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+The Project Gutenberg EBook of The Birth-Time of the World and Other
+Scientific Essays, by J. (John) Joly
+
+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: The Birth-Time of the World and Other Scientific Essays
+
+Author: J. (John) Joly
+
+Release Date: August 28, 2005 [EBook #16614]
+
+Language: English
+
+Character set encoding: ISO-8859-1
+
+*** START OF THIS PROJECT GUTENBERG EBOOK THE BIRTH-TIME OF THE WORLD ***
+
+
+
+
+Produced by Hugh Rance
+
+
+
+
+
+THE BIRTH-TIME OF THE WORLD AND OTHER SCIENTIFIC ESSAYS
+
+by
+
+J. JOLY, M.A., Sc.D., F.R.S.,
+PROFESSOR OF GEOLOGY AND MINERALOGY IN THE UNIVERSITY OF DUBLIN
+
+E. P. DUTTON AND COMPANY
+681 FIFTH AVENUE NEW YORK
+
+
+Cover
+
+Title page
+
+CONTENTS PAGE
+
+I. THE BIRTH-TIME OF THE WORLD - - - - - - - - - - - 1
+
+II. DENUDATION  - - - - - - - - - - - - - - - - - - 30
+
+III. THE ABUNDANCE OF LIFE  - - - - - - - - - - - - 60
+
+IV. THE BRIGHT COLOURS OF ALPINE FLOWERS - - - - - 102
+
+V. MOUNTAIN GENESIS  - - - - - - - - - - - - - - - 116
+
+VI. ALPINE STRUCTURE - - - - - - - - - - - - - - - 146
+
+VII. OTHER MINDS THAN OURS - - - - - - - - - - - - 162
+
+VIII. THE LATENT IMAGE - - - - - - - - - - - - - - 202
+
+IX. PLEOCHROIC HALOES  - - - - - - - - - - - - - - 214
+
+X. THE USE OF RADIUM IN MEDICINE - - - - - - - - - 244
+
+XI. SKATING  - - - - - - - - - - - - - - - - - - - 260
+
+XII. A SPECULATION AS TO A PRE-MATERIAL UNIVERSE - 288
+
+LIST OF ILLUSTRATIONS
+
+PLATE I. LAKE OF LUCERNE, LOOKING WEST FROM BRUNNEN -
+Frontispiece
+
+PLATE II. "UPLIFTED FROM THE SEAS." CLIFFS OF THE TITLIS,
+SWITZERLAND - to face p. 4
+
+PLATE III. AN ALPINE TORRENT AT WORK--VAL D'HERENS, SWITZERLAND -
+to face p. 31
+
+PLATE IV. EARTH PILLARS--VAL D'HERENS - to face p. 34
+
+PLATE V. "SCENES OF DESOLATION." THE WEISSHORN SEEN FROM BELLA
+TOLA, SWITZERLAND - to face p. 40
+
+PLATE VI. ALLUVIAL CONE--NICOLAI THAL, SWITZERLAND. MORAINE ON
+ALETSCH GLACIER SWITZERLAND - to face p. 50
+
+PLATE VII. IN THE REGION OF THE CROCI; DOLOMITES. THE ROTHWAND
+SEEN FROM MONTE PIANO - to face p. 60
+
+PLATE VIII. FIRS ASSAILING THE HEIGHTS OF THE MADERANER THAL,
+SWITZERLAND - to face p. 73
+
+PLATE IX. LIFE NEAR THE SNOW LINE; THE BOG-COTTON IN POSSESSION.
+NEAR THE TSCHINGEL PASS, SWITZERLAND - to face p. 80
+
+PLATE X. THE JOY OF LIFE. THE AMPEZZO THAL; DOLOMITES - to face
+p. 93
+
+PLATE XI. "PINES SOLEMNLY QUIET." DÜSSISTOCK; MADERANER THAL - to
+face p. 100
+
+PLATE XII. ALPINE FLOWERS IN THE VALLEYS - to face p. 105
+
+PLATE XIII. ALPINE FLOWERS ON THE HEIGHTS - to face p. 106
+
+PLATE XIV. MOUNTAIN SOLITUDES; VAL DE ZINAL. FROM LEFT TO RIGHT
+ROTHHORN; BESSO; OBERGABELHORN; MATTERHORN; PIC DE ZINAL (THROUGH
+CLOUD); DENT BLANCHE - to face p. 116
+
+ix
+
+PLATE XV. SECTOR OF THE EARTH RISE OF ISOGEOTHERMS INTO A DEPOSIT
+EVOLVING RADIOACTIVE HEAT - to face p. 118
+
+PLATE XVI. "THE MOUNTAINS COME AND GO." THE DENT BLANCHE SEEN
+FROM THE SASSENEIRE - to face p. 133
+
+PLATE XVII. DIAGRAMMATIC SECTIONS OF THE HIMALAYA - to face p.
+140
+
+PLATE XVIII. RESIDUES OF DENUDATION. THE MATTERHORN SEEN FROM THE
+SUMMIT OF THE ZINAL ROTHHORN - to face p. 148
+
+PLATE XIX. THE FOLDED ROCKS OF THE MATTERHORN, SEEN FROM NEAR
+HÖHBALM. SKETCH MADE IN 1906 - to face p. 156
+
+PLATE XX. SCHIAPARELLI'S MAP OF MARS OF 1882, AND ADDITIONS (IN
+RED) OF 1892 - to face p. 166
+
+PLATE XXI. GLOBE OF MARS SHOWING PATH OF IN-FALLING SATELLITE -
+to face p. 188
+
+PLATE XXII. CANALS MAPPED BY LOWELL COMPARED WITH CANALS FORMED
+BY IN-FALLING SATELLITES - to face p. 192
+
+PLATE XXIII. HALOES IN MICA; CO. CARLOW. HALO IN BIOTITE
+CONTAINED IN GRANITE - to face p. 224
+
+PLATE XXIV. RADIUM HALO, MUCH ENLARGED. THORIUM HALO AND RADIUM
+HALO IN MICA - to face p. 228
+
+PLATE XXV. HALO ROUND CAPILLARY IN GLASS TUBE. HALOES ROUND
+TUBULAR PASSAGES IN MICA - to face p. 230
+
+PLATE XXVI. ALETSCH GLACIER, SWITZERLAND - to face p. 282
+
+PLATE XXVII. THE MIDDLE ALETSCH GLACIER JOINING THE GREAT ALETSCH
+GLACIER. GLACIERS OF THE LAUTERBRUNNEN THAL - to face p. 285
+
+PLATE XXVIII. PERCHED BLOCK ON THE ALETSCH GLACIER. GRANITE
+ERRATIC NEAR ROUNDWOOD, CO. WICKLOW; NOW BROKEN UP AND REMOVED -
+to face p. 286
+
+And Fifteen Illustrations in the Text.
+
+x
+
+PREFACE
+
+Tins volume contains twelve essays written at various times
+during recent years. Many of them are studies contributed to
+Scientific Reviews or delivered as popular lectures. Some are
+expositions of views the scientific basis of which may be
+regarded as established. Others--the greater number--may be
+described as attempting the solution of problems which cannot be
+approached by direct observation.
+
+The essay on The Birth-time of the World is based on a lecture
+delivered before the Royal Dublin Society. The subject has
+attracted much attention within recent years. The age of the
+Earth is, indeed, of primary importance in our conception of the
+longevity of planetary systems. The essay deals with the
+evidence, derived from the investigation of purely terrestrial
+phenomena, as to the period which has elapsed since the ocean
+condensed upon the Earth's surface. Dr. Decker's recent addition
+to the subject appeared too late for inclusion in it. He finds
+that the movements (termed isostatic) which geologists recognise
+as taking place deep in the Earth's crust, indicate an age of the
+same order of magnitude
+
+xi
+
+as that which is inferred from the statistics of denudative
+history.[1]
+
+The subject of _Denudation_ naturally arises from the first essay.
+In thinking over the method of finding the age of the ocean by
+the accumulation of sodium therein, I perceived so long ago as
+1899, when my first paper was published, that this method
+afforded a means of ascertaining the grand total of denudative
+work effected on the Earth's surface since the beginning of
+geological time; the resulting knowledge in no way involving any
+assumption as to the duration of the period comprising the
+denudative actions. This idea has been elaborated in various
+publications since then, both by myself and by others.
+"Denudation," while including a survey of the subject generally,
+is mainly a popular account of this method and its results. It
+closes with a reference to the fascinating problems presented by
+the inner nature of sedimentation: a branch of science to which I
+endeavoured to contribute some years ago.
+
+_Mountain Genesis_ first brings in the subject of the geological
+intervention of radioactivity. There can, I believe, be no doubt
+as to the influence of transforming elements upon the
+developments of the surface features of the Earth; and, if I am
+right, this source of thermal energy is mainly responsible for
+that local accumulation of wrinkling which we term mountain
+chains. The
+
+[1] Bull. Geol. Soc. America, vol. xxvi, March 1915.
+
+xii
+
+paper on _Alpine Structure_ is a reprint from "Radioactivity and
+Geology," which for the sake of completeness is here included. It
+is directed to the elucidation of a detail of mountain genesis: a
+detail which enters into recent theories of Alpine development.
+The weakness of the theory of the "horst" is manifest, however,
+in many of its other applications; if not, indeed, in all.
+
+The foregoing essays on the physical influences affecting the
+surface features of the Earth are accompanied by one entitled _The
+Abundance of Life._ This originated amidst the overwhelming
+presentation of life which confronts us in the Swiss Alps. The
+subject is sufficiently inspiring. Can no fundamental reason be
+given for the urgency and aggressiveness of life? Vitality is an
+ever-extending phenomenon. It is plain that the great principles
+which have been enunciated in explanation of the origin of
+species do not really touch the problem. In the essay--which is an
+early one (1890)--the explanation of the whole great matter is
+sought--and as I believe found--in the attitude of the organism
+towards energy external to it; an attitude which results in its
+evasion of the retardative and dissipatory effects which prevail
+in lifeless dynamic systems of all kinds.
+
+_Other Minds than Ours_? attempts a solution of the vexed question
+of the origin of the Martian "canals." The essay is an abridgment
+of two popular lectures on the subject. I had previously written
+an account of my views which carried the enquiry as far as it was
+in
+
+xiii
+
+my power to go. This paper appeared in the "Transactions of the
+Royal Dublin Society, 1897." The theory put forward is a purely
+physical one, and, if justified, the view that intelligent beings
+exist in Mars derives no support from his visible surface
+features; but is, in fact, confronted with fresh difficulties.
+
+_Pleochroic Haloes_ is a popular exposition of an inconspicuous but
+very beautiful phenomenon of the rocks. Minute darkened spheres--a
+microscopic detail--appear everywhere in certain of the rock
+minerals. What are they? The discoveries of recent radioactive
+research--chiefly due to Rutherford--give the answer. The
+measurements applied to the little objects render the explanation
+beyond question. They turn out to be a quite extraordinary record
+of radioactive energy; a record accumulated since remote
+geological times, and assuring us, indirectly, of the stability
+of the chemical elements in general since the beginning of the
+world. This assurance is, without proof, often assumed in our
+views on the geological history of the Globe.
+
+Skating is a discourse, with a recent addition supporting the
+original thesis. It is an illustration of a common experience--the
+explanation of an unimportant action involving principles the
+most influential considered as a part of Nature's resources.
+
+The address on _The Latent Image_ deals with a subject which had
+been approached by various writers before the time of my essay;
+but, so far as I know, an explanation
+
+xiv
+
+based on the facts of photo-electricity had not been attempted.
+Students of this subject will notice that the views expressed are
+similar to those subsequently put forward by Lenard and Saeland
+in explanation of phosphorescence. The whole matter is of more
+practical importance than appears at first sight, for the
+photoelectric nature of the effects involved in the radiative
+treatment of many cruel diseases seems to be beyond doubt.
+
+It was in connection with photo-electric science that I was led
+to take an interest in the application of radioactivity in
+medicine. The lecture on _The Use of Radium in Medicine_ deals with
+this subject. Towards the conclusion of this essay reference will
+be found to a practical outcome of such studies which, by
+improving on the methods, and facilitating the application, of
+radioactive treatment, has, in the hands of skilled medical men,
+already resulted in the alleviation of suffering.
+
+Leaving out much which might well appear in a prefatory notice, a
+word should yet be added respecting the illustrations of scenery.
+They are a small selection from a considerable number of
+photographs taken during my summer wanderings in the Alps in
+company with Henry H. Dixon. An exception is Plate X, which is by
+the late Dr. Edward Stapleton. From what has been said above, it
+will be gathered that these illustrations are fitly included
+among pages which owe so much to Alpine inspiration. They
+illustrate the
+
+xv
+
+subjects dealt with, and, it is to be hoped, they will in some
+cases recall to the reader scenes which have in past times
+influenced his thoughts in the same manner; scenes which in their
+endless perspective seem to reduce to their proper insignificance
+the lesser things of life.
+
+My thanks are due to Mr. John Murray for kindly consenting to the
+reissue of the essay on _The Birth-time of the World_ from the
+pages of _Science Progress_; to Messrs. Constable & Co. for leave
+to reprint _Pleochroic Haloes_ from _Bedrock_, and also to make some
+extracts from _Radioactivity and Geology_; and to the Council of
+the Royal Dublin Society for permission to republish certain
+papers from the Proceedings of the Society.
+
+_Iveagh Geological Laboratory, Trinity College, Dublin._
+
+July, 1915.
+
+xvi
+
+THE BIRTH-TIME OF THE WORLD [1]
+
+LONG ago Lucretius wrote: "For lack of power to solve the
+question troubles the mind with doubts, whether there was ever a
+birth-time of the world and whether likewise there is to be any
+end." "And if" (he says in answer) "there was no birth-time of
+earth and heaven and they have been from everlasting, why before
+the Theban war and the destruction of Troy have not other poets
+as well sung other themes? Whither have so many deeds of men so
+often passed away, why live they nowhere embodied in lasting
+records of fame? The truth methinks is that the sum has but a
+recent date, and the nature of the world is new and has but
+lately had its commencement."[2]
+
+Thus spake Lucretius nearly 2,000 years ago. Since then we have
+attained another standpoint and found very different limitations.
+To Lucretius the world commenced with man, and the answer he
+would give to his questions was in accord with his philosophy: he
+would date the birth-time of the world from the time when
+
+[1] A lecture delivered before the Royal Dublin Society, February
+6th, 1914. _Science Progress_, vol. ix., p. 37
+
+[2] _De Rerum Natura_, translated by H. A. J. Munro (Cambridge,
+1886).
+
+1
+
+poets first sang upon the earth. Modern Science has along with
+the theory that the Earth dated its beginning with the advent of
+man, swept utterly away this beautiful imagining. We can, indeed,
+find no beginning of the world. We trace back events and come to
+barriers which close our vista--barriers which, for all we know,
+may for ever close it. They stand like the gates of ivory and of
+horn; portals from which only dreams proceed; and Science cannot
+as yet say of this or that dream if it proceeds from the gate of
+horn or from that of ivory.
+
+In short, of the Earth's origin we have no certain knowledge; nor
+can we assign any date to it. Possibly its formation was an event
+so gradual that the beginning was spread over immense periods. We
+can only trace the history back to certain events which may with
+considerable certainty be regarded as ushering in our geological
+era.
+
+Notwithstanding our limitations, the date of the birth-time of
+our geological era is the most important date in Science. For in
+taking into our minds the spacious history of the universe, the
+world's age must play the part of time-unit upon which all our
+conceptions depend. If we date the geological history of the
+Earth by thousands of years, as did our forerunners, we must
+shape our ideas of planetary time accordingly; and the duration
+of our solar system, and of the heavens, becomes comparable with
+that of the dynasties of ancient nations. If by millions of
+years, the sun and stars are proportionately venerable. If by
+hundreds or thousands of millions of
+
+2
+
+years the human mind must consent to correspondingly vast epochs
+for the duration of material changes. The geological age plays
+the same part in our views of the duration of the universe as the
+Earth's orbital radius does in our views of the immensity of
+space. Lucretius knew nothing of our time-unit: his unit was the
+life of a man. So also he knew nothing of our space-unit, and he
+marvels that so small a body as the sun can shed so much, heat
+and light upon the Earth.
+
+A study of the rocks shows us that the world was not always what
+it now is and long has been. We live in an epoch of denudation.
+The rains and frosts disintegrate the hills; and the rivers roll
+to the sea the finely divided particles into which they have been
+resolved; as well as the salts which have been leached from them.
+The sediments collect near the coasts of the continents; the
+dissolved matter mingles with the general ocean. The geologist
+has measured and mapped these deposits and traced them back into
+the past, layer by layer. He finds them ever the same;
+sandstones, slates, limestones, etc. But one thing is not the
+same. _Life_ grows ever less diversified in character as the
+sediments are traced downwards. Mammals and birds, reptiles,
+amphibians, fishes, die out successively in the past; and barren
+sediments ultimately succeed, leaving the first beginnings of
+life undecipherable. Beneath these barren sediments lie rocks
+collectively differing in character from those above: mainly
+volcanic or poured out from fissures in
+
+3
+
+the early crust of the Earth. Sediments are scarce among these
+materials.[1]
+
+There can be little doubt that in this underlying floor of
+igneous and metamorphic rocks we have reached those surface
+materials of the earth which existed before the long epoch of
+sedimentation began, and before the seas came into being. They
+formed the floor of a vaporised ocean upon which the waters
+condensed here and there from the hot and heavy atmosphere. Such
+were the probable conditions which preceded the birth-time of the
+ocean and of our era of life and its evolution.
+
+It is from this epoch we date our geological age. Our next
+purpose is to consider how long ago, measured in years, that
+birth-time was.
+
+That the geological age of the Earth is very great appears from
+what we have already reviewed. The sediments of the past are many
+miles in collective thickness: yet the feeble silt of the rivers
+built them all from base to summit. They have been uplifted from
+the seas and piled into mountains by movements so slow that
+during all the time man has been upon the Earth but little change
+would have been visible. The mountains have again been worn down
+into the ocean by denudation and again younger mountains built
+out of their redeposited materials. The contemplation of such
+vast events
+
+[1] For a description of these early rocks, see especially the
+monograph of Van Hise and Leith on the pre-Cambrian Geology of
+North America (Bulletin 360, U.S. Geol. Survey).
+
+4
+
+prepares our minds to accept many scores of millions of years or
+hundreds of millions of years, if such be yielded by our
+calculations.
+
+THE AGE AS INFERRED FROM THE THICKNESS OF THE SEDIMENTS
+
+The earliest recognised method of arriving at an estimate of the
+Earth's geological age is based upon the measurement of the
+collective sediments of geological periods. The method has
+undergone much revision from time to time. Let us briefly review
+it on the latest data.
+
+The method consists in measuring the depths of all the successive
+sedimentary deposits where these are best developed. We go all
+over the explored world, recognising the successive deposits by
+their fossils and by their stratigraphical relations, measuring
+their thickness and selecting as part of the data required those
+beds which we believe to most completely represent each
+formation. The total of these measurements would tell us the age
+of the Earth if their tale was indeed complete, and if we knew
+the average rate at which they have been deposited. We soon,
+however, find difficulties in arriving at the quantities we
+require. Thus it is not easy to measure the real thickness of a
+deposit. It may be folded back upon itself, and so we may measure
+it twice over. We may exaggerate its thickness by measuring it
+not quite straight across the bedding or by unwittingly including
+volcanic materials. On the other hand, there
+
+5
+
+may be deposits which are inaccessible to us; or, again, an
+entire absence of deposits; either because not laid down in the
+areas we examine, or, if laid down, again washed into the sea.
+These sources of error in part neutralise one another. Some make
+our resulting age too long, others make it out too short. But we
+do not know if a balance of error does not still remain. Here,
+however, is a table of deposits which summarises a great deal of
+our knowledge of the thickness of the stratigraphical
+accumulations. It is due to Sollas.[1]
+
+Feet.
+
+Recent and Pleistocene  - -    4,000
+Pliocene                 - -   13,000
+Miocene                  - -   14,000
+Oligocene                - -     2,000
+Eocene                   - -  20,000
+                              63,000
+
+Upper Cretaceous        - -   24,000
+Lower Cretaceous        - -   20,000
+Jurassic                 - -    8,000
+Trias                    - -   7,000
+                              69,000
+
+Permian                  - -    2,000
+Carboniferous           - -    29,000
+Devonian                 - -   22,000
+                              63,000
+
+Silurian                 - -   15,000
+Ordovician              - -   17,000
+Cambrian                 - -   6,000
+                              58,000
+
+Algonkian--Keeweenawan   - -   50,000
+Algonkian--Animikian     - -   14,000
+Algonkian--Huronian      - -   18,000
+                              82,000
+
+Archæan                   - -   ?
+
+Total                     - - 335,000 feet.
+
+[1] Address to the Geol. Soc. of London, 1509.
+
+6
+
+In the next place we require to know the average rate at which
+these rocks were laid down. This is really the weakest link in
+the chain. The most diverse results have been arrived at, which
+space does not permit us to consider. The value required is most
+difficult to determine, for it is different for the different
+classes of material, and varies from river to river according to
+the conditions of discharge to the sea. We may probably take it
+as between two and six inches in a century.
+
+Now the total depth of the sediments as we see is about 335,000
+feet (or 64 miles), and if we take the rate of collecting as
+three inches in a hundred years we get the time for all to
+collect as 134 millions of years. If the rate be four inches, the
+time is soo millions of years, which is the figure Geikie
+favoured, although his result was based on somewhat different
+data. Sollas most recently finds 80 millions of years.[1]
+
+THE AGE AS INFERRED FROM THE MASS OF THE SEDIMENTS
+
+In the above method we obtain our result by the measurement of
+the linear dimensions of the sediments. These measurements, as we
+have seen, are difficult to arrive at. We may, however, proceed
+by measurements of the mass of the sediments, and then the method
+becomes more definite. The new method is pursued as follows:
+
+[1] Geikie, _Text Book of Geology_ (Macmillan, 1903), vol. i., p.
+73, _et seq._ Sollas, _loc. cit._ Joly, _Radioactivity and Geology_
+(Constable, 1909), and Phil. Mag., Sept. 1911.
+
+7
+
+The total mass of the sediments formed since denudation began may
+be ascertained with comparative accuracy by a study of the
+chemical composition of the waters of the ocean. The salts in the
+ocean are undoubtedly derived from the rocks; increasing age by
+age as the latter are degraded from their original character
+under the action of the weather, etc., and converted to the
+sedimentary form. By comparing the average chemical composition
+of these two classes of material--the primary or igneous rocks and
+the sedimentary--it is easy to arrive at a knowledge of how much
+of this or that constituent was given to the ocean by each ton of
+primary rock which was denuded to the sedimentary form. This,
+however, will not assist us to our object unless the ocean has
+retained the salts shed into it. It has not generally done so. In
+the case of every substance but one the ocean continually gives
+up again more or less of the salts supplied to it by the rivers.
+The one exception is the element sodium. The great solubility of
+its salts has protected it from abstraction, and it has gone on
+collecting during geological time, practically in its entirety.
+This gives us the clue to the denudative history of the
+Earth.[1]
+
+The process is now simple. We estimate by chemical examination of
+igneous and sedimentary rocks the amount of sodium which has been
+supplied to the ocean per ton of sediment produced by denudation.
+We also calculate
+
+[1] _Trans. R.D.S._, May, 1899.
+
+8
+
+the amount of sodium contained in the ocean. We divide the one
+into the other (stated, of course, in the same units of mass),
+and the quotient gives us the number of tons of sediment. The
+most recent estimate of the sediments made in this manner affords
+56 x 1016 tonnes.[1]
+
+Now we are assured that all this sediment was transported by the
+rivers to the sea during geological time. Thus it follows that,
+if we can estimate the average annual rate of the river supply of
+sediments to the ocean over the past, we can calculate the
+required age. The land surface is at present largely covered with
+the sedimentary rocks themselves. Sediment derived from these
+rocks must be regarded as, for the most part, purely cyclical;
+that is, circulating from the sea to the land and back again. It
+does not go to increase the great body of detrital deposits. We
+cannot, therefore, take the present river supply of sediment as
+representing that obtaining over the long past. If the land was
+all covered still with primary rocks we might do so. It has been
+estimated that about 25 per cent. of the existing continental
+area is covered with archæan and igneous rocks, the remainder
+being sediments.[2] On this estimate we may find valuable
+
+[1] Clarke, _A Preliminary Study of Chemical Denudation_
+(Washington, 1910). My own estimate in 1899 (_loc. cit._) made as a
+test of yet another method of finding the age, showed that the
+sediments may be taken as sufficient to form a layer 1.1 mile
+deep if spread uniformly over the continents; and would amount to
+64 x 1018 tons.
+
+[2] Van Tillo, _Comptes Rendues_ (Paris), vol. cxiv., 1892.
+
+9
+
+major and minor limits to the geological age. If we take 25 per
+cent. only of the present river supply of sediment, we evidently
+fix a major limit to the age, for it is certain that over the
+past there must have been on the average a faster supply. If we
+take the entire river supply, on similar reasoning we have what
+is undoubtedly a minor limit to the age.
+
+The river supply of detrital sediment has not been very
+extensively investigated, although the quantities involved may be
+found with comparative ease and accuracy. The following table
+embodies the results obtained for some of the leading rivers.[1]
+
+               Mean annual   Total annual   Ratio of
+               discharge in  sediment in    sediment
+               cubic feet    thousands      to water
+               per second.   of tons.       by weight.
+
+Potomac -        20,160          5,557        1 : 3.575
+Mississippi -   610,000       406,250        1 : 1,500
+Rio Grande -      1,700          3,830        1 : 291
+Uruguay -       150,000         14,782       1 : 10,000
+Rhone -          65,850         36,000       1 : 1,775
+Po -             62,200         67,000       1 : 900
+Danube -        315,200        108,000       1 : 2,880
+Nile -          113,000         54,000       1 : 2,050
+Irrawaddy -     475,000       291,430        1 : 1,610
+
+Mean -          201,468        109,650       1 : 2,731
+
+We see that the ratio of the weight of water to the
+
+[1] Russell, _River Development_ (John Murray, 1888).
+
+10
+
+weight of transported sediment in six out of the nine rivers does
+not vary widely. The mean is 2,730 to 1. But this is not the
+required average. The water-discharge of each river has to be
+taken into account. If we ascribe to the ratio given for each
+river the weight proper to the amount of water it discharges, the
+proportion of weight of water to weight of sediment, for the
+whole quantity of water involved, comes out as 2,520 to 1.
+
+Now if this proportion holds for all the rivers of the
+world--which collectively discharge about 27 x 1012 tonnes of
+water per annum--the river-born detritus is 1.07 x 1010 tonnes. To
+this an addition of 11 per cent. has to be made for silt pushed
+along the river-bed.[1] On these figures the minor limit to the
+age comes out as 47 millions of years, and the major limit as 188
+millions. We are here going on rather deficient estimates, the
+rivers involved representing only some 6 per cent. of the total
+river supply of water to the ocean. But the result is probably
+not very far out.
+
+We may arrive at a probable age lying between the major and minor
+limits. If, first, we take the arithmetic mean of these limits,
+we get 117 millions of years. Now this is almost certainly
+excessive, for we here assume that the rate of covering of the
+primary rocks by sediments was uniform. It would not be so,
+however, for the rate of supply of original sediment must have
+been continually diminishing
+
+[1] According to observations made on the Mississippi (Russell,
+_loc. cit._).
+
+11
+
+during geological time, and hence we may assume that the rate of
+advance of the sediments on the primary rocks has also been
+diminishing. Now we may probably take, as a fair assumption, that
+the sediment-covered area was at any instant increasing at a rate
+proportionate to the rate of supply of sediment; that is, to the
+area of primary rocks then exposed. On this assumption the age is
+found to be 87 millions of years.
+
+THE AGE BY THE SODIUM OF THE OCEAN
+
+I have next to lay before you a quite different method. I have
+already touched upon the chemistry of the ocean, and on the
+remarkable fact that the sodium contained in it has been
+preserved, practically, in its entirety from the beginning of
+geological time.
+
+That the sea is one of the most beautiful and magnificent sights
+in Nature, all admit. But, I think, to those who know its story
+its beauty and magnificence are ten-fold increased. Its saltness
+it due to no magic mill. It is the dissolved rocks of the Earth
+which give it at once its brine, its strength, and its buoyancy.
+The rivers which we say flow with "fresh" water to the sea
+nevertheless contain those traces of salt which, collected over
+the long ages, occasion the saltness of the ocean. Each gallon of
+river water contributes to the final result; and this has been
+going on since the beginning of our era. The mighty total of the
+rivers is 6,500 cubic miles of water in the year!
+
+12
+
+There is little doubt that the primeval ocean was in the
+condition of a fresh-water lake. It can be shown that a primitive
+and more rapid solution of the original crust of the Earth by the
+slowly cooling ocean would have given rise to relatively small
+salinity. The fact is, the quantity of salts in the ocean is
+enormous. We are only now concerned with the sodium; but if we
+could extract all the rock-salt (the chloride of sodium) from the
+ocean we should have enough to cover the entire dry land of the
+Earth to a depth of 400 feet. It is this gigantic quantity which
+is going to enter into our estimate of the Earth's age. The
+calculated mass of sodium contained in this rock-salt is 14,130
+million million tonnes.
+
+If now we can determine the rate at which the rivers supply
+sodium to the ocean, we can determine the age.[1] As the result
+of many thousands of river analyses, the total amount of sodium
+annually discharged to the ocean
+
+[1] _Trans. R.D.S._, 1899. A paper by Edmund Halley, the
+astronomer, in the _Philosophical Transactions of the Royal
+Society_ for 1715, contains a suggestion for finding the age of
+the world by the following procedure. He proposes to make
+observations on the saltness of the seas and ocean at intervals
+of one or more centuries, and from the increment of saltness
+arrive at their age. The measurements, as a matter of fact, are
+impracticable. The salinity would only gain (if all remained in
+solution) one millionth part in Too years; and, of course, the
+continuous rejection of salts by the ocean would invalidate the
+method. The last objection also invalidates the calculation by T.
+Mellard Reade (_Proc. Liverpool Geol. Soc._, 1876) of a minor limit
+to the age by the calcium sulphate in the ocean. Both papers were
+quite unknown to me when working out my method. Halley's paper
+was, I think, only brought to light in 1908.
+
+13
+
+by all the rivers of the world is found to be probably not far
+from 175 million tonnes.[1] Dividing this into the mass of
+oceanic sodium we get the age as 80.7 millions of years. Certain
+corrections have to be applied to this figure which result in
+raising it to a little over 90 millions of years. Sollas, as the
+result of a careful review of the data, gets the age as between
+80 and 150 millions of years. My own result[2] was between 80 and
+90 millions of years; but I subsequently found that upon certain
+extreme assumptions a maximum age might be arrived at of 105
+millions of years.[3] Clarke regards the 80.7 millions of years
+as certainly a maximum in the light of certain calculations by
+Becker.[4]
+
+The order of magnitude of these results cannot be shaken unless
+on the assumption that there is something entirely misleading in
+the existing rate of solvent denudation. On the strength of the
+results of another and
+
+[1] F. W. Clarke, _A Preliminary Study of Chemical Denudation_
+(Smithsonian Miscellaneous Collections, 1910).
+
+[2] _Loc. cit._
+
+[3] "The Circulation of Salt and Geological Time" (Geol. Mag.,
+1901, p. 350).
+
+[4] Becker (loc. cit.), assuming that the exposed igneous and
+archæan rocks alone are responsible for the supply of sodium to
+the ocean, arrives at 74 millions of years as the geological age.
+This matter was discussed by me formerly (Trans. R.D.S., 1899,
+pp. 54 _et seq._). The assumption made is, I believe, inadmissible.
+It is not supported by river analyses, or by the chemical
+character of residual soils from sedimentary rocks. There may be
+some convergence in the rate of solvent denudation, but--as I
+think on the evidence--in our time unimportant.
+
+14
+
+entirely different method of approaching the question of the
+Earth's age (which shall be presently referred to), it has been
+contended that it is too low. It is even asserted that it is from
+nine to fourteen times too low. We have then to consider whether
+such an enormous error can enter into the method. The
+measurements involved cannot be seriously impugned. Corrections
+for possible errors applied to the quantities entering into this
+method have been considered by various writers. My own original
+corrections have been generally confirmed. I think the only point
+left open for discussion is the principle of uniformitarianism
+involved in this method and in the methods previously discussed.
+
+In order to appreciate the force of the evidence for uniformity
+in the geological history of the Earth, it is, of course,
+necessary to possess some acquaintance with geological science.
+Some of the most eminent geologists, among whom Lyell and
+Geikie[1] may be mentioned, have upheld the doctrine of
+uniformity. It must here suffice to dwell upon a few points
+having special reference to the matter under discussion.
+
+The mere extent of the land surface does not, within limits,
+affect the question of the rate of denudation. This arises from
+the fact that the rain supply is quite insufficient to denude the
+whole existing land surface. About 30 per cent. of it does not,
+in fact, drain to the
+
+[1] See especially Geikie's Address to Sect. C., Brit. Assoc.
+Rep., 1399.
+
+15
+
+ocean. If the continents become invaded by a great transgression
+of the ocean, this "rainless" area diminishes: and the denuded
+area advances inwards without diminution. If the ocean recedes
+from the present strand lines, the "rainless" area advances
+outwards, but, the rain supply being sensibly constant, no change
+in the river supply of salts is to be expected.
+
+Age-long submergence of the entire land, or of any very large
+proportion of what now exists, is negatived by the continuous
+sequence of vast areas of sediment in every geologic age from the
+earliest times. Now sediment-receiving areas always are but a
+small fraction of those exposed areas whence the sediments are
+supplied.[1] Hence in the continuous records of the sediments we
+have assurance of the continuous exposure of the continents above
+the ocean surface. The doctrine of the permanency of the
+continents has in its main features been accepted by the most
+eminent authorities. As to the actual amount of land which was
+exposed during past times to denudative effects, no data exist to
+show it was very different from what is now exposed. It has been
+estimated that the average area of the North American continent
+over geologic time was about eight-tenths of its existing
+area.[2] Restorations of other continents, so far as they have
+been attempted, would not
+
+[1] On the strength of the Mississippi measurements about 1 to 18
+(Magee, _Am. Jour. of Sc._, 1892, p. 188).
+
+[2] Schuchert, _Bull. Geol. Soc. Am._, vol. xx., 1910.
+
+16
+
+suggest any more serious divergency one way or the other.
+
+That climate in the oceans and upon the land was throughout much
+as it is now, the continuous chain of teeming life and the
+sensitive temperature limits of protoplasmic existence are
+sufficient evidence.[1] The influence at once of climate and of
+elevation of the land may be appraised at their true value by the
+ascertained facts of solvent denudation, as the following table
+shows.
+
+                 Tonnes removed in      Mean elevation.
+                 solution per square    Metres.
+                 mile per annum.
+North America -         79                  700
+South America -         50                  650
+Europe -                100                  300
+Asia -                   84                  950
+Africa -                 44                  650
+
+In this table the estimated number of tonnes of matter in
+solution, which for every square mile of area the rivers convey
+to the ocean in one year, is given in the first column. These
+results are compiled by Clarke from a very large number of
+analyses of river waters. The second column of the table gives
+the mean heights in metres above sea level of the several
+continents, as cited by Arrhenius.[2]
+
+Of all the denudation results given in the table, those relating
+to North America and to Europe are far the
+
+[1] See also Poulton, Address to Sect. D., Brit. Assoc. Rep.,
+1896.
+
+[2] _Lehybuch dev Kosmischen Physik_, vol. i., p. 347.
+
+17
+
+most reliable. Indeed these may be described as highly reliable,
+being founded on some thousands of analyses, many of which have
+been systematically pursued through every season of the year.
+These show that Europe with a mean altitude of less than half
+that of North America sheds to the ocean 25 per cent. more salts.
+A result which is to be expected when the more important factors
+of solvent denudation are given intelligent consideration and we
+discriminate between conditions favouring solvent and detrital
+denudation respectively: conditions in many cases
+antagonistic.[1] Hence if it is true, as has been stated, that we
+now live in a period of exceptionally high continental elevation,
+we must infer that the average supply of salts to the ocean by
+the rivers of the world is less than over the long past, and
+that, therefore, our estimate of the age of the Earth as already
+given is excessive.
+
+There is, however, one condition which will operate to unduly
+diminish our estimate of geologic time, and it is a condition
+which may possibly obtain at the present time. If the land is, on
+the whole, now sinking relatively to the ocean level, the
+denudation area tends, as we have seen, to move inwards. It will
+thus encroach upon regions which have not for long periods
+drained to the ocean. On such areas there is an accumulation of
+soluble salts which the deficient rivers have not been able to
+carry to the ocean. Thus the salt content of certain of
+
+[1] See the essay on Denudation.
+
+18
+
+the rivers draining to the ocean will be influenced not only by
+present denudative effects, but also by the stored results of
+past effects. Certain rivers appear to reveal this unduly
+increased salt supply those which flow through comparatively arid
+areas. However, the flowoff of such tributaries is relatively
+small and the final effects on the great rivers apparently
+unimportant--a result which might have been anticipated when the
+extremely slow rate of the land movements is taken into account.
+
+The difficulty of effecting any reconciliation of the methods
+already described and that now to be given increases the interest
+both of the former and the latter.
+
+THE AGE BY RADIOACTIVE TRANSFORMATIONS
+
+Rutherford suggested in 1905 that as helium was continually being
+evolved at a uniform rate by radioactive substances (in the form
+of the alpha rays) a determination of the age of minerals
+containing the radioactive elements might be made by measurements
+of the amount of the stored helium and of the radioactive
+elements giving rise to it, The parent radioactive substances
+are--according to present knowledge--uranium and thorium. An
+estimate of the amounts of these elements present enables the
+rate of production of the helium to be calculated. Rutherford
+shortly afterwards found by this method an age of 240 millions of
+years for a radioactive mineral of presumably remote age. Strutt,
+who carried
+
+19
+
+his measurements to a wonderful degree of refinement, found the
+following ages for mineral substances originating in different
+geological ages:
+
+Oligocene -               8.4 millions of years.
+Eocene -                 31    millions of years.
+Lower Carboniferous -  150   millions of years.
+Archæan -               750    millions of years.
+
+Periods of time much less than, and very inconsistent with, these
+were also found. The lower results are, however, easily explained
+if we assume that the helium--which is a gas under prevailing
+conditions--escapes in many cases slowly from the mineral.
+
+Another product of radioactive origin is lead. The suggestion
+that this substance might be made available to determine the age
+of the Earth also originated with Rutherford. We are at least
+assured that this element cannot escape by gaseous diffusion from
+the minerals. Boltwood's results on the amount of lead contained
+in minerals of various ages, taken in conjunction with the amount
+of uranium or parent substance present, afforded ages rising to
+1,640 millions of years for archæan and 1,200 millions for
+Algonkian time. Becker, applying the same method, obtained
+results rising to quite incredible periods: from 1,671 to 11,470
+millions of years. Becker maintained that original lead rendered
+the determinations indefinite. The more recent results of Mr. A.
+Holmes support the conclusion that "original" lead may be present
+and may completely falsify results derived
+
+20
+
+from minerals of low radioactivity in which the derived lead
+would be small in amount. By rejecting such results as appeared
+to be of this character, he arrives at 370 millions of years as
+the age of the Devonian.
+
+I must now describe a very recent method of estimating the age of
+the Earth. There are, in certain rock-forming minerals,
+colour-changes set up by radioactive causes. The minute and
+curious marks so produced are known as haloes; for they surround,
+in ringlike forms, minute particles of included substances which
+contain radioactive elements. It is now well known how these
+haloes are formed. The particle in the centre of the halo
+contains uranium or thorium, and, necessarily, along with the
+parent substance, the various elements derived from it. In the
+process of transformation giving rise to these several derived
+substances, atoms of helium--the alpha rays--projected with great
+velocity into the surrounding mineral, occasion the colour
+changes referred to. These changes are limited to the distance to
+which the alpha rays penetrate; hence the halo is a spherical
+volume surrounding the central substance.[1]
+
+The time required to form a halo could be found if on the one
+hand we could ascertain the number of alpha rays ejected from the
+nucleus of the halo in, say, one year, and, on the other, if we
+determined by experiment just how many alpha rays were required
+to produce the same
+
+[1] _Phil. Mag._, March, 1907 and February, 1910; also _Bedrock_,
+January, 1913. See _Pleochroic Haloes_ in this volume.
+
+21
+
+amount of colour alteration as we perceive to extend around the
+nucleus.
+
+The latter estimate is fairly easily and surely made. But to know
+the number of rays leaving the central particle in unit time we
+require to know the quantity of radioactive material in the
+nucleus. This cannot be directly determined. We can only, from
+known results obtained with larger specimens of just such a
+mineral substance as composes the nucleus, guess at the amount of
+uranium, or it may be thorium, which may be present.
+
+This method has been applied to the uranium haloes of the mica of
+County Carlow.[1] Results for the age of the halo of from 20 to
+400 millions of years have been obtained. This mica was probably
+formed in the granite of Leinster in late Silurian or in Devonian
+times.
+
+The higher results are probably the least in error, upon the data
+involved; for the assumption made as to the amount of uranium in
+the nuclei of the haloes was such as to render the higher results
+the more reliable.
+
+This method is, of course, a radioactive method, and similar to
+the method by helium storage, save that it is free of the risk of
+error by escape of the helium, the effects of which are, as it
+were, registered at the moment of its production, so that its
+subsequent escape is of no moment.
+
+[1] Joly and Rutherford, _Phil. Mag._, April, 1913.
+
+22
+
+REVIEW OF THE RESULTS
+
+We shall now briefly review the results on the geological age of
+the Earth.
+
+By methods based on the approximate uniformity of denudative
+effects in the past, a period of the order of 100 millions of
+years has been obtained as the duration of our geological age;
+and consistently whether we accept for measurement the sediments
+or the dissolved sodium. We can give reasons why these
+measurements might afford too great an age, but we can find
+absolutely no good reason why they should give one much too low.
+
+By measuring radioactive products ages have been found which,
+while they vary widely among themselves, yet claim to possess
+accuracy in their superior limits, and exceed those derived from
+denudation from nine to fourteen times.
+
+In this difficulty let us consider the claims of the radioactive
+method in any of its forms. In order to be trustworthy it must be
+true; (1) that the rate of transformation now shown by the parent
+substance has obtained throughout the entire past, and (2) that
+there were no other radioactive substances, either now or
+formerly existing, except uranium, which gave rise to lead. As
+regards methods based on the production of helium, what we have
+to say will largely apply to it also. If some unknown source of
+these elements exists we, of course, on our assumption
+overestimate the age.
+
+23
+
+As regards the first point: In ascribing a constant rate of
+change to the parent substance--which Becker (loc. cit.) describes
+as "a simple though tremendous extrapolation"--we reason upon
+analogy with the constant rate of decay observed in the derived
+radioactive bodies. If uranium and thorium are really primary
+elements, however, the analogy relied on may be misleading; at
+least, it is obviously incomplete. It is incomplete in a
+particular which may be very important: the mode of origin of
+these parent bodies--whatever it may have been--is different to
+that of the secondary elements with which we compare them. A
+convergence in their rate of transformation is not impossible, or
+even improbable, so far as we known.
+
+As regards the second point: It is assumed that uranium alone of
+the elements in radioactive minerals is ultimately transformed to
+lead by radioactive changes. We must consider this assumption.
+
+Recent advances in the chemistry of the radioactive elements has
+brought out evidence that all three lines of radioactive descent
+known to us--_i.e._ those beginning with uranium, with thorium,
+and with actinium--alike converge to lead.[1] There are
+difficulties in the way of believing that all the lead-like atoms
+so produced ("isotopes" of lead, as Soddy proposes to call them)
+actually remain as stable lead in the minerals. For one
+
+[1] See Soddy's _Chemistry of the Radioactive Elements_ (Longmans,
+Green & Co.).
+
+24
+
+thing there is sometimes, along with very large amounts of
+thorium, an almost entire absence of lead in thorianites and
+thorites. And in some urano--thorites the lead may be noticed to
+follow the uranium in approximate proportionality,
+notwithstanding the presence of large amounts of thorium.[1] This
+is in favour of the assumption that all the lead present is
+derived from the uranium. The actinium is present in negligibly
+small amounts.
+
+On the other hand, there is evidence arising from the atomic
+weight of lead which seems to involve some other parent than
+uranium. Soddy, in the work referred to, points this out. The
+atomic weight of radium is well known, and uranium in its descent
+has to change to this element. The loss of mass between radium
+and uranium-derived lead can be accurately estimated by the
+number of alpha rays given off. From this we get the atomic
+weight of uranium-derived lead as closely 206. Now the best
+determinations of the atomic weight of normal lead assign to this
+element an atomic weight of closely
+
+[1] It seems very difficult at present to suggest an end product
+for thorium, unless we assume that, by loss of electrons, thorium
+E, or thorium-lead, reverts to a substance chemically identical
+with thorium itself. Such a change--whether considered from the
+point of view of the periodic law or of the radioactive theory
+would involve many interesting consequences. It is, of course,
+quite possible that the nature of the conditions attending the
+deposition of the uranium ores, many of which are comparatively
+recent, are responsible for the difficulties observed. The
+thorium and uranium ores are, again, specially prone to
+alteration.
+
+25
+
+207. By a somewhat similar calculation it is deduced that
+thorium-derived lead would possess the atomic weight of 208. Thus
+normal lead might be an admixture of uranium- and thorium-derived
+lead. However, as we have seen, the view that thorium gives rise
+to stable lead is beset with some difficulties.
+
+If we are going upon reliable facts and figures, we must, then,
+assume: (a) That some other element than uranium, and genetically
+connected with it (probably as parent substance), gives rise, or
+formerly gave rise, to lead of heavier atomic weight than normal
+lead. It may be observed respecting this theory that there is
+some support for the view that a parent substance both to uranium
+and thorium has existed or possibly exists. The evidence is found
+in the proportionality frequently observed between the amounts of
+thorium and uranium in the primary rocks.[1] Or: (b) We may meet
+the difficulties in a simpler way, which may be stated as
+follows: If we assume that all stable lead is derived from
+uranium, and at the same time recognise that lead is not
+perfectly homogeneous in atomic weight, we must, of necessity,
+ascribe to uranium a similar want of homogeneity; heavy atoms of
+uranium giving rise to heavy
+
+[1] Compare results for the thorium content of such rocks
+(appearing in a paper by the author Cong. Int. _de Radiologie et
+d'Electricité_, vol. i., 1910, p. 373), and those for the radium
+content, as collected in _Phil. Mag._, October, 1912, p. 697.
+Also A. L. Fletcher, _Phil. Mag._, July, 1910; January, 1911, and
+June, 1911. J. H. J. Poole, _Phil. Mag._, April, 1915
+
+26
+
+atoms of lead and light atoms of uranium generating light atoms
+of lead. This assumption seems to be involved in the figures
+upon, which we are going. Still relying on these figures, we
+find, however, that existing uranium cannot give rise to lead of
+normal atomic weight. We can only conclude that the heavier atoms
+of uranium have decayed more rapidly than the lighter ones. In
+this connection it is of interest to note the complexity of
+uranium as recently established by Geiger, although in this case
+it is assumed that the shorter-lived isotope bears the relation
+of offspring to the longer-lived and largely preponderating
+constituent. However, there does not seem to be any direct proof
+of this as yet.
+
+From these considerations it would seem that unless the atomic
+weight of lead in uraninites, etc., is 206, the former complexity
+and more accelerated decay of uranium are indicated in the data
+respecting the atomic weights of radium and lead[1]. As an
+alternative view, we may assume, as in our first hypothesis, that
+some elementally different but genetically connected substance,
+decaying along branching lines of descent at a rate sufficient to
+practically remove the whole of it during geological time,
+formerly existed. Whichever hypothesis we adopt
+
+[1] Later investigation has shown that the atomic weight of lead
+in uranium-bearing ores is about 206.6 (see Richards and Lembert,
+_Journ. of Am. Claem. Soc._, July, 1914). This result gives support
+to the view expressed above.
+
+27
+
+we are confronted by probabilities which invalidate
+time-measurements based on the lead and helium ratio in minerals.
+We have, in short, grave reason to question the measure of
+uniformitarianism postulated in finding the age by any of the
+known radioactive methods.
+
+That we have much to learn respecting our assumptions, whether we
+pursue the geological or the radioactive methods of approaching
+the age of our era, is, indeed, probable. Whatever the issue it
+is certain that the reconciling facts will leave us with much
+more light than we at present possess either as respects the
+Earth's history or the history of the radioactive elements. With
+this necessary admission we leave our study of the Birth-Time of
+the World.
+
+It has led us a long way from Lucretius. We do not ask if other
+Iliads have perished; or if poets before Homer have vainly sung,
+becoming a prey to all-consuming time. We move in a greater
+history, the landmarks of which are not the birth and death of
+kings and poets, but of species, genera, orders. And we set out
+these organic events not according to the passing generations of
+man, but over scores or hundreds of millions of years.
+
+How much Lucretius has lost, and how much we have gained, is
+bound up with the question of the intrinsic value of knowledge
+and great ideas. Let us appraise knowledge as we would the
+Homeric poems, as some-
+
+28
+
+thing which ennobles life and makes it happier. Well, then, we
+are, as I think, in possession today of some of those lost Iliads
+and Odysseys for which Lucretius looked in vain.[1]
+
+[1] The duration in the past of Solar heat is necessarily bound
+up with the geological age. There is no known means (outside
+speculative science) of accounting for more than about 30 million
+years of the existing solar temperature in the past. In this
+direction the age seems certainly limited to 100 million years.
+See a review of the question by Dr. Lindemann in Nature, April
+5th, 1915.
+
+29
+
+DENUDATION
+
+THE subject of denudation is at once one of the most interesting
+and one of the most complicated with which the geologist has to
+deal. While its great results are apparent even to the most
+casual observer, the factors which have led to these results are
+in many cases so indeterminate, and in some cases apparently so
+variable in influence, that thoughtful writers have even claimed
+precisely opposite effects as originating from, the same cause.
+Indeed, it is almost impossible to deal with the subject without
+entering upon controversial matters. In the following pages I
+shall endeavour to keep to broad issues which are, at the present
+day, either conceded by the greater number of authorities on the
+subject, or are, from their strictly quantitative character, not
+open to controversy.
+
+It is evident, in the first place, that denudation--or the wearing
+away of the land surfaces of the earth--is mainly a result of the
+circulation of water from the ocean to the land, and back again
+to the ocean. An action entirely conditioned by solar heat, and
+without which it would completely cease and further change upon
+the land come to an end.
+
+To what actions, then, is so great a potency of the
+
+30
+
+circulating water to be traced? Broadly speaking, we may classify
+them as mechanical and chemical. The first involves the
+separation of rock masses into smaller fragments of all sizes,
+down to the finest dust. The second involves the actual solution
+in the water of the rock constituents, which may be regarded as
+the final act of disintegration. The rivers bear the burden both
+of the comminuted and the dissolved materials to the sea. The mud
+and sand carried by their currents, or gradually pushed along
+their beds, represent the former; the invisible dissolved matter,
+only to be demonstrated to the eye by evaporation of the water or
+by chemical precipitation, represents the latter.
+
+The results of these actions, integrated over geological time,
+are enormous. The entire bulk of the sedimentary rocks, such as
+sandstones, slates, shales, conglomerates, limestones, etc., and
+the salt content of the ocean, are due to the combined activity
+of mechanical and solvent denudation. We shall, later on, make an
+estimate of the magnitude of the quantities actually involved.
+
+In the Swiss valleys we see torrents of muddy water hurrying
+along, and if we follow them up, we trace them to glaciers high
+among the mountains. From beneath the foot of the glacier, we
+find, the torrent has birth. The first debris given to the river
+is derived from the wearing of the rocky bed along which the
+glacier moves. The river of ice bequeaths to the river of
+water--of which it is the parent--the spoils which it has won from
+the rocks
+
+31
+
+The work of mechanical disintegration is, however, not restricted
+to the glacier's bed. It proceeds everywhere over the surface of
+the rocks. It is aided by the most diverse actions. For instance,
+the freezing and expansion of water in the chinks and cracks in
+those alpine heights where between sunrise and sunset the heat of
+summer reigns, and between sunset and sunrise the cold of winter.
+Again, under these conditions the mere change of surface
+temperature from night to day severely stresses the surface
+layers of the rocks, and, on the same principles as we explain
+the fracture of an unequally heated glass vessel, the rocks
+cleave off in slabs which slip down the steeps of the mountain
+and collect as screes in the valley. At lower levels the
+expansive force of vegetable growth is not unimportant, as all
+will admit who have seen the strong roots of the pines
+penetrating the crannies of the rocks. Nor does the river which
+flows in the bed of the valley act as a carrier only. Listening
+carefully we may detect beneath the roar of the alpine torrent
+the crunching and knocking of descending boulders. And in the
+potholes scooped by its whirling waters we recognise the abrasive
+action of the suspended sand upon the river bed.
+
+A view from an Alpine summit reveals a scene of remarkable
+desolation (Pl. V, p. 40). Screes lie piled against the steep
+slopes. Cliffs stand shattered and ready to fall in ruins. And
+here the forces at work readily reveal themselves. An occasional
+wreath of white smoke among
+
+32
+
+the far-off peaks, followed by a rumbling reverberation, marks
+the fall of an avalanche. Water everywhere trickles through the
+shaly _débris_ scattered around. In the full sunshine the rocks are
+almost too hot to bear touching. A few hours later the cold is
+deadly, and all becomes a frozen silence. In such scenes of
+desolation and destruction, detrital sediments are actively being
+generated. As we descend into the valley we hear the deep voice
+of the torrents which are continually hurrying the disintegrated
+rocks to the ocean.
+
+A remarkable demonstration of the activity of mechanical
+denudation is shown by the phenomenon of "earth pillars." The
+photograph (Pl. IV.) of the earth pillars of the Val d'Hérens
+(Switzerland) shows the peculiar appearance these objects
+present. They arise under conditions where large stones or
+boulders are scattered in a deep deposit of clay, and where much
+of the denudation is due to water scour. The large boulders not
+only act as shelter against rain, but they bind and consolidate
+by their mere weight the clay upon which they rest. Hence the
+materials underlying the boulders become more resistant, and as
+the surrounding clays are gradually washed away and carried to
+the streams, these compacted parts persist, and, finally, stand
+like walls or pillars above the general level. After a time the
+great boulders fall off and the underlying clay becomes worn by
+the rainwash to fantastic spikes and ridges. In the Val d'Hérens
+the earth pillars are formed
+
+33
+
+of the deep moraine stuff which thickly overlies the slopes of
+the valley. The wall of pillars runs across the axis of the
+valley, down the slope of the hill, and crosses the road, so that
+it has to be tunnelled to permit the passage of traffic. It is
+not improbable that some additional influence--possibly the
+presence of lime--has hardened the material forming the pillars,
+and tended to their preservation.
+
+Denudation has, however, other methods of work than purely
+mechanical; methods more noiseless and gentle, but not less
+effective, as the victories of peace ate no less than those of
+war.
+
+Over the immense tracts of the continents chemical work proceeds
+relentlessly. The rock in general, more especially the primary
+igneous rock, is not stable in presence of the atmosphere and of
+water. Some of the minerals, such as certain silicates and
+carbonates, dissolve relatively fast, others with extreme
+slowness. In the process of solution chemical actions are
+involved; oxidation in presence of the free oxygen of the
+atmosphere; attack by the feeble acid arising from the solution
+of carbon dioxide in water; or, again, by the activity of certain
+acids--humous acids--which originate in the decomposition of
+vegetable remains. These chemical agents may in some instances,
+_e.g._ in the case of carbonates such as limestone or
+dolomite--bring practically the whole rock into solution. In other
+instances--_e.g._ granites, basalts, etc.--they may remove some of
+the
+
+34
+
+constituent minerals completely or partially, such as felspar,
+olivine, augite, and leave more resistant substances to be
+ultimately washed down as fine sand or mud into the river.
+
+It is often difficult or impossible to appraise the relative
+efficiency of mechanical and chemical denudation in removing the
+materials from a certain area. There can be, indeed, little doubt
+that in mountainous regions the mechanical effects are largely
+predominant. The silts of glacial rivers are little different
+from freshly-powdered rock. The water which carries them but
+little different from the pure rain or snow which falls from the
+sky. There has not been time for the chemical or solvent actions
+to take place. Now while gravitational forces favour sudden shock
+and violent motions in the hills, the effect of these on solvent
+and chemical denudation is but small. Nor is good drainage
+favourable to chemical actions, for water is the primary factor
+in every case. Water takes up and removes soluble combinations of
+molecules, and penetrates beneath residual insoluble substances.
+It carries the oxygen and acids downwards through the soils, and
+finally conveys the results of its own work to the rivers and
+streams. The lower mean temperature of the mountains as well as
+the perfect drainage diminishes chemical activities.
+
+Hence we conclude that the heights are not generally favourable
+to the purely solvent and chemical actions. It is on the
+lower-lying land that soils tend to accumulate,
+
+35
+
+and in these the chief solvent and the chief chemical denudation
+of the Earth are effected.
+
+The solvent and chemical effects which go on in the
+finely-divided materials of the soils may be observed in the
+laboratory. They proceed faster than would be anticipated. The
+observation is made by passing a measured quantity of water
+backwards and forwards for some months through a tube containing
+a few grammes of powdered rock. Finally the water is analysed,
+and in this manner the amount of dissolved matter it has taken up
+is estimated. The rock powder is examined under the microscope in
+order to determine the size of the grains, and so to calculate
+the total surface exposed to the action of the water. We must be
+careful in such experiments to permit free oxidation by the
+atmosphere. Results obtained in this way of course take no
+account of the chemical effects of organic acids such as exist in
+the soils. The quantities obtained in the laboratory will,
+therefore, be deficient as compared with the natural results.
+
+In this manner it has been found that fresh basalt exposed to
+continually moving water will lose about 0.20 gramme per square
+metre of surface per year. The mineral orthoclase, which enters
+largely into the constitution of many granites, was found to lose
+under the same conditions 0.025 gramme. A glassy lava (obsidian)
+rich in silica and in the chemical constituents of an average
+granite, was more resistant still; losing but 0.013 gramme per
+square metre per year. Hornblende, a mineral
+
+36
+
+abundant in many rocks, lost 0.075 gramme. The mean of the
+results showed that 0.08 gramme was washed in a year from each
+square metre. Such results give us some indication of the rate at
+which the work of solution goes on in the finely divided
+soils.[1]
+
+It might be urged that, as the mechanical break up of rocks, and
+the production in this way of large surfaces, must be at the
+basis of solvent and chemical denudation, these latter activities
+should be predominant in the mountains. The answer to this is
+that the soils rarely owe their existence to mechanical actions.
+The alluvium of the valleys constitutes only narrow margins to
+the rivers; the finer _débris_ from the mountains is rapidly
+brought into the ocean. The soils which cover the greater part of
+continental areas have had a very different origin.
+
+In any quarry where a section of the soil and of the underlying
+rock is visible, we may study the mode of formation of soils. Our
+observations are, we will suppose, pursued in a granite quarry.
+We first note that the material of the soil nearest the surface
+is intermixed with the roots of grasses, trees, or shrubs.
+Examining a handful of this soil, we see glistening flakes of
+mica which plainly are derived from the original granite. Washing
+off the finer particles, we find the largest remaining grains are
+composed of the all but indestructible quartz.
+
+[1] Proc. Roy. Irish Acad., VIII., Ser. A, p. 21.
+
+37
+
+This also is from the granite. Some few of the grains are of
+chalky-looking felspar; again a granitic mineral. What is the
+finer silt we have washed off? It, too, is composed of mineral
+particles to a great extent; rock dust stained with iron oxide
+and intermixed with organic remains, both animal and vegetable.
+But if we make a chemical analysis of the finer silt we find that
+the composition is by no means that of the granite beneath. The
+chemist is able to say, from a study of his results, that there
+has been, in the first place, a large loss of material attending
+the conversion of the granite to the soil. He finds a
+concentration of certain of the more resistant substances of the
+granite arising from the loss of the less resistant. Thus the
+percentage amount of alumina is increased. The percentage of iron
+is also increased. But silica and most other substances show a
+diminished percentage. Notably lime has nearly disappeared. Soda
+is much reduced; so is magnesia. Potash is not so completely
+abstracted. Finally, owing to hydration, there is much more
+combined water in the soil than in the rock. This is a typical
+result for rocks of this kind.
+
+Deeper in the soil we often observe a change of texture. It has
+become finer, and at the same time the clay is paler in colour.
+This subsoil represents the finer particles carried by rain from
+above. The change of colour is due to the state of the iron which
+is less oxidised low down in the soil. Beneath the subsoil the
+soil grows
+
+38
+
+again coarser. Finally, we recognise in it fragments of granite
+which ever grow larger as we descend, till the soil has become
+replaced by the loose and shattered rock. Beneath this the only
+sign of weathering apparent in the rock is the rusty hue imparted
+by the oxidised iron which the percolating rain has leached from
+iron-bearing minerals.
+
+The soil we have examined has plainly been derived in situ from
+the underlying rock. It represents the more insoluble residue
+after water and acids have done their work. Each year there must
+be a very slow sinking of the surface, but the ablation is
+infinitesimal.
+
+The depth of such a soil may be considerable. The total surface
+exposed by the countless grains of which it is composed is
+enormous. In a cubic foot of average soil the surface area of the
+grains may be 50,000 square feet or more. Hence a soil only two
+feet deep may expose 100,000 square feet for each square foot of
+surface area.
+
+It is true that soils formed in this manner by atmospheric and
+organic actions take a very long time to grow. It must be
+remembered, however, that the process is throughout attended by
+the removal in solution: of chemically altered materials.
+
+Considerations such as the foregoing must convince us that while
+the accumulation of the detrital sediments around the continents
+is largely the result of activities progressing on the steeper
+slopes of the land, that is,
+
+39
+
+among the mountainous regions, the feeding of the salts to the
+ocean arises from the slower work of meteorological and organic
+agencies attacking the molecular constitution of the rocks;
+processes which best proceed where the drainage is sluggish and
+the quiescent conditions permit of the development of abundant
+organic growth and decay.
+
+Statistics of the solvent denudation of the continents support
+this view. Within recent years a very large amount of work has
+been expended on the chemical investigation of river waters of
+America and of Europe. F. W. Clarke has, at the expense of much
+labour, collected and compared these results. They are expressed
+as so many tonnes removed in solution per square mile per annum.
+For North America the result shows 79 tonnes so removed; for
+Europe 100 tonnes. Now there is a notable difference between the
+mean elevations of these two continents. North America has a mean
+elevation of 700 metres over sea level, whereas the mean
+elevation of Europe is but 300 metres. We see in these figures
+that the more mountainous land supplies less dissolved matter to
+the ocean than the land of lower elevation, as our study has led
+us to expect.
+
+We have now considered the source of the detrital sediments, as
+well as of the dissolved matter which has given to the ocean, in
+the course of geological time, its present gigantic load of
+salts. It is true there are further solvent and chemical effects
+exerted by the sea water
+
+40
+
+upon the sediments discharged into it; but we are justified in
+concluding that, relatively to the similar actions taking place
+in the soils, the solvent and chemical work of the ocean is
+small. The fact is, the deposited detrital sediments around the
+continents occupy an area small when contrasted with the vast
+stretches of the land. The area of deposition is much less than
+that of denudation; probably hardly as much as one twentieth.
+And, again, the conditions of aeration and circulation which
+largely promote chemical and solvent denudation in the soils are
+relatively limited and ineffective in the detrital oceanic
+deposits.
+
+The summation of the amounts of dissolved and detrital materials
+which denudation has brought into the ocean during the long
+denudative history of the Earth, as we might anticipate, reveals
+quantities of almost unrealisable greatness. The facts are among
+the most impressive which geological science has brought to
+light. Elsewhere in this volume they have been mentioned when
+discussing the age of the Earth. In the present connection,
+however, they are deserving of separate consideration.
+
+The basis of our reasoning is that the ocean owes its saltness
+mainly if not entirely to the denudative activities we have been
+considering. We must establish this.
+
+We may, in the first place, say that any other view at once
+raises the greatest difficulties. The chemical composition of the
+detrital sediments which are spread over
+
+41
+
+the continents and which build up the mountains, differs on the
+average very considerably from that of the igneous rocks. We know
+the former have been derived from the latter, and we know that
+the difference in the composition of the two classes of materials
+is due to the removal in solution of certain of the constituents
+of the igneous rocks. But the ocean alone can have received this
+dissolved matter. We know of no other place in which to look for
+it. It is true that some part of this dissolved matter has been
+again rejected by the ocean; thus the formation of limestone is
+largely due to the abstraction of lime from sea water by organic
+and other agencies. This, however, in no way relieves us of the
+necessity of tracing to the ocean the substances dissolved from
+the igneous rocks. It follows that we have here a very causa for
+the saltness of the ocean. The view that the ocean "was salt from
+the first" is without one known fact to support it, and leaves us
+with the burden of the entire dissolved salts of geological time
+to dispose of--Where and how?
+
+The argument we have outlined above becomes convincingly strong
+when examined more closely. For this purpose we first compare the
+average chemical composition of the sedimentary and the igneous
+rocks. The following table gives the percentages of the chief
+chemical constituents: [1]
+
+[1] F. W. Clarke: _A Preliminary Study of Chemical Denudation_,
+p. 13
+
+42
+
+                          Igneous.        Sedimentary.
+Silica (SiO2) -             59.99           58.51
+Alumina (Al2O3) -           15.04           13.07
+Ferric oxide (F2O3) -       2.59            3.40
+Ferrous oxide (FeO) -       3.34            2.00
+Magnesia (MgO) -            3.89            2.52
+Lime (CaO) -                 4.81            5.42
+Soda (Na2O) -                3.41            1.12
+Potash (K2O) -               2.95            2.80
+Water (H2O) -                1.92            4.28
+Carbon dioxide (CO2) -       --             4.93
+Minor constituents -        2.06            1.95
+                          100.00          100.00
+
+In the derivation of the sediments from the igneous rocks there
+is a loss by solution of about 33 per cent; _i.e._ 100 tons of
+igneous rock yields rather less than 70 tons of sedimentary rock.
+This involves a concentration in the sediments of the more
+insoluble constituents. To this rule the lime-content appears to
+be an exception. It is not so in reality. Its high value in the
+sediments is due to its restoration from the ocean to the land.
+The magnesia and potash are, also, largely restored from the
+ocean; the former in dolomites and magnesian limestones; the
+latter in glauconite sands. The iron of the sediments shows
+increased oxidation. The most notable difference in the two
+analyses appears, however, in the soda percentages. This falls
+from 3.41 in the igneous rock to 1.12 in the average sediment.
+Indeed, this
+
+43
+
+deficiency of soda in sedimentary rocks is so characteristic of
+secondary rocks that it may with some safety be applied to
+discriminate between the two classes of substances in cases where
+petrological distinctions of other kinds break down.
+
+To what is this so marked deficiency of soda to be ascribed? It
+is a result of the extreme solubility of the salts of sodium in
+water. This has not only rendered its deposition by evaporation a
+relatively rare and unimportant incident of geological history,
+but also has protected it from abstraction from the ocean by
+organic agencies. The element sodium has, in fact, accumulated in
+the ocean during the whole of geological time.
+
+We can use the facts associated with the accumulation of sodium
+salts in the ocean as a means of obtaining additional support to
+the view, that the processes of solvent denudation are
+responsible for the saltness of the ocean. The new evidence may
+be stated as follows: Estimates of the amounts of sedimentary
+rock on the continents have repeatedly been made. It is true that
+these estimates are no more than approximations. But they
+undoubtedly _are_ approximations, and as such may legitimately be
+used in our argument; more especially as final agreement tends to
+check and to support the several estimates which enter into
+them.
+
+The most recent and probable estimates of the sediments on the
+land assign an average thickness of one mile of
+
+44
+
+secondary rocks over the land area of the world. To this some
+increase must be made to allow for similar materials concealed in
+the ocean, principally around the continental margins. If we add
+10 per cent. and assign a specific gravity of 2.5 we get as the
+mass of the sediments 64 x 1016 tonnes. But as this is about 67
+per cent. of the parent igneous rock--_i.e._ the average igneous
+rock from which the sediments are derived--we conclude that the
+primary denuded rock amounted to a mass of about 95 x 1016
+tonnes.
+
+Now from the mean chemical composition of the secondary rocks we
+calculate that the mass of sediments as above determined contains
+0.72 x1016 tonnes of the sodium oxide, Na2O. If to this amount we
+add the quantity of sodium oxide which must have been given to
+the ocean in order to account for the sodium salts contained
+therein, we arrive at a total quantity of oxide of sodium which
+must be that possessed by the primary rock before denudation
+began its work upon it. The mass of the ocean being well
+ascertained, we easily calculate that the sodium in the ocean
+converted to sodium oxide amounts to 2.1 x 1016 tonnes. Hence
+between the estimated sediments and the waters of the ocean we
+can account for 2.82 x 1016 tonnes of soda. When now we put this
+quantity back into the estimated mass of primary rock we find
+that it assigns to the primary rock a soda percentage of 3.0. On
+the average analysis given above this should be 3.41 per cent.
+The agreement,
+
+45
+
+all things considered, more especially the uncertainty in the
+estimate of the sediments, is plainly in support of the view that
+oceanic salts are derived from the rocks; if, indeed, it does not
+render it a certainty.
+
+A leading and fundamental inference in the denudative history of
+the Earth thus finds support: indeed, we may say, verification.
+In the light of this fact the whole work of denudation stands
+revealed. That the ocean began its history as a vast fresh-water
+envelope of the Globe is a view which accords with the evidence
+for the primitive high temperature of the Earth. Geological
+history opened with the condensation of an atmosphere of immense
+extent, which, after long fluctuations between the states of
+steam and water, finally settled upon the surface, almost free of
+matter in solution: an ocean of distilled water. The epoch of
+denudation then began. It will, probably, continue till the
+waters, undergoing further loss of thermal energy, suffer yet
+another change of state, when their circulation will cease and
+their attack upon the rocks come to an end.
+
+From what has been reviewed above it is evident that the sodium
+in the ocean is an index of the total activity of denudation
+integrated over geological time. From this the broad facts of the
+results of denudation admit of determination with considerable
+accuracy. We can estimate the amount of rock which has been
+degraded by solvent and chemical actions, and the amount of
+sediments which has been derived from it. We are,
+
+46
+
+thus, able to amend our estimate of the sediments which, as
+determined by direct observation, served to support the basis of
+our argument.
+
+We now go straight to the ocean for the amount of sodium of
+denudative origin. There may, indeed, have been some primitive
+sodium dissolved by a more rapid denudation while the Earth's
+surface was still falling in temperature. It can be shown,
+however, that this amount was relatively small. Neglecting it we
+may say with safety that the quantity of sodium carried into the
+ocean by the rivers must be between 14,000 and 15,000 million
+million tonnes: _i.e._ 14,500 x 1012 tonnes, say.
+
+Keeping the figures to round numbers we find that this amount of
+sodium involves the denudation of about 80 x 1016 tonnes of
+average igneous rock to 53 x 1016 tonnes of average sediment.
+From these vast quantities we know that the parent rock denuded
+during geological time amounted to some 300 million cubic
+kilometres or about seventy million cubic miles. The sediments
+derived therefrom possessed a bulk of 220 million cubic
+kilometres or fifty million cubic miles. The area of the land
+surface of the Globe is 144 million square kilometres. The parent
+rock would have covered this to a uniform depth of rather more
+than two kilometres, and the derived sediment to more than 1.5
+kilometres, or about one mile deep.
+
+The slow accomplishment of results so vast conveys some idea of
+the great duration of geological time.
+
+47
+
+The foregoing method of investigating the statistics of solvent
+denudation is capable of affording information not only as to the
+amount of sediments upon the land, but also as to the quantity
+which is spread over the floor of the ocean.
+
+We see this when we follow the fate of the 33 per cent. of
+dissolved salts which has been leached from the parent igneous
+rock, and the mass of which we calculate from the ascertained
+mass of the latter, to be 27 x 1016 tonnes. This quantity was at
+one time or another all in the ocean. But, as we saw above, a
+certain part of it has been again abstracted from solution,
+chiefly by organic agencies. Now the abstracted solids have not
+been altogether retained beneath the ocean. Movements of the land
+during geological time have resulted in some portion being
+uplifted along with other sediments. These substances constitute,
+mainly, the limestones.
+
+We see, then, that the 27 x 1016 tonnes of substances leached
+from the parent igneous rocks have had a threefold destination.
+One part is still in solution; a second part has been
+precipitated to the bottom of the ocean; a third part exists on
+the land in the form of calcareous rocks.
+
+Observation on the land sediments shows that the calcareous rocks
+amount to about 5 per cent. of the whole. From this we find that
+3 x 1016 tonnes, approximately, of such rocks have been taken
+from the ocean. This accounts for one of the three classes of
+material
+
+48
+
+into which the original dissolved matter has been divided.
+Another of the three quantities is easily estimated: the amount
+of matter still in solution in the ocean. The volume of the ocean
+is 1,414 million cubic kilometres and its mass is 145 x 1016
+tonnes. The dissolved salts in it constitute 3.4 per cent. of its
+mass; or, rather more than 5 x 1016 tonnes. The limestones on the
+land and the salts in the sea water together make up about 8 x
+1016 tonnes. If we, now, deduct this from the total of 27 x 1016
+tonnes, we find that about 19 x 1016 tonnes must exist as
+precipitated matter on the floor of the ocean.
+
+The area of the ocean is 367 x 1012 square metres, so that if the
+precipitated sediment possesses an average specific gravity of
+2.5, it would cover the entire floor to a uniform depth of 218
+metres; that is 715 feet. This assumes that there was uniform
+deposition of the abstracted matter over the floor of the ocean.
+Of course, this assumption is not justifiable. It is certain that
+the rate of deposition on the floor of the sea has varied
+enormously with various conditions--principally with the depth.
+Again, it must be remembered that this estimate takes no account
+of solid materials otherwise brought into the oceanic deposits;
+_e.g._, by wind-transported dust from the land or volcanic
+ejectamenta in the ocean depths. It is not probable, however,
+that any considerable addition to the estimated mean depth of
+deposit from such sources would be allowable.
+
+49
+
+The greatness of the quantities involved in these determinations
+is almost awe inspiring. Take the case of the dissolved salts in
+the ocean. They are but a fraction, as we have seen, of the total
+results of solvent denudation and represent the integration of
+the minute traces contributed by the river water. Yet the common
+salt (chloride of sodium) alone, contained in the ocean, would,
+if abstracted and spread over the dry land as a layer of rock
+salt having a specific gravity of 2.2, cover the whole to a depth
+of 107 metres or 354 feet. The total salts in solution in the
+ocean similarly spread over the land would increase the depth of
+the layer to 460 feet. After considering what this means we have
+to remember that this amount of matter now in solution in the
+seas is, in point of fact, less than a fifth part of the total
+dissolved from the rocks during geological time.
+
+The transport by denudation of detrital and dissolved matter from
+the land to the ocean has had a most important influence on the
+events of geological history. The existing surface features of
+the earth must have been largely conditioned by the dynamical
+effects arising therefrom. In dealing with the subject of
+mountain genesis we will, elsewhere, see that all the great
+mountain ranges have originated in the accumulation of the
+detrital sediments near the shore in areas which, in consequence
+of the load, gradually became depressed and developed into
+synclines of many thousands of feet in depth. The most impressive
+surface features of the Globe originated
+
+50
+
+in this manner. We will see too that these events were of a
+rhythmic character; the upraising of the mountains involving
+intensified mechanical denudation over the elevated area and in
+this way an accelerated transport of detritus to the sea; the
+formation of fresh deposits; renewed synclinal sinking of the sea
+floor, and, finally, the upheaval of a younger mountain range.
+This extraordinary sequence of events has been determined by the
+events of detrital denudation acting along with certain general
+conditions which have all along involved the growth of
+compressive stresses in the surface crust of the Earth.
+
+The effects of purely solvent denudation are less easily traced,
+but, very probably, they have been of not less importance. I
+refer here to the transport from the land to the sea of matter in
+solution.
+
+Solvent denudation, as observed above, takes place mainly in the
+soils and in this way over the more level continental areas. It
+has resulted in the removal from the land and transfer to the
+ocean of an amount of matter which represents a uniform layer of
+one half a kilometre; that is of more than 1,600 feet of rock.
+The continents have, during geological time, been lightened to
+this extent. On the other hand all this matter has for the
+greater part escaped the geosynclines and become uniformly
+diffused throughout the ocean or precipitated over its floor
+principally on the continental slopes before the great depths are
+reached. Of this material the ocean
+
+51
+
+waters contain in solution an amount sufficient to increase their
+specific gravity by 2.7 per cent.
+
+Taking the last point first, it is interesting to note the
+effects upon the bulk of the ocean which has resulted from the
+matter dissolved in it. From the known density of average sea
+water we find that 100 ccs. of it weigh just 102.7 grammes. Of
+this 3.5 per cent. by weight are solids in solution. That is to
+say, 3.594 grammes. Hence the weight of water present is 99.1
+grammes, or a volume of 99.1 ccs. From this we see that the salts
+present have increased the volume by 0.9 ccs. or 0.9 per cent.
+
+The average depth of the ocean is 2,000 fathoms or 3,700 metres.
+The increase of depth due to salts dissolved in the ocean has
+been, therefore, 108 feet or 33.24 metres. This result assumes
+that there has been no increased elastic compression due to the
+increased pressure, and no change of compressional elastic
+properties. We may be sure that the rise on the shore line of the
+land has not been less than 100 feet.
+
+We see then that as the result of solvent denudation we have to
+do with a heavier and a deeper ocean, expanded in volume by
+nearly one per cent. and the floor of which has become raised, on
+an average, about 700 feet by precipitated sediment.
+
+One of the first conceptions, which the student of geology has to
+dismiss from his mind, is that of the immobility or rigidity of
+the Earth's crust. The lane, we live on sways even to the gentle
+rise and fail of ocean tides
+
+52
+
+around the coasts. It suffers its own tidal oscillations due to
+the moon's attractions. Large tracts of semi-liquid matter
+underlie it. There is every evidence that the raised features of
+the Globe are sustained by such pressures acting over other and
+adjacent areas as serve to keep them in equilibrium against the
+force of gravity. This state of equilibrium, which was first
+recognised by Pratt, as part of the dynamics of the Earth's
+crust, has been named isostasy. The state of the crust is that of
+"mobile equilibrium."
+
+The transfer of matter from the exposed land surfaces to the
+sub-oceanic slopes of the continents and the increase in the
+density of the ocean, must all along have been attended by
+isostatic readjustment. We cannot take any other view. On the one
+hand the land was being lightened; on the other the sea was
+increasing in mass and depth and the flanks of the continents
+were being loaded with the matter removed from the land and borne
+in solution to the ocean. How important the resulting movements
+must have been may be gathered from the fact that the existing
+land of the Globe stands at a mean elevation of no more than
+2,000 feet above sea level. We have seen that solvent denudation
+removed over 1,600 feet of rock. But we have no evidence that on
+the whole the elevation of land in the past was ever very
+different from what it now is.
+
+We have, then, presented to our view the remarkable fact that
+throughout the past, and acting with extreme
+
+53
+
+slowness, the land has steadily been melted down into the sea and
+as steadily been upraised from the waters. It is possible that
+the increased bulk of the ocean has led to a certain diminution
+of the exposed land area. The point is a difficult one. One thing
+we may without much risk assume. The sub-aereal current of
+dissolved matter from the land to the ocean was accompanied by a
+sub-crustal flux from the ocean areas to the land areas; the
+heated viscous materials creeping from depths far beneath the
+ocean floor to depths beneath the roots of the mountains which
+arose around the oceans. Such movements took ages for their
+accomplishment. Indeed, they have been, probably, continuous all
+along and are still proceeding. A low degree of viscosity will
+suffice to permit of movements so slow. Superimposed upon these
+movements the rhythmic alternations of depression and elevation
+of the geosynclines probably resulted in releasing the crust from
+local accumulation of strains arising in the more rigid surface
+materials. The whole sequence of movements presents an
+extraordinary picture of pseudo-vitality--reminding us of the
+circulatory and respiratory systems of a vast organism.
+
+All great results in our universe are founded in motions and
+forces the most minute. In contemplating the Cause or the Effect
+we stand equally impressed with the spectacle presented to us. We
+shall now turn from the great effects of denudation upon the
+history and evolution of a world and consider for a moment
+activities
+
+54
+
+so minute in detail that their operations will probably for ever
+elude our bodily senses, but which nevertheless have necessarily
+affected and modified the great results we have been
+considering.
+
+The ocean a little way from the land is generally so free from
+suspended sediments that it has a blackness as of ink. This
+blackness is due to its absolute freedom from particles
+reflecting the sun's light. The beautiful blue of the Swiss and
+Italian lakes is due to the presence of very fine particles
+carried into them by the rivers; the finest flour of the
+glaciers, which remain almost indefinitely suspended in the
+water. But in the ocean it is only in those places where rapid
+currents running over shallows stir continually the sediments or
+where the fresh water of a great river is carried far from the
+land, that the presence of silt is to be observed. The beautiful
+phenomenon of the coal-black sea is familiar to every yachtsman
+who has sailed to the west of our Islands.[1]
+
+There is, in fact, a very remarkable difference in the manner of
+settlement of fine sediments in salt and in fresh water. We are
+here brought into contact with one of those subtle yet
+influential natural actions the explanation of which involves
+scientific advance along many apparently unconnected lines of
+investigation.
+
+[1] See Tyndall's Voyage to Algeria in _Fragments of Science._ The
+cause of the blue colour of the lakes has been discussed by
+various observers, not always with agreement.
+
+55
+
+It is easy to observe in the laboratory the fact of the different
+behaviour of salt and fresh water towards finely divided
+substances. The nature of the insoluble substance is not
+important.
+
+We place, in a good light, two glass vessels of equal dimensions;
+the one filled with sea water, the other with fresh water. Into
+each we stir the same weight of very finely powdered slate: just
+so much as will produce a cloudiness. In a few hours we find the
+sea water limpid. The fresh water is still cloudy, however; and,
+indeed, may be hardly different in appearance from what it was at
+starting. In itself this is a most extraordinary experiment. We
+would have anticipated quite the opposite result owing to the
+greater density of the sea water.
+
+But a still more interesting experiment remains to be carried
+out. In the sea water we have many different salts in solution.
+Let us see if these salts are equally responsible for the result
+we have obtained. For this purpose we measure out quantities of
+sodium chloride and magnesium chloride in the proportion in which
+they exist in sea water: that is about as seven to one. We add
+such an equal amount of water to each as represents the dilution
+of these salts in sea water. Then finally we stir a little of the
+finely powdered slate into each. It will be found that the
+magnesium chloride, although so much more dilute than the sodium
+chloride, is considerably more active in clearing out the
+suspension. We may now try such marine salts as magnesium
+sulphate,
+
+56
+
+or calcium sulphate against sodium chloride; keeping the marine
+proportions. Again we find that the magnesium and calcium salts
+are the most effective, although so much more dilute than the
+sodium salt.
+
+There is no visible clue to the explanation of these results. But
+we must conclude as most probable that some action is at work in
+the sea water and in the salt solutions which clumps or
+flocculates the sediment. For only by the gathering of the
+particles together in little aggregates can we explain their
+rapid fall to the bottom. It is not a question of viscosity
+(_i.e._ of resistance to the motion of the particles), for the
+salt solutions are rather more viscous than the fresh water.
+Still more remarkable is the fact that every dissolved substance
+will not bring about the result. Thus if we dissolve sugar in
+water we find that, if anything, the silt settles more slowly in
+the sugar solution than in fresh water.
+
+Now there is one effect produced by the solution of such salts as
+we have dealt with which is not produced by such bodies as sugar.
+The water is rendered a conductor of electricity. Long ago
+Faraday explained this as due to the presence of free atoms of
+the dissolved salt in the solution, carrying electric charges. We
+now speak of the salt as "ionised." That is it is partly split up
+into ions or free electrified atoms of chlorine, sodium,
+magnesium, etc., according to the particular salt in solution.
+This fact leads us to think that these electrified
+
+57
+
+atoms moving about in the solution may be the cause of the
+clumping or flocculation. Such electrified atoms are absent from
+the sugar solution: sugar does not become "ionised" when it is
+dissolved.
+
+The suspicion that the free electrified atoms play a part in the
+phenomenon is strengthened when we recall the remarkable
+difference in the action of sodium chloride and magnesium
+chloride. In each of the solutions of these substances there are
+free chlorine atoms each of which carries a single charge of
+negative electricity. As these atoms are alike in both solutions
+the different behaviour of the solutions cannot be due to the
+chlorine. But the metallic atom is very different in the two
+cases. The ionised sodium atom is known to be _monad_ or carries
+but _one_ positive charge; whereas the magnesium atom is _diad_ and
+carries _two_ positive charges. If, then, we assume that the
+metallic, positively electrified atom is in each case
+responsible, we have something to go on. It may be now stated
+that it has been found by experiment and supported by theory that
+the clumping power of an ion rises very rapidly with its valency;
+that is with the number of unit charges associated with it. Thus
+diads such as magnesium, calcium, barium, etc., are very much
+more efficient than monads such as sodium, potassium, etc., and
+again, triads such as aluminium are, similarly, very much more
+powerful than diad atoms. Here, in short, we have arrived at the
+active cause of the phenomenon. Its inner mechanism
+
+58
+
+is, however, harder to fathom. A plausible explanation can be
+offered, but a study of it would take us too far. Sufficient has
+been said to show the very subtile nature of the forces at work.
+
+We have here an effect due to the sea salts derived by denudation
+from the land which has been slowly augmenting during geological
+time. It is certain that the ocean was practically fresh water in
+remote ages. During those times the silt from the great rivers
+would have been carried very far from the land. A Mississippi of
+those ages would have sent its finer suspensions far abroad on a
+contemporary Gulf stream: not improbably right across the
+Atlantic. The earlier sediments of argillaceous type were not
+collected in the geosynclines and the genesis of the mountains
+was delayed proportionately. But it was, probably, not for very
+long that such conditions prevailed. For the accumulation of
+calcium salts must have been rapid, and although the great
+salinity due to sodium salts was of slow growth the salts of the
+diad element calcium must have soon introduced the cooperation of
+the ion in the work of building the mountain.
+
+59
+
+THE ABUNDANCE OF LIFE [1]
+
+WE had reached the Pass of Tre Croci[2]and from a point a little
+below the summit, looked eastward over the glorious Val Buona.
+The pines which clothed the floor and lower slopes of the valley,
+extended their multitudes into the furthest distance, among the
+many recesses of the mountains, and into the confluent Val di
+Misurina. In the sunshine the Alpine butterflies flitted from
+stone to stone. The ground at our feet and everywhere throughout
+the forests teamed with the countless millions of the small black
+ants.
+
+It was a magnificent display of vitality; of the aggressiveness
+of vitality, assailing the barren heights of the limestone,
+wringing a subsistence from dead things. And the question
+suggested itself with new force: why the abundance of life and
+its unending activity?
+
+In trying to answer this question, the present sketch
+originated.
+
+I propose to refer for an answer to dynamic considerations. It is
+apparent that natural selection can only be concerned in a
+secondary way. Natural selection defines
+
+[1] Proc. Roy. Dublin Soc., vol. vii., 1890.
+
+[2] In the Dolomites of Southeast Tyrol; during the summer of
+1890. Much of what follows was evolved in discussion with my
+fellow-traveller, Henry H. Dixon. Much of it is his.
+
+60
+
+a certain course of development for the organism; but very
+evidently some property of inherent progressiveness in the
+organism must be involved. The mineral is not affected by natural
+selection to enter on a course of continual variation and
+multiplication. The dynamic relations of the organism with the
+environment are evidently very different from those of inanimate
+nature.
+
+GENERAL DYNAMIC CONDITIONS ATTENDING INANIMATE ACTIONS
+
+It is necessary, in the first place, to refer briefly to the
+phenomena attending the transfer of energy within and into
+inanimate material systems. It is not assumed here that these
+phenomena are restricted in their sphere of action to inanimate
+nature. It is, in fact, very certain that they are not; but while
+they confer on dead nature its own dynamic tendencies, it will
+appear that their effects are by various means evaded in living
+nature. We, therefore, treat of them as characteristic of
+inanimate actions. We accept as fundamental to all the
+considerations which follow the truth of the principle of the
+Conservation of Energy.[1]
+
+[1] "The principle of the Conservation of Energy has acquired so
+much scientific weight during the last twenty years that no
+physiologist would feel any confidence in an experiment which
+showed a considerable difference between the work done by the
+animal and the balance of the account of Energy received and
+spent."--Clerk Maxwell, _Nature_, vol. xix., p. 142. See also
+Helmholtz _On the Conservation of Force._
+
+61
+
+Whatever speculations may be made as to the course of events very
+distant from us in space, it appears certain that dissipation of
+energy is at present actively progressing throughout our sphere
+of observation in inanimate nature. It follows, in fact, from the
+second law of thermodynamics, that whenever work is derived from
+heat, a certain quantity of heat falls in potential without doing
+work or, in short, is dissipated. On the other hand, work may be
+entirely converted into heat. The result is the heat-tendency of
+the universe. Heat, being an undirected form of energy, seeks, as
+it were, its own level, so that the result of this heat-tendency
+is continual approach to uniformity of potential.
+
+The heat-tendency of the universe is also revealed in the
+far-reaching "law of maximum work," which defines that chemical
+change, accomplished without the intervention of external energy,
+tends to the production of the body, or system of bodies, which
+disengage the greatest quantity of heat.[1] And, again, vast
+numbers of actions going on throughout nature are attended by
+dissipatory thermal effects, as those arising from the motions of
+proximate molecules (friction, viscosity), and from the fall of
+electrical potential.
+
+Thus, on all sides, the energy which was once most probably
+existent in the form of gravitational potential, is being
+dissipated into unavailable forms. We must
+
+[1] Berthelot, _Essai de Mécanique Chimique._
+
+62
+
+recognize dissipation as an inevitable attendant on inanimate
+transfer of energy.
+
+But when we come to consider inanimate actions in relation to
+time, or time-rate of change, we find a new feature in the
+phenomena attending transfer of energy; a feature which is really
+involved in general statements as to the laws of physical
+interactions.[1] It is seen, that the attitude of inanimate
+material systems is very generally, if not in all cases,
+retardative of change--opposing it by effects generated by the
+primary action, which may be called "secondary" for convenience.
+Further, it will be seen that these secondary effects are those
+concerned in bringing about the inevitable dissipation.
+
+As example, let us endeavour to transfer gravitational potential
+energy contained in a mass raised above the surface of the Earth
+into an elastic body, which we can put into compression by
+resting the weight upon it. In this way work is done against
+elastic force and stored as elastic potential energy. We may deal
+with a metal spring, or with a mass of gas contained in a
+cylinder fitted with a piston upon which the weight may be
+placed. In either case we find the effect of compression is to
+raise the temperature of the substance, thus causing its
+
+[1] Helmholtz, _Ice and Glaciers._ Atkinson's collection of his
+Popular Lectures. First Series, p.120. Quoted by Tate, _Heat_,
+p. 311.
+
+63
+
+expansion or increased resistance to the descent of the weight.
+And this resistance continues, with diminishing intensity, till
+all the heat generated is dissipated into the surrounding medium.
+The secondary effect thus delays the final transfer of energy.
+
+Again, if we suppose the gas in the cylinder replaced by a vapour
+in a state of saturation, the effect of increased pressure, as of
+a weight placed upon the piston, is to reduce the vapour to a
+liquid, thereby bringing about a great diminution of volume and
+proportional loss of gravitational potential by the weight. But
+this change will by no means be brought about instantaneously.
+When a little of the vapour is condensed, this portion parts with
+latent heat of vaporisation, increasing the tension of the
+remainder, or raising its point of saturation, so that before the
+weight descends any further, this heat has to escape from the
+cylinder.
+
+Many more such cases might be cited. The heating of india-rubber
+when expanded, its cooling when compressed, is a remarkable one;
+for at first sight it appears as if this must render it
+exceptional to the general law, most substances exhibiting the
+opposite thermal effects when stressed. However, here, too, the
+action of the stress is opposed by the secondary effects
+developed in the substance; for it is found that this substance
+contracts when heated, expands when cooled. Again, ice being a
+substance which contracts in melting, the effect of pressure is
+to facilitate melting, lowering its freezing point. But
+
+64
+
+so soon as a little melting occurs, the resulting liquid calls on
+the residual ice for an amount of heat equivalent to the latent
+heat of liquefaction, and so by cooling the whole, retards the
+change.
+
+Such particular cases illustrate a principle controlling the
+interaction of matter and energy which seems universal in
+application save when evaded, as we shall see, by the ingenuity
+of life. This principle is not only revealed in the researches of
+the laboratory; it is manifest in the history of worlds and solar
+systems. Thus, consider the effects arising from the aggregation
+of matter in space under the influence of the mutual attraction
+of the particles. The tendency here is loss of gravitational
+potential. The final approach is however retarded by the
+temperature, or vis viva of the parts attending collision and
+compression. From this cause the great suns of space radiate for
+ages before the final loss of potential is attained.
+
+Clerk Maxwell[1] observes on the general principle that less
+force is required to produce a change in a body when the change
+is unopposed by constraints than when it is subjected to such.
+From this if we assume the external forces acting upon a system
+not to rise above a certain potential (which is the order of
+nature), the constraints of secondary actions may, under certain
+circumstances, lead to final rejection of some of the energy, or,
+in any
+
+[1] _Theory of Heat_, p. 131.
+
+65
+
+case, to retardation of change in the system--dissipation of
+energy being the result.[1]
+
+As such constraints seem inherently present in the properties of
+matter, we may summarise as follows:
+
+_The transfer of energy into any inanimate material system is
+attended by effects retardative to the transfer and conducive to
+dissipation._
+
+Was this the only possible dynamic order ruling in material
+systems it is quite certain the myriads of ants and pines never
+could have been, except all generated by creative act at vast
+primary expenditure of energy. Growth and reproduction would have
+been impossible in systems which retarded change at every step
+and never proceeded in any direction but in that of dissipation.
+Once created, indeed, it is conceivable that, as heat engines,
+they might have dragged out an existence of alternate life and
+death; life in the hours of sunshine, death in hours of darkness:
+no final death, however, their lot, till their parts were simply
+worn out by long use, never made good by repair. But the
+sustained and increasing activity of organized nature is a fact;
+therefore some other order of events must be possible.
+
+[1] The law of Least Action, which has been applied, not alone in
+optics, but in many mechanical systems, appears physically based
+upon the restraint and retardation opposing the transfer of
+energy in material systems.
+
+66
+
+GENERAL DYNAMIC CONDITIONS ATTENDING ANIMATE ACTIONS
+
+What is the actual dynamic attitude of the primary organic
+engine--the vegetable organism? We consider, here, in the first
+place, not intervening, but resulting phenomena.
+
+The young leaf exposed to solar radiation is small at first, and
+the quantity of radiant energy it receives in unit of time cannot
+exceed that which falls upon its surface. But what is the effect
+of this energy? Not to produce a retardative reaction, but an
+accelerative response: for, in the enlarging of the leaf by
+growth, the plant opens for itself new channels of supply.
+
+If we refer to "the living protoplasm which, with its unknown
+molecular arrangement, is the only absolute test of the cell and
+of the organism in general,[1] we find a similar attitude towards
+external sources of available energy. In the act of growth
+increased rate of assimilation is involved, so that there is an
+acceleration of change till a bulk of maximum activity is
+attained. The surface, finally, becomes too small for the
+absorption of energy adequate to sustain further increase of mass
+(Spencer[2]), and the acceleration ceases. The waste going on in
+the central parts is then just balanced by the renewal at the
+surface. By division, by spreading of the mass, by
+
+[1] Claus, _Zoology_, p. 13.
+
+[2] Geddes and Thomson, _The Evolution of Sex_, p. 220.
+
+67
+
+out-flowing processes, the normal activity of growth may be
+restored. Till this moment nothing would be gained by any of
+these changes. One or other of them is now conducive to
+progressive absorption of energy by the organism, and one or
+other occurs, most generally the best of them, subdivision. Two
+units now exist; the total mass immediately on division is
+unaltered, but paths for the more abundant absorption of energy
+are laid open.
+
+The encystment of the protoplasm (occurring under conditions upon
+which naturalists do not seem agreed[1]) is to all appearance
+protective from an unfavourable environment, but it is often a
+period of internal change as well, resulting in a segregation
+within the mass of numerous small units, followed by a breakup of
+the whole into these units. It is thus an extension of the basis
+of supply, and in an impoverished medium, where unit of surface
+is less active, is evidently the best means of preserving a
+condition of progress.
+
+Thus, in the organism which forms the basis of all modes of life,
+a definite law of action is obeyed under various circumstances of
+reaction with the available energy of its environment.
+
+Similarly, in the case of the more complex leaf, we see, not only
+in the phenomenon of growth, but in its extension in a flattened
+form, and in the orientation of greatest surface towards the
+source of energy, an attitude towards
+
+[1] However, "In no way comparable with death." Weismann,
+_Biological Memoirs_, p. 158.
+
+68
+
+available energy causative of accelerated transfer. There is
+seemingly a principle at work, leading to the increase of organic
+activity.
+
+Many other examples might be adduced. The gastrula stage in the
+development of embryos, where by invagination such an arrangement
+of the multiplying cells is secured as to offer the greatest
+possible surface consistent with a first division of labour; the
+provision of cilia for drawing upon the energy supplies of the
+medium; and more generally the specialisation of organs in the
+higher developments of life, may alike be regarded as efforts of
+the organism directed to the absorption of energy. When any
+particular organ becomes unavailing in the obtainment of
+supplies, the organ in the course of time becomes aborted or
+disappears.[1] On the other hand, when a too ready and liberal
+supply renders exertion and specialisation unnecessary, a similar
+abortion of functionless organs takes place. This is seen in the
+degraded members of certain parasites.
+
+During certain epochs of geological history, the vegetable world
+developed enormously; in response probably to liberal supplies of
+carbon dioxide. A structural adaptation to the rich atmosphere
+occurred, such as was calculated to cooperate in rapidly
+consuming the supplies, and to this obedience to a law of
+progressive transfer of energy we owe the vast stores of energy
+now accumulated
+
+[1] Claus, _Zoology_, p. 157
+
+69
+
+in our coal fields. And when, further, we reflect that this store
+of energy had long since been dissipated into space but for the
+intervention of the organism, we see definitely another factor in
+organic transfer of energy--a factor acting conservatively of
+energy, or antagonistically to dissipation.
+
+The tendency of organized nature in the presence of unlimited
+supplies is to "run riot." This seems so universal a relation,
+that we are safe in seeing here cause and effect, and in drawing
+our conclusions as to the attitude of the organism towards
+available energy. New species, when they come on the field of
+geological history, armed with fresh adaptations, irresistible
+till the slow defences of the subjected organisms are completed,
+attain enormous sizes under the stimulus of abundant supply, till
+finally, the environment, living and dead, reacts upon them with
+restraining influence. The exuberance of the organism in presence
+of energy is often so abundant as to lead by deprivation to its
+self-destruction. Thus the growth of bacteria is often controlled
+by their own waste products. A moment's consideration shows that
+such progressive activity denotes an accelerative attitude on the
+part of the organism towards the transfer of energy into the
+organic material system. Finally, we are conscious in ourselves
+how, by use, our faculties are developed; and it is apparent that
+all such progressive developments must rest on actions which
+respond to supplies with fresh demands. Possibly in the present
+and ever-
+
+70
+
+increasing consumption of inanimate power by civilised races, we
+see revealed the dynamic attitude of the organism working through
+thought-processes.
+
+Whether this be so or not, we find generally in organised nature
+causes at work which in some way lead to a progressive transfer
+of energy into the organic system. And we notice, too, that all
+is not spent, but both immediately in the growth of the
+individual, and ultimately in the multiplication of the species,
+there are actions associated with vitality which retard the
+dissipation of energy. We proceed to state the dynamical
+principles involved in these manifestations, which appear
+characteristic of the organism, as follows:--
+
+_The transfer of energy into any animate material system is
+attended by effects conducive to the transfer, and retardative of
+dissipation._
+
+This statement is, I think, perfectly general. It has been in
+part advanced before, but from the organic more than the physical
+point of view. Thus, "hunger is an essential characteristic of
+living matter"; and again, "hunger is a dominant characteristic
+of living matter,"[1] are, in part, expressions of the statement.
+If it be objected against the generality of the statement, that
+there are periods in the life of individuals when stagnation and
+decay make their appearance, we may answer, that
+
+[1] _Evolution of Sex._ Geddes and Thomson, chap. xvi. See also a
+reference to Cope's theory of "Growth Force," in Wallace's
+_Darwinism_, p. 425.
+
+71
+
+such phenomena arise in phases of life developed under conditions
+of external constraint, as will be urged more fully further on,
+and that in fact the special conditions of old age do not and
+cannot express the true law and tendency of the dynamic relations
+of life in the face of its evident advance upon the Earth. The
+law of the unconstrained cell is growth on an ever increasing
+scale; and although we assume the organic configuration, whether
+somatic or reproductive, to be essentially unstable, so that
+continual inflow of energy is required merely to keep it in
+existence, this does not vitiate the fact that, when free of all
+external constraint, growth gains on waste. Indeed, even in the
+case of old age, the statement remains essentially true, for the
+phenomena then displayed point to a breakdown of the functioning
+power of the cell, an approximation to configurations incapable
+of assimilation. It is not as if life showed in these phenomena
+that its conditions could obtain in the midst of abundance, and
+yet its law be suspended; but as if they represented a
+degradation of the very conditions of life, a break up, under the
+laws of the inanimate, of the animate contrivance; so that energy
+is no longer available to it, or the primary condition, "the
+transfer of energy into the animate system," is imperfectly
+obeyed. It is to the perfect contrivance of life our statement
+refers.
+
+That the final end of all will be general non-availability there
+seems little reason to doubt, and the organism, itself dependent
+upon differences of potential, cannot
+
+72
+
+hope to carry on aggregation of energy beyond the period when
+differences of potential are not. The organism is not accountable
+for this. It is being affected by events external to it, by the
+actions going on through inanimate agents. And although there be
+only a part of the received energy preserved, there is a part
+preserved, and this amount is continually on the increase. To see
+this it is only necessary to reflect that the sum of animate
+energy--capability of doing work in any way through animate
+means--at present upon the Earth, is the result, although a small
+one, of energy reaching the Earth since a remote period, and
+which otherwise had been dissipated in space. In inanimate
+actions throughout nature, as we know it, the availability is
+continually diminishing. The change is all the one way. As,
+however, the supply of available energy in the universe is
+(probably) limited in amount, we must look upon the two as simply
+effecting the final dissipation of potential in very different
+ways. The animate system is aggressive on the energy available to
+it, spends with economy, and invests at interest till death
+finally deprives it of all. It has heirs, indeed, who inherit
+some of its gains, but they, too, must die, and ultimately there
+will be no successors, and the greater part must melt away as if
+it had never been. The inanimate system responds to the forces
+imposed upon it by sluggish changes; of that which is thrust upon
+it, it squanders uselessly. The path of the energy is very
+different in the two cases.
+
+73
+
+While it is true generally that both systems ultimately result in
+the dissipation of energy to uniform potential, the organism can,
+as we have seen, under particular circumstances evade the final
+doom altogether. It can lay up a store of potential energy which
+may be permanent. Thus, so long as there is free oxygen in the
+universe, our coalfields might, at any time in the remote future,
+generate light and heat in the universal grave.
+
+It is necessary to observe on the fundamental distinction between
+the growth of the protoplasm and the growth of the crystal. It is
+common to draw comparison between the two, and to point to
+metabolism as the chief distinction. But while this is the most
+obvious distinction the more fundamental one remains in the
+energy relations of the two with the environment.[1] The growth
+of the crystal is the result of loss of energy; that of the
+organism the result of gain of energy. The crystal represents a
+last position of stable equilibrium assumed by molecules upon a
+certain loss of kinetic energy, and the formation of the crystal
+by evaporation and concentration of a liquid does not, in its
+dynamic aspect, differ much from the precipitation of an
+amorphous sediment. The organism, on the other hand, represents a
+more or less unstable condition formed and maintained by inflow
+of energy; its formation, indeed, often attended with a loss of
+kinetic energy (fixation of carbon in plants), but, if so,
+accompanied by
+
+[1] It appears exceptional for the crystal line configuration to
+stand higher in the scale of energy than the amorphous.
+
+74
+
+a more than compensatory increase of potential molecular energy.
+
+Thus, between growth in the living world and growth in the dead
+world, the energy relations with the environment reveal a marked
+contrast. Again, in the phenomena of combustion, there are
+certain superficial resemblances which have led to comparison
+between the two. Here again, however, the attitudes towards the
+energy of the environment stand very much as + and -. The life
+absorbs, stores, and spends with economy. The flame only
+recklessly spends. The property of storage by the organism calls
+out a further distinction between the course of the two
+processes. It secures that the chemical activity of the organism
+can be propagated in a medium in which the supply of energy is
+discontinuous or localised. The chemical activity of the
+combustion can, strictly speaking, only be propagated among
+contiguous particles. I need not dwell on the latter fact; an
+example of the former is seen in the action of the roots of
+plants, which will often traverse a barren place or circumvent an
+obstacle in their search for energy. In this manner roots will
+find out spots of rich nutriment.
+
+Thus there is a dynamic distinction between the progress of the
+organism and the progress of the combustion, or of the chemical
+reaction generally. And although there be unstable chemical
+systems which absorb energy during reaction, these are
+(dynamically) no more than the expansion of the compressed gas.
+There is a certain
+
+75
+
+initial capacity in the system for a given quantity of energy;
+this satisfied, progress ceases. The progress of the organism in
+time is continual, and goes on from less to greater so long as
+its development is unconstrained and the supply of energy is
+unlimited.
+
+We must regard the organism as a configuration which is so
+contrived as to evade the tendency of the universal laws of
+nature. Except we are prepared to believe that a violation of the
+second law of thermodynamics occurs in the organism, that a
+"sorting demon" is at work within it, we must, I think, assume
+that the interactions going on among its molecules are
+accompanied by retardation and dissipation like the rest of
+nature. That such conditions are not incompatible with the
+definition of the dynamic attitude of the organism, can be shown
+by analogy with our inanimate machines which, by aid of
+hypotheses in keeping with the second law of thermodynamics, may
+be supposed to fulfil the energy-functions of the plant or
+animal, and, in fact, in all apparent respects conform to the
+definition of the organism.
+
+We may assume this accomplished by a contrivance of the nature of
+a steam-engine, driven by solar energy. It has a boiler, which we
+may suppose fed by the action of the engine. It has piston,
+cranks, and other movable parts, all subject to resistance from
+friction, etc. Now there is no reason why this engine should not
+expend its surplus energy in shaping, fitting, and starting into
+action other engines:--in fact, in reproductive sacrifice. All
+
+76
+
+these other engines represent a multiplied absorption of energy
+as the effects of the energy received by the parent engine, and
+may in time be supposed to reproduce themselves. Further, we may
+suppose the parent engine to be small and capable of developing
+very little power, but the whole series as increasing in power at
+each generation. Thus the primary energy relations of the
+vegetable organism are represented in these engines, and no
+violation of the second law of thermodynamics involved.
+
+We might extend the analogy, and assuming these engines to spend
+a portion of their surplus energy in doing work against chemical
+forces--as, for example, by decomposing water through the
+intervention of a dynamo--suppose them to lay up in this way a
+store of potential energy capable of heating the boilers of a
+second order of engines, representing the graminivorous animal.
+It is obvious without proceeding to a tertiary or carnivorous
+order, that the condition of energy in the animal world may be
+supposed fulfilled in these successive series of engines, and no
+violation of the principles governing the actions going on in our
+machines assumed. Organisms evolving on similar principles would
+experience loss at every transfer. Thus only a portion of the
+radiant energy absorbed by the leaf would be expended in actual
+work, chemical and gravitational, etc. It is very certain that
+this is, in fact, what takes place.
+
+It is, perhaps, worth passing observation that, from the
+nutritive dependence of the animal upon the vegetable,
+
+77
+
+and the fact that a conversion of the energy of the one to the
+purposes of the other cannot occur without loss, the mean energy
+absorbed daily by the vegetable for the purpose of growth must
+greatly exceed that used in animal growth; so that the chemical
+potential energy of vegetation upon the earth is much greater
+than the energy of all kinds represented in the animal
+configurations.[1] It appears, too, that in the power possessed
+by the vegetable of remaining comparatively inactive, of
+surviving hard times by the expenditure and absorption of but
+little, the vegetable constitutes a veritable reservoir for the
+uniform supply of the more unstable and active animal.
+
+Finally, on the question of the manner of origin of organic
+systems, it is to be observed that, while the life of the present
+is very surely the survival of the fittest of the tendencies and
+chances of the past, yet, in the initiation of the organised
+world, a single chance may have decided a whole course of events:
+for, once originated, its own law secures its increase, although
+within the new order of actions, the law of the fittest must
+assert itself. That such a progressive material system as an
+organism was possible, and at some remote period was initiated,
+is matter of knowledge; whether or not the initiatory living
+configuration was rare and fortuitous, or the probable result of
+the general action of physical laws acting among innumerable
+chances, must remain matter of
+
+[1] I find a similar conclusion arrived at in Semper's _Animal
+Life_, p. 52.
+
+78
+
+speculation. In the event of the former being the truth, it is
+evidently possible, in spite of a large finite number of
+habitable worlds, that life is non-existent elsewhere. If the
+latter is the truth, it is almost certain that there is life in
+all, or many of those worlds.
+
+EVOLUTION AND ACCELERATION OF ACTIVITY
+
+The primary factor in evolution is the "struggle for existence."
+This involves a "natural selection" among the many variations of
+the organism. If we seek the underlying causes of the struggle,
+we find that the necessity of food and (in a lesser degree) the
+desire for a mate are the principal causes of contention. The
+former is much the more important factor, and, accordingly, we
+find the greater degree of specialisation based upon it.
+
+The present view assumes a dynamic necessity for its demands
+involved in the nature of the organism as such. This assumption
+is based on observation of the outcome of its unconstrained
+growth, reproduction, and life-acts. We have the same right to
+assert this of the organism as we have to assert that retardation
+and degradation attend the actions of inanimate machines, which
+assertion, also, is based on observation of results. Thus we pass
+from the superficial statements that organisms require food in
+order to live, or that organisms desire food, to the more
+fundamental one that:
+
+_The organism is a configuration of matter which absorbs energy
+acceleratively, without limit, when unconstrained._
+
+79
+
+This is the dynamic basis for a "struggle for existence." The
+organism being a material system responding to accession of
+energy with fresh demands, and energy being limited in amount,
+the struggle follows as a necessity. Thus, evolution guiding' the
+steps of the energy-seeking organism, must presuppose and find
+its origin in that inherent property of the organism which
+determines its attitude in presence of available energy.
+
+Turning to the factor, "adaptation," we find that this also must
+presuppose, in order to be explicable, some quality of
+aggressiveness on the part of the organism. For adaptation in
+this or that direction is the result of repulse or victory, and,
+therefore, we must presuppose an attack. The attack is made by
+the organism in obedience to its law of demand; we see in the
+adaptation of the organism but the accumulated wisdom derived
+from past defeats and victories.
+
+Where the environment is active, that is living, adaptation
+occurs on both sides. Improved means of defence or improved means
+of attack, both presuppose activity. Thus the reactions to the
+environment, animate and inanimate, are at once the outcome of
+the eternal aggressiveness of the organism, and the source of
+fresh aggressiveness upon the resources of the medium.
+
+As concerns the "survival of the fittest" (or "natural
+selection"), we can, I think, at once conclude that the organism
+which best fulfils the organic law under the circumstances of
+supply is the "fittest," _ipso facto._ In many
+
+80
+
+cases this is contained in the commonsense consideration, that to
+be strong, consistent with concealment from enemies which are
+stronger, is best, as giving the organism mastery over foes which
+are weaker, and generally renders it better able to secure
+supplies. Weismann points out that natural selection favours
+early and abundant reproduction. But whether the qualifications
+of the "fittest" be strength, fertility, cunning, fleetness,
+imitation, or concealment, we are safe in concluding that growth
+and reproduction must be the primary qualities which at once
+determine selection and are fostered by it. Inherent in the
+nature of the organism is accelerated absorption of energy, but
+the qualifications of the "fittest" are various, for the supply
+of energy is limited, and there are many competitors for it. To
+secure that none be wasted is ultimately the object of natural
+selection, deciding among the eager competitors what is best for
+each.
+
+In short, the facts and generalisations concerning evolution must
+presuppose an organism endowed with the quality of progressive
+absorption of energy, and retentive of it. The continuity of
+organic activity in a world where supplies are intermittent is
+evidently only possible upon the latter condition. Thus it
+appears that the dynamic attitude of the organism, considered in
+these pages, occupies a fundamental position regarding its
+evolution.
+
+We turn to the consideration of old age and death, endeavouring
+to discover in what relation they stand to the innate
+progressiveness of the organism.
+
+81
+
+THE PERIODICITY OF THE ORGANISM AND THE LAW OF PROGRESSIVE
+ACTIVITY
+
+The organic system is essentially unstable. Its aggressive
+attitude is involved in the phenomenon of growth, and in
+reproduction which is a form of growth. But the energy absorbed
+is not only spent in growth. It partly goes, also, to make good
+the decay which arises from the instability of the organic unit.
+The cell is molecularly perishable. It possesses its entity much
+as a top keeps erect, by the continual inflow of energy.
+Metabolism is always taking place within it. Any other condition
+would, probably, involve the difficulties of perpetual motion.
+
+The phenomenon of old age is not evident in the case of the
+unicellular organism reproducing by fission. At any stage of its
+history all the individuals are of the same age: all contain a
+like portion of the original cell, so far as this can be regarded
+as persisting where there is continual flux of matter and energy.
+In the higher organisms death is universally evident. Why is
+this?
+
+The question is one of great complexity. Considered from the more
+fundamental molecular point of view we should perhaps look to
+failure of the power of cell division as the condition of
+mortality. For it is to this phenomenon--that of cell
+division--that the continued life of the protozoon is to be
+ascribed, as we have already seen. Reproduction is, in fact, the
+saving factor here.
+
+As we do not know the source or nature of the stimulus
+
+82
+
+responsible for cell division we cannot give a molecular account
+of death in the higher organisms. However we shall now see that,
+philosophically, we are entitled to consider reproduction as a
+saving factor in this case also; and to regard the death of the
+individual much as we regard the fall of the leaf from the tree:
+_i.e._ as the cessation of an outgrowth from a development
+extending from the past into the future. The phenomena of old age
+and natural death are, in short, not at variance with the
+progressive activity of the organism. We perceive this when we
+come to consider death from the evolutionary point of view.
+
+Professor Weismann, in his two essays, "The Duration of Life,"
+and "Life and Death,"[1] adopts and defends the view that "death
+is not a primary necessity but that it has been secondarily
+acquired by adaptation." The cell was not inherently limited in
+its number of cell-generations. The low unicellular organisms are
+potentially immortal, the higher multicellular forms with
+well-differentiated organs contain the germs of death within
+themselves.
+
+He finds the necessity of death in its utility to the species.
+Long life is a useless luxury. Early and abundant reproduction is
+best for the species. An immortal individual would gradually
+become injured and would be valueless or even harmful to the
+species by taking the place of those that are sound. Hence
+natural selection will shorten life.
+
+[1] See his _Biological Memoirs._ Oxford, 1888.
+
+83
+
+Weismann contends against the transmission of acquired characters
+as being unproved.[1] He bases the appearance of death on
+variations in the reproductive cells, encouraged by the ceaseless
+action of natural selection, which led to a differentiation into
+perishable somatic cells and immortal reproductive cells. The
+time-limit of any particular organism ultimately depends upon the
+number of somatic cell-generations and the duration of each
+generation. These quantities are "predestined in the germ itself"
+which gives rise to each individual. "The existence of immortal
+metazoan organisms is conceivable," but their capacity for
+existence is influenced by conditions of the external world; this
+renders necessary the process of adaptation. In fact, in the
+differentiation of somatic from reproductive cells, material was
+provided upon which natural selection could operate to shorten or
+to lengthen the life of the individual in accordance with the
+needs of the species. The soma is in a sense "a secondary
+appendage of the real bearer of life--the reproductive cells." The
+somatic cells probably lost their immortal qualities, on this
+immortality becoming useless to the species. Their mortality may
+have been a mere consequence of their differentiation (loc. cit.,
+p. 140), itself due to natural selection. "Natural death was
+not," in fact, "introduced from absolute intrinsic necessity
+inherent in the nature of living matter, but on grounds of
+utility,
+
+[1] Biological Memoirs, p. 142.
+
+84
+
+that is from necessities which sprang up, not from the general
+conditions of life, but from those special conditions which
+dominate the life of multicellular organisms."
+
+On the inherent immortality of life, Weismann finally states:
+"Reproduction is, in truth, an essential attribute of living
+matter, just as the growth which gives rise to it.... Life is
+continuous, and not periodically interrupted: ever since its
+first appearance upon the Earth in the lowest organism, it has
+continued without break; the forms in which it is manifest have
+alone undergone change. Every individual alive today--even the
+highest--is to be derived in an unbroken line from the first and
+lowest forms." [1]
+
+At the present day the view is very prevalent that the soma of
+higher organisms is, in a sense, but the carrier for a period of
+the immortal reproductive cells (Ray Lankester)[2]--an appendage
+due to adaptation, concerned in their supply, protection, and
+transmission. And whether we regard the time-limit of its
+functions as due to external constraints, recurrently acting till
+their effects become hereditary, or to variations more directly
+of internal origin, encouraged by natural selection, we see in
+old age and death phenomena ultimately brought about in obedience
+to the action of an environment. These are not inherent in the
+properties of living matter. But, in spite
+
+[1] Loc. cit., p. 159
+
+[2] Geddes and Thomson, The Evolution of Sex, chap. xviii.
+
+85
+
+of its mortality, the body remains a striking manifestation of
+the progressiveness of the organism, for to this it must be
+ascribed. To it energy is available which is denied to the
+protozoon. Ingenious adaptations to environment are more
+especially its privilege. A higher manifestation, however, was
+possible, and was found in the development of mind. This, too, is
+a servant of the cell, as the genii of the lamp. Through it
+energy is available which is denied to the body. This is the
+masterpiece of the cell. Its activity dates, as it were, but from
+yesterday, and today it inherits the most diverse energies of the
+Earth.
+
+Taking this view of organic succession, we may liken the
+individual to a particle vibrating for a moment and then coming
+to rest, but sweeping out in its motion one wave in the
+continuous organic vibration travelling from the past into the
+future. But as this vibration is one spreading with increased
+energy from each vibrating particle, its propagation involves a
+continual accelerated inflow of energy from the surrounding
+medium, a dynamic condition unknown in periodic effects
+transmitted by inanimate actions, and, indeed, marking the
+fundamental difference between the dynamic attitudes of the
+animate and inanimate.
+
+We can trace the periodic succession of individuals on a diagram
+of activity with some advantage. Considering, first, the case of
+the unicellular organism reproducing by subdivision and recalling
+that conditions, definite and inevitable, oppose a limit to the
+rate of growth, or, for our
+
+86
+
+present purpose, rate of consumption of energy, we proceed as
+follows:
+
+{Fig. 1}
+
+Along a horizontal axis units of time are measured; along a
+vertical axis units of energy. Then the life-history of the
+amoeba, for example, appears as a line such as A in Fig. 1.
+During the earlier stages of its growth the rate of absorption of
+energy is small; so that in the unit interval of time, t, the
+small quantity of energy, e1, is absorbed. As life advances, the
+activity of the organism augments, till finally this rate attains
+a maximum, when e2 units of energy are consumed in the unit of
+time.[1]
+
+[1] Reference to p. 76, where the organic system is treated as
+purely mechanical, may help readers to understand what is
+involved in this curve. The solar engine may, unquestionably,
+have its activity defined by such a curve. The organism is,
+indeed, more complex; but neither this fact nor our ignorance of
+its mechanism, affects the principles which justify the diagram.
+
+87
+
+On this diagram reproduction, on the part of the organism, is
+represented by a line which repeats the curvature of the parent
+organism originating at such a point as P in the path of the
+latter, when the rate of consumption of energy has become
+constant. The organism A has now ceased to act as a unit. The
+products of fission each carry on the vital development of
+
+{Fig. 2}
+
+the species along the curve B, which may be numbered (2), to
+signify that it represents the activity of two individuals, and
+so on, the numbering advancing in geometrical progression. The
+particular curvature adopted in the diagram is, of course,
+imaginary; but it is not of an indeterminate nature. Its course
+for any species is a characteristic of fundamental physical
+importance, regarding the part played in nature by the particular
+organism.
+
+88
+
+In Fig. 2 is represented the path of a primitive multicellular
+organism before the effects of competition produced or fostered
+its mortality. The lettering of Fig. 1 applies; the successive
+reproductive acts are marked P1, P2; Q1, Q2, etc., in the paths
+of the successive individuals.
+
+{Fig. 3}
+
+The next figure (Fig. 3) diagrammatically illustrates death in
+organic history. The path ever turns more and more from the axis
+of energy, till at length the point is reached when no more
+energy is available; a tangent to the curve at this point is at
+right angles to the axis of energy and parallel to the time axis.
+The death point is reached, and however great a length we measure
+along the axis of time, no further consumption of energy is
+
+89
+
+indicated by the path of the organism. Drawing the line beyond
+the death point is meaningless for our present purpose.
+
+It is observable that while the progress of animate nature finds
+its representation on this diagram by lines sloping _upwards_ from
+left to right, the course of events in inanimate nature--for
+example, the history of the organic configuration after death, or
+
+{Fig. 4}
+
+the changes progressing--let us say, in the solar system, or in
+the process of a crystallisation, would appear as lines sloping
+downwards from left to right.
+
+Whatever our views on the origin of death may be, we have to
+recognise a periodicity of functions in the life-history of the
+successive individuals of the present day; and whether or not we
+trace this directly or indirectly to
+
+90
+
+a sort of interference with the rising wave of life, imposed by
+the activity of a series of derived units, each seeking energy,
+and in virtue of its adaptation each being more fitted to obtain
+it than its predecessor, or even leave the idea of interference
+out of account altogether in the origination or perpetuation of
+death, the truth of the diagram (Fig. 4) holds in so far as it
+may be supposed to graphically represent the dynamic history of
+the individual. The point chosen on the curve for the origination
+of a derived unit is only applicable to certain organisms, many
+reproducing at the very close of life. A chain of units are
+supposed here represented.[1]
+
+THE LENGTH OF LIFE
+
+If we lay out waves as above to a common scale of time for
+different species, the difference of longevity is shown in the
+greater or less number of vibrations executed in a given time,
+_i.e._ in greater or less "frequency." We cannot indeed draw the
+curvature correctly, for this would necessitate a knowledge which
+we have not of the activity of the organism at different periods
+of its life-history, and so neither can we plot the direction of
+the organic line of propagation with respect to the
+
+[1] Projecting upon the axes of time and energy any one complete
+vibration, as in Fig. 4, the total energy consumed by the
+organism during life is the length E on the axis of energy, and
+its period of life is the length T on the time-axis. The mean
+activity is the quotient E/T.
+
+91
+
+axes of reference as this involves a knowledge of the mean
+activity.[1]
+
+The group of curves which follow, relating to typical animals
+possessing very different activities (Fig. 5), are therefore
+entirely diagrammatic, except in respect to the approximate
+
+{Fig. 5}
+
+longevity of the organisms. (1) might represent an animal of the
+length of life and of the activity of Man; (2), on the same scale
+of longevity,
+
+[1] In the relative food-supply at various periods of life the
+curvature is approximately determinable.
+
+92
+
+one of the smaller mammals; and (3), the life-history of a cold
+blooded animal living to a great age; _e.g._ certain of the
+reptilia.
+
+It is probable, that to conditions of structural development,
+under the influence of natural selection, the question of longer
+or shorter life is in a great degree referable. Thus, development
+along lines of large growth will tend to a slow rate of
+reproduction from the simple fact that unlimited energy to supply
+abundant reproduction is not procurable, whatever we may assume
+as to the strength or cunning exerted by the individual in its
+efforts to obtain its supplies. On the other hand, development
+along lines of small growth, in that reproduction is less costly,
+will probably lead to increased rate of reproduction. It is, in
+fact, matter of general observation that in the case of larger
+animals the rate of reproduction is generally slower than in the
+case of smaller animals. But the rate of reproduction might be
+expected to have an important influence in determining the
+particular periodicity of the organism. Were we to depict in the
+last diagram, on the same time-scale as Man, the vibrations of
+the smaller and shorter-lived living things, we would see but a
+straight line, save for secular variations in activity,
+representing the progress of the species in time: the tiny
+thrills of its units lost in comparison with the yet brief period
+of Man.
+
+The interdependence of the rate of reproduction and
+
+93
+
+the duration of the individual is, indeed, very probably revealed
+in the fact that short-lived animals most generally reproduce
+themselves rapidly and in great abundance, and vice versa. In
+many cases where this appears contradicted, it will be found that
+the young are exposed to such dangers that but few survive (_e.g._
+many of the reptilia, etc.), and so the rate of reproduction is
+actually slow.
+
+Death through the periodic rigour of the inanimate environment
+calls forth phenomena very different from death introduced or
+favoured by competition. A multiplicity of effects simulative of
+death occur. Organisms will, for example, learn to meet very
+rigorous conditions if slowly introduced, and not permanent. A
+transitory period of want can be tided over by contrivance. The
+lily withdrawing its vital forces into the bulb, protected from
+the greatest extremity of rigour by seclusion in the Earth; the
+trance of the hibernating animal; are instances of such
+contrivances.
+
+But there are organisms whose life-wave truly takes up the
+periodicity of the Earth in its orbit. Thus the smaller animals
+and plants, possessing less resources in themselves, die at the
+approach of winter, propagating themselves by units which,
+whether egg or seed, undergo a period of quiescence during the
+season of want. In these quiescent units the energy of the
+organism is potential, and the time-energy function is in
+abeyance. This condition is, perhaps, foreshadowed in the
+encyst-
+
+94
+
+ment of the amoeba in resistance to drought. In most cases of
+hibernation the time-energy function seems maintained at a loss
+of potential by the organism, a diminished vital consumption of
+energy being carried on at the expense of the stored energy of
+the tissues. So, too, even among the largest organisms there will
+be a diminution of activity periodically inspired by
+climatological conditions. Thus, wholly or in part, the activity
+of organisms is recurrently affected by the great energy--tides
+set up by the Earth's orbital motion.
+
+{Fig. 6}
+
+Similarly in the phenomenon of sleep the organism responds to the
+Earth's axial periodicity, for in the interval of night a period
+of impoverishment has to be endured. Thus the diurnal waves of
+energy also meet a response in the organism. These tides and
+waves of activity would appear as larger and smaller ripples
+
+95
+
+on the life-curve of the organism. But in some, in which life and
+death are encompassed in a day, this would not be so; and for the
+annual among plants, the seed rest divides the waves with lines
+of no activity (Fig. 6).
+
+Thus, finally, we regard the organism as a dynamic phenomenon
+passing through periodic variations of intensity. The material
+systems concerned in the transfer of the energy rise, flourish,
+and fall in endless succession, like cities of ancient dynasties.
+At points of similar phase upon the waves the rate of consumption
+of energy is approximately the same; the functions, too, which
+demand and expend the energy are of similar nature.
+
+That the rhythm of these events is ultimately based on harmony in
+the configuration and motion of the molecules within the germ
+seems an unavoidable conclusion. In the life of the individual
+rhythmic dynamic phenomena reappear which in some cases have no
+longer a parallel in the external world, or under conditions when
+the individual is no longer influenced by these external
+conditions.,, In many cases the periodic phenomena ultimately die
+out under new influences, like the oscillations of a body in a
+viscous medium; in others when they seem to be more deeply rooted
+in physiological conditions they persist.
+
+The "length of life is dependent upon the number
+
+[1] The _Descent of Man._
+
+96
+
+of generations of somatic cells which can succeed one another in
+the course of a single life, and furthermore the number as well
+as the duration of each single cell-generation is predestined in
+the germ itself."[1]
+
+Only in the vague conception of a harmonising or formative
+structural influence derived from the germ, perishing in each
+cell from internal causes, but handed from cell to cell till the
+formative influence itself degrades into molecular discords, does
+it seem possible to form any physical representation of the
+successive events of life. The degradation of the molecular
+formative influence might be supposed involved in its frequent
+transference according to some such dynamic actions as occur in
+inanimate nature. Thus, ultimately, to the waste within the cell,
+to the presence of a force retardative of its perpetual harmonic
+motions, the death of the individual is to be ascribed. Perhaps
+in protoplasmic waste the existence of a universal death should
+be recognised. It is here we seem to touch inanimate nature; and
+we are led back to a former conclusion that the organism in its
+unconstrained state is to be regarded as a contrivance for
+evading the dynamic tendencies of actions in which lifeless
+matter participates.[2]
+
+[1] Weismann, _Life and Death; Biological Memoirs_, p. 146.
+
+[2] In connection with the predestinating power and possible
+complexity of the germ, it is instructive to reflect on the very
+great molecular population of even the smallest spores--giving
+rise to very simple forms. Thus, the spores of the unicellular
+Schizomycetes are estimated to dimensions as low as 1/10,000 of a
+millimetre in diameter (Cornil et Babes, _Les Batteries_, 1. 37).
+From Lord Kelvin's estimate of the number of molecules in water,
+comprised within the length of a wave-length of yellow light
+(_The Size of Atoms_, Proc. R. I., vol. x., p. 185) it is
+probable that such spores contain some 500,000 molecules, while
+one hundred molecules range along a diameter.
+
+97
+
+THE NUMERICAL ABUNDANCE OF LIFE
+
+We began by seeking in various manifestations of life a dynamic
+principle sufficiently comprehensive to embrace its very various
+phenomena. This, to all appearance, found, we have been led to
+regard life, to a great extent, as a periodic dynamic phenomenon.
+Fundamentally, in that characteristic of the contrivance, which
+leads it to respond favourably to transfer of energy, its
+enormous extension is due. It is probable that to its instability
+its numerical abundance is to be traced--for this, necessitating
+the continual supply of all the parts already formed, renders
+large, undifferentiated growth, incompatible with the limited
+supplies of the environment. These are fundamental conditions of
+abundant life upon the Earth.
+
+Although we recognise in the instability of living systems the
+underlying reason for their numerical abundance, secondary
+evolutionary causes are at work. The most important of these is
+the self-favouring nature of the phenomenon of reproduction. Thus
+there is a tendency not only to favour reproductiveness, but
+early reproductiveness, in the form of one prolific
+reproductive.
+
+98
+
+act, after which the individual dies.[1] Hence the wavelength of
+the species diminishes, reproduction is more frequent, and
+correspondingly greater numbers come and go in an interval of
+time.
+
+Another cause of the numerical abundance of life exists, as
+already stated, in the conditions of nourishment. Energy is more
+readily conveyed to the various parts of the smaller mass, and
+hence the lesser organisms will more actively functionate; and
+this, as being the urging dynamic attitude, as well as that most
+generally favourable in the struggle, will multiply and favour
+such forms of life. On the other hand, however, these forms will
+have less resource within themselves, and less power of
+endurance, so that they are only suitable to fairly uniform
+conditions of supply; they cannot survive the long continued want
+of winter, and so we have the seasonal abundance of summer. Only
+the larger and more resistant organisms, whether animal or
+vegetable, will, in general, populate the Earth from year to
+year. From this we may conclude that, but for the seasonal
+energy-tides, the development of life upon the globe had gone
+along very different lines from those actually followed. It is,
+indeed, possible that the evolution of the larger organisms would
+not have occurred; there would have been no vacant place for
+their development, and a being so endowed as Man could hardly
+
+[1] Weismann, _The Duration of Life._
+
+99
+
+have been evolved. We may, too, apply this reasoning elsewhere,
+and regard as highly probable, that in worlds which are without
+seasonal influences, the higher developments of life have not
+appeared; except they have been evolved under other conditions,
+when they might for a period persist. We have, indeed, only to
+picture to ourselves what the consequence of a continuance of
+summer would be on insect life to arrive at an idea of the
+antagonistic influences obtaining in such worlds to the survival
+of larger organisms.
+
+It appears that to the dynamic attitude of life in the first
+place, and secondarily to the environmental conditions limiting
+undifferentiated growth, as well as to the action of heredity in
+transmitting the reproductive qualities of the parent to the
+offspring, the multitudes of the pines, and the hosts of ants,
+are to be ascribed. Other causes are very certainly at work, but
+these, I think, must remain primary causes.
+
+We well know that the abundance of the ants and pines is not a
+tithe of the abundance around us visible and invisible. It is a
+vain endeavour to realise the countless numbers of our
+fellow-citizens upon the Earth; but, for our purpose, the
+restless ants, and the pines solemnly quiet in the sunshine, have
+served as types of animate things. In the pine the gates of the
+organic have been thrown open that the vivifying river of energy
+may flow in. The ants and the butterflies sip for a brief moment
+of its waters, and again vanish into the
+
+100
+
+inorganic: life, love and death encompassed in a day.
+
+Whether the organism stands at rest and life comes to it on the
+material currents of the winds and waters, or in the vibratory
+energy of the æther; or, again, whether with restless craving it
+hurries hither and thither in search of it, matters nothing. The
+one principle--the accelerative law which is the law of the
+organic--urges all alike onward to development, reproduction and
+death. But although the individual dies death is not the end; for
+life is a rhythmic phenomenon. Through the passing ages the waves
+of life persist: waves which change in their form and in the
+frequency to which they are attuned from one geologic period to
+the next, but which still ever persist and still ever increase.
+And in the end the organism outlasts the generations of the
+hills.
+
+101
+
+THE BRIGHT COLOURS OF ALPINE FLOWERS [1]
+
+IT is admitted by all observers that many species of flowering
+plants growing on the higher alps of mountainous regions display
+a more vivid and richer colour in their bloom than is displayed
+in the same species growing in the valleys. That this is actually
+the case, and not merely an effect produced upon the observer by
+the scant foliage rendering the bloom more conspicuous, has been
+shown by comparative microscopic examination of the petals of
+species growing on the heights and in the valleys. Such
+examination has revealed that in many cases pigment granules are
+more numerous in the individuals growing at the higher altitudes.
+The difference is specially marked in Myosotis sylvatica,
+Campanula rotundifolia, Ranunculus sylvaticus, Galium cruciatum,
+and others. It is less marked in the case of Thymus serpyllum and
+Geranium sylvaticum; while in Rosa alpina and Erigeron alpinus no
+difference is observable.[2]
+
+In the following cases a difference of intensity of colour is,
+according to Kerner ("Pflanzenleben," 11. 504), especially
+noticeable:-- _Agrostemma githago, Campanula
+
+[1] _Proc. Royal Dublin Society_, 1893.
+
+[2] G. Bonnier, quoted by De Varigny, _Experimental Evolution_,
+p. 55.
+
+102
+
+pusilla, Dianthus inodorus (silvestris), Gypsophila repens, Lotus
+corniculatus, Saponaria ocymoides, Satureja hortensis, Taraxacumm
+officinale, Vicia cracca, and Vicia sepium._
+
+To my own observation this beautiful phenomenon has always
+appeared most obvious and impressive. It appears to have struck
+many unprofessional observers. Helmholtz offers the explanation
+that the vivid colours are the result of the brighter sunlight of
+the heights. It has been said, too, that they are the direct
+chemical effects of a more highly ozonized atmosphere. The latter
+explanation I am unable to refer to its author. The following
+pages contain a suggestion on the matter, which occurred to me
+while touring, along with Henry H. Dixon, in the Linthal district
+of Switzerland last summer.[1]
+
+If the bloom of these higher alpine flowers is especially
+pleasing to our own æsthetic instincts, and markedly conspicuous
+to us as observers, why not also especially attractive and
+conspicuous to the insect whose mission it is to wander from
+flower to flower over the pastures? The answer to this question
+involves the hypothesis I would advance as accounting for the
+bright colours of high-growing individuals. In short, I believe a
+satisfactory explanation is to be found in the conditions of
+insect life in the higher alps.
+
+In the higher pastures the summer begins late and
+
+[1] The summer of 1892.
+
+103
+
+closes early, and even in the middle of summer the day closes in
+with extreme cold, and the cold of night is only dispelled when
+the sun is well up. Again, clouds cover the heights when all is
+clear below, and cold winds sweep over them when there is warmth
+and shelter in the valleys. With these rigorous conditions the
+pollinating insects have to contend in their search for food, and
+that when the rival attractions of the valleys below are so many.
+I believe it is these rigorous conditions which are indirectly
+responsible for the bright colours of alpine flowers. For such
+conditions will bring about a comparative scarcity of insect
+activity on the heights; and a scarcity or uncertainty in the
+action of insect agency in effecting fertilization will intensify
+the competition to attract attention, and only the brightest
+blooms will be fertilized.[1]
+
+This will be a natural selection of the brightest, or the
+
+[1] Grant Allen, I have recently learned, advances in _Science in
+Arcady_ the theory that there is a natural selective cause
+fostering the bright blooms of alpines. The selective cause is,
+however, by him referred to the greater abundance of butterfly
+relatively to bee fertilizers. The former, he says, display more
+æsthetic instinct than bees. In the valley the bees secure the
+fertilization of all. I may observe that upon the Fridolins Alp
+all the fertilizers we observed were bees. I have always found
+butterflies very scarce at altitudes of 7,000 to 8,000 feet. The
+alpine bees are very light in body, like our hive bee, and I do
+not think rarefaction of the atmosphere can operate to hinder its
+ascent to the heights, as Grant Allen suggests. The observations
+on the death-rate of bees and butterflies on the glacier, to be
+referred to presently, seem to negative such a hypothesis, and to
+show that a large preponderance of bees over butterflies make
+their way to the heights.
+
+104
+
+brightest will be the fittest, and this condition, along with the
+influence of heredity, will encourage a race of vivid flowers. On
+the other hand, the more scant and uncertain root supply, and the
+severe atmospheric conditions, will not encourage the grosser
+struggle for existence which in the valleys is carried on so
+eagerly between leaves and branches--the normal offensive and
+defensive weapons of the plant--and so the struggle becomes
+refined into the more æsthetic one of colour and brightness
+between flower and flower. Hence the scant foliage and vivid
+bloom would be at once the result of a necessary economy, and a
+resort to the best method of securing reproduction under the
+circumstances of insect fertilizing agency. Or, in other words,
+while the luxuriant growth is forbidden by the conditions, and
+thus methods of offence and defence, based upon vigorous
+development, reduced in importance, it would appear that the
+struggle is mainly referred to rivalry for insect preference. It
+is probable that this is the more economical manner of carrying
+on the contest.
+
+In the valleys we see on every side the struggle between the
+vegetative organs of the plant; the soundless battle among the
+leaves and branches. The blossom here is carried aloft on a
+slender stem, or else, taking but a secondary part in the
+contest, it is relegated to obscurity (P1. XII.). Further up on
+the mountains, where the conditions are more severe and the
+supplies less abundant, the leaf and branch assume lesser
+dimensions, for they are costly weapons to provide and the
+elements are unfriendly
+
+105
+
+to their existence (Pl. XIII.). Still higher, approaching the
+climatic limit of vegetable life, the struggle for existence is
+mainly carried on by the æsthetic rivalry of lowly but
+conspicuous blossoms.
+
+As regards the conditions of insect life in the higher alps, it
+came to my notice in a very striking manner that vast numbers of
+such bees and butterflies as venture up perish in the cold of
+night time. It appears as if at the approach of dusk these are
+attracted by the gleam of the snow, and quitting the pastures,
+lose themselves upon the glaciers and firns, there to die in
+hundreds. Thus in an ascent of the Tödi from the Fridolinshüte we
+counted in the early dawn sixty-seven frozen bees, twenty-nine
+dead butterflies, and some half-dozen moths on the Biferten
+Glacier and Firn. These numbers, it is to be remembered, only
+included those lying to either side of our way over the snow, so
+that the number must have mounted up to thousands when integrated
+over the entire glacier and firn. Approaching the summit none
+were found. The bees resembled our hive bee in appearance, the
+butterflies resembled the small white variety common in our
+gardens, which has yellow and black upon its wings. One large
+moth, striped across the abdomen, and measuring nearly two inches
+in length of body, was found. Upon our return, long after the
+sun's rays had grown strong, we observed some of the butterflies
+showed signs of reanimation. We descended so quickly to avoid the
+inconvenience of the soft snow that we had time for no
+
+106
+
+close observation on the frozen bees. But dead bees are common
+objects upon the snows of the alps.
+
+These remarks I noted down roughly while at Linthal last summer,
+but quite recently I read in Natural Science[1] the following
+note:
+
+"Late Flowering Plants.--While we write, the ivy is in flower, and
+bees, wasps, and flies are jostling each other and struggling to
+find standing-room on the sweet-smelling plant. How great must be
+the advantage obtained by this plant through its exceptional
+habit of flowering in the late autumn, and ripening its fruit in
+the spring. To anyone who has watched the struggle to approach
+the ivy-blossom at a time when nearly all other plants are bare,
+it is evident that, as far as transport of pollen and
+cross-fertilization go, the plant could not flower at a more
+suitable time. The season is so late that most other plants are
+out of flower, but yet it is not too late for many insects to be
+brought out by each sunny day, and each insect, judging by its
+behaviour, must be exceptionally hungry.
+
+"Not only has the ivy the world to itself during its flowering
+season, but it delays to ripen its seed till the spring, a time
+when most other plants have shed their seed, and most edible
+fruits have been picked by the birds. Thus birds wanting fruit in
+the spring can obtain little but ivy, and how they appreciate the
+ivy berry is evident
+
+[1] For December, 1892, vol. i., p. 730.
+
+107
+
+by the purple stains everywhere visible within a short distance
+of the bush."
+
+These remarks suggest that the ivy adopts the converse attitude
+towards its visitors to that forced upon the alpine flower. The
+ivy bloom is small and inconspicuous, but then it has the season
+to itself, and its inconspicuousness is no disadvantage, _i.e._
+if one plant was more conspicuous than its neighbours, it would
+not have any decided advantage where the pollinating insect is
+abundant and otherwise unprovided for. Its dark-green berries in
+spring, which I would describe as very inconspicuous, have a
+similar advantage in relation to the necessities of bird life.
+
+The experiments of M. C. Flahault must be noticed. This
+naturalist grew seeds of coloured flowers which had ripened in
+Paris, part in Upsala, and part in Paris; and seed which had
+ripened in Upsala, part at Paris, and part at Upsala. The flowers
+opening in the more northern city were in most cases the
+brighter.[1] If this observation may be considered indisputable,
+as appears to be the case, the question arises, Are we to regard
+this as a direct effect of the more rigorous climate upon the
+development of colouring matter on the blooms opening at Upsala?
+If we suppose an affirmative answer, the theory of direct effect
+by sun brightness must I think be abandoned. But I venture to
+think that the explanation of the Upsala
+
+[1] Quoted by De Varigny, _Experimental Evolution_, p. 56.
+
+108
+
+experiment is not to be found in direct climatic influence upon
+the colour, but in causes which lie deeper, and involve some
+factors deducible from biological theory.
+
+The organism, as a result of the great facts of heredity and of
+the survival of the fittest, is necessarily a system which
+gathers experience with successive generations; and the principal
+lesson ever being impressed upon it by external events is
+economy. Its success depends upon the use it makes of its
+opportunities for the reception of energy and the economy
+attained in disposing of what is gained.
+
+With regard to using the passing opportunity the entire seasonal
+development of life is a manifestation of this attitude, and the
+fleetness, agility, etc., of higher organisms are developments in
+this direction. The higher vegetable organism is not locomotory,
+save in the transferences of pollen and seed, for its food comes
+to it, and the necessary relative motion between food and
+organism is preserved in the quick motion of radiated energy from
+the sun and the slower motion of the winds on the surface of the
+earth. But, even so, the vegetable organism must stand ever ready
+and waiting for its supplies. Its molecular parts must be ready
+to seize the prey offered to it, somewhat as the waiting spider
+the fly. Hence, the plant stands ready; and every cloud with
+moving shadow crossing the fields handicaps the shaded to the
+benefit of the unshaded plant in the adjoining field. The open
+bloom
+
+109
+
+is a manifestation of the generally expectant attitude of the
+plant, but in relation to reproduction.
+
+As regards economy, any principle of maximum economy, where many
+functions have to be fulfilled, will, we may very safely predict,
+involve as far as possible mutual helpfulness in the processes
+going on. Thus the process of the development towards meeting any
+particular external conditions, A, suppose, will, if possible,
+tend to forward the development towards meeting conditions B; so
+that, in short, where circumstances of morphology and physiology
+are favourable, the ideally economical system will be attained
+when in place of two separate processes, a, ß, the one process y,
+cheaper than a + ß, suffices to advance development
+simultaneously in both the directions A and B. The economy is as
+obvious as that involved in "killing two birds with the one
+stone"--if so crude a simile is permissible--and it is to be
+expected that to foster such economy will be the tendency of
+evolution in all organic systems subjected to restraints as those
+we are acquainted with invariably are.
+
+Such economy might be simply illustrated by considering the case
+of a reservoir of water elevated above two hydraulic motors, so
+that the elevated mass of water possessed gravitational
+potential. The available energy here represents the stored-up
+energy in the organism. How best may the water be conveyed to the
+two motors [the organic systems reacting towards conditions A and
+B] so
+
+110
+
+that as little energy as possible is lost in transit? If the
+motors are near together it is most economical to use the one
+conduit, which will distribute the requisite supply of water to
+both. If the motors are located far asunder it will be most
+economical to lay separate conduits. There is greatest economy in
+meeting a plurality of functions by the same train of
+physiological processes where this is consistent with meeting
+other demands necessitated by external or internal conditions.
+
+But an important and obvious consequence arises in the supply of
+the two motors from the one conduit. We cannot work one motor
+without working the other. If we open a valve in the conduit both
+motors start into motion and begin consuming the energy stored in
+the tank. And although they may both under one set of conditions
+be doing useful and necessary work, in some other set of
+conditions it may be needless for both to be driven.
+
+This last fact is an illustration of a consideration which must
+enter into the phenomenon which an eminent biologist speaks of as
+physiological or unconscious "memory,"[1] For the development of
+the organism from the ovum is but the starting of a train of
+interdependent events of a complexity depending upon the
+experience of the past.
+
+[1] Ewald Hering, quoted by Ray Lankaster, _The Advancement of
+Science_, p. 283.
+
+111
+
+In short, we may suppose the entire development of the plant,
+towards meeting certain groups of external conditions,
+physiologically knit together according as Nature tends to
+associate certain groups of conditions. Thus, in the case in
+point, climatic rigour and scarcity of pollinating agency will
+ever be associated; and in the long experience of the past the
+most economical physiological attitude towards both is, we may
+suppose, adopted; so that the presence of one condition excites
+the apparent unconscious memory of the other. In reality the
+process of meeting the one condition involves the process and
+development for meeting the other.
+
+And this consideration may be extended very generally to such
+organisms as can survive under the same associated natural
+conditions, for the history of evolution is so long, and the
+power of locomotion so essential to the organism at some period
+in its life history, that we cannot philosophically assume a
+local history for members of a species even if widely severed
+geographically at the present day. At some period in the past
+then, it is very possible that the individuals today thriving at
+Paris, acquired the experience called out at Upsala. The
+perfection of physiological memory inspires no limit to the date
+at which this may have occurred--possibly the result of a
+succession of severe seasons at Paris; possibly the result of
+migrations --and the seed of many flowering plants possess means
+of migration only inferior to those possessed by the flying and
+swimming animals. But, again, possibly the experi-
+
+112
+
+ence was acquired far back in the evolutionary history of the
+flower.[1]
+
+But a further consideration arises. Not only at each moment in
+the life of the individual must maximum income and most judicious
+expenditure be considered, but in its whole life history, and
+even over the history of its race, the efficiency must tend to be
+a maximum. This principle is even carried so far that when
+necessary it leads to the death of the individual, as in the case
+of those organisms which, having accomplished the reproductive
+act, almost immediately expire. This view of nature may be
+repellent, but it is, nevertheless, evident that we are parts of
+a system which ruthlessly sacrifices the individual on general
+grounds of economy. Thus, if the curve which defines the mean
+rate of reception of energy of all kinds at different periods in
+the life of the organism be opposed by a second curve, drawn
+below the axis along which time is measured, representing the
+mean rate of expenditure of energy on development, reproduction,
+etc. (Fig. 7), this latter curve, which is, of course,
+
+[1] The blooms of self-fertilising, and especially of
+cleistogamic plants (_e.g._ Viola), are examples of unconscious
+memory, or unconscious "association of ideas" leading to the
+development of organs now functionless. The _Pontederia crassipes_
+of the Amazon, which develops its floating bladders when grown in
+water, but aborts them rapidly when grown on land, and seems to
+retain this power of adaptation to the environment for an
+indefinite period of time, must act in each case upon an
+unconscious memory based upon past experience. Many other cases
+might be cited.
+
+113
+
+physiologically dependent on the former, must be of such a nature
+from its origin to its completion in death, that the condition is
+realized of the most economical rate of expenditure at each
+period of life.[1] The rate of expenditure of energy at any
+period of life is, of course, in such a curve defined by the
+slope of the curve towards the axis of time at the period in
+question; but this particular slope _must be led to by a previous
+part of the curve, and involves its past and future course to a
+very great extent_.
+
+{Fig. 7}
+
+There will, therefore, be impressed upon the
+organism by the factors of evolution a unified course of
+economical expenditure completed only by its death, and which
+will give to the developmental progress of the individual its
+prophetic character.
+
+In this way we look to the unified career of each organic unit,
+from its commencement in the ovum to the day
+
+[1] See _The Abundance of Life_.
+
+114
+
+when it is done with vitality, for that preparation for momentous
+organic events which is in progress throughout the entire course
+of development; and to the economy involved in the welding of
+physiological processes for the phenomenon of physiological
+memory, wherein we see reflected, as it were, in the development
+of the organism, the association of inorganic restraints
+occurring in nature which at some previous period impressed
+itself upon the plastic organism. We may picture the seedling at
+Upsala, swayed by organic memory and the inherited tendency to an
+economical preparation for future events, gradually developing
+towards the æsthetic climax of its career. In some such manner
+only does it appear possible to account for the prophetic
+development of organisms, not alone to be observed in the alpine
+flowers, but throughout nature.
+
+And thus, finally, to the effects of natural selection and to
+actions defined by general principles involved in biology, I
+would refer for explanation of the manner in which flowers on the
+Alps develop their peculiar beauty.
+
+115
+
+MOUNTAIN GENESIS
+
+OUR ancestors regarded mountainous regions with feelings of
+horror, mingled with commiseration for those whom an unkindly
+destiny had condemned to dwell therein. We, on the other hand,
+find in the contemplation of the great alps of the Earth such
+peaceful and elevated thoughts, and such rest to our souls, that
+it is to those very solitudes we turn to heal the wounds of ife.
+It is difficult to explain the cause of this very different point
+of view. It is probably, in part, to be referred to that cloud of
+superstitious horror which, throughout the Middle Ages, peopled
+the solitudes with unknown terrors; and, in part, to the
+asceticism which led the pious to regard the beauty and joy of
+life as snares to the soul's well-being. In those eternal
+solitudes where the overwhelming forces of Nature are most in
+evidence, an evil principle must dwell or a dragon's dreadful
+brood must find a home.
+
+But while in our time the aesthetic aspect of the hills appeals
+to all, there remains in the physical history of the mountains
+much that is lost to those who have not shared in the scientific
+studies of alpine structure and genesis. They lose a past history
+which for interest com-
+
+116
+
+petes with anything science has to tell of the changes of the
+Earth.
