by a beam of sunlight in a dark +room, under which circumstances the faintest signs of grayness +are easily discernible.</p> +<p>It may be mentioned here that if the surface is in any way +scratched the rouge will lodge in the scratches with great +persistence, and an expert can generally tell from the appearance +of scratches what kind of polishing powder has been employed.</p> +<p>The persistence with which rouge clings to a rough surface of +glass is rather remarkable. Some glass polishers prefer to use +putty powder as a polishing material, and it is sometimes said to +act more quickly than rouge; from my rather limited experience I +have not found this to be the case, but it may have merits that I +do not know of. Is it possible that its recommendation lies in +the fact that it does not render scratches so obtrusively obvious +as rouge does?</p> +<p>Rouge is generally made in two or more grades. The softer +grade is used for polishing silver, and is called jewellers' +rouge. The harder grade, suitable for glass polishing, is best +obtained from practical opticians (not mere sellers of optical +instruments). I mean people like Messrs. Cook of York. Many years +ago I prepared my own hard rouge by precipitating ferrous +sulphate solution by aqueous ammonia, washing the precipitate, +and heating it to a red heat. The product was ground up with +water, and washed to get rid of large particles. This answered +every purpose, and I could not find that it was in any way +inferior to hard rouge as purchased. The same precipitate heated +to a lower temperature is said to furnish a softer variety of +rouge; at all events, it gives one more suitable for polishing +speculum metal. Lord Rosso used rouge heated to a dull redness +for this purpose.</p> +<p>Rouge, whether made or bought, should always be washed to get +rid of grit. I ought to add that not the least remarkable fact +about the polishing is the extraordinarily small quantity of the +polishing material requisite, which suggests that the process of +polishing is not by any means the same as that of exceptionally +fine grinding. Is it possible that the chief proximate cause of +the utility of rouge is to be sought in its curious property of +adhering to a rough glass surface, causing it, so to speak, to +drag the glass off in minute quantities, and redeposit it after a +certain thickness has been attained on another part of the +surface?</p> +<p><b><a name="Toc158108925" id="Toc158108925"><font face= +"Bookman Old Style" size="4">§ 63. <i>Centering</i>. +—</font></a></b></p> +<p><font face="Bookman Old Style">When a lens is ground and +polished it will almost always happen that the axis of revolution +of its cylindrical edge is inclined to the axis of revolution of +its curved surfaces. Since in practice lenses have to be adjusted +by their edges, it is generally necessary to adjust the edge to a +cylinder about the axis of figure of the active surfaces. This is +best done on a lathe with a hollow mandrel.. The lens is chucked +on a chuck with a central aperture — generally by means of +pitch or Regnault's mastic, or "centering" cement for small +lenses — and a cross wire is fixed in the axis of +revolution of the lathe, and is illuminated by a lamp. This cross +wire is observed by an eye-piece (with cross wires only in the +case of a convex lens, or a telescope similarly furnished in the +case of a concave lens), also placed in the axis of rotation of +the lathe.</font></p> +<p>Both cross wires are thus in the axis of revolution of the +mandrel, and the distant one (B in the figure) is viewed through +the lens and referred to the fixed cross wires at A. In general, +as the lathe is rotated by turning the mandrel the image of the +illuminated cross wires will be observed to rotate also. The lens +is adjusted until the image remains steady on rotating the +mandrel and it is to give time for this operation that a +slow-setting cement is recommended. When the image remains +stationary we know that the optical centre of the lens is in the +axis of revolution, and that this axis is normal to both lens +surfaces, i.e. is the principal axis of the lens, or axis of +figure.</p> +<p>Fig. 54. <img src="images/Image97.gif" alt= +"images/Image97.gif" width="442" height="175"></p> +<p>A much readier method, and one, in general, good enough for +most purposes, is to put a candle on the end of the lathe-bed +where the back centre generally is, and observe the images of the +flame by reflection from both the lens surfaces. This method is +very handy with small lenses; the mandrel is turned, and the lens +adjusted by hand till the images are immovable. In both cases, of +course, the edge of the lens is turned or ground till it is truly +circular, the position of the lens remaining undisturbed on the +chuck. If the edge gauge has been properly used in the earlier +stages of figuring, it will be found that very little turning or +grinding is requisite to produce a true centering.</p> +<p>The particular defect due to want of centering in a lens may +be observed by using it as the objective of a telescope, and +observing a star slightly out of focus. The interference fringes +will not be concentric circles unless the lens is properly +centred. I ought to say that I have not looked into the theory of +this, but have merely taken it as a generally admitted fact. The +diseases of lenses and the modes of treating them are dealt with +in a book by Messrs. Cook of York, entitled <i>On the Adjustment +and Testing of Telescopic Objectives</i>.</p> +<p>The final process of figuring will be dealt with later on +(§ § 66 and 67), as it applies not only to lenses but +to mirrors, prisms, etc. If the instructions given have been +carefully carried out on a 2-inch lens, it should perform fairly +well, and possibly perfectly, without any further adjustment of +the glass.</p> +<p><b><font face="Bookman Old Style" size="4">§ 64. +<i>Preparation of Small Lenses, where great Accuracy is not of +the first Importance. —</i></font></b></p> +<p><font face="Bookman Old Style">Such lenses may generally be +made out of bits of good plate or sheet glass, and are of +constant use in the physical laboratory. They may be purchased so +cheaply, however, that only those who have the misfortune to work +in out-of-the-way places need be driven to make them.</font></p> +<p>Suitable glass having been obtained and the curves calculated +from the index of refraction, as obtained by any of the ordinary +methods applicable to plates (the microscope method, in general, +is quite good enough), squares circumscribing the desired circles +are cut out by the help of a diamond. <i>[Footnote:</i> +Glazebrook and Shaw's Practical Physics, p. 383 (4th +ed.).<i>]</i> The squares are roughly snipped by means of a pair +of pliers or spectacle-maker's shanks. The rough circles are then +mounted on the end of a brass or iron rod of rather greater +diameter than the finished lenses are to possess. This mounting +is best done by centering cement.</p> +<p>The discs are then dressed circular on a grindstone, the rod +serving both as a gauge and handle. A sufficient number of these +discs having been prepared, a pair of brass tools of the form +shown in the sketch (Fig. 55), and of about the proper radius of +curvature, are made. One of these tools is used as a support for +the glass discs.</p> +<p>Fig. <img src="images/Image98.gif" alt="images/Image98.gif" +width="401" height="116">55.</p> +<p>A compass being set to scribe circles of the same diameter as +the glass discs, centre marks are made on the surface of the +appropriate tool, circles are drawn on this, and facets are filed +or milled (for which the spiral head of the milling machine is +excellent). In the case of concave supporting surfaces, i.e. in +making concave lenses, I apprehend filing would be difficult, and +the facets would have to be made by a rose cutter or mill; but if +the discs are fairly round, then, in fact, no facets are +required.</p> +<p>The facets being ready, the glass discs are cemented to them +by centering cement, which may be used quite generally for small +lenses. When the cutting of facets has been omitted on a concave +surface, the best cement is hard pitch. The grinding tool is +generally rather larger than the nest of lenses. Coarse and fine +grinding is accomplished wholly on the lathe — the tool +being rotated at a fair speed (see infra), and the nest of lenses +moved about by its handle so as to grind all parts equally. It +must, of course, be held anywhere except "dead on," for then the +part round the axis would not get ground; this inoperative +portion of the rotating tool must therefore be allowed to +distribute its incapable efforts evenly over the nest of +lenses.</p> +<p>Polishing is accomplished by means of the grinding tool, +coated with paper and rouge as before; or the tool may be coated +with very thin cloth and used with rouge as before — in +this case the polishing goes on fastest when the surface of the +cloth is distinctly damp. In working by this method, each grade +of emery need only be applied from five to ten minutes. The glass +does not appear to get scratched when the emery is changed, +provided everything is well washed. A good polish may be got in +an hour. The lathe is run as for turning brass of the same +diameter as the tool.</p> +<p>One side of the lenses being thus prepared, they are reversed, +and the process gone through for the other side in a precisely +similar manner. <i>[Footnote:</i> Unless the radius of curvature +is very short and the lenses also convex, there is no necessity +to recess the facets, provided hard pitch is used as the cement. +See note on hard pitch.<i>]</i> To save trouble, it is usual, to +make such lenses of equal curvature on both faces; but of course +this is a matter of taste.</p> +<p>Fig. <img src="images/Image99.gif" alt="images/Image99.gif" +width="401" height="68"> 56.</p> +<p>For very common work, bits of good plate glass are employed, +and the manufacturer's surface treated as flat (Fig. 56). In this +way plano-convex lenses are easily and cheaply made. Finally the +lenses have to be centred, an essential operation in this case. +This is easily done by the reflection method — the edge +being turned off by the file and kerosene and the centering +cement being used in making the preliminary adjustment on the +chuck. I presume a lens made in this way is worth about a +shilling, so that laboratory manufacture is not very +remunerative. Fig. 56 shows the method of mounting small lenses +for lathe grinding, when only one lens is required. The tool is +generally rotated in the lathe and the lens held against it.</p> +<p><b><a name="Toc158108926" id="Toc158108926"><font face= +"Bookman Old Style" size="4">§ 65. Preparing Small Mirrors +for Galvanometers. —</font></a></b></p> +<p><font face="Bookman Old Style">To get good mirrors for +galvanometers, I have found the best plan is to grind and polish +a large number together, on a disc perhaps 8 or 10 inches in +diameter. I was led to this after inspecting and rejecting four +ounces of microscope cover slips, a most wearisome process. That +regular cover slips should be few and far between is not +unlikely, seeing that they are made (by one eminent firm at +least) simply by "pot" blowing a huge thin bulb, and then +smashing it on the floor and selecting the fragments. As in the +case of large mirrors, it is of course only necessary to grind +one side of the glass, theoretically at all events. The +objections to this course are:</font></p> +<ol> +<li>A silver surface cannot, in my experience, be polished +externally (on a minute object like a cover slip) to be anything +like so bright as the silver surface next the glass; and,</li> +<li>if one side only is ground, it will be found that the little +mirror hopelessly loses its figure directly it is detached from +the support on which it has been worked. Consequently, I +recommend that these small mirrors should be ground and polished +on both sides — enough may be made at one operation to last +for a very long time.</li> +</ol> +<p>A slate back is prepared of the same radius of curvature as it +is desired to impart to the mirrors. Bits of thin sheet glass are +then ground circular as described in the last section and +cemented to this surface by the smallest quantity of clean +archangel pitch, allowed to cool slowly and even to rest for a +day before the work is proceeded with. The whole surface is then +ground and polished as before.</p> +<p>The mirrors are now reversed, when they ought to nearly fit +the tool (assuming that flats are being made, and the fellow tool +in all other cases), and are recemented by pitch to the +appropriate backing ground, and polished. If very excellent +results are required, these processes may be preceded by a +preliminary rough grinding of one surface, so that the little +discs will "sit" exactly on the tool surface, and not run the +risk of being strained by capillary forces in the pitch. We have +always found this necessary for really good results.</p> +<p>On removing such mirrors from the backing, they generally, +more or less lose their figure, becoming (in general fairly +uniformly) more concave or convex. About 5 per cent of the +mirrors thus prepared will be found almost perfect if the work +has been well done, and the rest will probably be very fair, +unless the diameter is very large as compared with the thickness. +The best way of grinding and polishing such large surfaces (nests +10 inches in diameter) is on a grinding machine, such as will be +described below. The polishing is best done by means of paper, as +before described.</p> +<p>Having occasion to require hitherto unapproached lightness and +optical accuracy in such mirrors, I got my assistant to try +making them of fused quartz, slices being cut by a diamond wheel +from a rod of that material. Chips of natural quartz were also +obtained from broken "pebble" spectacles, and these were worked +at the same time. The resulting mirrors were certainly superior +to the best we could make from glass, but the labour of grinding +was greater, and the labour of polishing less, than in the latter +case. The pebble fragments gave practically as good mirrors as +the fused slices. For the future it will be better always to make +galvanometer mirrors from quartz crystals. These may be easily +sliced, as will be described in § 74. The slices are dressed +on a grindstone according to instructions already given for small +lenses.</p> +<p>The silvering of these mirrors is a point of great importance. +After trying nearly every formula published, we have settled down +to the following.</p> +<p>A solution of pure crystallised nitrate of silver in distilled +water is made up to a strength of 125 grams of the salt per +litre. This forms the stock solution and is kept in a dark +bottle.</p> +<p>Let the volume of silvering liquor required in any operation +be denoted by 4 <i>v</i>. The liquor is prepared as follows:</p> +<p>I. Measure out a volume <i>v</i> of the stock solution of +silver nitrate, and calculate the weight of salt which it +contains; let this be <i>w</i>. In another vessel dissolve pure +Rochelle salt to the amount of 2.6 <i>w</i>, and make up the +solution to the volume <i>v</i>. These two solutions are to be +mixed together at a temperature of 55° C., the vessels with +their contents being heated to this temperature on the water +bath. After mixing the liquids the temperature is to be kept +approximately constant for five minutes, after which the liquor +may be cooled. The white precipitate which first forms will +become gray or black and very dense as the liquid cools. If it +does not, the liquor must be reheated to 55° C., and kept at +that temperature for a few minutes and then again allowed to +cool. The solution is in good order when all the precipitate is +dense and gray or black and the liquor clear. The blacker and +denser the precipitate the better is the solution. The liquor is +decanted and filtered from the precipitate and brought up to the +volume 2 <i>v</i> by addition of some of the wash water.</p> +<p>II. Measure out a volume 0.118 <i>v</i> of the stock solution +into a separate vessel, and add to it a 5 per cent solution of +ammonium hydrate, with proper precautions, so that the +precipitate at first formed is all but redissolved after vigorous +shaking. It is very important that this condition should be +exactly attained. Therefore add the latter part of the ammonia +very carefully. Make up the volume to 2 <i>v</i>.</p> +<p>Mix the solutions I. and II. in a separate vessel and pour the +mixture into the depositing vessel. The surface to be silvered +should face downwards, and lie just beneath the free surface of +the liquid. Bubbles must of course be removed.</p> +<p>The silver deposit obtained in this manner is exceedingly +white and, bright on the surface next to the glass, but the back +is mat and requires polishing.</p> +<p>The detail of the process described above was worked out in my +laboratory by Mr. A. Pollock, to whom my thanks are due.</p> +<p>This process gives good deposits when the solutions are +freshly prepared, but the ammonia solution will not keep; The +surfaces to be silvered require to be absolutely clean. The +process is assisted by a summer temperature, say 70° Fahr., +and possibly by the action of light. Six or seven hours at least +are required for a good deposit; a good plan is to leave the +mirrors in the bath all night. On removal from the bath the +mirrors require to be well washed, and allowed to dry thoroughly +in sun heat for several hours before they are touched.</p> +<p>Care should be taken not to pull the mirrors out of shape when +they are mounted for the bath. A single drop of varnish or paint +(a mere speck) on the centre will suffice to hold them. The back +of the deposit requires to be varnished or painted as a rule to +preserve the silver. All paints and varnishes thus applied tend +to spoil the figure by expanding or contracting. On the whole, I +think boiled linseed oil and white or red lead — white or +red paint in fact — is less deleterious than other things I +have tried. Shellac varnish is the worst.</p> +<p>Of course, the best mirror can be easily spoiled by bad +mounting. I have tried a great number of methods and can +recommend as fairly successful the following:— A little +pure white lead, i.e. bought as pure as a chemical — not as +a paint — is mixed with an equal quantity of red lead and +made into a paste with a little linseed oil. I say a paste, not +putty. A trace of this is then worked on to the back of the +mirror at the centre as nearly as may be, and to this is attached +the support. The only objection to this is that nearly a week is +required for the paste to set. If people must use shellac let it +be remembered that it will go on changing its shape for months +after it has cooled (whether it has been dissolved in alcohol or +not).</p> +<p><b><a name="Toc158108927" id="Toc158108927"><font face= +"Bookman Old Style" size="4">§ 66. Preparation of Large +Mirrors or Lenses for Telescopes. —</font></a></b></p> +<p><font face="Bookman Old Style">So much has been written on +this subject by astronomers, generally in the <i>English +Mechanic</i> and in the <i>Philosophical Transactions</i> for +1840, that it might be thought nothing could be added. I will +only say here that the processes already described apply +perfectly to this case; but of course I only refer to silver on +glass mirrors. For any size over 6 inches in diameter, the +process of grinding and polishing by hand, particularly the +latter, will probably be found to involve too much labour, and a +machine will be required. A description of a modification of Mr. +Nasmyth's machine — as made by my assistant, Mr. Cook +— will be found below.</font></p> +<p>There is no difficulty in constructing or working such a +machine, and considered as an all round appliance, it possesses +solid advantages over the simple double pulley and crank +arrangement, which, however, from its simplicity deserves a note. +Two pulleys, A and B, of about 18 inches diameter by 4 inches on +the face, are arranged to rotate about vertical axes, and belted +together. The shaft of one of these pulleys is driven by a belt +in any convenient manner. Each pulley is provided on its upper +surface with a crank of adjustable length carrying a vertical +crank-pin.</p> +<p>Each crank-pin passes through a 3"X 2" wooden rod, say 3' 6" +long, and these rods are pinned together at their farther +extremities, and this pin carries the grinding or polishing tool, +or rather engages loosely with the back of this tool which lies +below the rod. It is clear that if the pulleys are of +commensurable diameters, and are rigidly connected — say by +belting which neither stretches nor slips — the polishing +tool will describe a closed curve. If, however, the belt is +arranged to slip slightly, or if the pulleys are of +incommensurable diameters, the curve traced out by the grinding +tool will be very complex, and in the case of the ratio of the +diameters being incommensurable, will always remain open; for +polishing purposes the consummation to be wished.</p> +<p>Mirror surfaces are ground spherical, the reduction to +parabolic form being attained in the process of polishing. A very +interesting account of the practice of dealing with very large +lenses will be found in Nature, May 1886, or the Journal of the +Society of Arts, same date (I presume), by Sir Howard Grubb. The +author considers that the final adjustment of surfaces by +"figuring" — of which more anon — is an art which +cannot be learned by inspection, any more than a man could learn +to paint by watching an artist. This is, no doubt, the case to +some extent; still, a person wishing to learn how to figure a +lens could not do better than take Sir Howard at his word, and +spend a month at his works. Meanwhile the following remarks must +suffice; it is not likely that anybody to whom these notes will +be of service would embark on such large work as is contemplated +by Sir Howard Grubb.</p> +<p>Fig. <img src="images/Image100.gif" alt="images/Image100.gif" +width="264" height="314">57.</p> +<p><i><font face="Bookman Old Style" size="3">Description of +Polishing Machine. </font></i>Power is applied through belting to the speed +cone A. By means of a bevel pinion rotation is communicated to +the wheel D, which is of solid metal and carries a T-slot, C. A +pedestal forming a crank-pin can be clamped so as to have any +desired radius of motion by the screw E. A train of wheels E F G +H K (ordinary cast lathe change wheels) communicate any desired +ratio of motion to the tool-holder, which simply consists of two +pins projecting vertically downwards from the spokes of wheel +K.</p> +<p>These pins form a fork, and each prong engages in a +corresponding hole in the back of the slate-grinding tool (not +shown in figure). The connection with the tool is purposely +loose. The wheel E, of course, cannot rotate about the crank-pin +D. Provision for changing the ratio of tool rotation is achieved +by mounting the wheels composing the train on pins capable of +sliding along a long slot in the bar supporting them. </p> +<p><font face="Bookman Old Style" size="3">The farther +end of this bar is caused to oscillate to and fro very slowly by +means of an additional crank-pin S and crank-shaft, the +projecting face of the bed-plate W being placed so as to allow V +to slide about easily and smoothly. Motion is communicated to +this part of the system by means of gears at 0 and P, and a belt +working from P to Q. Thus the vertical shaft R is set in motion +and communicates by gears with S. A pulley placed on the axle of +the wheel carrying the crank-pin S gives a slow rotation to the +work which is mounted on the table M. A small but important +feature is the tray L below the gear K. This prevents dirt +falling from the teeth of the wheel on to the work. The motion of +S is of course very much less than of B — say 100 times +less. The work can be conveniently adjusted as to height by means +of the screw N.</font></p> +<p>The machine must be on a steady foundation, and in a place as +free from dust as possible. Though it looks complicated it is +quite straight-forward to build and to operate.</p> +<p>It is explained in Lord Rayleigh's article on Optics in the +<i>Encyclopaedia Britannica</i> that a very minute change in the +form of the curvature of the surface of a lens will make a large +difference in the spherical aberration. This is to be expected, +seeing that spherical aberration is a phenomenon of a +differential sort, i.e. a measure of the difference between the +curvature actually attained, and the theoretical curvature at +each point of the lens, for given positions of point and image. +Sir H. Grubb gives an illustration of the minuteness of the +abrasion required in passing from a curve of one sort to a curve +of another, say from a spherical to a parabolic curve, +consequently the process of figuring by the slow action of a +polishing tool becomes quite intelligible. In making a large +mirror or lens all the processes hitherto described under +grinding and polishing, etc., have to be gone through and in the +manner described, and when all this is accomplished the final +process of correcting to test commences. This process is called +figuring.</p> +<p>§ 67. Of the actual operation of this process I have no +personal knowledge, and the following brief notes are drawn from +the article by Sir H. Grubb, from my assistant's (Mr. Cook) +experience, and from a small work On the Adjustment and Testing +of Telescopic Objectives, by T. Cook and Sons, Buckingham Works, +York (printed by Ben Johnson and Co., Micklegate, York). This +work has excellent photographs of the interference rings of star +images corresponding to various defects. It must be understood +that the following is a mere sketch. The art will probably hardly +ever be required in laboratory practice, and those who wish to +construct large telescopes should not be above looking up the +references.</p> +<p>The process is naturally divided for treatment into two +parts.</p> +<p>(1) The detection of errors, and the cause of these +errors.</p> +<p>(2) The application of a remedy.</p> +<p>(1) A lens, being mounted with its final adjustments, is +turned on to a star, which must not be too bright, and should be +fairly overhead. The following appearances may be noted:-</p> +<p>A. In focus, the star appears as a small disc with one or two +rings round it; inside and outside of the focus the rings +increase in number, are round, concentric with the disc, and the +bright and dark rings are apparently equally wide. The appearance +inside the focus exactly resembles that outside when allowance is +made for chromatic effects. Conclusion: objective good, and +correctly mounted.</p> +<p>B. The rings round the star in focus are not circular, nor is +the star at the centre of the system. In bad cases the fringes +are seen at one side only. Effects exaggerated outside and inside +the focus. Conclusion: the lens is astigmatic, or the objective +is not adjusted to be co-axial with the eyepiece.</p> +<p>C. When in focus the central disc is surrounded by an +intermittent diffraction pattern, i.e. for instance the system of +rings may appear along, and near, three or more radii. If these +shift when the points of support of the lens are shifted, flexure +may be suspected.</p> +<p>D. On observing inside and outside the focus, the rings are +not equally bright and dark. This may be due to uncorrected +spherical aberration, particularly to a fault known as "zonal +aberration," where different zones of the lens have different +foci, but each zone has a definite focus.</p> +<p>E. Irregular diffraction fringes point to bad annealing of the +glass. This may be checked by an examination of the lens in +polarised light.</p> +<p>F. If the disc appear blurred and coloured, however the focus +be adjusted, incomplete correction for chromatic aberration is +inferred. If in addition the colouring is unsymmetrical (in an +extreme case the star disc is drawn out to a coloured band), want +of centering is to be inferred. This will also show itself by the +interference fringes having the characteristics described in +C.</p> +<p>(2) The following steps may be taken in applying a remedy:</p> +<p>A. The adjusting screws of the cell mounting the object glass +may be worked until the best result is attained; this requires +great care and patience. Any errors left over are to be +attributed to other causes than the want of collinearity of the +axes of object glass and eyepiece.</p> +<p>B. Astigmatism is detected by rotating the object glass or +object glass cell. If the oval fringes still persist and the +longer axis follows the lens, astigmatism may be inferred. +Similarly, by rotating one lens on the other, astigmatism, or +want of centering (quite a different thing) may be localised to +the lens.</p> +<p>C. The presence of flexure may be confirmed by altering the +position of the points of support with respect to the eyepiece, +the lens maintaining its original position. The addition of more +points of support will in general reduce the ill effects. How to +get rid of them I do not know; they are only serious with large +lenses.</p> +<p>D. Spherical aberration may be located by using stops and +zonal screens, and observing the effect on the image. Sir H. +Grubb determines whether any point on the lens requires to be +raised or lowered, by touching the glass at that point with a +warm hand or cooling it by ether. The effects so produced are the +differential results of the change of figure and of refractive +index. By observing the effect of the heating or cooling of any +part, the operator will know whether to raise or lower that part, +provided that by a suitable preliminary experiment he has +determined the relation between the effect produced by the change +of figure, and that due to the temperature variation of the +refractive index. In general it is sufficient to consider the +change of shape only and neglect the change in refractive +power.</p> +<p>E. Marked astigmatism has never been noticed by me, but I +should think that the whole lens surface would require to be +repolished or perhaps reground in this case.</p> +<p>F. To decide in which surface faults exist is not easy. By +placing a film of oil between the two surfaces nearly in contact +these may be easily examined. Thus a mixture of nut and almond +oil of the right proportion, to be found by trial, for the +temperature, will have the same refractive index as the crown +glass, and will consequently reduce any errors of figure in the +interior crown surface, if properly applied between the surfaces. +Similarly the interior of the flint surface may have its +imperfections greatly reduced in effect by using almond oil +alone, or mixed with bisulphide of carbon. The outer surfaces, I +presume, must be examined by warming or cooling over suitable +areas or zones.</p> +<p>The defects being detected, a matter requiring a great deal of +skill and experience according to Sir H. Grubb, the next step is +to remedy them; and the remedial measures as applied to the glass +constitute the process of figuring. There are two ways of +correcting local defects, one by means of small paper or pitch +covered tools, which according to Sir H. Grubb is dangerous, and +according to the experience of Mr. Cook, and I think of many +French opticians, safe and advantageous.</p> +<p>Pitch polishing tools are generally used for figuring. They +are made by covering a slate backing with squares of pitch. The +backing is floated with pitch say one-eighth of an inch thick. +This is then scored into squares by a hot iron rod. The tool, +while slightly warm, is laid upon the lens surface, previously +slightly smeared with dilute glycerine, until the pitch takes the +figure of the glass. The polishing material is rouge and water. +Small tools are applied locally, and probably can only be so +applied with advantage for grave defects.</p> +<p>The other method is longer and probably safer. It consists in +furnishing the polishing tool with squares of pitch as before. +These being slightly warm, the lens is placed upon them so that +they will flow to the exact figure also as before. I presume that +the lens is to be slightly smeared with glycerine, or some +equivalent, to keep the pitch from sticking. The squares are most +thickly distributed where the abrasion is most required, i.e. +less pitch is melted out by the iron rod. This may be +supplemented by taking advantage of differences of hardness of +pitch, making some squares out of harder, others out of softer +pitch. The aim is to produce a polishing tool which will polish +unequally so as to remove the glass chiefly from predetermined +parts of the lens surface. The tool is worked over the surface of +the lens by the polishing machine, and part of the art consists +in adjusting the strokes to assist in the production of the local +variations required.</p> +<p>A source of difficulty and danger lies in the fact that the +pitch squares are rarely of the same hardness, so that some +abrade the glass more rapidly than others. This is particularly +likely to occur if the pitch has been overheated. +<i>[Footnote:</i> When pitch is heated till it evolves bubbles of +gas its hardness increases with the duration of the +process.<i>]</i> The reader must be good enough to regard these +remarks as of the barest possible kind, and not intended to +convey more than a general idea of the nature of the process of +figuring.</p> +<p>§ 68. A few remarks on cleaning lenses will fittingly +close this part of the subject. There is no need to go beyond the +following instructions given by Mr. Brashear in Popular +Astronomy, 1894, which are reproduced here verbatim.</p> +<p>"The writer does not advise the use of either fine chamois +skin, tissue paper, or an old soft silk handkerchief, nor any +other such material to wipe the lenses, as is usually advised. It +is not, however, these wiping materials that do the mischief, but +the dust particles on the lenses, many of them perhaps of a +silicious nature, which are always harder than optical glass, and +as these particles attach themselves to the wiping material they +cut microscopic or greater scratches on the surfaces of the +objective in the process of wiping.</p> +<p>"I write this article with the hope of helping to solve this +apparently difficult problem, but which in reality is a very +simple one.</p> +<p>"Let us commence by taking the object glass out of its cell. +Take out the screws that hold the ring in place, and lift out the +ring. Placing the fingers of both hands so as to grasp the +objective on opposite sides, reverse the cell, and with the +thumbs gently press the objective <i>squarely</i> out of the cell +on to a book, block of wood, or anything a little less in +diameter than the objective, which has had a cushion of muslin or +any soft substance laid upon it. One person can thus handle any +objective up to 12 inches in diameter.</p> +<p>"Before separating the lenses it should be carefully noted how +they were put together with relation to the cell, and to one +another, and if they art not marked they should be marked on the +edges conspicuously with a hard lead pencil, so that when +separated they may be put together in the same way, and placed in +the same relation to the cell. With only ordinary precaution this +should be an easy matter.</p> +<p>"Setting the objective on edge the two lenses may be readily +separated.</p> +<p>"And now as to the cleaning of the lenses. I have, on rare +occasions, found the inner surfaces of an object glass covered +with a curious film, not caused directly by moisture but by the +apparent oxidation of the tin-foil used to keep the lenses apart. +"A year or more ago a 7-inch objective made by Mr. Clark was +brought to me to clean. It had evidently been sadly neglected. +The inside of the lenses were covered with such a film as I have +mentioned, and I feared the glass was ruined. When taken apart it +was found that the tin-foil had oxidised totally and had +distributed itself all over the inner surfaces. I feared the +result, but was delighted to find that nitric acid and a tuft of +absorbent cotton cut all the deposit off, leaving no stains after +having passed through a subsequent washing with soap and +water.</p> +<p>"I mention this as others may have a similar case to deal +with.</p> +<p>"For the ordinary cleaning of an objective let a suitable +sized vessel, always a wooden one, be thoroughly cleaned with +soap and water, then half filled with clean water about the same +temperature as the glass. Slight differences of temperature are +of no moment. Great differences are dangerous in large +objectives.</p> +<p>"I usually put a teaspoonful of ammonia in half a pail of +water, and it is well to let a piece of washed 'cheese cloth' lie +in the pail, as then there is no danger if the lens slips away +from the hand, and, by the way, every observatory, indeed every +amateur owning a telescope, should have plenty of 'cheese cloth' +handy. It is cheap (about 3 cts. per yard) and is superior for +wiping purposes to any 'old soft silk handkerchief,' chamois +skin, etc. Before using it have it thoroughly washed with soap +and water, then rinsed in clean water, dried and laid away in a +box or other place where it can be kept clean. When you use a +piece to clean an objective throw it away, it is so cheap you can +afford to do so.</p> +<p>"If the lenses are very dirty or 'dusty,' a tuft of cotton or +a camel's-hair brush may be used to brush off the loose material +before placing the lenses in the water, but no pressure other +than the weight of the cotton or brush should be used. The writer +prefers to use the palms of the hand with plenty of good soap on +them to rub the surfaces, although the cheese cloth and the soap +answers nicely, and there seems to be absolutely no danger of +scratching when using the hands or the cheese cloth when plenty +of water is used; indeed when I wish to wipe off the front +surface of an objective in use, and the lens cannot well be taken +out, I first dust off the gross particles and then use the cheese +cloth with soap and water, and having gone over the surface +gently with one piece of cloth, throw it away and take another, +perhaps a third one, and then when the dirt is, as it were, all +lifted up from the surface, a piece of dry cheese cloth will +finish the work, leaving a clean brilliant surface, and no +scratches of any kind.</p> +<p>"In washing large objectives in water I generally use a 'tub' +and stand the lenses on their edge. When thoroughly washed they +are taken out and laid on a bundle of cheese cloth and several +pieces of the same used to dry them.</p> +<p>"I think it best not to leave them to drain dry; better take +up all moisture with the cloth, and vigorous rubbing will do no +harm if the surfaces have no abrading material on them. I have +yet to injure a glass cleaned in this way.</p> +<p>"This process may seem a rather long and tedious one, but it +is not so in practice, and it pays.</p> +<p>"In some places objectives must be frequently cleaned, not +only because they become covered with an adherent dust, but +because that dust produces so much diffused light in the field as +to ruin some kinds of telescope work. Mr. Hale of the Kenwood +Observatory tells me he cannot do any good prominence photography +unless his objective has a clean surface; indeed every observer +of faint objects or delicate planetary markings knows full well +the value of a dark field free from diffused light. The +object-glass maker uses his best efforts to produce the most +perfect polish on his lenses, aside from the accuracy of the +curves, both for high light value and freedom from diffused light +in the field, and if the surfaces are allowed to become covered +with dust, his good work counts for little.</p> +<p>"If only the front surface needs cleaning, the method of +cleaning with cheese cloth, soap and water, as described above, +answers very well, but always throw away the first and, if +necessary, the second cloth, then wipe dry with a third or fourth +cloth; but if the surfaces all need cleaning I know of no better +method than that of taking the objective out of its cell, always +using abundance of soap and water, and keep in a good humor."</p> +<p><b><a name="Toc158108928" id="Toc158108928"><font face= +"Bookman Old Style" size="4">§ 69. The Preparation of Flat +Surfaces of Rock Salt. —</font></a></b></p> +<p><font face="Bookman Old Style">The preliminary grinding is +accomplished as in the case of glass, except that it goes on +vastly faster. The polishing process is the only part of the +operation which presents any difficulty. The following is an +extract from a paper on the subject, by Mr. J. A. Brashear, +Pittsburg, Pa., U.S.A., from the Proceedings of the American +Association for the Advancement of Science, 1885. Practically the +same method was shown me by Mr. Cook some years earlier, so that +I can endorse all that Mr. Brashear says, with the following +exceptions. We consider that for small salt surfaces the pitch is +better scored into squares than provided with the holes +recommended by Mr. Brashear.</font></p> +<p>Mr. Brashear's instructions are as follows. After alluding to +the difficulty of drying the polished salt surface — which +is of course wet — Mr. Brashear says:-</p> +<p>"Happily I have no trouble in this respect now, and as my +method is easily carried out by any physicist who desires to work +with rock salt surfaces, it gives me pleasure to explain it. For +polishing a prism I make an ordinary pitch bed of about two and +one-half or three times the area of the surface of the prism to +be polished. While the pitch is still warm I press upon it any +approximately flat surface, such as a piece of ordinary plate +glass. The pitch bed is then cooled by a stream of water, and +conical holes are then drilled in the pitch with an ordinary +counter sink bit, say one-quarter of an inch in diameter, and at +intervals of half an inch over the entire surface. This is done +to relieve the atmospheric pressure in the final work. The upper +surface of the pitch is now very slightly warmed and a true plane +surface (usually a glass one, prepared by grinding and polishing +three surfaces in the ordinary way, previously wetted) is pressed +upon it until the pitch surface becomes an approximately true +plane itself. Fortunately, moderately hard pitch retains its +figure quite persistently through short periods and small changes +of temperature, and it always pays to spend a little time in the +preparation of the pitch bed.</p> +<p>"The polisher being now ready, a very small quantity of rouge +and water is taken upon a fine sponge and equally distributed +over its surface. The previously ground and fined salt surface +(this work is done the same as in glass working) is now placed +upon the polisher and motion instantly set up in diametral +strokes. I usually walk around the polisher while working a +surface. It is well to note that motion must be constant, for a +moment's rest is fatal to good results, for the reason that the +surface is quickly eaten away, and irregularly so, owing to the +holes that are in the pitch bed. Now comes the most important +part of this method. After a few minutes' work the moisture will +begin to evaporate quite rapidly. No new application of water is +to be made, but a careful watch must be kept upon the pitch bed, +and as the last vestige of moisture disappears the prism is to be +slipped off the polisher in a perfectly horizontal direction, and +if the work has been well done, a clean, bright, and dry surface +is the result. The surface is now tested by the well-known method +of interference from a perfect glass test plate (see Fig. +178).</p> +<p>"If an error of concavity presents itself the process of +polishing is gone over again, using short diametral strokes. If +the error is one of convexity, the polishing strokes are to be +made along the chords, extending over the edge of the polisher. +The one essential feature of this method is the fact that the +surface is wiped dry in the final strokes, thus getting rid of +the one great difficulty of pitch polishing, a method undoubtedly +far superior to that of polishing on broadcloth. If in the final +strokes the surface is not quite cleaned I usually breathe upon +the pitch bed, and thus by condensation place enough moisture +upon it to give a few more strokes, finishing just the same as +before. In ten minutes I have polished prisms of rock salt in +this manner that have not only shown the D line double, but +Professor Langley has informed me that his assistant, Mr. Keeler +(J. E.), has seen the nickel line clearly between the D lines. +This speaks for the superiority of the surfaces over those +polished on broadcloth.</p> +<p>"In polishing prisms I prefer to work them on top of the +polisher, as they can be easily held, but as it is difficult to +hold lenses or planes in this way without injuring the surfaces, +I usually support them in a block of soft wood, turned so as to +touch only at their edges, and work the polisher over them. +Though it takes considerable practice to succeed at first, the +results are so good that it well repays the few hours' work it +requires to master the few difficulties it presents."</p> +<p><img src="images/Image101.gif" alt="images/Image101.gif" +width="468" height="144">Fig. 58.</p> +<p><b><a name="Toc158108929" id="Toc158108929"><font face= +"Bookman Old Style" size="4">§ 70. Casting Specula for +Mirrors. —</font></a></b></p> +<p><font face="Bookman Old Style">According to Sir H. Grubb (loc. +cit.) the best alloy is made of four atoms of copper and one of +tin; this gives by weight, copper 252, tin 117.8.</font></p> +<p>The copper is melted first in a plumbago crucible; the tin is +added gradually. Of course, in the process of melting, even +though a little fine charcoal be sprinkled over the copper, some +loss of that metal will occur from oxidation. It is convenient in +practice, therefore to reserve a portion of the tin and test the +contents of the crucible by lifting a little of the alloy out and +examining it.</p> +<p>The following indications may be noted: When the copper is in +excess the tint of the alloy is slightly red, and the structure, +as shown at a fractured surface, is coarsely crystalline. As the +proper proportions are more nearly attained, the crystalline +structure becomes finer, the colour whiter, and the crystals +brighter. The alloy is ready for use when the maximum brightness +is attained and the grain is fine.</p> +<p>If too much tin be added, the lustre diminishes. The correct +proportion is, therefore, attained when a further small addition +of tin produces no apparent increase of brightness or fineness of +grain. About three-quarters of the tin may be added at first, and +the other quarter added with testing as described. The alloy is +allowed to cool until on skimming the surface the metal appears +bright and remains so without losing its lustre by oxidation for +a sensible time; it will still be quite red-hot.</p> +<p><img src="images/Image102.gif" alt="images/Image102.gif" +width="298" height="94">Fig. <img src="images/Image103.gif" alt= +"images/Image103.gif" width="280" height="288">59. Fig. 60.</p> +<p>As the speculum alloy is too difficult to work with ordinary +tools, it is best to cast the speculum of exactly the required +shape and size. This is done by means of a ring of iron turned +inside (and out) and on one edge. This ring is laid on a plate of +figured iron, and before the metal is poured the plate (G) (Figs +59 and 61) is heated to, say, 300° C. In order to avoid the +presence of oxide as far as possible, the following arrangements +for pouring are made. A portion of the lower surface of the ring +is removed by radial filing until a notch equal to, say, +one-twentieth of the whole circumference is produced. This is cut +to an axial depth of, say, half an inch.</p> +<p>A bar of iron is then dovetailed loosely into the notch (Fig. +60, B), so that it will rest on the iron plate, and half fill the +notch. The aperture thus left forms the port of ingress for the +hot metal (see Fig. 61, M). A bit of sheet iron is attached to +the upper surface of the ring, and lies as a sort of flap, shaped +like a deep shovel, against the outside of the ring overhanging +the port (Figs. 59 and 61 at F). This flap does not quite reach +the iron plate, and its sides are bent so as to be in contact +with the ring. A portion of a smaller ring is then applied in +such a manner as to form a pouring lip or pool on the outside of +the main ring at E, and the metal can only get into the main ring +by passing under the edge of the flap and up through the port. +This forms an efficient skimming arrangement. The process of +casting is carried out by pouring steadily into the lip.</p> +<p>To avoid air bubbles it is convenient to cause the metal to +spread slowly over the chill, and Mr. Nasmyth's method of +accomplishing this is shown in the figure (61). The chill rests +on three pins, A B C (Figs. 59 and 61). Before pouring begins the +chill is tilted up off C by means of the counterpoise D, which is +insufficient to tilt it after the speculum is poured. It is +important that the chill should be horizontal at the close of the +operation, in order that the speculum may be of even thickness +throughout. This is noted by means of levels placed on the ring +(at K for instance).</p> +<p><img src="images/Image104.gif" alt="images/Image104.gif" +width="452" height="169">Fig. 61.</p> +<p>This apparatus may appear unnecessarily complex, but it is +worth while to set it up, for it makes the operation of casting a +speculum fairly certain. If the metal is at the right temperature +it will form a uniformly liquid disc inside the ring. The mass +sets almost directly, and as soon as this occurs it is pushed to +the edge of the plate and the metal in the lip broken off by a +smart upward tap with a hammer. The dovetailed bit of iron is +knocked downwards and falls off, and the ring may then be lifted +clear of the casting. The object of the dovetail will now be +understood, for without it there is great risk of breaking into +the speculum in knocking the "tail" off.</p> +<p>A box of quite dry sawdust is prepared in readiness for the +process of annealing before the speculum is cast. The box must be +a sound wooden or metal box, and must be approximately air-tight. +For a speculum a foot in diameter the box must measure at least 3 +feet both ways in plan, and be 2 feet 6 inches deep. Half the +sawdust is in the box and is well pressed down so as to half fill +it. The other half must be conveniently ready to hand. As soon as +possible after casting, the speculum is thrown into the box, +covered over with the sawdust, and the lid is put on.</p> +<p>The object in having the box nearly air-tight is to avoid +air-currents, which would increase the rate of cooling. A +speculum a foot in diameter may conveniently take about three +days to anneal, and should be sensibly warm when the box is +opened on the fourth day. For larger sizes longer times will be +required. We will say that the sawdust thickness on each side +must be proportional to the dimensions of the speculum, or may +even increase faster with advantage if time is of no moment.</p> +<p>The process of annealing may be considered successful if the +disc does not fly to pieces in working; it is to be worked on the +chilled side. The object of giving the chill the approximate +counterpart form will now appear; it saves some rough grinding, +and causes the finished surface to be more homogeneous than it +would be if the centre were sunk by grinding through the chilled +surface.</p> +<p>In 1889 I learned from Mr. Schneider, Professor Row-land's +assistant at Baltimore, that in casting specula for concave +gratings a good deal of trouble had been saved by carrying out +the operation in an atmosphere consisting mostly of coal gas. It +was claimed that in this way the presence of specks of oxide was +avoided. I did not see the process in operation, but the results +attained are known and admired by all experimenters.</p> +<p><b><a name="Toc158108930" id="Toc158108930"><font face= +"Bookman Old Style" size="4">§ 71. Grinding and polishing +Specula. —</font></a></b></p> +<p><font face="Bookman Old Style">The rough grinding is +accomplished by means of a lead tool and coarse emery; the size +of grain may be such as will pass a sieve of 60 threads to the +inch. The process of grinding is quite similar to that previously +described, but it goes on comparatively quickly. The rough +grinding is checked by the spherometer, and is interrupted when +that instrument gives accordant and correct measurements all over +the surface.</font></p> +<p>The fine grinding may be proceeded with by means of a +glass-faced tool as before described, or the labour may be +reduced in the following manner. A slate tool, which must be free +from green spots (a source of uneven hardness), is prepared, and +this is brought nearly to the curvature of the roughly ground +speculum, by turning or otherwise. It is finished on the speculum +itself with a little flour of emery. The fine grinding is then +carried on by means of slate dust and water, the slate tool being +the grinder. The tool is, of course, scored into squares on the +surface.</p> +<p>If the casting process has been carried out successfully, the +rough grinding may take, say six hours, and the fine grinding say +thirty hours for a disc a foot in diameter. The greatest source +of trouble is want of homogeneity in the casting, as evidenced by +blowholes, etc. In general, the shortest way is to discard the +disc and start afresh if there is any serious want of perfection +in the continuity or homogeneity of the metal.</p> +<p>Fig. <img src="images/Image105.gif" alt="images/Image105.gif" +width="383" height="243"> 62.</p> +<p>The finely ground surface must, of course, be apparently +correct in so far as a spherometer (with 3 inches between the +legs for a disc 1 foot in diameter) will show. Polishing and +figuring are carried out simultaneously. Half an hour's polishing +with a slate-backed pitch tool and rouge and water will enable an +optical test to be made. The most convenient test is that of +Foucault, a simple appliance for the purpose being shown in the +figure (62). It essentially consists of a small lamp surrounded +by an opaque chimney (A) through which a minute aperture +(pin-hole) is made. A small lens may be used, of very great +curvature, or even a transparent marble to throw an image of the +flame on the pin-hole.</p> +<p>A screen (B) is placed close to the source, and is provided +with a rocking or tilting motion (C) in its own plane. The source +and screen are partly independent, and each is provided with a +fine adjustment which serves to place it in position near the +centre of curvature. The screen is so close to the pin-hole in +fact that both the source and a point on the edge of the screen +may be said to be at the centre of curvature of the mirror. The +mirror is temporarily mounted so as to have its axis horizontal, +in a cellar or other place of uniform temperature.</p> +<p>The final focussing to the centre of curvature is made by the +fine adjustment screws; the image may be received on a bit of +paper placed on the screen and overlapping the edge nearest the +source. The screws are worked till the image has its smallest +dimension and is bisected by the edge of the screen. The test +consists in observing the appearance of the mirror surface while +the screen is tilted to cut off the light, as seen by an eye +placed at the edge of the screen, a peephole or eye lens being +provided to facilitate placing the eye in a correct position. The +screen screws are worked so as to gradually cut off the light, +and the observer notes the appearance of the mirror surface. If +the curves are perfect and spherical, the transition from +complete illumination to darkness will be abrupt, and no part of +the mirror will remain illuminated after the rest.</p> +<p>For astronomical purposes a parabolic mirror is required. In +this case the disc may be partially screened by zonal screens, +and the position of the image for different zones noted; the +correctness or otherwise of the curvature may then be ascertained +by calculation. A shorter way is to place the source just outside +the focus, to be found by trial, and then, moving the extinction +screen (now a separate appliance) to, say, five times the radius +of curvature away, where the image should now appear, the +suddenness of extinction may be investigated. This, of course, +involves a corresponding modification of the apparatus.</p> +<p>Whether the tests indicate that a deepening of the Centre, +i.e. increase of the curvature, or a flattening of the edges is +required, at least two remedial processes are available. The +"chisel and mallet" method of altering the size of the pitch, +squares of the polisher may be employed, or paper or small pitch +tools may be used to deepen the centre. The "chisel and mallet" +method merely consists in removing pitch squares from a uniformly +divided tool surface by means of the instruments mentioned. This +removal is effected at those points at which the abrasion +requires to be reduced.</p> +<p>When some practice is attained, I understand that it is usual +to try for a parabolic form at once, as soon as the polishing +commences. This is done by dividing the pitch surface by V-shaped +grooves, the sides of the grooves being radii of the circular +surface, so that the central parts of the mirror get most of the +polishing action. If paper tools are used they must not be +allowed much overhang, or the edges of the mirror betray the +effects of paper elasticity. Most operators "sink" the middle, +but the late Mr. Lassell, a most accomplished worker, always +attained the parabolic form by reducing the curvature of the +edges of a spherical mirror.</p> +<p><b><a name="Toc158108931" id="Toc158108931"><font face= +"Bookman Old Style" size="4">§ 72. Preparation of Flat +Surfaces. —</font></a></b></p> +<p><font face="Bookman Old Style">As Sir H. Grubb has pointed +out, this operation only differs from those previously described +in that an additional condition has to be satisfied. This +condition refers to the mean curvature, which must be exact (in +the case of flats it is of course zero) to a degree which is +quite unnecessary in the manufacture of mirrors or +lenses.</font></p> +<p>A little consideration will show that to get a surface flat +the most straightforward method is to carry out the necessary and +sufficient condition for three surfaces to fit each other +impartially. If they each fit each other, they must clearly all +be flat. To carry out the process of producing a flat surface, +therefore, two tools are made, and the glass or speculum is +ground first on one and then on the other, the tools being kept +"in fit" by occasional mutual grinding. The grinding and +polishing go on as usual. If paper is employed, care must be +taken that the polisher is about the same size as the object to +be polished.</p> +<p>There is a slight tendency to polish most at the edges; but if +the sweeps are of the right shape and size, this may be corrected +approximately. The best surfaces which have come under my notice +are those prepared as "test surfaces" by Mr. Brashear of +Alleghany, Pa., U.S.A. These I believe to be pitch polished. A +pitch bed is prepared, I presume, in a manner similar to that +described for rocksalt surfaces; but the working of the glass is +an immense art, and one which I believe — if one may judge +by results — is only known to Mr. Brashear.</p> +<p>In general, the effect of polishing will be to produce a +convex or concave surface, quite good enough for most purposes, +but distinctly faulty when tested by the interference fringes +produced with the aid of the test plate. The following +information therefore — which I draw from Mr. 'Cook — +will not enable a student to emulate Mr. Brashear, but will +undoubtedly help him to get a very much better surface than he +usually buys at a high price, as exhibited on a spectroscope +prism.</p> +<p>The only difference between this process and the one described +for polishing lenses, lies in the fact that the rouge is put into +the paper surface while the latter is wet with a dilute gum +"mucilage." It is of course assumed that the object and the two +tools have been finely ground and fit each other impartially. The +paper is rubbed over with rouge and weak gum water. The tool, +when dry, is applied to the flat ground surface (of the object), +and is scraped with the three-cornered file chisel as formerly +described. This process must be very carefully carried out. The +paper must be of the quality mentioned, or may even be thinner +and harder. The cross strokes should be more employed than in the +case of the curved surfaces.</p> +<p>A good deal will depend on the method employed for supporting +the work; it is in general better to support the tool, which may +have a slate backing of any desired thickness, whereby the +difficulty resulting from strains is reduced. The work must be +mounted in such a way as to minimise the effect of changes of +temperature. If a pitch bed is selected, Mr. Brashear's +instructions for rock salt may be followed, with, of course, the +obvious necessary modifications. See also next section.</p> +<p><b><a name="Toc158108932" id="Toc158108932"><font face= +"Bookman Old Style" size="4">§ 73. Polishing Flat Surfaces +on Glass or on Speculum Metal. —</font></a></b></p> +<p><font face="Bookman Old Style">The above process may be +employed for speculum metal, or pitch may be used. In the latter +case a fresh tool must be prepared every hour or so, because the +metal begins to strip and leave bits on the polisher; this causes +a certain amount of scratching to take place. As against this +disadvantage, the process of polishing, in so far as the state of +the surface is concerned, need not take an hour if the fine +grinding has been well done.</font></p> +<p>For the finest work changes of temperature, as in the case of +glass, cause a good deal of trouble, and the operator must try to +arrange his method of holding the object so as to give rise to +the least possible communication of heat from the hand.</p> +<p>The partial elasticity of paper, which is its defect as a +polishing backing, is, I believe, partly counterbalanced by the +difficulty of forming with pitch an exact counterpart tool +without introducing a serious rise of temperature (i.e. warming +the pitch). The rate of subsidence of the latter is very slow at +temperatures where it is hard enough to work reliably as a +polisher.</p> +<p>A student interested in the matter of flat surfaces will do +well to read an account of Lord Rayleigh's work on the subject, +Nature, vol. xlviii., 1893, pp. 212, 526 (or B. A. Reports, +1893). In the first of these communications Lord Rayleigh +describes the method of using test plates, and shows how to +obtain the interference fringes in the clearest manner.</p> +<p>For the ordinary optician a dark room and a soda flame afford +all requisite information; and if a person succeeds in making +three glass discs, say 6 inches in diameter, so flat that, when +superposed in any manner, the interference fringes are parallel +and equidistant, even to the roughest observation, he has nothing +to learn from any book ever written on glass polishing. Lord +Rayleigh has also shown how to use the free clean surface of +water as a natural test plate.</p> +<p>Since the above was written the following details of his exact +course of procedure have been sent to me by Mr. Brashear, and I +hereby tender my thanks:—</p> +<p>"It really takes years to know just what to do when you reach +that point where another touch either gives you the most perfect +results attainable, or ruins the work you have already done. It +has taken us a long time to find out how to make a flat surface, +and when we were called upon to make the twenty-eight plane and +parallel surfaces for the investigation of the value of the metre +of the international standard, every one of which required an +accuracy of one-twentieth of a wave length, we had a difficult +task to perform. However, it was found that every surface had the +desired accuracy, and some of them went far beyond it.</p> +<p>It is an advantage in making flat surfaces to make more than +one at a time; it is better to make at least three, and in fact +we always grind and 'fine' three together. In making speculum +plates we get up ten or twelve at once on the lead lap. These +speculum plates we can test as we go on by means of our test +plane until we get them nearly flat. In polishing them we first +make quite a hard polisher, forming it on a large test plane that +is very nearly correct. We then polish a while on one surface and +test it, then on a second and test it, and after a while we +accumulate plates that are slightly concave and slightly convex. +By working upon these alternately with the same polisher, we +finally get our polisher into such shape that it approximates +more and more to a flat surface, and with extreme care and slow +procedure we finally attain the results desired.</p> +<p>All our flats are polished on a machine which has but little +virtue in itself, unmixed with brains. Any machine giving a +straight diametrical stroke will answer the purpose. The glass +should be mounted so as to be perfectly free to move in every +direction — that is to say, perfectly unconstrained. We +mount all our flats on a piece of body Brussels carpet, so that +every individual part of the woof acts as a yielding spring. The +flats are held in place by wooden clamps at the edges, which +never touch, but allow the bits of glass or metal to move slowly +around if they are circular; if they are rectangular we allow +them to tumble about as they please within the frame holding +them.</p> +<p>For making speculum metal plates either plane or concave we +use polishers so hard that they scratch the metal all over the +surface with fine microscopic scratches. We always work for +figure, and when we get a hard polisher that is in proper shape, +we can do ever so many surfaces with it if the environments of +temperature are all right. If we have fifty speculum flats to +make, and we recently made three times that number, we get them +all ready and of accurate surface with the hard polisher. Then we +prepare a very soft polisher, easily indented when cold with the +thumb nail. A drop of rouge and about three drops of water are +put on the plate, and with the soft polisher about one minute +suffices to clean up all the scratches and leave a beautiful +black polish on the metal. This final touch is given by hand; if +we do not get the polish in a few minutes the surface is +generally ruined for shape, and we have to resort to the hard +polisher again.</p> +<p>I assure you that nothing but patience and perseverance will +master the difficulty that one has to encounter, but with these +two elements ‘you are bound to get there.’"</p> +<p><a name="Toc158108933" id="Toc158108933">CHAPTER III</a></p> +<p><a name="Toc158108934" id="Toc158108934">MISCELLANEOUS +PROCESSES</a></p> +<p><a name="Toc158108935" id="Toc158108935"><font face= +"Bookman Old Style" size="4">§ 74. Coating Glass with +Aluminium and Soldering Aluminium. —</font></a></p> +<p><font face="Bookman Old Style">A process of coating glass with +aluminium has been lately discovered, which, if I mistake not, +may be of immense service in special cases where a strongly +adherent deposit is required. My attention was first attracted to +the matter by an article in the <i>Archives des Sciences +physiques et naturelles de Genève,</i> 1894, by M. Margot. +It appears that clean aluminium used as a pencil will leave a +mark on clean damp glass. If, instead of a pencil, a small wheel +of aluminium — say as big as a halfpenny and three times as +thick — is rotated on the lathe, and a piece of glass +pressed against it, the aluminium will form an adherent, though +not very continuous coating on the glass.</font></p> +<p>Working with a disc of the size described rotating about as +fast as for brass-turning, I covered about two square inches of +glass surface in about five minutes. The deposit was of very +uneven thickness, but was nearly all thick enough to be sensibly +opaque. By burnishing the brilliance is improved (I used an agate +burnisher and oil), but a little of the aluminium is rubbed off. +The fact that the burnisher does not entirely remove it is a sign +of the strength of the adherence which exists between the +aluminium and the glass. In making the experiment, care must be +taken to have the glass quite clean — or at all events free +from grease — in order to obtain the best results.</p> +<p>M. Margot has contributed further information to the Archives +des Sciences physiques et naturelles (February 1895). He finds +that adherence between aluminium and glass is promoted by dusting +the glass with powders, such as rouge. There is no doubt that a +considerable improvement is effected in this way; both rouge and +alumina have in my hands greatly increased the facility with +which the aluminium is deposited. M. Margot finds that zinc and +magnesium resemble aluminium in having properties of adherence to +glass, and, what is more, carry this property into their alloys +with tin. Thus an alloy of zinc and tin in the proportions of +about 92 per cent tin and 8 per cent zinc may be melted on +absolutely clean glass, and will adhere strongly to it if well +rubbed by an asbestos crayon.</p> +<p>A happy inspiration was to try whether these alloys would, +under similar circumstances, adhere to aluminium itself, and a +trial showed that this was indeed the case, provided that both +the aluminium and alloy are scrupulously clean and free from +oxide. In this way M. Margot has solved the problem of soldering +aluminium. I have satisfied myself by trial of the perfect ease +and absolute success of this method. The alloy of zinc and tin in +the proportions above mentioned is formed at the lowest possible +temperature by melting the constituents together. It is then +poured so as to form thin sticks.</p> +<p>The aluminium is carefully cleaned by rubbing with a cuttle +bone, or fine sand, and strong warm potash. It is then washed in +water and dried with a clean cloth. The aluminium is now held +over a clean flame and heated till it will melt the solder which +is rubbed against it. The solder sticks at once, especially if +rubbed with another bit of aluminium (an aluminium soldering bit) +similarly coated. To solder two bits of aluminium together it is +only necessary to tin the bits by this process and then sweat +them together.</p> +<p>The same process applies perfectly to aluminium caused to +adhere to glass by the previously mentioned process, and enables +strong soldered contacts to be made to glass. In one case, while +I was testing the method, the adhesion was so strong that the +solder on contracting while cooling actually chipped the surface +clean off the glass. In order to get over this I have endeavoured +to soften the solder by mixing in a little of the fusible metal +mercury amalgam; and though this prevents the glass from being so +much strained, it reduces the adherence of the solder. It is a +comfort to be able to solder aluminium after working for so many +years by way of electroplating, or filing under solder. An +alternative method of soldering aluminium will be described when +the electroplating of aluminium is discussed, § 138.</p> +<p><i>Gilding Glass</i>. — In looking over some volumes of +the Journal fuer praktische Chemie, I came across a method of +gilding glass due to Boettger (Journ. f. prakt. Chem. 103, p. +414). After many trials I believe I am in a position to give +definite instructions as to the best way of carrying out this +rather troublesome operation. The films of gold obtained by the +process are very thick, and the appearance of the gold +exceedingly fine. The difficulty lies in the exact apportionment +of the reducing solution. If too much of the reducing solution be +added, the gold deposits in a fine mud, and no coating is +obtained. If, on the other hand, too little of the reducing +solution be added, little or no gold is deposited. The secret of +success turns on exactly hitting the proper proportions.</p> +<p>The reducing solution consists of a mixture of aldehyde and +glucose, and the difficulty I have had in following Boettger's +instructions arose from his specifying "commercial aldehyde" of a +certain specific gravity which it was impossible to reproduce. I +did not wish to specify pure aldehyde, which is not very easily +got or stored, and consequently I have had to determine a +criterion as to when the proportion of reducing solution is +properly adjusted.</p> +<p>The aldehyde is best made as required. I employed the ordinary +process as described in Thorpe's Dictionary of Applied Chemistry, +by distilling alcohol, water, sulphuric acid, and manganese +dioxide together. The crude product is mixed with a large +quantity of calcium chloride (dry — not fused), and is +rectified once. The process is stopped when the specific gravity +of the product reaches 0.832 at 60° F. The specific gravity +of pure aldehyde is 0.79 nearly.</p> +<p>The following is a modification of Boettger's +formula:—</p> +<p><i>Solution I</i></p> +<p>1 gram of pure gold is converted into chloride — got +acid free — i.e. to the state represented by AuCl<sub>3</sub>, and +dissolved in 120 cc. of water.</p> +<p>This solution is the equivalent of one containing 6.5 grains +of trichloride to the ounce of water.</p> +<p><i>Solution II.</i></p> +<p>6 grams sodium hydrate.</p> +<p>100 grams water.</p> +<p><i>Solution III.</i></p> +<p>0.2 grams glucose (bought as pure).</p> +<p>12.6 cubic centimetres 95 per cent alcohol.</p> +<p>12.6 cubic centimetres water.</p> +<p>2.0 cubic centimetres aldehyde, sp. gr. 0.832.</p> +<p>To gild glass these solutions are used in the following +proportions by volume:-</p> +<p>16 parts of No. I.</p> +<p>4 parts of No. II.</p> +<p>0.8 parts of No. III.</p> +<p>The glass is first cleaned well with acid and washed with +water: it is then rinsed with Solution No. III. If it is desired +to gild the inside of a glass vessel, Solution No. III. may be +placed in the vessel first, and the walls of the vessel rinsed +round carefully. Solutions I. and II. are mixed separately and +then added to III. — after about two minutes the whole is +well shaken up.</p> +<p>If it be desired to gild a mirror of glass, the glass-plate is +suspended face downwards in a dish of the mixed solutions — +care being taken to rinse the glass with Solution III. first.</p> +<p>If the mixture darkens in from 7' to 10' in diffuse daylight +and at 60°F. it will gild well, and it generally pays to make +a few trials in a test tube to arrive at this. If too much +reducing solution is present the liquid will get dark more +rapidly, and vice versa. The gilding will require several hours +— as much as twelve hours may be needed.</p> +<p>The reaction is one of great chemical interest, being one of +that class of reactions which is greatly affected by capillarity. +Thus it occasionally happens that when the reducing solution is +not in the right proportion, gold will be deposited at the +surface of the liquid (so as to form a gilt ring on the inside of +a test tube), the remainder of the gold going down as mud. The +gold deposited is at first transparent to transmitted light and +is deeply blue. I thought this might be due to a trace of copper +or silver, but on carefully purifying the gold no change of +colour was noted. If the reducing solution is present in slightly +greater proportions than that given in the formula, the gold +comes down with a richer colour, and has a tendency to form a mat +surface and to separate from the glass. The gold which is +deposited more slowly has a less rich colour but a brighter +surface. The operation should be interrupted when a sufficient +deposit has been obtained, because it is found that the thicker +the deposit, the more lightly is it held to the glass +surface.</p> +<p><b><a name="Toc158108936" id="Toc158108936"><font face= +"Bookman Old Style" size="4">§ 75. The Use of the +Diamond-cutting Wheel. —</font></a></b></p> +<p><font face="Bookman Old Style">A matter which is not very well +known outside geological circles is the manipulation of the +diamond-cutting wheel, and as this is often of great use in the +physical laboratory, the following notes may not be out of place. +I first became acquainted with the art in connection with the +necessity which arose for me to make galvanometer mirrors out of +fused quartz, and it was then that I discovered with surprise how +difficult it is to obtain information on the point. I desire to +express my indebtedness to my colleagues, Professor David and Mr. +Smeeth, for the instruction they have given me. In what follows I +propose to describe their practice rather than my own, which has +been of a makeshift description. I will therefore select the +process of cutting a slice of rock for microscopical +investigation.</font></p> +<p><b><a name="Toc158108937" id="Toc158108937"><font face= +"Bookman Old Style" size="4">§ 76. Arming a Wheel. +—</font></a></b></p> +<p><font face="Bookman Old Style">Fig. <img src= +"images/Image106.gif" alt="images/Image106.gif" width="430" height="310">63.</font></p> +<p>A convenient wheel is made out of tin-plate, i.e. mild steel +sheet, about one-thirtieth of an inch thick and seven inches in +diameter. This wheel must be quite flat and true, as well as +round; too much pains cannot be taken in securing these +qualities. After the wheel is mounted, it is better to turn it +quite true by means of a watch-maker's "graver", or other +suitable tool. The general design of a rock-cutting machine will +be clear from the illustration (Fig. 63).</p> +<p>The wheel being set up correctly, the next step is to arm it +with diamond dust. For this purpose it is before all things +necessary that real diamond dust should be obtained. The best +plan is to procure a bit of "bort" which has been used in a +diamond drill, and whose properties have therefore been tested to +some extent. This is ground in a diamond mortar — or rather +hammered in one — and passed through a sieve having at +least 80 threads to the inch. The dust may be conveniently kept +in oil.</p> +<p>To arm the wheel, a little dust and oil is taken on the +finger, and laid on round the periphery of the wheel. A bit of +flint or agate is then held firmly against the edge of the wheel +and the latter is rotated two or three times by hand. The +rotation must be quite slow — say one turn in half a minute +— and the flint must be held firmly and steadily against +the wheel. Some operators prefer to hammer the diamond dust into +the wheel with a lump of flint, or agate, but there is a risk of +deforming the wheel in the process. When a new wheel is set up, +it may be necessary to repeat the above process once every half +hour or so till the cutting is satisfactory, but when once a +wheel is well armed it will work for a long time without further +attention.</p> +<p><b><a name="Toc158108938" id="Toc158108938"><font face= +"Bookman Old Style" size="4">§ 77. Cutting a Section. +—</font></a></b></p> +<p><font face="Bookman Old Style">A wheel 7 inches in diameter +may be rotated about 500 times per minute, and will give good +results at that speed. The work, as will be seen from the +diagram, is pressed against the edge of the wheel by a force, +which in the case quoted was about the weight of eleven ounces. +This was distributed along a cutting arc of three-quarters of an +inch.</font></p> +<p>A convenient cutting lubricant is a solution of Castile soap +in water, and this must be freely supplied; if the wheel gets dry +it is almost immediately spoiled owing to the diamond dust being +scraped off. In the figure the lubricant is supplied by a wick +running into the reservoir. I have used both clock oil and +ordinary gas-engine oil as lubricants, with equally satisfactory +results. As to the speed of cutting, in the experiment quoted a +bit of rather friable "gabbro," measuring three-quarters of an +inch on the face by five-eighths of an inch thick, was cut clean +through in six minutes, or by 3000 turns of the wheel. The travel +of the edge was thus between 5000 and 6000 feet, or say 9000 +feet, nearly 2 miles, per inch cut.</p> +<p>A good solid rock, like basalt, can be cut into slices of +about 3/32 inch thick. A very loose rock is best boiled in Canada +balsam, hard enough to set, before it is put against the +wheel.</p> +<p>Instead of a grinding machine a lathe may be employed. The +disc is, of course, mounted on the mandrel, and the work on the +slide-rest. The latter must be disconnected from its feed screws, +and a weight arranged over a pulley so as to keep the work +pressed against the wheel by a constant force.</p> +<p>It may, perhaps, occur to the reader to inquire whether any +clearance in the cut is necessary. The answer is that in all +probability, and in spite of every care, the wheel will wobble +enough to give clearance. If it does not, a little diamond dust +rubbed into the side of the wheel, as well as the edge, will do +all that is required. The edge also, after two or three armings, +"burrs" a little, and thus provides a clearance naturally. It is +not unlikely that in the near future the electric furnace will +furnish us with a number of products capable of replacing the +diamond as abrading agents. The cost of the small amount of +diamond dust; required in a laboratory is so small, however, that +it; is doubtful whether any appreciable economy will be, +effected.</p> +<p><b><a name="Toc158108939" id="Toc158108939"><font face= +"Bookman Old Style" size="4">§ 78. Grinding Rock Sections, +or Thin Slips of any Hard Material.—</font></a></b></p> +<p><font face="Bookman Old Style">A note on this is, perhaps, +worth making, for the same reasons as were given for note, § +75, which it naturally follows. Just as trout-fishing; is +described by Mr. Francis as the "art of fine and far off," +<i>[Footnote:</i> In the Badminton Library, volume on +Fishing.<i>]</i> section grinding may be called "the art of +Canada balsam cooking," as follows. A section of rock having been +cut from the lump as just described, it becomes; necessary to +grind it down for purposes of microscopical investigation. For +this purpose it is placed on a slip of glass, and cemented in +position by Canada, balsam. Success in the operation of grinding +the mounted section depends almost entirely on the way in which +the mounting is done, and this in its turn depends on the +condition to which the Canada has been brought.</font></p> +<p>To illustrate the operations, I will describe a specific case, +viz. that of grinding the section of "gabbro"' above described, +for microscopical purposes. One side of the section is probably +sufficiently smooth and plane from the operation of the diamond +wheel; if not, it must be ground by the finger on a slab of iron +or gun-metal with emery and water, the emery passing a sieve of +80 threads to the inch. The glass base on which the section is to +be mounted for grinding is placed on a bit of iron or copper +plate over a Bunsen burner, and three or four drops of natural +Canada balsam are placed upon it. The section is placed on the +plate to heat at the same time.</p> +<p>The temperature must not rise so high as to cause any visible +change in the Canada balsam, except a slight formation of +bubbles, which rise to the surface, and can be blown off. The +heating may require to be continued, say, up to twenty minutes. +The progress of the operation is tested by examining the balsam +as to its viscous properties.</p> +<p>An exceedingly simple and accurate way of testing is to dip a +pair of ordinary forceps in the balsam, which may be stirred a +little to secure uniformity. The forceps are introduced with the +jaws in contact, and, as soon as withdrawn, the jaws are allowed +to spring apart, thus drawing out a balsam thread. In a few +moments the thread is cold, and if the forceps be compressed, +this thread will bend.</p> +<p>The Canada must be heated until it is just in such a state +that on bringing the jaws together the thread breaks. The forceps +may open to about three-quarters of an inch. If the Canada is +more viscous, so that the thread does not break, the section when +cemented by it will most probably slip on the slide. On the other +hand, if the balsam is more brittle, it will crumble away during +the grinding.</p> +<p>Assuming that the proper point has been reached, the section +is mounted with the usual precautions to avoid air bubbles, i.e. +by dropping one edge on the balsam first. When all is cold, the +surface of the section may be ground on an iron plate with emery +passing the 80 sieve, till it is about 1/40 inch thick. From this +point it must be reduced on ground glass by flours of emery and +water; the rough particles of the former may be washed out for +fine work.</p> +<p>The process of grinding should not take more than half an hour +if the section is properly cut, etc. Beyond this point the +allowable thickness must depend on the nature of the rock; a good +general rule is to get the section just so thin that felspars +show the yellow of the first order in a polarising, microscope. +The section is then finished with, say, two minutes emery or +water of Ayr-stone dust. It is better not to have the surface too +smooth.</p> +<p>To transfer the section, the hard Canada round the sides is +scraped away, and the section itself covered with some fresh +Canada from the bottle. It is then warmed till it will slip off +when a pin, or the invaluable dentist's chisel, is pressed +against one side. If the section be very delicate, the cover slip +should be placed over it before it is moved to the proper slide. +The Canada used for mounting is not quite so hard as that +employed in grinding, but it should be hard when cold, i.e. not +sticky.</p> +<p>The art of preparing Canada balsam appears to consist in +heating it under such conditions as will ensure its being exposed +in thin layers. I have wasted a good deal of time in trying to +bake Canada in evaporating basins, with the invariable result +that it was either over or under-baked, and got dark in colour +during the process.</p> +<p>On reviewing the process of rock section-cutting and mounting +as just described, I cannot help thinking that, if properly +systematised, it could be made much more rapid by the +introduction of proper automatic grinding machinery. It also +seems not improbable that a proper overhaul of available gums and +cements would be found to lead to a cementing material less +troublesome than Canada balsam.</p> +<p><b><a name="Toc158108940" id="Toc158108940"><font face= +"Bookman Old Style" size="4">§ 79. Cutting Sections of Soft +Substances. —</font></a></b></p> +<p><font face="Bookman Old Style">Though this art is fully +treated of in books on practical biology, it is occasionally of +use to the physicist, and the following note treats of that part +of the subject which is not distinctly biological.</font></p> +<p>Soft materials, of which thin sections may be required, +generally require to be strengthened before they are cut. For +this purpose a variety of materials are available. The one most +generally used is hard paraffin. The only point requiring +attention is the embedding. The material must be dry.</p> +<p>This is accomplished by soaking in absolute alcohol, i.e. +really absolute alcohol made by shaking up rectified spirit with +potassium carbonate, previously dried, and then digesting for a +day with large excess of quick-lime, making use of an inverted +condenser and finally distilling off the alcohol without allowing +it to come in contact with undried air. After soaking for some +time in absolute alcohol, the material may be transferred to oil +of bergamot, or oil of cloves, or almost any essential oil. After +soaking in this long enough to allow the alcohol to diffuse out, +the material may be lifted into a bath of melted paraffin +(melting at, say, 51° C.). The process of soaking is in some +cases made to go more rapidly by exhausting, and, if the material +will stand it, by raising the temperature over 100° C. The +soaking process may require minutes, hours, or days, according to +the size and density of the material; but a few hours are usually +sufficient.