+
+Great as are the physical features of the mountains compared with
+the works of Man, and great as are the forces involved compared
+with those we can originate or control, the loftiest ranges are
+small contrasted with the dimensions of the Earth. It is well to
+bear this in mind. I give here (Pl. XV.) a measured drawing
+showing a sector cut from a sphere of 50 cms. radius; so much of
+it as to exhibit the convergence of its radial boundaries which
+if prolonged will meet at the centre. On the same scale as the
+radius the diagram shows the highest mountains and the deepest
+ocean. The average height of the land and the average depth of
+the ocean are also exhibited. We see how small a movement of the
+crust the loftiest elevation of the Himalaya represents and what
+a little depression holds the ocean.
+
+Nevertheless, it is not by any means easy to explain the genesis
+of those small elevations and depressions. It would lead us far
+from our immediate subject to discuss the various theoretical
+views which have been advanced to account for the facts. The idea
+that mountain folds, and the lesser rugosities of the Earth's
+surface, arose in a wrinkling of the crust under the influence of
+cooling and skrinkage of the subcrustal materials, is held by
+many eminent geologists, but not without dissent from others.
+
+The most striking observational fact connected with mountain
+structure is that, without exception, the ranges
+
+117
+
+of the Earth are built essentially of sedimentary rocks: that is
+of rocks which have been accumulated at some remote past time
+beneath the surface of the ocean. A volcanic core there may
+sometimes be--probably an attendant or consequence of the
+uplifting--or a core of plutonic igneous rocks which has arisen
+under the same compressive forces which have bowed and arched the
+strata from their original horizontal position. It is not
+uncommon to meet among unobservant people those who regard all
+mountain ranges as volcanic in origin. Volcanoes, however, do not
+build mountain ranges. They break out as more or less isolated
+cones or hills. Compare the map of the Auvergne with that of
+Switzerland; the volcanoes of South Italy with the Apennines.
+Such great ranges as those which border with triple walls the
+west coast of North America are in no sense volcanic: nor are the
+Pyrenees, the Caucasus, or the Himalaya. Volcanic materials are
+poured out from the summits of the Andes, but the range itself is
+built up of folded sediments on the same architecture as the
+other great ranges of the Earth.
+
+Before attempting an explanation of the origin of the mountains
+we must first become more closely acquainted with the phenomena
+attending mountain elevation.
+
+At the present day great accumulations of sediment are taking
+place along the margins of the continents where the rivers reach
+the ocean. Thus, the Gulf of Mexico receiving the sediment of the
+Mississippi and Rio Grande;
+
+118
+
+the northeast coast of South America receiving the sediments of
+the Amazons; the east coast of Asia receiving the detritus of the
+Chinese rivers; are instances of such areas of deposition. Year
+by year, century by century, the accumulation progresses, and as
+it grows the floor of the sea sinks under the load. Of the
+yielding of the crust under the burthen of the sediments we are
+assured; for otherwise the many miles of vertically piled strata
+which are uplifted to our view in the mountains, never could have
+been deposited in the coastal seas of the past. The flexure and
+sinking of the crust are undeniable realities.
+
+Such vast subsiding areas are known as geosynclines. From the
+accumulated sediments of the geosynclines the mountain ranges of
+the past have in every case originated; and the mountains of the
+future will assuredly arise and lofty ranges will stand where now
+the ocean waters close over the collecting sediments. Every
+mountain range upon the Earth enforces the certainty of this
+prediction.
+
+The mountain-forming movement takes place after a certain great
+depth of sediment is collected. It is most intense where the
+thickness of deposit is greatest. We see this when we examine the
+structure of our existing mountain ranges. At either side where
+the sediments thin out, the disturbance dies away, till we find
+the comparatively shallow and undisturbed level sediments which
+clothe the continental surface.
+
+Whatever be the connection between the deposition and
+
+119
+
+the subsequent upheaval, _the element of great depth of
+accumulation seems a necessary condition and must evidently enter
+as a factor into the Physical Processes involved_. The mountain
+range can only arise where the geosyncline is deeply filled by
+long ages of sedimentation.
+
+Dana's description of the events attending mountain building is
+impressive:
+
+"A mountain range of the common type, like that to which the
+Appalachians belong, is made out of the sedimentary formations of
+a long preceding era; beds that were laid down conformably, and
+in succession, until they had reached the needed thickness; beds
+spreading over a region tens of thousands of square miles in
+area. The region over which sedimentary formations were in
+progress in order to make, finally, the Appalachian range,
+reached from New York to Alabama, and had a breadth of 100 to 200
+miles, and the pile of horizontal beds along the middle was
+40,000 feet in depth. The pile for the Wahsatch Mountains was
+60,000 feet thick, according to King. The beds for the
+Appalachians were not laid down in a deep ocean, but in shallow
+waters, where a gradual subsidence was in progress; and they at
+last, when ready for the genesis, lay in a trough 40,000 feet
+deep, filling the trough to the brim. It thus appears that epochs
+of mountain-making have occurred only after long intervals of
+quiet in the history of a continent."[1]
+
+[1] Dana, _Manual of Geology_, third edition, p. 794
+
+120
+
+On the western side of North America the work of
+mountain-building was, indeed, on the grandest scale. For long
+ages and through a succession of geological epochs, sedimentation
+had proceeded so that the accumulations of Palaeozoic and
+Mesozoic times had collected in the geosyncline formed by their
+own ever increasing weight. The site of the future Laramide range
+was in late Cretaceous times occupied by some 50,000 feet of
+sedimentary deposits; but the limit had apparently been attained,
+and at this time the Laramide range, as well as its southerly
+continuation into the United States, the Rockies, had their
+beginning. Chamberlin and Salisbury[1] estimate that the height
+of the mountains developed in the Laramide range at this time was
+20,000 feet, and that, owing to the further elevation which has
+since taken place, from 32,000 to 35,000 feet would be their
+present height if erosion had not reduced them. Thus on either
+side of the American continent we have the same forces at work,
+throwing up mountain ridges where the sediments had formerly been
+shed into the ocean.
+
+These great events are of a rhythmic character; the crust, as it
+were, pulsating under the combined influences of sedimentation
+and denudation. The first involves downward movements under a
+gathering load, and ultimately a reversal of the movement to one
+of upheaval; the second factor, which throughout has been in
+
+[1] Chamberlin and Salisbury, _Geology_, 1906, iii., 163.
+
+121
+
+operation as creator of the sediments, then intervenes as an
+assailant of the newly-raised mountains, transporting their
+materials again to the ocean, when the rhythmic action is
+restored to its first phase, and the age-long sequence of events
+must begin all over again.
+
+It has long been inferred that compressive stress in the crust
+must be a primary condition of these movements. The wvork
+required to effect the upheavals must be derived from some
+preexisting source of energy. The phenomenon--intrinsically one of
+folding of the crust--suggests the adjustment of the earth-crust
+to a lessening radius; the fact that great mountain-building
+movements have simultaneously affected the entire earth is
+certainly in favour of the view that a generally prevailing cause
+is at the basis of the phenomenon.
+
+The compressive stresses must be confined to the upper few miles
+of the crust, for, in fact, the downward increase of temperature
+and pressure soon confers fluid properties on the medium, and
+slow tangential compression results in hydrostatic pressure
+rather than directed stresses. Thus the folding visible in the
+mountain range, and the lateral compression arising therefrom,
+are effects confined to the upper parts of the crust.
+
+The energy which uplifts the mountain is probably a surviving
+part of the original gravitational potential energy of the crust
+itself. It must be assumed that the crust in following downwards
+the shrinking subcrustal magma, develops immense compressive
+stresses in
+
+122
+
+its materials, vast geographical areas being involved. When
+folding at length takes place along the axis of the elongated
+syncline of deposition, the stresses find relief probably for
+some hundreds of miles, and the region of folding now becomes
+compressed in a transverse direction. As an illustration, the
+Laramide range, according to Dawson, represents the reduction of
+a surface-belt 50 miles wide to one of 25 miles. The marvellous
+translatory movements of crustal folds from south to north
+arising in the genesis of the Swiss Alps, which recent research
+has brought to light, is another example of these movements of
+relief, which continue to take place perhaps for many millions of
+years after they are initiated.
+
+The result of this yielding of the crust is a buckling of the
+surface which on the whole is directed upwards; but depression
+also is an attendant, in many cases at least, on mountain
+upheaval. Thus we find that the ocean floor is depressed into a
+syncline along the western coast of South America; a trough
+always parallel to the ranges of the Andes. The downward
+deflection of the crust is of course an outcome of the same
+compressive stresses which elevate the mountain.
+
+The fact that the yielding of the crust is always situated where
+the sediments have accumulated to the greatest depth, has led to
+attempts from time to time of establishing a physical connexion
+between the one and the other. The best-known of these theories
+is that of Babbage and Herschel. This seeks the connexion in the
+rise of the
+
+123
+
+geotherms into the sinking mass of sediment and the consequent
+increase of temperature of the earth-crust beneath. It will be
+understood that as these isogeotherms, or levels at which the
+temperature is the same, lie at a uniform distance from the
+surface all over the Earth, unless where special variations of
+conductivity may disturb them, the introduction of material
+pressed downwards from above must result in these materials
+partaking of the temperature proper to the depth to which they
+are depressed. In other words the geotherms rise into the sinking
+sediments, always, however, preserving their former average
+distance from the surface. The argument is that as this process
+undoubtedly involves the heating up of that portion of the crust
+which the sediments have displaced downwards, the result must be
+a local enfeeblement of the crust, and hence these areas become
+those of least resistance to the stresses in the crust.
+
+When this theory is examined closely, we see that it only amounts
+to saying that the bedded rocks, which have taken the place of
+the igneous materials beneath, as a part of the rigid crust of
+the Earth, must be less able to withstand compressive stress than
+the average crust. For there has been no absolute rise of the
+geotherms, the thermal conductivities of both classes of
+materials differing but little. Sedimentary rock has merely taken
+the place of average crust-rock, and is subjected to the same
+average temperature and pressure prevailing in the surrounding
+crust. But are there any grounds for the
+
+124
+
+assumption that the compressive resistance of a complex of
+sedimentary rocks is inferior to one of igneous materials? The
+metamorphosed siliceous sediments are among the strongest rocks
+known as regards resistance to compressive stress; and if
+limestones have indeed plastic qualities, it must be remembered
+that their average amount is only some 5 per cent. of the whole.
+Again, so far as rise of temperature in the upper crust may
+affect the question, a temperature which will soften an average
+igneous rock will not soften a sedimentary rock, for the reason
+that the effect of solvent denudation has been to remove those
+alkaline silicates which confer fusibility.
+
+When, however, we take into account the radioactive content of
+the sediments the matter assumes a different aspect.
+
+The facts as to the general distribution of radioactive
+substances at the surface, and in rocks which have come from
+considerable depths in the crust, lead us to regard as certain
+the widespread existence of heat-producing radioactive elements
+in the exterior crust of the Earth. We find, indeed, in this fact
+an explanation--at least in part--of the outflow of heat
+continually taking place at the surface as revealed by the rising
+temperature inwards. And we conclude that there must be a
+thickness of crust amounting to some miles, containing the
+radioactive elements.
+
+Some of the most recent measurements of the quantities of radium
+and thorium in the rocks of igneous origin--_e.g._ granites,
+syenites, diorites, basalts, etc., show that the
+
+125
+
+radioactive heat continually given out by such rocks amounts to
+about one millionth part of 0.6 calories per second per cubic
+metre of average igneous rock. As we have to account for the
+escape of about 0.0014 calorie[1] per square metre of the Earth's
+surface per second (assuming the rise of temperature downwards,
+_i.e._ the "gradient" of temperature, to be one degree centigrade
+in 35 metres) the downward extension of such rocks might, _prima
+facie_, be as much as 19 kilometres.
+
+About this calculation we have to observe that we assume the
+average radioactivity of the materials with which we have dealt
+at the surface to extend uniformly all the way down, _i.e._ that
+our experiments reveal the average radioactivity of a radioactive
+crust. There is much to be said for this assumption. The rocks
+which enter into the measurements come from all depths of the
+crust. It is highly probable that the less silicious, _i.e._ the
+more basic, rocks, mainly come from considerable depths; the more
+acid or silica-rich rocks, from higher levels in the crust. The
+radioactivity determined as the mean of the values for these two
+classes of rock closely agrees with that found for intermediate
+rocks, or rocks containing an intermediate amount of silica.
+Clarke contends that this last class of material probably
+represents the average composition of the Earth's crust so far as
+it has been explored by us.
+
+[1] The calorie referred to is the quantity of heat required to
+heat one gram of water, _i.e._ one cubic centimetre of
+water--through one degree centigrade.
+
+126
+
+It is therefore highly probable that the value found for the mean
+radioactivity of acid and basic rocks, or that found for
+intermediate rocks, truly represents the radioactive state of the
+crust to a considerable depth. But it is easy to show that we
+cannot with confidence speak of the thickness of this crust as
+determinable by equating the heat outflow at the surface with the
+heat production of this average rock.
+
+This appears in the failure of a radioactive layer, taken at a
+thickness of about 19-kilometres, to account for the deep-seated
+high temperatures which we find to be indicated by volcanic
+phenomena at many places on the surface. It is not hard to show
+that the 19-kilometre layer would account for a temperature no
+higher than about 270° >C. at its base.
+
+It is true that this will be augmented beneath the sedimentary
+deposits as we shall presently see; and that it is just in
+association with these deposits that deep-seated temperatures are
+most in evidence at the surface; but still the result that the
+maximum temperature beneath the crust in general attains a value
+no higher than 270° C. is hardly tenable. We conclude, then, that
+some other source of heat exists beneath. This may be radioactive
+in origin and may be easily accounted for if the radioactive
+materials are more sparsely distributed at the base of the upper
+crust. Or, again, the heat may be primeval or original heat,
+still escaping from a cooling world. For our present purpose it
+does not much matter which view
+
+127
+
+we adopt. But we must recognise that the calculated depth of 19
+kilometres of crust, possessing the average radioactivity of the
+surface, is excessive; for, in fact, we are compelled by the
+facts to recognise that some other source of heat exists
+beneath.
+
+If the observed surface gradient of temperature persisted
+uniformly downwards, at some 35 kilometres beneath the surface
+there would exist temperatures (of about 1000° C.) adequate to
+soften basic rocks. It is probable, however, that the gradient
+diminishes downwards, and that the level at which such
+temperatures exist lies rather deeper than this. It is,
+doubtless, somewhat variable according to local conditions; nor
+can we at all approximate closely to an estimate of the depth at
+which the fusion temperatures will be reached, for, in fact, the
+existence of the radioactive layer very much complicates our
+estimates. In what follows we assume the depth of softening to
+lie at about 40 kilometres beneath the surface of the normal
+crust; that is 25 miles down. It is to be observed that Prestwich
+and other eminent geologists, from a study of the facts of
+crust-folding, etc., have arrived at similar estimates.[1] As a
+further assumption we are probably not far wrong if we assign to
+the radioactive part of this crust a thickness of about 10 or 12
+kilometres; _i.e._ six or seven miles. This is necessarily a
+rough approximation only; but the conclusions at which
+
+[1] Prestwich, _Proc. Royal Soc._, xii., p. 158 _et seq._
+
+128
+
+we shall arrive are reached in their essential features allowing
+a wide latitude in our choice of data. We shall speak of this
+part of the crust as the normal radioactive layer.
+
+An important fact is evolved from the mathematical investigation
+of the temperature conditions arising from the presence of such a
+radioactive layer. It is found that the greatest temperature, due
+to the radioactive heat everywhere evolved in the layer--_i.e._
+the temperature at its base--is proportional to the square of the
+thickness of the layer. This fact has a direct bearing on the
+influence of radioactivity upon mountain elevation; as we shall
+now find.
+
+The normal radioactive layer of the Earth is composed of rocks
+extending--as we assume--approximately to a depth of 12 kilometres
+(7.5 miles). The temperature at the base of this layer due to the
+heat being continually evolved in it, is, say, t1°. Now, let us
+suppose, in the trough of the geosyncline, and upon the top of
+the normal layer, a deposit of, say, 10 kilometres (6.2 miles) of
+sediments is formed during a long period of continental
+denudation. What is the effect of this on the temperature at the
+base of the normal layer depressed beneath this load? The total
+thickness of radioactive rocks is now 22 kilometres. Accordingly
+we find the new temperature t2°, by the proportion t1° : t2° ::
+12° : 22° That is, as 144 to 484. In fact, the temperature is more
+than trebled. It is true we here assume the radioactivity of the
+sediments
+
+129
+
+and of the normal crust to be the same. The sediments are,
+however, less radioactive in the proportion of 4 to 3.
+Nevertheless the effects of the increased thickness will be
+considerable.
+
+Now this remarkable increase in the temperature arises entirely
+from the condition attending the radioactive heating; and
+involves something _additional_ to the temperature conditions
+determined by the mere depression and thickening of the crust as
+in the Babbage-Herschel theory. The latter theory only involves a
+_shifting_ of the temperature levels (or geotherms) into the
+deposited materials. The radioactive theory involves an actual
+rise in the temperature at any distance from the surface; so that
+_the level in the crust at which the rocks are softened is nearer
+to the surface in the geosynclines than it is elsewhere in the
+normal crust_ (Pl. XV, p. 118).
+
+In this manner the rigid part of the crust is reduced in
+thickness where the great sedimentary deposits have collected. A
+ten-kilometre layer of sediment might result in reducing the
+effective thickness of the crust by 30 per cent.; a
+fourteen-kilometre layer might reduce it by nearly 50 per cent.
+Even a four-kilometre deposit might reduce the effective
+resistance of the crust to compressive forces, by 10 per cent.
+
+Such results are, of course, approximate only. They show that as
+the sediments grow in depth there is a rising of the geotherm of
+plasticity--whatever its true temperature may be--gradually
+reducing the thickness of that part
+
+130
+
+of the upper crust which is bearing the simultaneously increasing
+compressive stresses. Below this geotherm long-continued stress
+resolves itself into hydrostatic pressure; above it (there is, of
+course, no sharp line of demarcation) the crust accumulates
+elastic energy. The final yielding and flexure occur when the
+resistant cross-section has been sufficiently diminished. It is
+probable that there is also some outward hydrostaitic thrust over
+the area of rising temperature, which assists in determining the
+upward throw of the folds.
+
+When yielding has begun in any geosyncline, and the materials are
+faulted and overthrust, there results a considerably increased
+thickness. As an instance, consider the piling up of sediments
+over the existing materials of the Alps, which resulted from the
+compressive force acting from south to north in the progress of
+Alpine upheaval. Schmidt of Basel has estimated that from 15 to
+20 kilometres of rock covered the materials of the Simplon as now
+exposed, at the time when the orogenic forces were actively at
+work folding and shearing the beds, and injecting into their
+folds the plastic gneisses coming from beneath.[1] The lateral
+compression of the area of deposition of the Laramide, already
+referred to, resulted in a great thickening of the deposits. Many
+other cases might be cited; the effect is always in some degree
+necessarily produced.
+
+[1] Schmidt, Ec. Geol. _Helvelix_, vol. ix., No. 4, p. 590
+
+131
+
+If time be given for the heat to accumulate in the lower depths
+of the crushed-up sediments, here is an additional source of
+increased temperature. The piled-up masses of the Simplon might
+have occasioned a rise due to radioactive heating of one or two
+hundred degrees, or even more; and if this be added to the
+interior heat, a total of from 800° to 1000° might have prevailed
+in the rocks now exposed at the surface of the mountain. Even a
+lesser temperature, accompanied by the intense pressure
+conditions, might well occasion the appearances of thermal
+metamorphism described by Weinschenk, and for which, otherwise,
+there is difficulty in accounting.[1]
+
+This increase upon the primarily developed temperature conditions
+takes place concurrently with the progress of compression; and
+while it cannot be taken into account in estimating the
+conditions of initial yielding of the crust, it adds an element
+of instability, inasmuch as any progressive thickening by lateral
+compression results in an accelerated rise of the goetherms. It
+is probable that time sufficient for these effects to develop, if
+not to their final, yet to a considerable extent, is often
+available. The viscous movements of siliceous materials, and the
+out-pouring of igneous rocks which often attend mountain
+elevation, would find an explanation in such temperatures.
+
+[1] Weinschenk, _Congrès Géol. Internat._, 1900, i., p. 332.
+
+132
+
+There is no more striking feature of the part here played by
+radioactivity than the fact that the rhythmic occurrence of
+depression and upheaval succeeding each other after great
+intervals of time, and often shifting their position but little
+from the first scene of sedimentation, becomes accounted for. The
+source of thermal energy, as we have already remarked, is in fact
+transported with the sediments--that energy which determines the
+place of yielding and upheaval, and ordains that the mountain
+ranges shall stand around the continental borders. Sedimentation
+from this point of view is a convection of energy.
+
+When the consolidated sediments are by these and by succeeding
+movements forced upwards into mountains, they are exposed to
+denudative effects greatly exceeding those which affect the
+plains. Witness the removal during late Tertiary times of the
+vast thickness of rock enveloping the Alps. Such great masses are
+hurried away by ice, rivers, and rain. The ocean receives them;
+and with infinite patience the world awaits the slow accumulation
+of the radioactive energy beginning afresh upon its work. The
+time for such events appears to us immense, for millions of years
+are required for the sediments to grow in thickness, and the
+geotherms to move upwards; but vast as it is, it is but a moment
+in the life of the parent radioactive substances, whose atoms,
+hardly diminished in numbers, pursue their changes while the
+mountains come and go, and the
+
+133
+
+rudiments of life develop into its highest consummations.
+
+To those unacquainted with the results of geological
+investigation the history of the mountains as deciphered in the
+rocks seems almost incredible.
+
+The recently published sections of the Himalaya, due to H. H.
+Hayden and the many distinguished men who have contributed to the
+Geological Survey of India, show these great ranges to be
+essentially formed of folded sediments penetrated by vast masses
+of granite and other eruptives. Their geological history may be
+summarised as follows
+
+The Himalayan area in pre-Cambrian times was, in its southwestern
+extension, part of the floor of a sea which covered much of what
+is now the Indian Peninsula. In the northern shallows of this sea
+were laid down beds of conglomerate, shale, sandstone and
+limestone, derived from the denudation of Archæan rocks, which,
+probably, rose as hills or mountains in parts of Peninsular India
+and along the Tibetan edge of the Himalayan region. These beds
+constitute the record of the long Purana Era[1] and are probably
+coeval with the Algonkian of North America. Even in these early
+times volcanic disturbances affected this area and the lower beds
+of the Purana deposits were penetrated by volcanic outflows,
+covered by sheets of lava, uplifted, denuded and again submerged
+
+[1] See footnote, p. 139.
+
+134
+
+beneath the waters. Two such periods of instability have left
+their records in the sediments of the Purana sea.
+
+The succeeding era--the Dravidian Era--opens with Haimanta
+(Cambrian) times. A shallow sea now extended over Kumaun, Garwal,
+and Spiti, as well as Kashmir and ultimately over the Salt Range
+region of the Punjab as is shown by deposits in these areas. This
+sea was not, however, connected with the Cambrian sea of Europe.
+The fossil faunas left by the two seas are distinct.
+
+After an interval of disturbance during closing Haimanta times,
+geographical changes attendant on further land movements
+occurred. The central sea of Asia, the Tethys, extended westwards
+and now joined with the European Paleozoic sea; and deposits of
+Ordovician and Silurian age were laid down:--the Muth deposits.
+
+The succeeding Devonian Period saw the whole Northern Himalayan
+area under the waters of the Tethys which, eastward, extended to
+Burma and China and, westward, covered Kashmir, the Hindu Kush
+and part of Afghanistan. Deposits continued to be formed in this
+area till middle Carboniferous times.
+
+Near. the close of the Dravidian Era Kashmir became convulsed by
+volcanic disturbance and the Penjal traps were ejected. It was a
+time of worldwide disturbance and of redistribution of land and
+water. Carboniferous times had begun, and the geographical
+changes in
+
+135
+
+the southern limits of the Tethys are regarded as ushering in a
+new and last era in Indian geological history the Aryan Bra.
+
+India was now part of Gondwanaland; that vanished continent which
+then reached westward to South Africa and eastward to Australia.
+A boulder-bed of glacial origin, the Talchir Boulder-bed, occurs
+in many surviving parts of this great land. It enters largely
+into the Salt Range deposits. There is evidence that extensive
+sheets of ice, wearing down the rocks of Rajputana, shoved their
+moraines northward into the Salt Range Sea; then, probably, a
+southern extension of the Tethys.
+
+Subsequent to this ice age the Indian coalfields of the Gondwana
+were laid down, with beds rich in the Glossopteris and
+Gangamopteris flora. This remarkable carboniferous flora extends
+to Southern Kashmir, so that it is to be inferred that this
+region was also part of the main Gondwanaland. But its emergence
+was but for a brief period. Upper Carboniferous marine deposits
+succeeded; and, in fact, there was no important discontinuity in
+the deposits in this area from Panjal times till the early
+Tertiaries. During the whole of which vast period Kashmir was
+covered with the waters of the Tethys.
+
+The closing Dravidian disturbances of the Kashmir region did not,
+apparently, extend to the eastern Himalayan area. But the
+Carboniferous Period was, in this
+
+136
+
+eastern area, one of instability, culminating, at the close of
+the Period, in a steady rise of the land and a northward retreat
+of the Tethys. Nearly the entire Himalaya east of Kashmir became
+a land surface and remained exposed to denudative forces for so
+long a time that in places the whole of the Carboniferous,
+Devonian, and a large part of the Silurian and Ordovician
+deposits were removed--some thousands of feet in thickness--before
+resubmergence in the Tethys occurred.
+
+Towards the end of the Palaeozoic Age the Aryan Tethys receded
+westwards, but still covered the Himalaya and was still connected
+with the European Palæozoic sea. The Himalayan area (as well as
+Kashmir) remained submerged in its waters throughout the entire
+Mesozoic Age.
+
+During Cretaceous times the Tethys became greatly extended,
+indicating a considerable subsidence of northwestern India,
+Afghanistan, Western Asia, and, probably, much of Tibet. The
+shallow-water character of the deposits of the Tibetan Himalaya
+indicates, however, a coast line near this region. Volcanic
+materials, now poured out, foreshadow the incoming of the great
+mountain-building epoch of the Tertiary Era. The enormous mass of
+the Deccan traps, possessing a volume which has been estimated at
+as much as 6,000 cubic miles, was probably extruded over the
+Northern Peninsular region during late Cretaceous times. The sea
+now began to retreat, and by the close of
+
+137
+
+the Eocene, it had moved westward to Sind and Baluchistan. The
+movements of the Earth's crust were attended by intense volcanic
+activity, and great volumes of granite were injected into the
+sediments, followed by dykes and outflows of basic lavas.
+
+The Tethys vanished to return no more. It survives in the
+Mediterranean of today. The mountain-building movements continued
+into Pliocene times. The Nummulite beds of the Eocene were, as
+the result, ultimately uplifted 18,500 feet over sea level, a
+total uplift of not less than 20,000 feet.
+
+Thus with many vicissitudes, involving intervals of volcanic
+activity, local uplifting, and extensive local denudation, the
+Himalaya, which had originated in the sediments of the ancient
+Purana sea, far back in pre-Cambrian times, and which had
+developed potentially in a long sequence of deposits collecting
+almost continuously throughout the whole of geological time,
+finally took their place high in the heavens, where only the
+winds--faint at such altitudes--and the lights of heaven can visit
+their eternal snows.[1]
+
+In this great history it is significant that the longest
+continuous series of sedimentary deposits which the world has
+known has become transfigured into the loftiest elevation upon
+its surface.
+
+[1] See A Sketch of the _Geography and Geology of the Himalaya
+Mountains and Tibet_. By Colonel S. G. Burrard, R.E., F.R.S., and
+H. H. Hayden, F.G.S., Part IV. Calcutta, 1908.
+
+138
+
+The diagrammatic sections of the Himalaya accompanying this brief
+description arc taken from the monograph of Burrard and Hayden
+(loc. cit.) on the Himalaya. Looking at the sections we see that
+some of the loftiest summits are sculptured in granite and other
+crystalline rocks. The appearance of these materials at the
+surface indicates the removal by denudation and the extreme
+metamorphism of much sedimentary deposit. The crystalline rocks,
+indeed, penetrate some of the oldest rocks in the world. They
+appear in contact with Archaean, Algonkian or early Palaeozoic
+rocks. A study of the sections reveals not only the severe earth
+movements, but also the immense amount of sedimentary deposits
+involved in the genesis of these alps. It will be noted that the
+vertical scale is not exaggerated relatively to the
+horizontal.[1] Although there is no evidence of mountain
+building
+
+[1] To those unacquainted with the terminology of Indian geology
+the following list of approximate equivalents in time will be of
+use
+
+Ngari Khorsum Beds - Pleistocene.
+Siwalik Series -     Miocene and Pliocene.
+Sirmur Series -      Oligocene.
+Kampa System -       Eocene and Cretaceous.
+Lilang System -      Triassic.
+Kuling System -      Permian.
+Gondwana System -    Carboniferous.
+Kenawar System -     Carboniferous and Devonian
+Muth System -        Silurian.
+Haimanta System -    Mid. and Lower Cambrian.
+Purana Group -       Algonkian.
+Vaikrita System -    Archæan.
+Daling Series -      Archæan.
+
+139
+
+on a large scale in the Himalayan area till the Tertiary
+upheaval, it is, in the majority of cases, literally correct to
+speak of the mountains as having their generations like organic
+beings, and passing through all the stages of birth, life, death
+and reproduction. The Alps, the Jura, the Pyrenees, the Andes,
+have been remade more than once in the course of geological time,
+the _débris_ of a worn-out range being again uplifted in succeeding
+ages.
+
+Thus to dwell for a moment on one case only: that of the
+Pyrenees. The Pyrenees arose as a range of older Palmozoic rocks
+in Devonian times. These early mountains, however, were
+sufficiently worn out and depressed by Carboniferous times to
+receive the deposits of that age laid down on the up-turned edges
+of the older rocks. And to Carboniferous succeeded Permian,
+Triassic, Jurassic and Lower Cretaceous sediments all laid down
+in conformable sequence. There was then fresh disturbance and
+upheaval followed by denudation, and these mountains, in turn,
+became worn out and depressed beneath the ocean so that Upper
+Greensand rocks were laid down unconforrnably on all beneath. To
+these now succeeded Upper Chalk, sediments of Danian age, and so
+on, till Eocene times, when the tale was completed and the
+existing ranges rose from the sea. Today we find the folded
+Nummulitic strata of Eocene age uplifted 11,000 feet, or within
+200 feet of the greatest heights of the Pyrenees. And so they
+stand awaiting
+
+140
+
+the time when once again they shall "fall into the portion of
+outworn faces."[1]
+
+Only mountains can beget mountains. Great accumulations of
+sediment are a necessary condition for the localisation of
+crust-flexure. The earliest mountains arose as purely igneous or
+volcanic elevations, but the generations of the hills soon
+originated in the collection of the _débris_, under the law of
+gravity, in the hollow places. And if a foundered range is
+exposed now to our view encumbered with thousands of feet of
+overlying sediments we know that while the one range was sinking,
+another, from which the sediments were derived, surely existed.
+Through the "windows" in the deep-cut rocks of the Swiss valleys
+we see the older Carboniferous Alps looking out, revisiting the
+sun light, after scores of millions of years of imprisonment. We
+know that just as surely as the Alps of today are founding by
+their muddy torrents ranges yet to arise, so other primeval Alps
+fed into the ocean the materials of these buried pre-Permian
+rocks.
+
+This succession of events only can cease when the rocks have been
+sufficiently impoverished of the heat-producing substances, or
+the forces of compression shall have died out in the surface
+crust of the earth.
+
+It seems impossible to escape the conclusion that in the great
+development of ocean-encircling areas of
+
+[1] See Prestwich, _Chemical and Physical Geology_, p. 302.
+
+141
+
+deposition and crustal folding, the heat of radioactivity has
+been a determining factor. We recognise in the movements of the
+sediments not only an influence localising and accelerating
+crustal movements, but one which, in subservience to the primal
+distribution of land and water, has determined some of the
+greatest geographical features of the globe.
+
+It is no more than a step to show that bound up with the
+radioactive energy are most of the earthquake and volcanic
+phenomena of the earth. The association of earthquakes with the
+great geosynclines is well known. The work of De Montessus showed
+that over 94 per cent. of all recorded shocks lie in the
+geosynclinal belts. There can be no doubt that these
+manifestations of instability are the results of the local
+weakness and flexure which originated in the accumulation of
+energy denuded from the continents. Similarly we may view in
+volcanoes phenomena referable to the same fundamental cause. The
+volcano was, in fact, long regarded as more intimately connected
+with earthquakes than it, probably, actually is; the association
+being regarded in a causative light, whereas the connexion is
+more that of possessing a common origin. The girdle of volcanoes
+around the Pacific and the earthquake belt coincide. Again, the
+ancient and modern volcanoes and earthquakes of Europe are
+associated with the geosyncline of the greater Mediterranean, the
+Tethys of Mesozoic times. There is no difficulty in understanding
+in a
+
+142
+
+general way the nature of the association. The earthquake is the
+manifestation of rupture and slip, and, as Suess has shown, the
+epicentres shift along that fault line where the crust has
+yielded.[1] The volcano marks the spot where the zone of fusion
+is brought so high in the fractured crust that the melted
+materials are poured out upon the surface.
+
+In a recent work on the subject of earthquakes Professor Hobbs
+writes: "One of the most interesting of the generalisations which
+De Montessus has reached as a result of his protracted studies,
+is that the earthquake districts on the land correspond almost
+exactly to those belts upon the globe which were the almost
+continuous ocean basins of the long Secondary era of geological
+history. Within these belts the sedimentary formations of the
+crust were laid down in the greatest thickness, and the
+formations follow each other in relatively complete succession.
+For almost or quite the whole of this long era it is therefore
+clear that the ocean covered these zones. About them the
+formations are found interrupted, and the lacuna indicate that
+the sea invaded the area only to recede from it, and again at
+some later period to transgress upon it. For a long time,
+therefore, these earthquake belts were the sea basins--the
+geosynclines. They became later the rising mountains of the
+Tertiary period, and mountains they
+
+[1] Suess, _The Face of the Earth_, vol. ii., chap. ii.
+
+143
+
+are today. The earthquake belts are hence those portions of the
+earth's crust which in recent times have suffered the greatest
+movements in a vertical direction--they are the most mobile
+portions of the earth's crust."[1] Whether the movements
+attending mountain elevation and denudation are a connected and
+integral part of those wide geographical changes which result in
+submergence and elevation of large continental areas, is an
+obscure and complex question. We seem, indeed, according to the
+views of some authorities, hardly in a position to affirm with
+certainty that such widespread movements of the land have
+actually occurred, and that the phenomena are not the outcome of
+fluctuations of oceanic level; that our observations go no
+further than the recognition of positive and negative movements
+of the strand. However this may be, the greater part of
+mechanical denudation during geological time has been done on the
+mountain ranges. It is, in short, indisputable that the orogenic
+movements which uplift the hills have been at the basis of
+geological history. To them the great accumulations of sediments
+which now form so large a part of continental land are mainly
+due. There can be no doubt of the fact that these movements have
+swayed the entire history, both inorganic and organic, of the
+world in which we live.
+
+[1] Hobbs, _Earthquakes_, p. 58.
+
+144
+
+To sum the contents of this essay in the most general terms, we
+find that in the conception of denudation as producing the
+convection and accumulation of radiothermal energy the surface
+features of the globe receive a new significance. The heat of the
+earth is not internal only, but rather a heat-source exists at
+the surface, which, as we have seen, cannot prevail to the same
+degree within; and when the conditions become favourable for the
+aggregation of the energy, the crust, heated both from beneath
+and from above, assumes properties more akin to those of its
+earlier stages of development, the secular heat-loss being
+restored in the radioactive supplies. These causes of local
+mobility have been in operation, shifting somewhat from place to
+place, and defined geographically by the continental masses
+undergoing denudation, since the earliest times.
+
+145
+
+ALPINE STRUCTURE
+
+AN intelligent observer of the geological changes progressing in
+southern Europe in Eocene times would have seen little to inspire
+him with a premonition of the events then developing. The
+Nummulitic limestones were being laid down in that enlarged
+Mediterranean which at this period, save for a few islands,
+covered most of south Europe. Of these stratified remains, as
+well as of the great beds of Cretaceous, Jurassic, Triassic, and
+Permian sediments beneath, our hypothetical observer would
+probably have been regardless; just as today we observe, with an
+indifference born of our transitoriness, the deposits rapidly
+gathering wherever river discharge is distributing the sediments
+over the sea-floor, or the lime-secreting organisms are actively
+at work. And yet it took but a few millions of years to uplift
+the deposits of the ancient Tethys; pile high its sediments in
+fold upon fold in the Alps, the Carpathians, and the Himalayas;
+and--exposing them to the rigours of denudation at altitudes where
+glaciation, landslip, and torrent prevail--inaugurate a new epoch
+of sedimentation and upheaval.
+
+146
+
+In the case of the Alps, to which we wish now specially to refer,
+the chief upheaval appears to have been in Oligocene times,
+although movement continued to the close of the Pliocene. There
+was thus a period of some millions of years within which the
+entire phenomena were comprised. Availing ourselves of Sollas'
+computations,[1] we may sum the maximum depths of sedimentary
+deposits of the geological periods concerned as follows:--
+
+Pliocene - - - - - 3,950 m.
+
+Miocene  - - - - - 4,250 m.
+
+Oligocene  - - - - 3,660 m.
+
+Eocene - - - - - - 6,100 m.
+
+and assuming that the orogenic forces began their work in the
+last quarter of the Eocene period, we have a total of 13,400 m.
+as some measure of the time which elapsed. At the rate of io
+centimetres in a century these deposits could not have collected
+in less than 13.4 millions of years. It would appear that not
+less than some ten millions of years were consumed in the genesis
+of the Alps before constructive movements finally ceased.
+
+The progress of the earth-movements was attended by the usual
+volcanic phenomena. The Oligocene and Miocene volcanoes extended
+in a band marked by the Auvergne, the Eiffel, the Bohemian, and
+the eastern Carpathian eruptions; and, later, towards the close
+of the movements in Pliocene times, the south border
+
+[1] Sollas, Anniversary Address, Geol. Soc., London, 1909.
+
+147
+
+regions of the Alps became the scene of eruptions such as those
+of Etna, Santorin, Somma (Vesuvius), etc.
+
+We have referred to these well-known episodes with two objects in
+view: to recall to mind the time-interval involved, and the
+evidence of intense crustal disturbance, both dynamic and
+thermal. According to views explained in a previous essay, the
+energetic effects of radium in the sediments and upper crust were
+a principal factor in localising and bringing about these
+results. We propose now to inquire if, also, in the more intimate
+structure of the Alps, the radioactive energy may not have borne
+a part.
+
+What we see today in the Alps is but a residue spared by
+denudation. It is certain that vast thicknesses of material have
+disappeared. Even while constructive effects were still in
+progress, denudative forces were not idle. Of this fact the
+shingle accumulations of the Molasse, where, on the northern
+borders of the Alps, they stand piled into mountains, bear
+eloquent testimony. In the sub-Apennine series of Italy, the
+great beds of clays, marls, and limestones afford evidence of
+these destructive processes continued into Pliocene times. We
+have already referred to Schmidt's estimate that the sedimentary
+covering must have in places amounted to from 15,000 to 20,000
+metres. The evidence for this is mainly tectonic or structural;
+but is partly forthcoming in the changes which the materials now
+open to our inspection plainly reveal. Thus it is impos-
+
+148
+
+sible to suppose that gneissic rocks can become so far plastic as
+to flow in and around the calcareous sediments, or be penetrated
+by the latter--as we see in the Jungfrau and elsewhere--unless
+great pressures and high temperatures prevailed. And, according
+to some writers, the temperatures revealed by the intimate
+structural changes of rock-forming minerals must have amounted to
+those of fusion. The existence of such conditions is supported by
+the observation that where the.crystallisation is now the most
+perfect, the phenomena of folding and injection are best
+developed.[1] These high temperatures would appear to be
+unaccountable without the intervention of radiothermal effects;
+and, indeed, have been regarded as enigmatic by observers of the
+phenomena in question. A covering of 20,000 metres in thickness
+would not occasion an earth-temperature exceeding 500° C. if the
+gradients were such as obtain in mountain regions generally; and
+600° is about the limit we could ascribe to the purely passive
+effects of such a layer in elevating the geotherms.
+
+Those who are still unacquainted with the recently published
+observations on the structure of the Alps may find it difficult
+to enter into what has now to be stated; for the facts are,
+indeed, very different from the generally preconceived ideas of
+mountain formation. Nor can we wonder that many geologists for
+long held
+
+[1] Weinschenk, C. R. _Congrès Géol._, 1900, p. 321, et seq.
+
+149
+
+back from admitting views which appeared so extreme. Receptivity
+is the first virtue of the scientific mind; but, with every
+desire to lay aside prejudice, many felt unequal to the
+acceptance of structural features involving a folding of the
+earth-crust in laps which lay for scores of miles from country to
+country, and the carriage of mountainous materials from the south
+of the Alps to the north, leaving them finally as Alpine ranges
+of ancient sediments reposing on foundations of more recent date.
+The historian of the subject will have to relate how some who
+finally were most active in advancing the new views were at first
+opposed to them. In the change of conviction of these eminent
+geologists we have the strongest proof of the convincing nature
+of the observations and the reality of the tectonic features upon
+which the recent views are founded.
+
+The lesser mountains which stand along the northern border of the
+great limestone Alps, those known as the Préalpes, present the
+strange characteristic of resting upon materials younger than
+themselves. Such mountains as the remarkable-looking Mythen, near
+Schwyz, for instance, are weathered from masses of Triassic and
+Jurassic rock, and repose on the much more recent Flysch. In
+sharp contrast to the Flysch scenery, they stand as abrupt and
+gigantic erratics, which have been transported from the central
+zone of the Alps lying far to the south. They are strangers
+petrologically,
+
+150
+
+stratigraphically, and geographically,[1] to the locality in
+which they now occur. The exotic materials may be dolomites,
+limestones, schists, sandstones, or rocks of igneous origin. They
+show in every case traces of the severe dynamic actions to which
+they have been subjected in transit. The igneous, like the
+sedimentary, klippen, can be traced to distant sources; to the
+massif of Belladonne, to Mont Blanc, Lugano, and the Tyrol. The
+Préalpes are, in fact, mountains without local roots.
+
+In this last-named essential feature, the Préalpes do not differ
+from the still greater limestone Alps which succeed them to the
+south. These giants, _e.g._ the Jungfrau, Wetterhorn, Eiger, etc.,
+are also without local foundations. They have been formed from
+the overthrown and drawn-out anticlines of great crust-folds,
+whose synclines or roots are traceable to the south side of the
+Rhone Valley. The Bernese Oberland originated in the piling-up of
+four great sheets or recumbent folds, one of which is continued
+into the Préalpes. With Lugeon[2] we may see in the phenomenon of
+the formation of the Préalpes a detail; regarding it as a normal
+expression of that mechanism which has created the Swiss Alps.
+For these limestone masses of the Oberland are not indications of
+a merely local shift of the sedimentary covering of the Alps.
+Almost the whole covering has
+
+[1] De Lapparent, _Traité de Géologie_, p. 1,785.
+
+[2] Lugeon, _Bulletin Soc. Géol. de France_, 1901, p. 772.
+
+151
+
+been pushed over and piled up to the north. Lugeon[l] concludes
+that, before denudation had done its work and cut off the
+Préalpes from their roots, there would have been found sheets, to
+the number of eight, superimposed and extending between the Mont
+Blanc massif and the massif of the Finsteraarhorn: these sheets
+being the overthrown folds of the wrinkled sedimentary covering.
+The general nature of the alpine structure
+
+{Fig. 8}
+
+will be understood from the presentation of it diagrammatically
+after Schmidt of Basel (Fig. 8).[2] The section extends from
+north to south, and brings out the relations of the several
+recumbent folds. We must imagine almost the whole of these
+superimposed folds now removed from the central regions of the
+Alps by denudation,
+
+[1] Lugeon, _loc. cit._
+
+[2] Schmidt, _Ec. Geol. Helvetiae_, vol. ix., No. 4.
+
+152
+
+and leaving the underlying gneisses rising through the remains of
+Permian, Triassic, and Jurassic sediments; while to the north the
+great limestone mountains and further north still, the Préalpes,
+carved from the remains of the recumbent folds, now stand with
+almost as little resemblance to the vanished mountains as the
+memories of the past have to its former intense reality.
+
+These views as to the origin of the Alps, which are shared at the
+present day by so many distinguished geologists, had their origin
+in the labours of many now gone; dating back to Studer; finding
+their inspiration in the work of Heim, Suess, and Marcel
+Bertrand; and their consummation in that of Lugeon, Schardt,
+Rothpletz, Schmidt, and many others. Nor must it be forgotten
+that nearer home, somewhat similar phenomena, necessarily on a
+smaller scale, were recognised by Lapworth, twenty-six years ago,
+in his work on the structure of the Scottish Highlands.
+
+An important tectonic principle underlies the development of the
+phenomena we have just been reviewing. The uppermost of the
+superimposed recumbent folds is more extended in its development
+than those which lie beneath. Passing downwards from the highest
+of the folds, they are found to be less and less extended both in
+the northerly and in the southerly direction, speaking of the
+special case--the Alps--now before us. This feature might be
+described somewhat differently. We might say that those folds
+which had their roots farther
+
+153
+
+to the south were the most drawn-out towards the north: or again
+we might say that the synclinal or deep-seated part of the fold
+has lagged behind the anticlinal or what was originally the
+highest part of the fold, in the advance of the latter to the
+north. The anticline has advanced relatively to the syncline. To
+this law one exception only is observed in the Swiss Alps; the
+sheet of the Brèche (_Byecciendecke_) falls short, in its northerly
+extension, of the underlying fold, which extends to form the
+Préalpes.
+
+Contemplating such a generalised section as Professor Schmidt's,
+or, indeed, more particular sections, such as those in the Mont
+Blanc Massif by Marcel Bertrand,[1] of the Dent de Morcles,
+Diablerets, Wildhorn, and Massif de la Brèche by Lugeon,[2] or
+finally Termier's section of the Pelvoux Massif,[3] one is
+reminded of the breaking of waves on a sloping beach. The wave,
+retarded at its base, is carried forward above by its momentum,
+and finally spreads far up on the strand; and if it could there
+remain, the succeeding wave must necessarily find itself
+superimposed upon the first. But no effects of inertia, no
+kinetic effects, may be called to our aid in explaining the
+formation of mountains. Some geologists have accordingly supposed
+that in order to account for
+
+[1] Marcel Bertrand, _Cong. Géol. Internat._, 1900, Guide Géol.,
+xiii. a, p. 41.
+
+[2] Lugeon, _loc. cit._, p. 773.
+
+[3] De Lapparent, _Traite de Géol._, p. 1,773.
+
+154
+
+the recumbent folds and the peculiar phenomena of increasing
+overlap, or _déferlement_, an obstacle, fixed and deep-seated, must
+have arrested the roots or synclines of the folds, and held them
+against translational motion, while a movement of the upper crust
+drew out and carried forward the anticlines. Others have
+contented themselves by recording the facts without advancing any
+explanatory hypothesis beyond that embodied in the incontestable
+statement that such phenomena must be referred to the effects of
+tangential forces acting in the Earth's crust.
+
+It would appear that the explanation of the phenomena of
+recumbent folds and their _déferlement_ is to be obtained directly
+from the temperature conditions prevailing throughout the
+stressed pile of rocks; and here the subject of mountain
+tectonics touches that with which we were elsewhere specially
+concerned--the geological influence of accumulated radioactive
+energy.
+
+As already shown[1], a rise of temperature due to this source of
+several hundred degrees might be added to such temperatures as
+would arise from the mere blanketing of the Earth, and the
+consequent upward movement of the geotherms. The time element is
+here the most important consideration. The whole sequence of
+events from the first orogenic movements to the final upheaval in
+Pliocene times must probably have occupied not less than ten
+million years.
+
+[1] _Mountain Genesis_, p. 129, et seq.
+
+155
+
+Unfortunately the full investigation of the distribution of
+temperature after any given time is beset with difficulties; the
+conditions being extremely complex. If the radioactive heating
+was strictly adiabatic--that is, if all the heat was conserved and
+none entered from without--the time required for the attainment of
+the equilibrium radioactive temperature would be just about six
+million years. The conditions are not, indeed, adiabatic; but, on
+the other hand, the rocks upraised by lateral pressure were by no
+means at 0° C. to start with. They must be assumed to have
+possessed such temperatures as the prior radiothermal effects,
+and the conducted heat from the Earth's interior, may have
+established.
+
+It would from this appear probable that if a duration of ten
+million years was involved, the equilibrium radioactive
+temperatures must nearly have been attained. The effects of heat
+conducted from the underlying earthcrust have to be added,
+leading to a further rise in temperature of not less than 500° or
+600° . In such considerations the observed indications of high
+temperatures in materials now laid bare by denudation, probably
+find their explanation (P1. XIX).
+
+The first fact that we infer from the former existence of such a
+temperature distribution is the improbability, indeed the
+impossibility, that anything resembling a rigid obstacle, or
+deep-seated "horst," can have existed beneath the present
+surface-level, and opposed the northerly movement of the
+deep-lying synclines. For
+
+156
+
+such a horst can only have been constituted of some siliceous
+rock-material such as we find everywhere rising through the
+worn-down sediments of the Alps; and the idea that this could
+retain rigidity under the prevailing temperature conditions, must
+be dismissed. There is no need to labour this question; the horst
+cannot have existed. To what, then, is the retardation of the
+lower parts of the folds, their overthrow, above, to the north,
+and their _déferlement_, to be ascribed?
+
+A little consideration shows that the very conditions of high
+temperature and viscosity, which render untenable the hypothesis
+of a rigid obstacle, suffice to afford a full explanation of the
+retardation of the roots of the folds. For directed translatory
+movements cannot be transmitted through a fluid, pressure in
+which is necessarily hydrostatic, and must be exerted equally in
+every direction. And this applies, not only to a fluid, but to a
+body which will yield viscously to an impressed force. There will
+be a gradation, according as viscosity gives place to rigidity,
+between the states in which the applied force resolves itself
+into a purely hydrostatic pressure, and in which it is
+transmitted through the material as a directed thrust. The nature
+of the force, in the most general case, of course, has to be
+considered; whether it is suddenly applied and of brief duration,
+or steady and long-continued. The latter conditions alone apply
+to the present case.
+
+It follows from this that, although a tangential force
+
+157
+
+or pressure be engendered by a crustal movement occurring to the
+south, and the resultant effects be transmitted northwards, these
+stresses can only mechanically affect the rigid parts of the
+crust into which they are carried. That is to say, they may
+result in folding and crushing, or horizontally transporting, the
+upper layers of the Earth's crust; but in the deeper-lying
+viscous materials they must be resolved into hydrostatic pressure
+which may act to upheave the overlying covering, but must refuse
+to transmit the horizontal translatory movements affecting the
+rigid materials above.
+
+Between the regions in which these two opposing conditions
+prevail there will be no hard and fast line; but with the
+downward increase of fluidity there will be a gradual failure of
+the mechanical conditions and an increase of the hydrostatic.
+Thus while the uppermost layers of the crust may be transported
+to the full amount of the crustal displacement acting from the
+south (speaking still of the Alps) deeper down there will be a
+lesser horizontal movement, and still deeper there is no
+influence to urge the viscous rock-materials in a northerly
+direction. The consequences of these conditions must be the
+recumbence of the folds formed under the crust-stress, and their
+_déferlement_ towards the north. To see this, we must follow the
+several stages of development.
+
+The earliest movements, we may suppose, result in flexures of the
+Jura-Mountain type--that is, in a
+
+158
+
+succession of undulations more or less symmetrical. As the
+orogenic force continues and develops, these undulations give
+place to folds, the limbs of which are approximately vertical,
+and the synclinal parts of which become ever more and more
+depressed into the deeper, and necessarily hotter, underlying
+materials; the anticlines being probably correspondingly
+elevated. These events are slowly developed, and the temperature
+beneath is steadily rising in consequence of the conducted
+interior heat, and the steady accumulation of radioactive energy
+in the sedimentary rocks and in the buried radioactive layer of
+the Earth. The work expended on the crushed and sheared rock also
+contributes to the developing temperature. Thus the geotherms
+must move upwards, and the viscous conditions extend from below;
+continually diminishing the downward range of the translatory
+movements progressing in the higher parts. While above the folded
+sediments are being carried northward, beneath they are becoming
+anchored in the growing viscosity of the medium. The anticlines
+will bend over, and the most southerly of the folds will
+gradually become pushed or bent over those lying to the north.
+Finally, the whole upper part of the sheaf will become
+horizontally recumbent; and as the uppermost folds will be those
+experiencing the greatest effects of the continued displacement,
+the _déferlement_ or overlap must necessarily arise.
+
+We may follow these stages of mountain evolution
+
+159
+
+in a diagram (Fig. 9) in which we eliminate intermediate
+conditions, and regard the early and final stages of development
+only. In the upper sketch we suppose the lateral compression much
+developed and the upward movement of the geotherms in progress.
+The dotted line may be assumed to be a geotherm having a
+temperature of viscosity. If the conditions here shown persist
+
+{Fig. 9}
+
+indefinitely, there is no doubt that the only further
+developments possible are the continued crushing of the sediments
+and the bodily displacement of the whole mass to the north. The
+second figure is intended to show in what manner these results
+are evaded. The geotherm of viscosity has risen. All above it is
+affected mechanically by the continuing stress, and borne
+northwards in varying
+
+160
+
+degree depending upon the rigidity. The folds have been
+overthrown and drawn out; those which lay originally most to the
+south have become the uppermost; and, experiencing the maximum
+amount of displacement, overlap those lying beneath. There has
+also been a certain amount of upthrow owing to the hydrostatic
+pressure. This last-mentioned element of the phenomena is of
+highly indeterminate character, for we know not the limits to
+which the hydrostatic pressure may be transmitted, and where it
+may most readily find relief. While, according to some of the
+published sections, the uplifting force would seem to have
+influenced the final results of the orogenic movements, a
+discussion of its effects would not be profitable.
+
+161
+
+OTHER MINDS THAN OURS?
+
+IN the year 1610 Galileo, looking through his telescope then
+newly perfected by his own hands, discovered that the planet
+Jupiter was attended by a train of tiny stars which went round
+and round him just as the moon goes round the Earth.
+
+It was a revelation too great to be credited by mankind. It was
+opposed to the doctrine of the centrality of the Earth, for it
+suggested that other worlds constituted like ours might exist in
+the heavens.
+
+Some said it was a mere optic illusion; others that he who looked
+through such a tube did it at the peril of his soul--it was but a
+delusion of Satan. Galileo converted a few of the unbelievers who
+had the courage to look through his telescope. To the others he
+said, he hoped they would see those moons on their way to heaven.
+Old as this story is it has never lost its pathos or its
+teaching.
+
+The spirit which assailed Galileo's discoveries and which finally
+was potent to overshadow his declining years, closed in former
+days the mouths of those who asked the question written at the
+head of this lecture: "Are we to believe that there are other
+minds than ours?"
+
+162
+
+Today we consider the question in a very different spirit. Few
+would regard it as either foolish or improper. Its intense
+interest would be admitted by all, and but for the limitations
+closing our way on every side it would, doubtless, attract the
+most earnest investigation. Even on the mere balance of judgment
+between the probable and the improbable, we have little to go on.
+We know nothing definitely as to the conditions under which life
+may originate: whether these are such as to be rare almost to
+impossibility, or common almost to certainty. Only within narrow
+limits of temperature and in presence of certain of the elements,
+can life like ours exist, and outside these conditions life, if
+such there be, must be different from ours. Once originated it is
+so constituted as to assail the energies around it and to advance
+from less to greater. Do we know more than these vague facts?
+Yes, we have in our experience one other fact and one involving
+much.
+
+We know that our world is very old; that life has been for many
+millions of years upon it; and that Man as a thinking being is
+but of yesterday. Here is then a condition to be fulfilled. To
+every world is physically assigned a limit to the period during
+which it is habitable according to our knowledge of life and its
+necessities. This limit passed and rationality missed, the chance
+for that world is gone for ever, and other minds than ours
+assuredly will not from it contemplate the universe. Looking at
+our own world we see that the tree of life has,
+
+163
+
+indeed, branched, leaved and, possibly, budded many times; it
+never bloomed but once.
+
+All difficulties dissolve and speculations become needless under
+one condition only: that in which rationality may be inferred
+directly or indirectly by our observations on some sister world
+in space, This is just the evidence which in recent years has
+been claimed as derived from a study of the surface of Mars. To
+that planet our hope of such evidence is restricted. Our survey
+in all other directions is barred by insurmountable difficulties.
+Unless some meteoric record reached our Earth, revelationary of
+intelligence on a perished world, our only hope of obtaining such
+evidence rests on the observation of Mars' surface features. To
+this subject we confine our attention in what follows.
+
+The observations made during recent years upon the surface
+features of Mars have, excusably enough, given rise to
+sensational reports. We must consider under what circumstances
+these observations have been made.
+
+Mars comes into particularly favourable conditions for
+observation every fifteen years. It is true that every two years
+and two months we overtake him in his orbit and he is then in
+"opposition." That is, the Earth is between him and the sun: he
+is therefore in the opposite part of the heavens to the sun. Now
+Mars' orbit is very excentric, sometimes he is 139 million miles
+from the sun, and sometimes he as as much as 154 million miles
+from the sun. The Earth's orbit is, by comparison, almost
+
+164
+
+a circle. Evidently if we pass him when he is nearest to the sun
+we see him at his best; not only because he is then nearest to
+us, but because he is then also most brightly lit. In such
+favourable oppositions we are within 35 million miles of him; if
+Mars was in aphelion we would pass him at a distance of 61
+million miles. Opposition occurs under the most favourable
+circumstances every fifteen years. There was one in 1862, another
+in 1877, one in 1892, and so on.
+
+When Mars is 35 million miles off and we apply a telescope
+magnifying 1,000 diameters, we see him as if placed 35,000 miles
+off. This would be seven times nearer than we see the moon with
+the naked eye. As Mars has a diameter about twice as great as
+that of the moon, at such a distance he would look fourteen times
+the diameter of the moon. Granting favourable conditions of
+atmosphere much should be seen.
+
+But these are just the conditions of atmosphere of which most of
+the European observatories cannot boast. It is to the honour of
+Schiaparelli, of Milan, that under comparatively unfavourable
+conditions and with a small instrument, he so far outstripped his
+contemporaries in the observation of the features of Mars that
+those contemporaries received much of his early discoveries with
+scepticism. Light and dark outlines and patches on the planet's
+surface had indeed been mapped by others, and even a couple of
+the canals sighted; but at the opposition of 1877 Schiaparelli
+first mapped any considerable
+
+165
+
+number of the celebrated "canals" and showed that these
+constituted an extraordinary and characteristic feature of the
+planet's geography. He called them "canali," meaning thereby
+"channels." It is remarkable indeed that a mistranslation appears
+really responsible for the initiation of the idea that these
+features are canals.
+
+In 1882 Schiaparelli startled the astronomical world by declaring
+that he saw some of the canals double--that is appearing as two
+parallel lines. As these lines span the planet's surface for
+distances of many thousands of miles the announcement naturally
+gave rise to much surprise and, as I have said, to much
+scepticism. But he resolutely stuck to his statement. Here is his
+map of 1882. It is sufficiently startling.
+
+In 1892 he drew a new map. It adds a little to the former map,
+but the doubling was not so well seen. It is just the strangest
+feature about this doubling that at times it is conspicuous, at
+times invisible. A line which is distinctly seen as a single line
+at one time, a few weeks later will appear distinctly to consist
+of two parallel lines; like railway tracks, but tracks perhaps
+200 miles apart and up to 3,000 or even 4,000 miles in length.
+
+Many speculations were, of course, made to account for the origin
+of such features. No known surface peculiarity on the Earth or
+moon at all resembles these features. The moon's surface as you
+know is cracked and
+
+166
+
+streaked. But the cracks are what we generally find cracks to
+be--either aimless, wandering lines, or, if radiating from a
+centre, then lines which contract in width as they leave the
+point of rupture. Where will we find cracks accurately parallel
+to one another sweeping round a planet's face with steady
+curvature for, 4,000 miles, and crossing each other as if quite
+unhampered by one another's presence? If the phenomenon on Mars
+be due to cracks they imply a uniformity in thickness and
+strength of crust, a homogeneity, quite beyond all anticipation.
+We will afterwards see that the course of the lines is itself
+further opposed to the theory that haphazard cracking of the
+crust of the planet is responsible for the lines. It was also
+suggested that the surface of the planet was covered with ice and
+that these were cracks in the ice. This theory has even greater
+difficulties than the last to contend with. Rivers have been
+suggested. A glance at our own maps at once disposes of this
+hypothesis. Rivers wander just as cracks do and parallel rivers
+like parallel cracks are unknown.
+
+In time the many suggestions were put aside. One only remained.
+That the lines are actually the work of intelligence; actually
+are canals, artificially made, constructed for irrigation
+purposes on a scale of which we can hardly form any conception
+based on our own earthly engineering structures.
+
+During the opposition of 1894, Percival Lowell, along with A. E.
+Douglass, and W. H. Pickering,
+
+167
+
+observed the planet from the summit of a mountain in Arizona,
+using an 18-inch refracting telescope and every resource of
+delicate measurement and spectroscopy. So superb a climate
+favoured them that for ten months the planet was kept under
+continual observation. Over 900 drawings were made and not only
+were Schiaparelli's channels confirmed, but they added 116 to his
+79, on that portion of the planet visible at that opposition.
+They made the further important discovery that the lines do not
+stop short at the dark regions of the planet's surface, as
+hitherto believed, but go right on in many cases; the curvature
+of the lines being unaltered.
+
+Lowell is an uncompromising advocate of the "canal" theory. If
+his arguments are correct we have at once an answer to our
+question, "Are there other minds than ours?"
+
+We must consider a moment Lowell's arguments; not that it is my
+intention to combat them. You must form your own conclusions. I
+shall lay before you another and, as I venture to think, more
+adequate hypothesis in explanation of the channels of
+Schiaparelli. We learn, however, much from Lowell's book--it is
+full of interest.[1]
+
+Lowell lays a deep foundation. He begins by showing that Mars has
+an atmosphere. This must be granted him till some counter
+observations are made.
+
+[1] _Mars_, by Percival Lowell (Longmans, Green & Co.), 1896,
+
+168
+
+It is generally accepted. What that atmosphere is, is another
+matter. He certainly has made out a good case for the presence of
+water as one of its constituents,
+
+It was long known that Mars possessed white regions at his poles,
+just as our Earth does. The waning of these polar snows--if indeed
+they are such--with the advance of the Martian summer, had often
+been observed. Lowell plots day by day this waning. It is evident
+from his observations that the snowfall must be light indeed. We
+see in his map the south pole turned towards us. Mars in
+perihelion always turns his south pole towards the sun and
+therefore towards the Earth. We see that between the dates June
+3rd to August 3rd--or in two months--the polar snow had almost
+completely vanished. This denotes a very scanty covering. It must
+be remembered that Mars even when nearest to the sun receives but
+half our supply of solar heat and light.
+
+But other evidence exists to show that Mars probably possesses
+but little water upon his surface. The dark places are not
+water-covered, although they have been named as if they were,
+indeed, seas and lakes. Various phenomena show this. The canals
+show it. It would never do to imagine canals crossing the seas.
+No great rivers are visible. There is a striking absence of
+clouds. The atmosphere of Mars seems as serene as that of Venus
+appears to be cloudy. Mists and clouds, however, sometime appear
+to veil his face and add to the difficulty of
+
+169
+
+making observations near the limb of the planet. Lowell concludes
+it must be a calm and serene atmosphere; probably only
+one-seventh of our own in density. The normal height of the
+barometer in Mars would then be but four and a half inches. This
+is a pressure far less than exists on the top of the highest
+terrestrial mountain. A mountain here must have an altitude of
+about ten miles to possess so low a pressure on its summit. Drops
+of water big enough to form rain can hardly collect in such a
+rarefied atmosphere. Moisture will fall as dew or frost upon the
+ground. The days will be hot owing to the unimpeded solar
+radiation; the nights bitterly cold owing to the free radiation
+into space.
+
+We may add that in such a climate the frost will descend
+principally upon the high ground at night time and in the
+advancing day it will melt. The freer radiation brings about this
+phenomenon among our own mountains in clear and calm weather.
+
+With the progressive melting of the snow upon the pole Lowell
+connected many phenomena upon the planet's surface of much
+interest. The dark spaces appear to grow darker and more
+greenish. The canals begin to show themselves and reveal their
+double nature. All this suggests that the moisture liberated by
+the melting of the polar snow with the advancing year, is
+carrying vitality and springtime over the surface of the planet.
+But how is the water conveyed?
+
+Lowell believes principally by the canals. These are
+
+170
+
+constructed triangulating the surface of the planet in all
+directions. What we see, according to Lowell, is not the canal
+itself, but the broad band of vegetation which springs up on the
+arrival of the water. This band is perhaps thirty or forty miles
+wide, but perhaps much less, for Lowell reports that the better
+the conditions of observation the finer the lines appeared, so
+that they may be as narrow, possibly, as fifteen miles. It is to
+be remarked that a just visible dot on the surface of Mars must
+possess a diameter of 30 miles. But a chain of much smaller dots
+will be visible, just as we can see such fine objects as spiders'
+webs. The widening of the canals is then accounted for, according
+to Lowell, by the growth of a band of vegetation, similar to that
+which springs into existence when the floods of the Nile irrigate
+the plains of Egypt.
+
+If no other explanation of the lines is forthcoming than that
+they are the work of intelligence, all this must be remembered.
+If all other theories fail us, much must be granted Lowell. We
+must not reason like fishes--as Lowell puts it--and deny that
+intelligent beings can thrive in an atmospheric pressure of four
+and half inches of mercury. Zurbriggen has recently got to the
+top of Aconcagua, a height of 24,000 feet. On the summit of such
+a mountain the barometer must stand at about ten inches. Why
+should not beings be developed by evolution with a lung capacity
+capable of living at two and a half times this altitude. Those
+steadily
+
+171
+
+curved parallel lines are, indeed, very unlike anything we have
+experience of. It would be rather to be expected that another
+civilisation than our own would present many wide differences in
+its development.
+
+What then is the picture we have before us according to Lowell?
+It is a sufficiently dramatic one.
+
+Mars is a world whose water supply, never probably very abundant,
+has through countless years been drying up, sinking into his
+surface. But the inhabitants are making a brave fight for it,
+They have constructed canals right round their world so that the
+water, which otherwise would run to waste over the vast deserts,
+is led from oasis to oasis. Here the great centres of
+civilisation are placed: their Londons, Viennas, New Yorks. These
+gigantic works are the works of despair. A great and civilised
+world finds death staring it in the face. They have had to triple
+their canals so that when the central canal has done its work the
+water is turned into the side canals, in order to utilise it as
+far as possible. Through their splendid telescopes they must view
+our seas and ample rivers; and must die like travellers in the
+desert seeing in a mirage the cool waters of a distant lake.
+
+Perhaps that lonely signal reported to have been seen in the
+twilight limb of Mars was the outcome of pride in their splendid
+and perishing civilisation. They would leave some memory of it:
+they would have us witness how great was that civilisation before
+they perish!
+
+I close this dramatic picture with the poor comfort
+
+172
+
+that several philanthropic people have suggested signalling to
+them as a mark of sympathy. It is said that a fortune was
+bequeathed to the French Academy for the purpose of communicating
+with the Martians. It has been suggested that we could flash
+signals to them by means of gigantic mirrors reflecting the light
+of our Sun. Or, again, that we might light bonfires on a
+sufficiently large scale. They would have to be about ten miles
+in diameter! A writer in the Pall Mall Gazette suggested that
+there need really be no difficulty in the matter. With the kind
+cooperation of the London Gas Companies (this was before the days
+of electric lighting) a signal might be sent without any
+additional expense if the gas companies would consent to
+simultaneously turn off the gas at intervals of five minutes over
+the whole of London, a signal which would be visible to the
+astronomers in Mars would result. He adds, naively: "If only
+tried for an hour each night some results might be obtained."
+
+II
+
+We have reviewed the theory of the artificial construction of the
+Martian lines. The amount of consideration we are disposed to
+give to the supposition that there are upon Mars other minds than
+ours will--as I have stated--necessarily depend upon whether or not
+we can assign a probable explanation of the lines upon purely
+physical grounds. If it is apparent that such
+
+173
+
+lines would be formed with great probability under certain
+conditions, which conditions are themselves probable, then the
+argument by exclusion for the existence of civilisation on Mars,
+at once breaks down.
+
+{Fig. 10}
+
+As a romance writer is sometimes under the necessity of
+transporting his readers to other scenes, so I must now ask you
+to consent to be transported some millions
+
+174
+
+of miles into the region of the heavens which lies outside Mars'
+orbit.
+
+Between Mars and Jupiter is a chasm of 341 millions of miles.
+This gap in the sequence of planets was long known to be quite
+out of keeping with the orderly succession of worlds outward from
+the Sun. A society was formed at the close of the last century
+for the detection of the missing world. On the first day of the
+last century, Piazzi--who, by the way, was not a member of the
+society--discovered a tiny world in the vacant gap. Although
+eagerly welcomed, as better than nothing, it was a disappointing
+find. The new world was a mere rock. A speck of about 160 miles
+in diameter. It was obviously never intended that such a body
+should have all this space to itself. And, sure enough, shortly
+after, another small world was discovered. Then another was
+found, and another, and so on; and now more than 400 of these
+strange little worlds are known.
+
+But whence came such bodies? The generally accepted belief is
+that these really represent a misbegotten world. When the Sun was
+younger he shed off the several worlds of our system as so many
+rings. Each ring then coalesced into a world. Neptune being the
+first born; Mercury the youngest born.
+
+After Jupiter was thrown off, and the Sun had shrunk away inwards
+some 20o million miles, he shed off another ring. Meaning that
+this offspring of his should grow up like the rest, develop into
+a stable world with the
+
+175
+
+potentiality even, it may be, of becoming the abode of rational
+beings. But something went wrong. It broke up into a ring of
+little bodies, circulating around him.
+
+It is probable on this hypothesis that the number we are
+acquainted with does not nearly represent the actual number of
+past and present asteroids. It would take 125,000 of the biggest
+of them to make up a globe as big as our world. They, so far as
+they are known, vary in size from 10 miles to 160 miles in
+diameter. It is probable then--on the assumption that this failure
+of a world was intended to be about the mass of our Earth--that
+they numbered, and possibly number, many hundreds of thousands.
+
+Some of these little bodies are very peculiar in respect to the
+orbits they move in. This peculiarity is sometimes in the
+eccentricity of their orbits, sometimes in the manner in which
+their orbits are tilted to the general plane of the ecliptic, in
+which all the other planets move.
+
+The eccentricity, according to Proctor, in some cases may attain
+such extremes as to bring the little world inside Mars' mean
+distance from the sun. This, as you will remember, is very much
+less than his greatest distance from the sun. The entire belt of
+asteroids--as known--lie much nearer to Mars than to Jupiter.
+
+As regards the tilt of their orbits, some are actually as much as
+34 degrees inclined to the ecliptic, so that in fact they are
+seen from the Earth among our polar constellations.
+
+176
+
+From all this you see that Mars occupies a rather hot comer in
+the solar system. Is it not possible that more than once in the
+remote past Mars may have encountered one of these wanderers? If
+he came within a certain distance of the small body his great
+mass would sway it from its orbit, and under certain conditions
+he would pick up a satellite in this manner. That his present
+satellites were actually so acquired is the suggestion of Newton,
+of Yale College.
+
+Mars' satellites are indeed suspiciously and most abnormally
+small. I have not time to prove this to you by comparison with
+the other worlds of the solar system. In fact, they were not
+discovered till 1877--although they were predicted in a most
+curious manner, with the most uncannily accurate details, by
+Swift.
+
+One of these bodies is about 36 miles in diameter. This is
+Phobos. Phobos is only 3.700 miles from the surface of Mars. The
+other is smaller and further off. He is named Deimos, and his
+diameter is only 10 miles. He is 12,500 miles from Mars' surface.
+With the exception of Phobos the next smallest satellite known in
+the solar system is one of Saturn's--Hyperion; almost 800 miles in
+diameter. The inner one goes all round Mars in 7½ hours. This is
+Phobos' month. Mars turns on his axis in 24 hours and 40 minutes,
+so that people in Mars would see the rise of Phobos twice in the
+course of a day and night; lie would apparently cross the sky
+
+177
+
+going against the other satellite; that is, he would move
+apparently from west to east.
+
+We may at least assume as probable that other satellites have
+been gathered by Mars in the past from the army of asteroids.
+
+Some of the satellites so picked up would be direct: that is,
+would move round the planet in the direction of his axial
+rotation. Others, on the chances, would be retrograde: that is,
+would move against his axial rotation. They would describe orbits
+making the same various angles with the ecliptic as do the
+asteroids; and we may be sure they would be of the same varying
+dimensions.
+
+We go on to inquire what would be the consequence to Mars of such
+captures.
+
+A satellite captured in this manner is very likely to be pulled
+into the Planet. This is a probable end of a satellite in any
+case. It will probably be the end of our satellite too. The
+satellite Phobos is indeed believed to be about to take this very
+plunge into his planet. But in the case when the satellite picked
+up happens to be rotating round the planet in the opposite
+direction to the axial rotation of the planet, it is pretty
+certain that its career as a satellite will be a brief one. The
+reasons for this I cannot now give. If, then, Mars picked up
+satellites he is very sure to have absorbed them sooner or later.
+Sooner if they happened to be retrograde satellites, later if
+direct satellites. His present satellites are recent additions.
+They are direct.
+
+178
+
+The path of an expiring satellite will be a slow spiral described
+round the planet. The spiral will at last, after many years,
+bring the satellite down upon the surface of the primary. Its
+final approach will be accelerated if the planet possesses an
+atmosphere, as Mars probably does. A satellite of the dimensions
+of Phobos--that is 36 miles in diameter--would hardly survive more
+than 30 to 60 years within seventy miles of Mars' surface. It
+will then be rotating round Mars in an hour and forty minutes,
+moving, in fact, at the rate of 2.2 miles per second. In the
+course of this 30 or 60 years it will, therefore, get round
+perhaps 200,000 times, before it finally crashes down upon the
+Martians. During this closing history of the satellite there is
+reason to believe, however, that it would by no means pursue
+continually the same path over the surface of the planet. There
+are many disturbing factors to be considered. Being so small any
+large surface features of Mars would probably act to perturb the
+orbit of the satellite.
+
+The explanation of Mars' lines which I suggest, is that they were
+formed by the approach of such satellites in former times. I do
+not mean that they are lines cut into his surface by the actual
+infall of a satellite. The final end of the satellite would be
+too rapid for this, I think. But I hope to be able to show you
+that there is reason to believe that the mere passage of the
+satellite, say at 70 miles above the surface of the planet, will,
+in itself, give rise to effects on the crust of the planet
+capable
+
+179
+
+of accounting for just such single or parallel lines as we see.
+
+In the first place we have to consider the stability of the
+satellite. Even in the case of a small satellite we cannot
+overlook the fact that the half of the satellite near the planet
+is pulled towards the planet by a gravitational force greater
+than that attracting the outer half, and that the centrifugal
+force is less on the inner than on the outer hemisphere. Hence
+there exists a force tending to tear the satellite asunder on the
+equatorial section tangential
+
+{Fig. 11}
+
+to the planet's surface. If in a fluid or plastic state, Phobos,
+for instance, could not possibly exist near the planet's surface.
+The forces referred to would decide its fate. It may be shown by
+calculation, however, that if Phobos has the strength of basalt
+or glass there would remain a considerable coefficient of safety
+in favour of the satellite's stability; even when the surfaces of
+planet and satellite were separated by only five miles.
+
+We have now to consider some things which we expect will happen
+before the satellite takes its final plunge into the planet.
+
+180
+
+This diagram (Fig. 11) shows you the satellite travelling above
+the surface of the planet. The satellite is advancing towards, or
+away from, the spectator. The planet is supposed to show its
+solid crust in cross section, which may be a few miles in
+thickness. Below this is such a hot plastic magma as we have
+reason to believe underlies much of the solid crust of our own
+Earth. Now there is an attraction between the satellite and the
+crust of the planet; the same gravitational attraction which
+exists between every particle of matter in the universe. Let us
+consider how this attraction will affect the planet's crust. I
+have drawn little arrows to show how we may consider the
+attraction of the satellite pulling the crust of the planet not
+only upwards, but also pulling it inwards beneath the satellite.
+I have made these arrows longer where calculation shows the
+stress is greater. You see that the greatest lifting stress is
+just beneath the satellite, whereas the greatest stress pulling
+the crust in under the satellite is at a point which lies out
+from under the satellite, at a considerable distance. At each
+side of the satellite there is a point where the stress pulling
+on the crust is the greatest. Of the two stresses the lifting
+stress will tend to raise the crust a little; the pulling stress
+may in certain cases actually tear the crust across; as at A and
+B.
+
+It is possible to calculate the amount of the stress at the point
+at each side of the satellite where the stress is at its
+greatest. We must assume the satellite to be a certain size and
+density; we must also assume the crust of
+
+181
+
+Mars to be of some certain density. To fix our ideas on these
+points I take the case of the present satellite Phobos. What
+amount of stress will he exert upon the crust of Mars when he
+approaches within, say, 40 miles of the planet's surface? We know
+his size approximately--he is about 36 miles in diameter. We can
+guess his density to be between four times that of water and
+eight times that of water. We may assume the density of Mars'
+surface to be about the same as that of our Earth's surface, that
+is three times as dense as water. We now find that the greatest
+stress tending to rend open the surface crust of Mars will be
+between 4,000 and 8,000 pounds to the square foot according to
+the density we assign to Phobos.
+
+Will such a stress actually tear open the crust? We are not able
+to answer this question with any certainty. Much will depend upon
+the nature and condition of the crust. Thus, suppose that we are
+here (Fig. 12) looking down upon the satellite which is moving
+along slowly relatively to Mars' surface, in the direction of the
+arrow. The satellite has just passed over a weak and cracked part
+of the planet's crust. Here the stress has been sufficient to
+start two cracks. Now you know how easy it is to tear a piece of
+cloth when you go to the edge of it in order to make a beginning.
+Here the stress from the satellite has got to the edge of the
+crust. It is greatly concentrated just at the extremities of the
+cracks. It will, unler such circumstances probably carry on the
+
+182
+
+tear. If it does not do so this time, remember the satellite will
+some hours later be coming over the same place again, and then
+again for, at least, many hundreds of times. Then also we are not
+limited to the assumption that the
+
+{Fig. 12}
+
+satellite is as small as Phobos. Suppose we consider the case of
+a satellite approaching Mars which has a diameter double that of
+Phobos; a diameter still much less than that of the larger class
+of asteroids. Even at the distance
+
+183
+
+of 65 miles the stress will now amount to as much as from 15 to
+30 tons per square foot. It is almost certain that such a stress
+repeated a comparatively few times over the same parts of the
+planet's surface would so rend the crust as to set up lines along
+which plutonic action would find a vent. That is, we might expect
+along these lines all the phenomena of upheaval and volcanic
+eruption which give rise to surface elevations.
+
+The probable effect of a satellite of this dimension travelling
+slowly relatively to the surface of Mars is, then, to leave a
+very conspicuous memorial of his presence behind him. You see
+from the diagram that this memorial will consist o: two parallel
+lines of disturbance.
+
+The linear character of the gravitational effects of the
+satellite is due entirely to the motion of the satellite
+relatively to the surface of the planet. If the satellite stood
+still above the surface the gravitational stress in the crust
+would, of course, be exerted radially outwards from the centre of
+the satellite. It would attain at the central point beneath the
+satellite its maximum vertical effect, and at some radial
+distance measured outwards from this point, which distance we can
+calculate, its maximum horizontal tearing effect. When the
+satellite moves relatively to the planet's crust, the horizontal
+tearing force acts differently according to whether it is
+directed in the line of motion or at right angles to this line.
+
+In the direction of motion we see that the satellite
+
+184
+
+creates as it passes over the crust a wave of rarefaction or
+tension as at D, followed by compression just beneath the
+satellite and by a reversed direction of gravitational pull as
+the satellite passes onwards. These stresses rapidly replace one
+another as the satellite travels along. They are resisted by the
+inertia of the crust, and are taken up by its elasticity. The
+nature of this succession of alternate compressions and
+rarefactions in the crust possess some resemblance to those
+arising in an earthquake shock.
+
+If we consider the effects taking place laterally to the line of
+motion we see that there are no such changes in the nature of the
+forces in the crust. At each passage of the satellite the
+horizontal tearing stress increases to a maximum, when it is
+exerted laterally, along the line passing through the horizontal
+projection of the satellite and at right angles to the line of
+motion, and again dies away. It is always a tearing stress,
+renewed again and again.
+
+This effect is at its maximum along two particular parallel lines
+which are tangents to the circle of maximum horizontal stress and
+which run parallel with the path of the satellite. The distance
+separating these lines depend upon the elevation of the satellite
+above the planet's surface. Such lines mark out the theoretical
+axes of the "double canals" which future crustal movements will
+more fully develop.
+
+It is interesting to consider what the effect of such
+
+185
+
+conditions would be if they arose at the surface of our own
+planet. We assume a horizontal force in the crust adequate to set
+up tensile stresses of the order, say, of fifteen tons to the
+square foot and these stresses to be repeated every few hours;
+our world being also subject to the dynamic effects we recognise
+in and beneath its crust.
+
+It is easy to see that the areas over which the satellite exerted
+its gravitational stresses must become the foci --foci of linear
+form--of tectonic developments or crust movements. The relief of
+stresses, from whatever cause arising, in and beneath the crust
+must surely take place in these regions of disturbance and along
+these linear areas. Here must become concentrated the folding
+movements, which are under existing conditions brought into the
+geosynclines, along with their attendant volcanic phenomena. In
+the case of Mars such a concentration of tectonic events would
+not, owing to the absence of extensive subaerial denudation and
+great oceans, be complicated by the existence of such synclinal
+accumulations as have controlled terrestrial surface development.
+With the passage of time the linear features would probably
+develop; the energetic substratum continually asserting its
+influence along such lines of weakness. It is in the highest
+degree probable that radioactivity plays no less a part in
+Martian history than in terrestrial. The fact of radioactive
+heating allows us to assume the thin surface crust and continued
+sub-crustal energy throughout the entire period of the planet's
+history.
+
+186
+
+How far willl these effects resemble the double canals of Mars?
+In this figure and in the calculations I have given you I have
+supposed the satellite engaged in marking the planet's surface
+with two lines separated by about the interval separating the
+wider double canals of Mars--that is about 220 miles apart. What
+the distance between the lines will be, as already stated, will
+depend upon the height of the satellite above the surface when it
+comes upon a part of the crust in a condition to be affected by
+the stresses it sets up in it. If the satellite does its work at
+a point lower down above the surface the canal produced will be
+narrower. The stresses, too, will then be much greater. I must
+also observe that once the crust has yielded to the pulling
+stress, there is great probability that in future revolutions of
+the satellite a central fracture will result. For then all the
+pulling force adds itself to the lifting force and tends to crush
+the crust inwards on the central line beneath the satellite. It
+is thus quite possible that the passage of a satellite may give
+rise to triple lines. There is reason to believe that the canals
+on Mars are in some cases triple.
+
+I have spoken all along of the satellite moving slowly over the
+surface of Mars. I have done so as I cannot at all pronounce so
+readily on what will happen when the satellite's velocity over
+the surface of Mars is very great. To account for all the lines
+mapped by Lowell some of them must have been produced by
+satellities moving relatively to the surface of Mars at
+velocities so great
+
+187
+
+as three miles a second or even rather more. The stresses set up
+are, in such cases, very difficult to estimate. It has not yet
+been done. Parallel lines of greatest stress or impulse ought to
+be formed as in the other case.
+
+I now ask your attention to another kind of evidence that the
+lines are due in some way to the motion of satellites passing
+over the surface of Mars.
+
+I may put the fresh evidence to which I refer, in this way: In
+Lowell's map (P1. XXII, p. 192), and in a less degree in
+Schiaparelli's map (ante p. 166), we are given the course of the
+lines as fragments of incomplete curves. Now these curves might
+have been anything at all. We must take them as they are,
+however, when we apply them as a test of the theory that the
+motion of a satellite round Mars can strike such lines. If it can
+be shown that satellites revolving round Mars might strike just
+such curves then we assume this as an added confirmation of the
+hypothesis.
+
+We must begin by realising what sort of curves a satellite which
+disturbs the surface of a planet would leave behind it after its
+demise. You might think that the satellite revolving round and
+round the planet must simply describe a circle upon the spherical
+surface of the planet: a "great circle" as it is called; that is
+the greatest circle which can be described upon a sphere. This
+great circle can, however, only be struck, as you will see, when
+the planet is not turning upon its axis: a condition not likely
+to be realised.
+
+This diagram (PI. XXI) shows the surface of a globe
+
+188
+
+covered with the usual imaginary lines of latitude and longitude.
+The orbit of a supposed satellite is shown by a line crossing the
+sphere at some assumed angle with the equator. Along this line
+the satellite always moves at uniform velocity, passing across
+and round the back of the sphere and again across. If the sphere
+is not turning on its polar axis then this satellite, which we
+will suppose armed with a pencil which draws a line upon the
+sphere, will strike a great circle right round the sphere. But
+the sphere is rotating. And it is to be expected that at
+different times in a planet's history the rate of rotation varies
+very much indeed. There is reason to believe that our own day was
+once only 2½ hours long, or thereabouts. After a preliminary rise
+in velocity of axial rotation, due to shrinkage attending rapid
+cooling, a planet as it advances in years rotates slower and
+slower. This phenomenon is due to tidal influences of the sun or
+of satellites. On the assumption that satellites fell into Mars
+there would in his case be a further action tending to shorten
+his day as time went on.
+
+The effect of the rotation of the planet will be, of course, that
+as the satellite advances with its pencil it finds the surface of
+the sphere being displaced from under it. The line struck ceases
+to be the great circle but wanders off in another curve--which is
+in fact not a circle at all.
+
+You will readily see how we find this curve. Suppose the sphere
+to be rotating at such a speed that while the satellite is
+advancing the distance _Oa_, the point _b_ on the
+
+189
+
+sphere will be carried into the path of the satellite. The pencil
+will mark this point. Similarly we find that all the points along
+this full curved line are points which will just find themselves
+under the satellite as it passes with its pencil. This curve is
+then the track marked out by the revolving satellite. You see it
+dotted round the back of the sphere to where it cuts the equator
+at a certain point. The course of the curve and the point where
+it cuts the equator, before proceeding on its way, entirely
+depend upon the rate at which we suppose the sphere to be
+rotating and the satellite to be describing the orbit. We may
+call the distance measured round the planet's equator separating
+the starting point of the curve from the point at which it again
+meets the equator, the "span" of the curve. The span then depends
+entirely upon the rate of rotation of the planet on its axis and
+of the satellite in its orbit round the planet.
+
+But the nature of events might have been somewhat different. The
+satellite is, in the figure, supposed to be rotating round the
+sphere in the same direction as that in which the sphere is
+turning. It might have been that Mars had picked up a satellite
+travelling in the opposite direction to that in which he was
+turning. With the velocity of planet on its axis and of satellite
+in its orbit the same as before, a different curve would have
+been described. The span of the curve due to a retrograde
+satellite will be greater than that due to a direct satellite.
+The retrograde satellite will have a span more than half
+
+190
+
+way round the planet, the direct satellite will describe a curve
+which will be less than half way round the planet: that is a span
+due to a retrograde satellite will be more than 180 degrees,
+while the span due to a direct satellite will be less than 180
+degrees upon the planet's equator.
+
+I would draw your attention to the fact that what the span will
+be does not depend upon how much the orbit of the satellite is
+inclined to the equator. This only decides how far the curve
+marked out by the satellite will recede from the equator.
+
+We find then, so far, that it is easy to distinguish between the
+direct and the retrograde curves. The span of one is less, of the
+other greater, than 180 degrees. The number of degrees which
+either sort of curve subtends upon the equator entirely depends
+upon the velocity of the satellite and the axial velocity of the
+planet.
+
+But of these two velocities that of the satellite may be taken as
+sensibly invariable, when close enough to use his pencil. This
+depends upon the law of centrifugal force, which teaches us that
+the mass of the planet alone decides the velocity of a satellite
+in its orbit at any fixed distance from the planet's centre. The
+other velocity--that of the planet upon its axis--was, as we have
+seen, not in the past what it is now. If then Mars, at various
+times in his past history, picked up satellites, these satellites
+will describe curves round him having different spans which will
+depend upon the velocity of axial rotation of Mars at the time
+and upon this only.
+
+191
+
+In what way now can we apply this knowledge of the curves
+described by a satellite as a test of the lunar origin of the
+lines on Mars?
+
+To do this we must apply to Lowell's map. We pick out preferably,
+of course, the most complete and definite curves. The chain of
+canals of which Acheron and Erebus are members mark out a fairly
+definite curve. We produce it by eye, preserving the curvature as
+far as possible, till it cuts the equator. Reading the span on
+the equator we find' it to be 255 degrees. In the first place we
+say then that this curve is due to a retrograde satellite. We
+also note on Lowell's map that the greatest rise of the curve is
+to a point about 32 degrees north of the equator. This gives the
+inclination of the satellite's orbit to the plane of Mars'
+equator.
+
+With these data we calculate the velocity which the planet must
+have possessed at the time the canal was formed on the hypothesis
+that the curve was indeed the work of a satellite. The final
+question now remains If we determine the curve due to this
+velocity of Mars on its axis, will this curve fit that one which
+appears on Lowell's map, and of which we have really availed
+ourselves of only three points? To answer this question we plot
+upon a sphere, the curve of a satellite, in the manner I have
+described, assigning to this sphere the velocity derived from the
+span of 255 degrees. Having plotted the curve on the sphere it
+only remains to transfer it to Lowell's map. This is easily
+done.
+
+192
+
+This map (Pl. XXII) shows you the result of treating this, as
+well as other curves, in the manner just described. You see that
+whether the fragmentary curves are steep and receding far from
+the equator; or whether they are flat and lying close along the
+equator; whether they span less or more than 180 degrees; the
+curves determined on the supposition that they are the work of
+satellites revolving round Mars agree with the mapped curves;
+following them with wonderful accuracy; possessing their
+properties, and, indeed, in some cases, actually coinciding with
+them.
+
+I may add that the inadmissible span of 180 degrees and spans
+very near this value, which are not well admissible, are so far
+as I can find, absent. The curves are not great circles.
+
+You will require of me that I should explain the centres of
+radiation so conspicuous here and there on Lowell's map. The
+meeting of more than two lines at the oases is a phenomenon
+possibly of the same nature and also requiring explanation.
+
+In the first place the curves to which I have but briefly
+referred actually give rise in most cases to nodal, or crossing
+points; sometimes on the equator, sometimes off the equator;
+through which the path of the satellite returns again and again.
+These nodal points will not, however, afford a general
+explanation of the many-branched radiants.
+
+It is probable that we should refer such an appearance
+
+193
+
+as is shown at the Sinus Titanum to the perturbations of the
+satellite's path due to the surface features on Mars. Observe
+that the principal radiants are situated upon the boundary of the
+dark regions or at the oases. Higher surface levels may be
+involved in both cases. Some marked difference in topography must
+characterise both these features. The latter may possibly
+originate in the destruction of satellites. Or again, they may
+arise in crustal disturbance of a volcanic nature, primarily
+induced or localised by the crossing of two canals. Whatever the
+origin of these features it is only necessary to assume that they
+represent elevated features of some magnitude to explain the
+multiplication of crossing lines. We must here recall what
+observers say of the multiplicity of the canals. According to
+Lowell, "What their number maybe lies quite beyond the
+possibility of count at present; for the better our own air, the
+more of them are visible."
+
+Such innumerable canals are just what the present theory
+requires. An in-falling satellite will, in the course of the last
+60 or 80 years of its career, circulate some 100,000 times over
+Mars' surface. Now what will determine the more conspicuous
+development of a particular canal? The mass of the satellite; the
+state of the surface crust; the proximity of the satellite; and
+the amount of repetition over the same ground. The after effects
+may be taken as proportional to the primary disturbance.
+
+194
+
+It is probable that elevated surface features will influence two
+of these conditions: the number of repetitions and the proximity
+to the surface. A tract 100 miles in diameter and elevated 5,000
+or 10,000 feet would seriously perturb the orbit of such a body as
+Phobos. It is to be expected that not only would it be effective
+in swaying the orbit of the satellite in the horizontal direction
+but also would draw it down closer to the surface. It is even to
+be considered if such a mass might not become nodal to the
+satellite's orbit, so that this passed through or above this
+point at various inclinations with its primary direction. If
+acting to bring down the orbit then this will quicken the speed
+and cause the satellite further on its path to attain a somewhat
+higher elevation above the surface. The lines most conspicuous in
+the telescope are, in short, those which have been favoured by a
+combination of circumstances as reviewed above, among which
+crustal features have, in some cases, played a part.
+
+I must briefly refer to what is one of the most interesting
+features of the Martian lines: the manner in which they appear to
+come and go like visions.
+
+Something going on in Mars determines the phenomenon. On a
+particular night a certain line looks single. A few nights later
+signs of doubling are perceived, and later still, when the seeing
+is particularly good, not one but two lines are seen. Thus, as an
+example, we may take the case of Phison and Euphrates. Faint
+glimpses of the dual state were detected in the summer
+
+195
+
+and autumn, but not till November did they appear as distinctly
+double. Observe that by this time the Antarctic snows had melted,
+and there was in addition, sufficient time for the moisture so
+liberated to become diffused in the planet's atmosphere.
+
+This increase in the definition and conspicuousness of certain
+details on Mars' surface is further brought into connection with
+the liberation of the polar snows and the diffusion of this water
+through the atmosphere, by the fact that the definition appeared
+progressively better from the south pole upwards as the snow
+disappeared. Lowell thinks this points to vegetation springing up
+under the influence of moisture; he considers, however, as we
+have seen, that the canals convey the moisture. He has to assume
+the construction of triple canals to explain the doubling of the
+lines.
+
+If we once admit the canals to be elevated ranges--not necessarily
+of great height--the difficulty of accounting for increased
+definition with increase of moisture vanishes. We need not
+necessarily even suppose vegetation concerned. With respect to
+this last possibility we may remark that the colour observations,
+upon which the idea of vegetation is based, are likely to be
+uncertain owing to possible fatigue effects where a dark object
+is seen against a reddish background.
+
+However this may be we have to consider what the effects of
+moisture increasing in the atmosphere of Mars will be with regard
+to the visibility of elevated ranges,
+
+196
+
+We assume a serene and rare atmosphere: the nights intensely
+cold, the days hot with the unveiled solar radiation. On the hill
+tops the cold of night will be still more intense and so, also,
+will the solar radiation by day. The result of this state of
+things will be that the moisture will be precipitated mainly on
+the mountains during the cold of night--in the form of frost--and
+during the day this covering of frost will melt; and, just as we
+see a heavy dew-fall darken the ground in summer, so the melting
+ice will set off the elevated land against the arid plains below.
+Our valleys are more moist than our mountains only because our
+moisture is so abundant that it drains off the mountains into the
+valleys. If moisture was scarce it would distil from the plains
+to the colder elevations of the hills. On this view the
+accentuation of a canal is the result of meteorological effects
+such as would arise in the Martian climate; effects which must be
+influenced by conditions of mountain elevation, atmospheric
+currents, etc. We, thus, follow Lowell in ascribing the
+accentuation of the canals to the circulation of water in Mars;
+but we assume a simple and natural mode of conveyance and do not
+postulate artificial structures of all but impossible magnitude.
+That vegetation may take part in the darkening of the elevated
+tracts is not improbable. Indeed we would expect that in the
+Martian climate these tracts would be the only fertile parts of
+the surface.
+
+Clouds also there certainly are. More recent observations
+
+197
+
+appear to have set this beyond doubt. Their presence obviously
+brings in other possible explanations of the coming and going of
+elevated surface features.
+
+Finally, we may ask what about the reliability of the maps? About
+this it is to be said that the most recent map--that by Lowell--has
+been confirmed by numerous drawings by different observers, and
+that it is,itself the result of over 900 drawings. It has become
+a standard chart of Mars, and while it would be rash to contend
+for absence of errors it appears certain that the trend of the
+principal canals may be relied on, as, also, the general features
+of the planet's surface.
+
+The question of the possibility of illusion has frequently been
+raised. What I have said above to a great extent answers such
+objections. The close agreement between the drawings of different
+observers ought really to set the matter at rest. Recently,
+however, photography has left no further room for scepticism.
+First photographed in 1905, the planet has since been
+photographed many thousands of times from various observatories.
+A majority of the canals have been so mapped. The doubling of the
+canals is stated to have been also so recorded.[1]
+
+The hypothesis which I have ventured to put before you involves
+no organic intervention to account for the
+
+[1] E. C. Slipher's paper in _Popular Astronomy_ for March, 1914,
+gives a good account of the recent work.
+
+198
+
+details on Mars' surface. They are physical surface features.
+Mars presents his history written upon his face in the scars of
+former encounters--like the shield of Sir Launcelot. Some of the
+most interesting inferences of mathematical and physical
+astronomy find a confirmation in his history. The slowing down in
+the rate of axial rotation of the primary; the final inevitable
+destruction of the satellite; the existence in the past of a far
+larger number of asteroids than we at present are acquainted
+with; all these great facts are involved in the theory now
+advanced. If justifiably, then is Mars' face a veritable
+Principia.
+
+To fully answer the question which heads these lectures, we
+should go out into the populous solitudes (if the term be
+permitted) which lie beyond our system. It is well that there is
+now no time left to do so; for, in fact, there we can only dream
+dreams wherein the limits of the possible and the impossible
+become lost.
+
+The marvel of the infinite number of stars is not so marvellous
+as the rationality that fain would comprehend them. In seeking
+other minds than ours we seek for what is almost infinitely
+complex and coordinated in a material universe relatively simple
+and heterogeneous. In our mental attitude towards the great
+question, this fact must be regarded as fundamental.
+
+I can only fitly close a discourse which has throughout weighed
+the question of the living thought against the unthinking laws of
+matter, by a paraphrase of the words
+
+199
+
+of a great poet when he, in higher and, perhaps, more philosophic
+language, also sought to place the one in comparison with the
+other.[1]
+
+Richter thought that he was--with his human heart
+unstrengthened--taken by an angel among the universe of stars.
+Then, as they journeyed, our solar system was sunken like a faint
+star in the abyss, and they travelled yet further, on the wings
+of thought, through mightier systems: through all the countless
+numbers of our galaxy. But at length these also were left behind,
+and faded like a mist into the past. But this was not all. The
+dawn of other galaxies appeared in the void. Stars more countless
+still with insufferable light emerged. And these also were
+passed. And so they went through galaxies without number till at
+length they stood in the great Cathedral of the Universe. Endless
+were the starry aisles; endless the starry columns; infinite the
+arches and the architraves of stars. And the poet saw the mighty
+galaxies as steps descending to infinity, and as steps going up
+to infinity.
+
+Then his human heart fainted and he longed for some narrow cell;
+longed to lie down in the grave that he might hide from infinity.
+And he said to the angel:
+
+"Angel, I can go with thee no farther. Is there, then, no end to
+the universe of stars?"
+
+[1] De Quincy in his _System of the Heavens_ gives a fine
+paraphrase of "Richter's Dream."
+
+200
+
+Then the angel flung up his glorious hands to the heaven of
+heavens, saying "End is there none to the universe of God? Lo!
+also there is no beginning."
+
+201
+
+THE LATENT IMAGE [1]
+
+My inclination has led me, in spite of a lively dread of
+incurring a charge of presumption, to address you principally on
+that profound and most subtle question, the nature and mode of
+formation of the photographic image. I am impelled to do so, not
+only because the subject is full of fascination and hopefulness,
+but because the wide topics of photographic methods or
+photographic applications would be quite unfittingly handled by
+the president you have chosen.
+
+I would first direct your attention to Sir James Dewar's
+remarkable result that the photographic plate retains
+considerable power of forming the latent image at temperatures
+approaching the absolute zero--a result which, as I submit,
+compels us to regard the fundamental effects progressing in the
+film under the stimulus of light undulations as other than those
+of a purely chemical nature. But few, if any, instances of
+chemical combination or decomposition are known at so low a
+temperature. Purely chemical actions cease, indeed, at far higher
+temperatures, fluorine being among the few bodies which still
+show
+
+[1] Presidential address to the Photographic Convention of the
+United Kingdom, July, 1905. _Nature_, Vol. 72, p. 308.
+
+202
+
+chemical activity at the comparatively elevated temperature of
+-180° C. In short, this result of Sir James Dewar's suggests that
+we must seek for the foundations of photographic action in some
+physical or intra-atomic effect which, as in the case of
+radioactivity or fluorescence, is not restricted to intervals of
+temperature over which active molecular vis viva prevails. It
+compels us to regard with doubt the role of oxidation or other
+chemical action as essential, but rather points to the view that
+such effects must be secondary or subsidiary. We feel, in a word,
+that we must turn for guidance to some purely photo-physical
+effect.
+
+Here, in the first place, we naturally recall the views of Bose.
+This physicist would refer the formation of the image to a strain
+of the bromide of silver molecule under the electric force in the
+light wave, converting it into what might be regarded as an
+allotropic modification of the normal bromide which subsequently
+responds specially to the attack of the developer. The function
+of the sensitiser, according to this view, is to retard the
+recovery from strain. Bose obtained many suggestive parallels
+between the strain phenomena he was able to observe in silver and
+other substances under electromagnetic radiation and the
+behaviour of the photographic plate when subjected to
+long-continued exposure to light.
+
+This theory, whatever it may have to recommend it, can hardly be
+regarded as offering a fundamental explanation. In the first
+place, we are left in the dark as to what
+
+203
+
+the strain may be. It may mean many and various things. We know
+nothing as to the inner mechanism of its effects upon subsequent
+chemical actions--or at least we cannot correlate it with what is
+known of the physics of chemical activity. Finally, as will be
+seen later, it is hardly adequate to account for the varying
+degrees of stability which may apparently characterise the latent
+image. Still, there is much in Bose's work deserving of careful
+consideration. He has by no means exhausted the line of
+investigation he has originated.
+
+Another theory has doubtless been in the minds of many. I have
+said we must seek guidance in some photo-physical phenomenon.
+There is one such which preeminently connects light and chemical
+phenomena through the intermediary of the effects of the former
+upon a component part of the atom. I refer to the phenomena of
+photo-electricity.
+
+It was ascertained by Hertz and his immediate successors that
+light has a remarkable power of discharging negative
+electrification from the surface of bodies--especially from
+certain substances. For long no explanation of the cause of this
+appeared. But the electron--the ubiquitous electron--is now known
+with considerable certainty to be responsible. The effect of the
+electric force in the light wave is to direct or assist the
+electrons contained in the substance to escape from the surface
+of the body. Each electron carries away a very small charge of
+negative electrification. If, then, a body is
+
+204
+
+originally charged negatively, it will be gradually discharged by
+this convective process. If it is not charged to start with, the
+electrons will still be liberated at the surface of the body, and
+this will acquire a positive charge. If the body is positively
+charged at first, we cannot discharge it by illumination.
+
+It would be superfluous for me to speak here of the nature of
+electrons or of the various modes in which their presence may be
+detected. Suffice it to say, in further connection with the Hertz
+effect, that when projected among gaseous molecules the electron
+soon attaches itself to one of these. In other words, it ionises
+a molecule of the gas or confers its electric charge upon it. The
+gaseous molecule may even be itself disrupted by impact of the
+electron, if this is moving fast enough, and left bereft of an
+electron.
+
+We must note that such ionisation may be regarded as conferring
+potential chemical properties upon the molecules of the gas and
+upon the substance whence the electrons are derived. Similar
+ionisation under electric forces enters, as we now believe, into
+all the chemical effects progressing in the galvanic cell, and,
+indeed, generally in ionised solutes.
+
+An experiment will best illustrate the principles I wish to
+remind you of. A clean aluminium plate, carefully insulated by a
+sulphur support, is faced by a sheet of copper-wire-gauze placed
+a couple of centimetres away from it. The gauze is maintained at
+a high positive
+
+205
+
+potential by this dry pile. A sensitive gold-leaf electroscope is
+attached to the aluminium plate, and its image thrown upon the
+screen. I now turn the light from this arc lamp upon the wire
+gauze, through which it in part passes and shines upon the
+aluminium plate. The electroscope at once charges up rapidly.
+There is a liberation of negative electrons at the surface of the
+aluminium; these, under the attraction of the positive body, are
+rapidly removed as ions, and the electroscope charges up
+positively.
+
+Again, if I simply electrify negatively this aluminium plate so
+that the leaves of the attached electroscope diverge widely, and
+now expose it to the rays from the arc lamp, the charge, as you
+see, is very rapidly dissipated. With positive electrification of
+the aluminium there is no effect attendant on the illumination.
+
+Thus from the work of Hertz and his successors we know that
+light, and more particularly what we call actinic light, is an
+effective means of setting free electrons from certain
+substances. In short, our photographic agent, light, has the
+power of expelling from certain substances the electron which is
+so potent a factor in most, if not in all, chemical effects. I
+have not time here to refer to the work of Elster and Geitel
+whereby they have shown that this action is to be traced to the
+electric force in the light wave, but must turn to the probable
+bearing of this phenomenon on the familiar facts of photography.
+I assume that the experiment I have shown you is the most
+
+206
+
+fundamental photographic experiment which it is now in our power
+to make.
+
+We must first ask from what substances can light liberate
+electrons. There are many--metals as well as non-metals and
+liquids. It is a very general phenomenon and must operate widely
+throughout nature. But what chiefly concerns the present
+consideration is the fact that the haloid salts of silver are
+vigorously photo-electric, and, it is suggestive, possess,
+according to Schmidt, an activity in the descending order
+bromide, chloride, iodide. This is, in other words, their order
+of activity as ionisers (under the proper conditions) when
+exposed to ultra-violet light. Photographers will recognise that
+this is also the order of their photographic sensitiveness.
+
+Another class of bodies also concerns our subject: the special
+sensitisers used by the photographer to modify the spectral
+distribution of sensibility of the haloid salts, _e.g._ eosine,
+fuchsine, cyanine. These again are electron-producers under light
+stimulus. Now it has been shown by Stoletow, Hallwachs, and
+Elster and Geitel that there is an intimate connection between
+photo-electric activity and the absorption of light by the
+substance, and, indeed, that the particular wave-lengths absorbed
+by the substance are those which are effective in liberating the
+electrons. Thus we have strong reason for believing that the
+vigorous photo-electric activity displayed by the special
+sensitisers must be dependent upon their colour absorption. You
+will recognise that this is just
+
+207
+
+the connection between their photographic effects and their
+behaviour towards light.
+
+There is yet another suggestive parallel. I referred to the
+observation of Sir James Dewar as to the continued sensitiveness
+of the photographic film at the lowest attained extreme of
+temperature, and drew the inference that the fundamental
+photographic action must be of intra-atomic nature, and not
+dependent upon the vis viva of the molecule or atom. In then
+seeking the origin of photographic action in photo-electric
+phenomena we naturally ask, Are these latter phenomena also
+traceable at low temperatures? If they are, we are entitled to
+look upon this fact as a qualifying characteristic or as another
+link in the chain of evidence connecting photographic with
+photo-electric activity.
+
+I have quite recently, with the aid of liquid air supplied to me
+from the laboratory of the Royal Dublin Society, tested the
+photo-sensibility of aluminium and also of silver bromide down to
+temperatures approaching that of the liquid air. The mode of
+observation is essentially that of Schmidt--what he terms his
+static method. The substance undergoing observation is, however,
+contained at the bottom of a thin copper tube, 5 cm. in diameter,
+which is immersed to a depth of about 10 cm in liquid air. The
+tube is closed above by a paraffin stopper which carries a thin
+quartz window as well as the sulphur tubes through which the
+connections pass. The air within is very carefully dried by
+phosphorus
+
+208
+
+pentoxide before the experiment. The arc light is used as source
+of illumination. It is found that a vigorous photo-electric
+effect continues in the case of the clean aluminium. In the case
+of the silver bromide a distinct photo-electric effect is still
+observed. I have not had leisure to make, as yet, any trustworthy
+estimate of the percentage effect at this temperature in the case
+of either substance. Nor have I determined the temperature
+accurately. The latter may be taken as roughly about -150° C,
+
+Sir James Dewar's actual measilrements afforded twenty per cent.
+of the normal photographic effect at -180° C. and ten per cent.
+at the temperature of -252.5° C.
+
+With this much to go upon, and the important additional fact that
+the electronic discharge--as from the X-ray tube or from
+radium--generates the latent image, I think we are fully entitled
+to suggest, as a legitimate lead to experiment, the hypothesis
+that the beginnings of photographic action involve an electronic
+discharge from the light-sensitive molecule; in other words that
+the latent image is built up of ionised atoms or molecules the
+result of the photo-electric effect on the illuminated silver
+haloid, and it is upon these ionised atoms that the chemical
+effects of the developer are subsequently directed. It may be
+that the liberated electrons ionise molecules not directly
+affected, or it may be that in their liberation they disrupt
+complex molecules built up in the ripening of the
+
+209
+
+emulsion. With the amount we have to go upon we cannot venture to
+particularise. It will be said that such an action must be in
+part of the nature of a chemical effect. This must be admitted,
+and, in so far as the rearrangement of molecular fabrics is
+involved, the result will doubtless be controlled by temperature
+conditions. The facts observed by Sir James Dewar support this.
+But there is involved a fundamental process--the liberation of the
+electron by the electric force in the light wave, which is a
+physical effect, and which, upon the hypothesis of its reality as
+a factor in forming the latent image, appears to explain
+completely the outstanding photographic sensitiveness of the film
+at temperatures far below those at which chemical actions in
+general cease.
+
+Again, we may assume that the electron--producing power of the
+special sensitiser or dye for the particular ray it absorbs is
+responsible, or responsible in part, for the special
+sensitiveness it confers upon the film. Sir Wm. Abney has shown
+that these sensitisers are active even if laid on as a varnish on
+the sensitive surface and removed before development. It must be
+remembered, however, that at temperatures of about -50° these
+sensitisers lose much of their influence on the film; as I have
+pointed out in a paper read before the Photographic Convention of
+1894.
+
+It. appears to me that on these views the curious phenomenon of
+recurrent reversals does not present a problem hopeless of
+explanation. The process of photo-
+
+210
+
+ionisation constituting the latent image, where the ion is
+probably not immediately neutralised by chemical combination,
+presents features akin to the charging of a capacity--say a Leyden
+jar. There may be a rising potential between the groups of ions
+until ultimately a point is attained when there is a spontaneous
+neutralisation. I may observe that the phenomena of reversal
+appear to indicate that the change in the silver bromide
+molecule, whatever be its nature, is one of gradually increasing
+intensity, and finally attains a maximum when a return to the
+original condition occurs. The maximum is the point of most
+intense developable image. It is probable that the sensitiser--in
+this case the gelatin in which the bromide of silver is
+immersed--plays a part in the conditions of stability which are
+involved.
+
+Of great interest in all our considerations and theories is the
+recent work of Wood on photographic reversal. The result of this
+work is--as I take it--to show that the stability of the latent
+image may be very various according to the mode of its formation.
+Thus it appears that the sort of latent effect which is produced
+by pressure or friction is the least stable of any. This may be
+reversed or wiped out by the application of any other known form
+of photographic stimulus. Thus an exposure to X-rays will
+obliterate it, or a very brief exposure to light. The latent
+image arising from X-rays is next in order of increasing
+stability. Light action will remove this. Third in order is a
+very brief light-shock or sudden flash. This
+
+211
+
+cannot be reversed by any of the foregoing modes of stimulation,
+but a long-continued undulatory stimulus, as from lamp-light,
+will reverse it. Last and most stable of all is the gradually
+built-up configuration due to long-continued light exposure. This
+can only be reversed by overdoing it according to the known facts
+of recurrent reversal. Wood takes occasion to remark that these
+phenomena are in bad agreement with the strain theory of Bose. We
+have, in fact, but the one resource--the allotropic modification
+of the haloid--whereby to explain all these orders of stability.
+It appears to me that the elasticity of the electronic theory is
+greater. The state of the ionised system may be very various
+according as it arises from continued rhythmic effects or from
+unorganised shocks. The ionisation due to X-rays or to friction
+will probably be quite unorganised, that due to light more or
+less stable according to the gradual and gentle nature of the
+forces at work. I think we are entitled to conclude that on the
+whole there is nothing in Wood's beautiful experiments opposed to
+the photo-electric origin of photographic effects, but that they
+rather fall in with what might be anticipated according to that
+theory.
+
+When we look for further support to the views I have laid before
+you we are confronted with many difficulties. I have not as yet
+detected any electronic discharge from the film under light
+stimulus. This may be due to my defective experiments, or to a
+fact noted by Elster and Geitel concerning the photo-electric
+properties of gelatin.
+
+212
+
+They obtained a vigorous effect from Balmain's luminous paint,
+but when this was mixed in gelatin there was no external effect.
+Schmidt's results as to the continuance of photo-electric
+activity when bodies in general are dissolved in each other lead
+us to believe that an actual conservative property of the medium
+and not an effect of this on the luminous paint is here involved.
+This conservative effect of the gelatin may be concerned with its
+efficacy as a sensitiser.
+
+In the views I have laid before you I have endeavoured to show
+that the recent addition to our knowledge of the electron as an
+entity taking part in many physical and chemical effects should
+be kept in sight in seeking an explanation of the mode of origin
+of the latent image.[1]
+
+[1] For a more detailed account of the subject, and some
+ingenious extensions of the views expressed above, see
+_Photo-Electricity_, by H. Stanley Allen: Longmans, Green & Ca.,
+1913.
+
+213
+
+PLEOCHROIC HALOES [1]
+
+IT is now well established that a helium atom is expelled from
+certain of the radioactive elements at the moment of
+transformation. The helium atom or alpha ray leaves the
+transforming atom with a velocity which varies in the different
+radioactive elements, but which is always very great, attaining
+as much as 2 x 109 cms. per second; a velocity which, if
+unchecked, would carry the atom round the earth in less than two
+seconds. The alpha ray carries a positive charge of double the
+ionic amount.
+
+When an alpha ray is discharged from the transforming element
+into a gaseous medium its velocity is rapidly checked and its
+energy absorbed. A certain amount of energy is thus transferred
+from the transforming atom to the gas. We recognise this energy
+in the gas by the altered properties of the latter; chiefly by
+the fact that it becomes a conductor of electricity. The
+mechanism by which this change is effected is in part known. The
+atoms of the gas, which appear to be freely penetrated by the
+alpha ray, are so far dismembered as to yield charged electrons
+or ions; the atoms remaining charged with an equal and opposite
+charge. Such a medium of
+
+[1] Being the Huxley Lecture, delivered at the University of
+Birmingham on October 30th, 1912. Bedrock, Jan., 1913.
+
+214
+
+free electric charges becomes a conductor of electricity by
+convection when an electromotive force is applied. The gas also
+acquires other properties in virtue of its ionisation. Under
+certain conditions it may acquire chemical activity and new
+combinations may be formed or existing ones broken up. When its
+initial velocity is expended the helium atom gives up its
+properties as an alpha ray and thenceforth remains possessed of
+the ordinary varying velocity of thermal agitation. Bragg and
+Kleeman and others have investigated the career of the alpha ray
+when its path or range lies in a gas at ordinary or obtainable
+conditions of pressure and temperature. We will review some of
+the facts ascertained.
+
+The range or distance traversed in a gas at ordinary pressures is
+a few centimetres. The following table, compiled by Geiger, gives
+the range in air at the temperature of 15° C.:
+
+               cms.                   cms.                   cms.
+Uranium 1 -   2.50    Thorium -     2.72     Radioactinium  4.60
+Uranium 2 -   2.90    Radiothorium  3.87     Actinium X -   4.40
+Ionium -      3.00    Thorium X -   4.30     Act Emanation  5.70
+Radium -      3.30    Th Emanation  5.00     Actinium A -   6.50
+Ra Emanation  4.16    Thorium A -   5.70     Actinium C -   5.40
+Radium A -    4.75    Thorium C1 -  4.80
+Radium C -    6.94    Thorium C2 -  8.60
+Radium F -    3.77
+
+It will be seen that the ray of greatest range is that proceeding
+from thorium C2, which reaches a distance of 8.6 cms. In the
+uranium family the fastest ray is
+
+215
+
+that of radium C. It attains 6.94 cms. There is thus an
+appreciable difference between the ultimate distances traversed
+by the most energetic rays of the two families. The shortest
+ranges are those of uranium 1 and 2.
+
+The ionisation effected by these rays is by no means uniform
+along the path of the ray. By examining the conductivity of the
+gas at different points along the path of the ray, the ionisation
+at these points may be determined. At the limits of the range the
+ionisation
+
+{Fig. 13}
+
+ceases. In this manner the range is, in fact, determined. The
+dotted curve (Fig. 13) depicts the recent investigation of the
+ionisation effected by a sheaf of parallel rays of radium C in
+air, as determined by Geiger. The range is laid out horizontally
+in centimetres. The numbers of ions are laid out vertically. The
+remarkable nature of the results will be at once apparent. We
+should have expected that the ray at the beginning of its path,
+when its velocity and kinetic energy were greatest, would have
+been more effective than towards the end of its range
+
+216
+
+when its energy had almost run out. But the curve shows that it
+is just the other way. The lagging ray, about to resign its
+ionising properties, becomes a much more efficient ioniser than
+it was at first. The maximum efficiency is, however, in the case
+of a bundle of parallel rays, not quite at the end of the range,
+but about half a centimetre from it. The increase to the maximum
+is rapid, the fall from the maximum to nothing is much more
+rapid.
+
+It can be shown that the ionisation effected anywhere along the
+path of the ray is inversely proportional to the velocity of the
+ray at that point. But this evidently does not apply to the last
+5 or 10 mms. of the range where the rate of ionisation and of the
+speed of the ray change most rapidly. To what are the changing
+properties of the rays near the end of their path to be ascribed?
+It is only recently that this matter has been elucidated.
+
+When the alpha ray has sufficiently slowed down, its power of
+passing right through atoms, without appreciably experiencing any
+effects from them, diminishes. The opposing atoms begin to exert
+an influence on the path of the ray, deflecting it a little. The
+heavier atoms will deflect it most. This effect has been very
+successfully investigated by Geiger. It is known as "scattering."
+The angle of scattering increases rapidly with the decrease of
+velocity. Now the effect of the scattering will be to cause some
+of the rays to complete their ranges
+
+217
+
+or, more accurately, to leave their direct line of advance a
+little sooner than others. In the beautiful experiments of C. T.
+R. Wilson we are enabled to obtain ocular demonstration of the
+scattering. The photograph (Fig. 14.), which I owe to the
+kindness of Mr. Wilson, shows the deflection of the ray towards
+the end of its path. In
+
+{Fig. 14}
+
+this case the path of the ray has been rendered visible by the
+condensation of water particles under the influence of the
+ionisation; the atmosphere in which the ray travels being in a
+state of supersaturation with water vapour at the instant of the
+passage of the ray. It is evident that if we were observing the
+ionisation along a sheaf of parallel rays, all starting with
+equal velocity,
+
+218
+
+the effect of the bending of some of the rays near the end of
+their range must be to cause a decrease in the aggregate
+ionisation near the very end of the ultimate range. For, in fact,
+some of the rays complete their work of ionising at points in the
+gas before the end is reached. This is the cause, or at least an
+important contributory cause, of the decline in the ionisation
+near the end of the range, when the effects of a bundle of rays
+are being observed. The explanation does not suggest that the
+ionising power of any one ray is actually diminished before it
+finally ceases to be an alpha ray.
+
+The full line in Fig. 13 gives the ionisation curve which it may
+be expected would be struck out by a single alpha ray. In it the
+ionisation goes on increasing till it abruptly ceases altogether,
+with the entire loss of the initial kinetic energy of the
+particle.
+
+A highly remarkable fact was found out by Bragg. The effect of
+the atom traversed by the ray in checking the velocity of the ray
+is independent of the physical and chemical condition of the
+atom. He measured the "stopping power" of a medium by the
+distance the ray can penetrate into it compared with the distance
+to which it can penetrate in air. The less the ratio the greater
+is the stopping power. The stopping power of a substance is
+proportional to the square root of its atomic weight. The
+stopping power of an atom is not altered if it is in chemical
+union with another atom. The atomic weight is the one quality of
+importance. The physical
+
+219
+
+state, whether the element is in the solid, liquid or gaseous
+state, is unimportant. And when we deal with molecules the
+stopping power is simply proportional to the sum of the square
+roots of the atomic weights of the atoms entering into the
+molecule. This is the "additive law," and it obviously enables us
+to calculate what the range in any substance of known chemical
+composition and density will be, compared with the range in air.
+
+This is of special importance in connection with phenomena we
+have presently to consider. It means that, knowing the chemical
+composition and density of any medium whatsoever, solid, liquid
+or gaseous, we can calculate accurately the distance to which any
+particular alpha ray will penetrate. Nor have the temperature and
+pressure to which the medium is subjected any influence save in
+so far as they may affect the proximity of one atom to another.
+The retardation of the alpha ray in the atom is not affected.
+
+This valuable additive law, however, cannot be applied in
+strictness to the amount of ionisation attending the ray. The
+form of the molecule, or more generally its volume, may have an
+influence upon this. Bragg draws the conclusion, from this fact
+as well as from the notable increase of ionisation with loss of
+speed, that the ionisation is dependent upon the time the ray
+spends in the molecule. The energy of the ray is, indeed, found
+to be less efficient in producing ionisation in the smaller
+atomm.
+
+220
+
+Before leaving our review of the general laws governing the
+passage of alpha rays through matter, another point of interest
+must be referred to. We have hitherto spoken in general terms of
+the fact that ionisation attends the passage of the ray. We have
+said nothing as to the nature of the ionisation so produced. But
+in point of fact the ionisation due to an alpha ray is sui
+generis. A glance at one of Wilson's photographs (Fig. 14.)
+illustrates this. The white streak of water particles marks the
+path of the ray. The ions produced are evidently closely crowded
+along the track of the ray. They have been called into existence
+in a very minute instant of time. Now we know that ions of
+opposite sign if left to themselves recombine. The rate of
+recombination depends upon the product of the number of each sign
+present in unit volume. Here the numbers are very great and the
+volume very small. The ionic density is therefore high, and
+recombination very rapidly removes the ions after they are
+formed. We see here a peculiarity of the ionisation effected by
+alpha rays. It is linear in distribution and very local. Much of
+the ionisation in gases is again undone by recombination before
+diffusion leads to the separation of the ions. This "initial
+recombination" is greatest towards the end of the path of the ray
+where the ionisation is a maximum. Here it may be so effective
+that the form of the curve is completely lost unless a very large
+electromotive force is used to separate the ions when the
+ionisation is being investigated.
+
+221
+
+We have now reviewed recent work at sufficient length to
+understand something of the nature of the most important advance
+ever made in our knowledge of the atom. Let us glance briefly at
+what we have learned. The radioactive atom in sinking to a lower
+atomic weight casts out with enormous velocity an atom of helium.
+It thus loses a definite portion of its mass and of its energy.
+Helium which is chemically one of the most inert of the elements,
+is, when possessed of such great kinetic energy, able to
+penetrate and ionise the atoms which it meets in its path. It
+spends its energy in the act of ionising them, coming to rest,
+when it moves in air, in a few centimetres. Its initial velocity
+depends upon the particular radioactive element which has given
+rise to it. The length of its path is therefore different
+according to the radioactive element from which it proceeds. The
+retardation which it experiences in its path depends entirely
+upon the atomic weight of the atoms which it traverses. As it
+advances in its path its effectiveness in ionising the atom
+rapidly increases and attains a very marked maximum. In a gas the
+ions produced being much crowded together recombine rapidly; so
+rapidly that the actual ionisation may be quite concealed unless
+a sufficiently strong electric force is applied to separate them.
+Such is a brief summary of the climax of radioactive
+discovery:--the birth, life and death of the alpha ray. Its advent
+into Science has altered fundamentally our conception of
+
+222
+
+matter. It is fraught with momentous bearings upon Geological
+Science. How the work of the alpha ray is sometimes recorded
+visibly in the rocks and what we may learn from that record, I
+propose now to bring before you.
+
+In certain minerals, notably the brown variety of mica known as
+biotite, the microscope reveals minute circular marks occurring
+here and there, quite irregularly. The most usual appearance is
+that of a circular area darker in colour than the surrounding
+mineral. The radii of these little disc-shaped marks when well
+defined are found to be remarkably uniform, in some cases four
+hundredths of a millimetre and in others three hundredths, about.
+These are the measurements in biotite. In other minerals the
+measurements are not quite the same as in biotite. Such minute
+objects are quite invisible to the naked eye. In some rocks they
+are very abundant, indeed they may be crowded together in such
+numbers as to darken the colour of the mineral containing them.
+They have long been a mystery to petrologists.
+
+Close examination shows that there is always a small speck of a
+foreign body at the centre of the circle, and it is often
+possible to identify the nature of this central substance, small
+though it be. Most generally it is found to be the mineral
+zircon. Now this mineral was shown by Strutt to contain radium in
+quantities much exceeding those found in ordinary rock
+substances.
+
+223
+
+Some other mineral may occasionally form the nucleus, but we
+never find any which is not known to be specially likely to
+contain a radioactive substance. Another circumstance we notice.
+The smaller this central nucleus the more perfect in form is the
+darkened circular area surrounding it. When the circle is very
+perfect and the central mineral clearly defined at its centre we
+find by measurement that the radius of the darkened area is
+generally 0.033 mm. It may sometimes be 0.040 mm. These are
+always the measurements in biotite. In other minerals the radii
+are a little different.
+
+We see in the photograph (Pl. XXIII, lower figure), much
+magnified, a halo contained in biotite. We are looking at a
+region in a rock-section, the rock being ground down to such a
+thickness that light freely passes through it. The biotite is in
+the centre of the field. Quartz and felspar surround it. The rock
+is a granite. The biotite is not all one crystal. Two crystals,
+mutually inclined, are cut across. The halo extends across both
+crystals, but owing to the fact that polarised light is used in
+taking the photograph it appears darker in one crystal than in
+the other. We see the zircon which composes the nucleus. The fine
+striated appearance of the biotite is due to the cleavage of that
+mineral, which is cut across in the section.
+
+The question arises whether the darkened area surrounding the
+zircon may not be due to the influence of the radioactive
+substances contained in the zircon. The
+
+224
+
+extraordinary uniformity of the radial measurements of perfectly
+formed haloes (to use the name by which they have long been
+known) suggests that they may be the result of alpha radiation.
+For in that case, as we have seen, we can at once account for the
+definite radius as simply representing the range of the ray in
+biotite. The furthest-reaching ray will define the radius of the
+halo. In the case of the uranium family this will be radium C,
+and in the case of thorium it will be thorium C. Now here we
+possess a means of at once confirming or rejecting the view that
+the halo is a radioactive phenomenon and occasioned by alpha
+radiation; for we can calculate what the range of these rays will
+be in biotite, availing ourselves of Bragg's additive law,
+already referred to. When we make this calculation we find that
+radium C just penetrates 0.033 mm. and thorium C 0.040 mm. The
+proof is complete that we are dealing with the effects of alpha
+rays. Observe now that not only is the coincidence of measurement
+and calculation a proof of the view that alpha radiation has
+occasioned the halo, but it is a very complete verification of
+the important fact stated by Bragg, that the stopping power
+depends solely on the atomic weight of the atoms traversed by the
+ray.
+
+We have seen that our examination of the rocks reveals only the
+two sorts of halo: the radium halo and the thorium halo. This is
+not without teaching. For why not find an actinium halo? Now
+Rutherford long ago suggested that this element and its
+derivatives were
+
+225
+
+probably an offspring of the uranium family; a side branch, as it
+were, in the formation of which relatively few transforming atoms
+took part. On Rutherford's theory then, actinium should always
+accompany uranium and radium, but in very subordinate amount. The
+absence of actinium haloes clearly supports this view. For if
+actinium was an independent element we would be sure to find
+actinium haloes. The difference in radius should be noticeable.
+If, on the other hand, actinium
+
+was always associated with uranium and radium, then its effects
+would be submerged in those of the much more potent effects of
+the uranium series of elements.
+
+It will have occurred to you already that if the radioactive
+origin of the halo is assured the shape of a halo is not really
+circular, but spherical. This is so. There is no such thing as a
+disc-shaped halo. The halo is a spherical volume containing the
+radioactive nucleus at its centre. The true radius of the halo
+may, therefore, only be measured on sections passing through the
+nucleus.
+
+226
+
+In order to understand the mode of formation of a halo we may
+profitably study on a diagram the events which go on within the
+halo-sphere. Such a diagram is seen in Fig. 15. It shows to
+relatively correct scale the limiting range of all the alpha-ray
+producing members of the uranium and thorium families. We know
+that each member of a family will exist in equilibrium amount
+within the nucleus possessing the parent element. Each alpha ray
+leaving the nucleus will just attain its range and then cease to
+affect the mica. Within the halosphere, there must be, therefore,
+the accumulated effects of the influences of all the rays. Each
+has its own sphere of influence, and the spheres are all
+concentric.
+
+The radii in biotite of the several spheres are given in the
+following table
+
+URANIUM FAMILY.
+Radium C -       0.0330 mm.
+Radium A -       0.0224 mm.
+Ra Emanation -   0.0196 mm.
+Radium F -       0.0177 mm.
+Radium -         0.0156 mm.
+Ionium -         0.0141 mm.
+Uranium 1 -      0.0137 mm.
+Uranium 2 -      0.0118 mm.
+
+THORIUM FAMILY.
+Thorium CE -     0.040 mm.
+Thorium A -      0.026 mm.
+Th Emanation -   0.023 mm.
+Thorium Ci -     0.022 mm.
+Thorium X -      0.020 mm.
+Radiothorium -   0.119 mm.
+Thorium -        0.013 mm.
+
+In the photograph (Pl. XXIV, lower figure), we see a uranium and
+a thorium halo in the same crystal of mica. The mica is contained
+in a rock-section and is cut across the cleavage. The effects of
+thorium Ca are clearly shown
+
+227
+
+as a lighter border surrounding the accumulated inner darkening
+due to the other thorium rays. The uranium halo (to the right)
+similarly shows the effects of radium C, but less distinctly.
+
+Haloes which are uniformly dark all over as described above are,
+in point of fact, "over-exposed"; to borrow a familiar
+photographic term. Haloes are found which show much beautiful
+internal detail. Too vigorous action obscures this detail just as
+detail is lost in an over-exposed photograph. We may again have
+"under-exposed" haloes in which the action of the several rays is
+incomplete or in which the action of certain of the rays has left
+little if any trace. Beginning at the most under-exposed haloes
+we find circular dark marks having the radius 0.012 or 0.013 mm.
+These haloes are due to uranium, although their inner darkening
+is doubtless aided by the passage of rays which were too few to
+extend the darkening beyond the vigorous effects of the two
+uranium rays. Then we find haloes carried out to the radii 0.016,
+0.018 and 0.019 mm. The last sometimes show very beautiful outer
+rings having radial dimensions such as would be produced by
+radium A and radium C. Finally we may have haloes in which
+interior detail is lost so far out as the radius due to emanation
+or radium A, while outside this floats the ring due to radium C.
+Certain variations of these effects may occur, marking,
+apparently, different stages of exposure. Plates XXIII and XXIV
+(upper figure) illustrate some of these stages;
+
+228
+
+the latter photograph being greatly enlarged to show clearly the
+halo-sphere of radium A.
+
+In most of the cases mentioned above the structure evidently
+shows the existence of concentric spherical shells of darkened
+biotite. This is a very interesting fact. For it proves that in
+the mineral the alpha ray gives rise to the same increased
+ionisation towards the end of its range, as Bragg determined in
+the case of gases. And we must conclude that the halo in every
+case grows in this manner. A spherical shell of darkened biotite
+is first produced and the inner colouration is only effected as
+the more feeble ionisation along the track of the ray in course
+of ages gives rise to sufficient alteration of the mineral. This
+more feeble ionisation is, near the nucleus, enhanced in its
+effects by the fact that there all the rays combine to increase
+the ionisation and, moreover, the several tracks are there
+crowded by the convergency to the centre. Hence the most
+elementary haloes seldom show definite rings due to uranium,
+etc., but appear as embryonic disc-like markings. The photographs
+illustrate many of the phases of halo development.
+
+Rutherford succeeded in making a halo artificially by compressing
+into a capillary glass tube a quantity of the emanation of
+radium. As the emanation decayed the various derived products
+came into existence and all the several alpha rays penetrated the
+glass, darkening the walls of the capillary out to the limit of
+the range of radium C in glass. Plate XXV shows a magnified
+section of the
+
+229
+
+tube. The dark central part is the capillary. The tubular halo
+surrounds it. This experiment has, however, been anticipated by
+some scores of millions of years, for here is the same effect in
+a biotite crystal (Pl. XXV). Along what are apparently tubular
+passages or cracks in the mica, a solution, rich in radioactive
+substances, has moved; probably during the final consolidation of
+the granite in which the mica occurs. A continuous and very
+regular halo has developed along these conduits. A string of
+halo-spheres may lie along such passages. We must infer that
+solutions or gases able to establish the radioactive nuclei moved
+along these conduits, and we are entitled to ask if all the
+haloes in this biotite are not, in this sense, of secondary
+origin. There is, I may add, much to support such a conclusion.
+
+The widespread distribution of radioactive substances is most
+readily appreciated by examination of sections of rocks cut thin
+enough for microscopic investigation. It is, indeed, difficult to
+find, in the older rocks of granitic type, mica which does not
+show haloes, or traces of haloes. Often we find that every one of
+the inclusions in the mica--that is, every one of the earlier
+formed substances--contain radioactive elements, as indicated by
+the presence of darkened borders. As will be seen presently the
+quantities involved are generally vanishingly small. For example
+it was found by direct determination that in one gram of the
+halo-rich mica of Co. Carlow there was rather less than twelve
+billionths of a gram of radium, We are
+
+230
+
+entitled to infer that other rare elements are similarly widely
+distributed but remain undetectable because of their more stable
+properties.
+
+It must not be thought that the under-exposed halo is a recent
+creation. By no means. All are old, appallingly old; and in the
+same rock all are, probably, of the same, or neatly the same,
+age. The under-exposure is simply due to a lesser quantity of the
+radioactive elements in the nucleus. They are under-exposed, in
+short, not because of lesser duration of exposure, but because of
+insufficient action; as when in taking a photograph the stop is
+not open enough for the time of the exposure.
+
+The halo has, so far, told us that the additive law is obeyed in
+solid media, and that the increased ionisation attending the
+slowing down of the ray obtaining in gases, also obtains in
+solids; for, otherwise, the halo would not commence its
+development as a spherical shell or envelope. But here we learn
+that there is probably a certain difference in the course of
+events attending the immediate passage of the ray in the gas and
+in the solid. In the former, initial recombination may obscure
+the intense ionisation near the end of the range. We can only
+detect the true end-effects by artificially separating the ions
+by a strong electric force. If this recombination happened in the
+mineral we should not have the concentric spheres so well defined
+as we see them to be. What, then, hinders the initial
+recombination in the solid? The answer probably is that the newly
+formed
+
+231
+
+ion is instantly used up in a fresh chemical combination. Nor is
+it free to change its place as in the gas. There is simply a new
+equilibrium brought about by its sudden production. In this
+manner the conditions in the complex molecule of biotite,
+tourmaline, etc., may be quite as effective in preventing initial
+recombination as the most effective electric force we could
+apply. The final result is that we find the Bragg curve
+reproduced most accurately in the delicate shading of the rings
+making up the perfectly exposed halo.
+
+That the shading of the rings reproduces the form of the Bragg
+curve, projected, as it were, upon the line of advance of the ray
+and reproduced in depth of shading, shows that in yet another
+particular the alpha ray behaves much the same in the solid as in
+the gas. A careful examination of the outer edge of the circles
+always reveals a steep but not abrupt cessation of the action of
+the ray. Now Geiger has investigated and proved the existence of
+scattering of the alpha ray by solids. We may, therefore, suppose
+with much probability that there is the same scattering within
+the mineral near the end of the range. The heavy iron atom of the
+biotite is, doubtless, chiefly responsible for this in biotite
+haloes. I may observe that this shading of the outer bounding
+surface of the sphere of action is found however minute the
+central nucleus. In the case of a nucleus of considerable size
+another effect comes in which tends to produce an enhanced
+shading. This will
+
+232
+
+result if rays proceed from different depths in the nucleus. If
+the nucleus were of the same density and atomic weight as the
+surrounding mica, there would be little effect. But its density
+and molecular weight are generally greater, hence the retardation
+is greater, and rays proceeding from deep in the nucleus
+experience more retardation than those which proceed from points
+near to the surface. The distances reached by the rays in the
+mica will vary accordingly, and so there will be a gradual
+cessation of the effects of the rays.
+
+The result of our study of the halo may be summed up in the
+statement that in nearly every particular we have the phenomena,
+which have been measured and observed in the gas, reproduced on a
+minute scale in the halo. Initial recombination seems, however,
+to be absent or diminished in effectiveness; probably because of
+the new stability instantly assumed by the ionised atoms.
+
+One of the most interesting points about the halo remains to be
+referred to. The halo is always uniformly darkened all round its
+circumference and is perfectly spherical. Sections, whether taken
+in the plane of cleavage of the mica or across it, show the same
+exactly circular form, and the same radius. Of course, if there
+was any appreciable increase of range along or across the
+cleavage the form of the halo on the section across the cleavage
+should be elliptical. The fact that there is no measurable
+ellipticity is, I think, one which on first consideration would
+not be expected.
+
+233
+
+For what are the conditions attending the passage of the ray in a
+medium such as mica? According to crystallographic conceptions we
+have here an orderly arrangement of molecules, the units
+composing the crystal being alike in mass, geometrically spaced,
+and polarised as regards the attractions they exert one upon
+another. Mica, more especially, has the cleavage phenomenon
+developed to a degree which transcends its development in any
+other known substance. We can cleave it and again cleave it till
+its flakes float in the air, and we may yet go on cleaving it by
+special means till the flakes no longer reflect visible light.
+And not less remarkable is the uniplanar nature of its cleavage.
+There is little cleavage in any plane but the one, although it is
+easy to show that the molecules in the plane of the flake are in
+orderly arrangement and are more easily parted in some directions
+than in others. In such a medium beyond all others we must look
+with surprise upon the perfect sphere struck out by the alpha
+rays, because it seems certain that the cleavage is due to lesser
+attraction, and, probably, further spacing of the molecules, in a
+direction perpendicular to the cleavage.
+
+It may turn out that the spacing of the molecules will influence
+but little the average number per unit distance encountered by
+rays moving in divergent paths. If this is so, we seem left to
+conclude that, in spite of its unequal and polarised attractions,
+there is equal retardation and equal ionisation in the molecule
+in whatever
+
+234
+
+direction it is approached. Or, again, if the encounters indeed
+differ in number, then some compensating effect must exist
+whereby a direction of lesser linear density involves greater
+stopping power in the molecule encountered, and vice versa.
+
+The nature of the change produced by the alpha rays is unknown.
+But the formation of the halo is not, at least in its earlier
+stages, attended by destruction of the crystallographic and
+optical properties of the medium. The optical properties are
+unaltered in nature but are increased in intensity. This applies
+till the halo has become so darkened that light is no longer
+transmitted under the conditions of thickness obtaining in rock
+sections. It is well known that there is in biotite a maximum
+absorption of a plane-polarised light ray, when the plane of
+vibration coincides with the plane of cleavage. A section across
+the cleavage then shows a maximum amount of absorption. A halo
+seen on this section simply produces this effect in a more
+intense degree. This is well shown in Plate XXIII (lower figure),
+on a portion of the halo-sphere. The descriptive name "Pleochroic
+Halo" has originated from this fact. We must conclude that the
+effect of the ionisation due to the alpha ray has not been to
+alter fundamentally the conditions which give rise to the optical
+properties of the medium. The increased absorption is probably
+associated with some change in the chemical state of the iron
+present. Haloes are, I believe, not found in minerals from which
+this
+
+235
+
+element is absent. One thing is quite certain. The colouration is
+not due to an accumulation of helium atoms, _i.e._ of spent alpha
+rays. The evidence for this is conclusive. If helium was
+responsible we should have haloes produced in all sorts of
+colourless minerals. Now we sometimes see zircons in felspars and
+in quartz, etc., but in no such case is a halo produced. And
+halo-spheres formed within and sufficiently close to the edge of
+a crystal of mica are abruptly truncated by neighbouring areas of
+fclspar or quartz, although we know that the rays must pass
+freely across the boundary. Again it is easy to show that even in
+the oldest haloes the quantity of helium involved is so small
+that one might say the halo-sphere was a tolerably good vacuum as
+regards helium. There is, finally, no reason to suppose that the
+imprisoned helium would exhibit such a colouration, or, indeed,
+any at all.
+
+I have already referred to the great age of the halo. Haloes are
+not found in the younger igneous rocks. It is probable that a
+halo less than a million years old has never been seen. This,
+primâ facie, indicates an extremely slow rate of formation. And
+our calculations quite support the conclusions that the growth of
+a halo, if this has been uniform, proceeds at a rate of almost
+unimaginable slowness.
+
+Let us calculate the number of alpha rays which may have gone to
+form a halo in the Devonian granite of Leinster.
+
+236
+
+It is common to find haloes developed perfectly in this granite,
+and having a nucleus of zircon less than 5 x 10-4 cms. in
+diameter. The volume of zircon is 65 x 10-12 c.cs. and the mass
+3 x 10-10 grm.; and if there was in this zircon 10-8 grm. radium
+per gram (a quantity about five times the greatest amount
+measured by Strutt), the mass of radium involved is 3 x 10-18
+grm. From this and from the fact ascertained by Rutherford that
+the number of alpha rays expelled by a gram of radium in one
+second is 3.4 x 1010, we find that three rays are shot from the
+nucleus in a year. If, now, geological time since the Devonian is
+50 millions of years, then 150 millions of rays built up the
+halo. If geological time since the Devonian is 400 millions of
+years, then 1,200 millions of alpha rays are concerned in its
+genesis. The number of ions involved, of course, greatly exceeds
+these numbers. A single alpha ray fired from radium C will
+produce 2.37 x 105 ions in air.
+
+But haloes may be found quite clearly defined and fairly dark out
+to the range of the emanation ray and derived from much less
+quantities of radioactive materials. Thus a zircon nucleus with a
+diameter of but 3.4 x 10-4 cms. formed a halo strongly darkened
+within, and showing radium A and radium C as clear smoky rings.
+Such a nucleus, on the assumption made above as to its radium
+content, expels one ray in a year. But, again, haloes are
+observed with less blackened pupils and with faint ring due to
+radium C, formed round nuclei
+
+237
+
+of rather less than 2 x 10-4 cms. diameter. Such nuclei would
+expel one ray in five years. And even lesser nuclei will generate
+in these old rocks haloes with their earlier characteristic
+features clearly developed. In the case of the most minute
+nuclei, if my assumption as to the uranium content is correct, an
+alpha ray is expelled, probably, no oftener than once in a
+century; and possibly at still longer intervals.
+
+The equilibrium amount of radium contained in some nuclei may
+amount to only a few atoms. Even in the case of the larger nuclei
+and more perfectly developed haloes the quantity of radium
+involved is many millions of times less than the least amount we
+can recognise by any other means. But the delicacy of the
+observation is not adequately set forth in this statement. We can
+not only tell the nature of the radioactive family with which we
+are dealing; but we can recognise the presence of some of its
+constituent members. I may say that it is not probable the
+zircons are richer in radium than I have assumed. My assumption
+involves about 3 per cent. of uranium. I know of no analyses
+ascribing so great an amount of uranium to zircon. The variety
+cyrtolite has been found to contain half this amount, about. But
+even if we doubled our estimate of radium content, the remarkable
+nature of our conclusions is hardly lessened.
+
+It may appear strange that the ever-interesting question of the
+Earth's age should find elucidation from the
+
+238
+
+study of haloes. Nevertheless the subjects are closely connected.
+The circumstances are as follows. Geologists have estimated the
+age of the Earth since denudation began, by measurements of the
+integral effects of denudation. These methods agree in showing an
+age of about rob years. On the other hand, measurements have been
+made of the accumulation in minerals of radioactive _débris_--the
+helium and lead--and results obtained which, although they do not
+agree very well among themselves, are concordant in assigning a
+very much greater age to the rocks. If the radioactive estimate
+is correct, then we are now living in a time when the denudative
+forces of the Earth are about eight or nine times as active as
+they have been on the average over the past. Such a state of
+things is absolutely unaccountable. And all the more
+unaccountable because from all we know we would expect a somewhat
+_lesser_ rate of solvent denudation as the world gets older and the
+land gets more and more loaded with the washed-out materials of
+the rocks.
+
+Both the methods referred to of finding the age assume the
+principle of uniformity. The geologist contends for uniformity
+throughout the past physical history of the Earth. The physicist
+claims the like for the change-rates of the radioactive elements.
+Now the study of the rocks enables us to infer something as to
+the past history of our Globe. Nothing is, on the other hand,
+known respecting the origin of uranium or thorium--the parent
+radioactive bodies. And while not questioning the law
+
+239
+
+and regularity which undoubtedly prevail in the periods of the
+members of the radioactive families, it appears to me that it is
+allowable to ask if the change rate of uranium has been always
+what we now believe it to be. This comes to much the same thing
+as supposing that atoms possessing a faster change rate once were
+associated with it which were capable of yielding both helium and
+lead to the rocks. Such atoms might have been collateral in
+origin with uranium from some antecedent element. Like helium,
+lead may be a derivative from more than one sequence of
+radioactive changes. In the present state of our knowledge the
+possibilities are many. The rate of change is known to be
+connected with the range of the alpha ray expelled by the
+transforming element; and the conformity of the halo with our
+existing knowledge of the ranges is reason for assuming that,
+whatever the origin of the more active associate of uranium, this
+passed through similar elemental changes in the progress of its
+disintegration. There may, however, have been differences in the
+ranges which the halo would not reveal. It is remarkable that
+uranium at the present time is apparently responsible for two
+alpha rays of very different ranges. If these proceed from
+different elements, one should be faster in its change rate than
+the other. Some guidance may yet be forthcoming from the study of
+the more obscure problems of radioactivity.
+
+Now it is not improbable that the halo may contribute directly to
+this discussion. We can evidently attack
+
+240
+
+the biotite with a known number of alpha rays and determine how
+many are required to produce a certain intensity of darkening,
+corresponding to that of a halo with a nucleus of measurable
+dimensions. On certain assumptions, which are correct within
+defined limits, we can calculate, as I have done above, the
+number of rays concerned in forming the halo. In doing so we
+assume some value for the age of the halo. Let us take the
+maximum radioactive value. A halo originating in Devonian times
+may attain a certain central blackening from the effects of, say,
+rob rays. But now suppose we find that we cannot produce the same
+degree of blackening with this number of rays applied in the
+laboratory. What are we to conclude? I think there is only the
+one conclusion open to us; that some other source of alpha rays,
+or a faster rate of supply, existed in the past. And this
+conclusion would explain the absence of haloes from the younger
+rocks; which, in view of the vast range of effects possible in
+the development of haloes, is, otherwise, not easy to account
+for. It is apparent that the experiment on the biotite has a
+direct bearing on the validity of the radioactive method of
+estimating the age of the rocks. It is now being carried out by
+Professor Rutherford under reliable conditions.
+
+Finally, there is one very certain and valuable fact to be
+learned from the halo. The halo has established the extreme
+rarity of radioactivity as an atomic phenomenon. One and all of
+the speculations as to
+
+241
+
+the slow breakdown of the commoner elements may be dismissed. The
+halo shows that the mica of the rocks is radioactively sensitive.
+The fundamental criterion of radioactive change is the expulsion
+of the alpha ray. The molecular system of the mica and of many
+other minerals is unstable in presence of these rays, just as a
+photographic plate is unstable in presence of light. Moreover,
+the mineral integrates the radioactive effects in the same way as
+a photographic salt integrates the effects of light. In both
+cases the feeblest activities become ultimately apparent to our
+inspection. We have seen that one ray in each year since the
+Devonian period will build the fully formed halo: an object
+unlike any other appearance in the rocks. And we have been able
+to allocate all the haloes so far investigated to one or the
+other of the known radioactive families. We are evidently
+justified in the belief that had other elements been radioactive
+we must either find characteristic haloes produced by them, or
+else find a complete darkening of the mica. The feeblest alpha
+rays emitted by the relatively enormous quantities of the
+prevailing elements, acting over the whole duration of geological
+time--and it must be remembered that the haloes we have been
+studying are comparatively young--must have registered their
+effects on the sensitive minerals. And thus we are safe in
+concluding that the common elements, and, indeed, many which
+would be called rare, are possessed of a degree of stability
+which has preserved them un
+
+242
+
+changed since the beginning of geological time. Each unaffected
+flake of mica is, thus, unassailable proof of a fact which but
+for the halo would, probably, have been for ever beyond our
+cognisance.
+
+THE USE OF RADIUM IN MEDICINE [1]
+
+IT has been unfortunate for the progress of the radioactive
+treatment of disease that its methods and claims involve much of
+the marvellous. Up till recently, indeed, a large part of
+radioactive therapeutics could only be described as bordering on
+the occult. It is not surprising that when, in addition to its
+occult and marvellous characters, claims were made on its behalf
+which in many cases could not be supported, many medical men came
+to regard it with a certain amount of suspicion.
+
+Today, I believe, we are in a better position. I think it is
+possible to ascribe a rational scientific basis to its legitimate
+claims, and to show, in fact, that in radioactive treatment we
+are pursuing methods which have been already tried extensively
+and found to be of definite value; and that new methods differ
+from the old mainly in their power and availability, and little,
+or not at all, in kind.
+
+Let us briefly review the basis of the science. Radium is a
+metallic element chemically resembling barium. It
+
+[1] A Lecture to Postgraduate Students of Medicine in connection
+with the founding of the Dublin Radium Institute, delivered in
+the School of Physic in Ireland, Trinity College, on October 2nd,
+1914
+
+244
+
+possesses, however, a remarkable property which barium does not.
+Its atoms are not equally stable. In a given quantity of radium a
+certain very small percentage of the total number of atoms
+present break up per second. By "breaking up" we mean their
+transmutation to another element. Radium, which is a solid
+element under ordinary conditions, gives rise by transmutation to
+a gaseous element--the emanation of radium. The new element is a
+heavy gas at ordinary temperatures and, like other gases, can be
+liquified by extreme cold. The extraordinary property of
+transmutation is entirely automatic. No influence which chemist
+or physicist can apply can affect the rate of transformation.
+
+The emanation inherits the property of instability, but in its
+case the instability is more pronounced. A relatively large
+fraction of its atoms transmute per second to a solid element
+designated Radium A. In turn this new generation of atoms breaks
+up--even faster than the emanation--becoming yet another element
+with specific chemical properties. And so on for a whole sequence
+of transmutations, till finally a stable substance is formed,
+identical with ordinary lead in chemical and physical properties,
+but possessing a slightly lower atomic weight.
+
+The genealogy of the radium series of elements shows that radium
+is not the starting point. It possesses ancestors which have been
+traced back to the element uranium.
+
+Now what bearing has this series of transmutations
+
+245
+
+upon medical science? Radium or emanation, &c., are not in the
+Pharmacopoeia as are, say, arsenic or bismuth. The whole
+medicinal value of these elements resides in the very wonderful
+phenomena of their radiations. They radiate in the process of
+transmuting.