</p> +<p>When cold, the sections may be cut in any of the ordinary +forms of microtome.</p> +<p>Another way of embedding is to soak in collodion, and then +precipitate the latter in the material and around it by plunging +into nearly absolute alcohol. The collodion yields a harder +matrix than the paraffin.</p> +<p>Whatever form of cutting machine is employed, the art of +sharpening the knife is the only one requiring any particular +notice. The easiest way of obtaining a knife hard enough to +sharpen, is to use a razor of good quality. If it has to be +ground, it is best to do this on a fine Turkey stone which is +conveniently rested on two bits of rubber tubing, to avoid +jarring the blade. Many stones are slightly cracked, but on no +account must the razor be dragged across a crack, or the edge +will suffer.</p> +<p>The necessary and sufficient condition is that the razor must +be worked in little sweeps over the stone, and pressed against +the latter by little more than its own weight, and the grinding +must be regular. The edge may be inspected under a microscope, +and it must be perfectly smooth and even before it will cut +sections. A finishing touch may be given on a leather strap, but +it must be done skilfully, otherwise it is better omitted.</p> +<p>The necessity for providing exceptionally keen and sharp edges +arose in the manufacture of phonographs, where the knife used to +turn up the wax cylinders must leave a perfectly smooth surface. +In 1889 this was being accomplished on an ivory lap fed with a +trace of very fine diamond dust.</p> +<p>I have had this method in mind as a possible solution of the +difficulty of razor-grinding, but have not tried it. I imagine +one would use a soft steel or ivory slip rubbed over with fine +diamond dust and oil by means of an agate. The lap used in the +phonograph works was rotated at a high speed.</p> +<p><b><a name="Toc158108941" id="Toc158108941"><font face= +"Bookman Old Style" size="4">§ 80. On the Production of +Quartz Threads.' —</font></a></b></p> +<p><i><font face="Bookman Old Style">[Footnote:</font> Since this +was written an article on the same subject by Mr. Boys appeared +in the Electrician for 1896. The instructions therein given are +in accordance with what I had written, and I have made no +alteration in the text.]</i></p> +<p>In 1887 the important properties of fused quartz were +discovered by Mr. Vernon Boys (Philosophical Magazine, June 1887, +p. 489, "On the Production, Properties, and Some Suggested Uses +of the Finest Threads"). A detailed study of the properties of +quartz threads was made by Mr. Boys and communicated to the +Society of Arts in 1889 (Journal of the Society of Arts, 1889). +An independent study of the subject was made by the present +writer in 1889 (Philosophical Magazine, July 1890, "On the +Elastic Constants of Quartz Threads ").</p> +<p>There is also a paper in the Philosophical Magazine for 1894 +(vol. xxxvii. p. 463), by Mr. Boys, on "The Attachment of Quartz +Fibres." This paper also appeared in the Journal of the Physical +Society at about the same date, together with an interesting +discussion of the matter. In the American Journal, Electric +Power, for 1894, there is a series of articles by Professor +Nichols on "Galvanometers," in which a particular method of +producing quartz threads is recommended. The method was +originally discovered by Mr. Boys, but he seems to have made no +use of it. A hunt through French and German literature on the +subject has disclosed nothing of interest — nothing indeed +which cannot be found in the papers mentioned.</p> +<p>§ 81. Quartz fibres have two great advantages over other +forms of suspension when employed for any kind of torsion +balance, from an ordinary more or less "astatic" galvanometer to +the Cavendish apparatus. In the first place the actual strength +of the fibres under longitudinal stress is remarkably high, +ranging from fifty to seventy tons weight per square inch of +section, and even more than this in the case of very fine +threads; the second and more important point in favour of quartz +depends on the wide limits within which cylindrical threads of +this material obey the simplest possible law of torsion, i.e. the +law that for a given thread carrying a given weight at a given +temperature and having one end clamped, the twist about the axis +of figure produced by a turning moment applied at the free end is +proportional simply to the moment of the twisting forces, and is +independent of the previous history of the thread.</p> +<p>It is to be noted, however, that the torsional resilience of +quartz as tested by the above law is not so perfect but that our +instrumental means allow us to detect its imperfections, and thus +to satisfy ourselves that threads made of quartz are not things +standing apart from all other materials, except in the sense that +the limits within which they may be twisted without deviating in +their behaviour from the law of strict proportionality by more +than some unassigned small quantity, are phenomenally wide.</p> +<p>A torsion balance — if we except the case of certain +spiral springs — is almost always called upon for +information as to the magnitude of very small forces, and for +this purpose it is not essential merely that some law of twisting +should be exactly obeyed, but also that the resistance to +twisting of the suspension should be small.</p> +<p>Now, regarded merely as a substance possessing elastic +rigidity, quartz is markedly inferior to the majority of +materials, for it is very stiff indeed; its utility depends as +much as anything upon its great strength, for this allows us to, +use threads of exceeding fineness. In addition to this it must be +possible, and moreover readily possible, to obtain threads of +uniform section over a sufficient length, or the rate of twist +per unit length of the thread will vary in practice from point to +point, so that the limits of allowable twist averaged over the +whole thread may not be exceeded, and yet they may be greatly +overpassed at particular points of the thread.</p> +<p>It is interesting to note that in the case of quartz we not +only have a means for readily producing very uniform cylindrical +threads, but that the limits of allowable rate of twist are so +wide that a small departure from uniformity of section produces +much less inconvenience than in the case of any other known +substance.</p> +<p>§ 82. There are three methods generally in use for +drawing quartz fibres, all depending on the fact that quartz when +fused is so viscous that it may be drawn into threads of great +length, without these threads breaking up into drops, or indeed +without their showing any sign of doing so. The surface tension +of the melted quartz must, however, be very considerable, as may +be seen by examining the shape of a drop of the molten material, +and this suffices to impress a rigidly cylindrical form upon the +thread, the great viscosity apparently damping down all +oscillation.</p> +<p>The first method is the one originally employed by Mr. Boys. A +needle of quartz is melted somewhere in its length and is then +drawn out rapidly by a light arrow, to which one end of the +needle is attached, and which is projected from a kind of +crossbow.</p> +<p>A modification of this method, which the writer has found of +service when very thick threads are required, is to replace the +bow and arrow by a kind of catapult.</p> +<p>The third method, which yields threads of almost unmanageable +fineness, depends on the experimental fact that when a fine point +of quartz is held in a high pressure oxygen gas blow-pipe flame, +the friction of the flame gases suffices to overcome the tendency +of the capillary forces to produce a spherical drop, and actually +causes a fine thread to be projected outwards in the direction of +the flame.</p> +<p>§ 83. A preliminary operation to any method is the +production of a stick of fused quartz. This is managed as +follows. A rock crystal or quartz pebble is selected and +examined. It must be perfectly white, transparent, and free from +dirt. Surface impurity can of course be got rid of by means of a +grindstone. The crystal is placed in a perfectly clean +Stourbridge clay crucible, furnished with a cover, and heated to +bright redness for about an hour in a clean fire or in a +Fletcher's gas furnace. The contents of the crucible are turned +out when sufficiently cool on to a clean brick or bit of slate. +It will be found that the crystal is completely broken up and the +fragments must be examined in case any of them have become +contaminated by the crucible, but this will not have happened if +the temperature did not rise beyond a bright red heat.</p> +<p>The heap of fragments being found satisfactory, the next thing +is to fuse some of the pieces together. Unless the preliminary +heating has been efficiently carried out this will prove an +annoying task, because a rock crystal generally contains so much +water that it splinters under the blow-pipe in a very persistent +manner. There are two ways of assembling the fragments. One is to +place two tiles or bricks on edge about the heap of quartz lying +upon a third tile, so that the heap occupies the angular corner +or nook formed by the tiles (Fig. 64).</p> +<p>The oxygas blow-pipe previously described is adjusted to give +its hottest flame, the bags being weighted by at least two +hundredweight, if of the size described (see § 15).</p> +<p>The tip of the inner cone of the blow-pipe is brought to bear +directly upon one of the fragments, and if the operation is +performed boldly it will be found that the surface of the +fragment can be fused, and the fragment thus caused to hold +together before the lower side gets hot enough to suffer any +contamination from the tile or brick. A second fragment may be +treated in the same way, and then a third, and so on.</p> +<p>Finally, the fragments may be fused together slightly at the +corners, and a stick may thus be formed. Of course a good deal of +cracking and splitting of the fragments takes place in the +process; the best pieces to operate upon are those which are well +cracked to begin with, and that in such a way that the little +fragments are interlocked.</p> +<p>An alternative method which has some advantages is to arm a +pair of forceps with two stout platinum jaws, say an inch and a +half long, and flattened a little at the ends. The fragments are +held in these platinum forceps and the blow-pipe applied as +before. This method works very well in adding to a rod which has +already been partly formed, but the jaws require constant +renewals. The first fragment which is fused sufficiently to +cohere may also be fused to a bit of tobacco pipe, or hard glass +tube or rod, and the quartz stick gradually built up by fusing +fresh pieces on to the one already in position.</p> +<p>Fig. <img src="images/Image107.gif" alt="images/Image107.gif" +width="415" height="200">64.</p> +<p>Since the glass or pipeclay will contaminate the quartz which +has been fused on to it, it is necessary to discard the end +pieces at the close of the operation. A string of fragments +having been collected and stuck together, the next step is to +fuse them down into a uniform rod. This is easily done by holding +the string in the blow-pipe flame and allowing it to fuse down. +Twisting the fused part has a good effect in assisting the +operation. It is desirable to use a large jet and as powerful a +flame as can be obtained during this part of the operation.</p> +<p>The final result should be a rod, say two or three inches long +and one-eighth of an inch thick, which will in most cases contain +a large number of air bubbles. Since the presence of drawn-out +bubbles cannot be advantageous, it is often desirable to get rid +of them, and this can conveniently be done at the present stage. +The process at best is rather tedious; it consists in drawing the +quartz down very fine before an intense flame, in order to allow +the bubbles to get close enough to the surface to burst. A +considerable loss of material invariably occurs during the +process; for whenever the thin rod separates into two bits the +process of flame-drawing of threads goes on, and entails a +certain waste; moreover, the quartz in fine filaments is probably +partially volatilised.</p> +<p>Sooner or later, however, a sufficient length of bubble-free +quartz can be obtained. It must not be supposed that it is always +necessary to eliminate bubbles as perfectly as is contemplated in +the foregoing description of the treatment, but for special +purposes it may be essential to do so, and in any case the +reader's attention is directed to a possible source of error.</p> +<p>It may be mentioned in connection with this matter that +crystals of quartz may look perfectly white and clear, and yet +contain impurity. For instance, traces of sodium are generally +present, and lithium was found in large spectroscopic quantity in +five out of six samples of the purest crystals in my laboratory. +The presence of lithium in rock crystal has also been detected by +Tegetmeier (Vied. Ann., xli. p. 19, 1890).</p> +<p>After some practice in preparing rods and freeing them of +bubbles the operator will notice a distinct difference in the +fusibility of the samples of quartz he investigates, though all +may appear equally pure to the unaided eye. It should be +mentioned, however, that high infusibility cannot always be taken +as a test of purity, for the most infusible, or rather most +viscous, sample examined by the writer contained more lithium +than some less viscous samples.</p> +<p>Fig. <img src="images/Image108.gif" alt="images/Image108.gif" +width="237" height="21"> 65.</p> +<p>During the process of freeing the quartz from bubbles the +lithium and sodium will be found to burn away, or at all events +to disappear.</p> +<p>A rod of quartz, say three inches long, one-sixteenth of an +inch in diameter, and free from bubbles for half an inch of its +length, even when examined by a strong lens, is suitable for +drawing into threads. The rod is manipulated exactly in the +manner described under GLASS-BLOWING, and is finally drawn down +at the bubble free part into a needle, say 0.02 inch in diameter +(No. 25 on the Birmingham wire gauge), and 2 inches long.</p> +<p>Fig. <img src="images/Image109.gif" alt="images/Image109.gif" +width="343" height="24"> 66.</p> +<p>There is one peculiarity about fused quartz which renders its +manipulation easier than that of glass — it is impossible +to break fused quartz, however suddenly it be thrust into the +blow-pipe flame. A rod having a diameter of three-sixteenths of +an inch — and perhaps much more — may be brought +right up to the tip of the inner cone of the oxy-gas flame and +held there-till one side fuses, the other being comparatively +cool, without the slightest fear of precipitating a smash. In +seven years' experience I have never seen a bit of once fused +quartz broken by sudden heating; whether it might be done if +sufficient precautions were taken I do not know.</p> +<p>The reason of the fortunate peculiarity of quartz in this +respect is, I presume, to be found in the fact that quartz once +it has been fused is really a very strong material indeed, and is +also probably the least expansible substance known. From some +experiments of the writer upon the subject, it may be concluded +that at the most quartz which has been fused expands only about +one-fifth as fast as flint-glass, at all events between 20° +and 70° C.</p> +<p><b><a name="Toc158108942" id="Toc158108942"><font face= +"Bookman Old Style" size="4">§ 84. Drawing Quartz Threads. +—</font></a></b></p> +<p><font face="Bookman Old Style">The thick end of the rod of +quartz is held in the fingers or occasionally in a clip. The end +of the fine point is attached to a straw arrow by means of a +little sealing-wax. The arrow is laid on the stock of a crossbow +in the proper position for firing. See Figs. 67 and 68, which +practically explain themselves.</font></p> +<p>The needle is heated by the blow-pipe till a minute length is +in a state of uniform fusion; the arrow is then let fly, when it +draws a thread out with it. The arrow is preferably allowed to +strike a wooden target placed, say, 30 feet away from the bow, +and a width of black glazed calico is laid under the line of fire +to catch the thread or arrow if it falls short. The general +arrangements will be obvious from the figure.</p> +<p>The bow is of pine in the case where very long thin threads +are required, though for ordinary purposes I have found a bow of +lance-wood succeed quite as well. The trigger of the bow consists +of a simple pin passing through the stock and fastened at its +lower end to a string connected with a board which can be +depressed by foot. In the figure an ordinary trigger is shown, +but the pin does just as well.</p> +<p>Fig 67<img src="images/Image110.gif" alt= +"images/Image110.gif" width="299" height="382">.</p> +<p>The arrow is made out of about 6 inches of straw, plugged up +aft by a small plug of pine or willow fastened in with +sealing-wax, and projecting backwards one-eighth of an inch. This +projection serves a double purpose: it gives a point of +attachment for the quartz needle, and on firing the bow it forms +a resisting anvil on which the string of the bow impinges. The +head of the arrow is formed by a large needle stuck in with +sealing-wax, and heavy enough to bring the centre of gravity of +the arrow forward of one-third of its length, the condition of +stability in flight.</p> +<p>Fig. 68<img src="images/Image111.gif" alt= +"images/Image111.gif" width="396" height="252">.</p> +<p>It is not necessary to employ any feathering for these arrows; +though I have occasionally used feathers or mica to "wing the +shaft" no advantage has resulted therefrom.</p> +<p>To get fine threads a high velocity is essential. This is +obtained by considering (and acting upon) the principles +involved. The bow may be regarded as a doubly-tapering rod +clamped at the middle. After deflection it returns towards its +equilibrium position at a rate depending in general terms on the +elastic forces brought into play, directly, and on the effective +moment of inertia of the rod, inversely (see Rayleigh, Sound, +vol. ii. chap. viii.) If the mass of the arrow is negligible +compared with the bow, the rate at which the arrow moves is +practically determined by that attained by the end of the bow, +which is a maximum in crossing its equilibrium position.</p> +<p>The extent to which the arrow profits by this velocity depends +on the way the bow is strung. It will be greatest when the string +is perpendicular to the bow when passing its equilibrium +position; or in other words, when the string is infinitely long. +Since the string has mass, however, it is not permissible to make +it too long, or its weight begins to make itself felt, and a +point is soon reached at which the geometrical gain in string +velocity is compensated for by the total loss of velocity due to +the inertia of the string. In practice it is sufficient to use a +string 10 per cent longer than the bow.</p> +<p>It is well to use a light fiddle string, served with waxed +silk at the trigger catch; if this be omitted the gut gets worn +through very quickly. In order to decide how far it is +permissible to bend the bow, the quickest way is to make a rough +experiment on a bit of the same plank from which the bow is to be +cut, and then to allow a small factor of safety. In the figure +the bow is of lance-wood and is more bent than would be suitable +for pine.</p> +<p>The bow itself is tapered from the middle outwards just like +any other bow. If thick threads are required, the above +considerations are modified by the fact that quartz opposes a +considerable resistance to drawing, and that consequently the +arrow must not only have a high velocity, but a fair supply of +energy as well; in other words, it must be heavy. A thin pine +arrow instead of a straw generally does very well, but in this +case the advantage of using pine for the bow vanishes; and in +fact lance-wood does better, owing to the greater displacement +which it will stand without breaking. This of course only means +that a greater store of energy can be accumulated at one +bending.</p> +<p>I had occasion to investigate whether the unavoidable spin of +an arrow about its axis produces any effect on the thread, and +for this purpose made arrows with inertia bars thrust through the +head, i.e. an arrow with a bit of wire run through it, +perpendicular to its length — forming a cross in fact +— the arms of the cross being weighted at the extreme ends +by shot. This form of arrow has a considerable moment of inertia +about its longer axis, and consequently rotates less than a mere +straw, provided that the couples tending to produce rotation are +not increased by the cross arm, or the velocity too much reduced. +Shooting one of these arrows slowly, I could see that it did not +rotate, and when fired at a high velocity, it generally arrived +at the target (placed at varying distances front bow) with the +arms nearly horizontal, thus showing that it probably did not +rotate much.</p> +<p>I did not succeed in this at the first trial, by any means. +The threads got in this way were no better than those made with a +single straw, whence we may conclude very provisionally that the +spin of the arrow has only a small effect, if any, on the quality +of the threads.</p> +<p>Feathering the arrow, in my experience, tends, if anything to +make it spin more; for one thing, because it is practically +impossible to lay the feathering on straight.</p> +<p>After the arrow is shot, it remains to gather in the thread, +and if the latter is at all thin, we have a rather troublesome +job. In a thread thirty or forty feet long, the most uniform part +generally lies in the middle if the thread is thin, i.e. of the +order of a ten-thousandth of an inch in diameter. If the thread +is thick the most uniform part may be anywhere. The part of the +thread required is generally best isolated by passing a slip of +paper under it at each end and cementing the thread to the paper +by means of a little paraffin or soft wax, and then cutting off +the outer portions. One bit of paper may then be lifted off the +calico, and the thread will carry the other bit. In this way the +thread may be taken to a blackened board, where it may be mounted +for stock.</p> +<p>By passing the two ends of the thread under a microscope, or +rather by breaking bits off the two ends and examining them +together, it is easy to form an Opinion as to uniformity.</p> +<p>Mr. Boys has employed an optical method of examining threads, +but the writer has invariably found a high-power microscope more +convenient and capable of giving more exact information as to the +diameter of the threads.</p> +<p>The beginner — or indeed the practised hand — need +not expect to get a thread of the exact dimensions required at +the first shot. A little experience is necessary to enable one to +judge of the right thickness of the needle for a thread of given +diameter. The threads are so easily shot, however, that a few +trials take up very little time and generally afford quite +sufficient experience to enable a thread of any required diameter +to be prepared.</p> +<p>It is no use attempting to heat an appreciable length of +needle; if this be done the thread almost invariably has a thick +part about the middle of its length.. It is sufficient to fuse at +most about one-twentieth of an inch along the needle before +firing off the bow. This can be done by means of the smaller +oxygas blow-pipe jet described in the article on blow-pipes for +GLASS-BLOWING, § 14. The flame must of course be turned down +so as to be of a suitable size. A sufficiently small flame may be +got from almost any jet.</p> +<p>If the needle be not equally heated all round, the thread +tends to be curly; indeed by means of the catapult, threads may +be pulled which, when broken, tend to coil up like the +balance-springs of watches, if only care be taken to have one +side of the needle much hotter than the other.</p> +<p>§ 85. When examining bits of threads, say thicker than +the two-thousandth of an inch, under the microscope it is +convenient to use a film of glycerine stained with some kind of +dye, in order to render the thread more sharply visible. The +thread is mounted beneath a cover slip, and a drop of the stained +glycerine allowed to run in. Such a treatment gives the image of +the thread a sharply defined edge 3 and the contrast between the +whiteness of the thread and the colour of the background allows +measurements to be made with great ease.</p> +<p>On the whole the easiest way of measuring the diameter of a +thick thread is to use a measuring microscope, i.e. one in which +the lens system can be displaced along a plane bed by means of a +finely cut micrometer screw. The instruments made by the +Cambridge Scientific Instrument Company do fairly well. Direct +measurements up to 0.0001 inch are easily made by means of a +microscope provided with a Zeiss "A" objective, and rather +smaller differences of thickness can be made out by it. For thin +threads the method next to be described is more fitting, because +higher powers can be more conveniently used.</p> +<p>In this method an ordinary microscope is employed together +with a scale micrometer, and either an eyepiece micrometer, or a +camera and subsidiary scale. The eyepiece micrometer is the more +convenient. If a camera be employed, i.e. such an one as is +supplied by Zeiss, it is astonishing how the accuracy of +observation may be increased by attending carefully to the +illumination of both the subsidiary scale and of the thread. The +two images should be as far as possible of equal brightness, and +for this purpose it will be found requisite to employ small +screens.</p> +<p>The detail of making a measurement by means of the micrometer +eyepiece is very simple. The thread is arranged on the stage so +as to point towards the observer, and the apparent diameter is +read off on the eyepiece scale. In order to calibrate the latter +it is only necessary to replace the thread by the stage +micrometer, and to observe the number of stage micrometer +divisions occupying the space in the eyepiece micrometer formerly +occupied by the thread. It is essential that both thread and +stage micrometer should occupy the same position in the field, +for errors due to unequal distortion may otherwise become of +importance. For this reason it is best to utilise the centre of +the field only.</p> +<p>The same remark applies to measurements by means of the +camera, where the image of the thread is projected against the +reflected image of the subsidiary scale laid alongside the +microscope. In this case the value of the subsidiary scale +divisions must be obtained from the divisions of the stage +micrometer, coinciding as nearly as possible with the position +occupied by the thread. Before commencing a measurement the +screens are moved about till both images appear equally +bright.</p> +<p>Threads up to about one twenty-thousandth of an inch in +diameter may be sufficiently well measured by means of a Zeiss "4 +centimetre apochromatic object-glass" and an eyepiece "No. 6" +with sixteen centimetre tube length. <i>[Footnote:</i> The +objective certainly had "4 cm." marked on it, but the focal +length appeared to be about I.5 mm. only.<i>]</i></p> +<p><b><a name="Toc158108943" id="Toc158108943"><font face= +"Bookman Old Style" size="4">§ 86. Drawing Threads by the +Catapult. —</font></a></b></p> +<p><font face="Bookman Old Style">The bow-and-arrow method fails +when threads of a greater diameter than about 0.0015 inch are +required — at least if any reasonable uniformity be +demanded, and no radical change in the bow and arrow be carried +out.</font></p> +<p>Thus in the writer's laboratory a thread of about this +diameter, within 1/10000 of an inch-13 inches long and free from +air bubbles — was required. A fortnight's work by a most +skilful operator only resulted in the production of two lengths +satisfying the conditions.</p> +<p>The greatest loss of time occurs in the examination of the +thread by means of the microscope.</p> +<p>Threads for galvanometer suspensions are conveniently from +0.0001 to 0.0004 inch in diameter, and are much more easily made +and got uniform than thicker threads, to the production of which +the catapult method applies.</p> +<p>A reference to the diagram will make the construction of the +instrument quite clear. The moving end of the quartz is attached +to a small boxwood slider working on a tubular girder or between +wires. The quartz is secured in position by clamps shown at A and +B, and motion is imparted to the slider by a stretched piece of +catapult elastic (C). An easy means of regulating the pull of the +elastic is to hold it back by a loop of string whose length can +be varied by twisting it round a pin.</p> +<p>Fig. 69. <img src="images/Image112.gif" alt= +"images/Image112.gif" width="490" height="223"> <i>[Footnote:</i> +For greater clearness of drawing, the tube carrying the slider is +shown somewhat higher above the base than is convenient in +practice; and the slide itself is shown too thin in the direction +of the hole through it.<i>]</i></p> +<p>Since it is not permissible to allow the slider to rebound at +the end of its journey, some such arrangement of breaks as is +shown must be adopted. In the diagram the bottom of the slider +runs on to a brass spring between the girder and the base of the +appliance, and so gets jammed; the spiral spring acts merely as +an additional guard. The diagram does not show the lower spring +very clearly; it is a mere strip lying in the groove.</p> +<p>A rod of quartz, with a needle at one end, is prepared as +before and secured in the clamps. During the operation of +fastening down the clamps, there is some danger of breaking the +needle, and consequently it is advisable to soften the latter +before and while adjusting the second clamp.</p> +<p>The process of drawing a thread by this method is exactly +similar to the operation already described in connection with the +arrow method. Though short thick threads form the product +generally obtained from the catapult, it must not be supposed +that thin threads cannot be obtained in this way. If a short +length of a very fine needle be heated, it will be found to yield +threads quite fine enough for ordinary suspension purposes, but +naturally not so uniform as those obtained from the 40-foot +lengths obtainable by the bow-and-arrow method.</p> +<p>It is easy to make spiral quartz springs resembling watch +balance-springs by means of the catapult. All that is necessary +is to see that the quartz is rather unequally heated before the +shot is fired. In the future it is by no means impossible that +such springs may have a real value, for the rigidity of quartz is +known to increase as temperature rises. Hence it is probable that +the springs would become stiffer as temperature rises, even +though they work chiefly by bending, and little or not at all by +twisting. As this is the kind of temperature variation required +to compensate an uncompensated watch balance wheel, it may turn +out to have some value.</p> +<p><b><a name="Toc158108944" id="Toc158108944"><font face= +"Bookman Old Style" size="4">§ 87. Drawing Threads by the +Flame alone. —</font></a></b></p> +<p><font face="Bookman Old Style">A stick of quartz is drawn down +to a fine point, and the tip of this point is held in the +blow-pipe flame in the position shown in Fig. 70.</font></p> +<p><img src="images/Image113.gif" alt="images/Image113.gif" +width="348" height="469">Fig. 70.</p> +<p>The friction of the flame gases is found to be sufficient to +carry forward the fused quartz and to draw it into threads in +spite of the influence of the capillary forces. If a sheet of +paper be suspended at a distance of two or three feet in front of +the blow-pipe flame, it will be found to be covered with fine +threads tangled together into a cobwebby mass. As this method is +an exceedingly simple one of obtaining threads, I have +endeavoured to reduce it to a systematic operation.</p> +<p>A sheet of cardboard, about two feet square, is painted dead +black and suspended horizontally, painted side downwards (Fig. +70, A), at a height of about two feet above the blow-pipe flame. +The latter is adjusted so as to point almost vertically upwards +and towards the centre of the cardboard. A few half-inch pins are +thrust through the card from the upper surface and pushed home; +about one dozen pins scattered over the surface will be +sufficient. Their object is to prevent the threads being carried +away round the edge of the screen.</p> +<p>The flame from the jet described so often is fed from gas bags +weighted to about eighty pounds per square foot of (one) surface, +i.e. "4-foot" bags require from three to four hundredweight to +give an advantageous pressure. <i>[Footnote:</i> The resulting +threads were really too fine for convenient manipulation, so that +unless extremely fine threads are required it will be better to +reduce the pressure of the gases considerably.<i>]</i></p> +<p>Two sticks of quartz are introduced and caused to meet just in +front of the inner cone — the hottest part of the flame. +They are then drawn apart so as to form a fine neck, which +softens and is bent in the direction of motion of the flame +gases. When fusion is complete the neck separates into two parts, +and a thread is drawn from each of them. By alternately lightly +touching the rods together, and drawing them apart, quite a mass +of threads may be obtained in two or three minutes, when the +process should be stopped. If too many threads get entangled in +the pins, one gives one's self the unnecessary trouble of +separating them. On taking down the card it will be found that +the threads have been caught by the pins; but the card now being +laid black side upwards, the former easily slip off the +points.</p> +<p>Threads at least a foot long, and perhaps vastly longer, may +be obtained by this method, and are extraordinarily fine. When I +first read Professor Nichols' statement (Electric Power, 1894) as +to the value of these fibres for galvanometer purposes, I was +rather sceptical on the ground that the threads would tend to get +annealed by being drawn gradually, instead of suddenly, from a +place of intense heat to regions of lower temperature.</p> +<p>Now annealing threads by a Bunsen makes them rotten. The +threads being immersed in the hot flame gases could only cool at +the same rate as the gas, and it was not — and is not +— clear to me that annealing of the threads can be avoided. +On the other hand, it may be possible that a thread cooled slowly +from the first does not suffer in the same way as a cold thread +would do when annealed in a Bunsen flame.</p> +<p>Again the velocity of the gases is beyond doubt exceedingly +high, so that the annealing, even supposing it to be deleterious, +might not be carried very far. Threads drawn by this method and +measured "dry," i.e. by mounting them on a slide without the +addition of any liquid, turned out to have a diameter of about +1/20000 of an inch.</p> +<p>I do not think I could manage to mount such fine threads +without very special trouble. All the threads lying on the board, +however, were found in reality to consist of three or four +separate threads, and there is no reason why several threads +should not be mounted in parallel, provided, of course, that they +are equally stretched and touching each other. Equality of +tension in the mounting could be secured by making one attachment +good, then cementing the other attachment to the other end of the +threads, and "drawing" the two attachments slightly apart at the +moment the cement commences to set. This method may turn out to +be very valuable, for, so far as I can see, the carrying power +would be increased without an increase of torsional stiffness of +anything like so high an order as would be the case were one +thread only employed. On the other hand, the law of torsion could +hardly be quite so simple, at all events, to the second order of +approximations.</p> +<p><b><a name="Toc158108945" id="Toc158108945"><font face= +"Bookman Old Style" size="4">§ 88. Properties of Threads. +—</font></a></b></p> +<p><font face="Bookman Old Style">A large number of experiments +on the numerical values of the elastic constants of quartz +threads have been made by Mr. Boys and his students, and by the +writer. As the methods employed were quite distinct and the +results wholly independent, and yet in good agreement with each +other, a rounded average may be accepted with considerable +confidence.</font></p> +<table border summary="" cellspacing="1" cellpadding="7" width= +"593"> +<tr> +<td valign="top" colspan="4"> +<p><font face="Bookman Old Style">TENACITY OF QUARTZ FIBRES +(BOYS).</font></p> +</td> +</tr> +<tr> +<td width="35%" valign="top" colspan="2"> +<p><font face="Bookman Old Style">Diameter of Thread.</font></p> +</td> +<td width="29%" valign="top" rowspan="2"> +<p><font face="Bookman Old Style">Tenacity in Tons' Weight per +Square Inch of Section.</font></p> +</td> +<td width="36%" valign="top" rowspan="2"> +<p><font face="Bookman Old Style">Tenacity in Dynes per Square +Centimetre.</font></p> +</td> +</tr> +<tr> +<td width="16%" valign="top" height="54"> +<p><font face="Bookman Old Style">Inches</font></p> +</td> +<td width="19%" valign="top" height="54"> +<p><font face="Bookman Old Style">Centimetres</font></p> +</td> +</tr> +<tr> +<td width="16%" valign="top"> +<p align="right"><font face= +"Bookman Old Style">0.00069</font></p> +</td> +<td width="19%" valign="top"> +<p align="right"><font face= +"Bookman Old Style">0.00175</font></p> +</td> +<td width="29%" valign="top"> +<p align="right"><font face="Bookman Old Style">51.7</font></p> +</td> +<td width="36%" valign="top"> +<p align="right"><font face="Bookman Old Style">8 X +10<sup>9</sup></font></p> +</td> +</tr> +<tr> +<td width="16%" valign="top"> +<p align="right"><font face= +"Bookman Old Style">0.00019</font></p> +</td> +<td width="19%" valign="top"> +<p align="right"><font face= +"Bookman Old Style">0.00048</font></p> +</td> +<td width="29%" valign="top"> +<p align="right"><font face="Bookman Old Style">74.5</font></p> +</td> +<td width="36%" valign="top"> +<p align="right"><font face="Bookman Old Style">11.5 X +10<sup>9</sup></font></p> +</td> +</tr> +</table> +<table border summary="" cellspacing="1" cellpadding="7" width= +"568"> +<tr> +<td valign="top" colspan="2"> +<p><font face="Bookman Old Style">Rounded mean of Boys' and +Threlfall's results:</font></p> +</td> +</tr> +<tr> +<td width="58%" valign="top"> +<p><font face="Bookman Old Style">Young's Modulus at 20° +C.,</font></p> +</td> +<td width="42%" valign="top"> +<p align="right"><font face="Bookman Old Style">5.6 X +10<sup>11</sup> C.G.S.</font></p> +</td> +</tr> +<tr> +<td width="58%" valign="top"> +<p><font face="Bookman Old Style">Modulus of Simple Rigidity at +20° C.,</font></p> +</td> +<td width="42%" valign="top"> +<p align="right"><font face="Bookman Old Style">2.65 X +10<sup>11</sup> C.G.S.</font></p> +</td> +</tr> +<tr> +<td width="58%" valign="top"> +<p><font face="Bookman Old Style">Modulus of +Incompressibility,</font></p> +</td> +<td width="42%" valign="top"> +<p align="right"><font face="Bookman Old Style">1.4 X +10<sup>11</sup> C.G.S.</font></p> +</td> +</tr> +<tr> +<td width="58%" valign="top"> +<p><font face="Bookman Old Style">Modulus of Torsion,</font></p> +</td> +<td width="42%" valign="top"> +<p align="right"><font face="Bookman Old Style">3.7 X +10<sup>11</sup> C.G.S.</font></p> +</td> +</tr> +</table> +<p>Approximate coefficient of linear expansion of quartz per +degree between 80° C. and 30° C. is 0.0000017 +(<i>Threlfall = loc. cit</i>.).</p> +<p>This must be regarded with some suspicion, as the data were +not concordant. There is no doubt, however, about the extreme +inexpansibility of quartz.</p> +<p>Temperature coefficient of modulus of torsional rigidity per +degree centigrade, 22° to 98° C., 0.000133</p> +<p>Ditto, absolute simple rigidity, 0.000128 +(<i>Threlfall</i>).</p> +<p>Limit of allowable rate of twist in round numbers is, +one-third turn per centimetre, in a fibre 0.01 cm. diameter.</p> +<p>The limiting rate is probably roughly inversely as the +diameter.</p> +<p>Attention must be called to the rapid increase in the +torsional rigidity of these threads as the temperature rises. A +quartz spiral spring-balance will be appreciably stronger in hot +weather.</p> +<p>§ 89. In the majority of instances in which quartz +threads are applied in the laboratory, it is desirable to keep +the coefficient of torsion as small as possible, and hence +threads are used as fine as possible.</p> +<p>It is convenient to remember that a thread 0.0014 cm. or +0.0007 inch in diameter breaks with a weight of about ten +grammes, and may conveniently be employed to carry, say, five +grammes. With threads three times finer the breaking strength per +unit area increases, say, 50 per cent. In ordinary practice +— galvanometric work for instance — where it is +desirable to use a thread as fine and short as possible to +sustain a weight up to, say, half a gramme, it will be found that +fibres five centimetres long or over give no trouble through +defect of elastic properties. A factor of safety of two is a fair +allowance when loading threads.</p> +<p>No difficulty will be experienced in mounting threads having a +diameter of 0.0002 inch or over. With finer threads it is +necessary to employ very dark backgrounds (Mr. Boys uses the +darkness of a slightly opened drawer), or the threads cannot be +sufficiently well seen.</p> +<p>In the case of instruments in which threads remain highly +twisted for long periods of time, the above rule as to the safe +limit of twist does not allow of a sufficient margin; it is only +applicable to galvanometric and similar purposes.</p> +<p>The cause of the increase in tenacity as the diameter +diminishes is at present unknown. It is due neither to an effect +of annealing (annealed threads are rotten), nor is it a skin +effect, nor is it due to the cooling of the thread under higher +capillary pressure. It is, however, possible that it may be +associated with some kind of permanent set taken by the fibres +during the stage of passage from the liquid to the solid +state.</p> +<p><b><a name="Toc158108946" id="Toc158108946"><font face= +"Bookman Old Style" size="4">§ 90. On the Attachment of +Quartz Fibres. —</font></a></b></p> +<p><font face="Bookman Old Style">For many purposes it is +sufficient to cement the fibres in position by means of ordinary +yellow shellac, but where very great accuracy is aimed at, the +shellac (being itself imperfectly elastic and exposed to shearing +stress) imposes its imperfections on the whole system. This +source of error can be got over by soldering the threads in +position. Attempts were made by the writer in this direction, +with fair success, in 1889, but as Mr. Boys has carried the art +to a high degree of perfection, I will suppress the description +of my own method and describe his in preference. It has, of +course, been frequently repeated in my laboratory.</font></p> +<p>In many cases, however, if not in all, it may be replaced by +Margot soldering, as already described, a note on the application +of which to this purpose will follow.</p> +<p>A thread of the proper diameter having been selected, it is +cut to the right length. With fine threads this is not always a +perfectly easy matter. The best way is for the operator to +station himself facing a good light, not sunlight, which is too +tiring to the eye, but bright diffused light. The thread will be +furnished with bits of paper stuck on with paraffin at both ends, +as already described.</p> +<p>A rough sketch of the apparatus — or, at all events, two +lines showing the exact length which the free part of the thread +must have — are marked on a smooth board, and this is +supported with its plane vertical. The thread is held against the +board, and the upper piece of paper is stuck lightly to the board +with a trace of soft wax, so that the lower edge of the paper is +at any desired height above the upper mark. This distance is +measured, and forms the length of thread allowed to overlap the +support. A second bit of paper is attached below the lower mark, +a margin for the attachment of the lower end being measured and +left as before. The thread will be most easily seen if the board +is painted a dead black.</p> +<p>If it is desired to attach the thread to its supports merely +by shellac, this is practically all that needs to be done. The +supports should resemble large pins. The upper support will be a +brass wire in most cases, and will require to be filed away as +shown in the sketch (Fig. 71). It is then coated with shellac by +heating and rubbing upon the shellac. As previously noted, the +shellac must not be overheated.</p> +<p>The thread is cut off below the lower slip of paper, and the +upper support being conveniently laid in a horizontal position on +another dead-black surface, the thread is carried to it and laid +as designed against the shellac, which is now cold. When the +thread is in place, a soldering iron is put against the brass +wire, and the shellac gradually melted till it closes over the +thread.</p> +<p><img src="images/Image114.gif" alt="images/Image114.gif" +width="209" height="33">Fig. 71.</p> +<p>The iron is then withdrawn and the thread pulled away from the +point for one-twentieth of an inch or less. This ensures that the +thread makes proper contact with the cement, and also that it is +free from kinks; of course, it must leave the cement in the +proper direction. A similar process is next carried out with +respect to the lower attachment, and the ends of the thread are +neatly trimmed off.</p> +<p>Both ends of the thread being secured, the next step is to +transfer the upper support to a clip stand, the suspended parts +being held by hand, so that the weight comes on the thread very +gradually. In this way it will be easily seen whether the thread +is bent where it enters the shellac, and should this be the case, +a hot iron must be brought up to the shellac and the error +rectified.</p> +<p>When both the support and the suspended parts are brought +nearly to the required bearing, the hot iron is held for a moment +close up to each attachment, the hand being held close below but +not touching the suspended parts, and both attachments are +allowed to straighten themselves out naturally.</p> +<p>These details may appear tiresome, and so they are when +written out at length, but the time occupied in carrying them out +is very short, and quartz threads break easily, unless the pull +upon them is accurately in the direction of their length at all +points.</p> +<p>In the event of its being decided to attach the thread by +soldering, the process is rather more expensive in time, but not +otherwise more troublesome.</p> +<p><img src="images/Image115.gif" alt="images/Image115.gif" +width="152" height="296">Fig. 7<img src="images/Image116.gif" +alt="images/Image116.gif" width="161" height="286">2. Fig. +73.</p> +<p>The thread being cut as before to the proper length, little +bits of aluminium foil are smeared all over with melted shellac +and suspended from the thread replacing the paper slips before +described. It is important that no paraffin should be allowed to +touch the thread anywhere near a point intended to be soldered. +The thread is hung up from a clip stand by one of the bits of +foil, and the lower end is washed by dipping it into strong +nitric acid for a moment and thence into water. The object of +smearing the foils all over with shellac is to prevent them being +acted upon by the acid. The threads are not very easily washed +acid free, but the process may be assisted by means of a fine +camel's-hair pencil.</p> +<p>Some silvering solution made as described (§ 65) is put +into a test tube; the thread, after rinsing with distilled water, +is lowered into the solution so far as is required, and is +allowed to receive a coating of silver. It has been observed that +the coating of silver must not be too thick — not +sufficiently thick to be opaque. A watch may be kept on the +process by immersing a minute strip of mica alongside the +thread.</p> +<p>The silvered thread is rinsed with distilled water and allowed +to dry.</p> +<p>Meanwhile the other end of the thread may be silvered. When +both ends are silvered the process of coppering by electro +deposit is commenced. A test tube is partially filled with a ten +per cent solution of sulphate of copper, and several copper wires +are dipped into it to form an anode. The thread is lowered +carefully into the solution so as not to introduce air bubbles, +and the silvered part is allowed to project far enough above the +surface of the solution to come in contact with a fine copper +wire. The circuit is closed through a Leclanché cell and a +resistance box.</p> +<p>It is as well to begin with a fair resistance, say 100 ohms +out in the box, and the progress of the deposit is watched by +means of a low-power microscope set up in front of the thread. If +the copper appears to come down in a granular form, the +resistance is too small and must be increased; if no headway +appears to be made, the resistance must be diminished.</p> +<p>As soon as a fair coat of copper has come down, i.e. when the +diameter of the thread is about doubled, the process is +interrupted. The thread is withdrawn, washed, dipped in a +solution of chloride of zinc, and carefully tinned by dragging it +over a small clean drop of solder on a soldering bit.</p> +<p>During this part of the process the shellac is apt to get +melted if the iron is held too close, so that it is advisable to +begin by making the thread somewhat over long. The end of the +thread must only be trimmed off at the conclusion of the +operation, i.e. after the thread is soldered up. The thread is +attached to the previously tinned supports much in the same way +as has been described under the head of shellac attachments. It +does not very much matter whether both ends are coppered before +one is soldered up or not. At the conclusion of the whole process +the superfluous copper and silver are dissolved off by a little +hot strong nitric acid applied on a glass hair pencil. This is +best done by holding the thread horizontally with the assistance +of clip stands.</p> +<p>If the thread is too delicate to bear brushing, the nitric +acid may be applied by pouring out a big drop into a bit of +platinum foil and holding this below the thread so as to touch it +lightly. The dissolving of the copper and silver is, of course, +followed by copious washing with hot water. This process is more +laborious than might be imagined, but it may be shortened by +heating the platinum foil supporting the water (Fig. 74).</p> +<p>Fig. <img src="images/Image117.gif" alt="images/Image117.gif" +width="209" height="259"> 74</p> +<p>The washing part of the process is, in the opinion of the +writer, the most difficult part of the whole business, and it +requires to be very thorough, or the thread will end by drawing +out of the solder. In many cases it is better to try to do +without any application of nitric acid at all, but, of course, +this involves silvering and coppering to exact distances from the +ends of the thread — at all events, in apparatus where the +effective length of the thread is narrowly prescribed.</p> +<p>It is important not to leave the active parts of the thread +appreciably silvered, for the sake of avoiding zero changes due +to the imperfect elasticity of the silver. In this soldering +process ordinary tinman's solder may be employed; it must be +applied very free from dust or oxide.</p> +<p><b><a name="Toc158108947" id="Toc158108947"><font face= +"Bookman Old Style" size="4">§ 91. Other Modes of soldering +Quartz. —</font></a></b></p> +<p><font face="Bookman Old Style">Thick rods of quartz may be +treated for attachment by solder in the same way as glass was +treated by Professor Kundt to get a foundation for his +electrolytically deposited prisms. <i>[Footnote:</i> See Appendix +at end of book.<i>]</i></font></p> +<p>The application of a drop of a strong solution of platinum +tetrachloride to the rod will, on drying, give rise to a film of +the dry salt, and this may be reduced in the luminous gas flame. +During the process, however, the quartz is apt to get rotten, +especially if the temperature has been anything approaching a +full red heat. The resulting platinum deposit adheres very +strongly to the quartz, and may be soldered to as before. This +method has been employed by the writer with success since 1887, +and may even be extended to thick threads.</p> +<p>It was also found that fusible metal either stuck to or +contracted upon clean quartz so as to make a firm joint. In the +light of M. Margot's researches (already described), it occurred +to me that perhaps my experience was only a special case of the +phenomena of adhesion investigated with so much success by M. +Margot. I therefore tried whether the alloy of tin and zinc used +for soldering aluminium would stick to quartz, and instantly +found that this was indeed the case.</p> +<p>Adhesion between the alloy and perfectly clean quartz takes +place almost without rubbing. A rod of quartz thus "tinned" can +be soldered up to anything to which solder will stick, at once. +On applying the method to thick quartz threads, success was +instantaneous (the threads were some preserved for ordinary +galvanometer suspensions); but when the method was applied to +very fine threads, great difficulty in tinning the threads was +experienced. The operation is best performed by having the alloy +on the end of an aluminium soldering bit, and taking care that it +is <i>perfectly free from oxide</i> before the thread is drawn +across it. There was no difficulty in soldering a thread "tinned" +in this manner to a copper wire with tinman's solder, and the +joint appeared perfect, the thread breaking finally at about an +inch away from the joint.</p> +<p>I allow Mr. Boys' method to stand as I have written it, simply +because I have not had time as yet to make thorough tests of the +durability of "Margot" joints on the finest threads; but I have +practically no doubt as to its perfect applicability, provided +always that the solder can be got clean enough when melted on the +bit. Very fine threads will require to be stretched before +tinning, in order to enable them to break through the capillary +barrier of the surface of the melted solder.</p> +<p><b><a name="Toc158108948" id="Toc158108948"><font face= +"Bookman Old Style" size="4">§ 92. Soldering. +—</font></a></b></p> +<p><font face="Bookman Old Style">It is almost unfair to the arts +of the glass-blower or optician to describe them side by side +with the humble trade of soldering. Nevertheless, no +accomplishment of a mechanical kind is so serviceable to the +physicist as handiness with the soldering bit; and, as a rule, +there is no other exercise in which the average student shows +such lamentable incapacity. The following remarks on the subject +are therefore addressed to persons presumably quite ignorant of +the way in which soldering is carried out, and do not profess to +be more than of the most elementary character.</font></p> +<p>For laboratory purposes three kinds of solder are in general +sufficient. One is the ordinary tinman's solder composed of lead +and tin. The second is "spelter," or soft fusible brass, and the +third is an alloy of silver and brass called silver solder.</p> +<p>Tinman's solder is used for most purposes where high +temperatures are not required, or where the apparatus is intended +to be temporary. The "spelter," which is really only finely +granulated fusible brass, is used for brazing iron joints. The +silver solder is convenient for most purposes where permanency is +required, and is especially suited to the joining of small +objects.</p> +<p>§ 93. Soft tinman's solder is made by melting together +two parts of grain tin and one of soft lead — the exact +proportions are not of consequence — but, on the other +hand, the purer the constituents the better the solder. Within +certain limits, the greater the proportion of tin the cleaner and +more fusible is the solder. It is usually worth while to prepare +the solder in the laboratory, for in this way a uniform and +dependable product is assured. Good soft lead is melted in an +iron ladle and skimmed; the temperature is allowed to rise very +little above the melting-point. The tin is then added little by +little, the alloy stirred vigorously and skimmed, and sticks of +solder conveniently cast by sweeping the ladle over a clean iron +plate, so as to pour out a thin stream of solder. If the solder +be properly made it will have a mat and bright mottled surface, +and will "crackle" when held up to the ear and bent.</p> +<p>Perhaps the chief precaution necessary in making solder is to +exclude zinc. The presence of a very small percentage of this +metal entirely spoils the solder for tinman's work by preventing +its "running" or flowing smoothly under the soldering bit.</p> +<p>Fig. <img src="images/Image118.gif" alt="images/Image118.gif" +width="499" height="84"> 75.</p> +<p>Fig. <img src="images/Image119.gif" alt="images/Image119.gif" +width="499" height="70"> 76.</p> +<p>Fig. <img src="images/Image120.gif" alt="images/Image120.gif" +width="463" height="89"> 77.</p> +<p><b><a name="Toc158108949" id="Toc158108949"><font face= +"Bookman Old Style" size="4">§ 94. Preparing a Soldering +Bit. —</font></a></b></p> +<p><font face="Bookman Old Style">The wedge-shaped edge of one of +the forms of bit shown in the sketch is filed to shape and the +bit heated in a fire or on a gas heater. A bit of rough +sandstone, or even a clean soft brick, or a bit of tin plate +having some sand sprinkled over it, is placed in a convenient +position and sprinkled with resin.</font></p> +<p>As soon as the bit is hot enough to melt solder it is +withdrawn and a few drops of solder melted on to the brick or its +equivalent. The iron or bit is then rubbed to and fro over the +solder and resin till the former adheres to and tins the copper +head. It will be found advisable to tin every side of the point +of the bit and to carry the tinning back at least half an inch +from the edge.</p> +<p>If the solder obstinately refuses to adhere, the cause is to +be sought in the oxidation of the copper, or of the solder, or +both — in either case the result of too high a temperature +or too prolonged heating. The simple remedy is to get the iron +hot, and then to dress it with an old file, so as to expose a +bright surface, which is instantly passed over the resin as a +means of preserving it from oxidation. If the process above +described be now carried out, it will be found that the +difficulty disappears.</p> +<p>Before using the iron, wipe off any soot or coke or burned +resin by means of an old rag. An iron tinned in this way is much +to be preferred to one tinned by means of chloride of zinc.</p> +<p>A shorter and more usual method is carried out as follows: The +solution of chloride of zinc is prepared by adding bits of zinc +to some commercial hydrochloric acid diluted with a little (say +25 per cent) of water. The acid may conveniently be placed in a +small glazed white jar (a jam pot does excellently), and this +should only be filled to about one-quarter of its capacity. An +excess of zinc may be added.</p> +<p>It may be fancy, but I prefer a soldering solution made in +this way to a solution of chloride of zinc bought as a chemical +product. The jar is generally mounted on a heavy leaden base, so +as to avoid any danger of its getting knocked over, for nothing +is so nasty or bad for tools as a bench on which this noxious +liquid has been upset (Fig. 78).</p> +<p>Fig. <img src="images/Image121.gif" alt="images/Image121.gif" +width="161" height="146">78.</p> +<p>To tin a soldering bit, a little of the fluid is dipped out of +the jar on to a bit of tin plate bent up at the edges--a few +drops is sufficient — and the iron is heated and rubbed +about in the liquid with a drop of solder. If the iron is +anything like clean it will tin at once and exhibit a very bright +surface, but quite dirty copper may be tinned by dipping it for a +moment in the liquid in the pot and then working it about over +the solder. An iron so tinned remains covered with chloride of +zinc, and this must be carefully wiped off if it is intended to +use the iron with a resin or tallow flux in lead soldering.</p> +<p>One disadvantage of this process is that the copper bit soon +gets eaten into holes and requires to be dressed up afresh. On +the other hand, an iron so tinned always presents a nice clean +solder surface until the next time it is heated, when it +generally becomes very dirty and requires to be carefully wiped +before using.</p> +<p>In my experience also an iron so tinned is more easily spoiled +as to the state of its surface, "detinned," in fact, by +overheating than when the tinning is carried out by resin and +friction. When this happens, the shortest way out of the +difficulty is the application of the old file so as to obtain a +perfectly fresh surface. No one who knows his business ever uses +an iron that is not perfectly clean and well tinned.</p> +<p>The iron may be cleaned from time to time by heating it red +hot and quenching it in water to get rid of the oxide, which +scales off in the process.</p> +<p><b><a name="Toc158108950" id="Toc158108950"><font face= +"Bookman Old Style" size="4">§ 95. Soft Soldering. +—</font></a></b></p> +<p><font face="Bookman Old Style">In the laboratory the chief +application of the process is to copper soldering during the +construction of electrical apparatus and to zinc soldering for +general purposes.</font></p> +<p>In ninety-nine cases out of every hundred where difficulties +occur their origin is to be traced to dirt. There seems to be +some inexplicable kink in the human mind which renders it callous +to repeated proofs of the necessity for cleaning surfaces which +it is intended to solder. The slightest trace of albuminous or +gelatinous matter or shellac will prevent solder adhering to most +metals and the same remark applies in a measure to the presence +of oxides, although these may be removed by chloride of zinc or +prevented from forming by resin or tallow. A touch with an +ordinarily dirty hand — I refer to a solderer's hand +— will often soil work sufficiently to make the adherence +of solder difficult.</p> +<p>The fluxes most generally employed are tallow for lead, resin +or Venice turpentine for copper, chloride of zinc for anything +except lead, which never requires it. The latter flux has the +property (also possessed by borax at a red heat) of dissolving +any traces of oxide which may be formed, as well as acting as a +protecting layer to the metal.</p> +<p>We may now turn to the consideration of a simple case of +soldering, say the joining of two copper wires. The wires are +first cleaned either by dipping in a bath of sulphuric and nitric +acids — a thing no laboratory should be without — or +by any suitable mechanical means. The cleaned wires are then +twisted together — there is a regulation way of doing this, +but it presents no advantage in laboratory practice — and +the joint is sprinkled over with resin, or painted with a +solution of resin in alcohol.</p> +<p>The iron, being heated and floated with solder, is held +against the joint, the latter being supported on a brick, and the +solder is allowed to "sweat" into the joint. Enough solder must +be present to penetrate right through the joint. Nothing is +gained by rubbing violently with the iron. If the copper is clean +it will tin, and if it is dirty it won't, and there the matter +ends.</p> +<p>Beginners generally use too small or too cold a bit, and +produce a ragged, dirty joint in consequence. If the saving of +time be an object, the joint may be twisted together on +ordinarily dirty oxidised wires and heated to, say, 200° C. +It is then painted with chloride of zinc and soldered with the +bit.</p> +<p>There is a difference of opinion as to the relative merits of +chloride of zinc and of resin as a flux in soldering copper. Thus +the standing German practice is, or was, to employ the former +flux in every case for soldering electric light wires, while in +England the custom used to be to specify that soldering should be +done by resin, and this custom may still prevail; it lingers in +Australia at all events.</p> +<p>However, it is agreed on all hands that when chloride of zinc +is used it must be carefully washed off. I have known of an +electrical engineer insisting on his workmen "licking" joints +with their tongues to ensure the total removal of chloride of +zinc; it has a horrible taste; and I have occasionally pursued +the same plan myself when the soldering of fine wires was in +question.</p> +<p>In any case, it is very certain that chloride of zinc left in +a joint will ruin it sooner or later by loosening the contact +between copper and solder.</p> +<p>Very often it is requisite to solder together two extensive +flat surfaces — for instance, in "chucking" certain kinds +of brass work. The surfaces to be soldered must be carefully +tinned, most conveniently by the help of the blow-pipe and +chloride of zinc. After tinning, the surfaces are laid together +and heated so as to "sweat" them together; the phrase, though +inelegant, is expressive.</p> +<p>96. <i>Soldering Tin Plate.</i> —</p> +<p>If the plate be new and clean, a little resin or its solution +in alcohol is all that is necessary as a flux. If the tin plate +is rusty the rust must be removed and the clean iron, or rather +mild steel, surface exposed. The use of chloride of zinc is +practically essential in this case. Tin plate is often spotted +with rust long before it becomes rusty as a whole, when, of +course, it may be regarded as worn out, and such rust spots are +most conveniently removed by means of the plumber's shave-hook. +The shave-hook is merely a peculiarly shaped hard steel scraping +knife on a handle (Fig. 79).</p> +<p>Fig. <a name="LastCursor" id="LastCursor"></a><img src= +"images/Image122.gif" alt="images/Image122.gif" width="279" height="80"> 79</p> +<p>With tin plate the soldering of long joints is often +necessary. The plate must be temporarily held in position either +by binding with iron wire, fastening by clamps, or holding by an +assistant. The flux is applied and the iron run slowly along the +joint. Enough solder is used to completely float the tip of the +iron. By arranging the joint so that it slopes downward slightly, +and commencing at the upper end, the solder may be caused to flow +after the iron, and will leave a joint with the minimum +permissible amount of solder in it. By regulating the slope, heat +of iron, etc., any desired quantity of solder may be run into the +joint.</p> +<p><b><a name="Toc158108951" id="Toc158108951"><font face= +"Bookman Old Style" size="4">§ 97. Soldering Zinc. +—</font></a></b></p> +<p><font face="Bookman Old Style">Zinc alloys with soft solder +very easily, and by so doing entirely spoils it, making, it +"crumbly," dirty, and preventing it running. Consequently, in +soldering up zinc great care must be taken to prevent the solder +becoming appreciably contaminated by the zinc. To this end the +zinc surfaces are cleaned by means of a little hydrochloric acid, +which is painted on instead of chloride of zinc. Plenty of solder +is melted on to the work, and is drawn along over the joint by a +single slow motion of the soldering bit. The iron must be just +hot enough to make the solder flow freely, and it must never be +rubbed violently on the zinc or allowed to linger in one spot; +the result of the latter action will be to melt a hole through +the zinc, owing to the tendency of this metal to form an easily +fusible alloy with the solder.</font></p> +<p>The art of soldering zinc is a very useful one in the +laboratory. The majority of physicists appear to overlook the +advantages of zinc considered as a material for apparatus +construction. It is light, fairly strong, cheap, easily fusible, +and yet hard and elastic when cold. It may be worked as easily as +lead at a temperature of, say, 150° to 200° C., and +slightly below the melting-point (423° C.) it is brittle and +may & powdered. The property of softening at a moderate +temperature is invaluable as a means of flattening zinc plate or +shaping it in any way. During the work it may be held by means of +an old cloth. Zinc sheet which has been heated between iron +plates and flattened by pressure retains its flatness very fairly +well after cooling.</p> +<p><b><a name="Toc158108952" id="Toc158108952"><font face= +"Bookman Old Style" size="4">§ 98. Soldering other Metals +—</font></a></b></p> +<p><i><font face="Bookman Old Style">Iron</font>. —</i></p> +<p>The iron must be filed clean and then brushed with chloride of +zinc solution. Some people add a little sat ammoniac to the +chloride of zinc, but the improvement thus made is practically +inappreciable. If the iron is clean it tins quite easily, and the +process of soldering it is perfectly easy and requires no special +comment.</p> +<p><i>Brass. —</i></p> +<p>The same method as described for iron succeeds perfectly. The +brass, if not exceedingly dirty, may be cleaned by heating to the +temperature at which solder melts (below 200° C.), and +painting it over with chloride of zinc, or dipping it in the +liquor. If now the brass be heated again in the blow-pipe flame, +it will be found to tin perfectly well when rubbed over with +solder.</p> +<p><i>German Silver, Platinoid, Silver, and Platinum</i></p> +<p>are treated like iron. With regard to silver and platinum the +same precautions as recommended in the case of zinc must be +observed, for both these metals form fusible alloys with +solder.</p> +<p><i>Gold</i></p> +<p>when pure requires no flux. Standard gold, which contains +copper, solders better with a little chloride of zinc.</p> +<p><i>Lead</i></p> +<p>must be pared absolutely clean and then soldered quickly with +a hot iron, using tallow as a flux. Since solder if over hot will +adhere to lead almost anywhere, plumbers are in the habit of +specially soiling those parts to which it is not intended that +solder shall adhere. The "soiling" paint consists of very thin +glue, called size, mixed with lampblack; on an emergency a raw +potato may be cut in half, and the work to be soiled may be +rubbed over with the cut surface of the potato.</p> +<p><i>Hard Carbon or gas coke</i></p> +<p>may be soldered after coating with copper by an electrolytic +process, as will be described.</p> +<p><b><a name="Toc158108953" id="Toc158108953"><font face= +"Bookman Old Style" size="4">§ 99. +<i>Brazing</i>.</font></a></b></p> +<p><font face="Bookman Old Style">Soldering at a red heat by +means of spelter is called brazing. Spelter is soft brass, and is +generally made from zinc one part, copper one part; an alloy +easily granulated at a red heat; it is purchased in the granular +form.</font></p> +<p>The art of brazing is applied to metals which will withstand a +red heat, and the joints so soldered have the strength of +brass.</p> +<p>The pieces to be jointed by this method must be carefully +cleaned and held in their proper relative positions by means of +iron wire. It is generally necessary to soften iron wire as +purchased by heating it red hot and allowing it to cool in the +air; if this is not done the wire is usually too hard to be +employed satisfactorily for binding.</p> +<p>Very thin wire--i.e. above No. 20 on the Birmingham wire gauge +— does not do, for it gets burned through, and perhaps +allows the work to fall apart at a critical moment.</p> +<p>The work being securely fastened, the next step is to cover +the cleaned parts with flux in order to prevent oxidation. For +this purpose "glass borax" is employed. "Glass" borax is simply +ordinary borax which has been fused for the purpose of getting +rid of water of crystallisation. The glass borax is reduced to +powder in an iron mortar, for it is very hard, and is then made +up into a cream with a little water. This cream is painted on to +the parts of the work which are destined to receive the +solder.</p> +<p>The next step is to prepare the spelter, and this is easily +done by mixing it with the cream, taking care to stir thoroughly +with a flattened iron wire till each particle of spelter is +perfectly covered with the borax. The mixture should not be too +wet to behave as a granular mass, and may then be lifted on to +the work by means of the iron spatula.</p> +<p>Care must be taken to place the spelter on those parts only +which are intended to receive it, and when this is done, the +joint may be lightly powdered over with the dry borax, and will +then be ready for heating.</p> +<p>If the object is of considerable size it is most conveniently +heated on the forge; if small the blowpipe is more convenient. In +the latter case, place the work on a firebrick, and arrange two +other bricks on edge about it, so that it lies more or less in a +corner. A few bits of coke may also be placed on and about the +work to increase the temperature by their combustion, and to +concentrate the flame and prevent radiation. The temperature is +gradually raised to a bright red heat, when the spelter will be +observed to fuse or "run," as it is technically said to do.</p> +<p>If the cleaning and distribution of flux has been successful, +the spelter will "run" along the joint very freely, and the work +should be tapped gently to make sure that the spelter has really +run into the joint. The heating may be interrupted when the +spelter is observed to have melted into a continuous mass. As +soon as the work has fallen below a red heat it may be plunged +into water, a process which has the effect of cracking off the +glass-like layer of borax.</p> +<p>There is, however, some risk of causing the work to buckle by +this violent treatment, which must of course be modified so as to +suit the circumstances of the case. If the joint is in such a +position that the borax cannot be filed off, a very convenient +instrument for its removal by scraping is the watchmaker's +graver, a square rod of hard steel ground to a bevelled point +(Fig. 80).</p> +<p>Fig. <img src="images/Image123.gif" alt="images/Image123.gif" +width="300" height="72"> 80.</p> +<p>Several precautions require to be mentioned. In the first +place, spelter is merely rather soft brass, and consequently it +often cannot be fused without endangering the rest of the work. A +good protection is a layer of fireclay laid upon the more +delicate parts, such for instance as any screwed part.</p> +<p>Gun-metal and tap-metal do not lend themselves to brazing so +readily as iron or yellow brass, and are usually more +conveniently treated by means of silver solder.</p> +<p>Spelter tends to run very freely when it melts, and if the +brass surface in the neighbourhood of the joint is at all clean, +may run where it is not wanted. Of course some control may be +exercised by "soiling" with fireclay or using an oxidising flame; +but the erratic behaviour of spelter in this respect is the +greatest drawback to its use in apparatus construction. The +secret of success in brazing lies in properly cleaning up the +work to begin with, and in disposing the borax so as to prevent +subsequent oxidation.</p> +<p><b><a name="Toc158108954" id="Toc158108954"><font face= +"Bookman Old Style" size="4">§ 100. Silver Soldering. +—</font></a></b></p> +<p><font face="Bookman Old Style">This process resembles that +last described, but instead of spelter an alloy of silver, +copper, and zinc is employed. The solder, as prepared by +jewellers to meet special cases, varies a good deal in +composition, but for the laboratory the usual proportions are +—</font></p> +<p>For soft silver solder</p> +<div style="margin-left: 4em"> +<p>Fine silver 2 parts</p> +<p>Brass wire 1 part</p> +</div> +<p>For hard silver solder</p> +<div style="margin-left: 4em"> +<p>Sterling silver 3 parts</p> +<p>Brass wire 1 part</p> +</div> +<p>The latter is, perhaps, generally the more convenient.</p> +<p>Silver solders may, of course, be purchased at watchmakers' +supply shops, and as thus obtained, are generally in thin sheet. +This is snipped fine with a pair of shears preparatory to +use.</p> +<p>As odds and ends of silver (from old anodes and silver +residues) generally accumulate in the laboratory, it is often +more convenient to make the solder one's self. In this case it +must be remembered in making hard solder by the second receipt +that standard silver contains about one-twelfth of its weight of +copper — exactly 18 parts copper to 220 silver.</p> +<p>The silver is first melted in a plumbago crucible in a small +furnace together with a little borax; if any copper is required +this is then added, and finally the brass is introduced. When +fusion is complete, the contents of the crucible are poured into +any suitable mould.</p> +<p>The quickest and most convenient way of preparing the alloy +for use is to convert it into filings with the assistance of a +coarse file, or by milling it, if a milling machine is +available.</p> +<p>Equal volumes of filings and powdered glass borax are made +into a thin paste with water, and applied in an exactly similar +manner to that described under the head of "brazing." In fact all +the processes there described may be applied equally to the case +under discussion, the substitution of silver for spelter being +the only variation.</p> +<p>The silver solder is more manageable than spelter, and does +not tend to run wild over the work: a property which makes it +much more convenient both for delicate joints and in cases where +it is desired to restrict the solder to a single point or line. +Small objects are almost invariably soldered with silver solder, +and are held by forceps or on charcoal in the pointed flame of an +ordinary blow-pipe.</p> +<p><b><a name="Toc158108955" id="Toc158108955"><font face= +"Bookman Old Style" size="4">§ 101. On the Construction of +Electrical Apparatus - Insulators. —</font></a></b></p> +<p><font face="Bookman Old Style">It is not intended to deal in +any way with the design of special examples of electrical +apparatus, but merely to describe a rather miscellaneous set of +materials and processes constantly required in its +construction.</font></p> +<p>It is not known whether there is such a thing as a perfect +insulator, even if we presuppose ideal circumstances. Materials +as they exist must be regarded merely as of high specific +resistance, that is if we allow ourselves to use such a term in +connection with substances, conduction through which is neither +independent of electromotive force per unit length, nor of +previous history.</p> +<p>Even the best of these substances generally get coated with a +layer of moisture when exposed to the air, and this as a rule +conducts fairly well. Very pure crystalline sulphur and fused +quartz suffer from this defect less than any other substances +with which the writer is acquainted, but even with them the +surface conductivity soon grows to such an extent as totally to +mask the internal conduction.</p> +<p>It is proposed to give a brief account of the properties of +some insulating substances and their application in electrical +construction, and at the same time to indicate the appliances and +methods requisite for working them.</p> +<p>With regard to the specific resistances which will be quoted, +the numbers must not be taken to mean too much, partly for the +reason already given. It is also in general doubtful whether +sufficient care has been taken to distinguish the body from the +surface conductivity, and consequently numerical estimates are to +be regarded with suspicion. The question of "sampling" also +arises, for it must be remembered that a change in composition +amounting to, say, 1/10000 per cent may be accompanied by a +million-fold change in specific resistance.</p> +<p><b><a name="Toc158108956" id="Toc158108956"><font face= +"Bookman Old Style" size="4">§ 102. Sulphur. +—</font></a></b></p> +<p><font face="Bookman Old Style">This element exists in several +allotropic forms, which have very different electric properties. +After melting at about 125° C., and annealing at 110° for +several hours, the soluble crystalline modification is formed. +After keeping for some days — especially if exposed to +light — the crystals lose their optical properties, but +remain of the same melting-point, and are perfectly soluble in +carbon bisulphide. The change is accompanied by a change in +colour, or rather in brightness, as the transparency +changes.</font></p> +<p>The "specific resistance" of sulphur in this condition is +above 10<sup>28</sup> C.G.S.E.M. units, or 10<sup>13</sup> +megohms per cubic centimetre for an electric intensity of say +12,000 volts per centimetre. This is at ordinary temperatures. At +75° C. the specific resistance falls to about 10<sup>25</sup> +under similar conditions as to voltage.</p> +<p>In all cases the conductivity appears to increase with the +electric intensity, or at all events with an increase in voltage, +the thickness of the layer of sulphur remaining the same.</p> +<p>The specific inductive capacity is 3.162 at ordinary +temperatures, and increases very slightly with rise of +temperature. <i>[Footnote:</i> March 1897. — It is now the +opinion of the writer that though the specific inductive capacity +of a given sample of a solid element is perfectly definite, yet +it is very difficult to obtain two samples having exactly the +same value for this constant, even in the case of a material so +well defined as sulphur.<i>]</i></p> +<p>The total residual charge, after ten minutes' charging with an +intensity of 12,000 volts per centimetre, is not more than 4 +parts in 10,000 of the original charge. In making this +measurement the discharge occupied a fraction of a second. The +electric strength for a homogeneous plate of crystalline sulphur +is not less than 33,000 volts per centimetre, and probably a good +deal more. If the sulphur is contaminated with up to 3 per cent +of the amorphous variety, as is the case if it is cooled fairly +quickly from a temperature of 170° C. or over, the specific +resistance falls to from 10^25 to 10^26 at ordinary temperatures; +and the specific inductive capacity increases up to 3.75, +according to the amount of insoluble sulphur present.</p> +<p>The residual charge under circumstances similar to those +described above, but with an intensity of about 4000 volts per +centimetre is, say, 2 per cent of the initial charge. So far as +the writer is aware sulphur is the only solid non-conductor which +can be easily obtained in a condition of approximate purity and +in samples sufficiently exactly comparable with one another; it +is the only one, therefore, that repays any detail of +description.</p> +<p>Very pure sulphur can be bought by the ton if necessary from +the United Alkali Company of Newcastle-on-Tyne. It is recovered +from sulphur waste by the Chance process, which consists in +converting the sulphur into hydrogen sulphide, and burning the +latter with insufficient air for complete combustion. The sulphur +is thrown out of combination, and forms a crystalline mass on the +walls and floor of the chamber.</p> +<p>The sulphur which comes into the market consists of this mass +broken up into convenient fragments. In order to purify it +sufficiently for use as an insulator, the sulphur may be melted +at a temperature of 120° to 140° C., and filtered through +a plug of glass wool in a zinc funnel; as thus prepared it is an +excellent insulator. To obtain the results mentioned in the table +it is, however, necessary to conduct a further purification +(chiefly from water) by distillation in a glass retort.</p> +<p>The sulphur thus obtained may be cast of any desired form in +zinc moulds, the castings and moulds being immediately removed to +an annealing oven at a temperature of from 100° to 110° +C., where they are left for several hours. If the sulphur is kept +melted for some time at 125° C. the annealing is not so +important.</p> +<p>The castings may be removed from the mould by slightly heating +the latter, but many breakages result. Insulators made on this +plan are much less affected by the condensation of moisture from +the air than anything except fused quartz. They are, however, +very weak mechanically, and apt to crack by exposure to such +changes of temperature as go on from day to day. It is clear, +however, that in spite of this their magnificent electrical +properties fit them for many important uses.</p> +<p>If the sulphur be cooled rapidly from 170° C. or over, a +mixture of the crystalline and amorphous varieties of sulphur is +obtained. This mixture is very much stronger and tougher than the +purely crystalline substance, and may be worked with ordinary +hardwood tools into fairly permanent plates, rods, etc. Sheets of +pure thick filter paper may also be dipped into sulphur at +170° C., at which temperature air and moisture are mostly +expelled, and such sheets show a very considerable insulating +power. The sulphur does not penetrate the paper, which therefore +merely forms a nucleus.</p> +<p>Cakes of the crystalline or mixed varieties may be made by +grinding up some purified sulphur, moistening it with redistilled +carbon bisulphide, or toluene, or even benzene +(C<sub>6</sub>H<sub>6</sub>), and pressing it in a suitable mould +under the hydraulic press. The plates thus formed are porous, but +are splendid insulators, especially if made from the crystalline +variety of sulphur, and they appear to keep their shape very +well, and do not crack with ordinary temperature changes.</p> +<p>The metals which resist the action of sulphur best are gold +and aluminium; while platinum and zinc are practically unacted +upon at temperatures below a red heat — in the former case, +— and below the boiling-point of sulphur in the latter.</p> +<p>A very convenient test of the purity of sulphur is the colour +assumed by it when suddenly cooled from the temperature at which +it is viscous. Quite pure sulphur remains of a pale lemon yellow +under this treatment, but the slightest trace of impurity, such +as arises from dust containing organic matter, stains the +sulphur, and renders it darker in colour.</p> +<p><b><a name="Toc158108957" id="Toc158108957"><font face= +"Bookman Old Style" size="4">§ 103. <i>Fused Quartz</i>. +—</font></a></b></p> +<p><font face="Bookman Old Style">This is on the whole the most +reliable and most perfect insulator for general purposes. No +exact numerical data have been obtained, but the resistivity must +certainly be of the same order as that of pure sulphur at its +best. The influence of the moisture of the air also reaches its +minimum in the case of quartz, as was originally observed by +Boys.</font></p> +<p>As yet, however, the material can only be obtained in the form +of rods or threads. For most purposes rods of about one-eighth of +an inch in diameter are the most convenient. These rods may be +used as insulating supports, and succeed perfectly even if they +interpose less than an inch of their length to electrical +conduction. The sketch (Figs. 81 and 81A) shows (to a scale of +about one-quarter full size) a complete outfit for elementary +electrostatic experiments, such as has been in use in the +writer's laboratory for five years. With these appliances all the +fundamental experiments may be performed, and the apparatus is +always ready at a moment's notice.</p> +<p>Fig. <img src="images/Image124.gif" alt="images/Image124.gif" +width="283" height="304"> 81.</p> +<p>Though quartz does not condense moisture or gas to form a +conducting layer of anything like the same conductivity as in the +case of glass or ebonite, still it is well to heat it if the best +results are to be obtained. For this purpose a small pointed +blow-pipe flame may be used, and the rods may be got red-hot +without the slightest danger of breaking them. They then remain +perfectly good and satisfactory for several hours at least, even +when exposed to damp and dusty air.</p> +<p>The rods are conveniently held in position by small brass +ferrules, into which they are fastened by a little plaster of +Paris. Sealing-wax must be avoided, on account of the +inconvenience it causes when the heating of the rods is being +carried out.