+
+The changing atom may radiate a part of its own mass. The
+"alpha"-ray (a-ray) is such a material ray. It is an electrified
+helium atom cast out of the parent atom with enormous
+velocity--such a velocity as would carry it, if not impeded, all
+round the earth in two seconds. All alpha-rays are positively
+electrified atoms of the element helium, which thereby is shown
+to be an integral constituent of many elements. The alpha-ray is
+not of much value to medical science, for, in spite of its great
+velocity, it is soon stopped by encounter with other atoms. It
+can penetrate only a minute fraction of a millimetre into
+ordinary soft tissues. We shall not further consider it.
+
+Transmuting atoms give out also material rays of another kind:
+the ß-rays. The ß-ray is in mass but a very small fraction of,
+even, a hydrogen atom. Its speed may approach that of light. As
+cast out by radioactive elements it starts with speeds which vary
+with the element, and may be from one-third to nine-tenths the
+velocity of light. The ß-ray is negatively electrified. It has
+long been known to science as the electron. It is also identical
+with the cathode ray of the vacuum tube.
+
+246
+
+Another and quite different kind of radiation is given out by
+many of the transmuting elements:--the y-ray. This is not
+material, it is ethereal. It is known now with certainty that the
+y-ray is in kind identical with light, but of very much shorter
+wave length than even the extreme ultraviolet light of the solar
+spectrum. The y-ray is flashed from the transmuting atom along
+with the ß-ray. It is identical in character with the x-ray but
+of even shorter wave length.
+
+There is a very interesting connection between the y-ray and the
+ß-ray which it is important for the medical man to understand--as
+far as it is practicable on our present knowledge.
+
+When y-rays or x-rays fall on matter they give rise to ß-rays.
+The mechanism involved is not known but it is possibly a result
+of the resonance of the atom, or of parts of it, to the short
+light waves. And it is remarkable that the y-rays which, as we
+have seen, are shorter and more penetrating waves than the
+x-rays, give rise to ß-rays possessed of greater velocity and
+penetration than ß-rays excited by the x-rays. Indeed the ß-rays
+originated by y-rays may attain a velocity nearly approaching
+that of light and as great as that of any ß-rays emitted by
+transmuting atoms. Again there is demonstrable evidence that
+ß-rays impinging on matter may give rise to y-rays. The most
+remarkable demonstration of this is seen in the x-ray tube. Here
+the x-rays originate where the stream of ß- or cathode-rays
+
+247
+
+are arrested on the anode. But the first relation is at present
+of most importance to us--_i.e._ that the y-or x-rays give rise to
+ß-rays.
+
+This relation gives us additional evidence of the identity of the
+physical effects of y-, x-, and light-rays --using the term light
+rays in the usual sense of spectral rays. For it has long been
+known that light waves liberate electrons from atoms. It has been
+found that these electrons possess a certain initial velocity
+which is the greater the shorter the wave length of the light
+concerned in their liberation. The whole science of
+"photo-electricity" centres round this phenomenon. The action of
+light on the photographic plate, as well as many other physical
+and chemical phenomena, find an explanation in this liberation of
+the electron by the light wave.
+
+Here, then, we have spectral light waves liberating
+electrons--_i.e._ very minute negatively-charged particles, and we
+find that, as we use shorter light waves, the initial velocity of
+these particles increases. Again, we have x-rays which are far
+smaller in wave length than spectral light, liberating much
+faster negatively electrified particles. Finally, we have
+y-rays--the shortest nether waves of all-liberating negative
+particles of the highest velocity known. Plainly the whole series
+of phenomena is continuous.
+
+We can now look closer at the actions involved in the therapeutic
+influence of the several rays and in
+
+248
+
+this way, also, see further the correlation between what may be
+called photo-therapeutics and radioactive therapeutics.
+
+The ß-ray, whether we obtain it directly from the transforming
+radioactive atom or whether we obtain it as a result of the
+effects of the y- or x-rays upon the atom, is an ionising agent
+of wonderful power. What is meant by this? In its physical aspect
+this means that the atoms through which it passes acquire free
+electric charges; some becoming positive, some negative. This can
+only be due to the loss of an electron by the affected atom. The
+loss of the small negative charge carried in the electron leaves
+the atom positively electrified or creates a positive ion. The
+fixing of the wandering electron to a neutral atom creates a
+negative ion. Before further consideration of the importance of
+the phenomenon of ionisation we must fix in our minds that the
+agent, which brings this about, is the ß-ray. There is little
+evidence that the y-ray can directly create ions to any large
+extent. But the action of liberating high-speed ß-rays results in
+the creation of many thousands of ions by each ß-ray liberated.
+As an agent in the hands of the medical man we must regard the
+y-ray as a light wave of extremely penetrating character, which
+creates high-speed ß-rays in the tissues which it penetrates,
+these ß-rays being most potent ionising agents. The ß-rays
+directly obtained from radioactive atoms assist in the work of
+ionisation. ß-rays do not
+
+249
+
+penetrate far from their source. The fastest of them would not
+probably penetrate one centimetre in soft tissues.
+
+We must now return to the phenomenon of ionisation. Ionisation is
+revealed to observation most conspicuously when it takes place in
+a gas. The + and - electric charges on the gas particles endow it
+with the properties of a conductor of electricity, the + ions
+moving freely in one direction and the - ions in the opposite
+direction under an electric potential. But there are effects
+brought about by ionisation of more importance to the medical man
+than this. The chemist has long come to recognise that in the ion
+he is concerned with the inner mechanism of a large number of
+chemical phenomena. For with the electrification of the atom
+attractive and repulsive forces arise. We can directly show the
+chemical effects of the ionising ß-rays. Water exposed to their
+bombardment splits up into hydrogen and oxygen. And, again, the
+separated atoms may be in part recombined under the influence of
+the radiation. Ammonia splits up into hydrogen and nitrogen.
+Carbon dioxide forms carbon, carbon monoxide, and oxygen;
+hydrochloric acid forms chlorine and hydrogen. In these cases,
+also, recombination can be partially effected by the rays.
+
+We can be quite sure that within the complex structure of the
+living cell the ionising effects which everywhere accompany the
+ß-rays must exert a profound influence. The sequence of chemical
+events which as yet seem
+
+250
+
+beyond the ken of science and which are involved in metabolism
+cannot fail to be affected. Any, it is not surprising that as the
+result of eaperinient it is found that the radiations are agents
+which may be used either for the stimulation of the natural
+events of growth or used for the actual destruction of the cell.
+It is easy to see that the feeble radiation should produce the
+one effect, the strong the other. In a similar way by a moderate
+light stimulus we create the latent image in the photographic
+plate; by an intense light we again destroy this image. The inner
+mechanism in this last case can be logically stated.[1]
+
+_There is plainly a true physical basis here for the efficacy of
+radioactive treatment and, what is more, we find when we examine
+it, that it is in kind not different from that underlying
+treatment by spectral radiations. But in degree it is very
+different and here is the reason for the special importance of
+radioactivity as a therapeutic agent._ The Finsen light is capable
+of influencing the soft tissues to a short depth only. The reason
+is that the wave length of the light used is too great to pass
+without rapid absorption through the tissues; and, further, the
+electrons it gives rise to--_i.e._ the ß-rays it liberates--are too
+slow-moving to be very efficient ionisers. X-rays penetrate in
+some cases quite freely and give rise to much faster and more
+powerful ß-rays
+
+[1] See _The Latent Image_, p. 202.
+
+251
+
+than can the Finsen light. But far more penetrating than x-rays
+are the y-rays emitted in certain of the radioactive changes.
+These give rise to ß-rays having a velocity approximate to that
+of light.
+
+The y-rays are, therefore, very penetrating and powerfully
+ionising light waves; light waves which are quite invisible to
+the eye and can beam right through the tissues of the body. To
+the mind's eye only are they visible. And a very wonderful
+picture they make. We see the transmuting atom flashing out this
+light for an inconceivably short instant as it throws off the
+ß-ray. And "so far this little candle throws his beams" in the
+complex system of the cells, so far atoms shaken by the rays send
+out ß-rays; these in turn are hurled against other atomic
+systems; fresh separations of electrons arise and new attractions
+and repulsions spring up and the most important chemical changes
+are brought about. Our mental picture can claim to be no more
+than diagrammatic of the reality. Still we are here dealing with
+recognised physical and chemical phenomena, and their description
+as "occult" in the derogatory sense is certainly not
+justifiable.
+
+Having now briefly reviewed the nature of the rays arising in
+radioactive substances and the rationale of their influence, we
+must turn to more especially practical considerations.
+
+The Table given opposite shows that radium itself is responsible
+for a- and ß-rays only. It happens that
+
+252
+
+Period in whioh ½ element is transformed.
+
+URANIUM 1 & 2 { a 6 } x 109 years.
+
+URANIUM X { a ß } 24.6 days.
+
+IONIUM { a 8 } x 104 years.
+
+RADIUM { a ß } 2 x 102 years.
+
+EMANATION { a } 8.85 days.
+
+RADIUM A { a 8 } minutes.
+
+RADIUM B { ß y } 26.7 minutes.
+
+RADIUM C { a ß y } 13.5 minutes.
+
+RADIUM D { ß } 15 years.
+
+RADIUM E { ß y } 4.8 days.
+
+RADIUM (Polonium) F { a } 140 days.
+
+Table showing the successive generations of the elements of the
+Uranium-radium family, the character of their radiations and
+their longevity.
+
+253
+
+the ß-rays emitted by radium are very "soft"--_i.e._ slow and
+easily absorbed. The a-ray is in no case available for more than
+mere surface application. Hence we see that, contrary to what is
+generally believed, radium itself is of little direct therapeutic
+value. Nor is the next body in succession--the emanation, for it
+gives only a-rays. In fact, to be brief, it is not till we come
+to Radium B that ß-rays of a relatively high penetrative quality
+are reached; and it is not till we come to Radium C that highly
+penetrative y-rays are obtained.
+
+It is around this element, Radium C, that the chief medical
+importance of radioactive treatment by this family of radioactive
+bodies centres. Not only are ß-rays of Radium C very penetrating,
+but the y-rays are perhaps the most energetic rays of the, kind
+known. Further in the list there is no very special medical
+interest.
+
+Now, how can we get a supply of this valuable element Radium C?
+We can obtain it from radium itself. For even if radium has been
+deprived of its emanation (which is easily done by heating it or
+bringing it into solution) in a few weeks we get back the Radium
+C. One thing here we must be clear about. With a given quantity
+of Radium only a certain definitely limited amount of Radium C,
+or of emanation, or any other of the derived bodies, will be
+associated. Why is this? The answer is because the several
+successive elements are themselves decaying --_i.e._ changing one
+into the other. The atomic per-
+
+254
+
+centage of each, which decays in a second, is a fixed quantity
+which we cannot alter. Now if we picture radium which has been
+completely deprived of its emanation, again accumulating by
+automatic transmutation a fresh store of this element, we have to
+remember:-- (i) That the rate of creation of emanation by the
+radium is practically constant; and (2) that the absolute amount
+of the emanation decaying per second increases as the stock of
+emanation increases. Finally, when the amount of accumulated
+emanation has increased to such an extent that the number of
+emanation atoms transmuting per second becomes exactly equal to
+the number being generated per second, the amount of emanation
+present cannot increase. This is called the equilibrium amount.
+If fifteen members are elected steadily each year into a
+newly-founded society the number of members will increase for the
+first few years; finally, when the losses by death of the members
+equal about fifteen per annum the society can get no bigger. It
+has attained the equilibrium number of members.
+
+This applies to every one of the successive elements. It takes
+twenty-one days for the equilibrium quantity of emanation to be
+formed in radium which has been completely de-emanated; and it
+takes 3.8 days for half the equilibrium amount to be formed.
+Again, if we start with a stock of emanation it takes just three
+hours for the equilibrium amount of Radium C to be formed.
+
+255
+
+We can evidently grow Radium C either from radium itself or from
+the emanation of radium. If we use a tube of radium we have an
+almost perfectly constant quantity of Radium C present, for as
+fast as the Radium C and intervening elements decay the Radium,
+which only diminishes very slowly in amount, makes up the loss.
+But, if we start off with a tube of emanation, we do not possess
+a constant supply of Radium C, because the emanation is decaying
+fairly rapidly and there is no radium to make good its loss. In
+3.8 days about one half the emanation is transmuted and the
+Radium C decreases proportionately and, of course, with the
+Radium C the valuable radiations also decrease. In another 3.8
+days--that is in about a week from the start--the radioactive value
+of the tube has fallen to one-fourth of its original value.
+
+But in spite of the inconstant character of the emanation tube
+there are many reasons for preferring its use to the use of the
+radium tube. Chief of these is the fact that we can keep the
+precious radium safely locked up in the laboratory and not
+exposed to the thousand-and-one risks of the hospital. Then,
+secondly, the emanation, being a gas, is very convenient for
+subdivision into a large number of very small tubes according to
+the dosage required.
+
+In fact the volume of the emanation is exceedingly minute. The
+amount of emanation in equilibrium with one gramme of radium is
+called the curie, and with one
+
+256
+
+milligramme the millicurie. Now, the volume of the curie is only
+a little more than one half a cubic millimetre. Hence in dealing
+with emanation from twenty or forty milligrammes of radium we are
+dealing with very small volumes.
+
+How may the emanation be obtained? The process is an easy one in
+skilled and practised hands. The salt of radium--generally the
+bromide or chloride--is brought into acid solution. This causes
+the emanation to be freely given off as fast as it is formed. At
+intervals we pump it off with a mercury pump.
+
+Let us see how many millicuries we will in future be able to turn
+out in the week in our new Dublin Radium Institute.[1] We shall
+have about 130 milligrammes of radium. In 3.8 days we get 65
+millicuries from this--_i.e._ half the equilibrium amount of 130
+millicuries. Hence in the week, we shall have about 130
+millicuries.
+
+This is not much. Many experts consider this little enough for
+one tube. But here in Dublin we have been using the emanation in
+a more economical and effective manner than is the usage
+elsewhere; according to a method which has been worked out and
+developed in our own Radium Institute. The economy is obtained by
+the very simple expedient of minutely subdividing the' dose. The
+system in vogue, generally, is to treat the tumour by inserting
+into it one or two very active
+
+[1] Then recently established by the Royal Dublin Society.
+
+257
+
+tubes, containing, perhaps, up to 200 millicuries, or even more,
+per tube. Now these very heavily charged tubes give a radiation
+so intense at points close to the tube, due to the greater
+density of the rays near the tube, and, also, to the action of
+the softer and more easily absorbable rays, that it has been
+found necessary to stop these softer rays--both the y and ß--by
+wrapping lead or platinum round the tube. In this lead or
+platinum some thirty per cent. or more of the rays is absorbed
+and, of course, wasted. But in the absence of the screen there is
+extensive necrosis of the tissues near the tubes.
+
+If, however, in place of one or two such tubes we use ten or
+twenty, each containing one-tenth or one-twentieth of the dose,
+we can avail ourselves of the softer rays around each tube with
+benefit. Thus a wasteful loss is avoided. Moreover a more uniform
+"illumination" of the tissues results, just as we can illuminate
+a hall more uniformly by the use of many lesser centres of light
+than by the use of one intense centre of radiation. Also we get
+what is called "cross-radiation,"which is found to be beneficial.
+The surgeon knows far better what he is doing by this method.
+Thus it may be arranged for the effects to go on with approximate
+uniformity throughout the tumour instead of varying rapidly
+around a central point or--and this may be very important-- the
+effects may be readily concentrated locally.
+
+Finally, not the least of the benefit arises in the easy
+technique of this new method. The quantities of
+
+258
+
+emanation employed can fit in the finest capillary glass tubing
+and the hairlike tubes can in turn be placed in fine exploring
+needles. There is comparatively little inconvenience to the
+patient in inserting these needles, and there is the most perfect
+control of the dosage in the number and strength of these tubes
+and the duration of exposure.[1]
+
+The first Radium Institute in Ireland has already done good work
+for the relief of human suffering. It will have, I hope, a great
+future before it, for I venture, with diffidence, to hold the
+opinion, that with increased study the applications and claims of
+radioactive treatment will increase.
+
+[1] For particulars of the new technique and of some of the work
+already accomplished, see papers, by Dr. Walter C. Stevenson,
+_British Medical Journal_, July 4th, 1914, and March 20th, 1915.
+
+259
+
+SKATING [1]
+
+IT is now many years ago since, as a student, I was present at a
+college lecture delivered by a certain learned professor on the
+subject of friction. At this lecture a discussion arose out of a
+question addressed to our teacher: "How is it we can skate on ice
+and on no other substance?"
+
+The answer came back without hesitation: "Because the ice is so
+smooth."
+
+It was at once objected: "But you can skate on ice which is not
+smooth."
+
+This put the professor in a difficulty. Obviously it is not on
+account of the smoothness of the ice. A piece of polished plate
+glass is far smoother than a surface of ice after the latter is
+cut up by a day's skating. Nevertheless, on the scratched and
+torn ice-surface skating is still quite possible; on the smooth
+plate glass we know we could not skate.
+
+Some little time after this discussion, the connection between
+skating and a somewhat abstruse fact in physical science occurred
+to me. As the fact itself is one which has played a part in the
+geological history of the earth,
+
+[1] A lecture delivered before the Royal Dublin Society in 1905.
+
+260
+
+and a part of no little importance, the subject of skating,
+whereby it is perhaps best brought home to every one, is
+deserving of our careful attention. Let not, then, the title of
+this lecture mislead the reader as to the importance of its
+subject matter.
+
+Before going on to the explanation of the wonderful freedom of
+the skater's movements, I wish to verify what I have inferred as
+to the great difference in the slipperiness of glass and the
+slipperiness of ice. Here is a slab of polished glass. I can
+raise it to any angle I please so that at length this brass
+weight of 250 grams just slips down when started with a slight
+shove. The angle is, as you see, about 12½ degrees. I now
+transfer the weight on to this large slab of ice which I first
+rapidly dry with soft linen. Observe that the weight slips down
+the surface of ice at a much lower angle. It is a very low angle
+indeed: I read it as between 4 and 5 degrees. We see by this
+experiment that there is a great difference between the
+slipperiness of the two surfaces as measured by what is called
+"the angle of friction." In this experiment, too, the glass
+possesses by far the smoother surface although I have rubbed the
+deeper rugosities out of the ice by smoothing it with a glass
+surface. Notwithstanding this, its surface is spotted with small
+cavities due to bubbles and imperfections. It is certain that if
+the glass was equally rough, its angle of friction towards the
+brass weight would be higher.
+
+261
+
+We have, however, another comparative experiment to carry out. I
+made as you saw a determination of the angle at which this weight
+of 250 grams just slipped on the ice. The lower surface of the
+weight, the part which presses on the ice, consists of a light,
+brass curtain ring. This can be detached. Its mass is only 6½
+grams, the curtain ring being, in fact, hollow and made of very
+thin metal. We have, therefore, in it a very small weight which
+presents exactly the same surface beneath as did the weight of
+250 grams. You see, now, that this light weight will not slip on
+ice at 5 or 6 degrees of slope, but first does so at about io
+degrees.
+
+This is a very important experiment as regards our present
+inquiry. Ice appears to possess more than one angle of friction
+according as a heavy or a light weight is used to press upon it.
+We will make the same experiment with the plate of glass. You see
+that there is little or no difference in the angle of friction of
+brass on glass when we press the surfaces together with a heavy
+or with a light weight. The light weight requires the same slope
+of 12½ degrees to make it slip.
+
+This last result is in accordance with the laws of friction. We
+say that when solid presses on solid, for each pair of substances
+pressed together there is a constant ratio between the force
+required to keep one in motion over the other, and the force
+pressing the solids together. This ratio is called"the
+coefficient of friction."The coefficient is, in fact, constant or
+approximately
+
+262
+
+so. I can determine the coefficient of friction from the angle of
+friction by taking the tangent of the angle. The tangent of the
+angle of friction is the coefficient of friction. If, then, the
+coefficient is constant, so, of course, must the angle of
+friction be constant. We have seen that it is so in the case of
+metal on glass, but not so in the case of metal on ice. This
+curious result shows that there is something abnormal about the
+slipperiness of ice.
+
+The experiments we have hitherto made are open to the reproach
+that the surface of the ice is probably damp owing to the warmth
+of the air in contact with it. I have here a means of dealing
+with a surface of cold, dry ice. This shallow copper tank about
+18 inches (45 cms.) long, and 4 inches (10 cms.) wide, is filled
+with a freezing 'mixture circulated through it from a larger
+vessel containing ice melting in hydrochloric acid at a
+temperature of about -18° C. This keeps the tank below the
+melting point of ice. The upper surface of the tank is provided
+with raised edges so that it can be flooded with water. The water
+is now frozen and its temperature is below 0° C. It is about
+10° C. I can place over the ice a roof-shaped cover made of two
+inclined slabs of thick plate glass. This acts to keep out warm
+air, and to do away with any possibility of the surface of the
+ice being wet with water thawed from the ice. The whole tank
+along with its roof of glass can be adjusted to any angle, and a,
+scale at the
+
+263
+
+raised end of the tank gives the angle of slope in degrees. A
+weight placed on the ice can be easily seen through the glass
+cover.
+
+The weight we shall use consists of a very light ring of
+aluminium wire which is rendered plainly visible by a ping-pong
+ball attached above it. The weight rests now on a copper plate
+provided for the purpose at the upper end of the tank. The plate
+being in direct contact beneath with the freezing mixture we are
+sure that the aluminium ring is no hotter than the ice. A light
+jerk suffices to shake the weight on to the surface of the ice.
+
+We find that this ring loaded with only the ping-pong ball, and
+weighing a total of 2.55 grams does not slip at the low angles. I
+have the surface of the ice at an angle of rather over 13½, and
+only by continuous tapping of the apparatus can it be induced to
+slip down. This is a coefficient of 0.24, and compares with the
+coefficient of hard and smooth solids on one another. I now
+replace the empty ping-pong ball by a similar ball filled with
+lead shot. The total weight is now 155 grams. You see the angle
+of slipping has fallen to 7°.
+
+Every one who has made friction experiments knows how
+unsatisfactory and inconsistent they often are. We can only
+discuss notable quantities and broad results, unless the most
+conscientious care be taken to eliminate errors. The net result
+here is that ice at about -10° C. when pressed on by a very light
+weight possesses a
+
+264
+
+coefficient of friction comparable with the usual coefficients of
+solids on solids, but when the pressure is increased, the
+coefficient falls to about half this value.
+
+The following table embodies some results obtained on the
+friction of ice and glass, using the methods I have shown you. I
+add some of the more carefully determined coefficients of other
+observers.
+
+          Wt. in      On Plate         On Ice         On Ice
+          Grams.      Glass.           at 0° C.       at 10° C.
+
+                   Angle. Coeff.    Angle. Coeff.  Angle. Coeff
+Aluminium   2.55    12½°  0.22        12°   0.21     13½°   0.24
+Same      155       12½°   0.22        6°   0.11       7°   0.12
+Brass       6.5     12½°  0.22        10°   0.17     10½°   0.18
+Same      107       12½°   0.22        5°   0.09       6°   0.10
+
+Steel on steel (Morin) - - - - 0.14
+Brass on cast iron (Morin) - - 0.19
+Steel on cast iron (Morin) - - 0.20
+Skate on ice (J. Müller) - - - 0.016--0.032
+Best-greased surfaces (Perry) - 0.03--0.036
+
+You perceive from the table that while the friction of brass or
+aluminium on glass is quite independent of the weight used, that
+of brass or aluminium on ice depends in some way upon the weight,
+and falls in a very marked degree when the weight is heavy. Now,
+I think that if we had been on the look out for any abnormality
+in the friction of hard substances on ice, we would have rather
+anticipated a variation in the
+
+265
+
+other direction. We would have, perhaps, expected that a heavy
+weight would have given rise to the greater friction. I now turn
+to the explanation of this extraordinary result.
+
+You are aware that it requires an expenditure of heat merely to
+convert ice to water, the water produced being at the temperature
+of the ice, _i.e._ at 0° C., from which it is derived. The heat
+required to change the ice from the solid to the liquid state is
+the latent heat of water. We take the unit quantity of heat to be
+that which is required to heat 1 kilogram of water 1° C. Then if
+we melt 1 kilogram of ice, we must supply it with 80 such units
+of heat. While melting is going on, there is no change of
+temperature if the experiment is carefully conducted. The melting
+ice and the water coming from it remain at 0° C. throughout the
+operation, and neither the thermometer nor your own sensations
+would tell you of the amount of heat which was flowing in. The
+heat is latent or hidden in the liquid produced, and has gone to
+do molecular work in the substance. Observe that if we supply
+only 40 thermal units, we get only one-half the ice melted. If
+only 10 units are supplied, then we get only one eighth of a
+kilogram of water, and no more nor less.
+
+I have ventured to recall to you these commonplaces of science
+before considering a mode of melting ice which is less generally
+known, and which involves no supply of heat on your part. This
+method involves for its
+
+266
+
+understanding a careful consideration of the thermal properties
+of water in the solid state.
+
+It must have been observed a very long time ago that water
+expands when it freezes. Otherwise ice would not float on water;
+and, what is perhaps more important in your eyes, your water
+pipes would not burst in winter when the water freezes therein.
+But although the important fact of the expansion of water on
+freezing was so long presented to the observation of mankind, it
+was not till almost exactly the middle of the last century that
+James Thomson, a gifted Irishman, predicted many important
+consequences arising from the fact of the expansion of water on
+becoming solid. The principles lie enunciated are perfectly
+general, and apply in every case of change of volume attending
+change of state. We are here only concerned with the case of
+water and ice.
+
+James Thomson, following a train of thought which we cannot here
+pursue, predicted that owing to the fact of the expansion of
+water on becoming solid, pressure will lower the melting point of
+ice or the freezing point of water. Normally, as you are aware,
+the temperature is 0° C. or 32° F. Thomson said that this would
+be found to be the freezing point only at atmospheric pressure.
+He calculated how much it would change with change of pressure.
+He predicted that the freezing point would fall 0.0075 of a
+degree Centigrade for each additional atmosphere of pressure
+applied to the water. Suppose,
+
+267
+
+for instance, our earth possessed an atmosphere so heavy to as
+exert a thousand times the pressure of the existing atmosphere,
+then water would not freeze at 0° C., but at -7.5° C. or about
+18° F. Again, in vacuo, that is when the pressure has been
+reduced to the relatively small vapour pressure of the water, the
+freezing point is above 0° C., _i.e._ at 0.0075° C. In parts of
+the ocean depths the pressure is much over a thousand
+atmospheres. Fresh water would remain liquid there at
+temperatures much below 0° C.
+
+It will be evident enough, even to those not possessed of the
+scientific insight of James Thomson, that some such fact is to be
+anticipated. It is, however, easy to be wise after the event. It
+appeals to us in a general way that as water expands on freezing,
+pressure will tend to resist the turning of it to ice. The water
+will try to remain liquid in obedience to the pressure. It will,
+therefore, require a lower temperature to induce it to become
+ice.
+
+James Thomson left his thesis as a prediction. But he predicted
+exactly what his distinguished brother, Sir William Thomson--later
+Lord Kelvin--found to happen when the matter was put to the test
+of experiment. We must consider the experiment made by Lord
+Kelvin.
+
+According to Thomson's views, if a quantity of ice and water are
+compressed, there must be _a fall of temperature_. The nature of
+his argument is as follows:
+
+268
+
+Let the ice and water be exactly at 0° C. to start with. Then
+suppose we apply, say, one thousand atmospheres pressure. The
+melting point of the ice is lowered to -7.5° C. That is, it will
+require a temperature so low as -7.5° C. to keep it solid. It
+will therefore at once set about melting, for as we have seen,
+its actual temperature is not -7.5° C., but a higher temperature,
+_i.e._ 0° C. In other words, it is 7.5° above its melting point.
+But as soon as it begins melting it also begins to absorb heat to
+supply the 80 thermal units which, as we know, are required to
+turn each kilogram of the ice to water. Where can it get this
+heat? We assume that we give it none. It has only two sources,
+the ice can take heat from itself, and it can take heat from the
+water. It does both in this case, and both ice and water drop in
+temperature. They fall in temperature till -7.5° is reached. Then
+the ice has got to its melting point under the pressure of one
+thousand atmospheres, or, as we may put it, the water has reached
+its freezing point. There can be no more melting. The whole mass
+is down to -7.5° C., and will stay there if we keep heat from
+flowing either into or out of the vessel. There is now more water
+and less ice in the vessel than when we started, and the
+temperature has fallen to -7.5° C. The fall of temperature to the
+amount predicted by the theory was verified by Lord Kelvin.
+
+Suppose we now suddenly remove the pressure; what will happen? We
+have water and ice at -7.5° C.
+
+269
+
+and at the normal pressure. Water at -7.5° and at the normal
+pressure of course turns to ice. The water will, therefore,
+instantly freeze in the vessel, and the whole process will be
+reversed. In freezing, the water will give up its latent heat,
+and this will warm up the whole mass till once again 0° C. is
+attained. Then there will be no more freezing, for again the ice
+is at its melting point. This is the remarkable series of events
+which James Thomson predicted. And these are the events which
+Lord Kelvin by a delicate series of experiments, verified in
+every respect.
+
+Suppose we had nothing but solid ice in the vessel at starting,
+would the experiment result in the same way? Yes, it assuredly
+would. The ice under the increased pressure would melt a little
+everywhere throughout its mass, taking the requisite latent heat
+from itself at the expense of its sensible heat, and the
+temperature of the ice would fall to the new melting point.
+
+Could we melt the whole of the ice in this manner? Again the
+answer is "yes." But the pressure must be very great. If we
+assume that all the heat is obtained at the expense of the
+sensible heat of the ice, the cooling must be such as to supply
+the latent heat of the whole mass of water produced. However, the
+latent heat diminishes as the melting point is lowered, and at a
+rate which would reduce it to nothing at about 18,000
+atmospheres. Mousson, operating on ice enclosed in a conducting
+cylinder and cooled to -18° at starting
+
+270
+
+appears to have obtained very complete liquefaction. Mousson must
+have attained a pressure of at least an amount adequate to lower
+the melting point below -18°. The degree of liquefaction actually
+attained may have been due in part to the passage of heat through
+the walls of the vessel. He proved the more or less complete
+liquefaction of the ice within the vessel by the fall of a copper
+index from the top to the bottom of the vessel while the pressure
+was on.
+
+I have here a simple way of demonstrating to you the fall of
+temperature attending the compression of ice. In this mould,
+which is strongly made of steel, lined with boxwood to diminish
+the passage of conducted heat, is a quantity of ice which I
+compress when I force in this plunger. In the ice is a
+thermoelectric junction, the wires leading to which are in
+communication with a reflecting galvanometer. The thermocouple is
+of copper and nickel, and is of such sensitiveness as to show by
+motion of the spot of light on the screen even a small fraction
+of a degree. On applying the pressure, you see the spot of light
+is displaced, and in such a direction as to indicate cooling. The
+balancing thermocouple is all the time imbedded in a block of ice
+so that its temperature remains unaltered. On taking off the
+pressure, the spot of light returns to its first position. I can
+move the spot of light backwards and forwards on the screen by
+taking off and putting on the pressure. The effects are quite
+instantaneous.
+
+271
+
+The fact last referred to is very important. The ice, in fact, is
+as it were automatically turned to water. It is not a matter of
+the conduction of heat from point to point in the ice. Its own
+sensible heat is immediately absorbed throughout the mass. This
+would be the theoretical result, but it is probable that owing to
+imperfections throughout the ice and failure in uniformity in the
+distribution of the stress, the melting would not take place
+quite uniformly or homogeneously.
+
+Before applying our new ideas to skating, I want you to notice a
+fact which I have inferentially stated, but not specifically
+mentioned. Pressure will only lead to the melting of ice if the
+new melting point, _i.e._ that due to the pressure, is below the
+prevailing temperature. Let us take figures. The ice to start
+with is, say, at -3° C. Suppose we apply such a pressure to this
+ice as will confer a melting point of -2° C. on it. Obviously,
+there will be no melting. For why should ice which is at -3° C.
+melt when its melting point is -2° C.? The ice is, in fact,
+colder than its melting point. Hence, you note this fact: The
+pressure must be sufficiently intense to bring the melting point
+below the prevailing temperature, or there will be no melting;
+and the further we reduce the melting point by pressure below the
+prevailing temperature, the more ice will be melted.
+
+We come at length to the object of our remarks I don't know who
+invented skating or skates. It is said that in the thirteenth
+century the inhabitants of
+
+272
+
+England used to amuse themselves by fastening the bones of an
+animal beneath their feet, and pushing themselves about on the
+ice by means of a stick pointed with iron. With such skates, any
+performance either on inside or outside edge was impossible. We
+are a conservative people. This exhilarating amusement appears to
+have served the people of England for three centuries. Not till
+1660 were wooden skates shod with iron introduced from the
+Netherlands. It is certain that skating was a fashionable
+amusement in Pepys' time. He writes in 1662 to the effect: "It
+being a great frost, did see people sliding with their skates,
+which is a very pretty art." It is remarkable that it was the
+German poet Klopstock who made skating fashionable in Germany.
+Until his time, the art was considered a pastime, only fit for
+very young or silly people.
+
+I wish now to dwell upon that beautiful contrivance the modern
+skate. It is a remarkable example of how an appliance can develop
+towards perfection in the absence of a really intelligent
+understanding of the principles underlying its development. For
+what are the principles underlying the proper construction of the
+skate? After what I have said, I think you will readily
+understand. The object is to produce such a pressure under the
+blade that the ice will melt. We wish to establish such a
+pressure under the skate that even on a day when the ice is below
+zero, its melting
+
+273
+
+point is so reduced just under the edge of the skate that the ice
+turns to water.
+
+It is this melting of the ice under the skate which secures the
+condition essential to skating. In the first place, the skate no
+longer rests on a solid. It rests on a liquid. You are aware how
+in cases where we want to reduce friction--say at the bearing of a
+wheel or under a pivot--we introduce a liquid. Look at the
+bearings of a steam engine. A continuous stream of oil is fed in
+to interpose itself between the solid surfaces. I need not
+illustrate so well-known a principle by experiment. Solid
+friction disappears when the liquid intervenes. In its place we
+substitute the lesser difficulty of shearing one layer of the
+liquid over the other; and if we keep up the supply of oil the
+work required to do this is not very different, no matter how
+great we make the pressure upon the bearings. Compared with the
+resistance of solid friction, the resistance of fluid friction is
+trifling. Here under the skate the lubrication is perhaps the
+most perfect which it is possible to conceive. J. Müller has
+determined the coefficient by towing a skater holding on by a
+spring balance. The coefficient is between 0.016 and 0.032. In
+other words, the skater would run down an incline so little as 1
+or 2 degrees; an inclination not perceivable by the eye. Now
+observe that the larger of these coefficients is almost exactly
+the same as that which Perry found in the case of well-greased
+surfaces. But evidently no
+
+274
+
+artificial system of lubrication could hope to equal that which
+exists between the skate and the ice. For the lubrication here
+is, as it were, automatic. In the machine if the lubricant gets
+squeezed out there instantly ensues solid friction. Under the
+skate this cannot happen for the squeezing out of the lubricant
+is instantly followed by the formation of another film of water.
+The conditions of pressure which may lead to solid friction in
+the machine here automatically call the lubricant into
+existence.
+
+Just under the edge of the skate the pressure is enormous.
+Consider that the whole weight of the skater is born upon a mere
+knife edge. The skater alternately throws his whole weight upon
+the edge of each skate. But not only is the weight thus
+concentrated upon one edge, further concentration is secured in
+the best skates by making the skate hollow-ground, _i.e._
+increasing the keenness of the edge by making it less than a
+right angle. Still greater pressure is obtained by diminishing
+the length of that part of the blade which is in contact with the
+ice. This is done by putting curvature on the blade or making it
+what is called "hog-backed." You see that everything is done to
+diminish the area in contact with the ice, and thus to increase
+the pressure. The result is a very great compression of the ice
+beneath the edge of the skate. Even in the very coldest weather
+melting must take place to some extent.
+
+As we observed before, the melting is instantaneous,
+
+275
+
+Heat has not to travel from one point of the ice to another;
+immediately the pressure comes on the ice it turns to water. It
+takes the requisite heat from itself in order that the change of
+state may be accomplished. So soon as the skate passes on, the
+water resumes the solid state. It is probable that there is an
+instantaneous escape, and re-freezing of some of the water from
+beneath the skate, the skate instantly taking a fresh bearing and
+melting more ice. The temperature of the water escaping from
+beneath the skate, or left behind by it, immediately becomes what
+it was before the skate pressed upon it.
+
+Thus, a most wonderful and complex series of molecular events
+takes place beneath the skate. Swift as it passes, the whole
+sequence of events which James Thomson predicted has to take
+place beneath the blade Compression; lowering of the melting
+point below the temperature of the surrounding ice; melting;
+absorption of heat; and cooling to the new melting point, _i.e._
+to that proper to the pressure beneath the blade. The skate now
+passes on. Then follow: Relief of pressure; re-solidification of
+the water; restoration of the borrowed heat from the congealing
+water and reversion of the ice to the original temperature.
+
+If we reflect for a moment on all this, we see that we do not
+skate on ice but on water. We could not skate on ice any more
+than we could skate on glass. We saw that with light weights and
+when the pressure
+
+276
+
+{Diagram}
+
+Diagram showing successive states obtaining in ice, before,
+during, and after the passage of the skate. The temperatures and
+pressures selected for illustration are such as might occur under
+ordinary conditions. The edge of the skate is shown in magnified
+cross-section.
+
+277
+
+Was not sufficient to melt the ice, the friction was much the
+same as that of metal on glass. Ice is not slippery. It is an
+error to say that it is. The learned professor was very much
+astray when he said that you could skate on ice because it is so
+smooth. The smoothness of the ice has nothing to do with the
+matter. In short, owing to the action of gravity upon your body,
+you escape the normal resistance of solid on solid, and glide
+about with feet winged like the messenger of the Gods; but on
+water.
+
+A second condition essential to the art of skating is also
+involved in the melting of the ice. The sinking of the skate
+gives the skater "bite." This it is which enables him to urge
+himself forward. So long as skates consisted of the rounded bones
+of animals, the skater had to use a pointed staff to propel
+himself. In creating bite, the skater again unconsciously appeals
+to the peculiar physical properties of ice. The pressure required
+for the propulsion of the skater is spread all along the length
+of the groove he has cut in the ice, and obliquely downwards. The
+skate will not slip away laterally, for the horizontal component
+of the pressure is not enough to melt the ice. He thus gets the
+resistance he requires.
+
+You see what a very perfect contrivance the skate is; and what a
+similitude of intelligence there is in its evolution. Blind
+intelligence, because it is certain the true physics of skating
+was never held in view by
+
+278
+
+the makers of skates. The evolution of the skate has been truly
+organic. The skater selected the fittest skate, and hence the fit
+skate survived.
+
+In a word, the possibility of skating depends on the dynamical
+melting of ice under pressure. And observe the whole matter turns
+upon the apparently unrelated fact that the freezing of water
+results in a solid more bulky than the water which gives rise to
+it. If ice was less bulky than the water from which it was
+derived, pressure would not melt it; it would be all the more
+solid for the pressure, as it were. The melting point would rise
+instead of falling. Most substances behave in this manner, and
+hence we cannot skate upon them. Only quite a few substances
+expand on freezing, and it happens that their particular melting
+temperatures or other properties render them unsuitable to
+skating. The most abundant fluid substance on the earth, and the
+most abundant substance of any one kind on its surface, thus
+possesses the ideally correct and suitable properties for the art
+of skating.
+
+I have pointed out that the pressure must be such as to bring the
+temperature of melting below that prevailing in the ice at the
+time. We have seen also, that one atmosphere lowers the melting
+point of ice by the 1/140 of a degree Centigrade; more exactly by
+0.0075°. Let us now assume that the skate is so far sunken in the
+ice as to bear for a length of two inches, and for a width of
+one-hundredth of an inch. The skater weighs,
+
+279
+
+let us say--150 pounds. If this weight was borne on one square
+inch, the pressure would be ten atmospheres. But the skater rests
+his weight, in fact, upon an area of one-fiftieth of an inch. The
+pressure is, therefore, fifty times as great. The ice is
+subjected to a pressure of 500 atmospheres. This lowers the
+melting point to -3.75° C. Hence, on a day when the ice is at
+this temperature, the skate will sink in the ice till the weight
+of the skater is concentrated as we have assumed. His skate can
+sink no further, for any lesser concentration of the pressure
+will not bring the melting point below the prevailing
+temperature. We can calculate the theoretical bite for any state
+of the ice. If the ice is colder the bite will not be so deep. If
+the temperature was twice as far below zero, then the area over
+which the skater's weight will be distributed, when the skate has
+penetrated its maximum depth, will be only half the former area,
+and the pressure will be one thousand atmospheres.
+
+An important consideration arises from the fact that under the
+very extreme edge of the skate the pressure is indefinitely
+great. For this involves that there will always be some bite,
+however cold the ice may be. That is, the narrow strip of ice
+which first receives the skater's weight must partially liquefy
+however cold the ice.
+
+It must have happened to many here to be on ice which was too
+cold to skate on with comfort. The
+
+280
+
+skater in this case speaks of the ice as too hard. In the
+Engadine, the ice on the large lakes gets so cold that skaters
+complain of this. On the rinks, which are chiefly used there, the
+ice is frequently renewed by flooding with water at the close of
+the day. It thus never gets so very cold as on the lakes. I have
+been on ice in North France, which, in the early morning, was too
+hard to afford sufficient bite for comfort. The cause of this is
+easily understood from what we have been considering.
+
+We may now return to the experimental results which we obtained
+early in the lecture. The heavy weights slip off the ice at a low
+angle because just at the points of contact with the ice the
+latter melts, and they, in fact, slip not on ice but on water.
+The light weights on cold, dry ice do not lower the melting point
+below the temperature of the ice, _i.e._ below -10° C., and so
+they slip on dry ice. They therefore give us the true coefficient
+of friction of metal on ice.
+
+This subject has, more recently been investigated by H. Morphy,
+of Trinity College, Dublin. The refinement of a closed vessel at
+uniform temperature, in which the ice is formed and the
+experiment carried out, is introduced. Thermocouples give the
+temperatures, not only of the ice but of the aluminium sleigh
+which slips upon it under various loads. In this way we may be
+certain that the metal runners are truly at the temperature of
+the ice. I now quote from Morphy's paper
+
+281
+
+"The angle of friction was found to remain constant until a
+certain stage of the loading, when it suddenly fell to about half
+of its original value. It then remained constant for further
+increases in the load.
+
+"These results, which confirmed those obtained previously with
+less satisfactory apparatus, are shown in the table below. In the
+first column is shown the load, _i.e._ the weight of sleigh +
+weight of shot added. In the second and third columns are shown,
+respectively, the coefficient and angle of friction, whilst the
+fourth gives the temperature of the ice as determined from the
+galvanometer deflexions.
+
+Load.        Tan y.          y.        Temp.
+
+5.68 grams.  0.36±.01     20°±30'    -5.65° C.
+10.39                                -5.65°
+11.96                                -5.75°
+12.74                                -5.60°
+13.53                                -5.65°
+14.31                                -5.65°
+15.10 grams. 0.17±.01    9°.30'±30'  -5.60°
+16.67                                -5.55°
+19.81                                -5.60°
+24.52                                -5.60°
+5.68 grams. 0.36±.01      20°±30'    -5.60°
+
+"These experiments were repeated on another occasion with the same
+result and similar results had been obtained with different
+apparatus.
+
+"As a result of the investigation the following points are
+clearly shown:--
+
+282
+
+"(1) The coefficient of friction for ice at constant temperature
+may have either of two constant values according to the pressure
+per unit surface of contact.
+
+"(2) For small pressures, and up to a certain well defined limit
+of pressure, the coefficient is fairly large, having the value
+0.36±.01 in the case investigated.
+
+"(3) For pressures greater than the above limit the coefficient
+is relatively small, having the value 0.17±.01 in the case
+investigated."
+
+It will be seen that Morphy's results are similar to those
+arrived at in the first experimental consideration of our
+subject; but from the manner in which the experiments have been
+carried out, they are more accurate and reliable.
+
+A great deal more might be said about skating, and the allied
+sports of tobogganing, sleighing, curling, ice yachting, and
+last, but by no means least, sliding--that unpretentious pastime
+of the million. Happy the boy who has nails in his boots when
+Jack-Frost appears in his white garment, and congeals the
+neighbouring pond. But I must turn away at the threshold of the
+humorous aspect of my subject (for the victim of the street
+"slide" owes his injured dignity to the abstruse laws we have
+been discussing) and pass to other and graver subjects intimately
+connected with skating.
+
+James Thomson pointed out that if we apply compressional stress
+to an ice crystal contained in a vessel
+
+283
+
+which also contains other ice crystals, and water at 0° C., then
+the stressed crystal will melt and become water, but its
+counterpart or equivalent quantity of ice will reappear elsewhere
+in the vessel. This is, obviously, but a deduction from the
+principles we have been examining. The phenomenon is commonly
+called "regelation." I have already made the usual regelation
+experiment before you when I compressed broken ice in this mould.
+The result was a clear, hard and almost flawless lens of ice. Now
+in this operation we must figure to ourselves the pieces of ice
+when pressed against one another melting away where compressed,
+and the water produced escaping into the spaces between the
+fragments, and there solidifying in virtue of its temperature
+being below the freezing point of unstressed water. The final
+result is the uniform lens of ice. The same process goes on in a
+less perfect manner when you make--or shall I better say--when you
+made snowballs.
+
+We now come to theories of glacier motion; of which there are
+two. The one refers it mainly to regelation; the other to a real
+viscosity of the ice.
+
+The late J. C. M'Connel established the fact that ice possesses
+viscosity; that is, it will slowly yield and change its shape
+under long continued stresses. His observations, indeed, raise a
+difficulty in applying this viscosity to explain glacier motion,
+for he showed that an ice crystal is only viscous in a certain
+structural
+
+284
+
+direction. A complex mixture of crystals such, as we know glacier
+ice to be, ought, we would imagine, to display a nett or
+resultant rigidity. A mass of glacier ice when distorted by
+application of a force must, however, undergo precisely the
+transformations which took place in forming the lens from the
+fragments of ice. In fact, regelation will confer upon it all the
+appearance of viscosity.
+
+Let us picture to ourselves a glacier pressing its enormous mass
+down a Swiss valley. At any point suppose it to be hindered in
+its downward path by a rocky obstacle. At that point the ice
+turns to water just as it does beneath the skate. The cold water
+escapes and solidifies elsewhere. But note this, only where there
+is freedom from pressure. In escaping, it carries away its latent
+heat of liquefaction, and this we must assume, is lost to the
+region of ice lately under pressure. This region will, however,
+again warm up by conduction of heat from the surrounding ice, or
+by the circulation of water from the suxface. Meanwhile, the
+pressure at that point has been relieved. The mechanical
+resistance is transferred elsewhere. At this new point there is
+again melting and relief of pressure. In this manner the glacier
+may be supposed to move down. There is continual flux of
+conducted heat and converted latent heat, hither and thither, to
+and from the points of resistance. The final motion of the whole
+mass is necessarily slow; a few feet in the day or, in winter,
+
+285
+
+even only a few inches. And as we might expect, perfect silence
+attends the downward slipping of the gigantic mass. The motion
+is, I believe, sufficiently explained as a skating motion. The
+skate is, however, fixed, the ice moves. The great Aletsch
+Glacier collects its snows among the highest summits of the
+Oberland. Thence, the consolidated ice makes its way into the
+Rhone Valley, travelling a distance of some 20 miles. The ice now
+melting into the youthful Rhone fell upon the Monch, the Jungfrau
+or the Eiger in the days when Elizabeth ruled in England and
+Shakespeare lived.
+
+The ice-fall is a common sight on the glacier. In great lumps and
+broken pinnacles it topples over some rocky obstacle and falls
+shattered on to the glacier below. But a little further down the
+wound is healed again, and regelation has restored the smooth
+surface of the glacier. All such phenomena are explained on James
+Thomson's exposition of the behaviour of a substance which
+expands on passing from the liquid to the solid state.
+
+We thus have arrived at very far-reaching considerations arising
+out of skating and its science. The tendency for snow to
+accumulate on the highest regions of the Earth depends on
+principles which we cannot stop to consider. We know it collects
+above a certain level even at the Equator. We may consider, then,
+that but for the operation of the laws which James Thomson
+brought to light, and which his illustrious brother,
+
+286
+
+Lord Kelvin, made manifest, the uplands of the Earth could not
+have freed themselves of the burthen of ice. The geological
+history of the Earth must have been profoundly modified. The
+higher levels must have been depressed; the general level of the
+ocean relatively to the land thereby raised, and, it is even
+possible, that such a mean level might have been attained as
+would result in general submergence.
+
+During the last great glacial period, we may say the fate of the
+world hung on the operation of those laws which have concerned us
+throughout this lecture. It is believed the ice was piled up to a
+height of some 6,000 feet over the region of Scandinavia. Under
+the influence of the pressure and fusion at points of resistance,
+the accumulation was stayed, and it flowed southwards the
+accumulation was stayed, and it flowed southwards over Northern
+Europe. The Highlands of Scotland were covered with, perhaps,
+three or four thousand feet of ice. Ireland was covered from
+north to south, and mighty ice-bergs floated from our western and
+southern shores.
+
+The transported or erratic stones, often of great size, which are
+found in many parts of Ireland, are records of these long past
+events: events which happened before Man, as a rational being,
+appeared upon the Earth.
+
+287
+
+A SPECULATION AS TO A PREMATERIAL UNIVERSE [1]
+
+"And therefore...these things likewise had a birth; for things
+which are of mortal body could not for an infinite time back...
+have been able to set at naught the puissant strength of
+immeasurable age."--LUCRETIUS, _De Rerum Natura._
+
+"O fearful meditation! Where, alack! Shall Time's best jewel
+from Time's chest lie hid?" --SHAKESPEARE.
+
+IN the material universe we find presented to our senses a
+physical development continually progressing, extending to all,
+even the most minute, material configurations. Some fundamental
+distinctions existing between this development as apparent in the
+organic and the inorganic systems of the present day are referred
+to elsewhere in this volume.[2] In the present essay, these
+systems as having a common origin and common ending, are merged
+in the same consideration as to the nature of the origin of
+material systems in general. This present essay is occupied by
+the consideration of the necessity of limiting material
+interactions in past time. The speculation originated in the
+difficulties which present themselves when we ascribe to these
+interactions infinite duration in the past. These difficulties
+first claim our consideration.
+
+[1] Proc. Royal Dublin Soc., vol. vii., Part V, 1892.
+
+[2] _The Abundance of Life._
+
+288
+
+Accepting the hypothesis of Kant and Laplace in its widest
+extension, we are referred to a primitive condition of wide
+material diffusion, and necessarily too of material instability.
+The hypothesis is, in fact, based upon this material instability.
+We may pursue the sequence of events assumed in this hypothesis
+into the future, and into the past.
+
+In the future we find finality to progress clearly indicated. The
+hypothesis points to a time when there will be no more
+progressive change but a mere sequence of unfruitful events, such
+as the eternal uniform motion of a mass of matter no longer
+gaining or losing heat in an ether possessed of a uniform
+distribution of energy in all its parts. Or, again, if the ether
+absorb the energy of material motion, this vast and dark
+aggregation eternally poised and at rest within it. The action is
+transferred to the subtle parts of the ether which suffer none of
+the energy to degrade. This is, physically, a thinkable future.
+Our minds suggest no change, and demand none. More than this,
+change is unthinkable according to our present ideas of energy.
+Of progress there is an end.
+
+This finality _â parte post_ is instructive. Abstract
+considerations, based on geometrical or analytical illustrations,
+question the finiteness of some physical developments. Thus our
+sun may require eternal time to attain the temperature of the
+ether around it, the approach to this condition being assumed to
+be asymptotic in
+
+289
+
+character. But consider the legitimate _reductio ad absurdum_ of
+an ember raked from a fire 1000 years ago. Is it not yet cooled
+down to the constant temperature of its surroundings? And we may
+evidently increase the time a million-fold if we please. It
+appears as if we must regard eternity as outliving every
+progressive change, For there is no convergence or enfeeblement
+of time. The ever-flowing present moves no differently for the
+occurrence of the mightiest or the most insignificant events. And
+even if we say that time is only the attendant upon events, yet
+this attendant waits patiently for the end, however long
+deferred.
+
+Does the essentially material hypothesis of Kant and Laplace
+account for an infinite past as thinkably as it accounts for the
+infinite future? As this hypothesis is based upon material
+instability the question resolves itself into this:-- Is the
+assumption of an infinitely prolonged past instability a probable
+or possible account of the past? There are, it appears to me,
+great difficulties involved in accepting the hypothesis of
+infinitely prolonged material instability. I will refer here to
+three principal objections. The first may be called a
+metaphysical objection; the second is partly metaphysical and
+partly physical, the third may be considered a physical
+objection, as it is involved directly in the phenomena presented
+by our universe.
+
+The metaphysical objection must have presented itself to every
+one who has considered the question. It may
+
+290
+
+be put thus:--If present events are merely one stage in an
+infinite progress, why is not the present stage long ago passed
+over? We are evidently at liberty to push back any stage of
+progress to as remote a period as we like by putting back first
+the one before this and next the stage preceding this, and so on,
+for, by hypothesis, there is no beginning to the progress.
+
+Thus, the sum of passing events constituting the present universe
+should long ago have been accomplished and passed away. If we
+consider alternative hypotheses not involving this difficulty, we
+are at once struck by the fact that the future of material
+development is free of the objection. For the eternity of
+unprogressive events involved in the future on Kant's hypothesis,
+is not only thinkable, but any change is, as observed,
+irreconcilable with our ideas of energy. As in the future so in
+the past we look to a cessation to progress. But as we believe
+the activity of the present universe must in some form have
+existed all along, the only refuge in the past is to imagine an
+active but unprogressive eternity, the unprogressive activity at
+some period becoming a progressive activity--that progressive
+activity of which we are spectators. To the unprogressive
+activity there was no beginning; in fact, beginning is as
+unthinkable and uncalled for to the unprogressive activity of the
+past as ending is to the unprogressive activity of the future,
+when all developmental actions shall have ceased. There is no
+beginning or ending to the activity of the universe.
+
+291
+
+There is beginning and ending to present progressive activity.
+Looking through the realm of nature we seek beginning and ending,
+but "passing through nature to eternity" we find neither. Both
+are justified; the questioning of the ancient poet regarding the
+past, and of the modern regarding the future, quoted at the head
+of this essay.
+
+The next objection, which is in part metaphysical, is founded on
+the difficulty of ascribing any ultimate reality or potency to
+forces diminishing through eternal time. Thus, against the
+assumption that our universe is the result of material
+aggregation progressing over eternal time, which involves the
+primitive infinite separation of the particles, we may ask, what
+force can have acted between particles sundered by infinite
+distance? The gravitational force falling off as the square of
+the distance, must vanish at infinity if we mean what we say when
+we ascribe infinite separation to them. Their condition is then
+one of neutral stability, a finite movement of the particles
+neither increasing nor diminishing interaction. They had then
+remained eternally in their separated condition, there being no
+cause to render such condition finite. The difficulty involved
+here appears to me of the same nature as the difficulty of
+ascribing any residual heat to the sun after eternal time has
+elapsed. In both cases we are bound to prolong the time, from our
+very idea of time, till progress is no more, when in the one case
+we can imagine no mutual approximation of the
+
+292
+
+particles, in the other no further cooling of the body. However,
+I will riot dwell further upon this objection, as it does not, I
+believe, present itself with equal force to every mind. A reason
+less open to dispute, as being less subjective, against the
+aggregation of infinitely remote particles as the origin of our
+universe, is contained in the physical objection.
+
+In this objection we consider that the appearance presented by
+our universe negatives the hypothesis of infinitely prolonged
+aggregation. We base this negation upon the appearance of
+simultaneity ~ presented by the heavens, contending that this
+simultaneity is contrary to what we would expect to find in the
+case of particles gathered from infinitely remote distances.
+Whether these particles were endowed with relative motions or not
+is unimportant to the consideration. In what respects do the
+phenomena of our universe present the appearance of simultaneous
+phenomena? We must remember that the suns in space are as fires
+which brighten only for a moment and are then extinguished. It is
+in this sense we must regard the longest burning of the stars.
+Whether just lit or just expiring counts little in eternity. The
+light and heat of the star is being absorbed by the ether of
+space as effectually and rapidly as the ocean swallows the ripple
+from the wings of an expiring insect. Sir William Herschel says
+of the galaxy of the milky way:-- "We do not know the rate of
+progress of this mysterious chronometer, but it is nevertheless
+certain that it cannot
+
+293
+
+last for ever, and its past duration cannot be infinite." We do
+not know, indeed, the rate of progress of the chronometer, but if
+the dial be one divided into eternal durations the consummation
+of any finite physical change represents such a movement of the
+hand as is accomplished in a single vibration of the balance
+wheel.
+
+Hence we must regard the hosts of glittering stars as a
+conflagration that has been simultaneously lighted up in the
+heavens. The enormous (to our ideas) thermal energy of the stars
+resembles the scintillation of iron dust in a jar of oxygen when
+a pinch of the dust is thrown in. Although some particles be
+burnt up before others become alight, and some linger yet a
+little longer than the others, in our day's work the
+scintillation of the iron dust is the work of a single instant,
+and so in the long night of eternity the scintillation of the
+mightiest suns of space is over in a moment. A little longer,
+indeed, in duration than the life which stirs a moment in
+response to the diffusion of the energy, but only very little. So
+must an Eternal Being regard the scintillation of the stars and
+the periodic vibration of life in our geological time and the
+most enduring efforts of thought. The latter indeed are no more
+lasting than
+
+"... the labour of ants In the light of a million million of
+suns."
+
+But the myriad suns themselves, with their generations, are the
+momentary gleam of lights for ever after extinguished.
+
+294
+
+Again, science suggests that the present process of material
+aggregation is not finished, and possibly will only be when it
+prevails universally. Hence the very distribution of the stars,
+as we observe them, as isolated aggregations, indicates a
+development which in the infinite duration must be regarded as
+equally advanced in all parts of stellar space and essentially a
+simultaneous phenomenon. For were we spectators of a system in
+which any very great difference of age prevailed, this very great
+difference would be attended by some such appearance as the
+following:--
+
+The aupearance of but one star, other generations being long
+extinct or no others yet come into being; or, perhaps, a faint
+nebulous wreath of aggregating matter somewhere solitary in the
+heavens; or no sign of matter beyond our system, either because
+ungathered or long passed away into darkness.[1]
+
+Some such appearances were to be expected had the aggregation of
+matter depended solely on chance encounters of particles
+scattered through infinite space.
+
+For as, by hypothesis, the aggregation occupies an infinite time
+in consummation it is nearly a certainty that each particle
+encountered after immeasurable time, and then for the first time
+endowed with actual gravitational potential energy, would have
+long expended this energy
+
+[1] It is interesting to reflect upon the effect which an entire
+absence of luminaries outside our solar system would have had
+upon the views of our philosophers and upon our outlook on life.
+
+295
+
+before another particle was gathered. But the fact that so many
+fires which we know to be of brief duration are scattered through
+a region of space, and the fact of a configuration which we
+believe to be a transitory ore, suggest their simultaneous
+aggregation here and there. And in the nebulous wreaths situated
+amidst the stars there is evidence that these actually originated
+where they now are, for in such no relative motion, I believe,
+has as yet been detected by the spectroscope. All this, too, is
+in keeping with the nebular hypothesis of Kant and Laplace so
+long as this does not assume a primitive infinite dispersion of
+matter, but the gathering of matter from finite distances first
+into nebulous patches which aggregating with each other have
+given rise to our system of stars. But if we extend this
+hypothesis throughout an infinite past by the supposition of
+aggregation of infinitely remote particles we replace the
+simultaneous approach required in order to accotnt for the
+simultaneous phenomena visible in the heavens, by a succession of
+aggregative events, by hypothesis at intervals of nearly infinite
+duration, when the events of the universe had consisted of fitful
+gleams lighted after eternities of time and extinguished for yet
+other eternities.
+
+Finally, if we seek to replace the eternal instability involved
+in Kant's hypothesis when extended over an infinite past, by any
+hypothesis of material stability, we at once find ourselves in
+the difficulty that from the known properties of matter such
+stability must have been
+
+296
+
+permanent if ever existent, which is contrary to fact. Thus the
+kinetic inertia expressed in Newton's first law of motion might
+well be supposed to secure equilibrium with material attraction,
+but if primevally diffused matter had ever thus been held in
+equilibrium it must have remained so, or it was maintained so
+imperfectly, which brings us back to endless evolution.
+
+On these grounds I contend that the present gravitational
+properties of matter cannot be supposed to have acted for all
+past duration. Universal equilibrium of gravitating particles
+would have been indestructible by internal causes. Perpetual
+instability or evolution is alike unthinkable and contrary to the
+phenomena of the universe of which we are cognisant. We therefore
+turn from gravitating matter as affording no rational account of
+the past. We do so of necessity, however much we feel our
+ignorance of the nature of the unknown actions to which we have
+recourse.
+
+A prematerial condition of the universe was, we assume, a
+condition in which uniformity as regards the average distribution
+of energy in space prevailed, but neterogeneity and instability
+were possible. The realization of that possibility was the
+beginning we seek, and we today are witnesses of the train of
+events involved in the breakdown of an eternal past equilibrium.
+We are witnesses on this hypothesis, of a catastrophe possibly
+confined to certain regions of space, but which is, to the
+motions and configurations concerned, absolutely unique,
+reversible to
+
+297
+
+its former condition of potential by no process of which we can
+have any conception.
+
+Our speculation is that we, as spectators of evolution, are
+witnessing the interaction of forces which have not always been
+acting. A prematerial state of the universe was one of unfruitful
+motions, that is, motions unattended by progressing changes, in
+our region of the ether. How extended we cannot say; the nature
+of the motions we know not; but the kinetic entities differed
+from matter in the one important particular of not possessing
+gravitational attraction. Such kinetic configurations we cannot
+consider to be matter. It was _possible_ to construct matter by
+their summation or linkage as the configuration of the crystal is
+possible in the clear supersaturated liquid.
+
+Duration in an ether filled with such motions would pass in a
+succession of mere unfruitful events; as duration, we may
+imagine, even now passes in parts of the ether similar to our
+own. An endless (it may be) succession of unprogressive,
+fruitless events. But at one moment in the infinite duration the
+requisite configuration of the elementary motions is attained;
+solely by the one chance disposition the stability of all must
+go, spreading from the fateful point.
+
+Possibly the material segregation was confined to one part of
+space, the elementary motions condensing upon transformation, and
+so impoverishing the ether around till the action ceased. Again
+in the same sense as the
+
+298
+
+stars are simultaneous, so also they may be regarded as uniform
+in size, for the difference in magnitude might have been anything
+we please to imagine, if at the same time we ascribe sufficient
+distance sundering great and small. So, too;, will a dilute
+solution of acetate of soda build a crystal at one point, and the
+impoverishment of the medium checking the growth in this region,
+another centre will begin at the furthest extremities of the
+first crystal till the liquid is filled with loose feathery
+aggregations comparable in size with one another. In a similar
+way the crystallizing out of matter may have given rise, not to a
+uniform nebula in space, but to detached nebula, approximately of
+equal mass, from which ultimately were formed the stars.
+
+That an all-knowing Being might have foretold the ultimate event
+at any preceding period by observing the motions of the parts
+then occurring, and reasoning as to the train of consequences
+arising from these nations, is supposable. But considerations
+arising from this involve no difficulty in ascribing to this
+prematerial train of events infinite duration. For progress there
+is none, and we can quite as easily conceive of some part of
+space where the same Infinite Intelligence, contemplating a
+similar train of unfruitful motions, finds that at no time in the
+future will the equilibrium be disturbed. But where evolution is
+progressing this is no longer conceivable, as being contradictory
+to the very idea of progressive development. In this case
+Infinite Intelligence
+
+299
+
+_necessarily_ finds, as the result of his contemplation, the
+aggregation of matter, and the consequences arising therefrom.
+
+The negation of so primary a material property as gravitation to
+these primitive motions of (or in) the ether, probably involves
+the negation of many properties we find associated with matter.
+Possibly the quality of inertia, equally primary, is involved
+with that of gravitation, and we may suppose that these two
+properties so intimately associated in determining the motions of
+bodies in space were conferred upon the primitive motions as
+crystallographic attraction and rigidity are first conferred upon
+the solid growing from the supersaturated liquid. But in some
+degree less speculative is the supposition that the new order of
+motions involved the transformation of much energy into the form
+of heat vibrations; so that the newly generated matter, like the
+newly formed crystal, began its existence in a medium richly fed
+with thermal radiant energy. We may consider that the thermal
+conditions were such as would account for a primitive
+dissociation of the elements. And, again, we recall how the
+physicist finds his estimate of the energy involved in mere
+gravitational aggregation inadequate to afford explanation of
+past solar heat. It is supposable, on such a hypothesis as we
+have been dwelling on, that the entire subsequent gravitational
+condensation and conversion of material potential energy, dating
+from the first formation of matter to the stage of star
+formation
+
+300
+
+may be insignificant in amount compared with the conversion of
+etherial energy attending the crystallizing out of matter from
+the primitive motions. And thus possibly the conditions then
+obtaining involved a progressively increasing complexity of
+material structure the genesis of the elements, from an
+infra-hydrogen possessing the simplest material configuration,
+resulting ultimately in such self-luminous nebula as we yet see
+in the heavens.
+
+The late James Croll, in his _Stellar Evolution_, finds objections
+to an eternal evolution, one of which is similar to the
+"metaphysical" objection urged in this paper. His way out of the
+difficulty is in the speculation that our stellar system
+originated by the collision of two masses endowed with relative
+motion, eternal in past duration, their meeting ushering in the
+dawn of evolution. However, the state of aggregation here
+assumed, from the known laws of matter and from analogy, calls
+for explanation as probably the result of prior diffusion, when,
+of course, the difficulty is only put back, not set at rest. Nor
+do I think the primitive collision in harmony with the number of
+relatively stationary nebula visible in space.
+
+The metaphysical objection is, I find, also urged by George
+Salmon, late Provost of Trinity College, in favour of the
+creation of the universe.--(_Sermons on Agnosticism_.)
+
+A. Winchell, in _World Life_, says: "We have not
+
+301
+
+the slightest scientific grounds for assuming that matter existed
+in a certain condition from all eternity. The essential activity
+of the powers ascribed to it forbids the thought; for all that we
+know, and, indeed, as the _conclusion_ from all that we know,
+primal matter began its progressive changes on the morning of its
+existence."
+
+Finally, in reference to the hypothesis of a unique determination
+of matter after eternal duration in the past, it may not be out
+of place to remind the reader of the complexity which modern
+research ascribes to the structure of the atom.
+
+302
+
+INDEX
+
+A.
+
+Abney, Sir Wm., on sensitisers, 210.
+
+Abundance of life, numerical, 98-100.
+
+Adaptation and aggressiveness of the organism, 80.
+
+Additive law, the, with reference to alpha rays, 220.
+
+Age of Earth, comparison of denudative and radioactive methods of
+finding, 23-29.
+
+Aletsch glacier, 286.
+
+Allen, Grant, on colour of Alpine plants, 104.
+
+Allen, H. Stanley, on photo-electricity, 203.
+
+Alpha rays, nature of, 214; velocity of, 214; effects of, on
+gases, 214; range of, in air, 215; visualised, 218; ionisation
+curve of, 216; number of, from one gram of radium, 237; number of
+ions made by, 237.
+
+Alpine flowers, intensity of colour of, 102.
+
+Alps, history of, 141; Tertiary denudation of, 148; depth of
+sedimentary covering of, 148; evidence of high pressures and
+temperatures in, 149; recent theories of formation of, 150 _et
+seq._; upheaval of, 147; age of, 147; volcanic phenomena
+attending elevation of, 147.
+
+Andes, trough parallel to, 123; not volcanic in origin, 118.
+
+Angle of friction on ice, 261-265, 281-283; on glass, 261-265.
+
+Animate systems, dynamic conditions of, 67; and transfer of
+energy, 71; and old age, 72; mechanical imitation of, 76, 77.
+
+Animate and inanimate systems compared, 73-75.
+
+Appalachian range, formation of, 120.
+
+Arrhenius, on elevation of continents, 17.
+
+Aryan Era of India, 136.
+
+Asteroids, probable origin of, 175; discovery of, 175; dimensions
+of, 176; orbits of, 176; Mars' moons derived from, 177.
+
+B.
+
+Babbage and Herschel, theory of mountain building, 123.
+
+Babes (and Cornil), size of spores, 98.
+
+Becker, G. F., age of Earth by sodium collection, 14; age of
+minerals by lead ratio, 20.
+
+Berthelot, law of maximum work, 62.
+
+Bertrand, Marcel, section of Mont Blanc Massif, 154.
+
+Beta rays, nature of, 246; accompanied by gamma rays, 247;
+production of, by gamma rays, 247; as ionising agents, 249.
+
+Biotite, containing haloes, 223; pleochroism of, 235; intensified
+pleochroism in halo, 235.
+
+Body and mind, as manifestations of progressiveness of the
+organism, 86.
+
+Boltwood, age of minerals by lead ratio, 20.
+
+Bose, theory of latent image, 203.
+
+Bragg and Kleeman, on path of the alpha ray, 215; stopping power,
+219; laws affecting ionisation by alpha rays, 220; curve of
+ionisation and structure of the halo, 232.
+
+Brecciendecke, sheet of the, 154.
+
+Brdche, sheet of the, 154.
+
+Burrard and Hayden on the Himalaya, 138; sections of the
+Himalaya, 139.
+
+C.
+
+Canals and "canali," 166; curvature of, and path of a satellite,
+188 _et seq._; double and triple accounted for, 186, 187;
+doubling of, 195; disappearance and reappearance of, 196-198;
+photography of, 198; not due to cracks, 167; not due to rivers,
+167; of Mars, double nature of, 166, 170; crossing dark regions
+of planet's surface, 168; of Mars, Lowell's views on, 168 _et
+seq._; shown on Lowell's map, investigation of, 192 _et seq._;
+radiating, explanation of, 193, 194; number of, 194; developed by
+secondary disturbances, 194; nodal development of, due to raised
+surface features, 195.
+
+Chamberlin and Salisbury, the Laramide range, 121.
+
+Clarke, F. W., estimate of mass of sediments, 9; age of Earth by
+sodium collection, 14; average composition of sedimentary and
+igneous rocks, 42; on average composition of the crust, 126;
+solvent denudation of the continents, 17, 40.
+
+Claus, protoplasm the test of the cell, 67; abortion of useless
+organs, 69.
+
+Coefficient of friction, definition of, 262; deduction of, from
+angle of friction, 263; abnormal values on ice, 261-265, 282; for
+various substances, 265.
+
+Continental areas, movements of, 144.
+
+Cornil and Babes, size of spores, 98.
+
+Croll, James, dawn of evolution, 301.
+
+Crust of the Earth, average composition of, 126; depth of
+softening in, 128.
+
+Curie, definition of the, 256.
+
+D.
+
+Dana, on mountain building, 120.
+
+Dawson, reduction of surface represented by Laramide range, 123.
+
+Deccan traps, 137
+
+_déferlement_, theory of, 155; explanation of, 155 _et seq._;
+temperature involved in, 156.
+
+Deimos, dimensions of, 177; orbit of, 577.
+
+De Lapparent, exotic nature of the Préalpes, 150.
+
+De Montessus and the association of earthquakes with
+geosynclines, 142.
+
+Denudation as affected by continental elevation, 17; factors
+promoting, 30 _et seg._; relative activity in mountains and on
+plains, 35-40; solvent, by the sea, 40; the sodium index of,
+46-50; thickness of rock-layer removed from the land, 51.
+
+De Quincy, System of the Heavens, 200.
+
+Dewar, Sir James, latent image formed at low temperatures, 202.
+
+Dixon, H. H., and AGnadance of Life, 60.
+
+Double canals, formation by attraction of a satellite, 585-187.
+
+Douglass, A. E., observations on Mars, 167.
+
+Dravidian Era of India, 135.
+
+E.
+
+Earth, early history of, 3, 4; dimensions of, relative to surface
+features, 117.
+
+Earth's age determined by thickness of sediments, 5; determined
+by mass of the sediments, 7; determined by sodium in the ocean,
+12; determined by radioactive transformations, 19; significance
+of, 2.
+
+Earthquakes associated with geosynclincs, 142.
+
+Efficiency, tendency to maximum, in organisms, 113, 114.
+
+Elements, probable wide diffusion of rare, 230; rarity of
+radioactive, 241.
+
+Elster and Geitel, photo-electric activity and absorption, 207;
+photo-electric properties of gelatin, 212; Emanation of radium,
+therapeutic use of, 256-259; advantages of, in medicine, 256;
+volume of, 257; how obtained, 257; use of, in needles, 258.
+
+Equilibrium amount, meaning of, 254, 255.
+
+Evolution and acceleration of activity, 79; of the universe not
+eternal a pane ante, 298.
+
+F.
+
+Faraday and ionisation, 57.
+
+Finality of progress a part, post, 289.
+
+Flahault, experiments on colour of flowers, 108.
+
+Fletcher, A. L., proportionality of thorium and uranium, 26,
+
+G.
+
+Galileo, discovery of Jupiter's moons, 162.
+
+Gamma rays, nature of, 247: production of, by beta rays, 247; as
+ionising agents, 249.
+
+Geddes and Thomson, hunger and living matter, 71.
+
+Geiger, range of alpha rays in air, 215; ionisation affected by
+alpha rays in air, 216; on "scattering," 217; scattering and the
+structure of the halo, 232.
+
+Geikie, Sir A., uniformity in geological history, 15.
+
+Geosynclines, 119; association with earthquakes and volcanoes,
+142; of the tethys, 142; radioactive heat in, due to sediments,
+130; temperature effects due to lateral compression of, 131.
+
+Glacial epoch, phenomena of, 287.
+
+Glacier motion, cause of. 285.
+
+Glossopteris and Gangamopteris flora, 136.
+
+Gondwanaland, 136.
+
+Gradient of temperature in Earth's surface crust, 126.
+
+H.
+
+Haimanta period of India, 135.
+
+Halley, Edmund, finding age by saltness of ocean, 13.
+
+Hallwachs, photo-electric activity and absorption, 207.
+
+Haloes, pleochroic, finding age of rocks by, 21; due to uranium
+and thorium families, 227; radii of, 227; over-exposed and
+underexposed, 228; intimate structure of, 229 _et seq._;
+artificial, 229; tubular, in mica, 230; extreme age of, 231;
+effect of nucleus on structure of, 232; inference from spherical
+form of, in crystals, 233; structure of, unaffected by cleavage,
+235; origin of the name "pleochroic,"235; colouration due to
+iron, 235; colouration not due to helium, 236; age Of, 236; slow
+formation of, 237, 238; number of rays required to build, 237;
+and age of the Earth, 238-241.
+
+Hayden, H.H., geology of the Himalaya, 134, 138, 139.
+
+Heat-tendency of the universe, 62.
+
+Heat emission from the Earth's surface, 126; from average igneous
+rock due to radioactivity, 126.
+
+Helium and the alpha ray, 214, 222; colouration of halo not due
+to, 236.
+
+Hering, E., and physiological or unconscious memory, 111.
+
+Herschel and Babbage theory of mountain building, 123.
+
+Herschel, Sir W., on galaxy of milky way, 293.
+
+Hertz, negative electrification discharged by light, 204.
+
+Himalaya, geological history of, 134-139.
+
+Hobbs, on association of earthquakes and geosynclines, 143.
+
+Holmes, A., original lead in minerals, 20; age of Devonian, 21.
+
+Horst concerned in Alpine _déferlement_, objections to, 156.
+
+Hyperion, dimensions of, 177.
+
+I.
+
+Ice, melting of, by pressure, 267 _et seq._; expansion of water
+in becoming, 267; lowering of melting-point by pressure, 267;
+fall of temperature under pressure, 268 _et seq._; viscosity of,
+284.
+
+Igneous rocks, average composition of, 43.
+
+Inanimate actions, dynamic conditions of, 61.
+
+Inanimate systems, secondary effects in, 63-65; transfer of
+energy into, 66.
+
+Indian geology, equivalent nomenclature of, 139.
+
+Initial recombination of ions due to alpha rays, 221, 222, 231;
+and structure of the halo, 231.
+
+Insect life in the higher Alps, 104, 105; destruction of, on the
+Alpine snows, 106.
+
+Ionisation by alpha ray, density of, 221; importance in chemical
+actions, 250; in living cell, 250.
+
+Ions, number of, produced by an alpha ray, 237.
+
+Isostasy, 53; and preservation of continents, 53.
+
+Ivy, inconspicuous blossoms of, 107; delay in ripening seed,
+107.
+
+K.
+
+Kant and Laplace, material hypothesis of, does not account for
+the past, 290.
+
+Kelvin, Lord, experiment on effects of pressure on ice, 268-270.
+
+Kleeman and Bragg. See Bragg.
+
+Klopstock introduces skating into Germany, 273.
+
+L.
+
+Lakes, cause of blue colour of, 55.
+
+Land, movements of the, 53, 54.
+
+Laukester, Ray, the soma and reproductive cells, 85.
+
+Lapworth, structure of the Scottish Highlauds, 153.
+
+Latent heat of water, 266.
+
+Latent image, formed at low temperatures, 202; Bose's theory of,
+203; photo-electric theory of, 204, 209 _et seq._
+
+Least action, law of, 66.
+
+Lembert and Richards, atomic weight of lead, 27.
+
+Length of life dependent on conditions of structural development,
+93; dependent on rate of reproduction, 94.
+
+Life-curves of organisms having different activities, 92.
+
+Life, length of, 91.
+
+Life waves of a cerial, 95; of Ausaeba, 87; of a species, 90.
+
+Light, effects of, in discharging negative electrification, 204;
+chemical effects of, 205; experiment showing effect of, in
+discharging electrified body, 205.
+
+Lindemann, Dr., duration of solar heat, 29.
+
+Lowell, Percival, observations on Mars, 167 _et seq._; map of
+Mars, reliability of, 198.
+
+Lucretius, birth-time of the world, 1.
+
+Lugeon, formation of the Préalpes, 171; sections in the Alps,
+154.
+
+Lyell, uniformity in geological history, 15.
+
+M.
+
+Magee, relative areas of deposition and denudation, 16.
+
+Mars, climate of, 170; position in solar system, 174, 175;
+dimensions of satellites of, 177; snow on, 169; water on, 169;
+clouds on, 169; atmosphere of, 170; melting of snow on, 170;
+dimensions of canals, 171; signal on, 172; times of opposition,
+164; orbit of, 165; distance from the Earth, 165; eccentricity of
+his orbit, 165; observations of, by Schiaparelli, 165, 166;
+Lowell's observations on, 167 _et seq._
+
+Maxwell, Clerk, changes made under constraints, 65; on
+conservation of energy, 61.
+
+M'Connel, J. C., viscosity and rigidity of ice, 284.
+
+Memory, physiological, 111, 112.
+
+Metamorphism, thermal, in Alpine rocks, 132, 149
+
+Millicurie, definition of, 256.
+
+Molasse, accumulations of, 148.
+
+Morin, coefficients of friction, 265.
+
+Morphy, H., experiments on coefficient of friction of ice, 281.
+
+Mountain-building and the geosynclines, 119-121; conditioned by
+radioactive energy, 125; energy for, due to gravitation, 122;
+reduction of surface attending, 123; depression attending, 123;
+instability due to thermal effects of compression, 132; igneous
+phenomena attending, 132; rhythmic character of, accounted for,
+133; movements confined to upper crust, 122; movements due to
+compressive stresses in crust, 122; movements, rhythmic character
+of, 121.
+
+Mountain ranges built of sedimentary materials, 118.
+
+Müller, J., coefficient of friction of skate on ice, 265, 274.
+
+Muth deposits of India, 135.
+
+N.
+
+Newton, Professor, of Yale, on origin of Mars' satellites, 177.
+
+Nucleus, dimensions of, 237; amount of radium in, 238.
+
+Nummulitic beds of Himalaya, 138.
+
+O.
+
+Ocean, amount of rock salt in, 50; cause of black colour of, 55;
+estimated mass of sediments in, 48; increase of bulk due to
+solvent denudation, 52; its saltness due to denudation, 41.
+
+Old age and death, 82-85; not at variance with progressive
+activity, 83.
+
+Organic systems, origin of, 78.
+
+Organic vibrations, 86 _et seq._
+
+Organism and accelerative absorption of energy, 79; and economy,
+109-111; and periodic rigour of the environment, 94,95.
+
+Organism and sleep, 95; ultimate explanation of rythmic events
+in, 96, 97; law of action of, 68 _et seq._; periodicity of; and
+law of progressive activity, 82 _et seq._
+
+P.
+
+Penjal traps, 135.
+
+Pepys and skating, 273.
+
+Perry, coefficient of friction of greased surfaces, 265.
+
+Phobos, dimensions of, 177; orbit of, 177.
+
+Photoelectric activity and absorption, 207; persists at low
+temperatures, 208, 209; not affected by solution, 213.
+
+Photo-electric experiment, 205; sensitiveness of the hands, 207;
+theory of latent image, 204, 209 _et seq._
+
+Photographic reversal, experiments on, by Wood, 211; theory of,
+210.
+
+Piazzi, discovery of first Asteroid, 175.
+
+Pickering, W. H., observations on Mars, 167.
+
+Planet, slowing of axial rotation of, 189.
+
+Plant, expectant attitude of, 109.
+
+Pleochroic haloes, measurements of, 224; theory of, 224 _et
+seq._; true form of, 226; radius of, and the additive law, 225;
+absence of actinium haloes, 225; see _also_ Haloes; mode of
+occurrence of, 223 _et seq._
+
+Poole, J. H. J., proportionality of thorium and uranium, 26.
+
+Poulton, uniformity of past climate, 17.
+
+Pratt, Archdeacon, and isostasy, 53.
+
+Préalpes, exotic nature of, 150, 151.
+
+Prematerial universe, nature of a, 297, 300.
+
+Prestwich and thickness of rigid crust, 128; history of the
+Pyrenees, 140.
+
+Primitive organisms, interference of, 89; life-curves of, 88.
+
+Proctor and orbits of Asteroids, 176.
+
+Protoplasm, encystment of, 68.
+
+Purana Era of India, 134.
+
+Pyrenees, history of, 140.
+
+R.
+
+Radioactive elements concerned in mountain building, 125.
+
+Radioactive layer, failure to account for deep-seated
+temperatures, 127; assumed thickness of, 128; temperature at base
+of, due to radioactivity, 129; in the upper crust of the Earth,
+125; thickness of, 126-128.
+
+Radioactive treatment, physical basis of, 251.
+
+Radioactivity and heat emission from average igneous rock, 126;
+rarity of, established by haloes, 241, 243.
+
+Radium, chemical nature and transmutation of, 244-245; emanation
+of, 245; rays from, 253, 254; table of family of, 253; period of,
+253; small therapeutic value of, 254.
+
+Radium C, therapeutic value of, 254; rays from. 254; generation
+of, 254.
+
+Rationality, conditions for development of, 163.
+
+Rays, similarity in nature of gamma, X, and light rays, 248;
+effects on living cell, 251; penetration of, 251.
+
+Reade, T. Mellard, finding age of ocean by calcium sulphate, 13.
+
+Recumbent folds, formation of, 155 _et seq._
+
+Regelation, 284; affecting glacier motion, 285.
+
+Reversal, photographic, explanation of, 211.
+
+Richards and Lembert, atomic weight of lead, 27.
+
+Richter, Jean Paul, Dream of the Universe, 200.
+
+Rock salt in the ocean, amount of, 13.
+
+Rocks, average composition of, 43; radioactive heat from, 126;
+rate of solution of, 36.
+
+Russell, I. C., river supply of sediments, 10.
+
+Rutherford, Sir E., determination of age of minerals, 19, 20; age
+of rocks by haloes, 22; derivation of actinium, 226; artificial
+halo, 229; number of alpha rays from one gram of radium, 237.
+
+S.
+
+Salt range deposits of India, 134. 135.
+
+Saltness of the ocean due to denudation, 41-46.
+
+Salisbury (and Chamberlin), the Larimide range, 121.
+
+Salmon, Rev. George, on creation, 301.
+
+Satellite, velocity of, in its orbit, 191; method of finding path
+of, over a rotating primary, 189 _et seq._; direct and
+retrograde, 178; ultimate end of, 178; path of, when falling into
+primary, 179; effect of Mars' atmosphere on infalling satellite,
+179; stability of close to primary, 180; effects of, on crust of
+primary, 180 _et seq._
+
+Schiaparelli, observations on Mars, 165 166.
+
+Schmidt, C., original depth of Alpine layer, 131-148; structure
+of the Alps, 152.
+
+Schmidt, G. C., on photo-electricity, 207, 208; effect of
+solution on photo-electric activity, 213.
+
+Schuchert, C., average area of N. America during geological time,
+16.
+
+Sedimentary rocks, average composition of, 43; mass of,
+determined by sodium index, 47.
+
+Sedimentation a convection of energy, 133.
+
+Sediments, average river supply of, 11; on ocean floor, mass of,
+48; average thickness of, 49; precipitation of, by dissolved
+salts, 56-58; radioactivity of 130; radioactive heat of,
+influential in mountain building, 130, 131; rate of collecting,
+7; determination of mass of, 8; river supply of, 10; total
+thickness of, 6.
+
+Semper, energy absorption of vegetable and animal systems, 78.
+
+Sensitisers, effects of low temperature on, 210.
+
+Simplon, radioactive temperature in rocks of, before denudation,
+132.
+
+Skates, early forms of, 273; principles of construction of, 273
+_et seq._; action of, on ice, 276; bite of, 278-280.
+
+Skating not dependent on smoothness of ice, 260; history of,
+273.
+
+Skating only possible on very few substances, 279.
+
+Soddy, F., on isotopes, 24.
+
+Sodium, deficiency of, in sediments, 44; discharge of rivers,
+14.
+
+Soils, formation of, 37-39; surface area exposed in, 39.
+
+Sollas, W. J., age of Earth by sodium in ocean, 14; thickness of
+sediments, 6.
+
+Spencer, on division of protoplasm, 67.
+
+Spores, number of molecules in, 97.
+
+Stevenson, Dr. Walter C., and technique of radioactive treatment,
+259.
+
+Stoletow, photo-electric activity anal absorption, 207.
+
+Stopping power of substances with reference to alpha rays, 219.
+
+Struggle for existence, dynamic basis of, 80.
+
+Strutt, Prof. the Hon. R. J., age of geological periods, 20;
+radioactivity of zircon, 223.
+
+Sub-Apennine series of Italy, 148.
+
+Suess, nature of earthquakes. 143.
+
+Survival of the fittest and the organic law, 80.
+
+T.
+
+Talchir boulder-bed, 136.
+
+Temperature gradient in Earth's crust, 126.
+
+Termier, section of the Pelvoux Massif, 254.
+
+Tethys, early extent of, 135-137; geosynclines of, 142.
+
+Thermal metamorphism in Alpine rocks, 132, 149.
+
+Thomson, James, prediction of melting of ice by pressure, 267.
+
+Thorium and uranium, proportionality of, in older rocks, 26.
+
+Triple canals, formation of, by attraction of a satellite, 187.
+
+Tyndall, colour of ocean water, 55.
+
+U.
+
+Uniformitarian view of geological history, 15-18.
+
+Universe, simultaneity of the, 293-295.
+
+Uranium-radium family of elements, table of, 253.
+
+V.
+
+Val d'Hérens, earth pillars of, 33.
+
+Van Tillo, nature of continental rock covering, 9.
+
+Vegetable and animal systems, relative absorption of energy of,
+78.
+
+Vegetative organs, struggle between, 105, 106.
+
+Volcanoes and mountain ranges, 118; associated with geosynclines,
+142; Oligocene and Miocene of Europe, 147.
+
+W.
+
+Weinschenk and thermal metamorphism, 132,
+
+149.
+
+Weismaun, encystment of protoplasm, 68; length of life and
+somatic cells, 96; origin of death, 83; tendency to early
+reproductiveness, 98.
+
+Wilson, C. T. R., visualised alpha rays, 218.
+
+Winchell, progressive changes of matter not eternal, 302.
+
+Wood, R. W., on photographic reversal, 211.
+
+Z.
+
+Zircon, radioactivity of, 223; as nucleus of halo, 223.
+
+
+
+
+
+End of the Project Gutenberg EBook of The Birth-Time of the World and Other
+Scientific Essays, by J. (John) Joly
+
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+
+<pre>
+
+The Project Gutenberg EBook of The Birth-Time of the World and Other
+Scientific Essays, by J. (John) Joly
+
+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: The Birth-Time of the World and Other Scientific Essays
+
+Author: J. (John) Joly
+
+Release Date: August 28, 2005 [EBook #16614]
+
+Language: English
+
+Character set encoding: ISO-8859-1
+
+*** START OF THIS PROJECT GUTENBERG EBOOK THE BIRTH-TIME OF THE WORLD ***
+
+
+
+
+Produced by Hugh Rance
+
+
+
+
+
+</pre>
+
+
+<p>THE BIRTH-TIME OF THE WORLD AND OTHER SCIENTIFIC ESSAYS</p>
+<p>by</p>
+<p>J. JOLY, M.A., Sc.D., F.R.S.,<br>
+PROFESSOR OF GEOLOGY AND MINERALOGY IN THE UNIVERSITY OF
+DUBLIN</p>
+<p>E. P. DUTTON AND COMPANY<br>
+681 FIFTH AVENUE NEW YORK</p>
+<p>Produced by Hugh Rance, 2005</p>
+<p>Cover</p>
+<p>Title page</p>
+<p>CONTENTS PAGE</p>
+<p>I. THE BIRTH-TIME OF THE WORLD - - - - - - - - - - - 1</p>
+<p>II. DENUDATION  - - - - - - - - - - - - - - - - - - 30</p>
+<p>III. THE ABUNDANCE OF LIFE  - - - - - - - - - - - - 60</p>
+<p>IV. THE BRIGHT COLOURS OF ALPINE FLOWERS - - - - - 102</p>
+<p>V. MOUNTAIN GENESIS  - - - - - - - - - - - - - - - 116</p>
+<p>VI. ALPINE STRUCTURE - - - - - - - - - - - - - - - 146</p>
+<p>VII. OTHER MINDS THAN OURS - - - - - - - - - - - - 162</p>
+<p>VIII. THE LATENT IMAGE - - - - - - - - - - - - - - 202</p>
+<p>IX. PLEOCHROIC HALOES  - - - - - - - - - - - - - - 214</p>
+<p>X. THE USE OF RADIUM IN MEDICINE - - - - - - - - - 244</p>
+<p>XI. SKATING  - - - - - - - - - - - - - - - - - - - 260</p>
+<p>XII. A SPECULATION AS TO A PRE-MATERIAL UNIVERSE - 288</p>
+<p>LIST OF ILLUSTRATIONS</p>
+<p>PLATE I. LAKE OF LUCERNE, LOOKING WEST FROM BRUNNEN -<br>
+Frontispiece</p>
+<p>PLATE II. "UPLIFTED FROM THE SEAS." CLIFFS OF THE TITLIS,<br>
+SWITZERLAND - to face p. 4</p>
+<p>PLATE III. AN ALPINE TORRENT AT WORK&mdash;VAL D'HERENS,
+SWITZERLAND -<br>
+to face p. 31</p>
+<p>PLATE IV. EARTH PILLARS&mdash;VAL D'HERENS - to face p. 34</p>
+<p>PLATE V. "SCENES OF DESOLATION." THE WEISSHORN SEEN FROM
+BELLA<br>
+TOLA, SWITZERLAND - to face p. 40</p>
+<p>PLATE VI. ALLUVIAL CONE&mdash;NICOLAI THAL, SWITZERLAND.
+MORAINE ON<br>
+ALETSCH GLACIER SWITZERLAND - to face p. 50</p>
+<p>PLATE VII. IN THE REGION OF THE CROCI; DOLOMITES. THE
+ROTHWAND<br>
+SEEN FROM MONTE PIANO - to face p. 60</p>
+<p>PLATE VIII. FIRS ASSAILING THE HEIGHTS OF THE MADERANER
+THAL,<br>
+SWITZERLAND - to face p. 73</p>
+<p>PLATE IX. LIFE NEAR THE SNOW LINE; THE BOG-COTTON IN
+POSSESSION.<br>
+NEAR THE TSCHINGEL PASS, SWITZERLAND - to face p. 80</p>
+<p>PLATE X. THE JOY OF LIFE. THE AMPEZZO THAL; DOLOMITES - to
+face<br>
+p. 93</p>
+<p>PLATE XI. "PINES SOLEMNLY QUIET." D&Uuml;SSISTOCK; MADERANER
+THAL - to<br>
+face p. 100</p>
+<p>PLATE XII. ALPINE FLOWERS IN THE VALLEYS - to face p. 105</p>
+<p>PLATE XIII. ALPINE FLOWERS ON THE HEIGHTS - to face p. 106</p>
+<p>PLATE XIV. MOUNTAIN SOLITUDES; VAL DE ZINAL. FROM LEFT TO
+RIGHT<br>
+ROTHHORN; BESSO; OBERGABELHORN; MATTERHORN; PIC DE ZINAL
+(THROUGH<br>
+CLOUD); DENT BLANCHE - to face p. 116</p>
+<p>ix</p>
+<p>PLATE XV. SECTOR OF THE EARTH RISE OF ISOGEOTHERMS INTO A
+DEPOSIT<br>
+EVOLVING RADIOACTIVE HEAT - to face p. 118</p>
+<p>PLATE XVI. "THE MOUNTAINS COME AND GO." THE DENT BLANCHE
+SEEN<br>
+FROM THE SASSENEIRE - to face p. 133</p>
+<p>PLATE XVII. DIAGRAMMATIC SECTIONS OF THE HIMALAYA - to face
+p.<br>
+140</p>
+<p>PLATE XVIII. RESIDUES OF DENUDATION. THE MATTERHORN SEEN FROM
+THE<br>
+SUMMIT OF THE ZINAL ROTHHORN - to face p. 148</p>
+<p>PLATE XIX. THE FOLDED ROCKS OF THE MATTERHORN, SEEN FROM
+NEAR<br>
+H&Ouml;HBALM. SKETCH MADE IN 1906 - to face p. 156</p>
+<p>PLATE XX. SCHIAPARELLI'S MAP OF MARS OF 1882, AND ADDITIONS
+(IN<br>
+RED) OF 1892 - to face p. 166</p>
+<p>PLATE XXI. GLOBE OF MARS SHOWING PATH OF IN-FALLING SATELLITE
+-<br>
+to face p. 188</p>
+<p>PLATE XXII. CANALS MAPPED BY LOWELL COMPARED WITH CANALS
+FORMED<br>
+BY IN-FALLING SATELLITES - to face p. 192</p>
+<p>PLATE XXIII. HALOES IN MICA; CO. CARLOW. HALO IN BIOTITE<br>
+CONTAINED IN GRANITE - to face p. 224</p>
+<p>PLATE XXIV. RADIUM HALO, MUCH ENLARGED. THORIUM HALO AND
+RADIUM<br>
+HALO IN MICA - to face p. 228</p>
+<p>PLATE XXV. HALO ROUND CAPILLARY IN GLASS TUBE. HALOES
+ROUND<br>
+TUBULAR PASSAGES IN MICA - to face p. 230</p>
+<p>PLATE XXVI. ALETSCH GLACIER, SWITZERLAND - to face p. 282</p>
+<p>PLATE XXVII. THE MIDDLE ALETSCH GLACIER JOINING THE GREAT
+ALETSCH<br>
+GLACIER. GLACIERS OF THE LAUTERBRUNNEN THAL - to face p. 285</p>
+<p>PLATE XXVIII. PERCHED BLOCK ON THE ALETSCH GLACIER.
+GRANITE<br>
+ERRATIC NEAR ROUNDWOOD, CO. WICKLOW; NOW BROKEN UP AND REMOVED
+-<br>
+to face p. 286</p>
+<p>And Fifteen Illustrations in the Text.</p>
+<p>x</p>
+<p>PREFACE</p>
+<p>Tins volume contains twelve essays written at various
+times<br>
+during recent years. Many of them are studies contributed to<br>
+Scientific Reviews or delivered as popular lectures. Some are<br>
+expositions of views the scientific basis of which may be<br>
+regarded as established. Others&mdash;the greater
+number&mdash;may be<br>
+described as attempting the solution of problems which cannot
+be<br>
+approached by direct observation.</p>
+<p>The essay on The Birth-time of the World is based on a
+lecture<br>
+delivered before the Royal Dublin Society. The subject has<br>
+attracted much attention within recent years. The age of the<br>
+Earth is, indeed, of primary importance in our conception of
+the<br>
+longevity of planetary systems. The essay deals with the<br>
+evidence, derived from the investigation of purely
+terrestrial<br>
+phenomena, as to the period which has elapsed since the ocean<br>
+condensed upon the Earth's surface. Dr. Decker's recent
+addition<br>
+to the subject appeared too late for inclusion in it. He
+finds<br>
+that the movements (termed isostatic) which geologists
+recognise<br>
+as taking place deep in the Earth's crust, indicate an age of
+the<br>
+same order of magnitude</p>
+<p>xi</p>
+<p>as that which is inferred from the statistics of
+denudative<br>
+history.[1]</p>
+<p>The subject of _Denudation_ naturally arises from the first
+essay.<br>
+In thinking over the method of finding the age of the ocean
+by<br>
+the accumulation of sodium therein, I perceived so long ago
+as<br>
+1899, when my first paper was published, that this method<br>
+afforded a means of ascertaining the grand total of
+denudative<br>
+work effected on the Earth's surface since the beginning of<br>
+geological time; the resulting knowledge in no way involving
+any<br>
+assumption as to the duration of the period comprising the<br>
+denudative actions. This idea has been elaborated in various<br>
+publications since then, both by myself and by others.<br>
+"Denudation," while including a survey of the subject
+generally,<br>
+is mainly a popular account of this method and its results.
+It<br>
+closes with a reference to the fascinating problems presented
+by<br>
+the inner nature of sedimentation: a branch of science to which
+I<br>
+endeavoured to contribute some years ago.</p>
+<p>_Mountain Genesis_ first brings in the subject of the
+geological<br>
+intervention of radioactivity. There can, I believe, be no
+doubt<br>
+as to the influence of transforming elements upon the<br>
+developments of the surface features of the Earth; and, if I
+am<br>
+right, this source of thermal energy is mainly responsible
+for<br>
+that local accumulation of wrinkling which we term mountain<br>
+chains. The</p>
+<p>[1] Bull. Geol. Soc. America, vol. xxvi, March 1915.</p>
+<p>xii</p>
+<p>paper on _Alpine Structure_ is a reprint from "Radioactivity
+and<br>
+Geology," which for the sake of completeness is here included.
+It<br>
+is directed to the elucidation of a detail of mountain genesis:
+a<br>
+detail which enters into recent theories of Alpine
+development.<br>
+The weakness of the theory of the "horst" is manifest,
+however,<br>
+in many of its other applications; if not, indeed, in all.</p>
+<p>The foregoing essays on the physical influences affecting
+the<br>
+surface features of the Earth are accompanied by one entitled
+_The<br>
+Abundance of Life._ This originated amidst the overwhelming<br>
+presentation of life which confronts us in the Swiss Alps.
+The<br>
+subject is sufficiently inspiring. Can no fundamental reason
+be<br>
+given for the urgency and aggressiveness of life? Vitality is
+an<br>
+ever-extending phenomenon. It is plain that the great
+principles<br>
+which have been enunciated in explanation of the origin of<br>
+species do not really touch the problem. In the essay&mdash;which
+is an<br>
+early one (1890)&mdash;the explanation of the whole great matter
+is<br>
+sought&mdash;and as I believe found&mdash;in the attitude of the
+organism<br>
+towards energy external to it; an attitude which results in
+its<br>
+evasion of the retardative and dissipatory effects which
+prevail<br>
+in lifeless dynamic systems of all kinds.</p>
+<p>_Other Minds than Ours_? attempts a solution of the vexed
+question<br>
+of the origin of the Martian "canals." The essay is an
+abridgment<br>
+of two popular lectures on the subject. I had previously
+written<br>
+an account of my views which carried the enquiry as far as it
+was<br>
+in</p>
+<p>xiii</p>
+<p>my power to go. This paper appeared in the "Transactions of
+the<br>
+Royal Dublin Society, 1897." The theory put forward is a
+purely<br>
+physical one, and, if justified, the view that intelligent
+beings<br>
+exist in Mars derives no support from his visible surface<br>
+features; but is, in fact, confronted with fresh
+difficulties.</p>
+<p>_Pleochroic Haloes_ is a popular exposition of an
+inconspicuous but<br>
+very beautiful phenomenon of the rocks. Minute darkened
+spheres&mdash;a<br>
+microscopic detail&mdash;appear everywhere in certain of the
+rock<br>
+minerals. What are they? The discoveries of recent
+radioactive<br>
+research&mdash;chiefly due to Rutherford&mdash;give the answer.
+The<br>
+measurements applied to the little objects render the
+explanation<br>
+beyond question. They turn out to be a quite extraordinary
+record<br>
+of radioactive energy; a record accumulated since remote<br>
+geological times, and assuring us, indirectly, of the
+stability<br>
+of the chemical elements in general since the beginning of
+the<br>
+world. This assurance is, without proof, often assumed in our<br>
+views on the geological history of the Globe.</p>
+<p>Skating is a discourse, with a recent addition supporting
+the<br>
+original thesis. It is an illustration of a common
+experience&mdash;the<br>
+explanation of an unimportant action involving principles the<br>
+most influential considered as a part of Nature's resources.</p>
+<p>The address on _The Latent Image_ deals with a subject which
+had<br>
+been approached by various writers before the time of my
+essay;<br>
+but, so far as I know, an explanation</p>
+<p>xiv</p>
+<p>based on the facts of photo-electricity had not been
+attempted.<br>
+Students of this subject will notice that the views expressed
+are<br>
+similar to those subsequently put forward by Lenard and
+Saeland<br>
+in explanation of phosphorescence. The whole matter is of
+more<br>
+practical importance than appears at first sight, for the<br>
+photoelectric nature of the effects involved in the radiative<br>
+treatment of many cruel diseases seems to be beyond doubt.</p>
+<p>It was in connection with photo-electric science that I was
+led<br>
+to take an interest in the application of radioactivity in<br>
+medicine. The lecture on _The Use of Radium in Medicine_ deals
+with<br>
+this subject. Towards the conclusion of this essay reference
+will<br>
+be found to a practical outcome of such studies which, by<br>
+improving on the methods, and facilitating the application,
+of<br>
+radioactive treatment, has, in the hands of skilled medical
+men,<br>
+already resulted in the alleviation of suffering.</p>
+<p>Leaving out much which might well appear in a prefatory
+notice, a<br>
+word should yet be added respecting the illustrations of
+scenery.<br>
+They are a small selection from a considerable number of<br>
+photographs taken during my summer wanderings in the Alps in<br>
+company with Henry H. Dixon. An exception is Plate X, which is
+by<br>
+the late Dr. Edward Stapleton. From what has been said above,
+it<br>
+will be gathered that these illustrations are fitly included<br>
+among pages which owe so much to Alpine inspiration. They<br>
+illustrate the</p>
+<p>xv</p>
+<p>subjects dealt with, and, it is to be hoped, they will in
+some<br>
+cases recall to the reader scenes which have in past times<br>
+influenced his thoughts in the same manner; scenes which in
+their<br>
+endless perspective seem to reduce to their proper
+insignificance<br>
+the lesser things of life.</p>
+<p>My thanks are due to Mr. John Murray for kindly consenting to
+the<br>
+reissue of the essay on _The Birth-time of the World_ from
+the<br>
+pages of _Science Progress_; to Messrs. Constable &amp; Co. for
+leave<br>
+to reprint _Pleochroic Haloes_ from _Bedrock_, and also to make
+some<br>
+extracts from _Radioactivity and Geology_; and to the Council
+of<br>
+the Royal Dublin Society for permission to republish certain<br>
+papers from the Proceedings of the Society.</p>
+<p>_Iveagh Geological Laboratory, Trinity College, Dublin._</p>
+<p>July, 1915.</p>
+<p>xvi</p>
+<p><u>THE BIRTH-TIME OF THE WORLD</u> [1]</p>
+<p>LONG ago Lucretius wrote: "For lack of power to solve the<br>
+question troubles the mind with doubts, whether there was ever
+a<br>
+birth-time of the world and whether likewise there is to be
+any<br>
+end." "And if" (he says in answer) "there was no birth-time
+of<br>
+earth and heaven and they have been from everlasting, why
+before<br>
+the Theban war and the destruction of Troy have not other
+poets<br>
+as well sung other themes? Whither have so many deeds of men
+so<br>
+often passed away, why live they nowhere embodied in lasting<br>
+records of fame? The truth methinks is that the sum has but a<br>
+recent date, and the nature of the world is new and has but<br>
+lately had its commencement."[2]</p>
+<p>Thus spake Lucretius nearly 2,000 years ago. Since then we
+have<br>
+attained another standpoint and found very different
+limitations.<br>
+To Lucretius the world commenced with man, and the answer he<br>
+would give to his questions was in accord with his philosophy:
+he<br>
+would date the birth-time of the world from the time when</p>
+<p>[1] A lecture delivered before the Royal Dublin Society,
+February<br>
+6th, 1914. _Science Progress_, vol. ix., p. 37</p>
+<p>[2] _De Rerum Natura_, translated by H. A. J. Munro
+(Cambridge,<br>
+1886).</p>
+<p>1</p>
+<p>poets first sang upon the earth. Modern Science has along
+with<br>
+the theory that the Earth dated its beginning with the advent
+of<br>
+man, swept utterly away this beautiful imagining. We can,
+indeed,<br>
+find no beginning of the world. We trace back events and come
+to<br>
+barriers which close our vista&mdash;barriers which, for all we
+know,<br>
+may for ever close it. They stand like the gates of ivory and
+of<br>
+horn; portals from which only dreams proceed; and Science
+cannot<br>
+as yet say of this or that dream if it proceeds from the gate
+of<br>
+horn or from that of ivory.</p>
+<p>In short, of the Earth's origin we have no certain knowledge;
+nor<br>
+can we assign any date to it. Possibly its formation was an
+event<br>
+so gradual that the beginning was spread over immense periods.
+We<br>
+can only trace the history back to certain events which may
+with<br>
+considerable certainty be regarded as ushering in our
+geological<br>
+era.</p>
+<p>Notwithstanding our limitations, the date of the birth-time
+of<br>
+our geological era is the most important date in Science. For
+in<br>
+taking into our minds the spacious history of the universe,
+the<br>
+world's age must play the part of time-unit upon which all
+our<br>
+conceptions depend. If we date the geological history of the<br>
+Earth by thousands of years, as did our forerunners, we must<br>
+shape our ideas of planetary time accordingly; and the
+duration<br>
+of our solar system, and of the heavens, becomes comparable
+with<br>
+that of the dynasties of ancient nations. If by millions of<br>
+years, the sun and stars are proportionately venerable. If by<br>
+hundreds or thousands of millions of</p>
+<p>2</p>
+<p>years the human mind must consent to correspondingly vast
+epochs<br>
+for the duration of material changes. The geological age
+plays<br>
+the same part in our views of the duration of the universe as
+the<br>
+Earth's orbital radius does in our views of the immensity of<br>
+space. Lucretius knew nothing of our time-unit: his unit was
+the<br>
+life of a man. So also he knew nothing of our space-unit, and
+he<br>
+marvels that so small a body as the sun can shed so much,
+heat<br>
+and light upon the Earth.</p>
+<p>A study of the rocks shows us that the world was not always
+what<br>
+it now is and long has been. We live in an epoch of
+denudation.<br>
+The rains and frosts disintegrate the hills; and the rivers
+roll<br>
+to the sea the finely divided particles into which they have
+been<br>
+resolved; as well as the salts which have been leached from
+them.<br>
+The sediments collect near the coasts of the continents; the<br>
+dissolved matter mingles with the general ocean. The
+geologist<br>
+has measured and mapped these deposits and traced them back
+into<br>
+the past, layer by layer. He finds them ever the same;<br>
+sandstones, slates, limestones, etc. But one thing is not the<br>
+same. _Life_ grows ever less diversified in character as the<br>
+sediments are traced downwards. Mammals and birds, reptiles,<br>
+amphibians, fishes, die out successively in the past; and
+barren<br>
+sediments ultimately succeed, leaving the first beginnings of<br>
+life undecipherable. Beneath these barren sediments lie rocks<br>
+collectively differing in character from those above: mainly<br>
+volcanic or poured out from fissures in</p>
+<p>3</p>
+<p>the early crust of the Earth. Sediments are scarce among
+these<br>
+materials.[1]</p>
+<p>There can be little doubt that in this underlying floor of<br>
+igneous and metamorphic rocks we have reached those surface<br>
+materials of the earth which existed before the long epoch of<br>
+sedimentation began, and before the seas came into being.
+They<br>
+formed the floor of a vaporised ocean upon which the waters<br>
+condensed here and there from the hot and heavy atmosphere.
+Such<br>
+were the probable conditions which preceded the birth-time of
+the<br>
+ocean and of our era of life and its evolution.</p>
+<p>It is from this epoch we date our geological age. Our next<br>
+purpose is to consider how long ago, measured in years, that<br>
+birth-time was.</p>
+<p>That the geological age of the Earth is very great appears
+from<br>
+what we have already reviewed. The sediments of the past are
+many<br>
+miles in collective thickness: yet the feeble silt of the
+rivers<br>
+built them all from base to summit. They have been uplifted
+from<br>
+the seas and piled into mountains by movements so slow that<br>
+during all the time man has been upon the Earth but little
+change<br>
+would have been visible. The mountains have again been worn
+down<br>
+into the ocean by denudation and again younger mountains
+built<br>
+out of their redeposited materials. The contemplation of such<br>
+vast events</p>
+<p>[1] For a description of these early rocks, see especially
+the<br>
+monograph of Van Hise and Leith on the pre-Cambrian Geology
+of<br>
+North America (Bulletin 360, U.S. Geol. Survey).</p>
+<p>4</p>
+<p>prepares our minds to accept many scores of millions of years
+or<br>
+hundreds of millions of years, if such be yielded by our<br>
+calculations.</p>
+<p>THE AGE AS INFERRED FROM THE THICKNESS OF THE SEDIMENTS</p>
+<p>The earliest recognised method of arriving at an estimate of
+the<br>
+Earth's geological age is based upon the measurement of the<br>
+collective sediments of geological periods. The method has<br>
+undergone much revision from time to time. Let us briefly
+review<br>
+it on the latest data.</p>
+<p>The method consists in measuring the depths of all the
+successive<br>
+sedimentary deposits where these are best developed. We go
+all<br>
+over the explored world, recognising the successive deposits
+by<br>
+their fossils and by their stratigraphical relations,
+measuring<br>
+their thickness and selecting as part of the data required
+those<br>
+beds which we believe to most completely represent each<br>
+formation. The total of these measurements would tell us the
+age<br>
+of the Earth if their tale was indeed complete, and if we
+knew<br>
+the average rate at which they have been deposited. We soon,<br>
+however, find difficulties in arriving at the quantities we<br>
+require. Thus it is not easy to measure the real thickness of
+a<br>
+deposit. It may be folded back upon itself, and so we may
+measure<br>
+it twice over. We may exaggerate its thickness by measuring
+it<br>
+not quite straight across the bedding or by unwittingly
+including<br>
+volcanic materials. On the other hand, there</p>
+<p>5</p>
+<p>may be deposits which are inaccessible to us; or, again,
+an<br>
+entire absence of deposits; either because not laid down in
+the<br>
+areas we examine, or, if laid down, again washed into the
+sea.<br>
+These sources of error in part neutralise one another. Some
+make<br>
+our resulting age too long, others make it out too short. But
+we<br>
+do not know if a balance of error does not still remain.
+Here,<br>
+however, is a table of deposits which summarises a great deal
+of<br>
+our knowledge of the thickness of the stratigraphical<br>
+accumulations. It is due to Sollas.[1]</p>
+<p>Feet.<br>
+<br>
+Recent and Pleistocene  - -    4,000<br>
+Pliocene                 - -   13,000<br>
+Miocene                  - -   14,000<br>
+Oligocene                - -     2,000<br>
+Eocene                   - -  <u>20,000</u><br>
+                              63,000<br>
+<br>
+Upper Cretaceous        - -   24,000<br>
+Lower Cretaceous        - -   20,000<br>
+Jurassic                 - -    8,000<br>
+Trias                    - -   <u>7,000</u><br>
+                              69,000<br>
+<br>
+Permian                  - -    2,000<br>
+Carboniferous           - -    29,000<br>
+Devonian                 - -   <u>22,000</u><br>
+                              63,000<br>
+<br>
+Silurian                 - -   15,000<br>
+Ordovician              - -   17,000<br>
+Cambrian                 - -   <u>6,000</u><br>
+                              58,000<br>
+<br>
+Algonkian&mdash;Keeweenawan   - -   50,000<br>
+Algonkian&mdash;Animikian     - -   14,000<br>
+Algonkian&mdash;Huronian      - -   <u>18,000</u><br>
+                              82,000<br>
+<br>
+Arch&aelig;an                   - -   ?<br>
+                             <u>      </u><br>
+Total                     - - 335,000 feet.</p>
+<p>[1] Address to the Geol. Soc. of London, 1509.</p>
+<p>6</p>
+<p>In the next place we require to know the average rate at
+which<br>
+these rocks were laid down. This is really the weakest link
+in<br>
+the chain. The most diverse results have been arrived at,
+which<br>
+space does not permit us to consider. The value required is
+most<br>
+difficult to determine, for it is different for the different<br>
+classes of material, and varies from river to river according
+to<br>
+the conditions of discharge to the sea. We may probably take
+it<br>
+as between two and six inches in a century.</p>
+<p>Now the total depth of the sediments as we see is about
+335,000<br>
+feet (or 64 miles), and if we take the rate of collecting as<br>
+three inches in a hundred years we get the time for all to<br>
+collect as 134 millions of years. If the rate be four inches,
+the<br>
+time is soo millions of years, which is the figure Geikie<br>
+favoured, although his result was based on somewhat different<br>
+data. Sollas most recently finds 80 millions of years.[1]</p>
+<p>THE AGE AS INFERRED FROM THE MASS OF THE SEDIMENTS</p>
+<p>In the above method we obtain our result by the measurement
+of<br>
+the linear dimensions of the sediments. These measurements, as
+we<br>
+have seen, are difficult to arrive at. We may, however,
+proceed<br>
+by measurements of the mass of the sediments, and then the
+method<br>
+becomes more definite. The new method is pursued as follows:</p>
+<p>[1] Geikie, _Text Book of Geology_ (Macmillan, 1903), vol. i.,
+p.<br>
+73, _et seq._ Sollas, _loc. cit._ Joly, _Radioactivity and
+Geology_<br>
+(Constable, 1909), and Phil. Mag., Sept. 1911.</p>
+<p>7</p>
+<p>The total mass of the sediments formed since denudation began
+may<br>
+be ascertained with comparative accuracy by a study of the<br>
+chemical composition of the waters of the ocean. The salts in
+the<br>
+ocean are undoubtedly derived from the rocks; increasing age
+by<br>
+age as the latter are degraded from their original character<br>
+under the action of the weather, etc., and converted to the<br>
+sedimentary form. By comparing the average chemical
+composition<br>
+of these two classes of material&mdash;the primary or igneous
+rocks and<br>
+the sedimentary&mdash;it is easy to arrive at a knowledge of how
+much<br>
+of this or that constituent was given to the ocean by each ton
+of<br>
+primary rock which was denuded to the sedimentary form. This,<br>
+however, will not assist us to our object unless the ocean
+has<br>
+retained the salts shed into it. It has not generally done so.
+In<br>
+the case of every substance but one the ocean continually
+gives<br>
+up again more or less of the salts supplied to it by the
+rivers.<br>
+The one exception is the element sodium. The great solubility
+of<br>
+its salts has protected it from abstraction, and it has gone
+on<br>
+collecting during geological time, practically in its
+entirety.<br>
+This gives us the clue to the denudative history of the<br>
+Earth.[1]</p>
+<p>The process is now simple. We estimate by chemical examination
+of<br>
+igneous and sedimentary rocks the amount of sodium which has
+been<br>
+supplied to the ocean per ton of sediment produced by
+denudation.<br>
+We also calculate</p>
+<p>[1] _Trans. R.D.S._, May, 1899.</p>
+<p>8</p>
+<p>the amount of sodium contained in the ocean. We divide the
+one<br>
+into the other (stated, of course, in the same units of
+mass),<br>
+and the quotient gives us the number of tons of sediment. The<br>
+most recent estimate of the sediments made in this manner
+affords<br>
+56 x 10<sup>16</sup> tonnes.[1]</p>
+<p>Now we are assured that all this sediment was transported by
+the<br>
+rivers to the sea during geological time. Thus it follows
+that,<br>
+if we can estimate the average annual rate of the river supply
+of<br>
+sediments to the ocean over the past, we can calculate the<br>
+required age. The land surface is at present largely covered
+with<br>
+the sedimentary rocks themselves. Sediment derived from these<br>
+rocks must be regarded as, for the most part, purely
+cyclical;<br>
+that is, circulating from the sea to the land and back again.
+It<br>
+does not go to increase the great body of detrital deposits.
+We<br>
+cannot, therefore, take the present river supply of sediment
+as<br>
+representing that obtaining over the long past. If the land
+was<br>
+all covered still with primary rocks we might do so. It has
+been<br>
+estimated that about 25 per cent. of the existing continental<br>
+area is covered with arch&aelig;an and igneous rocks, the
+remainder<br>
+being sediments.[2] On this estimate we may find valuable</p>
+<p>[1] Clarke, _A Preliminary Study of Chemical Denudation_<br>
+(Washington, 1910). My own estimate in 1899 (_loc. cit._) made as
+a<br>
+test of yet another method of finding the age, showed that
+the<br>
+sediments may be taken as sufficient to form a layer 1.1 mile<br>
+deep if spread uniformly over the continents; and would amount
+to<br>
+64 x 10<sup>18</sup> tons.</p>
+<p>[2] Van Tillo, _Comptes Rendues_ (Paris), vol. cxiv.,
+1892.</p>
+<p>9</p>
+<p>major and minor limits to the geological age. If we take 25
+per<br>
+cent. only of the present river supply of sediment, we
+evidently<br>
+fix a major limit to the age, for it is certain that over the<br>
+past there must have been on the average a faster supply. If
+we<br>
+take the entire river supply, on similar reasoning we have
+what<br>
+is undoubtedly a minor limit to the age.</p>
+<p>The river supply of detrital sediment has not been very<br>
+extensively investigated, although the quantities involved may
+be<br>
+found with comparative ease and accuracy. The following table<br>
+embodies the results obtained for some of the leading
+rivers.[1]</p>
+<p>               Mean annual   Total annual   Ratio of<br>
+               discharge in  sediment in    sediment<br>
+               cubic feet    thousands      to water<br>
+               per second.   of tons.       by weight.<br>
+<br>
+Potomac -        20,160          5,557        1 : 3.575<br>
+Mississippi -   610,000       406,250        1 : 1,500<br>
+Rio Grande -      1,700          3,830        1 : 291<br>
+Uruguay -       150,000         14,782       1 : 10,000<br>
+Rhone -          65,850         36,000       1 : 1,775<br>
+Po -             62,200         67,000       1 : 900<br>
+Danube -        315,200        108,000       1 : 2,880<br>
+Nile -          113,000         54,000       1 : 2,050<br>
+Irrawaddy -     475,000       291,430        1 : 1,610<br>
+<br>
+Mean -          201,468        109,650       1 : 2,731</p>
+<p>We see that the ratio of the weight of water to the</p>
+<p>[1] Russell, _River Development_ (John Murray, 1888).</p>
+<p>10</p>
+<p>weight of transported sediment in six out of the nine rivers
+does<br>
+not vary widely. The mean is 2,730 to 1. But this is not the<br>
+required average. The water-discharge of each river has to be<br>
+taken into account. If we ascribe to the ratio given for each<br>
+river the weight proper to the amount of water it discharges,
+the<br>
+proportion of weight of water to weight of sediment, for the<br>
+whole quantity of water involved, comes out as 2,520 to 1.</p>
+<p>Now if this proportion holds for all the rivers of the<br>
+world&mdash;which collectively discharge about 27 x 1012 tonnes
+of<br>
+water per annum&mdash;the river-born detritus is 1.07 x 1010
+tonnes. To<br>
+this an addition of 11 per cent. has to be made for silt
+pushed<br>
+along the river-bed.[1] On these figures the minor limit to
+the<br>
+age comes out as 47 millions of years, and the major limit as
+188<br>
+millions. We are here going on rather deficient estimates,
+the<br>
+rivers involved representing only some 6 per cent. of the
+total<br>
+river supply of water to the ocean. But the result is
+probably<br>
+not very far out.</p>
+<p>We may arrive at a probable age lying between the major and
+minor<br>
+limits. If, first, we take the arithmetic mean of these
+limits,<br>
+we get 117 millions of years. Now this is almost certainly<br>
+excessive, for we here assume that the rate of covering of
+the<br>
+primary rocks by sediments was uniform. It would not be so,<br>
+however, for the rate of supply of original sediment must
+have<br>
+been continually diminishing</p>
+<p>[1] According to observations made on the Mississippi
+(Russell,<br>
+_loc. cit._).</p>
+<p>11</p>
+<p>during geological time, and hence we may assume that the rate
+of<br>
+advance of the sediments on the primary rocks has also been<br>
+diminishing. Now we may probably take, as a fair assumption,
+that<br>
+the sediment-covered area was at any instant increasing at a
+rate<br>
+proportionate to the rate of supply of sediment; that is, to
+the<br>
+area of primary rocks then exposed. On this assumption the age
+is<br>
+found to be 87 millions of years.</p>
+<p>THE AGE BY THE SODIUM OF THE OCEAN</p>
+<p>I have next to lay before you a quite different method. I
+have<br>
+already touched upon the chemistry of the ocean, and on the<br>
+remarkable fact that the sodium contained in it has been<br>
+preserved, practically, in its entirety from the beginning of<br>
+geological time.</p>
+<p>That the sea is one of the most beautiful and magnificent
+sights<br>
+in Nature, all admit. But, I think, to those who know its
+story<br>
+its beauty and magnificence are ten-fold increased. Its
+saltness<br>
+it due to no magic mill. It is the dissolved rocks of the
+Earth<br>
+which give it at once its brine, its strength, and its
+buoyancy.<br>
+The rivers which we say flow with "fresh" water to the sea<br>
+nevertheless contain those traces of salt which, collected
+over<br>
+the long ages, occasion the saltness of the ocean. Each gallon
+of<br>
+river water contributes to the final result; and this has
+been<br>
+going on since the beginning of our era. The mighty total of
+the<br>
+rivers is 6,500 cubic miles of water in the year!</p>
+<p>12</p>
+<p>There is little doubt that the primeval ocean was in the<br>
+condition of a fresh-water lake. It can be shown that a
+primitive<br>
+and more rapid solution of the original crust of the Earth by
+the<br>
+slowly cooling ocean would have given rise to relatively
+small<br>
+salinity. The fact is, the quantity of salts in the ocean is<br>
+enormous. We are only now concerned with the sodium; but if
+we<br>
+could extract all the rock-salt (the chloride of sodium) from
+the<br>
+ocean we should have enough to cover the entire dry land of
+the<br>
+Earth to a depth of 400 feet. It is this gigantic quantity
+which<br>
+is going to enter into our estimate of the Earth's age. The<br>
+calculated mass of sodium contained in this rock-salt is
+14,130<br>
+million million tonnes.</p>
+<p>If now we can determine the rate at which the rivers
+supply<br>
+sodium to the ocean, we can determine the age.[1] As the
+result<br>
+of many thousands of river analyses, the total amount of
+sodium<br>
+annually discharged to the ocean</p>
+<p>[1] _Trans. R.D.S._, 1899. A paper by Edmund Halley, the<br>
+astronomer, in the _Philosophical Transactions of the Royal<br>
+Society_ for 1715, contains a suggestion for finding the age
+of<br>
+the world by the following procedure. He proposes to make<br>
+observations on the saltness of the seas and ocean at
+intervals<br>
+of one or more centuries, and from the increment of saltness<br>
+arrive at their age. The measurements, as a matter of fact,
+are<br>
+impracticable. The salinity would only gain (if all remained
+in<br>
+solution) one millionth part in Too years; and, of course,
+the<br>
+continuous rejection of salts by the ocean would invalidate
+the<br>
+method. The last objection also invalidates the calculation by
+T.<br>
+Mellard Reade (_Proc. Liverpool Geol. Soc._, 1876) of a minor
+limit<br>
+to the age by the calcium sulphate in the ocean. Both papers
+were<br>
+quite unknown to me when working out my method. Halley's
+paper<br>
+was, I think, only brought to light in 1908.</p>
+<p>13</p>
+<p>by all the rivers of the world is found to be probably not
+far<br>
+from 175 million tonnes.[1] Dividing this into the mass of<br>
+oceanic sodium we get the age as 80.7 millions of years.
+Certain<br>
+corrections have to be applied to this figure which result in<br>
+raising it to a little over 90 millions of years. Sollas, as
+the<br>
+result of a careful review of the data, gets the age as
+between<br>
+80 and 150 millions of years. My own result[2] was between 80
+and<br>
+90 millions of years; but I subsequently found that upon
+certain<br>
+extreme assumptions a maximum age might be arrived at of 105<br>
+millions of years.[3] Clarke regards the 80.7 millions of
+years<br>
+as certainly a maximum in the light of certain calculations
+by<br>
+Becker.[4]</p>
+<p>The order of magnitude of these results cannot be shaken
+unless<br>
+on the assumption that there is something entirely misleading
+in<br>
+the existing rate of solvent denudation. On the strength of
+the<br>
+results of another and</p>
+<p>[1] F. W. Clarke, _A Preliminary Study of Chemical
+Denudation_<br>
+(Smithsonian Miscellaneous Collections, 1910).</p>
+<p>[2] _Loc. cit._</p>
+<p>[3] "The Circulation of Salt and Geological Time" (Geol.
+Mag.,<br>
+1901, p. 350).</p>
+<p>[4] Becker (loc. cit.), assuming that the exposed igneous
+and<br>
+arch&aelig;an rocks alone are responsible for the supply of
+sodium to<br>
+the ocean, arrives at 74 millions of years as the geological
+age.<br>
+This matter was discussed by me formerly (Trans. R.D.S.,
+1899,<br>
+pp. 54 _et seq._). The assumption made is, I believe,
+inadmissible.<br>
+It is not supported by river analyses, or by the chemical<br>
+character of residual soils from sedimentary rocks. There may
+be<br>
+some convergence in the rate of solvent denudation, but&mdash;as
+I<br>
+think on the evidence&mdash;in our time unimportant.</p>
+<p>14</p>
+<p>entirely different method of approaching the question of
+the<br>
+Earth's age (which shall be presently referred to), it has
+been<br>
+contended that it is too low. It is even asserted that it is
+from<br>
+nine to fourteen times too low. We have then to consider
+whether<br>
+such an enormous error can enter into the method. The<br>
+measurements involved cannot be seriously impugned.
+Corrections<br>
+for possible errors applied to the quantities entering into
+this<br>
+method have been considered by various writers. My own
+original<br>
+corrections have been generally confirmed. I think the only
+point<br>
+left open for discussion is the principle of
+uniformitarianism<br>
+involved in this method and in the methods previously
+discussed.</p>
+<p>In order to appreciate the force of the evidence for
+uniformity<br>
+in the geological history of the Earth, it is, of course,<br>
+necessary to possess some acquaintance with geological
+science.<br>
+Some of the most eminent geologists, among whom Lyell and<br>
+Geikie[1] may be mentioned, have upheld the doctrine of<br>
+uniformity. It must here suffice to dwell upon a few points<br>
+having special reference to the matter under discussion.</p>
+<p>The mere extent of the land surface does not, within
+limits,<br>
+affect the question of the rate of denudation. This arises
+from<br>
+the fact that the rain supply is quite insufficient to denude
+the<br>
+whole existing land surface. About 30 per cent. of it does
+not,<br>
+in fact, drain to the</p>
+<p>[1] See especially Geikie's Address to Sect. C., Brit.
+Assoc.<br>
+Rep., 1399.</p>
+<p>15</p>
+<p>ocean. If the continents become invaded by a great
+transgression<br>
+of the ocean, this "rainless" area diminishes: and the
+denuded<br>
+area advances inwards without diminution. If the ocean
+recedes<br>
+from the present strand lines, the "rainless" area advances<br>
+outwards, but, the rain supply being sensibly constant, no
+change<br>
+in the river supply of salts is to be expected.</p>
+<p>Age-long submergence of the entire land, or of any very
+large<br>
+proportion of what now exists, is negatived by the continuous<br>
+sequence of vast areas of sediment in every geologic age from
+the<br>
+earliest times. Now sediment-receiving areas always are but a<br>
+small fraction of those exposed areas whence the sediments
+are<br>
+supplied.[1] Hence in the continuous records of the sediments
+we<br>
+have assurance of the continuous exposure of the continents
+above<br>
+the ocean surface. The doctrine of the permanency of the<br>
+continents has in its main features been accepted by the most<br>
+eminent authorities. As to the actual amount of land which
+was<br>
+exposed during past times to denudative effects, no data exist
+to<br>
+show it was very different from what is now exposed. It has
+been<br>
+estimated that the average area of the North American
+continent<br>
+over geologic time was about eight-tenths of its existing<br>
+area.[2] Restorations of other continents, so far as they
+have<br>
+been attempted, would not</p>
+<p>[1] On the strength of the Mississippi measurements about 1 to
+18<br>
+(Magee, _Am. Jour. of Sc._, 1892, p. 188).</p>
+<p>[2] Schuchert, _Bull. Geol. Soc. Am._, vol. xx., 1910.</p>
+<p>16</p>
+<p>suggest any more serious divergency one way or the other.</p>
+<p>That climate in the oceans and upon the land was throughout
+much<br>
+as it is now, the continuous chain of teeming life and the<br>
+sensitive temperature limits of protoplasmic existence are<br>
+sufficient evidence.[1] The influence at once of climate and
+of<br>
+elevation of the land may be appraised at their true value by
+the<br>
+ascertained facts of solvent denudation, as the following
+table<br>
+shows.</p>
+<p>                 Tonnes removed in      Mean elevation.<br>
+                 solution per square    Metres.<br>
+                 mile per annum.<br>
+North America -         79                  700<br>
+South America -         50                  650<br>
+Europe -                100                  300<br>
+Asia -                   84                  950<br>
+Africa -                 44                  650</p>
+<p>In this table the estimated number of tonnes of matter in<br>
+solution, which for every square mile of area the rivers
+convey<br>
+to the ocean in one year, is given in the first column. These<br>
+results are compiled by Clarke from a very large number of<br>
+analyses of river waters. The second column of the table
+gives<br>
+the mean heights in metres above sea level of the several<br>
+continents, as cited by Arrhenius.[2]</p>
+<p>Of all the denudation results given in the table, those
+relating<br>
+to North America and to Europe are far the</p>
+<p>[1] See also Poulton, Address to Sect. D., Brit. Assoc.
+Rep.,<br>
+1896.</p>
+<p>[2] _Lehybuch dev Kosmischen Physik_, vol. i., p. 347.</p>
+<p>17</p>
+<p>most reliable. Indeed these may be described as highly
+reliable,<br>
+being founded on some thousands of analyses, many of which
+have<br>
+been systematically pursued through every season of the year.<br>
+These show that Europe with a mean altitude of less than half<br>
+that of North America sheds to the ocean 25 per cent. more
+salts.<br>
+A result which is to be expected when the more important
+factors<br>
+of solvent denudation are given intelligent consideration and
+we<br>
+discriminate between conditions favouring solvent and
+detrital<br>
+denudation respectively: conditions in many cases<br>
+antagonistic.[1] Hence if it is true, as has been stated, that
+we<br>
+now live in a period of exceptionally high continental
+elevation,<br>
+we must infer that the average supply of salts to the ocean
+by<br>
+the rivers of the world is less than over the long past, and<br>
+that, therefore, our estimate of the age of the Earth as
+already<br>
+given is excessive.</p>
+<p>There is, however, one condition which will operate to
+unduly<br>
+diminish our estimate of geologic time, and it is a condition<br>
+which may possibly obtain at the present time. If the land is,
+on<br>
+the whole, now sinking relatively to the ocean level, the<br>
+denudation area tends, as we have seen, to move inwards. It
+will<br>
+thus encroach upon regions which have not for long periods<br>
+drained to the ocean. On such areas there is an accumulation
+of<br>
+soluble salts which the deficient rivers have not been able
+to<br>
+carry to the ocean. Thus the salt content of certain of</p>
+<p>[1] See the essay on Denudation.</p>
+<p>18</p>
+<p>the rivers draining to the ocean will be influenced not only
+by<br>
+present denudative effects, but also by the stored results of<br>
+past effects. Certain rivers appear to reveal this unduly<br>
+increased salt supply those which flow through comparatively
+arid<br>
+areas. However, the flowoff of such tributaries is relatively<br>
+small and the final effects on the great rivers apparently<br>
+unimportant&mdash;a result which might have been anticipated when
+the<br>
+extremely slow rate of the land movements is taken into
+account.</p>
+<p>The difficulty of effecting any reconciliation of the
+methods<br>
+already described and that now to be given increases the
+interest<br>
+both of the former and the latter.</p>
+<p>THE AGE BY RADIOACTIVE TRANSFORMATIONS</p>
+<p>Rutherford suggested in 1905 that as helium was continually
+being<br>
+evolved at a uniform rate by radioactive substances (in the
+form<br>
+of the alpha rays) a determination of the age of minerals<br>
+containing the radioactive elements might be made by
+measurements<br>
+of the amount of the stored helium and of the radioactive<br>
+elements giving rise to it, The parent radioactive substances<br>
+are&mdash;according to present knowledge&mdash;uranium and
+thorium. An<br>
+estimate of the amounts of these elements present enables the<br>
+rate of production of the helium to be calculated. Rutherford<br>
+shortly afterwards found by this method an age of 240 millions
+of<br>
+years for a radioactive mineral of presumably remote age.
+Strutt,<br>
+who carried</p>
+<p>19</p>
+<p>his measurements to a wonderful degree of refinement, found
+the<br>
+following ages for mineral substances originating in
+different<br>
+geological ages:</p>
+<p>Oligocene -               8.4 millions of years.<br>
+Eocene -                 31    millions of years.<br>
+Lower Carboniferous -  150   millions of years.<br>
+Arch&aelig;an -               750    millions of years.</p>
+<p>Periods of time much less than, and very inconsistent with,
+these<br>
+were also found. The lower results are, however, easily
+explained<br>
+if we assume that the helium&mdash;which is a gas under
+prevailing<br>
+conditions&mdash;escapes in many cases slowly from the
+mineral.</p>
+<p>Another product of radioactive origin is lead. The
+suggestion<br>
+that this substance might be made available to determine the
+age<br>
+of the Earth also originated with Rutherford. We are at least<br>
+assured that this element cannot escape by gaseous diffusion
+from<br>
+the minerals. Boltwood's results on the amount of lead
+contained<br>
+in minerals of various ages, taken in conjunction with the
+amount<br>
+of uranium or parent substance present, afforded ages rising
+to<br>
+1,640 millions of years for arch&aelig;an and 1,200 millions
+for<br>
+Algonkian time. Becker, applying the same method, obtained<br>
+results rising to quite incredible periods: from 1,671 to
+11,470<br>
+millions of years. Becker maintained that original lead
+rendered<br>
+the determinations indefinite. The more recent results of Mr.
+A.<br>
+Holmes support the conclusion that "original" lead may be
+present<br>
+and may completely falsify results derived</p>
+<p>20</p>
+<p>from minerals of low radioactivity in which the derived
+lead<br>
+would be small in amount. By rejecting such results as
+appeared<br>
+to be of this character, he arrives at 370 millions of years
+as<br>
+the age of the Devonian.</p>
+<p>I must now describe a very recent method of estimating the age
+of<br>
+the Earth. There are, in certain rock-forming minerals,<br>
+colour-changes set up by radioactive causes. The minute and<br>
+curious marks so produced are known as haloes; for they
+surround,<br>
+in ringlike forms, minute particles of included substances
+which<br>
+contain radioactive elements. It is now well known how these<br>
+haloes are formed. The particle in the centre of the halo<br>
+contains uranium or thorium, and, necessarily, along with the<br>
+parent substance, the various elements derived from it. In
+the<br>
+process of transformation giving rise to these several
+derived<br>
+substances, atoms of helium&mdash;the alpha rays&mdash;projected
+with great<br>
+velocity into the surrounding mineral, occasion the colour<br>
+changes referred to. These changes are limited to the distance
+to<br>
+which the alpha rays penetrate; hence the halo is a spherical<br>
+volume surrounding the central substance.[1]</p>
+<p>The time required to form a halo could be found if on the
+one<br>
+hand we could ascertain the number of alpha rays ejected from
+the<br>
+nucleus of the halo in, say, one year, and, on the other, if
+we<br>
+determined by experiment just how many alpha rays were
+required<br>
+to produce the same</p>
+<p>[1] _Phil. Mag._, March, 1907 and February, 1910; also
+_Bedrock_,<br>
+January, 1913. See _Pleochroic Haloes_ in this volume.</p>
+<p>21</p>
+<p>amount of colour alteration as we perceive to extend around
+the<br>
+nucleus.</p>
+<p>The latter estimate is fairly easily and surely made. But to
+know<br>
+the number of rays leaving the central particle in unit time
+we<br>
+require to know the quantity of radioactive material in the<br>
+nucleus. This cannot be directly determined. We can only,
+from<br>
+known results obtained with larger specimens of just such a<br>
+mineral substance as composes the nucleus, guess at the amount
+of<br>
+uranium, or it may be thorium, which may be present.</p>
+<p>This method has been applied to the uranium haloes of the mica
+of<br>
+County Carlow.[1] Results for the age of the halo of from 20
+to<br>
+400 millions of years have been obtained. This mica was
+probably<br>
+formed in the granite of Leinster in late Silurian or in
+Devonian<br>
+times.</p>
+<p>The higher results are probably the least in error, upon the
+data<br>
+involved; for the assumption made as to the amount of uranium
+in<br>
+the nuclei of the haloes was such as to render the higher
+results<br>
+the more reliable.</p>
+<p>This method is, of course, a radioactive method, and similar
+to<br>
+the method by helium storage, save that it is free of the risk
+of<br>
+error by escape of the helium, the effects of which are, as
+it<br>
+were, registered at the moment of its production, so that its<br>
+subsequent escape is of no moment.</p>
+<p>[1] Joly and Rutherford, _Phil. Mag._, April, 1913.</p>
+<p>22</p>
+<p>REVIEW OF THE RESULTS</p>
+<p>We shall now briefly review the results on the geological age
+of<br>
+the Earth.</p>
+<p>By methods based on the approximate uniformity of
+denudative<br>
+effects in the past, a period of the order of 100 millions of<br>
+years has been obtained as the duration of our geological
+age;<br>
+and consistently whether we accept for measurement the
+sediments<br>
+or the dissolved sodium. We can give reasons why these<br>
+measurements might afford too great an age, but we can find<br>
+absolutely no good reason why they should give one much too
+low.</p>
+<p>By measuring radioactive products ages have been found
+which,<br>
+while they vary widely among themselves, yet claim to possess<br>
+accuracy in their superior limits, and exceed those derived
+from<br>
+denudation from nine to fourteen times.</p>
+<p>In this difficulty let us consider the claims of the
+radioactive<br>
+method in any of its forms. In order to be trustworthy it must
+be<br>
+true; (1) that the rate of transformation now shown by the
+parent<br>
+substance has obtained throughout the entire past, and (2)
+that<br>
+there were no other radioactive substances, either now or<br>
+formerly existing, except uranium, which gave rise to lead.
+As<br>
+regards methods based on the production of helium, what we
+have<br>
+to say will largely apply to it also. If some unknown source
+of<br>
+these elements exists we, of course, on our assumption<br>
+overestimate the age.</p>
+<p>23</p>
+<p>As regards the first point: In ascribing a constant rate
+of<br>
+change to the parent substance&mdash;which Becker (loc. cit.)
+describes<br>
+as "a simple though tremendous extrapolation"&mdash;we reason
+upon<br>
+analogy with the constant rate of decay observed in the
+derived<br>
+radioactive bodies. If uranium and thorium are really primary<br>
+elements, however, the analogy relied on may be misleading;
+at<br>
+least, it is obviously incomplete. It is incomplete in a<br>
+particular which may be very important: the mode of origin of<br>
+these parent bodies&mdash;whatever it may have been&mdash;is
+different to<br>
+that of the secondary elements with which we compare them. A<br>
+convergence in their rate of transformation is not impossible,
+or<br>
+even improbable, so far as we known.</p>
+<p>As regards the second point: It is assumed that uranium alone
+of<br>
+the elements in radioactive minerals is ultimately transformed
+to<br>
+lead by radioactive changes. We must consider this
+assumption.</p>
+<p>Recent advances in the chemistry of the radioactive elements
+has<br>
+brought out evidence that all three lines of radioactive
+descent<br>
+known to us&mdash;_i.e._ those beginning with uranium, with
+thorium,<br>
+and with actinium&mdash;alike converge to lead.[1] There are<br>
+difficulties in the way of believing that all the lead-like
+atoms<br>
+so produced ("isotopes" of lead, as Soddy proposes to call
+them)<br>
+actually remain as stable lead in the minerals. For one</p>
+<p>[1] See Soddy's _Chemistry of the Radioactive Elements_
+(Longmans,<br>
+Green &amp; Co.).</p>
+<p>24</p>
+<p>thing there is sometimes, along with very large amounts of<br>
+thorium, an almost entire absence of lead in thorianites and<br>
+thorites. And in some urano&mdash;thorites the lead may be
+noticed to<br>
+follow the uranium in approximate proportionality,<br>
+notwithstanding the presence of large amounts of thorium.[1]
+This<br>
+is in favour of the assumption that all the lead present is<br>
+derived from the uranium. The actinium is present in
+negligibly<br>
+small amounts.</p>
+<p>On the other hand, there is evidence arising from the
+atomic<br>
+weight of lead which seems to involve some other parent than<br>
+uranium. Soddy, in the work referred to, points this out. The<br>
+atomic weight of radium is well known, and uranium in its
+descent<br>
+has to change to this element. The loss of mass between
+radium<br>
+and uranium-derived lead can be accurately estimated by the<br>
+number of alpha rays given off. From this we get the atomic<br>
+weight of uranium-derived lead as closely 206. Now the best<br>
+determinations of the atomic weight of normal lead assign to
+this<br>
+element an atomic weight of closely</p>
+<p>[1] It seems very difficult at present to suggest an end
+product<br>
+for thorium, unless we assume that, by loss of electrons,
+thorium<br>
+E, or thorium-lead, reverts to a substance chemically
+identical<br>
+with thorium itself. Such a change&mdash;whether considered from
+the<br>
+point of view of the periodic law or of the radioactive
+theory<br>
+would involve many interesting consequences. It is, of
+course,<br>
+quite possible that the nature of the conditions attending
+the<br>
+deposition of the uranium ores, many of which are
+comparatively<br>
+recent, are responsible for the difficulties observed. The<br>
+thorium and uranium ores are, again, specially prone to<br>
+alteration.</p>
+<p>25</p>
+<p>207. By a somewhat similar calculation it is deduced that<br>
+thorium-derived lead would possess the atomic weight of 208.
+Thus<br>
+normal lead might be an admixture of uranium- and
+thorium-derived<br>
+lead. However, as we have seen, the view that thorium gives
+rise<br>
+to stable lead is beset with some difficulties.</p>
+<p>If we are going upon reliable facts and figures, we must,
+then,<br>
+assume: (a) That some other element than uranium, and
+genetically<br>
+connected with it (probably as parent substance), gives rise,
+or<br>
+formerly gave rise, to lead of heavier atomic weight than
+normal<br>
+lead. It may be observed respecting this theory that there is<br>
+some support for the view that a parent substance both to
+uranium<br>
+and thorium has existed or possibly exists. The evidence is
+found<br>
+in the proportionality frequently observed between the amounts
+of<br>
+thorium and uranium in the primary rocks.[1] Or: (b) We may
+meet<br>
+the difficulties in a simpler way, which may be stated as<br>
+follows: If we assume that all stable lead is derived from<br>
+uranium, and at the same time recognise that lead is not<br>
+perfectly homogeneous in atomic weight, we must, of
+necessity,<br>
+ascribe to uranium a similar want of homogeneity; heavy atoms
+of<br>
+uranium giving rise to heavy</p>
+<p>[1] Compare results for the thorium content of such rocks<br>
+(appearing in a paper by the author Cong. Int. _de Radiologie
+et<br>
+d'Electricit&eacute;_, vol. i., 1910, p. 373), and those for the
+radium<br>
+content, as collected in _Phil. Mag._, October, 1912, p. 697.<br>
+Also A. L. Fletcher, _Phil. Mag._, July, 1910; January, 1911,
+and<br>
+June, 1911. J. H. J. Poole, _Phil. Mag._, April, 1915</p>
+<p>26</p>
+<p>atoms of lead and light atoms of uranium generating light
+atoms<br>
+of lead. This assumption seems to be involved in the figures<br>
+upon, which we are going. Still relying on these figures, we<br>
+find, however, that existing uranium cannot give rise to lead
+of<br>
+normal atomic weight. We can only conclude that the heavier
+atoms<br>
+of uranium have decayed more rapidly than the lighter ones.
+In<br>
+this connection it is of interest to note the complexity of<br>
+uranium as recently established by Geiger, although in this
+case<br>
+it is assumed that the shorter-lived isotope bears the
+relation<br>
+of offspring to the longer-lived and largely preponderating<br>
+constituent. However, there does not seem to be any direct
+proof<br>
+of this as yet.</p>
+<p>From these considerations it would seem that unless the
+atomic<br>
+weight of lead in uraninites, etc., is 206, the former
+complexity<br>
+and more accelerated decay of uranium are indicated in the
+data<br>
+respecting the atomic weights of radium and lead[1]. As an<br>
+alternative view, we may assume, as in our first hypothesis,
+that<br>
+some elementally different but genetically connected
+substance,<br>
+decaying along branching lines of descent at a rate sufficient
+to<br>
+practically remove the whole of it during geological time,<br>
+formerly existed. Whichever hypothesis we adopt</p>
+<p>[1] Later investigation has shown that the atomic weight of
+lead<br>
+in uranium-bearing ores is about 206.6 (see Richards and
+Lembert,<br>
+_Journ. of Am. Claem. Soc._, July, 1914). This result gives
+support<br>
+to the view expressed above.</p>
+<p>27</p>
+<p>we are confronted by probabilities which invalidate<br>
+time-measurements based on the lead and helium ratio in
+minerals.<br>
+We have, in short, grave reason to question the measure of<br>
+uniformitarianism postulated in finding the age by any of the<br>
+known radioactive methods.</p>
+<p>That we have much to learn respecting our assumptions, whether
+we<br>
+pursue the geological or the radioactive methods of
+approaching<br>
+the age of our era, is, indeed, probable. Whatever the issue
+it<br>
+is certain that the reconciling facts will leave us with much<br>
+more light than we at present possess either as respects the<br>
+Earth's history or the history of the radioactive elements.
+With<br>
+this necessary admission we leave our study of the Birth-Time
+of<br>
+the World.</p>
+<p>It has led us a long way from Lucretius. We do not ask if
+other<br>
+Iliads have perished; or if poets before Homer have vainly
+sung,<br>
+becoming a prey to all-consuming time. We move in a greater<br>
+history, the landmarks of which are not the birth and death
+of<br>
+kings and poets, but of species, genera, orders. And we set
+out<br>
+these organic events not according to the passing generations
+of<br>
+man, but over scores or hundreds of millions of years.</p>
+<p>How much Lucretius has lost, and how much we have gained,
+is<br>
+bound up with the question of the intrinsic value of
+knowledge<br>
+and great ideas. Let us appraise knowledge as we would the<br>
+Homeric poems, as some-</p>
+<p>28</p>
+<p>thing which ennobles life and makes it happier. Well, then,
+we<br>
+are, as I think, in possession today of some of those lost
+Iliads<br>
+and Odysseys for which Lucretius looked in vain.[1]</p>
+<p>[1] The duration in the past of Solar heat is necessarily
+bound<br>
+up with the geological age. There is no known means (outside<br>
+speculative science) of accounting for more than about 30
+million<br>
+years of the existing solar temperature in the past. In this<br>
+direction the age seems certainly limited to 100 million
+years.<br>
+See a review of the question by Dr. Lindemann in Nature,
+April<br>
+5th, 1915.</p>
+<p>29</p>
+<p><u>DENUDATION</u></p>
+<p>THE subject of denudation is at once one of the most
+interesting<br>
+and one of the most complicated with which the geologist has
+to<br>
+deal. While its great results are apparent even to the most<br>
+casual observer, the factors which have led to these results
+are<br>
+in many cases so indeterminate, and in some cases apparently
+so<br>
+variable in influence, that thoughtful writers have even
+claimed<br>
+precisely opposite effects as originating from, the same
+cause.<br>
+Indeed, it is almost impossible to deal with the subject
+without<br>
+entering upon controversial matters. In the following pages I<br>
+shall endeavour to keep to broad issues which are, at the
+present<br>
+day, either conceded by the greater number of authorities on
+the<br>
+subject, or are, from their strictly quantitative character,
+not<br>
+open to controversy.</p>
+<p>It is evident, in the first place, that denudation&mdash;or
+the wearing<br>
+away of the land surfaces of the earth&mdash;is mainly a result
+of the<br>
+circulation of water from the ocean to the land, and back
+again<br>
+to the ocean. An action entirely conditioned by solar heat,
+and<br>
+without which it would completely cease and further change
+upon<br>
+the land come to an end.</p>
+<p>To what actions, then, is so great a potency of the</p>
+<p>30</p>
+<p>circulating water to be traced? Broadly speaking, we may
+classify<br>
+them as mechanical and chemical. The first involves the<br>
+separation of rock masses into smaller fragments of all
+sizes,<br>
+down to the finest dust. The second involves the actual
+solution<br>
+in the water of the rock constituents, which may be regarded
+as<br>
+the final act of disintegration. The rivers bear the burden
+both<br>
+of the comminuted and the dissolved materials to the sea. The
+mud<br>
+and sand carried by their currents, or gradually pushed along<br>
+their beds, represent the former; the invisible dissolved
+matter,<br>
+only to be demonstrated to the eye by evaporation of the water
+or<br>
+by chemical precipitation, represents the latter.</p>
+<p>The results of these actions, integrated over geological
+time,<br>
+are enormous. The entire bulk of the sedimentary rocks, such
+as<br>
+sandstones, slates, shales, conglomerates, limestones, etc.,
+and<br>
+the salt content of the ocean, are due to the combined
+activity<br>
+of mechanical and solvent denudation. We shall, later on, make
+an<br>
+estimate of the magnitude of the quantities actually
+involved.</p>
+<p>In the Swiss valleys we see torrents of muddy water
+hurrying<br>
+along, and if we follow them up, we trace them to glaciers
+high<br>
+among the mountains. From beneath the foot of the glacier, we<br>
+find, the torrent has birth. The first debris given to the
+river<br>
+is derived from the wearing of the rocky bed along which the<br>
+glacier moves. The river of ice bequeaths to the river of<br>
+water&mdash;of which it is the parent&mdash;the spoils which it
+has won from<br>
+the rocks</p>
+<p>31</p>
+<p>The work of mechanical disintegration is, however, not
+restricted<br>
+to the glacier's bed. It proceeds everywhere over the surface
+of<br>
+the rocks. It is aided by the most diverse actions. For
+instance,<br>
+the freezing and expansion of water in the chinks and cracks
+in<br>
+those alpine heights where between sunrise and sunset the heat
+of<br>
+summer reigns, and between sunset and sunrise the cold of
+winter.<br>
+Again, under these conditions the mere change of surface<br>
+temperature from night to day severely stresses the surface<br>
+layers of the rocks, and, on the same principles as we
+explain<br>
+the fracture of an unequally heated glass vessel, the rocks<br>
+cleave off in slabs which slip down the steeps of the
+mountain<br>
+and collect as screes in the valley. At lower levels the<br>
+expansive force of vegetable growth is not unimportant, as
+all<br>
+will admit who have seen the strong roots of the pines<br>
+penetrating the crannies of the rocks. Nor does the river
+which<br>
+flows in the bed of the valley act as a carrier only.
+Listening<br>
+carefully we may detect beneath the roar of the alpine
+torrent<br>
+the crunching and knocking of descending boulders. And in the<br>
+potholes scooped by its whirling waters we recognise the
+abrasive<br>
+action of the suspended sand upon the river bed.</p>
+<p>A view from an Alpine summit reveals a scene of remarkable<br>
+desolation (Pl. V, p. 40). Screes lie piled against the steep<br>
+slopes. Cliffs stand shattered and ready to fall in ruins.
+And<br>
+here the forces at work readily reveal themselves. An
+occasional<br>
+wreath of white smoke among</p>
+<p>32</p>
+<p>the far-off peaks, followed by a rumbling reverberation,
+marks<br>
+the fall of an avalanche. Water everywhere trickles through
+the<br>
+shaly _d&eacute;bris_ scattered around. In the full sunshine the
+rocks are<br>
+almost too hot to bear touching. A few hours later the cold
+is<br>
+deadly, and all becomes a frozen silence. In such scenes of<br>
+desolation and destruction, detrital sediments are actively
+being<br>
+generated. As we descend into the valley we hear the deep
+voice<br>
+of the torrents which are continually hurrying the
+disintegrated<br>
+rocks to the ocean.</p>
+<p>A remarkable demonstration of the activity of mechanical<br>
+denudation is shown by the phenomenon of "earth pillars." The<br>
+photograph (Pl. IV.) of the earth pillars of the Val
+d'H&eacute;rens<br>
+(Switzerland) shows the peculiar appearance these objects<br>
+present. They arise under conditions where large stones or<br>
+boulders are scattered in a deep deposit of clay, and where
+much<br>
+of the denudation is due to water scour. The large boulders
+not<br>
+only act as shelter against rain, but they bind and
+consolidate<br>
+by their mere weight the clay upon which they rest. Hence the<br>
+materials underlying the boulders become more resistant, and
+as<br>
+the surrounding clays are gradually washed away and carried
+to<br>
+the streams, these compacted parts persist, and, finally,
+stand<br>
+like walls or pillars above the general level. After a time
+the<br>
+great boulders fall off and the underlying clay becomes worn
+by<br>
+the rainwash to fantastic spikes and ridges. In the Val
+d'H&eacute;rens<br>
+the earth pillars are formed</p>
+<p>33</p>
+<p>of the deep moraine stuff which thickly overlies the slopes
+of<br>
+the valley. The wall of pillars runs across the axis of the<br>
+valley, down the slope of the hill, and crosses the road, so
+that<br>
+it has to be tunnelled to permit the passage of traffic. It
+is<br>
+not improbable that some additional influence&mdash;possibly
+the<br>
+presence of lime&mdash;has hardened the material forming the
+pillars,<br>
+and tended to their preservation.</p>
+<p>Denudation has, however, other methods of work than purely<br>
+mechanical; methods more noiseless and gentle, but not less<br>
+effective, as the victories of peace ate no less than those
+of<br>
+war.</p>
+<p>Over the immense tracts of the continents chemical work
+proceeds<br>
+relentlessly. The rock in general, more especially the
+primary<br>
+igneous rock, is not stable in presence of the atmosphere and
+of<br>
+water. Some of the minerals, such as certain silicates and<br>
+carbonates, dissolve relatively fast, others with extreme<br>
+slowness. In the process of solution chemical actions are<br>
+involved; oxidation in presence of the free oxygen of the<br>
+atmosphere; attack by the feeble acid arising from the
+solution<br>
+of carbon dioxide in water; or, again, by the activity of
+certain<br>
+acids&mdash;humous acids&mdash;which originate in the
+decomposition of<br>
+vegetable remains. These chemical agents may in some
+instances,<br>
+_e.g._ in the case of carbonates such as limestone or<br>
+dolomite&mdash;bring practically the whole rock into solution. In
+other<br>
+instances&mdash;_e.g._ granites, basalts, etc.&mdash;they may
+remove some of<br>
+the</p>
+<p>34</p>
+<p>constituent minerals completely or partially, such as
+felspar,<br>
+olivine, augite, and leave more resistant substances to be<br>
+ultimately washed down as fine sand or mud into the river.</p>
+<p>It is often difficult or impossible to appraise the
+relative<br>
+efficiency of mechanical and chemical denudation in removing
+the<br>
+materials from a certain area. There can be, indeed, little
+doubt<br>
+that in mountainous regions the mechanical effects are
+largely<br>
+predominant. The silts of glacial rivers are little different<br>
+from freshly-powdered rock. The water which carries them but<br>
+little different from the pure rain or snow which falls from
+the<br>
+sky. There has not been time for the chemical or solvent
+actions<br>
+to take place. Now while gravitational forces favour sudden
+shock<br>
+and violent motions in the hills, the effect of these on
+solvent<br>
+and chemical denudation is but small. Nor is good drainage<br>
+favourable to chemical actions, for water is the primary
+factor<br>
+in every case. Water takes up and removes soluble combinations
+of<br>
+molecules, and penetrates beneath residual insoluble
+substances.<br>
+It carries the oxygen and acids downwards through the soils,
+and<br>
+finally conveys the results of its own work to the rivers and<br>
+streams. The lower mean temperature of the mountains as well
+as<br>
+the perfect drainage diminishes chemical activities.</p>
+<p>Hence we conclude that the heights are not generally
+favourable<br>
+to the purely solvent and chemical actions. It is on the<br>
+lower-lying land that soils tend to accumulate,</p>
+<p>35</p>
+<p>and in these the chief solvent and the chief chemical
+denudation<br>
+of the Earth are effected.</p>
+<p>The solvent and chemical effects which go on in the<br>
+finely-divided materials of the soils may be observed in the<br>
+laboratory. They proceed faster than would be anticipated.
+The<br>
+observation is made by passing a measured quantity of water<br>
+backwards and forwards for some months through a tube
+containing<br>
+a few grammes of powdered rock. Finally the water is
+analysed,<br>
+and in this manner the amount of dissolved matter it has taken
+up<br>
+is estimated. The rock powder is examined under the microscope
+in<br>
+order to determine the size of the grains, and so to
+calculate<br>
+the total surface exposed to the action of the water. We must
+be<br>
+careful in such experiments to permit free oxidation by the<br>
+atmosphere. Results obtained in this way of course take no<br>
+account of the chemical effects of organic acids such as exist
+in<br>
+the soils. The quantities obtained in the laboratory will,<br>
+therefore, be deficient as compared with the natural results.</p>
+<p>In this manner it has been found that fresh basalt exposed
+to<br>
+continually moving water will lose about 0.20 gramme per
+square<br>
+metre of surface per year. The mineral orthoclase, which
+enters<br>
+largely into the constitution of many granites, was found to
+lose<br>
+under the same conditions 0.025 gramme. A glassy lava
+(obsidian)<br>
+rich in silica and in the chemical constituents of an average<br>
+granite, was more resistant still; losing but 0.013 gramme
+per<br>
+square metre per year. Hornblende, a mineral</p>
+<p>36</p>
+<p>abundant in many rocks, lost 0.075 gramme. The mean of the<br>
+results showed that 0.08 gramme was washed in a year from
+each<br>
+square metre. Such results give us some indication of the rate
+at<br>
+which the work of solution goes on in the finely divided<br>
+soils.[1]</p>
+<p>It might be urged that, as the mechanical break up of rocks,
+and<br>
+the production in this way of large surfaces, must be at the<br>
+basis of solvent and chemical denudation, these latter
+activities<br>
+should be predominant in the mountains. The answer to this is<br>
+that the soils rarely owe their existence to mechanical
+actions.<br>
+The alluvium of the valleys constitutes only narrow margins
+to<br>
+the rivers; the finer _d&eacute;bris_ from the mountains is
+rapidly<br>
+brought into the ocean. The soils which cover the greater part
+of<br>
+continental areas have had a very different origin.</p>
+<p>In any quarry where a section of the soil and of the
+underlying<br>
+rock is visible, we may study the mode of formation of soils.
+Our<br>
+observations are, we will suppose, pursued in a granite
+quarry.<br>
+We first note that the material of the soil nearest the
+surface<br>
+is intermixed with the roots of grasses, trees, or shrubs.<br>
+Examining a handful of this soil, we see glistening flakes of<br>
+mica which plainly are derived from the original granite.
+Washing<br>
+off the finer particles, we find the largest remaining grains
+are<br>
+composed of the all but indestructible quartz.</p>
+<p>[1] Proc. Roy. Irish Acad., VIII., Ser. A, p. 21.</p>
+<p>37</p>
+<p>This also is from the granite. Some few of the grains are
+of<br>
+chalky-looking felspar; again a granitic mineral. What is the<br>
+finer silt we have washed off? It, too, is composed of
+mineral<br>
+particles to a great extent; rock dust stained with iron
+oxide<br>
+and intermixed with organic remains, both animal and
+vegetable.<br>
+But if we make a chemical analysis of the finer silt we find
+that<br>
+the composition is by no means that of the granite beneath.
+The<br>
+chemist is able to say, from a study of his results, that
+there<br>
+has been, in the first place, a large loss of material
+attending<br>
+the conversion of the granite to the soil. He finds a<br>
+concentration of certain of the more resistant substances of
+the<br>
+granite arising from the loss of the less resistant. Thus the<br>
+percentage amount of alumina is increased. The percentage of
+iron<br>
+is also increased. But silica and most other substances show
+a<br>
+diminished percentage. Notably lime has nearly disappeared.
+Soda<br>
+is much reduced; so is magnesia. Potash is not so completely<br>
+abstracted. Finally, owing to hydration, there is much more<br>
+combined water in the soil than in the rock. This is a
+typical<br>
+result for rocks of this kind.</p>
+<p>Deeper in the soil we often observe a change of texture. It
+has<br>
+become finer, and at the same time the clay is paler in
+colour.<br>
+This subsoil represents the finer particles carried by rain
+from<br>
+above. The change of colour is due to the state of the iron
+which<br>
+is less oxidised low down in the soil. Beneath the subsoil
+the<br>
+soil grows</p>
+<p>38</p>
+<p>again coarser. Finally, we recognise in it fragments of
+granite<br>
+which ever grow larger as we descend, till the soil has
+become<br>
+replaced by the loose and shattered rock. Beneath this the
+only<br>
+sign of weathering apparent in the rock is the rusty hue
+imparted<br>
+by the oxidised iron which the percolating rain has leached
+from<br>
+iron-bearing minerals.</p>
+<p>The soil we have examined has plainly been derived in situ
+from<br>
+the underlying rock. It represents the more insoluble residue<br>
+after water and acids have done their work. Each year there
+must<br>
+be a very slow sinking of the surface, but the ablation is<br>
+infinitesimal.</p>
+<p>The depth of such a soil may be considerable. The total
+surface<br>
+exposed by the countless grains of which it is composed is<br>
+enormous. In a cubic foot of average soil the surface area of
+the<br>
+grains may be 50,000 square feet or more. Hence a soil only
+two<br>
+feet deep may expose 100,000 square feet for each square foot
+of<br>
+surface area.</p>
+<p>It is true that soils formed in this manner by atmospheric
+and<br>
+organic actions take a very long time to grow. It must be<br>
+remembered, however, that the process is throughout attended
+by<br>
+the removal in solution: of chemically altered materials.</p>
+<p>Considerations such as the foregoing must convince us that
+while<br>
+the accumulation of the detrital sediments around the
+continents<br>
+is largely the result of activities progressing on the
+steeper<br>
+slopes of the land, that is,</p>
+<p>39</p>
+<p>among the mountainous regions, the feeding of the salts to
+the<br>
+ocean arises from the slower work of meteorological and
+organic<br>
+agencies attacking the molecular constitution of the rocks;<br>
+processes which best proceed where the drainage is sluggish
+and<br>
+the quiescent conditions permit of the development of
+abundant<br>
+organic growth and decay.</p>
+<p>Statistics of the solvent denudation of the continents
+support<br>
+this view. Within recent years a very large amount of work
+has<br>
+been expended on the chemical investigation of river waters
+of<br>
+America and of Europe. F. W. Clarke has, at the expense of
+much<br>
+labour, collected and compared these results. They are
+expressed<br>
+as so many tonnes removed in solution per square mile per
+annum.<br>
+For North America the result shows 79 tonnes so removed; for<br>
+Europe 100 tonnes. Now there is a notable difference between
+the<br>
+mean elevations of these two continents. North America has a
+mean<br>
+elevation of 700 metres over sea level, whereas the mean<br>
+elevation of Europe is but 300 metres. We see in these
+figures<br>
+that the more mountainous land supplies less dissolved matter
+to<br>
+the ocean than the land of lower elevation, as our study has
+led<br>
+us to expect.</p>
+<p>We have now considered the source of the detrital sediments,
+as<br>
+well as of the dissolved matter which has given to the ocean,
+in<br>
+the course of geological time, its present gigantic load of<br>
+salts. It is true there are further solvent and chemical
+effects<br>
+exerted by the sea water</p>
+<p>40</p>
+<p>upon the sediments discharged into it; but we are justified
+in<br>
+concluding that, relatively to the similar actions taking
+place<br>
+in the soils, the solvent and chemical work of the ocean is<br>
+small. The fact is, the deposited detrital sediments around
+the<br>
+continents occupy an area small when contrasted with the vast<br>
+stretches of the land. The area of deposition is much less
+than<br>
+that of denudation; probably hardly as much as one twentieth.<br>
+And, again, the conditions of aeration and circulation which<br>
+largely promote chemical and solvent denudation in the soils
+are<br>
+relatively limited and ineffective in the detrital oceanic<br>
+deposits.</p>
+<p>The summation of the amounts of dissolved and detrital
+materials<br>
+which denudation has brought into the ocean during the long<br>
+denudative history of the Earth, as we might anticipate,
+reveals<br>
+quantities of almost unrealisable greatness. The facts are
+among<br>
+the most impressive which geological science has brought to<br>
+light. Elsewhere in this volume they have been mentioned when<br>
+discussing the age of the Earth. In the present connection,<br>
+however, they are deserving of separate consideration.</p>
+<p>The basis of our reasoning is that the ocean owes its
+saltness<br>
+mainly if not entirely to the denudative activities we have
+been<br>
+considering. We must establish this.</p>
+<p>We may, in the first place, say that any other view at
+once<br>
+raises the greatest difficulties. The chemical composition of
+the<br>
+detrital sediments which are spread over</p>
+<p>41</p>
+<p>the continents and which build up the mountains, differs on
+the<br>
+average very considerably from that of the igneous rocks. We
+know<br>
+the former have been derived from the latter, and we know
+that<br>
+the difference in the composition of the two classes of
+materials<br>
+is due to the removal in solution of certain of the
+constituents<br>
+of the igneous rocks. But the ocean alone can have received
+this<br>
+dissolved matter. We know of no other place in which to look
+for<br>
+it. It is true that some part of this dissolved matter has
+been<br>
+again rejected by the ocean; thus the formation of limestone
+is<br>
+largely due to the abstraction of lime from sea water by
+organic<br>
+and other agencies. This, however, in no way relieves us of
+the<br>
+necessity of tracing to the ocean the substances dissolved
+from<br>
+the igneous rocks. It follows that we have here a very causa
+for<br>
+the saltness of the ocean. The view that the ocean "was salt
+from<br>
+the first" is without one known fact to support it, and leaves
+us<br>
+with the burden of the entire dissolved salts of geological
+time<br>
+to dispose of&mdash;Where and how?</p>
+<p>The argument we have outlined above becomes convincingly
+strong<br>
+when examined more closely. For this purpose we first compare
+the<br>
+average chemical composition of the sedimentary and the
+igneous<br>
+rocks. The following table gives the percentages of the chief<br>
+chemical constituents: [1]</p>
+<p>[1] F. W. Clarke: _A Preliminary Study of Chemical
+Denudation_,<br>
+p. 13</p>
+<p>42</p>
+<p>                          Igneous.        Sedimentary.<br>
+Silica (SiO<sub>2</sub>) -             59.99           58.51<br>
+Alumina (Al<sub>2</sub>O<sub>3</sub>) -           15.04          
+13.07<br>
+Ferric oxide (F<sub>2</sub>O<sub>3</sub>) -       2.59           
+3.40<br>
+Ferrous oxide (FeO) -       3.34            2.00<br>
+Magnesia (MgO) -            3.89            2.52<br>
+Lime (CaO) -                 4.81            5.42<br>
+Soda (Na<sub>2</sub>O) -                3.41            1.12<br>
+Potash (K<sub>2</sub>O) -               2.95            2.80<br>
+Water (H<sub>2</sub>O) -                1.92            4.28<br>
+Carbon dioxide (CO<sub>2</sub>) -       --             4.93<br>
+Minor constituents -      <u>  2.06</u>          <u> 
+1.95</u><br>
+                          100.00          100.00</p>
+<p>In the derivation of the sediments from the igneous rocks
+there<br>
+is a loss by solution of about 33 per cent; _i.e._ 100 tons
+of<br>
+igneous rock yields rather less than 70 tons of sedimentary
+rock.<br>
+This involves a concentration in the sediments of the more<br>
+insoluble constituents. To this rule the lime-content appears
+to<br>
+be an exception. It is not so in reality. Its high value in
+the<br>
+sediments is due to its restoration from the ocean to the
+land.<br>
+The magnesia and potash are, also, largely restored from the<br>
+ocean; the former in dolomites and magnesian limestones; the<br>
+latter in glauconite sands. The iron of the sediments shows<br>
+increased oxidation. The most notable difference in the two<br>
+analyses appears, however, in the soda percentages. This
+falls<br>
+from 3.41 in the igneous rock to 1.12 in the average
+sediment.<br>
+Indeed, this</p>
+<p>43</p>
+<p>deficiency of soda in sedimentary rocks is so characteristic
+of<br>
+secondary rocks that it may with some safety be applied to<br>
+discriminate between the two classes of substances in cases
+where<br>
+petrological distinctions of other kinds break down.</p>
+<p>To what is this so marked deficiency of soda to be ascribed?
+It<br>
+is a result of the extreme solubility of the salts of sodium
+in<br>
+water. This has not only rendered its deposition by evaporation
+a<br>
+relatively rare and unimportant incident of geological
+history,<br>
+but also has protected it from abstraction from the ocean by<br>
+organic agencies. The element sodium has, in fact, accumulated
+in<br>
+the ocean during the whole of geological time.</p>
+<p>We can use the facts associated with the accumulation of
+sodium<br>
+salts in the ocean as a means of obtaining additional support
+to<br>
+the view, that the processes of solvent denudation are<br>
+responsible for the saltness of the ocean. The new evidence
+may<br>
+be stated as follows: Estimates of the amounts of sedimentary<br>
+rock on the continents have repeatedly been made. It is true
+that<br>
+these estimates are no more than approximations. But they<br>
+undoubtedly _are_ approximations, and as such may legitimately
+be<br>
+used in our argument; more especially as final agreement tends
+to<br>
+check and to support the several estimates which enter into<br>
+them.</p>
+<p>The most recent and probable estimates of the sediments on
+the<br>
+land assign an average thickness of one mile of</p>
+<p>44</p>
+<p>secondary rocks over the land area of the world. To this
+some<br>
+increase must be made to allow for similar materials concealed
+in<br>
+the ocean, principally around the continental margins. If we
+add<br>
+10 per cent. and assign a specific gravity of 2.5 we get as
+the<br>
+mass of the sediments 64 x 10<sup>16</sup> tonnes. But as this is
+about 67<br>
+per cent. of the parent igneous rock&mdash;_i.e._ the average
+igneous<br>
+rock from which the sediments are derived&mdash;we conclude that
+the<br>
+primary denuded rock amounted to a mass of about 95 x
+10<sup>16</sup><br>
+tonnes.</p>
+<p>Now from the mean chemical composition of the secondary rocks
+we<br>
+calculate that the mass of sediments as above determined
+contains<br>
+0.72 x10<sup>16</sup> tonnes of the sodium oxide,
+Na<sub>2</sub>O. If to this amount we<br>
+add the quantity of sodium oxide which must have been given
+to<br>
+the ocean in order to account for the sodium salts contained<br>
+therein, we arrive at a total quantity of oxide of sodium
+which<br>
+must be that possessed by the primary rock before denudation<br>
+began its work upon it. The mass of the ocean being well<br>
+ascertained, we easily calculate that the sodium in the ocean<br>
+converted to sodium oxide amounts to 2.1 x 10<sup>16</sup>
+tonnes. Hence<br>
+between the estimated sediments and the waters of the ocean
+we<br>
+can account for 2.82 x 10<sup>16</sup> tonnes of soda. When now
+we put this<br>
+quantity back into the estimated mass of primary rock we find<br>
+that it assigns to the primary rock a soda percentage of 3.0.
+On<br>
+the average analysis given above this should be 3.41 per
+cent.<br>
+The agreement,</p>
+<p>45</p>
+<p>all things considered, more especially the uncertainty in
+the<br>
+estimate of the sediments, is plainly in support of the view
+that<br>
+oceanic salts are derived from the rocks; if, indeed, it does
+not<br>
+render it a certainty.</p>
+<p>A leading and fundamental inference in the denudative history
+of<br>
+the Earth thus finds support: indeed, we may say,
+verification.<br>
+In the light of this fact the whole work of denudation stands<br>
+revealed. That the ocean began its history as a vast
+fresh-water<br>
+envelope of the Globe is a view which accords with the
+evidence<br>
+for the primitive high temperature of the Earth. Geological<br>
+history opened with the condensation of an atmosphere of
+immense<br>
+extent, which, after long fluctuations between the states of<br>
+steam and water, finally settled upon the surface, almost free
+of<br>
+matter in solution: an ocean of distilled water. The epoch of<br>
+denudation then began. It will, probably, continue till the<br>
+waters, undergoing further loss of thermal energy, suffer yet<br>
+another change of state, when their circulation will cease
+and<br>
+their attack upon the rocks come to an end.</p>
+<p>From what has been reviewed above it is evident that the
+sodium<br>
+in the ocean is an index of the total activity of denudation<br>
+integrated over geological time. From this the broad facts of
+the<br>
+results of denudation admit of determination with
+considerable<br>
+accuracy. We can estimate the amount of rock which has been<br>
+degraded by solvent and chemical actions, and the amount of<br>
+sediments which has been derived from it. We are,</p>
+<p>46</p>
+<p>thus, able to amend our estimate of the sediments which,
+as<br>
+determined by direct observation, served to support the basis
+of<br>
+our argument.</p>
+<p>We now go straight to the ocean for the amount of sodium
+of<br>
+denudative origin. There may, indeed, have been some
+primitive<br>
+sodium dissolved by a more rapid denudation while the Earth's<br>
+surface was still falling in temperature. It can be shown,<br>
+however, that this amount was relatively small. Neglecting it
+we<br>
+may say with safety that the quantity of sodium carried into
+the<br>
+ocean by the rivers must be between 14,000 and 15,000 million<br>
+million tonnes: _i.e._ 14,500 x 10<sup>12</sup> tonnes, say.</p>
+<p>Keeping the figures to round numbers we find that this amount
+of<br>
+sodium involves the denudation of about 80 x 10<sup>16</sup>
+tonnes of<br>
+average igneous rock to 53 x 1016 tonnes of average sediment.<br>
+From these vast quantities we know that the parent rock
+denuded<br>
+during geological time amounted to some 300 million cubic<br>
+kilometres or about seventy million cubic miles. The
+sediments<br>
+derived therefrom possessed a bulk of 220 million cubic<br>
+kilometres or fifty million cubic miles. The area of the land<br>
+surface of the Globe is 144 million square kilometres. The
+parent<br>
+rock would have covered this to a uniform depth of rather
+more<br>
+than two kilometres, and the derived sediment to more than
+1.5<br>
+kilometres, or about one mile deep.</p>
+<p>The slow accomplishment of results so vast conveys some idea
+of<br>
+the great duration of geological time.</p>
+<p>47</p>
+<p>The foregoing method of investigating the statistics of
+solvent<br>
+denudation is capable of affording information not only as to
+the<br>
+amount of sediments upon the land, but also as to the
+quantity<br>
+which is spread over the floor of the ocean.</p>
+<p>We see this when we follow the fate of the 33 per cent. of<br>
+dissolved salts which has been leached from the parent
+igneous<br>
+rock, and the mass of which we calculate from the ascertained<br>
+mass of the latter, to be 27 x 10<sup>16</sup> tonnes. This
+quantity was at<br>
+one time or another all in the ocean. But, as we saw above, a<br>
+certain part of it has been again abstracted from solution,<br>
+chiefly by organic agencies. Now the abstracted solids have
+not<br>
+been altogether retained beneath the ocean. Movements of the
+land<br>
+during geological time have resulted in some portion being<br>
+uplifted along with other sediments. These substances
+constitute,<br>
+mainly, the limestones.</p>
+<p>We see, then, that the 27 x 10<sup>16</sup> tonnes of
+substances leached<br>
+from the parent igneous rocks have had a threefold
+destination.<br>
+One part is still in solution; a second part has been<br>
+precipitated to the bottom of the ocean; a third part exists
+on<br>
+the land in the form of calcareous rocks.</p>
+<p>Observation on the land sediments shows that the calcareous
+rocks<br>
+amount to about 5 per cent. of the whole. From this we find
+that<br>
+3 x 10<sup>16</sup> tonnes, approximately, of such rocks have
+been taken<br>
+from the ocean. This accounts for one of the three classes of<br>
+material</p>
+<p>48</p>
+<p>into which the original dissolved matter has been divided.<br>
+Another of the three quantities is easily estimated: the
+amount<br>
+of matter still in solution in the ocean. The volume of the
+ocean<br>
+is 1,414 million cubic kilometres and its mass is 145 x
+10<sup>16</sup><br>
+tonnes. The dissolved salts in it constitute 3.4 per cent. of
+its<br>
+mass; or, rather more than 5 x 10<sup>16</sup> tonnes. The
+limestones on the<br>
+land and the salts in the sea water together make up about 8
+x<br>
+10<sup>16</sup> tonnes. If we, now, deduct this from the total of
+27 x 10<sup>16</sup><br>
+tonnes, we find that about 19 x 10<sup>16</sup> tonnes must exist
+as<br>
+precipitated matter on the floor of the ocean.</p>
+<p>The area of the ocean is 367 x 10<sup>12</sup> square metres,
+so that if the<br>
+precipitated sediment possesses an average specific gravity
+of<br>
+2.5, it would cover the entire floor to a uniform depth of
+218<br>
+metres; that is 715 feet. This assumes that there was uniform<br>
+deposition of the abstracted matter over the floor of the
+ocean.<br>
+Of course, this assumption is not justifiable. It is certain
+that<br>
+the rate of deposition on the floor of the sea has varied<br>
+enormously with various conditions&mdash;principally with the
+depth.<br>
+Again, it must be remembered that this estimate takes no
+account<br>
+of solid materials otherwise brought into the oceanic
+deposits;<br>
+_e.g._, by wind-transported dust from the land or volcanic<br>
+ejectamenta in the ocean depths. It is not probable, however,<br>
+that any considerable addition to the estimated mean depth of<br>
+deposit from such sources would be allowable.</p>
+<p>49</p>
+<p>The greatness of the quantities involved in these
+determinations<br>
+is almost awe inspiring. Take the case of the dissolved salts
+in<br>
+the ocean. They are but a fraction, as we have seen, of the
+total<br>
+results of solvent denudation and represent the integration
+of<br>
+the minute traces contributed by the river water. Yet the
+common<br>
+salt (chloride of sodium) alone, contained in the ocean,
+would,<br>
+if abstracted and spread over the dry land as a layer of rock<br>
+salt having a specific gravity of 2.2, cover the whole to a
+depth<br>
+of 107 metres or 354 feet. The total salts in solution in the<br>
+ocean similarly spread over the land would increase the depth
+of<br>
+the layer to 460 feet. After considering what this means we
+have<br>
+to remember that this amount of matter now in solution in the<br>
+seas is, in point of fact, less than a fifth part of the
+total<br>
+dissolved from the rocks during geological time.</p>
+<p>The transport by denudation of detrital and dissolved matter
+from<br>
+the land to the ocean has had a most important influence on
+the<br>
+events of geological history. The existing surface features
+of<br>
+the earth must have been largely conditioned by the dynamical<br>
+effects arising therefrom. In dealing with the subject of<br>
+mountain genesis we will, elsewhere, see that all the great<br>
+mountain ranges have originated in the accumulation of the<br>
+detrital sediments near the shore in areas which, in
+consequence<br>
+of the load, gradually became depressed and developed into<br>
+synclines of many thousands of feet in depth. The most
+impressive<br>
+surface features of the Globe originated</p>
+<p>50</p>
+<p>in this manner. We will see too that these events were of
+a<br>
+rhythmic character; the upraising of the mountains involving<br>
+intensified mechanical denudation over the elevated area and
+in<br>
+this way an accelerated transport of detritus to the sea; the<br>
+formation of fresh deposits; renewed synclinal sinking of the
+sea<br>
+floor, and, finally, the upheaval of a younger mountain
+range.<br>
+This extraordinary sequence of events has been determined by
+the<br>
+events of detrital denudation acting along with certain
+general<br>
+conditions which have all along involved the growth of<br>
+compressive stresses in the surface crust of the Earth.</p>
+<p>The effects of purely solvent denudation are less easily
+traced,<br>
+but, very probably, they have been of not less importance. I<br>
+refer here to the transport from the land to the sea of matter
+in<br>
+solution.</p>
+<p>Solvent denudation, as observed above, takes place mainly in
+the<br>
+soils and in this way over the more level continental areas.
+It<br>
+has resulted in the removal from the land and transfer to the<br>
+ocean of an amount of matter which represents a uniform layer
+of<br>
+one half a kilometre; that is of more than 1,600 feet of
+rock.<br>
+The continents have, during geological time, been lightened
+to<br>
+this extent. On the other hand all this matter has for the<br>
+greater part escaped the geosynclines and become uniformly<br>
+diffused throughout the ocean or precipitated over its floor<br>
+principally on the continental slopes before the great depths
+are<br>
+reached. Of this material the ocean</p>
+<p>51</p>
+<p>waters contain in solution an amount sufficient to increase
+their<br>
+specific gravity by 2.7 per cent.</p>
+<p>Taking the last point first, it is interesting to note the<br>
+effects upon the bulk of the ocean which has resulted from
+the<br>
+matter dissolved in it. From the known density of average sea<br>
+water we find that 100 ccs. of it weigh just 102.7 grammes.
+Of<br>
+this 3.5 per cent. by weight are solids in solution. That is
+to<br>
+say, 3.594 grammes. Hence the weight of water present is 99.1<br>
+grammes, or a volume of 99.1 ccs. From this we see that the
+salts<br>
+present have increased the volume by 0.9 ccs. or 0.9 per
+cent.</p>
+<p>The average depth of the ocean is 2,000 fathoms or 3,700
+metres.<br>
+The increase of depth due to salts dissolved in the ocean has<br>
+been, therefore, 108 feet or 33.24 metres. This result
+assumes<br>
+that there has been no increased elastic compression due to
+the<br>
+increased pressure, and no change of compressional elastic<br>
+properties. We may be sure that the rise on the shore line of
+the<br>
+land has not been less than 100 feet.</p>
+<p>We see then that as the result of solvent denudation we have
+to<br>
+do with a heavier and a deeper ocean, expanded in volume by<br>
+nearly one per cent. and the floor of which has become raised,
+on<br>
+an average, about 700 feet by precipitated sediment.</p>
+<p>One of the first conceptions, which the student of geology has
+to<br>
+dismiss from his mind, is that of the immobility or rigidity
+of<br>
+the Earth's crust. The lane, we live on sways even to the
+gentle<br>
+rise and fail of ocean tides</p>
+<p>52</p>
+<p>around the coasts. It suffers its own tidal oscillations due
+to<br>
+the moon's attractions. Large tracts of semi-liquid matter<br>
+underlie it. There is every evidence that the raised features
+of<br>
+the Globe are sustained by such pressures acting over other
+and<br>
+adjacent areas as serve to keep them in equilibrium against
+the<br>
+force of gravity. This state of equilibrium, which was first<br>
+recognised by Pratt, as part of the dynamics of the Earth's<br>
+crust, has been named isostasy. The state of the crust is that
+of<br>
+"mobile equilibrium."</p>
+<p>The transfer of matter from the exposed land surfaces to
+the<br>
+sub-oceanic slopes of the continents and the increase in the<br>
+density of the ocean, must all along have been attended by<br>
+isostatic readjustment. We cannot take any other view. On the
+one<br>
+hand the land was being lightened; on the other the sea was<br>
+increasing in mass and depth and the flanks of the continents<br>
+were being loaded with the matter removed from the land and
+borne<br>
+in solution to the ocean. How important the resulting
+movements<br>
+must have been may be gathered from the fact that the
+existing<br>
+land of the Globe stands at a mean elevation of no more than<br>
+2,000 feet above sea level. We have seen that solvent
+denudation<br>
+removed over 1,600 feet of rock. But we have no evidence that
+on<br>
+the whole the elevation of land in the past was ever very<br>
+different from what it now is.</p>
+<p>We have, then, presented to our view the remarkable fact
+that<br>
+throughout the past, and acting with extreme</p>
+<p>53</p>
+<p>slowness, the land has steadily been melted down into the sea
+and<br>
+as steadily been upraised from the waters. It is possible
+that<br>
+the increased bulk of the ocean has led to a certain
+diminution<br>
+of the exposed land area. The point is a difficult one. One
+thing<br>
+we may without much risk assume. The sub-aereal current of<br>
+dissolved matter from the land to the ocean was accompanied by
+a<br>
+sub-crustal flux from the ocean areas to the land areas; the<br>
+heated viscous materials creeping from depths far beneath the<br>
+ocean floor to depths beneath the roots of the mountains
+which<br>
+arose around the oceans. Such movements took ages for their<br>
+accomplishment. Indeed, they have been, probably, continuous
+all<br>
+along and are still proceeding. A low degree of viscosity
+will<br>
+suffice to permit of movements so slow. Superimposed upon
+these<br>
+movements the rhythmic alternations of depression and
+elevation<br>
+of the geosynclines probably resulted in releasing the crust
+from<br>
+local accumulation of strains arising in the more rigid
+surface<br>
+materials. The whole sequence of movements presents an<br>
+extraordinary picture of pseudo-vitality&mdash;reminding us of
+the<br>
+circulatory and respiratory systems of a vast organism.</p>
+<p>All great results in our universe are founded in motions
+and<br>
+forces the most minute. In contemplating the Cause or the
+Effect<br>
+we stand equally impressed with the spectacle presented to us.
+We<br>
+shall now turn from the great effects of denudation upon the<br>
+history and evolution of a world and consider for a moment<br>
+activities</p>
+<p>54</p>
+<p>so minute in detail that their operations will probably for
+ever<br>
+elude our bodily senses, but which nevertheless have
+necessarily<br>
+affected and modified the great results we have been<br>
+considering.</p>
+<p>The ocean a little way from the land is generally so free
+from<br>
+suspended sediments that it has a blackness as of ink. This<br>
+blackness is due to its absolute freedom from particles<br>
+reflecting the sun's light. The beautiful blue of the Swiss
+and<br>
+Italian lakes is due to the presence of very fine particles<br>
+carried into them by the rivers; the finest flour of the<br>
+glaciers, which remain almost indefinitely suspended in the<br>
+water. But in the ocean it is only in those places where
+rapid<br>
+currents running over shallows stir continually the sediments
+or<br>
+where the fresh water of a great river is carried far from
+the<br>
+land, that the presence of silt is to be observed. The
+beautiful<br>
+phenomenon of the coal-black sea is familiar to every
+yachtsman<br>
+who has sailed to the west of our Islands.[1]</p>
+<p>There is, in fact, a very remarkable difference in the manner
+of<br>
+settlement of fine sediments in salt and in fresh water. We
+are<br>
+here brought into contact with one of those subtle yet<br>
+influential natural actions the explanation of which involves<br>
+scientific advance along many apparently unconnected lines of<br>
+investigation.</p>
+<p>[1] See Tyndall's Voyage to Algeria in _Fragments of Science._
+The<br>
+cause of the blue colour of the lakes has been discussed by<br>
+various observers, not always with agreement.</p>
+<p>55</p>
+<p>It is easy to observe in the laboratory the fact of the
+different<br>
+behaviour of salt and fresh water towards finely divided<br>
+substances. The nature of the insoluble substance is not<br>
+important.</p>
+<p>We place, in a good light, two glass vessels of equal
+dimensions;<br>
+the one filled with sea water, the other with fresh water.
+Into<br>
+each we stir the same weight of very finely powdered slate:
+just<br>
+so much as will produce a cloudiness. In a few hours we find
+the<br>
+sea water limpid. The fresh water is still cloudy, however;
+and,<br>
+indeed, may be hardly different in appearance from what it was
+at<br>
+starting. In itself this is a most extraordinary experiment.
+We<br>
+would have anticipated quite the opposite result owing to the<br>
+greater density of the sea water.</p>
+<p>But a still more interesting experiment remains to be
+carried<br>
+out. In the sea water we have many different salts in
+solution.<br>
+Let us see if these salts are equally responsible for the
+result<br>
+we have obtained. For this purpose we measure out quantities
+of<br>
+sodium chloride and magnesium chloride in the proportion in
+which<br>
+they exist in sea water: that is about as seven to one. We
+add<br>
+such an equal amount of water to each as represents the
+dilution<br>
+of these salts in sea water. Then finally we stir a little of
+the<br>
+finely powdered slate into each. It will be found that the<br>
+magnesium chloride, although so much more dilute than the
+sodium<br>
+chloride, is considerably more active in clearing out the<br>
+suspension. We may now try such marine salts as magnesium<br>
+sulphate,</p>
+<p>56</p>
+<p>or calcium sulphate against sodium chloride; keeping the
+marine<br>
+proportions. Again we find that the magnesium and calcium
+salts<br>
+are the most effective, although so much more dilute than the<br>
+sodium salt.</p>
+<p>There is no visible clue to the explanation of these results.
+But<br>
+we must conclude as most probable that some action is at work
+in<br>
+the sea water and in the salt solutions which clumps or<br>
+flocculates the sediment. For only by the gathering of the<br>
+particles together in little aggregates can we explain their<br>
+rapid fall to the bottom. It is not a question of viscosity<br>
+(_i.e._ of resistance to the motion of the particles), for
+the<br>
+salt solutions are rather more viscous than the fresh water.<br>
+Still more remarkable is the fact that every dissolved
+substance<br>
+will not bring about the result. Thus if we dissolve sugar in<br>
+water we find that, if anything, the silt settles more slowly
+in<br>
+the sugar solution than in fresh water.</p>
+<p>Now there is one effect produced by the solution of such salts
+as<br>
+we have dealt with which is not produced by such bodies as
+sugar.<br>
+The water is rendered a conductor of electricity. Long ago<br>
+Faraday explained this as due to the presence of free atoms
+of<br>
+the dissolved salt in the solution, carrying electric charges.
+We<br>
+now speak of the salt as "ionised." That is it is partly split
+up<br>
+into ions or free electrified atoms of chlorine, sodium,<br>
+magnesium, etc., according to the particular salt in
+solution.<br>
+This fact leads us to think that these electrified</p>
+<p>57</p>
+<p>atoms moving about in the solution may be the cause of the<br>
+clumping or flocculation. Such electrified atoms are absent
+from<br>
+the sugar solution: sugar does not become "ionised" when it
+is<br>
+dissolved.</p>
+<p>The suspicion that the free electrified atoms play a part in
+the<br>
+phenomenon is strengthened when we recall the remarkable<br>
+difference in the action of sodium chloride and magnesium<br>
+chloride. In each of the solutions of these substances there
+are<br>
+free chlorine atoms each of which carries a single charge of<br>
+negative electricity. As these atoms are alike in both
+solutions<br>
+the different behaviour of the solutions cannot be due to the<br>
+chlorine. But the metallic atom is very different in the two<br>
+cases. The ionised sodium atom is known to be _monad_ or
+carries<br>
+but _one_ positive charge; whereas the magnesium atom is _diad_
+and<br>
+carries _two_ positive charges. If, then, we assume that the<br>
+metallic, positively electrified atom is in each case<br>
+responsible, we have something to go on. It may be now stated<br>
+that it has been found by experiment and supported by theory
+that<br>
+the clumping power of an ion rises very rapidly with its
+valency;<br>
+that is with the number of unit charges associated with it.
+Thus<br>
+diads such as magnesium, calcium, barium, etc., are very much<br>
+more efficient than monads such as sodium, potassium, etc.,
+and<br>
+again, triads such as aluminium are, similarly, very much
+more<br>
+powerful than diad atoms. Here, in short, we have arrived at
+the<br>
+active cause of the phenomenon. Its inner mechanism</p>
+<p>58</p>
+<p>is, however, harder to fathom. A plausible explanation can
+be<br>
+offered, but a study of it would take us too far. Sufficient
+has<br>
+been said to show the very subtile nature of the forces at
+work.</p>
+<p>We have here an effect due to the sea salts derived by
+denudation<br>
+from the land which has been slowly augmenting during
+geological<br>
+time. It is certain that the ocean was practically fresh water
+in<br>
+remote ages. During those times the silt from the great
+rivers<br>
+would have been carried very far from the land. A Mississippi
+of<br>
+those ages would have sent its finer suspensions far abroad on
+a<br>
+contemporary Gulf stream: not improbably right across the<br>
+Atlantic. The earlier sediments of argillaceous type were not<br>
+collected in the geosynclines and the genesis of the
+mountains<br>
+was delayed proportionately. But it was, probably, not for
+very<br>
+long that such conditions prevailed. For the accumulation of<br>
+calcium salts must have been rapid, and although the great<br>
+salinity due to sodium salts was of slow growth the salts of
+the<br>
+diad element calcium must have soon introduced the cooperation
+of<br>
+the ion in the work of building the mountain.</p>
+<p>59</p>
+<p><u>THE ABUNDANCE OF LIFE</u> [1]</p>
+<p>WE had reached the Pass of Tre Croci[2]and from a point a
+little<br>
+below the summit, looked eastward over the glorious Val
+Buona.<br>
+The pines which clothed the floor and lower slopes of the
+valley,<br>
+extended their multitudes into the furthest distance, among
+the<br>
+many recesses of the mountains, and into the confluent Val di<br>
+Misurina. In the sunshine the Alpine butterflies flitted from<br>
+stone to stone. The ground at our feet and everywhere
+throughout<br>
+the forests teamed with the countless millions of the small
+black<br>
+ants.</p>
+<p>It was a magnificent display of vitality; of the
+aggressiveness<br>
+of vitality, assailing the barren heights of the limestone,<br>
+wringing a subsistence from dead things. And the question<br>
+suggested itself with new force: why the abundance of life
+and<br>
+its unending activity?</p>
+<p>In trying to answer this question, the present sketch<br>
+originated.</p>
+<p>I propose to refer for an answer to dynamic considerations. It
+is<br>
+apparent that natural selection can only be concerned in a<br>
+secondary way. Natural selection defines</p>
+<p>[1] Proc. Roy. Dublin Soc., vol. vii., 1890.</p>
+<p>[2] In the Dolomites of Southeast Tyrol; during the summer
+of<br>
+1890. Much of what follows was evolved in discussion with my<br>
+fellow-traveller, Henry H. Dixon. Much of it is his.</p>
+<p>60</p>
+<p>a certain course of development for the organism; but very<br>
+evidently some property of inherent progressiveness in the<br>
+organism must be involved. The mineral is not affected by
+natural<br>
+selection to enter on a course of continual variation and<br>
+multiplication. The dynamic relations of the organism with
+the<br>
+environment are evidently very different from those of
+inanimate<br>
+nature.</p>
+<p>GENERAL DYNAMIC CONDITIONS ATTENDING INANIMATE ACTIONS</p>
+<p>It is necessary, in the first place, to refer briefly to
+the<br>
+phenomena attending the transfer of energy within and into<br>
+inanimate material systems. It is not assumed here that these<br>
+phenomena are restricted in their sphere of action to
+inanimate<br>
+nature. It is, in fact, very certain that they are not; but
+while<br>
+they confer on dead nature its own dynamic tendencies, it
+will<br>
+appear that their effects are by various means evaded in
+living<br>
+nature. We, therefore, treat of them as characteristic of<br>
+inanimate actions. We accept as fundamental to all the<br>
+considerations which follow the truth of the principle of the<br>
+Conservation of Energy.[1]</p>
+<p>[1] "The principle of the Conservation of Energy has acquired
+so<br>
+much scientific weight during the last twenty years that no<br>
+physiologist would feel any confidence in an experiment which<br>
+showed a considerable difference between the work done by the<br>
+animal and the balance of the account of Energy received and<br>
+spent."&mdash;Clerk Maxwell, _Nature_, vol. xix., p. 142. See
+also<br>
+Helmholtz _On the Conservation of Force._</p>
+<p>61</p>
+<p>Whatever speculations may be made as to the course of events
+very<br>
+distant from us in space, it appears certain that dissipation
+of<br>
+energy is at present actively progressing throughout our
+sphere<br>
+of observation in inanimate nature. It follows, in fact, from
+the<br>
+second law of thermodynamics, that whenever work is derived
+from<br>
+heat, a certain quantity of heat falls in potential without
+doing<br>
+work or, in short, is dissipated. On the other hand, work may
+be<br>
+entirely converted into heat. The result is the heat-tendency
+of<br>
+the universe. Heat, being an undirected form of energy, seeks,
+as<br>
+it were, its own level, so that the result of this
+heat-tendency<br>
+is continual approach to uniformity of potential.</p>
+<p>The heat-tendency of the universe is also revealed in the<br>
+far-reaching "law of maximum work," which defines that
+chemical<br>
+change, accomplished without the intervention of external
+energy,<br>
+tends to the production of the body, or system of bodies,
+which<br>
+disengage the greatest quantity of heat.[1] And, again, vast<br>
+numbers of actions going on throughout nature are attended by<br>
+dissipatory thermal effects, as those arising from the motions
+of<br>
+proximate molecules (friction, viscosity), and from the fall
+of<br>
+electrical potential.</p>
+<p>Thus, on all sides, the energy which was once most
+probably<br>
+existent in the form of gravitational potential, is being<br>
+dissipated into unavailable forms. We must</p>
+<p>[1] Berthelot, _Essai de M&eacute;canique Chimique._</p>
+<p>62</p>
+<p>recognize dissipation as an inevitable attendant on
+inanimate<br>
+transfer of energy.</p>
+<p>But when we come to consider inanimate actions in relation
+to<br>
+time, or time-rate of change, we find a new feature in the<br>
+phenomena attending transfer of energy; a feature which is
+really<br>
+involved in general statements as to the laws of physical<br>
+interactions.[1] It is seen, that the attitude of inanimate<br>
+material systems is very generally, if not in all cases,<br>
+retardative of change&mdash;opposing it by effects generated by
+the<br>
+primary action, which may be called "secondary" for
+convenience.<br>
+Further, it will be seen that these secondary effects are
+those<br>
+concerned in bringing about the inevitable dissipation.</p>
+<p>As example, let us endeavour to transfer gravitational
+potential<br>
+energy contained in a mass raised above the surface of the
+Earth<br>
+into an elastic body, which we can put into compression by<br>
+resting the weight upon it. In this way work is done against<br>
+elastic force and stored as elastic potential energy. We may
+deal<br>
+with a metal spring, or with a mass of gas contained in a<br>
+cylinder fitted with a piston upon which the weight may be<br>
+placed. In either case we find the effect of compression is
+to<br>
+raise the temperature of the substance, thus causing its</p>
+<p>[1] Helmholtz, _Ice and Glaciers._ Atkinson's collection of
+his<br>
+Popular Lectures. First Series, p.120. Quoted by Tate,
+_Heat_,<br>
+p. 311.</p>
+<p>63</p>
+<p>expansion or increased resistance to the descent of the
+weight.<br>
+And this resistance continues, with diminishing intensity,
+till<br>
+all the heat generated is dissipated into the surrounding
+medium.<br>
+The secondary effect thus delays the final transfer of
+energy.</p>
+<p>Again, if we suppose the gas in the cylinder replaced by a
+vapour<br>
+in a state of saturation, the effect of increased pressure, as
+of<br>
+a weight placed upon the piston, is to reduce the vapour to a<br>
+liquid, thereby bringing about a great diminution of volume
+and<br>
+proportional loss of gravitational potential by the weight.
+But<br>
+this change will by no means be brought about
+instantaneously.<br>
+When a little of the vapour is condensed, this portion parts
+with<br>
+latent heat of vaporisation, increasing the tension of the<br>
+remainder, or raising its point of saturation, so that before
+the<br>
+weight descends any further, this heat has to escape from the<br>
+cylinder.</p>
+<p>Many more such cases might be cited. The heating of
+india-rubber<br>
+when expanded, its cooling when compressed, is a remarkable
+one;<br>
+for at first sight it appears as if this must render it<br>
+exceptional to the general law, most substances exhibiting
+the<br>
+opposite thermal effects when stressed. However, here, too,
+the<br>
+action of the stress is opposed by the secondary effects<br>
+developed in the substance; for it is found that this
+substance<br>
+contracts when heated, expands when cooled. Again, ice being
+a<br>
+substance which contracts in melting, the effect of pressure
+is<br>
+to facilitate melting, lowering its freezing point. But</p>
+<p>64</p>
+<p>so soon as a little melting occurs, the resulting liquid calls
+on<br>
+the residual ice for an amount of heat equivalent to the
+latent<br>
+heat of liquefaction, and so by cooling the whole, retards
+the<br>
+change.</p>
+<p>Such particular cases illustrate a principle controlling
+the<br>
+interaction of matter and energy which seems universal in<br>
+application save when evaded, as we shall see, by the
+ingenuity<br>
+of life. This principle is not only revealed in the researches
+of<br>
+the laboratory; it is manifest in the history of worlds and
+solar<br>
+systems. Thus, consider the effects arising from the
+aggregation<br>
+of matter in space under the influence of the mutual
+attraction<br>
+of the particles. The tendency here is loss of gravitational<br>
+potential. The final approach is however retarded by the<br>
+temperature, or vis viva of the parts attending collision and<br>
+compression. From this cause the great suns of space radiate
+for<br>
+ages before the final loss of potential is attained.</p>
+<p>Clerk Maxwell[1] observes on the general principle that
+less<br>
+force is required to produce a change in a body when the
+change<br>
+is unopposed by constraints than when it is subjected to
+such.<br>
+From this if we assume the external forces acting upon a
+system<br>
+not to rise above a certain potential (which is the order of<br>
+nature), the constraints of secondary actions may, under
+certain<br>
+circumstances, lead to final rejection of some of the energy,
+or,<br>
+in any</p>
+<p>[1] _Theory of Heat_, p. 131.</p>
+<p>65</p>
+<p>case, to retardation of change in the system&mdash;dissipation
+of<br>
+energy being the result.[1]</p>
+<p>As such constraints seem inherently present in the properties
+of<br>
+matter, we may summarise as follows:</p>
+<p>_The transfer of energy into any inanimate material system
+is<br>
+attended by effects retardative to the transfer and conducive
+to<br>
+dissipation._</p>
+<p>Was this the only possible dynamic order ruling in
+material<br>
+systems it is quite certain the myriads of ants and pines
+never<br>
+could have been, except all generated by creative act at vast<br>
+primary expenditure of energy. Growth and reproduction would
+have<br>
+been impossible in systems which retarded change at every
+step<br>
+and never proceeded in any direction but in that of
+dissipation.<br>
+Once created, indeed, it is conceivable that, as heat
+engines,<br>
+they might have dragged out an existence of alternate life
+and<br>
+death; life in the hours of sunshine, death in hours of
+darkness:<br>
+no final death, however, their lot, till their parts were
+simply<br>
+worn out by long use, never made good by repair. But the<br>
+sustained and increasing activity of organized nature is a
+fact;<br>
+therefore some other order of events must be possible.</p>
+<p>[1] The law of Least Action, which has been applied, not alone
+in<br>
+optics, but in many mechanical systems, appears physically
+based<br>
+upon the restraint and retardation opposing the transfer of<br>
+energy in material systems.</p>
+<p>66</p>
+<p>GENERAL DYNAMIC CONDITIONS ATTENDING ANIMATE ACTIONS</p>
+<p>What is the actual dynamic attitude of the primary organic<br>
+engine&mdash;the vegetable organism? We consider, here, in the
+first<br>
+place, not intervening, but resulting phenomena.</p>
+<p>The young leaf exposed to solar radiation is small at first,
+and<br>
+the quantity of radiant energy it receives in unit of time
+cannot<br>
+exceed that which falls upon its surface. But what is the
+effect<br>
+of this energy? Not to produce a retardative reaction, but an<br>
+accelerative response: for, in the enlarging of the leaf by<br>
+growth, the plant opens for itself new channels of supply.</p>
+<p>If we refer to "the living protoplasm which, with its
+unknown<br>
+molecular arrangement, is the only absolute test of the cell
+and<br>
+of the organism in general,[1] we find a similar attitude
+towards<br>
+external sources of available energy. In the act of growth<br>
+increased rate of assimilation is involved, so that there is
+an<br>
+acceleration of change till a bulk of maximum activity is<br>
+attained. The surface, finally, becomes too small for the<br>
+absorption of energy adequate to sustain further increase of
+mass<br>
+(Spencer[2]), and the acceleration ceases. The waste going on
+in<br>
+the central parts is then just balanced by the renewal at the<br>
+surface. By division, by spreading of the mass, by</p>
+<p>[1] Claus, _Zoology_, p. 13.</p>
+<p>[2] Geddes and Thomson, _The Evolution of Sex_, p. 220.</p>
+<p>67</p>
+<p>out-flowing processes, the normal activity of growth may
+be<br>
+restored. Till this moment nothing would be gained by any of<br>
+these changes. One or other of them is now conducive to<br>
+progressive absorption of energy by the organism, and one or<br>
+other occurs, most generally the best of them, subdivision.
+Two<br>
+units now exist; the total mass immediately on division is<br>
+unaltered, but paths for the more abundant absorption of
+energy<br>
+are laid open.</p>
+<p>The encystment of the protoplasm (occurring under conditions
+upon<br>
+which naturalists do not seem agreed[1]) is to all appearance<br>
+protective from an unfavourable environment, but it is often
+a<br>
+period of internal change as well, resulting in a segregation<br>
+within the mass of numerous small units, followed by a breakup
+of<br>
+the whole into these units. It is thus an extension of the
+basis<br>
+of supply, and in an impoverished medium, where unit of
+surface<br>
+is less active, is evidently the best means of preserving a<br>
+condition of progress.</p>
+<p>Thus, in the organism which forms the basis of all modes of
+life,<br>
+a definite law of action is obeyed under various circumstances
+of<br>
+reaction with the available energy of its environment.</p>
+<p>Similarly, in the case of the more complex leaf, we see, not
+only<br>
+in the phenomenon of growth, but in its extension in a
+flattened<br>
+form, and in the orientation of greatest surface towards the<br>
+source of energy, an attitude towards</p>
+<p>[1] However, "In no way comparable with death." Weismann,<br>
+_Biological Memoirs_, p. 158.</p>
+<p>68</p>
+<p>available energy causative of accelerated transfer. There
+is<br>
+seemingly a principle at work, leading to the increase of
+organic<br>
+activity.</p>
+<p>Many other examples might be adduced. The gastrula stage in
+the<br>
+development of embryos, where by invagination such an
+arrangement<br>
+of the multiplying cells is secured as to offer the greatest<br>
+possible surface consistent with a first division of labour;
+the<br>
+provision of cilia for drawing upon the energy supplies of
+the<br>
+medium; and more generally the specialisation of organs in
+the<br>
+higher developments of life, may alike be regarded as efforts
+of<br>
+the organism directed to the absorption of energy. When any<br>
+particular organ becomes unavailing in the obtainment of<br>
+supplies, the organ in the course of time becomes aborted or<br>
+disappears.[1] On the other hand, when a too ready and
+liberal<br>
+supply renders exertion and specialisation unnecessary, a
+similar<br>
+abortion of functionless organs takes place. This is seen in
+the<br>
+degraded members of certain parasites.</p>
+<p>During certain epochs of geological history, the vegetable
+world<br>
+developed enormously; in response probably to liberal supplies
+of<br>
+carbon dioxide. A structural adaptation to the rich
+atmosphere<br>
+occurred, such as was calculated to cooperate in rapidly<br>
+consuming the supplies, and to this obedience to a law of<br>
+progressive transfer of energy we owe the vast stores of
+energy<br>
+now accumulated</p>
+<p>[1] Claus, _Zoology_, p. 157</p>
+<p>69</p>
+<p>in our coal fields. And when, further, we reflect that this
+store<br>
+of energy had long since been dissipated into space but for
+the<br>
+intervention of the organism, we see definitely another factor
+in<br>
+organic transfer of energy&mdash;a factor acting conservatively
+of<br>
+energy, or antagonistically to dissipation.</p>
+<p>The tendency of organized nature in the presence of
+unlimited<br>
+supplies is to "run riot." This seems so universal a
+relation,<br>
+that we are safe in seeing here cause and effect, and in
+drawing<br>
+our conclusions as to the attitude of the organism towards<br>
+available energy. New species, when they come on the field of<br>
+geological history, armed with fresh adaptations,
+irresistible<br>
+till the slow defences of the subjected organisms are
+completed,<br>
+attain enormous sizes under the stimulus of abundant supply,
+till<br>
+finally, the environment, living and dead, reacts upon them
+with<br>
+restraining influence. The exuberance of the organism in
+presence<br>
+of energy is often so abundant as to lead by deprivation to
+its<br>
+self-destruction. Thus the growth of bacteria is often
+controlled<br>
+by their own waste products. A moment's consideration shows
+that<br>
+such progressive activity denotes an accelerative attitude on
+the<br>
+part of the organism towards the transfer of energy into the<br>
+organic material system. Finally, we are conscious in
+ourselves<br>
+how, by use, our faculties are developed; and it is apparent
+that<br>
+all such progressive developments must rest on actions which<br>
+respond to supplies with fresh demands. Possibly in the
+present<br>
+and ever-</p>
+<p>70</p>
+<p>increasing consumption of inanimate power by civilised races,
+we<br>
+see revealed the dynamic attitude of the organism working
+through<br>
+thought-processes.</p>
+<p>Whether this be so or not, we find generally in organised
+nature<br>
+causes at work which in some way lead to a progressive
+transfer<br>
+of energy into the organic system. And we notice, too, that
+all<br>
+is not spent, but both immediately in the growth of the<br>
+individual, and ultimately in the multiplication of the
+species,<br>
+there are actions associated with vitality which retard the<br>
+dissipation of energy. We proceed to state the dynamical<br>
+principles involved in these manifestations, which appear<br>
+characteristic of the organism, as follows:&mdash;</p>
+<p>_The transfer of energy into any animate material system
+is<br>
+attended by effects conducive to the transfer, and retardative
+of<br>
+dissipation._</p>
+<p>This statement is, I think, perfectly general. It has been
+in<br>
+part advanced before, but from the organic more than the
+physical<br>
+point of view. Thus, "hunger is an essential characteristic
+of<br>
+living matter"; and again, "hunger is a dominant
+characteristic<br>
+of living matter,"[1] are, in part, expressions of the
+statement.<br>
+If it be objected against the generality of the statement,
+that<br>
+there are periods in the life of individuals when stagnation
+and<br>
+decay make their appearance, we may answer, that</p>
+<p>[1] _Evolution of Sex._ Geddes and Thomson, chap. xvi. See
+also a<br>
+reference to Cope's theory of "Growth Force," in Wallace's<br>
+_Darwinism_, p. 425.</p>
+<p>71</p>
+<p>such phenomena arise in phases of life developed under
+conditions<br>
+of external constraint, as will be urged more fully further
+on,<br>
+and that in fact the special conditions of old age do not and<br>
+cannot express the true law and tendency of the dynamic
+relations<br>
+of life in the face of its evident advance upon the Earth.
+The<br>
+law of the unconstrained cell is growth on an ever increasing<br>
+scale; and although we assume the organic configuration,
+whether<br>
+somatic or reproductive, to be essentially unstable, so that<br>
+continual inflow of energy is required merely to keep it in<br>
+existence, this does not vitiate the fact that, when free of
+all<br>
+external constraint, growth gains on waste. Indeed, even in
+the<br>
+case of old age, the statement remains essentially true, for
+the<br>
+phenomena then displayed point to a breakdown of the
+functioning<br>
+power of the cell, an approximation to configurations
+incapable<br>
+of assimilation. It is not as if life showed in these
+phenomena<br>
+that its conditions could obtain in the midst of abundance,
+and<br>
+yet its law be suspended; but as if they represented a<br>
+degradation of the very conditions of life, a break up, under
+the<br>
+laws of the inanimate, of the animate contrivance; so that
+energy<br>
+is no longer available to it, or the primary condition, "the<br>
+transfer of energy into the animate system," is imperfectly<br>
+obeyed. It is to the perfect contrivance of life our
+statement<br>
+refers.</p>
+<p>That the final end of all will be general non-availability
+there<br>
+seems little reason to doubt, and the organism, itself
+dependent<br>
+upon differences of potential, cannot</p>
+<p>72</p>
+<p>hope to carry on aggregation of energy beyond the period
+when<br>
+differences of potential are not. The organism is not
+accountable<br>
+for this. It is being affected by events external to it, by
+the<br>
+actions going on through inanimate agents. And although there
+be<br>
+only a part of the received energy preserved, there is a part<br>
+preserved, and this amount is continually on the increase. To
+see<br>
+this it is only necessary to reflect that the sum of animate<br>
+energy&mdash;capability of doing work in any way through
+animate<br>
+means&mdash;at present upon the Earth, is the result, although a
+small<br>
+one, of energy reaching the Earth since a remote period, and<br>
+which otherwise had been dissipated in space. In inanimate<br>
+actions throughout nature, as we know it, the availability is<br>
+continually diminishing. The change is all the one way. As,<br>
+however, the supply of available energy in the universe is<br>
+(probably) limited in amount, we must look upon the two as
+simply<br>
+effecting the final dissipation of potential in very
+different<br>
+ways. The animate system is aggressive on the energy available
+to<br>
+it, spends with economy, and invests at interest till death<br>
+finally deprives it of all. It has heirs, indeed, who inherit<br>
+some of its gains, but they, too, must die, and ultimately
+there<br>
+will be no successors, and the greater part must melt away as
+if<br>
+it had never been. The inanimate system responds to the
+forces<br>
+imposed upon it by sluggish changes; of that which is thrust
+upon<br>
+it, it squanders uselessly. The path of the energy is very<br>
+different in the two cases.</p>
+<p>73</p>
+<p>While it is true generally that both systems ultimately result
+in<br>
+the dissipation of energy to uniform potential, the organism
+can,<br>
+as we have seen, under particular circumstances evade the
+final<br>
+doom altogether. It can lay up a store of potential energy
+which<br>
+may be permanent. Thus, so long as there is free oxygen in
+the<br>
+universe, our coalfields might, at any time in the remote
+future,<br>
+generate light and heat in the universal grave.</p>
+<p>It is necessary to observe on the fundamental distinction
+between<br>
+the growth of the protoplasm and the growth of the crystal. It
+is<br>
+common to draw comparison between the two, and to point to<br>
+metabolism as the chief distinction. But while this is the
+most<br>
+obvious distinction the more fundamental one remains in the<br>
+energy relations of the two with the environment.[1] The
+growth<br>
+of the crystal is the result of loss of energy; that of the<br>
+organism the result of gain of energy. The crystal represents
+a<br>
+last position of stable equilibrium assumed by molecules upon
+a<br>
+certain loss of kinetic energy, and the formation of the
+crystal<br>
+by evaporation and concentration of a liquid does not, in its<br>
+dynamic aspect, differ much from the precipitation of an<br>
+amorphous sediment. The organism, on the other hand, represents
+a<br>
+more or less unstable condition formed and maintained by
+inflow<br>
+of energy; its formation, indeed, often attended with a loss
+of<br>
+kinetic energy (fixation of carbon in plants), but, if so,<br>
+accompanied by</p>
+<p>[1] It appears exceptional for the crystal line configuration
+to<br>
+stand higher in the scale of energy than the amorphous.</p>
+<p>74</p>
+<p>a more than compensatory increase of potential molecular
+energy.</p>
+<p>Thus, between growth in the living world and growth in the
+dead<br>
+world, the energy relations with the environment reveal a
+marked<br>
+contrast. Again, in the phenomena of combustion, there are<br>
+certain superficial resemblances which have led to comparison<br>
+between the two. Here again, however, the attitudes towards
+the<br>
+energy of the environment stand very much as + and -. The
+life<br>
+absorbs, stores, and spends with economy. The flame only<br>
+recklessly spends. The property of storage by the organism
+calls<br>
+out a further distinction between the course of the two<br>
+processes. It secures that the chemical activity of the
+organism<br>
+can be propagated in a medium in which the supply of energy
+is<br>
+discontinuous or localised. The chemical activity of the<br>
+combustion can, strictly speaking, only be propagated among<br>
+contiguous particles. I need not dwell on the latter fact; an<br>
+example of the former is seen in the action of the roots of<br>
+plants, which will often traverse a barren place or circumvent
+an<br>
+obstacle in their search for energy. In this manner roots
+will<br>
+find out spots of rich nutriment.</p>
+<p>Thus there is a dynamic distinction between the progress of
+the<br>
+organism and the progress of the combustion, or of the
+chemical<br>
+reaction generally. And although there be unstable chemical<br>
+systems which absorb energy during reaction, these are<br>
+(dynamically) no more than the expansion of the compressed
+gas.<br>
+There is a certain</p>
+<p>75</p>
+<p>initial capacity in the system for a given quantity of
+energy;<br>
+this satisfied, progress ceases. The progress of the organism
+in<br>
+time is continual, and goes on from less to greater so long
+as<br>
+its development is unconstrained and the supply of energy is<br>
+unlimited.</p>
+<p>We must regard the organism as a configuration which is so<br>
+contrived as to evade the tendency of the universal laws of<br>
+nature. Except we are prepared to believe that a violation of
+the<br>
+second law of thermodynamics occurs in the organism, that a<br>
+"sorting demon" is at work within it, we must, I think,
+assume<br>
+that the interactions going on among its molecules are<br>
+accompanied by retardation and dissipation like the rest of<br>
+nature. That such conditions are not incompatible with the<br>
+definition of the dynamic attitude of the organism, can be
+shown<br>
+by analogy with our inanimate machines which, by aid of<br>
+hypotheses in keeping with the second law of thermodynamics,
+may<br>
+be supposed to fulfil the energy-functions of the plant or<br>
+animal, and, in fact, in all apparent respects conform to the<br>
+definition of the organism.</p>
+<p>We may assume this accomplished by a contrivance of the nature
+of<br>
+a steam-engine, driven by solar energy. It has a boiler, which
+we<br>
+may suppose fed by the action of the engine. It has piston,<br>
+cranks, and other movable parts, all subject to resistance
+from<br>
+friction, etc. Now there is no reason why this engine should
+not<br>
+expend its surplus energy in shaping, fitting, and starting
+into<br>
+action other engines:&mdash;in fact, in reproductive sacrifice.
+All</p>
+<p>76</p>
+<p>these other engines represent a multiplied absorption of
+energy<br>
+as the effects of the energy received by the parent engine,
+and<br>
+may in time be supposed to reproduce themselves. Further, we
+may<br>
+suppose the parent engine to be small and capable of
+developing<br>
+very little power, but the whole series as increasing in power
+at<br>
+each generation. Thus the primary energy relations of the<br>
+vegetable organism are represented in these engines, and no<br>
+violation of the second law of thermodynamics involved.</p>
+<p>We might extend the analogy, and assuming these engines to
+spend<br>
+a portion of their surplus energy in doing work against
+chemical<br>
+forces&mdash;as, for example, by decomposing water through
+the<br>
+intervention of a dynamo&mdash;suppose them to lay up in this way
+a<br>
+store of potential energy capable of heating the boilers of a<br>
+second order of engines, representing the graminivorous
+animal.<br>
+It is obvious without proceeding to a tertiary or carnivorous<br>
+order, that the condition of energy in the animal world may
+be<br>
+supposed fulfilled in these successive series of engines, and
+no<br>
+violation of the principles governing the actions going on in
+our<br>
+machines assumed. Organisms evolving on similar principles
+would<br>
+experience loss at every transfer. Thus only a portion of the<br>
+radiant energy absorbed by the leaf would be expended in
+actual<br>
+work, chemical and gravitational, etc. It is very certain
+that<br>
+this is, in fact, what takes place.</p>
+<p>It is, perhaps, worth passing observation that, from the<br>
+nutritive dependence of the animal upon the vegetable,</p>
+<p>77</p>
+<p>and the fact that a conversion of the energy of the one to
+the<br>
+purposes of the other cannot occur without loss, the mean
+energy<br>
+absorbed daily by the vegetable for the purpose of growth
+must<br>
+greatly exceed that used in animal growth; so that the
+chemical<br>
+potential energy of vegetation upon the earth is much greater<br>
+than the energy of all kinds represented in the animal<br>
+configurations.[1] It appears, too, that in the power
+possessed<br>
+by the vegetable of remaining comparatively inactive, of<br>
+surviving hard times by the expenditure and absorption of but<br>
+little, the vegetable constitutes a veritable reservoir for
+the<br>
+uniform supply of the more unstable and active animal.</p>
+<p>Finally, on the question of the manner of origin of
+organic<br>
+systems, it is to be observed that, while the life of the
+present<br>
+is very surely the survival of the fittest of the tendencies
+and<br>
+chances of the past, yet, in the initiation of the organised<br>
+world, a single chance may have decided a whole course of
+events:<br>
+for, once originated, its own law secures its increase,
+although<br>
+within the new order of actions, the law of the fittest must<br>
+assert itself. That such a progressive material system as an<br>
+organism was possible, and at some remote period was
+initiated,<br>
+is matter of knowledge; whether or not the initiatory living<br>
+configuration was rare and fortuitous, or the probable result
+of<br>
+the general action of physical laws acting among innumerable<br>
+chances, must remain matter of</p>
+<p>[1] I find a similar conclusion arrived at in Semper's
+_Animal<br>
+Life_, p. 52.</p>
+<p>78</p>
+<p>speculation. In the event of the former being the truth, it
+is<br>
+evidently possible, in spite of a large finite number of<br>
+habitable worlds, that life is non-existent elsewhere. If the<br>
+latter is the truth, it is almost certain that there is life
+in<br>
+all, or many of those worlds.</p>
+<p>EVOLUTION AND ACCELERATION OF ACTIVITY</p>
+<p>The primary factor in evolution is the "struggle for
+existence."<br>
+This involves a "natural selection" among the many variations
+of<br>
+the organism. If we seek the underlying causes of the
+struggle,<br>
+we find that the necessity of food and (in a lesser degree)
+the<br>
+desire for a mate are the principal causes of contention. The<br>
+former is much the more important factor, and, accordingly,
+we<br>
+find the greater degree of specialisation based upon it.</p>
+<p>The present view assumes a dynamic necessity for its
+demands<br>
+involved in the nature of the organism as such. This
+assumption<br>
+is based on observation of the outcome of its unconstrained<br>
+growth, reproduction, and life-acts. We have the same right
+to<br>
+assert this of the organism as we have to assert that
+retardation<br>
+and degradation attend the actions of inanimate machines,
+which<br>
+assertion, also, is based on observation of results. Thus we
+pass<br>
+from the superficial statements that organisms require food
+in<br>
+order to live, or that organisms desire food, to the more<br>
+fundamental one that:</p>
+<p>_The organism is a configuration of matter which absorbs
+energy<br>
+acceleratively, without limit, when unconstrained._</p>
+<p>79</p>
+<p>This is the dynamic basis for a "struggle for existence."
+The<br>
+organism being a material system responding to accession of<br>
+energy with fresh demands, and energy being limited in
+amount,<br>
+the struggle follows as a necessity. Thus, evolution guiding'
+the<br>
+steps of the energy-seeking organism, must presuppose and
+find<br>
+its origin in that inherent property of the organism which<br>
+determines its attitude in presence of available energy.</p>
+<p>Turning to the factor, "adaptation," we find that this also
+must<br>
+presuppose, in order to be explicable, some quality of<br>
+aggressiveness on the part of the organism. For adaptation in<br>
+this or that direction is the result of repulse or victory,
+and,<br>
+therefore, we must presuppose an attack. The attack is made
+by<br>
+the organism in obedience to its law of demand; we see in the<br>
+adaptation of the organism but the accumulated wisdom derived<br>
+from past defeats and victories.</p>
+<p>Where the environment is active, that is living,
+adaptation<br>
+occurs on both sides. Improved means of defence or improved
+means<br>
+of attack, both presuppose activity. Thus the reactions to
+the<br>
+environment, animate and inanimate, are at once the outcome
+of<br>
+the eternal aggressiveness of the organism, and the source of<br>
+fresh aggressiveness upon the resources of the medium.</p>
+<p>As concerns the "survival of the fittest" (or "natural<br>
+selection"), we can, I think, at once conclude that the
+organism<br>
+which best fulfils the organic law under the circumstances of<br>
+supply is the "fittest," _ipso facto._ In many</p>
+<p>80</p>
+<p>cases this is contained in the commonsense consideration, that
+to<br>
+be strong, consistent with concealment from enemies which are<br>
+stronger, is best, as giving the organism mastery over foes
+which<br>
+are weaker, and generally renders it better able to secure<br>
+supplies. Weismann points out that natural selection favours<br>
+early and abundant reproduction. But whether the
+qualifications<br>
+of the "fittest" be strength, fertility, cunning, fleetness,<br>
+imitation, or concealment, we are safe in concluding that
+growth<br>
+and reproduction must be the primary qualities which at once<br>
+determine selection and are fostered by it. Inherent in the<br>
+nature of the organism is accelerated absorption of energy,
+but<br>
+the qualifications of the "fittest" are various, for the
+supply<br>
+of energy is limited, and there are many competitors for it.
+To<br>
+secure that none be wasted is ultimately the object of
+natural<br>
+selection, deciding among the eager competitors what is best
+for<br>
+each.</p>
+<p>In short, the facts and generalisations concerning evolution
+must<br>
+presuppose an organism endowed with the quality of
+progressive<br>
+absorption of energy, and retentive of it. The continuity of<br>
+organic activity in a world where supplies are intermittent
+is<br>
+evidently only possible upon the latter condition. Thus it<br>
+appears that the dynamic attitude of the organism, considered
+in<br>
+these pages, occupies a fundamental position regarding its<br>
+evolution.</p>
+<p>We turn to the consideration of old age and death,
+endeavouring<br>
+to discover in what relation they stand to the innate<br>
+progressiveness of the organism.</p>
+<p>81</p>
+<p>THE PERIODICITY OF THE ORGANISM AND THE LAW OF PROGRESSIVE<br>
+ACTIVITY</p>
+<p>The organic system is essentially unstable. Its aggressive<br>
+attitude is involved in the phenomenon of growth, and in<br>
+reproduction which is a form of growth. But the energy
+absorbed<br>
+is not only spent in growth. It partly goes, also, to make
+good<br>
+the decay which arises from the instability of the organic
+unit.<br>
+The cell is molecularly perishable. It possesses its entity
+much<br>
+as a top keeps erect, by the continual inflow of energy.<br>
+Metabolism is always taking place within it. Any other
+condition<br>
+would, probably, involve the difficulties of perpetual
+motion.</p>
+<p>The phenomenon of old age is not evident in the case of
+the<br>
+unicellular organism reproducing by fission. At any stage of
+its<br>
+history all the individuals are of the same age: all contain
+a<br>
+like portion of the original cell, so far as this can be
+regarded<br>
+as persisting where there is continual flux of matter and
+energy.<br>
+In the higher organisms death is universally evident. Why is<br>
+this?</p>
+<p>The question is one of great complexity. Considered from the
+more<br>
+fundamental molecular point of view we should perhaps look to<br>
+failure of the power of cell division as the condition of<br>
+mortality. For it is to this phenomenon&mdash;that of cell<br>
+division&mdash;that the continued life of the protozoon is to
+be<br>
+ascribed, as we have already seen. Reproduction is, in fact,
+the<br>
+saving factor here.</p>
+<p>As we do not know the source or nature of the stimulus</p>
+<p>82</p>
+<p>responsible for cell division we cannot give a molecular
+account<br>
+of death in the higher organisms. However we shall now see
+that,<br>
+philosophically, we are entitled to consider reproduction as
+a<br>
+saving factor in this case also; and to regard the death of
+the<br>
+individual much as we regard the fall of the leaf from the
+tree:<br>
+_i.e._ as the cessation of an outgrowth from a development<br>
+extending from the past into the future. The phenomena of old
+age<br>
+and natural death are, in short, not at variance with the<br>
+progressive activity of the organism. We perceive this when
+we<br>
+come to consider death from the evolutionary point of view.</p>
+<p>Professor Weismann, in his two essays, "The Duration of
+Life,"<br>
+and "Life and Death,"[1] adopts and defends the view that
+"death<br>
+is not a primary necessity but that it has been secondarily<br>
+acquired by adaptation." The cell was not inherently limited
+in<br>
+its number of cell-generations. The low unicellular organisms
+are<br>
+potentially immortal, the higher multicellular forms with<br>
+well-differentiated organs contain the germs of death within<br>
+themselves.</p>
+<p>He finds the necessity of death in its utility to the
+species.<br>
+Long life is a useless luxury. Early and abundant reproduction
+is<br>
+best for the species. An immortal individual would gradually<br>
+become injured and would be valueless or even harmful to the<br>
+species by taking the place of those that are sound. Hence<br>
+natural selection will shorten life.</p>
+<p>[1] See his _Biological Memoirs._ Oxford, 1888.</p>
+<p>83</p>
+<p>Weismann contends against the transmission of acquired
+characters<br>
+as being unproved.[1] He bases the appearance of death on<br>
+variations in the reproductive cells, encouraged by the
+ceaseless<br>
+action of natural selection, which led to a differentiation
+into<br>
+perishable somatic cells and immortal reproductive cells. The<br>
+time-limit of any particular organism ultimately depends upon
+the<br>
+number of somatic cell-generations and the duration of each<br>
+generation. These quantities are "predestined in the germ
+itself"<br>
+which gives rise to each individual. "The existence of
+immortal<br>
+metazoan organisms is conceivable," but their capacity for<br>
+existence is influenced by conditions of the external world;
+this<br>
+renders necessary the process of adaptation. In fact, in the<br>
+differentiation of somatic from reproductive cells, material
+was<br>
+provided upon which natural selection could operate to shorten
+or<br>
+to lengthen the life of the individual in accordance with the<br>
+needs of the species. The soma is in a sense "a secondary<br>
+appendage of the real bearer of life&mdash;the reproductive
+cells." The<br>
+somatic cells probably lost their immortal qualities, on this<br>
+immortality becoming useless to the species. Their mortality
+may<br>
+have been a mere consequence of their differentiation (loc.
+cit.,<br>
+p. 140), itself due to natural selection. "Natural death was<br>
+not," in fact, "introduced from absolute intrinsic necessity<br>
+inherent in the nature of living matter, but on grounds of<br>
+utility,</p>
+<p>[1] Biological Memoirs, p. 142.</p>
+<p>84</p>
+<p>that is from necessities which sprang up, not from the
+general<br>
+conditions of life, but from those special conditions which<br>
+dominate the life of multicellular organisms."</p>
+<p>On the inherent immortality of life, Weismann finally
+states:<br>
+"Reproduction is, in truth, an essential attribute of living<br>
+matter, just as the growth which gives rise to it.... Life is<br>
+continuous, and not periodically interrupted: ever since its<br>
+first appearance upon the Earth in the lowest organism, it
+has<br>
+continued without break; the forms in which it is manifest
+have<br>
+alone undergone change. Every individual alive today&mdash;even
+the<br>
+highest&mdash;is to be derived in an unbroken line from the first
+and<br>
+lowest forms." [1]</p>
+<p>At the present day the view is very prevalent that the soma
+of<br>
+higher organisms is, in a sense, but the carrier for a period
+of<br>
+the immortal reproductive cells (Ray Lankester)[2]&mdash;an
+appendage<br>
+due to adaptation, concerned in their supply, protection, and<br>
+transmission. And whether we regard the time-limit of its<br>
+functions as due to external constraints, recurrently acting
+till<br>
+their effects become hereditary, or to variations more
+directly<br>
+of internal origin, encouraged by natural selection, we see
+in<br>
+old age and death phenomena ultimately brought about in
+obedience<br>
+to the action of an environment. These are not inherent in
+the<br>
+properties of living matter. But, in spite</p>
+<p>[1] Loc. cit., p. 159</p>
+<p>[2] Geddes and Thomson, The Evolution of Sex, chap. xviii.</p>
+<p>85</p>
+<p>of its mortality, the body remains a striking manifestation
+of<br>
+the progressiveness of the organism, for to this it must be<br>
+ascribed. To it energy is available which is denied to the<br>
+protozoon. Ingenious adaptations to environment are more<br>
+especially its privilege. A higher manifestation, however,
+was<br>
+possible, and was found in the development of mind. This, too,
+is<br>
+a servant of the cell, as the genii of the lamp. Through it<br>
+energy is available which is denied to the body. This is the<br>
+masterpiece of the cell. Its activity dates, as it were, but
+from<br>
+yesterday, and today it inherits the most diverse energies of
+the<br>
+Earth.</p>
+<p>Taking this view of organic succession, we may liken the<br>
+individual to a particle vibrating for a moment and then
+coming<br>
+to rest, but sweeping out in its motion one wave in the<br>
+continuous organic vibration travelling from the past into
+the<br>
+future. But as this vibration is one spreading with increased<br>
+energy from each vibrating particle, its propagation involves
+a<br>
+continual accelerated inflow of energy from the surrounding<br>
+medium, a dynamic condition unknown in periodic effects<br>
+transmitted by inanimate actions, and, indeed, marking the<br>
+fundamental difference between the dynamic attitudes of the<br>
+animate and inanimate.</p>
+<p>We can trace the periodic succession of individuals on a
+diagram<br>
+of activity with some advantage. Considering, first, the case
+of<br>
+the unicellular organism reproducing by subdivision and
+recalling<br>
+that conditions, definite and inevitable, oppose a limit to
+the<br>
+rate of growth, or, for our</p>
+<p>86</p>
+<p>present purpose, rate of consumption of energy, we proceed
+as<br>
+follows:</p>
+<p>{Fig. 1}</p>
+<p>Along a horizontal axis units of time are measured; along
+a<br>
+vertical axis units of energy. Then the life-history of the<br>
+amoeba, for example, appears as a line such as A in Fig. 1.<br>
+During the earlier stages of its growth the rate of absorption
+of<br>
+energy is small; so that in the unit interval of time, t, the<br>
+small quantity of energy, e1, is absorbed. As life advances,
+the<br>
+activity of the organism augments, till finally this rate
+attains<br>
+a maximum, when e2 units of energy are consumed in the unit
+of<br>
+time.[1]</p>
+<p>[1] Reference to p. 76, where the organic system is treated
+as<br>
+purely mechanical, may help readers to understand what is<br>
+involved in this curve. The solar engine may, unquestionably,<br>
+have its activity defined by such a curve. The organism is,<br>
+indeed, more complex; but neither this fact nor our ignorance
+of<br>
+its mechanism, affects the principles which justify the
+diagram.</p>
+<p>87</p>
+<p>On this diagram reproduction, on the part of the organism,
+is<br>
+represented by a line which repeats the curvature of the
+parent<br>
+organism originating at such a point as P in the path of the<br>
+latter, when the rate of consumption of energy has become<br>
+constant. The organism A has now ceased to act as a unit. The<br>
+products of fission each carry on the vital development of</p>
+<p>{Fig. 2}</p>
+<p>the species along the curve B, which may be numbered (2),
+to<br>
+signify that it represents the activity of two individuals,
+and<br>
+so on, the numbering advancing in geometrical progression.
+The<br>
+particular curvature adopted in the diagram is, of course,<br>
+imaginary; but it is not of an indeterminate nature. Its
+course<br>
+for any species is a characteristic of fundamental physical<br>
+importance, regarding the part played in nature by the
+particular<br>
+organism.</p>
+<p>88</p>
+<p>In Fig. 2 is represented the path of a primitive
+multicellular<br>
+organism before the effects of competition produced or
+fostered<br>
+its mortality. The lettering of Fig. 1 applies; the
+successive<br>
+reproductive acts are marked P1, P2; Q1, Q2, etc., in the
+paths<br>
+of the successive individuals.</p>
+<p>{Fig. 3}</p>
+<p>The next figure (Fig. 3) diagrammatically illustrates death
+in<br>
+organic history. The path ever turns more and more from the
+axis<br>
+of energy, till at length the point is reached when no more<br>
+energy is available; a tangent to the curve at this point is
+at<br>
+right angles to the axis of energy and parallel to the time
+axis.<br>
+The death point is reached, and however great a length we
+measure<br>
+along the axis of time, no further consumption of energy is</p>
+<p>89</p>
+<p>indicated by the path of the organism. Drawing the line
+beyond<br>
+the death point is meaningless for our present purpose.</p>
+<p>It is observable that while the progress of animate nature
+finds<br>
+its representation on this diagram by lines sloping _upwards_
+from<br>
+left to right, the course of events in inanimate
+nature&mdash;for<br>
+example, the history of the organic configuration after death,
+or</p>
+<p>{Fig. 4}</p>
+<p>the changes progressing&mdash;let us say, in the solar system,
+or in<br>
+the process of a crystallisation, would appear as lines
+sloping<br>
+downwards from left to right.</p>
+<p>Whatever our views on the origin of death may be, we have
+to<br>
+recognise a periodicity of functions in the life-history of
+the<br>
+successive individuals of the present day; and whether or not
+we<br>
+trace this directly or indirectly to</p>
+<p>90</p>
+<p>a sort of interference with the rising wave of life, imposed
+by<br>
+the activity of a series of derived units, each seeking
+energy,<br>
+and in virtue of its adaptation each being more fitted to
+obtain<br>
+it than its predecessor, or even leave the idea of
+interference<br>
+out of account altogether in the origination or perpetuation
+of<br>
+death, the truth of the diagram (Fig. 4) holds in so far as
+it<br>
+may be supposed to graphically represent the dynamic history
+of<br>
+the individual. The point chosen on the curve for the
+origination<br>
+of a derived unit is only applicable to certain organisms,
+many<br>
+reproducing at the very close of life. A chain of units are<br>
+supposed here represented.[1]</p>
+<p>THE LENGTH OF LIFE</p>
+<p>If we lay out waves as above to a common scale of time for<br>
+different species, the difference of longevity is shown in
+the<br>
+greater or less number of vibrations executed in a given
+time,<br>
+_i.e._ in greater or less "frequency." We cannot indeed draw
+the<br>
+curvature correctly, for this would necessitate a knowledge
+which<br>
+we have not of the activity of the organism at different
+periods<br>
+of its life-history, and so neither can we plot the direction
+of<br>
+the organic line of propagation with respect to the</p>
+<p>[1] Projecting upon the axes of time and energy any one
+complete<br>
+vibration, as in Fig. 4, the total energy consumed by the<br>
+organism during life is the length E on the axis of energy,
+and<br>
+its period of life is the length T on the time-axis. The mean<br>
+activity is the quotient E/T.</p>
+<p>91</p>
+<p>axes of reference as this involves a knowledge of the mean<br>
+activity.[1]</p>
+<p>The group of curves which follow, relating to typical
+animals<br>
+possessing very different activities (Fig. 5), are therefore<br>
+entirely diagrammatic, except in respect to the approximate</p>
+<p>{Fig. 5}</p>
+<p>longevity of the organisms. (1) might represent an animal of
+the<br>
+length of life and of the activity of Man; (2), on the same
+scale<br>
+of longevity,</p>
+<p>[1] In the relative food-supply at various periods of life
+the<br>
+curvature is approximately determinable.</p>
+<p>92</p>
+<p>one of the smaller mammals; and (3), the life-history of a
+cold<br>
+blooded animal living to a great age; _e.g._ certain of the<br>
+reptilia.</p>
+<p>It is probable, that to conditions of structural
+development,<br>
+under the influence of natural selection, the question of
+longer<br>
+or shorter life is in a great degree referable. Thus,
+development<br>
+along lines of large growth will tend to a slow rate of<br>
+reproduction from the simple fact that unlimited energy to
+supply<br>
+abundant reproduction is not procurable, whatever we may
+assume<br>
+as to the strength or cunning exerted by the individual in
+its<br>
+efforts to obtain its supplies. On the other hand,
+development<br>
+along lines of small growth, in that reproduction is less
+costly,<br>
+will probably lead to increased rate of reproduction. It is,
+in<br>
+fact, matter of general observation that in the case of
+larger<br>
+animals the rate of reproduction is generally slower than in
+the<br>
+case of smaller animals. But the rate of reproduction might
+be<br>
+expected to have an important influence in determining the<br>
+particular periodicity of the organism. Were we to depict in
+the<br>
+last diagram, on the same time-scale as Man, the vibrations
+of<br>
+the smaller and shorter-lived living things, we would see but
+a<br>
+straight line, save for secular variations in activity,<br>
+representing the progress of the species in time: the tiny<br>
+thrills of its units lost in comparison with the yet brief
+period<br>
+of Man.</p>
+<p>The interdependence of the rate of reproduction and</p>
+<p>93</p>
+<p>the duration of the individual is, indeed, very probably
+revealed<br>
+in the fact that short-lived animals most generally reproduce<br>
+themselves rapidly and in great abundance, and vice versa. In<br>
+many cases where this appears contradicted, it will be found
+that<br>
+the young are exposed to such dangers that but few survive
+(_e.g._<br>
+many of the reptilia, etc.), and so the rate of reproduction
+is<br>
+actually slow.</p>
+<p>Death through the periodic rigour of the inanimate
+environment<br>
+calls forth phenomena very different from death introduced or<br>
+favoured by competition. A multiplicity of effects simulative
+of<br>
+death occur. Organisms will, for example, learn to meet very<br>
+rigorous conditions if slowly introduced, and not permanent.
+A<br>
+transitory period of want can be tided over by contrivance.
+The<br>
+lily withdrawing its vital forces into the bulb, protected
+from<br>
+the greatest extremity of rigour by seclusion in the Earth;
+the<br>
+trance of the hibernating animal; are instances of such<br>
+contrivances.</p>
+<p>But there are organisms whose life-wave truly takes up the<br>
+periodicity of the Earth in its orbit. Thus the smaller
+animals<br>
+and plants, possessing less resources in themselves, die at
+the<br>
+approach of winter, propagating themselves by units which,<br>
+whether egg or seed, undergo a period of quiescence during
+the<br>
+season of want. In these quiescent units the energy of the<br>
+organism is potential, and the time-energy function is in<br>
+abeyance. This condition is, perhaps, foreshadowed in the<br>
+encyst-</p>
+<p>94</p>
+<p>ment of the amoeba in resistance to drought. In most cases
+of<br>
+hibernation the time-energy function seems maintained at a
+loss<br>
+of potential by the organism, a diminished vital consumption
+of<br>
+energy being carried on at the expense of the stored energy
+of<br>
+the tissues. So, too, even among the largest organisms there
+will<br>
+be a diminution of activity periodically inspired by<br>
+climatological conditions. Thus, wholly or in part, the
+activity<br>
+of organisms is recurrently affected by the great
+energy&mdash;tides<br>
+set up by the Earth's orbital motion.</p>
+<p>{Fig. 6}</p>
+<p>Similarly in the phenomenon of sleep the organism responds to
+the<br>
+Earth's axial periodicity, for in the interval of night a
+period<br>
+of impoverishment has to be endured. Thus the diurnal waves
+of<br>
+energy also meet a response in the organism. These tides and<br>
+waves of activity would appear as larger and smaller ripples</p>
+<p>95</p>
+<p>on the life-curve of the organism. But in some, in which life
+and<br>
+death are encompassed in a day, this would not be so; and for
+the<br>
+annual among plants, the seed rest divides the waves with
+lines<br>
+of no activity (Fig. 6).</p>
+<p>Thus, finally, we regard the organism as a dynamic
+phenomenon<br>
+passing through periodic variations of intensity. The
+material<br>
+systems concerned in the transfer of the energy rise,
+flourish,<br>
+and fall in endless succession, like cities of ancient
+dynasties.<br>
+At points of similar phase upon the waves the rate of
+consumption<br>
+of energy is approximately the same; the functions, too,
+which<br>
+demand and expend the energy are of similar nature.</p>
+<p>That the rhythm of these events is ultimately based on harmony
+in<br>
+the configuration and motion of the molecules within the germ<br>
+seems an unavoidable conclusion. In the life of the
+individual<br>
+rhythmic dynamic phenomena reappear which in some cases have
+no<br>
+longer a parallel in the external world, or under conditions
+when<br>
+the individual is no longer influenced by these external<br>
+conditions.,, In many cases the periodic phenomena ultimately
+die<br>
+out under new influences, like the oscillations of a body in
+a<br>
+viscous medium; in others when they seem to be more deeply
+rooted<br>
+in physiological conditions they persist.</p>
+<p>The "length of life is dependent upon the number</p>
+<p>[1] The _Descent of Man._</p>
+<p>96</p>
+<p>of generations of somatic cells which can succeed one another
+in<br>
+the course of a single life, and furthermore the number as
+well<br>
+as the duration of each single cell-generation is predestined
+in<br>
+the germ itself."[1]</p>
+<p>Only in the vague conception of a harmonising or formative<br>
+structural influence derived from the germ, perishing in each<br>
+cell from internal causes, but handed from cell to cell till
+the<br>
+formative influence itself degrades into molecular discords,
+does<br>
+it seem possible to form any physical representation of the<br>
+successive events of life. The degradation of the molecular<br>
+formative influence might be supposed involved in its
+frequent<br>
+transference according to some such dynamic actions as occur
+in<br>
+inanimate nature. Thus, ultimately, to the waste within the
+cell,<br>
+to the presence of a force retardative of its perpetual
+harmonic<br>
+motions, the death of the individual is to be ascribed.
+Perhaps<br>
+in protoplasmic waste the existence of a universal death
+should<br>
+be recognised. It is here we seem to touch inanimate nature;
+and<br>
+we are led back to a former conclusion that the organism in
+its<br>
+unconstrained state is to be regarded as a contrivance for<br>
+evading the dynamic tendencies of actions in which lifeless<br>
+matter participates.[2]</p>
+<p>[1] Weismann, _Life and Death; Biological Memoirs_, p.
+146.</p>
+<p>[2] In connection with the predestinating power and
+possible<br>
+complexity of the germ, it is instructive to reflect on the
+very<br>
+great molecular population of even the smallest
+spores&mdash;giving<br>
+rise to very simple forms. Thus, the spores of the
+unicellular<br>
+Schizomycetes are estimated to dimensions as low as 1/10,000 of
+a<br>
+millimetre in diameter (Cornil et Babes, _Les Batteries_, 1.
+37).<br>
+From Lord Kelvin's estimate of the number of molecules in
+water,<br>
+comprised within the length of a wave-length of yellow light<br>
+(_The Size of Atoms_, Proc. R. I., vol. x., p. 185) it is<br>
+probable that such spores contain some 500,000 molecules,
+while<br>
+one hundred molecules range along a diameter.</p>
+<p>97</p>
+<p>THE NUMERICAL ABUNDANCE OF LIFE</p>
+<p>We began by seeking in various manifestations of life a
+dynamic<br>
+principle sufficiently comprehensive to embrace its very
+various<br>
+phenomena. This, to all appearance, found, we have been led
+to<br>
+regard life, to a great extent, as a periodic dynamic
+phenomenon.<br>
+Fundamentally, in that characteristic of the contrivance,
+which<br>
+leads it to respond favourably to transfer of energy, its<br>
+enormous extension is due. It is probable that to its
+instability<br>
+its numerical abundance is to be traced&mdash;for this,
+necessitating<br>
+the continual supply of all the parts already formed, renders<br>
+large, undifferentiated growth, incompatible with the limited<br>
+supplies of the environment. These are fundamental conditions
+of<br>
+abundant life upon the Earth.</p>
+<p>Although we recognise in the instability of living systems
+the<br>
+underlying reason for their numerical abundance, secondary<br>
+evolutionary causes are at work. The most important of these
+is<br>
+the self-favouring nature of the phenomenon of reproduction.
+Thus<br>
+there is a tendency not only to favour reproductiveness, but<br>
+early reproductiveness, in the form of one prolific<br>
+reproductive.</p>
+<p>98</p>
+<p>act, after which the individual dies.[1] Hence the wavelength
+of<br>
+the species diminishes, reproduction is more frequent, and<br>
+correspondingly greater numbers come and go in an interval of<br>
+time.</p>
+<p>Another cause of the numerical abundance of life exists,
+as<br>
+already stated, in the conditions of nourishment. Energy is
+more<br>
+readily conveyed to the various parts of the smaller mass,
+and<br>
+hence the lesser organisms will more actively functionate;
+and<br>
+this, as being the urging dynamic attitude, as well as that
+most<br>
+generally favourable in the struggle, will multiply and
+favour<br>
+such forms of life. On the other hand, however, these forms
+will<br>
+have less resource within themselves, and less power of<br>
+endurance, so that they are only suitable to fairly uniform<br>
+conditions of supply; they cannot survive the long continued
+want<br>
+of winter, and so we have the seasonal abundance of summer.
+Only<br>
+the larger and more resistant organisms, whether animal or<br>
+vegetable, will, in general, populate the Earth from year to<br>
+year. From this we may conclude that, but for the seasonal<br>
+energy-tides, the development of life upon the globe had gone<br>
+along very different lines from those actually followed. It
+is,<br>
+indeed, possible that the evolution of the larger organisms
+would<br>
+not have occurred; there would have been no vacant place for<br>
+their development, and a being so endowed as Man could hardly</p>
+<p>[1] Weismann, _The Duration of Life._</p>
+<p>99</p>
+<p>have been evolved. We may, too, apply this reasoning
+elsewhere,<br>
+and regard as highly probable, that in worlds which are
+without<br>
+seasonal influences, the higher developments of life have not<br>
+appeared; except they have been evolved under other
+conditions,<br>
+when they might for a period persist. We have, indeed, only
+to<br>
+picture to ourselves what the consequence of a continuance of<br>
+summer would be on insect life to arrive at an idea of the<br>
+antagonistic influences obtaining in such worlds to the
+survival<br>
+of larger organisms.</p>
+<p>It appears that to the dynamic attitude of life in the
+first<br>
+place, and secondarily to the environmental conditions
+limiting<br>
+undifferentiated growth, as well as to the action of heredity
+in<br>
+transmitting the reproductive qualities of the parent to the<br>
+offspring, the multitudes of the pines, and the hosts of
+ants,<br>
+are to be ascribed. Other causes are very certainly at work,
+but<br>
+these, I think, must remain primary causes.</p>
+<p>We well know that the abundance of the ants and pines is not
+a<br>
+tithe of the abundance around us visible and invisible. It is
+a<br>
+vain endeavour to realise the countless numbers of our<br>
+fellow-citizens upon the Earth; but, for our purpose, the<br>
+restless ants, and the pines solemnly quiet in the sunshine,
+have<br>
+served as types of animate things. In the pine the gates of
+the<br>
+organic have been thrown open that the vivifying river of
+energy<br>
+may flow in. The ants and the butterflies sip for a brief
+moment<br>
+of its waters, and again vanish into the</p>
+<p>100</p>
+<p>inorganic: life, love and death encompassed in a day.</p>
+<p>Whether the organism stands at rest and life comes to it on
+the<br>
+material currents of the winds and waters, or in the
+vibratory<br>
+energy of the &aelig;ther; or, again, whether with restless
+craving it<br>
+hurries hither and thither in search of it, matters nothing.
+The<br>
+one principle&mdash;the accelerative law which is the law of
+the<br>
+organic&mdash;urges all alike onward to development, reproduction
+and<br>
+death. But although the individual dies death is not the end;
+for<br>
+life is a rhythmic phenomenon. Through the passing ages the
+waves<br>
+of life persist: waves which change in their form and in the<br>
+frequency to which they are attuned from one geologic period
+to<br>
+the next, but which still ever persist and still ever
+increase.<br>
+And in the end the organism outlasts the generations of the<br>
+hills.</p>
+<p>101</p>
+<p><u>THE BRIGHT COLOURS OF ALPINE FLOWERS</u> [1]</p>
+<p>IT is admitted by all observers that many species of
+flowering<br>
+plants growing on the higher alps of mountainous regions
+display<br>
+a more vivid and richer colour in their bloom than is
+displayed<br>
+in the same species growing in the valleys. That this is
+actually<br>
+the case, and not merely an effect produced upon the observer
+by<br>
+the scant foliage rendering the bloom more conspicuous, has
+been<br>
+shown by comparative microscopic examination of the petals of<br>
+species growing on the heights and in the valleys. Such<br>
+examination has revealed that in many cases pigment granules
+are<br>
+more numerous in the individuals growing at the higher
+altitudes.<br>
+The difference is specially marked in Myosotis sylvatica,<br>
+Campanula rotundifolia, Ranunculus sylvaticus, Galium
+cruciatum,<br>
+and others. It is less marked in the case of Thymus serpyllum
+and<br>
+Geranium sylvaticum; while in Rosa alpina and Erigeron alpinus
+no<br>
+difference is observable.[2]</p>
+<p>In the following cases a difference of intensity of colour
+is,<br>
+according to Kerner ("Pflanzenleben," 11. 504), especially<br>
+noticeable:&mdash; _Agrostemma githago, Campanula</p>
+<p>[1] _Proc. Royal Dublin Society_, 1893.</p>
+<p>[2] G. Bonnier, quoted by De Varigny, _Experimental
+Evolution_,<br>
+p. 55.</p>
+<p>102</p>
+<p>pusilla, Dianthus inodorus (silvestris), Gypsophila repens,
+Lotus<br>
+corniculatus, Saponaria ocymoides, Satureja hortensis,
+Taraxacumm<br>
+officinale, Vicia cracca, and Vicia sepium._</p>
+<p>To my own observation this beautiful phenomenon has always<br>
+appeared most obvious and impressive. It appears to have
+struck<br>
+many unprofessional observers. Helmholtz offers the
+explanation<br>
+that the vivid colours are the result of the brighter sunlight
+of<br>
+the heights. It has been said, too, that they are the direct<br>
+chemical effects of a more highly ozonized atmosphere. The
+latter<br>
+explanation I am unable to refer to its author. The following<br>
+pages contain a suggestion on the matter, which occurred to
+me<br>
+while touring, along with Henry H. Dixon, in the Linthal
+district<br>
+of Switzerland last summer.[1]</p>
+<p>If the bloom of these higher alpine flowers is especially<br>
+pleasing to our own &aelig;sthetic instincts, and markedly
+conspicuous<br>
+to us as observers, why not also especially attractive and<br>
+conspicuous to the insect whose mission it is to wander from<br>
+flower to flower over the pastures? The answer to this
+question<br>
+involves the hypothesis I would advance as accounting for the<br>
+bright colours of high-growing individuals. In short, I believe
+a<br>
+satisfactory explanation is to be found in the conditions of<br>
+insect life in the higher alps.</p>
+<p>In the higher pastures the summer begins late and</p>
+<p>[1] The summer of 1892.</p>
+<p>103</p>
+<p>closes early, and even in the middle of summer the day closes
+in<br>
+with extreme cold, and the cold of night is only dispelled
+when<br>
+the sun is well up. Again, clouds cover the heights when all
+is<br>
+clear below, and cold winds sweep over them when there is
+warmth<br>
+and shelter in the valleys. With these rigorous conditions
+the<br>
+pollinating insects have to contend in their search for food,
+and<br>
+that when the rival attractions of the valleys below are so
+many.<br>
+I believe it is these rigorous conditions which are
+indirectly<br>
+responsible for the bright colours of alpine flowers. For
+such<br>
+conditions will bring about a comparative scarcity of insect<br>
+activity on the heights; and a scarcity or uncertainty in the<br>
+action of insect agency in effecting fertilization will
+intensify<br>
+the competition to attract attention, and only the brightest<br>
+blooms will be fertilized.[1]</p>
+<p>This will be a natural selection of the brightest, or the</p>
+<p>[1] Grant Allen, I have recently learned, advances in _Science
+in<br>
+Arcady_ the theory that there is a natural selective cause<br>
+fostering the bright blooms of alpines. The selective cause
+is,<br>
+however, by him referred to the greater abundance of
+butterfly<br>
+relatively to bee fertilizers. The former, he says, display
+more<br>
+&aelig;sthetic instinct than bees. In the valley the bees secure
+the<br>
+fertilization of all. I may observe that upon the Fridolins
+Alp<br>
+all the fertilizers we observed were bees. I have always
+found<br>
+butterflies very scarce at altitudes of 7,000 to 8,000 feet.
+The<br>
+alpine bees are very light in body, like our hive bee, and I
+do<br>
+not think rarefaction of the atmosphere can operate to hinder
+its<br>
+ascent to the heights, as Grant Allen suggests. The
+observations<br>
+on the death-rate of bees and butterflies on the glacier, to
+be<br>
+referred to presently, seem to negative such a hypothesis, and
+to<br>
+show that a large preponderance of bees over butterflies make<br>
+their way to the heights.</p>
+<p>104</p>
+<p>brightest will be the fittest, and this condition, along with
+the<br>
+influence of heredity, will encourage a race of vivid flowers.
+On<br>
+the other hand, the more scant and uncertain root supply, and
+the<br>
+severe atmospheric conditions, will not encourage the grosser<br>
+struggle for existence which in the valleys is carried on so<br>
+eagerly between leaves and branches&mdash;the normal offensive
+and<br>
+defensive weapons of the plant&mdash;and so the struggle
+becomes<br>
+refined into the more &aelig;sthetic one of colour and
+brightness<br>
+between flower and flower. Hence the scant foliage and vivid<br>
+bloom would be at once the result of a necessary economy, and
+a<br>
+resort to the best method of securing reproduction under the<br>
+circumstances of insect fertilizing agency. Or, in other
+words,<br>
+while the luxuriant growth is forbidden by the conditions,
+and<br>
+thus methods of offence and defence, based upon vigorous<br>
+development, reduced in importance, it would appear that the<br>
+struggle is mainly referred to rivalry for insect preference.
+It<br>
+is probable that this is the more economical manner of
+carrying<br>
+on the contest.</p>
+<p>In the valleys we see on every side the struggle between
+the<br>
+vegetative organs of the plant; the soundless battle among
+the<br>
+leaves and branches. The blossom here is carried aloft on a<br>
+slender stem, or else, taking but a secondary part in the<br>
+contest, it is relegated to obscurity (P1. XII.). Further up
+on<br>
+the mountains, where the conditions are more severe and the<br>
+supplies less abundant, the leaf and branch assume lesser<br>
+dimensions, for they are costly weapons to provide and the<br>
+elements are unfriendly</p>
+<p>105</p>
+<p>to their existence (Pl. XIII.). Still higher, approaching
+the<br>
+climatic limit of vegetable life, the struggle for existence
+is<br>
+mainly carried on by the &aelig;sthetic rivalry of lowly but<br>
+conspicuous blossoms.</p>
+<p>As regards the conditions of insect life in the higher alps,
+it<br>
+came to my notice in a very striking manner that vast numbers
+of<br>
+such bees and butterflies as venture up perish in the cold of<br>
+night time. It appears as if at the approach of dusk these
+are<br>
+attracted by the gleam of the snow, and quitting the
+pastures,<br>
+lose themselves upon the glaciers and firns, there to die in<br>
+hundreds. Thus in an ascent of the T&ouml;di from the
+Fridolinsh&uuml;te we<br>
+counted in the early dawn sixty-seven frozen bees,
+twenty-nine<br>
+dead butterflies, and some half-dozen moths on the Biferten<br>
+Glacier and Firn. These numbers, it is to be remembered, only<br>
+included those lying to either side of our way over the snow,
+so<br>
+that the number must have mounted up to thousands when
+integrated<br>
+over the entire glacier and firn. Approaching the summit none<br>
+were found. The bees resembled our hive bee in appearance,
+the<br>
+butterflies resembled the small white variety common in our<br>
+gardens, which has yellow and black upon its wings. One large<br>
+moth, striped across the abdomen, and measuring nearly two
+inches<br>
+in length of body, was found. Upon our return, long after the<br>
+sun's rays had grown strong, we observed some of the
+butterflies<br>
+showed signs of reanimation. We descended so quickly to avoid
+the<br>
+inconvenience of the soft snow that we had time for no</p>
+<p>106</p>
+<p>close observation on the frozen bees. But dead bees are
+common<br>
+objects upon the snows of the alps.</p>
+<p>These remarks I noted down roughly while at Linthal last
+summer,<br>
+but quite recently I read in Natural Science[1] the following<br>
+note:</p>
+<p>"Late Flowering Plants.&mdash;While we write, the ivy is in
+flower, and<br>
+bees, wasps, and flies are jostling each other and struggling
+to<br>
+find standing-room on the sweet-smelling plant. How great must
+be<br>
+the advantage obtained by this plant through its exceptional<br>
+habit of flowering in the late autumn, and ripening its fruit
+in<br>
+the spring. To anyone who has watched the struggle to
+approach<br>
+the ivy-blossom at a time when nearly all other plants are
+bare,<br>
+it is evident that, as far as transport of pollen and<br>
+cross-fertilization go, the plant could not flower at a more<br>
+suitable time. The season is so late that most other plants
+are<br>
+out of flower, but yet it is not too late for many insects to
+be<br>
+brought out by each sunny day, and each insect, judging by
+its<br>
+behaviour, must be exceptionally hungry.</p>
+<p>"Not only has the ivy the world to itself during its
+flowering<br>
+season, but it delays to ripen its seed till the spring, a
+time<br>
+when most other plants have shed their seed, and most edible<br>
+fruits have been picked by the birds. Thus birds wanting fruit
+in<br>
+the spring can obtain little but ivy, and how they appreciate
+the<br>
+ivy berry is evident</p>
+<p>[1] For December, 1892, vol. i., p. 730.</p>
+<p>107</p>
+<p>by the purple stains everywhere visible within a short
+distance<br>
+of the bush."</p>
+<p>These remarks suggest that the ivy adopts the converse
+attitude<br>
+towards its visitors to that forced upon the alpine flower.
+The<br>
+ivy bloom is small and inconspicuous, but then it has the
+season<br>
+to itself, and its inconspicuousness is no disadvantage,
+_i.e._<br>
+if one plant was more conspicuous than its neighbours, it
+would<br>
+not have any decided advantage where the pollinating insect
+is<br>
+abundant and otherwise unprovided for. Its dark-green berries
+in<br>
+spring, which I would describe as very inconspicuous, have a<br>
+similar advantage in relation to the necessities of bird
+life.</p>
+<p>The experiments of M. C. Flahault must be noticed. This<br>
+naturalist grew seeds of coloured flowers which had ripened
+in<br>
+Paris, part in Upsala, and part in Paris; and seed which had<br>
+ripened in Upsala, part at Paris, and part at Upsala. The
+flowers<br>
+opening in the more northern city were in most cases the<br>
+brighter.[1] If this observation may be considered
+indisputable,<br>
+as appears to be the case, the question arises, Are we to
+regard<br>
+this as a direct effect of the more rigorous climate upon the<br>
+development of colouring matter on the blooms opening at
+Upsala?<br>
+If we suppose an affirmative answer, the theory of direct
+effect<br>
+by sun brightness must I think be abandoned. But I venture to<br>
+think that the explanation of the Upsala</p>
+<p>[1] Quoted by De Varigny, _Experimental Evolution_, p. 56.</p>
+<p>108</p>
+<p>experiment is not to be found in direct climatic influence
+upon<br>
+the colour, but in causes which lie deeper, and involve some<br>
+factors deducible from biological theory.</p>
+<p>The organism, as a result of the great facts of heredity and
+of<br>
+the survival of the fittest, is necessarily a system which<br>
+gathers experience with successive generations; and the
+principal<br>
+lesson ever being impressed upon it by external events is<br>
+economy. Its success depends upon the use it makes of its<br>
+opportunities for the reception of energy and the economy<br>
+attained in disposing of what is gained.</p>
+<p>With regard to using the passing opportunity the entire
+seasonal<br>
+development of life is a manifestation of this attitude, and
+the<br>
+fleetness, agility, etc., of higher organisms are developments
+in<br>
+this direction. The higher vegetable organism is not
+locomotory,<br>
+save in the transferences of pollen and seed, for its food
+comes<br>
+to it, and the necessary relative motion between food and<br>
+organism is preserved in the quick motion of radiated energy
+from<br>
+the sun and the slower motion of the winds on the surface of
+the<br>
+earth. But, even so, the vegetable organism must stand ever
+ready<br>
+and waiting for its supplies. Its molecular parts must be
+ready<br>
+to seize the prey offered to it, somewhat as the waiting
+spider<br>
+the fly. Hence, the plant stands ready; and every cloud with<br>
+moving shadow crossing the fields handicaps the shaded to the<br>
+benefit of the unshaded plant in the adjoining field. The
+open<br>
+bloom</p>
+<p>109</p>
+<p>is a manifestation of the generally expectant attitude of
+the<br>
+plant, but in relation to reproduction.</p>
+<p>As regards economy, any principle of maximum economy, where
+many<br>
+functions have to be fulfilled, will, we may very safely
+predict,<br>
+involve as far as possible mutual helpfulness in the
+processes<br>
+going on. Thus the process of the development towards meeting
+any<br>
+particular external conditions, A, suppose, will, if
+possible,<br>
+tend to forward the development towards meeting conditions B;
+so<br>
+that, in short, where circumstances of morphology and
+physiology<br>
+are favourable, the ideally economical system will be
+attained<br>
+when in place of two separate processes, a, &szlig;, the one
+process y,<br>
+cheaper than a + &szlig;, suffices to advance development<br>
+simultaneously in both the directions A and B. The economy is
+as<br>
+obvious as that involved in "killing two birds with the one<br>
+stone"&mdash;if so crude a simile is permissible&mdash;and it is
+to be<br>
+expected that to foster such economy will be the tendency of<br>
+evolution in all organic systems subjected to restraints as
+those<br>
+we are acquainted with invariably are.</p>
+<p>Such economy might be simply illustrated by considering the
+case<br>
+of a reservoir of water elevated above two hydraulic motors,
+so<br>
+that the elevated mass of water possessed gravitational<br>
+potential. The available energy here represents the stored-up<br>
+energy in the organism. How best may the water be conveyed to
+the<br>
+two motors [the organic systems reacting towards conditions A
+and<br>
+B] so</p>
+<p>110</p>
+<p>that as little energy as possible is lost in transit? If
+the<br>
+motors are near together it is most economical to use the one<br>
+conduit, which will distribute the requisite supply of water
+to<br>
+both. If the motors are located far asunder it will be most<br>
+economical to lay separate conduits. There is greatest economy
+in<br>
+meeting a plurality of functions by the same train of<br>
+physiological processes where this is consistent with meeting<br>
+other demands necessitated by external or internal
+conditions.</p>
+<p>But an important and obvious consequence arises in the supply
+of<br>
+the two motors from the one conduit. We cannot work one motor<br>
+without working the other. If we open a valve in the conduit
+both<br>
+motors start into motion and begin consuming the energy stored
+in<br>
+the tank. And although they may both under one set of
+conditions<br>
+be doing useful and necessary work, in some other set of<br>
+conditions it may be needless for both to be driven.</p>
+<p>This last fact is an illustration of a consideration which
+must<br>
+enter into the phenomenon which an eminent biologist speaks of
+as<br>
+physiological or unconscious "memory,"[1] For the development
+of<br>
+the organism from the ovum is but the starting of a train of<br>
+interdependent events of a complexity depending upon the<br>
+experience of the past.</p>
+<p>[1] Ewald Hering, quoted by Ray Lankaster, _The Advancement
+of<br>
+Science_, p. 283.</p>
+<p>111</p>
+<p>In short, we may suppose the entire development of the
+plant,<br>
+towards meeting certain groups of external conditions,<br>
+physiologically knit together according as Nature tends to<br>
+associate certain groups of conditions. Thus, in the case in<br>
+point, climatic rigour and scarcity of pollinating agency
+will<br>
+ever be associated; and in the long experience of the past
+the<br>
+most economical physiological attitude towards both is, we
+may<br>
+suppose, adopted; so that the presence of one condition
+excites<br>
+the apparent unconscious memory of the other. In reality the<br>
+process of meeting the one condition involves the process and<br>
+development for meeting the other.</p>
+<p>And this consideration may be extended very generally to
+such<br>
+organisms as can survive under the same associated natural<br>
+conditions, for the history of evolution is so long, and the<br>
+power of locomotion so essential to the organism at some
+period<br>
+in its life history, that we cannot philosophically assume a<br>
+local history for members of a species even if widely severed<br>
+geographically at the present day. At some period in the past<br>
+then, it is very possible that the individuals today thriving
+at<br>
+Paris, acquired the experience called out at Upsala. The<br>
+perfection of physiological memory inspires no limit to the
+date<br>
+at which this may have occurred&mdash;possibly the result of
+a<br>
+succession of severe seasons at Paris; possibly the result of<br>
+migrations &mdash;and the seed of many flowering plants possess
+means<br>
+of migration only inferior to those possessed by the flying
+and<br>
+swimming animals. But, again, possibly the experi-</p>
+<p>112</p>
+<p>ence was acquired far back in the evolutionary history of
+the<br>
+flower.[1]</p>
+<p>But a further consideration arises. Not only at each moment
+in<br>
+the life of the individual must maximum income and most
+judicious<br>
+expenditure be considered, but in its whole life history, and<br>
+even over the history of its race, the efficiency must tend to
+be<br>
+a maximum. This principle is even carried so far that when<br>
+necessary it leads to the death of the individual, as in the
+case<br>
+of those organisms which, having accomplished the
+reproductive<br>
+act, almost immediately expire. This view of nature may be<br>
+repellent, but it is, nevertheless, evident that we are parts
+of<br>
+a system which ruthlessly sacrifices the individual on
+general<br>
+grounds of economy. Thus, if the curve which defines the mean<br>
+rate of reception of energy of all kinds at different periods
+in<br>
+the life of the organism be opposed by a second curve, drawn<br>
+below the axis along which time is measured, representing the<br>
+mean rate of expenditure of energy on development,
+reproduction,<br>
+etc. (Fig. 7), this latter curve, which is, of course,</p>
+<p>[1] The blooms of self-fertilising, and especially of<br>
+cleistogamic plants (_e.g._ Viola), are examples of
+unconscious<br>
+memory, or unconscious "association of ideas" leading to the<br>
+development of organs now functionless. The _Pontederia
+crassipes_<br>
+of the Amazon, which develops its floating bladders when grown
+in<br>
+water, but aborts them rapidly when grown on land, and seems
+to<br>
+retain this power of adaptation to the environment for an<br>
+indefinite period of time, must act in each case upon an<br>
+unconscious memory based upon past experience. Many other
+cases<br>
+might be cited.</p>
+<p>113</p>
+<p>physiologically dependent on the former, must be of such a
+nature<br>
+from its origin to its completion in death, that the condition
+is<br>
+realized of the most economical rate of expenditure at each<br>
+period of life.[1] The rate of expenditure of energy at any<br>
+period of life is, of course, in such a curve defined by the<br>
+slope of the curve towards the axis of time at the period in<br>
+question; but this particular slope _must be led to by a
+previous<br>
+part of the curve, and involves its past and future course to
+a<br>
+very great extent_.</p>
+<p>{Fig. 7}</p>
+<p>There will, therefore, be impressed upon the<br>
+organism by the factors of evolution a unified course of<br>
+economical expenditure completed only by its death, and which<br>
+will give to the developmental progress of the individual its<br>
+prophetic character.</p>
+<p>In this way we look to the unified career of each organic
+unit,<br>
+from its commencement in the ovum to the day</p>
+<p>[1] See _The Abundance of Life_.</p>
+<p>114</p>
+<p>when it is done with vitality, for that preparation for
+momentous<br>
+organic events which is in progress throughout the entire
+course<br>
+of development; and to the economy involved in the welding of<br>
+physiological processes for the phenomenon of physiological<br>
+memory, wherein we see reflected, as it were, in the
+development<br>
+of the organism, the association of inorganic restraints<br>
+occurring in nature which at some previous period impressed<br>
+itself upon the plastic organism. We may picture the seedling
+at<br>
+Upsala, swayed by organic memory and the inherited tendency to
+an<br>
+economical preparation for future events, gradually
+developing<br>
+towards the &aelig;sthetic climax of its career. In some such
+manner<br>
+only does it appear possible to account for the prophetic<br>
+development of organisms, not alone to be observed in the
+alpine<br>
+flowers, but throughout nature.</p>
+<p>And thus, finally, to the effects of natural selection and
+to<br>
+actions defined by general principles involved in biology, I<br>
+would refer for explanation of the manner in which flowers on
+the<br>
+Alps develop their peculiar beauty.</p>
+<p>115</p>
+<p><u>MOUNTAIN GENESIS</u></p>
+<p>OUR ancestors regarded mountainous regions with feelings
+of<br>
+horror, mingled with commiseration for those whom an unkindly<br>
+destiny had condemned to dwell therein. We, on the other
+hand,<br>
+find in the contemplation of the great alps of the Earth such<br>
+peaceful and elevated thoughts, and such rest to our souls,
+that<br>
+it is to those very solitudes we turn to heal the wounds of
+ife.<br>
+It is difficult to explain the cause of this very different
+point<br>
+of view. It is probably, in part, to be referred to that cloud
+of<br>
+superstitious horror which, throughout the Middle Ages,
+peopled<br>
+the solitudes with unknown terrors; and, in part, to the<br>
+asceticism which led the pious to regard the beauty and joy
+of<br>
+life as snares to the soul's well-being. In those eternal<br>
+solitudes where the overwhelming forces of Nature are most in<br>
+evidence, an evil principle must dwell or a dragon's dreadful<br>
+brood must find a home.</p>
+<p>But while in our time the aesthetic aspect of the hills
+appeals<br>
+to all, there remains in the physical history of the
+mountains<br>
+much that is lost to those who have not shared in the
+scientific<br>
+studies of alpine structure and genesis. They lose a past
+history<br>
+which for interest com-</p>
+<p>116</p>
+<p>petes with anything science has to tell of the changes of
+the<br>
+Earth.</p>
+<p>Great as are the physical features of the mountains compared
+with<br>
+the works of Man, and great as are the forces involved
+compared<br>
+with those we can originate or control, the loftiest ranges
+are<br>
+small contrasted with the dimensions of the Earth. It is well
+to<br>
+bear this in mind. I give here (Pl. XV.) a measured drawing<br>
+showing a sector cut from a sphere of 50 cms. radius; so much
+of<br>
+it as to exhibit the convergence of its radial boundaries
+which<br>
+if prolonged will meet at the centre. On the same scale as
+the<br>
+radius the diagram shows the highest mountains and the
+deepest<br>
+ocean. The average height of the land and the average depth
+of<br>
+the ocean are also exhibited. We see how small a movement of
+the<br>
+crust the loftiest elevation of the Himalaya represents and
+what<br>
+a little depression holds the ocean.</p>
+<p>Nevertheless, it is not by any means easy to explain the
+genesis<br>
+of those small elevations and depressions. It would lead us
+far<br>
+from our immediate subject to discuss the various theoretical<br>
+views which have been advanced to account for the facts. The
+idea<br>
+that mountain folds, and the lesser rugosities of the Earth's<br>
+surface, arose in a wrinkling of the crust under the influence
+of<br>
+cooling and skrinkage of the subcrustal materials, is held by<br>
+many eminent geologists, but not without dissent from others.</p>
+<p>The most striking observational fact connected with
+mountain<br>
+structure is that, without exception, the ranges</p>
+<p>117</p>
+<p>of the Earth are built essentially of sedimentary rocks: that
+is<br>
+of rocks which have been accumulated at some remote past time<br>
+beneath the surface of the ocean. A volcanic core there may<br>
+sometimes be&mdash;probably an attendant or consequence of
+the<br>
+uplifting&mdash;or a core of plutonic igneous rocks which has
+arisen<br>
+under the same compressive forces which have bowed and arched
+the<br>
+strata from their original horizontal position. It is not<br>
+uncommon to meet among unobservant people those who regard
+all<br>
+mountain ranges as volcanic in origin. Volcanoes, however, do
+not<br>
+build mountain ranges. They break out as more or less
+isolated<br>
+cones or hills. Compare the map of the Auvergne with that of<br>
+Switzerland; the volcanoes of South Italy with the Apennines.<br>
+Such great ranges as those which border with triple walls the<br>
+west coast of North America are in no sense volcanic: nor are
+the<br>
+Pyrenees, the Caucasus, or the Himalaya. Volcanic materials
+are<br>
+poured out from the summits of the Andes, but the range itself
+is<br>
+built up of folded sediments on the same architecture as the<br>
+other great ranges of the Earth.</p>
+<p>Before attempting an explanation of the origin of the
+mountains<br>
+we must first become more closely acquainted with the
+phenomena<br>
+attending mountain elevation.</p>
+<p>At the present day great accumulations of sediment are
+taking<br>
+place along the margins of the continents where the rivers
+reach<br>
+the ocean. Thus, the Gulf of Mexico receiving the sediment of
+the<br>
+Mississippi and Rio Grande;</p>
+<p>118</p>
+<p>the northeast coast of South America receiving the sediments
+of<br>
+the Amazons; the east coast of Asia receiving the detritus of
+the<br>
+Chinese rivers; are instances of such areas of deposition.
+Year<br>
+by year, century by century, the accumulation progresses, and
+as<br>
+it grows the floor of the sea sinks under the load. Of the<br>
+yielding of the crust under the burthen of the sediments we
+are<br>
+assured; for otherwise the many miles of vertically piled
+strata<br>
+which are uplifted to our view in the mountains, never could
+have<br>
+been deposited in the coastal seas of the past. The flexure
+and<br>
+sinking of the crust are undeniable realities.</p>
+<p>Such vast subsiding areas are known as geosynclines. From
+the<br>
+accumulated sediments of the geosynclines the mountain ranges
+of<br>
+the past have in every case originated; and the mountains of
+the<br>
+future will assuredly arise and lofty ranges will stand where
+now<br>
+the ocean waters close over the collecting sediments. Every<br>
+mountain range upon the Earth enforces the certainty of this<br>
+prediction.</p>
+<p>The mountain-forming movement takes place after a certain
+great<br>
+depth of sediment is collected. It is most intense where the<br>
+thickness of deposit is greatest. We see this when we examine
+the<br>
+structure of our existing mountain ranges. At either side
+where<br>
+the sediments thin out, the disturbance dies away, till we
+find<br>
+the comparatively shallow and undisturbed level sediments
+which<br>
+clothe the continental surface.</p>
+<p>Whatever be the connection between the deposition and</p>
+<p>119</p>
+<p>the subsequent upheaval, _the element of great depth of<br>
+accumulation seems a necessary condition and must evidently
+enter<br>
+as a factor into the Physical Processes involved_. The
+mountain<br>
+range can only arise where the geosyncline is deeply filled
+by<br>
+long ages of sedimentation.</p>
+<p>Dana's description of the events attending mountain building
+is<br>
+impressive:</p>
+<p>"A mountain range of the common type, like that to which
+the<br>
+Appalachians belong, is made out of the sedimentary formations
+of<br>
+a long preceding era; beds that were laid down conformably,
+and<br>
+in succession, until they had reached the needed thickness;
+beds<br>
+spreading over a region tens of thousands of square miles in<br>
+area. The region over which sedimentary formations were in<br>
+progress in order to make, finally, the Appalachian range,<br>
+reached from New York to Alabama, and had a breadth of 100 to
+200<br>
+miles, and the pile of horizontal beds along the middle was<br>
+40,000 feet in depth. The pile for the Wahsatch Mountains was<br>
+60,000 feet thick, according to King. The beds for the<br>
+Appalachians were not laid down in a deep ocean, but in
+shallow<br>
+waters, where a gradual subsidence was in progress; and they
+at<br>
+last, when ready for the genesis, lay in a trough 40,000 feet<br>
+deep, filling the trough to the brim. It thus appears that
+epochs<br>
+of mountain-making have occurred only after long intervals of<br>
+quiet in the history of a continent."[1]</p>
+<p>[1] Dana, _Manual of Geology_, third edition, p. 794</p>
+<p>120</p>
+<p>On the western side of North America the work of<br>
+mountain-building was, indeed, on the grandest scale. For
+long<br>
+ages and through a succession of geological epochs,
+sedimentation<br>
+had proceeded so that the accumulations of Palaeozoic and<br>
+Mesozoic times had collected in the geosyncline formed by
+their<br>
+own ever increasing weight. The site of the future Laramide
+range<br>
+was in late Cretaceous times occupied by some 50,000 feet of<br>
+sedimentary deposits; but the limit had apparently been
+attained,<br>
+and at this time the Laramide range, as well as its southerly<br>
+continuation into the United States, the Rockies, had their<br>
+beginning. Chamberlin and Salisbury[1] estimate that the
+height<br>
+of the mountains developed in the Laramide range at this time
+was<br>
+20,000 feet, and that, owing to the further elevation which
+has<br>
+since taken place, from 32,000 to 35,000 feet would be their<br>
+present height if erosion had not reduced them. Thus on
+either<br>
+side of the American continent we have the same forces at
+work,<br>
+throwing up mountain ridges where the sediments had formerly
+been<br>
+shed into the ocean.</p>
+<p>These great events are of a rhythmic character; the crust, as
+it<br>
+were, pulsating under the combined influences of
+sedimentation<br>
+and denudation. The first involves downward movements under a<br>
+gathering load, and ultimately a reversal of the movement to
+one<br>
+of upheaval; the second factor, which throughout has been in</p>
+<p>[1] Chamberlin and Salisbury, _Geology_, 1906, iii., 163.</p>
+<p>121</p>
+<p>operation as creator of the sediments, then intervenes as
+an<br>
+assailant of the newly-raised mountains, transporting their<br>
+materials again to the ocean, when the rhythmic action is<br>
+restored to its first phase, and the age-long sequence of
+events<br>
+must begin all over again.</p>
+<p>It has long been inferred that compressive stress in the
+crust<br>
+must be a primary condition of these movements. The wvork<br>
+required to effect the upheavals must be derived from some<br>
+preexisting source of energy. The phenomenon&mdash;intrinsically
+one of<br>
+folding of the crust&mdash;suggests the adjustment of the
+earth-crust<br>
+to a lessening radius; the fact that great mountain-building<br>
+movements have simultaneously affected the entire earth is<br>
+certainly in favour of the view that a generally prevailing
+cause<br>
+is at the basis of the phenomenon.</p>
+<p>The compressive stresses must be confined to the upper few
+miles<br>
+of the crust, for, in fact, the downward increase of
+temperature<br>
+and pressure soon confers fluid properties on the medium, and<br>
+slow tangential compression results in hydrostatic pressure<br>
+rather than directed stresses. Thus the folding visible in
+the<br>
+mountain range, and the lateral compression arising
+therefrom,<br>
+are effects confined to the upper parts of the crust.</p>
+<p>The energy which uplifts the mountain is probably a
+surviving<br>
+part of the original gravitational potential energy of the
+crust<br>
+itself. It must be assumed that the crust in following
+downwards<br>
+the shrinking subcrustal magma, develops immense compressive<br>
+stresses in</p>
+<p>122</p>
+<p>its materials, vast geographical areas being involved.
+When<br>
+folding at length takes place along the axis of the elongated<br>
+syncline of deposition, the stresses find relief probably for<br>
+some hundreds of miles, and the region of folding now becomes<br>
+compressed in a transverse direction. As an illustration, the<br>
+Laramide range, according to Dawson, represents the reduction
+of<br>
+a surface-belt 50 miles wide to one of 25 miles. The
+marvellous<br>
+translatory movements of crustal folds from south to north<br>
+arising in the genesis of the Swiss Alps, which recent
+research<br>
+has brought to light, is another example of these movements
+of<br>
+relief, which continue to take place perhaps for many millions
+of<br>
+years after they are initiated.</p>
+<p>The result of this yielding of the crust is a buckling of
+the<br>
+surface which on the whole is directed upwards; but
+depression<br>
+also is an attendant, in many cases at least, on mountain<br>
+upheaval. Thus we find that the ocean floor is depressed into
+a<br>
+syncline along the western coast of South America; a trough<br>
+always parallel to the ranges of the Andes. The downward<br>
+deflection of the crust is of course an outcome of the same<br>
+compressive stresses which elevate the mountain.</p>
+<p>The fact that the yielding of the crust is always situated
+where<br>
+the sediments have accumulated to the greatest depth, has led
+to<br>
+attempts from time to time of establishing a physical
+connexion<br>
+between the one and the other. The best-known of these
+theories<br>
+is that of Babbage and Herschel. This seeks the connexion in
+the<br>
+rise of the</p>
+<p>123</p>
+<p>geotherms into the sinking mass of sediment and the
+consequent<br>
+increase of temperature of the earth-crust beneath. It will
+be<br>
+understood that as these isogeotherms, or levels at which the<br>
+temperature is the same, lie at a uniform distance from the<br>
+surface all over the Earth, unless where special variations
+of<br>
+conductivity may disturb them, the introduction of material<br>
+pressed downwards from above must result in these materials<br>
+partaking of the temperature proper to the depth to which
+they<br>
+are depressed. In other words the geotherms rise into the
+sinking<br>
+sediments, always, however, preserving their former average<br>
+distance from the surface. The argument is that as this
+process<br>
+undoubtedly involves the heating up of that portion of the
+crust<br>
+which the sediments have displaced downwards, the result must
+be<br>
+a local enfeeblement of the crust, and hence these areas
+become<br>
+those of least resistance to the stresses in the crust.</p>
+<p>When this theory is examined closely, we see that it only
+amounts<br>
+to saying that the bedded rocks, which have taken the place
+of<br>
+the igneous materials beneath, as a part of the rigid crust
+of<br>
+the Earth, must be less able to withstand compressive stress
+than<br>
+the average crust. For there has been no absolute rise of the<br>
+geotherms, the thermal conductivities of both classes of<br>
+materials differing but little. Sedimentary rock has merely
+taken<br>
+the place of average crust-rock, and is subjected to the same<br>
+average temperature and pressure prevailing in the
+surrounding<br>
+crust. But are there any grounds for the</p>
+<p>124</p>
+<p>assumption that the compressive resistance of a complex of<br>
+sedimentary rocks is inferior to one of igneous materials?
+The<br>
+metamorphosed siliceous sediments are among the strongest
+rocks<br>
+known as regards resistance to compressive stress; and if<br>
+limestones have indeed plastic qualities, it must be
+remembered<br>
+that their average amount is only some 5 per cent. of the
+whole.<br>
+Again, so far as rise of temperature in the upper crust may<br>
+affect the question, a temperature which will soften an
+average<br>
+igneous rock will not soften a sedimentary rock, for the
+reason<br>
+that the effect of solvent denudation has been to remove
+those<br>
+alkaline silicates which confer fusibility.</p>
+<p>When, however, we take into account the radioactive content
+of<br>
+the sediments the matter assumes a different aspect.</p>
+<p>The facts as to the general distribution of radioactive<br>
+substances at the surface, and in rocks which have come from<br>
+considerable depths in the crust, lead us to regard as
+certain<br>
+the widespread existence of heat-producing radioactive
+elements<br>
+in the exterior crust of the Earth. We find, indeed, in this
+fact<br>
+an explanation&mdash;at least in part&mdash;of the outflow of
+heat<br>
+continually taking place at the surface as revealed by the
+rising<br>
+temperature inwards. And we conclude that there must be a<br>
+thickness of crust amounting to some miles, containing the<br>
+radioactive elements.</p>
+<p>Some of the most recent measurements of the quantities of
+radium<br>
+and thorium in the rocks of igneous origin&mdash;_e.g._
+granites,<br>
+syenites, diorites, basalts, etc., show that the</p>
+<p>125</p>
+<p>radioactive heat continually given out by such rocks amounts
+to<br>
+about one millionth part of 0.6 calories per second per cubic<br>
+metre of average igneous rock. As we have to account for the<br>
+escape of about 0.0014 calorie[1] per square metre of the
+Earth's<br>
+surface per second (assuming the rise of temperature
+downwards,<br>
+_i.e._ the "gradient" of temperature, to be one degree
+centigrade<br>
+in 35 metres) the downward extension of such rocks might,
+_prima<br>
+facie_, be as much as 19 kilometres.</p>
+<p>About this calculation we have to observe that we assume
+the<br>
+average radioactivity of the materials with which we have
+dealt<br>
+at the surface to extend uniformly all the way down, _i.e._
+that<br>
+our experiments reveal the average radioactivity of a
+radioactive<br>
+crust. There is much to be said for this assumption. The
+rocks<br>
+which enter into the measurements come from all depths of the<br>
+crust. It is highly probable that the less silicious, _i.e._
+the<br>
+more basic, rocks, mainly come from considerable depths; the
+more<br>
+acid or silica-rich rocks, from higher levels in the crust.
+The<br>
+radioactivity determined as the mean of the values for these
+two<br>
+classes of rock closely agrees with that found for
+intermediate<br>
+rocks, or rocks containing an intermediate amount of silica.<br>
+Clarke contends that this last class of material probably<br>
+represents the average composition of the Earth's crust so far
+as<br>
+it has been explored by us.</p>
+<p>[1] The calorie referred to is the quantity of heat required
+to<br>
+heat one gram of water, _i.e._ one cubic centimetre of<br>
+water&mdash;through one degree centigrade.</p>
+<p>126</p>
+<p>It is therefore highly probable that the value found for the
+mean<br>
+radioactivity of acid and basic rocks, or that found for<br>
+intermediate rocks, truly represents the radioactive state of
+the<br>
+crust to a considerable depth. But it is easy to show that we<br>
+cannot with confidence speak of the thickness of this crust
+as<br>
+determinable by equating the heat outflow at the surface with
+the<br>
+heat production of this average rock.</p>
+<p>This appears in the failure of a radioactive layer, taken at
+a<br>
+thickness of about 19-kilometres, to account for the
+deep-seated<br>
+high temperatures which we find to be indicated by volcanic<br>
+phenomena at many places on the surface. It is not hard to
+show<br>
+that the 19-kilometre layer would account for a temperature
+no<br>
+higher than about 270&deg; &gt;C. at its base.</p>
+<p>It is true that this will be augmented beneath the
+sedimentary<br>
+deposits as we shall presently see; and that it is just in<br>
+association with these deposits that deep-seated temperatures
+are<br>
+most in evidence at the surface; but still the result that
+the<br>
+maximum temperature beneath the crust in general attains a
+value<br>
+no higher than 270&deg; C. is hardly tenable. We conclude, then,
+that<br>
+some other source of heat exists beneath. This may be
+radioactive<br>
+in origin and may be easily accounted for if the radioactive<br>
+materials are more sparsely distributed at the base of the
+upper<br>
+crust. Or, again, the heat may be primeval or original heat,<br>
+still escaping from a cooling world. For our present purpose
+it<br>
+does not much matter which view</p>
+<p>127</p>
+<p>we adopt. But we must recognise that the calculated depth of
+19<br>
+kilometres of crust, possessing the average radioactivity of
+the<br>
+surface, is excessive; for, in fact, we are compelled by the<br>
+facts to recognise that some other source of heat exists<br>
+beneath.</p>
+<p>If the observed surface gradient of temperature persisted<br>
+uniformly downwards, at some 35 kilometres beneath the
+surface<br>
+there would exist temperatures (of about 1000&deg; C.) adequate
+to<br>
+soften basic rocks. It is probable, however, that the
+gradient<br>
+diminishes downwards, and that the level at which such<br>
+temperatures exist lies rather deeper than this. It is,<br>
+doubtless, somewhat variable according to local conditions;
+nor<br>
+can we at all approximate closely to an estimate of the depth
+at<br>
+which the fusion temperatures will be reached, for, in fact,
+the<br>
+existence of the radioactive layer very much complicates our<br>
+estimates. In what follows we assume the depth of softening
+to<br>
+lie at about 40 kilometres beneath the surface of the normal<br>
+crust; that is 25 miles down. It is to be observed that
+Prestwich<br>
+and other eminent geologists, from a study of the facts of<br>
+crust-folding, etc., have arrived at similar estimates.[1] As
+a<br>
+further assumption we are probably not far wrong if we assign
+to<br>
+the radioactive part of this crust a thickness of about 10 or
+12<br>
+kilometres; _i.e._ six or seven miles. This is necessarily a<br>
+rough approximation only; but the conclusions at which</p>
+<p>[1] Prestwich, _Proc. Royal Soc._, xii., p. 158 _et seq._</p>
+<p>128</p>
+<p>we shall arrive are reached in their essential features
+allowing<br>
+a wide latitude in our choice of data. We shall speak of this<br>
+part of the crust as the normal radioactive layer.</p>
+<p>An important fact is evolved from the mathematical
+investigation<br>
+of the temperature conditions arising from the presence of such
+a<br>
+radioactive layer. It is found that the greatest temperature,
+due<br>
+to the radioactive heat everywhere evolved in the
+layer&mdash;_i.e._<br>
+the temperature at its base&mdash;is proportional to the square
+of the<br>
+thickness of the layer. This fact has a direct bearing on the<br>
+influence of radioactivity upon mountain elevation; as we
+shall<br>
+now find.</p>
+<p>The normal radioactive layer of the Earth is composed of
+rocks<br>
+extending&mdash;as we assume&mdash;approximately to a depth of 12
+kilometres<br>
+(7.5 miles). The temperature at the base of this layer due to
+the<br>
+heat being continually evolved in it, is, say,
+t<sub>1</sub>&deg;. Now, let us<br>
+suppose, in the trough of the geosyncline, and upon the top
+of<br>
+the normal layer, a deposit of, say, 10 kilometres (6.2 miles)
+of<br>
+sediments is formed during a long period of continental<br>
+denudation. What is the effect of this on the temperature at
+the<br>
+base of the normal layer depressed beneath this load? The
+total<br>
+thickness of radioactive rocks is now 22 kilometres.
+Accordingly<br>
+we find the new temperature t<sub>2</sub>&deg;, by the proportion
+t<sub>1</sub>&deg; : t<sub>2</sub>&deg; ::<br>
+12&deg; : 22&deg; That is, as 144 to 484. In fact, the
+temperature is more<br>
+than trebled. It is true we here assume the radioactivity of
+the<br>
+sediments</p>
+<p>129</p>
+<p>and of the normal crust to be the same. The sediments are,<br>
+however, less radioactive in the proportion of 4 to 3.<br>
+Nevertheless the effects of the increased thickness will be<br>
+considerable.</p>
+<p>Now this remarkable increase in the temperature arises
+entirely<br>
+from the condition attending the radioactive heating; and<br>
+involves something _additional_ to the temperature conditions<br>
+determined by the mere depression and thickening of the crust
+as<br>
+in the Babbage-Herschel theory. The latter theory only involves
+a<br>
+_shifting_ of the temperature levels (or geotherms) into the<br>
+deposited materials. The radioactive theory involves an
+actual<br>
+rise in the temperature at any distance from the surface; so
+that<br>
+_the level in the crust at which the rocks are softened is
+nearer<br>
+to the surface in the geosynclines than it is elsewhere in
+the<br>
+normal crust_ (Pl. XV, p. 118).</p>
+<p>In this manner the rigid part of the crust is reduced in<br>
+thickness where the great sedimentary deposits have collected.
+A<br>
+ten-kilometre layer of sediment might result in reducing the<br>
+effective thickness of the crust by 30 per cent.; a<br>
+fourteen-kilometre layer might reduce it by nearly 50 per
+cent.<br>
+Even a four-kilometre deposit might reduce the effective<br>
+resistance of the crust to compressive forces, by 10 per
+cent.</p>
+<p>Such results are, of course, approximate only. They show that
+as<br>
+the sediments grow in depth there is a rising of the geotherm
+of<br>
+plasticity&mdash;whatever its true temperature may
+be&mdash;gradually<br>
+reducing the thickness of that part</p>
+<p>130</p>
+<p>of the upper crust which is bearing the simultaneously
+increasing<br>
+compressive stresses. Below this geotherm long-continued
+stress<br>
+resolves itself into hydrostatic pressure; above it (there is,
+of<br>
+course, no sharp line of demarcation) the crust accumulates<br>
+elastic energy. The final yielding and flexure occur when the<br>
+resistant cross-section has been sufficiently diminished. It
+is<br>
+probable that there is also some outward hydrostaitic thrust
+over<br>
+the area of rising temperature, which assists in determining
+the<br>
+upward throw of the folds.</p>
+<p>When yielding has begun in any geosyncline, and the materials
+are<br>
+faulted and overthrust, there results a considerably
+increased<br>
+thickness. As an instance, consider the piling up of
+sediments<br>
+over the existing materials of the Alps, which resulted from
+the<br>
+compressive force acting from south to north in the progress
+of<br>
+Alpine upheaval. Schmidt of Basel has estimated that from 15
+to<br>
+20 kilometres of rock covered the materials of the Simplon as
+now<br>
+exposed, at the time when the orogenic forces were actively
+at<br>
+work folding and shearing the beds, and injecting into their<br>
+folds the plastic gneisses coming from beneath.[1] The
+lateral<br>
+compression of the area of deposition of the Laramide,
+already<br>
+referred to, resulted in a great thickening of the deposits.
+Many<br>
+other cases might be cited; the effect is always in some
+degree<br>
+necessarily produced.</p>
+<p>[1] Schmidt, Ec. Geol. _Helvelix_, vol. ix., No. 4, p. 590</p>
+<p>131</p>
+<p>If time be given for the heat to accumulate in the lower
+depths<br>
+of the crushed-up sediments, here is an additional source of<br>
+increased temperature. The piled-up masses of the Simplon
+might<br>
+have occasioned a rise due to radioactive heating of one or
+two<br>
+hundred degrees, or even more; and if this be added to the<br>
+interior heat, a total of from 800&deg; to 1000&deg; might have
+prevailed<br>
+in the rocks now exposed at the surface of the mountain. Even
+a<br>
+lesser temperature, accompanied by the intense pressure<br>
+conditions, might well occasion the appearances of thermal<br>
+metamorphism described by Weinschenk, and for which,
+otherwise,<br>
+there is difficulty in accounting.[1]</p>
+<p>This increase upon the primarily developed temperature
+conditions<br>
+takes place concurrently with the progress of compression;
+and<br>
+while it cannot be taken into account in estimating the<br>
+conditions of initial yielding of the crust, it adds an
+element<br>
+of instability, inasmuch as any progressive thickening by
+lateral<br>
+compression results in an accelerated rise of the goetherms.
+It<br>
+is probable that time sufficient for these effects to develop,
+if<br>
+not to their final, yet to a considerable extent, is often<br>
+available. The viscous movements of siliceous materials, and
+the<br>
+out-pouring of igneous rocks which often attend mountain<br>
+elevation, would find an explanation in such temperatures.</p>
+<p>[1] Weinschenk, _Congr&egrave;s G&eacute;ol. Internat._, 1900,
+i., p. 332.</p>
+<p>132</p>
+<p>There is no more striking feature of the part here played
+by<br>
+radioactivity than the fact that the rhythmic occurrence of<br>
+depression and upheaval succeeding each other after great<br>
+intervals of time, and often shifting their position but
+little<br>
+from the first scene of sedimentation, becomes accounted for.
+The<br>
+source of thermal energy, as we have already remarked, is in
+fact<br>
+transported with the sediments&mdash;that energy which determines
+the<br>
+place of yielding and upheaval, and ordains that the mountain<br>
+ranges shall stand around the continental borders.
+Sedimentation<br>
+from this point of view is a convection of energy.</p>
+<p>When the consolidated sediments are by these and by
+succeeding<br>
+movements forced upwards into mountains, they are exposed to<br>
+denudative effects greatly exceeding those which affect the<br>
+plains. Witness the removal during late Tertiary times of the<br>
+vast thickness of rock enveloping the Alps. Such great masses
+are<br>
+hurried away by ice, rivers, and rain. The ocean receives
+them;<br>
+and with infinite patience the world awaits the slow
+accumulation<br>
+of the radioactive energy beginning afresh upon its work. The<br>
+time for such events appears to us immense, for millions of
+years<br>
+are required for the sediments to grow in thickness, and the<br>
+geotherms to move upwards; but vast as it is, it is but a
+moment<br>
+in the life of the parent radioactive substances, whose
+atoms,<br>
+hardly diminished in numbers, pursue their changes while the<br>
+mountains come and go, and the</p>
+<p>133</p>
+<p>rudiments of life develop into its highest consummations.</p>
+<p>To those unacquainted with the results of geological<br>
+investigation the history of the mountains as deciphered in
+the<br>
+rocks seems almost incredible.</p>
+<p>The recently published sections of the Himalaya, due to H.
+H.<br>
+Hayden and the many distinguished men who have contributed to
+the<br>
+Geological Survey of India, show these great ranges to be<br>
+essentially formed of folded sediments penetrated by vast
+masses<br>
+of granite and other eruptives. Their geological history may
+be<br>
+summarised as follows</p>
+<p>The Himalayan area in pre-Cambrian times was, in its
+southwestern<br>
+extension, part of the floor of a sea which covered much of
+what<br>
+is now the Indian Peninsula. In the northern shallows of this
+sea<br>
+were laid down beds of conglomerate, shale, sandstone and<br>
+limestone, derived from the denudation of Arch&aelig;an rocks,
+which,<br>
+probably, rose as hills or mountains in parts of Peninsular
+India<br>
+and along the Tibetan edge of the Himalayan region. These
+beds<br>
+constitute the record of the long Purana Era[1] and are
+probably<br>
+coeval with the Algonkian of North America. Even in these
+early<br>
+times volcanic disturbances affected this area and the lower
+beds<br>
+of the Purana deposits were penetrated by volcanic outflows,<br>
+covered by sheets of lava, uplifted, denuded and again
+submerged</p>
+<p>[1] See footnote, p. 139.</p>
+<p>134</p>
+<p>beneath the waters. Two such periods of instability have
+left<br>
+their records in the sediments of the Purana sea.</p>
+<p>The succeeding era&mdash;the Dravidian Era&mdash;opens with
+Haimanta<br>
+(Cambrian) times. A shallow sea now extended over Kumaun,
+Garwal,<br>
+and Spiti, as well as Kashmir and ultimately over the Salt
+Range<br>
+region of the Punjab as is shown by deposits in these areas.