</p> +<p>One useful application of fused quartz is to the insulation of +galvanometer coils (Fig. 82), another to the manufacture of +highly insulating keys (Fig. 83); while as an insulating +suspension it has all the virtues. If it is desired to render the +threads conducting they may be lightly silvered, and will be +found to conduct well enough for electrometer work before the +silver coating is thick enough to sensibly impair their elastic +properties.</p> +<p>Fig. <img src="images/Image125.gif" alt="images/Image125.gif" +width="449" height="373"> 81A.</p> +<p>Fig. 82 is a full-size working drawing of a particular form of +mounting for galvanometer coils. The objects sought to be +attained are</p> +<p>(1) high insulation of the coils,</p> +<p>(2) easy adjustment of the coils to the suspended system.</p> +<p>The first object is attained as follows. The ebonite ring A is +bored with four radial holes, through which are slipped from the +inside the fused quartz bolt-headed pins B. The coil already +soaked in hard paraffin is placed concentrically in the ring A by +means of a special temporary centering stand. The space between +the coil and the ring is filled up with hard paraffin, and this +holds the quartz pins in position. The system of ebonite ring, +coil, and pins is then fastened into the gun-metal coil carrier, +which is cut away entirely, except near the edges, where it +carries the pin brackets C. These brackets can swivel about the +lower fastening at E before the latter is tightened up.</p> +<p>The coil is now adjusted in the adjusting stand to be +concentric with the axis of symmetry of the coil carrier, and the +supporting pins are slipped into slot holes cut in the brackets, +the brackets being swivelled as much as necessary to allow of +this. When the pins are all inserted the brackets are screwed up +by the screws at E. The pins are then cemented firmly to the +brackets by a little plaster of Paris. The coil carrier can now +be adjusted to the galvanometer frame by means of screws at D, +which pass through wide holes in the carrier and bold the latter +in position by their heads. In the sectional plan the parts of +the galvanometer frame are shown shaded. The front of the frame +at F F is of glass, and the back of the frame is also made of +glass, though this is not shown in the section.</p> +<p>A represents an ebonite ring into which the wire coil is +cemented by means of paraffin. B B B B are quartz pins, with +heads inside the ebonite ring. C C C are slotted brackets +adjustable to the pins and capable of rotation by releasing the +screws E E. D D are the screws holding the coil carriage to the +galvanometer framework. These screws pass through large holes in +the carriage so as to allow of some adjustment.</p> +<p>Fig. <img src="images/Image126.gif" alt="images/Image126.gif" +width="549" height="124"> 82.</p> +<p>Fig. <img src="images/Image127.gif" alt="images/Image127.gif" +width="261" height="255"> 83.</p> +<p><b><a name="Toc158108958" id="Toc158108958"><font face= +"Bookman Old Style" size="4">§ 104. <i>Glass.</i> +—</font></a></b></p> +<p><font face="Bookman Old Style">When glass is properly chosen +and perfectly dry it has insulating properties possibly equal to +those possessed by quartz or crystalline sulphur. For many +purposes, however, its usefulness is seriously reduced by the +persistence with which it exhibits the phenomena of residual +charge, and the difficulty that is experienced in keeping it +dry.</font></p> +<p>The insulating power of white flint glass is much in excess of +that of soft soda glass, which is a poor insulator, and of +ordinary green bottle glass. The jars of Lord Kelvin's +electrometers, which insulate very well, are made of white flint +glass manufactured in Glasgow, but it is found that occasionally +a particular jar has to be rejected on account of its refusing to +insulate, and this, if I understand aright, even when it exhibits +no visible defects.</p> +<p>A large number of varieties of glass were tested by Dr. +Hopkinson at Messrs. Chance Bros. Works, in 1875 and 1876 (Phil. +Trans., 1877), and in 1887 (Proc. Roy. Soc. xli. 453), chiefly +with a view to the elucidation of the laws regulating the +residual charge; and incidentally some extraordinarily high +insulations were noted among the flint glasses. The glass which +gave the smallest residual charge was an "opal" glass; and flint +glasses were found to insulate 10<sup>5</sup> times as well as +soda lime glasses. The plates of Wimshurst machines are made of +ordinary sheet window glass, but as the insulating property of +this material appears to vary, it is generally necessary to +clean, dry, and test a sheet before using it. With regard to hard +Bohemian glass, this is stated by Koeller (Wien Bericht) to +insulate ten times as well as the ordinary Thuringian soft soda +glass.</p> +<p>On the whole the most satisfactory laboratory practice is to +employ good white flint glass. The only point requiring attention +is the preparation of the glass by cleaning and drying. Of course +all grease or visible dirt must be removed as described in an +earlier chapter (§ 13), but this is only a beginning. The +glass after being treated as described and got into such a state +as to its surface that clean water no longer tends to dry off +unequally, must be subjected to a further scrub with bibulous +paper and a clear solution of oleate of soda. The glass is then +to be well rinsed with distilled water and allowed to drain on a +sheet of filter paper.</p> +<p>A very common cause of failure lies in the contamination of +the glass with grease from the operator's fingers. Before setting +out to clean the glass the student will do well to wash his hands +with soap and water, then with dilute ammonia and finally with +distilled water.</p> +<p>In the case of an electrometer jar which has become conducting +but is not perceptibly dirty, rubbing with a little oleate of +soda and a silk ribbon, followed, of course, by copious washing, +does very well. If there is any tin-foil on the jar, great care +must be taken not to allow the glass surface to become +contaminated by t<a name="lastbookmark" id="lastbookmark"></a>he +shellac varnish or gum used to stick the tin-foil in +position.</p> +<p>Finally, the glass should be dried by radiant heat and raised +to a temperature of 100° C. at least, and kept at it for at +least half an hour. Before drying it is of course advisable to +allow the water to drain away as far as possible, and if the +water is only the ordinary distilled water of the laboratory, the +glass is preferably wiped with a clean bit of filter paper; any +hairs which may be left upon the glass will brush off easily when +the glass is dry.</p> +<p>In order to obtain satisfactory results the glass must be +placed in dry air before it has appreciably cooled. This is +easily done in the case of electrometer jars, and so long as the +air remains perfectly dry through the action of sulphuric acid or +phosphorus pentoxide, the jar will insulate. The slightest whiff +of ordinarily damp air will, however, enormously reduce the +insulating power of the glass, so that unvarnished glass surfaces +must be kept for apparatus which is practically air-tight.</p> +<p>For outside or imperfectly protected uses the glass does +better when varnished. It is a fact, however, that varnished +glass is rarely if ever so good as unvarnished glass at its best. +Too much care cannot be taken over the preparation of the +varnish; French polish, or carelessly made shellac varnish, is +likely to do more harm than good.</p> +<p>The best orange shellac must be dissolved in good cold alcohol +by shaking the materials together in a bottle. The alcohol is +made sufficiently pure by starting with rectified spirit and +digesting it in a tin flask over quick-lime for several days, a +reversed condenser being attached. A large excess of lime must be +employed, and this leads to a considerable loss of alcohol, a +misfortune which must be put up with.</p> +<p>After, say, thirty hours' digestion, the alcohol may be +distilled off and employed to act on the shellac. In making +varnish, time and trouble are saved by making a good deal at one +operation — a Winchester full is a reasonable quantity. The +bottle may be filled three-quarters full of the shellac flakes +and then filled up with alcohol; this gives a solution of a +convenient strength.</p> +<p>The solution, however, is by no means perfect, for the shellac +contains insoluble matter, and this must be got rid off.`' One +way of doing this is to filter the solution through the thick +filtering paper made by Schleicher and Schuell for the purpose, +but the filtering is a slow process, and hence requires to be +conducted by a filter paper held in a clip (not a funnel) under a +bell jar to avoid evaporation.</p> +<p>Another and generally more convenient way in the laboratory is +to allow the muddy varnish to settle — a process requiring +at least a month — and to decant the clear solution off +into another bottle, where it is kept for use. The muddy residue +works up with the next lot of shellac and alcohol, which may be +added at once for future use.</p> +<p>The glass to be varnished is warmed to a temperature of, say, +50° C., and the varnish put on with a lacquering brush; a +thin uniform coat is required. The glass is left to dry long +enough for the shellac to get nearly hard and to allow most of +the alcohol to evaporate. It is then heated before a fire, or +even over a Bunsen, till the shellac softens and begins to yield +its fragrant characteristic smell.</p> +<p>If the coating is too heavy, or if the heating is commenced +before the shellac is sufficiently dry, the latter will draw up +into "tears," which are unsightly and difficult to dry properly. +On no account must the shellac be allowed to get overheated. If +the varnish is not quite hard when cold it may be assumed to be +doing more harm than good.</p> +<p>In varnishing glass tubes for insulating purposes it must be +remembered that the inside of the tube is seldom closed perfectly +as against the external air, and consequently it also requires to +be varnished. This is best done by pouring in a little varnish +considerably more dilute than that described, and allowing it to +drain away as far as possible, after seeing that it has flooded +every part of the tube.</p> +<p>During this part of the process the upper end of the tube must +be closed, or evaporation will go on so fast that moisture will +be deposited from the air upon the varnished surface. Afterwards +the tube may be gently warmed and a current of air allowed to +pass, so as to prevent alcohol distilling from one part of the +tube to another. The tube is finally heated to the softening +point of shellac, and if possible closed as far as is practicable +at once.</p> +<p><b><a name="Toc158108959" id="Toc158108959"><font face= +"Bookman Old Style" size="4">§ 105. Ebonite or Hard Rubber. +—</font></a></b></p> +<p><font face="Bookman Old Style">This exceedingly useful +substance can be bought of a perfectly useless quality. Much of +the ebonite formerly used to cover induction coils for instance, +deteriorates so rapidly when exposed to the air that it requires +to have its surface renewed every few weeks.</font></p> +<p>The very best quality of ebonite obtainable should be solely +employed in constructing electric works. It is possible to +purchase good ebonite from the Silvertown Rubber Company (and +probably from other firms), but the price is necessarily high, +about four shillings per pound or over.</p> +<p>At ordinary temperatures ebonite is hard and brittle and +breaks with a well-marked conchoidal fracture. At the temperature +of boiling water the ebonite becomes somewhat softened, so that +it is readily bent into any desired shape; on cooling it resumes +its original hardness.</p> +<p>This property of softening at the temperature of boiling water +is a very valuable one. The ebonite to be bent or flattened is +merely boiled for half an hour or so in water, taken out, brought +to the required shape as quickly as possible, and left to cool +clamped in position.</p> +<p>The sheet ebonite as it comes from the makers is generally far +from flat. It is often necessary to flatten a sheet of ebonite, +and of course this is the more easily accomplished the smaller +the sheet. Consequently a bit of ebonite of about the required +size is first cut from the stock sheet by a hack-saw such as is +generally used for metals. This piece is then boiled and pressed +between two planed iron plates previously warmed to near 100° +C.</p> +<p>With pieces of ebonite such as are used for the tops of +resistance boxes, measuring, say, 20 X 8 X 11 inches, very little +trouble is experienced. The sheets when cold are found to retain +the flatness which has been forced upon them perfectly well. It +is otherwise with large thin sheets such as are used for Holtz +machines. I have succeeded fairly, but only fairly, by pressing +them in a "gluing press," consisting of heavy planed iron slabs +previously heated to 100° C.</p> +<p>I do not know exactly how best to flatten very thin and large +sheets. It is easy to make large tubes out of sheet ebonite by +taking advantage of the softening which ebonite undergoes in +boiling water. A wooden mandrel is prepared of the proper size +and shape. The ebonite is softened and bent round it; this may +require two or three operations, for the ebonite gets stiff very +quickly after it is taken out of the water. Finally the tube is +bound round the mandrel with sufficient force to bring it to the +proper shape and boiled in water, mandrel and all. The bath and +its contents are allowed to cool together, so that the ebonite +cools uniformly.</p> +<p>Tubes made in this way are of course subject to the drawback +of having an unwelded seam, but they do well enough to wind wire +upon if very great accuracy of form is not required. If very +accurate spools are needed the mandrel is better made of iron or +slate and the spool is turned up afterwards. The seam may be +strapped inside or at the ends by bits of ebonite acting as +bridges, and the seam itself may be caulked with melted paraffin +or anthracene.</p> +<p><i>Working Ebonite.</i> —</p> +<p>Ebonite is best worked as if it were brass, with ordinary +brass-turning or planing tools. These tools should be as hard as +possible, for the edges are apt to suffer severely, and blunt +tools leave a very undesirable woolly surface on the ebonite. In +turning or shaping ebonite sheets it is as well to begin by +taking the skin off one side first, and then reversing the sheet, +finishing the second side, and then returning to the first. This +is on account of the fact that ebonite sometimes springs a little +out of shape when the skin is removed.</p> +<p>Turned work in ebonite, if well done, requires no +sand-papering, but may be sufficiently polished by a handful of +its own shavings and a little vaseline. The advantage of using a +polished ebonite surface is that such a surface deteriorates more +slowly under the influence of light and air than a surface left +rough from the tool. If very highly polished surfaces are +required, the ebonite after tooling is worked with fine pumice +dust and water, applied on felt, or where possible by means of a +felt buff on the lathe, and finally polished with rouge and +water, applied on felt or cloth.</p> +<p>Ebonite works particularly well under a spiral milling cutter, +and sufficiently well under an ordinary rounded planing tool, +with cutting angle the same as for brass, and hardened to the +lightest straw colour.</p> +<p>It is not possible, on the other hand, to use the carpenter's +plane with success, for the angle of the tool is too acute and +causes the ebonite to chip.</p> +<p>In boring ebonite the drill should be withdrawn from the hole +pretty often and well lubricated, for if the borings jam, as they +are apt to do, the heat developed is very great and the temper of +the drill gets spoiled. Ebonite will spoil a drill by heating as +quickly as anything known; on the other hand, it may be drilled +very fast if proper precaution is taken.</p> +<p>It is advisable to expose ebonite to the light as little as +possible, especially if the surface is unpolished, for under the +combined action of light and air the sulphur at the surface of +the ebonite rapidly oxidises, and the ebonite becomes covered +with a thin but highly conducting layer of sulphurous or +sulphuric acid or their compounds. When this happens the ebonite +may be improved by scrubbing with hot water, or washing freely +with alcohol rubbed on with cotton waste in the case of apparatus +that cannot be dismounted.</p> +<p>A complete cure, however, can only be effected by scraping off +the outer layer of ebonite so as to expose a fresh surface. For +this purpose a bit of sheet glass broken so as to leave a curved +edge is very useful, and the ebonite is then scraped like a +cricket bat. In designing apparatus for laboratory use it is as +well to bear in mind that sooner or later the ebonite parts will +require to be taken down and scraped up. Rods or tubes are, of +course, most quickly treated on the lathe with rough glass cloth, +and may be finished with fine sandpaper, then pumice dust and +water, applied on felt. After cleaning the pumice off by means of +water and a rag, the final touch may be given by means of +vaseline, applied on cloth or on ebonite shavings.</p> +<p><b><a name="Toc158108960" id="Toc158108960"><font face= +"Bookman Old Style" size="4">§ 106. <i>Mica</i>. +—</font></a></b></p> +<p><font face="Bookman Old Style">A great variety of minerals go +under this name. Speaking generally, the Russian micas coming +into commerce are potash micas, and mica purchased in England may +be taken to be potash mica, especially if it is in large +sheets.</font></p> +<p>At ordinary temperatures "mica" of the kind found in commerce +is an excellent insulator. Schultze (<i>Wied. Ann</i>. vol. +xxxvi. p. 655) comes to the conclusion that both at high and at +low temperatures mica (of all kinds?) is a better insulator than +white "mirror glass," the composition of which is not stated. The +experiments of the author referred to were apparently left +unfinished, and altogether too much stress must not be laid on +the results obtained, one of which was that mica conducts +electrolytically to some extent at high temperatures.</p> +<p>Bouty (<i>Journal de Physique</i>, 1890 [9], 288) and J. Curie +(<i>Thèse de Doctorat</i>, Paris, 1888) agree in making +the final conductivity of the mica used in Carpentier's +condensers exceedingly small — at all events at ordinary +temperatures. Bearing in mind that for such substances the term +specific resistance has no very definite meaning, M. Bouty +considers it is not less than 3.19 x 10<sup>28</sup> E.M. units +at ordinary temperatures. M. Bouty gives a note or illustration +of what such numbers mean — a precaution not superfluous in +cases where magnitudes are denoted logarithmically. Referring to +the value quoted, viz. 3.19 x 10<sup>28</sup>, M. Bouty says, "Ce +serait la resistance d'une colonne de mercure de 1<sup>mmq</sup> +de section et de longueur telle que la lumière se +propageant dans le vide, mettrait plus de 3000 ans A se +transmettre d'une extrémité à I'autre de la +colonne."</p> +<p>M. Bouty returns to the study of mica (muscovite) in the +<i>Journal de Physique</i> for 1892, p. 5, and there deals with +the specific inductive capacity, which for a very small period of +charge he finds has the value 8 — an enormous value for +such a good insulator, and one that it would be desirable to +verify by some totally distinct method. This remark is enforced +by the fact that M. Klemencic finds the number 6 for the same +constant. The temperature coefficient of this constant was too +small for M. Bouty to determine. The electric intensity was of +the order of 100 volts per centimetre, and the experiments seem +to indicate that the specific inductive capacity would be only +slightly less if referred to a period of charge indefinitely +short.</p> +<p>I have found that the residual charge in a mica condenser, +made according to Carpentier's method (to be described below), is +about 1 per cent of the original charge under the following +circumstances.</p> +<p>Voltage 300 volts on a plate 0.2 mm. thick, duration of charge +ten minutes, temperature about 20° C. To get this result the +mica must be most carefully dried. This and other facts indicate +that the so-called residual charge on ordinary condensers is, to +a very large extent, due to the creeping of the charge from the +armatures over the more or less conducting varnished surfaces of +the mica, and its slow return on discharge.</p> +<p>This source of residual charge was carefully guarded against +by Rowland and Nichols (<i>Phil. Mag</i>. 1881) in their work on +quartz, and is referred to by M. Bouty, who adduces some +experiments to show that his own results are not vitiated by it. +On the other hand, M. Bouty shows that a small rise in +temperature enormously affects the state of a mica surface, and +that the surface gets changed in such a way as to become very +fairly conducting at 300° C. Also anybody can easily try for +himself whether exposing a mica condenser plate which has been +examined in presence of phosphorus pentoxide to ordinary air for +five minutes will not enormously increase the residual charge, as +has always been the case in the writer's experience, and if so, +it is open to him to suggest some cause other than surface +creeping as an explanation.</p> +<p>M. Bouty, using less perfectly dried mica, did not get so good +a result as to smallness of residual charge as the one above +quoted.</p> +<p>The chief use of mica for laboratory purposes depends on the +ease with which it can be split, and also upon the fact that it +may be considerably crumpled and bent without breaking. It +therefore makes an excellent dielectric in so far as convenience +of construction is concerned in the preparation of condensers, +and lends itself freely to the construction of insulating washers +or separators of any kind. Its success as a fair insulator at +moderate temperatures has led to its use in resistance +thermometers, where it appears to have given satisfaction up to, +at all events, 400° C.</p> +<p>It is worth a note that according to Werner Siemens, who had +immense experience (<i>Wied. Ann</i>. vol. clix.), soapstone is +the only reliable insulator at a red heat, but, no doubt, a good +deal depends on the particular specimen investigated.</p> +<p><b><a name="Toc158108961" id="Toc158108961"><font face= +"Bookman Old Style" size="4">§ 107. Use of Mica in +Condensers. —</font></a></b></p> +<p><font face="Bookman Old Style">If good results are desired it +is essential to select the mica very carefully. Pieces +appreciably stained, — particularly if the stain is not +uniformly distributed, — cracked pieces, and pieces tending +to flake off in patches should be rejected. The best samples of +mica that have come under the writer's observation are those +sheets sold for the purpose of giving to silver photographic +prints that hideous glazed surface which some years ago was so +popular.</font></p> +<p>Sheets of mica about 0.1 to 0.2 mm. thick form good +serviceable condenser plates, and will certainly stand a pressure +of 300 volts, and most likely a good deal more. The general +practice in England seems to have been to build up condensers of +alternate sheets of varnished or paraffined-mica and +tin-foil.</p> +<p>This practice is open to several objections. In the first +place, the capacity of a condenser made in this way varies with +the pressure binding the plates together. In the second place, +the amount of mica and tin-foil required is often excessive in +consequence of the imperfect contact of these substances. Again, +the inevitable air film between the mica and tin-foil renders +condensers so made unsuitable for use with alternating currents, +owing to the heating set up through air discharges, and which is +generally, though often (if not always) wrongly, attributed to +dielectric hysteresis.</p> +<p>These imperfections are to a great extent got over by M. +Carpentier's method of construction, which is, however, rather +more costly both in material and labour. On the other hand, +wonderful capacities are obtained with quite small amounts of +mica. M. Bouty mentions a condenser of one microfarad capacity +weighing 1500 grms. and contained in a square box measuring 12 +centimetres on the side, and about 3 centimetres thick.</p> +<p>The relation between the capacity and surface of doubly-coated +plates is in electro-static units —</p> +<p>Capacity = (sp. ind. capacity X area of one surface)/(4pi X +thickness)</p> +<p>This may be reduced to electro-magnetic units by dividing by +9x10^20, and to microfarads by further multiplying by 10^15.</p> +<p>M. Carpentier begins, of course, by having his mica +scrupulously clean and well selected. It is then silvered by one +of the silvering processes (§ 65) on both sides, for which +purpose the sheets may be suspended in a paraffined wood rack, so +as to lie horizontally in the silvering solution, a space of +about half an inch being allowed between the sheets. The +silvering being finished, the sheets are dipped along two +parallel edges in 75 per cent nitric acid. With regard to the +third and fourth edges of the sheet, the silver is removed on one +side only, using a spun glass brush; if we agree to call the two +surfaces of the mica A and B respectively, and the two edges in +question C and D, then the silver is removed from the A side +along edge C, and from the B side along edge D. The silvered part +is shown shaded in Fig. 84. By this arrangement the silver and +mica plates may be built up together so as to form the same +mutual arrangement of contacts as in an ordinary mica tin-foil +condenser.</p> +<p><img src="images/Image128.gif" alt="images/Image128.gif" +width="396" height="237">Fig. 84.</p> +<p>It need hardly be said that the sheets require very complete +washing after treatment with nitric acid, followed by a +varnishing of the edges as already described in the case of +glass, and baking at a temperature of 140° C. in air free +from flame gases, till the shellac begins to emit its +characteristic odour and is absolutely hard when cold.</p> +<p>The plates are then built up so as to connect the sheets which +require to be connected, and to insulate the other set. General +contact is, if necessary, secured by means of a little silver +leaf looped across from plate to plate — a part of the +construction which requires particular attention and clean hands, +for it is by no means so easy to make an unimpeachable contact as +might at first appear.</p> +<p>The condenser, having been built up, may be clamped solid and +placed in its case; the capacity will not depend appreciably on +the tightness of the clamp screws — a great feature of the +construction. Such a condenser will not give its best results +unless absolutely dry. I have kept one very conveniently in a +vacuum desiccator over phosphorus pentoxide, but if of any size, +the condenser deserves a box to itself, and this must be +air-tight and provided with a drying reagent, so arranged that it +can be removed through a manhole of some sort.</p> +<p>Contact to the brass-work on the lid may be made by pressing +spring contacts tightly down upon the ends of the silver foils +and carrying the connections through the lid. This also serves to +secure the condenser in position.</p> +<p><b><a name="Toc158108962" id="Toc158108962"><font face= +"Bookman Old Style" size="4">§ 108. <i>Micanite</i>. +—</font></a></b></p> +<p><font face="Bookman Old Style">This substance, though probably +comparing somewhat unfavourably with the insulators already +enumerated, and being subject to the uncertainties of +manufacture, has during the last few years achieved a +considerable success in American electrical engineering +construction. It is composed of scrap mica and shellac varnish +worked under pressure to the desired shape, and may be obtained +in sheets, plates, and rods, or in any of the forms for which a +die happens to have been constructed.</font></p> +<p>Of course, in special cases it would be worth while to prepare +a die, and then the attainable forms would be limited by moulding +considerations only. The writer's experience is very limited in +this matter, but Dr. Kennelly, with whom he communicated on the +subject, was good enough to reply in favour of micanite for +engineering work.</p> +<p><b><a name="Toc158108963" id="Toc158108963"><font face= +"Bookman Old Style" size="4">§ 109. <i>Celluloid</i>. +—</font></a></b></p> +<p><font face="Bookman Old Style">This material is composed of +nitrocellulose and camphor.</font></p> +<p>It has fair insulating properties, and may be obtained in a +variety of forms, but has now been generally abandoned for +electrical work on account of its inflammability.</p> +<p><b><a name="Toc158108964" id="Toc158108964"><font face= +"Bookman Old Style" size="4">§ 110. +<i>Paper</i>.</font></a></b></p> +<p><font face="Bookman Old Style">Pure white filter paper, +perfectly dry, is probably a very fair insulator; the misfortune +is that in practice it cannot be kept dry. Under the most +favourable circumstances its specific resistance may approach +10<sup>24</sup> E.M. units. It must therefore be considered +rather as a partial conductor than as an insulator. The only case +of the use of dry paper as an insulator in machine construction +which has come under the writer's notice is in building up the +commutators of the small motors which used to drive the Edison +phonographs.</font></p> +<p>Its advantages in this connection are to be traced to the fact +that a commutator so built up is durable and keeps a clean +surface. Of course, the use of paper as an insulator for +telephone wires is well known, but its success in this direction +depends less upon its insulating properties than upon the fact +that it can be arranged in such a way as to allow of the wires +being partially air insulated, an arrangement tending to reduce +the electrostatic capacity of the wire system.</p> +<p>At one time it was the custom of instrument makers to employ +ordinary printed paper in the shape of leaves torn from books or +the folios of old ledgers to form the dielectric of the +condensers used in connection with the contact breakers of +induction coils. This practice has nothing but economy to +recommend it, for cases often occur in which the paper, by +gradual absorption of moisture from the air, comes to insulate so +badly that it practically short circuits the spark gap, and so +stops the action of the coil. Three separate cases have come +within the writer's experience.</p> +<p>Some measurements of the resistance of paper have been made by +F. Uppenborn (<i>Centralblatt fuer Electrotechnik</i>, Vol. xi. +p. 215, 1889). There is an abstract of the paper also in +<i>Wiedemann's Beiblaetter</i> (1889, vol. xiii. P. 711). +Uppenborn examined the samples of paper under normal conditions +as to moisture and obtained the following results: —</p> +<table border summary="" cellspacing="1" cellpadding="1" width= +"567"> +<tr> +<td width="20%" valign="top"> +<p><font face="Bookman Old Style" size="1">Description of +Paper</font></p> +</td> +<td width="20%" valign="top"> +<p><font face="Bookman Old Style" size="1">I</font></p> +<p>Pressure Intensity</p> +</td> +<td width="20%" valign="top"> +<p><font face="Bookman Old Style" size="1">II.</font></p> +<p>Specific Resistance corresponding to pressures as in Column I. +Ohms.</p> +</td> +<td width="20%" valign="top"> +<p><font face="Bookman Old Style" size="1">III</font></p> +<p>Pressure Intensity.</p> +</td> +<td width="20%" valign="top"> +<p><font face="Bookman Old Style" size="1">IV.</font></p> +<p>Specific Resistance corresponding to Column III. Ohms.</p> +</td> +</tr> +<tr> +<td width="20%" valign="top"> +<p><font face="Bookman Old Style" size="1">Common cardboard 2.3 +mm. thick</font></p> +</td> +<td width="20%" valign="top"> +<p><font face="Bookman Old Style" size="1">0.05 kilo. per 6000 +sq. mm.</font></p> +</td> +<td width="20%" valign="top"> +<p><font face="Bookman Old Style" size="1">4.85 x +10<sup>15</sup></font></p> +</td> +<td width="20%" valign="top"> +<p><font face="Bookman Old Style" size="1">20 kg. per 6000 sq. +mm.</font></p> +</td> +<td width="20%" valign="top"> +<p><font face="Bookman Old Style" size="1">4.7 x +10<sup>14</sup></font></p> +</td> +</tr> +<tr> +<td width="20%" valign="top"> +<p><font face="Bookman Old Style" size="1">Gray paper, 0.26 mm. +thick</font></p> +</td> +<td width="20%" valign="top"> +<p><font face="Bookman Old Style" size="1">0.05 kilo. per 5000 +sq. mm.</font></p> +</td> +<td width="20%" valign="top"> +<p><font face="Bookman Old Style" size="1">3.1 x +10^<sup>15</sup></font></p> +</td> +<td width="20%" valign="top"> +<p><font face="Bookman Old Style" size="1">20 kg. per 5000 sq. +mm.</font></p> +</td> +<td width="20%" valign="top"> +<p><font face="Bookman Old Style" size="1">8 x +10<sup>14</sup></font></p> +</td> +</tr> +<tr> +<td width="20%" valign="top"> +<p><font face="Bookman Old Style" size="1">Yellow parchment +paper-09 mm. thick</font></p> +</td> +<td width="20%" valign="top"> +<p><font face="Bookman Old Style" size="1">0.05 kilo. per 5300 +sq. mm.</font></p> +</td> +<td width="20%" valign="top"> +<p><font face="Bookman Old Style" size="1">3.05 x +10<sup>16</sup></font></p> +</td> +<td width="20%" valign="top"> +<p><font face="Bookman Old Style" size="1">20 kg. per 5300 sq. +mm.</font></p> +</td> +<td width="20%" valign="top"> +<p><font face="Bookman Old Style" size="1">8 x +10<sup>16</sup></font></p> +</td> +</tr> +<tr> +<td width="20%" valign="top"> +<p><font face="Bookman Old Style" size="1">Linen tracing +cloth</font></p> +</td> +<td width="20%" valign="top"> +<p><font face="Bookman Old Style" size="1">0.05 kilo. per 6000 +sq. mm.</font></p> +</td> +<td width="20%" valign="top"> +<p><font face="Bookman Old Style" size="1">1.35 x +10<sup>16</sup></font></p> +</td> +<td width="20%" valign="top"> +<p><font face="Bookman Old Style" size="1">20 kg. per 33,000 sq. +mm.</font></p> +</td> +<td width="20%" valign="top"> +<p><font face="Bookman Old Style" size="1">1.86 x +10^<sup>15</sup></font></p> +</td> +</tr> +</table> +<p><b><a name="Toc158108965" id="Toc158108965"><font face= +"Bookman Old Style" size="4">§ 111. Paraffined Paper. +—</font></a></b></p> +<p><font face="Bookman Old Style">Like wood and other +semiconductors, paper can be vastly improved as an insulator by +saturating it with melted paraffin. To get the best results a +pure paper free from size must be employed — gray Swedish +filter paper does well. This is dried at a temperature above +100° C. for, say, half an hour, and the sheets are then +floated on the top of paraffin, kept melted at 140° C. or +thereabout in a baking dish. As soon as the paper is placed upon +the melted paraffin the latter begins to soak through, in virtue +of capillary action, and drives before it the air and moisture, +causing a strongly marked effervescence.</font></p> +<p>After about one minute the paper may be thrust below the +paraffin to soak. When a sufficient number of papers have +accumulated, and when no more gas comes off, the tray may be +placed in a vacuum box (Fig. 85), and the pressure reduced by the +filter pump. As the removal of the air takes time, provision must +be made for keeping the bath hot.</p> +<p>A vacuum may be maintained for about an hour, and air then +readmitted. Repeated exhaustions and readmissions of air, which +appear to improve wood, do not give anything like such a good +result with paper. In using a vacuum box provision must be made +in the shape of a cool bottle between the air pump and the box. +If this precaution be omitted, and if any paraffin splashes on to +the hot surface of the box, it volatilises with decomposition and +the products go to stop up the pump. Paraffin with a +melting-point of 50° C. or upwards does well.</p> +<p>The bath should be allowed to cool to about 60° C. before +the papers are removed, so that enough paraffin may be carried +out to thoroughly coat the paper and prevent the entrance of +air.</p> +<p>Fig. <img src="images/Image129.gif" alt="images/Image129.gif" +width="259" height="238"> 85.</p> +<p>Fig. 85 is a section of a vacuum vessel which has been found +very convenient. It measures about two feet in diameter at the +top. It is round, because it is much easier to turn one circular +surface than to plane up four surfaces, which has to be done if +the box is square. Both the rim of the vessel and the +approximating part of the cover require to be truly turned and +smoothly finished. A very good packing is made of solid +indiarubber core about half an inch thick. This is carefully +spliced — cemented by means of a solution of rubber in +naphtha, and the splice sewed by thick thread. The lid ought to +have a rim fitting inside the vessel, for this keeps the rubber +packing in place; the rim has been accidentally omitted in Fig. +85. The bolts should not be more than five inches apart, and +should lie at least half an inch in diameter, and the rim and lid +should be each half an inch thick.</p> +<p>Condensers may now be built up of sheets of this prepared +paper interleaved with tin-foil in the ordinary way. If good +results are required, the condenser when finished is compressed +between wooden or glass end-pieces by means of suitable clamps. +It can then be put in a box of melted paraffin, heated up to +140° C., and exhausted by means of the water pump for several +hours.</p> +<p>In this process the air rushes out from between the paper and +foils with such vehemence that the paraffin is generally thrown +entirely out of the box. To guard against this the box must be +provided with a loosely fitting and temporary lid, pierced with +several holes.</p> +<p>The real test as to when exhaustion is complete would be the +cessation of any yield of air or water. Since it is not generally +convenient to make the vacuum box so air-tight that there are +absolutely no leaks at all, and as the paraffin itself is, I +think, inclined to "crack" slightly at the temperature of +140° C., this test or criterion cannot be conveniently +applied.</p> +<p>Two exhaustions, each of about two hours' duration, have, +however, in my experience succeeded very well, provided, of +course, that the dielectric is prepared as suggested. At the end +of the exhaustion process the clamping screws are tightened as +far as possible, the condenser remaining in its bath until the +paraffin is pasty.</p> +<p>Condensers made in this way resist the application of +alternating currents perfectly, as the following tests will show. +The dielectric consisted of about equal parts of hard paraffin +and vaseline. A condenser of about 0.123 microfarads capacity and +an insulation resistance of 2000 megohms, <i>[Footnote:</i> As +tested by a small voltage.<i>]</i> having a tin-foil area of 4.23 +square metres (about), and double papers each about 0.2 mm. +thick, designed to run at 2000 volts with a frequency of 63 +complete periods, was tested at this frequency.</p> +<p>The condenser was thoroughly packed all round in cotton-wool +to a thickness of 6 inches, and its temperature was indicated +more or less by a thermometer plunged through a hole in the lid +of the containing box and of the condenser box, and resting on +the upper surface of one set of tin-foil electrodes, from which +the soft paraffin mixture had been purposely scraped away. The +following were the results of a four hours' run at a voltage 50 +per cent higher than that for which the condenser was designed +— i.e. 3000 volts.</p> +<table border summary="" cellspacing="1" cellpadding="7" width= +"525"> +<tr> +<td width="22%" valign="top" colspan="2"> +<p align="center"><font face= +"Bookman Old Style">Times.</font></p> +</td> +<td width="14%" valign="top" colspan="2"> +<p align="center"><font face= +"Bookman Old Style">Voltage</font></p> +</td> +<td width="23%" valign="top" colspan="2"> +<p><font face="Bookman Old Style">Temperature in +Condenser.</font></p> +</td> +<td width="23%" valign="top" colspan="2"> +<p><font face="Bookman Old Style">Temperature in Air.</font></p> +</td> +<td width="18%" valign="top" colspan="2"> +<p><font face="Bookman Old Style">Difference</font></p> +</td> +</tr> +<tr> +<td width="11%" valign="top"> +<p><font face="Bookman Old Style">Hrs.</font></p> +</td> +<td width="11%" valign="top" colspan="2"> +<p><font face="Bookman Old Style">Min.</font></p> +</td> +<td width="14%" valign="top" colspan="2"></td> +<td width="23%" valign="top" colspan="2"></td> +<td width="23%" valign="top" colspan="2"></td> +<td width="18%" valign="top"></td> +</tr> +<tr> +<td width="11%" valign="top"> +<p><font face="Bookman Old Style">2</font></p> +</td> +<td width="11%" valign="top" colspan="2"> +<p><font face="Bookman Old Style">10</font></p> +</td> +<td width="14%" valign="top" colspan="2"> +<p align="center"><font face="Bookman Old Style">2750</font></p> +</td> +<td width="23%" valign="top" colspan="2"> +<p><font face="Bookman Old Style">22.8° C.</font></p> +</td> +<td width="23%" valign="top" colspan="2"> +<p><font face="Bookman Old Style">23.0° C.</font></p> +</td> +<td width="18%" valign="top"> +<p><font face="Bookman Old Style">+ 0.2°</font></p> +</td> +</tr> +<tr> +<td width="11%" valign="top"> +<p><font face="Bookman Old Style">3</font></p> +</td> +<td width="11%" valign="top" colspan="2"> +<p><font face="Bookman Old Style">10</font></p> +</td> +<td width="14%" valign="top" colspan="2"> +<p align="center"><font face="Bookman Old Style">2700</font></p> +</td> +<td width="23%" valign="top" colspan="2"> +<p><font face="Bookman Old Style">23.0° C.</font></p> +</td> +<td width="23%" valign="top" colspan="2"> +<p><font face="Bookman Old Style">23.3° C.</font></p> +</td> +<td width="18%" valign="top"> +<p><font face="Bookman Old Style">+ 0.3°</font></p> +</td> +</tr> +<tr> +<td width="11%" valign="top"> +<p><font face="Bookman Old Style">3</font></p> +</td> +<td width="11%" valign="top" colspan="2"> +<p><font face="Bookman Old Style">18</font></p> +</td> +<td width="14%" valign="top" colspan="2"> +<p align="center"><font face="Bookman Old Style">3200</font></p> +</td> +<td width="23%" valign="top" colspan="2"> +<p><font face="Bookman Old Style">23.1° C.</font></p> +</td> +<td width="23%" valign="top" colspan="2"> +<p><font face="Bookman Old Style">23.0° C.</font></p> +</td> +<td width="18%" valign="top"> +<p><font face="Bookman Old Style">-0.1°</font></p> +</td> +</tr> +<tr> +<td width="11%" valign="top"> +<p><font face="Bookman Old Style">4</font></p> +</td> +<td width="11%" valign="top" colspan="2"> +<p><font face="Bookman Old Style">10</font></p> +</td> +<td width="14%" valign="top" colspan="2"> +<p align="center"><font face="Bookman Old Style">3200</font></p> +</td> +<td width="23%" valign="top" colspan="2"> +<p><font face="Bookman Old Style">23.3° C.</font></p> +</td> +<td width="23%" valign="top" colspan="2"> +<p><font face="Bookman Old Style">23.7° C.</font></p> +</td> +<td width="18%" valign="top"> +<p><font face="Bookman Old Style">+ 0.4°</font></p> +</td> +</tr> +<tr> +<td width="11%" valign="top"> +<p><font face="Bookman Old Style">5</font></p> +</td> +<td width="11%" valign="top" colspan="2"> +<p><font face="Bookman Old Style">10</font></p> +</td> +<td width="14%" valign="top" colspan="2"> +<p align="center"><font face="Bookman Old Style">3100</font></p> +</td> +<td width="23%" valign="top" colspan="2"> +<p><font face="Bookman Old Style">23.6° C.</font></p> +</td> +<td width="23%" valign="top" colspan="2"> +<p><font face="Bookman Old Style">23.4° C.</font></p> +</td> +<td width="18%" valign="top"> +<p><font face="Bookman Old Style">-0.2°</font></p> +</td> +</tr> +<tr> +<td width="11%" valign="top"> +<p><font face="Bookman Old Style">6</font></p> +</td> +<td width="11%" valign="top" colspan="2"> +<p><font face="Bookman Old Style">10</font></p> +</td> +<td width="14%" valign="top" colspan="2"> +<p align="center"><font face="Bookman Old Style">3000</font></p> +</td> +<td width="23%" valign="top" colspan="2"> +<p><font face="Bookman Old Style">23.8° C.