+This<br>
+sea was not, however, connected with the Cambrian sea of
+Europe.<br>
+The fossil faunas left by the two seas are distinct.</p>
+<p>After an interval of disturbance during closing Haimanta
+times,<br>
+geographical changes attendant on further land movements<br>
+occurred. The central sea of Asia, the Tethys, extended
+westwards<br>
+and now joined with the European Paleozoic sea; and deposits
+of<br>
+Ordovician and Silurian age were laid down:&mdash;the Muth
+deposits.</p>
+<p>The succeeding Devonian Period saw the whole Northern
+Himalayan<br>
+area under the waters of the Tethys which, eastward, extended
+to<br>
+Burma and China and, westward, covered Kashmir, the Hindu
+Kush<br>
+and part of Afghanistan. Deposits continued to be formed in
+this<br>
+area till middle Carboniferous times.</p>
+<p>Near. the close of the Dravidian Era Kashmir became convulsed
+by<br>
+volcanic disturbance and the Penjal traps were ejected. It was
+a<br>
+time of worldwide disturbance and of redistribution of land
+and<br>
+water. Carboniferous times had begun, and the geographical<br>
+changes in</p>
+<p>135</p>
+<p>the southern limits of the Tethys are regarded as ushering in
+a<br>
+new and last era in Indian geological history the Aryan Bra.</p>
+<p>India was now part of Gondwanaland; that vanished continent
+which<br>
+then reached westward to South Africa and eastward to
+Australia.<br>
+A boulder-bed of glacial origin, the Talchir Boulder-bed,
+occurs<br>
+in many surviving parts of this great land. It enters largely<br>
+into the Salt Range deposits. There is evidence that
+extensive<br>
+sheets of ice, wearing down the rocks of Rajputana, shoved
+their<br>
+moraines northward into the Salt Range Sea; then, probably, a<br>
+southern extension of the Tethys.</p>
+<p>Subsequent to this ice age the Indian coalfields of the
+Gondwana<br>
+were laid down, with beds rich in the Glossopteris and<br>
+Gangamopteris flora. This remarkable carboniferous flora
+extends<br>
+to Southern Kashmir, so that it is to be inferred that this<br>
+region was also part of the main Gondwanaland. But its
+emergence<br>
+was but for a brief period. Upper Carboniferous marine
+deposits<br>
+succeeded; and, in fact, there was no important discontinuity
+in<br>
+the deposits in this area from Panjal times till the early<br>
+Tertiaries. During the whole of which vast period Kashmir was<br>
+covered with the waters of the Tethys.</p>
+<p>The closing Dravidian disturbances of the Kashmir region did
+not,<br>
+apparently, extend to the eastern Himalayan area. But the<br>
+Carboniferous Period was, in this</p>
+<p>136</p>
+<p>eastern area, one of instability, culminating, at the close
+of<br>
+the Period, in a steady rise of the land and a northward
+retreat<br>
+of the Tethys. Nearly the entire Himalaya east of Kashmir
+became<br>
+a land surface and remained exposed to denudative forces for
+so<br>
+long a time that in places the whole of the Carboniferous,<br>
+Devonian, and a large part of the Silurian and Ordovician<br>
+deposits were removed&mdash;some thousands of feet in
+thickness&mdash;before<br>
+resubmergence in the Tethys occurred.</p>
+<p>Towards the end of the Palaeozoic Age the Aryan Tethys
+receded<br>
+westwards, but still covered the Himalaya and was still
+connected<br>
+with the European Pal&aelig;ozoic sea. The Himalayan area (as
+well as<br>
+Kashmir) remained submerged in its waters throughout the
+entire<br>
+Mesozoic Age.</p>
+<p>During Cretaceous times the Tethys became greatly
+extended,<br>
+indicating a considerable subsidence of northwestern India,<br>
+Afghanistan, Western Asia, and, probably, much of Tibet. The<br>
+shallow-water character of the deposits of the Tibetan
+Himalaya<br>
+indicates, however, a coast line near this region. Volcanic<br>
+materials, now poured out, foreshadow the incoming of the
+great<br>
+mountain-building epoch of the Tertiary Era. The enormous mass
+of<br>
+the Deccan traps, possessing a volume which has been estimated
+at<br>
+as much as 6,000 cubic miles, was probably extruded over the<br>
+Northern Peninsular region during late Cretaceous times. The
+sea<br>
+now began to retreat, and by the close of</p>
+<p>137</p>
+<p>the Eocene, it had moved westward to Sind and Baluchistan.
+The<br>
+movements of the Earth's crust were attended by intense
+volcanic<br>
+activity, and great volumes of granite were injected into the<br>
+sediments, followed by dykes and outflows of basic lavas.</p>
+<p>The Tethys vanished to return no more. It survives in the<br>
+Mediterranean of today. The mountain-building movements
+continued<br>
+into Pliocene times. The Nummulite beds of the Eocene were,
+as<br>
+the result, ultimately uplifted 18,500 feet over sea level, a<br>
+total uplift of not less than 20,000 feet.</p>
+<p>Thus with many vicissitudes, involving intervals of
+volcanic<br>
+activity, local uplifting, and extensive local denudation,
+the<br>
+Himalaya, which had originated in the sediments of the
+ancient<br>
+Purana sea, far back in pre-Cambrian times, and which had<br>
+developed potentially in a long sequence of deposits
+collecting<br>
+almost continuously throughout the whole of geological time,<br>
+finally took their place high in the heavens, where only the<br>
+winds&mdash;faint at such altitudes&mdash;and the lights of
+heaven can visit<br>
+their eternal snows.[1]</p>
+<p>In this great history it is significant that the longest<br>
+continuous series of sedimentary deposits which the world has<br>
+known has become transfigured into the loftiest elevation
+upon<br>
+its surface.</p>
+<p>[1] See A Sketch of the _Geography and Geology of the
+Himalaya<br>
+Mountains and Tibet_. By Colonel S. G. Burrard, R.E., F.R.S.,
+and<br>
+H. H. Hayden, F.G.S., Part IV. Calcutta, 1908.</p>
+<p>138</p>
+<p>The diagrammatic sections of the Himalaya accompanying this
+brief<br>
+description arc taken from the monograph of Burrard and
+Hayden<br>
+(loc. cit.) on the Himalaya. Looking at the sections we see
+that<br>
+some of the loftiest summits are sculptured in granite and
+other<br>
+crystalline rocks. The appearance of these materials at the<br>
+surface indicates the removal by denudation and the extreme<br>
+metamorphism of much sedimentary deposit. The crystalline
+rocks,<br>
+indeed, penetrate some of the oldest rocks in the world. They<br>
+appear in contact with Archaean, Algonkian or early
+Palaeozoic<br>
+rocks. A study of the sections reveals not only the severe
+earth<br>
+movements, but also the immense amount of sedimentary
+deposits<br>
+involved in the genesis of these alps. It will be noted that
+the<br>
+vertical scale is not exaggerated relatively to the<br>
+horizontal.[1] Although there is no evidence of mountain<br>
+building</p>
+<p>[1] To those unacquainted with the terminology of Indian
+geology<br>
+the following list of approximate equivalents in time will be
+of<br>
+use</p>
+<p>Ngari Khorsum Beds - Pleistocene.<br>
+Siwalik Series -     Miocene and Pliocene.<br>
+Sirmur Series -      Oligocene.<br>
+Kampa System -       Eocene and Cretaceous.<br>
+Lilang System -      Triassic.<br>
+Kuling System -      Permian.<br>
+Gondwana System -    Carboniferous.<br>
+Kenawar System -     Carboniferous and Devonian<br>
+Muth System -        Silurian.<br>
+Haimanta System -    Mid. and Lower Cambrian.<br>
+Purana Group -       Algonkian.<br>
+Vaikrita System -    Arch&aelig;an.<br>
+Daling Series -      Arch&aelig;an.</p>
+<p>139</p>
+<p>on a large scale in the Himalayan area till the Tertiary<br>
+upheaval, it is, in the majority of cases, literally correct
+to<br>
+speak of the mountains as having their generations like
+organic<br>
+beings, and passing through all the stages of birth, life,
+death<br>
+and reproduction. The Alps, the Jura, the Pyrenees, the
+Andes,<br>
+have been remade more than once in the course of geological
+time,<br>
+the _d&eacute;bris_ of a worn-out range being again uplifted in
+succeeding<br>
+ages.</p>
+<p>Thus to dwell for a moment on one case only: that of the<br>
+Pyrenees. The Pyrenees arose as a range of older Palmozoic
+rocks<br>
+in Devonian times. These early mountains, however, were<br>
+sufficiently worn out and depressed by Carboniferous times to<br>
+receive the deposits of that age laid down on the up-turned
+edges<br>
+of the older rocks. And to Carboniferous succeeded Permian,<br>
+Triassic, Jurassic and Lower Cretaceous sediments all laid
+down<br>
+in conformable sequence. There was then fresh disturbance and<br>
+upheaval followed by denudation, and these mountains, in
+turn,<br>
+became worn out and depressed beneath the ocean so that Upper<br>
+Greensand rocks were laid down unconforrnably on all beneath.
+To<br>
+these now succeeded Upper Chalk, sediments of Danian age, and
+so<br>
+on, till Eocene times, when the tale was completed and the<br>
+existing ranges rose from the sea. Today we find the folded<br>
+Nummulitic strata of Eocene age uplifted 11,000 feet, or
+within<br>
+200 feet of the greatest heights of the Pyrenees. And so they<br>
+stand awaiting</p>
+<p>140</p>
+<p>the time when once again they shall "fall into the portion
+of<br>
+outworn faces."[1]</p>
+<p>Only mountains can beget mountains. Great accumulations of<br>
+sediment are a necessary condition for the localisation of<br>
+crust-flexure. The earliest mountains arose as purely igneous
+or<br>
+volcanic elevations, but the generations of the hills soon<br>
+originated in the collection of the _d&eacute;bris_, under the
+law of<br>
+gravity, in the hollow places. And if a foundered range is<br>
+exposed now to our view encumbered with thousands of feet of<br>
+overlying sediments we know that while the one range was
+sinking,<br>
+another, from which the sediments were derived, surely
+existed.<br>
+Through the "windows" in the deep-cut rocks of the Swiss
+valleys<br>
+we see the older Carboniferous Alps looking out, revisiting
+the<br>
+sun light, after scores of millions of years of imprisonment.
+We<br>
+know that just as surely as the Alps of today are founding by<br>
+their muddy torrents ranges yet to arise, so other primeval
+Alps<br>
+fed into the ocean the materials of these buried pre-Permian<br>
+rocks.</p>
+<p>This succession of events only can cease when the rocks have
+been<br>
+sufficiently impoverished of the heat-producing substances,
+or<br>
+the forces of compression shall have died out in the surface<br>
+crust of the earth.</p>
+<p>It seems impossible to escape the conclusion that in the
+great<br>
+development of ocean-encircling areas of</p>
+<p>[1] See Prestwich, _Chemical and Physical Geology_, p.
+302.</p>
+<p>141</p>
+<p>deposition and crustal folding, the heat of radioactivity
+has<br>
+been a determining factor. We recognise in the movements of
+the<br>
+sediments not only an influence localising and accelerating<br>
+crustal movements, but one which, in subservience to the
+primal<br>
+distribution of land and water, has determined some of the<br>
+greatest geographical features of the globe.</p>
+<p>It is no more than a step to show that bound up with the<br>
+radioactive energy are most of the earthquake and volcanic<br>
+phenomena of the earth. The association of earthquakes with
+the<br>
+great geosynclines is well known. The work of De Montessus
+showed<br>
+that over 94 per cent. of all recorded shocks lie in the<br>
+geosynclinal belts. There can be no doubt that these<br>
+manifestations of instability are the results of the local<br>
+weakness and flexure which originated in the accumulation of<br>
+energy denuded from the continents. Similarly we may view in<br>
+volcanoes phenomena referable to the same fundamental cause.
+The<br>
+volcano was, in fact, long regarded as more intimately
+connected<br>
+with earthquakes than it, probably, actually is; the
+association<br>
+being regarded in a causative light, whereas the connexion is<br>
+more that of possessing a common origin. The girdle of
+volcanoes<br>
+around the Pacific and the earthquake belt coincide. Again,
+the<br>
+ancient and modern volcanoes and earthquakes of Europe are<br>
+associated with the geosyncline of the greater Mediterranean,
+the<br>
+Tethys of Mesozoic times. There is no difficulty in
+understanding<br>
+in a</p>
+<p>142</p>
+<p>general way the nature of the association. The earthquake is
+the<br>
+manifestation of rupture and slip, and, as Suess has shown,
+the<br>
+epicentres shift along that fault line where the crust has<br>
+yielded.[1] The volcano marks the spot where the zone of
+fusion<br>
+is brought so high in the fractured crust that the melted<br>
+materials are poured out upon the surface.</p>
+<p>In a recent work on the subject of earthquakes Professor
+Hobbs<br>
+writes: "One of the most interesting of the generalisations
+which<br>
+De Montessus has reached as a result of his protracted
+studies,<br>
+is that the earthquake districts on the land correspond
+almost<br>
+exactly to those belts upon the globe which were the almost<br>
+continuous ocean basins of the long Secondary era of
+geological<br>
+history. Within these belts the sedimentary formations of the<br>
+crust were laid down in the greatest thickness, and the<br>
+formations follow each other in relatively complete
+succession.<br>
+For almost or quite the whole of this long era it is
+therefore<br>
+clear that the ocean covered these zones. About them the<br>
+formations are found interrupted, and the lacuna indicate
+that<br>
+the sea invaded the area only to recede from it, and again at<br>
+some later period to transgress upon it. For a long time,<br>
+therefore, these earthquake belts were the sea
+basins&mdash;the<br>
+geosynclines. They became later the rising mountains of the<br>
+Tertiary period, and mountains they</p>
+<p>[1] Suess, _The Face of the Earth_, vol. ii., chap. ii.</p>
+<p>143</p>
+<p>are today. The earthquake belts are hence those portions of
+the<br>
+earth's crust which in recent times have suffered the
+greatest<br>
+movements in a vertical direction&mdash;they are the most
+mobile<br>
+portions of the earth's crust."[1] Whether the movements<br>
+attending mountain elevation and denudation are a connected
+and<br>
+integral part of those wide geographical changes which result
+in<br>
+submergence and elevation of large continental areas, is an<br>
+obscure and complex question. We seem, indeed, according to
+the<br>
+views of some authorities, hardly in a position to affirm
+with<br>
+certainty that such widespread movements of the land have<br>
+actually occurred, and that the phenomena are not the outcome
+of<br>
+fluctuations of oceanic level; that our observations go no<br>
+further than the recognition of positive and negative
+movements<br>
+of the strand. However this may be, the greater part of<br>
+mechanical denudation during geological time has been done on
+the<br>
+mountain ranges. It is, in short, indisputable that the
+orogenic<br>
+movements which uplift the hills have been at the basis of<br>
+geological history. To them the great accumulations of
+sediments<br>
+which now form so large a part of continental land are mainly<br>
+due. There can be no doubt of the fact that these movements
+have<br>
+swayed the entire history, both inorganic and organic, of the<br>
+world in which we live.</p>
+<p>[1] Hobbs, _Earthquakes_, p. 58.</p>
+<p>144</p>
+<p>To sum the contents of this essay in the most general terms,
+we<br>
+find that in the conception of denudation as producing the<br>
+convection and accumulation of radiothermal energy the
+surface<br>
+features of the globe receive a new significance. The heat of
+the<br>
+earth is not internal only, but rather a heat-source exists
+at<br>
+the surface, which, as we have seen, cannot prevail to the
+same<br>
+degree within; and when the conditions become favourable for
+the<br>
+aggregation of the energy, the crust, heated both from
+beneath<br>
+and from above, assumes properties more akin to those of its<br>
+earlier stages of development, the secular heat-loss being<br>
+restored in the radioactive supplies. These causes of local<br>
+mobility have been in operation, shifting somewhat from place
+to<br>
+place, and defined geographically by the continental masses<br>
+undergoing denudation, since the earliest times.</p>
+<p>145</p>
+<p><u>ALPINE STRUCTURE</u></p>
+<p>AN intelligent observer of the geological changes progressing
+in<br>
+southern Europe in Eocene times would have seen little to
+inspire<br>
+him with a premonition of the events then developing. The<br>
+Nummulitic limestones were being laid down in that enlarged<br>
+Mediterranean which at this period, save for a few islands,<br>
+covered most of south Europe. Of these stratified remains, as<br>
+well as of the great beds of Cretaceous, Jurassic, Triassic,
+and<br>
+Permian sediments beneath, our hypothetical observer would<br>
+probably have been regardless; just as today we observe, with
+an<br>
+indifference born of our transitoriness, the deposits rapidly<br>
+gathering wherever river discharge is distributing the
+sediments<br>
+over the sea-floor, or the lime-secreting organisms are
+actively<br>
+at work. And yet it took but a few millions of years to
+uplift<br>
+the deposits of the ancient Tethys; pile high its sediments
+in<br>
+fold upon fold in the Alps, the Carpathians, and the
+Himalayas;<br>
+and&mdash;exposing them to the rigours of denudation at altitudes
+where<br>
+glaciation, landslip, and torrent prevail&mdash;inaugurate a new
+epoch<br>
+of sedimentation and upheaval.</p>
+<p>146</p>
+<p>In the case of the Alps, to which we wish now specially to
+refer,<br>
+the chief upheaval appears to have been in Oligocene times,<br>
+although movement continued to the close of the Pliocene.
+There<br>
+was thus a period of some millions of years within which the<br>
+entire phenomena were comprised. Availing ourselves of
+Sollas'<br>
+computations,[1] we may sum the maximum depths of sedimentary<br>
+deposits of the geological periods concerned as
+follows:&mdash;</p>
+<p>Pliocene - - - - - 3,950 m.</p>
+<p>Miocene  - - - - - 4,250 m.</p>
+<p>Oligocene  - - - - 3,660 m.</p>
+<p>Eocene - - - - - - 6,100 m.</p>
+<p>and assuming that the orogenic forces began their work in
+the<br>
+last quarter of the Eocene period, we have a total of 13,400
+m.<br>
+as some measure of the time which elapsed. At the rate of io<br>
+centimetres in a century these deposits could not have
+collected<br>
+in less than 13.4 millions of years. It would appear that not<br>
+less than some ten millions of years were consumed in the
+genesis<br>
+of the Alps before constructive movements finally ceased.</p>
+<p>The progress of the earth-movements was attended by the
+usual<br>
+volcanic phenomena. The Oligocene and Miocene volcanoes
+extended<br>
+in a band marked by the Auvergne, the Eiffel, the Bohemian,
+and<br>
+the eastern Carpathian eruptions; and, later, towards the
+close<br>
+of the movements in Pliocene times, the south border</p>
+<p>[1] Sollas, Anniversary Address, Geol. Soc., London, 1909.</p>
+<p>147</p>
+<p>regions of the Alps became the scene of eruptions such as
+those<br>
+of Etna, Santorin, Somma (Vesuvius), etc.</p>
+<p>We have referred to these well-known episodes with two objects
+in<br>
+view: to recall to mind the time-interval involved, and the<br>
+evidence of intense crustal disturbance, both dynamic and<br>
+thermal. According to views explained in a previous essay,
+the<br>
+energetic effects of radium in the sediments and upper crust
+were<br>
+a principal factor in localising and bringing about these<br>
+results. We propose now to inquire if, also, in the more
+intimate<br>
+structure of the Alps, the radioactive energy may not have
+borne<br>
+a part.</p>
+<p>What we see today in the Alps is but a residue spared by<br>
+denudation. It is certain that vast thicknesses of material
+have<br>
+disappeared. Even while constructive effects were still in<br>
+progress, denudative forces were not idle. Of this fact the<br>
+shingle accumulations of the Molasse, where, on the northern<br>
+borders of the Alps, they stand piled into mountains, bear<br>
+eloquent testimony. In the sub-Apennine series of Italy, the<br>
+great beds of clays, marls, and limestones afford evidence of<br>
+these destructive processes continued into Pliocene times. We<br>
+have already referred to Schmidt's estimate that the
+sedimentary<br>
+covering must have in places amounted to from 15,000 to
+20,000<br>
+metres. The evidence for this is mainly tectonic or
+structural;<br>
+but is partly forthcoming in the changes which the materials
+now<br>
+open to our inspection plainly reveal. Thus it is impos-</p>
+<p>148</p>
+<p>sible to suppose that gneissic rocks can become so far plastic
+as<br>
+to flow in and around the calcareous sediments, or be
+penetrated<br>
+by the latter&mdash;as we see in the Jungfrau and
+elsewhere&mdash;unless<br>
+great pressures and high temperatures prevailed. And,
+according<br>
+to some writers, the temperatures revealed by the intimate<br>
+structural changes of rock-forming minerals must have amounted
+to<br>
+those of fusion. The existence of such conditions is supported
+by<br>
+the observation that where the.crystallisation is now the
+most<br>
+perfect, the phenomena of folding and injection are best<br>
+developed.[1] These high temperatures would appear to be<br>
+unaccountable without the intervention of radiothermal
+effects;<br>
+and, indeed, have been regarded as enigmatic by observers of
+the<br>
+phenomena in question. A covering of 20,000 metres in
+thickness<br>
+would not occasion an earth-temperature exceeding 500&deg; C. if
+the<br>
+gradients were such as obtain in mountain regions generally;
+and<br>
+600&deg; is about the limit we could ascribe to the purely
+passive<br>
+effects of such a layer in elevating the geotherms.</p>
+<p>Those who are still unacquainted with the recently
+published<br>
+observations on the structure of the Alps may find it
+difficult<br>
+to enter into what has now to be stated; for the facts are,<br>
+indeed, very different from the generally preconceived ideas
+of<br>
+mountain formation. Nor can we wonder that many geologists
+for<br>
+long held</p>
+<p>[1] Weinschenk, C. R. _Congr&egrave;s G&eacute;ol._, 1900, p.
+321, et seq.</p>
+<p>149</p>
+<p>back from admitting views which appeared so extreme.
+Receptivity<br>
+is the first virtue of the scientific mind; but, with every<br>
+desire to lay aside prejudice, many felt unequal to the<br>
+acceptance of structural features involving a folding of the<br>
+earth-crust in laps which lay for scores of miles from country
+to<br>
+country, and the carriage of mountainous materials from the
+south<br>
+of the Alps to the north, leaving them finally as Alpine
+ranges<br>
+of ancient sediments reposing on foundations of more recent
+date.<br>
+The historian of the subject will have to relate how some who<br>
+finally were most active in advancing the new views were at
+first<br>
+opposed to them. In the change of conviction of these eminent<br>
+geologists we have the strongest proof of the convincing
+nature<br>
+of the observations and the reality of the tectonic features
+upon<br>
+which the recent views are founded.</p>
+<p>The lesser mountains which stand along the northern border of
+the<br>
+great limestone Alps, those known as the Pr&eacute;alpes, present
+the<br>
+strange characteristic of resting upon materials younger than<br>
+themselves. Such mountains as the remarkable-looking Mythen,
+near<br>
+Schwyz, for instance, are weathered from masses of Triassic
+and<br>
+Jurassic rock, and repose on the much more recent Flysch. In<br>
+sharp contrast to the Flysch scenery, they stand as abrupt
+and<br>
+gigantic erratics, which have been transported from the
+central<br>
+zone of the Alps lying far to the south. They are strangers<br>
+petrologically,</p>
+<p>150</p>
+<p>stratigraphically, and geographically,[1] to the locality
+in<br>
+which they now occur. The exotic materials may be dolomites,<br>
+limestones, schists, sandstones, or rocks of igneous origin.
+They<br>
+show in every case traces of the severe dynamic actions to
+which<br>
+they have been subjected in transit. The igneous, like the<br>
+sedimentary, klippen, can be traced to distant sources; to
+the<br>
+massif of Belladonne, to Mont Blanc, Lugano, and the Tyrol.
+The<br>
+Pr&eacute;alpes are, in fact, mountains without local roots.</p>
+<p>In this last-named essential feature, the Pr&eacute;alpes do
+not differ<br>
+from the still greater limestone Alps which succeed them to
+the<br>
+south. These giants, _e.g._ the Jungfrau, Wetterhorn, Eiger,
+etc.,<br>
+are also without local foundations. They have been formed
+from<br>
+the overthrown and drawn-out anticlines of great crust-folds,<br>
+whose synclines or roots are traceable to the south side of
+the<br>
+Rhone Valley. The Bernese Oberland originated in the piling-up
+of<br>
+four great sheets or recumbent folds, one of which is
+continued<br>
+into the Pr&eacute;alpes. With Lugeon[2] we may see in the
+phenomenon of<br>
+the formation of the Pr&eacute;alpes a detail; regarding it as a
+normal<br>
+expression of that mechanism which has created the Swiss
+Alps.<br>
+For these limestone masses of the Oberland are not indications
+of<br>
+a merely local shift of the sedimentary covering of the Alps.<br>
+Almost the whole covering has</p>
+<p>[1] De Lapparent, _Trait&eacute; de G&eacute;ologie_, p.
+1,785.</p>
+<p>[2] Lugeon, _Bulletin Soc. G&eacute;ol. de France_, 1901, p.
+772.</p>
+<p>151</p>
+<p>been pushed over and piled up to the north. Lugeon[l]
+concludes<br>
+that, before denudation had done its work and cut off the<br>
+Pr&eacute;alpes from their roots, there would have been found
+sheets, to<br>
+the number of eight, superimposed and extending between the
+Mont<br>
+Blanc massif and the massif of the Finsteraarhorn: these
+sheets<br>
+being the overthrown folds of the wrinkled sedimentary
+covering.<br>
+The general nature of the alpine structure</p>
+<p>{Fig. 8}</p>
+<p>will be understood from the presentation of it
+diagrammatically<br>
+after Schmidt of Basel (Fig. 8).[2] The section extends from<br>
+north to south, and brings out the relations of the several<br>
+recumbent folds. We must imagine almost the whole of these<br>
+superimposed folds now removed from the central regions of
+the<br>
+Alps by denudation,</p>
+<p>[1] Lugeon, _loc. cit._</p>
+<p>[2] Schmidt, _Ec. Geol. Helvetiae_, vol. ix., No. 4.</p>
+<p>152</p>
+<p>and leaving the underlying gneisses rising through the remains
+of<br>
+Permian, Triassic, and Jurassic sediments; while to the north
+the<br>
+great limestone mountains and further north still, the
+Pr&eacute;alpes,<br>
+carved from the remains of the recumbent folds, now stand
+with<br>
+almost as little resemblance to the vanished mountains as the<br>
+memories of the past have to its former intense reality.</p>
+<p>These views as to the origin of the Alps, which are shared at
+the<br>
+present day by so many distinguished geologists, had their
+origin<br>
+in the labours of many now gone; dating back to Studer;
+finding<br>
+their inspiration in the work of Heim, Suess, and Marcel<br>
+Bertrand; and their consummation in that of Lugeon, Schardt,<br>
+Rothpletz, Schmidt, and many others. Nor must it be forgotten<br>
+that nearer home, somewhat similar phenomena, necessarily on
+a<br>
+smaller scale, were recognised by Lapworth, twenty-six years
+ago,<br>
+in his work on the structure of the Scottish Highlands.</p>
+<p>An important tectonic principle underlies the development of
+the<br>
+phenomena we have just been reviewing. The uppermost of the<br>
+superimposed recumbent folds is more extended in its
+development<br>
+than those which lie beneath. Passing downwards from the
+highest<br>
+of the folds, they are found to be less and less extended both
+in<br>
+the northerly and in the southerly direction, speaking of the<br>
+special case&mdash;the Alps&mdash;now before us. This feature
+might be<br>
+described somewhat differently. We might say that those folds<br>
+which had their roots farther</p>
+<p>153</p>
+<p>to the south were the most drawn-out towards the north: or
+again<br>
+we might say that the synclinal or deep-seated part of the
+fold<br>
+has lagged behind the anticlinal or what was originally the<br>
+highest part of the fold, in the advance of the latter to the<br>
+north. The anticline has advanced relatively to the syncline.
+To<br>
+this law one exception only is observed in the Swiss Alps;
+the<br>
+sheet of the Br&egrave;che (_Byecciendecke_) falls short, in its
+northerly<br>
+extension, of the underlying fold, which extends to form the<br>
+Pr&eacute;alpes.</p>
+<p>Contemplating such a generalised section as Professor
+Schmidt's,<br>
+or, indeed, more particular sections, such as those in the
+Mont<br>
+Blanc Massif by Marcel Bertrand,[1] of the Dent de Morcles,<br>
+Diablerets, Wildhorn, and Massif de la Br&egrave;che by
+Lugeon,[2] or<br>
+finally Termier's section of the Pelvoux Massif,[3] one is<br>
+reminded of the breaking of waves on a sloping beach. The
+wave,<br>
+retarded at its base, is carried forward above by its
+momentum,<br>
+and finally spreads far up on the strand; and if it could
+there<br>
+remain, the succeeding wave must necessarily find itself<br>
+superimposed upon the first. But no effects of inertia, no<br>
+kinetic effects, may be called to our aid in explaining the<br>
+formation of mountains. Some geologists have accordingly
+supposed<br>
+that in order to account for</p>
+<p>[1] Marcel Bertrand, _Cong. G&eacute;ol. Internat._, 1900,
+Guide G&eacute;ol.,<br>
+xiii. a, p. 41.</p>
+<p>[2] Lugeon, _loc. cit._, p. 773.</p>
+<p>[3] De Lapparent, _Traite de G&eacute;ol._, p. 1,773.</p>
+<p>154</p>
+<p>the recumbent folds and the peculiar phenomena of
+increasing<br>
+overlap, or _d&eacute;ferlement_, an obstacle, fixed and
+deep-seated, must<br>
+have arrested the roots or synclines of the folds, and held
+them<br>
+against translational motion, while a movement of the upper
+crust<br>
+drew out and carried forward the anticlines. Others have<br>
+contented themselves by recording the facts without advancing
+any<br>
+explanatory hypothesis beyond that embodied in the
+incontestable<br>
+statement that such phenomena must be referred to the effects
+of<br>
+tangential forces acting in the Earth's crust.</p>
+<p>It would appear that the explanation of the phenomena of<br>
+recumbent folds and their _d&eacute;ferlement_ is to be obtained
+directly<br>
+from the temperature conditions prevailing throughout the<br>
+stressed pile of rocks; and here the subject of mountain<br>
+tectonics touches that with which we were elsewhere specially<br>
+concerned&mdash;the geological influence of accumulated
+radioactive<br>
+energy.</p>
+<p>As already shown[1], a rise of temperature due to this source
+of<br>
+several hundred degrees might be added to such temperatures
+as<br>
+would arise from the mere blanketing of the Earth, and the<br>
+consequent upward movement of the geotherms. The time element
+is<br>
+here the most important consideration. The whole sequence of<br>
+events from the first orogenic movements to the final upheaval
+in<br>
+Pliocene times must probably have occupied not less than ten<br>
+million years.</p>
+<p>[1] _Mountain Genesis_, p. 129, et seq.</p>
+<p>155</p>
+<p>Unfortunately the full investigation of the distribution
+of<br>
+temperature after any given time is beset with difficulties;
+the<br>
+conditions being extremely complex. If the radioactive
+heating<br>
+was strictly adiabatic&mdash;that is, if all the heat was
+conserved and<br>
+none entered from without&mdash;the time required for the
+attainment of<br>
+the equilibrium radioactive temperature would be just about
+six<br>
+million years. The conditions are not, indeed, adiabatic; but,
+on<br>
+the other hand, the rocks upraised by lateral pressure were by
+no<br>
+means at 0&deg; C. to start with. They must be assumed to
+have<br>
+possessed such temperatures as the prior radiothermal
+effects,<br>
+and the conducted heat from the Earth's interior, may have<br>
+established.</p>
+<p>It would from this appear probable that if a duration of
+ten<br>
+million years was involved, the equilibrium radioactive<br>
+temperatures must nearly have been attained. The effects of
+heat<br>
+conducted from the underlying earthcrust have to be added,<br>
+leading to a further rise in temperature of not less than
+500&deg; or<br>
+600&deg; . In such considerations the observed indications of
+high<br>
+temperatures in materials now laid bare by denudation,
+probably<br>
+find their explanation (P1. XIX).</p>
+<p>The first fact that we infer from the former existence of such
+a<br>
+temperature distribution is the improbability, indeed the<br>
+impossibility, that anything resembling a rigid obstacle, or<br>
+deep-seated "horst," can have existed beneath the present<br>
+surface-level, and opposed the northerly movement of the<br>
+deep-lying synclines. For</p>
+<p>156</p>
+<p>such a horst can only have been constituted of some
+siliceous<br>
+rock-material such as we find everywhere rising through the<br>
+worn-down sediments of the Alps; and the idea that this could<br>
+retain rigidity under the prevailing temperature conditions,
+must<br>
+be dismissed. There is no need to labour this question; the
+horst<br>
+cannot have existed. To what, then, is the retardation of the<br>
+lower parts of the folds, their overthrow, above, to the
+north,<br>
+and their _d&eacute;ferlement_, to be ascribed?</p>
+<p>A little consideration shows that the very conditions of
+high<br>
+temperature and viscosity, which render untenable the
+hypothesis<br>
+of a rigid obstacle, suffice to afford a full explanation of
+the<br>
+retardation of the roots of the folds. For directed
+translatory<br>
+movements cannot be transmitted through a fluid, pressure in<br>
+which is necessarily hydrostatic, and must be exerted equally
+in<br>
+every direction. And this applies, not only to a fluid, but to
+a<br>
+body which will yield viscously to an impressed force. There
+will<br>
+be a gradation, according as viscosity gives place to
+rigidity,<br>
+between the states in which the applied force resolves itself<br>
+into a purely hydrostatic pressure, and in which it is<br>
+transmitted through the material as a directed thrust. The
+nature<br>
+of the force, in the most general case, of course, has to be<br>
+considered; whether it is suddenly applied and of brief
+duration,<br>
+or steady and long-continued. The latter conditions alone
+apply<br>
+to the present case.</p>
+<p>It follows from this that, although a tangential force</p>
+<p>157</p>
+<p>or pressure be engendered by a crustal movement occurring to
+the<br>
+south, and the resultant effects be transmitted northwards,
+these<br>
+stresses can only mechanically affect the rigid parts of the<br>
+crust into which they are carried. That is to say, they may<br>
+result in folding and crushing, or horizontally transporting,
+the<br>
+upper layers of the Earth's crust; but in the deeper-lying<br>
+viscous materials they must be resolved into hydrostatic
+pressure<br>
+which may act to upheave the overlying covering, but must
+refuse<br>
+to transmit the horizontal translatory movements affecting
+the<br>
+rigid materials above.</p>
+<p>Between the regions in which these two opposing conditions<br>
+prevail there will be no hard and fast line; but with the<br>
+downward increase of fluidity there will be a gradual failure
+of<br>
+the mechanical conditions and an increase of the hydrostatic.<br>
+Thus while the uppermost layers of the crust may be
+transported<br>
+to the full amount of the crustal displacement acting from
+the<br>
+south (speaking still of the Alps) deeper down there will be
+a<br>
+lesser horizontal movement, and still deeper there is no<br>
+influence to urge the viscous rock-materials in a northerly<br>
+direction. The consequences of these conditions must be the<br>
+recumbence of the folds formed under the crust-stress, and
+their<br>
+_d&eacute;ferlement_ towards the north. To see this, we must
+follow the<br>
+several stages of development.</p>
+<p>The earliest movements, we may suppose, result in flexures of
+the<br>
+Jura-Mountain type&mdash;that is, in a</p>
+<p>158</p>
+<p>succession of undulations more or less symmetrical. As the<br>
+orogenic force continues and develops, these undulations give<br>
+place to folds, the limbs of which are approximately
+vertical,<br>
+and the synclinal parts of which become ever more and more<br>
+depressed into the deeper, and necessarily hotter, underlying<br>
+materials; the anticlines being probably correspondingly<br>
+elevated. These events are slowly developed, and the
+temperature<br>
+beneath is steadily rising in consequence of the conducted<br>
+interior heat, and the steady accumulation of radioactive
+energy<br>
+in the sedimentary rocks and in the buried radioactive layer
+of<br>
+the Earth. The work expended on the crushed and sheared rock
+also<br>
+contributes to the developing temperature. Thus the geotherms<br>
+must move upwards, and the viscous conditions extend from
+below;<br>
+continually diminishing the downward range of the translatory<br>
+movements progressing in the higher parts. While above the
+folded<br>
+sediments are being carried northward, beneath they are
+becoming<br>
+anchored in the growing viscosity of the medium. The
+anticlines<br>
+will bend over, and the most southerly of the folds will<br>
+gradually become pushed or bent over those lying to the
+north.<br>
+Finally, the whole upper part of the sheaf will become<br>
+horizontally recumbent; and as the uppermost folds will be
+those<br>
+experiencing the greatest effects of the continued
+displacement,<br>
+the _d&eacute;ferlement_ or overlap must necessarily arise.</p>
+<p>We may follow these stages of mountain evolution</p>
+<p>159</p>
+<p>in a diagram (Fig. 9) in which we eliminate intermediate<br>
+conditions, and regard the early and final stages of
+development<br>
+only. In the upper sketch we suppose the lateral compression
+much<br>
+developed and the upward movement of the geotherms in
+progress.<br>
+The dotted line may be assumed to be a geotherm having a<br>
+temperature of viscosity. If the conditions here shown
+persist</p>
+<p>{Fig. 9}</p>
+<p>indefinitely, there is no doubt that the only further<br>
+developments possible are the continued crushing of the
+sediments<br>
+and the bodily displacement of the whole mass to the north.
+The<br>
+second figure is intended to show in what manner these
+results<br>
+are evaded. The geotherm of viscosity has risen. All above it
+is<br>
+affected mechanically by the continuing stress, and borne<br>
+northwards in varying</p>
+<p>160</p>
+<p>degree depending upon the rigidity. The folds have been<br>
+overthrown and drawn out; those which lay originally most to
+the<br>
+south have become the uppermost; and, experiencing the
+maximum<br>
+amount of displacement, overlap those lying beneath. There
+has<br>
+also been a certain amount of upthrow owing to the
+hydrostatic<br>
+pressure. This last-mentioned element of the phenomena is of<br>
+highly indeterminate character, for we know not the limits to<br>
+which the hydrostatic pressure may be transmitted, and where
+it<br>
+may most readily find relief. While, according to some of the<br>
+published sections, the uplifting force would seem to have<br>
+influenced the final results of the orogenic movements, a<br>
+discussion of its effects would not be profitable.</p>
+<p>161</p>
+<p><u>OTHER MINDS THAN OURS?</u></p>
+<p>IN the year 1610 Galileo, looking through his telescope
+then<br>
+newly perfected by his own hands, discovered that the planet<br>
+Jupiter was attended by a train of tiny stars which went
+round<br>
+and round him just as the moon goes round the Earth.</p>
+<p>It was a revelation too great to be credited by mankind. It
+was<br>
+opposed to the doctrine of the centrality of the Earth, for
+it<br>
+suggested that other worlds constituted like ours might exist
+in<br>
+the heavens.</p>
+<p>Some said it was a mere optic illusion; others that he who
+looked<br>
+through such a tube did it at the peril of his soul&mdash;it was
+but a<br>
+delusion of Satan. Galileo converted a few of the unbelievers
+who<br>
+had the courage to look through his telescope. To the others
+he<br>
+said, he hoped they would see those moons on their way to
+heaven.<br>
+Old as this story is it has never lost its pathos or its<br>
+teaching.</p>
+<p>The spirit which assailed Galileo's discoveries and which
+finally<br>
+was potent to overshadow his declining years, closed in
+former<br>
+days the mouths of those who asked the question written at
+the<br>
+head of this lecture: "Are we to believe that there are other<br>
+minds than ours?"</p>
+<p>162</p>
+<p>Today we consider the question in a very different spirit.
+Few<br>
+would regard it as either foolish or improper. Its intense<br>
+interest would be admitted by all, and but for the
+limitations<br>
+closing our way on every side it would, doubtless, attract
+the<br>
+most earnest investigation. Even on the mere balance of
+judgment<br>
+between the probable and the improbable, we have little to go
+on.<br>
+We know nothing definitely as to the conditions under which
+life<br>
+may originate: whether these are such as to be rare almost to<br>
+impossibility, or common almost to certainty. Only within
+narrow<br>
+limits of temperature and in presence of certain of the
+elements,<br>
+can life like ours exist, and outside these conditions life,
+if<br>
+such there be, must be different from ours. Once originated it
+is<br>
+so constituted as to assail the energies around it and to
+advance<br>
+from less to greater. Do we know more than these vague facts?<br>
+Yes, we have in our experience one other fact and one
+involving<br>
+much.</p>
+<p>We know that our world is very old; that life has been for
+many<br>
+millions of years upon it; and that Man as a thinking being
+is<br>
+but of yesterday. Here is then a condition to be fulfilled.
+To<br>
+every world is physically assigned a limit to the period
+during<br>
+which it is habitable according to our knowledge of life and
+its<br>
+necessities. This limit passed and rationality missed, the
+chance<br>
+for that world is gone for ever, and other minds than ours<br>
+assuredly will not from it contemplate the universe. Looking
+at<br>
+our own world we see that the tree of life has,</p>
+<p>163</p>
+<p>indeed, branched, leaved and, possibly, budded many times;
+it<br>
+never bloomed but once.</p>
+<p>All difficulties dissolve and speculations become needless
+under<br>
+one condition only: that in which rationality may be inferred<br>
+directly or indirectly by our observations on some sister
+world<br>
+in space, This is just the evidence which in recent years has<br>
+been claimed as derived from a study of the surface of Mars.
+To<br>
+that planet our hope of such evidence is restricted. Our
+survey<br>
+in all other directions is barred by insurmountable
+difficulties.<br>
+Unless some meteoric record reached our Earth, revelationary
+of<br>
+intelligence on a perished world, our only hope of obtaining
+such<br>
+evidence rests on the observation of Mars' surface features.
+To<br>
+this subject we confine our attention in what follows.</p>
+<p>The observations made during recent years upon the surface<br>
+features of Mars have, excusably enough, given rise to<br>
+sensational reports. We must consider under what
+circumstances<br>
+these observations have been made.</p>
+<p>Mars comes into particularly favourable conditions for<br>
+observation every fifteen years. It is true that every two
+years<br>
+and two months we overtake him in his orbit and he is then in<br>
+"opposition." That is, the Earth is between him and the sun:
+he<br>
+is therefore in the opposite part of the heavens to the sun.
+Now<br>
+Mars' orbit is very excentric, sometimes he is 139 million
+miles<br>
+from the sun, and sometimes he as as much as 154 million
+miles<br>
+from the sun. The Earth's orbit is, by comparison, almost</p>
+<p>164</p>
+<p>a circle. Evidently if we pass him when he is nearest to the
+sun<br>
+we see him at his best; not only because he is then nearest
+to<br>
+us, but because he is then also most brightly lit. In such<br>
+favourable oppositions we are within 35 million miles of him;
+if<br>
+Mars was in aphelion we would pass him at a distance of 61<br>
+million miles. Opposition occurs under the most favourable<br>
+circumstances every fifteen years. There was one in 1862,
+another<br>
+in 1877, one in 1892, and so on.</p>
+<p>When Mars is 35 million miles off and we apply a telescope<br>
+magnifying 1,000 diameters, we see him as if placed 35,000
+miles<br>
+off. This would be seven times nearer than we see the moon
+with<br>
+the naked eye. As Mars has a diameter about twice as great as<br>
+that of the moon, at such a distance he would look fourteen
+times<br>
+the diameter of the moon. Granting favourable conditions of<br>
+atmosphere much should be seen.</p>
+<p>But these are just the conditions of atmosphere of which most
+of<br>
+the European observatories cannot boast. It is to the honour
+of<br>
+Schiaparelli, of Milan, that under comparatively unfavourable<br>
+conditions and with a small instrument, he so far outstripped
+his<br>
+contemporaries in the observation of the features of Mars
+that<br>
+those contemporaries received much of his early discoveries
+with<br>
+scepticism. Light and dark outlines and patches on the
+planet's<br>
+surface had indeed been mapped by others, and even a couple
+of<br>
+the canals sighted; but at the opposition of 1877
+Schiaparelli<br>
+first mapped any considerable</p>
+<p>165</p>
+<p>number of the celebrated "canals" and showed that these<br>
+constituted an extraordinary and characteristic feature of
+the<br>
+planet's geography. He called them "canali," meaning thereby<br>
+"channels." It is remarkable indeed that a mistranslation
+appears<br>
+really responsible for the initiation of the idea that these<br>
+features are canals.</p>
+<p>In 1882 Schiaparelli startled the astronomical world by
+declaring<br>
+that he saw some of the canals double&mdash;that is appearing as
+two<br>
+parallel lines. As these lines span the planet's surface for<br>
+distances of many thousands of miles the announcement
+naturally<br>
+gave rise to much surprise and, as I have said, to much<br>
+scepticism. But he resolutely stuck to his statement. Here is
+his<br>
+map of 1882. It is sufficiently startling.</p>
+<p>In 1892 he drew a new map. It adds a little to the former
+map,<br>
+but the doubling was not so well seen. It is just the
+strangest<br>
+feature about this doubling that at times it is conspicuous,
+at<br>
+times invisible. A line which is distinctly seen as a single
+line<br>
+at one time, a few weeks later will appear distinctly to
+consist<br>
+of two parallel lines; like railway tracks, but tracks
+perhaps<br>
+200 miles apart and up to 3,000 or even 4,000 miles in
+length.</p>
+<p>Many speculations were, of course, made to account for the
+origin<br>
+of such features. No known surface peculiarity on the Earth
+or<br>
+moon at all resembles these features. The moon's surface as
+you<br>
+know is cracked and</p>
+<p>166</p>
+<p>streaked. But the cracks are what we generally find cracks
+to<br>
+be&mdash;either aimless, wandering lines, or, if radiating from
+a<br>
+centre, then lines which contract in width as they leave the<br>
+point of rupture. Where will we find cracks accurately
+parallel<br>
+to one another sweeping round a planet's face with steady<br>
+curvature for, 4,000 miles, and crossing each other as if
+quite<br>
+unhampered by one another's presence? If the phenomenon on
+Mars<br>
+be due to cracks they imply a uniformity in thickness and<br>
+strength of crust, a homogeneity, quite beyond all
+anticipation.<br>
+We will afterwards see that the course of the lines is itself<br>
+further opposed to the theory that haphazard cracking of the<br>
+crust of the planet is responsible for the lines. It was also<br>
+suggested that the surface of the planet was covered with ice
+and<br>
+that these were cracks in the ice. This theory has even
+greater<br>
+difficulties than the last to contend with. Rivers have been<br>
+suggested. A glance at our own maps at once disposes of this<br>
+hypothesis. Rivers wander just as cracks do and parallel
+rivers<br>
+like parallel cracks are unknown.</p>
+<p>In time the many suggestions were put aside. One only
+remained.<br>
+That the lines are actually the work of intelligence;
+actually<br>
+are canals, artificially made, constructed for irrigation<br>
+purposes on a scale of which we can hardly form any
+conception<br>
+based on our own earthly engineering structures.</p>
+<p>During the opposition of 1894, Percival Lowell, along with A.
+E.<br>
+Douglass, and W. H. Pickering,</p>
+<p>167</p>
+<p>observed the planet from the summit of a mountain in
+Arizona,<br>
+using an 18-inch refracting telescope and every resource of<br>
+delicate measurement and spectroscopy. So superb a climate<br>
+favoured them that for ten months the planet was kept under<br>
+continual observation. Over 900 drawings were made and not
+only<br>
+were Schiaparelli's channels confirmed, but they added 116 to
+his<br>
+79, on that portion of the planet visible at that opposition.<br>
+They made the further important discovery that the lines do
+not<br>
+stop short at the dark regions of the planet's surface, as<br>
+hitherto believed, but go right on in many cases; the
+curvature<br>
+of the lines being unaltered.</p>
+<p>Lowell is an uncompromising advocate of the "canal" theory.
+If<br>
+his arguments are correct we have at once an answer to our<br>
+question, "Are there other minds than ours?"</p>
+<p>We must consider a moment Lowell's arguments; not that it is
+my<br>
+intention to combat them. You must form your own conclusions.
+I<br>
+shall lay before you another and, as I venture to think, more<br>
+adequate hypothesis in explanation of the channels of<br>
+Schiaparelli. We learn, however, much from Lowell's book&mdash;it
+is<br>
+full of interest.[1]</p>
+<p>Lowell lays a deep foundation. He begins by showing that Mars
+has<br>
+an atmosphere. This must be granted him till some counter<br>
+observations are made.</p>
+<p>[1] _Mars_, by Percival Lowell (Longmans, Green &amp; Co.),
+1896,</p>
+<p>168</p>
+<p>It is generally accepted. What that atmosphere is, is
+another<br>
+matter. He certainly has made out a good case for the presence
+of<br>
+water as one of its constituents,</p>
+<p>It was long known that Mars possessed white regions at his
+poles,<br>
+just as our Earth does. The waning of these polar snows&mdash;if
+indeed<br>
+they are such&mdash;with the advance of the Martian summer, had
+often<br>
+been observed. Lowell plots day by day this waning. It is
+evident<br>
+from his observations that the snowfall must be light indeed.
+We<br>
+see in his map the south pole turned towards us. Mars in<br>
+perihelion always turns his south pole towards the sun and<br>
+therefore towards the Earth. We see that between the dates
+June<br>
+3rd to August 3rd&mdash;or in two months&mdash;the polar snow had
+almost<br>
+completely vanished. This denotes a very scanty covering. It
+must<br>
+be remembered that Mars even when nearest to the sun receives
+but<br>
+half our supply of solar heat and light.</p>
+<p>But other evidence exists to show that Mars probably
+possesses<br>
+but little water upon his surface. The dark places are not<br>
+water-covered, although they have been named as if they were,<br>
+indeed, seas and lakes. Various phenomena show this. The
+canals<br>
+show it. It would never do to imagine canals crossing the
+seas.<br>
+No great rivers are visible. There is a striking absence of<br>
+clouds. The atmosphere of Mars seems as serene as that of
+Venus<br>
+appears to be cloudy. Mists and clouds, however, sometime
+appear<br>
+to veil his face and add to the difficulty of</p>
+<p>169</p>
+<p>making observations near the limb of the planet. Lowell
+concludes<br>
+it must be a calm and serene atmosphere; probably only<br>
+one-seventh of our own in density. The normal height of the<br>
+barometer in Mars would then be but four and a half inches.
+This<br>
+is a pressure far less than exists on the top of the highest<br>
+terrestrial mountain. A mountain here must have an altitude
+of<br>
+about ten miles to possess so low a pressure on its summit.
+Drops<br>
+of water big enough to form rain can hardly collect in such a<br>
+rarefied atmosphere. Moisture will fall as dew or frost upon
+the<br>
+ground. The days will be hot owing to the unimpeded solar<br>
+radiation; the nights bitterly cold owing to the free
+radiation<br>
+into space.</p>
+<p>We may add that in such a climate the frost will descend<br>
+principally upon the high ground at night time and in the<br>
+advancing day it will melt. The freer radiation brings about
+this<br>
+phenomenon among our own mountains in clear and calm weather.</p>
+<p>With the progressive melting of the snow upon the pole
+Lowell<br>
+connected many phenomena upon the planet's surface of much<br>
+interest. The dark spaces appear to grow darker and more<br>
+greenish. The canals begin to show themselves and reveal
+their<br>
+double nature. All this suggests that the moisture liberated
+by<br>
+the melting of the polar snow with the advancing year, is<br>
+carrying vitality and springtime over the surface of the
+planet.<br>
+But how is the water conveyed?</p>
+<p>Lowell believes principally by the canals. These are</p>
+<p>170</p>
+<p>constructed triangulating the surface of the planet in all<br>
+directions. What we see, according to Lowell, is not the
+canal<br>
+itself, but the broad band of vegetation which springs up on
+the<br>
+arrival of the water. This band is perhaps thirty or forty
+miles<br>
+wide, but perhaps much less, for Lowell reports that the
+better<br>
+the conditions of observation the finer the lines appeared,
+so<br>
+that they may be as narrow, possibly, as fifteen miles. It is
+to<br>
+be remarked that a just visible dot on the surface of Mars
+must<br>
+possess a diameter of 30 miles. But a chain of much smaller
+dots<br>
+will be visible, just as we can see such fine objects as
+spiders'<br>
+webs. The widening of the canals is then accounted for,
+according<br>
+to Lowell, by the growth of a band of vegetation, similar to
+that<br>
+which springs into existence when the floods of the Nile
+irrigate<br>
+the plains of Egypt.</p>
+<p>If no other explanation of the lines is forthcoming than
+that<br>
+they are the work of intelligence, all this must be
+remembered.<br>
+If all other theories fail us, much must be granted Lowell.
+We<br>
+must not reason like fishes&mdash;as Lowell puts it&mdash;and
+deny that<br>
+intelligent beings can thrive in an atmospheric pressure of
+four<br>
+and half inches of mercury. Zurbriggen has recently got to
+the<br>
+top of Aconcagua, a height of 24,000 feet. On the summit of
+such<br>
+a mountain the barometer must stand at about ten inches. Why<br>
+should not beings be developed by evolution with a lung
+capacity<br>
+capable of living at two and a half times this altitude.
+Those<br>
+steadily</p>
+<p>171</p>
+<p>curved parallel lines are, indeed, very unlike anything we
+have<br>
+experience of. It would be rather to be expected that another<br>
+civilisation than our own would present many wide differences
+in<br>
+its development.</p>
+<p>What then is the picture we have before us according to
+Lowell?<br>
+It is a sufficiently dramatic one.</p>
+<p>Mars is a world whose water supply, never probably very
+abundant,<br>
+has through countless years been drying up, sinking into his<br>
+surface. But the inhabitants are making a brave fight for it,<br>
+They have constructed canals right round their world so that
+the<br>
+water, which otherwise would run to waste over the vast
+deserts,<br>
+is led from oasis to oasis. Here the great centres of<br>
+civilisation are placed: their Londons, Viennas, New Yorks.
+These<br>
+gigantic works are the works of despair. A great and
+civilised<br>
+world finds death staring it in the face. They have had to
+triple<br>
+their canals so that when the central canal has done its work
+the<br>
+water is turned into the side canals, in order to utilise it
+as<br>
+far as possible. Through their splendid telescopes they must
+view<br>
+our seas and ample rivers; and must die like travellers in
+the<br>
+desert seeing in a mirage the cool waters of a distant lake.</p>
+<p>Perhaps that lonely signal reported to have been seen in
+the<br>
+twilight limb of Mars was the outcome of pride in their
+splendid<br>
+and perishing civilisation. They would leave some memory of
+it:<br>
+they would have us witness how great was that civilisation
+before<br>
+they perish!</p>
+<p>I close this dramatic picture with the poor comfort</p>
+<p>172</p>
+<p>that several philanthropic people have suggested signalling
+to<br>
+them as a mark of sympathy. It is said that a fortune was<br>
+bequeathed to the French Academy for the purpose of
+communicating<br>
+with the Martians. It has been suggested that we could flash<br>
+signals to them by means of gigantic mirrors reflecting the
+light<br>
+of our Sun. Or, again, that we might light bonfires on a<br>
+sufficiently large scale. They would have to be about ten
+miles<br>
+in diameter! A writer in the Pall Mall Gazette suggested that<br>
+there need really be no difficulty in the matter. With the
+kind<br>
+cooperation of the London Gas Companies (this was before the
+days<br>
+of electric lighting) a signal might be sent without any<br>
+additional expense if the gas companies would consent to<br>
+simultaneously turn off the gas at intervals of five minutes
+over<br>
+the whole of London, a signal which would be visible to the<br>
+astronomers in Mars would result. He adds, naively: "If only<br>
+tried for an hour each night some results might be obtained."</p>
+<p>II</p>
+<p>We have reviewed the theory of the artificial construction of
+the<br>
+Martian lines. The amount of consideration we are disposed to<br>
+give to the supposition that there are upon Mars other minds
+than<br>
+ours will&mdash;as I have stated&mdash;necessarily depend upon
+whether or not<br>
+we can assign a probable explanation of the lines upon purely<br>
+physical grounds. If it is apparent that such</p>
+<p>173</p>
+<p>lines would be formed with great probability under certain<br>
+conditions, which conditions are themselves probable, then
+the<br>
+argument by exclusion for the existence of civilisation on
+Mars,<br>
+at once breaks down.</p>
+<p>{Fig. 10}</p>
+<p>As a romance writer is sometimes under the necessity of<br>
+transporting his readers to other scenes, so I must now ask
+you<br>
+to consent to be transported some millions</p>
+<p>174</p>
+<p>of miles into the region of the heavens which lies outside
+Mars'<br>
+orbit.</p>
+<p>Between Mars and Jupiter is a chasm of 341 millions of
+miles.<br>
+This gap in the sequence of planets was long known to be
+quite<br>
+out of keeping with the orderly succession of worlds outward
+from<br>
+the Sun. A society was formed at the close of the last
+century<br>
+for the detection of the missing world. On the first day of
+the<br>
+last century, Piazzi&mdash;who, by the way, was not a member of
+the<br>
+society&mdash;discovered a tiny world in the vacant gap.
+Although<br>
+eagerly welcomed, as better than nothing, it was a
+disappointing<br>
+find. The new world was a mere rock. A speck of about 160
+miles<br>
+in diameter. It was obviously never intended that such a body<br>
+should have all this space to itself. And, sure enough,
+shortly<br>
+after, another small world was discovered. Then another was<br>
+found, and another, and so on; and now more than 400 of these<br>
+strange little worlds are known.</p>
+<p>But whence came such bodies? The generally accepted belief
+is<br>
+that these really represent a misbegotten world. When the Sun
+was<br>
+younger he shed off the several worlds of our system as so
+many<br>
+rings. Each ring then coalesced into a world. Neptune being
+the<br>
+first born; Mercury the youngest born.</p>
+<p>After Jupiter was thrown off, and the Sun had shrunk away
+inwards<br>
+some 20o million miles, he shed off another ring. Meaning
+that<br>
+this offspring of his should grow up like the rest, develop
+into<br>
+a stable world with the</p>
+<p>175</p>
+<p>potentiality even, it may be, of becoming the abode of
+rational<br>
+beings. But something went wrong. It broke up into a ring of<br>
+little bodies, circulating around him.</p>
+<p>It is probable on this hypothesis that the number we are<br>
+acquainted with does not nearly represent the actual number
+of<br>
+past and present asteroids. It would take 125,000 of the
+biggest<br>
+of them to make up a globe as big as our world. They, so far
+as<br>
+they are known, vary in size from 10 miles to 160 miles in<br>
+diameter. It is probable then&mdash;on the assumption that this
+failure<br>
+of a world was intended to be about the mass of our
+Earth&mdash;that<br>
+they numbered, and possibly number, many hundreds of
+thousands.</p>
+<p>Some of these little bodies are very peculiar in respect to
+the<br>
+orbits they move in. This peculiarity is sometimes in the<br>
+eccentricity of their orbits, sometimes in the manner in
+which<br>
+their orbits are tilted to the general plane of the ecliptic,
+in<br>
+which all the other planets move.</p>
+<p>The eccentricity, according to Proctor, in some cases may
+attain<br>
+such extremes as to bring the little world inside Mars' mean<br>
+distance from the sun. This, as you will remember, is very
+much<br>
+less than his greatest distance from the sun. The entire belt
+of<br>
+asteroids&mdash;as known&mdash;lie much nearer to Mars than to
+Jupiter.</p>
+<p>As regards the tilt of their orbits, some are actually as much
+as<br>
+34 degrees inclined to the ecliptic, so that in fact they are<br>
+seen from the Earth among our polar constellations.</p>
+<p>176</p>
+<p>From all this you see that Mars occupies a rather hot comer
+in<br>
+the solar system. Is it not possible that more than once in
+the<br>
+remote past Mars may have encountered one of these wanderers?
+If<br>
+he came within a certain distance of the small body his great<br>
+mass would sway it from its orbit, and under certain
+conditions<br>
+he would pick up a satellite in this manner. That his present<br>
+satellites were actually so acquired is the suggestion of
+Newton,<br>
+of Yale College.</p>
+<p>Mars' satellites are indeed suspiciously and most
+abnormally<br>
+small. I have not time to prove this to you by comparison
+with<br>
+the other worlds of the solar system. In fact, they were not<br>
+discovered till 1877&mdash;although they were predicted in a
+most<br>
+curious manner, with the most uncannily accurate details, by<br>
+Swift.</p>
+<p>One of these bodies is about 36 miles in diameter. This is<br>
+Phobos. Phobos is only 3.700 miles from the surface of Mars.
+The<br>
+other is smaller and further off. He is named Deimos, and his<br>
+diameter is only 10 miles. He is 12,500 miles from Mars'
+surface.<br>
+With the exception of Phobos the next smallest satellite known
+in<br>
+the solar system is one of Saturn's&mdash;Hyperion; almost 800
+miles in<br>
+diameter. The inner one goes all round Mars in 7&frac12; hours.
+This is<br>
+Phobos' month. Mars turns on his axis in 24 hours and 40
+minutes,<br>
+so that people in Mars would see the rise of Phobos twice in
+the<br>
+course of a day and night; lie would apparently cross the sky</p>
+<p>177</p>
+<p>going against the other satellite; that is, he would move<br>
+apparently from west to east.</p>
+<p>We may at least assume as probable that other satellites
+have<br>
+been gathered by Mars in the past from the army of asteroids.</p>
+<p>Some of the satellites so picked up would be direct: that
+is,<br>
+would move round the planet in the direction of his axial<br>
+rotation. Others, on the chances, would be retrograde: that
+is,<br>
+would move against his axial rotation. They would describe
+orbits<br>
+making the same various angles with the ecliptic as do the<br>
+asteroids; and we may be sure they would be of the same
+varying<br>
+dimensions.</p>
+<p>We go on to inquire what would be the consequence to Mars of
+such<br>
+captures.</p>
+<p>A satellite captured in this manner is very likely to be
+pulled<br>
+into the Planet. This is a probable end of a satellite in any<br>
+case. It will probably be the end of our satellite too. The<br>
+satellite Phobos is indeed believed to be about to take this
+very<br>
+plunge into his planet. But in the case when the satellite
+picked<br>
+up happens to be rotating round the planet in the opposite<br>
+direction to the axial rotation of the planet, it is pretty<br>
+certain that its career as a satellite will be a brief one.
+The<br>
+reasons for this I cannot now give. If, then, Mars picked up<br>
+satellites he is very sure to have absorbed them sooner or
+later.<br>
+Sooner if they happened to be retrograde satellites, later if<br>
+direct satellites. His present satellites are recent
+additions.<br>
+They are direct.</p>
+<p>178</p>
+<p>The path of an expiring satellite will be a slow spiral
+described<br>
+round the planet. The spiral will at last, after many years,<br>
+bring the satellite down upon the surface of the primary. Its<br>
+final approach will be accelerated if the planet possesses an<br>
+atmosphere, as Mars probably does. A satellite of the
+dimensions<br>
+of Phobos&mdash;that is 36 miles in diameter&mdash;would hardly
+survive more<br>
+than 30 to 60 years within seventy miles of Mars' surface. It<br>
+will then be rotating round Mars in an hour and forty
+minutes,<br>
+moving, in fact, at the rate of 2.2 miles per second. In the<br>
+course of this 30 or 60 years it will, therefore, get round<br>
+perhaps 200,000 times, before it finally crashes down upon
+the<br>
+Martians. During this closing history of the satellite there
+is<br>
+reason to believe, however, that it would by no means pursue<br>
+continually the same path over the surface of the planet.
+There<br>
+are many disturbing factors to be considered. Being so small
+any<br>
+large surface features of Mars would probably act to perturb
+the<br>
+orbit of the satellite.</p>
+<p>The explanation of Mars' lines which I suggest, is that they
+were<br>
+formed by the approach of such satellites in former times. I
+do<br>
+not mean that they are lines cut into his surface by the
+actual<br>
+infall of a satellite. The final end of the satellite would
+be<br>
+too rapid for this, I think. But I hope to be able to show
+you<br>
+that there is reason to believe that the mere passage of the<br>
+satellite, say at 70 miles above the surface of the planet,
+will,<br>
+in itself, give rise to effects on the crust of the planet<br>
+capable</p>
+<p>179</p>
+<p>of accounting for just such single or parallel lines as we
+see.</p>
+<p>In the first place we have to consider the stability of
+the<br>
+satellite. Even in the case of a small satellite we cannot<br>
+overlook the fact that the half of the satellite near the
+planet<br>
+is pulled towards the planet by a gravitational force greater<br>
+than that attracting the outer half, and that the centrifugal<br>
+force is less on the inner than on the outer hemisphere.
+Hence<br>
+there exists a force tending to tear the satellite asunder on
+the<br>
+equatorial section tangential</p>
+<p>{Fig. 11}</p>
+<p>to the planet's surface. If in a fluid or plastic state,
+Phobos,<br>
+for instance, could not possibly exist near the planet's
+surface.<br>
+The forces referred to would decide its fate. It may be shown
+by<br>
+calculation, however, that if Phobos has the strength of
+basalt<br>
+or glass there would remain a considerable coefficient of
+safety<br>
+in favour of the satellite's stability; even when the surfaces
+of<br>
+planet and satellite were separated by only five miles.</p>
+<p>We have now to consider some things which we expect will
+happen<br>
+before the satellite takes its final plunge into the planet.</p>
+<p>180</p>
+<p>This diagram (Fig. 11) shows you the satellite travelling
+above<br>
+the surface of the planet. The satellite is advancing towards,
+or<br>
+away from, the spectator. The planet is supposed to show its<br>
+solid crust in cross section, which may be a few miles in<br>
+thickness. Below this is such a hot plastic magma as we have<br>
+reason to believe underlies much of the solid crust of our
+own<br>
+Earth. Now there is an attraction between the satellite and
+the<br>
+crust of the planet; the same gravitational attraction which<br>
+exists between every particle of matter in the universe. Let
+us<br>
+consider how this attraction will affect the planet's crust.
+I<br>
+have drawn little arrows to show how we may consider the<br>
+attraction of the satellite pulling the crust of the planet
+not<br>
+only upwards, but also pulling it inwards beneath the
+satellite.<br>
+I have made these arrows longer where calculation shows the<br>
+stress is greater. You see that the greatest lifting stress
+is<br>
+just beneath the satellite, whereas the greatest stress
+pulling<br>
+the crust in under the satellite is at a point which lies out<br>
+from under the satellite, at a considerable distance. At each<br>
+side of the satellite there is a point where the stress
+pulling<br>
+on the crust is the greatest. Of the two stresses the lifting<br>
+stress will tend to raise the crust a little; the pulling
+stress<br>
+may in certain cases actually tear the crust across; as at A
+and<br>
+B.</p>
+<p>It is possible to calculate the amount of the stress at the
+point<br>
+at each side of the satellite where the stress is at its<br>
+greatest. We must assume the satellite to be a certain size
+and<br>
+density; we must also assume the crust of</p>
+<p>181</p>
+<p>Mars to be of some certain density. To fix our ideas on
+these<br>
+points I take the case of the present satellite Phobos. What<br>
+amount of stress will he exert upon the crust of Mars when he<br>
+approaches within, say, 40 miles of the planet's surface? We
+know<br>
+his size approximately&mdash;he is about 36 miles in diameter. We
+can<br>
+guess his density to be between four times that of water and<br>
+eight times that of water. We may assume the density of Mars'<br>
+surface to be about the same as that of our Earth's surface,
+that<br>
+is three times as dense as water. We now find that the
+greatest<br>
+stress tending to rend open the surface crust of Mars will be<br>
+between 4,000 and 8,000 pounds to the square foot according
+to<br>
+the density we assign to Phobos.</p>
+<p>Will such a stress actually tear open the crust? We are not
+able<br>
+to answer this question with any certainty. Much will depend
+upon<br>
+the nature and condition of the crust. Thus, suppose that we
+are<br>
+here (Fig. 12) looking down upon the satellite which is
+moving<br>
+along slowly relatively to Mars' surface, in the direction of
+the<br>
+arrow. The satellite has just passed over a weak and cracked
+part<br>
+of the planet's crust. Here the stress has been sufficient to<br>
+start two cracks. Now you know how easy it is to tear a piece
+of<br>
+cloth when you go to the edge of it in order to make a
+beginning.<br>
+Here the stress from the satellite has got to the edge of the<br>
+crust. It is greatly concentrated just at the extremities of
+the<br>
+cracks. It will, unler such circumstances probably carry on
+the</p>
+<p>182</p>
+<p>tear. If it does not do so this time, remember the satellite
+will<br>
+some hours later be coming over the same place again, and
+then<br>
+again for, at least, many hundreds of times. Then also we are
+not<br>
+limited to the assumption that the</p>
+<p>{Fig. 12}</p>
+<p>satellite is as small as Phobos. Suppose we consider the case
+of<br>
+a satellite approaching Mars which has a diameter double that
+of<br>
+Phobos; a diameter still much less than that of the larger
+class<br>
+of asteroids. Even at the distance</p>
+<p>183</p>
+<p>of 65 miles the stress will now amount to as much as from 15
+to<br>
+30 tons per square foot. It is almost certain that such a
+stress<br>
+repeated a comparatively few times over the same parts of the<br>
+planet's surface would so rend the crust as to set up lines
+along<br>
+which plutonic action would find a vent. That is, we might
+expect<br>
+along these lines all the phenomena of upheaval and volcanic<br>
+eruption which give rise to surface elevations.</p>
+<p>The probable effect of a satellite of this dimension
+travelling<br>
+slowly relatively to the surface of Mars is, then, to leave a<br>
+very conspicuous memorial of his presence behind him. You see<br>
+from the diagram that this memorial will consist o: two
+parallel<br>
+lines of disturbance.</p>
+<p>The linear character of the gravitational effects of the<br>
+satellite is due entirely to the motion of the satellite<br>
+relatively to the surface of the planet. If the satellite
+stood<br>
+still above the surface the gravitational stress in the crust<br>
+would, of course, be exerted radially outwards from the centre
+of<br>
+the satellite. It would attain at the central point beneath
+the<br>
+satellite its maximum vertical effect, and at some radial<br>
+distance measured outwards from this point, which distance we
+can<br>
+calculate, its maximum horizontal tearing effect. When the<br>
+satellite moves relatively to the planet's crust, the
+horizontal<br>
+tearing force acts differently according to whether it is<br>
+directed in the line of motion or at right angles to this
+line.</p>
+<p>In the direction of motion we see that the satellite</p>
+<p>184</p>
+<p>creates as it passes over the crust a wave of rarefaction
+or<br>
+tension as at D, followed by compression just beneath the<br>
+satellite and by a reversed direction of gravitational pull
+as<br>
+the satellite passes onwards. These stresses rapidly replace
+one<br>
+another as the satellite travels along. They are resisted by
+the<br>
+inertia of the crust, and are taken up by its elasticity. The<br>
+nature of this succession of alternate compressions and<br>
+rarefactions in the crust possess some resemblance to those<br>
+arising in an earthquake shock.</p>
+<p>If we consider the effects taking place laterally to the line
+of<br>
+motion we see that there are no such changes in the nature of
+the<br>
+forces in the crust. At each passage of the satellite the<br>
+horizontal tearing stress increases to a maximum, when it is<br>
+exerted laterally, along the line passing through the
+horizontal<br>
+projection of the satellite and at right angles to the line
+of<br>
+motion, and again dies away. It is always a tearing stress,<br>
+renewed again and again.</p>
+<p>This effect is at its maximum along two particular parallel
+lines<br>
+which are tangents to the circle of maximum horizontal stress
+and<br>
+which run parallel with the path of the satellite. The
+distance<br>
+separating these lines depend upon the elevation of the
+satellite<br>
+above the planet's surface. Such lines mark out the
+theoretical<br>
+axes of the "double canals" which future crustal movements
+will<br>
+more fully develop.</p>
+<p>It is interesting to consider what the effect of such</p>
+<p>185</p>
+<p>conditions would be if they arose at the surface of our
+own<br>
+planet. We assume a horizontal force in the crust adequate to
+set<br>
+up tensile stresses of the order, say, of fifteen tons to the<br>
+square foot and these stresses to be repeated every few
+hours;<br>
+our world being also subject to the dynamic effects we
+recognise<br>
+in and beneath its crust.</p>
+<p>It is easy to see that the areas over which the satellite
+exerted<br>
+its gravitational stresses must become the foci &mdash;foci of
+linear<br>
+form&mdash;of tectonic developments or crust movements. The
+relief of<br>
+stresses, from whatever cause arising, in and beneath the
+crust<br>
+must surely take place in these regions of disturbance and
+along<br>
+these linear areas. Here must become concentrated the folding<br>
+movements, which are under existing conditions brought into
+the<br>
+geosynclines, along with their attendant volcanic phenomena.
+In<br>
+the case of Mars such a concentration of tectonic events
+would<br>
+not, owing to the absence of extensive subaerial denudation
+and<br>
+great oceans, be complicated by the existence of such
+synclinal<br>
+accumulations as have controlled terrestrial surface
+development.<br>
+With the passage of time the linear features would probably<br>
+develop; the energetic substratum continually asserting its<br>
+influence along such lines of weakness. It is in the highest<br>
+degree probable that radioactivity plays no less a part in<br>
+Martian history than in terrestrial. The fact of radioactive<br>
+heating allows us to assume the thin surface crust and
+continued<br>
+sub-crustal energy throughout the entire period of the
+planet's<br>
+history.</p>
+<p>186</p>
+<p>How far willl these effects resemble the double canals of
+Mars?<br>
+In this figure and in the calculations I have given you I
+have<br>
+supposed the satellite engaged in marking the planet's
+surface<br>
+with two lines separated by about the interval separating the<br>
+wider double canals of Mars&mdash;that is about 220 miles apart.
+What<br>
+the distance between the lines will be, as already stated,
+will<br>
+depend upon the height of the satellite above the surface when
+it<br>
+comes upon a part of the crust in a condition to be affected
+by<br>
+the stresses it sets up in it. If the satellite does its work
+at<br>
+a point lower down above the surface the canal produced will
+be<br>
+narrower. The stresses, too, will then be much greater. I
+must<br>
+also observe that once the crust has yielded to the pulling<br>
+stress, there is great probability that in future revolutions
+of<br>
+the satellite a central fracture will result. For then all
+the<br>
+pulling force adds itself to the lifting force and tends to
+crush<br>
+the crust inwards on the central line beneath the satellite.
+It<br>
+is thus quite possible that the passage of a satellite may
+give<br>
+rise to triple lines. There is reason to believe that the
+canals<br>
+on Mars are in some cases triple.</p>
+<p>I have spoken all along of the satellite moving slowly over
+the<br>
+surface of Mars. I have done so as I cannot at all pronounce
+so<br>
+readily on what will happen when the satellite's velocity
+over<br>
+the surface of Mars is very great. To account for all the
+lines<br>
+mapped by Lowell some of them must have been produced by<br>
+satellities moving relatively to the surface of Mars at<br>
+velocities so great</p>
+<p>187</p>
+<p>as three miles a second or even rather more. The stresses set
+up<br>
+are, in such cases, very difficult to estimate. It has not
+yet<br>
+been done. Parallel lines of greatest stress or impulse ought
+to<br>
+be formed as in the other case.</p>
+<p>I now ask your attention to another kind of evidence that
+the<br>
+lines are due in some way to the motion of satellites passing<br>
+over the surface of Mars.</p>
+<p>I may put the fresh evidence to which I refer, in this way:
+In<br>
+Lowell's map (P1. XXII, p. 192), and in a less degree in<br>
+Schiaparelli's map (ante p. 166), we are given the course of
+the<br>
+lines as fragments of incomplete curves. Now these curves
+might<br>
+have been anything at all. We must take them as they are,<br>
+however, when we apply them as a test of the theory that the<br>
+motion of a satellite round Mars can strike such lines. If it
+can<br>
+be shown that satellites revolving round Mars might strike
+just<br>
+such curves then we assume this as an added confirmation of
+the<br>
+hypothesis.</p>
+<p>We must begin by realising what sort of curves a satellite
+which<br>
+disturbs the surface of a planet would leave behind it after
+its<br>
+demise. You might think that the satellite revolving round
+and<br>
+round the planet must simply describe a circle upon the
+spherical<br>
+surface of the planet: a "great circle" as it is called; that
+is<br>
+the greatest circle which can be described upon a sphere.
+This<br>
+great circle can, however, only be struck, as you will see,
+when<br>
+the planet is not turning upon its axis: a condition not
+likely<br>
+to be realised.</p>
+<p>This diagram (PI. XXI) shows the surface of a globe</p>
+<p>188</p>
+<p>covered with the usual imaginary lines of latitude and
+longitude.<br>
+The orbit of a supposed satellite is shown by a line crossing
+the<br>
+sphere at some assumed angle with the equator. Along this
+line<br>
+the satellite always moves at uniform velocity, passing
+across<br>
+and round the back of the sphere and again across. If the
+sphere<br>
+is not turning on its polar axis then this satellite, which
+we<br>
+will suppose armed with a pencil which draws a line upon the<br>
+sphere, will strike a great circle right round the sphere.
+But<br>
+the sphere is rotating. And it is to be expected that at<br>
+different times in a planet's history the rate of rotation
+varies<br>
+very much indeed. There is reason to believe that our own day
+was<br>
+once only 2&frac12; hours long, or thereabouts. After a
+preliminary rise<br>
+in velocity of axial rotation, due to shrinkage attending
+rapid<br>
+cooling, a planet as it advances in years rotates slower and<br>
+slower. This phenomenon is due to tidal influences of the sun
+or<br>
+of satellites. On the assumption that satellites fell into
+Mars<br>
+there would in his case be a further action tending to
+shorten<br>
+his day as time went on.</p>
+<p>The effect of the rotation of the planet will be, of course,
+that<br>
+as the satellite advances with its pencil it finds the surface
+of<br>
+the sphere being displaced from under it. The line struck
+ceases<br>
+to be the great circle but wanders off in another
+curve&mdash;which is<br>
+in fact not a circle at all.</p>
+<p>You will readily see how we find this curve. Suppose the
+sphere<br>
+to be rotating at such a speed that while the satellite is<br>
+advancing the distance _Oa_, the point _b_ on the</p>
+<p>189</p>
+<p>sphere will be carried into the path of the satellite. The
+pencil<br>
+will mark this point. Similarly we find that all the points
+along<br>
+this full curved line are points which will just find
+themselves<br>
+under the satellite as it passes with its pencil. This curve
+is<br>
+then the track marked out by the revolving satellite. You see
+it<br>
+dotted round the back of the sphere to where it cuts the
+equator<br>
+at a certain point. The course of the curve and the point
+where<br>
+it cuts the equator, before proceeding on its way, entirely<br>
+depend upon the rate at which we suppose the sphere to be<br>
+rotating and the satellite to be describing the orbit. We may<br>
+call the distance measured round the planet's equator
+separating<br>
+the starting point of the curve from the point at which it
+again<br>
+meets the equator, the "span" of the curve. The span then
+depends<br>
+entirely upon the rate of rotation of the planet on its axis
+and<br>
+of the satellite in its orbit round the planet.</p>
+<p>But the nature of events might have been somewhat different.
+The<br>
+satellite is, in the figure, supposed to be rotating round
+the<br>
+sphere in the same direction as that in which the sphere is<br>
+turning. It might have been that Mars had picked up a
+satellite<br>
+travelling in the opposite direction to that in which he was<br>
+turning. With the velocity of planet on its axis and of
+satellite<br>
+in its orbit the same as before, a different curve would have<br>
+been described. The span of the curve due to a retrograde<br>
+satellite will be greater than that due to a direct
+satellite.<br>
+The retrograde satellite will have a span more than half</p>
+<p>190</p>
+<p>way round the planet, the direct satellite will describe a
+curve<br>
+which will be less than half way round the planet: that is a
+span<br>
+due to a retrograde satellite will be more than 180 degrees,<br>
+while the span due to a direct satellite will be less than
+180<br>
+degrees upon the planet's equator.</p>
+<p>I would draw your attention to the fact that what the span
+will<br>
+be does not depend upon how much the orbit of the satellite
+is<br>
+inclined to the equator. This only decides how far the curve<br>
+marked out by the satellite will recede from the equator.</p>
+<p>We find then, so far, that it is easy to distinguish between
+the<br>
+direct and the retrograde curves. The span of one is less, of
+the<br>
+other greater, than 180 degrees. The number of degrees which<br>
+either sort of curve subtends upon the equator entirely
+depends<br>
+upon the velocity of the satellite and the axial velocity of
+the<br>
+planet.</p>
+<p>But of these two velocities that of the satellite may be taken
+as<br>
+sensibly invariable, when close enough to use his pencil.
+This<br>
+depends upon the law of centrifugal force, which teaches us
+that<br>
+the mass of the planet alone decides the velocity of a
+satellite<br>
+in its orbit at any fixed distance from the planet's centre.
+The<br>
+other velocity&mdash;that of the planet upon its axis&mdash;was,
+as we have<br>
+seen, not in the past what it is now. If then Mars, at
+various<br>
+times in his past history, picked up satellites, these
+satellites<br>
+will describe curves round him having different spans which
+will<br>
+depend upon the velocity of axial rotation of Mars at the
+time<br>
+and upon this only.</p>
+<p>191</p>
+<p>In what way now can we apply this knowledge of the curves<br>
+described by a satellite as a test of the lunar origin of the<br>
+lines on Mars?</p>
+<p>To do this we must apply to Lowell's map. We pick out
+preferably,<br>
+of course, the most complete and definite curves. The chain
+of<br>
+canals of which Acheron and Erebus are members mark out a
+fairly<br>
+definite curve. We produce it by eye, preserving the curvature
+as<br>
+far as possible, till it cuts the equator. Reading the span
+on<br>
+the equator we find' it to be 255 degrees. In the first place
+we<br>
+say then that this curve is due to a retrograde satellite. We<br>
+also note on Lowell's map that the greatest rise of the curve
+is<br>
+to a point about 32 degrees north of the equator. This gives
+the<br>
+inclination of the satellite's orbit to the plane of Mars'<br>
+equator.</p>
+<p>With these data we calculate the velocity which the planet
+must<br>
+have possessed at the time the canal was formed on the
+hypothesis<br>
+that the curve was indeed the work of a satellite. The final<br>
+question now remains If we determine the curve due to this<br>
+velocity of Mars on its axis, will this curve fit that one
+which<br>
+appears on Lowell's map, and of which we have really availed<br>
+ourselves of only three points? To answer this question we
+plot<br>
+upon a sphere, the curve of a satellite, in the manner I have<br>
+described, assigning to this sphere the velocity derived from
+the<br>
+span of 255 degrees. Having plotted the curve on the sphere
+it<br>
+only remains to transfer it to Lowell's map. This is easily<br>
+done.</p>
+<p>192</p>
+<p>This map (Pl. XXII) shows you the result of treating this,
+as<br>
+well as other curves, in the manner just described. You see
+that<br>
+whether the fragmentary curves are steep and receding far
+from<br>
+the equator; or whether they are flat and lying close along
+the<br>
+equator; whether they span less or more than 180 degrees; the<br>
+curves determined on the supposition that they are the work
+of<br>
+satellites revolving round Mars agree with the mapped curves;<br>
+following them with wonderful accuracy; possessing their<br>
+properties, and, indeed, in some cases, actually coinciding
+with<br>
+them.</p>
+<p>I may add that the inadmissible span of 180 degrees and
+spans<br>
+very near this value, which are not well admissible, are so
+far<br>
+as I can find, absent. The curves are not great circles.</p>
+<p>You will require of me that I should explain the centres
+of<br>
+radiation so conspicuous here and there on Lowell's map. The<br>
+meeting of more than two lines at the oases is a phenomenon<br>
+possibly of the same nature and also requiring explanation.</p>
+<p>In the first place the curves to which I have but briefly<br>
+referred actually give rise in most cases to nodal, or
+crossing<br>
+points; sometimes on the equator, sometimes off the equator;<br>
+through which the path of the satellite returns again and
+again.<br>
+These nodal points will not, however, afford a general<br>
+explanation of the many-branched radiants.</p>
+<p>It is probable that we should refer such an appearance</p>
+<p>193</p>
+<p>as is shown at the Sinus Titanum to the perturbations of
+the<br>
+satellite's path due to the surface features on Mars. Observe<br>
+that the principal radiants are situated upon the boundary of
+the<br>
+dark regions or at the oases. Higher surface levels may be<br>
+involved in both cases. Some marked difference in topography
+must<br>
+characterise both these features. The latter may possibly<br>
+originate in the destruction of satellites. Or again, they
+may<br>
+arise in crustal disturbance of a volcanic nature, primarily<br>
+induced or localised by the crossing of two canals. Whatever
+the<br>
+origin of these features it is only necessary to assume that
+they<br>
+represent elevated features of some magnitude to explain the<br>
+multiplication of crossing lines. We must here recall what<br>
+observers say of the multiplicity of the canals. According to<br>
+Lowell, "What their number maybe lies quite beyond the<br>
+possibility of count at present; for the better our own air,
+the<br>
+more of them are visible."</p>
+<p>Such innumerable canals are just what the present theory<br>
+requires. An in-falling satellite will, in the course of the
+last<br>
+60 or 80 years of its career, circulate some 100,000 times
+over<br>
+Mars' surface. Now what will determine the more conspicuous<br>
+development of a particular canal? The mass of the satellite;
+the<br>
+state of the surface crust; the proximity of the satellite;
+and<br>
+the amount of repetition over the same ground. The after
+effects<br>
+may be taken as proportional to the primary disturbance.</p>
+<p>194</p>
+<p>It is probable that elevated surface features will influence
+two<br>
+of these conditions: the number of repetitions and the
+proximity<br>
+to the surface. A tract 100 miles in diameter and elevated
+5,000<br>
+or 10,000 feet would seriously perturb the orbit of such a body
+as<br>
+Phobos. It is to be expected that not only would it be
+effective<br>
+in swaying the orbit of the satellite in the horizontal
+direction<br>
+but also would draw it down closer to the surface. It is even
+to<br>
+be considered if such a mass might not become nodal to the<br>
+satellite's orbit, so that this passed through or above this<br>
+point at various inclinations with its primary direction. If<br>
+acting to bring down the orbit then this will quicken the
+speed<br>
+and cause the satellite further on its path to attain a
+somewhat<br>
+higher elevation above the surface. The lines most conspicuous
+in<br>
+the telescope are, in short, those which have been favoured by
+a<br>
+combination of circumstances as reviewed above, among which<br>
+crustal features have, in some cases, played a part.</p>
+<p>I must briefly refer to what is one of the most
+interesting<br>
+features of the Martian lines: the manner in which they appear
+to<br>
+come and go like visions.</p>
+<p>Something going on in Mars determines the phenomenon. On a<br>
+particular night a certain line looks single. A few nights
+later<br>
+signs of doubling are perceived, and later still, when the
+seeing<br>
+is particularly good, not one but two lines are seen. Thus, as
+an<br>
+example, we may take the case of Phison and Euphrates. Faint<br>
+glimpses of the dual state were detected in the summer</p>
+<p>195</p>
+<p>and autumn, but not till November did they appear as
+distinctly<br>
+double. Observe that by this time the Antarctic snows had
+melted,<br>
+and there was in addition, sufficient time for the moisture
+so<br>
+liberated to become diffused in the planet's atmosphere.</p>
+<p>This increase in the definition and conspicuousness of
+certain<br>
+details on Mars' surface is further brought into connection
+with<br>
+the liberation of the polar snows and the diffusion of this
+water<br>
+through the atmosphere, by the fact that the definition
+appeared<br>
+progressively better from the south pole upwards as the snow<br>
+disappeared. Lowell thinks this points to vegetation springing
+up<br>
+under the influence of moisture; he considers, however, as we<br>
+have seen, that the canals convey the moisture. He has to
+assume<br>
+the construction of triple canals to explain the doubling of
+the<br>
+lines.</p>
+<p>If we once admit the canals to be elevated ranges&mdash;not
+necessarily<br>
+of great height&mdash;the difficulty of accounting for
+increased<br>
+definition with increase of moisture vanishes. We need not<br>
+necessarily even suppose vegetation concerned. With respect
+to<br>
+this last possibility we may remark that the colour
+observations,<br>
+upon which the idea of vegetation is based, are likely to be<br>
+uncertain owing to possible fatigue effects where a dark
+object<br>
+is seen against a reddish background.</p>
+<p>However this may be we have to consider what the effects
+of<br>
+moisture increasing in the atmosphere of Mars will be with
+regard<br>
+to the visibility of elevated ranges,</p>
+<p>196</p>
+<p>We assume a serene and rare atmosphere: the nights
+intensely<br>
+cold, the days hot with the unveiled solar radiation. On the
+hill<br>
+tops the cold of night will be still more intense and so,
+also,<br>
+will the solar radiation by day. The result of this state of<br>
+things will be that the moisture will be precipitated mainly
+on<br>
+the mountains during the cold of night&mdash;in the form of
+frost&mdash;and<br>
+during the day this covering of frost will melt; and, just as
+we<br>
+see a heavy dew-fall darken the ground in summer, so the
+melting<br>
+ice will set off the elevated land against the arid plains
+below.<br>
+Our valleys are more moist than our mountains only because
+our<br>
+moisture is so abundant that it drains off the mountains into
+the<br>
+valleys. If moisture was scarce it would distil from the
+plains<br>
+to the colder elevations of the hills. On this view the<br>
+accentuation of a canal is the result of meteorological
+effects<br>
+such as would arise in the Martian climate; effects which must
+be<br>
+influenced by conditions of mountain elevation, atmospheric<br>
+currents, etc. We, thus, follow Lowell in ascribing the<br>
+accentuation of the canals to the circulation of water in
+Mars;<br>
+but we assume a simple and natural mode of conveyance and do
+not<br>
+postulate artificial structures of all but impossible
+magnitude.<br>
+That vegetation may take part in the darkening of the
+elevated<br>
+tracts is not improbable. Indeed we would expect that in the<br>
+Martian climate these tracts would be the only fertile parts
+of<br>
+the surface.</p>
+<p>Clouds also there certainly are. More recent observations</p>
+<p>197</p>
+<p>appear to have set this beyond doubt. Their presence
+obviously<br>
+brings in other possible explanations of the coming and going
+of<br>
+elevated surface features.</p>
+<p>Finally, we may ask what about the reliability of the maps?
+About<br>
+this it is to be said that the most recent map&mdash;that by
+Lowell&mdash;has<br>
+been confirmed by numerous drawings by different observers,
+and<br>
+that it is,itself the result of over 900 drawings. It has
+become<br>
+a standard chart of Mars, and while it would be rash to
+contend<br>
+for absence of errors it appears certain that the trend of
+the<br>
+principal canals may be relied on, as, also, the general
+features<br>
+of the planet's surface.</p>
+<p>The question of the possibility of illusion has frequently
+been<br>
+raised. What I have said above to a great extent answers such<br>
+objections. The close agreement between the drawings of
+different<br>
+observers ought really to set the matter at rest. Recently,<br>
+however, photography has left no further room for scepticism.<br>
+First photographed in 1905, the planet has since been<br>
+photographed many thousands of times from various
+observatories.<br>
+A majority of the canals have been so mapped. The doubling of
+the<br>
+canals is stated to have been also so recorded.[1]</p>
+<p>The hypothesis which I have ventured to put before you
+involves<br>
+no organic intervention to account for the</p>
+<p>[1] E. C. Slipher's paper in _Popular Astronomy_ for March,
+1914,<br>
+gives a good account of the recent work.</p>
+<p>198</p>
+<p>details on Mars' surface. They are physical surface
+features.<br>
+Mars presents his history written upon his face in the scars
+of<br>
+former encounters&mdash;like the shield of Sir Launcelot. Some of
+the<br>
+most interesting inferences of mathematical and physical<br>
+astronomy find a confirmation in his history. The slowing down
+in<br>
+the rate of axial rotation of the primary; the final
+inevitable<br>
+destruction of the satellite; the existence in the past of a
+far<br>
+larger number of asteroids than we at present are acquainted<br>
+with; all these great facts are involved in the theory now<br>
+advanced. If justifiably, then is Mars' face a veritable<br>
+Principia.</p>
+<p>To fully answer the question which heads these lectures,
+we<br>
+should go out into the populous solitudes (if the term be<br>
+permitted) which lie beyond our system. It is well that there
+is<br>
+now no time left to do so; for, in fact, there we can only
+dream<br>
+dreams wherein the limits of the possible and the impossible<br>
+become lost.</p>
+<p>The marvel of the infinite number of stars is not so
+marvellous<br>
+as the rationality that fain would comprehend them. In
+seeking<br>
+other minds than ours we seek for what is almost infinitely<br>
+complex and coordinated in a material universe relatively
+simple<br>
+and heterogeneous. In our mental attitude towards the great<br>
+question, this fact must be regarded as fundamental.</p>
+<p>I can only fitly close a discourse which has throughout
+weighed<br>
+the question of the living thought against the unthinking laws
+of<br>
+matter, by a paraphrase of the words</p>
+<p>199</p>
+<p>of a great poet when he, in higher and, perhaps, more
+philosophic<br>
+language, also sought to place the one in comparison with the<br>
+other.[1]</p>
+<p>Richter thought that he was&mdash;with his human heart<br>
+unstrengthened&mdash;taken by an angel among the universe of
+stars.<br>
+Then, as they journeyed, our solar system was sunken like a
+faint<br>
+star in the abyss, and they travelled yet further, on the
+wings<br>
+of thought, through mightier systems: through all the
+countless<br>
+numbers of our galaxy. But at length these also were left
+behind,<br>
+and faded like a mist into the past. But this was not all.
+The<br>
+dawn of other galaxies appeared in the void. Stars more
+countless<br>
+still with insufferable light emerged. And these also were<br>
+passed. And so they went through galaxies without number till
+at<br>
+length they stood in the great Cathedral of the Universe.
+Endless<br>
+were the starry aisles; endless the starry columns; infinite
+the<br>
+arches and the architraves of stars. And the poet saw the
+mighty<br>
+galaxies as steps descending to infinity, and as steps going
+up<br>
+to infinity.</p>
+<p>Then his human heart fainted and he longed for some narrow
+cell;<br>
+longed to lie down in the grave that he might hide from
+infinity.<br>
+And he said to the angel:</p>
+<p>"Angel, I can go with thee no farther. Is there, then, no end
+to<br>
+the universe of stars?"</p>
+<p>[1] De Quincy in his _System of the Heavens_ gives a fine<br>
+paraphrase of "Richter's Dream."</p>
+<p>200</p>
+<p>Then the angel flung up his glorious hands to the heaven
+of<br>
+heavens, saying "End is there none to the universe of God?
+Lo!<br>
+also there is no beginning."</p>
+<p>201</p>
+<p><u>THE LATENT IMAGE</u> [1]</p>
+<p>My inclination has led me, in spite of a lively dread of<br>
+incurring a charge of presumption, to address you principally
+on<br>
+that profound and most subtle question, the nature and mode
+of<br>
+formation of the photographic image. I am impelled to do so,
+not<br>
+only because the subject is full of fascination and
+hopefulness,<br>
+but because the wide topics of photographic methods or<br>
+photographic applications would be quite unfittingly handled
+by<br>
+the president you have chosen.</p>
+<p>I would first direct your attention to Sir James Dewar's<br>
+remarkable result that the photographic plate retains<br>
+considerable power of forming the latent image at
+temperatures<br>
+approaching the absolute zero&mdash;a result which, as I
+submit,<br>
+compels us to regard the fundamental effects progressing in
+the<br>
+film under the stimulus of light undulations as other than
+those<br>
+of a purely chemical nature. But few, if any, instances of<br>
+chemical combination or decomposition are known at so low a<br>
+temperature. Purely chemical actions cease, indeed, at far
+higher<br>
+temperatures, fluorine being among the few bodies which still<br>
+show</p>
+<p>[1] Presidential address to the Photographic Convention of
+the<br>
+United Kingdom, July, 1905. _Nature_, Vol. 72, p. 308.</p>
+<p>202</p>
+<p>chemical activity at the comparatively elevated temperature
+of<br>
+-180&deg; C. In short, this result of Sir James Dewar's suggests
+that<br>
+we must seek for the foundations of photographic action in
+some<br>
+physical or intra-atomic effect which, as in the case of<br>
+radioactivity or fluorescence, is not restricted to intervals
+of<br>
+temperature over which active molecular vis viva prevails. It<br>
+compels us to regard with doubt the role of oxidation or
+other<br>
+chemical action as essential, but rather points to the view
+that<br>
+such effects must be secondary or subsidiary. We feel, in a
+word,<br>
+that we must turn for guidance to some purely photo-physical<br>
+effect.</p>
+<p>Here, in the first place, we naturally recall the views of
+Bose.<br>
+This physicist would refer the formation of the image to a
+strain<br>
+of the bromide of silver molecule under the electric force in
+the<br>
+light wave, converting it into what might be regarded as an<br>
+allotropic modification of the normal bromide which
+subsequently<br>
+responds specially to the attack of the developer. The
+function<br>
+of the sensitiser, according to this view, is to retard the<br>
+recovery from strain. Bose obtained many suggestive parallels<br>
+between the strain phenomena he was able to observe in silver
+and<br>
+other substances under electromagnetic radiation and the<br>
+behaviour of the photographic plate when subjected to<br>
+long-continued exposure to light.</p>
+<p>This theory, whatever it may have to recommend it, can hardly
+be<br>
+regarded as offering a fundamental explanation. In the first<br>
+place, we are left in the dark as to what</p>
+<p>203</p>
+<p>the strain may be. It may mean many and various things. We
+know<br>
+nothing as to the inner mechanism of its effects upon
+subsequent<br>
+chemical actions&mdash;or at least we cannot correlate it with
+what is<br>
+known of the physics of chemical activity. Finally, as will
+be<br>
+seen later, it is hardly adequate to account for the varying<br>
+degrees of stability which may apparently characterise the
+latent<br>
+image. Still, there is much in Bose's work deserving of
+careful<br>
+consideration. He has by no means exhausted the line of<br>
+investigation he has originated.</p>
+<p>Another theory has doubtless been in the minds of many. I
+have<br>
+said we must seek guidance in some photo-physical phenomenon.<br>
+There is one such which preeminently connects light and
+chemical<br>
+phenomena through the intermediary of the effects of the
+former<br>
+upon a component part of the atom. I refer to the phenomena
+of<br>
+photo-electricity.</p>
+<p>It was ascertained by Hertz and his immediate successors
+that<br>
+light has a remarkable power of discharging negative<br>
+electrification from the surface of bodies&mdash;especially
+from<br>
+certain substances. For long no explanation of the cause of
+this<br>
+appeared. But the electron&mdash;the ubiquitous electron&mdash;is
+now known<br>
+with considerable certainty to be responsible. The effect of
+the<br>
+electric force in the light wave is to direct or assist the<br>
+electrons contained in the substance to escape from the
+surface<br>
+of the body. Each electron carries away a very small charge
+of<br>
+negative electrification. If, then, a body is</p>
+<p>204</p>
+<p>originally charged negatively, it will be gradually discharged
+by<br>
+this convective process. If it is not charged to start with,
+the<br>
+electrons will still be liberated at the surface of the body,
+and<br>
+this will acquire a positive charge. If the body is
+positively<br>
+charged at first, we cannot discharge it by illumination.</p>
+<p>It would be superfluous for me to speak here of the nature
+of<br>
+electrons or of the various modes in which their presence may
+be<br>
+detected. Suffice it to say, in further connection with the
+Hertz<br>
+effect, that when projected among gaseous molecules the
+electron<br>
+soon attaches itself to one of these. In other words, it
+ionises<br>
+a molecule of the gas or confers its electric charge upon it.
+The<br>
+gaseous molecule may even be itself disrupted by impact of
+the<br>
+electron, if this is moving fast enough, and left bereft of
+an<br>
+electron.</p>
+<p>We must note that such ionisation may be regarded as
+conferring<br>
+potential chemical properties upon the molecules of the gas
+and<br>
+upon the substance whence the electrons are derived. Similar<br>
+ionisation under electric forces enters, as we now believe,
+into<br>
+all the chemical effects progressing in the galvanic cell,
+and,<br>
+indeed, generally in ionised solutes.</p>
+<p>An experiment will best illustrate the principles I wish
+to<br>
+remind you of. A clean aluminium plate, carefully insulated by
+a<br>
+sulphur support, is faced by a sheet of copper-wire-gauze
+placed<br>
+a couple of centimetres away from it. The gauze is maintained
+at<br>
+a high positive</p>
+<p>205</p>
+<p>potential by this dry pile. A sensitive gold-leaf electroscope
+is<br>
+attached to the aluminium plate, and its image thrown upon
+the<br>
+screen. I now turn the light from this arc lamp upon the wire<br>
+gauze, through which it in part passes and shines upon the<br>
+aluminium plate. The electroscope at once charges up rapidly.<br>
+There is a liberation of negative electrons at the surface of
+the<br>
+aluminium; these, under the attraction of the positive body,
+are<br>
+rapidly removed as ions, and the electroscope charges up<br>
+positively.</p>
+<p>Again, if I simply electrify negatively this aluminium plate
+so<br>
+that the leaves of the attached electroscope diverge widely,
+and<br>
+now expose it to the rays from the arc lamp, the charge, as
+you<br>
+see, is very rapidly dissipated. With positive electrification
+of<br>
+the aluminium there is no effect attendant on the
+illumination.</p>
+<p>Thus from the work of Hertz and his successors we know
+that<br>
+light, and more particularly what we call actinic light, is
+an<br>
+effective means of setting free electrons from certain<br>
+substances. In short, our photographic agent, light, has the<br>
+power of expelling from certain substances the electron which
+is<br>
+so potent a factor in most, if not in all, chemical effects.
+I<br>
+have not time here to refer to the work of Elster and Geitel<br>
+whereby they have shown that this action is to be traced to
+the<br>
+electric force in the light wave, but must turn to the
+probable<br>
+bearing of this phenomenon on the familiar facts of
+photography.<br>
+I assume that the experiment I have shown you is the most</p>
+<p>206</p>
+<p>fundamental photographic experiment which it is now in our
+power<br>
+to make.</p>
+<p>We must first ask from what substances can light liberate<br>
+electrons. There are many&mdash;metals as well as non-metals
+and<br>
+liquids. It is a very general phenomenon and must operate
+widely<br>
+throughout nature. But what chiefly concerns the present<br>
+consideration is the fact that the haloid salts of silver are<br>
+vigorously photo-electric, and, it is suggestive, possess,<br>
+according to Schmidt, an activity in the descending order<br>
+bromide, chloride, iodide. This is, in other words, their
+order<br>
+of activity as ionisers (under the proper conditions) when<br>
+exposed to ultra-violet light. Photographers will recognise
+that<br>
+this is also the order of their photographic sensitiveness.</p>
+<p>Another class of bodies also concerns our subject: the
+special<br>
+sensitisers used by the photographer to modify the spectral<br>
+distribution of sensibility of the haloid salts, _e.g._
+eosine,<br>
+fuchsine, cyanine. These again are electron-producers under
+light<br>
+stimulus. Now it has been shown by Stoletow, Hallwachs, and<br>
+Elster and Geitel that there is an intimate connection
+between<br>
+photo-electric activity and the absorption of light by the<br>
+substance, and, indeed, that the particular wave-lengths
+absorbed<br>
+by the substance are those which are effective in liberating
+the<br>
+electrons. Thus we have strong reason for believing that the<br>
+vigorous photo-electric activity displayed by the special<br>
+sensitisers must be dependent upon their colour absorption.
+You<br>
+will recognise that this is just</p>
+<p>207</p>
+<p>the connection between their photographic effects and
+their<br>
+behaviour towards light.</p>
+<p>There is yet another suggestive parallel. I referred to
+the<br>
+observation of Sir James Dewar as to the continued
+sensitiveness<br>
+of the photographic film at the lowest attained extreme of<br>
+temperature, and drew the inference that the fundamental<br>
+photographic action must be of intra-atomic nature, and not<br>
+dependent upon the vis viva of the molecule or atom. In then<br>
+seeking the origin of photographic action in photo-electric<br>
+phenomena we naturally ask, Are these latter phenomena also<br>
+traceable at low temperatures? If they are, we are entitled
+to<br>
+look upon this fact as a qualifying characteristic or as
+another<br>
+link in the chain of evidence connecting photographic with<br>
+photo-electric activity.</p>
+<p>I have quite recently, with the aid of liquid air supplied to
+me<br>
+from the laboratory of the Royal Dublin Society, tested the<br>
+photo-sensibility of aluminium and also of silver bromide down
+to<br>
+temperatures approaching that of the liquid air. The mode of<br>
+observation is essentially that of Schmidt&mdash;what he terms
+his<br>
+static method. The substance undergoing observation is,
+however,<br>
+contained at the bottom of a thin copper tube, 5 cm. in
+diameter,<br>
+which is immersed to a depth of about 10 cm in liquid air.
+The<br>
+tube is closed above by a paraffin stopper which carries a
+thin<br>
+quartz window as well as the sulphur tubes through which the<br>
+connections pass. The air within is very carefully dried by<br>
+phosphorus</p>
+<p>208</p>
+<p>pentoxide before the experiment. The arc light is used as
+source<br>
+of illumination. It is found that a vigorous photo-electric<br>
+effect continues in the case of the clean aluminium. In the
+case<br>
+of the silver bromide a distinct photo-electric effect is
+still<br>
+observed. I have not had leisure to make, as yet, any
+trustworthy<br>
+estimate of the percentage effect at this temperature in the
+case<br>
+of either substance. Nor have I determined the temperature<br>
+accurately. The latter may be taken as roughly about -150&deg;
+C,</p>
+<p>Sir James Dewar's actual measilrements afforded twenty per
+cent.<br>
+of the normal photographic effect at -180&deg; C. and ten per
+cent.<br>
+at the temperature of -252.5&deg; C.</p>
+<p>With this much to go upon, and the important additional fact
+that<br>
+the electronic discharge&mdash;as from the X-ray tube or from<br>
+radium&mdash;generates the latent image, I think we are fully
+entitled<br>
+to suggest, as a legitimate lead to experiment, the
+hypothesis<br>
+that the beginnings of photographic action involve an
+electronic<br>
+discharge from the light-sensitive molecule; in other words
+that<br>
+the latent image is built up of ionised atoms or molecules
+the<br>
+result of the photo-electric effect on the illuminated silver<br>
+haloid, and it is upon these ionised atoms that the chemical<br>
+effects of the developer are subsequently directed. It may be<br>
+that the liberated electrons ionise molecules not directly<br>
+affected, or it may be that in their liberation they disrupt<br>
+complex molecules built up in the ripening of the</p>
+<p>209</p>
+<p>emulsion. With the amount we have to go upon we cannot venture
+to<br>
+particularise. It will be said that such an action must be in<br>
+part of the nature of a chemical effect. This must be
+admitted,<br>
+and, in so far as the rearrangement of molecular fabrics is<br>
+involved, the result will doubtless be controlled by
+temperature<br>
+conditions. The facts observed by Sir James Dewar support
+this.<br>
+But there is involved a fundamental process&mdash;the liberation
+of the<br>
+electron by the electric force in the light wave, which is a<br>
+physical effect, and which, upon the hypothesis of its reality
+as<br>
+a factor in forming the latent image, appears to explain<br>
+completely the outstanding photographic sensitiveness of the
+film<br>
+at temperatures far below those at which chemical actions in<br>
+general cease.</p>
+<p>Again, we may assume that the electron&mdash;producing power
+of the<br>
+special sensitiser or dye for the particular ray it absorbs
+is<br>
+responsible, or responsible in part, for the special<br>
+sensitiveness it confers upon the film. Sir Wm. Abney has
+shown<br>
+that these sensitisers are active even if laid on as a varnish
+on<br>
+the sensitive surface and removed before development. It must
+be<br>
+remembered, however, that at temperatures of about -50&deg;
+these<br>
+sensitisers lose much of their influence on the film; as I
+have<br>
+pointed out in a paper read before the Photographic Convention
+of<br>
+1894.</p>
+<p>It. appears to me that on these views the curious phenomenon
+of<br>
+recurrent reversals does not present a problem hopeless of<br>
+explanation. The process of photo-</p>
+<p>210</p>
+<p>ionisation constituting the latent image, where the ion is<br>
+probably not immediately neutralised by chemical combination,<br>
+presents features akin to the charging of a capacity&mdash;say a
+Leyden<br>
+jar. There may be a rising potential between the groups of
+ions<br>
+until ultimately a point is attained when there is a
+spontaneous<br>
+neutralisation. I may observe that the phenomena of reversal<br>
+appear to indicate that the change in the silver bromide<br>
+molecule, whatever be its nature, is one of gradually
+increasing<br>
+intensity, and finally attains a maximum when a return to the<br>
+original condition occurs. The maximum is the point of most<br>
+intense developable image. It is probable that the
+sensitiser&mdash;in<br>
+this case the gelatin in which the bromide of silver is<br>
+immersed&mdash;plays a part in the conditions of stability which
+are<br>
+involved.</p>
+<p>Of great interest in all our considerations and theories is
+the<br>
+recent work of Wood on photographic reversal. The result of
+this<br>
+work is&mdash;as I take it&mdash;to show that the stability of
+the latent<br>
+image may be very various according to the mode of its
+formation.<br>
+Thus it appears that the sort of latent effect which is
+produced<br>
+by pressure or friction is the least stable of any. This may
+be<br>
+reversed or wiped out by the application of any other known
+form<br>
+of photographic stimulus. Thus an exposure to X-rays will<br>
+obliterate it, or a very brief exposure to light. The latent<br>
+image arising from X-rays is next in order of increasing<br>
+stability. Light action will remove this. Third in order is a<br>
+very brief light-shock or sudden flash. This</p>
+<p>211</p>
+<p>cannot be reversed by any of the foregoing modes of
+stimulation,<br>
+but a long-continued undulatory stimulus, as from lamp-light,<br>
+will reverse it. Last and most stable of all is the gradually<br>
+built-up configuration due to long-continued light exposure.
+This<br>
+can only be reversed by overdoing it according to the known
+facts<br>
+of recurrent reversal. Wood takes occasion to remark that
+these<br>
+phenomena are in bad agreement with the strain theory of Bose.
+We<br>
+have, in fact, but the one resource&mdash;the allotropic
+modification<br>
+of the haloid&mdash;whereby to explain all these orders of
+stability.<br>
+It appears to me that the elasticity of the electronic theory
+is<br>
+greater. The state of the ionised system may be very various<br>
+according as it arises from continued rhythmic effects or
+from<br>
+unorganised shocks. The ionisation due to X-rays or to
+friction<br>
+will probably be quite unorganised, that due to light more or<br>
+less stable according to the gradual and gentle nature of the<br>
+forces at work. I think we are entitled to conclude that on
+the<br>
+whole there is nothing in Wood's beautiful experiments opposed
+to<br>
+the photo-electric origin of photographic effects, but that
+they<br>
+rather fall in with what might be anticipated according to
+that<br>
+theory.</p>
+<p>When we look for further support to the views I have laid
+before<br>
+you we are confronted with many difficulties. I have not as
+yet<br>
+detected any electronic discharge from the film under light<br>
+stimulus. This may be due to my defective experiments, or to
+a<br>
+fact noted by Elster and Geitel concerning the photo-electric<br>
+properties of gelatin.</p>
+<p>212</p>
+<p>They obtained a vigorous effect from Balmain's luminous
+paint,<br>
+but when this was mixed in gelatin there was no external
+effect.<br>
+Schmidt's results as to the continuance of photo-electric<br>
+activity when bodies in general are dissolved in each other
+lead<br>
+us to believe that an actual conservative property of the
+medium<br>
+and not an effect of this on the luminous paint is here
+involved.<br>
+This conservative effect of the gelatin may be concerned with
+its<br>
+efficacy as a sensitiser.</p>
+<p>In the views I have laid before you I have endeavoured to
+show<br>
+that the recent addition to our knowledge of the electron as
+an<br>
+entity taking part in many physical and chemical effects
+should<br>
+be kept in sight in seeking an explanation of the mode of
+origin<br>
+of the latent image.[1]</p>
+<p>[1] For a more detailed account of the subject, and some<br>
+ingenious extensions of the views expressed above, see<br>
+_Photo-Electricity_, by H. Stanley Allen: Longmans, Green &amp;
+Ca.,<br>
+1913.</p>
+<p>213</p>
+<p><u>PLEOCHROIC HALOES</u> [1]</p>
+<p>IT is now well established that a helium atom is expelled
+from<br>
+certain of the radioactive elements at the moment of<br>
+transformation. The helium atom or alpha ray leaves the<br>
+transforming atom with a velocity which varies in the
+different<br>
+radioactive elements, but which is always very great,
+attaining<br>
+as much as 2 x 109 cms. per second; a velocity which, if<br>
+unchecked, would carry the atom round the earth in less than
+two<br>
+seconds. The alpha ray carries a positive charge of double
+the<br>
+ionic amount.</p>
+<p>When an alpha ray is discharged from the transforming
+element<br>
+into a gaseous medium its velocity is rapidly checked and its<br>
+energy absorbed. A certain amount of energy is thus
+transferred<br>
+from the transforming atom to the gas. We recognise this
+energy<br>
+in the gas by the altered properties of the latter; chiefly
+by<br>
+the fact that it becomes a conductor of electricity. The<br>
+mechanism by which this change is effected is in part known.
+The<br>
+atoms of the gas, which appear to be freely penetrated by the<br>
+alpha ray, are so far dismembered as to yield charged
+electrons<br>
+or ions; the atoms remaining charged with an equal and
+opposite<br>
+charge. Such a medium of</p>
+<p>[1] Being the Huxley Lecture, delivered at the University
+of<br>
+Birmingham on October 30th, 1912. Bedrock, Jan., 1913.</p>
+<p>214</p>
+<p>free electric charges becomes a conductor of electricity
+by<br>
+convection when an electromotive force is applied. The gas
+also<br>
+acquires other properties in virtue of its ionisation. Under<br>
+certain conditions it may acquire chemical activity and new<br>
+combinations may be formed or existing ones broken up. When
+its<br>
+initial velocity is expended the helium atom gives up its<br>
+properties as an alpha ray and thenceforth remains possessed
+of<br>
+the ordinary varying velocity of thermal agitation. Bragg and<br>
+Kleeman and others have investigated the career of the alpha
+ray<br>
+when its path or range lies in a gas at ordinary or
+obtainable<br>
+conditions of pressure and temperature. We will review some
+of<br>
+the facts ascertained.</p>
+<p>The range or distance traversed in a gas at ordinary pressures
+is<br>
+a few centimetres. The following table, compiled by Geiger,
+gives<br>
+the range in air at the temperature of 15&deg; C.:</p>
+<p>               cms.                   cms.                  
+cms.<br>
+Uranium 1 -   2.50    Thorium -     2.72     Radioactinium 
+4.60<br>
+Uranium 2 -   2.90    Radiothorium  3.87     Actinium X -  
+4.40<br>
+Ionium -      3.00    Thorium X -   4.30     Act Emanation 
+5.70<br>
+Radium -      3.30    Th Emanation  5.00     Actinium A -  
+6.50<br>
+Ra Emanation  4.16    Thorium A -   5.70     Actinium C -  
+5.40<br>
+Radium A -    4.75    Thorium C1 -  4.80<br>
+Radium C -    6.94    Thorium C2 -  8.60<br>
+Radium F -    3.77</p>
+<p>It will be seen that the ray of greatest range is that
+proceeding<br>
+from thorium C2, which reaches a distance of 8.6 cms. In the<br>
+uranium family the fastest ray is</p>
+<p>215</p>
+<p>that of radium C. It attains 6.94 cms. There is thus an<br>
+appreciable difference between the ultimate distances
+traversed<br>
+by the most energetic rays of the two families. The shortest<br>
+ranges are those of uranium 1 and 2.</p>
+<p>The ionisation effected by these rays is by no means
+uniform<br>
+along the path of the ray. By examining the conductivity of
+the<br>
+gas at different points along the path of the ray, the
+ionisation<br>
+at these points may be determined. At the limits of the range
+the<br>
+ionisation</p>
+<p>{Fig. 13}</p>
+<p>ceases. In this manner the range is, in fact, determined.
+The<br>
+dotted curve (Fig. 13) depicts the recent investigation of
+the<br>
+ionisation effected by a sheaf of parallel rays of radium C
+in<br>
+air, as determined by Geiger. The range is laid out
+horizontally<br>
+in centimetres. The numbers of ions are laid out vertically.
+The<br>
+remarkable nature of the results will be at once apparent. We<br>
+should have expected that the ray at the beginning of its
+path,<br>
+when its velocity and kinetic energy were greatest, would
+have<br>
+been more effective than towards the end of its range</p>
+<p>216</p>
+<p>when its energy had almost run out. But the curve shows that
+it<br>
+is just the other way. The lagging ray, about to resign its<br>
+ionising properties, becomes a much more efficient ioniser
+than<br>
+it was at first. The maximum efficiency is, however, in the
+case<br>
+of a bundle of parallel rays, not quite at the end of the
+range,<br>
+but about half a centimetre from it. The increase to the
+maximum<br>
+is rapid, the fall from the maximum to nothing is much more<br>
+rapid.</p>
+<p>It can be shown that the ionisation effected anywhere along
+the<br>
+path of the ray is inversely proportional to the velocity of
+the<br>
+ray at that point. But this evidently does not apply to the
+last<br>
+5 or 10 mms. of the range where the rate of ionisation and of
+the<br>
+speed of the ray change most rapidly. To what are the
+changing<br>
+properties of the rays near the end of their path to be
+ascribed?<br>
+It is only recently that this matter has been elucidated.</p>
+<p>When the alpha ray has sufficiently slowed down, its power
+of<br>
+passing right through atoms, without appreciably experiencing
+any<br>
+effects from them, diminishes. The opposing atoms begin to
+exert<br>
+an influence on the path of the ray, deflecting it a little.
+The<br>
+heavier atoms will deflect it most. This effect has been very<br>
+successfully investigated by Geiger. It is known as
+"scattering."<br>
+The angle of scattering increases rapidly with the decrease
+of<br>
+velocity. Now the effect of the scattering will be to cause
+some<br>
+of the rays to complete their ranges</p>
+<p>217</p>
+<p>or, more accurately, to leave their direct line of advance
+a<br>
+little sooner than others. In the beautiful experiments of C.
+T.<br>
+R. Wilson we are enabled to obtain ocular demonstration of
+the<br>
+scattering. The photograph (Fig. 14.), which I owe to the<br>
+kindness of Mr. Wilson, shows the deflection of the ray
+towards<br>
+the end of its path. In</p>
+<p>{Fig. 14}</p>
+<p>this case the path of the ray has been rendered visible by
+the<br>
+condensation of water particles under the influence of the<br>
+ionisation; the atmosphere in which the ray travels being in
+a<br>
+state of supersaturation with water vapour at the instant of
+the<br>
+passage of the ray. It is evident that if we were observing
+the<br>
+ionisation along a sheaf of parallel rays, all starting with<br>
+equal velocity,</p>
+<p>218</p>
+<p>the effect of the bending of some of the rays near the end
+of<br>
+their range must be to cause a decrease in the aggregate<br>
+ionisation near the very end of the ultimate range. For, in
+fact,<br>
+some of the rays complete their work of ionising at points in
+the<br>
+gas before the end is reached. This is the cause, or at least
+an<br>
+important contributory cause, of the decline in the
+ionisation<br>
+near the end of the range, when the effects of a bundle of
+rays<br>
+are being observed. The explanation does not suggest that the<br>
+ionising power of any one ray is actually diminished before
+it<br>
+finally ceases to be an alpha ray.</p>
+<p>The full line in Fig. 13 gives the ionisation curve which it
+may<br>
+be expected would be struck out by a single alpha ray. In it
+the<br>
+ionisation goes on increasing till it abruptly ceases
+altogether,<br>
+with the entire loss of the initial kinetic energy of the<br>
+particle.</p>
+<p>A highly remarkable fact was found out by Bragg. The effect
+of<br>
+the atom traversed by the ray in checking the velocity of the
+ray<br>
+is independent of the physical and chemical condition of the<br>
+atom. He measured the "stopping power" of a medium by the<br>
+distance the ray can penetrate into it compared with the
+distance<br>
+to which it can penetrate in air. The less the ratio the
+greater<br>
+is the stopping power. The stopping power of a substance is<br>
+proportional to the square root of its atomic weight. The<br>
+stopping power of an atom is not altered if it is in chemical<br>
+union with another atom. The atomic weight is the one quality
+of<br>
+importance. The physical</p>
+<p>219</p>
+<p>state, whether the element is in the solid, liquid or
+gaseous<br>
+state, is unimportant. And when we deal with molecules the<br>
+stopping power is simply proportional to the sum of the
+square<br>
+roots of the atomic weights of the atoms entering into the<br>
+molecule. This is the "additive law," and it obviously enables
+us<br>
+to calculate what the range in any substance of known
+chemical<br>
+composition and density will be, compared with the range in
+air.</p>
+<p>This is of special importance in connection with phenomena
+we<br>
+have presently to consider. It means that, knowing the
+chemical<br>
+composition and density of any medium whatsoever, solid,
+liquid<br>
+or gaseous, we can calculate accurately the distance to which
+any<br>
+particular alpha ray will penetrate. Nor have the temperature
+and<br>
+pressure to which the medium is subjected any influence save
+in<br>
+so far as they may affect the proximity of one atom to
+another.<br>
+The retardation of the alpha ray in the atom is not affected.</p>
+<p>This valuable additive law, however, cannot be applied in<br>
+strictness to the amount of ionisation attending the ray. The<br>
+form of the molecule, or more generally its volume, may have
+an<br>
+influence upon this. Bragg draws the conclusion, from this
+fact<br>
+as well as from the notable increase of ionisation with loss
+of<br>
+speed, that the ionisation is dependent upon the time the ray<br>
+spends in the molecule. The energy of the ray is, indeed,
+found<br>
+to be less efficient in producing ionisation in the smaller<br>
+atomm.</p>
+<p>220</p>
+<p>Before leaving our review of the general laws governing
+the<br>
+passage of alpha rays through matter, another point of
+interest<br>
+must be referred to. We have hitherto spoken in general terms
+of<br>
+the fact that ionisation attends the passage of the ray. We
+have<br>
+said nothing as to the nature of the ionisation so produced.
+But<br>
+in point of fact the ionisation due to an alpha ray is sui<br>
+generis. A glance at one of Wilson's photographs (Fig. 14.)<br>
+illustrates this. The white streak of water particles marks
+the<br>
+path of the ray. The ions produced are evidently closely
+crowded<br>
+along the track of the ray. They have been called into
+existence<br>
+in a very minute instant of time. Now we know that ions of<br>
+opposite sign if left to themselves recombine. The rate of<br>
+recombination depends upon the product of the number of each
+sign<br>
+present in unit volume. Here the numbers are very great and
+the<br>
+volume very small. The ionic density is therefore high, and<br>
+recombination very rapidly removes the ions after they are<br>
+formed. We see here a peculiarity of the ionisation effected
+by<br>
+alpha rays. It is linear in distribution and very local. Much
+of<br>
+the ionisation in gases is again undone by recombination
+before<br>
+diffusion leads to the separation of the ions. This "initial<br>
+recombination" is greatest towards the end of the path of the
+ray<br>
+where the ionisation is a maximum. Here it may be so
+effective<br>
+that the form of the curve is completely lost unless a very
+large<br>
+electromotive force is used to separate the ions when the<br>
+ionisation is being investigated.</p>
+<p>221</p>
+<p>We have now reviewed recent work at sufficient length to<br>
+understand something of the nature of the most important
+advance<br>
+ever made in our knowledge of the atom. Let us glance briefly
+at<br>
+what we have learned. The radioactive atom in sinking to a
+lower<br>
+atomic weight casts out with enormous velocity an atom of
+helium.<br>
+It thus loses a definite portion of its mass and of its
+energy.<br>
+Helium which is chemically one of the most inert of the
+elements,<br>
+is, when possessed of such great kinetic energy, able to<br>
+penetrate and ionise the atoms which it meets in its path. It<br>
+spends its energy in the act of ionising them, coming to
+rest,<br>
+when it moves in air, in a few centimetres. Its initial
+velocity<br>
+depends upon the particular radioactive element which has
+given<br>
+rise to it. The length of its path is therefore different<br>
+according to the radioactive element from which it proceeds.
+The<br>
+retardation which it experiences in its path depends entirely<br>
+upon the atomic weight of the atoms which it traverses. As it<br>
+advances in its path its effectiveness in ionising the atom<br>
+rapidly increases and attains a very marked maximum. In a gas
+the<br>
+ions produced being much crowded together recombine rapidly;
+so<br>
+rapidly that the actual ionisation may be quite concealed
+unless<br>
+a sufficiently strong electric force is applied to separate
+them.<br>
+Such is a brief summary of the climax of radioactive<br>
+discovery:&mdash;the birth, life and death of the alpha ray. Its
+advent<br>
+into Science has altered fundamentally our conception of</p>
+<p>222</p>
+<p>matter. It is fraught with momentous bearings upon
+Geological<br>
+Science. How the work of the alpha ray is sometimes recorded<br>
+visibly in the rocks and what we may learn from that record,
+I<br>
+propose now to bring before you.</p>
+<p>In certain minerals, notably the brown variety of mica known
+as<br>
+biotite, the microscope reveals minute circular marks
+occurring<br>
+here and there, quite irregularly. The most usual appearance
+is<br>
+that of a circular area darker in colour than the surrounding<br>
+mineral. The radii of these little disc-shaped marks when
+well<br>
+defined are found to be remarkably uniform, in some cases
+four<br>
+hundredths of a millimetre and in others three hundredths,
+about.<br>
+These are the measurements in biotite. In other minerals the<br>
+measurements are not quite the same as in biotite. Such
+minute<br>
+objects are quite invisible to the naked eye. In some rocks
+they<br>
+are very abundant, indeed they may be crowded together in
+such<br>
+numbers as to darken the colour of the mineral containing
+them.<br>
+They have long been a mystery to petrologists.</p>
+<p>Close examination shows that there is always a small speck of
+a<br>
+foreign body at the centre of the circle, and it is often<br>
+possible to identify the nature of this central substance,
+small<br>
+though it be. Most generally it is found to be the mineral<br>
+zircon. Now this mineral was shown by Strutt to contain radium
+in<br>
+quantities much exceeding those found in ordinary rock<br>
+substances.</p>
+<p>223</p>
+<p>Some other mineral may occasionally form the nucleus, but
+we<br>
+never find any which is not known to be specially likely to<br>
+contain a radioactive substance. Another circumstance we
+notice.<br>
+The smaller this central nucleus the more perfect in form is
+the<br>
+darkened circular area surrounding it. When the circle is
+very<br>
+perfect and the central mineral clearly defined at its centre
+we<br>
+find by measurement that the radius of the darkened area is<br>
+generally 0.033 mm. It may sometimes be 0.040 mm. These are<br>
+always the measurements in biotite. In other minerals the
+radii<br>
+are a little different.</p>
+<p>We see in the photograph (Pl. XXIII, lower figure), much<br>
+magnified, a halo contained in biotite. We are looking at a<br>
+region in a rock-section, the rock being ground down to such
+a<br>
+thickness that light freely passes through it. The biotite is
+in<br>
+the centre of the field. Quartz and felspar surround it. The
+rock<br>
+is a granite. The biotite is not all one crystal. Two
+crystals,<br>
+mutually inclined, are cut across. The halo extends across
+both<br>
+crystals, but owing to the fact that polarised light is used
+in<br>
+taking the photograph it appears darker in one crystal than
+in<br>
+the other. We see the zircon which composes the nucleus. The
+fine<br>
+striated appearance of the biotite is due to the cleavage of
+that<br>
+mineral, which is cut across in the section.</p>
+<p>The question arises whether the darkened area surrounding
+the<br>
+zircon may not be due to the influence of the radioactive<br>
+substances contained in the zircon. The</p>
+<p>224</p>
+<p>extraordinary uniformity of the radial measurements of
+perfectly<br>
+formed haloes (to use the name by which they have long been<br>
+known) suggests that they may be the result of alpha
+radiation.<br>
+For in that case, as we have seen, we can at once account for
+the<br>
+definite radius as simply representing the range of the ray
+in<br>
+biotite. The furthest-reaching ray will define the radius of
+the<br>
+halo. In the case of the uranium family this will be radium
+C,<br>
+and in the case of thorium it will be thorium C. Now here we<br>
+possess a means of at once confirming or rejecting the view
+that<br>
+the halo is a radioactive phenomenon and occasioned by alpha<br>
+radiation; for we can calculate what the range of these rays
+will<br>
+be in biotite, availing ourselves of Bragg's additive law,<br>
+already referred to. When we make this calculation we find
+that<br>
+radium C just penetrates 0.033 mm. and thorium C 0.040 mm.
+The<br>
+proof is complete that we are dealing with the effects of
+alpha<br>
+rays. Observe now that not only is the coincidence of
+measurement<br>
+and calculation a proof of the view that alpha radiation has<br>
+occasioned the halo, but it is a very complete verification
+of<br>
+the important fact stated by Bragg, that the stopping power<br>
+depends solely on the atomic weight of the atoms traversed by
+the<br>
+ray.</p>
+<p>We have seen that our examination of the rocks reveals only
+the<br>
+two sorts of halo: the radium halo and the thorium halo. This
+is<br>
+not without teaching. For why not find an actinium halo? Now<br>
+Rutherford long ago suggested that this element and its<br>
+derivatives were</p>
+<p>225</p>
+<p>probably an offspring of the uranium family; a side branch, as
+it<br>
+were, in the formation of which relatively few transforming
+atoms<br>
+took part. On Rutherford's theory then, actinium should
+always<br>
+accompany uranium and radium, but in very subordinate amount.
+The<br>
+absence of actinium haloes clearly supports this view. For if<br>
+actinium was an independent element we would be sure to find<br>
+actinium haloes. The difference in radius should be
+noticeable.<br>
+If, on the other hand, actinium</p>
+<p>was always associated with uranium and radium, then its
+effects<br>
+would be submerged in those of the much more potent effects
+of<br>
+the uranium series of elements.</p>
+<p>It will have occurred to you already that if the
+radioactive<br>
+origin of the halo is assured the shape of a halo is not
+really<br>
+circular, but spherical. This is so. There is no such thing as
+a<br>
+disc-shaped halo. The halo is a spherical volume containing
+the<br>
+radioactive nucleus at its centre. The true radius of the
+halo<br>
+may, therefore, only be measured on sections passing through
+the<br>
+nucleus.</p>
+<p>226</p>
+<p>In order to understand the mode of formation of a halo we
+may<br>
+profitably study on a diagram the events which go on within
+the<br>
+halo-sphere. Such a diagram is seen in Fig. 15. It shows to<br>
+relatively correct scale the limiting range of all the
+alpha-ray<br>
+producing members of the uranium and thorium families. We
+know<br>
+that each member of a family will exist in equilibrium amount<br>
+within the nucleus possessing the parent element. Each alpha
+ray<br>
+leaving the nucleus will just attain its range and then cease
+to<br>
+affect the mica. Within the halosphere, there must be,
+therefore,<br>
+the accumulated effects of the influences of all the rays.
+Each<br>
+has its own sphere of influence, and the spheres are all<br>
+concentric.</p>
+<p>The radii in biotite of the several spheres are given in
+the<br>
+following table</p>
+<p>URANIUM FAMILY.<br>
+Radium C -       0.0330 mm.<br>
+Radium A -       0.0224 mm.<br>
+Ra Emanation -   0.0196 mm.<br>
+Radium F -       0.0177 mm.<br>
+Radium -         0.0156 mm.<br>
+Ionium -         0.0141 mm.<br>
+Uranium 1 -      0.0137 mm.<br>
+Uranium 2 -      0.0118 mm.</p>
+<p>THORIUM FAMILY.<br>
+Thorium CE -     0.040 mm.<br>
+Thorium A -      0.026 mm.<br>
+Th Emanation -   0.023 mm.<br>
+Thorium Ci -     0.022 mm.<br>
+Thorium X -      0.020 mm.<br>
+Radiothorium -   0.119 mm.<br>
+Thorium -        0.013 mm.</p>
+<p>In the photograph (Pl. XXIV, lower figure), we see a uranium
+and<br>
+a thorium halo in the same crystal of mica. The mica is
+contained<br>
+in a rock-section and is cut across the cleavage. The effects
+of<br>
+thorium Ca are clearly shown</p>
+<p>227</p>
+<p>as a lighter border surrounding the accumulated inner
+darkening<br>
+due to the other thorium rays. The uranium halo (to the
+right)<br>
+similarly shows the effects of radium C, but less distinctly.</p>
+<p>Haloes which are uniformly dark all over as described above
+are,<br>
+in point of fact, "over-exposed"; to borrow a familiar<br>
+photographic term. Haloes are found which show much beautiful<br>
+internal detail. Too vigorous action obscures this detail just
+as<br>
+detail is lost in an over-exposed photograph. We may again
+have<br>
+"under-exposed" haloes in which the action of the several rays
+is<br>
+incomplete or in which the action of certain of the rays has
+left<br>
+little if any trace. Beginning at the most under-exposed
+haloes<br>
+we find circular dark marks having the radius 0.012 or 0.013
+mm.<br>
+These haloes are due to uranium, although their inner
+darkening<br>
+is doubtless aided by the passage of rays which were too few
+to<br>
+extend the darkening beyond the vigorous effects of the two<br>
+uranium rays. Then we find haloes carried out to the radii
+0.016,<br>
+0.018 and 0.019 mm. The last sometimes show very beautiful
+outer<br>
+rings having radial dimensions such as would be produced by<br>
+radium A and radium C. Finally we may have haloes in which<br>
+interior detail is lost so far out as the radius due to
+emanation<br>
+or radium A, while outside this floats the ring due to radium
+C.<br>
+Certain variations of these effects may occur, marking,<br>
+apparently, different stages of exposure. Plates XXIII and
+XXIV<br>
+(upper figure) illustrate some of these stages;</p>
+<p>228</p>
+<p>the latter photograph being greatly enlarged to show clearly
+the<br>
+halo-sphere of radium A.</p>
+<p>In most of the cases mentioned above the structure
+evidently<br>
+shows the existence of concentric spherical shells of
+darkened<br>
+biotite. This is a very interesting fact. For it proves that
+in<br>
+the mineral the alpha ray gives rise to the same increased<br>
+ionisation towards the end of its range, as Bragg determined
+in<br>
+the case of gases. And we must conclude that the halo in
+every<br>
+case grows in this manner. A spherical shell of darkened
+biotite<br>
+is first produced and the inner colouration is only effected
+as<br>
+the more feeble ionisation along the track of the ray in
+course<br>
+of ages gives rise to sufficient alteration of the mineral.
+This<br>
+more feeble ionisation is, near the nucleus, enhanced in its<br>
+effects by the fact that there all the rays combine to
+increase<br>
+the ionisation and, moreover, the several tracks are there<br>
+crowded by the convergency to the centre. Hence the most<br>
+elementary haloes seldom show definite rings due to uranium,<br>
+etc., but appear as embryonic disc-like markings. The
+photographs<br>
+illustrate many of the phases of halo development.</p>
+<p>Rutherford succeeded in making a halo artificially by
+compressing<br>
+into a capillary glass tube a quantity of the emanation of<br>
+radium. As the emanation decayed the various derived products<br>
+came into existence and all the several alpha rays penetrated
+the<br>
+glass, darkening the walls of the capillary out to the limit
+of<br>
+the range of radium C in glass. Plate XXV shows a magnified<br>
+section of the</p>
+<p>229</p>
+<p>tube. The dark central part is the capillary. The tubular
+halo<br>
+surrounds it. This experiment has, however, been anticipated
+by<br>
+some scores of millions of years, for here is the same effect
+in<br>
+a biotite crystal (Pl. XXV). Along what are apparently
+tubular<br>
+passages or cracks in the mica, a solution, rich in
+radioactive<br>
+substances, has moved; probably during the final consolidation
+of<br>
+the granite in which the mica occurs. A continuous and very<br>
+regular halo has developed along these conduits. A string of<br>
+halo-spheres may lie along such passages. We must infer that<br>
+solutions or gases able to establish the radioactive nuclei
+moved<br>
+along these conduits, and we are entitled to ask if all the<br>
+haloes in this biotite are not, in this sense, of secondary<br>
+origin. There is, I may add, much to support such a
+conclusion.</p>
+<p>The widespread distribution of radioactive substances is
+most<br>
+readily appreciated by examination of sections of rocks cut
+thin<br>
+enough for microscopic investigation. It is, indeed, difficult
+to<br>
+find, in the older rocks of granitic type, mica which does
+not<br>
+show haloes, or traces of haloes. Often we find that every one
+of<br>
+the inclusions in the mica&mdash;that is, every one of the
+earlier<br>
+formed substances&mdash;contain radioactive elements, as
+indicated by<br>
+the presence of darkened borders. As will be seen presently
+the<br>
+quantities involved are generally vanishingly small. For
+example<br>
+it was found by direct determination that in one gram of the<br>
+halo-rich mica of Co. Carlow there was rather less than
+twelve<br>
+billionths of a gram of radium, We are</p>
+<p>230</p>
+<p>entitled to infer that other rare elements are similarly
+widely<br>
+distributed but remain undetectable because of their more
+stable<br>
+properties.</p>
+<p>It must not be thought that the under-exposed halo is a
+recent<br>
+creation. By no means. All are old, appallingly old; and in
+the<br>
+same rock all are, probably, of the same, or neatly the same,<br>
+age. The under-exposure is simply due to a lesser quantity of
+the<br>
+radioactive elements in the nucleus. They are under-exposed,
+in<br>
+short, not because of lesser duration of exposure, but because
+of<br>
+insufficient action; as when in taking a photograph the stop
+is<br>
+not open enough for the time of the exposure.</p>
+<p>The halo has, so far, told us that the additive law is obeyed
+in<br>
+solid media, and that the increased ionisation attending the<br>
+slowing down of the ray obtaining in gases, also obtains in<br>
+solids; for, otherwise, the halo would not commence its<br>
+development as a spherical shell or envelope. But here we
+learn<br>
+that there is probably a certain difference in the course of<br>
+events attending the immediate passage of the ray in the gas
+and<br>
+in the solid. In the former, initial recombination may
+obscure<br>
+the intense ionisation near the end of the range. We can only<br>
+detect the true end-effects by artificially separating the
+ions<br>
+by a strong electric force. If this recombination happened in
+the<br>
+mineral we should not have the concentric spheres so well
+defined<br>
+as we see them to be. What, then, hinders the initial<br>
+recombination in the solid? The answer probably is that the
+newly<br>
+formed</p>
+<p>231</p>
+<p>ion is instantly used up in a fresh chemical combination. Nor
+is<br>
+it free to change its place as in the gas. There is simply a
+new<br>
+equilibrium brought about by its sudden production. In this<br>
+manner the conditions in the complex molecule of biotite,<br>
+tourmaline, etc., may be quite as effective in preventing
+initial<br>
+recombination as the most effective electric force we could<br>
+apply. The final result is that we find the Bragg curve<br>
+reproduced most accurately in the delicate shading of the
+rings<br>
+making up the perfectly exposed halo.</p>
+<p>That the shading of the rings reproduces the form of the
+Bragg<br>
+curve, projected, as it were, upon the line of advance of the
+ray<br>
+and reproduced in depth of shading, shows that in yet another<br>
+particular the alpha ray behaves much the same in the solid as
+in<br>
+the gas. A careful examination of the outer edge of the
+circles<br>
+always reveals a steep but not abrupt cessation of the action
+of<br>
+the ray. Now Geiger has investigated and proved the existence
+of<br>
+scattering of the alpha ray by solids. We may, therefore,
+suppose<br>
+with much probability that there is the same scattering
+within<br>
+the mineral near the end of the range. The heavy iron atom of
+the<br>
+biotite is, doubtless, chiefly responsible for this in
+biotite<br>
+haloes. I may observe that this shading of the outer bounding<br>
+surface of the sphere of action is found however minute the<br>
+central nucleus. In the case of a nucleus of considerable
+size<br>
+another effect comes in which tends to produce an enhanced<br>
+shading. This will</p>
+<p>232</p>
+<p>result if rays proceed from different depths in the nucleus.
+If<br>
+the nucleus were of the same density and atomic weight as the<br>
+surrounding mica, there would be little effect. But its
+density<br>
+and molecular weight are generally greater, hence the
+retardation<br>
+is greater, and rays proceeding from deep in the nucleus<br>
+experience more retardation than those which proceed from
+points<br>
+near to the surface. The distances reached by the rays in the<br>
+mica will vary accordingly, and so there will be a gradual<br>
+cessation of the effects of the rays.</p>
+<p>The result of our study of the halo may be summed up in
+the<br>
+statement that in nearly every particular we have the
+phenomena,<br>
+which have been measured and observed in the gas, reproduced on
+a<br>
+minute scale in the halo. Initial recombination seems,
+however,<br>
+to be absent or diminished in effectiveness; probably because
+of<br>
+the new stability instantly assumed by the ionised atoms.</p>
+<p>One of the most interesting points about the halo remains to
+be<br>
+referred to. The halo is always uniformly darkened all round
+its<br>
+circumference and is perfectly spherical. Sections, whether
+taken<br>
+in the plane of cleavage of the mica or across it, show the
+same<br>
+exactly circular form, and the same radius. Of course, if
+there<br>
+was any appreciable increase of range along or across the<br>
+cleavage the form of the halo on the section across the
+cleavage<br>
+should be elliptical. The fact that there is no measurable<br>
+ellipticity is, I think, one which on first consideration
+would<br>
+not be expected.</p>
+<p>233</p>
+<p>For what are the conditions attending the passage of the ray
+in a<br>
+medium such as mica? According to crystallographic conceptions
+we<br>
+have here an orderly arrangement of molecules, the units<br>
+composing the crystal being alike in mass, geometrically
+spaced,<br>
+and polarised as regards the attractions they exert one upon<br>
+another. Mica, more especially, has the cleavage phenomenon<br>
+developed to a degree which transcends its development in any<br>
+other known substance. We can cleave it and again cleave it
+till<br>
+its flakes float in the air, and we may yet go on cleaving it
+by<br>
+special means till the flakes no longer reflect visible
+light.<br>
+And not less remarkable is the uniplanar nature of its
+cleavage.<br>
+There is little cleavage in any plane but the one, although it
+is<br>
+easy to show that the molecules in the plane of the flake are
+in<br>
+orderly arrangement and are more easily parted in some
+directions<br>
+than in others. In such a medium beyond all others we must
+look<br>
+with surprise upon the perfect sphere struck out by the alpha<br>
+rays, because it seems certain that the cleavage is due to
+lesser<br>
+attraction, and, probably, further spacing of the molecules, in
+a<br>
+direction perpendicular to the cleavage.</p>
+<p>It may turn out that the spacing of the molecules will
+influence<br>
+but little the average number per unit distance encountered
+by<br>
+rays moving in divergent paths. If this is so, we seem left
+to<br>
+conclude that, in spite of its unequal and polarised
+attractions,<br>
+there is equal retardation and equal ionisation in the
+molecule<br>
+in whatever</p>
+<p>234</p>
+<p>direction it is approached. Or, again, if the encounters
+indeed<br>
+differ in number, then some compensating effect must exist<br>
+whereby a direction of lesser linear density involves greater<br>
+stopping power in the molecule encountered, and vice versa.</p>
+<p>The nature of the change produced by the alpha rays is
+unknown.<br>
+But the formation of the halo is not, at least in its earlier<br>
+stages, attended by destruction of the crystallographic and<br>
+optical properties of the medium. The optical properties are<br>
+unaltered in nature but are increased in intensity. This
+applies<br>
+till the halo has become so darkened that light is no longer<br>
+transmitted under the conditions of thickness obtaining in
+rock<br>
+sections. It is well known that there is in biotite a maximum<br>
+absorption of a plane-polarised light ray, when the plane of<br>
+vibration coincides with the plane of cleavage. A section
+across<br>
+the cleavage then shows a maximum amount of absorption. A
+halo<br>
+seen on this section simply produces this effect in a more<br>
+intense degree. This is well shown in Plate XXIII (lower
+figure),<br>
+on a portion of the halo-sphere. The descriptive name
+"Pleochroic<br>
+Halo" has originated from this fact. We must conclude that
+the<br>
+effect of the ionisation due to the alpha ray has not been to<br>
+alter fundamentally the conditions which give rise to the
+optical<br>
+properties of the medium. The increased absorption is
+probably<br>
+associated with some change in the chemical state of the iron<br>
+present. Haloes are, I believe, not found in minerals from
+which<br>
+this</p>
+<p>235</p>
+<p>element is absent. One thing is quite certain. The colouration
+is<br>
+not due to an accumulation of helium atoms, _i.e._ of spent
+alpha<br>
+rays. The evidence for this is conclusive. If helium was<br>
+responsible we should have haloes produced in all sorts of<br>
+colourless minerals. Now we sometimes see zircons in felspars
+and<br>
+in quartz, etc., but in no such case is a halo produced. And<br>
+halo-spheres formed within and sufficiently close to the edge
+of<br>
+a crystal of mica are abruptly truncated by neighbouring areas
+of<br>
+fclspar or quartz, although we know that the rays must pass<br>
+freely across the boundary. Again it is easy to show that even
+in<br>
+the oldest haloes the quantity of helium involved is so small<br>
+that one might say the halo-sphere was a tolerably good vacuum
+as<br>
+regards helium. There is, finally, no reason to suppose that
+the<br>
+imprisoned helium would exhibit such a colouration, or,
+indeed,<br>
+any at all.</p>
+<p>I have already referred to the great age of the halo. Haloes
+are<br>
+not found in the younger igneous rocks. It is probable that a<br>
+halo less than a million years old has never been seen. This,<br>
+prim&acirc; facie, indicates an extremely slow rate of formation.
+And<br>
+our calculations quite support the conclusions that the growth
+of<br>
+a halo, if this has been uniform, proceeds at a rate of
+almost<br>
+unimaginable slowness.</p>
+<p>Let us calculate the number of alpha rays which may have gone
+to<br>
+form a halo in the Devonian granite of Leinster.</p>
+<p>236</p>
+<p>It is common to find haloes developed perfectly in this
+granite,<br>
+and having a nucleus of zircon less than 5 x 10<sup>-4</sup> cms.
+in<br>
+diameter. The volume of zircon is 65 x 10<sup>-12</sup> c.cs. and
+the mass<br>
+3 x 10<sup>-10</sup> grm.; and if there was in this zircon
+10<sup>-8</sup> grm. radium<br>
+per gram (a quantity about five times the greatest amount<br>
+measured by Strutt), the mass of radium involved is 3 x
+10<sup>-18</sup><br>
+grm. From this and from the fact ascertained by Rutherford
+that<br>
+the number of alpha rays expelled by a gram of radium in one<br>
+second is 3.4 x 10<sup>10</sup>, we find that three rays are shot
+from the<br>
+nucleus in a year. If, now, geological time since the Devonian
+is<br>
+50 millions of years, then 150 millions of rays built up the<br>
+halo. If geological time since the Devonian is 400 millions
+of<br>
+years, then 1,200 millions of alpha rays are concerned in its<br>
+genesis. The number of ions involved, of course, greatly
+exceeds<br>
+these numbers. A single alpha ray fired from radium C will<br>
+produce 2.37 x 10<sup>5</sup> ions in air.</p>
+<p>But haloes may be found quite clearly defined and fairly dark
+out<br>
+to the range of the emanation ray and derived from much less<br>
+quantities of radioactive materials. Thus a zircon nucleus with
+a<br>
+diameter of but 3.4 x 10<sup>-4</sup> cms. formed a halo strongly
+darkened<br>
+within, and showing radium A and radium C as clear smoky
+rings.<br>
+Such a nucleus, on the assumption made above as to its radium<br>
+content, expels one ray in a year. But, again, haloes are<br>
+observed with less blackened pupils and with faint ring due
+to<br>
+radium C, formed round nuclei</p>
+<p>237</p>
+<p>of rather less than 2 x 10<sup>-4</sup> cms. diameter. Such
+nuclei would<br>
+expel one ray in five years. And even lesser nuclei will
+generate<br>
+in these old rocks haloes with their earlier characteristic<br>
+features clearly developed. In the case of the most minute<br>
+nuclei, if my assumption as to the uranium content is correct,
+an<br>
+alpha ray is expelled, probably, no oftener than once in a<br>
+century; and possibly at still longer intervals.</p>
+<p>The equilibrium amount of radium contained in some nuclei
+may<br>
+amount to only a few atoms. Even in the case of the larger
+nuclei<br>
+and more perfectly developed haloes the quantity of radium<br>
+involved is many millions of times less than the least amount
+we<br>
+can recognise by any other means. But the delicacy of the<br>
+observation is not adequately set forth in this statement. We
+can<br>
+not only tell the nature of the radioactive family with which
+we<br>
+are dealing; but we can recognise the presence of some of its<br>
+constituent members. I may say that it is not probable the<br>
+zircons are richer in radium than I have assumed. My
+assumption<br>
+involves about 3 per cent. of uranium. I know of no analyses<br>
+ascribing so great an amount of uranium to zircon. The
+variety<br>
+cyrtolite has been found to contain half this amount, about.
+But<br>
+even if we doubled our estimate of radium content, the
+remarkable<br>
+nature of our conclusions is hardly lessened.</p>
+<p>It may appear strange that the ever-interesting question of
+the<br>
+Earth's age should find elucidation from the</p>
+<p>238</p>
+<p>study of haloes. Nevertheless the subjects are closely
+connected.<br>
+The circumstances are as follows. Geologists have estimated
+the<br>
+age of the Earth since denudation began, by measurements of
+the<br>
+integral effects of denudation. These methods agree in showing
+an<br>
+age of about rob years. On the other hand, measurements have
+been<br>
+made of the accumulation in minerals of radioactive
+_d&eacute;bris_&mdash;the<br>
+helium and lead&mdash;and results obtained which, although they
+do not<br>
+agree very well among themselves, are concordant in assigning
+a<br>
+very much greater age to the rocks. If the radioactive
+estimate<br>
+is correct, then we are now living in a time when the
+denudative<br>
+forces of the Earth are about eight or nine times as active
+as<br>
+they have been on the average over the past. Such a state of<br>
+things is absolutely unaccountable. And all the more<br>
+unaccountable because from all we know we would expect a
+somewhat<br>
+_lesser_ rate of solvent denudation as the world gets older and
+the<br>
+land gets more and more loaded with the washed-out materials
+of<br>
+the rocks.</p>
+<p>Both the methods referred to of finding the age assume the<br>
+principle of uniformity. The geologist contends for
+uniformity<br>
+throughout the past physical history of the Earth. The
+physicist<br>
+claims the like for the change-rates of the radioactive
+elements.<br>
+Now the study of the rocks enables us to infer something as
+to<br>
+the past history of our Globe. Nothing is, on the other hand,<br>
+known respecting the origin of uranium or thorium&mdash;the
+parent<br>
+radioactive bodies. And while not questioning the law</p>
+<p>239</p>
+<p>and regularity which undoubtedly prevail in the periods of
+the<br>
+members of the radioactive families, it appears to me that it
+is<br>
+allowable to ask if the change rate of uranium has been
+always<br>
+what we now believe it to be. This comes to much the same
+thing<br>
+as supposing that atoms possessing a faster change rate once
+were<br>
+associated with it which were capable of yielding both helium
+and<br>
+lead to the rocks. Such atoms might have been collateral in<br>
+origin with uranium from some antecedent element. Like
+helium,<br>
+lead may be a derivative from more than one sequence of<br>
+radioactive changes. In the present state of our knowledge
+the<br>
+possibilities are many. The rate of change is known to be<br>
+connected with the range of the alpha ray expelled by the<br>
+transforming element; and the conformity of the halo with our<br>
+existing knowledge of the ranges is reason for assuming that,<br>
+whatever the origin of the more active associate of uranium,
+this<br>
+passed through similar elemental changes in the progress of
+its<br>
+disintegration. There may, however, have been differences in
+the<br>
+ranges which the halo would not reveal. It is remarkable that<br>
+uranium at the present time is apparently responsible for two<br>
+alpha rays of very different ranges. If these proceed from<br>
+different elements, one should be faster in its change rate
+than<br>
+the other. Some guidance may yet be forthcoming from the study
+of<br>
+the more obscure problems of radioactivity.</p>
+<p>Now it is not improbable that the halo may contribute directly
+to<br>
+this discussion. We can evidently attack</p>
+<p>240</p>
+<p>the biotite with a known number of alpha rays and determine
+how<br>
+many are required to produce a certain intensity of
+darkening,<br>
+corresponding to that of a halo with a nucleus of measurable<br>
+dimensions. On certain assumptions, which are correct within<br>
+defined limits, we can calculate, as I have done above, the<br>
+number of rays concerned in forming the halo. In doing so we<br>
+assume some value for the age of the halo. Let us take the<br>
+maximum radioactive value. A halo originating in Devonian
+times<br>
+may attain a certain central blackening from the effects of,
+say,<br>
+rob rays. But now suppose we find that we cannot produce the
+same<br>
+degree of blackening with this number of rays applied in the<br>
+laboratory. What are we to conclude? I think there is only
+the<br>
+one conclusion open to us; that some other source of alpha
+rays,<br>
+or a faster rate of supply, existed in the past. And this<br>
+conclusion would explain the absence of haloes from the
+younger<br>
+rocks; which, in view of the vast range of effects possible
+in<br>
+the development of haloes, is, otherwise, not easy to account<br>
+for. It is apparent that the experiment on the biotite has a<br>
+direct bearing on the validity of the radioactive method of<br>
+estimating the age of the rocks. It is now being carried out
+by<br>
+Professor Rutherford under reliable conditions.</p>
+<p>Finally, there is one very certain and valuable fact to be<br>
+learned from the halo. The halo has established the extreme<br>
+rarity of radioactivity as an atomic phenomenon. One and all
+of<br>
+the speculations as to</p>
+<p>241</p>
+<p>the slow breakdown of the commoner elements may be dismissed.
+The<br>
+halo shows that the mica of the rocks is radioactively
+sensitive.<br>
+The fundamental criterion of radioactive change is the
+expulsion<br>
+of the alpha ray. The molecular system of the mica and of
+many<br>
+other minerals is unstable in presence of these rays, just as
+a<br>
+photographic plate is unstable in presence of light.
+Moreover,<br>
+the mineral integrates the radioactive effects in the same way
+as<br>
+a photographic salt integrates the effects of light. In both<br>
+cases the feeblest activities become ultimately apparent to
+our<br>
+inspection. We have seen that one ray in each year since the<br>
+Devonian period will build the fully formed halo: an object<br>
+unlike any other appearance in the rocks. And we have been
+able<br>
+to allocate all the haloes so far investigated to one or the<br>
+other of the known radioactive families. We are evidently<br>
+justified in the belief that had other elements been
+radioactive<br>
+we must either find characteristic haloes produced by them,
+or<br>
+else find a complete darkening of the mica. The feeblest
+alpha<br>
+rays emitted by the relatively enormous quantities of the<br>
+prevailing elements, acting over the whole duration of
+geological<br>
+time&mdash;and it must be remembered that the haloes we have
+been<br>
+studying are comparatively young&mdash;must have registered
+their<br>
+effects on the sensitive minerals. And thus we are safe in<br>
+concluding that the common elements, and, indeed, many which<br>
+would be called rare, are possessed of a degree of stability<br>
+which has preserved them un</p>
+<p>242</p>
+<p>changed since the beginning of geological time. Each
+unaffected<br>
+flake of mica is, thus, unassailable proof of a fact which
+but<br>
+for the halo would, probably, have been for ever beyond our<br>
+cognisance.</p>
+<p><u>THE USE OF RADIUM IN MEDICINE</u> [1]</p>
+<p>IT has been unfortunate for the progress of the
+radioactive<br>
+treatment of disease that its methods and claims involve much
+of<br>
+the marvellous. Up till recently, indeed, a large part of<br>
+radioactive therapeutics could only be described as bordering
+on<br>
+the occult. It is not surprising that when, in addition to
+its<br>
+occult and marvellous characters, claims were made on its
+behalf<br>
+which in many cases could not be supported, many medical men
+came<br>
+to regard it with a certain amount of suspicion.</p>
+<p>Today, I believe, we are in a better position. I think it
+is<br>
+possible to ascribe a rational scientific basis to its
+legitimate<br>
+claims, and to show, in fact, that in radioactive treatment
+we<br>
+are pursuing methods which have been already tried
+extensively<br>
+and found to be of definite value; and that new methods
+differ<br>
+from the old mainly in their power and availability, and
+little,<br>
+or not at all, in kind.</p>
+<p>Let us briefly review the basis of the science. Radium is
+a<br>
+metallic element chemically resembling barium. It</p>
+<p>[1] A Lecture to Postgraduate Students of Medicine in
+connection<br>
+with the founding of the Dublin Radium Institute, delivered
+in<br>
+the School of Physic in Ireland, Trinity College, on October
+2nd,<br>
+1914</p>
+<p>244</p>
+<p>possesses, however, a remarkable property which barium does
+not.<br>
+Its atoms are not equally stable. In a given quantity of radium
+a<br>
+certain very small percentage of the total number of atoms<br>
+present break up per second. By "breaking up" we mean their<br>
+transmutation to another element. Radium, which is a solid<br>
+element under ordinary conditions, gives rise by transmutation
+to<br>
+a gaseous element&mdash;the emanation of radium. The new element
+is a<br>
+heavy gas at ordinary temperatures and, like other gases, can
+be<br>
+liquified by extreme cold. The extraordinary property of<br>
+transmutation is entirely automatic. No influence which
+chemist<br>
+or physicist can apply can affect the rate of transformation.</p>
+<p>The emanation inherits the property of instability, but in
+its<br>
+case the instability is more pronounced. A relatively large<br>
+fraction of its atoms transmute per second to a solid element<br>
+designated Radium A. In turn this new generation of atoms
+breaks<br>
+up&mdash;even faster than the emanation&mdash;becoming yet
+another element<br>
+with specific chemical properties. And so on for a whole
+sequence<br>
+of transmutations, till finally a stable substance is formed,<br>
+identical with ordinary lead in chemical and physical
+properties,<br>
+but possessing a slightly lower atomic weight.</p>
+<p>The genealogy of the radium series of elements shows that
+radium<br>
+is not the starting point. It possesses ancestors which have
+been<br>
+traced back to the element uranium.</p>
+<p>Now what bearing has this series of transmutations</p>
+<p>245</p>
+<p>upon medical science? Radium or emanation, &amp;c., are not in
+the<br>
+Pharmacopoeia as are, say, arsenic or bismuth. The whole<br>
+medicinal value of these elements resides in the very
+wonderful<br>
+phenomena of their radiations. They radiate in the process of<br>
+transmuting.</p>
+<p>The changing atom may radiate a part of its own mass. The<br>
+"alpha"-ray (a-ray) is such a material ray. It is an
+electrified<br>
+helium atom cast out of the parent atom with enormous<br>
+velocity&mdash;such a velocity as would carry it, if not impeded,
+all<br>
+round the earth in two seconds. All alpha-rays are positively<br>
+electrified atoms of the element helium, which thereby is
+shown<br>
+to be an integral constituent of many elements. The alpha-ray
+is<br>
+not of much value to medical science, for, in spite of its
+great<br>
+velocity, it is soon stopped by encounter with other atoms.
+It<br>
+can penetrate only a minute fraction of a millimetre into<br>
+ordinary soft tissues. We shall not further consider it.</p>
+<p>Transmuting atoms give out also material rays of another
+kind:<br>
+the &szlig;-rays. The &szlig;-ray is in mass but a very small
+fraction of,<br>
+even, a hydrogen atom. Its speed may approach that of light.
+As<br>
+cast out by radioactive elements it starts with speeds which
+vary<br>
+with the element, and may be from one-third to nine-tenths
+the<br>
+velocity of light. The &szlig;-ray is negatively electrified. It
+has<br>
+long been known to science as the electron. It is also
+identical<br>
+with the cathode ray of the vacuum tube.</p>
+<p>246</p>
+<p>Another and quite different kind of radiation is given out
+by<br>
+many of the transmuting elements:&mdash;the y-ray. This is
+not<br>
+material, it is ethereal. It is known now with certainty that
+the<br>
+y-ray is in kind identical with light, but of very much
+shorter<br>
+wave length than even the extreme ultraviolet light of the
+solar<br>
+spectrum. The y-ray is flashed from the transmuting atom
+along<br>
+with the &szlig;-ray. It is identical in character with the x-ray
+but<br>
+of even shorter wave length.</p>
+<p>There is a very interesting connection between the y-ray and
+the<br>
+&szlig;-ray which it is important for the medical man to
+understand&mdash;as<br>
+far as it is practicable on our present knowledge.</p>
+<p>When y-rays or x-rays fall on matter they give rise to
+&szlig;-rays.<br>
+The mechanism involved is not known but it is possibly a
+result<br>
+of the resonance of the atom, or of parts of it, to the short<br>
+light waves. And it is remarkable that the y-rays which, as
+we<br>
+have seen, are shorter and more penetrating waves than the<br>
+x-rays, give rise to &szlig;-rays possessed of greater velocity
+and<br>
+penetration than &szlig;-rays excited by the x-rays. Indeed the
+&szlig;-rays<br>
+originated by y-rays may attain a velocity nearly approaching<br>
+that of light and as great as that of any &szlig;-rays emitted
+by<br>
+transmuting atoms. Again there is demonstrable evidence that<br>
+&szlig;-rays impinging on matter may give rise to y-rays. The
+most<br>
+remarkable demonstration of this is seen in the x-ray tube.
+Here<br>
+the x-rays originate where the stream of &szlig;- or
+cathode-rays</p>
+<p>247</p>
+<p>are arrested on the anode. But the first relation is at
+present<br>
+of most importance to us&mdash;_i.e._ that the y-or x-rays give
+rise to<br>
+&szlig;-rays.</p>
+<p>This relation gives us additional evidence of the identity of
+the<br>
+physical effects of y-, x-, and light-rays &mdash;using the term
+light<br>
+rays in the usual sense of spectral rays. For it has long
+been<br>
+known that light waves liberate electrons from atoms. It has
+been<br>
+found that these electrons possess a certain initial velocity<br>
+which is the greater the shorter the wave length of the light<br>
+concerned in their liberation. The whole science of<br>
+"photo-electricity" centres round this phenomenon. The action
+of<br>
+light on the photographic plate, as well as many other
+physical<br>
+and chemical phenomena, find an explanation in this liberation
+of<br>
+the electron by the light wave.</p>
+<p>Here, then, we have spectral light waves liberating<br>
+electrons&mdash;_i.e._ very minute negatively-charged particles,
+and we<br>
+find that, as we use shorter light waves, the initial velocity
+of<br>
+these particles increases. Again, we have x-rays which are
+far<br>
+smaller in wave length than spectral light, liberating much<br>
+faster negatively electrified particles. Finally, we have<br>
+y-rays&mdash;the shortest nether waves of all-liberating
+negative<br>
+particles of the highest velocity known. Plainly the whole
+series<br>
+of phenomena is continuous.</p>
+<p>We can now look closer at the actions involved in the
+therapeutic<br>
+influence of the several rays and in</p>
+<p>248</p>
+<p>this way, also, see further the correlation between what may
+be<br>
+called photo-therapeutics and radioactive therapeutics.</p>
+<p>The &szlig;-ray, whether we obtain it directly from the
+transforming<br>
+radioactive atom or whether we obtain it as a result of the<br>
+effects of the y- or x-rays upon the atom, is an ionising
+agent<br>
+of wonderful power. What is meant by this? In its physical
+aspect<br>
+this means that the atoms through which it passes acquire
+free<br>
+electric charges; some becoming positive, some negative. This
+can<br>
+only be due to the loss of an electron by the affected atom.
+The<br>
+loss of the small negative charge carried in the electron
+leaves<br>
+the atom positively electrified or creates a positive ion.
+The<br>
+fixing of the wandering electron to a neutral atom creates a<br>
+negative ion. Before further consideration of the importance
+of<br>
+the phenomenon of ionisation we must fix in our minds that
+the<br>
+agent, which brings this about, is the &szlig;-ray. There is
+little<br>
+evidence that the y-ray can directly create ions to any large<br>
+extent. But the action of liberating high-speed &szlig;-rays
+results in<br>
+the creation of many thousands of ions by each &szlig;-ray
+liberated.<br>
+As an agent in the hands of the medical man we must regard
+the<br>
+y-ray as a light wave of extremely penetrating character,
+which<br>
+creates high-speed &szlig;-rays in the tissues which it
+penetrates,<br>
+these &szlig;-rays being most potent ionising agents. The
+&szlig;-rays<br>
+directly obtained from radioactive atoms assist in the work
+of<br>
+ionisation. &szlig;-rays do not</p>
+<p>249</p>
+<p>penetrate far from their source. The fastest of them would
+not<br>
+probably penetrate one centimetre in soft tissues.</p>
+<p>We must now return to the phenomenon of ionisation. Ionisation
+is<br>
+revealed to observation most conspicuously when it takes place
+in<br>
+a gas. The + and - electric charges on the gas particles endow
+it<br>
+with the properties of a conductor of electricity, the + ions<br>
+moving freely in one direction and the - ions in the opposite<br>
+direction under an electric potential. But there are effects<br>
+brought about by ionisation of more importance to the medical
+man<br>
+than this. The chemist has long come to recognise that in the
+ion<br>
+he is concerned with the inner mechanism of a large number of<br>
+chemical phenomena. For with the electrification of the atom<br>
+attractive and repulsive forces arise. We can directly show
+the<br>
+chemical effects of the ionising &szlig;-rays. Water exposed to
+their<br>
+bombardment splits up into hydrogen and oxygen. And, again,
+the<br>
+separated atoms may be in part recombined under the influence
+of<br>
+the radiation. Ammonia splits up into hydrogen and nitrogen.<br>
+Carbon dioxide forms carbon, carbon monoxide, and oxygen;<br>
+hydrochloric acid forms chlorine and hydrogen. In these
+cases,<br>
+also, recombination can be partially effected by the rays.</p>
+<p>We can be quite sure that within the complex structure of
+the<br>
+living cell the ionising effects which everywhere accompany
+the<br>
+&szlig;-rays must exert a profound influence. The sequence of
+chemical<br>
+events which as yet seem</p>
+<p>250</p>
+<p>beyond the ken of science and which are involved in
+metabolism<br>
+cannot fail to be affected. Any, it is not surprising that as
+the<br>
+result of eaperinient it is found that the radiations are
+agents<br>
+which may be used either for the stimulation of the natural<br>
+events of growth or used for the actual destruction of the
+cell.<br>
+It is easy to see that the feeble radiation should produce
+the<br>
+one effect, the strong the other. In a similar way by a
+moderate<br>
+light stimulus we create the latent image in the photographic<br>
+plate; by an intense light we again destroy this image. The
+inner<br>
+mechanism in this last case can be logically stated.[1]</p>
+<p>_There is plainly a true physical basis here for the efficacy
+of<br>
+radioactive treatment and, what is more, we find when we
+examine<br>
+it, that it is in kind not different from that underlying<br>
+treatment by spectral radiations. But in degree it is very<br>
+different and here is the reason for the special importance
+of<br>
+radioactivity as a therapeutic agent._ The Finsen light is
+capable<br>
+of influencing the soft tissues to a short depth only. The
+reason<br>
+is that the wave length of the light used is too great to
+pass<br>
+without rapid absorption through the tissues; and, further,
+the<br>
+electrons it gives rise to&mdash;_i.e._ the &szlig;-rays it
+liberates&mdash;are too<br>
+slow-moving to be very efficient ionisers. X-rays penetrate
+in<br>
+some cases quite freely and give rise to much faster and more<br>
+powerful &szlig;-rays</p>
+<p>[1] See _The Latent Image_, p. 202.</p>
+<p>251</p>
+<p>than can the Finsen light. But far more penetrating than
+x-rays<br>
+are the y-rays emitted in certain of the radioactive changes.<br>
+These give rise to &szlig;-rays having a velocity approximate to
+that<br>
+of light.</p>
+<p>The y-rays are, therefore, very penetrating and powerfully<br>
+ionising light waves; light waves which are quite invisible
+to<br>
+the eye and can beam right through the tissues of the body.
+To<br>
+the mind's eye only are they visible. And a very wonderful<br>
+picture they make. We see the transmuting atom flashing out
+this<br>
+light for an inconceivably short instant as it throws off the<br>
+&szlig;-ray. And "so far this little candle throws his beams" in
+the<br>
+complex system of the cells, so far atoms shaken by the rays
+send<br>
+out &szlig;-rays; these in turn are hurled against other
+atomic<br>
+systems; fresh separations of electrons arise and new
+attractions<br>
+and repulsions spring up and the most important chemical
+changes<br>
+are brought about. Our mental picture can claim to be no more<br>
+than diagrammatic of the reality. Still we are here dealing
+with<br>
+recognised physical and chemical phenomena, and their
+description<br>
+as "occult" in the derogatory sense is certainly not<br>
+justifiable.</p>
+<p>Having now briefly reviewed the nature of the rays arising
+in<br>
+radioactive substances and the rationale of their influence,
+we<br>
+must turn to more especially practical considerations.</p>
+<p>The Table given opposite shows that radium itself is
+responsible<br>
+for a- and &szlig;-rays only. It happens that</p>
+<p>252</p>
+<p>Period in whioh &frac12; element is transformed.</p>
+<p>URANIUM 1 &amp; 2 { a 6 } x 10<sup>9</sup> years.</p>
+<p>URANIUM X { a &szlig; } 24.6 days.</p>
+<p>IONIUM { a 8 } x 104 years.</p>
+<p>RADIUM { a &szlig; } 2 x 10<sup>2</sup> years.</p>
+<p>EMANATION { a } 8.85 days.</p>
+<p>RADIUM A { a 8 } minutes.</p>
+<p>RADIUM B { &szlig; y } 26.7 minutes.</p>
+<p>RADIUM C { a &szlig; y } 13.5 minutes.</p>
+<p>RADIUM D { &szlig; } 15 years.</p>
+<p>RADIUM E { &szlig; y } 4.8 days.</p>
+<p>RADIUM (Polonium) F { a } 140 days.</p>
+<p>Table showing the successive generations of the elements of
+the<br>
+Uranium-radium family, the character of their radiations and<br>
+their longevity.</p>
+<p>253</p>
+<p>the &szlig;-rays emitted by radium are very
+"soft"&mdash;_i.e._ slow and<br>
+easily absorbed. The a-ray is in no case available for more
+than<br>
+mere surface application. Hence we see that, contrary to what
+is<br>
+generally believed, radium itself is of little direct
+therapeutic<br>
+value. Nor is the next body in succession&mdash;the emanation,
+for it<br>
+gives only a-rays. In fact, to be brief, it is not till we
+come<br>
+to Radium B that &szlig;-rays of a relatively high penetrative
+quality<br>
+are reached; and it is not till we come to Radium C that
+highly<br>
+penetrative y-rays are obtained.</p>
+<p>It is around this element, Radium C, that the chief
+medical<br>
+importance of radioactive treatment by this family of
+radioactive<br>
+bodies centres. Not only are &szlig;-rays of Radium C very
+penetrating,<br>
+but the y-rays are perhaps the most energetic rays of the,
+kind<br>
+known. Further in the list there is no very special medical<br>
+interest.</p>
+<p>Now, how can we get a supply of this valuable element Radium
+C?<br>
+We can obtain it from radium itself. For even if radium has
+been<br>
+deprived of its emanation (which is easily done by heating it
+or<br>
+bringing it into solution) in a few weeks we get back the
+Radium<br>
+C. One thing here we must be clear about. With a given
+quantity<br>
+of Radium only a certain definitely limited amount of Radium
+C,<br>
+or of emanation, or any other of the derived bodies, will be<br>
+associated. Why is this? The answer is because the several<br>
+successive elements are themselves decaying &mdash;_i.e._
+changing one<br>
+into the other. The atomic per-</p>
+<p>254</p>
+<p>centage of each, which decays in a second, is a fixed
+quantity<br>
+which we cannot alter. Now if we picture radium which has
+been<br>
+completely deprived of its emanation, again accumulating by<br>
+automatic transmutation a fresh store of this element, we have
+to<br>
+remember:&mdash; (i) That the rate of creation of emanation by
+the<br>
+radium is practically constant; and (2) that the absolute
+amount<br>
+of the emanation decaying per second increases as the stock
+of<br>
+emanation increases. Finally, when the amount of accumulated<br>
+emanation has increased to such an extent that the number of<br>
+emanation atoms transmuting per second becomes exactly equal
+to<br>
+the number being generated per second, the amount of
+emanation<br>
+present cannot increase. This is called the equilibrium
+amount.<br>
+If fifteen members are elected steadily each year into a<br>
+newly-founded society the number of members will increase for
+the<br>
+first few years; finally, when the losses by death of the
+members<br>
+equal about fifteen per annum the society can get no bigger.
+It<br>
+has attained the equilibrium number of members.</p>
+<p>This applies to every one of the successive elements. It
+takes<br>
+twenty-one days for the equilibrium quantity of emanation to
+be<br>
+formed in radium which has been completely de-emanated; and
+it<br>
+takes 3.8 days for half the equilibrium amount to be formed.<br>
+Again, if we start with a stock of emanation it takes just
+three<br>
+hours for the equilibrium amount of Radium C to be formed.</p>
+<p>255</p>
+<p>We can evidently grow Radium C either from radium itself or
+from<br>
+the emanation of radium. If we use a tube of radium we have
+an<br>
+almost perfectly constant quantity of Radium C present, for
+as<br>
+fast as the Radium C and intervening elements decay the
+Radium,<br>
+which only diminishes very slowly in amount, makes up the
+loss.<br>
+But, if we start off with a tube of emanation, we do not
+possess<br>
+a constant supply of Radium C, because the emanation is
+decaying<br>
+fairly rapidly and there is no radium to make good its loss.
+In<br>
+3.8 days about one half the emanation is transmuted and the<br>
+Radium C decreases proportionately and, of course, with the<br>
+Radium C the valuable radiations also decrease. In another
+3.8<br>
+days&mdash;that is in about a week from the start&mdash;the
+radioactive value<br>
+of the tube has fallen to one-fourth of its original value.</p>
+<p>But in spite of the inconstant character of the emanation
+tube<br>
+there are many reasons for preferring its use to the use of
+the<br>
+radium tube. Chief of these is the fact that we can keep the<br>
+precious radium safely locked up in the laboratory and not<br>
+exposed to the thousand-and-one risks of the hospital. Then,<br>
+secondly, the emanation, being a gas, is very convenient for<br>
+subdivision into a large number of very small tubes according
+to<br>
+the dosage required.</p>
+<p>In fact the volume of the emanation is exceedingly minute.
+The<br>
+amount of emanation in equilibrium with one gramme of radium
+is<br>
+called the curie, and with one</p>
+<p>256</p>
+<p>milligramme the millicurie. Now, the volume of the curie is
+only<br>
+a little more than one half a cubic millimetre. Hence in
+dealing<br>
+with emanation from twenty or forty milligrammes of radium we
+are<br>
+dealing with very small volumes.</p>
+<p>How may the emanation be obtained? The process is an easy one
+in<br>
+skilled and practised hands. The salt of radium&mdash;generally
+the<br>
+bromide or chloride&mdash;is brought into acid solution. This
+causes<br>
+the emanation to be freely given off as fast as it is formed.
+At<br>
+intervals we pump it off with a mercury pump.</p>
+<p>Let us see how many millicuries we will in future be able to
+turn<br>
+out in the week in our new Dublin Radium Institute.[1] We
+shall<br>
+have about 130 milligrammes of radium. In 3.8 days we get 65<br>
+millicuries from this&mdash;_i.e._ half the equilibrium amount of
+130<br>
+millicuries. Hence in the week, we shall have about 130<br>
+millicuries.</p>
+<p>This is not much. Many experts consider this little enough
+for<br>
+one tube. But here in Dublin we have been using the emanation
+in<br>
+a more economical and effective manner than is the usage<br>
+elsewhere; according to a method which has been worked out
+and<br>
+developed in our own Radium Institute. The economy is obtained
+by<br>
+the very simple expedient of minutely subdividing the' dose.
+The<br>
+system in vogue, generally, is to treat the tumour by
+inserting<br>
+into it one or two very active</p>
+<p>[1] Then recently established by the Royal Dublin Society.</p>
+<p>257</p>
+<p>tubes, containing, perhaps, up to 200 millicuries, or even
+more,<br>
+per tube. Now these very heavily charged tubes give a
+radiation<br>
+so intense at points close to the tube, due to the greater<br>
+density of the rays near the tube, and, also, to the action
+of<br>
+the softer and more easily absorbable rays, that it has been<br>
+found necessary to stop these softer rays&mdash;both the y and
+&szlig;&mdash;by<br>
+wrapping lead or platinum round the tube. In this lead or<br>
+platinum some thirty per cent. or more of the rays is
+absorbed<br>
+and, of course, wasted. But in the absence of the screen there
+is<br>
+extensive necrosis of the tissues near the tubes.</p>
+<p>If, however, in place of one or two such tubes we use ten
+or<br>
+twenty, each containing one-tenth or one-twentieth of the
+dose,<br>
+we can avail ourselves of the softer rays around each tube
+with<br>
+benefit. Thus a wasteful loss is avoided. Moreover a more
+uniform<br>
+"illumination" of the tissues results, just as we can
+illuminate<br>
+a hall more uniformly by the use of many lesser centres of
+light<br>
+than by the use of one intense centre of radiation. Also we
+get<br>
+what is called "cross-radiation,"which is found to be
+beneficial.<br>
+The surgeon knows far better what he is doing by this method.<br>
+Thus it may be arranged for the effects to go on with
+approximate<br>
+uniformity throughout the tumour instead of varying rapidly<br>
+around a central point or&mdash;and this may be very
+important&mdash; the<br>
+effects may be readily concentrated locally.</p>
+<p>Finally, not the least of the benefit arises in the easy<br>
+technique of this new method. The quantities of</p>
+<p>258</p>
+<p>emanation employed can fit in the finest capillary glass
+tubing<br>
+and the hairlike tubes can in turn be placed in fine
+exploring<br>
+needles. There is comparatively little inconvenience to the<br>
+patient in inserting these needles, and there is the most
+perfect<br>
+control of the dosage in the number and strength of these
+tubes<br>
+and the duration of exposure.[1]</p>
+<p>The first Radium Institute in Ireland has already done good
+work<br>
+for the relief of human suffering. It will have, I hope, a
+great<br>
+future before it, for I venture, with diffidence, to hold the<br>
+opinion, that with increased study the applications and claims
+of<br>
+radioactive treatment will increase.</p>
+<p>[1] For particulars of the new technique and of some of the
+work<br>
+already accomplished, see papers, by Dr. Walter C. Stevenson,<br>
+_British Medical Journal_, July 4th, 1914, and March 20th,
+1915.</p>
+<p>259</p>
+<p><u>SKATING</u> [1]</p>
+<p>IT is now many years ago since, as a student, I was present at
+a<br>
+college lecture delivered by a certain learned professor on
+the<br>
+subject of friction. At this lecture a discussion arose out of
+a<br>
+question addressed to our teacher: "How is it we can skate on
+ice<br>
+and on no other substance?"</p>
+<p>The answer came back without hesitation: "Because the ice is
+so<br>
+smooth."</p>
+<p>It was at once objected: "But you can skate on ice which is
+not<br>
+smooth."</p>
+<p>This put the professor in a difficulty. Obviously it is not
+on<br>
+account of the smoothness of the ice. A piece of polished
+plate<br>
+glass is far smoother than a surface of ice after the latter
+is<br>
+cut up by a day's skating. Nevertheless, on the scratched and<br>
+torn ice-surface skating is still quite possible; on the
+smooth<br>
+plate glass we know we could not skate.</p>
+<p>Some little time after this discussion, the connection
+between<br>
+skating and a somewhat abstruse fact in physical science
+occurred<br>
+to me. As the fact itself is one which has played a part in
+the<br>
+geological history of the earth,</p>
+<p>[1] A lecture delivered before the Royal Dublin Society in
+1905.</p>
+<p>260</p>
+<p>and a part of no little importance, the subject of
+skating,<br>
+whereby it is perhaps best brought home to every one, is<br>
+deserving of our careful attention. Let not, then, the title
+of<br>
+this lecture mislead the reader as to the importance of its<br>
+subject matter.</p>
+<p>Before going on to the explanation of the wonderful freedom
+of<br>
+the skater's movements, I wish to verify what I have inferred
+as<br>
+to the great difference in the slipperiness of glass and the<br>
+slipperiness of ice. Here is a slab of polished glass. I can<br>
+raise it to any angle I please so that at length this brass<br>
+weight of 250 grams just slips down when started with a
+slight<br>
+shove. The angle is, as you see, about 12&frac12; degrees. I
+now<br>
+transfer the weight on to this large slab of ice which I
+first<br>
+rapidly dry with soft linen. Observe that the weight slips
+down<br>
+the surface of ice at a much lower angle. It is a very low
+angle<br>
+indeed: I read it as between 4 and 5 degrees. We see by this<br>
+experiment that there is a great difference between the<br>
+slipperiness of the two surfaces as measured by what is
+called<br>
+"the angle of friction." In this experiment, too, the glass<br>
+possesses by far the smoother surface although I have rubbed
+the<br>
+deeper rugosities out of the ice by smoothing it with a glass<br>
+surface. Notwithstanding this, its surface is spotted with
+small<br>
+cavities due to bubbles and imperfections. It is certain that
+if<br>
+the glass was equally rough, its angle of friction towards
+the<br>
+brass weight would be higher.</p>
+<p>261</p>
+<p>We have, however, another comparative experiment to carry out.
+I<br>
+made as you saw a determination of the angle at which this
+weight<br>
+of 250 grams just slipped on the ice. The lower surface of
+the<br>
+weight, the part which presses on the ice, consists of a
+light,<br>
+brass curtain ring. This can be detached. Its mass is only
+6&frac12;<br>
+grams, the curtain ring being, in fact, hollow and made of
+very<br>
+thin metal. We have, therefore, in it a very small weight
+which<br>
+presents exactly the same surface beneath as did the weight
+of<br>
+250 grams. You see, now, that this light weight will not slip
+on<br>
+ice at 5 or 6 degrees of slope, but first does so at about io<br>
+degrees.</p>
+<p>This is a very important experiment as regards our present<br>
+inquiry. Ice appears to possess more than one angle of
+friction<br>
+according as a heavy or a light weight is used to press upon
+it.<br>
+We will make the same experiment with the plate of glass. You
+see<br>
+that there is little or no difference in the angle of friction
+of<br>
+brass on glass when we press the surfaces together with a
+heavy<br>
+or with a light weight. The light weight requires the same
+slope<br>
+of 12&frac12; degrees to make it slip.</p>
+<p>This last result is in accordance with the laws of friction.
+We<br>
+say that when solid presses on solid, for each pair of
+substances<br>
+pressed together there is a constant ratio between the force<br>
+required to keep one in motion over the other, and the force<br>
+pressing the solids together. This ratio is called"the<br>
+coefficient of friction."The coefficient is, in fact, constant
+or<br>
+approximately</p>
+<p>262</p>
+<p>so. I can determine the coefficient of friction from the angle
+of<br>
+friction by taking the tangent of the angle. The tangent of
+the<br>
+angle of friction is the coefficient of friction. If, then,
+the<br>
+coefficient is constant, so, of course, must the angle of<br>
+friction be constant. We have seen that it is so in the case
+of<br>
+metal on glass, but not so in the case of metal on ice. This<br>
+curious result shows that there is something abnormal about
+the<br>
+slipperiness of ice.</p>
+<p>The experiments we have hitherto made are open to the
+reproach<br>
+that the surface of the ice is probably damp owing to the
+warmth<br>
+of the air in contact with it. I have here a means of dealing<br>
+with a surface of cold, dry ice. This shallow copper tank
+about<br>
+18 inches (45 cms.) long, and 4 inches (10 cms.) wide, is
+filled<br>
+with a freezing 'mixture circulated through it from a larger<br>
+vessel containing ice melting in hydrochloric acid at a<br>
+temperature of about -18&deg; C. This keeps the tank below
+the<br>
+melting point of ice. The upper surface of the tank is
+provided<br>
+with raised edges so that it can be flooded with water. The
+water<br>
+is now frozen and its temperature is below 0&deg; C. It is
+about<br>
+10&deg; C. I can place over the ice a roof-shaped cover made of
+two<br>
+inclined slabs of thick plate glass. This acts to keep out
+warm<br>
+air, and to do away with any possibility of the surface of
+the<br>
+ice being wet with water thawed from the ice. The whole tank<br>
+along with its roof of glass can be adjusted to any angle, and
+a,<br>
+scale at the</p>
+<p>263</p>
+<p>raised end of the tank gives the angle of slope in degrees.
+A<br>
+weight placed on the ice can be easily seen through the glass<br>
+cover.</p>
+<p>The weight we shall use consists of a very light ring of<br>
+aluminium wire which is rendered plainly visible by a
+ping-pong<br>
+ball attached above it. The weight rests now on a copper
+plate<br>
+provided for the purpose at the upper end of the tank. The
+plate<br>
+being in direct contact beneath with the freezing mixture we
+are<br>
+sure that the aluminium ring is no hotter than the ice. A
+light<br>
+jerk suffices to shake the weight on to the surface of the
+ice.</p>
+<p>We find that this ring loaded with only the ping-pong ball,
+and<br>
+weighing a total of 2.55 grams does not slip at the low angles.
+I<br>
+have the surface of the ice at an angle of rather over
+13&frac12;, and<br>
+only by continuous tapping of the apparatus can it be induced
+to<br>
+slip down. This is a coefficient of 0.24, and compares with
+the<br>
+coefficient of hard and smooth solids on one another. I now<br>
+replace the empty ping-pong ball by a similar ball filled
+with<br>
+lead shot. The total weight is now 155 grams. You see the
+angle<br>
+of slipping has fallen to 7&deg;.</p>
+<p>Every one who has made friction experiments knows how<br>
+unsatisfactory and inconsistent they often are. We can only<br>
+discuss notable quantities and broad results, unless the most<br>
+conscientious care be taken to eliminate errors. The net
+result<br>
+here is that ice at about -10&deg; C. when pressed on by a very
+light<br>
+weight possesses a</p>
+<p>264</p>
+<p>coefficient of friction comparable with the usual coefficients
+of<br>
+solids on solids, but when the pressure is increased, the<br>
+coefficient falls to about half this value.</p>
+<p>The following table embodies some results obtained on the<br>
+friction of ice and glass, using the methods I have shown you.
+I<br>
+add some of the more carefully determined coefficients of
+other<br>
+observers.</p>
+<p>          Wt. in      On Plate         On Ice         On
+Ice<br>
+          Grams.      Glass.           at 0&deg; C.       at
+10&deg; C.</p>
+<p>                   Angle. Coeff.    Angle. Coeff.  Angle.
+Coeff<br>
+Aluminium   2.55    12&frac12;&deg;  0.22        12&deg;  
+0.21     13&frac12;&deg;   0.24<br>
+Same      155       12&frac12;&deg;   0.22        6&deg;   0.11
+      7&deg;   0.12<br>
+Brass       6.5     12&frac12;&deg;  0.22        10&deg;  
+0.17     10&frac12;&deg;   0.18<br>
+Same      107       12&frac12;&deg;   0.22        5&deg;   0.09
+      6&deg;   0.10</p>
+<p>Steel on steel (Morin) - - - - 0.14<br>
+Brass on cast iron (Morin) - - 0.19<br>
+Steel on cast iron (Morin) - - 0.20<br>
+Skate on ice (J. M&uuml;ller) - - - 0.016&mdash;0.032<br>
+Best-greased surfaces (Perry) - 0.03&mdash;0.036</p>
+<p>You perceive from the table that while the friction of brass
+or<br>
+aluminium on glass is quite independent of the weight used,
+that<br>
+of brass or aluminium on ice depends in some way upon the
+weight,<br>
+and falls in a very marked degree when the weight is heavy.
+Now,<br>
+I think that if we had been on the look out for any
+abnormality<br>
+in the friction of hard substances on ice, we would have
+rather<br>
+anticipated a variation in the</p>
+<p>265</p>
+<p>other direction. We would have, perhaps, expected that a
+heavy<br>
+weight would have given rise to the greater friction. I now
+turn<br>
+to the explanation of this extraordinary result.</p>
+<p>You are aware that it requires an expenditure of heat merely
+to<br>
+convert ice to water, the water produced being at the
+temperature<br>
+of the ice, _i.e._ at 0&deg; C., from which it is derived. The
+heat<br>
+required to change the ice from the solid to the liquid state
+is<br>
+the latent heat of water. We take the unit quantity of heat to
+be<br>
+that which is required to heat 1 kilogram of water 1&deg; C. Then
+if<br>
+we melt 1 kilogram of ice, we must supply it with 80 such
+units<br>
+of heat. While melting is going on, there is no change of<br>
+temperature if the experiment is carefully conducted. The
+melting<br>
+ice and the water coming from it remain at 0&deg; C. throughout
+the<br>
+operation, and neither the thermometer nor your own
+sensations<br>
+would tell you of the amount of heat which was flowing in.
+The<br>
+heat is latent or hidden in the liquid produced, and has gone
+to<br>
+do molecular work in the substance. Observe that if we supply<br>
+only 40 thermal units, we get only one-half the ice melted.
+If<br>
+only 10 units are supplied, then we get only one eighth of a<br>
+kilogram of water, and no more nor less.</p>
+<p>I have ventured to recall to you these commonplaces of
+science<br>
+before considering a mode of melting ice which is less
+generally<br>
+known, and which involves no supply of heat on your part.
+This<br>
+method involves for its</p>
+<p>266</p>
+<p>understanding a careful consideration of the thermal
+properties<br>
+of water in the solid state.</p>
+<p>It must have been observed a very long time ago that water<br>
+expands when it freezes. Otherwise ice would not float on
+water;<br>
+and, what is perhaps more important in your eyes, your water<br>
+pipes would not burst in winter when the water freezes
+therein.<br>
+But although the important fact of the expansion of water on<br>
+freezing was so long presented to the observation of mankind,
+it<br>
+was not till almost exactly the middle of the last century
+that<br>
+James Thomson, a gifted Irishman, predicted many important<br>
+consequences arising from the fact of the expansion of water
+on<br>
+becoming solid. The principles lie enunciated are perfectly<br>
+general, and apply in every case of change of volume
+attending<br>
+change of state. We are here only concerned with the case of<br>
+water and ice.</p>
+<p>James Thomson, following a train of thought which we cannot
+here<br>
+pursue, predicted that owing to the fact of the expansion of<br>
+water on becoming solid, pressure will lower the melting point
+of<br>
+ice or the freezing point of water. Normally, as you are
+aware,<br>
+the temperature is 0&deg; C. or 32&deg; F. Thomson said that this
+would<br>
+be found to be the freezing point only at atmospheric
+pressure.<br>
+He calculated how much it would change with change of
+pressure.<br>
+He predicted that the freezing point would fall 0.0075 of a<br>
+degree Centigrade for each additional atmosphere of pressure<br>
+applied to the water. Suppose,</p>
+<p>267</p>
+<p>for instance, our earth possessed an atmosphere so heavy to
+as<br>
+exert a thousand times the pressure of the existing
+atmosphere,<br>
+then water would not freeze at 0&deg; C., but at -7.5&deg; C. or
+about<br>
+18&deg; F. Again, in vacuo, that is when the pressure has
+been<br>
+reduced to the relatively small vapour pressure of the water,
+the<br>
+freezing point is above 0&deg; C., _i.e._ at 0.0075&deg; C. In
+parts of<br>
+the ocean depths the pressure is much over a thousand<br>
+atmospheres. Fresh water would remain liquid there at<br>
+temperatures much below 0&deg; C.</p>
+<p>It will be evident enough, even to those not possessed of
+the<br>
+scientific insight of James Thomson, that some such fact is to
+be<br>
+anticipated. It is, however, easy to be wise after the event.
+It<br>
+appeals to us in a general way that as water expands on
+freezing,<br>
+pressure will tend to resist the turning of it to ice. The
+water<br>
+will try to remain liquid in obedience to the pressure. It
+will,<br>
+therefore, require a lower temperature to induce it to become<br>
+ice.</p>
+<p>James Thomson left his thesis as a prediction. But he
+predicted<br>
+exactly what his distinguished brother, Sir William
+Thomson&mdash;later<br>
+Lord Kelvin&mdash;found to happen when the matter was put to the
+test<br>
+of experiment. We must consider the experiment made by Lord<br>
+Kelvin.</p>
+<p>According to Thomson's views, if a quantity of ice and water
+are<br>
+compressed, there must be _a fall of temperature_. The nature
+of<br>
+his argument is as follows:</p>
+<p>268</p>
+<p>Let the ice and water be exactly at 0&deg; C. to start with.
+Then<br>
+suppose we apply, say, one thousand atmospheres pressure. The<br>
+melting point of the ice is lowered to -7.5&deg; C. That is, it
+will<br>
+require a temperature so low as -7.5&deg; C. to keep it solid.
+It<br>
+will therefore at once set about melting, for as we have
+seen,<br>
+its actual temperature is not -7.5&deg; C., but a higher
+temperature,<br>
+_i.e._ 0&deg; C. In other words, it is 7.5&deg; above its melting
+point.<br>
+But as soon as it begins melting it also begins to absorb heat
+to<br>
+supply the 80 thermal units which, as we know, are required
+to<br>
+turn each kilogram of the ice to water. Where can it get this<br>
+heat? We assume that we give it none. It has only two
+sources,<br>
+the ice can take heat from itself, and it can take heat from
+the<br>
+water. It does both in this case, and both ice and water drop
+in<br>
+temperature. They fall in temperature till -7.5&deg; is reached.
+Then<br>
+the ice has got to its melting point under the pressure of
+one<br>
+thousand atmospheres, or, as we may put it, the water has
+reached<br>
+its freezing point. There can be no more melting. The whole
+mass<br>
+is down to -7.5&deg; C., and will stay there if we keep heat
+from<br>
+flowing either into or out of the vessel. There is now more
+water<br>
+and less ice in the vessel than when we started, and the<br>
+temperature has fallen to -7.5&deg; C. The fall of temperature to
+the<br>
+amount predicted by the theory was verified by Lord Kelvin.</p>
+<p>Suppose we now suddenly remove the pressure; what will happen?
+We<br>
+have water and ice at -7.5&deg; C.</p>
+<p>269</p>
+<p>and at the normal pressure. Water at -7.5&deg; and at the
+normal<br>
+pressure of course turns to ice. The water will, therefore,<br>
+instantly freeze in the vessel, and the whole process will be<br>
+reversed. In freezing, the water will give up its latent
+heat,<br>
+and this will warm up the whole mass till once again 0&deg; C.
+is<br>
+attained. Then there will be no more freezing, for again the
+ice<br>
+is at its melting point. This is the remarkable series of
+events<br>
+which James Thomson predicted. And these are the events which<br>
+Lord Kelvin by a delicate series of experiments, verified in<br>
+every respect.</p>
+<p>Suppose we had nothing but solid ice in the vessel at
+starting,<br>
+would the experiment result in the same way? Yes, it
+assuredly<br>
+would. The ice under the increased pressure would melt a
+little<br>
+everywhere throughout its mass, taking the requisite latent
+heat<br>
+from itself at the expense of its sensible heat, and the<br>
+temperature of the ice would fall to the new melting point.</p>
+<p>Could we melt the whole of the ice in this manner? Again
+the<br>
+answer is "yes." But the pressure must be very great. If we<br>
+assume that all the heat is obtained at the expense of the<br>
+sensible heat of the ice, the cooling must be such as to
+supply<br>
+the latent heat of the whole mass of water produced. However,
+the<br>
+latent heat diminishes as the melting point is lowered, and at
+a<br>
+rate which would reduce it to nothing at about 18,000<br>
+atmospheres. Mousson, operating on ice enclosed in a
+conducting<br>
+cylinder and cooled to -18&deg; at starting</p>
+<p>270</p>
+<p>appears to have obtained very complete liquefaction. Mousson
+must<br>
+have attained a pressure of at least an amount adequate to
+lower<br>
+the melting point below -18&deg;. The degree of liquefaction
+actually<br>
+attained may have been due in part to the passage of heat
+through<br>
+the walls of the vessel. He proved the more or less complete<br>
+liquefaction of the ice within the vessel by the fall of a
+copper<br>
+index from the top to the bottom of the vessel while the
+pressure<br>
+was on.</p>
+<p>I have here a simple way of demonstrating to you the fall
+of<br>
+temperature attending the compression of ice. In this mould,<br>
+which is strongly made of steel, lined with boxwood to
+diminish<br>
+the passage of conducted heat, is a quantity of ice which I<br>
+compress when I force in this plunger. In the ice is a<br>
+thermoelectric junction, the wires leading to which are in<br>
+communication with a reflecting galvanometer. The thermocouple
+is<br>
+of copper and nickel, and is of such sensitiveness as to show
+by<br>
+motion of the spot of light on the screen even a small
+fraction<br>
+of a degree. On applying the pressure, you see the spot of
+light<br>
+is displaced, and in such a direction as to indicate cooling.
+The<br>
+balancing thermocouple is all the time imbedded in a block of
+ice<br>
+so that its temperature remains unaltered. On taking off the<br>
+pressure, the spot of light returns to its first position. I
+can<br>
+move the spot of light backwards and forwards on the screen
+by<br>
+taking off and putting on the pressure. The effects are quite<br>
+instantaneous.</p>
+<p>271</p>
+<p>The fact last referred to is very important. The ice, in fact,
+is<br>
+as it were automatically turned to water. It is not a matter
+of<br>
+the conduction of heat from point to point in the ice. Its
+own<br>
+sensible heat is immediately absorbed throughout the mass.
+This<br>
+would be the theoretical result, but it is probable that owing
+to<br>
+imperfections throughout the ice and failure in uniformity in
+the<br>
+distribution of the stress, the melting would not take place<br>
+quite uniformly or homogeneously.</p>
+<p>Before applying our new ideas to skating, I want you to notice
+a<br>
+fact which I have inferentially stated, but not specifically<br>
+mentioned. Pressure will only lead to the melting of ice if
+the<br>
+new melting point, _i.e._ that due to the pressure, is below
+the<br>
+prevailing temperature. Let us take figures. The ice to start<br>
+with is, say, at -3&deg; C. Suppose we apply such a pressure to
+this<br>
+ice as will confer a melting point of -2&deg; C. on it.
+Obviously,<br>
+there will be no melting. For why should ice which is at -3&deg;
+C.<br>
+melt when its melting point is -2&deg; C.? The ice is, in
+fact,<br>
+colder than its melting point. Hence, you note this fact: The<br>
+pressure must be sufficiently intense to bring the melting
+point<br>
+below the prevailing temperature, or there will be no
+melting;<br>
+and the further we reduce the melting point by pressure below
+the<br>
+prevailing temperature, the more ice will be melted.</p>
+<p>We come at length to the object of our remarks I don't know
+who<br>
+invented skating or skates. It is said that in the thirteenth<br>
+century the inhabitants of</p>
+<p>272</p>
+<p>England used to amuse themselves by fastening the bones of
+an<br>
+animal beneath their feet, and pushing themselves about on
+the<br>
+ice by means of a stick pointed with iron. With such skates,
+any<br>
+performance either on inside or outside edge was impossible.
+We<br>
+are a conservative people. This exhilarating amusement appears
+to<br>
+have served the people of England for three centuries. Not
+till<br>
+1660 were wooden skates shod with iron introduced from the<br>
+Netherlands. It is certain that skating was a fashionable<br>
+amusement in Pepys' time. He writes in 1662 to the effect:
+"It<br>
+being a great frost, did see people sliding with their
+skates,<br>
+which is a very pretty art." It is remarkable that it was the<br>
+German poet Klopstock who made skating fashionable in
+Germany.<br>
+Until his time, the art was considered a pastime, only fit
+for<br>
+very young or silly people.</p>
+<p>I wish now to dwell upon that beautiful contrivance the
+modern<br>
+skate. It is a remarkable example of how an appliance can
+develop<br>
+towards perfection in the absence of a really intelligent<br>
+understanding of the principles underlying its development.
+For<br>
+what are the principles underlying the proper construction of
+the<br>
+skate? After what I have said, I think you will readily<br>
+understand. The object is to produce such a pressure under
+the<br>
+blade that the ice will melt. We wish to establish such a<br>
+pressure under the skate that even on a day when the ice is
+below<br>
+zero, its melting</p>
+<p>273</p>
+<p>point is so reduced just under the edge of the skate that the
+ice<br>
+turns to water.</p>
+<p>It is this melting of the ice under the skate which secures
+the<br>
+condition essential to skating. In the first place, the skate
+no<br>
+longer rests on a solid. It rests on a liquid. You are aware
+how<br>
+in cases where we want to reduce friction&mdash;say at the
+bearing of a<br>
+wheel or under a pivot&mdash;we introduce a liquid. Look at
+the<br>
+bearings of a steam engine. A continuous stream of oil is fed
+in<br>
+to interpose itself between the solid surfaces. I need not<br>
+illustrate so well-known a principle by experiment. Solid<br>
+friction disappears when the liquid intervenes. In its place
+we<br>
+substitute the lesser difficulty of shearing one layer of the<br>
+liquid over the other; and if we keep up the supply of oil
+the<br>
+work required to do this is not very different, no matter how<br>
+great we make the pressure upon the bearings. Compared with
+the<br>
+resistance of solid friction, the resistance of fluid friction
+is<br>
+trifling. Here under the skate the lubrication is perhaps the<br>
+most perfect which it is possible to conceive. J. M&uuml;ller
+has<br>
+determined the coefficient by towing a skater holding on by a<br>
+spring balance. The coefficient is between 0.016 and 0.032.
+In<br>
+other words, the skater would run down an incline so little as
+1<br>
+or 2 degrees; an inclination not perceivable by the eye. Now<br>
+observe that the larger of these coefficients is almost
+exactly<br>
+the same as that which Perry found in the case of
+well-greased<br>
+surfaces. But evidently no</p>
+<p>274</p>
+<p>artificial system of lubrication could hope to equal that
+which<br>
+exists between the skate and the ice. For the lubrication
+here<br>
+is, as it were, automatic. In the machine if the lubricant
+gets<br>
+squeezed out there instantly ensues solid friction. Under the<br>
+skate this cannot happen for the squeezing out of the
+lubricant<br>
+is instantly followed by the formation of another film of
+water.<br>
+The conditions of pressure which may lead to solid friction
+in<br>
+the machine here automatically call the lubricant into<br>
+existence.</p>
+<p>Just under the edge of the skate the pressure is enormous.<br>
+Consider that the whole weight of the skater is born upon a
+mere<br>
+knife edge. The skater alternately throws his whole weight
+upon<br>
+the edge of each skate. But not only is the weight thus<br>
+concentrated upon one edge, further concentration is secured
+in<br>
+the best skates by making the skate hollow-ground, _i.e._<br>
+increasing the keenness of the edge by making it less than a<br>
+right angle. Still greater pressure is obtained by
+diminishing<br>
+the length of that part of the blade which is in contact with
+the<br>
+ice. This is done by putting curvature on the blade or making
+it<br>
+what is called "hog-backed." You see that everything is done
+to<br>
+diminish the area in contact with the ice, and thus to
+increase<br>
+the pressure. The result is a very great compression of the
+ice<br>
+beneath the edge of the skate. Even in the very coldest
+weather<br>
+melting must take place to some extent.</p>
+<p>As we observed before, the melting is instantaneous,</p>
+<p>275</p>
+<p>Heat has not to travel from one point of the ice to
+another;<br>
+immediately the pressure comes on the ice it turns to water.
+It<br>
+takes the requisite heat from itself in order that the change
+of<br>
+state may be accomplished. So soon as the skate passes on,
+the<br>
+water resumes the solid state. It is probable that there is
+an<br>
+instantaneous escape, and re-freezing of some of the water
+from<br>
+beneath the skate, the skate instantly taking a fresh bearing
+and<br>
+melting more ice. The temperature of the water escaping from<br>
+beneath the skate, or left behind by it, immediately becomes
+what<br>
+it was before the skate pressed upon it.</p>
+<p>Thus, a most wonderful and complex series of molecular
+events<br>
+takes place beneath the skate. Swift as it passes, the whole<br>
+sequence of events which James Thomson predicted has to take<br>
+place beneath the blade Compression; lowering of the melting<br>
+point below the temperature of the surrounding ice; melting;<br>
+absorption of heat; and cooling to the new melting point,
+_i.e._<br>
+to that proper to the pressure beneath the blade. The skate
+now<br>
+passes on. Then follow: Relief of pressure; re-solidification
+of<br>
+the water; restoration of the borrowed heat from the
+congealing<br>
+water and reversion of the ice to the original temperature.</p>
+<p>If we reflect for a moment on all this, we see that we do
+not<br>
+skate on ice but on water. We could not skate on ice any more<br>
+than we could skate on glass. We saw that with light weights
+and<br>
+when the pressure</p>
+<p>276</p>
+<p>{Diagram}</p>
+<p>Diagram showing successive states obtaining in ice,
+before,<br>
+during, and after the passage of the skate. The temperatures
+and<br>
+pressures selected for illustration are such as might occur
+under<br>
+ordinary conditions. The edge of the skate is shown in
+magnified<br>
+cross-section.</p>
+<p>277</p>
+<p>Was not sufficient to melt the ice, the friction was much
+the<br>
+same as that of metal on glass. Ice is not slippery. It is an<br>
+error to say that it is. The learned professor was very much<br>
+astray when he said that you could skate on ice because it is
+so<br>
+smooth. The smoothness of the ice has nothing to do with the<br>
+matter. In short, owing to the action of gravity upon your
+body,<br>
+you escape the normal resistance of solid on solid, and glide<br>
+about with feet winged like the messenger of the Gods; but on<br>
+water.</p>
+<p>A second condition essential to the art of skating is also<br>
+involved in the melting of the ice. The sinking of the skate<br>
+gives the skater "bite." This it is which enables him to urge<br>
+himself forward. So long as skates consisted of the rounded
+bones<br>
+of animals, the skater had to use a pointed staff to propel<br>
+himself. In creating bite, the skater again unconsciously
+appeals<br>
+to the peculiar physical properties of ice. The pressure
+required<br>
+for the propulsion of the skater is spread all along the
+length<br>
+of the groove he has cut in the ice, and obliquely downwards.
+The<br>
+skate will not slip away laterally, for the horizontal
+component<br>
+of the pressure is not enough to melt the ice. He thus gets
+the<br>
+resistance he requires.</p>
+<p>You see what a very perfect contrivance the skate is; and what
+a<br>
+similitude of intelligence there is in its evolution. Blind<br>
+intelligence, because it is certain the true physics of
+skating<br>
+was never held in view by</p>
+<p>278</p>
+<p>the makers of skates. The evolution of the skate has been
+truly<br>
+organic. The skater selected the fittest skate, and hence the
+fit<br>
+skate survived.</p>
+<p>In a word, the possibility of skating depends on the
+dynamical<br>
+melting of ice under pressure. And observe the whole matter
+turns<br>
+upon the apparently unrelated fact that the freezing of water<br>
+results in a solid more bulky than the water which gives rise
+to<br>
+it. If ice was less bulky than the water from which it was<br>
+derived, pressure would not melt it; it would be all the more<br>
+solid for the pressure, as it were. The melting point would
+rise<br>
+instead of falling. Most substances behave in this manner,
+and<br>
+hence we cannot skate upon them. Only quite a few substances<br>
+expand on freezing, and it happens that their particular
+melting<br>
+temperatures or other properties render them unsuitable to<br>
+skating. The most abundant fluid substance on the earth, and
+the<br>
+most abundant substance of any one kind on its surface, thus<br>
+possesses the ideally correct and suitable properties for the
+art<br>
+of skating.</p>
+<p>I have pointed out that the pressure must be such as to bring
+the<br>
+temperature of melting below that prevailing in the ice at
+the<br>
+time. We have seen also, that one atmosphere lowers the
+melting<br>
+point of ice by the 1/140 of a degree Centigrade; more exactly
+by<br>
+0.0075&deg;. Let us now assume that the skate is so far sunken in
+the<br>
+ice as to bear for a length of two inches, and for a width of<br>
+one-hundredth of an inch. The skater weighs,</p>
+<p>279</p>
+<p>let us say&mdash;150 pounds. If this weight was borne on one
+square<br>
+inch, the pressure would be ten atmospheres. But the skater
+rests<br>
+his weight, in fact, upon an area of one-fiftieth of an inch.
+The<br>
+pressure is, therefore, fifty times as great. The ice is<br>
+subjected to a pressure of 500 atmospheres. This lowers the<br>
+melting point to -3.75&deg; C. Hence, on a day when the ice is
+at<br>
+this temperature, the skate will sink in the ice till the
+weight<br>
+of the skater is concentrated as we have assumed. His skate
+can<br>
+sink no further, for any lesser concentration of the pressure<br>
+will not bring the melting point below the prevailing<br>
+temperature. We can calculate the theoretical bite for any
+state<br>
+of the ice. If the ice is colder the bite will not be so deep.
+If<br>
+the temperature was twice as far below zero, then the area
+over<br>
+which the skater's weight will be distributed, when the skate
+has<br>
+penetrated its maximum depth, will be only half the former
+area,<br>
+and the pressure will be one thousand atmospheres.</p>
+<p>An important consideration arises from the fact that under
+the<br>
+very extreme edge of the skate the pressure is indefinitely<br>
+great. For this involves that there will always be some bite,<br>
+however cold the ice may be. That is, the narrow strip of ice<br>
+which first receives the skater's weight must partially
+liquefy<br>
+however cold the ice.</p>
+<p>It must have happened to many here to be on ice which was
+too<br>
+cold to skate on with comfort. The</p>
+<p>280</p>
+<p>skater in this case speaks of the ice as too hard. In the<br>
+Engadine, the ice on the large lakes gets so cold that
+skaters<br>
+complain of this. On the rinks, which are chiefly used there,
+the<br>
+ice is frequently renewed by flooding with water at the close
+of<br>
+the day. It thus never gets so very cold as on the lakes. I
+have<br>
+been on ice in North France, which, in the early morning, was
+too<br>
+hard to afford sufficient bite for comfort. The cause of this
+is<br>
+easily understood from what we have been considering.</p>
+<p>We may now return to the experimental results which we
+obtained<br>
+early in the lecture. The heavy weights slip off the ice at a
+low<br>
+angle because just at the points of contact with the ice the<br>
+latter melts, and they, in fact, slip not on ice but on
+water.<br>
+The light weights on cold, dry ice do not lower the melting
+point<br>
+below the temperature of the ice, _i.e._ below -10&deg; C., and
+so<br>
+they slip on dry ice. They therefore give us the true
+coefficient<br>
+of friction of metal on ice.</p>
+<p>This subject has, more recently been investigated by H.
+Morphy,<br>
+of Trinity College, Dublin. The refinement of a closed vessel
+at<br>
+uniform temperature, in which the ice is formed and the<br>
+experiment carried out, is introduced. Thermocouples give the<br>
+temperatures, not only of the ice but of the aluminium sleigh<br>
+which slips upon it under various loads. In this way we may
+be<br>
+certain that the metal runners are truly at the temperature
+of<br>
+the ice. I now quote from Morphy's paper</p>
+<p>281</p>
+<p>"The angle of friction was found to remain constant until
+a<br>
+certain stage of the loading, when it suddenly fell to about
+half<br>
+of its original value. It then remained constant for further<br>
+increases in the load.</p>
+<p>"These results, which confirmed those obtained previously
+with<br>
+less satisfactory apparatus, are shown in the table below. In
+the<br>
+first column is shown the load, _i.e._ the weight of sleigh +<br>
+weight of shot added. In the second and third columns are
+shown,<br>
+respectively, the coefficient and angle of friction, whilst
+the<br>
+fourth gives the temperature of the ice as determined from
+the<br>
+galvanometer deflexions.</p>
+<p>Load.        Tan y.          y.        Temp.</p>
+<p>5.68 grams.  0.36&plusmn;.01     20&deg;&plusmn;30'   
+-5.65&deg; C.<br>
+10.39                                -5.65&deg;<br>
+11.96                                -5.75&deg;<br>
+12.74                                -5.60&deg;<br>
+13.53                                -5.65&deg;<br>
+14.31                                -5.65&deg;<br>
+15.10 grams. 0.17&plusmn;.01    9&deg;.30'&plusmn;30' 
+-5.60&deg;<br>
+16.67                                -5.55&deg;<br>
+19.81                                -5.60&deg;<br>
+24.52                                -5.60&deg;<br>
+5.68 grams. 0.36&plusmn;.01      20&deg;&plusmn;30'   
+-5.60&deg;</p>
+<p>"These experiments were repeated on another occasion with the
+same<br>
+result and similar results had been obtained with different<br>
+apparatus.</p>
+<p>"As a result of the investigation the following points are<br>
+clearly shown:&mdash;</p>
+<p>282</p>
+<p>"(1) The coefficient of friction for ice at constant
+temperature<br>
+may have either of two constant values according to the
+pressure<br>
+per unit surface of contact.</p>
+<p>"(2) For small pressures, and up to a certain well defined
+limit<br>
+of pressure, the coefficient is fairly large, having the
+value<br>
+0.36&plusmn;.01 in the case investigated.</p>
+<p>"(3) For pressures greater than the above limit the
+coefficient<br>
+is relatively small, having the value 0.17&plusmn;.01 in the
+case<br>
+investigated."</p>
+<p>It will be seen that Morphy's results are similar to those<br>
+arrived at in the first experimental consideration of our<br>
+subject; but from the manner in which the experiments have
+been<br>
+carried out, they are more accurate and reliable.</p>
+<p>A great deal more might be said about skating, and the
+allied<br>
+sports of tobogganing, sleighing, curling, ice yachting, and<br>
+last, but by no means least, sliding&mdash;that unpretentious
+pastime<br>
+of the million. Happy the boy who has nails in his boots when<br>
+Jack-Frost appears in his white garment, and congeals the<br>
+neighbouring pond. But I must turn away at the threshold of
+the<br>
+humorous aspect of my subject (for the victim of the street<br>
+"slide" owes his injured dignity to the abstruse laws we have<br>
+been discussing) and pass to other and graver subjects
+intimately<br>
+connected with skating.</p>
+<p>James Thomson pointed out that if we apply compressional
+stress<br>
+to an ice crystal contained in a vessel</p>
+<p>283</p>
+<p>which also contains other ice crystals, and water at 0&deg;
+C., then<br>
+the stressed crystal will melt and become water, but its<br>
+counterpart or equivalent quantity of ice will reappear
+elsewhere<br>
+in the vessel. This is, obviously, but a deduction from the<br>
+principles we have been examining. The phenomenon is commonly<br>
+called "regelation." I have already made the usual regelation<br>
+experiment before you when I compressed broken ice in this
+mould.<br>
+The result was a clear, hard and almost flawless lens of ice.
+Now<br>
+in this operation we must figure to ourselves the pieces of
+ice<br>
+when pressed against one another melting away where
+compressed,<br>
+and the water produced escaping into the spaces between the<br>
+fragments, and there solidifying in virtue of its temperature<br>
+being below the freezing point of unstressed water. The final<br>
+result is the uniform lens of ice. The same process goes on in
+a<br>
+less perfect manner when you make&mdash;or shall I better
+say&mdash;when you<br>
+made snowballs.</p>
+<p>We now come to theories of glacier motion; of which there
+are<br>
+two. The one refers it mainly to regelation; the other to a
+real<br>
+viscosity of the ice.</p>
+<p>The late J. C. M'Connel established the fact that ice
+possesses<br>
+viscosity; that is, it will slowly yield and change its shape<br>
+under long continued stresses. His observations, indeed, raise
+a<br>
+difficulty in applying this viscosity to explain glacier
+motion,<br>
+for he showed that an ice crystal is only viscous in a
+certain<br>
+structural</p>
+<p>284</p>
+<p>direction. A complex mixture of crystals such, as we know
+glacier<br>
+ice to be, ought, we would imagine, to display a nett or<br>
+resultant rigidity. A mass of glacier ice when distorted by<br>
+application of a force must, however, undergo precisely the<br>
+transformations which took place in forming the lens from the<br>
+fragments of ice. In fact, regelation will confer upon it all
+the<br>
+appearance of viscosity.</p>
+<p>Let us picture to ourselves a glacier pressing its enormous
+mass<br>
+down a Swiss valley. At any point suppose it to be hindered
+in<br>
+its downward path by a rocky obstacle. At that point the ice<br>
+turns to water just as it does beneath the skate. The cold
+water<br>
+escapes and solidifies elsewhere. But note this, only where
+there<br>
+is freedom from pressure. In escaping, it carries away its
+latent<br>
+heat of liquefaction, and this we must assume, is lost to the<br>
+region of ice lately under pressure. This region will,
+however,<br>
+again warm up by conduction of heat from the surrounding ice,
+or<br>
+by the circulation of water from the suxface. Meanwhile, the<br>
+pressure at that point has been relieved. The mechanical<br>
+resistance is transferred elsewhere. At this new point there
+is<br>
+again melting and relief of pressure. In this manner the
+glacier<br>
+may be supposed to move down. There is continual flux of<br>
+conducted heat and converted latent heat, hither and thither,
+to<br>
+and from the points of resistance. The final motion of the
+whole<br>
+mass is necessarily slow; a few feet in the day or, in
+winter,</p>
+<p>285</p>
+<p>even only a few inches. And as we might expect, perfect
+silence<br>
+attends the downward slipping of the gigantic mass. The
+motion<br>
+is, I believe, sufficiently explained as a skating motion.
+The<br>
+skate is, however, fixed, the ice moves. The great Aletsch<br>
+Glacier collects its snows among the highest summits of the<br>
+Oberland. Thence, the consolidated ice makes its way into the<br>
+Rhone Valley, travelling a distance of some 20 miles. The ice
+now<br>
+melting into the youthful Rhone fell upon the Monch, the
+Jungfrau<br>
+or the Eiger in the days when Elizabeth ruled in England and<br>
+Shakespeare lived.</p>
+<p>The ice-fall is a common sight on the glacier. In great lumps
+and<br>
+broken pinnacles it topples over some rocky obstacle and
+falls<br>
+shattered on to the glacier below. But a little further down
+the<br>
+wound is healed again, and regelation has restored the smooth<br>
+surface of the glacier. All such phenomena are explained on
+James<br>
+Thomson's exposition of the behaviour of a substance which<br>
+expands on passing from the liquid to the solid state.</p>
+<p>We thus have arrived at very far-reaching considerations
+arising<br>
+out of skating and its science. The tendency for snow to<br>
+accumulate on the highest regions of the Earth depends on<br>
+principles which we cannot stop to consider. We know it
+collects<br>
+above a certain level even at the Equator. We may consider,
+then,<br>
+that but for the operation of the laws which James Thomson<br>
+brought to light, and which his illustrious brother,</p>
+<p>286</p>
+<p>Lord Kelvin, made manifest, the uplands of the Earth could
+not<br>
+have freed themselves of the burthen of ice. The geological<br>
+history of the Earth must have been profoundly modified. The<br>
+higher levels must have been depressed; the general level of
+the<br>
+ocean relatively to the land thereby raised, and, it is even<br>
+possible, that such a mean level might have been attained as<br>
+would result in general submergence.</p>
+<p>During the last great glacial period, we may say the fate of
+the<br>
+world hung on the operation of those laws which have concerned
+us<br>
+throughout this lecture. It is believed the ice was piled up to
+a<br>
+height of some 6,000 feet over the region of Scandinavia.
+Under<br>
+the influence of the pressure and fusion at points of
+resistance,<br>
+the accumulation was stayed, and it flowed southwards the<br>
+accumulation was stayed, and it flowed southwards over
+Northern<br>
+Europe. The Highlands of Scotland were covered with, perhaps,<br>
+three or four thousand feet of ice. Ireland was covered from<br>
+north to south, and mighty ice-bergs floated from our western
+and<br>
+southern shores.</p>
+<p>The transported or erratic stones, often of great size, which
+are<br>
+found in many parts of Ireland, are records of these long
+past<br>
+events: events which happened before Man, as a rational
+being,<br>
+appeared upon the Earth.</p>
+<p>287</p>
+<p><u>A SPECULATION AS TO A PREMATERIAL UNIVERSE</u> [1]</p>
+<p>"And therefore...these things likewise had a birth; for
+things<br>
+which are of mortal body could not for an infinite time
+back...<br>
+have been able to set at naught the puissant strength of<br>
+immeasurable age."&mdash;LUCRETIUS, _De Rerum Natura._</p>
+<p>"O fearful meditation! Where, alack! Shall Time's best
+jewel<br>
+from Time's chest lie hid?" &mdash;SHAKESPEARE.</p>
+<p>IN the material universe we find presented to our senses a<br>
+physical development continually progressing, extending to
+all,<br>
+even the most minute, material configurations. Some
+fundamental<br>
+distinctions existing between this development as apparent in
+the<br>
+organic and the inorganic systems of the present day are
+referred<br>
+to elsewhere in this volume.[2] In the present essay, these<br>
+systems as having a common origin and common ending, are
+merged<br>
+in the same consideration as to the nature of the origin of<br>
+material systems in general. This present essay is occupied
+by<br>
+the consideration of the necessity of limiting material<br>
+interactions in past time. The speculation originated in the<br>
+difficulties which present themselves when we ascribe to
+these<br>
+interactions infinite duration in the past. These
+difficulties<br>
+first claim our consideration.</p>
+<p>[1] Proc. Royal Dublin Soc., vol. vii., Part V, 1892.</p>
+<p>[2] _The Abundance of Life._</p>
+<p>288</p>
+<p>Accepting the hypothesis of Kant and Laplace in its widest<br>
+extension, we are referred to a primitive condition of wide<br>
+material diffusion, and necessarily too of material
+instability.<br>
+The hypothesis is, in fact, based upon this material
+instability.<br>
+We may pursue the sequence of events assumed in this
+hypothesis<br>
+into the future, and into the past.</p>
+<p>In the future we find finality to progress clearly indicated.
+The<br>
+hypothesis points to a time when there will be no more<br>
+progressive change but a mere sequence of unfruitful events,
+such<br>
+as the eternal uniform motion of a mass of matter no longer<br>
+gaining or losing heat in an ether possessed of a uniform<br>
+distribution of energy in all its parts. Or, again, if the
+ether<br>
+absorb the energy of material motion, this vast and dark<br>
+aggregation eternally poised and at rest within it. The action
+is<br>
+transferred to the subtle parts of the ether which suffer none
+of<br>
+the energy to degrade. This is, physically, a thinkable
+future.<br>
+Our minds suggest no change, and demand none. More than this,<br>
+change is unthinkable according to our present ideas of
+energy.<br>
+Of progress there is an end.</p>
+<p>This finality _&acirc; parte post_ is instructive.
+Abstract<br>
+considerations, based on geometrical or analytical
+illustrations,<br>
+question the finiteness of some physical developments. Thus
+our<br>
+sun may require eternal time to attain the temperature of the<br>
+ether around it, the approach to this condition being assumed
+to<br>
+be asymptotic in</p>
+<p>289</p>
+<p>character. But consider the legitimate _reductio ad absurdum_
+of<br>
+an ember raked from a fire 1000 years ago. Is it not yet
+cooled<br>
+down to the constant temperature of its surroundings? And we
+may<br>
+evidently increase the time a million-fold if we please. It<br>
+appears as if we must regard eternity as outliving every<br>
+progressive change, For there is no convergence or
+enfeeblement<br>
+of time. The ever-flowing present moves no differently for
+the<br>
+occurrence of the mightiest or the most insignificant events.
+And<br>
+even if we say that time is only the attendant upon events,
+yet<br>
+this attendant waits patiently for the end, however long<br>
+deferred.</p>
+<p>Does the essentially material hypothesis of Kant and
+Laplace<br>
+account for an infinite past as thinkably as it accounts for
+the<br>
+infinite future? As this hypothesis is based upon material<br>
+instability the question resolves itself into this:&mdash; Is
+the<br>
+assumption of an infinitely prolonged past instability a
+probable<br>
+or possible account of the past? There are, it appears to me,<br>
+great difficulties involved in accepting the hypothesis of<br>
+infinitely prolonged material instability. I will refer here
+to<br>
+three principal objections. The first may be called a<br>
+metaphysical objection; the second is partly metaphysical and<br>
+partly physical, the third may be considered a physical<br>
+objection, as it is involved directly in the phenomena
+presented<br>
+by our universe.</p>
+<p>The metaphysical objection must have presented itself to
+every<br>
+one who has considered the question. It may</p>
+<p>290</p>
+<p>be put thus:&mdash;If present events are merely one stage in
+an<br>
+infinite progress, why is not the present stage long ago
+passed<br>
+over? We are evidently at liberty to push back any stage of<br>
+progress to as remote a period as we like by putting back
+first<br>
+the one before this and next the stage preceding this, and so
+on,<br>
+for, by hypothesis, there is no beginning to the progress.</p>
+<p>Thus, the sum of passing events constituting the present
+universe<br>
+should long ago have been accomplished and passed away. If we<br>
+consider alternative hypotheses not involving this difficulty,
+we<br>
+are at once struck by the fact that the future of material<br>
+development is free of the objection. For the eternity of<br>
+unprogressive events involved in the future on Kant's
+hypothesis,<br>
+is not only thinkable, but any change is, as observed,<br>
+irreconcilable with our ideas of energy. As in the future so
+in<br>
+the past we look to a cessation to progress. But as we
+believe<br>
+the activity of the present universe must in some form have<br>
+existed all along, the only refuge in the past is to imagine
+an<br>
+active but unprogressive eternity, the unprogressive activity
+at<br>
+some period becoming a progressive activity&mdash;that
+progressive<br>
+activity of which we are spectators. To the unprogressive<br>
+activity there was no beginning; in fact, beginning is as<br>
+unthinkable and uncalled for to the unprogressive activity of
+the<br>
+past as ending is to the unprogressive activity of the
+future,<br>
+when all developmental actions shall have ceased. There is no<br>
+beginning or ending to the activity of the universe.</p>
+<p>291</p>
+<p>There is beginning and ending to present progressive
+activity.<br>
+Looking through the realm of nature we seek beginning and
+ending,<br>
+but "passing through nature to eternity" we find neither.
+Both<br>
+are justified; the questioning of the ancient poet regarding
+the<br>
+past, and of the modern regarding the future, quoted at the
+head<br>
+of this essay.</p>
+<p>The next objection, which is in part metaphysical, is founded
+on<br>
+the difficulty of ascribing any ultimate reality or potency
+to<br>
+forces diminishing through eternal time. Thus, against the<br>
+assumption that our universe is the result of material<br>
+aggregation progressing over eternal time, which involves the<br>
+primitive infinite separation of the particles, we may ask,
+what<br>
+force can have acted between particles sundered by infinite<br>
+distance? The gravitational force falling off as the square
+of<br>
+the distance, must vanish at infinity if we mean what we say
+when<br>
+we ascribe infinite separation to them. Their condition is
+then<br>
+one of neutral stability, a finite movement of the particles<br>
+neither increasing nor diminishing interaction. They had then<br>
+remained eternally in their separated condition, there being
+no<br>
+cause to render such condition finite. The difficulty
+involved<br>
+here appears to me of the same nature as the difficulty of<br>
+ascribing any residual heat to the sun after eternal time has<br>
+elapsed. In both cases we are bound to prolong the time, from
+our<br>
+very idea of time, till progress is no more, when in the one
+case<br>
+we can imagine no mutual approximation of the</p>
+<p>292</p>
+<p>particles, in the other no further cooling of the body.
+However,<br>
+I will riot dwell further upon this objection, as it does not,
+I<br>
+believe, present itself with equal force to every mind. A
+reason<br>
+less open to dispute, as being less subjective, against the<br>
+aggregation of infinitely remote particles as the origin of
+our<br>
+universe, is contained in the physical objection.</p>
+<p>In this objection we consider that the appearance presented
+by<br>
+our universe negatives the hypothesis of infinitely prolonged<br>
+aggregation. We base this negation upon the appearance of<br>
+simultaneity ~ presented by the heavens, contending that this<br>
+simultaneity is contrary to what we would expect to find in
+the<br>
+case of particles gathered from infinitely remote distances.<br>
+Whether these particles were endowed with relative motions or
+not<br>
+is unimportant to the consideration. In what respects do the<br>
+phenomena of our universe present the appearance of
+simultaneous<br>
+phenomena? We must remember that the suns in space are as
+fires<br>
+which brighten only for a moment and are then extinguished. It
+is<br>
+in this sense we must regard the longest burning of the
+stars.<br>
+Whether just lit or just expiring counts little in eternity.
+The<br>
+light and heat of the star is being absorbed by the ether of<br>
+space as effectually and rapidly as the ocean swallows the
+ripple<br>
+from the wings of an expiring insect. Sir William Herschel
+says<br>
+of the galaxy of the milky way:&mdash; "We do not know the rate
+of<br>
+progress of this mysterious chronometer, but it is
+nevertheless<br>
+certain that it cannot</p>
+<p>293</p>
+<p>last for ever, and its past duration cannot be infinite." We
+do<br>
+not know, indeed, the rate of progress of the chronometer, but
+if<br>
+the dial be one divided into eternal durations the
+consummation<br>
+of any finite physical change represents such a movement of
+the<br>
+hand as is accomplished in a single vibration of the balance<br>
+wheel.</p>
+<p>Hence we must regard the hosts of glittering stars as a<br>
+conflagration that has been simultaneously lighted up in the<br>
+heavens. The enormous (to our ideas) thermal energy of the
+stars<br>
+resembles the scintillation of iron dust in a jar of oxygen
+when<br>
+a pinch of the dust is thrown in. Although some particles be<br>
+burnt up before others become alight, and some linger yet a<br>
+little longer than the others, in our day's work the<br>
+scintillation of the iron dust is the work of a single
+instant,<br>
+and so in the long night of eternity the scintillation of the<br>
+mightiest suns of space is over in a moment. A little longer,<br>
+indeed, in duration than the life which stirs a moment in<br>
+response to the diffusion of the energy, but only very little.
+So<br>
+must an Eternal Being regard the scintillation of the stars
+and<br>
+the periodic vibration of life in our geological time and the<br>
+most enduring efforts of thought. The latter indeed are no
+more<br>
+lasting than</p>
+<p>"... the labour of ants In the light of a million million
+of<br>
+suns."</p>
+<p>But the myriad suns themselves, with their generations, are
+the<br>
+momentary gleam of lights for ever after extinguished.</p>
+<p>294</p>
+<p>Again, science suggests that the present process of
+material<br>
+aggregation is not finished, and possibly will only be when
+it<br>
+prevails universally. Hence the very distribution of the
+stars,<br>
+as we observe them, as isolated aggregations, indicates a<br>
+development which in the infinite duration must be regarded
+as<br>
+equally advanced in all parts of stellar space and essentially
+a<br>
+simultaneous phenomenon. For were we spectators of a system
+in<br>
+which any very great difference of age prevailed, this very
+great<br>
+difference would be attended by some such appearance as the<br>
+following:&mdash;</p>
+<p>The aupearance of but one star, other generations being
+long<br>
+extinct or no others yet come into being; or, perhaps, a
+faint<br>
+nebulous wreath of aggregating matter somewhere solitary in
+the<br>
+heavens; or no sign of matter beyond our system, either
+because<br>
+ungathered or long passed away into darkness.[1]</p>
+<p>Some such appearances were to be expected had the aggregation
+of<br>
+matter depended solely on chance encounters of particles<br>
+scattered through infinite space.</p>
+<p>For as, by hypothesis, the aggregation occupies an infinite
+time<br>
+in consummation it is nearly a certainty that each particle<br>
+encountered after immeasurable time, and then for the first
+time<br>
+endowed with actual gravitational potential energy, would
+have<br>
+long expended this energy</p>
+<p>[1] It is interesting to reflect upon the effect which an
+entire<br>
+absence of luminaries outside our solar system would have had<br>
+upon the views of our philosophers and upon our outlook on
+life.</p>
+<p>295</p>
+<p>before another particle was gathered. But the fact that so
+many<br>
+fires which we know to be of brief duration are scattered
+through<br>
+a region of space, and the fact of a configuration which we<br>
+believe to be a transitory ore, suggest their simultaneous<br>
+aggregation here and there. And in the nebulous wreaths
+situated<br>
+amidst the stars there is evidence that these actually
+originated<br>
+where they now are, for in such no relative motion, I
+believe,<br>
+has as yet been detected by the spectroscope. All this, too,
+is<br>
+in keeping with the nebular hypothesis of Kant and Laplace so<br>
+long as this does not assume a primitive infinite dispersion
+of<br>
+matter, but the gathering of matter from finite distances
+first<br>
+into nebulous patches which aggregating with each other have<br>
+given rise to our system of stars. But if we extend this<br>
+hypothesis throughout an infinite past by the supposition of<br>
+aggregation of infinitely remote particles we replace the<br>
+simultaneous approach required in order to accotnt for the<br>
+simultaneous phenomena visible in the heavens, by a succession
+of<br>
+aggregative events, by hypothesis at intervals of nearly
+infinite<br>
+duration, when the events of the universe had consisted of
+fitful<br>
+gleams lighted after eternities of time and extinguished for
+yet<br>
+other eternities.</p>
+<p>Finally, if we seek to replace the eternal instability
+involved<br>
+in Kant's hypothesis when extended over an infinite past, by
+any<br>
+hypothesis of material stability, we at once find ourselves
+in<br>
+the difficulty that from the known properties of matter such<br>
+stability must have been</p>
+<p>296</p>
+<p>permanent if ever existent, which is contrary to fact. Thus
+the<br>
+kinetic inertia expressed in Newton's first law of motion
+might<br>
+well be supposed to secure equilibrium with material
+attraction,<br>
+but if primevally diffused matter had ever thus been held in<br>
+equilibrium it must have remained so, or it was maintained so<br>
+imperfectly, which brings us back to endless evolution.</p>
+<p>On these grounds I contend that the present gravitational<br>
+properties of matter cannot be supposed to have acted for all<br>
+past duration. Universal equilibrium of gravitating particles<br>
+would have been indestructible by internal causes. Perpetual<br>
+instability or evolution is alike unthinkable and contrary to
+the<br>
+phenomena of the universe of which we are cognisant. We
+therefore<br>
+turn from gravitating matter as affording no rational account
+of<br>
+the past. We do so of necessity, however much we feel our<br>
+ignorance of the nature of the unknown actions to which we
+have<br>
+recourse.</p>
+<p>A prematerial condition of the universe was, we assume, a<br>
+condition in which uniformity as regards the average
+distribution<br>
+of energy in space prevailed, but neterogeneity and
+instability<br>
+were possible. The realization of that possibility was the<br>
+beginning we seek, and we today are witnesses of the train of<br>
+events involved in the breakdown of an eternal past
+equilibrium.<br>
+We are witnesses on this hypothesis, of a catastrophe
+possibly<br>
+confined to certain regions of space, but which is, to the<br>
+motions and configurations concerned, absolutely unique,<br>
+reversible to</p>
+<p>297</p>
+<p>its former condition of potential by no process of which we
+can<br>
+have any conception.</p>
+<p>Our speculation is that we, as spectators of evolution,
+are<br>
+witnessing the interaction of forces which have not always
+been<br>
+acting. A prematerial state of the universe was one of
+unfruitful<br>
+motions, that is, motions unattended by progressing changes,
+in<br>
+our region of the ether. How extended we cannot say; the
+nature<br>
+of the motions we know not; but the kinetic entities differed<br>
+from matter in the one important particular of not possessing<br>
+gravitational attraction. Such kinetic configurations we
+cannot<br>
+consider to be matter. It was _possible_ to construct matter
+by<br>
+their summation or linkage as the configuration of the crystal
+is<br>
+possible in the clear supersaturated liquid.</p>
+<p>Duration in an ether filled with such motions would pass in
+a<br>
+succession of mere unfruitful events; as duration, we may<br>
+imagine, even now passes in parts of the ether similar to our<br>
+own. An endless (it may be) succession of unprogressive,<br>
+fruitless events. But at one moment in the infinite duration
+the<br>
+requisite configuration of the elementary motions is
+attained;<br>
+solely by the one chance disposition the stability of all
+must<br>
+go, spreading from the fateful point.</p>
+<p>Possibly the material segregation was confined to one part
+of<br>
+space, the elementary motions condensing upon transformation,
+and<br>
+so impoverishing the ether around till the action ceased.
+Again<br>
+in the same sense as the</p>
+<p>298</p>
+<p>stars are simultaneous, so also they may be regarded as
+uniform<br>
+in size, for the difference in magnitude might have been
+anything<br>
+we please to imagine, if at the same time we ascribe
+sufficient<br>
+distance sundering great and small. So, too;, will a dilute<br>
+solution of acetate of soda build a crystal at one point, and
+the<br>
+impoverishment of the medium checking the growth in this
+region,<br>
+another centre will begin at the furthest extremities of the<br>
+first crystal till the liquid is filled with loose feathery<br>
+aggregations comparable in size with one another. In a
+similar<br>
+way the crystallizing out of matter may have given rise, not to
+a<br>
+uniform nebula in space, but to detached nebula, approximately
+of<br>
+equal mass, from which ultimately were formed the stars.</p>
+<p>That an all-knowing Being might have foretold the ultimate
+event<br>
+at any preceding period by observing the motions of the parts<br>
+then occurring, and reasoning as to the train of consequences<br>
+arising from these nations, is supposable. But considerations<br>
+arising from this involve no difficulty in ascribing to this<br>
+prematerial train of events infinite duration. For progress
+there<br>
+is none, and we can quite as easily conceive of some part of<br>
+space where the same Infinite Intelligence, contemplating a<br>
+similar train of unfruitful motions, finds that at no time in
+the<br>
+future will the equilibrium be disturbed. But where evolution
+is<br>
+progressing this is no longer conceivable, as being
+contradictory<br>
+to the very idea of progressive development. In this case<br>
+Infinite Intelligence</p>
+<p>299</p>
+<p>_necessarily_ finds, as the result of his contemplation,
+the<br>
+aggregation of matter, and the consequences arising
+therefrom.</p>
+<p>The negation of so primary a material property as gravitation
+to<br>
+these primitive motions of (or in) the ether, probably
+involves<br>
+the negation of many properties we find associated with
+matter.<br>
+Possibly the quality of inertia, equally primary, is involved<br>
+with that of gravitation, and we may suppose that these two<br>
+properties so intimately associated in determining the motions
+of<br>
+bodies in space were conferred upon the primitive motions as<br>
+crystallographic attraction and rigidity are first conferred
+upon<br>
+the solid growing from the supersaturated liquid. But in some<br>
+degree less speculative is the supposition that the new order
+of<br>
+motions involved the transformation of much energy into the
+form<br>
+of heat vibrations; so that the newly generated matter, like
+the<br>
+newly formed crystal, began its existence in a medium richly
+fed<br>
+with thermal radiant energy. We may consider that the thermal<br>
+conditions were such as would account for a primitive<br>
+dissociation of the elements. And, again, we recall how the<br>
+physicist finds his estimate of the energy involved in mere<br>
+gravitational aggregation inadequate to afford explanation of<br>
+past solar heat. It is supposable, on such a hypothesis as we<br>
+have been dwelling on, that the entire subsequent
+gravitational<br>
+condensation and conversion of material potential energy,
+dating<br>
+from the first formation of matter to the stage of star<br>
+formation</p>
+<p>300</p>
+<p>may be insignificant in amount compared with the conversion
+of<br>
+etherial energy attending the crystallizing out of matter
+from<br>
+the primitive motions. And thus possibly the conditions then<br>
+obtaining involved a progressively increasing complexity of<br>
+material structure the genesis of the elements, from an<br>
+infra-hydrogen possessing the simplest material
+configuration,<br>
+resulting ultimately in such self-luminous nebula as we yet
+see<br>
+in the heavens.</p>
+<p>The late James Croll, in his _Stellar Evolution_, finds
+objections<br>
+to an eternal evolution, one of which is similar to the<br>
+"metaphysical" objection urged in this paper. His way out of
+the<br>
+difficulty is in the speculation that our stellar system<br>
+originated by the collision of two masses endowed with
+relative<br>
+motion, eternal in past duration, their meeting ushering in
+the<br>
+dawn of evolution. However, the state of aggregation here<br>
+assumed, from the known laws of matter and from analogy,
+calls<br>
+for explanation as probably the result of prior diffusion,
+when,<br>
+of course, the difficulty is only put back, not set at rest.
+Nor<br>
+do I think the primitive collision in harmony with the number
+of<br>
+relatively stationary nebula visible in space.</p>
+<p>The metaphysical objection is, I find, also urged by
+George<br>
+Salmon, late Provost of Trinity College, in favour of the<br>
+creation of the universe.&mdash;(_Sermons on Agnosticism_.)</p>
+<p>A. Winchell, in _World Life_, says: "We have not</p>
+<p>301</p>
+<p>the slightest scientific grounds for assuming that matter
+existed<br>
+in a certain condition from all eternity. The essential
+activity<br>
+of the powers ascribed to it forbids the thought; for all that
+we<br>
+know, and, indeed, as the _conclusion_ from all that we know,<br>
+primal matter began its progressive changes on the morning of
+its<br>
+existence."</p>
+<p>Finally, in reference to the hypothesis of a unique
+determination<br>
+of matter after eternal duration in the past, it may not be
+out<br>
+of place to remind the reader of the complexity which modern<br>
+research ascribes to the structure of the atom.</p>
+<p>302</p>
+<p><u>INDEX</u></p>
+<p>A.</p>
+<p>Abney, Sir Wm., on sensitisers, 210.</p>
+<p>Abundance of life, numerical, 98-100.</p>
+<p>Adaptation and aggressiveness of the organism, 80.</p>
+<p>Additive law, the, with reference to alpha rays, 220.</p>
+<p>Age of Earth, comparison of denudative and radioactive methods
+of<br>
+finding, 23-29.</p>
+<p>Aletsch glacier, 286.</p>
+<p>Allen, Grant, on colour of Alpine plants, 104.</p>
+<p>Allen, H. Stanley, on photo-electricity, 203.</p>
+<p>Alpha rays, nature of, 214; velocity of, 214; effects of,
+on<br>
+gases, 214; range of, in air, 215; visualised, 218;
+ionisation<br>
+curve of, 216; number of, from one gram of radium, 237; number
+of<br>
+ions made by, 237.</p>
+<p>Alpine flowers, intensity of colour of, 102.</p>
+<p>Alps, history of, 141; Tertiary denudation of, 148; depth
+of<br>
+sedimentary covering of, 148; evidence of high pressures and<br>
+temperatures in, 149; recent theories of formation of, 150
+_et<br>
+seq._; upheaval of, 147; age of, 147; volcanic phenomena<br>
+attending elevation of, 147.</p>
+<p>Andes, trough parallel to, 123; not volcanic in origin,
+118.</p>
+<p>Angle of friction on ice, 261-265, 281-283; on glass,
+261-265.</p>
+<p>Animate systems, dynamic conditions of, 67; and transfer
+of<br>
+energy, 71; and old age, 72; mechanical imitation of, 76, 77.</p>
+<p>Animate and inanimate systems compared, 73-75.</p>
+<p>Appalachian range, formation of, 120.</p>
+<p>Arrhenius, on elevation of continents, 17.</p>
+<p>Aryan Era of India, 136.</p>
+<p>Asteroids, probable origin of, 175; discovery of, 175;
+dimensions<br>
+of, 176; orbits of, 176; Mars' moons derived from, 177.</p>
+<p>B.</p>
+<p>Babbage and Herschel, theory of mountain building, 123.</p>
+<p>Babes (and Cornil), size of spores, 98.</p>
+<p>Becker, G. F., age of Earth by sodium collection, 14; age
+of<br>
+minerals by lead ratio, 20.</p>
+<p>Berthelot, law of maximum work, 62.</p>
+<p>Bertrand, Marcel, section of Mont Blanc Massif, 154.</p>
+<p>Beta rays, nature of, 246; accompanied by gamma rays, 247;<br>
+production of, by gamma rays, 247; as ionising agents, 249.</p>
+<p>Biotite, containing haloes, 223; pleochroism of, 235;
+intensified<br>
+pleochroism in halo, 235.</p>
+<p>Body and mind, as manifestations of progressiveness of the<br>
+organism, 86.</p>
+<p>Boltwood, age of minerals by lead ratio, 20.</p>
+<p>Bose, theory of latent image, 203.</p>
+<p>Bragg and Kleeman, on path of the alpha ray, 215; stopping
+power,<br>
+219; laws affecting ionisation by alpha rays, 220; curve of<br>
+ionisation and structure of the halo, 232.</p>
+<p>Brecciendecke, sheet of the, 154.</p>
+<p>Brdche, sheet of the, 154.</p>
+<p>Burrard and Hayden on the Himalaya, 138; sections of the<br>
+Himalaya, 139.</p>
+<p>C.</p>
+<p>Canals and "canali," 166; curvature of, and path of a
+satellite,<br>
+188 _et seq._; double and triple accounted for, 186, 187;<br>
+doubling of, 195; disappearance and reappearance of, 196-198;<br>
+photography of, 198; not due to cracks, 167; not due to
+rivers,<br>
+167; of Mars, double nature of, 166, 170; crossing dark
+regions<br>
+of planet's surface, 168; of Mars, Lowell's views on, 168 _et<br>
+seq._; shown on Lowell's map, investigation of, 192 _et
+seq._;<br>
+radiating, explanation of, 193, 194; number of, 194; developed
+by<br>
+secondary disturbances, 194; nodal development of, due to
+raised<br>
+surface features, 195.</p>
+<p>Chamberlin and Salisbury, the Laramide range, 121.</p>
+<p>Clarke, F. W., estimate of mass of sediments, 9; age of Earth
+by<br>
+sodium collection, 14; average composition of sedimentary and<br>
+igneous rocks, 42; on average composition of the crust, 126;<br>
+solvent denudation of the continents, 17, 40.</p>
+<p>Claus, protoplasm the test of the cell, 67; abortion of
+useless<br>
+organs, 69.</p>
+<p>Coefficient of friction, definition of, 262; deduction of,
+from<br>
+angle of friction, 263; abnormal values on ice, 261-265, 282;
+for<br>
+various substances, 265.</p>
+<p>Continental areas, movements of, 144.</p>
+<p>Cornil and Babes, size of spores, 98.</p>
+<p>Croll, James, dawn of evolution, 301.</p>
+<p>Crust of the Earth, average composition of, 126; depth of<br>
+softening in, 128.</p>
+<p>Curie, definition of the, 256.</p>
+<p>D.</p>
+<p>Dana, on mountain building, 120.</p>
+<p>Dawson, reduction of surface represented by Laramide range,
+123.</p>
+<p>Deccan traps, 137</p>
+<p>_d&eacute;ferlement_, theory of, 155; explanation of, 155 _et
+seq._;<br>
+temperature involved in, 156.</p>
+<p>Deimos, dimensions of, 177; orbit of, 577.</p>
+<p>De Lapparent, exotic nature of the Pr&eacute;alpes, 150.</p>
+<p>De Montessus and the association of earthquakes with<br>
+geosynclines, 142.</p>
+<p>Denudation as affected by continental elevation, 17;
+factors<br>
+promoting, 30 _et seg._; relative activity in mountains and
+on<br>
+plains, 35-40; solvent, by the sea, 40; the sodium index of,<br>
+46-50; thickness of rock-layer removed from the land, 51.</p>
+<p>De Quincy, System of the Heavens, 200.</p>
+<p>Dewar, Sir James, latent image formed at low temperatures,
+202.</p>
+<p>Dixon, H. H., and AGnadance of Life, 60.</p>
+<p>Double canals, formation by attraction of a satellite,
+585-187.</p>
+<p>Douglass, A. E., observations on Mars, 167.</p>
+<p>Dravidian Era of India, 135.</p>
+<p>E.</p>
+<p>Earth, early history of, 3, 4; dimensions of, relative to
+surface<br>
+features, 117.</p>
+<p>Earth's age determined by thickness of sediments, 5;
+determined<br>
+by mass of the sediments, 7; determined by sodium in the
+ocean,<br>
+12; determined by radioactive transformations, 19;
+significance<br>
+of, 2.</p>
+<p>Earthquakes associated with geosynclincs, 142.</p>
+<p>Efficiency, tendency to maximum, in organisms, 113, 114.</p>
+<p>Elements, probable wide diffusion of rare, 230; rarity of<br>
+radioactive, 241.</p>
+<p>Elster and Geitel, photo-electric activity and absorption,
+207;<br>
+photo-electric properties of gelatin, 212; Emanation of
+radium,<br>
+therapeutic use of, 256-259; advantages of, in medicine, 256;<br>
+volume of, 257; how obtained, 257; use of, in needles, 258.</p>
+<p>Equilibrium amount, meaning of, 254, 255.</p>
+<p>Evolution and acceleration of activity, 79; of the universe
+not<br>
+eternal a pane ante, 298.</p>
+<p>F.</p>
+<p>Faraday and ionisation, 57.</p>
+<p>Finality of progress a part, post, 289.</p>
+<p>Flahault, experiments on colour of flowers, 108.</p>
+<p>Fletcher, A. L., proportionality of thorium and uranium,
+26,</p>
+<p>G.</p>
+<p>Galileo, discovery of Jupiter's moons, 162.</p>
+<p>Gamma rays, nature of, 247: production of, by beta rays, 247;
+as<br>
+ionising agents, 249.</p>
+<p>Geddes and Thomson, hunger and living matter, 71.</p>
+<p>Geiger, range of alpha rays in air, 215; ionisation affected
+by<br>
+alpha rays in air, 216; on "scattering," 217; scattering and
+the<br>
+structure of the halo, 232.</p>
+<p>Geikie, Sir A., uniformity in geological history, 15.</p>
+<p>Geosynclines, 119; association with earthquakes and
+volcanoes,<br>
+142; of the tethys, 142; radioactive heat in, due to
+sediments,<br>
+130; temperature effects due to lateral compression of, 131.</p>
+<p>Glacial epoch, phenomena of, 287.</p>
+<p>Glacier motion, cause of. 285.</p>
+<p>Glossopteris and Gangamopteris flora, 136.</p>
+<p>Gondwanaland, 136.</p>
+<p>Gradient of temperature in Earth's surface crust, 126.</p>
+<p>H.</p>
+<p>Haimanta period of India, 135.</p>
+<p>Halley, Edmund, finding age by saltness of ocean, 13.</p>
+<p>Hallwachs, photo-electric activity and absorption, 207.</p>
+<p>Haloes, pleochroic, finding age of rocks by, 21; due to
+uranium<br>
+and thorium families, 227; radii of, 227; over-exposed and<br>
+underexposed, 228; intimate structure of, 229 _et seq._;<br>
+artificial, 229; tubular, in mica, 230; extreme age of, 231;<br>
+effect of nucleus on structure of, 232; inference from
+spherical<br>
+form of, in crystals, 233; structure of, unaffected by
+cleavage,<br>
+235; origin of the name "pleochroic,"235; colouration due to<br>
+iron, 235; colouration not due to helium, 236; age Of, 236;
+slow<br>
+formation of, 237, 238; number of rays required to build,
+237;<br>
+and age of the Earth, 238-241.</p>
+<p>Hayden, H.H., geology of the Himalaya, 134, 138, 139.</p>
+<p>Heat-tendency of the universe, 62.</p>
+<p>Heat emission from the Earth's surface, 126; from average
+igneous<br>
+rock due to radioactivity, 126.</p>
+<p>Helium and the alpha ray, 214, 222; colouration of halo not
+due<br>
+to, 236.</p>
+<p>Hering, E., and physiological or unconscious memory, 111.</p>
+<p>Herschel and Babbage theory of mountain building, 123.</p>
+<p>Herschel, Sir W., on galaxy of milky way, 293.</p>
+<p>Hertz, negative electrification discharged by light, 204.</p>
+<p>Himalaya, geological history of, 134-139.</p>
+<p>Hobbs, on association of earthquakes and geosynclines,
+143.</p>
+<p>Holmes, A., original lead in minerals, 20; age of Devonian,
+21.</p>
+<p>Horst concerned in Alpine _d&eacute;ferlement_, objections to,
+156.</p>
+<p>Hyperion, dimensions of, 177.</p>
+<p>I.</p>
+<p>Ice, melting of, by pressure, 267 _et seq._; expansion of
+water<br>
+in becoming, 267; lowering of melting-point by pressure, 267;<br>
+fall of temperature under pressure, 268 _et seq._; viscosity
+of,<br>
+284.</p>
+<p>Igneous rocks, average composition of, 43.</p>
+<p>Inanimate actions, dynamic conditions of, 61.</p>
+<p>Inanimate systems, secondary effects in, 63-65; transfer
+of<br>
+energy into, 66.</p>
+<p>Indian geology, equivalent nomenclature of, 139.</p>
+<p>Initial recombination of ions due to alpha rays, 221, 222,
+231;<br>
+and structure of the halo, 231.</p>
+<p>Insect life in the higher Alps, 104, 105; destruction of, on
+the<br>
+Alpine snows, 106.</p>
+<p>Ionisation by alpha ray, density of, 221; importance in
+chemical<br>
+actions, 250; in living cell, 250.</p>
+<p>Ions, number of, produced by an alpha ray, 237.</p>
+<p>Isostasy, 53; and preservation of continents, 53.</p>
+<p>Ivy, inconspicuous blossoms of, 107; delay in ripening
+seed,<br>
+107.</p>
+<p>K.</p>
+<p>Kant and Laplace, material hypothesis of, does not account
+for<br>
+the past, 290.</p>
+<p>Kelvin, Lord, experiment on effects of pressure on ice,
+268-270.</p>
+<p>Kleeman and Bragg. See Bragg.</p>
+<p>Klopstock introduces skating into Germany, 273.</p>
+<p>L.</p>
+<p>Lakes, cause of blue colour of, 55.</p>
+<p>Land, movements of the, 53, 54.</p>
+<p>Laukester, Ray, the soma and reproductive cells, 85.</p>
+<p>Lapworth, structure of the Scottish Highlauds, 153.</p>
+<p>Latent heat of water, 266.</p>
+<p>Latent image, formed at low temperatures, 202; Bose's theory
+of,<br>
+203; photo-electric theory of, 204, 209 _et seq._</p>
+<p>Least action, law of, 66.</p>
+<p>Lembert and Richards, atomic weight of lead, 27.</p>
+<p>Length of life dependent on conditions of structural
+development,<br>
+93; dependent on rate of reproduction, 94.</p>
+<p>Life-curves of organisms having different activities, 92.</p>
+<p>Life, length of, 91.</p>
+<p>Life waves of a cerial, 95; of Ausaeba, 87; of a species,
+90.</p>
+<p>Light, effects of, in discharging negative electrification,
+204;<br>
+chemical effects of, 205; experiment showing effect of, in<br>
+discharging electrified body, 205.</p>
+<p>Lindemann, Dr., duration of solar heat, 29.</p>
+<p>Lowell, Percival, observations on Mars, 167 _et seq._; map
+of<br>
+Mars, reliability of, 198.</p>
+<p>Lucretius, birth-time of the world, 1.</p>
+<p>Lugeon, formation of the Pr&eacute;alpes, 171; sections in the
+Alps,<br>
+154.</p>
+<p>Lyell, uniformity in geological history, 15.</p>
+<p>M.</p>
+<p>Magee, relative areas of deposition and denudation, 16.</p>
+<p>Mars, climate of, 170; position in solar system, 174, 175;<br>
+dimensions of satellites of, 177; snow on, 169; water on,
+169;<br>
+clouds on, 169; atmosphere of, 170; melting of snow on, 170;<br>
+dimensions of canals, 171; signal on, 172; times of
+opposition,<br>
+164; orbit of, 165; distance from the Earth, 165; eccentricity
+of<br>
+his orbit, 165; observations of, by Schiaparelli, 165, 166;<br>
+Lowell's observations on, 167 _et seq._</p>
+<p>Maxwell, Clerk, changes made under constraints, 65; on<br>
+conservation of energy, 61.</p>
+<p>M'Connel, J. C., viscosity and rigidity of ice, 284.</p>
+<p>Memory, physiological, 111, 112.</p>
+<p>Metamorphism, thermal, in Alpine rocks, 132, 149</p>
+<p>Millicurie, definition of, 256.</p>
+<p>Molasse, accumulations of, 148.</p>
+<p>Morin, coefficients of friction, 265.</p>
+<p>Morphy, H., experiments on coefficient of friction of ice,
+281.</p>
+<p>Mountain-building and the geosynclines, 119-121; conditioned
+by<br>
+radioactive energy, 125; energy for, due to gravitation, 122;<br>
+reduction of surface attending, 123; depression attending,
+123;<br>
+instability due to thermal effects of compression, 132;
+igneous<br>
+phenomena attending, 132; rhythmic character of, accounted
+for,<br>
+133; movements confined to upper crust, 122; movements due to<br>
+compressive stresses in crust, 122; movements, rhythmic
+character<br>
+of, 121.</p>
+<p>Mountain ranges built of sedimentary materials, 118.</p>
+<p>M&uuml;ller, J., coefficient of friction of skate on ice, 265,
+274.</p>
+<p>Muth deposits of India, 135.</p>
+<p>N.</p>
+<p>Newton, Professor, of Yale, on origin of Mars' satellites,
+177.</p>
+<p>Nucleus, dimensions of, 237; amount of radium in, 238.</p>
+<p>Nummulitic beds of Himalaya, 138.</p>
+<p>O.</p>
+<p>Ocean, amount of rock salt in, 50; cause of black colour of,
+55;<br>
+estimated mass of sediments in, 48; increase of bulk due to<br>
+solvent denudation, 52; its saltness due to denudation, 41.</p>
+<p>Old age and death, 82-85; not at variance with progressive<br>
+activity, 83.</p>
+<p>Organic systems, origin of, 78.</p>
+<p>Organic vibrations, 86 _et seq._</p>
+<p>Organism and accelerative absorption of energy, 79; and
+economy,<br>
+109-111; and periodic rigour of the environment, 94,95.</p>
+<p>Organism and sleep, 95; ultimate explanation of rythmic
+events<br>
+in, 96, 97; law of action of, 68 _et seq._; periodicity of;
+and<br>
+law of progressive activity, 82 _et seq._</p>
+<p>P.</p>
+<p>Penjal traps, 135.</p>
+<p>Pepys and skating, 273.</p>
+<p>Perry, coefficient of friction of greased surfaces, 265.</p>
+<p>Phobos, dimensions of, 177; orbit of, 177.</p>
+<p>Photoelectric activity and absorption, 207; persists at
+low<br>
+temperatures, 208, 209; not affected by solution, 213.</p>
+<p>Photo-electric experiment, 205; sensitiveness of the hands,
+207;<br>
+theory of latent image, 204, 209 _et seq._</p>
+<p>Photographic reversal, experiments on, by Wood, 211; theory
+of,<br>
+210.</p>
+<p>Piazzi, discovery of first Asteroid, 175.</p>
+<p>Pickering, W. H., observations on Mars, 167.</p>
+<p>Planet, slowing of axial rotation of, 189.</p>
+<p>Plant, expectant attitude of, 109.</p>
+<p>Pleochroic haloes, measurements of, 224; theory of, 224
+_et<br>
+seq._; true form of, 226; radius of, and the additive law,
+225;<br>
+absence of actinium haloes, 225; see _also_ Haloes; mode of<br>
+occurrence of, 223 _et seq._</p>
+<p>Poole, J. H. J., proportionality of thorium and uranium,
+26.</p>
+<p>Poulton, uniformity of past climate, 17.</p>
+<p>Pratt, Archdeacon, and isostasy, 53.</p>
+<p>Pr&eacute;alpes, exotic nature of, 150, 151.</p>
+<p>Prematerial universe, nature of a, 297, 300.</p>
+<p>Prestwich and thickness of rigid crust, 128; history of
+the<br>
+Pyrenees, 140.</p>
+<p>Primitive organisms, interference of, 89; life-curves of,
+88.</p>
+<p>Proctor and orbits of Asteroids, 176.</p>
+<p>Protoplasm, encystment of, 68.</p>
+<p>Purana Era of India, 134.</p>
+<p>Pyrenees, history of, 140.</p>
+<p>R.</p>
+<p>Radioactive elements concerned in mountain building, 125.</p>
+<p>Radioactive layer, failure to account for deep-seated<br>
+temperatures, 127; assumed thickness of, 128; temperature at
+base<br>
+of, due to radioactivity, 129; in the upper crust of the
+Earth,<br>
+125; thickness of, 126-128.</p>
+<p>Radioactive treatment, physical basis of, 251.</p>
+<p>Radioactivity and heat emission from average igneous rock,
+126;<br>
+rarity of, established by haloes, 241, 243.</p>
+<p>Radium, chemical nature and transmutation of, 244-245;
+emanation<br>
+of, 245; rays from, 253, 254; table of family of, 253; period
+of,<br>
+253; small therapeutic value of, 254.</p>
+<p>Radium C, therapeutic value of, 254; rays from. 254;
+generation<br>
+of, 254.</p>
+<p>Rationality, conditions for development of, 163.</p>
+<p>Rays, similarity in nature of gamma, X, and light rays,
+248;<br>
+effects on living cell, 251; penetration of, 251.</p>
+<p>Reade, T. Mellard, finding age of ocean by calcium sulphate,
+13.</p>
+<p>Recumbent folds, formation of, 155 _et seq._</p>
+<p>Regelation, 284; affecting glacier motion, 285.</p>
+<p>Reversal, photographic, explanation of, 211.</p>
+<p>Richards and Lembert, atomic weight of lead, 27.</p>
+<p>Richter, Jean Paul, Dream of the Universe, 200.</p>
+<p>Rock salt in the ocean, amount of, 13.</p>
+<p>Rocks, average composition of, 43; radioactive heat from,
+126;<br>
+rate of solution of, 36.</p>
+<p>Russell, I. C., river supply of sediments, 10.</p>
+<p>Rutherford, Sir E., determination of age of minerals, 19, 20;
+age<br>
+of rocks by haloes, 22; derivation of actinium, 226;
+artificial<br>
+halo, 229; number of alpha rays from one gram of radium, 237.</p>
+<p>S.</p>
+<p>Salt range deposits of India, 134. 135.</p>
+<p>Saltness of the ocean due to denudation, 41-46.</p>
+<p>Salisbury (and Chamberlin), the Larimide range, 121.</p>
+<p>Salmon, Rev. George, on creation, 301.</p>
+<p>Satellite, velocity of, in its orbit, 191; method of finding
+path<br>
+of, over a rotating primary, 189 _et seq._; direct and<br>
+retrograde, 178; ultimate end of, 178; path of, when falling
+into<br>
+primary, 179; effect of Mars' atmosphere on infalling
+satellite,<br>
+179; stability of close to primary, 180; effects of, on crust
+of<br>
+primary, 180 _et seq._</p>
+<p>Schiaparelli, observations on Mars, 165 166.</p>
+<p>Schmidt, C., original depth of Alpine layer, 131-148;
+structure<br>
+of the Alps, 152.</p>
+<p>Schmidt, G. C., on photo-electricity, 207, 208; effect of<br>
+solution on photo-electric activity, 213.</p>
+<p>Schuchert, C., average area of N. America during geological
+time,<br>
+16.</p>
+<p>Sedimentary rocks, average composition of, 43; mass of,<br>
+determined by sodium index, 47.</p>
+<p>Sedimentation a convection of energy, 133.</p>
+<p>Sediments, average river supply of, 11; on ocean floor, mass
+of,<br>
+48; average thickness of, 49; precipitation of, by dissolved<br>
+salts, 56-58; radioactivity of 130; radioactive heat of,<br>
+influential in mountain building, 130, 131; rate of
+collecting,<br>
+7; determination of mass of, 8; river supply of, 10; total<br>
+thickness of, 6.</p>
+<p>Semper, energy absorption of vegetable and animal systems,
+78.</p>
+<p>Sensitisers, effects of low temperature on, 210.</p>
+<p>Simplon, radioactive temperature in rocks of, before
+denudation,<br>
+132.</p>
+<p>Skates, early forms of, 273; principles of construction of,
+273<br>
+_et seq._; action of, on ice, 276; bite of, 278-280.</p>
+<p>Skating not dependent on smoothness of ice, 260; history
+of,<br>
+273.</p>
+<p>Skating only possible on very few substances, 279.</p>
+<p>Soddy, F., on isotopes, 24.</p>
+<p>Sodium, deficiency of, in sediments, 44; discharge of
+rivers,<br>
+14.</p>
+<p>Soils, formation of, 37-39; surface area exposed in, 39.</p>
+<p>Sollas, W. J., age of Earth by sodium in ocean, 14; thickness
+of<br>
+sediments, 6.</p>
+<p>Spencer, on division of protoplasm, 67.</p>
+<p>Spores, number of molecules in, 97.</p>
+<p>Stevenson, Dr. Walter C., and technique of radioactive
+treatment,<br>
+259.</p>
+<p>Stoletow, photo-electric activity anal absorption, 207.</p>
+<p>Stopping power of substances with reference to alpha rays,
+219.</p>
+<p>Struggle for existence, dynamic basis of, 80.</p>
+<p>Strutt, Prof. the Hon. R. J., age of geological periods,
+20;<br>
+radioactivity of zircon, 223.</p>
+<p>Sub-Apennine series of Italy, 148.</p>
+<p>Suess, nature of earthquakes. 143.</p>
+<p>Survival of the fittest and the organic law, 80.</p>
+<p>T.</p>
+<p>Talchir boulder-bed, 136.</p>
+<p>Temperature gradient in Earth's crust, 126.</p>
+<p>Termier, section of the Pelvoux Massif, 254.</p>
+<p>Tethys, early extent of, 135-137; geosynclines of, 142.</p>
+<p>Thermal metamorphism in Alpine rocks, 132, 149.</p>
+<p>Thomson, James, prediction of melting of ice by pressure,
+267.</p>
+<p>Thorium and uranium, proportionality of, in older rocks,
+26.</p>
+<p>Triple canals, formation of, by attraction of a satellite,
+187.</p>
+<p>Tyndall, colour of ocean water, 55.</p>
+<p>U.</p>
+<p>Uniformitarian view of geological history, 15-18.</p>
+<p>Universe, simultaneity of the, 293-295.</p>
+<p>Uranium-radium family of elements, table of, 253.</p>
+<p>V.</p>
+<p>Val d'H&eacute;rens, earth pillars of, 33.</p>
+<p>Van Tillo, nature of continental rock covering, 9.</p>
+<p>Vegetable and animal systems, relative absorption of energy
+of,<br>
+78.</p>
+<p>Vegetative organs, struggle between, 105, 106.</p>
+<p>Volcanoes and mountain ranges, 118; associated with
+geosynclines,<br>
+142; Oligocene and Miocene of Europe, 147.</p>
+<p>W.</p>
+<p>Weinschenk and thermal metamorphism, 132,</p>
+<p>149.</p>
+<p>Weismaun, encystment of protoplasm, 68; length of life and<br>
+somatic cells, 96; origin of death, 83; tendency to early<br>
+reproductiveness, 98.</p>
+<p>Wilson, C. T. R., visualised alpha rays, 218.</p>
+<p>Winchell, progressive changes of matter not eternal, 302.</p>
+<p>Wood, R. W., on photographic reversal, 211.</p>
+<p>Z.</p>
+<p>Zircon, radioactivity of, 223; as nucleus of halo, 223.</p>
+
+
+
+
+
+
+
+<pre>
+
+
+
+
+
+End of the Project Gutenberg EBook of The Birth-Time of the World and Other
+Scientific Essays, by J. (John) Joly
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+</pre>
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+</body>
+</html>
diff --git a/16614.txt b/16614.txt
new file mode 100644
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+++ b/16614.txt
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+The Project Gutenberg EBook of The Birth-Time of the World and Other
+Scientific Essays, by J. (John) Joly
+
+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: The Birth-Time of the World and Other Scientific Essays
+
+Author: J. (John) Joly
+
+Release Date: August 28, 2005 [EBook #16614]
+
+Language: English
+
+Character set encoding: ASCII
+
+*** START OF THIS PROJECT GUTENBERG EBOOK THE BIRTH-TIME OF THE WORLD ***
+
+
+
+
+Produced by Hugh Rance
+
+
+
+
+
+THE BIRTH-TIME OF THE WORLD AND OTHER SCIENTIFIC ESSAYS
+
+by
+
+J. JOLY, M.A., Sc.D., F.R.S.,
+PROFESSOR OF GEOLOGY AND MINERALOGY IN THE UNIVERSITY OF DUBLIN
+
+E. P. DUTTON AND COMPANY
+681 FIFTH AVENUE NEW YORK
+
+
+Cover
+
+Title page
+
+CONTENTS PAGE
+
+I. THE BIRTH-TIME OF THE WORLD - - - - - - - - - - - 1
+
+II. DENUDATION  - - - - - - - - - - - - - - - - - - 30
+
+III. THE ABUNDANCE OF LIFE  - - - - - - - - - - - - 60
+
+IV. THE BRIGHT COLOURS OF ALPINE FLOWERS - - - - - 102
+
+V. MOUNTAIN GENESIS  - - - - - - - - - - - - - - - 116
+
+VI. ALPINE STRUCTURE - - - - - - - - - - - - - - - 146
+
+VII. OTHER MINDS THAN OURS - - - - - - - - - - - - 162
+
+VIII. THE LATENT IMAGE - - - - - - - - - - - - - - 202
+
+IX. PLEOCHROIC HALOES  - - - - - - - - - - - - - - 214
+
+X. THE USE OF RADIUM IN MEDICINE - - - - - - - - - 244
+
+XI. SKATING  - - - - - - - - - - - - - - - - - - - 260
+
+XII. A SPECULATION AS TO A PRE-MATERIAL UNIVERSE - 288
+
+LIST OF ILLUSTRATIONS
+
+PLATE I. LAKE OF LUCERNE, LOOKING WEST FROM BRUNNEN -
+Frontispiece
+
+PLATE II. "UPLIFTED FROM THE SEAS." CLIFFS OF THE TITLIS,
+SWITZERLAND - to face p. 4
+
+PLATE III. AN ALPINE TORRENT AT WORK--VAL D'HERENS, SWITZERLAND -
+to face p. 31
+
+PLATE IV. EARTH PILLARS--VAL D'HERENS - to face p. 34
+
+PLATE V. "SCENES OF DESOLATION." THE WEISSHORN SEEN FROM BELLA
+TOLA, SWITZERLAND - to face p. 40
+
+PLATE VI. ALLUVIAL CONE--NICOLAI THAL, SWITZERLAND. MORAINE ON
+ALETSCH GLACIER SWITZERLAND - to face p. 50
+
+PLATE VII. IN THE REGION OF THE CROCI; DOLOMITES. THE ROTHWAND
+SEEN FROM MONTE PIANO - to face p. 60
+
+PLATE VIII. FIRS ASSAILING THE HEIGHTS OF THE MADERANER THAL,
+SWITZERLAND - to face p. 73
+
+PLATE IX. LIFE NEAR THE SNOW LINE; THE BOG-COTTON IN POSSESSION.
+NEAR THE TSCHINGEL PASS, SWITZERLAND - to face p. 80
+
+PLATE X. THE JOY OF LIFE. THE AMPEZZO THAL; DOLOMITES - to face
+p. 93
+
+PLATE XI. "PINES SOLEMNLY QUIET." DUeSSISTOCK; MADERANER THAL - to
+face p. 100
+
+PLATE XII. ALPINE FLOWERS IN THE VALLEYS - to face p. 105
+
+PLATE XIII. ALPINE FLOWERS ON THE HEIGHTS - to face p. 106
+
+PLATE XIV. MOUNTAIN SOLITUDES; VAL DE ZINAL. FROM LEFT TO RIGHT
+ROTHHORN; BESSO; OBERGABELHORN; MATTERHORN; PIC DE ZINAL (THROUGH
+CLOUD); DENT BLANCHE - to face p. 116
+
+ix
+
+PLATE XV. SECTOR OF THE EARTH RISE OF ISOGEOTHERMS INTO A DEPOSIT
+EVOLVING RADIOACTIVE HEAT - to face p. 118
+
+PLATE XVI. "THE MOUNTAINS COME AND GO." THE DENT BLANCHE SEEN
+FROM THE SASSENEIRE - to face p. 133
+
+PLATE XVII. DIAGRAMMATIC SECTIONS OF THE HIMALAYA - to face p.
+140
+
+PLATE XVIII. RESIDUES OF DENUDATION. THE MATTERHORN SEEN FROM THE
+SUMMIT OF THE ZINAL ROTHHORN - to face p. 148
+
+PLATE XIX. THE FOLDED ROCKS OF THE MATTERHORN, SEEN FROM NEAR
+HOeHBALM. SKETCH MADE IN 1906 - to face p. 156
+
+PLATE XX. SCHIAPARELLI'S MAP OF MARS OF 1882, AND ADDITIONS (IN
+RED) OF 1892 - to face p. 166
+
+PLATE XXI. GLOBE OF MARS SHOWING PATH OF IN-FALLING SATELLITE -
+to face p. 188
+
+PLATE XXII. CANALS MAPPED BY LOWELL COMPARED WITH CANALS FORMED
+BY IN-FALLING SATELLITES - to face p. 192
+
+PLATE XXIII. HALOES IN MICA; CO. CARLOW. HALO IN BIOTITE
+CONTAINED IN GRANITE - to face p. 224
+
+PLATE XXIV. RADIUM HALO, MUCH ENLARGED. THORIUM HALO AND RADIUM
+HALO IN MICA - to face p. 228
+
+PLATE XXV. HALO ROUND CAPILLARY IN GLASS TUBE. HALOES ROUND
+TUBULAR PASSAGES IN MICA - to face p. 230
+
+PLATE XXVI. ALETSCH GLACIER, SWITZERLAND - to face p. 282
+
+PLATE XXVII. THE MIDDLE ALETSCH GLACIER JOINING THE GREAT ALETSCH
+GLACIER. GLACIERS OF THE LAUTERBRUNNEN THAL - to face p. 285
+
+PLATE XXVIII. PERCHED BLOCK ON THE ALETSCH GLACIER. GRANITE
+ERRATIC NEAR ROUNDWOOD, CO. WICKLOW; NOW BROKEN UP AND REMOVED -
+to face p. 286
+
+And Fifteen Illustrations in the Text.
+
+x
+
+PREFACE
+
+Tins volume contains twelve essays written at various times
+during recent years. Many of them are studies contributed to
+Scientific Reviews or delivered as popular lectures. Some are
+expositions of views the scientific basis of which may be
+regarded as established. Others--the greater number--may be
+described as attempting the solution of problems which cannot be
+approached by direct observation.
+
+The essay on The Birth-time of the World is based on a lecture
+delivered before the Royal Dublin Society. The subject has
+attracted much attention within recent years. The age of the
+Earth is, indeed, of primary importance in our conception of the
+longevity of planetary systems. The essay deals with the
+evidence, derived from the investigation of purely terrestrial
+phenomena, as to the period which has elapsed since the ocean
+condensed upon the Earth's surface. Dr. Decker's recent addition
+to the subject appeared too late for inclusion in it. He finds
+that the movements (termed isostatic) which geologists recognise
+as taking place deep in the Earth's crust, indicate an age of the
+same order of magnitude
+
+xi
+
+as that which is inferred from the statistics of denudative
+history.[1]
+
+The subject of _Denudation_ naturally arises from the first essay.
+In thinking over the method of finding the age of the ocean by
+the accumulation of sodium therein, I perceived so long ago as
+1899, when my first paper was published, that this method
+afforded a means of ascertaining the grand total of denudative
+work effected on the Earth's surface since the beginning of
+geological time; the resulting knowledge in no way involving any
+assumption as to the duration of the period comprising the
+denudative actions. This idea has been elaborated in various
+publications since then, both by myself and by others.
+"Denudation," while including a survey of the subject generally,
+is mainly a popular account of this method and its results. It
+closes with a reference to the fascinating problems presented by
+the inner nature of sedimentation: a branch of science to which I
+endeavoured to contribute some years ago.
+
+_Mountain Genesis_ first brings in the subject of the geological
+intervention of radioactivity. There can, I believe, be no doubt
+as to the influence of transforming elements upon the
+developments of the surface features of the Earth; and, if I am
+right, this source of thermal energy is mainly responsible for
+that local accumulation of wrinkling which we term mountain
+chains. The
+
+[1] Bull. Geol. Soc. America, vol. xxvi, March 1915.
+
+xii
+
+paper on _Alpine Structure_ is a reprint from "Radioactivity and
+Geology," which for the sake of completeness is here included. It
+is directed to the elucidation of a detail of mountain genesis: a
+detail which enters into recent theories of Alpine development.
+The weakness of the theory of the "horst" is manifest, however,
+in many of its other applications; if not, indeed, in all.
+
+The foregoing essays on the physical influences affecting the
+surface features of the Earth are accompanied by one entitled _The
+Abundance of Life._ This originated amidst the overwhelming
+presentation of life which confronts us in the Swiss Alps. The
+subject is sufficiently inspiring. Can no fundamental reason be
+given for the urgency and aggressiveness of life? Vitality is an
+ever-extending phenomenon. It is plain that the great principles
+which have been enunciated in explanation of the origin of
+species do not really touch the problem. In the essay--which is an
+early one (1890)--the explanation of the whole great matter is
+sought--and as I believe found--in the attitude of the organism
+towards energy external to it; an attitude which results in its
+evasion of the retardative and dissipatory effects which prevail
+in lifeless dynamic systems of all kinds.
+
+_Other Minds than Ours_? attempts a solution of the vexed question
+of the origin of the Martian "canals." The essay is an abridgment
+of two popular lectures on the subject. I had previously written
+an account of my views which carried the enquiry as far as it was
+in
+
+xiii
+
+my power to go. This paper appeared in the "Transactions of the
+Royal Dublin Society, 1897." The theory put forward is a purely
+physical one, and, if justified, the view that intelligent beings
+exist in Mars derives no support from his visible surface
+features; but is, in fact, confronted with fresh difficulties.
+
+_Pleochroic Haloes_ is a popular exposition of an inconspicuous but
+very beautiful phenomenon of the rocks. Minute darkened spheres--a
+microscopic detail--appear everywhere in certain of the rock
+minerals. What are they? The discoveries of recent radioactive
+research--chiefly due to Rutherford--give the answer. The
+measurements applied to the little objects render the explanation
+beyond question. They turn out to be a quite extraordinary record
+of radioactive energy; a record accumulated since remote
+geological times, and assuring us, indirectly, of the stability
+of the chemical elements in general since the beginning of the
+world. This assurance is, without proof, often assumed in our
+views on the geological history of the Globe.
+
+Skating is a discourse, with a recent addition supporting the
+original thesis. It is an illustration of a common experience--the
+explanation of an unimportant action involving principles the
+most influential considered as a part of Nature's resources.
+
+The address on _The Latent Image_ deals with a subject which had
+been approached by various writers before the time of my essay;
+but, so far as I know, an explanation
+
+xiv
+
+based on the facts of photo-electricity had not been attempted.
+Students of this subject will notice that the views expressed are
+similar to those subsequently put forward by Lenard and Saeland
+in explanation of phosphorescence. The whole matter is of more
+practical importance than appears at first sight, for the
+photoelectric nature of the effects involved in the radiative
+treatment of many cruel diseases seems to be beyond doubt.
+
+It was in connection with photo-electric science that I was led
+to take an interest in the application of radioactivity in
+medicine. The lecture on _The Use of Radium in Medicine_ deals with
+this subject. Towards the conclusion of this essay reference will
+be found to a practical outcome of such studies which, by
+improving on the methods, and facilitating the application, of
+radioactive treatment, has, in the hands of skilled medical men,
+already resulted in the alleviation of suffering.
+
+Leaving out much which might well appear in a prefatory notice, a
+word should yet be added respecting the illustrations of scenery.
+They are a small selection from a considerable number of
+photographs taken during my summer wanderings in the Alps in
+company with Henry H. Dixon. An exception is Plate X, which is by
+the late Dr. Edward Stapleton. From what has been said above, it
+will be gathered that these illustrations are fitly included
+among pages which owe so much to Alpine inspiration. They
+illustrate the
+
+xv
+
+subjects dealt with, and, it is to be hoped, they will in some
+cases recall to the reader scenes which have in past times
+influenced his thoughts in the same manner; scenes which in their
+endless perspective seem to reduce to their proper insignificance
+the lesser things of life.
+
+My thanks are due to Mr. John Murray for kindly consenting to the
+reissue of the essay on _The Birth-time of the World_ from the
+pages of _Science Progress_; to Messrs. Constable & Co. for leave
+to reprint _Pleochroic Haloes_ from _Bedrock_, and also to make some
+extracts from _Radioactivity and Geology_; and to the Council of
+the Royal Dublin Society for permission to republish certain
+papers from the Proceedings of the Society.
+
+_Iveagh Geological Laboratory, Trinity College, Dublin._
+
+July, 1915.
+
+xvi
+
+THE BIRTH-TIME OF THE WORLD [1]
+
+LONG ago Lucretius wrote: "For lack of power to solve the
+question troubles the mind with doubts, whether there was ever a
+birth-time of the world and whether likewise there is to be any
+end." "And if" (he says in answer) "there was no birth-time of
+earth and heaven and they have been from everlasting, why before
+the Theban war and the destruction of Troy have not other poets
+as well sung other themes? Whither have so many deeds of men so
+often passed away, why live they nowhere embodied in lasting
+records of fame? The truth methinks is that the sum has but a
+recent date, and the nature of the world is new and has but
+lately had its commencement."[2]
+
+Thus spake Lucretius nearly 2,000 years ago. Since then we have
+attained another standpoint and found very different limitations.
+To Lucretius the world commenced with man, and the answer he
+would give to his questions was in accord with his philosophy: he
+would date the birth-time of the world from the time when
+
+[1] A lecture delivered before the Royal Dublin Society, February
+6th, 1914. _Science Progress_, vol. ix., p. 37
+
+[2] _De Rerum Natura_, translated by H. A. J. Munro (Cambridge,
+1886).
+
+1
+
+poets first sang upon the earth. Modern Science has along with
+the theory that the Earth dated its beginning with the advent of
+man, swept utterly away this beautiful imagining. We can, indeed,
+find no beginning of the world. We trace back events and come to
+barriers which close our vista--barriers which, for all we know,
+may for ever close it. They stand like the gates of ivory and of
+horn; portals from which only dreams proceed; and Science cannot
+as yet say of this or that dream if it proceeds from the gate of
+horn or from that of ivory.
+
+In short, of the Earth's origin we have no certain knowledge; nor
+can we assign any date to it. Possibly its formation was an event
+so gradual that the beginning was spread over immense periods. We
+can only trace the history back to certain events which may with
+considerable certainty be regarded as ushering in our geological
+era.
+
+Notwithstanding our limitations, the date of the birth-time of
+our geological era is the most important date in Science. For in
+taking into our minds the spacious history of the universe, the
+world's age must play the part of time-unit upon which all our
+conceptions depend. If we date the geological history of the
+Earth by thousands of years, as did our forerunners, we must
+shape our ideas of planetary time accordingly; and the duration
+of our solar system, and of the heavens, becomes comparable with
+that of the dynasties of ancient nations. If by millions of
+years, the sun and stars are proportionately venerable. If by
+hundreds or thousands of millions of
+
+2
+
+years the human mind must consent to correspondingly vast epochs
+for the duration of material changes. The geological age plays
+the same part in our views of the duration of the universe as the
+Earth's orbital radius does in our views of the immensity of
+space. Lucretius knew nothing of our time-unit: his unit was the
+life of a man. So also he knew nothing of our space-unit, and he
+marvels that so small a body as the sun can shed so much, heat
+and light upon the Earth.
+
+A study of the rocks shows us that the world was not always what
+it now is and long has been. We live in an epoch of denudation.
+The rains and frosts disintegrate the hills; and the rivers roll
+to the sea the finely divided particles into which they have been
+resolved; as well as the salts which have been leached from them.
+The sediments collect near the coasts of the continents; the
+dissolved matter mingles with the general ocean. The geologist
+has measured and mapped these deposits and traced them back into
+the past, layer by layer. He finds them ever the same;
+sandstones, slates, limestones, etc. But one thing is not the
+same. _Life_ grows ever less diversified in character as the
+sediments are traced downwards. Mammals and birds, reptiles,
+amphibians, fishes, die out successively in the past; and barren
+sediments ultimately succeed, leaving the first beginnings of
+life undecipherable. Beneath these barren sediments lie rocks
+collectively differing in character from those above: mainly
+volcanic or poured out from fissures in
+
+3
+
+the early crust of the Earth. Sediments are scarce among these
+materials.[1]
+
+There can be little doubt that in this underlying floor of
+igneous and metamorphic rocks we have reached those surface
+materials of the earth which existed before the long epoch of
+sedimentation began, and before the seas came into being. They
+formed the floor of a vaporised ocean upon which the waters
+condensed here and there from the hot and heavy atmosphere. Such
+were the probable conditions which preceded the birth-time of the
+ocean and of our era of life and its evolution.
+
+It is from this epoch we date our geological age. Our next
+purpose is to consider how long ago, measured in years, that
+birth-time was.
+
+That the geological age of the Earth is very great appears from
+what we have already reviewed. The sediments of the past are many
+miles in collective thickness: yet the feeble silt of the rivers
+built them all from base to summit. They have been uplifted from
+the seas and piled into mountains by movements so slow that
+during all the time man has been upon the Earth but little change
+would have been visible. The mountains have again been worn down
+into the ocean by denudation and again younger mountains built
+out of their redeposited materials. The contemplation of such
+vast events
+
+[1] For a description of these early rocks, see especially the
+monograph of Van Hise and Leith on the pre-Cambrian Geology of
+North America (Bulletin 360, U.S. Geol. Survey).
+
+4
+
+prepares our minds to accept many scores of millions of years or
+hundreds of millions of years, if such be yielded by our
+calculations.
+
+THE AGE AS INFERRED FROM THE THICKNESS OF THE SEDIMENTS
+
+The earliest recognised method of arriving at an estimate of the
+Earth's geological age is based upon the measurement of the
+collective sediments of geological periods. The method has
+undergone much revision from time to time. Let us briefly review
+it on the latest data.
+
+The method consists in measuring the depths of all the successive
+sedimentary deposits where these are best developed. We go all
+over the explored world, recognising the successive deposits by
+their fossils and by their stratigraphical relations, measuring
+their thickness and selecting as part of the data required those
+beds which we believe to most completely represent each
+formation. The total of these measurements would tell us the age
+of the Earth if their tale was indeed complete, and if we knew
+the average rate at which they have been deposited. We soon,
+however, find difficulties in arriving at the quantities we
+require. Thus it is not easy to measure the real thickness of a
+deposit. It may be folded back upon itself, and so we may measure
+it twice over. We may exaggerate its thickness by measuring it
+not quite straight across the bedding or by unwittingly including
+volcanic materials. On the other hand, there
+
+5
+
+may be deposits which are inaccessible to us; or, again, an
+entire absence of deposits; either because not laid down in the
+areas we examine, or, if laid down, again washed into the sea.
+These sources of error in part neutralise one another. Some make
+our resulting age too long, others make it out too short. But we
+do not know if a balance of error does not still remain. Here,
+however, is a table of deposits which summarises a great deal of
+our knowledge of the thickness of the stratigraphical
+accumulations. It is due to Sollas.[1]
+
+Feet.
+
+Recent and Pleistocene  - -    4,000
+Pliocene                 - -   13,000
+Miocene                  - -   14,000
+Oligocene                - -     2,000
+Eocene                   - -  20,000
+                              63,000
+
+Upper Cretaceous        - -   24,000
+Lower Cretaceous        - -   20,000
+Jurassic                 - -    8,000
+Trias                    - -   7,000
+                              69,000
+
+Permian                  - -    2,000
+Carboniferous           - -    29,000
+Devonian                 - -   22,000
+                              63,000
+
+Silurian                 - -   15,000
+Ordovician              - -   17,000
+Cambrian                 - -   6,000
+                              58,000
+
+Algonkian--Keeweenawan   - -   50,000
+Algonkian--Animikian     - -   14,000
+Algonkian--Huronian      - -   18,000
+                              82,000
+
+Archaean                   - -   ?
+
+Total                     - - 335,000 feet.
+
+[1] Address to the Geol. Soc. of London, 1509.
+
+6
+
+In the next place we require to know the average rate at which
+these rocks were laid down. This is really the weakest link in
+the chain. The most diverse results have been arrived at, which
+space does not permit us to consider. The value required is most
+difficult to determine, for it is different for the different
+classes of material, and varies from river to river according to
+the conditions of discharge to the sea. We may probably take it
+as between two and six inches in a century.
+
+Now the total depth of the sediments as we see is about 335,000
+feet (or 64 miles), and if we take the rate of collecting as
+three inches in a hundred years we get the time for all to
+collect as 134 millions of years. If the rate be four inches, the
+time is soo millions of years, which is the figure Geikie
+favoured, although his result was based on somewhat different
+data. Sollas most recently finds 80 millions of years.[1]
+
+THE AGE AS INFERRED FROM THE MASS OF THE SEDIMENTS
+
+In the above method we obtain our result by the measurement of
+the linear dimensions of the sediments. These measurements, as we
+have seen, are difficult to arrive at. We may, however, proceed
+by measurements of the mass of the sediments, and then the method
+becomes more definite. The new method is pursued as follows:
+
+[1] Geikie, _Text Book of Geology_ (Macmillan, 1903), vol. i., p.
+73, _et seq._ Sollas, _loc. cit._ Joly, _Radioactivity and Geology_
+(Constable, 1909), and Phil. Mag., Sept. 1911.
+
+7
+
+The total mass of the sediments formed since denudation began may
+be ascertained with comparative accuracy by a study of the
+chemical composition of the waters of the ocean. The salts in the
+ocean are undoubtedly derived from the rocks; increasing age by
+age as the latter are degraded from their original character
+under the action of the weather, etc., and converted to the
+sedimentary form. By comparing the average chemical composition
+of these two classes of material--the primary or igneous rocks and
+the sedimentary--it is easy to arrive at a knowledge of how much
+of this or that constituent was given to the ocean by each ton of
+primary rock which was denuded to the sedimentary form. This,
+however, will not assist us to our object unless the ocean has
+retained the salts shed into it. It has not generally done so. In
+the case of every substance but one the ocean continually gives
+up again more or less of the salts supplied to it by the rivers.
+The one exception is the element sodium. The great solubility of
+its salts has protected it from abstraction, and it has gone on
+collecting during geological time, practically in its entirety.
+This gives us the clue to the denudative history of the
+Earth.[1]
+
+The process is now simple. We estimate by chemical examination of
+igneous and sedimentary rocks the amount of sodium which has been
+supplied to the ocean per ton of sediment produced by denudation.
+We also calculate
+
+[1] _Trans. R.D.S._, May, 1899.
+
+8
+
+the amount of sodium contained in the ocean. We divide the one
+into the other (stated, of course, in the same units of mass),
+and the quotient gives us the number of tons of sediment. The
+most recent estimate of the sediments made in this manner affords
+56 x 1016 tonnes.[1]
+
+Now we are assured that all this sediment was transported by the
+rivers to the sea during geological time. Thus it follows that,
+if we can estimate the average annual rate of the river supply of
+sediments to the ocean over the past, we can calculate the
+required age. The land surface is at present largely covered with
+the sedimentary rocks themselves. Sediment derived from these
+rocks must be regarded as, for the most part, purely cyclical;
+that is, circulating from the sea to the land and back again. It
+does not go to increase the great body of detrital deposits. We
+cannot, therefore, take the present river supply of sediment as
+representing that obtaining over the long past. If the land was
+all covered still with primary rocks we might do so. It has been
+estimated that about 25 per cent. of the existing continental
+area is covered with archaean and igneous rocks, the remainder
+being sediments.[2] On this estimate we may find valuable
+
+[1] Clarke, _A Preliminary Study of Chemical Denudation_
+(Washington, 1910). My own estimate in 1899 (_loc. cit._) made as a
+test of yet another method of finding the age, showed that the
+sediments may be taken as sufficient to form a layer 1.1 mile
+deep if spread uniformly over the continents; and would amount to
+64 x 1018 tons.
+
+[2] Van Tillo, _Comptes Rendues_ (Paris), vol. cxiv., 1892.
+
+9
+
+major and minor limits to the geological age. If we take 25 per
+cent. only of the present river supply of sediment, we evidently
+fix a major limit to the age, for it is certain that over the
+past there must have been on the average a faster supply. If we
+take the entire river supply, on similar reasoning we have what
+is undoubtedly a minor limit to the age.
+
+The river supply of detrital sediment has not been very
+extensively investigated, although the quantities involved may be
+found with comparative ease and accuracy. The following table
+embodies the results obtained for some of the leading rivers.[1]
+
+               Mean annual   Total annual   Ratio of
+               discharge in  sediment in    sediment
+               cubic feet    thousands      to water
+               per second.   of tons.       by weight.
+
+Potomac -        20,160          5,557        1 : 3.575
+Mississippi -   610,000       406,250        1 : 1,500
+Rio Grande -      1,700          3,830        1 : 291
+Uruguay -       150,000         14,782       1 : 10,000
+Rhone -          65,850         36,000       1 : 1,775
+Po -             62,200         67,000       1 : 900
+Danube -        315,200        108,000       1 : 2,880
+Nile -          113,000         54,000       1 : 2,050
+Irrawaddy -     475,000       291,430        1 : 1,610
+
+Mean -          201,468        109,650       1 : 2,731
+
+We see that the ratio of the weight of water to the
+
+[1] Russell, _River Development_ (John Murray, 1888).
+
+10
+
+weight of transported sediment in six out of the nine rivers does
+not vary widely. The mean is 2,730 to 1. But this is not the
+required average. The water-discharge of each river has to be
+taken into account. If we ascribe to the ratio given for each
+river the weight proper to the amount of water it discharges, the
+proportion of weight of water to weight of sediment, for the
+whole quantity of water involved, comes out as 2,520 to 1.
+
+Now if this proportion holds for all the rivers of the
+world--which collectively discharge about 27 x 1012 tonnes of
+water per annum--the river-born detritus is 1.07 x 1010 tonnes. To
+this an addition of 11 per cent. has to be made for silt pushed
+along the river-bed.[1] On these figures the minor limit to the
+age comes out as 47 millions of years, and the major limit as 188
+millions. We are here going on rather deficient estimates, the
+rivers involved representing only some 6 per cent. of the total
+river supply of water to the ocean. But the result is probably
+not very far out.
+
+We may arrive at a probable age lying between the major and minor
+limits. If, first, we take the arithmetic mean of these limits,
+we get 117 millions of years. Now this is almost certainly
+excessive, for we here assume that the rate of covering of the
+primary rocks by sediments was uniform. It would not be so,
+however, for the rate of supply of original sediment must have
+been continually diminishing
+
+[1] According to observations made on the Mississippi (Russell,
+_loc. cit._).
+
+11
+
+during geological time, and hence we may assume that the rate of
+advance of the sediments on the primary rocks has also been
+diminishing. Now we may probably take, as a fair assumption, that
+the sediment-covered area was at any instant increasing at a rate
+proportionate to the rate of supply of sediment; that is, to the
+area of primary rocks then exposed. On this assumption the age is
+found to be 87 millions of years.
+
+THE AGE BY THE SODIUM OF THE OCEAN
+
+I have next to lay before you a quite different method. I have
+already touched upon the chemistry of the ocean, and on the
+remarkable fact that the sodium contained in it has been
+preserved, practically, in its entirety from the beginning of
+geological time.
+
+That the sea is one of the most beautiful and magnificent sights
+in Nature, all admit. But, I think, to those who know its story
+its beauty and magnificence are ten-fold increased. Its saltness
+it due to no magic mill. It is the dissolved rocks of the Earth
+which give it at once its brine, its strength, and its buoyancy.
+The rivers which we say flow with "fresh" water to the sea
+nevertheless contain those traces of salt which, collected over
+the long ages, occasion the saltness of the ocean. Each gallon of
+river water contributes to the final result; and this has been
+going on since the beginning of our era. The mighty total of the
+rivers is 6,500 cubic miles of water in the year!
+
+12
+
+There is little doubt that the primeval ocean was in the
+condition of a fresh-water lake. It can be shown that a primitive
+and more rapid solution of the original crust of the Earth by the
+slowly cooling ocean would have given rise to relatively small
+salinity. The fact is, the quantity of salts in the ocean is
+enormous. We are only now concerned with the sodium; but if we
+could extract all the rock-salt (the chloride of sodium) from the
+ocean we should have enough to cover the entire dry land of the
+Earth to a depth of 400 feet. It is this gigantic quantity which
+is going to enter into our estimate of the Earth's age. The
+calculated mass of sodium contained in this rock-salt is 14,130
+million million tonnes.
+
+If now we can determine the rate at which the rivers supply
+sodium to the ocean, we can determine the age.[1] As the result
+of many thousands of river analyses, the total amount of sodium
+annually discharged to the ocean
+
+[1] _Trans. R.D.S._, 1899. A paper by Edmund Halley, the
+astronomer, in the _Philosophical Transactions of the Royal
+Society_ for 1715, contains a suggestion for finding the age of
+the world by the following procedure. He proposes to make
+observations on the saltness of the seas and ocean at intervals
+of one or more centuries, and from the increment of saltness
+arrive at their age. The measurements, as a matter of fact, are
+impracticable. The salinity would only gain (if all remained in
+solution) one millionth part in Too years; and, of course, the
+continuous rejection of salts by the ocean would invalidate the
+method. The last objection also invalidates the calculation by T.
+Mellard Reade (_Proc. Liverpool Geol. Soc._, 1876) of a minor limit
+to the age by the calcium sulphate in the ocean. Both papers were
+quite unknown to me when working out my method. Halley's paper
+was, I think, only brought to light in 1908.
+
+13
+
+by all the rivers of the world is found to be probably not far
+from 175 million tonnes.[1] Dividing this into the mass of
+oceanic sodium we get the age as 80.7 millions of years. Certain
+corrections have to be applied to this figure which result in
+raising it to a little over 90 millions of years. Sollas, as the
+result of a careful review of the data, gets the age as between
+80 and 150 millions of years. My own result[2] was between 80 and
+90 millions of years; but I subsequently found that upon certain
+extreme assumptions a maximum age might be arrived at of 105
+millions of years.[3] Clarke regards the 80.7 millions of years
+as certainly a maximum in the light of certain calculations by
+Becker.[4]
+
+The order of magnitude of these results cannot be shaken unless
+on the assumption that there is something entirely misleading in
+the existing rate of solvent denudation. On the strength of the
+results of another and
+
+[1] F. W. Clarke, _A Preliminary Study of Chemical Denudation_
+(Smithsonian Miscellaneous Collections, 1910).
+
+[2] _Loc. cit._
+
+[3] "The Circulation of Salt and Geological Time" (Geol. Mag.,
+1901, p. 350).
+
+[4] Becker (loc. cit.), assuming that the exposed igneous and
+archaean rocks alone are responsible for the supply of sodium to
+the ocean, arrives at 74 millions of years as the geological age.
+This matter was discussed by me formerly (Trans. R.D.S., 1899,
+pp. 54 _et seq._). The assumption made is, I believe, inadmissible.
+It is not supported by river analyses, or by the chemical
+character of residual soils from sedimentary rocks. There may be
+some convergence in the rate of solvent denudation, but--as I
+think on the evidence--in our time unimportant.
+
+14
+
+entirely different method of approaching the question of the
+Earth's age (which shall be presently referred to), it has been
+contended that it is too low. It is even asserted that it is from
+nine to fourteen times too low. We have then to consider whether
+such an enormous error can enter into the method. The
+measurements involved cannot be seriously impugned. Corrections
+for possible errors applied to the quantities entering into this
+method have been considered by various writers. My own original
+corrections have been generally confirmed. I think the only point
+left open for discussion is the principle of uniformitarianism
+involved in this method and in the methods previously discussed.
+
+In order to appreciate the force of the evidence for uniformity
+in the geological history of the Earth, it is, of course,
+necessary to possess some acquaintance with geological science.
+Some of the most eminent geologists, among whom Lyell and
+Geikie[1] may be mentioned, have upheld the doctrine of
+uniformity. It must here suffice to dwell upon a few points
+having special reference to the matter under discussion.
+
+The mere extent of the land surface does not, within limits,
+affect the question of the rate of denudation. This arises from
+the fact that the rain supply is quite insufficient to denude the
+whole existing land surface. About 30 per cent. of it does not,
+in fact, drain to the
+
+[1] See especially Geikie's Address to Sect. C., Brit. Assoc.
+Rep., 1399.
+
+15
+
+ocean. If the continents become invaded by a great transgression
+of the ocean, this "rainless" area diminishes: and the denuded
+area advances inwards without diminution. If the ocean recedes
+from the present strand lines, the "rainless" area advances
+outwards, but, the rain supply being sensibly constant, no change
+in the river supply of salts is to be expected.
+
+Age-long submergence of the entire land, or of any very large
+proportion of what now exists, is negatived by the continuous
+sequence of vast areas of sediment in every geologic age from the
+earliest times. Now sediment-receiving areas always are but a
+small fraction of those exposed areas whence the sediments are
+supplied.[1] Hence in the continuous records of the sediments we
+have assurance of the continuous exposure of the continents above
+the ocean surface. The doctrine of the permanency of the
+continents has in its main features been accepted by the most
+eminent authorities. As to the actual amount of land which was
+exposed during past times to denudative effects, no data exist to
+show it was very different from what is now exposed. It has been
+estimated that the average area of the North American continent
+over geologic time was about eight-tenths of its existing
+area.[2] Restorations of other continents, so far as they have
+been attempted, would not
+
+[1] On the strength of the Mississippi measurements about 1 to 18
+(Magee, _Am. Jour. of Sc._, 1892, p. 188).
+
+[2] Schuchert, _Bull. Geol. Soc. Am._, vol. xx., 1910.
+
+16
+
+suggest any more serious divergency one way or the other.
+
+That climate in the oceans and upon the land was throughout much
+as it is now, the continuous chain of teeming life and the
+sensitive temperature limits of protoplasmic existence are
+sufficient evidence.[1] The influence at once of climate and of
+elevation of the land may be appraised at their true value by the
+ascertained facts of solvent denudation, as the following table
+shows.
+
+                 Tonnes removed in      Mean elevation.
+                 solution per square    Metres.
+                 mile per annum.
+North America -         79                  700
+South America -         50                  650
+Europe -                100                  300
+Asia -                   84                  950
+Africa -                 44                  650
+
+In this table the estimated number of tonnes of matter in
+solution, which for every square mile of area the rivers convey
+to the ocean in one year, is given in the first column. These
+results are compiled by Clarke from a very large number of
+analyses of river waters. The second column of the table gives
+the mean heights in metres above sea level of the several
+continents, as cited by Arrhenius.[2]
+
+Of all the denudation results given in the table, those relating
+to North America and to Europe are far the
+
+[1] See also Poulton, Address to Sect. D., Brit. Assoc. Rep.,
+1896.
+
+[2] _Lehybuch dev Kosmischen Physik_, vol. i., p. 347.
+
+17
+
+most reliable. Indeed these may be described as highly reliable,
+being founded on some thousands of analyses, many of which have
+been systematically pursued through every season of the year.
+These show that Europe with a mean altitude of less than half
+that of North America sheds to the ocean 25 per cent. more salts.
+A result which is to be expected when the more important factors
+of solvent denudation are given intelligent consideration and we
+discriminate between conditions favouring solvent and detrital
+denudation respectively: conditions in many cases
+antagonistic.[1] Hence if it is true, as has been stated, that we
+now live in a period of exceptionally high continental elevation,
+we must infer that the average supply of salts to the ocean by
+the rivers of the world is less than over the long past, and
+that, therefore, our estimate of the age of the Earth as already
+given is excessive.
+
+There is, however, one condition which will operate to unduly
+diminish our estimate of geologic time, and it is a condition
+which may possibly obtain at the present time. If the land is, on
+the whole, now sinking relatively to the ocean level, the
+denudation area tends, as we have seen, to move inwards. It will
+thus encroach upon regions which have not for long periods
+drained to the ocean. On such areas there is an accumulation of
+soluble salts which the deficient rivers have not been able to
+carry to the ocean. Thus the salt content of certain of
+
+[1] See the essay on Denudation.
+
+18
+
+the rivers draining to the ocean will be influenced not only by
+present denudative effects, but also by the stored results of
+past effects. Certain rivers appear to reveal this unduly
+increased salt supply those which flow through comparatively arid
+areas. However, the flowoff of such tributaries is relatively
+small and the final effects on the great rivers apparently
+unimportant--a result which might have been anticipated when the
+extremely slow rate of the land movements is taken into account.