</font></p> +</td> +<td width="23%" valign="top" colspan="2"> +<p><font face="Bookman Old Style">23.35° C.</font></p> +</td> +<td width="18%" valign="top"> +<p><font face="Bookman Old Style">-0.45°</font></p> +</td> +</tr> +</table> +<p><font face="Bookman Old Style">An idea of the order of the +amount of waste may be formed from the following additional +experiment.</font></p> +<p>A condenser similar to the one described was filled with oil +of a low insulating power. It was tested calorimetrically, and +also by the three voltmeter method, which, however, proved to be +too insensitive. The temperature rise in the non-conducting box +in air was about 0.3° C. per hour, and the loss of power was +found to be less than 0.1 per cent. In the present case the +actual rise was only 1° in four hours, and the integral give +and take between the condenser and the air is practically +nothing; consequently we may consider with safety that the rate +of rise is certainly less than 1 degree per three hours. The +voltage and frequency were about the same in both experiments, +consequently the energy passed is about proportional to the +capacity used in the two experiments.</p> +<p>From this it follows that since the specific heat of both +condensers was the same (nearly), the loss in the present case is +a good deal less than one-tenth per cent. The residual charge is +also much less than when the condenser is simply built up of +paper paraffined in an unsystematic manner, and from which the +air and water have been imperfectly extracted, as by baking the +condenser first, and then immersing it in paraffin or oil.</p> +<p>It is usual to consider that the phenomena of residual charge +and heating in condensers, to which alternating voltages are +applied, are closely allied. This is true, but the alliance is +not one between cause and effect — at all events, with +regard to the greater part of the heating. The imperfect +exclusion of air and moisture, particularly the latter, certainly +increases the residual charge by allowing surface creeping to +occur; but it also acts directly in producing heating, both by +lowering the insulation of the condenser and by allowing of air +discharges between the condenser plates.</p> +<p>Of these causes of heating, the discharges in air or water +vapour are probably the more important. Long ago a theory of +residual charge was given by Maxwell, based on the consideration +of a laminated dielectric, the inductivity and resistance of +which varied from layer to layer. It was shown that such an +arrangement, and hence generally any want of homogeneity in a +direction inclined to the lines of force leading to a change of +value of the product of specific resistance and specific +inductive capacity, would account for residual charge.</p> +<p>This possible explanation has been generally accepted as the +actual explanation, and many cases of residual charge attributed +to want of homogeneity, which are certainly to be explained in a +simpler manner. For instance, the residual charge in a silvered +mica plate condenser, carefully dried, can be increased at least +tenfold by an exposure of a few minutes to ordinarily damp air. +The same result occurs with condensers of paraffined or sulphured +paper; and these are the residual changes generally observed. The +greater part must be due to creeping.</p> +<p><b><a name="Toc158108966" id="Toc158108966"><font face= +"Bookman Old Style" size="4">§ 112. <i>Paraffin</i> +—</font></a></b></p> +<p><font face="Bookman Old Style">This substance has long enjoyed +great popularity in the physical laboratory. Its specific +resistance is given by Ayrton and Perry as more than +10<sup>25</sup>, but it is probably much higher in selected +samples. The most serviceable kind of paraffin is the hardest +obtainable, melting at a temperature of not less than 52° C. +It is a good plan to remelt the commercial paraffin and keep it +at a temperature of, say, 120° C. for an hour, stirring it +carefully with a glass rod so that it does not get overheated; +this helps to get rid of traces of water vapour.</font></p> +<p>Hard paraffin, when melted, has an enormous rate of expansion +with temperature, so great, indeed, that care must be taken not +to overfill the vessels in which it is to be heated. Castings can +only be prepared by cooling the mould slowly from the bottom, +keeping the rest of the mould warm, and adding-paraffin from time +to time to make up for the contraction. The cooling is gradually +allowed to spread up to the free surface.</p> +<p>The chief use of paraffin in the laboratory is in the +construction of complicated connection boards, which are easily +made by drilling holes in a slab of paraffin, half filling them +with mercury, and using them as mercury cups.</p> +<p>Since paraffin is a great collector of dust, it should be +screened by paper, otherwise the blocks require to be scraped at +frequent intervals, which, of course, electrifies them +considerably. This electrification is often difficult to remove +without injuring the insulating power of the paraffin. A light +touch with a clean Bunsen flame is the readiest method, and does +not appear to reduce the insulation so much as might be expected. +The safest way, however, is to leave the key covered by a clean +cloth, which, however, must not touch the surface, for a +sufficient time to allow of the charges getting away.</p> +<p>The paraffin often becomes electrified itself by the friction +of the key contacts, so that in electrometer work it is often +convenient to form the cups by lining them with a little roll of +copper foil twisted up at the bottom. In this case the connecting +wires should, of course, be copper. Small steel staples are +convenient for fastening the collecting wires upon the paraffin; +or, in the case where these wires have to be often removed and +changed about, drawing-pins are very handy.</p> +<p>With mercury cups simply bored in paraffin great trouble will +often be experienced in electrometer work, owing to a potential +difference appearing between the cups — at all events when +the contacts are inserted and however carefully this be done. A +few drops of very pure alcohol poured in above the mercury often +cures this defect. The surface of paraffin is by no means exempt +from the defect of losing its insulating power when exposed to +damp air, but it is not so sensitive as glass, nor does the +insulating power fall so far.</p> +<p>Two useful appliances are figured.</p> +<p><img src="images/Image130.gif" alt="images/Image130.gif" +width="95" height="150">Fig. <img src="images/Image131.gif" alt= +"images/Image131.gif" width="142" height="283"> 86. Fig. 87.</p> +<p>One, in which paraffin appears as a cement, is an insulating +stand made out of a bit of glass or ebonite tube cemented into an +Erlenmeyer flask, having its neck protected from dust when out of +use by a rubber washer, the other a "petticoat" insulator made by cementing a flint glass bottle into a glass +dish with paraffin. In course of time the paraffin will be found +to have separated from the glass. When this occurs the apparatus +may be melted together again by placing it on the water bath for +a few minutes.</p> +<p><b><a name="Toc158108967" id="Toc158108967"><font face= +"Bookman Old Style" size="4">§ 113. Vaseline, Vaseline Oil, +and Kerosene Oil. —</font></a></b></p> +<p><font face="Bookman Old Style">These, when dry, insulate +almost, but not quite as well as solid paraffin. H. Koeller +(<i>Wien Berichte</i>, 98, ii. 201, 1889; <i>Beibl</i>. +<i>Wied</i>. <i>Ann</i>. 1890, p. 186), working with very small +voltages, places the final(?) specific resistance of commercial +petroleum, ether, and vaseline oil at about 2 X 10<sup>27</sup> +C.G.S. This is ten times higher than the value assigned to +commercial benzene (C<sub>6</sub>H<sub>6</sub>), and a hundred +times higher than the value for commercial toluene.</font></p> +<p>All these numbers mean little or nothing, but the petroleum +and vaseline oil were the best fluid insulators examined by +Koeller. By mixing vaseline with paraffin a soft wax may be made +of any desired degree of softness, and by dissolving vaseline in +kerosene an insulating liquid of any degree of viscidity may be +obtained.</p> +<p>Hard paraffin may be softened somewhat by the addition of +kerosene, and an apparently homogeneous mass cast from the +mixture. It will be found, however, that in course of time the +kerosene oozes out, unless present in very small quantity. +Koeller has found (<i>loc. cit</i>.) that some samples of +vaseline oil conducted "vollstaendig gut," but I have not come +across such samples. Vaseline oil, however, is sold at a price +much above its value for insulating purposes.</p> +<p>Kerosene oil is best obtained dry by drawing it directly from +a new tin and exposing it to air as little as possible. Of +course, it may be dried by chemical means and distillation, but +this is usually (or always) unnecessary.</p> +<p>Fig <img src="images/Image132.gif" alt="images/Image132.gif" +width="282" height="253">88.</p> +<p>There is some danger of kerosene containing minute traces of +sulphuric acid, and it and other oils may be conveniently tested +for insulation in the following manner. The quartz electroscope +is taken, and the insulating rod heated in the blow-pipe. The +electroscope will now insulate well enough to show no appreciable +collapse of the leaves in one or two hours' time. Upon the plate +of the electroscope is put a platinum or copper cylinder, and +this is filled with kerosene (say) up to a fixed mark.</p> +<p>The electroscope is placed on a surface plate, or, at all +events, on a sheet of plate glass, and a "scribing block" is +placed along side it and the scriber adjusted to dip into the +kerosene to any required depth. This is done by twisting a bit of +wire round the scribing point and allowing it to project +downwards. The point itself serves to give an idea of the height +to which the vessel may be filled. The liquid is adjusted till +its surface is in contact with the end of the scribing point, and +the wire then projects into the liquid and forms an electrode of +constant area of surface. The scribing block is put to earth. A +charge is given to the electroscope, and the time required for a +given degree of collapse of the leaves noted.</p> +<p>The kerosene is then removed and its place taken by vaseline +or paraffin, known to insulate well as a standard for comparison. +The experiment is then repeated, and the time noted for the same +degree of collapse. This test, though of course rough, is +generally quite sufficient for workshop purposes, and is easily +applied. Moreover, it does not require correction for +electrometer leakage, as generally happens when more elaborate +appliances are used.</p> +<p>The actual resistance of insulating oils depends so much on +the electrical intensity, on the duration of that intensity, and +on the previous history of the oil as to the direction of the +voltage to which it has been subjected — to say nothing of +the effects of traces of moisture — that quantitative +experiments are of no value unless they are extremely elaborate. +I shall therefore only append the following numbers due to Bouty, +<i>Ann. de Chemie et de Physique</i> (6), vol. xxvii. p. 62, +1892, in which the effect of the conductivity on the +determination of the specific inductive capacity was properly +allowed for:—</p> +<table border summary="" cellspacing="1" cellpadding="3" width= +"661"> +<tr> +<td width="23%" valign="top"></td> +<td width="17%" valign="top"> +<p><font face="Bookman Old Style">Carbon<br> +Bisulphide.</font></p> +</td> +<td width="20%" valign="top"> +<p><font face="Bookman Old Style">Turpentine.</font></p> +</td> +<td width="20%" valign="top"> +<p><font face="Bookman Old Style">Benzene +(C<sub>6</sub>H<sub>6</sub>) at 20° C.</font></p> +</td> +<td width="20%" valign="top"> +<p><font face="Bookman Old Style">Benzene at<br> +60° C.</font></p> +</td> +</tr> +<tr> +<td width="23%" valign="top"> +<p><font face="Bookman Old Style">Specific inductive +capacity</font></p> +</td> +<td width="17%" valign="top"> +<p align="right"><font face="Bookman Old Style">2.715</font></p> +</td> +<td width="20%" valign="top"> +<p align="right"><font face="Bookman Old Style">2.314</font></p> +</td> +<td width="20%" valign="top"> +<p align="right"><font face="Bookman Old Style">2.21</font></p> +</td> +<td width="20%" valign="top"> +<p align="right"><font face="Bookman Old Style">2.22</font></p> +</td> +</tr> +<tr> +<td width="23%" valign="top"> +<p><a name="lastbookmark1" id="lastbookmark1"><font face= +"Bookman Old Style">Specific resistance in ohms per cubic +centimetre</font></a></p> +</td> +<td width="17%" valign="top"> +<p align="right"><font face="Bookman Old Style">1.5 x +10<sup>13</sup>,</font></p> +</td> +<td width="20%" valign="top"> +<p align="right"><font face="Bookman Old Style">1.75 x +10<sup>12</sup></font></p> +</td> +<td width="20%" valign="top"> +<p align="right"><font face="Bookman Old Style">1.56 x +10<sup>11</sup></font></p> +</td> +<td width="20%" valign="top"> +<p align="right"><font face="Bookman Old Style">7.9 x +10<sup>11</sup></font></p> +</td> +</tr> +</table> +<p><i><font face="Bookman Old Style">[Footnote:</font> Professor +J. J. Thomson, and Newall (Phil. Proc. 1886) consider that carbon +bisulphide showed traces of a "residual charge" effect; hence, +until this point is cleared up, we must regard Bouty's value as +corresponding only to a very short, but not indefinitely short, +period of charge. On this point the paper must be +consulted.</i></p> +<p>March 1897 — The writer has investigated this point by +an independent method, but found no traces of "residual +charge."<i>]</i></p> +<p>Information as to the specific inductive capacity of a large +number of oils may be found in a paper by Hopkinson, <i>Phil. +Proc</i>. 1887, and in a paper by Quincke in Wiedemann's +<i>Annalen</i>, 1883.</p> +<p><b><a name="Toc158108968" id="Toc158108968"><font face= +"Bookman Old Style" size="4">§ 114. Imperfect Conductors. +—</font></a></b></p> +<p><font face="Bookman Old Style">Under this heading may be +grouped such things as wood, slate, marble, etc. — in fact, +materials generally used for switchboard insulation. An +examination of the insulating power of these substances has +recently been made by B. O. Peirce (<i>Electrical Review</i>, +11th January 1895) with quite sufficient accuracy, having in view +the impossibility of being certain beforehand as to the character +of any particular sample. The tests were made by means of holes +drilled in slabs of the material to be examined. These holes were +three-eighths of an inch in diameter, and from five-eighths to +three-quarters of an inch deep, and one inch apart, centre to +centre. A voltage of about 15 volts was employed. The following +general results were arrived at:-</font></p> +<p>(1) Heating in a paraffin bath always increases the resistance +of wood, though only slightly if the wood be hard and dense.</p> +<p>(2) Frequent exhaustion and readmission of air above the +surface of the paraffin always has a good effect in increasing +the resistance of wood.</p> +<p>(3) When wood is once dry, impregnating it with paraffin tends +to keep it dry.</p> +<p>(4) Red vulcanised fibre, like wood, absorbs paraffin, but it +cannot be entirely waterproofed in this way.</p> +<p>(5) The resistance of wood with stream lines along the grain +is twenty to fifty per cent lower than when the stream lines +cross the grain.</p> +<p>(6) The "contact" resistance between slabs of wood pressed +together is always very high.</p> +<p>The following table will sufficiently illustrate the results +obtained. The stone was dried in the sun for three weeks in the +summer (United States), and the wood is described as having been +well seasoned:—</p> +<p align="center"><b>CURRENT WITH THE GRAIN</b></p> +<table border summary="" cellspacing="1" cellpadding="7" width= +"567"> +<tr> +<td width="18%" valign="top" height="4"></td> +<td width="22%" valign="top" height="4"> +<p><font face="Bookman Old Style">Lowest Resistance between two +Cups in Megohms.</font></p> +</td> +<td width="20%" valign="top" height="4"> +<p><font face="Bookman Old Style">Highest Resistance between two +Cups in Megohms.</font></p> +</td> +<td width="20%" valign="top" height="4"> +<p><font face="Bookman Old Style">Lowest Specific Resistance in +Megohms.</font></p> +</td> +<td width="20%" valign="top" height="4"> +<p><font face="Bookman Old Style">Highest Specific Resistance in +Megohms.</font></p> +</td> +</tr> +<tr> +<td width="18%" valign="top" height="4"> +<p><font face="Bookman Old Style">Ash.</font></p> +</td> +<td width="22%" valign="top" height="4"> +<p align="right"><font face="Bookman Old Style">550</font></p> +</td> +<td width="20%" valign="top" height="4"> +<p align="right"><font face="Bookman Old Style">920</font></p> +</td> +<td width="20%" valign="top" height="4"> +<p align="right"><font face="Bookman Old Style">380</font></p> +</td> +<td width="20%" valign="top" height="4"> +<p align="right"><font face="Bookman Old Style">700</font></p> +</td> +</tr> +<tr> +<td width="18%" valign="top" height="4"> +<p><font face="Bookman Old Style">Cherry</font></p> +</td> +<td width="22%" valign="top" height="4"> +<p align="right"><font face="Bookman Old Style">1100</font></p> +</td> +<td width="20%" valign="top" height="4"> +<p align="right"><font face="Bookman Old Style">4000</font></p> +</td> +<td width="20%" valign="top" height="4"> +<p align="right"><font face="Bookman Old Style">2800</font></p> +</td> +<td width="20%" valign="top" height="4"> +<p align="right"><font face="Bookman Old Style">6000</font></p> +</td> +</tr> +<tr> +<td width="18%" valign="top" height="4"> +<p><font face="Bookman Old Style">Mahogany</font></p> +</td> +<td width="22%" valign="top" height="4"> +<p align="right"><font face="Bookman Old Style">430</font></p> +</td> +<td width="20%" valign="top" height="4"> +<p align="right"><font face="Bookman Old Style">730</font></p> +</td> +<td width="20%" valign="top" height="4"> +<p align="right"><font face="Bookman Old Style">310</font></p> +</td> +<td width="20%" valign="top" height="4"> +<p align="right"><font face="Bookman Old Style">610</font></p> +</td> +</tr> +<tr> +<td width="18%" valign="top" height="4"> +<p><font face="Bookman Old Style">Oak</font></p> +</td> +<td width="22%" valign="top" height="4"> +<p align="right"><font face="Bookman Old Style">220</font></p> +</td> +<td width="20%" valign="top" height="4"> +<p align="right"><font face="Bookman Old Style">420</font></p> +</td> +<td width="20%" valign="top" height="4"> +<p align="right"><font face="Bookman Old Style">1050</font></p> +</td> +<td width="20%" valign="top" height="4"> +<p align="right"><font face="Bookman Old Style">2200</font></p> +</td> +</tr> +<tr> +<td width="18%" valign="top" height="4"> +<p><font face="Bookman Old Style">Pine.</font></p> +</td> +<td width="22%" valign="top" height="4"> +<p align="right"><font face="Bookman Old Style">330</font></p> +</td> +<td width="20%" valign="top" height="4"> +<p align="right"><font face="Bookman Old Style">630</font></p> +</td> +<td width="20%" valign="top" height="4"> +<p align="right"><font face="Bookman Old Style">360</font></p> +</td> +<td width="20%" valign="top" height="4"> +<p align="right"><font face="Bookman Old Style">1470</font></p> +</td> +</tr> +<tr> +<td width="18%" valign="top" height="4"> +<p><font face="Bookman Old Style">Hard pine.</font></p> +</td> +<td width="22%" valign="top" height="4"> +<p align="right"><font face="Bookman Old Style">10</font></p> +</td> +<td width="20%" valign="top" height="4"> +<p align="right"><font face="Bookman Old Style">48</font></p> +</td> +<td width="20%" valign="top" height="4"> +<p align="right"><font face="Bookman Old Style">17</font></p> +</td> +<td width="20%" valign="top" height="4"> +<p align="right"><font face="Bookman Old Style">1050</font></p> +</td> +</tr> +<tr> +<td width="18%" valign="top" height="4"> +<p><font face="Bookman Old Style">Black walnut</font></p> +</td> +<td width="22%" valign="top" height="4"> +<p align="right"><font face="Bookman Old Style">1100</font></p> +</td> +<td width="20%" valign="top" height="4"> +<p align="right"><font face="Bookman Old Style">3000</font></p> +</td> +<td width="20%" valign="top" height="4"> +<p align="right"><font face="Bookman Old Style">320</font></p> +</td> +<td width="20%" valign="top" height="4"> +<p align="right"><font face="Bookman Old Style">2100</font></p> +</td> +</tr> +<tr> +<td width="18%" valign="top" height="4"> +<p><font face="Bookman Old Style">Red fibre</font></p> +</td> +<td width="22%" valign="top" height="4"> +<p align="right"><font face="Bookman Old Style">2</font></p> +</td> +<td width="20%" valign="top" height="4"> +<p align="right"><font face="Bookman Old Style">4</font></p> +</td> +<td width="20%" valign="top" height="4"> +<p align="right"><font face="Bookman Old Style">3</font></p> +</td> +<td width="20%" valign="top" height="4"> +<p align="right"><font face="Bookman Old Style">60</font></p> +</td> +</tr> +<tr> +<td width="18%" valign="top" height="4"> +<p><font face="Bookman Old Style">Slate</font></p> +</td> +<td width="22%" valign="top" height="4"></td> +<td width="20%" valign="top" height="4"></td> +<td width="20%" valign="top" height="4"> +<p align="right"><font face="Bookman Old Style">184</font></p> +</td> +<td width="20%" valign="top" height="4"> +<p align="right"><font face="Bookman Old Style">280</font></p> +</td> +</tr> +<tr> +<td width="18%" valign="top" height="4"> +<p><font face="Bookman Old Style">Soapstone.</font></p> +</td> +<td width="22%" valign="top" height="4"></td> +<td width="20%" valign="top" height="4"></td> +<td width="20%" valign="top" height="4"> +<p align="right"><font face="Bookman Old Style">330</font></p> +</td> +<td width="20%" valign="top" height="4"> +<p align="right"><font face="Bookman Old Style">500</font></p> +</td> +</tr> +<tr> +<td width="18%" valign="top" height="4"> +<p><font face="Bookman Old Style">White marble</font></p> +</td> +<td width="22%" valign="top" height="4"></td> +<td width="20%" valign="top" height="4"></td> +<td width="20%" valign="top" height="4"> +<p align="right"><font face="Bookman Old Style">2000</font></p> +</td> +<td width="20%" valign="top" height="4"> +<p align="right"><font face="Bookman Old Style">8800</font></p> +</td> +</tr> +</table> +<p>§ 115. As to working the materials very little need be +said.</p> +<p>Fibre is worked like wood, but has the disadvantage of rapidly +taking the edge off the tools. In turning it, therefore, +brass-turning tools, though leaving not quite such a perfect +finish as wood-turning tools, last much longer, and really do +well enough. Fibre will not bear heating much above 100° C. +— at all events in paraffin. At 140° C. it becomes +perfectly brittle. Its chief merit lies in its great strength. So +far as insulation is concerned, Mr. Peirce's experiments show +that it is far below most kinds of wood.</p> +<p><i>Slate</i>. — This is a vastly more useful substance +than it is generally credited with being. It is very easily +worked at a slow speed, either on the shaping machine or on the +lathe, with tools adjusted for cutting brass, and it keeps its +figure, which is more than can be said for most materials. It +forms a splendid base for instruments, especially when ground +with a little emery by iron or glass grinders, fined with its own +dust, and French polished in the ordinary way. Spools for coils +of considerable radial dimension may be most conveniently made of +slate instead of wood or brass, both because it keeps its shape, +and because it insulates sufficiently well to stop eddy currents +— at all events, sufficiently for ordinary practice. An +appreciable advantage is that slate may be purchased at a +reasonable rate in large slabs of any desired thickness. It is +generally cut in the laboratory by means of an old cross-cut saw, +but it does not do much damage to a hard hack saw such as is used +for iron or brass.</p> +<p><i>Marble</i>. — According to Holtzapffell, marble may +be easily turned by means of simple pointed tools of good steel +tempered to a straw colour. The cutting point is ground on both +edges like a wood-turning tool, and held up to the work "at an +angle of twenty or thirty degrees" (?with the horizontal). The +marble is cut wet to save the tool. As soon as the point gets, by +grinding, to be about one-eighth of an inch broad it must either +be drawn down or reground; a flat tool will not turn marble at +all.</p> +<p>A convenient saw for marble is easily made on the principle of +the frame saw. A bit of hoop iron forms a convenient blade, and +is sharpened by being hammered into notches along one edge, using +the sharp end of a hammer head. The saw is liberally supplied +with sand and water — or emery and water, where economy of +time is an object. The sawing of marble is thus really a grinding +process, but it goes on rapidly. Marble is ground very easily +with sand and water; it is fined with emery and polished with +putty powder, which, I understand, is best used with water on +cloth or felt. As grinding processes have already been fully +described, there is no need to go into them here. I have no +personal knowledge of polishing marble.</p> +<p><b><a name="Toc158108969" id="Toc158108969"><font face= +"Bookman Old Style" size="4">§ 116. <i>Conductors</i>. +—</font></a></b></p> +<p><font face="Bookman Old Style">The properties of conductors, +more particularly of metals, have been so frequently examined, +that the literature of the subject is appallingly heavy. In what +follows I have endeavoured to keep clear of what might properly +appear in a treatise on electricity on the one hand, and in a +wiring table on the other. The most important work on the subject +of the experimental resistance properties of metals has been done +by Matthieson, <i>Phil. Trans</i>. 1860 and 1862, and <i>British +Association Reports</i> (1864); Callender, <i>Phil. Trans</i>. +vol. clxxiii.; Callender and Griffiths, <i>Phil. Trans</i>. vol. +clxxxii.; <i>The Committee of the British Association on +Electrical Standards from 1862 to Present Time;</i> Dewar and +Fleming, <i>Phil. Mag</i>. vol. xxxvi. (1893);</font></p> +<p>Klemencic, <i>Wiener Sitzungsberichte</i> (Denkschrift), 1888, +vol. xcvii. p. 838; Feussner and St. Lindeck, <i>Zeitsch. fuer +Inst. 'Kunde</i>, ix. 1889, p. 233, and <i>B. A. Reports</i>, +1892, p. 139. Of these, Matthieson, and Dewar and Fleming treat +of resistance generally, the latter particularly at low +temperatures.</p> +<p><i>[Footnote:</i> The following is a list of Dr. Matthieson's +chief papers on the subject of the electrical resistance of +metals and alloys: <i>Phil. Mag</i>. xvi. 1858, pp. 219-223; +<i>Phil. Trans</i>. 1858, pp. 383-388 <i>Phil. Trans</i>. 1860, +pp. 161-176; <i>Phil. Trans</i>. 1862, pp. 1-27 <i>Phil. Mag</i>. +xxi. (1861), pp. 107-115; <i>Phil. Mag</i>. xxiii. (1862), pp. +171-179; <i>Electrician</i>, iv. 1863, pp. 285-296; <i>British +Association Reports</i>, 1863, p. 351.<i>]</i></p> +<p>Matthieson, and Matthieson and Hockin, Klemencic, Feussner, +and St. Lindeck deal with the choice of metals for resistance +standards. Callender's, and Callender and Griffiths' work is +devoted to the study of platinum for thermometric purposes.</p> +<p>The bibliography referring to special points will be given +later. The simplest way of exhibiting the relative resistances of +metals is by means of a diagram published by Dewar and Fleming +(<i>loc. cit</i>.), which is reproduced on a suitable scale on +the opposite page. For very accurate work, in which corrections +for the volumes occupied by the metals at different temperatures +are of importance, the reader is referred to the discussion in +the original paper, which will be found most pleasant reading. +From this diagram both the specific resistance and the +temperature coefficient may be deduced with sufficient accuracy +for workshop purposes. In interpreting the diagram the following +notes will be of assistance. The diagram is drawn to a scale of +so-called "platinum temperatures" — that is to say, let +R<sub>0</sub>, R<sub>100</sub>, R<sub>t</sub> be the resistances +of pure platinum at 0°, 100°, and t° C. respectively, +then the platinum temperature p<sub>t</sub> is defined as</p> +<p>p<sub>t</sub> = 100 X +(R<sub>t</sub>-R<sub>0</sub>)/(R<sub>100</sub>-R<sub>0</sub>)</p> +<p>This amounts to making the temperature scale such that the +temperature at any point is proportional to the resistance of +platinum at that point. Consequently on a resistance temperature +diagram the straight line showing the relation between platinum +resistance and platinum temperature will "run out" at the +platinum absolute zero, which coincides more or less with the +thermodynamic absolute zero, and also with the "perfect gas" +absolute zero. Platinum temperatures may be taken for workshop +purposes over ordinary ranges as almost coinciding with air +thermometer temperatures. The metals used by Professors Dewar and +Fleming were, with some exceptions, not absolutely pure, but in +general represent the best that can be got by the most refined +process of practical metallurgy. We may note further that the +specific resistance is only correct for a temperature of about +15° C., since no correction for the expansion or contraction +of material has been applied.</p> +<p><img src="images/Image133.gif" alt="images/Image133.gif" +width="410" height="910"> </p><p>The following notes on alloys suitable +for resistance coils will probably be found sufficient.</p> +<p><b><a name="Toc158108970" id="Toc158108970"><font face= +"Bookman Old Style" size="4">§ 117. <i>Platinoid</i>. +—</font></a></b></p> +<p><font face="Bookman Old Style">This substance, discovered by +Martino and described by Bottomley (<i>Phil. Proc. Roy. Soc</i>. +1885), is an alloy of nickel, zinc, copper, and 1 per cent to 2 +per cent of tungsten, but I have not been able to obtain an +analysis of its exact composition. It appears to be difficult to +get the tungsten to alloy, and it has to be added to part of the +copper as phosphide of tungsten, in considerably greater quantity +than is finally required. The nickel is added to part of the +copper and the phosphide of tungsten, then the zinc, and then the +rest of the copper. The alloy requires to be remelted several +times, and a good deal of tungsten is lost by +oxidation.</font></p> +<p>The alloy is of a fine white colour, and is very little +affected by air — in fact, it is to some extent +untarnishable. The specific resistance will be seen to be about +one and a half times greater than that of German silver, and the +temperature coefficient is about 0.021 per cent per degree C. +(i.e. about nineteen times less than copper, and half that of +German silver). To all intents and purposes it may be regarded as +German silver with 1 per cent to 2 per cent of tungsten. It does +not appear to have been particularly examined for secular changes +of resistance.</p> +<p>118. <i>German Silver</i>. — This material has been +exhaustively examined of late years by Klemencic and by Feussner +and St. Lindeck. Everybody agrees that German silver, as +ordinarily used for resistances, and composed of copper four +parts, zinc two parts, nickel one part, is very ill-fitted for +the purpose of making resistance standards. This is due</p> +<p>(1) to its experiencing a considerable increase in resistance +on winding. Feussner and St. Lindeck found an increase of 1 per +cent when German silver was wound on a core of ten wire +diameters.</p> +<p>(2) To the fact that the change goes on, though with gradually +decreasing rate, for months or years;</p> +<p>(3) to the fact that the resistance is permanently changed +(increased) by heating to 40° C. or over. By "artificially +ageing" coils of German silver by heating to 150° C., say for +five or six hours, its permanency is greatly improved, and it +becomes fit for ordinary resistance coils where changes of, say, +1/5000 do not matter.</p> +<p>It is a remarkable property of all nickel alloys containing +zinc that their specific resistance is permanently increased by +heating, whereas alloys which do not contain zinc suffer a change +in the opposite direction. The manufacturers of German silver +appear to take very little care as to the uniformity of the +product put on the market; some so-called German silver is +distinctly yellow, while other samples are bright and white.</p> +<p>It is noted by Price (<i>Measurements of Electrical +Resistance</i>, p. 24) that German silver wire is apt to exhibit +great differences of resistance within quite short lengths. This +has been my own experience as well, and is a great drawback to +the use of German silver in the laboratory, for it makes it +useless to measure off definite lengths of wire with a view to +obtaining an approximate resistance. In England German silver +coils are generally soaked in melted hard paraffin. In Germany, +at all events at the Charlottenburg Institute, according to St. +Lindeck — coils are shellac-varnished and baked. In any +case it appears to be essential to thoroughly protect the metal +against atmospheric influence.</p> +<p><b><a name="Toc158108971" id="Toc158108971"><font face= +"Bookman Old Style" size="4">§ 119. Platinum Silver. +—</font></a></b></p> +<p><font face="Bookman Old Style">In the opinion of Matthieson +and of Klemencic the 10 per cent silver, 90 per cent platinum +alloy is the one most suitable for resistance standards. At all +events, it has stood the test of time, for, with the following +exceptions, all the British Association coils constructed of it +from 1867 to the present day have continued to agree well +together. The exceptions were three one-ohm coils, which +permanently increased between 1888 and 1890, probably through +some straining when immersed in ice. One coil changed by 0.0006 +in 1 between the years 1867 and 1891. According to Klemencic, +absolute permanency is not to be expected even from this +alloy.</font></p> +<p>Its recommendation as a standard depends on its chemical +inertness, its small temperature coefficient (0.00027 per +degree), and its small thermo-voltage against copper, as the +following table (taken from Klemencic) will show:—</p> +<p align="center">Thermo-voltages in Micro-volts per degree +against Copper<br> +over the Range 0° to 17° C.</p> +<table border summary="" cellspacing="1" cellpadding="7" width= +"568"> +<tr> +<td width="50%" valign="top"> +<p><font face="Bookman Old Style">Platinum iridium</font></p> +</td> +<td width="50%" valign="top"> +<p><font face="Bookman Old Style">7.14 micro-volts per degree +C.</font></p> +</td> +</tr> +<tr> +<td width="50%" valign="top"> +<p><font face="Bookman Old Style">Platinum silver</font></p> +</td> +<td width="50%" valign="top"> +<p><font face="Bookman Old Style">6.62 micro-volts per degree +C.</font></p> +</td> +</tr> +<tr> +<td width="50%" valign="top"> +<p><font face="Bookman Old Style">Nickelin .</font></p> +</td> +<td width="50%" valign="top"> +<p><font face="Bookman Old Style">28.5 micro-volts per degree +C.</font></p> +</td> +</tr> +<tr> +<td width="50%" valign="top"> +<p><font face="Bookman Old Style">German silver</font></p> +</td> +<td width="50%" valign="top"> +<p><font face="Bookman Old Style">10.43 micro-volts per degree +C.</font></p> +</td> +</tr> +<tr> +<td width="50%" valign="top"> +<p><font face="Bookman Old Style">Manganin (St. +Lindeck)</font></p> +</td> +<td width="50%" valign="top"> +<p><font face="Bookman Old Style">1.5 micro-volts per degree +C.</font></p> +</td> +</tr> +</table> +<p><font face="Bookman Old Style">Mechanically, the platinum +silver is weak, and is greatly affected as to its resistance by +mechanical strains — in fact, Klemencic considers it the +worst substance he examined from this point of view — a +conclusion rather borne out by Mr. Glazebrook's experience with +the British Association standards already referred to (<i>B. A. +Reports</i>, 1891 and 1892).</font></p> +<p>Taking everything into account, it will probably be well to +construct standards either with oil insulation only, or to bake +the coils in shellac before testing, instead of soaking in +paraffin. Fig. 89 illustrates a form of an oil immersed standard +which is in use in my laboratory, and through which a +considerable current may be passed. The oil is stirred by means +of a screw propeller.</p> +<p>Fig. <img src="images/Image134.gif" alt="images/Image134.gif" +width="552" height="323"> 89.</p> +<p>Fig. 89 represents a standard resistance for making Clerk cell +comparisons by the silver voltameter method. The framework on +which the coils are wound consists of a base and top of slate. +The pillars are of flint glass tube surrounding brass bolts, and +cemented to the latter by raw shellac. Grooves are cut in the +glass sleeves to hold the wires well apart. These grooves were +cut by means of a file working with kerosene lubrication. A screw +stirrer is provided, and the whole apparatus is immersed in +kerosene in the glass box of a storage cell. The apparatus is +aged to begin with by heating to a temperature a good deal higher +than any temperature it is expected to reach in actual work. +After this the rigidity of the frame is intended to prevent any +further straining of the wire. The apparatus as figured is not +intended to be cooled to 0° C., so that it is put in as large +a box as possible to gain the advantage of having a large volume +of liquid. The top and bottom slates measure seven inches by +seven inches, and the distance between them is seven inches. The +inner coil is wound on first, and the loop which constitutes the +end of the winding is brought up to a suitable position for +adjustment. The insulation of the heavy copper connectors is by +means of ebonite.</p> +<p><b><a name="Toc158108972" id="Toc158108972"><font face= +"Bookman Old Style" size="4">§ 120. Platinum Iridium. +—</font></a></b></p> +<p><font face="Bookman Old Style">Platinum 90 per cent, iridium +10 per cent. This material was prepared in some quantity at the +cost of the French Government, and distributed for test about +1886. Klemencic got some of it as representing Austria, and found +it behaved very like the platinum silver alloy just discussed. +The temperature coefficient is, however, higher than for platinum +silver (0.00126 as against 0.00027). The mechanical properties of +the alloy are, however, much better than those of the silver +alloy; and in view of the experience with B. A. standards above +quoted, it remains an open question whether, on the whole, it +would not be the better material for standards, in spite of its +higher price. Improvements in absolute measurements of +resistance, however, may render primary standards +superfluous.</font></p> +<p><b><a name="Toc158108973" id="Toc158108973"><font face= +"Bookman Old Style" size="4">§ 121. <i>Manganin</i>. +—</font></a></b></p> +<p><font face="Bookman Old Style">Discovered by Weston — at +all events as to its application to resistance coils. A glance at +the diagram will exhibit its unique properties, on account of +which it has been adopted by the Physikalisch Technischen +Reichsanstalt for resistance standards. The composition of the +alloy is copper 84 per cent, manganese 12 per cent, nickel 4 per +cent., and it is described as of a steel-gray colour. +Unfortunately it is apt to oxidise in the air, or rather the +manganese it contains does so, so that it wants a very perfect +protection against the atmosphere.</font></p> +<p>Like German silver, manganin changes in resistance on winding, +and coils made of it require to be artificially aged by heating +to 150° for five hours before final adjustment. The annealing +cannot be carried out in air, owing to the tendency to oxidation. +The method adopted by St. Lindeck (at all events up to 1892) is +to treat the coil with thick alcoholic shellac varnish till the +insulation is thoroughly saturated, and then to bake the coil as +described. The baking not only anneals the wire, but reduces the +shellac to a hard and highly insulating mass.</p> +<p>Whether stresses of sufficient magnitude to produce serious +mechanical effects can be set up by unequal expansion of wire and +shellac during heating and cooling is not yet known, but so far +as tested (and it must be presumed that the Reichsanstalt tests +are thorough) no difficulty seems to have been met with. In +course of time, however, probably the best shellac coating will +crack, and then adieu to the permanency of the coil! This might, +of course, be obviated by keeping the coil in kerosene, which has +no action on shellac, but which decomposes somewhat itself.</p> +<p>The method of treatment above described suffices to render +coils of manganin constant for at least a year (in 1892 the tests +had only been made for this time) within a few thousands per +cent. Manganin can be obtained in sheets, and from this material +standards of 10<sup>-2</sup>, 10<sup>-3</sup>, and +10<sup>-4</sup> ohms are made by soldering strips between stout +copper bars, and these are adjusted by gradually increasing their +resistance by boring small holes through them. The solder +employed is said to be "silver."</p> +<p>Mr. Griffiths (<i>Phil. Trans</i>. vol. clxxxiv. [1893], A, p. +390) has had some experience with manganin carrying comparatively +heavy currents, under which circumstances its resistance when +immersed in water was found to rise in spite of the varnish which +coated it. Other experiments in which the manganin wire was +immersed in paraffin oil did not exhibit this effect, though +stronger currents were passed.</p> +<p>On the whole, manganin appears to be the best material for +coil boxes and "secondary" resistance standards. Whether it is +fit to rank with the platinum alloys as regards permanency must +be treated as an open question.</p> +<p><b><a name="Toc158108974" id="Toc158108974"><font face= +"Bookman Old Style" size="4">§ 122. <i>Other Alloys</i>. +—</font></a></b></p> +<p><font face="Bookman Old Style">The following tables, taken +from the work of Feussner and St. Lindeck, <i>Zeitschrift fuer +Instrumenten Kunde</i>, 1889, vol. ix. p. 233, together with the +following notes, will suffice.</font></p> +<p><b><a name="Toc158108975" id="Toc158108975"><font face= +"Bookman Old Style" size="4">§ 123. <i>Nickelin</i>. +—</font></a></b></p> +<p>This is only German silver with a little less zinc, a little +more nickel, and traces of cobalt and manganese. It behaves like +German silver, but is an improvement on the latter in that all +the faults of German silver appear upon a reduced scale in +nickelin.</p> +<p><b><a name="Toc158108976" id="Toc158108976"><font face= +"Bookman Old Style" size="4">§ 124. Patent Nickel. +—</font></a></b></p> +<p>Practically a copper nickel alloy, used to some extent by +Siemens and Halske. It stands pretty well in the same relation to +nickelin as the latter does to German silver. After annealing as +for manganin it can be made into serviceable standards which do +not change more than a few thousandths per cent. I have not come +across a statement of its thermo-voltage against copper.</p> +<p><a name="Toc158108977" id="Toc158108977">§ 125. +<i>Constantin</i>. —</a></p> +<p><font face="Bookman Old Style">Another nickel copper alloy +containing 50 per cent of each constituent. It appears to be a +serviceable substance, having a temperature coefficient of 0.003 +per cent per degree only, but an exceedingly high thermo-voltage, +viz. 40 micro-volts per degree against copper.</font></p> +<table border summary="" cellspacing="1" cellpadding="7" width= +"612"> +<tr> +<td width="11%" valign="top"></td> +<td width="11%" valign="top"> +<p><font face="Bookman Old Style" size="2">1</font></p> +<p>German Silver.</p> +</td> +<td width="22%" valign="top" colspan="2"> +<p><font face="Bookman Old Style" size="2">2</font> <font face= +"Bookman Old Style"> __________ 3</font></p> +<p><font face="Bookman Old Style" size="2">Nickelin made by +Obermaier</font></p> +</td> +<td width="11%" valign="top"> +<p><font face="Bookman Old Style" size="2">4</font></p> +<p>Rheo-tane.</p> +</td> +<td width="22%" valign="top" colspan="2"> +<p><font face="Bookman Old Style" size="2">5</font> <font face= +"Bookman Old Style"> ___________6</font></p> +<p><font face="Bookman Old Style" size="2">Patent +Nickel</font></p> +</td> +<td width="11%" valign="top"> +<p><font face="Bookman Old Style" size="2">7. Manga-nese +Copper.