+
+The difficulty of effecting any reconciliation of the methods
+already described and that now to be given increases the interest
+both of the former and the latter.
+
+THE AGE BY RADIOACTIVE TRANSFORMATIONS
+
+Rutherford suggested in 1905 that as helium was continually being
+evolved at a uniform rate by radioactive substances (in the form
+of the alpha rays) a determination of the age of minerals
+containing the radioactive elements might be made by measurements
+of the amount of the stored helium and of the radioactive
+elements giving rise to it, The parent radioactive substances
+are--according to present knowledge--uranium and thorium. An
+estimate of the amounts of these elements present enables the
+rate of production of the helium to be calculated. Rutherford
+shortly afterwards found by this method an age of 240 millions of
+years for a radioactive mineral of presumably remote age. Strutt,
+who carried
+
+19
+
+his measurements to a wonderful degree of refinement, found the
+following ages for mineral substances originating in different
+geological ages:
+
+Oligocene -               8.4 millions of years.
+Eocene -                 31    millions of years.
+Lower Carboniferous -  150   millions of years.
+Archaean -               750    millions of years.
+
+Periods of time much less than, and very inconsistent with, these
+were also found. The lower results are, however, easily explained
+if we assume that the helium--which is a gas under prevailing
+conditions--escapes in many cases slowly from the mineral.
+
+Another product of radioactive origin is lead. The suggestion
+that this substance might be made available to determine the age
+of the Earth also originated with Rutherford. We are at least
+assured that this element cannot escape by gaseous diffusion from
+the minerals. Boltwood's results on the amount of lead contained
+in minerals of various ages, taken in conjunction with the amount
+of uranium or parent substance present, afforded ages rising to
+1,640 millions of years for archaean and 1,200 millions for
+Algonkian time. Becker, applying the same method, obtained
+results rising to quite incredible periods: from 1,671 to 11,470
+millions of years. Becker maintained that original lead rendered
+the determinations indefinite. The more recent results of Mr. A.
+Holmes support the conclusion that "original" lead may be present
+and may completely falsify results derived
+
+20
+
+from minerals of low radioactivity in which the derived lead
+would be small in amount. By rejecting such results as appeared
+to be of this character, he arrives at 370 millions of years as
+the age of the Devonian.
+
+I must now describe a very recent method of estimating the age of
+the Earth. There are, in certain rock-forming minerals,
+colour-changes set up by radioactive causes. The minute and
+curious marks so produced are known as haloes; for they surround,
+in ringlike forms, minute particles of included substances which
+contain radioactive elements. It is now well known how these
+haloes are formed. The particle in the centre of the halo
+contains uranium or thorium, and, necessarily, along with the
+parent substance, the various elements derived from it. In the
+process of transformation giving rise to these several derived
+substances, atoms of helium--the alpha rays--projected with great
+velocity into the surrounding mineral, occasion the colour
+changes referred to. These changes are limited to the distance to
+which the alpha rays penetrate; hence the halo is a spherical
+volume surrounding the central substance.[1]
+
+The time required to form a halo could be found if on the one
+hand we could ascertain the number of alpha rays ejected from the
+nucleus of the halo in, say, one year, and, on the other, if we
+determined by experiment just how many alpha rays were required
+to produce the same
+
+[1] _Phil. Mag._, March, 1907 and February, 1910; also _Bedrock_,
+January, 1913. See _Pleochroic Haloes_ in this volume.
+
+21
+
+amount of colour alteration as we perceive to extend around the
+nucleus.
+
+The latter estimate is fairly easily and surely made. But to know
+the number of rays leaving the central particle in unit time we
+require to know the quantity of radioactive material in the
+nucleus. This cannot be directly determined. We can only, from
+known results obtained with larger specimens of just such a
+mineral substance as composes the nucleus, guess at the amount of
+uranium, or it may be thorium, which may be present.
+
+This method has been applied to the uranium haloes of the mica of
+County Carlow.[1] Results for the age of the halo of from 20 to
+400 millions of years have been obtained. This mica was probably
+formed in the granite of Leinster in late Silurian or in Devonian
+times.
+
+The higher results are probably the least in error, upon the data
+involved; for the assumption made as to the amount of uranium in
+the nuclei of the haloes was such as to render the higher results
+the more reliable.
+
+This method is, of course, a radioactive method, and similar to
+the method by helium storage, save that it is free of the risk of
+error by escape of the helium, the effects of which are, as it
+were, registered at the moment of its production, so that its
+subsequent escape is of no moment.
+
+[1] Joly and Rutherford, _Phil. Mag._, April, 1913.
+
+22
+
+REVIEW OF THE RESULTS
+
+We shall now briefly review the results on the geological age of
+the Earth.
+
+By methods based on the approximate uniformity of denudative
+effects in the past, a period of the order of 100 millions of
+years has been obtained as the duration of our geological age;
+and consistently whether we accept for measurement the sediments
+or the dissolved sodium. We can give reasons why these
+measurements might afford too great an age, but we can find
+absolutely no good reason why they should give one much too low.
+
+By measuring radioactive products ages have been found which,
+while they vary widely among themselves, yet claim to possess
+accuracy in their superior limits, and exceed those derived from
+denudation from nine to fourteen times.
+
+In this difficulty let us consider the claims of the radioactive
+method in any of its forms. In order to be trustworthy it must be
+true; (1) that the rate of transformation now shown by the parent
+substance has obtained throughout the entire past, and (2) that
+there were no other radioactive substances, either now or
+formerly existing, except uranium, which gave rise to lead. As
+regards methods based on the production of helium, what we have
+to say will largely apply to it also. If some unknown source of
+these elements exists we, of course, on our assumption
+overestimate the age.
+
+23
+
+As regards the first point: In ascribing a constant rate of
+change to the parent substance--which Becker (loc. cit.) describes
+as "a simple though tremendous extrapolation"--we reason upon
+analogy with the constant rate of decay observed in the derived
+radioactive bodies. If uranium and thorium are really primary
+elements, however, the analogy relied on may be misleading; at
+least, it is obviously incomplete. It is incomplete in a
+particular which may be very important: the mode of origin of
+these parent bodies--whatever it may have been--is different to
+that of the secondary elements with which we compare them. A
+convergence in their rate of transformation is not impossible, or
+even improbable, so far as we known.
+
+As regards the second point: It is assumed that uranium alone of
+the elements in radioactive minerals is ultimately transformed to
+lead by radioactive changes. We must consider this assumption.
+
+Recent advances in the chemistry of the radioactive elements has
+brought out evidence that all three lines of radioactive descent
+known to us--_i.e._ those beginning with uranium, with thorium,
+and with actinium--alike converge to lead.[1] There are
+difficulties in the way of believing that all the lead-like atoms
+so produced ("isotopes" of lead, as Soddy proposes to call them)
+actually remain as stable lead in the minerals. For one
+
+[1] See Soddy's _Chemistry of the Radioactive Elements_ (Longmans,
+Green & Co.).
+
+24
+
+thing there is sometimes, along with very large amounts of
+thorium, an almost entire absence of lead in thorianites and
+thorites. And in some urano--thorites the lead may be noticed to
+follow the uranium in approximate proportionality,
+notwithstanding the presence of large amounts of thorium.[1] This
+is in favour of the assumption that all the lead present is
+derived from the uranium. The actinium is present in negligibly
+small amounts.
+
+On the other hand, there is evidence arising from the atomic
+weight of lead which seems to involve some other parent than
+uranium. Soddy, in the work referred to, points this out. The
+atomic weight of radium is well known, and uranium in its descent
+has to change to this element. The loss of mass between radium
+and uranium-derived lead can be accurately estimated by the
+number of alpha rays given off. From this we get the atomic
+weight of uranium-derived lead as closely 206. Now the best
+determinations of the atomic weight of normal lead assign to this
+element an atomic weight of closely
+
+[1] It seems very difficult at present to suggest an end product
+for thorium, unless we assume that, by loss of electrons, thorium
+E, or thorium-lead, reverts to a substance chemically identical
+with thorium itself. Such a change--whether considered from the
+point of view of the periodic law or of the radioactive theory
+would involve many interesting consequences. It is, of course,
+quite possible that the nature of the conditions attending the
+deposition of the uranium ores, many of which are comparatively
+recent, are responsible for the difficulties observed. The
+thorium and uranium ores are, again, specially prone to
+alteration.
+
+25
+
+207. By a somewhat similar calculation it is deduced that
+thorium-derived lead would possess the atomic weight of 208. Thus
+normal lead might be an admixture of uranium- and thorium-derived
+lead. However, as we have seen, the view that thorium gives rise
+to stable lead is beset with some difficulties.
+
+If we are going upon reliable facts and figures, we must, then,
+assume: (a) That some other element than uranium, and genetically
+connected with it (probably as parent substance), gives rise, or
+formerly gave rise, to lead of heavier atomic weight than normal
+lead. It may be observed respecting this theory that there is
+some support for the view that a parent substance both to uranium
+and thorium has existed or possibly exists. The evidence is found
+in the proportionality frequently observed between the amounts of
+thorium and uranium in the primary rocks.[1] Or: (b) We may meet
+the difficulties in a simpler way, which may be stated as
+follows: If we assume that all stable lead is derived from
+uranium, and at the same time recognise that lead is not
+perfectly homogeneous in atomic weight, we must, of necessity,
+ascribe to uranium a similar want of homogeneity; heavy atoms of
+uranium giving rise to heavy
+
+[1] Compare results for the thorium content of such rocks
+(appearing in a paper by the author Cong. Int. _de Radiologie et
+d'Electricite_, vol. i., 1910, p. 373), and those for the radium
+content, as collected in _Phil. Mag._, October, 1912, p. 697.
+Also A. L. Fletcher, _Phil. Mag._, July, 1910; January, 1911, and
+June, 1911. J. H. J. Poole, _Phil. Mag._, April, 1915
+
+26
+
+atoms of lead and light atoms of uranium generating light atoms
+of lead. This assumption seems to be involved in the figures
+upon, which we are going. Still relying on these figures, we
+find, however, that existing uranium cannot give rise to lead of
+normal atomic weight. We can only conclude that the heavier atoms
+of uranium have decayed more rapidly than the lighter ones. In
+this connection it is of interest to note the complexity of
+uranium as recently established by Geiger, although in this case
+it is assumed that the shorter-lived isotope bears the relation
+of offspring to the longer-lived and largely preponderating
+constituent. However, there does not seem to be any direct proof
+of this as yet.
+
+From these considerations it would seem that unless the atomic
+weight of lead in uraninites, etc., is 206, the former complexity
+and more accelerated decay of uranium are indicated in the data
+respecting the atomic weights of radium and lead[1]. As an
+alternative view, we may assume, as in our first hypothesis, that
+some elementally different but genetically connected substance,
+decaying along branching lines of descent at a rate sufficient to
+practically remove the whole of it during geological time,
+formerly existed. Whichever hypothesis we adopt
+
+[1] Later investigation has shown that the atomic weight of lead
+in uranium-bearing ores is about 206.6 (see Richards and Lembert,
+_Journ. of Am. Claem. Soc._, July, 1914). This result gives support
+to the view expressed above.
+
+27
+
+we are confronted by probabilities which invalidate
+time-measurements based on the lead and helium ratio in minerals.
+We have, in short, grave reason to question the measure of
+uniformitarianism postulated in finding the age by any of the
+known radioactive methods.
+
+That we have much to learn respecting our assumptions, whether we
+pursue the geological or the radioactive methods of approaching
+the age of our era, is, indeed, probable. Whatever the issue it
+is certain that the reconciling facts will leave us with much
+more light than we at present possess either as respects the
+Earth's history or the history of the radioactive elements. With
+this necessary admission we leave our study of the Birth-Time of
+the World.
+
+It has led us a long way from Lucretius. We do not ask if other
+Iliads have perished; or if poets before Homer have vainly sung,
+becoming a prey to all-consuming time. We move in a greater
+history, the landmarks of which are not the birth and death of
+kings and poets, but of species, genera, orders. And we set out
+these organic events not according to the passing generations of
+man, but over scores or hundreds of millions of years.
+
+How much Lucretius has lost, and how much we have gained, is
+bound up with the question of the intrinsic value of knowledge
+and great ideas. Let us appraise knowledge as we would the
+Homeric poems, as some-
+
+28
+
+thing which ennobles life and makes it happier. Well, then, we
+are, as I think, in possession today of some of those lost Iliads
+and Odysseys for which Lucretius looked in vain.[1]
+
+[1] The duration in the past of Solar heat is necessarily bound
+up with the geological age. There is no known means (outside
+speculative science) of accounting for more than about 30 million
+years of the existing solar temperature in the past. In this
+direction the age seems certainly limited to 100 million years.
+See a review of the question by Dr. Lindemann in Nature, April
+5th, 1915.
+
+29
+
+DENUDATION
+
+THE subject of denudation is at once one of the most interesting
+and one of the most complicated with which the geologist has to
+deal. While its great results are apparent even to the most
+casual observer, the factors which have led to these results are
+in many cases so indeterminate, and in some cases apparently so
+variable in influence, that thoughtful writers have even claimed
+precisely opposite effects as originating from, the same cause.
+Indeed, it is almost impossible to deal with the subject without
+entering upon controversial matters. In the following pages I
+shall endeavour to keep to broad issues which are, at the present
+day, either conceded by the greater number of authorities on the
+subject, or are, from their strictly quantitative character, not
+open to controversy.
+
+It is evident, in the first place, that denudation--or the wearing
+away of the land surfaces of the earth--is mainly a result of the
+circulation of water from the ocean to the land, and back again
+to the ocean. An action entirely conditioned by solar heat, and
+without which it would completely cease and further change upon
+the land come to an end.
+
+To what actions, then, is so great a potency of the
+
+30
+
+circulating water to be traced? Broadly speaking, we may classify
+them as mechanical and chemical. The first involves the
+separation of rock masses into smaller fragments of all sizes,
+down to the finest dust. The second involves the actual solution
+in the water of the rock constituents, which may be regarded as
+the final act of disintegration. The rivers bear the burden both
+of the comminuted and the dissolved materials to the sea. The mud
+and sand carried by their currents, or gradually pushed along
+their beds, represent the former; the invisible dissolved matter,
+only to be demonstrated to the eye by evaporation of the water or
+by chemical precipitation, represents the latter.
+
+The results of these actions, integrated over geological time,
+are enormous. The entire bulk of the sedimentary rocks, such as
+sandstones, slates, shales, conglomerates, limestones, etc., and
+the salt content of the ocean, are due to the combined activity
+of mechanical and solvent denudation. We shall, later on, make an
+estimate of the magnitude of the quantities actually involved.
+
+In the Swiss valleys we see torrents of muddy water hurrying
+along, and if we follow them up, we trace them to glaciers high
+among the mountains. From beneath the foot of the glacier, we
+find, the torrent has birth. The first debris given to the river
+is derived from the wearing of the rocky bed along which the
+glacier moves. The river of ice bequeaths to the river of
+water--of which it is the parent--the spoils which it has won from
+the rocks
+
+31
+
+The work of mechanical disintegration is, however, not restricted
+to the glacier's bed. It proceeds everywhere over the surface of
+the rocks. It is aided by the most diverse actions. For instance,
+the freezing and expansion of water in the chinks and cracks in
+those alpine heights where between sunrise and sunset the heat of
+summer reigns, and between sunset and sunrise the cold of winter.
+Again, under these conditions the mere change of surface
+temperature from night to day severely stresses the surface
+layers of the rocks, and, on the same principles as we explain
+the fracture of an unequally heated glass vessel, the rocks
+cleave off in slabs which slip down the steeps of the mountain
+and collect as screes in the valley. At lower levels the
+expansive force of vegetable growth is not unimportant, as all
+will admit who have seen the strong roots of the pines
+penetrating the crannies of the rocks. Nor does the river which
+flows in the bed of the valley act as a carrier only. Listening
+carefully we may detect beneath the roar of the alpine torrent
+the crunching and knocking of descending boulders. And in the
+potholes scooped by its whirling waters we recognise the abrasive
+action of the suspended sand upon the river bed.
+
+A view from an Alpine summit reveals a scene of remarkable
+desolation (Pl. V, p. 40). Screes lie piled against the steep
+slopes. Cliffs stand shattered and ready to fall in ruins. And
+here the forces at work readily reveal themselves. An occasional
+wreath of white smoke among
+
+32
+
+the far-off peaks, followed by a rumbling reverberation, marks
+the fall of an avalanche. Water everywhere trickles through the
+shaly _debris_ scattered around. In the full sunshine the rocks are
+almost too hot to bear touching. A few hours later the cold is
+deadly, and all becomes a frozen silence. In such scenes of
+desolation and destruction, detrital sediments are actively being
+generated. As we descend into the valley we hear the deep voice
+of the torrents which are continually hurrying the disintegrated
+rocks to the ocean.
+
+A remarkable demonstration of the activity of mechanical
+denudation is shown by the phenomenon of "earth pillars." The
+photograph (Pl. IV.) of the earth pillars of the Val d'Herens
+(Switzerland) shows the peculiar appearance these objects
+present. They arise under conditions where large stones or
+boulders are scattered in a deep deposit of clay, and where much
+of the denudation is due to water scour. The large boulders not
+only act as shelter against rain, but they bind and consolidate
+by their mere weight the clay upon which they rest. Hence the
+materials underlying the boulders become more resistant, and as
+the surrounding clays are gradually washed away and carried to
+the streams, these compacted parts persist, and, finally, stand
+like walls or pillars above the general level. After a time the
+great boulders fall off and the underlying clay becomes worn by
+the rainwash to fantastic spikes and ridges. In the Val d'Herens
+the earth pillars are formed
+
+33
+
+of the deep moraine stuff which thickly overlies the slopes of
+the valley. The wall of pillars runs across the axis of the
+valley, down the slope of the hill, and crosses the road, so that
+it has to be tunnelled to permit the passage of traffic. It is
+not improbable that some additional influence--possibly the
+presence of lime--has hardened the material forming the pillars,
+and tended to their preservation.
+
+Denudation has, however, other methods of work than purely
+mechanical; methods more noiseless and gentle, but not less
+effective, as the victories of peace ate no less than those of
+war.
+
+Over the immense tracts of the continents chemical work proceeds
+relentlessly. The rock in general, more especially the primary
+igneous rock, is not stable in presence of the atmosphere and of
+water. Some of the minerals, such as certain silicates and
+carbonates, dissolve relatively fast, others with extreme
+slowness. In the process of solution chemical actions are
+involved; oxidation in presence of the free oxygen of the
+atmosphere; attack by the feeble acid arising from the solution
+of carbon dioxide in water; or, again, by the activity of certain
+acids--humous acids--which originate in the decomposition of
+vegetable remains. These chemical agents may in some instances,
+_e.g._ in the case of carbonates such as limestone or
+dolomite--bring practically the whole rock into solution. In other
+instances--_e.g._ granites, basalts, etc.--they may remove some of
+the
+
+34
+
+constituent minerals completely or partially, such as felspar,
+olivine, augite, and leave more resistant substances to be
+ultimately washed down as fine sand or mud into the river.
+
+It is often difficult or impossible to appraise the relative
+efficiency of mechanical and chemical denudation in removing the
+materials from a certain area. There can be, indeed, little doubt
+that in mountainous regions the mechanical effects are largely
+predominant. The silts of glacial rivers are little different
+from freshly-powdered rock. The water which carries them but
+little different from the pure rain or snow which falls from the
+sky. There has not been time for the chemical or solvent actions
+to take place. Now while gravitational forces favour sudden shock
+and violent motions in the hills, the effect of these on solvent
+and chemical denudation is but small. Nor is good drainage
+favourable to chemical actions, for water is the primary factor
+in every case. Water takes up and removes soluble combinations of
+molecules, and penetrates beneath residual insoluble substances.
+It carries the oxygen and acids downwards through the soils, and
+finally conveys the results of its own work to the rivers and
+streams. The lower mean temperature of the mountains as well as
+the perfect drainage diminishes chemical activities.
+
+Hence we conclude that the heights are not generally favourable
+to the purely solvent and chemical actions. It is on the
+lower-lying land that soils tend to accumulate,
+
+35
+
+and in these the chief solvent and the chief chemical denudation
+of the Earth are effected.
+
+The solvent and chemical effects which go on in the
+finely-divided materials of the soils may be observed in the
+laboratory. They proceed faster than would be anticipated. The
+observation is made by passing a measured quantity of water
+backwards and forwards for some months through a tube containing
+a few grammes of powdered rock. Finally the water is analysed,
+and in this manner the amount of dissolved matter it has taken up
+is estimated. The rock powder is examined under the microscope in
+order to determine the size of the grains, and so to calculate
+the total surface exposed to the action of the water. We must be
+careful in such experiments to permit free oxidation by the
+atmosphere. Results obtained in this way of course take no
+account of the chemical effects of organic acids such as exist in
+the soils. The quantities obtained in the laboratory will,
+therefore, be deficient as compared with the natural results.
+
+In this manner it has been found that fresh basalt exposed to
+continually moving water will lose about 0.20 gramme per square
+metre of surface per year. The mineral orthoclase, which enters
+largely into the constitution of many granites, was found to lose
+under the same conditions 0.025 gramme. A glassy lava (obsidian)
+rich in silica and in the chemical constituents of an average
+granite, was more resistant still; losing but 0.013 gramme per
+square metre per year. Hornblende, a mineral
+
+36
+
+abundant in many rocks, lost 0.075 gramme. The mean of the
+results showed that 0.08 gramme was washed in a year from each
+square metre. Such results give us some indication of the rate at
+which the work of solution goes on in the finely divided
+soils.[1]
+
+It might be urged that, as the mechanical break up of rocks, and
+the production in this way of large surfaces, must be at the
+basis of solvent and chemical denudation, these latter activities
+should be predominant in the mountains. The answer to this is
+that the soils rarely owe their existence to mechanical actions.
+The alluvium of the valleys constitutes only narrow margins to
+the rivers; the finer _debris_ from the mountains is rapidly
+brought into the ocean. The soils which cover the greater part of
+continental areas have had a very different origin.
+
+In any quarry where a section of the soil and of the underlying
+rock is visible, we may study the mode of formation of soils. Our
+observations are, we will suppose, pursued in a granite quarry.
+We first note that the material of the soil nearest the surface
+is intermixed with the roots of grasses, trees, or shrubs.
+Examining a handful of this soil, we see glistening flakes of
+mica which plainly are derived from the original granite. Washing
+off the finer particles, we find the largest remaining grains are
+composed of the all but indestructible quartz.
+
+[1] Proc. Roy. Irish Acad., VIII., Ser. A, p. 21.
+
+37
+
+This also is from the granite. Some few of the grains are of
+chalky-looking felspar; again a granitic mineral. What is the
+finer silt we have washed off? It, too, is composed of mineral
+particles to a great extent; rock dust stained with iron oxide
+and intermixed with organic remains, both animal and vegetable.
+But if we make a chemical analysis of the finer silt we find that
+the composition is by no means that of the granite beneath. The
+chemist is able to say, from a study of his results, that there
+has been, in the first place, a large loss of material attending
+the conversion of the granite to the soil. He finds a
+concentration of certain of the more resistant substances of the
+granite arising from the loss of the less resistant. Thus the
+percentage amount of alumina is increased. The percentage of iron
+is also increased. But silica and most other substances show a
+diminished percentage. Notably lime has nearly disappeared. Soda
+is much reduced; so is magnesia. Potash is not so completely
+abstracted. Finally, owing to hydration, there is much more
+combined water in the soil than in the rock. This is a typical
+result for rocks of this kind.
+
+Deeper in the soil we often observe a change of texture. It has
+become finer, and at the same time the clay is paler in colour.
+This subsoil represents the finer particles carried by rain from
+above. The change of colour is due to the state of the iron which
+is less oxidised low down in the soil. Beneath the subsoil the
+soil grows
+
+38
+
+again coarser. Finally, we recognise in it fragments of granite
+which ever grow larger as we descend, till the soil has become
+replaced by the loose and shattered rock. Beneath this the only
+sign of weathering apparent in the rock is the rusty hue imparted
+by the oxidised iron which the percolating rain has leached from
+iron-bearing minerals.
+
+The soil we have examined has plainly been derived in situ from
+the underlying rock. It represents the more insoluble residue
+after water and acids have done their work. Each year there must
+be a very slow sinking of the surface, but the ablation is
+infinitesimal.
+
+The depth of such a soil may be considerable. The total surface
+exposed by the countless grains of which it is composed is
+enormous. In a cubic foot of average soil the surface area of the
+grains may be 50,000 square feet or more. Hence a soil only two
+feet deep may expose 100,000 square feet for each square foot of
+surface area.
+
+It is true that soils formed in this manner by atmospheric and
+organic actions take a very long time to grow. It must be
+remembered, however, that the process is throughout attended by
+the removal in solution: of chemically altered materials.
+
+Considerations such as the foregoing must convince us that while
+the accumulation of the detrital sediments around the continents
+is largely the result of activities progressing on the steeper
+slopes of the land, that is,
+
+39
+
+among the mountainous regions, the feeding of the salts to the
+ocean arises from the slower work of meteorological and organic
+agencies attacking the molecular constitution of the rocks;
+processes which best proceed where the drainage is sluggish and
+the quiescent conditions permit of the development of abundant
+organic growth and decay.
+
+Statistics of the solvent denudation of the continents support
+this view. Within recent years a very large amount of work has
+been expended on the chemical investigation of river waters of
+America and of Europe. F. W. Clarke has, at the expense of much
+labour, collected and compared these results. They are expressed
+as so many tonnes removed in solution per square mile per annum.
+For North America the result shows 79 tonnes so removed; for
+Europe 100 tonnes. Now there is a notable difference between the
+mean elevations of these two continents. North America has a mean
+elevation of 700 metres over sea level, whereas the mean
+elevation of Europe is but 300 metres. We see in these figures
+that the more mountainous land supplies less dissolved matter to
+the ocean than the land of lower elevation, as our study has led
+us to expect.
+
+We have now considered the source of the detrital sediments, as
+well as of the dissolved matter which has given to the ocean, in
+the course of geological time, its present gigantic load of
+salts. It is true there are further solvent and chemical effects
+exerted by the sea water
+
+40
+
+upon the sediments discharged into it; but we are justified in
+concluding that, relatively to the similar actions taking place
+in the soils, the solvent and chemical work of the ocean is
+small. The fact is, the deposited detrital sediments around the
+continents occupy an area small when contrasted with the vast
+stretches of the land. The area of deposition is much less than
+that of denudation; probably hardly as much as one twentieth.
+And, again, the conditions of aeration and circulation which
+largely promote chemical and solvent denudation in the soils are
+relatively limited and ineffective in the detrital oceanic
+deposits.
+
+The summation of the amounts of dissolved and detrital materials
+which denudation has brought into the ocean during the long
+denudative history of the Earth, as we might anticipate, reveals
+quantities of almost unrealisable greatness. The facts are among
+the most impressive which geological science has brought to
+light. Elsewhere in this volume they have been mentioned when
+discussing the age of the Earth. In the present connection,
+however, they are deserving of separate consideration.
+
+The basis of our reasoning is that the ocean owes its saltness
+mainly if not entirely to the denudative activities we have been
+considering. We must establish this.
+
+We may, in the first place, say that any other view at once
+raises the greatest difficulties. The chemical composition of the
+detrital sediments which are spread over
+
+41
+
+the continents and which build up the mountains, differs on the
+average very considerably from that of the igneous rocks. We know
+the former have been derived from the latter, and we know that
+the difference in the composition of the two classes of materials
+is due to the removal in solution of certain of the constituents
+of the igneous rocks. But the ocean alone can have received this
+dissolved matter. We know of no other place in which to look for
+it. It is true that some part of this dissolved matter has been
+again rejected by the ocean; thus the formation of limestone is
+largely due to the abstraction of lime from sea water by organic
+and other agencies. This, however, in no way relieves us of the
+necessity of tracing to the ocean the substances dissolved from
+the igneous rocks. It follows that we have here a very causa for
+the saltness of the ocean. The view that the ocean "was salt from
+the first" is without one known fact to support it, and leaves us
+with the burden of the entire dissolved salts of geological time
+to dispose of--Where and how?
+
+The argument we have outlined above becomes convincingly strong
+when examined more closely. For this purpose we first compare the
+average chemical composition of the sedimentary and the igneous
+rocks. The following table gives the percentages of the chief
+chemical constituents: [1]
+
+[1] F. W. Clarke: _A Preliminary Study of Chemical Denudation_,
+p. 13
+
+42
+
+                          Igneous.        Sedimentary.
+Silica (SiO2) -             59.99           58.51
+Alumina (Al2O3) -           15.04           13.07
+Ferric oxide (F2O3) -       2.59            3.40
+Ferrous oxide (FeO) -       3.34            2.00
+Magnesia (MgO) -            3.89            2.52
+Lime (CaO) -                 4.81            5.42
+Soda (Na2O) -                3.41            1.12
+Potash (K2O) -               2.95            2.80
+Water (H2O) -                1.92            4.28
+Carbon dioxide (CO2) -       --             4.93
+Minor constituents -        2.06            1.95
+                          100.00          100.00
+
+In the derivation of the sediments from the igneous rocks there
+is a loss by solution of about 33 per cent; _i.e._ 100 tons of
+igneous rock yields rather less than 70 tons of sedimentary rock.
+This involves a concentration in the sediments of the more
+insoluble constituents. To this rule the lime-content appears to
+be an exception. It is not so in reality. Its high value in the
+sediments is due to its restoration from the ocean to the land.
+The magnesia and potash are, also, largely restored from the
+ocean; the former in dolomites and magnesian limestones; the
+latter in glauconite sands. The iron of the sediments shows
+increased oxidation. The most notable difference in the two
+analyses appears, however, in the soda percentages. This falls
+from 3.41 in the igneous rock to 1.12 in the average sediment.
+Indeed, this
+
+43
+
+deficiency of soda in sedimentary rocks is so characteristic of
+secondary rocks that it may with some safety be applied to
+discriminate between the two classes of substances in cases where
+petrological distinctions of other kinds break down.
+
+To what is this so marked deficiency of soda to be ascribed? It
+is a result of the extreme solubility of the salts of sodium in
+water. This has not only rendered its deposition by evaporation a
+relatively rare and unimportant incident of geological history,
+but also has protected it from abstraction from the ocean by
+organic agencies. The element sodium has, in fact, accumulated in
+the ocean during the whole of geological time.
+
+We can use the facts associated with the accumulation of sodium
+salts in the ocean as a means of obtaining additional support to
+the view, that the processes of solvent denudation are
+responsible for the saltness of the ocean. The new evidence may
+be stated as follows: Estimates of the amounts of sedimentary
+rock on the continents have repeatedly been made. It is true that
+these estimates are no more than approximations. But they
+undoubtedly _are_ approximations, and as such may legitimately be
+used in our argument; more especially as final agreement tends to
+check and to support the several estimates which enter into
+them.
+
+The most recent and probable estimates of the sediments on the
+land assign an average thickness of one mile of
+
+44
+
+secondary rocks over the land area of the world. To this some
+increase must be made to allow for similar materials concealed in
+the ocean, principally around the continental margins. If we add
+10 per cent. and assign a specific gravity of 2.5 we get as the
+mass of the sediments 64 x 1016 tonnes. But as this is about 67
+per cent. of the parent igneous rock--_i.e._ the average igneous
+rock from which the sediments are derived--we conclude that the
+primary denuded rock amounted to a mass of about 95 x 1016
+tonnes.
+
+Now from the mean chemical composition of the secondary rocks we
+calculate that the mass of sediments as above determined contains
+0.72 x1016 tonnes of the sodium oxide, Na2O. If to this amount we
+add the quantity of sodium oxide which must have been given to
+the ocean in order to account for the sodium salts contained
+therein, we arrive at a total quantity of oxide of sodium which
+must be that possessed by the primary rock before denudation
+began its work upon it. The mass of the ocean being well
+ascertained, we easily calculate that the sodium in the ocean
+converted to sodium oxide amounts to 2.1 x 1016 tonnes. Hence
+between the estimated sediments and the waters of the ocean we
+can account for 2.82 x 1016 tonnes of soda. When now we put this
+quantity back into the estimated mass of primary rock we find
+that it assigns to the primary rock a soda percentage of 3.0. On
+the average analysis given above this should be 3.41 per cent.
+The agreement,
+
+45
+
+all things considered, more especially the uncertainty in the
+estimate of the sediments, is plainly in support of the view that
+oceanic salts are derived from the rocks; if, indeed, it does not
+render it a certainty.
+
+A leading and fundamental inference in the denudative history of
+the Earth thus finds support: indeed, we may say, verification.
+In the light of this fact the whole work of denudation stands
+revealed. That the ocean began its history as a vast fresh-water
+envelope of the Globe is a view which accords with the evidence
+for the primitive high temperature of the Earth. Geological
+history opened with the condensation of an atmosphere of immense
+extent, which, after long fluctuations between the states of
+steam and water, finally settled upon the surface, almost free of
+matter in solution: an ocean of distilled water. The epoch of
+denudation then began. It will, probably, continue till the
+waters, undergoing further loss of thermal energy, suffer yet
+another change of state, when their circulation will cease and
+their attack upon the rocks come to an end.
+
+From what has been reviewed above it is evident that the sodium
+in the ocean is an index of the total activity of denudation
+integrated over geological time. From this the broad facts of the
+results of denudation admit of determination with considerable
+accuracy. We can estimate the amount of rock which has been
+degraded by solvent and chemical actions, and the amount of
+sediments which has been derived from it. We are,
+
+46
+
+thus, able to amend our estimate of the sediments which, as
+determined by direct observation, served to support the basis of
+our argument.
+
+We now go straight to the ocean for the amount of sodium of
+denudative origin. There may, indeed, have been some primitive
+sodium dissolved by a more rapid denudation while the Earth's
+surface was still falling in temperature. It can be shown,
+however, that this amount was relatively small. Neglecting it we
+may say with safety that the quantity of sodium carried into the
+ocean by the rivers must be between 14,000 and 15,000 million
+million tonnes: _i.e._ 14,500 x 1012 tonnes, say.
+
+Keeping the figures to round numbers we find that this amount of
+sodium involves the denudation of about 80 x 1016 tonnes of
+average igneous rock to 53 x 1016 tonnes of average sediment.
+From these vast quantities we know that the parent rock denuded
+during geological time amounted to some 300 million cubic
+kilometres or about seventy million cubic miles. The sediments
+derived therefrom possessed a bulk of 220 million cubic
+kilometres or fifty million cubic miles. The area of the land
+surface of the Globe is 144 million square kilometres. The parent
+rock would have covered this to a uniform depth of rather more
+than two kilometres, and the derived sediment to more than 1.5
+kilometres, or about one mile deep.
+
+The slow accomplishment of results so vast conveys some idea of
+the great duration of geological time.
+
+47
+
+The foregoing method of investigating the statistics of solvent
+denudation is capable of affording information not only as to the
+amount of sediments upon the land, but also as to the quantity
+which is spread over the floor of the ocean.
+
+We see this when we follow the fate of the 33 per cent. of
+dissolved salts which has been leached from the parent igneous
+rock, and the mass of which we calculate from the ascertained
+mass of the latter, to be 27 x 1016 tonnes. This quantity was at
+one time or another all in the ocean. But, as we saw above, a
+certain part of it has been again abstracted from solution,
+chiefly by organic agencies. Now the abstracted solids have not
+been altogether retained beneath the ocean. Movements of the land
+during geological time have resulted in some portion being
+uplifted along with other sediments. These substances constitute,
+mainly, the limestones.
+
+We see, then, that the 27 x 1016 tonnes of substances leached
+from the parent igneous rocks have had a threefold destination.
+One part is still in solution; a second part has been
+precipitated to the bottom of the ocean; a third part exists on
+the land in the form of calcareous rocks.
+
+Observation on the land sediments shows that the calcareous rocks
+amount to about 5 per cent. of the whole. From this we find that
+3 x 1016 tonnes, approximately, of such rocks have been taken
+from the ocean. This accounts for one of the three classes of
+material
+
+48
+
+into which the original dissolved matter has been divided.
+Another of the three quantities is easily estimated: the amount
+of matter still in solution in the ocean. The volume of the ocean
+is 1,414 million cubic kilometres and its mass is 145 x 1016
+tonnes. The dissolved salts in it constitute 3.4 per cent. of its
+mass; or, rather more than 5 x 1016 tonnes. The limestones on the
+land and the salts in the sea water together make up about 8 x
+1016 tonnes. If we, now, deduct this from the total of 27 x 1016
+tonnes, we find that about 19 x 1016 tonnes must exist as
+precipitated matter on the floor of the ocean.
+
+The area of the ocean is 367 x 1012 square metres, so that if the
+precipitated sediment possesses an average specific gravity of
+2.5, it would cover the entire floor to a uniform depth of 218
+metres; that is 715 feet. This assumes that there was uniform
+deposition of the abstracted matter over the floor of the ocean.
+Of course, this assumption is not justifiable. It is certain that
+the rate of deposition on the floor of the sea has varied
+enormously with various conditions--principally with the depth.
+Again, it must be remembered that this estimate takes no account
+of solid materials otherwise brought into the oceanic deposits;
+_e.g._, by wind-transported dust from the land or volcanic
+ejectamenta in the ocean depths. It is not probable, however,
+that any considerable addition to the estimated mean depth of
+deposit from such sources would be allowable.
+
+49
+
+The greatness of the quantities involved in these determinations
+is almost awe inspiring. Take the case of the dissolved salts in
+the ocean. They are but a fraction, as we have seen, of the total
+results of solvent denudation and represent the integration of
+the minute traces contributed by the river water. Yet the common
+salt (chloride of sodium) alone, contained in the ocean, would,
+if abstracted and spread over the dry land as a layer of rock
+salt having a specific gravity of 2.2, cover the whole to a depth
+of 107 metres or 354 feet. The total salts in solution in the
+ocean similarly spread over the land would increase the depth of
+the layer to 460 feet. After considering what this means we have
+to remember that this amount of matter now in solution in the
+seas is, in point of fact, less than a fifth part of the total
+dissolved from the rocks during geological time.
+
+The transport by denudation of detrital and dissolved matter from
+the land to the ocean has had a most important influence on the
+events of geological history. The existing surface features of
+the earth must have been largely conditioned by the dynamical
+effects arising therefrom. In dealing with the subject of
+mountain genesis we will, elsewhere, see that all the great
+mountain ranges have originated in the accumulation of the
+detrital sediments near the shore in areas which, in consequence
+of the load, gradually became depressed and developed into
+synclines of many thousands of feet in depth. The most impressive
+surface features of the Globe originated
+
+50
+
+in this manner. We will see too that these events were of a
+rhythmic character; the upraising of the mountains involving
+intensified mechanical denudation over the elevated area and in
+this way an accelerated transport of detritus to the sea; the
+formation of fresh deposits; renewed synclinal sinking of the sea
+floor, and, finally, the upheaval of a younger mountain range.
+This extraordinary sequence of events has been determined by the
+events of detrital denudation acting along with certain general
+conditions which have all along involved the growth of
+compressive stresses in the surface crust of the Earth.
+
+The effects of purely solvent denudation are less easily traced,
+but, very probably, they have been of not less importance. I
+refer here to the transport from the land to the sea of matter in
+solution.
+
+Solvent denudation, as observed above, takes place mainly in the
+soils and in this way over the more level continental areas. It
+has resulted in the removal from the land and transfer to the
+ocean of an amount of matter which represents a uniform layer of
+one half a kilometre; that is of more than 1,600 feet of rock.
+The continents have, during geological time, been lightened to
+this extent. On the other hand all this matter has for the
+greater part escaped the geosynclines and become uniformly
+diffused throughout the ocean or precipitated over its floor
+principally on the continental slopes before the great depths are
+reached. Of this material the ocean
+
+51
+
+waters contain in solution an amount sufficient to increase their
+specific gravity by 2.7 per cent.
+
+Taking the last point first, it is interesting to note the
+effects upon the bulk of the ocean which has resulted from the
+matter dissolved in it. From the known density of average sea
+water we find that 100 ccs. of it weigh just 102.7 grammes. Of
+this 3.5 per cent. by weight are solids in solution. That is to
+say, 3.594 grammes. Hence the weight of water present is 99.1
+grammes, or a volume of 99.1 ccs. From this we see that the salts
+present have increased the volume by 0.9 ccs. or 0.9 per cent.
+
+The average depth of the ocean is 2,000 fathoms or 3,700 metres.
+The increase of depth due to salts dissolved in the ocean has
+been, therefore, 108 feet or 33.24 metres. This result assumes
+that there has been no increased elastic compression due to the
+increased pressure, and no change of compressional elastic
+properties. We may be sure that the rise on the shore line of the
+land has not been less than 100 feet.
+
+We see then that as the result of solvent denudation we have to
+do with a heavier and a deeper ocean, expanded in volume by
+nearly one per cent. and the floor of which has become raised, on
+an average, about 700 feet by precipitated sediment.
+
+One of the first conceptions, which the student of geology has to
+dismiss from his mind, is that of the immobility or rigidity of
+the Earth's crust. The lane, we live on sways even to the gentle
+rise and fail of ocean tides
+
+52
+
+around the coasts. It suffers its own tidal oscillations due to
+the moon's attractions. Large tracts of semi-liquid matter
+underlie it. There is every evidence that the raised features of
+the Globe are sustained by such pressures acting over other and
+adjacent areas as serve to keep them in equilibrium against the
+force of gravity. This state of equilibrium, which was first
+recognised by Pratt, as part of the dynamics of the Earth's
+crust, has been named isostasy. The state of the crust is that of
+"mobile equilibrium."
+
+The transfer of matter from the exposed land surfaces to the
+sub-oceanic slopes of the continents and the increase in the
+density of the ocean, must all along have been attended by
+isostatic readjustment. We cannot take any other view. On the one
+hand the land was being lightened; on the other the sea was
+increasing in mass and depth and the flanks of the continents
+were being loaded with the matter removed from the land and borne
+in solution to the ocean. How important the resulting movements
+must have been may be gathered from the fact that the existing
+land of the Globe stands at a mean elevation of no more than
+2,000 feet above sea level. We have seen that solvent denudation
+removed over 1,600 feet of rock. But we have no evidence that on
+the whole the elevation of land in the past was ever very
+different from what it now is.
+
+We have, then, presented to our view the remarkable fact that
+throughout the past, and acting with extreme
+
+53
+
+slowness, the land has steadily been melted down into the sea and
+as steadily been upraised from the waters. It is possible that
+the increased bulk of the ocean has led to a certain diminution
+of the exposed land area. The point is a difficult one. One thing
+we may without much risk assume. The sub-aereal current of
+dissolved matter from the land to the ocean was accompanied by a
+sub-crustal flux from the ocean areas to the land areas; the
+heated viscous materials creeping from depths far beneath the
+ocean floor to depths beneath the roots of the mountains which
+arose around the oceans. Such movements took ages for their
+accomplishment. Indeed, they have been, probably, continuous all
+along and are still proceeding. A low degree of viscosity will
+suffice to permit of movements so slow. Superimposed upon these
+movements the rhythmic alternations of depression and elevation
+of the geosynclines probably resulted in releasing the crust from
+local accumulation of strains arising in the more rigid surface
+materials. The whole sequence of movements presents an
+extraordinary picture of pseudo-vitality--reminding us of the
+circulatory and respiratory systems of a vast organism.
+
+All great results in our universe are founded in motions and
+forces the most minute. In contemplating the Cause or the Effect
+we stand equally impressed with the spectacle presented to us. We
+shall now turn from the great effects of denudation upon the
+history and evolution of a world and consider for a moment
+activities
+
+54
+
+so minute in detail that their operations will probably for ever
+elude our bodily senses, but which nevertheless have necessarily
+affected and modified the great results we have been
+considering.
+
+The ocean a little way from the land is generally so free from
+suspended sediments that it has a blackness as of ink. This
+blackness is due to its absolute freedom from particles
+reflecting the sun's light. The beautiful blue of the Swiss and
+Italian lakes is due to the presence of very fine particles
+carried into them by the rivers; the finest flour of the
+glaciers, which remain almost indefinitely suspended in the
+water. But in the ocean it is only in those places where rapid
+currents running over shallows stir continually the sediments or
+where the fresh water of a great river is carried far from the
+land, that the presence of silt is to be observed. The beautiful
+phenomenon of the coal-black sea is familiar to every yachtsman
+who has sailed to the west of our Islands.[1]
+
+There is, in fact, a very remarkable difference in the manner of
+settlement of fine sediments in salt and in fresh water. We are
+here brought into contact with one of those subtle yet
+influential natural actions the explanation of which involves
+scientific advance along many apparently unconnected lines of
+investigation.
+
+[1] See Tyndall's Voyage to Algeria in _Fragments of Science._ The
+cause of the blue colour of the lakes has been discussed by
+various observers, not always with agreement.
+
+55
+
+It is easy to observe in the laboratory the fact of the different
+behaviour of salt and fresh water towards finely divided
+substances. The nature of the insoluble substance is not
+important.
+
+We place, in a good light, two glass vessels of equal dimensions;
+the one filled with sea water, the other with fresh water. Into
+each we stir the same weight of very finely powdered slate: just
+so much as will produce a cloudiness. In a few hours we find the
+sea water limpid. The fresh water is still cloudy, however; and,
+indeed, may be hardly different in appearance from what it was at
+starting. In itself this is a most extraordinary experiment. We
+would have anticipated quite the opposite result owing to the
+greater density of the sea water.
+
+But a still more interesting experiment remains to be carried
+out. In the sea water we have many different salts in solution.
+Let us see if these salts are equally responsible for the result
+we have obtained. For this purpose we measure out quantities of
+sodium chloride and magnesium chloride in the proportion in which
+they exist in sea water: that is about as seven to one. We add
+such an equal amount of water to each as represents the dilution
+of these salts in sea water. Then finally we stir a little of the
+finely powdered slate into each. It will be found that the
+magnesium chloride, although so much more dilute than the sodium
+chloride, is considerably more active in clearing out the
+suspension. We may now try such marine salts as magnesium
+sulphate,
+
+56
+
+or calcium sulphate against sodium chloride; keeping the marine
+proportions. Again we find that the magnesium and calcium salts
+are the most effective, although so much more dilute than the
+sodium salt.
+
+There is no visible clue to the explanation of these results. But
+we must conclude as most probable that some action is at work in
+the sea water and in the salt solutions which clumps or
+flocculates the sediment. For only by the gathering of the
+particles together in little aggregates can we explain their
+rapid fall to the bottom. It is not a question of viscosity
+(_i.e._ of resistance to the motion of the particles), for the
+salt solutions are rather more viscous than the fresh water.
+Still more remarkable is the fact that every dissolved substance
+will not bring about the result. Thus if we dissolve sugar in
+water we find that, if anything, the silt settles more slowly in
+the sugar solution than in fresh water.
+
+Now there is one effect produced by the solution of such salts as
+we have dealt with which is not produced by such bodies as sugar.
+The water is rendered a conductor of electricity. Long ago
+Faraday explained this as due to the presence of free atoms of
+the dissolved salt in the solution, carrying electric charges. We
+now speak of the salt as "ionised." That is it is partly split up
+into ions or free electrified atoms of chlorine, sodium,
+magnesium, etc., according to the particular salt in solution.
+This fact leads us to think that these electrified
+
+57
+
+atoms moving about in the solution may be the cause of the
+clumping or flocculation. Such electrified atoms are absent from
+the sugar solution: sugar does not become "ionised" when it is
+dissolved.
+
+The suspicion that the free electrified atoms play a part in the
+phenomenon is strengthened when we recall the remarkable
+difference in the action of sodium chloride and magnesium
+chloride. In each of the solutions of these substances there are
+free chlorine atoms each of which carries a single charge of
+negative electricity. As these atoms are alike in both solutions
+the different behaviour of the solutions cannot be due to the
+chlorine. But the metallic atom is very different in the two
+cases. The ionised sodium atom is known to be _monad_ or carries
+but _one_ positive charge; whereas the magnesium atom is _diad_ and
+carries _two_ positive charges. If, then, we assume that the
+metallic, positively electrified atom is in each case
+responsible, we have something to go on. It may be now stated
+that it has been found by experiment and supported by theory that
+the clumping power of an ion rises very rapidly with its valency;
+that is with the number of unit charges associated with it. Thus
+diads such as magnesium, calcium, barium, etc., are very much
+more efficient than monads such as sodium, potassium, etc., and
+again, triads such as aluminium are, similarly, very much more
+powerful than diad atoms. Here, in short, we have arrived at the
+active cause of the phenomenon. Its inner mechanism
+
+58
+
+is, however, harder to fathom. A plausible explanation can be
+offered, but a study of it would take us too far. Sufficient has
+been said to show the very subtile nature of the forces at work.
+
+We have here an effect due to the sea salts derived by denudation
+from the land which has been slowly augmenting during geological
+time. It is certain that the ocean was practically fresh water in
+remote ages. During those times the silt from the great rivers
+would have been carried very far from the land. A Mississippi of
+those ages would have sent its finer suspensions far abroad on a
+contemporary Gulf stream: not improbably right across the
+Atlantic. The earlier sediments of argillaceous type were not
+collected in the geosynclines and the genesis of the mountains
+was delayed proportionately. But it was, probably, not for very
+long that such conditions prevailed. For the accumulation of
+calcium salts must have been rapid, and although the great
+salinity due to sodium salts was of slow growth the salts of the
+diad element calcium must have soon introduced the cooperation of
+the ion in the work of building the mountain.
+
+59
+
+THE ABUNDANCE OF LIFE [1]
+
+WE had reached the Pass of Tre Croci[2]and from a point a little
+below the summit, looked eastward over the glorious Val Buona.
+The pines which clothed the floor and lower slopes of the valley,
+extended their multitudes into the furthest distance, among the
+many recesses of the mountains, and into the confluent Val di
+Misurina. In the sunshine the Alpine butterflies flitted from
+stone to stone. The ground at our feet and everywhere throughout
+the forests teamed with the countless millions of the small black
+ants.
+
+It was a magnificent display of vitality; of the aggressiveness
+of vitality, assailing the barren heights of the limestone,
+wringing a subsistence from dead things. And the question
+suggested itself with new force: why the abundance of life and
+its unending activity?
+
+In trying to answer this question, the present sketch
+originated.
+
+I propose to refer for an answer to dynamic considerations. It is
+apparent that natural selection can only be concerned in a
+secondary way. Natural selection defines
+
+[1] Proc. Roy. Dublin Soc., vol. vii., 1890.
+
+[2] In the Dolomites of Southeast Tyrol; during the summer of
+1890. Much of what follows was evolved in discussion with my
+fellow-traveller, Henry H. Dixon. Much of it is his.
+
+60
+
+a certain course of development for the organism; but very
+evidently some property of inherent progressiveness in the
+organism must be involved. The mineral is not affected by natural
+selection to enter on a course of continual variation and
+multiplication. The dynamic relations of the organism with the
+environment are evidently very different from those of inanimate
+nature.
+
+GENERAL DYNAMIC CONDITIONS ATTENDING INANIMATE ACTIONS
+
+It is necessary, in the first place, to refer briefly to the
+phenomena attending the transfer of energy within and into
+inanimate material systems. It is not assumed here that these
+phenomena are restricted in their sphere of action to inanimate
+nature. It is, in fact, very certain that they are not; but while
+they confer on dead nature its own dynamic tendencies, it will
+appear that their effects are by various means evaded in living
+nature. We, therefore, treat of them as characteristic of
+inanimate actions. We accept as fundamental to all the
+considerations which follow the truth of the principle of the
+Conservation of Energy.[1]
+
+[1] "The principle of the Conservation of Energy has acquired so
+much scientific weight during the last twenty years that no
+physiologist would feel any confidence in an experiment which
+showed a considerable difference between the work done by the
+animal and the balance of the account of Energy received and
+spent."--Clerk Maxwell, _Nature_, vol. xix., p. 142. See also
+Helmholtz _On the Conservation of Force._
+
+61
+
+Whatever speculations may be made as to the course of events very
+distant from us in space, it appears certain that dissipation of
+energy is at present actively progressing throughout our sphere
+of observation in inanimate nature. It follows, in fact, from the
+second law of thermodynamics, that whenever work is derived from
+heat, a certain quantity of heat falls in potential without doing
+work or, in short, is dissipated. On the other hand, work may be
+entirely converted into heat. The result is the heat-tendency of
+the universe. Heat, being an undirected form of energy, seeks, as
+it were, its own level, so that the result of this heat-tendency
+is continual approach to uniformity of potential.
+
+The heat-tendency of the universe is also revealed in the
+far-reaching "law of maximum work," which defines that chemical
+change, accomplished without the intervention of external energy,
+tends to the production of the body, or system of bodies, which
+disengage the greatest quantity of heat.[1] And, again, vast
+numbers of actions going on throughout nature are attended by
+dissipatory thermal effects, as those arising from the motions of
+proximate molecules (friction, viscosity), and from the fall of
+electrical potential.
+
+Thus, on all sides, the energy which was once most probably
+existent in the form of gravitational potential, is being
+dissipated into unavailable forms. We must
+
+[1] Berthelot, _Essai de Mecanique Chimique._
+
+62
+
+recognize dissipation as an inevitable attendant on inanimate
+transfer of energy.
+
+But when we come to consider inanimate actions in relation to
+time, or time-rate of change, we find a new feature in the
+phenomena attending transfer of energy; a feature which is really
+involved in general statements as to the laws of physical
+interactions.[1] It is seen, that the attitude of inanimate
+material systems is very generally, if not in all cases,
+retardative of change--opposing it by effects generated by the
+primary action, which may be called "secondary" for convenience.
+Further, it will be seen that these secondary effects are those
+concerned in bringing about the inevitable dissipation.
+
+As example, let us endeavour to transfer gravitational potential
+energy contained in a mass raised above the surface of the Earth
+into an elastic body, which we can put into compression by
+resting the weight upon it. In this way work is done against
+elastic force and stored as elastic potential energy. We may deal
+with a metal spring, or with a mass of gas contained in a
+cylinder fitted with a piston upon which the weight may be
+placed. In either case we find the effect of compression is to
+raise the temperature of the substance, thus causing its
+
+[1] Helmholtz, _Ice and Glaciers._ Atkinson's collection of his
+Popular Lectures. First Series, p.120. Quoted by Tate, _Heat_,
+p. 311.
+
+63
+
+expansion or increased resistance to the descent of the weight.
+And this resistance continues, with diminishing intensity, till
+all the heat generated is dissipated into the surrounding medium.
+The secondary effect thus delays the final transfer of energy.
+
+Again, if we suppose the gas in the cylinder replaced by a vapour
+in a state of saturation, the effect of increased pressure, as of
+a weight placed upon the piston, is to reduce the vapour to a
+liquid, thereby bringing about a great diminution of volume and
+proportional loss of gravitational potential by the weight. But
+this change will by no means be brought about instantaneously.
+When a little of the vapour is condensed, this portion parts with
+latent heat of vaporisation, increasing the tension of the
+remainder, or raising its point of saturation, so that before the
+weight descends any further, this heat has to escape from the
+cylinder.
+
+Many more such cases might be cited. The heating of india-rubber
+when expanded, its cooling when compressed, is a remarkable one;
+for at first sight it appears as if this must render it
+exceptional to the general law, most substances exhibiting the
+opposite thermal effects when stressed. However, here, too, the
+action of the stress is opposed by the secondary effects
+developed in the substance; for it is found that this substance
+contracts when heated, expands when cooled. Again, ice being a
+substance which contracts in melting, the effect of pressure is
+to facilitate melting, lowering its freezing point. But
+
+64
+
+so soon as a little melting occurs, the resulting liquid calls on
+the residual ice for an amount of heat equivalent to the latent
+heat of liquefaction, and so by cooling the whole, retards the
+change.
+
+Such particular cases illustrate a principle controlling the
+interaction of matter and energy which seems universal in
+application save when evaded, as we shall see, by the ingenuity
+of life. This principle is not only revealed in the researches of
+the laboratory; it is manifest in the history of worlds and solar
+systems. Thus, consider the effects arising from the aggregation
+of matter in space under the influence of the mutual attraction
+of the particles. The tendency here is loss of gravitational
+potential. The final approach is however retarded by the
+temperature, or vis viva of the parts attending collision and
+compression. From this cause the great suns of space radiate for
+ages before the final loss of potential is attained.
+
+Clerk Maxwell[1] observes on the general principle that less
+force is required to produce a change in a body when the change
+is unopposed by constraints than when it is subjected to such.
+From this if we assume the external forces acting upon a system
+not to rise above a certain potential (which is the order of
+nature), the constraints of secondary actions may, under certain
+circumstances, lead to final rejection of some of the energy, or,
+in any
+
+[1] _Theory of Heat_, p. 131.
+
+65
+
+case, to retardation of change in the system--dissipation of
+energy being the result.[1]
+
+As such constraints seem inherently present in the properties of
+matter, we may summarise as follows:
+
+_The transfer of energy into any inanimate material system is
+attended by effects retardative to the transfer and conducive to
+dissipation._
+
+Was this the only possible dynamic order ruling in material
+systems it is quite certain the myriads of ants and pines never
+could have been, except all generated by creative act at vast
+primary expenditure of energy. Growth and reproduction would have
+been impossible in systems which retarded change at every step
+and never proceeded in any direction but in that of dissipation.
+Once created, indeed, it is conceivable that, as heat engines,
+they might have dragged out an existence of alternate life and
+death; life in the hours of sunshine, death in hours of darkness:
+no final death, however, their lot, till their parts were simply
+worn out by long use, never made good by repair. But the
+sustained and increasing activity of organized nature is a fact;
+therefore some other order of events must be possible.
+
+[1] The law of Least Action, which has been applied, not alone in
+optics, but in many mechanical systems, appears physically based
+upon the restraint and retardation opposing the transfer of
+energy in material systems.
+
+66
+
+GENERAL DYNAMIC CONDITIONS ATTENDING ANIMATE ACTIONS
+
+What is the actual dynamic attitude of the primary organic
+engine--the vegetable organism? We consider, here, in the first
+place, not intervening, but resulting phenomena.
+
+The young leaf exposed to solar radiation is small at first, and
+the quantity of radiant energy it receives in unit of time cannot
+exceed that which falls upon its surface. But what is the effect
+of this energy? Not to produce a retardative reaction, but an
+accelerative response: for, in the enlarging of the leaf by
+growth, the plant opens for itself new channels of supply.
+
+If we refer to "the living protoplasm which, with its unknown
+molecular arrangement, is the only absolute test of the cell and
+of the organism in general,[1] we find a similar attitude towards
+external sources of available energy. In the act of growth
+increased rate of assimilation is involved, so that there is an
+acceleration of change till a bulk of maximum activity is
+attained. The surface, finally, becomes too small for the
+absorption of energy adequate to sustain further increase of mass
+(Spencer[2]), and the acceleration ceases. The waste going on in
+the central parts is then just balanced by the renewal at the
+surface. By division, by spreading of the mass, by
+
+[1] Claus, _Zoology_, p. 13.
+
+[2] Geddes and Thomson, _The Evolution of Sex_, p. 220.
+
+67
+
+out-flowing processes, the normal activity of growth may be
+restored. Till this moment nothing would be gained by any of
+these changes. One or other of them is now conducive to
+progressive absorption of energy by the organism, and one or
+other occurs, most generally the best of them, subdivision. Two
+units now exist; the total mass immediately on division is
+unaltered, but paths for the more abundant absorption of energy
+are laid open.
+
+The encystment of the protoplasm (occurring under conditions upon
+which naturalists do not seem agreed[1]) is to all appearance
+protective from an unfavourable environment, but it is often a
+period of internal change as well, resulting in a segregation
+within the mass of numerous small units, followed by a breakup of
+the whole into these units. It is thus an extension of the basis
+of supply, and in an impoverished medium, where unit of surface
+is less active, is evidently the best means of preserving a
+condition of progress.
+
+Thus, in the organism which forms the basis of all modes of life,
+a definite law of action is obeyed under various circumstances of
+reaction with the available energy of its environment.
+
+Similarly, in the case of the more complex leaf, we see, not only
+in the phenomenon of growth, but in its extension in a flattened
+form, and in the orientation of greatest surface towards the
+source of energy, an attitude towards
+
+[1] However, "In no way comparable with death." Weismann,
+_Biological Memoirs_, p. 158.
+
+68
+
+available energy causative of accelerated transfer. There is
+seemingly a principle at work, leading to the increase of organic
+activity.
+
+Many other examples might be adduced. The gastrula stage in the
+development of embryos, where by invagination such an arrangement
+of the multiplying cells is secured as to offer the greatest
+possible surface consistent with a first division of labour; the
+provision of cilia for drawing upon the energy supplies of the
+medium; and more generally the specialisation of organs in the
+higher developments of life, may alike be regarded as efforts of
+the organism directed to the absorption of energy. When any
+particular organ becomes unavailing in the obtainment of
+supplies, the organ in the course of time becomes aborted or
+disappears.[1] On the other hand, when a too ready and liberal
+supply renders exertion and specialisation unnecessary, a similar
+abortion of functionless organs takes place. This is seen in the
+degraded members of certain parasites.
+
+During certain epochs of geological history, the vegetable world
+developed enormously; in response probably to liberal supplies of
+carbon dioxide. A structural adaptation to the rich atmosphere
+occurred, such as was calculated to cooperate in rapidly
+consuming the supplies, and to this obedience to a law of
+progressive transfer of energy we owe the vast stores of energy
+now accumulated
+
+[1] Claus, _Zoology_, p. 157
+
+69
+
+in our coal fields. And when, further, we reflect that this store
+of energy had long since been dissipated into space but for the
+intervention of the organism, we see definitely another factor in
+organic transfer of energy--a factor acting conservatively of
+energy, or antagonistically to dissipation.
+
+The tendency of organized nature in the presence of unlimited
+supplies is to "run riot." This seems so universal a relation,
+that we are safe in seeing here cause and effect, and in drawing
+our conclusions as to the attitude of the organism towards
+available energy. New species, when they come on the field of
+geological history, armed with fresh adaptations, irresistible
+till the slow defences of the subjected organisms are completed,
+attain enormous sizes under the stimulus of abundant supply, till
+finally, the environment, living and dead, reacts upon them with
+restraining influence. The exuberance of the organism in presence
+of energy is often so abundant as to lead by deprivation to its
+self-destruction. Thus the growth of bacteria is often controlled
+by their own waste products. A moment's consideration shows that
+such progressive activity denotes an accelerative attitude on the
+part of the organism towards the transfer of energy into the
+organic material system. Finally, we are conscious in ourselves
+how, by use, our faculties are developed; and it is apparent that
+all such progressive developments must rest on actions which
+respond to supplies with fresh demands. Possibly in the present
+and ever-
+
+70
+
+increasing consumption of inanimate power by civilised races, we
+see revealed the dynamic attitude of the organism working through
+thought-processes.
+
+Whether this be so or not, we find generally in organised nature
+causes at work which in some way lead to a progressive transfer
+of energy into the organic system. And we notice, too, that all
+is not spent, but both immediately in the growth of the
+individual, and ultimately in the multiplication of the species,
+there are actions associated with vitality which retard the
+dissipation of energy. We proceed to state the dynamical
+principles involved in these manifestations, which appear
+characteristic of the organism, as follows:--
+
+_The transfer of energy into any animate material system is
+attended by effects conducive to the transfer, and retardative of
+dissipation._
+
+This statement is, I think, perfectly general. It has been in
+part advanced before, but from the organic more than the physical
+point of view. Thus, "hunger is an essential characteristic of
+living matter"; and again, "hunger is a dominant characteristic
+of living matter,"[1] are, in part, expressions of the statement.
+If it be objected against the generality of the statement, that
+there are periods in the life of individuals when stagnation and
+decay make their appearance, we may answer, that
+
+[1] _Evolution of Sex._ Geddes and Thomson, chap. xvi. See also a
+reference to Cope's theory of "Growth Force," in Wallace's
+_Darwinism_, p. 425.
+
+71
+
+such phenomena arise in phases of life developed under conditions
+of external constraint, as will be urged more fully further on,
+and that in fact the special conditions of old age do not and
+cannot express the true law and tendency of the dynamic relations
+of life in the face of its evident advance upon the Earth. The
+law of the unconstrained cell is growth on an ever increasing
+scale; and although we assume the organic configuration, whether
+somatic or reproductive, to be essentially unstable, so that
+continual inflow of energy is required merely to keep it in
+existence, this does not vitiate the fact that, when free of all
+external constraint, growth gains on waste. Indeed, even in the
+case of old age, the statement remains essentially true, for the
+phenomena then displayed point to a breakdown of the functioning
+power of the cell, an approximation to configurations incapable
+of assimilation. It is not as if life showed in these phenomena
+that its conditions could obtain in the midst of abundance, and
+yet its law be suspended; but as if they represented a
+degradation of the very conditions of life, a break up, under the
+laws of the inanimate, of the animate contrivance; so that energy
+is no longer available to it, or the primary condition, "the
+transfer of energy into the animate system," is imperfectly
+obeyed. It is to the perfect contrivance of life our statement
+refers.
+
+That the final end of all will be general non-availability there
+seems little reason to doubt, and the organism, itself dependent
+upon differences of potential, cannot
+
+72
+
+hope to carry on aggregation of energy beyond the period when
+differences of potential are not. The organism is not accountable
+for this. It is being affected by events external to it, by the
+actions going on through inanimate agents. And although there be
+only a part of the received energy preserved, there is a part
+preserved, and this amount is continually on the increase. To see
+this it is only necessary to reflect that the sum of animate
+energy--capability of doing work in any way through animate
+means--at present upon the Earth, is the result, although a small
+one, of energy reaching the Earth since a remote period, and
+which otherwise had been dissipated in space. In inanimate
+actions throughout nature, as we know it, the availability is
+continually diminishing. The change is all the one way. As,
+however, the supply of available energy in the universe is
+(probably) limited in amount, we must look upon the two as simply
+effecting the final dissipation of potential in very different
+ways. The animate system is aggressive on the energy available to
+it, spends with economy, and invests at interest till death
+finally deprives it of all. It has heirs, indeed, who inherit
+some of its gains, but they, too, must die, and ultimately there
+will be no successors, and the greater part must melt away as if
+it had never been. The inanimate system responds to the forces
+imposed upon it by sluggish changes; of that which is thrust upon
+it, it squanders uselessly. The path of the energy is very
+different in the two cases.
+
+73
+
+While it is true generally that both systems ultimately result in
+the dissipation of energy to uniform potential, the organism can,
+as we have seen, under particular circumstances evade the final
+doom altogether. It can lay up a store of potential energy which
+may be permanent. Thus, so long as there is free oxygen in the
+universe, our coalfields might, at any time in the remote future,
+generate light and heat in the universal grave.
+
+It is necessary to observe on the fundamental distinction between
+the growth of the protoplasm and the growth of the crystal. It is
+common to draw comparison between the two, and to point to
+metabolism as the chief distinction. But while this is the most
+obvious distinction the more fundamental one remains in the
+energy relations of the two with the environment.[1] The growth
+of the crystal is the result of loss of energy; that of the
+organism the result of gain of energy. The crystal represents a
+last position of stable equilibrium assumed by molecules upon a
+certain loss of kinetic energy, and the formation of the crystal
+by evaporation and concentration of a liquid does not, in its
+dynamic aspect, differ much from the precipitation of an
+amorphous sediment. The organism, on the other hand, represents a
+more or less unstable condition formed and maintained by inflow
+of energy; its formation, indeed, often attended with a loss of
+kinetic energy (fixation of carbon in plants), but, if so,
+accompanied by
+
+[1] It appears exceptional for the crystal line configuration to
+stand higher in the scale of energy than the amorphous.
+
+74
+
+a more than compensatory increase of potential molecular energy.
+
+Thus, between growth in the living world and growth in the dead
+world, the energy relations with the environment reveal a marked
+contrast. Again, in the phenomena of combustion, there are
+certain superficial resemblances which have led to comparison
+between the two. Here again, however, the attitudes towards the
+energy of the environment stand very much as + and -. The life
+absorbs, stores, and spends with economy. The flame only
+recklessly spends. The property of storage by the organism calls
+out a further distinction between the course of the two
+processes. It secures that the chemical activity of the organism
+can be propagated in a medium in which the supply of energy is
+discontinuous or localised. The chemical activity of the
+combustion can, strictly speaking, only be propagated among
+contiguous particles. I need not dwell on the latter fact; an
+example of the former is seen in the action of the roots of
+plants, which will often traverse a barren place or circumvent an
+obstacle in their search for energy. In this manner roots will
+find out spots of rich nutriment.
+
+Thus there is a dynamic distinction between the progress of the
+organism and the progress of the combustion, or of the chemical
+reaction generally. And although there be unstable chemical
+systems which absorb energy during reaction, these are
+(dynamically) no more than the expansion of the compressed gas.
+There is a certain
+
+75
+
+initial capacity in the system for a given quantity of energy;
+this satisfied, progress ceases. The progress of the organism in
+time is continual, and goes on from less to greater so long as
+its development is unconstrained and the supply of energy is
+unlimited.
+
+We must regard the organism as a configuration which is so
+contrived as to evade the tendency of the universal laws of
+nature. Except we are prepared to believe that a violation of the
+second law of thermodynamics occurs in the organism, that a
+"sorting demon" is at work within it, we must, I think, assume
+that the interactions going on among its molecules are
+accompanied by retardation and dissipation like the rest of
+nature. That such conditions are not incompatible with the
+definition of the dynamic attitude of the organism, can be shown
+by analogy with our inanimate machines which, by aid of
+hypotheses in keeping with the second law of thermodynamics, may
+be supposed to fulfil the energy-functions of the plant or
+animal, and, in fact, in all apparent respects conform to the
+definition of the organism.
+
+We may assume this accomplished by a contrivance of the nature of
+a steam-engine, driven by solar energy. It has a boiler, which we
+may suppose fed by the action of the engine. It has piston,
+cranks, and other movable parts, all subject to resistance from
+friction, etc. Now there is no reason why this engine should not
+expend its surplus energy in shaping, fitting, and starting into
+action other engines:--in fact, in reproductive sacrifice. All
+
+76
+
+these other engines represent a multiplied absorption of energy
+as the effects of the energy received by the parent engine, and
+may in time be supposed to reproduce themselves. Further, we may
+suppose the parent engine to be small and capable of developing
+very little power, but the whole series as increasing in power at
+each generation. Thus the primary energy relations of the
+vegetable organism are represented in these engines, and no
+violation of the second law of thermodynamics involved.
+
+We might extend the analogy, and assuming these engines to spend
+a portion of their surplus energy in doing work against chemical
+forces--as, for example, by decomposing water through the
+intervention of a dynamo--suppose them to lay up in this way a
+store of potential energy capable of heating the boilers of a
+second order of engines, representing the graminivorous animal.
+It is obvious without proceeding to a tertiary or carnivorous
+order, that the condition of energy in the animal world may be
+supposed fulfilled in these successive series of engines, and no
+violation of the principles governing the actions going on in our
+machines assumed. Organisms evolving on similar principles would
+experience loss at every transfer. Thus only a portion of the
+radiant energy absorbed by the leaf would be expended in actual
+work, chemical and gravitational, etc. It is very certain that
+this is, in fact, what takes place.
+
+It is, perhaps, worth passing observation that, from the
+nutritive dependence of the animal upon the vegetable,
+
+77
+
+and the fact that a conversion of the energy of the one to the
+purposes of the other cannot occur without loss, the mean energy
+absorbed daily by the vegetable for the purpose of growth must
+greatly exceed that used in animal growth; so that the chemical
+potential energy of vegetation upon the earth is much greater
+than the energy of all kinds represented in the animal
+configurations.[1] It appears, too, that in the power possessed
+by the vegetable of remaining comparatively inactive, of
+surviving hard times by the expenditure and absorption of but
+little, the vegetable constitutes a veritable reservoir for the
+uniform supply of the more unstable and active animal.
+
+Finally, on the question of the manner of origin of organic
+systems, it is to be observed that, while the life of the present
+is very surely the survival of the fittest of the tendencies and
+chances of the past, yet, in the initiation of the organised
+world, a single chance may have decided a whole course of events:
+for, once originated, its own law secures its increase, although
+within the new order of actions, the law of the fittest must
+assert itself. That such a progressive material system as an
+organism was possible, and at some remote period was initiated,
+is matter of knowledge; whether or not the initiatory living
+configuration was rare and fortuitous, or the probable result of
+the general action of physical laws acting among innumerable
+chances, must remain matter of
+
+[1] I find a similar conclusion arrived at in Semper's _Animal
+Life_, p. 52.
+
+78
+
+speculation. In the event of the former being the truth, it is
+evidently possible, in spite of a large finite number of
+habitable worlds, that life is non-existent elsewhere. If the
+latter is the truth, it is almost certain that there is life in
+all, or many of those worlds.
+
+EVOLUTION AND ACCELERATION OF ACTIVITY
+
+The primary factor in evolution is the "struggle for existence."
+This involves a "natural selection" among the many variations of
+the organism. If we seek the underlying causes of the struggle,
+we find that the necessity of food and (in a lesser degree) the
+desire for a mate are the principal causes of contention. The
+former is much the more important factor, and, accordingly, we
+find the greater degree of specialisation based upon it.
+
+The present view assumes a dynamic necessity for its demands
+involved in the nature of the organism as such. This assumption
+is based on observation of the outcome of its unconstrained
+growth, reproduction, and life-acts. We have the same right to
+assert this of the organism as we have to assert that retardation
+and degradation attend the actions of inanimate machines, which
+assertion, also, is based on observation of results. Thus we pass
+from the superficial statements that organisms require food in
+order to live, or that organisms desire food, to the more
+fundamental one that:
+
+_The organism is a configuration of matter which absorbs energy
+acceleratively, without limit, when unconstrained._
+
+79
+
+This is the dynamic basis for a "struggle for existence." The
+organism being a material system responding to accession of
+energy with fresh demands, and energy being limited in amount,
+the struggle follows as a necessity. Thus, evolution guiding' the
+steps of the energy-seeking organism, must presuppose and find
+its origin in that inherent property of the organism which
+determines its attitude in presence of available energy.
+
+Turning to the factor, "adaptation," we find that this also must
+presuppose, in order to be explicable, some quality of
+aggressiveness on the part of the organism. For adaptation in
+this or that direction is the result of repulse or victory, and,
+therefore, we must presuppose an attack. The attack is made by
+the organism in obedience to its law of demand; we see in the
+adaptation of the organism but the accumulated wisdom derived
+from past defeats and victories.
+
+Where the environment is active, that is living, adaptation
+occurs on both sides. Improved means of defence or improved means
+of attack, both presuppose activity. Thus the reactions to the
+environment, animate and inanimate, are at once the outcome of
+the eternal aggressiveness of the organism, and the source of
+fresh aggressiveness upon the resources of the medium.
+
+As concerns the "survival of the fittest" (or "natural
+selection"), we can, I think, at once conclude that the organism
+which best fulfils the organic law under the circumstances of
+supply is the "fittest," _ipso facto._ In many
+
+80
+
+cases this is contained in the commonsense consideration, that to
+be strong, consistent with concealment from enemies which are
+stronger, is best, as giving the organism mastery over foes which
+are weaker, and generally renders it better able to secure
+supplies. Weismann points out that natural selection favours
+early and abundant reproduction. But whether the qualifications
+of the "fittest" be strength, fertility, cunning, fleetness,
+imitation, or concealment, we are safe in concluding that growth
+and reproduction must be the primary qualities which at once
+determine selection and are fostered by it. Inherent in the
+nature of the organism is accelerated absorption of energy, but
+the qualifications of the "fittest" are various, for the supply
+of energy is limited, and there are many competitors for it. To
+secure that none be wasted is ultimately the object of natural
+selection, deciding among the eager competitors what is best for
+each.
+
+In short, the facts and generalisations concerning evolution must
+presuppose an organism endowed with the quality of progressive
+absorption of energy, and retentive of it. The continuity of
+organic activity in a world where supplies are intermittent is
+evidently only possible upon the latter condition. Thus it
+appears that the dynamic attitude of the organism, considered in
+these pages, occupies a fundamental position regarding its
+evolution.
+
+We turn to the consideration of old age and death, endeavouring
+to discover in what relation they stand to the innate
+progressiveness of the organism.
+
+81
+
+THE PERIODICITY OF THE ORGANISM AND THE LAW OF PROGRESSIVE
+ACTIVITY
+
+The organic system is essentially unstable. Its aggressive
+attitude is involved in the phenomenon of growth, and in
+reproduction which is a form of growth. But the energy absorbed
+is not only spent in growth. It partly goes, also, to make good
+the decay which arises from the instability of the organic unit.
+The cell is molecularly perishable. It possesses its entity much
+as a top keeps erect, by the continual inflow of energy.
+Metabolism is always taking place within it. Any other condition
+would, probably, involve the difficulties of perpetual motion.
+
+The phenomenon of old age is not evident in the case of the
+unicellular organism reproducing by fission. At any stage of its
+history all the individuals are of the same age: all contain a
+like portion of the original cell, so far as this can be regarded
+as persisting where there is continual flux of matter and energy.
+In the higher organisms death is universally evident. Why is
+this?
+
+The question is one of great complexity. Considered from the more
+fundamental molecular point of view we should perhaps look to
+failure of the power of cell division as the condition of
+mortality. For it is to this phenomenon--that of cell
+division--that the continued life of the protozoon is to be
+ascribed, as we have already seen. Reproduction is, in fact, the
+saving factor here.
+
+As we do not know the source or nature of the stimulus
+
+82
+
+responsible for cell division we cannot give a molecular account
+of death in the higher organisms. However we shall now see that,
+philosophically, we are entitled to consider reproduction as a
+saving factor in this case also; and to regard the death of the
+individual much as we regard the fall of the leaf from the tree:
+_i.e._ as the cessation of an outgrowth from a development
+extending from the past into the future. The phenomena of old age
+and natural death are, in short, not at variance with the
+progressive activity of the organism. We perceive this when we
+come to consider death from the evolutionary point of view.
+
+Professor Weismann, in his two essays, "The Duration of Life,"
+and "Life and Death,"[1] adopts and defends the view that "death
+is not a primary necessity but that it has been secondarily
+acquired by adaptation." The cell was not inherently limited in
+its number of cell-generations. The low unicellular organisms are
+potentially immortal, the higher multicellular forms with
+well-differentiated organs contain the germs of death within
+themselves.
+
+He finds the necessity of death in its utility to the species.
+Long life is a useless luxury. Early and abundant reproduction is
+best for the species. An immortal individual would gradually
+become injured and would be valueless or even harmful to the
+species by taking the place of those that are sound. Hence
+natural selection will shorten life.
+
+[1] See his _Biological Memoirs._ Oxford, 1888.
+
+83
+
+Weismann contends against the transmission of acquired characters
+as being unproved.[1] He bases the appearance of death on
+variations in the reproductive cells, encouraged by the ceaseless
+action of natural selection, which led to a differentiation into
+perishable somatic cells and immortal reproductive cells. The
+time-limit of any particular organism ultimately depends upon the
+number of somatic cell-generations and the duration of each
+generation. These quantities are "predestined in the germ itself"
+which gives rise to each individual. "The existence of immortal
+metazoan organisms is conceivable," but their capacity for
+existence is influenced by conditions of the external world; this
+renders necessary the process of adaptation. In fact, in the
+differentiation of somatic from reproductive cells, material was
+provided upon which natural selection could operate to shorten or
+to lengthen the life of the individual in accordance with the
+needs of the species. The soma is in a sense "a secondary
+appendage of the real bearer of life--the reproductive cells." The
+somatic cells probably lost their immortal qualities, on this
+immortality becoming useless to the species. Their mortality may
+have been a mere consequence of their differentiation (loc. cit.,
+p. 140), itself due to natural selection. "Natural death was
+not," in fact, "introduced from absolute intrinsic necessity
+inherent in the nature of living matter, but on grounds of
+utility,
+
+[1] Biological Memoirs, p. 142.
+
+84
+
+that is from necessities which sprang up, not from the general
+conditions of life, but from those special conditions which
+dominate the life of multicellular organisms."
+
+On the inherent immortality of life, Weismann finally states:
+"Reproduction is, in truth, an essential attribute of living
+matter, just as the growth which gives rise to it.... Life is
+continuous, and not periodically interrupted: ever since its
+first appearance upon the Earth in the lowest organism, it has
+continued without break; the forms in which it is manifest have
+alone undergone change. Every individual alive today--even the
+highest--is to be derived in an unbroken line from the first and
+lowest forms." [1]
+
+At the present day the view is very prevalent that the soma of
+higher organisms is, in a sense, but the carrier for a period of
+the immortal reproductive cells (Ray Lankester)[2]--an appendage
+due to adaptation, concerned in their supply, protection, and
+transmission. And whether we regard the time-limit of its
+functions as due to external constraints, recurrently acting till
+their effects become hereditary, or to variations more directly
+of internal origin, encouraged by natural selection, we see in
+old age and death phenomena ultimately brought about in obedience
+to the action of an environment. These are not inherent in the
+properties of living matter. But, in spite
+
+[1] Loc. cit., p. 159
+
+[2] Geddes and Thomson, The Evolution of Sex, chap. xviii.
+
+85
+
+of its mortality, the body remains a striking manifestation of
+the progressiveness of the organism, for to this it must be
+ascribed. To it energy is available which is denied to the
+protozoon. Ingenious adaptations to environment are more
+especially its privilege. A higher manifestation, however, was
+possible, and was found in the development of mind. This, too, is
+a servant of the cell, as the genii of the lamp. Through it
+energy is available which is denied to the body. This is the
+masterpiece of the cell. Its activity dates, as it were, but from
+yesterday, and today it inherits the most diverse energies of the
+Earth.
+
+Taking this view of organic succession, we may liken the
+individual to a particle vibrating for a moment and then coming
+to rest, but sweeping out in its motion one wave in the
+continuous organic vibration travelling from the past into the
+future. But as this vibration is one spreading with increased
+energy from each vibrating particle, its propagation involves a
+continual accelerated inflow of energy from the surrounding
+medium, a dynamic condition unknown in periodic effects
+transmitted by inanimate actions, and, indeed, marking the
+fundamental difference between the dynamic attitudes of the
+animate and inanimate.
+
+We can trace the periodic succession of individuals on a diagram
+of activity with some advantage. Considering, first, the case of
+the unicellular organism reproducing by subdivision and recalling
+that conditions, definite and inevitable, oppose a limit to the
+rate of growth, or, for our
+
+86
+
+present purpose, rate of consumption of energy, we proceed as
+follows:
+
+{Fig. 1}
+
+Along a horizontal axis units of time are measured; along a
+vertical axis units of energy. Then the life-history of the
+amoeba, for example, appears as a line such as A in Fig. 1.
+During the earlier stages of its growth the rate of absorption of
+energy is small; so that in the unit interval of time, t, the
+small quantity of energy, e1, is absorbed. As life advances, the
+activity of the organism augments, till finally this rate attains
+a maximum, when e2 units of energy are consumed in the unit of
+time.[1]
+
+[1] Reference to p. 76, where the organic system is treated as
+purely mechanical, may help readers to understand what is
+involved in this curve. The solar engine may, unquestionably,
+have its activity defined by such a curve. The organism is,
+indeed, more complex; but neither this fact nor our ignorance of
+its mechanism, affects the principles which justify the diagram.
+
+87
+
+On this diagram reproduction, on the part of the organism, is
+represented by a line which repeats the curvature of the parent
+organism originating at such a point as P in the path of the
+latter, when the rate of consumption of energy has become
+constant. The organism A has now ceased to act as a unit. The
+products of fission each carry on the vital development of
+
+{Fig. 2}
+
+the species along the curve B, which may be numbered (2), to
+signify that it represents the activity of two individuals, and
+so on, the numbering advancing in geometrical progression. The
+particular curvature adopted in the diagram is, of course,
+imaginary; but it is not of an indeterminate nature. Its course
+for any species is a characteristic of fundamental physical
+importance, regarding the part played in nature by the particular
+organism.
+
+88
+
+In Fig. 2 is represented the path of a primitive multicellular
+organism before the effects of competition produced or fostered
+its mortality. The lettering of Fig. 1 applies; the successive
+reproductive acts are marked P1, P2; Q1, Q2, etc., in the paths
+of the successive individuals.
+
+{Fig. 3}
+
+The next figure (Fig. 3) diagrammatically illustrates death in
+organic history. The path ever turns more and more from the axis
+of energy, till at length the point is reached when no more
+energy is available; a tangent to the curve at this point is at
+right angles to the axis of energy and parallel to the time axis.
+The death point is reached, and however great a length we measure
+along the axis of time, no further consumption of energy is
+
+89
+
+indicated by the path of the organism. Drawing the line beyond
+the death point is meaningless for our present purpose.
+
+It is observable that while the progress of animate nature finds
+its representation on this diagram by lines sloping _upwards_ from
+left to right, the course of events in inanimate nature--for
+example, the history of the organic configuration after death, or
+
+{Fig. 4}
+
+the changes progressing--let us say, in the solar system, or in
+the process of a crystallisation, would appear as lines sloping
+downwards from left to right.
+
+Whatever our views on the origin of death may be, we have to
+recognise a periodicity of functions in the life-history of the
+successive individuals of the present day; and whether or not we
+trace this directly or indirectly to
+
+90
+
+a sort of interference with the rising wave of life, imposed by
+the activity of a series of derived units, each seeking energy,
+and in virtue of its adaptation each being more fitted to obtain
+it than its predecessor, or even leave the idea of interference
+out of account altogether in the origination or perpetuation of
+death, the truth of the diagram (Fig. 4) holds in so far as it
+may be supposed to graphically represent the dynamic history of
+the individual. The point chosen on the curve for the origination
+of a derived unit is only applicable to certain organisms, many
+reproducing at the very close of life. A chain of units are
+supposed here represented.[1]
+
+THE LENGTH OF LIFE
+
+If we lay out waves as above to a common scale of time for
+different species, the difference of longevity is shown in the
+greater or less number of vibrations executed in a given time,
+_i.e._ in greater or less "frequency." We cannot indeed draw the
+curvature correctly, for this would necessitate a knowledge which
+we have not of the activity of the organism at different periods
+of its life-history, and so neither can we plot the direction of
+the organic line of propagation with respect to the
+
+[1] Projecting upon the axes of time and energy any one complete
+vibration, as in Fig. 4, the total energy consumed by the
+organism during life is the length E on the axis of energy, and
+its period of life is the length T on the time-axis. The mean
+activity is the quotient E/T.
+
+91
+
+axes of reference as this involves a knowledge of the mean
+activity.[1]
+
+The group of curves which follow, relating to typical animals
+possessing very different activities (Fig. 5), are therefore
+entirely diagrammatic, except in respect to the approximate
+
+{Fig. 5}
+
+longevity of the organisms. (1) might represent an animal of the
+length of life and of the activity of Man; (2), on the same scale
+of longevity,
+
+[1] In the relative food-supply at various periods of life the
+curvature is approximately determinable.
+
+92
+
+one of the smaller mammals; and (3), the life-history of a cold
+blooded animal living to a great age; _e.g._ certain of the
+reptilia.
+
+It is probable, that to conditions of structural development,
+under the influence of natural selection, the question of longer
+or shorter life is in a great degree referable. Thus, development
+along lines of large growth will tend to a slow rate of
+reproduction from the simple fact that unlimited energy to supply
+abundant reproduction is not procurable, whatever we may assume
+as to the strength or cunning exerted by the individual in its
+efforts to obtain its supplies. On the other hand, development
+along lines of small growth, in that reproduction is less costly,
+will probably lead to increased rate of reproduction. It is, in
+fact, matter of general observation that in the case of larger
+animals the rate of reproduction is generally slower than in the
+case of smaller animals. But the rate of reproduction might be
+expected to have an important influence in determining the
+particular periodicity of the organism. Were we to depict in the
+last diagram, on the same time-scale as Man, the vibrations of
+the smaller and shorter-lived living things, we would see but a
+straight line, save for secular variations in activity,
+representing the progress of the species in time: the tiny
+thrills of its units lost in comparison with the yet brief period
+of Man.
+
+The interdependence of the rate of reproduction and
+
+93
+
+the duration of the individual is, indeed, very probably revealed
+in the fact that short-lived animals most generally reproduce
+themselves rapidly and in great abundance, and vice versa. In
+many cases where this appears contradicted, it will be found that
+the young are exposed to such dangers that but few survive (_e.g._
+many of the reptilia, etc.), and so the rate of reproduction is
+actually slow.
+
+Death through the periodic rigour of the inanimate environment
+calls forth phenomena very different from death introduced or
+favoured by competition. A multiplicity of effects simulative of
+death occur. Organisms will, for example, learn to meet very
+rigorous conditions if slowly introduced, and not permanent. A
+transitory period of want can be tided over by contrivance. The
+lily withdrawing its vital forces into the bulb, protected from
+the greatest extremity of rigour by seclusion in the Earth; the
+trance of the hibernating animal; are instances of such
+contrivances.
+
+But there are organisms whose life-wave truly takes up the
+periodicity of the Earth in its orbit. Thus the smaller animals
+and plants, possessing less resources in themselves, die at the
+approach of winter, propagating themselves by units which,
+whether egg or seed, undergo a period of quiescence during the
+season of want. In these quiescent units the energy of the
+organism is potential, and the time-energy function is in
+abeyance. This condition is, perhaps, foreshadowed in the
+encyst-
+
+94
+
+ment of the amoeba in resistance to drought. In most cases of
+hibernation the time-energy function seems maintained at a loss
+of potential by the organism, a diminished vital consumption of
+energy being carried on at the expense of the stored energy of
+the tissues. So, too, even among the largest organisms there will
+be a diminution of activity periodically inspired by
+climatological conditions. Thus, wholly or in part, the activity
+of organisms is recurrently affected by the great energy--tides
+set up by the Earth's orbital motion.
+
+{Fig. 6}
+
+Similarly in the phenomenon of sleep the organism responds to the
+Earth's axial periodicity, for in the interval of night a period
+of impoverishment has to be endured. Thus the diurnal waves of
+energy also meet a response in the organism. These tides and
+waves of activity would appear as larger and smaller ripples
+
+95
+
+on the life-curve of the organism. But in some, in which life and
+death are encompassed in a day, this would not be so; and for the
+annual among plants, the seed rest divides the waves with lines
+of no activity (Fig. 6).
+
+Thus, finally, we regard the organism as a dynamic phenomenon
+passing through periodic variations of intensity. The material
+systems concerned in the transfer of the energy rise, flourish,
+and fall in endless succession, like cities of ancient dynasties.
+At points of similar phase upon the waves the rate of consumption
+of energy is approximately the same; the functions, too, which
+demand and expend the energy are of similar nature.
+
+That the rhythm of these events is ultimately based on harmony in
+the configuration and motion of the molecules within the germ
+seems an unavoidable conclusion. In the life of the individual
+rhythmic dynamic phenomena reappear which in some cases have no
+longer a parallel in the external world, or under conditions when
+the individual is no longer influenced by these external
+conditions.,, In many cases the periodic phenomena ultimately die
+out under new influences, like the oscillations of a body in a
+viscous medium; in others when they seem to be more deeply rooted
+in physiological conditions they persist.
+
+The "length of life is dependent upon the number
+
+[1] The _Descent of Man._
+
+96
+
+of generations of somatic cells which can succeed one another in
+the course of a single life, and furthermore the number as well
+as the duration of each single cell-generation is predestined in
+the germ itself."[1]
+
+Only in the vague conception of a harmonising or formative
+structural influence derived from the germ, perishing in each
+cell from internal causes, but handed from cell to cell till the
+formative influence itself degrades into molecular discords, does
+it seem possible to form any physical representation of the
+successive events of life. The degradation of the molecular
+formative influence might be supposed involved in its frequent
+transference according to some such dynamic actions as occur in
+inanimate nature. Thus, ultimately, to the waste within the cell,
+to the presence of a force retardative of its perpetual harmonic
+motions, the death of the individual is to be ascribed. Perhaps
+in protoplasmic waste the existence of a universal death should
+be recognised. It is here we seem to touch inanimate nature; and
+we are led back to a former conclusion that the organism in its
+unconstrained state is to be regarded as a contrivance for
+evading the dynamic tendencies of actions in which lifeless
+matter participates.[2]
+
+[1] Weismann, _Life and Death; Biological Memoirs_, p. 146.
+
+[2] In connection with the predestinating power and possible
+complexity of the germ, it is instructive to reflect on the very
+great molecular population of even the smallest spores--giving
+rise to very simple forms. Thus, the spores of the unicellular
+Schizomycetes are estimated to dimensions as low as 1/10,000 of a
+millimetre in diameter (Cornil et Babes, _Les Batteries_, 1. 37).
+From Lord Kelvin's estimate of the number of molecules in water,
+comprised within the length of a wave-length of yellow light
+(_The Size of Atoms_, Proc. R. I., vol. x., p. 185) it is
+probable that such spores contain some 500,000 molecules, while
+one hundred molecules range along a diameter.
+
+97
+
+THE NUMERICAL ABUNDANCE OF LIFE
+
+We began by seeking in various manifestations of life a dynamic
+principle sufficiently comprehensive to embrace its very various
+phenomena. This, to all appearance, found, we have been led to
+regard life, to a great extent, as a periodic dynamic phenomenon.
+Fundamentally, in that characteristic of the contrivance, which
+leads it to respond favourably to transfer of energy, its
+enormous extension is due. It is probable that to its instability
+its numerical abundance is to be traced--for this, necessitating
+the continual supply of all the parts already formed, renders
+large, undifferentiated growth, incompatible with the limited
+supplies of the environment. These are fundamental conditions of
+abundant life upon the Earth.
+
+Although we recognise in the instability of living systems the
+underlying reason for their numerical abundance, secondary
+evolutionary causes are at work. The most important of these is
+the self-favouring nature of the phenomenon of reproduction. Thus
+there is a tendency not only to favour reproductiveness, but
+early reproductiveness, in the form of one prolific
+reproductive.
+
+98
+
+act, after which the individual dies.[1] Hence the wavelength of
+the species diminishes, reproduction is more frequent, and
+correspondingly greater numbers come and go in an interval of
+time.
+
+Another cause of the numerical abundance of life exists, as
+already stated, in the conditions of nourishment. Energy is more
+readily conveyed to the various parts of the smaller mass, and
+hence the lesser organisms will more actively functionate; and
+this, as being the urging dynamic attitude, as well as that most
+generally favourable in the struggle, will multiply and favour
+such forms of life. On the other hand, however, these forms will
+have less resource within themselves, and less power of
+endurance, so that they are only suitable to fairly uniform
+conditions of supply; they cannot survive the long continued want
+of winter, and so we have the seasonal abundance of summer. Only
+the larger and more resistant organisms, whether animal or
+vegetable, will, in general, populate the Earth from year to
+year. From this we may conclude that, but for the seasonal
+energy-tides, the development of life upon the globe had gone
+along very different lines from those actually followed. It is,
+indeed, possible that the evolution of the larger organisms would
+not have occurred; there would have been no vacant place for
+their development, and a being so endowed as Man could hardly
+
+[1] Weismann, _The Duration of Life._
+
+99
+
+have been evolved. We may, too, apply this reasoning elsewhere,
+and regard as highly probable, that in worlds which are without
+seasonal influences, the higher developments of life have not
+appeared; except they have been evolved under other conditions,
+when they might for a period persist. We have, indeed, only to
+picture to ourselves what the consequence of a continuance of
+summer would be on insect life to arrive at an idea of the
+antagonistic influences obtaining in such worlds to the survival
+of larger organisms.
+
+It appears that to the dynamic attitude of life in the first
+place, and secondarily to the environmental conditions limiting
+undifferentiated growth, as well as to the action of heredity in
+transmitting the reproductive qualities of the parent to the
+offspring, the multitudes of the pines, and the hosts of ants,
+are to be ascribed. Other causes are very certainly at work, but
+these, I think, must remain primary causes.
+
+We well know that the abundance of the ants and pines is not a
+tithe of the abundance around us visible and invisible. It is a
+vain endeavour to realise the countless numbers of our
+fellow-citizens upon the Earth; but, for our purpose, the
+restless ants, and the pines solemnly quiet in the sunshine, have
+served as types of animate things. In the pine the gates of the
+organic have been thrown open that the vivifying river of energy
+may flow in. The ants and the butterflies sip for a brief moment
+of its waters, and again vanish into the
+
+100
+
+inorganic: life, love and death encompassed in a day.
+
+Whether the organism stands at rest and life comes to it on the
+material currents of the winds and waters, or in the vibratory
+energy of the aether; or, again, whether with restless craving it
+hurries hither and thither in search of it, matters nothing. The
+one principle--the accelerative law which is the law of the
+organic--urges all alike onward to development, reproduction and
+death. But although the individual dies death is not the end; for
+life is a rhythmic phenomenon. Through the passing ages the waves
+of life persist: waves which change in their form and in the
+frequency to which they are attuned from one geologic period to
+the next, but which still ever persist and still ever increase.
+And in the end the organism outlasts the generations of the
+hills.
+
+101
+
+THE BRIGHT COLOURS OF ALPINE FLOWERS [1]
+
+IT is admitted by all observers that many species of flowering
+plants growing on the higher alps of mountainous regions display
+a more vivid and richer colour in their bloom than is displayed
+in the same species growing in the valleys. That this is actually
+the case, and not merely an effect produced upon the observer by
+the scant foliage rendering the bloom more conspicuous, has been
+shown by comparative microscopic examination of the petals of
+species growing on the heights and in the valleys. Such
+examination has revealed that in many cases pigment granules are
+more numerous in the individuals growing at the higher altitudes.
+The difference is specially marked in Myosotis sylvatica,
+Campanula rotundifolia, Ranunculus sylvaticus, Galium cruciatum,
+and others. It is less marked in the case of Thymus serpyllum and
+Geranium sylvaticum; while in Rosa alpina and Erigeron alpinus no
+difference is observable.[2]
+
+In the following cases a difference of intensity of colour is,
+according to Kerner ("Pflanzenleben," 11. 504), especially
+noticeable:-- _Agrostemma githago, Campanula
+
+[1] _Proc. Royal Dublin Society_, 1893.
+
+[2] G. Bonnier, quoted by De Varigny, _Experimental Evolution_,
+p. 55.
+
+102
+
+pusilla, Dianthus inodorus (silvestris), Gypsophila repens, Lotus
+corniculatus, Saponaria ocymoides, Satureja hortensis, Taraxacumm
+officinale, Vicia cracca, and Vicia sepium._
+
+To my own observation this beautiful phenomenon has always
+appeared most obvious and impressive. It appears to have struck
+many unprofessional observers. Helmholtz offers the explanation
+that the vivid colours are the result of the brighter sunlight of
+the heights. It has been said, too, that they are the direct
+chemical effects of a more highly ozonized atmosphere. The latter
+explanation I am unable to refer to its author. The following
+pages contain a suggestion on the matter, which occurred to me
+while touring, along with Henry H. Dixon, in the Linthal district
+of Switzerland last summer.[1]
+
+If the bloom of these higher alpine flowers is especially
+pleasing to our own aesthetic instincts, and markedly conspicuous
+to us as observers, why not also especially attractive and
+conspicuous to the insect whose mission it is to wander from
+flower to flower over the pastures? The answer to this question
+involves the hypothesis I would advance as accounting for the
+bright colours of high-growing individuals. In short, I believe a
+satisfactory explanation is to be found in the conditions of
+insect life in the higher alps.
+
+In the higher pastures the summer begins late and
+
+[1] The summer of 1892.
+
+103
+
+closes early, and even in the middle of summer the day closes in
+with extreme cold, and the cold of night is only dispelled when
+the sun is well up. Again, clouds cover the heights when all is
+clear below, and cold winds sweep over them when there is warmth
+and shelter in the valleys. With these rigorous conditions the
+pollinating insects have to contend in their search for food, and
+that when the rival attractions of the valleys below are so many.
+I believe it is these rigorous conditions which are indirectly
+responsible for the bright colours of alpine flowers. For such
+conditions will bring about a comparative scarcity of insect
+activity on the heights; and a scarcity or uncertainty in the
+action of insect agency in effecting fertilization will intensify
+the competition to attract attention, and only the brightest
+blooms will be fertilized.[1]
+
+This will be a natural selection of the brightest, or the
+
+[1] Grant Allen, I have recently learned, advances in _Science in
+Arcady_ the theory that there is a natural selective cause
+fostering the bright blooms of alpines. The selective cause is,
+however, by him referred to the greater abundance of butterfly
+relatively to bee fertilizers. The former, he says, display more
+aesthetic instinct than bees. In the valley the bees secure the
+fertilization of all. I may observe that upon the Fridolins Alp
+all the fertilizers we observed were bees. I have always found
+butterflies very scarce at altitudes of 7,000 to 8,000 feet. The
+alpine bees are very light in body, like our hive bee, and I do
+not think rarefaction of the atmosphere can operate to hinder its
+ascent to the heights, as Grant Allen suggests. The observations
+on the death-rate of bees and butterflies on the glacier, to be
+referred to presently, seem to negative such a hypothesis, and to
+show that a large preponderance of bees over butterflies make
+their way to the heights.
+
+104
+
+brightest will be the fittest, and this condition, along with the
+influence of heredity, will encourage a race of vivid flowers. On
+the other hand, the more scant and uncertain root supply, and the
+severe atmospheric conditions, will not encourage the grosser
+struggle for existence which in the valleys is carried on so
+eagerly between leaves and branches--the normal offensive and
+defensive weapons of the plant--and so the struggle becomes
+refined into the more aesthetic one of colour and brightness
+between flower and flower. Hence the scant foliage and vivid
+bloom would be at once the result of a necessary economy, and a
+resort to the best method of securing reproduction under the
+circumstances of insect fertilizing agency. Or, in other words,
+while the luxuriant growth is forbidden by the conditions, and
+thus methods of offence and defence, based upon vigorous
+development, reduced in importance, it would appear that the
+struggle is mainly referred to rivalry for insect preference. It
+is probable that this is the more economical manner of carrying
+on the contest.
+
+In the valleys we see on every side the struggle between the
+vegetative organs of the plant; the soundless battle among the
+leaves and branches. The blossom here is carried aloft on a
+slender stem, or else, taking but a secondary part in the
+contest, it is relegated to obscurity (P1. XII.). Further up on
+the mountains, where the conditions are more severe and the
+supplies less abundant, the leaf and branch assume lesser
+dimensions, for they are costly weapons to provide and the
+elements are unfriendly
+
+105
+
+to their existence (Pl. XIII.). Still higher, approaching the
+climatic limit of vegetable life, the struggle for existence is
+mainly carried on by the aesthetic rivalry of lowly but
+conspicuous blossoms.
+
+As regards the conditions of insect life in the higher alps, it
+came to my notice in a very striking manner that vast numbers of
+such bees and butterflies as venture up perish in the cold of
+night time. It appears as if at the approach of dusk these are
+attracted by the gleam of the snow, and quitting the pastures,
+lose themselves upon the glaciers and firns, there to die in
+hundreds. Thus in an ascent of the Toedi from the Fridolinshuete we
+counted in the early dawn sixty-seven frozen bees, twenty-nine
+dead butterflies, and some half-dozen moths on the Biferten
+Glacier and Firn. These numbers, it is to be remembered, only
+included those lying to either side of our way over the snow, so
+that the number must have mounted up to thousands when integrated
+over the entire glacier and firn. Approaching the summit none
+were found. The bees resembled our hive bee in appearance, the
+butterflies resembled the small white variety common in our
+gardens, which has yellow and black upon its wings. One large
+moth, striped across the abdomen, and measuring nearly two inches
+in length of body, was found. Upon our return, long after the
+sun's rays had grown strong, we observed some of the butterflies
+showed signs of reanimation. We descended so quickly to avoid the
+inconvenience of the soft snow that we had time for no
+
+106
+
+close observation on the frozen bees. But dead bees are common
+objects upon the snows of the alps.
+
+These remarks I noted down roughly while at Linthal last summer,
+but quite recently I read in Natural Science[1] the following
+note:
+
+"Late Flowering Plants.--While we write, the ivy is in flower, and
+bees, wasps, and flies are jostling each other and struggling to
+find standing-room on the sweet-smelling plant. How great must be
+the advantage obtained by this plant through its exceptional
+habit of flowering in the late autumn, and ripening its fruit in
+the spring. To anyone who has watched the struggle to approach
+the ivy-blossom at a time when nearly all other plants are bare,
+it is evident that, as far as transport of pollen and
+cross-fertilization go, the plant could not flower at a more
+suitable time. The season is so late that most other plants are
+out of flower, but yet it is not too late for many insects to be
+brought out by each sunny day, and each insect, judging by its
+behaviour, must be exceptionally hungry.
+
+"Not only has the ivy the world to itself during its flowering
+season, but it delays to ripen its seed till the spring, a time
+when most other plants have shed their seed, and most edible
+fruits have been picked by the birds. Thus birds wanting fruit in
+the spring can obtain little but ivy, and how they appreciate the
+ivy berry is evident
+
+[1] For December, 1892, vol. i., p. 730.
+
+107
+
+by the purple stains everywhere visible within a short distance
+of the bush."
+
+These remarks suggest that the ivy adopts the converse attitude
+towards its visitors to that forced upon the alpine flower. The
+ivy bloom is small and inconspicuous, but then it has the season
+to itself, and its inconspicuousness is no disadvantage, _i.e._
+if one plant was more conspicuous than its neighbours, it would
+not have any decided advantage where the pollinating insect is
+abundant and otherwise unprovided for. Its dark-green berries in
+spring, which I would describe as very inconspicuous, have a
+similar advantage in relation to the necessities of bird life.
+
+The experiments of M. C. Flahault must be noticed. This
+naturalist grew seeds of coloured flowers which had ripened in
+Paris, part in Upsala, and part in Paris; and seed which had
+ripened in Upsala, part at Paris, and part at Upsala. The flowers
+opening in the more northern city were in most cases the
+brighter.[1] If this observation may be considered indisputable,
+as appears to be the case, the question arises, Are we to regard
+this as a direct effect of the more rigorous climate upon the
+development of colouring matter on the blooms opening at Upsala?
+If we suppose an affirmative answer, the theory of direct effect
+by sun brightness must I think be abandoned. But I venture to
+think that the explanation of the Upsala
+
+[1] Quoted by De Varigny, _Experimental Evolution_, p. 56.
+
+108
+
+experiment is not to be found in direct climatic influence upon
+the colour, but in causes which lie deeper, and involve some
+factors deducible from biological theory.
+
+The organism, as a result of the great facts of heredity and of
+the survival of the fittest, is necessarily a system which
+gathers experience with successive generations; and the principal
+lesson ever being impressed upon it by external events is
+economy. Its success depends upon the use it makes of its
+opportunities for the reception of energy and the economy
+attained in disposing of what is gained.
+
+With regard to using the passing opportunity the entire seasonal
+development of life is a manifestation of this attitude, and the
+fleetness, agility, etc., of higher organisms are developments in
+this direction. The higher vegetable organism is not locomotory,
+save in the transferences of pollen and seed, for its food comes
+to it, and the necessary relative motion between food and
+organism is preserved in the quick motion of radiated energy from
+the sun and the slower motion of the winds on the surface of the
+earth. But, even so, the vegetable organism must stand ever ready
+and waiting for its supplies. Its molecular parts must be ready
+to seize the prey offered to it, somewhat as the waiting spider
+the fly. Hence, the plant stands ready; and every cloud with
+moving shadow crossing the fields handicaps the shaded to the
+benefit of the unshaded plant in the adjoining field. The open
+bloom
+
+109
+
+is a manifestation of the generally expectant attitude of the
+plant, but in relation to reproduction.
+
+As regards economy, any principle of maximum economy, where many
+functions have to be fulfilled, will, we may very safely predict,
+involve as far as possible mutual helpfulness in the processes
+going on. Thus the process of the development towards meeting any
+particular external conditions, A, suppose, will, if possible,
+tend to forward the development towards meeting conditions B; so
+that, in short, where circumstances of morphology and physiology
+are favourable, the ideally economical system will be attained
+when in place of two separate processes, a, ss, the one process y,
+cheaper than a + ss, suffices to advance development
+simultaneously in both the directions A and B. The economy is as
+obvious as that involved in "killing two birds with the one
+stone"--if so crude a simile is permissible--and it is to be
+expected that to foster such economy will be the tendency of
+evolution in all organic systems subjected to restraints as those
+we are acquainted with invariably are.
+
+Such economy might be simply illustrated by considering the case
+of a reservoir of water elevated above two hydraulic motors, so
+that the elevated mass of water possessed gravitational
+potential. The available energy here represents the stored-up
+energy in the organism. How best may the water be conveyed to the
+two motors [the organic systems reacting towards conditions A and
+B] so
+
+110
+
+that as little energy as possible is lost in transit? If the
+motors are near together it is most economical to use the one
+conduit, which will distribute the requisite supply of water to
+both. If the motors are located far asunder it will be most
+economical to lay separate conduits. There is greatest economy in
+meeting a plurality of functions by the same train of
+physiological processes where this is consistent with meeting
+other demands necessitated by external or internal conditions.
+
+But an important and obvious consequence arises in the supply of
+the two motors from the one conduit. We cannot work one motor
+without working the other. If we open a valve in the conduit both
+motors start into motion and begin consuming the energy stored in
+the tank. And although they may both under one set of conditions
+be doing useful and necessary work, in some other set of
+conditions it may be needless for both to be driven.
+
+This last fact is an illustration of a consideration which must
+enter into the phenomenon which an eminent biologist speaks of as
+physiological or unconscious "memory,"[1] For the development of
+the organism from the ovum is but the starting of a train of
+interdependent events of a complexity depending upon the
+experience of the past.
+
+[1] Ewald Hering, quoted by Ray Lankaster, _The Advancement of
+Science_, p. 283.
+
+111
+
+In short, we may suppose the entire development of the plant,
+towards meeting certain groups of external conditions,
+physiologically knit together according as Nature tends to
+associate certain groups of conditions. Thus, in the case in
+point, climatic rigour and scarcity of pollinating agency will
+ever be associated; and in the long experience of the past the
+most economical physiological attitude towards both is, we may
+suppose, adopted; so that the presence of one condition excites
+the apparent unconscious memory of the other. In reality the
+process of meeting the one condition involves the process and
+development for meeting the other.
+
+And this consideration may be extended very generally to such
+organisms as can survive under the same associated natural
+conditions, for the history of evolution is so long, and the
+power of locomotion so essential to the organism at some period
+in its life history, that we cannot philosophically assume a
+local history for members of a species even if widely severed
+geographically at the present day. At some period in the past
+then, it is very possible that the individuals today thriving at
+Paris, acquired the experience called out at Upsala. The
+perfection of physiological memory inspires no limit to the date
+at which this may have occurred--possibly the result of a
+succession of severe seasons at Paris; possibly the result of
+migrations --and the seed of many flowering plants possess means
+of migration only inferior to those possessed by the flying and
+swimming animals. But, again, possibly the experi-
+
+112
+
+ence was acquired far back in the evolutionary history of the
+flower.[1]
+
+But a further consideration arises. Not only at each moment in
+the life of the individual must maximum income and most judicious
+expenditure be considered, but in its whole life history, and
+even over the history of its race, the efficiency must tend to be
+a maximum. This principle is even carried so far that when
+necessary it leads to the death of the individual, as in the case
+of those organisms which, having accomplished the reproductive
+act, almost immediately expire. This view of nature may be
+repellent, but it is, nevertheless, evident that we are parts of
+a system which ruthlessly sacrifices the individual on general
+grounds of economy. Thus, if the curve which defines the mean
+rate of reception of energy of all kinds at different periods in
+the life of the organism be opposed by a second curve, drawn
+below the axis along which time is measured, representing the
+mean rate of expenditure of energy on development, reproduction,
+etc. (Fig. 7), this latter curve, which is, of course,
+
+[1] The blooms of self-fertilising, and especially of
+cleistogamic plants (_e.g._ Viola), are examples of unconscious
+memory, or unconscious "association of ideas" leading to the
+development of organs now functionless. The _Pontederia crassipes_
+of the Amazon, which develops its floating bladders when grown in
+water, but aborts them rapidly when grown on land, and seems to
+retain this power of adaptation to the environment for an
+indefinite period of time, must act in each case upon an
+unconscious memory based upon past experience. Many other cases
+might be cited.
+
+113
+
+physiologically dependent on the former, must be of such a nature
+from its origin to its completion in death, that the condition is
+realized of the most economical rate of expenditure at each
+period of life.[1] The rate of expenditure of energy at any
+period of life is, of course, in such a curve defined by the
+slope of the curve towards the axis of time at the period in
+question; but this particular slope _must be led to by a previous
+part of the curve, and involves its past and future course to a
+very great extent_.
+
+{Fig. 7}
+
+There will, therefore, be impressed upon the
+organism by the factors of evolution a unified course of
+economical expenditure completed only by its death, and which
+will give to the developmental progress of the individual its
+prophetic character.
+
+In this way we look to the unified career of each organic unit,
+from its commencement in the ovum to the day
+
+[1] See _The Abundance of Life_.
+
+114
+
+when it is done with vitality, for that preparation for momentous
+organic events which is in progress throughout the entire course
+of development; and to the economy involved in the welding of
+physiological processes for the phenomenon of physiological
+memory, wherein we see reflected, as it were, in the development
+of the organism, the association of inorganic restraints
+occurring in nature which at some previous period impressed
+itself upon the plastic organism. We may picture the seedling at
+Upsala, swayed by organic memory and the inherited tendency to an
+economical preparation for future events, gradually developing
+towards the aesthetic climax of its career. In some such manner
+only does it appear possible to account for the prophetic
+development of organisms, not alone to be observed in the alpine
+flowers, but throughout nature.
+
+And thus, finally, to the effects of natural selection and to
+actions defined by general principles involved in biology, I
+would refer for explanation of the manner in which flowers on the
+Alps develop their peculiar beauty.
+
+115
+
+MOUNTAIN GENESIS
+
+OUR ancestors regarded mountainous regions with feelings of
+horror, mingled with commiseration for those whom an unkindly
+destiny had condemned to dwell therein. We, on the other hand,
+find in the contemplation of the great alps of the Earth such
+peaceful and elevated thoughts, and such rest to our souls, that
+it is to those very solitudes we turn to heal the wounds of ife.
+It is difficult to explain the cause of this very different point
+of view. It is probably, in part, to be referred to that cloud of
+superstitious horror which, throughout the Middle Ages, peopled
+the solitudes with unknown terrors; and, in part, to the
+asceticism which led the pious to regard the beauty and joy of
+life as snares to the soul's well-being. In those eternal
+solitudes where the overwhelming forces of Nature are most in
+evidence, an evil principle must dwell or a dragon's dreadful
+brood must find a home.
+
+But while in our time the aesthetic aspect of the hills appeals
+to all, there remains in the physical history of the mountains
+much that is lost to those who have not shared in the scientific
+studies of alpine structure and genesis. They lose a past history
+which for interest com-
+
+116
+
+petes with anything science has to tell of the changes of the
+Earth.
+
+Great as are the physical features of the mountains compared with
+the works of Man, and great as are the forces involved compared
+with those we can originate or control, the loftiest ranges are
+small contrasted with the dimensions of the Earth. It is well to
+bear this in mind. I give here (Pl. XV.) a measured drawing
+showing a sector cut from a sphere of 50 cms. radius; so much of
+it as to exhibit the convergence of its radial boundaries which
+if prolonged will meet at the centre. On the same scale as the
+radius the diagram shows the highest mountains and the deepest
+ocean. The average height of the land and the average depth of
+the ocean are also exhibited. We see how small a movement of the
+crust the loftiest elevation of the Himalaya represents and what
+a little depression holds the ocean.
+
+Nevertheless, it is not by any means easy to explain the genesis
+of those small elevations and depressions. It would lead us far
+from our immediate subject to discuss the various theoretical
+views which have been advanced to account for the facts. The idea
+that mountain folds, and the lesser rugosities of the Earth's
+surface, arose in a wrinkling of the crust under the influence of
+cooling and skrinkage of the subcrustal materials, is held by
+many eminent geologists, but not without dissent from others.
+
+The most striking observational fact connected with mountain
+structure is that, without exception, the ranges
+
+117
+
+of the Earth are built essentially of sedimentary rocks: that is
+of rocks which have been accumulated at some remote past time
+beneath the surface of the ocean. A volcanic core there may
+sometimes be--probably an attendant or consequence of the
+uplifting--or a core of plutonic igneous rocks which has arisen
+under the same compressive forces which have bowed and arched the
+strata from their original horizontal position. It is not
+uncommon to meet among unobservant people those who regard all
+mountain ranges as volcanic in origin. Volcanoes, however, do not
+build mountain ranges. They break out as more or less isolated
+cones or hills. Compare the map of the Auvergne with that of
+Switzerland; the volcanoes of South Italy with the Apennines.
+Such great ranges as those which border with triple walls the
+west coast of North America are in no sense volcanic: nor are the
+Pyrenees, the Caucasus, or the Himalaya. Volcanic materials are
+poured out from the summits of the Andes, but the range itself is
+built up of folded sediments on the same architecture as the
+other great ranges of the Earth.
+
+Before attempting an explanation of the origin of the mountains
+we must first become more closely acquainted with the phenomena
+attending mountain elevation.
+
+At the present day great accumulations of sediment are taking
+place along the margins of the continents where the rivers reach
+the ocean. Thus, the Gulf of Mexico receiving the sediment of the
+Mississippi and Rio Grande;
+
+118
+
+the northeast coast of South America receiving the sediments of
+the Amazons; the east coast of Asia receiving the detritus of the
+Chinese rivers; are instances of such areas of deposition. Year
+by year, century by century, the accumulation progresses, and as
+it grows the floor of the sea sinks under the load. Of the
+yielding of the crust under the burthen of the sediments we are
+assured; for otherwise the many miles of vertically piled strata
+which are uplifted to our view in the mountains, never could have
+been deposited in the coastal seas of the past. The flexure and
+sinking of the crust are undeniable realities.
+
+Such vast subsiding areas are known as geosynclines. From the
+accumulated sediments of the geosynclines the mountain ranges of
+the past have in every case originated; and the mountains of the
+future will assuredly arise and lofty ranges will stand where now
+the ocean waters close over the collecting sediments. Every
+mountain range upon the Earth enforces the certainty of this
+prediction.
+
+The mountain-forming movement takes place after a certain great
+depth of sediment is collected. It is most intense where the
+thickness of deposit is greatest. We see this when we examine the
+structure of our existing mountain ranges. At either side where
+the sediments thin out, the disturbance dies away, till we find
+the comparatively shallow and undisturbed level sediments which
+clothe the continental surface.
+
+Whatever be the connection between the deposition and
+
+119
+
+the subsequent upheaval, _the element of great depth of
+accumulation seems a necessary condition and must evidently enter
+as a factor into the Physical Processes involved_. The mountain
+range can only arise where the geosyncline is deeply filled by
+long ages of sedimentation.
+
+Dana's description of the events attending mountain building is
+impressive:
+
+"A mountain range of the common type, like that to which the
+Appalachians belong, is made out of the sedimentary formations of
+a long preceding era; beds that were laid down conformably, and
+in succession, until they had reached the needed thickness; beds
+spreading over a region tens of thousands of square miles in
+area. The region over which sedimentary formations were in
+progress in order to make, finally, the Appalachian range,
+reached from New York to Alabama, and had a breadth of 100 to 200
+miles, and the pile of horizontal beds along the middle was
+40,000 feet in depth. The pile for the Wahsatch Mountains was
+60,000 feet thick, according to King. The beds for the
+Appalachians were not laid down in a deep ocean, but in shallow
+waters, where a gradual subsidence was in progress; and they at
+last, when ready for the genesis, lay in a trough 40,000 feet
+deep, filling the trough to the brim. It thus appears that epochs
+of mountain-making have occurred only after long intervals of
+quiet in the history of a continent."[1]
+
+[1] Dana, _Manual of Geology_, third edition, p. 794
+
+120
+
+On the western side of North America the work of
+mountain-building was, indeed, on the grandest scale. For long
+ages and through a succession of geological epochs, sedimentation
+had proceeded so that the accumulations of Palaeozoic and
+Mesozoic times had collected in the geosyncline formed by their
+own ever increasing weight. The site of the future Laramide range
+was in late Cretaceous times occupied by some 50,000 feet of
+sedimentary deposits; but the limit had apparently been attained,
+and at this time the Laramide range, as well as its southerly
+continuation into the United States, the Rockies, had their
+beginning. Chamberlin and Salisbury[1] estimate that the height
+of the mountains developed in the Laramide range at this time was
+20,000 feet, and that, owing to the further elevation which has
+since taken place, from 32,000 to 35,000 feet would be their
+present height if erosion had not reduced them. Thus on either
+side of the American continent we have the same forces at work,
+throwing up mountain ridges where the sediments had formerly been
+shed into the ocean.
+
+These great events are of a rhythmic character; the crust, as it
+were, pulsating under the combined influences of sedimentation
+and denudation. The first involves downward movements under a
+gathering load, and ultimately a reversal of the movement to one
+of upheaval; the second factor, which throughout has been in
+
+[1] Chamberlin and Salisbury, _Geology_, 1906, iii., 163.
+
+121
+
+operation as creator of the sediments, then intervenes as an
+assailant of the newly-raised mountains, transporting their
+materials again to the ocean, when the rhythmic action is
+restored to its first phase, and the age-long sequence of events
+must begin all over again.
+
+It has long been inferred that compressive stress in the crust
+must be a primary condition of these movements. The wvork
+required to effect the upheavals must be derived from some
+preexisting source of energy. The phenomenon--intrinsically one of
+folding of the crust--suggests the adjustment of the earth-crust
+to a lessening radius; the fact that great mountain-building
+movements have simultaneously affected the entire earth is
+certainly in favour of the view that a generally prevailing cause
+is at the basis of the phenomenon.
+
+The compressive stresses must be confined to the upper few miles
+of the crust, for, in fact, the downward increase of temperature
+and pressure soon confers fluid properties on the medium, and
+slow tangential compression results in hydrostatic pressure
+rather than directed stresses. Thus the folding visible in the
+mountain range, and the lateral compression arising therefrom,
+are effects confined to the upper parts of the crust.
+
+The energy which uplifts the mountain is probably a surviving
+part of the original gravitational potential energy of the crust
+itself. It must be assumed that the crust in following downwards
+the shrinking subcrustal magma, develops immense compressive
+stresses in
+
+122
+
+its materials, vast geographical areas being involved. When
+folding at length takes place along the axis of the elongated
+syncline of deposition, the stresses find relief probably for
+some hundreds of miles, and the region of folding now becomes
+compressed in a transverse direction. As an illustration, the
+Laramide range, according to Dawson, represents the reduction of
+a surface-belt 50 miles wide to one of 25 miles. The marvellous
+translatory movements of crustal folds from south to north
+arising in the genesis of the Swiss Alps, which recent research
+has brought to light, is another example of these movements of
+relief, which continue to take place perhaps for many millions of
+years after they are initiated.
+
+The result of this yielding of the crust is a buckling of the
+surface which on the whole is directed upwards; but depression
+also is an attendant, in many cases at least, on mountain
+upheaval. Thus we find that the ocean floor is depressed into a
+syncline along the western coast of South America; a trough
+always parallel to the ranges of the Andes. The downward
+deflection of the crust is of course an outcome of the same
+compressive stresses which elevate the mountain.
+
+The fact that the yielding of the crust is always situated where
+the sediments have accumulated to the greatest depth, has led to
+attempts from time to time of establishing a physical connexion
+between the one and the other. The best-known of these theories
+is that of Babbage and Herschel. This seeks the connexion in the
+rise of the
+
+123
+
+geotherms into the sinking mass of sediment and the consequent
+increase of temperature of the earth-crust beneath. It will be
+understood that as these isogeotherms, or levels at which the
+temperature is the same, lie at a uniform distance from the
+surface all over the Earth, unless where special variations of
+conductivity may disturb them, the introduction of material
+pressed downwards from above must result in these materials
+partaking of the temperature proper to the depth to which they
+are depressed. In other words the geotherms rise into the sinking
+sediments, always, however, preserving their former average
+distance from the surface. The argument is that as this process
+undoubtedly involves the heating up of that portion of the crust
+which the sediments have displaced downwards, the result must be
+a local enfeeblement of the crust, and hence these areas become
+those of least resistance to the stresses in the crust.
+
+When this theory is examined closely, we see that it only amounts
+to saying that the bedded rocks, which have taken the place of
+the igneous materials beneath, as a part of the rigid crust of
+the Earth, must be less able to withstand compressive stress than
+the average crust. For there has been no absolute rise of the
+geotherms, the thermal conductivities of both classes of
+materials differing but little. Sedimentary rock has merely taken
+the place of average crust-rock, and is subjected to the same
+average temperature and pressure prevailing in the surrounding
+crust. But are there any grounds for the
+
+124
+
+assumption that the compressive resistance of a complex of
+sedimentary rocks is inferior to one of igneous materials? The
+metamorphosed siliceous sediments are among the strongest rocks
+known as regards resistance to compressive stress; and if
+limestones have indeed plastic qualities, it must be remembered
+that their average amount is only some 5 per cent. of the whole.
+Again, so far as rise of temperature in the upper crust may
+affect the question, a temperature which will soften an average
+igneous rock will not soften a sedimentary rock, for the reason
+that the effect of solvent denudation has been to remove those
+alkaline silicates which confer fusibility.
+
+When, however, we take into account the radioactive content of
+the sediments the matter assumes a different aspect.
+
+The facts as to the general distribution of radioactive
+substances at the surface, and in rocks which have come from
+considerable depths in the crust, lead us to regard as certain
+the widespread existence of heat-producing radioactive elements
+in the exterior crust of the Earth. We find, indeed, in this fact
+an explanation--at least in part--of the outflow of heat
+continually taking place at the surface as revealed by the rising
+temperature inwards. And we conclude that there must be a
+thickness of crust amounting to some miles, containing the
+radioactive elements.
+
+Some of the most recent measurements of the quantities of radium
+and thorium in the rocks of igneous origin--_e.g._ granites,
+syenites, diorites, basalts, etc., show that the
+
+125
+
+radioactive heat continually given out by such rocks amounts to
+about one millionth part of 0.6 calories per second per cubic
+metre of average igneous rock. As we have to account for the
+escape of about 0.0014 calorie[1] per square metre of the Earth's
+surface per second (assuming the rise of temperature downwards,
+_i.e._ the "gradient" of temperature, to be one degree centigrade
+in 35 metres) the downward extension of such rocks might, _prima
+facie_, be as much as 19 kilometres.
+
+About this calculation we have to observe that we assume the
+average radioactivity of the materials with which we have dealt
+at the surface to extend uniformly all the way down, _i.e._ that
+our experiments reveal the average radioactivity of a radioactive
+crust. There is much to be said for this assumption. The rocks
+which enter into the measurements come from all depths of the
+crust. It is highly probable that the less silicious, _i.e._ the
+more basic, rocks, mainly come from considerable depths; the more
+acid or silica-rich rocks, from higher levels in the crust. The
+radioactivity determined as the mean of the values for these two
+classes of rock closely agrees with that found for intermediate
+rocks, or rocks containing an intermediate amount of silica.
+Clarke contends that this last class of material probably
+represents the average composition of the Earth's crust so far as
+it has been explored by us.
+
+[1] The calorie referred to is the quantity of heat required to
+heat one gram of water, _i.e._ one cubic centimetre of
+water--through one degree centigrade.
+
+126
+
+It is therefore highly probable that the value found for the mean
+radioactivity of acid and basic rocks, or that found for
+intermediate rocks, truly represents the radioactive state of the
+crust to a considerable depth. But it is easy to show that we
+cannot with confidence speak of the thickness of this crust as
+determinable by equating the heat outflow at the surface with the
+heat production of this average rock.
+
+This appears in the failure of a radioactive layer, taken at a
+thickness of about 19-kilometres, to account for the deep-seated
+high temperatures which we find to be indicated by volcanic
+phenomena at many places on the surface. It is not hard to show
+that the 19-kilometre layer would account for a temperature no
+higher than about 270 deg. >C. at its base.
+
+It is true that this will be augmented beneath the sedimentary
+deposits as we shall presently see; and that it is just in
+association with these deposits that deep-seated temperatures are
+most in evidence at the surface; but still the result that the
+maximum temperature beneath the crust in general attains a value
+no higher than 270 deg. C. is hardly tenable. We conclude, then, that
+some other source of heat exists beneath. This may be radioactive
+in origin and may be easily accounted for if the radioactive
+materials are more sparsely distributed at the base of the upper
+crust. Or, again, the heat may be primeval or original heat,
+still escaping from a cooling world. For our present purpose it
+does not much matter which view
+
+127
+
+we adopt. But we must recognise that the calculated depth of 19
+kilometres of crust, possessing the average radioactivity of the
+surface, is excessive; for, in fact, we are compelled by the
+facts to recognise that some other source of heat exists
+beneath.
+
+If the observed surface gradient of temperature persisted
+uniformly downwards, at some 35 kilometres beneath the surface
+there would exist temperatures (of about 1000 deg. C.) adequate to
+soften basic rocks. It is probable, however, that the gradient
+diminishes downwards, and that the level at which such
+temperatures exist lies rather deeper than this. It is,
+doubtless, somewhat variable according to local conditions; nor
+can we at all approximate closely to an estimate of the depth at
+which the fusion temperatures will be reached, for, in fact, the
+existence of the radioactive layer very much complicates our
+estimates. In what follows we assume the depth of softening to
+lie at about 40 kilometres beneath the surface of the normal
+crust; that is 25 miles down. It is to be observed that Prestwich
+and other eminent geologists, from a study of the facts of
+crust-folding, etc., have arrived at similar estimates.[1] As a
+further assumption we are probably not far wrong if we assign to
+the radioactive part of this crust a thickness of about 10 or 12
+kilometres; _i.e._ six or seven miles. This is necessarily a
+rough approximation only; but the conclusions at which
+
+[1] Prestwich, _Proc. Royal Soc._, xii., p. 158 _et seq._
+
+128
+
+we shall arrive are reached in their essential features allowing
+a wide latitude in our choice of data. We shall speak of this
+part of the crust as the normal radioactive layer.
+
+An important fact is evolved from the mathematical investigation
+of the temperature conditions arising from the presence of such a
+radioactive layer. It is found that the greatest temperature, due
+to the radioactive heat everywhere evolved in the layer--_i.e._
+the temperature at its base--is proportional to the square of the
+thickness of the layer. This fact has a direct bearing on the
+influence of radioactivity upon mountain elevation; as we shall
+now find.
+
+The normal radioactive layer of the Earth is composed of rocks
+extending--as we assume--approximately to a depth of 12 kilometres
+(7.5 miles). The temperature at the base of this layer due to the
+heat being continually evolved in it, is, say, t1 deg.. Now, let us
+suppose, in the trough of the geosyncline, and upon the top of
+the normal layer, a deposit of, say, 10 kilometres (6.2 miles) of
+sediments is formed during a long period of continental
+denudation. What is the effect of this on the temperature at the
+base of the normal layer depressed beneath this load? The total
+thickness of radioactive rocks is now 22 kilometres. Accordingly
+we find the new temperature t2 deg., by the proportion t1 deg. : t2 deg. ::
+12 deg. : 22 deg. That is, as 144 to 484. In fact, the temperature is more
+than trebled. It is true we here assume the radioactivity of the
+sediments
+
+129
+
+and of the normal crust to be the same. The sediments are,
+however, less radioactive in the proportion of 4 to 3.
+Nevertheless the effects of the increased thickness will be
+considerable.
+
+Now this remarkable increase in the temperature arises entirely
+from the condition attending the radioactive heating; and
+involves something _additional_ to the temperature conditions
+determined by the mere depression and thickening of the crust as
+in the Babbage-Herschel theory. The latter theory only involves a
+_shifting_ of the temperature levels (or geotherms) into the
+deposited materials. The radioactive theory involves an actual
+rise in the temperature at any distance from the surface; so that
+_the level in the crust at which the rocks are softened is nearer
+to the surface in the geosynclines than it is elsewhere in the
+normal crust_ (Pl. XV, p. 118).
+
+In this manner the rigid part of the crust is reduced in
+thickness where the great sedimentary deposits have collected. A
+ten-kilometre layer of sediment might result in reducing the
+effective thickness of the crust by 30 per cent.; a
+fourteen-kilometre layer might reduce it by nearly 50 per cent.
+Even a four-kilometre deposit might reduce the effective
+resistance of the crust to compressive forces, by 10 per cent.
+
+Such results are, of course, approximate only. They show that as
+the sediments grow in depth there is a rising of the geotherm of
+plasticity--whatever its true temperature may be--gradually
+reducing the thickness of that part
+
+130
+
+of the upper crust which is bearing the simultaneously increasing
+compressive stresses. Below this geotherm long-continued stress
+resolves itself into hydrostatic pressure; above it (there is, of
+course, no sharp line of demarcation) the crust accumulates
+elastic energy. The final yielding and flexure occur when the
+resistant cross-section has been sufficiently diminished. It is
+probable that there is also some outward hydrostaitic thrust over
+the area of rising temperature, which assists in determining the
+upward throw of the folds.
+
+When yielding has begun in any geosyncline, and the materials are
+faulted and overthrust, there results a considerably increased
+thickness. As an instance, consider the piling up of sediments
+over the existing materials of the Alps, which resulted from the
+compressive force acting from south to north in the progress of
+Alpine upheaval. Schmidt of Basel has estimated that from 15 to
+20 kilometres of rock covered the materials of the Simplon as now
+exposed, at the time when the orogenic forces were actively at
+work folding and shearing the beds, and injecting into their
+folds the plastic gneisses coming from beneath.[1] The lateral
+compression of the area of deposition of the Laramide, already
+referred to, resulted in a great thickening of the deposits. Many
+other cases might be cited; the effect is always in some degree
+necessarily produced.
+
+[1] Schmidt, Ec. Geol. _Helvelix_, vol. ix., No. 4, p. 590
+
+131
+
+If time be given for the heat to accumulate in the lower depths
+of the crushed-up sediments, here is an additional source of
+increased temperature. The piled-up masses of the Simplon might
+have occasioned a rise due to radioactive heating of one or two
+hundred degrees, or even more; and if this be added to the
+interior heat, a total of from 800 deg. to 1000 deg. might have prevailed
+in the rocks now exposed at the surface of the mountain. Even a
+lesser temperature, accompanied by the intense pressure
+conditions, might well occasion the appearances of thermal
+metamorphism described by Weinschenk, and for which, otherwise,
+there is difficulty in accounting.[1]
+
+This increase upon the primarily developed temperature conditions
+takes place concurrently with the progress of compression; and
+while it cannot be taken into account in estimating the
+conditions of initial yielding of the crust, it adds an element
+of instability, inasmuch as any progressive thickening by lateral
+compression results in an accelerated rise of the goetherms. It
+is probable that time sufficient for these effects to develop, if
+not to their final, yet to a considerable extent, is often
+available. The viscous movements of siliceous materials, and the
+out-pouring of igneous rocks which often attend mountain
+elevation, would find an explanation in such temperatures.
+
+[1] Weinschenk, _Congres Geol. Internat._, 1900, i., p. 332.
+
+132
+
+There is no more striking feature of the part here played by
+radioactivity than the fact that the rhythmic occurrence of
+depression and upheaval succeeding each other after great
+intervals of time, and often shifting their position but little
+from the first scene of sedimentation, becomes accounted for. The
+source of thermal energy, as we have already remarked, is in fact
+transported with the sediments--that energy which determines the
+place of yielding and upheaval, and ordains that the mountain
+ranges shall stand around the continental borders. Sedimentation
+from this point of view is a convection of energy.
+
+When the consolidated sediments are by these and by succeeding
+movements forced upwards into mountains, they are exposed to
+denudative effects greatly exceeding those which affect the
+plains. Witness the removal during late Tertiary times of the
+vast thickness of rock enveloping the Alps. Such great masses are
+hurried away by ice, rivers, and rain. The ocean receives them;
+and with infinite patience the world awaits the slow accumulation
+of the radioactive energy beginning afresh upon its work. The
+time for such events appears to us immense, for millions of years
+are required for the sediments to grow in thickness, and the
+geotherms to move upwards; but vast as it is, it is but a moment
+in the life of the parent radioactive substances, whose atoms,
+hardly diminished in numbers, pursue their changes while the
+mountains come and go, and the
+
+133
+
+rudiments of life develop into its highest consummations.
+
+To those unacquainted with the results of geological
+investigation the history of the mountains as deciphered in the
+rocks seems almost incredible.
+
+The recently published sections of the Himalaya, due to H. H.
+Hayden and the many distinguished men who have contributed to the
+Geological Survey of India, show these great ranges to be
+essentially formed of folded sediments penetrated by vast masses
+of granite and other eruptives. Their geological history may be
+summarised as follows
+
+The Himalayan area in pre-Cambrian times was, in its southwestern
+extension, part of the floor of a sea which covered much of what
+is now the Indian Peninsula. In the northern shallows of this sea
+were laid down beds of conglomerate, shale, sandstone and
+limestone, derived from the denudation of Archaean rocks, which,
+probably, rose as hills or mountains in parts of Peninsular India
+and along the Tibetan edge of the Himalayan region. These beds
+constitute the record of the long Purana Era[1] and are probably
+coeval with the Algonkian of North America. Even in these early
+times volcanic disturbances affected this area and the lower beds
+of the Purana deposits were penetrated by volcanic outflows,
+covered by sheets of lava, uplifted, denuded and again submerged
+
+[1] See footnote, p. 139.
+
+134
+
+beneath the waters. Two such periods of instability have left
+their records in the sediments of the Purana sea.
+
+The succeeding era--the Dravidian Era--opens with Haimanta
+(Cambrian) times. A shallow sea now extended over Kumaun, Garwal,
+and Spiti, as well as Kashmir and ultimately over the Salt Range
+region of the Punjab as is shown by deposits in these areas. This
+sea was not, however, connected with the Cambrian sea of Europe.
+The fossil faunas left by the two seas are distinct.
+
+After an interval of disturbance during closing Haimanta times,
+geographical changes attendant on further land movements
+occurred. The central sea of Asia, the Tethys, extended westwards
+and now joined with the European Paleozoic sea; and deposits of
+Ordovician and Silurian age were laid down:--the Muth deposits.
+
+The succeeding Devonian Period saw the whole Northern Himalayan
+area under the waters of the Tethys which, eastward, extended to
+Burma and China and, westward, covered Kashmir, the Hindu Kush
+and part of Afghanistan. Deposits continued to be formed in this
+area till middle Carboniferous times.
+
+Near. the close of the Dravidian Era Kashmir became convulsed by
+volcanic disturbance and the Penjal traps were ejected. It was a
+time of worldwide disturbance and of redistribution of land and
+water. Carboniferous times had begun, and the geographical
+changes in
+
+135
+
+the southern limits of the Tethys are regarded as ushering in a
+new and last era in Indian geological history the Aryan Bra.
+
+India was now part of Gondwanaland; that vanished continent which
+then reached westward to South Africa and eastward to Australia.
+A boulder-bed of glacial origin, the Talchir Boulder-bed, occurs
+in many surviving parts of this great land. It enters largely
+into the Salt Range deposits. There is evidence that extensive
+sheets of ice, wearing down the rocks of Rajputana, shoved their
+moraines northward into the Salt Range Sea; then, probably, a
+southern extension of the Tethys.
+
+Subsequent to this ice age the Indian coalfields of the Gondwana
+were laid down, with beds rich in the Glossopteris and
+Gangamopteris flora. This remarkable carboniferous flora extends
+to Southern Kashmir, so that it is to be inferred that this
+region was also part of the main Gondwanaland. But its emergence
+was but for a brief period. Upper Carboniferous marine deposits
+succeeded; and, in fact, there was no important discontinuity in
+the deposits in this area from Panjal times till the early
+Tertiaries. During the whole of which vast period Kashmir was
+covered with the waters of the Tethys.
+
+The closing Dravidian disturbances of the Kashmir region did not,
+apparently, extend to the eastern Himalayan area. But the
+Carboniferous Period was, in this
+
+136
+
+eastern area, one of instability, culminating, at the close of
+the Period, in a steady rise of the land and a northward retreat
+of the Tethys. Nearly the entire Himalaya east of Kashmir became
+a land surface and remained exposed to denudative forces for so
+long a time that in places the whole of the Carboniferous,
+Devonian, and a large part of the Silurian and Ordovician
+deposits were removed--some thousands of feet in thickness--before
+resubmergence in the Tethys occurred.
+
+Towards the end of the Palaeozoic Age the Aryan Tethys receded
+westwards, but still covered the Himalaya and was still connected
+with the European Palaeozoic sea. The Himalayan area (as well as
+Kashmir) remained submerged in its waters throughout the entire
+Mesozoic Age.
+
+During Cretaceous times the Tethys became greatly extended,
+indicating a considerable subsidence of northwestern India,
+Afghanistan, Western Asia, and, probably, much of Tibet. The
+shallow-water character of the deposits of the Tibetan Himalaya
+indicates, however, a coast line near this region. Volcanic
+materials, now poured out, foreshadow the incoming of the great
+mountain-building epoch of the Tertiary Era. The enormous mass of
+the Deccan traps, possessing a volume which has been estimated at
+as much as 6,000 cubic miles, was probably extruded over the
+Northern Peninsular region during late Cretaceous times. The sea
+now began to retreat, and by the close of
+
+137
+
+the Eocene, it had moved westward to Sind and Baluchistan. The
+movements of the Earth's crust were attended by intense volcanic
+activity, and great volumes of granite were injected into the
+sediments, followed by dykes and outflows of basic lavas.
+
+The Tethys vanished to return no more. It survives in the
+Mediterranean of today. The mountain-building movements continued
+into Pliocene times. The Nummulite beds of the Eocene were, as
+the result, ultimately uplifted 18,500 feet over sea level, a
+total uplift of not less than 20,000 feet.
+
+Thus with many vicissitudes, involving intervals of volcanic
+activity, local uplifting, and extensive local denudation, the
+Himalaya, which had originated in the sediments of the ancient
+Purana sea, far back in pre-Cambrian times, and which had
+developed potentially in a long sequence of deposits collecting
+almost continuously throughout the whole of geological time,
+finally took their place high in the heavens, where only the
+winds--faint at such altitudes--and the lights of heaven can visit
+their eternal snows.[1]
+
+In this great history it is significant that the longest
+continuous series of sedimentary deposits which the world has
+known has become transfigured into the loftiest elevation upon
+its surface.
+
+[1] See A Sketch of the _Geography and Geology of the Himalaya
+Mountains and Tibet_. By Colonel S. G. Burrard, R.E., F.R.S., and
+H. H. Hayden, F.G.S., Part IV. Calcutta, 1908.
+
+138
+
+The diagrammatic sections of the Himalaya accompanying this brief
+description arc taken from the monograph of Burrard and Hayden
+(loc. cit.) on the Himalaya. Looking at the sections we see that
+some of the loftiest summits are sculptured in granite and other
+crystalline rocks. The appearance of these materials at the
+surface indicates the removal by denudation and the extreme
+metamorphism of much sedimentary deposit. The crystalline rocks,
+indeed, penetrate some of the oldest rocks in the world. They
+appear in contact with Archaean, Algonkian or early Palaeozoic
+rocks. A study of the sections reveals not only the severe earth
+movements, but also the immense amount of sedimentary deposits
+involved in the genesis of these alps. It will be noted that the
+vertical scale is not exaggerated relatively to the
+horizontal.[1] Although there is no evidence of mountain
+building
+
+[1] To those unacquainted with the terminology of Indian geology
+the following list of approximate equivalents in time will be of
+use
+
+Ngari Khorsum Beds - Pleistocene.
+Siwalik Series -     Miocene and Pliocene.
+Sirmur Series -      Oligocene.
+Kampa System -       Eocene and Cretaceous.
+Lilang System -      Triassic.
+Kuling System -      Permian.
+Gondwana System -    Carboniferous.
+Kenawar System -     Carboniferous and Devonian
+Muth System -        Silurian.
+Haimanta System -    Mid. and Lower Cambrian.
+Purana Group -       Algonkian.
+Vaikrita System -    Archaean.
+Daling Series -      Archaean.
+
+139
+
+on a large scale in the Himalayan area till the Tertiary
+upheaval, it is, in the majority of cases, literally correct to
+speak of the mountains as having their generations like organic
+beings, and passing through all the stages of birth, life, death
+and reproduction. The Alps, the Jura, the Pyrenees, the Andes,
+have been remade more than once in the course of geological time,
+the _debris_ of a worn-out range being again uplifted in succeeding
+ages.
+
+Thus to dwell for a moment on one case only: that of the
+Pyrenees. The Pyrenees arose as a range of older Palmozoic rocks
+in Devonian times. These early mountains, however, were
+sufficiently worn out and depressed by Carboniferous times to
+receive the deposits of that age laid down on the up-turned edges
+of the older rocks. And to Carboniferous succeeded Permian,
+Triassic, Jurassic and Lower Cretaceous sediments all laid down
+in conformable sequence. There was then fresh disturbance and
+upheaval followed by denudation, and these mountains, in turn,
+became worn out and depressed beneath the ocean so that Upper
+Greensand rocks were laid down unconforrnably on all beneath. To
+these now succeeded Upper Chalk, sediments of Danian age, and so
+on, till Eocene times, when the tale was completed and the
+existing ranges rose from the sea. Today we find the folded
+Nummulitic strata of Eocene age uplifted 11,000 feet, or within
+200 feet of the greatest heights of the Pyrenees. And so they
+stand awaiting
+
+140
+
+the time when once again they shall "fall into the portion of
+outworn faces."[1]
+
+Only mountains can beget mountains. Great accumulations of
+sediment are a necessary condition for the localisation of
+crust-flexure. The earliest mountains arose as purely igneous or
+volcanic elevations, but the generations of the hills soon
+originated in the collection of the _debris_, under the law of
+gravity, in the hollow places. And if a foundered range is
+exposed now to our view encumbered with thousands of feet of
+overlying sediments we know that while the one range was sinking,
+another, from which the sediments were derived, surely existed.
+Through the "windows" in the deep-cut rocks of the Swiss valleys
+we see the older Carboniferous Alps looking out, revisiting the
+sun light, after scores of millions of years of imprisonment. We
+know that just as surely as the Alps of today are founding by
+their muddy torrents ranges yet to arise, so other primeval Alps
+fed into the ocean the materials of these buried pre-Permian
+rocks.
+
+This succession of events only can cease when the rocks have been
+sufficiently impoverished of the heat-producing substances, or
+the forces of compression shall have died out in the surface
+crust of the earth.
+
+It seems impossible to escape the conclusion that in the great
+development of ocean-encircling areas of
+
+[1] See Prestwich, _Chemical and Physical Geology_, p. 302.
+
+141
+
+deposition and crustal folding, the heat of radioactivity has
+been a determining factor. We recognise in the movements of the
+sediments not only an influence localising and accelerating
+crustal movements, but one which, in subservience to the primal
+distribution of land and water, has determined some of the
+greatest geographical features of the globe.
+
+It is no more than a step to show that bound up with the
+radioactive energy are most of the earthquake and volcanic
+phenomena of the earth. The association of earthquakes with the
+great geosynclines is well known. The work of De Montessus showed
+that over 94 per cent. of all recorded shocks lie in the
+geosynclinal belts. There can be no doubt that these
+manifestations of instability are the results of the local
+weakness and flexure which originated in the accumulation of
+energy denuded from the continents. Similarly we may view in
+volcanoes phenomena referable to the same fundamental cause. The
+volcano was, in fact, long regarded as more intimately connected
+with earthquakes than it, probably, actually is; the association
+being regarded in a causative light, whereas the connexion is
+more that of possessing a common origin. The girdle of volcanoes
+around the Pacific and the earthquake belt coincide. Again, the
+ancient and modern volcanoes and earthquakes of Europe are
+associated with the geosyncline of the greater Mediterranean, the
+Tethys of Mesozoic times. There is no difficulty in understanding
+in a
+
+142
+
+general way the nature of the association. The earthquake is the
+manifestation of rupture and slip, and, as Suess has shown, the
+epicentres shift along that fault line where the crust has
+yielded.[1] The volcano marks the spot where the zone of fusion
+is brought so high in the fractured crust that the melted
+materials are poured out upon the surface.
+
+In a recent work on the subject of earthquakes Professor Hobbs
+writes: "One of the most interesting of the generalisations which
+De Montessus has reached as a result of his protracted studies,
+is that the earthquake districts on the land correspond almost
+exactly to those belts upon the globe which were the almost
+continuous ocean basins of the long Secondary era of geological
+history. Within these belts the sedimentary formations of the
+crust were laid down in the greatest thickness, and the
+formations follow each other in relatively complete succession.
+For almost or quite the whole of this long era it is therefore
+clear that the ocean covered these zones. About them the
+formations are found interrupted, and the lacuna indicate that
+the sea invaded the area only to recede from it, and again at
+some later period to transgress upon it. For a long time,
+therefore, these earthquake belts were the sea basins--the
+geosynclines. They became later the rising mountains of the
+Tertiary period, and mountains they
+
+[1] Suess, _The Face of the Earth_, vol. ii., chap. ii.
+
+143
+
+are today. The earthquake belts are hence those portions of the
+earth's crust which in recent times have suffered the greatest
+movements in a vertical direction--they are the most mobile
+portions of the earth's crust."[1] Whether the movements
+attending mountain elevation and denudation are a connected and
+integral part of those wide geographical changes which result in
+submergence and elevation of large continental areas, is an
+obscure and complex question. We seem, indeed, according to the
+views of some authorities, hardly in a position to affirm with
+certainty that such widespread movements of the land have
+actually occurred, and that the phenomena are not the outcome of
+fluctuations of oceanic level; that our observations go no
+further than the recognition of positive and negative movements
+of the strand. However this may be, the greater part of
+mechanical denudation during geological time has been done on the
+mountain ranges. It is, in short, indisputable that the orogenic
+movements which uplift the hills have been at the basis of
+geological history. To them the great accumulations of sediments
+which now form so large a part of continental land are mainly
+due. There can be no doubt of the fact that these movements have
+swayed the entire history, both inorganic and organic, of the
+world in which we live.
+
+[1] Hobbs, _Earthquakes_, p. 58.
+
+144
+
+To sum the contents of this essay in the most general terms, we
+find that in the conception of denudation as producing the
+convection and accumulation of radiothermal energy the surface
+features of the globe receive a new significance. The heat of the
+earth is not internal only, but rather a heat-source exists at
+the surface, which, as we have seen, cannot prevail to the same
+degree within; and when the conditions become favourable for the
+aggregation of the energy, the crust, heated both from beneath
+and from above, assumes properties more akin to those of its
+earlier stages of development, the secular heat-loss being
+restored in the radioactive supplies. These causes of local
+mobility have been in operation, shifting somewhat from place to
+place, and defined geographically by the continental masses
+undergoing denudation, since the earliest times.
+
+145
+
+ALPINE STRUCTURE
+
+AN intelligent observer of the geological changes progressing in
+southern Europe in Eocene times would have seen little to inspire
+him with a premonition of the events then developing. The
+Nummulitic limestones were being laid down in that enlarged
+Mediterranean which at this period, save for a few islands,
+covered most of south Europe. Of these stratified remains, as
+well as of the great beds of Cretaceous, Jurassic, Triassic, and
+Permian sediments beneath, our hypothetical observer would
+probably have been regardless; just as today we observe, with an
+indifference born of our transitoriness, the deposits rapidly
+gathering wherever river discharge is distributing the sediments
+over the sea-floor, or the lime-secreting organisms are actively
+at work. And yet it took but a few millions of years to uplift
+the deposits of the ancient Tethys; pile high its sediments in
+fold upon fold in the Alps, the Carpathians, and the Himalayas;
+and--exposing them to the rigours of denudation at altitudes where
+glaciation, landslip, and torrent prevail--inaugurate a new epoch
+of sedimentation and upheaval.
+
+146
+
+In the case of the Alps, to which we wish now specially to refer,
+the chief upheaval appears to have been in Oligocene times,
+although movement continued to the close of the Pliocene. There
+was thus a period of some millions of years within which the
+entire phenomena were comprised. Availing ourselves of Sollas'
+computations,[1] we may sum the maximum depths of sedimentary
+deposits of the geological periods concerned as follows:--
+
+Pliocene - - - - - 3,950 m.