</font></p> +</td> +<td width="11%" valign="top"> +<p><font face="Bookman Old Style" size="2">8. Nickel Manga-nese +Copper.</font></p> +</td> +</tr> +<tr> +<td width="11%" valign="top"></td> +<td width="11%" valign="top"></td> +<td width="11%" valign="top"> +<p><font face="Bookman Old Style" size="2">Dia-meter 1.0 +mm</font></p> +</td> +<td width="11%" valign="top"> +<p><font face="Bookman Old Style" size="2">Dia-meter 0.1 +mm</font></p> +</td> +<td width="11%" valign="top"></td> +<td width="11%" valign="top"> +<p><font face="Bookman Old Style" size="2">Dia-meter 0.6 +mm</font></p> +</td> +<td width="11%" valign="top"> +<p><font face="Bookman Old Style" size="2">Dia-meter 1.0 +mm</font></p> +</td> +<td width="11%" valign="top"></td> +<td width="11%" valign="top"></td> +</tr> +<tr> +<td width="11%" valign="top"> +<p align="justify"><font face="Bookman Old Style" size= +"2">Copper</font></p> +</td> +<td width="11%" valign="top"> +<p><font face="Bookman Old Style" size="2">60.16</font></p> +</td> +<td width="11%" valign="top"> +<p><font face="Bookman Old Style" size="2">61.63</font></p> +</td> +<td width="11%" valign="top"> +<p><font face="Bookman Old Style" size="2">54.57</font></p> +</td> +<td width="11%" valign="top"> +<p><font face="Bookman Old Style" size="2">53.28</font></p> +</td> +<td width="11%" valign="top"> +<p><font face="Bookman Old Style" size="2">74.41</font></p> +</td> +<td width="11%" valign="top"> +<p><font face="Bookman Old Style" size="2">74.71</font></p> +</td> +<td width="11%" valign="top"> +<p><font face="Bookman Old Style" size="2">70</font></p> +</td> +<td width="11%" valign="top"> +<p><font face="Bookman Old Style" size="2">73</font></p> +</td> +</tr> +<tr> +<td width="11%" valign="top"> +<p align="justify"><font face="Bookman Old Style" size= +"2">Zinc</font></p> +</td> +<td width="11%" valign="top"> +<p><font face="Bookman Old Style" size="2">25.37</font></p> +</td> +<td width="11%" valign="top"> +<p><font face="Bookman Old Style" size="2">19.67</font></p> +</td> +<td width="11%" valign="top"> +<p><font face="Bookman Old Style" size="2">20.44</font></p> +</td> +<td width="11%" valign="top"> +<p><font face="Bookman Old Style" size="2">16.89</font></p> +</td> +<td width="11%" valign="top"> +<p><font face="Bookman Old Style" size="2">0.23</font></p> +</td> +<td width="11%" valign="top"> +<p><font face="Bookman Old Style" size="2">0.52</font></p> +</td> +<td width="11%" valign="top"> +<p><font face="Bookman Old Style" size="2">...</font></p> +</td> +<td width="11%" valign="top"> +<p><font face="Bookman Old Style" size="2">...</font></p> +</td> +</tr> +<tr> +<td width="11%" valign="top"> +<p align="justify"><font face="Bookman Old Style" size= +"2">Tin</font></p> +</td> +<td width="11%" valign="top"></td> +<td width="11%" valign="top"> +<p><font face="Bookman Old Style" size="2">...</font></p> +</td> +<td width="11%" valign="top"> +<p><font face="Bookman Old Style" size="2">...</font></p> +</td> +<td width="11%" valign="top"> +<p><font face="Bookman Old Style" size="2">...</font></p> +</td> +<td width="11%" valign="top"> +<p><font face="Bookman Old Style" size="2">...</font></p> +</td> +<td width="11%" valign="top"> +<p><font face="Bookman Old Style" size="2">trace</font></p> +</td> +<td width="11%" valign="top"> +<p><font face="Bookman Old Style" size="2">...</font></p> +</td> +<td width="11%" valign="top"> +<p><font face="Bookman Old Style" size="2">...</font></p> +</td> +</tr> +<tr> +<td width="11%" valign="top"> +<p align="justify"><font face="Bookman Old Style" size= +"2">Nickel</font></p> +</td> +<td width="11%" valign="top"> +<p><font face="Bookman Old Style" size="2">14.03</font></p> +</td> +<td width="11%" valign="top"> +<p><font face="Bookman Old Style" size="2">18.46</font></p> +</td> +<td width="11%" valign="top"> +<p><font face="Bookman Old Style" size="2">24.48</font></p> +</td> +<td width="11%" valign="top"> +<p><font face="Bookman Old Style" size="2">25.31</font></p> +</td> +<td width="11%" valign="top"> +<p><font face="Bookman Old Style" size="2">25.10</font></p> +</td> +<td width="11%" valign="top"> +<p><font face="Bookman Old Style" size="2">24.14</font></p> +</td> +<td width="11%" valign="top"> +<p><font face="Bookman Old Style" size="2">...</font></p> +</td> +<td width="11%" valign="top"> +<p><font face="Bookman Old Style" size="2">3</font></p> +</td> +</tr> +<tr> +<td width="11%" valign="top"> +<p align="justify"><font face="Bookman Old Style" size= +"2">Iron</font></p> +</td> +<td width="11%" valign="top"> +<p><font face="Bookman Old Style" size="2">0.30</font></p> +</td> +<td width="11%" valign="top"> +<p><font face="Bookman Old Style" size="2">0.24</font></p> +</td> +<td width="11%" valign="top"> +<p><font face="Bookman Old Style" size="2">0.64</font></p> +</td> +<td width="11%" valign="top"> +<p><font face="Bookman Old Style" size="2">4.46</font></p> +</td> +<td width="11%" valign="top"> +<p><font face="Bookman Old Style" size="2">0.42</font></p> +</td> +<td width="11%" valign="top"> +<p><font face="Bookman Old Style" size="2">0.70</font></p> +</td> +<td width="11%" valign="top"> +<p><font face="Bookman Old Style" size="2">...</font></p> +</td> +<td width="11%" valign="top"> +<p><font face="Bookman Old Style" size="2">...</font></p> +</td> +</tr> +<tr> +<td width="11%" valign="top"> +<p align="justify"><font face="Bookman Old Style" size= +"2">Cobalt</font></p> +</td> +<td width="11%" valign="top"> +<p><font face="Bookman Old Style" size="2">trace</font></p> +</td> +<td width="11%" valign="top"> +<p><font face="Bookman Old Style" size="2">0.19</font></p> +</td> +<td width="11%" valign="top"> +<p><font face="Bookman Old Style" size="2">...</font></p> +</td> +<td width="11%" valign="top"> +<p><font face="Bookman Old Style" size="2">...</font></p> +</td> +<td width="11%" valign="top"> +<p><font face="Bookman Old Style" size="2">trace</font></p> +</td> +<td width="11%" valign="top"> +<p><font face="Bookman Old Style" size="2">trace</font></p> +</td> +<td width="11%" valign="top"> +<p><font face="Bookman Old Style" size="2">...</font></p> +</td> +<td width="11%" valign="top"> +<p><font face="Bookman Old Style" size="2">...</font></p> +</td> +</tr> +<tr> +<td width="11%" valign="top"> +<p align="justify"><font face="Bookman Old Style" size= +"2">Mang-anese</font></p> +</td> +<td width="11%" valign="top"> +<p><font face="Bookman Old Style" size="2">trace</font></p> +</td> +<td width="11%" valign="top"> +<p><font face="Bookman Old Style" size="2">0.18</font></p> +</td> +<td width="11%" valign="top"> +<p><font face="Bookman Old Style" size="2">0.27</font></p> +</td> +<td width="11%" valign="top"> +<p><font face="Bookman Old Style" size="2">0.37</font></p> +</td> +<td width="11%" valign="top"> +<p><font face="Bookman Old Style" size="2">0.13</font></p> +</td> +<td width="11%" valign="top"> +<p><font face="Bookman Old Style" size="2">0.17</font></p> +</td> +<td width="11%" valign="top"> +<p><font face="Bookman Old Style" size="2">30</font></p> +</td> +<td width="11%" valign="top"> +<p><font face="Bookman Old Style" size="2">24</font></p> +</td> +</tr> +<tr> +<td width="11%" valign="top"></td> +<td width="11%" valign="top"> +<p><font face="Bookman Old Style" size="2">99.86</font></p> +</td> +<td width="11%" valign="top"> +<p><font face="Bookman Old Style" size="2">100.37</font></p> +</td> +<td width="11%" valign="top"> +<p><font face="Bookman Old Style" size="2">100.40</font></p> +</td> +<td width="11%" valign="top"> +<p><font face="Bookman Old Style" size="2">100.31</font></p> +</td> +<td width="11%" valign="top"> +<p><font face="Bookman Old Style" size="2">100.24</font></p> +</td> +<td width="11%" valign="top"> +<p><font face="Bookman Old Style" size="2">100.24</font></p> +</td> +<td width="11%" valign="top"> +<p><font face="Bookman Old Style" size="2">...</font></p> +</td> +<td width="11%" valign="top"> +<p><font face="Bookman Old Style" size="2">...</font></p> +</td> +</tr> +<tr> +<td width="11%" valign="top"> +<p align="justify"><font face="Bookman Old Style" size= +"2">Specific resist-ance</font></p> +</td> +<td width="11%" valign="top"> +<p><font face="Bookman Old Style" size="2">30.0</font></p> +</td> +<td width="11%" valign="top"> +<p><font face="Bookman Old Style" size="2">33.2</font></p> +</td> +<td width="11%" valign="top"> +<p><font face="Bookman Old Style" size="2">44.8</font></p> +</td> +<td width="11%" valign="top"> +<p><font face="Bookman Old Style" size="2">52.5</font></p> +</td> +<td width="11%" valign="top"> +<p><font face="Bookman Old Style" size="2">34.2</font></p> +</td> +<td width="11%" valign="top"> +<p><font face="Bookman Old Style" size="2">32.8</font></p> +</td> +<td width="11%" valign="top"> +<p><font face="Bookman Old Style" size="2">100.6</font></p> +</td> +<td width="11%" valign="top"> +<p><font face="Bookman Old Style" size="2">47.7</font></p> +</td> +</tr> +<tr> +<td width="11%" valign="top"> +<p align="justify"><font face="Bookman Old Style" size= +"2">Temp-erature co-efficient</font></p> +</td> +<td width="11%" valign="top"> +<p><font face="Bookman Old Style" size="1">0.00036</font></p> +</td> +<td width="11%" valign="top"> +<p><font face="Bookman Old Style" size="1">0.00030</font></p> +</td> +<td width="11%" valign="top"> +<p><font face="Bookman Old Style" size="1">0.00033</font></p> +</td> +<td width="11%" valign="top"> +<p><font face="Bookman Old Style" size="1">0.00041</font></p> +</td> +<td width="11%" valign="top"> +<p><font face="Bookman Old Style" size="1">0.00019</font></p> +</td> +<td width="11%" valign="top"> +<p><font face="Bookman Old Style" size="1">0.00021</font></p> +</td> +<td width="11%" valign="top"> +<p><font face="Bookman Old Style" size="1">0.00004</font></p> +</td> +<td width="11%" valign="top"> +<p><font face="Bookman Old Style" size="1">0.00003</font></p> +</td> +</tr> +</table> +<p>The specific resistance is in "microhms, i.e. 10<sup>-6</sup> +ohms per cubic centimetre, and the temperature coefficient in +degrees centigrade.</p> +<p><a name="Toc158108978" id="Toc158108978">126. Nickel Manganese +Copper. —</a></p> +<p><font face="Bookman Old Style">I can find no other reference +with regard to this alloy mentioned by Lindeck. Nicholls, however +(Silliman's Journal [3], 39, 171, 1890), gives some particulars +of alloys of copper and ferromanganese. The following table is +taken from Wiedemann's <i>Beiblatter</i> (abstract of Nicholl's +paper, 1890, p. 811). All these alloys appear to require +annealing at a red heat before their resistances are anything +like constant.</font></p> +<p>Let x be percentage of copper, then 100 — x is +percentage of "ferromanganese."</p> +<table border summary="" cellspacing="1" cellpadding="7" width= +"567"> +<tr> +<td width="20%" valign="top"> +<p><font face="Bookman Old Style" size="2">Values of +x.</font></p> +</td> +<td width="13%" valign="top"> +<p align="right"><font face="Bookman Old Style" size= +"2">100</font></p> +</td> +<td width="13%" valign="top"> +<p align="right"><font face="Bookman Old Style" size= +"2">99.26</font></p> +</td> +<td width="13%" valign="top"> +<p align="right"><font face="Bookman Old Style" size="2">91 +.88</font></p> +</td> +<td width="13%" valign="top"> +<p align="right"><font face="Bookman Old Style" size= +"2">86.98</font></p> +</td> +<td width="13%" valign="top"> +<p align="right"><font face="Bookman Old Style" size= +"2">80.4</font></p> +</td> +<td width="13%" valign="top"> +<p align="right"><font face="Bookman Old Style" size= +"2">70.65</font></p> +</td> +</tr> +<tr> +<td width="20%" valign="top"> +<p><font face="Bookman Old Style" size="2">Specific resistance +with respect to copper (? pure)</font></p> +</td> +<td width="13%" valign="top"> +<p align="right"><font face="Bookman Old Style" size= +"2">1</font></p> +</td> +<td width="13%" valign="top"> +<p align="right"><font face="Bookman Old Style" size= +"2">1.19</font></p> +</td> +<td width="13%" valign="top"> +<p align="right"><font face="Bookman Old Style" size= +"2">11.28</font></p> +</td> +<td width="13%" valign="top"> +<p align="right"><font face="Bookman Old Style" size= +"2">20.4</font></p> +</td> +<td width="13%" valign="top"> +<p align="right"><font face="Bookman Old Style" size= +"2">27.5</font></p> +</td> +<td width="13%" valign="top"> +<p align="right"><font face="Bookman Old Style" size= +"2">45.1</font></p> +</td> +</tr> +<tr> +<td width="20%" valign="top"> +<p><font face="Bookman Old Style" size="2">Temperature +coefficient per degree x 10<sup>6</sup> (hard)</font></p> +</td> +<td width="13%" valign="top"> +<p align="right"><font face="Bookman Old Style" size= +"2">3202</font></p> +</td> +<td width="13%" valign="top"> +<p align="right"><font face="Bookman Old Style" size= +"2">2167</font></p> +</td> +<td width="13%" valign="top"> +<p align="right"><font face="Bookman Old Style" size= +"2">138</font></p> +</td> +<td width="13%" valign="top"> +<p align="right"><font face="Bookman Old Style" size= +"2">16</font></p> +</td> +<td width="13%" valign="top"> +<p align="right"><font face="Bookman Old Style" size= +"2">22</font></p> +</td> +<td width="13%" valign="top"> +<p align="right"><font face="Bookman Old Style" size= +"2">-24</font></p> +</td> +</tr> +<tr> +<td width="20%" valign="top"> +<p><font face="Bookman Old Style" size="2">Ditto +(soft)</font></p> +</td> +<td width="13%" valign="top"></td> +<td width="13%" valign="top"></td> +<td width="13%" valign="top"> +<p align="right"><font face="Bookman Old Style" size= +"2">184</font></p> +</td> +<td width="13%" valign="top"> +<p align="right"><font face="Bookman Old Style" size= +"2">80</font></p> +</td> +<td width="13%" valign="top"> +<p align="right"><font face="Bookman Old Style" size= +"2">66</font></p> +</td> +<td width="13%" valign="top"> +<p align="right"><font face="Bookman Old Style" size= +"2">21</font></p> +</td> +</tr> +</table> +<p>If nickel is added, alloys of much the same character are +obtained, some with negative temperature coefficients — for +instance, one containing 52.51 per cent copper, 31.27 per cent +ferromanganese, and 16.22 nickel.</p> +<p>A detailed account of several alloys will be found in a paper +by Griffiths (Phil. Trans. 1894, p. 390), but as the constants +were determined to a higher order of accuracy than the +composition of the material — or, at all events, to a +higher degree of accuracy than that to which the materials can be +reproduced — there is no advantage in quoting them +here.</p> +<p><a name="Toc158108979" id="Toc158108979">CHAPTER IV</a></p> +<p><a name="Toc158108980" id="Toc158108980">ELECTROPLATING AND +ALLIED ARTS</a></p> +<p><a name="Toc158108981" id="Toc158108981"><font face= +"Bookman Old Style" size="4">§ 127. Electroplating. +—</font></a></p> +<p><font face="Bookman Old Style">This is an art which is usually +deemed worthy of a treatise to itself, but for ordinary +laboratory purposes it is a very simple matter — so simple, +indeed, that the multiplicity of receipts as given in treatises +are rather a source of embarrassment than otherwise.</font></p> +<p>The fundamental principles of the art are:-</p> +<p>(1) Dirty work cannot be electroplated.</p> +<p>(2) Electroplated surfaces may be rougher, but will not be +smoother than the original unplated surface.</p> +<p>(3) The art of electroplating being in advance of the science, +it is necessary to be careful as to carrying out instructions in +detail. This particularly applies to the conditions which +determine whether a metallic deposit shall come down in a +reguline or in a crystalline manner.</p> +<p><b><a name="Toc158108982" id="Toc158108982"><font face= +"Bookman Old Style" size="4">§ 128. The Dipping Bath. +—</font></a></b></p> +<p><font face="Bookman Old Style">An acid dipping bath is one of +the most useful adjuncts to the laboratory, not only for +cleansing metals for electroplating, but for cleaning up +apparatus made out of bits of brass tube and sheet, and +particularly for quickly cleaning binding screws, etc., where it +is necessary to ensure good electrical contact.</font></p> +<p>The cheapest and most satisfactory way in the end is to make +up two or three rather large baths to begin with. The glass boxes +of storage batteries do very nicely for the purpose, and being +generally ground pretty flat at the top, they may be covered by +sheets of patent plate glass, and thus preserved from the action +of the air.</p> +<p><i>First Bath</i>. — A 30 or 40 per cent solution of +commercial caustic soda. Objects may be cleansed from grease in +this bath by heating them as hot as is consistent with individual +circumstances, and plunging them into it.</p> +<p>It is a considerable advantage to begin by removing grease +from articles subsequently to be dipped in an acid bath, both +because it saves time and acid, and because more uniform results +are obtainable when this is done than when it is omitted. It is a +great advantage to have the caustic soda solution hot. This is +always done in factories where nickel-plating is carried on, but +it is inconvenient in the laboratory. The articles after dipping +in the alkali are swilled with water, and may even be scrubbed +with a brush, so as to remove greasy matters that have been +softened but not entirely removed.</p> +<p><i>Acid Bath</i>. — A convenient bath for laboratory +purposes is made by mixing two volumes of strong commercial +nitric acid with one of strong sulphuric acid in a cell +measuring, say, 12 X 10 X 15 inches.</p> +<p>Copper or brass articles are dipped in this bath for a few +seconds, then rinsed with water, then dipped again for a second +or two, or until they appear equally white all over, and then +withdrawn as rapidly as possible and plunged into a large +quantity of clean water. Care must be taken to transfer the +articles from the bath to the water as quickly as possible, for +if time be allowed for gas to be evolved, the surfaces become mat +instead of bright.</p> +<p>In order to save acid it is advisable to make up a third bath, +using those odds and ends of acids which gradually accumulate in +the laboratory. Sulphuric acid from the balance cases, for +instance, mixed with its own volume of commercial nitric acid, +does very well.</p> +<p>The objects to be dipped receive a preliminary cleansing by a +dip in this bath, the strong bath being reserved for the final +dip. Sheet brass and drawn tube, as it comes from the makers, +possesses a really fine surface, though this is generally +obscured by grease and oxide. Work executed in these materials, +cleaned in alkali, and dipped in really strong acid, will be +found to present a much better appearance than work which has +been filed, unless the latter be afterwards elaborately +polished.</p> +<p>On no account must paraffin be allowed to get into any of the +baths. When the final bath gets weak it must be relegated to a +subordinate position and a new bath set up. A weak acid bath +leaves an ugly mottled surface on brass work.</p> +<p>§ 129. A metallic surface which it is intended to +electroplate must, as has been mentioned, be scrupulously clean. +If the metal is not too valuable or delicate, cleaning by dipping +is easy and effectual. The following notes will be found to apply +to special cases which often occur.</p> +<p>(1) <i>Silver Surfaces intended to be gilt</i>. — These +are first washed clean with soap and hot water, and polished with +whitening. They are then dipped for a moment in a boiling +solution of potassium cyanide. A 20 per cent solution of common +commercial cyanide does well, but the exact strength is quite +immaterial. The cyanide is washed away in a large volume of soft +water, and the articles are kept under water till they are +scratch-brushed.</p> +<p>Mat surfaces are readily produced on standard silver by +dipping in hot strong sulphuric acid. The appearance of new +silver coins, which is familiar to everybody, is obtained by this +process.</p> +<p>(2) <i>Finely turned and finished Brass Work</i>. — If +it is intended to nickel-plate such work, and if it is desirable +to obtain brightly polished nickel surfaces, the work must be +perfectly polished to begin with. Full details as to polishing +may be found in workshop books or treatises on watch-making. It +will suffice here to say that the brass work is first smoothed by +the application of successive grades of emery and oil, or by very +fine "dead" smooth files covered with chalk. Polishing is carried +out by means of rotten stone and oil applied on leather.</p> +<p>In polishing turned work care must be taken to move the file, +emery, or rotten stone to and fro over the work with great +regularity, or the surface will end by looking scratchy and +irregular. The first process of cleaning is, of course, to remove +grease, and this is accomplished best by dipping in a bath of +strong hot caustic soda solution, and less perfectly by heating +the work and dipping it in the cold caustic soda bath.</p> +<p>During this process a certain amount of chemical action often +occurs leading to the brass surface exhibiting some +discoloration. The best way of remedying this is to dip the brass +into a hot bath of cyanide of potassium solution. If it is +inconvenient to employ hot baths or to heat the brass work, good +results may be obtained by rubbing the articles over with a large +rough cork plentifully lubricated with a strong solution of an +alkali.</p> +<p>If the surfaces are very soiled or dirty, a paste of alkali +and fine slaked lime may be applied on a cork rubber, and this in +my experience has always been most effective and satisfactory in +every way, except that it is difficult to get into crevices. If +the alkali stains the work, a little cyanide of potassium may be +rubbed over the surface in a similar manner.</p> +<p>Brass work treated by either of these methods is to be washed +in clean water till the alkali is entirely removed, and may then +be nickel-plated without any preliminary scratch-brushing. The +treatment in hot baths of alkali and cyanide is the method +generally employed in American factories as a preliminary to the +nickelling of small brass work for sewing machines, etc.</p> +<p>(3) <i>Copper</i> either for use as the kathode in +electrolysis calibration experiments or otherwise is most +conveniently prepared by dipping in the acid bath, rinsing +quickly in cold water, scratch-brushing under cold water, and +transferring at once to the plating bath. In the case where the +copper plates require to be weighed they are dipped into very hot +distilled water after scratch-brushing, and then dried at once by +means of a clean glass cloth.</p> +<p>(4) <i>Aluminium</i> (which, however, does not readily lend +itself to plating operations <i>[Footnote:</i> This difficulty +has now been overcome. See note, section 138.<i>]</i> ) is best +treated by alkali rubbed on with a cork, or by a hot alkaline +carbonate where rubbing is inexpedient. The clean aluminium is +scratch-brushed under water, and at once transferred to the +plating bath.</p> +<p>(5) <i>Iron for Nickel-plating</i>. — According to Dr. +Gore (Electra-metallurgy, p. 319) the best bath for cleaning iron +is made as follows: "One gallon of water and one pound of +sulphuric acid are mixed with one or two ounces of zinc (which of +course dissolves); to this is added half a pound of nitric acid." +The writer has been accustomed to clean iron by mechanical means, +to deprive it of grease by caustic alkali, and to finish it off +by, means of a hard scratch brush. This process has always worked +satisfactorily.</p> +<p>(6) <i>Articles soldered with soft solder containing lead and +tin</i> do not readily lend themselves to electrolytic processes, +the solder generally becoming black and refusing to be coated +with the electro-deposit. Moreover, if soldered articles are +boiled for any length of time in caustic alkali during the +preliminary cleansing, enough tin will dissolve to form a +solution of stannate of potash or soda — strong enough to +deposit tin on brass or copper. A method of coppering soldered +articles will be described later on.</p> +<p><b><a name="Toc158108983" id="Toc158108983"><font face= +"Bookman Old Style" size="4">§ 130. Scratch-brushing. +—</font></a></b></p> +<p><font face="Bookman Old Style">This process is generally +indispensable, and to its omission is to be traced most +laboratory failures in electroplating. Scratch-brushes may be +bought at those interesting shops where "watchmakers' supplies" +are sold. It will be well, therefore, to purchase a selection of +scratch brushes, for they are made to suit particular kinds of +work. They are all made of brass wire, and vary both in hardness +and in the fineness of the wire. The simplest kind of scratch +brush consists merely of a bundle of wires bound up tightly by +another wire, and somewhat "frizzed" out at the ends (Fig. 90). A +more useful kind is made just like a rotating brush, and has to +be mounted on a lathe (Fig. 91).</font></p> +<p><img src="images/Image135.gif" alt="images/Image135.gif" +width="225" height="213"><img src="images/Image136.gif" alt= +"images/Image136.gif" width="54" height="329">Fig. 90. Fig. +91.</p> +<p>The scratch brush is generally, if not always, applied wet; +the lubricant generally recommended is stale beer, but this may +be replaced by water containing a small quantity of glue, or any +other form of gelatine in solution — a mere trace (say .1 +per cent) is quite sufficient. Very fair results may be got by +using either pure or soapy water. The rotating brushes require to +be mounted on a lathe, and may be run at the same speed as would +be employed for turning wooden objects of the same +dimensions.</p> +<p>Since the brush has to be kept wet by allowing water or its +equivalent to drip upon it, it is usual to make a tin trough over +which the brush can revolve, and to further protect this by a tin +hood to keep the liquid from being thrown all over the room. In +many works the brush is arranged to lie partly in the liquid, and +this does very well if the hood is effective.</p> +<p>There is a superstition that electro-deposits stick better to +scratch-brushed surfaces than to surfaces which have not been so +treated, and consequently it is usual to scratch-brush surfaces +before electro-deposit. However this may be, there is no doubt +that adherence and solidity are promoted by frequent +scratch-brushing during the process of depositing metal, +especially when the latter tends to come down in a spongy +manner.</p> +<p><i>Gilt surfaces</i> — if the gilding is at all heavy +— are generally dull yellow, or even brown, when they come +from the bath, and require the scratch brush to cause the gold to +brighten, an office which it performs in a quite striking manner. +The same remark applies to silvered surfaces, which generally +leave the bath a dead white — at all events if the deposit +is thick, and if ordinary solutions are employed. In either case +the touch of the scratch brush is magical.</p> +<p><b><a name="Toc158108984" id="Toc158108984"><font face= +"Bookman Old Style" size="4">§ 131. Burnishing. +—</font></a></b></p> +<p><font face="Bookman Old Style">Burnishers of steel, agate, or +bloodstone can be bought at the shops where scratch brushes are +sold, and are used to produce the same brightening effect as can +be got by scratch-brushing. The same solutions are employed, but +rather stronger, and the burnisher is swept over the surface so +as to compress the deposited metal. Burnishing is rather an art, +but when well done gives a harder and more brilliant (because +smoother) surface than the scratch brush. On the whole, steel +burnishers are the most convenient if in constant use.</font></p> +<p>If the burnishing tools have to lie about, steel is apt to +rust, unless carefully protected by being plunged in quicklime or +thickly smeared with vaseline, and the least speck of rust is +fatal to a burnisher. In any case the steel requires to be +occasionally repolished by rouge and water on a bit of cloth or +felt. The process of burnishing is necessarily somewhat slow and +tedious, and as a rule is not worth troubling about except in +cases where great permanence is required.</p> +<p>The burnisher is moved over the work somewhat like a pencil +with considerable pressure, and care is taken to make the strokes +as uniform in direction as possible; otherwise the surface looks +non-uniform, and has to be further polished by tripoli, +whitening, etc., before it is presentable.</p> +<p><b><a name="Toc158108985" id="Toc158108985"><font face= +"Bookman Old Style" size="4">§ 132. Silver-plating. +—</font></a></b></p> +<p><font face="Bookman Old Style">The most convenient solution +for general purposes is an 8 to 10 per cent solution of the +double cyanide of silver and potassium together with 1 or 2 per +cent of "free" potassium cyanide. Great latitude is permissible +in the strength of solution and density of current. As commercial +cyanide of potassium generally contains an unknown percentage of +other salts, which, however, do not interfere with its value for +the purpose of silver-plating, the simplest procedure is as +follows.</font></p> +<p>For every 100 c.c. of plating solution about 7 grms. of dry +crystallised silver nitrate are required. The equivalent amount +of potassium cyanide (if dry and pure) is 5.2 grms., but +commercial cyanide may contain from 50 per cent upwards to 96 per +cent in the best fused cyanide made from ferrocyanide only. An +approximate idea of the cyanide content can be obtained from the +dealers when the salt is purchased, and this is all that is +required.</p> +<p>A quantity slightly in excess of the computed amount of +cyanide is dissolved in distilled water, and this is cautiously +added to the solution of the silver nitrate till precipitation is +just complete. The supernatant liquors are then drained away, and +the precipitate dissolved by adding a sufficiency of the +remaining cyanide; this process is assisted by warming and +stirring.</p> +<p>An allowance of about one-tenth of the whole cyanide employed +may be added to form "free" cyanide, and the solution made up to +the strength named. It is advisable to begin with the cyanide in +a moderately strong solution, for the sake of ease in dissolving +the precipitate.</p> +<p>This solution will deposit silver upon articles of copper or +brass immersed in it even without the battery, but the coat will +be thin. The solution is used cold, with a current density of +about 10 to 20 ampères per square foot. The articles to be +silvered are scratch-brushed, washed, and electroplated, till +they begin to look undesirably rough. They are then taken out of +the bath, rebrushed, and the process continued till a sufficiency +of silver is deposited. Four grammes weight of silver (nearly) is +deposited per ampère hour. It is best to use a fine silver +anode, so that the solution, does not get contaminated by +copper.</p> +<p>In most factories it is usual to "quicken" the objects to be +silvered before placing them in the electrolysis vats, because +the deposit is said to adhere better in consequence of this +treatment. I have never found it any improvement for laboratory +purposes, but it is easy to do. A dilute (say 2 per cent) +solution of cyanide of mercury is required containing a little +free cyanide. The objects to be "quickened" are scratch-brushed +and dipped into the cyanide of mercury solution till they are +uniformly white; it is generally agreed that the less the mercury +deposited the better, so long as a perfect coating is obtained. +The objects are rinsed after quickening, and put in the +depositing bath at once.</p> +<p>The mat surface of silver obtained by electrolysis of the +cyanide is very beautiful — one of the most beautiful +things in nature — shining with incomparable crystalline +whiteness. So delicate is it, however, for so great is the +surface it exposes, that it is generally rapidly deteriorated by +exposure to the air. It may be protected to some extent by +lacquering with pale lacquer, but it loses some of its brilliancy +and purity in the process. The deposit is generally +scratch-brushed or burnished down to a regular reflecting +surface.</p> +<p><b><a name="Toc158108986" id="Toc158108986"><font face= +"Bookman Old Style" size="4">§ 133. Cold Silvering. +—</font></a></b></p> +<p><font face="Bookman Old Style">A thin but brilliant coat of +silver may be readily applied to small articles of brass or +copper in the following way. A saturated solution of sodium +sulphite (neutral) is prepared, and into this a 10 per cent +solution of nitrate of silver is poured so long as the +precipitate formed is redissolved. A good deal of silver may be +got into solution in this way. Articles to be silvered need only +to be cleaned, brushed, and dipped in this solution till a coat +of the required thickness is obtained.</font></p> +<p>I must admit, however, that the coating thus laid on does not +appear to be so permanent as one deposited by simple immersion +from the cyanide solution, even though it is thicker. The cyanide +plating solution will itself give a good coat of silver if it is +used boiling, and if a little potassium cyanide be added.</p> +<p>For purposes of instrument construction, however, a thin coat +of silver is seldom to be recommended, on account of its +liability to tarnish and its rapid destruction when any attempt +is made to repolish it. For these reasons, nickel or gold plating +is much to be preferred.</p> +<p><b><a name="Toc158108987" id="Toc158108987"><font face= +"Bookman Old Style" size="4">§ 134. <i>Gilding</i>. +—</font></a></b></p> +<p><font face="Bookman Old Style">This art deserves to be much +more widely practised than is usual in laboratories. Regarded as +a means of preserving brass, copper, or steel, it is not +appreciably more "time robbing" than lacquering, and gives +infinitely better results. Moreover, it is not much more +expensive. Strange as it may seem, the costliness of gilding +seldom lies in the value of the gold deposited; the chief cost is +in the chemicals employed to clean the work, and in interest on +the not inconsiderable outlay on the solution and +anode.</font></p> +<p>The easiest metal to gild is silver, and it is not unusual to +give base metals a thin coating of silver or copper, or both, one +after the other, before gilding, in order to secure uniformity. +To illustrate the virtue of a thin layer of gold, I will mention +the following experiment. About three years ago I learned for the +first time that to "clean" the silver used in a small household +required at least an hour's labour <i>per diem</i>. I further +ascertained that most of this time is spent on the polishing part +of the process.</p> +<p>As this seemed a waste of labour, I decided to try the effect +of gilding. In order to give the proposal a fair trial I gilt the +following articles: half a dozen table spoons and forks, a dozen +dessert forks and spoons, and a dozen tea spoons. These were all +common electroplated ware. They were weighed before and after +gilding, and it was with difficulty that the increase of weight +was detected, even though a fine bullion balance was employed. On +calculating back to money, it appeared that the value of the gold +deposited was about threepence. Assuming that an equal weight of +silver had been accidentally dissolved by the free cyanide during +the plating — which is unlikely — the total amount of +gold deposited would be worth, say, sixpence.</p> +<p>After three years' continuous use the gilding is still +perfect, except at the points on which the spoons and forks rest, +where it is certainly rather shabby. Meanwhile the "gold" plate +only requires to be washed with hot water and soap to keep it in +perfect order, a much more cleanly and expeditious process than +that of silver cleaning.</p> +<p><b><a name="Toc158108988" id="Toc158108988"><font face= +"Bookman Old Style" size="4">§ 135. Preparing Surfaces for +Gilding. —</font></a></b></p> +<p><font face="Bookman Old Style">Ordinary brass work — +rough or smooth — may for purposes of preservation be +dipped, scratch-brushed, and gilt at once. Seven years ago the +writer gilt the inside of the head of a copper water still, and +simply scratch-brushed it; it is to-day in as good order as when +it was first done. If it is intended to gild work from the first, +with the view of making an exceptionally fine job of it, "gilding +metal," i.e. brass containing one to one and a quarter ounces of +zinc to the pound of copper may be specified. From its +costliness, however, this is only desirable for small +work.</font></p> +<p>Iron and steel are generally given a preliminary coating of +copper, but this may be dispensed with though with no advantage +— by using a particular process of gilding.</p> +<p>Base metals, zinc, pewter, lead, etc., are first coppered in a +cyanide of copper solution, as will be described under the head +of Copper-plating. If it is intended to gild soldered articles, +the preliminary coating of copper is essential.</p> +<p>The most convenient vessel for holding a gilding solution is +undoubtedly one formed of enamelled iron. Particularly useful are +the buckets and "billies" (i.e. cylindrical cans) made of this +material. These vessels may be heated without any fear of a +smash, and do not appear to be appreciably affected by gilding +solutions — at all events during several days or weeks. The +avoidance of all risk of breakage when twenty or thirty pounds' +worth of solution is in question is a matter of importance.</p> +<p>Under no circumstances is it desirable to use anything but the +purest gold and best fused cyanide (called "gold" cyanide) in the +preparation of the solutions. The appearance of a pure gold +deposit is far richer than of one containing silver, and its +resistance to the atmosphere is perfect; moreover, in +chemico-physical processes one has the satisfaction of knowing +what one is dealing with.</p> +<p><b><a name="Toc158108989" id="Toc158108989"><font face= +"Bookman Old Style" size="4">§ 136. Gilding Solutions. +—</font></a></b></p> +<p><font face="Bookman Old Style">The strength of solution +necessary for gilding brass, copper, and silver is not very +material. About one to two pounds of "gold" potassium cyanide (? +96 per cent KCN) per gallon does very well. The gold is best +introduced by electrolysing from a large to a small gold +electrode. One purchases a plate of pure gold either from the +mint or from reliable metallurgists (say Messrs. Johnson and +Matthey of London), and from this electrodes are cut.</font></p> +<p>The relative areas of the electrodes do not really much +matter. I have used an anode of four times the area of the +cathode. The solution is preferably heated to a temperature of +about 50° C., and a strong current is sent through it, say +twenty amperes to the square foot of anode. The electrodes must +be suspended below the surface of the solution by means of +platinum wires. If the gold plates are only partly immersed, they +dissolve much more rapidly where they cut the surface, possibly +on account of the effect of convection currents, though so far as +the writer is aware no proper explanation has yet been given.</p> +<p>After a time gold begins to be deposited on the cathode in a +powdery form, for which reason it is a good plan to begin by +wrapping the latter in filter paper. The process has gone on for +a sufficient time when a clean bit of platinum foil immersed in +the place of the cathode becomes properly gilt at a current +density of about ten amperes per square foot.</p> +<p>The powdery gold deposited on the cathode while preparing the +solution can be scraped off and melted for further use, or the +whole cathode may now be used as an anode. The platinum foil +testing cathode may also be "stripped" by making it an anode, and +is for this reason preferable to German silver or copper, which +would contaminate the solution while the "stripping" process was +in progress.</p> +<p>For general purposes a current density of say ten to fifteen +amperes per square foot may be used, but this may be considerably +varied, so long as the upper limit is not greatly overpassed. +During gold-plating there is a considerable advantage in keeping +the electrodes moving or the solution stirred.</p> +<p>After immersing the cleaned and scratch-brushed articles, +depositing may go on for about three minutes, after which they +are removed from the bath and examined, in order to detect any +want of uniformity in the deposit.</p> +<p>The articles should be entirely immersed; if this is not done, +irregularity is apt to appear at the surface. Platinum wires +employed as suspenders, and coated along with the articles to be +gilt, may also be cleaned without loss by making them anodes. If, +on examination, all is found to be going on well, reimmerse the +cathodes, and continue plating till they appear of a dull +yellowish brown (this will occur in about four minutes), then +remove them, rinse and scratch-brush them, and replace them in +the bath.</p> +<p>When a second coat appears to be getting rather brown than +yellowish brown, i.e. of the colour of wet wash-leather, the +removal, followed by scratch-brushing, may be repeated, and for +nearly all laboratory purposes, the articles are now fully +gilt.</p> +<p>The coating of gold deposited from a hot cyanide solution is +spongy in the extreme, and if the maximum wear-resisting effect +is to be obtained, it is advisable to burnish the gold rather +than to rely upon the scratch brush alone.</p> +<p>If the area of the cathode exceeds that of the anode the +solution is said to grow weaker, and vice versa. This may be +remedied in the former case by an obvious readjustment; the +latter introduces no difficulty so far as I know except when +plating iron or steel.</p> +<p>The student need not be troubled at the poor appearance of the +deposit before it is scratch-brushed. Heavy gold deposits are +almost always dull, not to say dirty, in appearance till the +burnisher or scratch brush is applied. On the other hand, the +deposit ought not to get anything like black in colour.</p> +<p>The following indications of defects may be noted--they are +taken from Gore. I have never been really troubled with them.</p> +<p>The deposit is blackish. This is caused by too strong a +current in too weak a bath. This may be remedied to some extent +by stirring or keeping the cathode in motion. The obvious remedy +is to add a little cyanide of gold.</p> +<p>The gold anode gets incrusted. This is a sign that the bath is +deficient in potassium cyanide. The gold anode gets black and +gives off gas. The solution is deficient in cyanide, and too +large a current is being passed.</p> +<p>If a bright surface is desired direct from the bath, some +caustic potash (say 2 per cent) may, according to Gore, be added, +or the articles may be plated only slightly by using a weak +current and taking them out directly they show signs of getting +dull. By a weak current I mean one of about five amperes per +square foot.</p> +<p>The deposit is said to be denser if the solution be heated as +directed; but the bath will gild, though not quite so freely when +cold.</p> +<p>To gild iron or steel directly, dilute the bath as above +recommended some five or six times, add about 1 per cent of +potassium cyanide, and gild with a very weak current (say two or +three amperes per square foot) in the cold. Frequent +scratch-brushing will be found requisite to secure proper +adherence.</p> +<p>It is generally recommended to gild brass or German silver in +solutions which are rather weak, but in the small practice which +occurs in the laboratory a solution prepared as suggested does +perfectly for everything except iron or steel. The +scratch-brushing should be done over a large photographic +developing dish to avoid loss of gold. It is a good plan to rinse +the articles after leaving the bath in a limited quantity of +distilled water, which is afterwards placed in a "residue" +bottle, and then to scratch-brush them by hand over the dish to +catch fine gold. When any loose dust is removed the articles may +be scratched in the lathe without appreciable further loss.</p> +<p>Silver-gilt articles tend to get discoloured by use, but this +discoloration can be removed by soap and water. After long use a +gold cyanide bath tends to alter greatly in composition, In +general, the bath tends to grow weaker, from the fact that there +is a strong temptation to gild as many articles at once as +possible.</p> +<p>It is therefore a good plan to keep a rough profit and loss +account of the gold in order to find the quantity in solution. +Fifty dwts. per gallon (or 78 grms. per 4.5 litres) is +recommended. A gallon of solution of this strength is worth about +eleven pounds sterling in gold and cyanide, and a serviceable +anode will be worth about 10 pounds. (Fine gold is worth +nominally four pounds four shillings and eleven pence ha'penny +per oz.) Gold may be easily obtained containing less impurity +than one part in ten thousand.</p> +<p><b><a name="Toc158108990" id="Toc158108990"><font face= +"Bookman Old Style" size="4">§ 137. Plating with Copper. +—</font></a></b></p> +<p><font face="Bookman Old Style">Copper may be deposited from +almost any of its salts in reguline form, the sulphate and +nitrate being most usually employed. In the laboratory a nearly +saturated solution of sulphate of copper with 1 or 2 per cent of +sulphuric acid will answer most purposes. A current density of, +at most, fifteen amperes per square foot may be used, either for +obtaining solid deposits for constructional purposes or for +calibrating current measuring instruments by electrolysis. A +copper anode is of course employed.</font></p> +<p>When coppering with a view to obtaining thick deposits it is a +good plan to place the electrodes several inches apart, and, if +possible, to keep the liquid stirred, as there is a considerable +tendency on the part of copper deposits to grow out into mossy +masses wherever the current density exceeds the limit mentioned. +As the masses grow towards the anode the defect naturally tends +to increase of itself, hence the necessity for care. The +phenomenon is particularly marked at the edges and corners of the +cathode.</p> +<p>If the deposit becomes markedly irregular, the best plan is to +stop the process and file the face of the deposit down to +approximate smoothness. In coppering it is of the utmost +importance that the cathode be clean and free from grease; it +must never be touched (by the finger, for instance) from the time +it is scratch-brushed till it is immersed in the plating bath. +Any grease or oxidation tends to prevent the copper deposit +adhering properly.</p> +<p>A copper deposit oxidises very easily when exposed to the air. +Consequently if the surface be required free from oxide, as, for +instance, when it is to be silvered or gilt, it must be quickly +washed when withdrawn from the coppering bath, scratch-brushed, +and transferred immediately to the silvering or gilding bath.</p> +<p>If the surface is to be dried, as in electrolysis +calibrations, it must be rinsed quickly with boiling water and +pressed between sheets of filter paper. Another method which has +been recommended is to rinse the copper in water slightly +acidulated with sulphuric acid (which prevents oxidation), then +in distilled water, and to dry by blotting paper and in front of +a fire, taking care not to make the plate too hot. The wash water +is sufficiently acidulated by the addition of two or three drops +of acid per litre. So far as I know, the method of washing in +acidulated water was first proposed by Mr. T. Gray.</p> +<p><b><a name="Toc158108991" id="Toc158108991"><font face= +"Bookman Old Style" size="4">§ 138. Coppering Aluminium. +—</font></a></b></p> +<p><font face="Bookman Old Style">A good adherent deposit of +copper on aluminium used to be considered a desideratum in the +days when it afforded the only means of soldering the latter. +Many receipts have been published from time to time, and I have +tried, I think, most of them. On no occasion, however, till this +year (1896), have I succeeded in obtaining a deposit which would +not strip after it was tinned and soldered, though it is not +difficult to get apparently adherent deposits so long as they are +not operated upon by the soldering iron. The best of the many +solutions which have been proposed in years gone by is very +dilute cupric nitrate with about 5 per cent of free nitric +acid.</font></p> +<p>The problem of electroplating aluminium which I have indicated +as awaiting a solution has at last found one. In the <i>Archives +des Sciences physiques et naturelles de Genève</i> for +December 1895 (vol. xxxiv. p. 563) there is a paper by M. Margot +on the subject, which discloses a perfectly successful method of +plating aluminium with copper. The paper itself deals in an +interesting way with the theory of the matter — however, +the result is as follows.</p> +<p>(1) The aluminium articles are boiled for a few minutes in a +strong solution of ordinary washing soda. The aluminium surface +is thus corroded somewhat, and rendered favourable to the deposit +of an adherent film of copper. After removal from the soda +solution the aluminium is well washed and brushed in running +water.</p> +<p>(2) The articles are dipped for thirty seconds or so in a hot +5 per cent solution of pure hydrochloric acid.</p> +<p>(3) After dipping in the hydrochloric acid, the work is +instantly plunged into clean water for about one second, so as to +remove nearly, but not quite, all of the aluminium chloride.</p> +<p>(4) The work is transferred to a cold dilute (say 5 per cent) +solution of cupric sulphate slightly acidulated with sulphuric +acid. The degree of acidulation does not appear to be very +important, but about one-tenth per cent of strong acid does +well.</p> +<p>If the preliminary processes have been properly carried out +the aluminium will become coated with copper, and the process is +accompanied by the disengagement of gas. It appears to be a rule +that if gas is not given off, the film of copper deposited is +non-adherent. The work must be left in the copper sulphate +solution till it has received a uniform coating of copper.</p> +<p>(5) When this is the case the work is removed — well +washed so as to get rid of the rest of the aluminium chloride, +and then electroplated by the battery in the ordinary copper +sulphate bath.</p> +<p>If the operation (4) does not appear to give a uniform coat, +or if gas is not evolved from every part of the aluminium +surface, I find that operations (2) and (3) may be repeated +without danger, provided that the dip in the hydrochloric acid +is shortened to two or three seconds.</p> +<p>The copper layer obtained by Margot's method is perfectly +adherent — even when used as a base for ordinary solder +— though in this case it can be stripped if sufficient +force is applied.</p> +<p>Since the solder recommended by M. Margot for aluminium +contains zinc, it does not run well when used to unite aluminium +to copper, brass, iron, etc. In this case, therefore, I have +found the most advantageous method of soldering to be by way of a +preliminary copper-plating.</p> +<p>The success of M. Margot's method depends in my experience on +obtaining just the proper amount of aluminium chloride in contact +with the aluminium when the latter is immersed in the copper +sulphate solution.</p> +<p>§ 139. The process of copper-plating from sulphate or +nitrate may, according to Mr. Swan (Journal of the Royal +Institution, 1892, p. 630), be considerably accelerated by the +addition of a trace of gelatine to the solution. As success +appears to depend upon hitting the exact percentage amount of the +gelatine, which must in any case be but a fraction of one per +cent, and as Mr. Swan refrains from stating what the amount is, I +am unable to give more precise instructions. A few experiments +made on the subject failed, doubtless through the gelatine +content not having been rightly adjusted. Mr. Swan claims to be +able to get a hard deposit of copper with a current density of +1000 amperes per square foot, but seems to recommend about +one-tenth of that amount for general use.</p> +<p>The solution employed is a mixture of nitrate of copper and +ammonium chloride — proportions not stated. Electrolytic +copper, as generally prepared, is very pure, but this is a mere +accident depending on the impurities which, as a rule, have to be +got rid of. Electrolysis seems to have no effect in purifying +from arsenic, for instance.</p> +<p>Roughly speaking, about 11 grms. of copper are deposited per +ampere hour from cupric salt solutions. When the current density +is too high the anode suffers by oxidation, and this introduces a +large and very variable resistance into the circuit.</p> +<p><b><a name="Toc158108992" id="Toc158108992"><font face= +"Bookman Old Style" size="4">§ 140. Alkaline Coppering +Solution —</font></a></b></p> +<p><font face="Bookman Old Style">Coppering Base Metals. — +It is often desirable to coat lead, zinc, pewter, iron, etc., +with a firm and uniform layer of copper preparatory to gilding or +silvering. If copper or brass articles are soldered with soft +solder it is found that the solder does not become silvered or +gilt along with the rest of the material, but remains uncoated +and of an ugly dark colour. This defect is got over by giving a +preliminary coating of copper.</font></p> +<p>This is done in an alkaline solution, generally containing +cyanogen and ammonia. The following method has succeeded +remarkably well with me. The receipt was taken originally from +Gore's <i>Electro-metallurgy</i>, p. 208. A solution is made of +50 grms. of potassium cyanide (ordinary commercial, say, 75 per +cent) and 30 grms. of sodium bisulphite in I.5 litres of water. +Thirty-five grammes of cupric acetate are dissolved in a litre of +water, and 20 cubic centimetres of the strongest liquid ammonia +are added. The precipitate formed must be more or less dissolved +to a strong blue solution. The cyanide and bisulphite solution is +then added with warming till the blue colour is destroyed. This +usually requires the exact amount of cyanide and bisulphite +mentioned, but I have not found it essential to entirely destroy +the colour.</p> +<p>The solution contains cuprocyanide of sodium and ammonium (?), +which is not very soluble, and this salt tends to be deposited in +granular crystalline masses on standing. However, at a +temperature of 50° C. the above receipt gives an excellent +coppering liquid, which will coat zinc with a fine reguline +deposit. Brass or copper partly smeared with solder will receive +a deposit of copper on the latter as well as on the former, and, +moreover, a deposit which appears to be perfectly uniform.</p> +<p>In using the bath the anode tends, as a rule, to become +incrusted, and this rapidly increases the resistance of the cell, +so that the current falls off quickly. The articles should be +scratch-brushed and plated for about two minutes with a current +density of about ten ampères per square foot.</p> +<p>As soon as the deposit begins to look red the articles are to +be removed and rebrushed, after which the process may be +continued. About five minutes' plating will give a copper deposit +quite thick enough after scratch-brushing to allow of a very even +gilding or silvering.</p> +<p>Aluminium appears to be fairly coated, but, as usual, the +copper strips after soldering. Iron receives an excellent and +adherent coat.</p> +<p>I do not think that the formation of a crust upon the anode +can be entirely prevented. According to Gore, its formation is +due to the solution being too poor in copper, but I have added a +solution of the acetate of copper and ammonium till the colour +was bright blue without in any way reducing the incrustation. If +the solutions become violently blue it is perhaps as well to add +a little more cyanide and bisulphite, but I have not found such +an addition necessary. The process is one of the easiest and most +satisfactory in electro-metallurgy.</p> +<p><b><a name="Toc158108993" id="Toc158108993"><font face= +"Bookman Old Style" size="4">§ 141. +Nickel-plating.—</font></a></b></p> +<p><font face="Bookman Old Style">An examination of several +American samples of nickel-plated goods has disclosed that the +coating of nickel is, as a rule, exceedingly thin. This is what +one would expect from laboratory repetition of the processes +employed.</font></p> +<p>Commercial practice in the matter of the composition of +nickelling solutions appears to vary a good deal. Thin coatings +of nickel may be readily given in a solution of the double +sulphate of nickel and ammonia, which does rather better if +slightly alkaline. Deposits from this solution, however, become +gray if of any thickness, and, moreover, are-apt to flake off the +work. The following solution has given very good results with me. +It is mentioned, together with others, in the <i>Electrical +Review</i>, 7th June 1895.</p> +<p>The ingredients are:-</p> +<div style="margin-left: 4em"> +<p>Nickel sulphate 5 parts</p> +<p>Ammonia sufficient to neutralise the nickel salt.</p> +<p>Ammonium tartrate 3.75 parts</p> +<p>Tannin 0.025 parts</p> +<p>Water 100 parts</p> +</div> +<p>The nickel sulphate and ammonia are dissolved in half the +water, the ammonium tartrate in the other half with the tannin. +The solutions are mixed and filtered at about 40<a name= +"lastbookmark2" id="lastbookmark2"></a>° C. This solution +works well at ordinary temperatures, or slightly warm, with a +current density of ten ampères per square foot. In an +experiment made for the purpose I found that plating may go on +for an hour in this solution before the deposit begins to show +signs of flaking off. The deposit is of a fine white colour.</p> +<p>The resistance of the bath is rather high and rather variable, +consequently it is as well to have a current indicator in +circuit, and it may well happen that five or six volts will be +found requisite to get the current up to the value stated. For +nickelling small objects of brass, such as binding screws, etc., +it is very necessary to be careful as to the state of polish and +uniformity of their surfaces before placing them in the plating +bath. A polished surface will appear when coated as a polished +surface, and a mat surface as a mat surface; moreover, any local +irregularity, such as a speck of a foreign metal, will give rise +to an ugly spot in the nickelling bath. For this reason it is +often advisable to commence with a coat of copper laid on in an +alkaline solution and scratch-brushed to absolute uniformity.</p> +<p>An examination of the work will, however, disclose whether +such a course is desirable or not; it is not done in American +practice, at all events for small brass objects. These are +cleaned in alkali and in boiling cyanide, which does not render a +polished surface mat, as weak acid is apt to do, and are then +coated with a current density of about ten ampères per +square foot.</p> +<p>In spite of what is to be found in books as to the ease with +which nickel deposits may be polished, I find that the mat +surface obtained by plating on an imperfectly polished cathode of +iron is by no means easily polished either by fine emery, +tripoli, or rouge. Consequently, as in the case of brass, if a +polished surface is desired, it must be first prepared on the +unplated cathode. In this case, even if the deposit appears dull, +but not gray, it may be easily polished by tripoli and water, +using a cork as the polisher. Scratch-brushing with brass wire, +however, though possibly not with German silver wire, brightens +the deposit, but discolours it. When the deposit becomes gray I +have not succeeded in polishing it satisfactorily.</p> +<p>Soldered brass or iron may be satisfactorily coated with +nickel by giving it a preliminary coating of copper in the +cyanide bath. On the whole, I recommend in general that iron be +first coated with copper in the alkaline bath, scratch-brushed, +and then nickel-plated, and this whether the iron appears to be +uniform or not. Much smoother, thicker, and stronger coats of +nickel are obtained upon the copper-plated surface than on the +iron one, and the coating does not become discoloured (? by iron +rust) in the same way that a coating on bare iron does. The +copper surface may be plated for at least an hour at a density of +ten ampères per square foot without scaling.</p> +<p>Scales or circles divided on brass may be greatly improved in +durability by nickel-plating. For this purpose the brass must +be highly polished and divided before it is nickelled.</p> +<p>The plating should be continued for a few minutes only, when a +very bright but thin coat of nickel will be deposited; it then +only remains to wash and dry the work, and this must be done at +once. If the nickel is deposited before the scale or circle is +engraved, very fine and legible divisions are obtained, but there +is a risk that flakes of nickel may become detached here and +there in the process of engraving.</p> +<p><b><a name="Toc158108994" id="Toc158108994"><font face= +"Bookman Old Style" size="4">142. Miscellaneous Notes on +Electroplating.</font></a></b></p> +<p><font face="Bookman Old Style">Occasionally it is desirable to +make a metallic mould or other object of complex shape. The +quickest way to do this is to carve the object out of hard +paraffin, and then copy it by electrotyping. Electrotype moulds +can be made in many ways. The easiest way perhaps is to take a +casting in plaster of Paris, or by means of pressure in warm +gutta-percha.</font></p> +<p>In cases where the mould will not draw, recourse must be had +to the devices of iron-founders, i.e. the plaster cast must be +made in suitable pieces, and these afterwards fitted together. +This process can occasionally be replaced by another in which the +moulding material is a mixture of treacle and glue. The glue is +soaked in cold water till it is completely soft. The superfluous +water thrown away, one-fourth part by volume of thick treacle is +added, and the mixture is melted on the water bath; during which +process stirring has to be resorted to, to produce a uniform +mixture.</p> +<p>This liquid forms the moulding mixture, and it is allowed to +flow round the object to be copied, contained in a suitable box, +whose sides have been slightly oiled. The object to be copied +should also be oiled. After some hours, when the glue mixture has +set, it will be found to be highly elastic, so that it may be +pulled away from the mould, and afterwards resume very nearly its +original form.</p> +<p>One drawback to the use of these moulds lies in the fact that +the gelatine will rarely stand the plating solution without +undergoing change, but this may be partially obviated by dipping +it for a few seconds in a 10 per cent solution of bichromate of +potash, exposing it to the sunlight for a few minutes, and then +rinsing it.</p> +<p>In order to render the surface conducting, it is washed over +with a solution of a gold or silver salt, and the latter reduced +in situ to metal by a suitable reagent. A solution of phosphorus +is the most usual one (see Gore, Electro-metallurgy, p. 216). +Such a mould may be copper-plated in the sulphate bath, +connection being made by wires suitably thrust into the +material.</p> +<p>Plaster of Paris moulds require to be dried and waxed by +standing on a hot plate in melted wax before they are immersed in +the plating bath. In this case the surface is best made +conducting either by silvering it by the silvering process used +for mirrors, or by brushing it over with good black lead rendered +more conducting by moistening with an ethereal solution of +chloride of gold and then drying in the sun.</p> +<p>The brushing requires a stiff camel's-hair pencil of large +size cut so that the hairs project to a distance of about a +quarter of an inch from the holder. The brushing must continue +till the surface is bright, and is often a lengthy process.</p> +<p>The same process of blackleading may be employed to get a coat +of deposited metal which will strip easily from the cathode.</p> +<p>In all cases where extensive deposits of copper are required, +the growth takes place too rapidly at the corners. Consequently +it is often desirable to localise the action of the deposit. A +"stopping" of ordinary copal varnish seems to be the usual thing, +but a thin coat of wax or paraffin or photographic (black) +varnish does practically as well.</p> +<p>I do not propose to deal with the subject of electrotyping to +any extent, for if practised as an art, a good many little +precautions are required, as the student may read in Gore's +Electro-metallurgy. The above instructions will be found +sufficient for the occasional use of the process in the +construction of apparatus, etc. There is no advantage in +attempting to hurry the process, a current density of about ten +ampères per square foot being quite suitable and +sufficiently low to ensure a solid deposit.</p> +<p><b><a name="Toc158108995" id="Toc158108995"><font face= +"Bookman Old Style" size="4">§ 143. Blacking Brass Surfaces. +—</font></a></b></p> +<p><font face="Bookman Old Style">A really uniform dead-black +surface is difficult to produce on brass by chemical means. A +paste of nitrate of copper and nitrate of silver heated on the +brass is said to give a dead-black surface, but I have not +succeeded in making it act uniformly. For optical purposes the +best plan is to use a paint made up of "drop" black, ground very +fine with a little shellac varnish, and diluted for use with +alcohol. No more varnish than is necessary to cause the black to +hold together should be employed.</font></p> +<p>In general, if the paint be ground to the consistency of very +thick cream with ordinary shellac varnish it will be found to +work well when reduced by alcohol to a free painting +consistency.</p> +<p>A very fine gray and black finish, with a rather metallic +lustre, may be easily given to brass work. For this purpose a +dilute solution of platinum tetrachloride (not stronger than 1 +per cent) is prepared by dissolving the salt in distilled water. +The polished brass work is cleaned by rubbing with a cork and +strong potash till all grease has disappeared, as shown by water +standing uniformly on the metal and draining away without +gathering into drops.</p> +<p>After copious washing the work is wholly immersed in a +considerable volume of the platinum tetrachloride solution at the +ordinary temperature. After about a quarter of an hour the brass +may be taken out and washed. The surface will be found to be +nicely and uniformly coated if the above instructions have been +carried out, but any finger-marks or otherwise dirty places will +cause irregularity of deposit. If the process has been successful +it will be found that the deposit adheres perfectly, hardly any +of it being removed by vigorous rubbing with a cloth. If the +deposit is allowed to thicken — either by leaving the +articles in the solution too long or heating the solution, or +having it too strong — it will merely rub off and leave an +irregular surface.</p> +<p>This process succeeds well with yellow brass and Muntz metal, +either cast or rolled, but it does not give quite such uniform +(though still good) results with gun-metal, on which, however, +the deposit is darker and deader in appearance.</p> +<p>A book might be written (several have been written) on the art +of metal colouring, but though doubtless a beautiful and delicate +art, it is of little service in the laboratory. For further +information the reader may consult a work by Hiorns.</p> +<p><b><a name="Toc158108996" id="Toc158108996"><font face= +"Bookman Old Style" size="4">§ 144. <i>Sieves</i>. +—</font></a></b></p> +<p><font face="Bookman Old Style">Properly graded sieves with +meshes of a reliable size are often of great use. They should be +made out of proper "bolting" cloth, a beautiful material made for +flour-millers. Messrs. Henry Simon and Company of Manchester have +kindly furnished me with the following table of materials used in +flour-milling.</font></p> +<p>Sieves made of these materials will be found to work much more +quickly and satisfactorily than those made from ordinary muslin +or wire gauze.</p> +<table border summary="" cellspacing="1" cellpadding="7" width= +"568"> +<tr> +<td valign="top" colspan="4"> +<p align="center"><b><font face="Bookman Old Style">Relative +Bolting Value of Silk, Wire, and Grit Gauze</font></b></p> +</td> +</tr> +<tr> +<td width="25%" valign="top"> +<p align="center"><font face="Bookman Old Style" size="2">Threads +per inch Approximate.</font></p> +</td> +<td width="25%" valign="top"> +<p align="center"><font face="Bookman Old Style" size="2">Trade +No. of Silk.</font></p> +</td> +<td width="25%" valign="top"> +<p align="center"><font face="Bookman Old Style" size="2">Trade +No. of Wire.</font></p> +</td> +<td width="25%" valign="top"> +<p align="center"><font face="Bookman Old Style" size="2">Trade +No. of Grit Gauze.</font></p> +</td> +</tr> +<tr> +<td width="25%" valign="top"> +<p align="right"><font face="Bookman Old Style">18</font></p> +</td> +<td width="25%" valign="top"> +<p align="right"><font face="Bookman Old Style">0000</font></p> +</td> +<td width="25%" valign="top"> +<p align="right"><font face="Bookman Old Style">18</font></p> +</td> +<td width="25%" valign="top"> +<p align="right"><font face="Bookman Old Style">16</font></p> +</td> +</tr> +<tr> +<td width="25%" valign="top"> +<p align="right"><font face="Bookman Old Style">22</font></p> +</td> +<td width="25%" valign="top"> +<p align="right"><font face="Bookman Old Style">000</font></p> +</td> +<td width="25%" valign="top"> +<p align="right"><font face="Bookman Old Style">20</font></p> +</td> +<td width="25%" valign="top"> +<p align="right"><font face="Bookman Old Style">20</font></p> +</td> +</tr> +<tr> +<td width="25%" valign="top"> +<p align="right"><font face="Bookman Old Style">28</font></p> +</td> +<td width="25%" valign="top"> +<p align="right"><font face="Bookman Old Style">00</font></p> +</td> +<td width="25%" valign="top"> +<p align="right"><font face="Bookman Old Style">26</font></p> +</td> +<td width="25%" valign="top"> +<p align="right"><font face="Bookman Old Style">26</font></p> +</td> +</tr> +<tr> +<td width="25%" valign="top"> +<p align="right"><font face="Bookman Old Style">38</font></p> +</td> +<td width="25%" valign="top"> +<p align="right"><font face="Bookman Old Style">0</font></p> +</td> +<td width="25%" valign="top"> +<p align="right"><font face="Bookman Old Style">32</font></p> +</td> +<td width="25%" valign="top"> +<p align="right"><font face="Bookman Old Style">34</font></p> +</td> +</tr> +<tr> +<td width="25%" valign="top"> +<p align="right"><font face="Bookman Old Style">48</font></p> +</td> +<td width="25%" valign="top"> +<p align="right"><font face="Bookman Old Style">1</font></p> +</td> +<td width="25%" valign="top"> +<p align="right"><font face="Bookman Old Style">40</font></p> +</td> +<td width="25%" valign="top"> +<p align="right"><font face="Bookman Old Style">44</font></p> +</td> +</tr> +<tr> +<td width="25%" valign="top"> +<p align="right"><font face="Bookman Old Style">52</font></p> +</td> +<td width="25%" valign="top"> +<p align="right"><font face="Bookman Old Style">2</font></p> +</td> +<td width="25%" valign="top"> +<p align="right"><font face="Bookman Old Style">45</font></p> +</td> +<td width="25%" valign="top"> +<p align="right"><font face="Bookman Old Style">50</font></p> +</td> +</tr> +<tr> +<td width="25%" valign="top"> +<p align="right"><font face="Bookman Old Style">56</font></p> +</td> +<td width="25%" valign="top"> +<p align="right"><font face="Bookman Old Style">3</font></p> +</td> +<td width="25%" valign="top"> +<p align="right"><font face="Bookman Old Style">50</font></p> +</td> +<td width="25%" valign="top"> +<p align="right"><font face="Bookman Old Style">54</font></p> +</td> +</tr> +<tr> +<td width="25%" valign="top"> +<p align="right"><font face="Bookman Old Style">60</font></p> +</td> +<td width="25%" valign="top"> +<p align="right"><font face="Bookman Old Style">4</font></p> +</td> +<td width="25%" valign="top"> +<p align="right"><font face="Bookman Old Style">56</font></p> +</td> +<td width="25%" valign="top"> +<p align="right"><font face="Bookman Old Style">58</font></p> +</td> +</tr> +<tr> +<td width="25%" valign="top"> +<p align="right"><font face="Bookman Old Style">64</font></p> +</td> +<td width="25%" valign="top"> +<p align="right"><font face="Bookman Old Style">5</font></p> +</td> +<td width="25%" valign="top"> +<p align="right"><font face="Bookman Old Style">60</font></p> +</td> +<td width="25%" valign="top"> +<p align="right"><font face="Bookman Old Style">60</font></p> +</td> +</tr> +<tr> +<td width="25%" valign="top"> +<p align="right"><font face="Bookman Old Style">72</font></p> +</td> +<td width="25%" valign="top"> +<p align="right"><font face="Bookman Old Style">6</font></p> +</td> +<td width="25%" valign="top"> +<p align="right"><font face="Bookman Old Style">64</font></p> +</td> +<td width="25%" valign="top"> +<p align="right"><font face="Bookman Old Style">66</font></p> +</td> +</tr> +<tr> +<td width="25%" valign="top"> +<p align="right"><font face="Bookman Old Style">80</font></p> +</td> +<td width="25%" valign="top"> +<p align="right"><font face="Bookman Old Style">7</font></p> +</td> +<td width="25%" valign="top"> +<p align="right"><font face="Bookman Old Style">70</font></p> +</td> +<td width="25%" valign="top"> +<p align="right"><font face="Bookman Old Style">70</font></p> +</td> +</tr> +<tr> +<td width="25%" valign="top"> +<p align="right"><font face="Bookman Old Style">84</font></p> +</td> +<td width="25%" valign="top"> +<p align="right"><font face="Bookman Old Style">8</font></p> +</td> +<td width="25%" valign="top"> +<p align="right"><font face="Bookman Old Style">80</font></p> +</td> +<td width="25%" valign="top"> +<p align="right"><font face="Bookman Old Style">80</font></p> +</td> +</tr> +<tr> +<td width="25%" valign="top"> +<p align="right"><font face="Bookman Old Style">94</font></p> +</td> +<td width="25%" valign="top"> +<p align="right"><font face="Bookman Old Style">9</font></p> +</td> +<td width="25%" valign="top"></td> +<td width="25%" valign="top"></td> +</tr> +<tr> +<td width="25%" valign="top"> +<p align="right"><font face="Bookman Old Style">106</font></p> +</td> +<td width="25%" valign="top"> +<p align="right"><font face="Bookman Old Style">10</font></p> +</td> +<td width="25%" valign="top"></td> +<td width="25%" valign="top"></td> +</tr> +<tr> +<td width="25%" valign="top"> +<p align="right"><font face="Bookman Old Style">114</font></p> +</td> +<td width="25%" valign="top"> +<p align="right"><font face="Bookman Old Style">11</font></p> +</td> +<td width="25%" valign="top"></td> +<td width="25%" valign="top"></td> +</tr> +<tr> +<td width="25%" valign="top"> +<p align="right"><font face="Bookman Old Style">124</font></p> +</td> +<td width="25%" valign="top"> +<p align="right"><font face="Bookman Old Style">12</font></p> +</td> +<td width="25%" valign="top"></td> +<td width="25%" valign="top"></td> +</tr> +<tr> +<td width="25%" valign="top"> +<p align="right"><font face="Bookman Old Style">130</font></p> +</td> +<td width="25%" valign="top"> +<p align="right"><font face="Bookman Old Style">13</font></p> +</td> +<td width="25%" valign="top"></td> +<td width="25%" valign="top"></td> +</tr> +<tr> +<td width="25%" valign="top"> +<p align="right"><font face="Bookman Old Style">139</font></p> +</td> +<td width="25%" valign="top"> +<p align="right"><font face="Bookman Old Style">14</font></p> +</td> +<td width="25%" valign="top"></td> +<td width="25%" valign="top"></td> +</tr> +<tr> +<td width="25%" valign="top"> +<p align="right"><font face="Bookman Old Style">148</font></p> +</td> +<td width="25%" valign="top"> +<p align="right"><font face="Bookman Old Style">15</font></p> +</td> +<td width="25%" valign="top"></td> +<td width="25%" valign="top"></td> +</tr> +<tr> +<td width="25%" valign="top"> +<p align="right"><font face="Bookman Old Style">156</font></p> +</td> +<td width="25%" valign="top"> +<p align="right"><font face="Bookman Old Style">16</font></p> +</td> +<td width="25%" valign="top"></td> +<td width="25%" valign="top"></td> +</tr> +<tr> +<td width="25%" valign="top"> +<p align="right"><font face="Bookman Old Style">163</font></p> +</td> +<td width="25%" valign="top"> +<p align="right"><font face="Bookman Old Style">17</font></p> +</td> +<td width="25%" valign="top"></td> +<td width="25%" valign="top"></td> +</tr> +<tr> +<td width="25%" valign="top"> +<p align="right"><font face="Bookman Old Style">167</font></p> +</td> +<td width="25%" valign="top"> +<p align="right"><font face="Bookman Old Style">18</font></p> +</td> +<td width="25%" valign="top"></td> +<td width="25%" valign="top"></td> +</tr> +<tr> +<td width="25%" valign="top"> +<p align="right"><font face="Bookman Old Style">170</font></p> +</td> +<td width="25%" valign="top"> +<p align="right"><font face="Bookman Old Style">19</font></p> +</td> +<td width="25%" valign="top"></td> +<td width="25%" valign="top"></td> +</tr> +<tr> +<td width="25%" valign="top"> +<p align="right"><font face="Bookman Old Style">173</font></p> +</td> +<td width="25%" valign="top"> +<p align="right"><font face="Bookman Old Style">20</font></p> +</td> +<td width="25%" valign="top"></td> +<td width="25%" valign="top"></td> +</tr> +</table> +<p><b><a name="Toc158108997" id="Toc158108997"><font face= +"Bookman Old Style" size="4">§ 145. Pottery making in the +Laboratory. —</font></a></b></p> +<p><font face="Bookman Old Style">When large pieces of +earthenware of any special design are required, recourse must be +had to a pottery. Small vessels, plates, parts of machines, etc., +can often be made in the laboratory in less time than it would +take to explain to the potter what is required. For this purpose +any good pipeclay may be employed. I have used a white pipe-clay +dug up in the laboratory garden with complete success.</font></p> +<p>The clay should be kneaded with water and squeezed through a +cloth to separate grit. It is then mixed with its own volume or +thereabouts of powdered porcelain evaporating basins, broken +basins being kept for this purpose. The smoothness of the +resulting earthenware will depend on the fineness to which the +porcelain fragments have been reduced. I have found that +fragments passing a sieve of sixty threads to the inch run, do +very well, though the resulting earthenware is decidedly +rough.</p> +<p>The porcelain and clay being thoroughly incorporated by +kneading, the articles are moulded, it being borne in mind that +they will contract somewhat on firing. <i>[Footnote:</i> The +contraction depends on the temperature attained as well as on the +time. An allowance of one part in twelve will be suitable in the +case considered.<i>]</i> The clay should be as stiff as is +convenient to work, and after moulding must be allowed to get +thoroughly dry by standing in an airy place; the drying must not +be forced, especially at first, or the clay will crack.</p> +<p>Small articles are readily fired in a Fletcher's crucible +furnace supplied with a gas blow-pipe; the furnace is heated +gradually to begin with. When a dull red heat is attained, the +full power of the blast may be turned on, and the furnace kept at +its maximum temperature for three or four hours at least, though +on an emergency shorter periods may be made to do.</p> +<p>The articles are supported on a bed of white sand; after +firing, the crucible furnace must be allowed to cool slowly. It +must be remembered that the furnace walls will get hot externally +after the first few hours, consequently the furnace must be +supported on bricks, to protect the bench.</p> +<p>The pottery when cold may be dressed on a grindstone if +necessary. This amateur pottery will be found of service in +making small fittings for switch-boards, commutators, and in +electrical work generally.</p> +<p>Pottery made as described is very hard and strong, the +hardness and strength depending in a great degree on the +proportion of powdered porcelain added to the clay, as well, of +course, as on the quality of both of these materials.</p> +<p>It is a good plan to knead a considerable quantity of the +mixture, which may then be placed in a well-covered jar, and kept +damp by the addition of a little water.</p> +<p>Pottery thus made does not require to be glazed, but, of +course, a glaze can be obtained by any of the methods described +in works on pottery manufacture. The following glaze has been +recommended to me by a very competent potter:—</p> +<table border summary="" cellspacing="1" cellpadding="7" width= +"375"> +<tr> +<td width="38%" valign="top"> +<p><font face="Bookman Old Style">Litharge</font></p> +</td> +<td width="62%" valign="top"> +<p align="right"><font face="Bookman Old Style">7 parts by +weight</font></p> +</td> +</tr> +<tr> +<td width="38%" valign="top"> +<p><font face="Bookman Old Style">Ground flint</font></p> +</td> +<td width="62%" valign="top"> +<p align="right"><font face="Bookman Old Style">2 parts by +weight</font></p> +</td> +</tr> +<tr> +<td width="38%" valign="top"> +<p><font face="Bookman Old Style">Cornish stone or +felspar</font></p> +</td> +<td width="62%" valign="top"> +<p align="right"><font face="Bookman Old Style">1 parts by +weight</font></p> +</td> +</tr> +</table> +<p><font face="Bookman Old Style">These ingredients are to be +ground up till they will pass the finest sieve — say 180 +threads to the inch. They are then mixed with water till they +form a paste of the consistency of cream. They must, of course, +be mixed together perfectly. The ware to be glazed is dipped into +the cream after the first firing; it is then dried as before and +refired. The glaze will melt at a bright red heat, but it will +crack if not fired harder; the harder it is fired the less likely +is it to crack.</font></p> +<p>If colouring matters are added they must be ground in a mill +free from iron till they are so fine that a thick blanket filter +will not filter them when suspended in water. This remark applies +particularly to oxide of cobalt.</p> +<p><b><a name="Toc158108998" id="Toc158108998"><font face= +"Bookman Old Style" size="4">APPENDIX</font></a></b></p> +<p><a name="Toc158108999" id="Toc158108999">PLATINISING +GLASS</a></p> +<p><font face="Bookman Old Style">IN the <i>Philosophical +Magazine</i> for July 1888 (vol. xxvi. p. 1) there is a paper by +Professor Kundt translated from the <i>Sitzungsberichte</i> of +the Prussian Academy. This paper deals with the indices of +refraction of metals. Thin prisms were obtained by depositing +metals electrolytically on glass surfaces coated with platinum. +The preparation of these surfaces is troublesome. Kundt recounts +that no less than two thousand trials were made before success +was attained. A detailed account of the preparation of these +surfaces is not given by Kundt, but one is promised — a +promise unfortunately unfulfilled so far as I am able to +discover. A hunt through the literature led to the discovery of +the following references: <i>Central Zeitung fuer Optik und +Mechanik</i>, p. 142 (1888); Dingler's <i>Polytechnik +Journal</i>, Vol. cxcv. p. 464; <i>Comptes Rendus</i>, vol. lxx. +(1870).</font></p> +<p>The original communication is a paper by Jouglet in the +Comptes Rendus, of which the other references are abstracts. The +account in Dingier is a literal translation of the original +paper, and the note in the <i>Central Zeitung</i> is abbreviated +sufficiently to be of no value. The details are briefly as +follows:-</p> +<p>One hundred grams of platinum are dissolved in aqua regia and +the solution is dried on the sand bath, without, however, +producing decomposition. Though the instructions are not +definite, I presume that the formation of PtCl<sub>4</sub> is +contemplated.</p> +<p>The dried salt is added little by little to rectified oil of +lavender, placed on a glass paint-grinding plate, and the salt +and oil are ground together with a muller. Care is required to +prevent any appreciable rise of temperature which would decompose +the compound aimed at, and it is for this reason that the salt is +to be added gradually. Of course the absorption of water from the +air must be prevented from taking place as far as possible. +Finally, the compound is diluted by adding oil of lavender up to +a total weight of 1400 grams (of oil).</p> +<p>The liquid is poured into a porcelain dish and left absolutely +at rest for eight days. It is then decanted and filtered, left +six days at rest, and again decanted (if necessary). The liquid +should have a specific gravity of 5° on the acid hydrometer. +(If by this the Baumé scale is intended, the corresponding +specific gravity would be 1.037.) A second liquid is prepared by +grinding up 25 grams of litharge with 25 grams of borate of lead +and 8 to 10 grams of oil of lavender. The grinding must be +thoroughly carried out.</p> +<p>This liquid is to be added to the one first described, and the +whole well mixed. The resulting fluid constitutes the platinising +liquid, and is applied as follows:-</p> +<p>A sheet of clean glass is held vertically, and the liquid is +painted over it, carrying the brush from the lower to the upper +edge. The layer of oil dries slowly, and when it is dry the +painting is again proceeded with, moving the brush this time from +right to left; and similarly the process is repeated twice, the +brush being carried from top to bottom and left to right. This is +with the object of securing great uniformity in the coating. +Nothing is said as to the manner in which the glass is to be +dried.</p> +<p>The dried glass is finally heated to a temperature of dull +redness in a muffle furnace. The resinous layer burns away +without running or bubbling, and leaves a dull metallic surface. +As the temperature rises this suddenly brightens, and we obtain +the desired surface (which probably consists of an alloy of lead +and platinum). It is bright only on the surface away from the +glass.</p> +<p>I have not had an opportunity of trying this process since I +discovered the detailed account given by Jouglet; but many +modifications have been tried in the laboratory of the Sydney +University by Mr. Pollock, starting from the imperfect note in +the Central Zeitung, which led to no real success.</p> +<p>It was found that it is perfectly easy to obtain brilliant +films of platinum by the following process, provided that the +presence of a few pin-holes does not matter.</p> +<p>The platinum salt employed is what is bought under the name of +platinic chloride; it is, however, probably a mixture of this +salt and hydro-chloro-platinic acid, and has all the appearance +of having been obtained by evaporating a solution of platinum in +aqua regia to dryness on the water bath. A solution of this salt +in distilled water is prepared; the strength does not seem to +matter very much, but perhaps one of salt to ninety-nine water +may be regarded as a standard proportion. To this solution is +added a few drops of ordinary gum water (i.e. a solution of +dextrin). The exact quantity does not matter, but perhaps about 2 +per cent may be mentioned as giving good results.</p> +<p>The glass is painted over with this solution and dried slowly +on the water bath. When the glass is dry, and covered with a +uniform hard film of gum and platinum salt free from bubble +holes, it is heated to redness in a muffle furnace. The necessary +and sufficient temperature is reached as soon as the glass is +just sensibly red-hot.</p> +<p>The mirrors obtained in this way are very brilliant on the +free platinum surface. If the gum be omitted, the platinum will +have a mat surface; and if too much gum be used, the platinum +will get spotty by bubbles bursting. There does not appear to be +any advantage in using lead.</p> +<p>It is quite essential that the film be dry and hard before the +glass is fired.</p> + + + + + + + +<pre> + + + + + +End of the Project Gutenberg EBook of On Laboratory Arts, by Richard Threlfall + +*** END OF THIS PROJECT GUTENBERG EBOOK ON LABORATORY ARTS *** + +***** This file should be named 22784-h.htm or 22784-h.zip ***** +This and all associated files of various formats will be found in: + http://www.gutenberg.org/2/2/7/8/22784/ + +Produced by Jon Richfield + +Updated editions will replace the previous one--the old editions +will be renamed. + +Creating the works from public domain print editions 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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