+
+Miocene  - - - - - 4,250 m.
+
+Oligocene  - - - - 3,660 m.
+
+Eocene - - - - - - 6,100 m.
+
+and assuming that the orogenic forces began their work in the
+last quarter of the Eocene period, we have a total of 13,400 m.
+as some measure of the time which elapsed. At the rate of io
+centimetres in a century these deposits could not have collected
+in less than 13.4 millions of years. It would appear that not
+less than some ten millions of years were consumed in the genesis
+of the Alps before constructive movements finally ceased.
+
+The progress of the earth-movements was attended by the usual
+volcanic phenomena. The Oligocene and Miocene volcanoes extended
+in a band marked by the Auvergne, the Eiffel, the Bohemian, and
+the eastern Carpathian eruptions; and, later, towards the close
+of the movements in Pliocene times, the south border
+
+[1] Sollas, Anniversary Address, Geol. Soc., London, 1909.
+
+147
+
+regions of the Alps became the scene of eruptions such as those
+of Etna, Santorin, Somma (Vesuvius), etc.
+
+We have referred to these well-known episodes with two objects in
+view: to recall to mind the time-interval involved, and the
+evidence of intense crustal disturbance, both dynamic and
+thermal. According to views explained in a previous essay, the
+energetic effects of radium in the sediments and upper crust were
+a principal factor in localising and bringing about these
+results. We propose now to inquire if, also, in the more intimate
+structure of the Alps, the radioactive energy may not have borne
+a part.
+
+What we see today in the Alps is but a residue spared by
+denudation. It is certain that vast thicknesses of material have
+disappeared. Even while constructive effects were still in
+progress, denudative forces were not idle. Of this fact the
+shingle accumulations of the Molasse, where, on the northern
+borders of the Alps, they stand piled into mountains, bear
+eloquent testimony. In the sub-Apennine series of Italy, the
+great beds of clays, marls, and limestones afford evidence of
+these destructive processes continued into Pliocene times. We
+have already referred to Schmidt's estimate that the sedimentary
+covering must have in places amounted to from 15,000 to 20,000
+metres. The evidence for this is mainly tectonic or structural;
+but is partly forthcoming in the changes which the materials now
+open to our inspection plainly reveal. Thus it is impos-
+
+148
+
+sible to suppose that gneissic rocks can become so far plastic as
+to flow in and around the calcareous sediments, or be penetrated
+by the latter--as we see in the Jungfrau and elsewhere--unless
+great pressures and high temperatures prevailed. And, according
+to some writers, the temperatures revealed by the intimate
+structural changes of rock-forming minerals must have amounted to
+those of fusion. The existence of such conditions is supported by
+the observation that where the.crystallisation is now the most
+perfect, the phenomena of folding and injection are best
+developed.[1] These high temperatures would appear to be
+unaccountable without the intervention of radiothermal effects;
+and, indeed, have been regarded as enigmatic by observers of the
+phenomena in question. A covering of 20,000 metres in thickness
+would not occasion an earth-temperature exceeding 500 deg. C. if the
+gradients were such as obtain in mountain regions generally; and
+600 deg. is about the limit we could ascribe to the purely passive
+effects of such a layer in elevating the geotherms.
+
+Those who are still unacquainted with the recently published
+observations on the structure of the Alps may find it difficult
+to enter into what has now to be stated; for the facts are,
+indeed, very different from the generally preconceived ideas of
+mountain formation. Nor can we wonder that many geologists for
+long held
+
+[1] Weinschenk, C. R. _Congres Geol._, 1900, p. 321, et seq.
+
+149
+
+back from admitting views which appeared so extreme. Receptivity
+is the first virtue of the scientific mind; but, with every
+desire to lay aside prejudice, many felt unequal to the
+acceptance of structural features involving a folding of the
+earth-crust in laps which lay for scores of miles from country to
+country, and the carriage of mountainous materials from the south
+of the Alps to the north, leaving them finally as Alpine ranges
+of ancient sediments reposing on foundations of more recent date.
+The historian of the subject will have to relate how some who
+finally were most active in advancing the new views were at first
+opposed to them. In the change of conviction of these eminent
+geologists we have the strongest proof of the convincing nature
+of the observations and the reality of the tectonic features upon
+which the recent views are founded.
+
+The lesser mountains which stand along the northern border of the
+great limestone Alps, those known as the Prealpes, present the
+strange characteristic of resting upon materials younger than
+themselves. Such mountains as the remarkable-looking Mythen, near
+Schwyz, for instance, are weathered from masses of Triassic and
+Jurassic rock, and repose on the much more recent Flysch. In
+sharp contrast to the Flysch scenery, they stand as abrupt and
+gigantic erratics, which have been transported from the central
+zone of the Alps lying far to the south. They are strangers
+petrologically,
+
+150
+
+stratigraphically, and geographically,[1] to the locality in
+which they now occur. The exotic materials may be dolomites,
+limestones, schists, sandstones, or rocks of igneous origin. They
+show in every case traces of the severe dynamic actions to which
+they have been subjected in transit. The igneous, like the
+sedimentary, klippen, can be traced to distant sources; to the
+massif of Belladonne, to Mont Blanc, Lugano, and the Tyrol. The
+Prealpes are, in fact, mountains without local roots.
+
+In this last-named essential feature, the Prealpes do not differ
+from the still greater limestone Alps which succeed them to the
+south. These giants, _e.g._ the Jungfrau, Wetterhorn, Eiger, etc.,
+are also without local foundations. They have been formed from
+the overthrown and drawn-out anticlines of great crust-folds,
+whose synclines or roots are traceable to the south side of the
+Rhone Valley. The Bernese Oberland originated in the piling-up of
+four great sheets or recumbent folds, one of which is continued
+into the Prealpes. With Lugeon[2] we may see in the phenomenon of
+the formation of the Prealpes a detail; regarding it as a normal
+expression of that mechanism which has created the Swiss Alps.
+For these limestone masses of the Oberland are not indications of
+a merely local shift of the sedimentary covering of the Alps.
+Almost the whole covering has
+
+[1] De Lapparent, _Traite de Geologie_, p. 1,785.
+
+[2] Lugeon, _Bulletin Soc. Geol. de France_, 1901, p. 772.
+
+151
+
+been pushed over and piled up to the north. Lugeon[l] concludes
+that, before denudation had done its work and cut off the
+Prealpes from their roots, there would have been found sheets, to
+the number of eight, superimposed and extending between the Mont
+Blanc massif and the massif of the Finsteraarhorn: these sheets
+being the overthrown folds of the wrinkled sedimentary covering.
+The general nature of the alpine structure
+
+{Fig. 8}
+
+will be understood from the presentation of it diagrammatically
+after Schmidt of Basel (Fig. 8).[2] The section extends from
+north to south, and brings out the relations of the several
+recumbent folds. We must imagine almost the whole of these
+superimposed folds now removed from the central regions of the
+Alps by denudation,
+
+[1] Lugeon, _loc. cit._
+
+[2] Schmidt, _Ec. Geol. Helvetiae_, vol. ix., No. 4.
+
+152
+
+and leaving the underlying gneisses rising through the remains of
+Permian, Triassic, and Jurassic sediments; while to the north the
+great limestone mountains and further north still, the Prealpes,
+carved from the remains of the recumbent folds, now stand with
+almost as little resemblance to the vanished mountains as the
+memories of the past have to its former intense reality.
+
+These views as to the origin of the Alps, which are shared at the
+present day by so many distinguished geologists, had their origin
+in the labours of many now gone; dating back to Studer; finding
+their inspiration in the work of Heim, Suess, and Marcel
+Bertrand; and their consummation in that of Lugeon, Schardt,
+Rothpletz, Schmidt, and many others. Nor must it be forgotten
+that nearer home, somewhat similar phenomena, necessarily on a
+smaller scale, were recognised by Lapworth, twenty-six years ago,
+in his work on the structure of the Scottish Highlands.
+
+An important tectonic principle underlies the development of the
+phenomena we have just been reviewing. The uppermost of the
+superimposed recumbent folds is more extended in its development
+than those which lie beneath. Passing downwards from the highest
+of the folds, they are found to be less and less extended both in
+the northerly and in the southerly direction, speaking of the
+special case--the Alps--now before us. This feature might be
+described somewhat differently. We might say that those folds
+which had their roots farther
+
+153
+
+to the south were the most drawn-out towards the north: or again
+we might say that the synclinal or deep-seated part of the fold
+has lagged behind the anticlinal or what was originally the
+highest part of the fold, in the advance of the latter to the
+north. The anticline has advanced relatively to the syncline. To
+this law one exception only is observed in the Swiss Alps; the
+sheet of the Breche (_Byecciendecke_) falls short, in its northerly
+extension, of the underlying fold, which extends to form the
+Prealpes.
+
+Contemplating such a generalised section as Professor Schmidt's,
+or, indeed, more particular sections, such as those in the Mont
+Blanc Massif by Marcel Bertrand,[1] of the Dent de Morcles,
+Diablerets, Wildhorn, and Massif de la Breche by Lugeon,[2] or
+finally Termier's section of the Pelvoux Massif,[3] one is
+reminded of the breaking of waves on a sloping beach. The wave,
+retarded at its base, is carried forward above by its momentum,
+and finally spreads far up on the strand; and if it could there
+remain, the succeeding wave must necessarily find itself
+superimposed upon the first. But no effects of inertia, no
+kinetic effects, may be called to our aid in explaining the
+formation of mountains. Some geologists have accordingly supposed
+that in order to account for
+
+[1] Marcel Bertrand, _Cong. Geol. Internat._, 1900, Guide Geol.,
+xiii. a, p. 41.
+
+[2] Lugeon, _loc. cit._, p. 773.
+
+[3] De Lapparent, _Traite de Geol._, p. 1,773.
+
+154
+
+the recumbent folds and the peculiar phenomena of increasing
+overlap, or _deferlement_, an obstacle, fixed and deep-seated, must
+have arrested the roots or synclines of the folds, and held them
+against translational motion, while a movement of the upper crust
+drew out and carried forward the anticlines. Others have
+contented themselves by recording the facts without advancing any
+explanatory hypothesis beyond that embodied in the incontestable
+statement that such phenomena must be referred to the effects of
+tangential forces acting in the Earth's crust.
+
+It would appear that the explanation of the phenomena of
+recumbent folds and their _deferlement_ is to be obtained directly
+from the temperature conditions prevailing throughout the
+stressed pile of rocks; and here the subject of mountain
+tectonics touches that with which we were elsewhere specially
+concerned--the geological influence of accumulated radioactive
+energy.
+
+As already shown[1], a rise of temperature due to this source of
+several hundred degrees might be added to such temperatures as
+would arise from the mere blanketing of the Earth, and the
+consequent upward movement of the geotherms. The time element is
+here the most important consideration. The whole sequence of
+events from the first orogenic movements to the final upheaval in
+Pliocene times must probably have occupied not less than ten
+million years.
+
+[1] _Mountain Genesis_, p. 129, et seq.
+
+155
+
+Unfortunately the full investigation of the distribution of
+temperature after any given time is beset with difficulties; the
+conditions being extremely complex. If the radioactive heating
+was strictly adiabatic--that is, if all the heat was conserved and
+none entered from without--the time required for the attainment of
+the equilibrium radioactive temperature would be just about six
+million years. The conditions are not, indeed, adiabatic; but, on
+the other hand, the rocks upraised by lateral pressure were by no
+means at 0 deg. C. to start with. They must be assumed to have
+possessed such temperatures as the prior radiothermal effects,
+and the conducted heat from the Earth's interior, may have
+established.
+
+It would from this appear probable that if a duration of ten
+million years was involved, the equilibrium radioactive
+temperatures must nearly have been attained. The effects of heat
+conducted from the underlying earthcrust have to be added,
+leading to a further rise in temperature of not less than 500 deg. or
+600 deg. . In such considerations the observed indications of high
+temperatures in materials now laid bare by denudation, probably
+find their explanation (P1. XIX).
+
+The first fact that we infer from the former existence of such a
+temperature distribution is the improbability, indeed the
+impossibility, that anything resembling a rigid obstacle, or
+deep-seated "horst," can have existed beneath the present
+surface-level, and opposed the northerly movement of the
+deep-lying synclines. For
+
+156
+
+such a horst can only have been constituted of some siliceous
+rock-material such as we find everywhere rising through the
+worn-down sediments of the Alps; and the idea that this could
+retain rigidity under the prevailing temperature conditions, must
+be dismissed. There is no need to labour this question; the horst
+cannot have existed. To what, then, is the retardation of the
+lower parts of the folds, their overthrow, above, to the north,
+and their _deferlement_, to be ascribed?
+
+A little consideration shows that the very conditions of high
+temperature and viscosity, which render untenable the hypothesis
+of a rigid obstacle, suffice to afford a full explanation of the
+retardation of the roots of the folds. For directed translatory
+movements cannot be transmitted through a fluid, pressure in
+which is necessarily hydrostatic, and must be exerted equally in
+every direction. And this applies, not only to a fluid, but to a
+body which will yield viscously to an impressed force. There will
+be a gradation, according as viscosity gives place to rigidity,
+between the states in which the applied force resolves itself
+into a purely hydrostatic pressure, and in which it is
+transmitted through the material as a directed thrust. The nature
+of the force, in the most general case, of course, has to be
+considered; whether it is suddenly applied and of brief duration,
+or steady and long-continued. The latter conditions alone apply
+to the present case.
+
+It follows from this that, although a tangential force
+
+157
+
+or pressure be engendered by a crustal movement occurring to the
+south, and the resultant effects be transmitted northwards, these
+stresses can only mechanically affect the rigid parts of the
+crust into which they are carried. That is to say, they may
+result in folding and crushing, or horizontally transporting, the
+upper layers of the Earth's crust; but in the deeper-lying
+viscous materials they must be resolved into hydrostatic pressure
+which may act to upheave the overlying covering, but must refuse
+to transmit the horizontal translatory movements affecting the
+rigid materials above.
+
+Between the regions in which these two opposing conditions
+prevail there will be no hard and fast line; but with the
+downward increase of fluidity there will be a gradual failure of
+the mechanical conditions and an increase of the hydrostatic.
+Thus while the uppermost layers of the crust may be transported
+to the full amount of the crustal displacement acting from the
+south (speaking still of the Alps) deeper down there will be a
+lesser horizontal movement, and still deeper there is no
+influence to urge the viscous rock-materials in a northerly
+direction. The consequences of these conditions must be the
+recumbence of the folds formed under the crust-stress, and their
+_deferlement_ towards the north. To see this, we must follow the
+several stages of development.
+
+The earliest movements, we may suppose, result in flexures of the
+Jura-Mountain type--that is, in a
+
+158
+
+succession of undulations more or less symmetrical. As the
+orogenic force continues and develops, these undulations give
+place to folds, the limbs of which are approximately vertical,
+and the synclinal parts of which become ever more and more
+depressed into the deeper, and necessarily hotter, underlying
+materials; the anticlines being probably correspondingly
+elevated. These events are slowly developed, and the temperature
+beneath is steadily rising in consequence of the conducted
+interior heat, and the steady accumulation of radioactive energy
+in the sedimentary rocks and in the buried radioactive layer of
+the Earth. The work expended on the crushed and sheared rock also
+contributes to the developing temperature. Thus the geotherms
+must move upwards, and the viscous conditions extend from below;
+continually diminishing the downward range of the translatory
+movements progressing in the higher parts. While above the folded
+sediments are being carried northward, beneath they are becoming
+anchored in the growing viscosity of the medium. The anticlines
+will bend over, and the most southerly of the folds will
+gradually become pushed or bent over those lying to the north.
+Finally, the whole upper part of the sheaf will become
+horizontally recumbent; and as the uppermost folds will be those
+experiencing the greatest effects of the continued displacement,
+the _deferlement_ or overlap must necessarily arise.
+
+We may follow these stages of mountain evolution
+
+159
+
+in a diagram (Fig. 9) in which we eliminate intermediate
+conditions, and regard the early and final stages of development
+only. In the upper sketch we suppose the lateral compression much
+developed and the upward movement of the geotherms in progress.
+The dotted line may be assumed to be a geotherm having a
+temperature of viscosity. If the conditions here shown persist
+
+{Fig. 9}
+
+indefinitely, there is no doubt that the only further
+developments possible are the continued crushing of the sediments
+and the bodily displacement of the whole mass to the north. The
+second figure is intended to show in what manner these results
+are evaded. The geotherm of viscosity has risen. All above it is
+affected mechanically by the continuing stress, and borne
+northwards in varying
+
+160
+
+degree depending upon the rigidity. The folds have been
+overthrown and drawn out; those which lay originally most to the
+south have become the uppermost; and, experiencing the maximum
+amount of displacement, overlap those lying beneath. There has
+also been a certain amount of upthrow owing to the hydrostatic
+pressure. This last-mentioned element of the phenomena is of
+highly indeterminate character, for we know not the limits to
+which the hydrostatic pressure may be transmitted, and where it
+may most readily find relief. While, according to some of the
+published sections, the uplifting force would seem to have
+influenced the final results of the orogenic movements, a
+discussion of its effects would not be profitable.
+
+161
+
+OTHER MINDS THAN OURS?
+
+IN the year 1610 Galileo, looking through his telescope then
+newly perfected by his own hands, discovered that the planet
+Jupiter was attended by a train of tiny stars which went round
+and round him just as the moon goes round the Earth.
+
+It was a revelation too great to be credited by mankind. It was
+opposed to the doctrine of the centrality of the Earth, for it
+suggested that other worlds constituted like ours might exist in
+the heavens.
+
+Some said it was a mere optic illusion; others that he who looked
+through such a tube did it at the peril of his soul--it was but a
+delusion of Satan. Galileo converted a few of the unbelievers who
+had the courage to look through his telescope. To the others he
+said, he hoped they would see those moons on their way to heaven.
+Old as this story is it has never lost its pathos or its
+teaching.
+
+The spirit which assailed Galileo's discoveries and which finally
+was potent to overshadow his declining years, closed in former
+days the mouths of those who asked the question written at the
+head of this lecture: "Are we to believe that there are other
+minds than ours?"
+
+162
+
+Today we consider the question in a very different spirit. Few
+would regard it as either foolish or improper. Its intense
+interest would be admitted by all, and but for the limitations
+closing our way on every side it would, doubtless, attract the
+most earnest investigation. Even on the mere balance of judgment
+between the probable and the improbable, we have little to go on.
+We know nothing definitely as to the conditions under which life
+may originate: whether these are such as to be rare almost to
+impossibility, or common almost to certainty. Only within narrow
+limits of temperature and in presence of certain of the elements,
+can life like ours exist, and outside these conditions life, if
+such there be, must be different from ours. Once originated it is
+so constituted as to assail the energies around it and to advance
+from less to greater. Do we know more than these vague facts?
+Yes, we have in our experience one other fact and one involving
+much.
+
+We know that our world is very old; that life has been for many
+millions of years upon it; and that Man as a thinking being is
+but of yesterday. Here is then a condition to be fulfilled. To
+every world is physically assigned a limit to the period during
+which it is habitable according to our knowledge of life and its
+necessities. This limit passed and rationality missed, the chance
+for that world is gone for ever, and other minds than ours
+assuredly will not from it contemplate the universe. Looking at
+our own world we see that the tree of life has,
+
+163
+
+indeed, branched, leaved and, possibly, budded many times; it
+never bloomed but once.
+
+All difficulties dissolve and speculations become needless under
+one condition only: that in which rationality may be inferred
+directly or indirectly by our observations on some sister world
+in space, This is just the evidence which in recent years has
+been claimed as derived from a study of the surface of Mars. To
+that planet our hope of such evidence is restricted. Our survey
+in all other directions is barred by insurmountable difficulties.
+Unless some meteoric record reached our Earth, revelationary of
+intelligence on a perished world, our only hope of obtaining such
+evidence rests on the observation of Mars' surface features. To
+this subject we confine our attention in what follows.
+
+The observations made during recent years upon the surface
+features of Mars have, excusably enough, given rise to
+sensational reports. We must consider under what circumstances
+these observations have been made.
+
+Mars comes into particularly favourable conditions for
+observation every fifteen years. It is true that every two years
+and two months we overtake him in his orbit and he is then in
+"opposition." That is, the Earth is between him and the sun: he
+is therefore in the opposite part of the heavens to the sun. Now
+Mars' orbit is very excentric, sometimes he is 139 million miles
+from the sun, and sometimes he as as much as 154 million miles
+from the sun. The Earth's orbit is, by comparison, almost
+
+164
+
+a circle. Evidently if we pass him when he is nearest to the sun
+we see him at his best; not only because he is then nearest to
+us, but because he is then also most brightly lit. In such
+favourable oppositions we are within 35 million miles of him; if
+Mars was in aphelion we would pass him at a distance of 61
+million miles. Opposition occurs under the most favourable
+circumstances every fifteen years. There was one in 1862, another
+in 1877, one in 1892, and so on.
+
+When Mars is 35 million miles off and we apply a telescope
+magnifying 1,000 diameters, we see him as if placed 35,000 miles
+off. This would be seven times nearer than we see the moon with
+the naked eye. As Mars has a diameter about twice as great as
+that of the moon, at such a distance he would look fourteen times
+the diameter of the moon. Granting favourable conditions of
+atmosphere much should be seen.
+
+But these are just the conditions of atmosphere of which most of
+the European observatories cannot boast. It is to the honour of
+Schiaparelli, of Milan, that under comparatively unfavourable
+conditions and with a small instrument, he so far outstripped his
+contemporaries in the observation of the features of Mars that
+those contemporaries received much of his early discoveries with
+scepticism. Light and dark outlines and patches on the planet's
+surface had indeed been mapped by others, and even a couple of
+the canals sighted; but at the opposition of 1877 Schiaparelli
+first mapped any considerable
+
+165
+
+number of the celebrated "canals" and showed that these
+constituted an extraordinary and characteristic feature of the
+planet's geography. He called them "canali," meaning thereby
+"channels." It is remarkable indeed that a mistranslation appears
+really responsible for the initiation of the idea that these
+features are canals.
+
+In 1882 Schiaparelli startled the astronomical world by declaring
+that he saw some of the canals double--that is appearing as two
+parallel lines. As these lines span the planet's surface for
+distances of many thousands of miles the announcement naturally
+gave rise to much surprise and, as I have said, to much
+scepticism. But he resolutely stuck to his statement. Here is his
+map of 1882. It is sufficiently startling.
+
+In 1892 he drew a new map. It adds a little to the former map,
+but the doubling was not so well seen. It is just the strangest
+feature about this doubling that at times it is conspicuous, at
+times invisible. A line which is distinctly seen as a single line
+at one time, a few weeks later will appear distinctly to consist
+of two parallel lines; like railway tracks, but tracks perhaps
+200 miles apart and up to 3,000 or even 4,000 miles in length.
+
+Many speculations were, of course, made to account for the origin
+of such features. No known surface peculiarity on the Earth or
+moon at all resembles these features. The moon's surface as you
+know is cracked and
+
+166
+
+streaked. But the cracks are what we generally find cracks to
+be--either aimless, wandering lines, or, if radiating from a
+centre, then lines which contract in width as they leave the
+point of rupture. Where will we find cracks accurately parallel
+to one another sweeping round a planet's face with steady
+curvature for, 4,000 miles, and crossing each other as if quite
+unhampered by one another's presence? If the phenomenon on Mars
+be due to cracks they imply a uniformity in thickness and
+strength of crust, a homogeneity, quite beyond all anticipation.
+We will afterwards see that the course of the lines is itself
+further opposed to the theory that haphazard cracking of the
+crust of the planet is responsible for the lines. It was also
+suggested that the surface of the planet was covered with ice and
+that these were cracks in the ice. This theory has even greater
+difficulties than the last to contend with. Rivers have been
+suggested. A glance at our own maps at once disposes of this
+hypothesis. Rivers wander just as cracks do and parallel rivers
+like parallel cracks are unknown.
+
+In time the many suggestions were put aside. One only remained.
+That the lines are actually the work of intelligence; actually
+are canals, artificially made, constructed for irrigation
+purposes on a scale of which we can hardly form any conception
+based on our own earthly engineering structures.
+
+During the opposition of 1894, Percival Lowell, along with A. E.
+Douglass, and W. H. Pickering,
+
+167
+
+observed the planet from the summit of a mountain in Arizona,
+using an 18-inch refracting telescope and every resource of
+delicate measurement and spectroscopy. So superb a climate
+favoured them that for ten months the planet was kept under
+continual observation. Over 900 drawings were made and not only
+were Schiaparelli's channels confirmed, but they added 116 to his
+79, on that portion of the planet visible at that opposition.
+They made the further important discovery that the lines do not
+stop short at the dark regions of the planet's surface, as
+hitherto believed, but go right on in many cases; the curvature
+of the lines being unaltered.
+
+Lowell is an uncompromising advocate of the "canal" theory. If
+his arguments are correct we have at once an answer to our
+question, "Are there other minds than ours?"
+
+We must consider a moment Lowell's arguments; not that it is my
+intention to combat them. You must form your own conclusions. I
+shall lay before you another and, as I venture to think, more
+adequate hypothesis in explanation of the channels of
+Schiaparelli. We learn, however, much from Lowell's book--it is
+full of interest.[1]
+
+Lowell lays a deep foundation. He begins by showing that Mars has
+an atmosphere. This must be granted him till some counter
+observations are made.
+
+[1] _Mars_, by Percival Lowell (Longmans, Green & Co.), 1896,
+
+168
+
+It is generally accepted. What that atmosphere is, is another
+matter. He certainly has made out a good case for the presence of
+water as one of its constituents,
+
+It was long known that Mars possessed white regions at his poles,
+just as our Earth does. The waning of these polar snows--if indeed
+they are such--with the advance of the Martian summer, had often
+been observed. Lowell plots day by day this waning. It is evident
+from his observations that the snowfall must be light indeed. We
+see in his map the south pole turned towards us. Mars in
+perihelion always turns his south pole towards the sun and
+therefore towards the Earth. We see that between the dates June
+3rd to August 3rd--or in two months--the polar snow had almost
+completely vanished. This denotes a very scanty covering. It must
+be remembered that Mars even when nearest to the sun receives but
+half our supply of solar heat and light.
+
+But other evidence exists to show that Mars probably possesses
+but little water upon his surface. The dark places are not
+water-covered, although they have been named as if they were,
+indeed, seas and lakes. Various phenomena show this. The canals
+show it. It would never do to imagine canals crossing the seas.
+No great rivers are visible. There is a striking absence of
+clouds. The atmosphere of Mars seems as serene as that of Venus
+appears to be cloudy. Mists and clouds, however, sometime appear
+to veil his face and add to the difficulty of
+
+169
+
+making observations near the limb of the planet. Lowell concludes
+it must be a calm and serene atmosphere; probably only
+one-seventh of our own in density. The normal height of the
+barometer in Mars would then be but four and a half inches. This
+is a pressure far less than exists on the top of the highest
+terrestrial mountain. A mountain here must have an altitude of
+about ten miles to possess so low a pressure on its summit. Drops
+of water big enough to form rain can hardly collect in such a
+rarefied atmosphere. Moisture will fall as dew or frost upon the
+ground. The days will be hot owing to the unimpeded solar
+radiation; the nights bitterly cold owing to the free radiation
+into space.
+
+We may add that in such a climate the frost will descend
+principally upon the high ground at night time and in the
+advancing day it will melt. The freer radiation brings about this
+phenomenon among our own mountains in clear and calm weather.
+
+With the progressive melting of the snow upon the pole Lowell
+connected many phenomena upon the planet's surface of much
+interest. The dark spaces appear to grow darker and more
+greenish. The canals begin to show themselves and reveal their
+double nature. All this suggests that the moisture liberated by
+the melting of the polar snow with the advancing year, is
+carrying vitality and springtime over the surface of the planet.
+But how is the water conveyed?
+
+Lowell believes principally by the canals. These are
+
+170
+
+constructed triangulating the surface of the planet in all
+directions. What we see, according to Lowell, is not the canal
+itself, but the broad band of vegetation which springs up on the
+arrival of the water. This band is perhaps thirty or forty miles
+wide, but perhaps much less, for Lowell reports that the better
+the conditions of observation the finer the lines appeared, so
+that they may be as narrow, possibly, as fifteen miles. It is to
+be remarked that a just visible dot on the surface of Mars must
+possess a diameter of 30 miles. But a chain of much smaller dots
+will be visible, just as we can see such fine objects as spiders'
+webs. The widening of the canals is then accounted for, according
+to Lowell, by the growth of a band of vegetation, similar to that
+which springs into existence when the floods of the Nile irrigate
+the plains of Egypt.
+
+If no other explanation of the lines is forthcoming than that
+they are the work of intelligence, all this must be remembered.
+If all other theories fail us, much must be granted Lowell. We
+must not reason like fishes--as Lowell puts it--and deny that
+intelligent beings can thrive in an atmospheric pressure of four
+and half inches of mercury. Zurbriggen has recently got to the
+top of Aconcagua, a height of 24,000 feet. On the summit of such
+a mountain the barometer must stand at about ten inches. Why
+should not beings be developed by evolution with a lung capacity
+capable of living at two and a half times this altitude. Those
+steadily
+
+171
+
+curved parallel lines are, indeed, very unlike anything we have
+experience of. It would be rather to be expected that another
+civilisation than our own would present many wide differences in
+its development.
+
+What then is the picture we have before us according to Lowell?
+It is a sufficiently dramatic one.
+
+Mars is a world whose water supply, never probably very abundant,
+has through countless years been drying up, sinking into his
+surface. But the inhabitants are making a brave fight for it,
+They have constructed canals right round their world so that the
+water, which otherwise would run to waste over the vast deserts,
+is led from oasis to oasis. Here the great centres of
+civilisation are placed: their Londons, Viennas, New Yorks. These
+gigantic works are the works of despair. A great and civilised
+world finds death staring it in the face. They have had to triple
+their canals so that when the central canal has done its work the
+water is turned into the side canals, in order to utilise it as
+far as possible. Through their splendid telescopes they must view
+our seas and ample rivers; and must die like travellers in the
+desert seeing in a mirage the cool waters of a distant lake.
+
+Perhaps that lonely signal reported to have been seen in the
+twilight limb of Mars was the outcome of pride in their splendid
+and perishing civilisation. They would leave some memory of it:
+they would have us witness how great was that civilisation before
+they perish!
+
+I close this dramatic picture with the poor comfort
+
+172
+
+that several philanthropic people have suggested signalling to
+them as a mark of sympathy. It is said that a fortune was
+bequeathed to the French Academy for the purpose of communicating
+with the Martians. It has been suggested that we could flash
+signals to them by means of gigantic mirrors reflecting the light
+of our Sun. Or, again, that we might light bonfires on a
+sufficiently large scale. They would have to be about ten miles
+in diameter! A writer in the Pall Mall Gazette suggested that
+there need really be no difficulty in the matter. With the kind
+cooperation of the London Gas Companies (this was before the days
+of electric lighting) a signal might be sent without any
+additional expense if the gas companies would consent to
+simultaneously turn off the gas at intervals of five minutes over
+the whole of London, a signal which would be visible to the
+astronomers in Mars would result. He adds, naively: "If only
+tried for an hour each night some results might be obtained."
+
+II
+
+We have reviewed the theory of the artificial construction of the
+Martian lines. The amount of consideration we are disposed to
+give to the supposition that there are upon Mars other minds than
+ours will--as I have stated--necessarily depend upon whether or not
+we can assign a probable explanation of the lines upon purely
+physical grounds. If it is apparent that such
+
+173
+
+lines would be formed with great probability under certain
+conditions, which conditions are themselves probable, then the
+argument by exclusion for the existence of civilisation on Mars,
+at once breaks down.
+
+{Fig. 10}
+
+As a romance writer is sometimes under the necessity of
+transporting his readers to other scenes, so I must now ask you
+to consent to be transported some millions
+
+174
+
+of miles into the region of the heavens which lies outside Mars'
+orbit.
+
+Between Mars and Jupiter is a chasm of 341 millions of miles.
+This gap in the sequence of planets was long known to be quite
+out of keeping with the orderly succession of worlds outward from
+the Sun. A society was formed at the close of the last century
+for the detection of the missing world. On the first day of the
+last century, Piazzi--who, by the way, was not a member of the
+society--discovered a tiny world in the vacant gap. Although
+eagerly welcomed, as better than nothing, it was a disappointing
+find. The new world was a mere rock. A speck of about 160 miles
+in diameter. It was obviously never intended that such a body
+should have all this space to itself. And, sure enough, shortly
+after, another small world was discovered. Then another was
+found, and another, and so on; and now more than 400 of these
+strange little worlds are known.
+
+But whence came such bodies? The generally accepted belief is
+that these really represent a misbegotten world. When the Sun was
+younger he shed off the several worlds of our system as so many
+rings. Each ring then coalesced into a world. Neptune being the
+first born; Mercury the youngest born.
+
+After Jupiter was thrown off, and the Sun had shrunk away inwards
+some 20o million miles, he shed off another ring. Meaning that
+this offspring of his should grow up like the rest, develop into
+a stable world with the
+
+175
+
+potentiality even, it may be, of becoming the abode of rational
+beings. But something went wrong. It broke up into a ring of
+little bodies, circulating around him.
+
+It is probable on this hypothesis that the number we are
+acquainted with does not nearly represent the actual number of
+past and present asteroids. It would take 125,000 of the biggest
+of them to make up a globe as big as our world. They, so far as
+they are known, vary in size from 10 miles to 160 miles in
+diameter. It is probable then--on the assumption that this failure
+of a world was intended to be about the mass of our Earth--that
+they numbered, and possibly number, many hundreds of thousands.
+
+Some of these little bodies are very peculiar in respect to the
+orbits they move in. This peculiarity is sometimes in the
+eccentricity of their orbits, sometimes in the manner in which
+their orbits are tilted to the general plane of the ecliptic, in
+which all the other planets move.
+
+The eccentricity, according to Proctor, in some cases may attain
+such extremes as to bring the little world inside Mars' mean
+distance from the sun. This, as you will remember, is very much
+less than his greatest distance from the sun. The entire belt of
+asteroids--as known--lie much nearer to Mars than to Jupiter.
+
+As regards the tilt of their orbits, some are actually as much as
+34 degrees inclined to the ecliptic, so that in fact they are
+seen from the Earth among our polar constellations.
+
+176
+
+From all this you see that Mars occupies a rather hot comer in
+the solar system. Is it not possible that more than once in the
+remote past Mars may have encountered one of these wanderers? If
+he came within a certain distance of the small body his great
+mass would sway it from its orbit, and under certain conditions
+he would pick up a satellite in this manner. That his present
+satellites were actually so acquired is the suggestion of Newton,
+of Yale College.
+
+Mars' satellites are indeed suspiciously and most abnormally
+small. I have not time to prove this to you by comparison with
+the other worlds of the solar system. In fact, they were not
+discovered till 1877--although they were predicted in a most
+curious manner, with the most uncannily accurate details, by
+Swift.
+
+One of these bodies is about 36 miles in diameter. This is
+Phobos. Phobos is only 3.700 miles from the surface of Mars. The
+other is smaller and further off. He is named Deimos, and his
+diameter is only 10 miles. He is 12,500 miles from Mars' surface.
+With the exception of Phobos the next smallest satellite known in
+the solar system is one of Saturn's--Hyperion; almost 800 miles in
+diameter. The inner one goes all round Mars in 71/2 hours. This is
+Phobos' month. Mars turns on his axis in 24 hours and 40 minutes,
+so that people in Mars would see the rise of Phobos twice in the
+course of a day and night; lie would apparently cross the sky
+
+177
+
+going against the other satellite; that is, he would move
+apparently from west to east.
+
+We may at least assume as probable that other satellites have
+been gathered by Mars in the past from the army of asteroids.
+
+Some of the satellites so picked up would be direct: that is,
+would move round the planet in the direction of his axial
+rotation. Others, on the chances, would be retrograde: that is,
+would move against his axial rotation. They would describe orbits
+making the same various angles with the ecliptic as do the
+asteroids; and we may be sure they would be of the same varying
+dimensions.
+
+We go on to inquire what would be the consequence to Mars of such
+captures.
+
+A satellite captured in this manner is very likely to be pulled
+into the Planet. This is a probable end of a satellite in any
+case. It will probably be the end of our satellite too. The
+satellite Phobos is indeed believed to be about to take this very
+plunge into his planet. But in the case when the satellite picked
+up happens to be rotating round the planet in the opposite
+direction to the axial rotation of the planet, it is pretty
+certain that its career as a satellite will be a brief one. The
+reasons for this I cannot now give. If, then, Mars picked up
+satellites he is very sure to have absorbed them sooner or later.
+Sooner if they happened to be retrograde satellites, later if
+direct satellites. His present satellites are recent additions.
+They are direct.
+
+178
+
+The path of an expiring satellite will be a slow spiral described
+round the planet. The spiral will at last, after many years,
+bring the satellite down upon the surface of the primary. Its
+final approach will be accelerated if the planet possesses an
+atmosphere, as Mars probably does. A satellite of the dimensions
+of Phobos--that is 36 miles in diameter--would hardly survive more
+than 30 to 60 years within seventy miles of Mars' surface. It
+will then be rotating round Mars in an hour and forty minutes,
+moving, in fact, at the rate of 2.2 miles per second. In the
+course of this 30 or 60 years it will, therefore, get round
+perhaps 200,000 times, before it finally crashes down upon the
+Martians. During this closing history of the satellite there is
+reason to believe, however, that it would by no means pursue
+continually the same path over the surface of the planet. There
+are many disturbing factors to be considered. Being so small any
+large surface features of Mars would probably act to perturb the
+orbit of the satellite.
+
+The explanation of Mars' lines which I suggest, is that they were
+formed by the approach of such satellites in former times. I do
+not mean that they are lines cut into his surface by the actual
+infall of a satellite. The final end of the satellite would be
+too rapid for this, I think. But I hope to be able to show you
+that there is reason to believe that the mere passage of the
+satellite, say at 70 miles above the surface of the planet, will,
+in itself, give rise to effects on the crust of the planet
+capable
+
+179
+
+of accounting for just such single or parallel lines as we see.
+
+In the first place we have to consider the stability of the
+satellite. Even in the case of a small satellite we cannot
+overlook the fact that the half of the satellite near the planet
+is pulled towards the planet by a gravitational force greater
+than that attracting the outer half, and that the centrifugal
+force is less on the inner than on the outer hemisphere. Hence
+there exists a force tending to tear the satellite asunder on the
+equatorial section tangential
+
+{Fig. 11}
+
+to the planet's surface. If in a fluid or plastic state, Phobos,
+for instance, could not possibly exist near the planet's surface.
+The forces referred to would decide its fate. It may be shown by
+calculation, however, that if Phobos has the strength of basalt
+or glass there would remain a considerable coefficient of safety
+in favour of the satellite's stability; even when the surfaces of
+planet and satellite were separated by only five miles.
+
+We have now to consider some things which we expect will happen
+before the satellite takes its final plunge into the planet.
+
+180
+
+This diagram (Fig. 11) shows you the satellite travelling above
+the surface of the planet. The satellite is advancing towards, or
+away from, the spectator. The planet is supposed to show its
+solid crust in cross section, which may be a few miles in
+thickness. Below this is such a hot plastic magma as we have
+reason to believe underlies much of the solid crust of our own
+Earth. Now there is an attraction between the satellite and the
+crust of the planet; the same gravitational attraction which
+exists between every particle of matter in the universe. Let us
+consider how this attraction will affect the planet's crust. I
+have drawn little arrows to show how we may consider the
+attraction of the satellite pulling the crust of the planet not
+only upwards, but also pulling it inwards beneath the satellite.
+I have made these arrows longer where calculation shows the
+stress is greater. You see that the greatest lifting stress is
+just beneath the satellite, whereas the greatest stress pulling
+the crust in under the satellite is at a point which lies out
+from under the satellite, at a considerable distance. At each
+side of the satellite there is a point where the stress pulling
+on the crust is the greatest. Of the two stresses the lifting
+stress will tend to raise the crust a little; the pulling stress
+may in certain cases actually tear the crust across; as at A and
+B.
+
+It is possible to calculate the amount of the stress at the point
+at each side of the satellite where the stress is at its
+greatest. We must assume the satellite to be a certain size and
+density; we must also assume the crust of
+
+181
+
+Mars to be of some certain density. To fix our ideas on these
+points I take the case of the present satellite Phobos. What
+amount of stress will he exert upon the crust of Mars when he
+approaches within, say, 40 miles of the planet's surface? We know
+his size approximately--he is about 36 miles in diameter. We can
+guess his density to be between four times that of water and
+eight times that of water. We may assume the density of Mars'
+surface to be about the same as that of our Earth's surface, that
+is three times as dense as water. We now find that the greatest
+stress tending to rend open the surface crust of Mars will be
+between 4,000 and 8,000 pounds to the square foot according to
+the density we assign to Phobos.
+
+Will such a stress actually tear open the crust? We are not able
+to answer this question with any certainty. Much will depend upon
+the nature and condition of the crust. Thus, suppose that we are
+here (Fig. 12) looking down upon the satellite which is moving
+along slowly relatively to Mars' surface, in the direction of the
+arrow. The satellite has just passed over a weak and cracked part
+of the planet's crust. Here the stress has been sufficient to
+start two cracks. Now you know how easy it is to tear a piece of
+cloth when you go to the edge of it in order to make a beginning.
+Here the stress from the satellite has got to the edge of the
+crust. It is greatly concentrated just at the extremities of the
+cracks. It will, unler such circumstances probably carry on the
+
+182
+
+tear. If it does not do so this time, remember the satellite will
+some hours later be coming over the same place again, and then
+again for, at least, many hundreds of times. Then also we are not
+limited to the assumption that the
+
+{Fig. 12}
+
+satellite is as small as Phobos. Suppose we consider the case of
+a satellite approaching Mars which has a diameter double that of
+Phobos; a diameter still much less than that of the larger class
+of asteroids. Even at the distance
+
+183
+
+of 65 miles the stress will now amount to as much as from 15 to
+30 tons per square foot. It is almost certain that such a stress
+repeated a comparatively few times over the same parts of the
+planet's surface would so rend the crust as to set up lines along
+which plutonic action would find a vent. That is, we might expect
+along these lines all the phenomena of upheaval and volcanic
+eruption which give rise to surface elevations.
+
+The probable effect of a satellite of this dimension travelling
+slowly relatively to the surface of Mars is, then, to leave a
+very conspicuous memorial of his presence behind him. You see
+from the diagram that this memorial will consist o: two parallel
+lines of disturbance.
+
+The linear character of the gravitational effects of the
+satellite is due entirely to the motion of the satellite
+relatively to the surface of the planet. If the satellite stood
+still above the surface the gravitational stress in the crust
+would, of course, be exerted radially outwards from the centre of
+the satellite. It would attain at the central point beneath the
+satellite its maximum vertical effect, and at some radial
+distance measured outwards from this point, which distance we can
+calculate, its maximum horizontal tearing effect. When the
+satellite moves relatively to the planet's crust, the horizontal
+tearing force acts differently according to whether it is
+directed in the line of motion or at right angles to this line.
+
+In the direction of motion we see that the satellite
+
+184
+
+creates as it passes over the crust a wave of rarefaction or
+tension as at D, followed by compression just beneath the
+satellite and by a reversed direction of gravitational pull as
+the satellite passes onwards. These stresses rapidly replace one
+another as the satellite travels along. They are resisted by the
+inertia of the crust, and are taken up by its elasticity. The
+nature of this succession of alternate compressions and
+rarefactions in the crust possess some resemblance to those
+arising in an earthquake shock.
+
+If we consider the effects taking place laterally to the line of
+motion we see that there are no such changes in the nature of the
+forces in the crust. At each passage of the satellite the
+horizontal tearing stress increases to a maximum, when it is
+exerted laterally, along the line passing through the horizontal
+projection of the satellite and at right angles to the line of
+motion, and again dies away. It is always a tearing stress,
+renewed again and again.
+
+This effect is at its maximum along two particular parallel lines
+which are tangents to the circle of maximum horizontal stress and
+which run parallel with the path of the satellite. The distance
+separating these lines depend upon the elevation of the satellite
+above the planet's surface. Such lines mark out the theoretical
+axes of the "double canals" which future crustal movements will
+more fully develop.
+
+It is interesting to consider what the effect of such
+
+185
+
+conditions would be if they arose at the surface of our own
+planet. We assume a horizontal force in the crust adequate to set
+up tensile stresses of the order, say, of fifteen tons to the
+square foot and these stresses to be repeated every few hours;
+our world being also subject to the dynamic effects we recognise
+in and beneath its crust.
+
+It is easy to see that the areas over which the satellite exerted
+its gravitational stresses must become the foci --foci of linear
+form--of tectonic developments or crust movements. The relief of
+stresses, from whatever cause arising, in and beneath the crust
+must surely take place in these regions of disturbance and along
+these linear areas. Here must become concentrated the folding
+movements, which are under existing conditions brought into the
+geosynclines, along with their attendant volcanic phenomena. In
+the case of Mars such a concentration of tectonic events would
+not, owing to the absence of extensive subaerial denudation and
+great oceans, be complicated by the existence of such synclinal
+accumulations as have controlled terrestrial surface development.
+With the passage of time the linear features would probably
+develop; the energetic substratum continually asserting its
+influence along such lines of weakness. It is in the highest
+degree probable that radioactivity plays no less a part in
+Martian history than in terrestrial. The fact of radioactive
+heating allows us to assume the thin surface crust and continued
+sub-crustal energy throughout the entire period of the planet's
+history.
+
+186
+
+How far willl these effects resemble the double canals of Mars?
+In this figure and in the calculations I have given you I have
+supposed the satellite engaged in marking the planet's surface
+with two lines separated by about the interval separating the
+wider double canals of Mars--that is about 220 miles apart. What
+the distance between the lines will be, as already stated, will
+depend upon the height of the satellite above the surface when it
+comes upon a part of the crust in a condition to be affected by
+the stresses it sets up in it. If the satellite does its work at
+a point lower down above the surface the canal produced will be
+narrower. The stresses, too, will then be much greater. I must
+also observe that once the crust has yielded to the pulling
+stress, there is great probability that in future revolutions of
+the satellite a central fracture will result. For then all the
+pulling force adds itself to the lifting force and tends to crush
+the crust inwards on the central line beneath the satellite. It
+is thus quite possible that the passage of a satellite may give
+rise to triple lines. There is reason to believe that the canals
+on Mars are in some cases triple.
+
+I have spoken all along of the satellite moving slowly over the
+surface of Mars. I have done so as I cannot at all pronounce so
+readily on what will happen when the satellite's velocity over
+the surface of Mars is very great. To account for all the lines
+mapped by Lowell some of them must have been produced by
+satellities moving relatively to the surface of Mars at
+velocities so great
+
+187
+
+as three miles a second or even rather more. The stresses set up
+are, in such cases, very difficult to estimate. It has not yet
+been done. Parallel lines of greatest stress or impulse ought to
+be formed as in the other case.
+
+I now ask your attention to another kind of evidence that the
+lines are due in some way to the motion of satellites passing
+over the surface of Mars.
+
+I may put the fresh evidence to which I refer, in this way: In
+Lowell's map (P1. XXII, p. 192), and in a less degree in
+Schiaparelli's map (ante p. 166), we are given the course of the
+lines as fragments of incomplete curves. Now these curves might
+have been anything at all. We must take them as they are,
+however, when we apply them as a test of the theory that the
+motion of a satellite round Mars can strike such lines. If it can
+be shown that satellites revolving round Mars might strike just
+such curves then we assume this as an added confirmation of the
+hypothesis.
+
+We must begin by realising what sort of curves a satellite which
+disturbs the surface of a planet would leave behind it after its
+demise. You might think that the satellite revolving round and
+round the planet must simply describe a circle upon the spherical
+surface of the planet: a "great circle" as it is called; that is
+the greatest circle which can be described upon a sphere. This
+great circle can, however, only be struck, as you will see, when
+the planet is not turning upon its axis: a condition not likely
+to be realised.
+
+This diagram (PI. XXI) shows the surface of a globe
+
+188
+
+covered with the usual imaginary lines of latitude and longitude.
+The orbit of a supposed satellite is shown by a line crossing the
+sphere at some assumed angle with the equator. Along this line
+the satellite always moves at uniform velocity, passing across
+and round the back of the sphere and again across. If the sphere
+is not turning on its polar axis then this satellite, which we
+will suppose armed with a pencil which draws a line upon the
+sphere, will strike a great circle right round the sphere. But
+the sphere is rotating. And it is to be expected that at
+different times in a planet's history the rate of rotation varies
+very much indeed. There is reason to believe that our own day was
+once only 21/2 hours long, or thereabouts. After a preliminary rise
+in velocity of axial rotation, due to shrinkage attending rapid
+cooling, a planet as it advances in years rotates slower and
+slower. This phenomenon is due to tidal influences of the sun or
+of satellites. On the assumption that satellites fell into Mars
+there would in his case be a further action tending to shorten
+his day as time went on.
+
+The effect of the rotation of the planet will be, of course, that
+as the satellite advances with its pencil it finds the surface of
+the sphere being displaced from under it. The line struck ceases
+to be the great circle but wanders off in another curve--which is
+in fact not a circle at all.
+
+You will readily see how we find this curve. Suppose the sphere
+to be rotating at such a speed that while the satellite is
+advancing the distance _Oa_, the point _b_ on the
+
+189
+
+sphere will be carried into the path of the satellite. The pencil
+will mark this point. Similarly we find that all the points along
+this full curved line are points which will just find themselves
+under the satellite as it passes with its pencil. This curve is
+then the track marked out by the revolving satellite. You see it
+dotted round the back of the sphere to where it cuts the equator
+at a certain point. The course of the curve and the point where
+it cuts the equator, before proceeding on its way, entirely
+depend upon the rate at which we suppose the sphere to be
+rotating and the satellite to be describing the orbit. We may
+call the distance measured round the planet's equator separating
+the starting point of the curve from the point at which it again
+meets the equator, the "span" of the curve. The span then depends
+entirely upon the rate of rotation of the planet on its axis and
+of the satellite in its orbit round the planet.
+
+But the nature of events might have been somewhat different. The
+satellite is, in the figure, supposed to be rotating round the
+sphere in the same direction as that in which the sphere is
+turning. It might have been that Mars had picked up a satellite
+travelling in the opposite direction to that in which he was
+turning. With the velocity of planet on its axis and of satellite
+in its orbit the same as before, a different curve would have
+been described. The span of the curve due to a retrograde
+satellite will be greater than that due to a direct satellite.
+The retrograde satellite will have a span more than half
+
+190
+
+way round the planet, the direct satellite will describe a curve
+which will be less than half way round the planet: that is a span
+due to a retrograde satellite will be more than 180 degrees,
+while the span due to a direct satellite will be less than 180
+degrees upon the planet's equator.
+
+I would draw your attention to the fact that what the span will
+be does not depend upon how much the orbit of the satellite is
+inclined to the equator. This only decides how far the curve
+marked out by the satellite will recede from the equator.
+
+We find then, so far, that it is easy to distinguish between the
+direct and the retrograde curves. The span of one is less, of the
+other greater, than 180 degrees. The number of degrees which
+either sort of curve subtends upon the equator entirely depends
+upon the velocity of the satellite and the axial velocity of the
+planet.
+
+But of these two velocities that of the satellite may be taken as
+sensibly invariable, when close enough to use his pencil. This
+depends upon the law of centrifugal force, which teaches us that
+the mass of the planet alone decides the velocity of a satellite
+in its orbit at any fixed distance from the planet's centre. The
+other velocity--that of the planet upon its axis--was, as we have
+seen, not in the past what it is now. If then Mars, at various
+times in his past history, picked up satellites, these satellites
+will describe curves round him having different spans which will
+depend upon the velocity of axial rotation of Mars at the time
+and upon this only.
+
+191
+
+In what way now can we apply this knowledge of the curves
+described by a satellite as a test of the lunar origin of the
+lines on Mars?
+
+To do this we must apply to Lowell's map. We pick out preferably,
+of course, the most complete and definite curves. The chain of
+canals of which Acheron and Erebus are members mark out a fairly
+definite curve. We produce it by eye, preserving the curvature as
+far as possible, till it cuts the equator. Reading the span on
+the equator we find' it to be 255 degrees. In the first place we
+say then that this curve is due to a retrograde satellite. We
+also note on Lowell's map that the greatest rise of the curve is
+to a point about 32 degrees north of the equator. This gives the
+inclination of the satellite's orbit to the plane of Mars'
+equator.
+
+With these data we calculate the velocity which the planet must
+have possessed at the time the canal was formed on the hypothesis
+that the curve was indeed the work of a satellite. The final
+question now remains If we determine the curve due to this
+velocity of Mars on its axis, will this curve fit that one which
+appears on Lowell's map, and of which we have really availed
+ourselves of only three points? To answer this question we plot
+upon a sphere, the curve of a satellite, in the manner I have
+described, assigning to this sphere the velocity derived from the
+span of 255 degrees. Having plotted the curve on the sphere it
+only remains to transfer it to Lowell's map. This is easily
+done.
+
+192
+
+This map (Pl. XXII) shows you the result of treating this, as
+well as other curves, in the manner just described. You see that
+whether the fragmentary curves are steep and receding far from
+the equator; or whether they are flat and lying close along the
+equator; whether they span less or more than 180 degrees; the
+curves determined on the supposition that they are the work of
+satellites revolving round Mars agree with the mapped curves;
+following them with wonderful accuracy; possessing their
+properties, and, indeed, in some cases, actually coinciding with
+them.
+
+I may add that the inadmissible span of 180 degrees and spans
+very near this value, which are not well admissible, are so far
+as I can find, absent. The curves are not great circles.
+
+You will require of me that I should explain the centres of
+radiation so conspicuous here and there on Lowell's map. The
+meeting of more than two lines at the oases is a phenomenon
+possibly of the same nature and also requiring explanation.
+
+In the first place the curves to which I have but briefly
+referred actually give rise in most cases to nodal, or crossing
+points; sometimes on the equator, sometimes off the equator;
+through which the path of the satellite returns again and again.
+These nodal points will not, however, afford a general
+explanation of the many-branched radiants.
+
+It is probable that we should refer such an appearance
+
+193
+
+as is shown at the Sinus Titanum to the perturbations of the
+satellite's path due to the surface features on Mars. Observe
+that the principal radiants are situated upon the boundary of the
+dark regions or at the oases. Higher surface levels may be
+involved in both cases. Some marked difference in topography must
+characterise both these features. The latter may possibly
+originate in the destruction of satellites. Or again, they may
+arise in crustal disturbance of a volcanic nature, primarily
+induced or localised by the crossing of two canals. Whatever the
+origin of these features it is only necessary to assume that they
+represent elevated features of some magnitude to explain the
+multiplication of crossing lines. We must here recall what
+observers say of the multiplicity of the canals. According to
+Lowell, "What their number maybe lies quite beyond the
+possibility of count at present; for the better our own air, the
+more of them are visible."
+
+Such innumerable canals are just what the present theory
+requires. An in-falling satellite will, in the course of the last
+60 or 80 years of its career, circulate some 100,000 times over
+Mars' surface. Now what will determine the more conspicuous
+development of a particular canal? The mass of the satellite; the
+state of the surface crust; the proximity of the satellite; and
+the amount of repetition over the same ground. The after effects
+may be taken as proportional to the primary disturbance.
+
+194
+
+It is probable that elevated surface features will influence two
+of these conditions: the number of repetitions and the proximity
+to the surface. A tract 100 miles in diameter and elevated 5,000
+or 10,000 feet would seriously perturb the orbit of such a body as
+Phobos. It is to be expected that not only would it be effective
+in swaying the orbit of the satellite in the horizontal direction
+but also would draw it down closer to the surface. It is even to
+be considered if such a mass might not become nodal to the
+satellite's orbit, so that this passed through or above this
+point at various inclinations with its primary direction. If
+acting to bring down the orbit then this will quicken the speed
+and cause the satellite further on its path to attain a somewhat
+higher elevation above the surface. The lines most conspicuous in
+the telescope are, in short, those which have been favoured by a
+combination of circumstances as reviewed above, among which
+crustal features have, in some cases, played a part.
+
+I must briefly refer to what is one of the most interesting
+features of the Martian lines: the manner in which they appear to
+come and go like visions.
+
+Something going on in Mars determines the phenomenon. On a
+particular night a certain line looks single. A few nights later
+signs of doubling are perceived, and later still, when the seeing
+is particularly good, not one but two lines are seen. Thus, as an
+example, we may take the case of Phison and Euphrates. Faint
+glimpses of the dual state were detected in the summer
+
+195
+
+and autumn, but not till November did they appear as distinctly
+double. Observe that by this time the Antarctic snows had melted,
+and there was in addition, sufficient time for the moisture so
+liberated to become diffused in the planet's atmosphere.
+
+This increase in the definition and conspicuousness of certain
+details on Mars' surface is further brought into connection with
+the liberation of the polar snows and the diffusion of this water
+through the atmosphere, by the fact that the definition appeared
+progressively better from the south pole upwards as the snow
+disappeared. Lowell thinks this points to vegetation springing up
+under the influence of moisture; he considers, however, as we
+have seen, that the canals convey the moisture. He has to assume
+the construction of triple canals to explain the doubling of the
+lines.
+
+If we once admit the canals to be elevated ranges--not necessarily
+of great height--the difficulty of accounting for increased
+definition with increase of moisture vanishes. We need not
+necessarily even suppose vegetation concerned. With respect to
+this last possibility we may remark that the colour observations,
+upon which the idea of vegetation is based, are likely to be
+uncertain owing to possible fatigue effects where a dark object
+is seen against a reddish background.
+
+However this may be we have to consider what the effects of
+moisture increasing in the atmosphere of Mars will be with regard
+to the visibility of elevated ranges,
+
+196
+
+We assume a serene and rare atmosphere: the nights intensely
+cold, the days hot with the unveiled solar radiation. On the hill
+tops the cold of night will be still more intense and so, also,
+will the solar radiation by day. The result of this state of
+things will be that the moisture will be precipitated mainly on
+the mountains during the cold of night--in the form of frost--and
+during the day this covering of frost will melt; and, just as we
+see a heavy dew-fall darken the ground in summer, so the melting
+ice will set off the elevated land against the arid plains below.
+Our valleys are more moist than our mountains only because our
+moisture is so abundant that it drains off the mountains into the
+valleys. If moisture was scarce it would distil from the plains
+to the colder elevations of the hills. On this view the
+accentuation of a canal is the result of meteorological effects
+such as would arise in the Martian climate; effects which must be
+influenced by conditions of mountain elevation, atmospheric
+currents, etc. We, thus, follow Lowell in ascribing the
+accentuation of the canals to the circulation of water in Mars;
+but we assume a simple and natural mode of conveyance and do not
+postulate artificial structures of all but impossible magnitude.
+That vegetation may take part in the darkening of the elevated
+tracts is not improbable. Indeed we would expect that in the
+Martian climate these tracts would be the only fertile parts of
+the surface.
+
+Clouds also there certainly are. More recent observations
+
+197
+
+appear to have set this beyond doubt. Their presence obviously
+brings in other possible explanations of the coming and going of
+elevated surface features.
+
+Finally, we may ask what about the reliability of the maps? About
+this it is to be said that the most recent map--that by Lowell--has
+been confirmed by numerous drawings by different observers, and
+that it is,itself the result of over 900 drawings. It has become
+a standard chart of Mars, and while it would be rash to contend
+for absence of errors it appears certain that the trend of the
+principal canals may be relied on, as, also, the general features
+of the planet's surface.
+
+The question of the possibility of illusion has frequently been
+raised. What I have said above to a great extent answers such
+objections. The close agreement between the drawings of different
+observers ought really to set the matter at rest. Recently,
+however, photography has left no further room for scepticism.
+First photographed in 1905, the planet has since been
+photographed many thousands of times from various observatories.
+A majority of the canals have been so mapped. The doubling of the
+canals is stated to have been also so recorded.[1]
+
+The hypothesis which I have ventured to put before you involves
+no organic intervention to account for the
+
+[1] E. C. Slipher's paper in _Popular Astronomy_ for March, 1914,
+gives a good account of the recent work.
+
+198
+
+details on Mars' surface. They are physical surface features.
+Mars presents his history written upon his face in the scars of
+former encounters--like the shield of Sir Launcelot. Some of the
+most interesting inferences of mathematical and physical
+astronomy find a confirmation in his history. The slowing down in
+the rate of axial rotation of the primary; the final inevitable
+destruction of the satellite; the existence in the past of a far
+larger number of asteroids than we at present are acquainted
+with; all these great facts are involved in the theory now
+advanced. If justifiably, then is Mars' face a veritable
+Principia.
+
+To fully answer the question which heads these lectures, we
+should go out into the populous solitudes (if the term be
+permitted) which lie beyond our system. It is well that there is
+now no time left to do so; for, in fact, there we can only dream
+dreams wherein the limits of the possible and the impossible
+become lost.
+
+The marvel of the infinite number of stars is not so marvellous
+as the rationality that fain would comprehend them. In seeking
+other minds than ours we seek for what is almost infinitely
+complex and coordinated in a material universe relatively simple
+and heterogeneous. In our mental attitude towards the great
+question, this fact must be regarded as fundamental.
+
+I can only fitly close a discourse which has throughout weighed
+the question of the living thought against the unthinking laws of
+matter, by a paraphrase of the words
+
+199
+
+of a great poet when he, in higher and, perhaps, more philosophic
+language, also sought to place the one in comparison with the
+other.[1]
+
+Richter thought that he was--with his human heart
+unstrengthened--taken by an angel among the universe of stars.
+Then, as they journeyed, our solar system was sunken like a faint
+star in the abyss, and they travelled yet further, on the wings
+of thought, through mightier systems: through all the countless
+numbers of our galaxy. But at length these also were left behind,
+and faded like a mist into the past. But this was not all. The
+dawn of other galaxies appeared in the void. Stars more countless
+still with insufferable light emerged. And these also were
+passed. And so they went through galaxies without number till at
+length they stood in the great Cathedral of the Universe. Endless
+were the starry aisles; endless the starry columns; infinite the
+arches and the architraves of stars. And the poet saw the mighty
+galaxies as steps descending to infinity, and as steps going up
+to infinity.
+
+Then his human heart fainted and he longed for some narrow cell;
+longed to lie down in the grave that he might hide from infinity.
+And he said to the angel:
+
+"Angel, I can go with thee no farther. Is there, then, no end to
+the universe of stars?"
+
+[1] De Quincy in his _System of the Heavens_ gives a fine
+paraphrase of "Richter's Dream."
+
+200
+
+Then the angel flung up his glorious hands to the heaven of
+heavens, saying "End is there none to the universe of God? Lo!
+also there is no beginning."
+
+201
+
+THE LATENT IMAGE [1]
+
+My inclination has led me, in spite of a lively dread of
+incurring a charge of presumption, to address you principally on
+that profound and most subtle question, the nature and mode of
+formation of the photographic image. I am impelled to do so, not
+only because the subject is full of fascination and hopefulness,
+but because the wide topics of photographic methods or
+photographic applications would be quite unfittingly handled by
+the president you have chosen.
+
+I would first direct your attention to Sir James Dewar's
+remarkable result that the photographic plate retains
+considerable power of forming the latent image at temperatures
+approaching the absolute zero--a result which, as I submit,
+compels us to regard the fundamental effects progressing in the
+film under the stimulus of light undulations as other than those
+of a purely chemical nature. But few, if any, instances of
+chemical combination or decomposition are known at so low a
+temperature. Purely chemical actions cease, indeed, at far higher
+temperatures, fluorine being among the few bodies which still
+show
+
+[1] Presidential address to the Photographic Convention of the
+United Kingdom, July, 1905. _Nature_, Vol. 72, p. 308.
+
+202
+
+chemical activity at the comparatively elevated temperature of
+-180 deg. C. In short, this result of Sir James Dewar's suggests that
+we must seek for the foundations of photographic action in some
+physical or intra-atomic effect which, as in the case of
+radioactivity or fluorescence, is not restricted to intervals of
+temperature over which active molecular vis viva prevails. It
+compels us to regard with doubt the role of oxidation or other
+chemical action as essential, but rather points to the view that
+such effects must be secondary or subsidiary. We feel, in a word,
+that we must turn for guidance to some purely photo-physical
+effect.
+
+Here, in the first place, we naturally recall the views of Bose.
+This physicist would refer the formation of the image to a strain
+of the bromide of silver molecule under the electric force in the
+light wave, converting it into what might be regarded as an
+allotropic modification of the normal bromide which subsequently
+responds specially to the attack of the developer. The function
+of the sensitiser, according to this view, is to retard the
+recovery from strain. Bose obtained many suggestive parallels
+between the strain phenomena he was able to observe in silver and
+other substances under electromagnetic radiation and the
+behaviour of the photographic plate when subjected to
+long-continued exposure to light.
+
+This theory, whatever it may have to recommend it, can hardly be
+regarded as offering a fundamental explanation. In the first
+place, we are left in the dark as to what
+
+203
+
+the strain may be. It may mean many and various things. We know
+nothing as to the inner mechanism of its effects upon subsequent
+chemical actions--or at least we cannot correlate it with what is
+known of the physics of chemical activity. Finally, as will be
+seen later, it is hardly adequate to account for the varying
+degrees of stability which may apparently characterise the latent
+image. Still, there is much in Bose's work deserving of careful
+consideration. He has by no means exhausted the line of
+investigation he has originated.
+
+Another theory has doubtless been in the minds of many. I have
+said we must seek guidance in some photo-physical phenomenon.
+There is one such which preeminently connects light and chemical
+phenomena through the intermediary of the effects of the former
+upon a component part of the atom. I refer to the phenomena of
+photo-electricity.
+
+It was ascertained by Hertz and his immediate successors that
+light has a remarkable power of discharging negative
+electrification from the surface of bodies--especially from
+certain substances. For long no explanation of the cause of this
+appeared. But the electron--the ubiquitous electron--is now known
+with considerable certainty to be responsible. The effect of the
+electric force in the light wave is to direct or assist the
+electrons contained in the substance to escape from the surface
+of the body. Each electron carries away a very small charge of
+negative electrification. If, then, a body is
+
+204
+
+originally charged negatively, it will be gradually discharged by
+this convective process. If it is not charged to start with, the
+electrons will still be liberated at the surface of the body, and
+this will acquire a positive charge. If the body is positively
+charged at first, we cannot discharge it by illumination.
+
+It would be superfluous for me to speak here of the nature of
+electrons or of the various modes in which their presence may be
+detected. Suffice it to say, in further connection with the Hertz
+effect, that when projected among gaseous molecules the electron
+soon attaches itself to one of these. In other words, it ionises
+a molecule of the gas or confers its electric charge upon it. The
+gaseous molecule may even be itself disrupted by impact of the
+electron, if this is moving fast enough, and left bereft of an
+electron.
+
+We must note that such ionisation may be regarded as conferring
+potential chemical properties upon the molecules of the gas and
+upon the substance whence the electrons are derived. Similar
+ionisation under electric forces enters, as we now believe, into
+all the chemical effects progressing in the galvanic cell, and,
+indeed, generally in ionised solutes.
+
+An experiment will best illustrate the principles I wish to
+remind you of. A clean aluminium plate, carefully insulated by a
+sulphur support, is faced by a sheet of copper-wire-gauze placed
+a couple of centimetres away from it. The gauze is maintained at
+a high positive
+
+205
+
+potential by this dry pile. A sensitive gold-leaf electroscope is
+attached to the aluminium plate, and its image thrown upon the
+screen. I now turn the light from this arc lamp upon the wire
+gauze, through which it in part passes and shines upon the
+aluminium plate. The electroscope at once charges up rapidly.
+There is a liberation of negative electrons at the surface of the
+aluminium; these, under the attraction of the positive body, are
+rapidly removed as ions, and the electroscope charges up
+positively.
+
+Again, if I simply electrify negatively this aluminium plate so
+that the leaves of the attached electroscope diverge widely, and
+now expose it to the rays from the arc lamp, the charge, as you
+see, is very rapidly dissipated. With positive electrification of
+the aluminium there is no effect attendant on the illumination.
+
+Thus from the work of Hertz and his successors we know that
+light, and more particularly what we call actinic light, is an
+effective means of setting free electrons from certain
+substances. In short, our photographic agent, light, has the
+power of expelling from certain substances the electron which is
+so potent a factor in most, if not in all, chemical effects. I
+have not time here to refer to the work of Elster and Geitel
+whereby they have shown that this action is to be traced to the
+electric force in the light wave, but must turn to the probable
+bearing of this phenomenon on the familiar facts of photography.
+I assume that the experiment I have shown you is the most
+
+206
+
+fundamental photographic experiment which it is now in our power
+to make.
+
+We must first ask from what substances can light liberate
+electrons. There are many--metals as well as non-metals and
+liquids. It is a very general phenomenon and must operate widely
+throughout nature. But what chiefly concerns the present
+consideration is the fact that the haloid salts of silver are
+vigorously photo-electric, and, it is suggestive, possess,
+according to Schmidt, an activity in the descending order
+bromide, chloride, iodide. This is, in other words, their order
+of activity as ionisers (under the proper conditions) when
+exposed to ultra-violet light. Photographers will recognise that
+this is also the order of their photographic sensitiveness.
+
+Another class of bodies also concerns our subject: the special
+sensitisers used by the photographer to modify the spectral
+distribution of sensibility of the haloid salts, _e.g._ eosine,
+fuchsine, cyanine. These again are electron-producers under light
+stimulus. Now it has been shown by Stoletow, Hallwachs, and
+Elster and Geitel that there is an intimate connection between
+photo-electric activity and the absorption of light by the
+substance, and, indeed, that the particular wave-lengths absorbed
+by the substance are those which are effective in liberating the
+electrons. Thus we have strong reason for believing that the
+vigorous photo-electric activity displayed by the special
+sensitisers must be dependent upon their colour absorption. You
+will recognise that this is just
+
+207
+
+the connection between their photographic effects and their
+behaviour towards light.
+
+There is yet another suggestive parallel. I referred to the
+observation of Sir James Dewar as to the continued sensitiveness
+of the photographic film at the lowest attained extreme of
+temperature, and drew the inference that the fundamental
+photographic action must be of intra-atomic nature, and not
+dependent upon the vis viva of the molecule or atom. In then
+seeking the origin of photographic action in photo-electric
+phenomena we naturally ask, Are these latter phenomena also
+traceable at low temperatures? If they are, we are entitled to
+look upon this fact as a qualifying characteristic or as another
+link in the chain of evidence connecting photographic with
+photo-electric activity.
+
+I have quite recently, with the aid of liquid air supplied to me
+from the laboratory of the Royal Dublin Society, tested the
+photo-sensibility of aluminium and also of silver bromide down to
+temperatures approaching that of the liquid air. The mode of
+observation is essentially that of Schmidt--what he terms his
+static method. The substance undergoing observation is, however,
+contained at the bottom of a thin copper tube, 5 cm. in diameter,
+which is immersed to a depth of about 10 cm in liquid air. The
+tube is closed above by a paraffin stopper which carries a thin
+quartz window as well as the sulphur tubes through which the
+connections pass. The air within is very carefully dried by
+phosphorus
+
+208
+
+pentoxide before the experiment. The arc light is used as source
+of illumination. It is found that a vigorous photo-electric
+effect continues in the case of the clean aluminium. In the case
+of the silver bromide a distinct photo-electric effect is still
+observed. I have not had leisure to make, as yet, any trustworthy
+estimate of the percentage effect at this temperature in the case
+of either substance. Nor have I determined the temperature
+accurately. The latter may be taken as roughly about -150 deg. C,
+
+Sir James Dewar's actual measilrements afforded twenty per cent.
+of the normal photographic effect at -180 deg. C. and ten per cent.
+at the temperature of -252.5 deg. C.
+
+With this much to go upon, and the important additional fact that
+the electronic discharge--as from the X-ray tube or from
+radium--generates the latent image, I think we are fully entitled
+to suggest, as a legitimate lead to experiment, the hypothesis
+that the beginnings of photographic action involve an electronic
+discharge from the light-sensitive molecule; in other words that
+the latent image is built up of ionised atoms or molecules the
+result of the photo-electric effect on the illuminated silver
+haloid, and it is upon these ionised atoms that the chemical
+effects of the developer are subsequently directed. It may be
+that the liberated electrons ionise molecules not directly
+affected, or it may be that in their liberation they disrupt
+complex molecules built up in the ripening of the
+
+209
+
+emulsion. With the amount we have to go upon we cannot venture to
+particularise. It will be said that such an action must be in
+part of the nature of a chemical effect. This must be admitted,
+and, in so far as the rearrangement of molecular fabrics is
+involved, the result will doubtless be controlled by temperature
+conditions. The facts observed by Sir James Dewar support this.
+But there is involved a fundamental process--the liberation of the
+electron by the electric force in the light wave, which is a
+physical effect, and which, upon the hypothesis of its reality as
+a factor in forming the latent image, appears to explain
+completely the outstanding photographic sensitiveness of the film
+at temperatures far below those at which chemical actions in
+general cease.
+
+Again, we may assume that the electron--producing power of the
+special sensitiser or dye for the particular ray it absorbs is
+responsible, or responsible in part, for the special
+sensitiveness it confers upon the film. Sir Wm. Abney has shown
+that these sensitisers are active even if laid on as a varnish on
+the sensitive surface and removed before development. It must be
+remembered, however, that at temperatures of about -50 deg. these
+sensitisers lose much of their influence on the film; as I have
+pointed out in a paper read before the Photographic Convention of
+1894.
+
+It. appears to me that on these views the curious phenomenon of
+recurrent reversals does not present a problem hopeless of
+explanation. The process of photo-
+
+210
+
+ionisation constituting the latent image, where the ion is
+probably not immediately neutralised by chemical combination,
+presents features akin to the charging of a capacity--say a Leyden
+jar. There may be a rising potential between the groups of ions
+until ultimately a point is attained when there is a spontaneous
+neutralisation. I may observe that the phenomena of reversal
+appear to indicate that the change in the silver bromide
+molecule, whatever be its nature, is one of gradually increasing
+intensity, and finally attains a maximum when a return to the
+original condition occurs. The maximum is the point of most
+intense developable image. It is probable that the sensitiser--in
+this case the gelatin in which the bromide of silver is
+immersed--plays a part in the conditions of stability which are
+involved.
+
+Of great interest in all our considerations and theories is the
+recent work of Wood on photographic reversal. The result of this
+work is--as I take it--to show that the stability of the latent
+image may be very various according to the mode of its formation.
+Thus it appears that the sort of latent effect which is produced
+by pressure or friction is the least stable of any. This may be
+reversed or wiped out by the application of any other known form
+of photographic stimulus. Thus an exposure to X-rays will
+obliterate it, or a very brief exposure to light. The latent
+image arising from X-rays is next in order of increasing
+stability. Light action will remove this. Third in order is a
+very brief light-shock or sudden flash. This
+
+211
+
+cannot be reversed by any of the foregoing modes of stimulation,
+but a long-continued undulatory stimulus, as from lamp-light,
+will reverse it. Last and most stable of all is the gradually
+built-up configuration due to long-continued light exposure. This
+can only be reversed by overdoing it according to the known facts
+of recurrent reversal. Wood takes occasion to remark that these
+phenomena are in bad agreement with the strain theory of Bose. We
+have, in fact, but the one resource--the allotropic modification
+of the haloid--whereby to explain all these orders of stability.
+It appears to me that the elasticity of the electronic theory is
+greater. The state of the ionised system may be very various
+according as it arises from continued rhythmic effects or from
+unorganised shocks. The ionisation due to X-rays or to friction
+will probably be quite unorganised, that due to light more or
+less stable according to the gradual and gentle nature of the
+forces at work. I think we are entitled to conclude that on the
+whole there is nothing in Wood's beautiful experiments opposed to
+the photo-electric origin of photographic effects, but that they
+rather fall in with what might be anticipated according to that
+theory.
+
+When we look for further support to the views I have laid before
+you we are confronted with many difficulties. I have not as yet
+detected any electronic discharge from the film under light
+stimulus. This may be due to my defective experiments, or to a
+fact noted by Elster and Geitel concerning the photo-electric
+properties of gelatin.
+
+212
+
+They obtained a vigorous effect from Balmain's luminous paint,
+but when this was mixed in gelatin there was no external effect.
+Schmidt's results as to the continuance of photo-electric
+activity when bodies in general are dissolved in each other lead
+us to believe that an actual conservative property of the medium
+and not an effect of this on the luminous paint is here involved.
+This conservative effect of the gelatin may be concerned with its
+efficacy as a sensitiser.
+
+In the views I have laid before you I have endeavoured to show
+that the recent addition to our knowledge of the electron as an
+entity taking part in many physical and chemical effects should
+be kept in sight in seeking an explanation of the mode of origin
+of the latent image.[1]
+
+[1] For a more detailed account of the subject, and some
+ingenious extensions of the views expressed above, see
+_Photo-Electricity_, by H. Stanley Allen: Longmans, Green & Ca.,
+1913.
+
+213
+
+PLEOCHROIC HALOES [1]
+
+IT is now well established that a helium atom is expelled from
+certain of the radioactive elements at the moment of
+transformation. The helium atom or alpha ray leaves the
+transforming atom with a velocity which varies in the different
+radioactive elements, but which is always very great, attaining
+as much as 2 x 109 cms. per second; a velocity which, if
+unchecked, would carry the atom round the earth in less than two
+seconds. The alpha ray carries a positive charge of double the
+ionic amount.
+
+When an alpha ray is discharged from the transforming element
+into a gaseous medium its velocity is rapidly checked and its
+energy absorbed. A certain amount of energy is thus transferred
+from the transforming atom to the gas. We recognise this energy
+in the gas by the altered properties of the latter; chiefly by
+the fact that it becomes a conductor of electricity. The
+mechanism by which this change is effected is in part known. The
+atoms of the gas, which appear to be freely penetrated by the
+alpha ray, are so far dismembered as to yield charged electrons
+or ions; the atoms remaining charged with an equal and opposite
+charge. Such a medium of
+
+[1] Being the Huxley Lecture, delivered at the University of
+Birmingham on October 30th, 1912. Bedrock, Jan., 1913.
+
+214
+
+free electric charges becomes a conductor of electricity by
+convection when an electromotive force is applied. The gas also
+acquires other properties in virtue of its ionisation. Under
+certain conditions it may acquire chemical activity and new
+combinations may be formed or existing ones broken up. When its
+initial velocity is expended the helium atom gives up its
+properties as an alpha ray and thenceforth remains possessed of
+the ordinary varying velocity of thermal agitation. Bragg and
+Kleeman and others have investigated the career of the alpha ray
+when its path or range lies in a gas at ordinary or obtainable
+conditions of pressure and temperature. We will review some of
+the facts ascertained.
+
+The range or distance traversed in a gas at ordinary pressures is
+a few centimetres. The following table, compiled by Geiger, gives
+the range in air at the temperature of 15 deg. C.:
+
+               cms.                   cms.                   cms.
+Uranium 1 -   2.50    Thorium -     2.72     Radioactinium  4.60
+Uranium 2 -   2.90    Radiothorium  3.87     Actinium X -   4.40
+Ionium -      3.00    Thorium X -   4.30     Act Emanation  5.70
+Radium -      3.30    Th Emanation  5.00     Actinium A -   6.50
+Ra Emanation  4.16    Thorium A -   5.70     Actinium C -   5.40
+Radium A -    4.75    Thorium C1 -  4.80
+Radium C -    6.94    Thorium C2 -  8.60
+Radium F -    3.77
+
+It will be seen that the ray of greatest range is that proceeding
+from thorium C2, which reaches a distance of 8.6 cms. In the
+uranium family the fastest ray is
+
+215
+
+that of radium C. It attains 6.94 cms. There is thus an
+appreciable difference between the ultimate distances traversed
+by the most energetic rays of the two families. The shortest
+ranges are those of uranium 1 and 2.
+
+The ionisation effected by these rays is by no means uniform
+along the path of the ray. By examining the conductivity of the
+gas at different points along the path of the ray, the ionisation
+at these points may be determined. At the limits of the range the
+ionisation
+
+{Fig. 13}
+
+ceases. In this manner the range is, in fact, determined. The
+dotted curve (Fig. 13) depicts the recent investigation of the
+ionisation effected by a sheaf of parallel rays of radium C in
+air, as determined by Geiger. The range is laid out horizontally
+in centimetres. The numbers of ions are laid out vertically. The
+remarkable nature of the results will be at once apparent. We
+should have expected that the ray at the beginning of its path,
+when its velocity and kinetic energy were greatest, would have
+been more effective than towards the end of its range
+
+216
+
+when its energy had almost run out. But the curve shows that it
+is just the other way. The lagging ray, about to resign its
+ionising properties, becomes a much more efficient ioniser than
+it was at first. The maximum efficiency is, however, in the case
+of a bundle of parallel rays, not quite at the end of the range,
+but about half a centimetre from it. The increase to the maximum
+is rapid, the fall from the maximum to nothing is much more
+rapid.
+
+It can be shown that the ionisation effected anywhere along the
+path of the ray is inversely proportional to the velocity of the
+ray at that point. But this evidently does not apply to the last
+5 or 10 mms. of the range where the rate of ionisation and of the
+speed of the ray change most rapidly. To what are the changing
+properties of the rays near the end of their path to be ascribed?
+It is only recently that this matter has been elucidated.
+
+When the alpha ray has sufficiently slowed down, its power of
+passing right through atoms, without appreciably experiencing any
+effects from them, diminishes. The opposing atoms begin to exert
+an influence on the path of the ray, deflecting it a little. The
+heavier atoms will deflect it most. This effect has been very
+successfully investigated by Geiger. It is known as "scattering."
+The angle of scattering increases rapidly with the decrease of
+velocity. Now the effect of the scattering will be to cause some
+of the rays to complete their ranges
+
+217
+
+or, more accurately, to leave their direct line of advance a
+little sooner than others. In the beautiful experiments of C. T.
+R. Wilson we are enabled to obtain ocular demonstration of the
+scattering. The photograph (Fig. 14.), which I owe to the
+kindness of Mr. Wilson, shows the deflection of the ray towards
+the end of its path. In
+
+{Fig. 14}
+
+this case the path of the ray has been rendered visible by the
+condensation of water particles under the influence of the
+ionisation; the atmosphere in which the ray travels being in a
+state of supersaturation with water vapour at the instant of the
+passage of the ray. It is evident that if we were observing the
+ionisation along a sheaf of parallel rays, all starting with
+equal velocity,
+
+218
+
+the effect of the bending of some of the rays near the end of
+their range must be to cause a decrease in the aggregate
+ionisation near the very end of the ultimate range. For, in fact,
+some of the rays complete their work of ionising at points in the
+gas before the end is reached. This is the cause, or at least an
+important contributory cause, of the decline in the ionisation
+near the end of the range, when the effects of a bundle of rays
+are being observed. The explanation does not suggest that the
+ionising power of any one ray is actually diminished before it
+finally ceases to be an alpha ray.
+
+The full line in Fig. 13 gives the ionisation curve which it may
+be expected would be struck out by a single alpha ray. In it the
+ionisation goes on increasing till it abruptly ceases altogether,
+with the entire loss of the initial kinetic energy of the
+particle.
+
+A highly remarkable fact was found out by Bragg. The effect of
+the atom traversed by the ray in checking the velocity of the ray
+is independent of the physical and chemical condition of the
+atom. He measured the "stopping power" of a medium by the
+distance the ray can penetrate into it compared with the distance
+to which it can penetrate in air. The less the ratio the greater
+is the stopping power. The stopping power of a substance is
+proportional to the square root of its atomic weight. The
+stopping power of an atom is not altered if it is in chemical
+union with another atom. The atomic weight is the one quality of
+importance. The physical
+
+219
+
+state, whether the element is in the solid, liquid or gaseous
+state, is unimportant. And when we deal with molecules the
+stopping power is simply proportional to the sum of the square
+roots of the atomic weights of the atoms entering into the
+molecule. This is the "additive law," and it obviously enables us
+to calculate what the range in any substance of known chemical
+composition and density will be, compared with the range in air.
+
+This is of special importance in connection with phenomena we
+have presently to consider. It means that, knowing the chemical
+composition and density of any medium whatsoever, solid, liquid
+or gaseous, we can calculate accurately the distance to which any
+particular alpha ray will penetrate. Nor have the temperature and
+pressure to which the medium is subjected any influence save in
+so far as they may affect the proximity of one atom to another.
+The retardation of the alpha ray in the atom is not affected.
+
+This valuable additive law, however, cannot be applied in
+strictness to the amount of ionisation attending the ray. The
+form of the molecule, or more generally its volume, may have an
+influence upon this. Bragg draws the conclusion, from this fact
+as well as from the notable increase of ionisation with loss of
+speed, that the ionisation is dependent upon the time the ray
+spends in the molecule. The energy of the ray is, indeed, found
+to be less efficient in producing ionisation in the smaller
+atomm.
+
+220
+
+Before leaving our review of the general laws governing the
+passage of alpha rays through matter, another point of interest
+must be referred to. We have hitherto spoken in general terms of
+the fact that ionisation attends the passage of the ray. We have
+said nothing as to the nature of the ionisation so produced. But
+in point of fact the ionisation due to an alpha ray is sui
+generis. A glance at one of Wilson's photographs (Fig. 14.)
+illustrates this. The white streak of water particles marks the
+path of the ray. The ions produced are evidently closely crowded
+along the track of the ray. They have been called into existence
+in a very minute instant of time. Now we know that ions of
+opposite sign if left to themselves recombine. The rate of
+recombination depends upon the product of the number of each sign
+present in unit volume. Here the numbers are very great and the
+volume very small. The ionic density is therefore high, and
+recombination very rapidly removes the ions after they are
+formed. We see here a peculiarity of the ionisation effected by
+alpha rays. It is linear in distribution and very local. Much of
+the ionisation in gases is again undone by recombination before
+diffusion leads to the separation of the ions. This "initial
+recombination" is greatest towards the end of the path of the ray
+where the ionisation is a maximum. Here it may be so effective
+that the form of the curve is completely lost unless a very large
+electromotive force is used to separate the ions when the
+ionisation is being investigated.
+
+221
+
+We have now reviewed recent work at sufficient length to
+understand something of the nature of the most important advance
+ever made in our knowledge of the atom. Let us glance briefly at
+what we have learned. The radioactive atom in sinking to a lower
+atomic weight casts out with enormous velocity an atom of helium.
+It thus loses a definite portion of its mass and of its energy.
+Helium which is chemically one of the most inert of the elements,
+is, when possessed of such great kinetic energy, able to
+penetrate and ionise the atoms which it meets in its path. It
+spends its energy in the act of ionising them, coming to rest,
+when it moves in air, in a few centimetres. Its initial velocity
+depends upon the particular radioactive element which has given
+rise to it. The length of its path is therefore different
+according to the radioactive element from which it proceeds. The
+retardation which it experiences in its path depends entirely
+upon the atomic weight of the atoms which it traverses. As it
+advances in its path its effectiveness in ionising the atom
+rapidly increases and attains a very marked maximum. In a gas the
+ions produced being much crowded together recombine rapidly; so
+rapidly that the actual ionisation may be quite concealed unless
+a sufficiently strong electric force is applied to separate them.
+Such is a brief summary of the climax of radioactive
+discovery:--the birth, life and death of the alpha ray. Its advent
+into Science has altered fundamentally our conception of
+
+222
+
+matter. It is fraught with momentous bearings upon Geological
+Science. How the work of the alpha ray is sometimes recorded
+visibly in the rocks and what we may learn from that record, I
+propose now to bring before you.
+
+In certain minerals, notably the brown variety of mica known as
+biotite, the microscope reveals minute circular marks occurring
+here and there, quite irregularly. The most usual appearance is
+that of a circular area darker in colour than the surrounding
+mineral. The radii of these little disc-shaped marks when well
+defined are found to be remarkably uniform, in some cases four
+hundredths of a millimetre and in others three hundredths, about.
+These are the measurements in biotite. In other minerals the
+measurements are not quite the same as in biotite. Such minute
+objects are quite invisible to the naked eye. In some rocks they
+are very abundant, indeed they may be crowded together in such
+numbers as to darken the colour of the mineral containing them.
+They have long been a mystery to petrologists.
+
+Close examination shows that there is always a small speck of a
+foreign body at the centre of the circle, and it is often
+possible to identify the nature of this central substance, small
+though it be. Most generally it is found to be the mineral
+zircon. Now this mineral was shown by Strutt to contain radium in
+quantities much exceeding those found in ordinary rock
+substances.
+
+223
+
+Some other mineral may occasionally form the nucleus, but we
+never find any which is not known to be specially likely to
+contain a radioactive substance. Another circumstance we notice.
+The smaller this central nucleus the more perfect in form is the
+darkened circular area surrounding it. When the circle is very
+perfect and the central mineral clearly defined at its centre we
+find by measurement that the radius of the darkened area is
+generally 0.033 mm. It may sometimes be 0.040 mm. These are
+always the measurements in biotite. In other minerals the radii
+are a little different.
+
+We see in the photograph (Pl. XXIII, lower figure), much
+magnified, a halo contained in biotite. We are looking at a
+region in a rock-section, the rock being ground down to such a
+thickness that light freely passes through it. The biotite is in
+the centre of the field. Quartz and felspar surround it. The rock
+is a granite. The biotite is not all one crystal. Two crystals,
+mutually inclined, are cut across. The halo extends across both
+crystals, but owing to the fact that polarised light is used in
+taking the photograph it appears darker in one crystal than in
+the other. We see the zircon which composes the nucleus. The fine
+striated appearance of the biotite is due to the cleavage of that
+mineral, which is cut across in the section.
+
+The question arises whether the darkened area surrounding the
+zircon may not be due to the influence of the radioactive
+substances contained in the zircon. The
+
+224
+
+extraordinary uniformity of the radial measurements of perfectly
+formed haloes (to use the name by which they have long been
+known) suggests that they may be the result of alpha radiation.
+For in that case, as we have seen, we can at once account for the
+definite radius as simply representing the range of the ray in
+biotite. The furthest-reaching ray will define the radius of the
+halo. In the case of the uranium family this will be radium C,
+and in the case of thorium it will be thorium C. Now here we
+possess a means of at once confirming or rejecting the view that
+the halo is a radioactive phenomenon and occasioned by alpha
+radiation; for we can calculate what the range of these rays will
+be in biotite, availing ourselves of Bragg's additive law,
+already referred to. When we make this calculation we find that
+radium C just penetrates 0.033 mm. and thorium C 0.040 mm. The
+proof is complete that we are dealing with the effects of alpha
+rays. Observe now that not only is the coincidence of measurement
+and calculation a proof of the view that alpha radiation has
+occasioned the halo, but it is a very complete verification of
+the important fact stated by Bragg, that the stopping power
+depends solely on the atomic weight of the atoms traversed by the
+ray.
+
+We have seen that our examination of the rocks reveals only the
+two sorts of halo: the radium halo and the thorium halo. This is
+not without teaching. For why not find an actinium halo? Now
+Rutherford long ago suggested that this element and its
+derivatives were
+
+225
+
+probably an offspring of the uranium family; a side branch, as it
+were, in the formation of which relatively few transforming atoms
+took part. On Rutherford's theory then, actinium should always
+accompany uranium and radium, but in very subordinate amount. The
+absence of actinium haloes clearly supports this view. For if
+actinium was an independent element we would be sure to find
+actinium haloes. The difference in radius should be noticeable.
+If, on the other hand, actinium
+
+was always associated with uranium and radium, then its effects
+would be submerged in those of the much more potent effects of
+the uranium series of elements.
+
+It will have occurred to you already that if the radioactive
+origin of the halo is assured the shape of a halo is not really
+circular, but spherical. This is so. There is no such thing as a
+disc-shaped halo. The halo is a spherical volume containing the
+radioactive nucleus at its centre. The true radius of the halo
+may, therefore, only be measured on sections passing through the
+nucleus.
+
+226
+
+In order to understand the mode of formation of a halo we may
+profitably study on a diagram the events which go on within the
+halo-sphere. Such a diagram is seen in Fig. 15. It shows to
+relatively correct scale the limiting range of all the alpha-ray
+producing members of the uranium and thorium families. We know
+that each member of a family will exist in equilibrium amount
+within the nucleus possessing the parent element. Each alpha ray
+leaving the nucleus will just attain its range and then cease to
+affect the mica. Within the halosphere, there must be, therefore,
+the accumulated effects of the influences of all the rays. Each
+has its own sphere of influence, and the spheres are all
+concentric.
+
+The radii in biotite of the several spheres are given in the
+following table
+
+URANIUM FAMILY.
+Radium C -       0.0330 mm.
+Radium A -       0.0224 mm.
+Ra Emanation -   0.0196 mm.
+Radium F -       0.0177 mm.
+Radium -         0.0156 mm.
+Ionium -         0.0141 mm.
+Uranium 1 -      0.0137 mm.
+Uranium 2 -      0.0118 mm.
+
+THORIUM FAMILY.
+Thorium CE -     0.040 mm.
+Thorium A -      0.026 mm.
+Th Emanation -   0.023 mm.
+Thorium Ci -     0.022 mm.
+Thorium X -      0.020 mm.
+Radiothorium -   0.119 mm.
+Thorium -        0.013 mm.
+
+In the photograph (Pl. XXIV, lower figure), we see a uranium and
+a thorium halo in the same crystal of mica. The mica is contained
+in a rock-section and is cut across the cleavage. The effects of
+thorium Ca are clearly shown
+
+227
+
+as a lighter border surrounding the accumulated inner darkening
+due to the other thorium rays. The uranium halo (to the right)
+similarly shows the effects of radium C, but less distinctly.
+
+Haloes which are uniformly dark all over as described above are,
+in point of fact, "over-exposed"; to borrow a familiar
+photographic term. Haloes are found which show much beautiful
+internal detail. Too vigorous action obscures this detail just as
+detail is lost in an over-exposed photograph. We may again have
+"under-exposed" haloes in which the action of the several rays is
+incomplete or in which the action of certain of the rays has left
+little if any trace. Beginning at the most under-exposed haloes
+we find circular dark marks having the radius 0.012 or 0.013 mm.
+These haloes are due to uranium, although their inner darkening
+is doubtless aided by the passage of rays which were too few to
+extend the darkening beyond the vigorous effects of the two
+uranium rays. Then we find haloes carried out to the radii 0.016,
+0.018 and 0.019 mm. The last sometimes show very beautiful outer
+rings having radial dimensions such as would be produced by
+radium A and radium C. Finally we may have haloes in which
+interior detail is lost so far out as the radius due to emanation
+or radium A, while outside this floats the ring due to radium C.
+Certain variations of these effects may occur, marking,
+apparently, different stages of exposure. Plates XXIII and XXIV
+(upper figure) illustrate some of these stages;
+
+228
+
+the latter photograph being greatly enlarged to show clearly the
+halo-sphere of radium A.
+
+In most of the cases mentioned above the structure evidently
+shows the existence of concentric spherical shells of darkened
+biotite. This is a very interesting fact. For it proves that in
+the mineral the alpha ray gives rise to the same increased
+ionisation towards the end of its range, as Bragg determined in
+the case of gases. And we must conclude that the halo in every
+case grows in this manner. A spherical shell of darkened biotite
+is first produced and the inner colouration is only effected as
+the more feeble ionisation along the track of the ray in course
+of ages gives rise to sufficient alteration of the mineral. This
+more feeble ionisation is, near the nucleus, enhanced in its
+effects by the fact that there all the rays combine to increase
+the ionisation and, moreover, the several tracks are there
+crowded by the convergency to the centre. Hence the most
+elementary haloes seldom show definite rings due to uranium,
+etc., but appear as embryonic disc-like markings. The photographs
+illustrate many of the phases of halo development.
+
+Rutherford succeeded in making a halo artificially by compressing
+into a capillary glass tube a quantity of the emanation of
+radium. As the emanation decayed the various derived products
+came into existence and all the several alpha rays penetrated the
+glass, darkening the walls of the capillary out to the limit of
+the range of radium C in glass. Plate XXV shows a magnified
+section of the
+
+229
+
+tube. The dark central part is the capillary. The tubular halo
+surrounds it. This experiment has, however, been anticipated by
+some scores of millions of years, for here is the same effect in
+a biotite crystal (Pl. XXV). Along what are apparently tubular
+passages or cracks in the mica, a solution, rich in radioactive
+substances, has moved; probably during the final consolidation of
+the granite in which the mica occurs. A continuous and very
+regular halo has developed along these conduits. A string of
+halo-spheres may lie along such passages. We must infer that
+solutions or gases able to establish the radioactive nuclei moved
+along these conduits, and we are entitled to ask if all the
+haloes in this biotite are not, in this sense, of secondary
+origin. There is, I may add, much to support such a conclusion.
+
+The widespread distribution of radioactive substances is most
+readily appreciated by examination of sections of rocks cut thin
+enough for microscopic investigation. It is, indeed, difficult to
+find, in the older rocks of granitic type, mica which does not
+show haloes, or traces of haloes. Often we find that every one of
+the inclusions in the mica--that is, every one of the earlier
+formed substances--contain radioactive elements, as indicated by
+the presence of darkened borders. As will be seen presently the
+quantities involved are generally vanishingly small. For example
+it was found by direct determination that in one gram of the
+halo-rich mica of Co. Carlow there was rather less than twelve
+billionths of a gram of radium, We are
+
+230
+
+entitled to infer that other rare elements are similarly widely
+distributed but remain undetectable because of their more stable
+properties.
+
+It must not be thought that the under-exposed halo is a recent
+creation. By no means. All are old, appallingly old; and in the
+same rock all are, probably, of the same, or neatly the same,
+age. The under-exposure is simply due to a lesser quantity of the
+radioactive elements in the nucleus. They are under-exposed, in
+short, not because of lesser duration of exposure, but because of
+insufficient action; as when in taking a photograph the stop is
+not open enough for the time of the exposure.
+
+The halo has, so far, told us that the additive law is obeyed in
+solid media, and that the increased ionisation attending the
+slowing down of the ray obtaining in gases, also obtains in
+solids; for, otherwise, the halo would not commence its
+development as a spherical shell or envelope. But here we learn
+that there is probably a certain difference in the course of
+events attending the immediate passage of the ray in the gas and
+in the solid. In the former, initial recombination may obscure
+the intense ionisation near the end of the range. We can only
+detect the true end-effects by artificially separating the ions
+by a strong electric force. If this recombination happened in the
+mineral we should not have the concentric spheres so well defined
+as we see them to be. What, then, hinders the initial
+recombination in the solid? The answer probably is that the newly
+formed
+
+231
+
+ion is instantly used up in a fresh chemical combination. Nor is
+it free to change its place as in the gas. There is simply a new
+equilibrium brought about by its sudden production. In this
+manner the conditions in the complex molecule of biotite,
+tourmaline, etc., may be quite as effective in preventing initial
+recombination as the most effective electric force we could
+apply. The final result is that we find the Bragg curve
+reproduced most accurately in the delicate shading of the rings
+making up the perfectly exposed halo.
+
+That the shading of the rings reproduces the form of the Bragg
+curve, projected, as it were, upon the line of advance of the ray
+and reproduced in depth of shading, shows that in yet another
+particular the alpha ray behaves much the same in the solid as in
+the gas. A careful examination of the outer edge of the circles
+always reveals a steep but not abrupt cessation of the action of
+the ray. Now Geiger has investigated and proved the existence of
+scattering of the alpha ray by solids. We may, therefore, suppose
+with much probability that there is the same scattering within
+the mineral near the end of the range. The heavy iron atom of the
+biotite is, doubtless, chiefly responsible for this in biotite
+haloes. I may observe that this shading of the outer bounding
+surface of the sphere of action is found however minute the
+central nucleus. In the case of a nucleus of considerable size
+another effect comes in which tends to produce an enhanced
+shading. This will
+
+232
+
+result if rays proceed from different depths in the nucleus. If
+the nucleus were of the same density and atomic weight as the
+surrounding mica, there would be little effect. But its density
+and molecular weight are generally greater, hence the retardation
+is greater, and rays proceeding from deep in the nucleus
+experience more retardation than those which proceed from points
+near to the surface. The distances reached by the rays in the
+mica will vary accordingly, and so there will be a gradual
+cessation of the effects of the rays.
+
+The result of our study of the halo may be summed up in the
+statement that in nearly every particular we have the phenomena,
+which have been measured and observed in the gas, reproduced on a
+minute scale in the halo. Initial recombination seems, however,
+to be absent or diminished in effectiveness; probably because of
+the new stability instantly assumed by the ionised atoms.
+
+One of the most interesting points about the halo remains to be
+referred to. The halo is always uniformly darkened all round its
+circumference and is perfectly spherical. Sections, whether taken
+in the plane of cleavage of the mica or across it, show the same
+exactly circular form, and the same radius. Of course, if there
+was any appreciable increase of range along or across the
+cleavage the form of the halo on the section across the cleavage
+should be elliptical. The fact that there is no measurable
+ellipticity is, I think, one which on first consideration would
+not be expected.
+
+233
+
+For what are the conditions attending the passage of the ray in a
+medium such as mica? According to crystallographic conceptions we
+have here an orderly arrangement of molecules, the units
+composing the crystal being alike in mass, geometrically spaced,
+and polarised as regards the attractions they exert one upon
+another. Mica, more especially, has the cleavage phenomenon
+developed to a degree which transcends its development in any
+other known substance. We can cleave it and again cleave it till
+its flakes float in the air, and we may yet go on cleaving it by
+special means till the flakes no longer reflect visible light.
+And not less remarkable is the uniplanar nature of its cleavage.
+There is little cleavage in any plane but the one, although it is
+easy to show that the molecules in the plane of the flake are in
+orderly arrangement and are more easily parted in some directions
+than in others. In such a medium beyond all others we must look
+with surprise upon the perfect sphere struck out by the alpha
+rays, because it seems certain that the cleavage is due to lesser
+attraction, and, probably, further spacing of the molecules, in a
+direction perpendicular to the cleavage.
+
+It may turn out that the spacing of the molecules will influence
+but little the average number per unit distance encountered by
+rays moving in divergent paths. If this is so, we seem left to
+conclude that, in spite of its unequal and polarised attractions,
+there is equal retardation and equal ionisation in the molecule
+in whatever
+
+234
+
+direction it is approached. Or, again, if the encounters indeed
+differ in number, then some compensating effect must exist
+whereby a direction of lesser linear density involves greater
+stopping power in the molecule encountered, and vice versa.
+
+The nature of the change produced by the alpha rays is unknown.
+But the formation of the halo is not, at least in its earlier
+stages, attended by destruction of the crystallographic and
+optical properties of the medium. The optical properties are
+unaltered in nature but are increased in intensity. This applies
+till the halo has become so darkened that light is no longer
+transmitted under the conditions of thickness obtaining in rock
+sections. It is well known that there is in biotite a maximum
+absorption of a plane-polarised light ray, when the plane of
+vibration coincides with the plane of cleavage. A section across
+the cleavage then shows a maximum amount of absorption. A halo
+seen on this section simply produces this effect in a more
+intense degree. This is well shown in Plate XXIII (lower figure),
+on a portion of the halo-sphere. The descriptive name "Pleochroic
+Halo" has originated from this fact. We must conclude that the
+effect of the ionisation due to the alpha ray has not been to
+alter fundamentally the conditions which give rise to the optical
+properties of the medium. The increased absorption is probably
+associated with some change in the chemical state of the iron
+present. Haloes are, I believe, not found in minerals from which
+this
+
+235
+
+element is absent. One thing is quite certain. The colouration is
+not due to an accumulation of helium atoms, _i.e._ of spent alpha
+rays. The evidence for this is conclusive. If helium was
+responsible we should have haloes produced in all sorts of
+colourless minerals. Now we sometimes see zircons in felspars and
+in quartz, etc., but in no such case is a halo produced. And
+halo-spheres formed within and sufficiently close to the edge of
+a crystal of mica are abruptly truncated by neighbouring areas of
+fclspar or quartz, although we know that the rays must pass
+freely across the boundary. Again it is easy to show that even in
+the oldest haloes the quantity of helium involved is so small
+that one might say the halo-sphere was a tolerably good vacuum as
+regards helium. There is, finally, no reason to suppose that the
+imprisoned helium would exhibit such a colouration, or, indeed,
+any at all.
+
+I have already referred to the great age of the halo. Haloes are
+not found in the younger igneous rocks. It is probable that a
+halo less than a million years old has never been seen. This,
+prima facie, indicates an extremely slow rate of formation. And
+our calculations quite support the conclusions that the growth of
+a halo, if this has been uniform, proceeds at a rate of almost
+unimaginable slowness.
+
+Let us calculate the number of alpha rays which may have gone to
+form a halo in the Devonian granite of Leinster.
+
+236
+
+It is common to find haloes developed perfectly in this granite,
+and having a nucleus of zircon less than 5 x 10-4 cms. in
+diameter. The volume of zircon is 65 x 10-12 c.cs. and the mass
+3 x 10-10 grm.; and if there was in this zircon 10-8 grm. radium
+per gram (a quantity about five times the greatest amount
+measured by Strutt), the mass of radium involved is 3 x 10-18
+grm. From this and from the fact ascertained by Rutherford that
+the number of alpha rays expelled by a gram of radium in one
+second is 3.4 x 1010, we find that three rays are shot from the
+nucleus in a year. If, now, geological time since the Devonian is
+50 millions of years, then 150 millions of rays built up the
+halo. If geological time since the Devonian is 400 millions of
+years, then 1,200 millions of alpha rays are concerned in its
+genesis. The number of ions involved, of course, greatly exceeds
+these numbers. A single alpha ray fired from radium C will
+produce 2.37 x 105 ions in air.
+
+But haloes may be found quite clearly defined and fairly dark out
+to the range of the emanation ray and derived from much less
+quantities of radioactive materials. Thus a zircon nucleus with a
+diameter of but 3.4 x 10-4 cms. formed a halo strongly darkened
+within, and showing radium A and radium C as clear smoky rings.
+Such a nucleus, on the assumption made above as to its radium
+content, expels one ray in a year. But, again, haloes are
+observed with less blackened pupils and with faint ring due to
+radium C, formed round nuclei
+
+237
+
+of rather less than 2 x 10-4 cms. diameter. Such nuclei would
+expel one ray in five years. And even lesser nuclei will generate
+in these old rocks haloes with their earlier characteristic
+features clearly developed. In the case of the most minute
+nuclei, if my assumption as to the uranium content is correct, an
+alpha ray is expelled, probably, no oftener than once in a
+century; and possibly at still longer intervals.
+
+The equilibrium amount of radium contained in some nuclei may
+amount to only a few atoms. Even in the case of the larger nuclei
+and more perfectly developed haloes the quantity of radium
+involved is many millions of times less than the least amount we
+can recognise by any other means. But the delicacy of the
+observation is not adequately set forth in this statement. We can
+not only tell the nature of the radioactive family with which we
+are dealing; but we can recognise the presence of some of its
+constituent members. I may say that it is not probable the
+zircons are richer in radium than I have assumed. My assumption
+involves about 3 per cent. of uranium. I know of no analyses
+ascribing so great an amount of uranium to zircon. The variety
+cyrtolite has been found to contain half this amount, about. But
+even if we doubled our estimate of radium content, the remarkable
+nature of our conclusions is hardly lessened.
+
+It may appear strange that the ever-interesting question of the
+Earth's age should find elucidation from the
+
+238
+
+study of haloes. Nevertheless the subjects are closely connected.
+The circumstances are as follows. Geologists have estimated the
+age of the Earth since denudation began, by measurements of the
+integral effects of denudation. These methods agree in showing an
+age of about rob years. On the other hand, measurements have been
+made of the accumulation in minerals of radioactive _debris_--the
+helium and lead--and results obtained which, although they do not
+agree very well among themselves, are concordant in assigning a
+very much greater age to the rocks. If the radioactive estimate
+is correct, then we are now living in a time when the denudative
+forces of the Earth are about eight or nine times as active as
+they have been on the average over the past. Such a state of
+things is absolutely unaccountable. And all the more
+unaccountable because from all we know we would expect a somewhat
+_lesser_ rate of solvent denudation as the world gets older and the
+land gets more and more loaded with the washed-out materials of
+the rocks.
+
+Both the methods referred to of finding the age assume the
+principle of uniformity. The geologist contends for uniformity
+throughout the past physical history of the Earth. The physicist
+claims the like for the change-rates of the radioactive elements.
+Now the study of the rocks enables us to infer something as to
+the past history of our Globe. Nothing is, on the other hand,
+known respecting the origin of uranium or thorium--the parent
+radioactive bodies. And while not questioning the law
+
+239
+
+and regularity which undoubtedly prevail in the periods of the
+members of the radioactive families, it appears to me that it is
+allowable to ask if the change rate of uranium has been always
+what we now believe it to be. This comes to much the same thing
+as supposing that atoms possessing a faster change rate once were
+associated with it which were capable of yielding both helium and
+lead to the rocks. Such atoms might have been collateral in
+origin with uranium from some antecedent element. Like helium,
+lead may be a derivative from more than one sequence of
+radioactive changes. In the present state of our knowledge the
+possibilities are many. The rate of change is known to be
+connected with the range of the alpha ray expelled by the
+transforming element; and the conformity of the halo with our
+existing knowledge of the ranges is reason for assuming that,
+whatever the origin of the more active associate of uranium, this
+passed through similar elemental changes in the progress of its
+disintegration. There may, however, have been differences in the
+ranges which the halo would not reveal. It is remarkable that
+uranium at the present time is apparently responsible for two
+alpha rays of very different ranges. If these proceed from
+different elements, one should be faster in its change rate than
+the other. Some guidance may yet be forthcoming from the study of
+the more obscure problems of radioactivity.
+
+Now it is not improbable that the halo may contribute directly to
+this discussion. We can evidently attack
+
+240
+
+the biotite with a known number of alpha rays and determine how
+many are required to produce a certain intensity of darkening,
+corresponding to that of a halo with a nucleus of measurable
+dimensions. On certain assumptions, which are correct within
+defined limits, we can calculate, as I have done above, the
+number of rays concerned in forming the halo. In doing so we
+assume some value for the age of the halo. Let us take the
+maximum radioactive value. A halo originating in Devonian times
+may attain a certain central blackening from the effects of, say,
+rob rays. But now suppose we find that we cannot produce the same
+degree of blackening with this number of rays applied in the
+laboratory. What are we to conclude? I think there is only the
+one conclusion open to us; that some other source of alpha rays,
+or a faster rate of supply, existed in the past. And this
+conclusion would explain the absence of haloes from the younger
+rocks; which, in view of the vast range of effects possible in
+the development of haloes, is, otherwise, not easy to account
+for. It is apparent that the experiment on the biotite has a
+direct bearing on the validity of the radioactive method of
+estimating the age of the rocks. It is now being carried out by
+Professor Rutherford under reliable conditions.
+
+Finally, there is one very certain and valuable fact to be
+learned from the halo. The halo has established the extreme
+rarity of radioactivity as an atomic phenomenon. One and all of
+the speculations as to
+
+241
+
+the slow breakdown of the commoner elements may be dismissed. The
+halo shows that the mica of the rocks is radioactively sensitive.
+The fundamental criterion of radioactive change is the expulsion
+of the alpha ray. The molecular system of the mica and of many
+other minerals is unstable in presence of these rays, just as a
+photographic plate is unstable in presence of light. Moreover,
+the mineral integrates the radioactive effects in the same way as
+a photographic salt integrates the effects of light. In both
+cases the feeblest activities become ultimately apparent to our
+inspection. We have seen that one ray in each year since the
+Devonian period will build the fully formed halo: an object
+unlike any other appearance in the rocks. And we have been able
+to allocate all the haloes so far investigated to one or the
+other of the known radioactive families. We are evidently
+justified in the belief that had other elements been radioactive
+we must either find characteristic haloes produced by them, or
+else find a complete darkening of the mica. The feeblest alpha
+rays emitted by the relatively enormous quantities of the
+prevailing elements, acting over the whole duration of geological
+time--and it must be remembered that the haloes we have been
+studying are comparatively young--must have registered their
+effects on the sensitive minerals. And thus we are safe in
+concluding that the common elements, and, indeed, many which
+would be called rare, are possessed of a degree of stability
+which has preserved them un
+
+242
+
+changed since the beginning of geological time. Each unaffected
+flake of mica is, thus, unassailable proof of a fact which but
+for the halo would, probably, have been for ever beyond our
+cognisance.
+
+THE USE OF RADIUM IN MEDICINE [1]
+
+IT has been unfortunate for the progress of the radioactive
+treatment of disease that its methods and claims involve much of
+the marvellous. Up till recently, indeed, a large part of
+radioactive therapeutics could only be described as bordering on
+the occult. It is not surprising that when, in addition to its
+occult and marvellous characters, claims were made on its behalf
+which in many cases could not be supported, many medical men came
+to regard it with a certain amount of suspicion.
+
+Today, I believe, we are in a better position. I think it is
+possible to ascribe a rational scientific basis to its legitimate
+claims, and to show, in fact, that in radioactive treatment we
+are pursuing methods which have been already tried extensively
+and found to be of definite value; and that new methods differ
+from the old mainly in their power and availability, and little,
+or not at all, in kind.
+
+Let us briefly review the basis of the science. Radium is a
+metallic element chemically resembling barium. It
+
+[1] A Lecture to Postgraduate Students of Medicine in connection
+with the founding of the Dublin Radium Institute, delivered in
+the School of Physic in Ireland, Trinity College, on October 2nd,
+1914
+
+244
+
+possesses, however, a remarkable property which barium does not.
+Its atoms are not equally stable. In a given quantity of radium a
+certain very small percentage of the total number of atoms
+present break up per second. By "breaking up" we mean their
+transmutation to another element. Radium, which is a solid
+element under ordinary conditions, gives rise by transmutation to
+a gaseous element--the emanation of radium. The new element is a
+heavy gas at ordinary temperatures and, like other gases, can be
+liquified by extreme cold. The extraordinary property of
+transmutation is entirely automatic. No influence which chemist
+or physicist can apply can affect the rate of transformation.
+
+The emanation inherits the property of instability, but in its
+case the instability is more pronounced. A relatively large
+fraction of its atoms transmute per second to a solid element
+designated Radium A. In turn this new generation of atoms breaks
+up--even faster than the emanation--becoming yet another element
+with specific chemical properties. And so on for a whole sequence
+of transmutations, till finally a stable substance is formed,
+identical with ordinary lead in chemical and physical properties,
+but possessing a slightly lower atomic weight.
+
+The genealogy of the radium series of elements shows that radium
+is not the starting point. It possesses ancestors which have been
+traced back to the element uranium.
+
+Now what bearing has this series of transmutations
+
+245
+
+upon medical science? Radium or emanation, &c., are not in the
+Pharmacopoeia as are, say, arsenic or bismuth. The whole
+medicinal value of these elements resides in the very wonderful
+phenomena of their radiations. They radiate in the process of
+transmuting.
+
+The changing atom may radiate a part of its own mass. The
+"alpha"-ray (a-ray) is such a material ray. It is an electrified
+helium atom cast out of the parent atom with enormous
+velocity--such a velocity as would carry it, if not impeded, all
+round the earth in two seconds. All alpha-rays are positively
+electrified atoms of the element helium, which thereby is shown
+to be an integral constituent of many elements. The alpha-ray is
+not of much value to medical science, for, in spite of its great
+velocity, it is soon stopped by encounter with other atoms. It
+can penetrate only a minute fraction of a millimetre into
+ordinary soft tissues. We shall not further consider it.
+
+Transmuting atoms give out also material rays of another kind:
+the ss-rays. The ss-ray is in mass but a very small fraction of,
+even, a hydrogen atom. Its speed may approach that of light. As
+cast out by radioactive elements it starts with speeds which vary
+with the element, and may be from one-third to nine-tenths the
+velocity of light. The ss-ray is negatively electrified. It has
+long been known to science as the electron. It is also identical
+with the cathode ray of the vacuum tube.
+
+246
+
+Another and quite different kind of radiation is given out by
+many of the transmuting elements:--the y-ray. This is not
+material, it is ethereal. It is known now with certainty that the
+y-ray is in kind identical with light, but of very much shorter
+wave length than even the extreme ultraviolet light of the solar
+spectrum. The y-ray is flashed from the transmuting atom along
+with the ss-ray. It is identical in character with the x-ray but
+of even shorter wave length.
+
+There is a very interesting connection between the y-ray and the
+ss-ray which it is important for the medical man to understand--as
+far as it is practicable on our present knowledge.
+
+When y-rays or x-rays fall on matter they give rise to ss-rays.
+The mechanism involved is not known but it is possibly a result
+of the resonance of the atom, or of parts of it, to the short
+light waves. And it is remarkable that the y-rays which, as we
+have seen, are shorter and more penetrating waves than the
+x-rays, give rise to ss-rays possessed of greater velocity and
+penetration than ss-rays excited by the x-rays. Indeed the ss-rays
+originated by y-rays may attain a velocity nearly approaching
+that of light and as great as that of any ss-rays emitted by
+transmuting atoms. Again there is demonstrable evidence that
+ss-rays impinging on matter may give rise to y-rays. The most
+remarkable demonstration of this is seen in the x-ray tube. Here
+the x-rays originate where the stream of ss- or cathode-rays
+
+247
+
+are arrested on the anode. But the first relation is at present
+of most importance to us--_i.e._ that the y-or x-rays give rise to
+ss-rays.
+
+This relation gives us additional evidence of the identity of the
+physical effects of y-, x-, and light-rays --using the term light
+rays in the usual sense of spectral rays. For it has long been
+known that light waves liberate electrons from atoms. It has been
+found that these electrons possess a certain initial velocity
+which is the greater the shorter the wave length of the light
+concerned in their liberation. The whole science of
+"photo-electricity" centres round this phenomenon. The action of
+light on the photographic plate, as well as many other physical
+and chemical phenomena, find an explanation in this liberation of
+the electron by the light wave.
+
+Here, then, we have spectral light waves liberating
+electrons--_i.e._ very minute negatively-charged particles, and we
+find that, as we use shorter light waves, the initial velocity of
+these particles increases. Again, we have x-rays which are far
+smaller in wave length than spectral light, liberating much
+faster negatively electrified particles. Finally, we have
+y-rays--the shortest nether waves of all-liberating negative
+particles of the highest velocity known. Plainly the whole series
+of phenomena is continuous.
+
+We can now look closer at the actions involved in the therapeutic
+influence of the several rays and in
+
+248
+
+this way, also, see further the correlation between what may be
+called photo-therapeutics and radioactive therapeutics.
+
+The ss-ray, whether we obtain it directly from the transforming
+radioactive atom or whether we obtain it as a result of the
+effects of the y- or x-rays upon the atom, is an ionising agent
+of wonderful power. What is meant by this? In its physical aspect
+this means that the atoms through which it passes acquire free
+electric charges; some becoming positive, some negative. This can
+only be due to the loss of an electron by the affected atom. The
+loss of the small negative charge carried in the electron leaves
+the atom positively electrified or creates a positive ion. The
+fixing of the wandering electron to a neutral atom creates a
+negative ion. Before further consideration of the importance of
+the phenomenon of ionisation we must fix in our minds that the
+agent, which brings this about, is the ss-ray. There is little
+evidence that the y-ray can directly create ions to any large
+extent. But the action of liberating high-speed ss-rays results in
+the creation of many thousands of ions by each ss-ray liberated.
+As an agent in the hands of the medical man we must regard the
+y-ray as a light wave of extremely penetrating character, which
+creates high-speed ss-rays in the tissues which it penetrates,
+these ss-rays being most potent ionising agents. The ss-rays
+directly obtained from radioactive atoms assist in the work of
+ionisation. ss-rays do not
+
+249
+
+penetrate far from their source. The fastest of them would not
+probably penetrate one centimetre in soft tissues.
+
+We must now return to the phenomenon of ionisation. Ionisation is
+revealed to observation most conspicuously when it takes place in
+a gas. The + and - electric charges on the gas particles endow it
+with the properties of a conductor of electricity, the + ions
+moving freely in one direction and the - ions in the opposite
+direction under an electric potential. But there are effects
+brought about by ionisation of more importance to the medical man
+than this. The chemist has long come to recognise that in the ion
+he is concerned with the inner mechanism of a large number of
+chemical phenomena. For with the electrification of the atom
+attractive and repulsive forces arise. We can directly show the
+chemical effects of the ionising ss-rays. Water exposed to their
+bombardment splits up into hydrogen and oxygen. And, again, the
+separated atoms may be in part recombined under the influence of
+the radiation. Ammonia splits up into hydrogen and nitrogen.
+Carbon dioxide forms carbon, carbon monoxide, and oxygen;
+hydrochloric acid forms chlorine and hydrogen. In these cases,
+also, recombination can be partially effected by the rays.
+
+We can be quite sure that within the complex structure of the
+living cell the ionising effects which everywhere accompany the
+ss-rays must exert a profound influence. The sequence of chemical
+events which as yet seem
+
+250
+
+beyond the ken of science and which are involved in metabolism
+cannot fail to be affected. Any, it is not surprising that as the
+result of eaperinient it is found that the radiations are agents
+which may be used either for the stimulation of the natural
+events of growth or used for the actual destruction of the cell.
+It is easy to see that the feeble radiation should produce the
+one effect, the strong the other. In a similar way by a moderate
+light stimulus we create the latent image in the photographic
+plate; by an intense light we again destroy this image. The inner
+mechanism in this last case can be logically stated.[1]
+
+_There is plainly a true physical basis here for the efficacy of
+radioactive treatment and, what is more, we find when we examine
+it, that it is in kind not different from that underlying
+treatment by spectral radiations. But in degree it is very
+different and here is the reason for the special importance of
+radioactivity as a therapeutic agent._ The Finsen light is capable
+of influencing the soft tissues to a short depth only. The reason
+is that the wave length of the light used is too great to pass
+without rapid absorption through the tissues; and, further, the
+electrons it gives rise to--_i.e._ the ss-rays it liberates--are too
+slow-moving to be very efficient ionisers. X-rays penetrate in
+some cases quite freely and give rise to much faster and more
+powerful ss-rays
+
+[1] See _The Latent Image_, p. 202.
+
+251
+
+than can the Finsen light. But far more penetrating than x-rays
+are the y-rays emitted in certain of the radioactive changes.
+These give rise to ss-rays having a velocity approximate to that
+of light.
+
+The y-rays are, therefore, very penetrating and powerfully
+ionising light waves; light waves which are quite invisible to
+the eye and can beam right through the tissues of the body. To
+the mind's eye only are they visible. And a very wonderful
+picture they make. We see the transmuting atom flashing out this
+light for an inconceivably short instant as it throws off the
+ss-ray. And "so far this little candle throws his beams" in the
+complex system of the cells, so far atoms shaken by the rays send
+out ss-rays; these in turn are hurled against other atomic
+systems; fresh separations of electrons arise and new attractions
+and repulsions spring up and the most important chemical changes
+are brought about. Our mental picture can claim to be no more
+than diagrammatic of the reality. Still we are here dealing with
+recognised physical and chemical phenomena, and their description
+as "occult" in the derogatory sense is certainly not
+justifiable.
+
+Having now briefly reviewed the nature of the rays arising in
+radioactive substances and the rationale of their influence, we
+must turn to more especially practical considerations.
+
+The Table given opposite shows that radium itself is responsible
+for a- and ss-rays only. It happens that
+
+252
+
+Period in whioh 1/2 element is transformed.
+
+URANIUM 1 & 2 { a 6 } x 109 years.
+
+URANIUM X { a ss } 24.6 days.
+
+IONIUM { a 8 } x 104 years.
+
+RADIUM { a ss } 2 x 102 years.
+
+EMANATION { a } 8.85 days.
+
+RADIUM A { a 8 } minutes.
+
+RADIUM B { ss y } 26.7 minutes.
+
+RADIUM C { a ss y } 13.5 minutes.
+
+RADIUM D { ss } 15 years.
+
+RADIUM E { ss y } 4.8 days.
+
+RADIUM (Polonium) F { a } 140 days.
+
+Table showing the successive generations of the elements of the
+Uranium-radium family, the character of their radiations and
+their longevity.
+
+253
+
+the ss-rays emitted by radium are very "soft"--_i.e._ slow and
+easily absorbed. The a-ray is in no case available for more than
+mere surface application. Hence we see that, contrary to what is
+generally believed, radium itself is of little direct therapeutic
+value. Nor is the next body in succession--the emanation, for it
+gives only a-rays. In fact, to be brief, it is not till we come
+to Radium B that ss-rays of a relatively high penetrative quality
+are reached; and it is not till we come to Radium C that highly
+penetrative y-rays are obtained.
+
+It is around this element, Radium C, that the chief medical
+importance of radioactive treatment by this family of radioactive
+bodies centres. Not only are ss-rays of Radium C very penetrating,
+but the y-rays are perhaps the most energetic rays of the, kind
+known. Further in the list there is no very special medical
+interest.
+
+Now, how can we get a supply of this valuable element Radium C?
+We can obtain it from radium itself. For even if radium has been
+deprived of its emanation (which is easily done by heating it or
+bringing it into solution) in a few weeks we get back the Radium
+C. One thing here we must be clear about. With a given quantity
+of Radium only a certain definitely limited amount of Radium C,
+or of emanation, or any other of the derived bodies, will be
+associated. Why is this? The answer is because the several
+successive elements are themselves decaying --_i.e._ changing one
+into the other. The atomic per-
+
+254
+
+centage of each, which decays in a second, is a fixed quantity
+which we cannot alter. Now if we picture radium which has been
+completely deprived of its emanation, again accumulating by
+automatic transmutation a fresh store of this element, we have to
+remember:-- (i) That the rate of creation of emanation by the
+radium is practically constant; and (2) that the absolute amount
+of the emanation decaying per second increases as the stock of
+emanation increases. Finally, when the amount of accumulated
+emanation has increased to such an extent that the number of
+emanation atoms transmuting per second becomes exactly equal to
+the number being generated per second, the amount of emanation
+present cannot increase. This is called the equilibrium amount.
+If fifteen members are elected steadily each year into a
+newly-founded society the number of members will increase for the
+first few years; finally, when the losses by death of the members
+equal about fifteen per annum the society can get no bigger. It
+has attained the equilibrium number of members.
+
+This applies to every one of the successive elements. It takes
+twenty-one days for the equilibrium quantity of emanation to be
+formed in radium which has been completely de-emanated; and it
+takes 3.8 days for half the equilibrium amount to be formed.
+Again, if we start with a stock of emanation it takes just three
+hours for the equilibrium amount of Radium C to be formed.
+
+255
+
+We can evidently grow Radium C either from radium itself or from
+the emanation of radium. If we use a tube of radium we have an
+almost perfectly constant quantity of Radium C present, for as
+fast as the Radium C and intervening elements decay the Radium,
+which only diminishes very slowly in amount, makes up the loss.
+But, if we start off with a tube of emanation, we do not possess
+a constant supply of Radium C, because the emanation is decaying
+fairly rapidly and there is no radium to make good its loss. In
+3.8 days about one half the emanation is transmuted and the
+Radium C decreases proportionately and, of course, with the
+Radium C the valuable radiations also decrease. In another 3.8
+days--that is in about a week from the start--the radioactive value
+of the tube has fallen to one-fourth of its original value.
+
+But in spite of the inconstant character of the emanation tube
+there are many reasons for preferring its use to the use of the
+radium tube. Chief of these is the fact that we can keep the
+precious radium safely locked up in the laboratory and not
+exposed to the thousand-and-one risks of the hospital. Then,
+secondly, the emanation, being a gas, is very convenient for
+subdivision into a large number of very small tubes according to
+the dosage required.
+
+In fact the volume of the emanation is exceedingly minute. The
+amount of emanation in equilibrium with one gramme of radium is
+called the curie, and with one
+
+256
+
+milligramme the millicurie. Now, the volume of the curie is only
+a little more than one half a cubic millimetre. Hence in dealing
+with emanation from twenty or forty milligrammes of radium we are
+dealing with very small volumes.
+
+How may the emanation be obtained? The process is an easy one in
+skilled and practised hands. The salt of radium--generally the
+bromide or chloride--is brought into acid solution. This causes
+the emanation to be freely given off as fast as it is formed. At
+intervals we pump it off with a mercury pump.
+
+Let us see how many millicuries we will in future be able to turn
+out in the week in our new Dublin Radium Institute.[1] We shall
+have about 130 milligrammes of radium. In 3.8 days we get 65
+millicuries from this--_i.e._ half the equilibrium amount of 130
+millicuries. Hence in the week, we shall have about 130
+millicuries.
+
+This is not much. Many experts consider this little enough for
+one tube. But here in Dublin we have been using the emanation in
+a more economical and effective manner than is the usage
+elsewhere; according to a method which has been worked out and
+developed in our own Radium Institute. The economy is obtained by
+the very simple expedient of minutely subdividing the' dose. The
+system in vogue, generally, is to treat the tumour by inserting
+into it one or two very active
+
+[1] Then recently established by the Royal Dublin Society.
+
+257
+
+tubes, containing, perhaps, up to 200 millicuries, or even more,
+per tube. Now these very heavily charged tubes give a radiation
+so intense at points close to the tube, due to the greater
+density of the rays near the tube, and, also, to the action of
+the softer and more easily absorbable rays, that it has been
+found necessary to stop these softer rays--both the y and ss--by
+wrapping lead or platinum round the tube. In this lead or
+platinum some thirty per cent. or more of the rays is absorbed
+and, of course, wasted. But in the absence of the screen there is
+extensive necrosis of the tissues near the tubes.
+
+If, however, in place of one or two such tubes we use ten or
+twenty, each containing one-tenth or one-twentieth of the dose,
+we can avail ourselves of the softer rays around each tube with
+benefit. Thus a wasteful loss is avoided. Moreover a more uniform
+"illumination" of the tissues results, just as we can illuminate
+a hall more uniformly by the use of many lesser centres of light
+than by the use of one intense centre of radiation. Also we get
+what is called "cross-radiation,"which is found to be beneficial.
+The surgeon knows far better what he is doing by this method.
+Thus it may be arranged for the effects to go on with approximate
+uniformity throughout the tumour instead of varying rapidly
+around a central point or--and this may be very important-- the
+effects may be readily concentrated locally.
+
+Finally, not the least of the benefit arises in the easy
+technique of this new method. The quantities of
+
+258
+
+emanation employed can fit in the finest capillary glass tubing
+and the hairlike tubes can in turn be placed in fine exploring
+needles. There is comparatively little inconvenience to the
+patient in inserting these needles, and there is the most perfect
+control of the dosage in the number and strength of these tubes
+and the duration of exposure.[1]
+
+The first Radium Institute in Ireland has already done good work
+for the relief of human suffering. It will have, I hope, a great
+future before it, for I venture, with diffidence, to hold the
+opinion, that with increased study the applications and claims of
+radioactive treatment will increase.
+
+[1] For particulars of the new technique and of some of the work
+already accomplished, see papers, by Dr. Walter C. Stevenson,
+_British Medical Journal_, July 4th, 1914, and March 20th, 1915.
+
+259
+
+SKATING [1]
+
+IT is now many years ago since, as a student, I was present at a
+college lecture delivered by a certain learned professor on the
+subject of friction. At this lecture a discussion arose out of a
+question addressed to our teacher: "How is it we can skate on ice
+and on no other substance?"
+
+The answer came back without hesitation: "Because the ice is so
+smooth."
+
+It was at once objected: "But you can skate on ice which is not
+smooth."
+
+This put the professor in a difficulty. Obviously it is not on
+account of the smoothness of the ice. A piece of polished plate
+glass is far smoother than a surface of ice after the latter is
+cut up by a day's skating. Nevertheless, on the scratched and
+torn ice-surface skating is still quite possible; on the smooth
+plate glass we know we could not skate.
+
+Some little time after this discussion, the connection between
+skating and a somewhat abstruse fact in physical science occurred
+to me. As the fact itself is one which has played a part in the
+geological history of the earth,
+
+[1] A lecture delivered before the Royal Dublin Society in 1905.
+
+260
+
+and a part of no little importance, the subject of skating,
+whereby it is perhaps best brought home to every one, is
+deserving of our careful attention. Let not, then, the title of
+this lecture mislead the reader as to the importance of its
+subject matter.
+
+Before going on to the explanation of the wonderful freedom of
+the skater's movements, I wish to verify what I have inferred as
+to the great difference in the slipperiness of glass and the
+slipperiness of ice. Here is a slab of polished glass. I can
+raise it to any angle I please so that at length this brass
+weight of 250 grams just slips down when started with a slight
+shove. The angle is, as you see, about 121/2 degrees. I now
+transfer the weight on to this large slab of ice which I first
+rapidly dry with soft linen. Observe that the weight slips down
+the surface of ice at a much lower angle. It is a very low angle
+indeed: I read it as between 4 and 5 degrees. We see by this
+experiment that there is a great difference between the
+slipperiness of the two surfaces as measured by what is called
+"the angle of friction." In this experiment, too, the glass
+possesses by far the smoother surface although I have rubbed the
+deeper rugosities out of the ice by smoothing it with a glass
+surface. Notwithstanding this, its surface is spotted with small
+cavities due to bubbles and imperfections. It is certain that if
+the glass was equally rough, its angle of friction towards the
+brass weight would be higher.
+
+261
+
+We have, however, another comparative experiment to carry out. I
+made as you saw a determination of the angle at which this weight
+of 250 grams just slipped on the ice. The lower surface of the
+weight, the part which presses on the ice, consists of a light,
+brass curtain ring. This can be detached. Its mass is only 61/2
+grams, the curtain ring being, in fact, hollow and made of very
+thin metal. We have, therefore, in it a very small weight which
+presents exactly the same surface beneath as did the weight of
+250 grams. You see, now, that this light weight will not slip on
+ice at 5 or 6 degrees of slope, but first does so at about io
+degrees.
+
+This is a very important experiment as regards our present
+inquiry. Ice appears to possess more than one angle of friction
+according as a heavy or a light weight is used to press upon it.
+We will make the same experiment with the plate of glass. You see
+that there is little or no difference in the angle of friction of
+brass on glass when we press the surfaces together with a heavy
+or with a light weight. The light weight requires the same slope
+of 121/2 degrees to make it slip.
+
+This last result is in accordance with the laws of friction. We
+say that when solid presses on solid, for each pair of substances
+pressed together there is a constant ratio between the force
+required to keep one in motion over the other, and the force
+pressing the solids together. This ratio is called"the
+coefficient of friction."The coefficient is, in fact, constant or
+approximately
+
+262
+
+so. I can determine the coefficient of friction from the angle of
+friction by taking the tangent of the angle. The tangent of the
+angle of friction is the coefficient of friction. If, then, the
+coefficient is constant, so, of course, must the angle of
+friction be constant. We have seen that it is so in the case of
+metal on glass, but not so in the case of metal on ice. This
+curious result shows that there is something abnormal about the
+slipperiness of ice.
+
+The experiments we have hitherto made are open to the reproach
+that the surface of the ice is probably damp owing to the warmth
+of the air in contact with it. I have here a means of dealing
+with a surface of cold, dry ice. This shallow copper tank about
+18 inches (45 cms.) long, and 4 inches (10 cms.) wide, is filled
+with a freezing 'mixture circulated through it from a larger
+vessel containing ice melting in hydrochloric acid at a
+temperature of about -18 deg. C. This keeps the tank below the
+melting point of ice. The upper surface of the tank is provided
+with raised edges so that it can be flooded with water. The water
+is now frozen and its temperature is below 0 deg. C. It is about
+10 deg. C. I can place over the ice a roof-shaped cover made of two
+inclined slabs of thick plate glass. This acts to keep out warm
+air, and to do away with any possibility of the surface of the
+ice being wet with water thawed from the ice. The whole tank
+along with its roof of glass can be adjusted to any angle, and a,
+scale at the
+
+263
+
+raised end of the tank gives the angle of slope in degrees. A
+weight placed on the ice can be easily seen through the glass
+cover.
+
+The weight we shall use consists of a very light ring of
+aluminium wire which is rendered plainly visible by a ping-pong
+ball attached above it. The weight rests now on a copper plate
+provided for the purpose at the upper end of the tank. The plate
+being in direct contact beneath with the freezing mixture we are
+sure that the aluminium ring is no hotter than the ice. A light
+jerk suffices to shake the weight on to the surface of the ice.
+
+We find that this ring loaded with only the ping-pong ball, and
+weighing a total of 2.55 grams does not slip at the low angles. I
+have the surface of the ice at an angle of rather over 131/2, and
+only by continuous tapping of the apparatus can it be induced to
+slip down. This is a coefficient of 0.24, and compares with the
+coefficient of hard and smooth solids on one another. I now
+replace the empty ping-pong ball by a similar ball filled with
+lead shot. The total weight is now 155 grams. You see the angle
+of slipping has fallen to 7 deg..
+
+Every one who has made friction experiments knows how
+unsatisfactory and inconsistent they often are. We can only
+discuss notable quantities and broad results, unless the most
+conscientious care be taken to eliminate errors. The net result
+here is that ice at about -10 deg. C. when pressed on by a very light
+weight possesses a
+
+264
+
+coefficient of friction comparable with the usual coefficients of
+solids on solids, but when the pressure is increased, the
+coefficient falls to about half this value.
+
+The following table embodies some results obtained on the
+friction of ice and glass, using the methods I have shown you. I
+add some of the more carefully determined coefficients of other
+observers.
+
+          Wt. in      On Plate         On Ice         On Ice
+          Grams.      Glass.           at 0 deg. C.       at 10 deg. C.
+
+                   Angle. Coeff.    Angle. Coeff.  Angle. Coeff
+Aluminium   2.55    121/2 deg.  0.22        12 deg.   0.21     131/2 deg.   0.24
+Same      155       121/2 deg.   0.22        6 deg.   0.11       7 deg.   0.12
+Brass       6.5     121/2 deg.  0.22        10 deg.   0.17     101/2 deg.   0.18
+Same      107       121/2 deg.   0.22        5 deg.   0.09       6 deg.   0.10
+
+Steel on steel (Morin) - - - - 0.14
+Brass on cast iron (Morin) - - 0.19
+Steel on cast iron (Morin) - - 0.20
+Skate on ice (J. Mueller) - - - 0.016--0.032
+Best-greased surfaces (Perry) - 0.03--0.036
+
+You perceive from the table that while the friction of brass or
+aluminium on glass is quite independent of the weight used, that
+of brass or aluminium on ice depends in some way upon the weight,
+and falls in a very marked degree when the weight is heavy. Now,
+I think that if we had been on the look out for any abnormality
+in the friction of hard substances on ice, we would have rather
+anticipated a variation in the
+
+265
+
+other direction. We would have, perhaps, expected that a heavy
+weight would have given rise to the greater friction. I now turn
+to the explanation of this extraordinary result.
+
+You are aware that it requires an expenditure of heat merely to
+convert ice to water, the water produced being at the temperature
+of the ice, _i.e._ at 0 deg. C., from which it is derived. The heat
+required to change the ice from the solid to the liquid state is
+the latent heat of water. We take the unit quantity of heat to be
+that which is required to heat 1 kilogram of water 1 deg. C. Then if
+we melt 1 kilogram of ice, we must supply it with 80 such units
+of heat. While melting is going on, there is no change of
+temperature if the experiment is carefully conducted. The melting
+ice and the water coming from it remain at 0 deg. C. throughout the
+operation, and neither the thermometer nor your own sensations
+would tell you of the amount of heat which was flowing in. The
+heat is latent or hidden in the liquid produced, and has gone to
+do molecular work in the substance. Observe that if we supply
+only 40 thermal units, we get only one-half the ice melted. If
+only 10 units are supplied, then we get only one eighth of a
+kilogram of water, and no more nor less.
+
+I have ventured to recall to you these commonplaces of science
+before considering a mode of melting ice which is less generally
+known, and which involves no supply of heat on your part. This
+method involves for its
+
+266
+
+understanding a careful consideration of the thermal properties
+of water in the solid state.
+
+It must have been observed a very long time ago that water
+expands when it freezes. Otherwise ice would not float on water;
+and, what is perhaps more important in your eyes, your water
+pipes would not burst in winter when the water freezes therein.
+But although the important fact of the expansion of water on
+freezing was so long presented to the observation of mankind, it
+was not till almost exactly the middle of the last century that
+James Thomson, a gifted Irishman, predicted many important
+consequences arising from the fact of the expansion of water on
+becoming solid. The principles lie enunciated are perfectly
+general, and apply in every case of change of volume attending
+change of state. We are here only concerned with the case of
+water and ice.
+
+James Thomson, following a train of thought which we cannot here
+pursue, predicted that owing to the fact of the expansion of
+water on becoming solid, pressure will lower the melting point of
+ice or the freezing point of water. Normally, as you are aware,
+the temperature is 0 deg. C. or 32 deg. F. Thomson said that this would
+be found to be the freezing point only at atmospheric pressure.
+He calculated how much it would change with change of pressure.
+He predicted that the freezing point would fall 0.0075 of a
+degree Centigrade for each additional atmosphere of pressure
+applied to the water. Suppose,
+
+267
+
+for instance, our earth possessed an atmosphere so heavy to as
+exert a thousand times the pressure of the existing atmosphere,
+then water would not freeze at 0 deg. C., but at -7.5 deg. C. or about
+18 deg. F. Again, in vacuo, that is when the pressure has been
+reduced to the relatively small vapour pressure of the water, the
+freezing point is above 0 deg. C., _i.e._ at 0.0075 deg. C. In parts of
+the ocean depths the pressure is much over a thousand
+atmospheres. Fresh water would remain liquid there at
+temperatures much below 0 deg. C.
+
+It will be evident enough, even to those not possessed of the
+scientific insight of James Thomson, that some such fact is to be
+anticipated. It is, however, easy to be wise after the event. It
+appeals to us in a general way that as water expands on freezing,
+pressure will tend to resist the turning of it to ice. The water
+will try to remain liquid in obedience to the pressure. It will,
+therefore, require a lower temperature to induce it to become
+ice.
+
+James Thomson left his thesis as a prediction. But he predicted
+exactly what his distinguished brother, Sir William Thomson--later
+Lord Kelvin--found to happen when the matter was put to the test
+of experiment. We must consider the experiment made by Lord
+Kelvin.
+
+According to Thomson's views, if a quantity of ice and water are
+compressed, there must be _a fall of temperature_. The nature of
+his argument is as follows:
+
+268
+
+Let the ice and water be exactly at 0 deg. C. to start with. Then
+suppose we apply, say, one thousand atmospheres pressure. The
+melting point of the ice is lowered to -7.5 deg. C. That is, it will
+require a temperature so low as -7.5 deg. C. to keep it solid. It
+will therefore at once set about melting, for as we have seen,
+its actual temperature is not -7.5 deg. C., but a higher temperature,
+_i.e._ 0 deg. C. In other words, it is 7.5 deg. above its melting point.
+But as soon as it begins melting it also begins to absorb heat to
+supply the 80 thermal units which, as we know, are required to
+turn each kilogram of the ice to water. Where can it get this
+heat? We assume that we give it none. It has only two sources,
+the ice can take heat from itself, and it can take heat from the
+water. It does both in this case, and both ice and water drop in
+temperature. They fall in temperature till -7.5 deg. is reached. Then
+the ice has got to its melting point under the pressure of one
+thousand atmospheres, or, as we may put it, the water has reached
+its freezing point. There can be no more melting. The whole mass
+is down to -7.5 deg. C., and will stay there if we keep heat from
+flowing either into or out of the vessel. There is now more water
+and less ice in the vessel than when we started, and the
+temperature has fallen to -7.5 deg. C. The fall of temperature to the
+amount predicted by the theory was verified by Lord Kelvin.
+
+Suppose we now suddenly remove the pressure; what will happen? We
+have water and ice at -7.5 deg. C.
+
+269
+
+and at the normal pressure. Water at -7.5 deg. and at the normal
+pressure of course turns to ice. The water will, therefore,
+instantly freeze in the vessel, and the whole process will be
+reversed. In freezing, the water will give up its latent heat,
+and this will warm up the whole mass till once again 0 deg. C. is
+attained. Then there will be no more freezing, for again the ice
+is at its melting point. This is the remarkable series of events
+which James Thomson predicted. And these are the events which
+Lord Kelvin by a delicate series of experiments, verified in
+every respect.
+
+Suppose we had nothing but solid ice in the vessel at starting,
+would the experiment result in the same way? Yes, it assuredly
+would. The ice under the increased pressure would melt a little
+everywhere throughout its mass, taking the requisite latent heat
+from itself at the expense of its sensible heat, and the
+temperature of the ice would fall to the new melting point.
+
+Could we melt the whole of the ice in this manner? Again the
+answer is "yes." But the pressure must be very great. If we
+assume that all the heat is obtained at the expense of the
+sensible heat of the ice, the cooling must be such as to supply
+the latent heat of the whole mass of water produced. However, the
+latent heat diminishes as the melting point is lowered, and at a
+rate which would reduce it to nothing at about 18,000
+atmospheres. Mousson, operating on ice enclosed in a conducting
+cylinder and cooled to -18 deg. at starting
+
+270
+
+appears to have obtained very complete liquefaction. Mousson must
+have attained a pressure of at least an amount adequate to lower
+the melting point below -18 deg.. The degree of liquefaction actually
+attained may have been due in part to the passage of heat through
+the walls of the vessel. He proved the more or less complete
+liquefaction of the ice within the vessel by the fall of a copper
+index from the top to the bottom of the vessel while the pressure
+was on.
+
+I have here a simple way of demonstrating to you the fall of
+temperature attending the compression of ice. In this mould,
+which is strongly made of steel, lined with boxwood to diminish
+the passage of conducted heat, is a quantity of ice which I
+compress when I force in this plunger. In the ice is a
+thermoelectric junction, the wires leading to which are in
+communication with a reflecting galvanometer. The thermocouple is
+of copper and nickel, and is of such sensitiveness as to show by
+motion of the spot of light on the screen even a small fraction
+of a degree. On applying the pressure, you see the spot of light
+is displaced, and in such a direction as to indicate cooling. The
+balancing thermocouple is all the time imbedded in a block of ice
+so that its temperature remains unaltered. On taking off the
+pressure, the spot of light returns to its first position. I can
+move the spot of light backwards and forwards on the screen by
+taking off and putting on the pressure. The effects are quite
+instantaneous.
+
+271
+
+The fact last referred to is very important. The ice, in fact, is
+as it were automatically turned to water. It is not a matter of
+the conduction of heat from point to point in the ice. Its own
+sensible heat is immediately absorbed throughout the mass. This
+would be the theoretical result, but it is probable that owing to
+imperfections throughout the ice and failure in uniformity in the
+distribution of the stress, the melting would not take place
+quite uniformly or homogeneously.
+
+Before applying our new ideas to skating, I want you to notice a
+fact which I have inferentially stated, but not specifically
+mentioned. Pressure will only lead to the melting of ice if the
+new melting point, _i.e._ that due to the pressure, is below the
+prevailing temperature. Let us take figures. The ice to start
+with is, say, at -3 deg. C. Suppose we apply such a pressure to this
+ice as will confer a melting point of -2 deg. C. on it. Obviously,
+there will be no melting. For why should ice which is at -3 deg. C.
+melt when its melting point is -2 deg. C.? The ice is, in fact,
+colder than its melting point. Hence, you note this fact: The
+pressure must be sufficiently intense to bring the melting point
+below the prevailing temperature, or there will be no melting;
+and the further we reduce the melting point by pressure below the
+prevailing temperature, the more ice will be melted.
+
+We come at length to the object of our remarks I don't know who
+invented skating or skates. It is said that in the thirteenth
+century the inhabitants of
+
+272
+
+England used to amuse themselves by fastening the bones of an
+animal beneath their feet, and pushing themselves about on the
+ice by means of a stick pointed with iron. With such skates, any
+performance either on inside or outside edge was impossible. We
+are a conservative people. This exhilarating amusement appears to
+have served the people of England for three centuries. Not till
+1660 were wooden skates shod with iron introduced from the
+Netherlands. It is certain that skating was a fashionable
+amusement in Pepys' time. He writes in 1662 to the effect: "It
+being a great frost, did see people sliding with their skates,
+which is a very pretty art." It is remarkable that it was the
+German poet Klopstock who made skating fashionable in Germany.
+Until his time, the art was considered a pastime, only fit for
+very young or silly people.
+
+I wish now to dwell upon that beautiful contrivance the modern
+skate. It is a remarkable example of how an appliance can develop
+towards perfection in the absence of a really intelligent
+understanding of the principles underlying its development. For
+what are the principles underlying the proper construction of the
+skate? After what I have said, I think you will readily
+understand. The object is to produce such a pressure under the
+blade that the ice will melt. We wish to establish such a
+pressure under the skate that even on a day when the ice is below
+zero, its melting
+
+273
+
+point is so reduced just under the edge of the skate that the ice
+turns to water.
+
+It is this melting of the ice under the skate which secures the
+condition essential to skating. In the first place, the skate no
+longer rests on a solid. It rests on a liquid. You are aware how
+in cases where we want to reduce friction--say at the bearing of a
+wheel or under a pivot--we introduce a liquid. Look at the
+bearings of a steam engine. A continuous stream of oil is fed in
+to interpose itself between the solid surfaces. I need not
+illustrate so well-known a principle by experiment. Solid
+friction disappears when the liquid intervenes. In its place we
+substitute the lesser difficulty of shearing one layer of the
+liquid over the other; and if we keep up the supply of oil the
+work required to do this is not very different, no matter how
+great we make the pressure upon the bearings. Compared with the
+resistance of solid friction, the resistance of fluid friction is
+trifling. Here under the skate the lubrication is perhaps the
+most perfect which it is possible to conceive. J. Mueller has
+determined the coefficient by towing a skater holding on by a
+spring balance. The coefficient is between 0.016 and 0.032. In
+other words, the skater would run down an incline so little as 1
+or 2 degrees; an inclination not perceivable by the eye. Now
+observe that the larger of these coefficients is almost exactly
+the same as that which Perry found in the case of well-greased
+surfaces. But evidently no
+
+274
+
+artificial system of lubrication could hope to equal that which
+exists between the skate and the ice. For the lubrication here
+is, as it were, automatic. In the machine if the lubricant gets
+squeezed out there instantly ensues solid friction. Under the
+skate this cannot happen for the squeezing out of the lubricant
+is instantly followed by the formation of another film of water.
+The conditions of pressure which may lead to solid friction in
+the machine here automatically call the lubricant into
+existence.
+
+Just under the edge of the skate the pressure is enormous.
+Consider that the whole weight of the skater is born upon a mere
+knife edge. The skater alternately throws his whole weight upon
+the edge of each skate. But not only is the weight thus
+concentrated upon one edge, further concentration is secured in
+the best skates by making the skate hollow-ground, _i.e._
+increasing the keenness of the edge by making it less than a
+right angle. Still greater pressure is obtained by diminishing
+the length of that part of the blade which is in contact with the
+ice. This is done by putting curvature on the blade or making it
+what is called "hog-backed." You see that everything is done to
+diminish the area in contact with the ice, and thus to increase
+the pressure. The result is a very great compression of the ice
+beneath the edge of the skate. Even in the very coldest weather
+melting must take place to some extent.
+
+As we observed before, the melting is instantaneous,
+
+275
+
+Heat has not to travel from one point of the ice to another;
+immediately the pressure comes on the ice it turns to water. It
+takes the requisite heat from itself in order that the change of
+state may be accomplished. So soon as the skate passes on, the
+water resumes the solid state. It is probable that there is an
+instantaneous escape, and re-freezing of some of the water from
+beneath the skate, the skate instantly taking a fresh bearing and
+melting more ice. The temperature of the water escaping from
+beneath the skate, or left behind by it, immediately becomes what
+it was before the skate pressed upon it.
+
+Thus, a most wonderful and complex series of molecular events
+takes place beneath the skate. Swift as it passes, the whole
+sequence of events which James Thomson predicted has to take
+place beneath the blade Compression; lowering of the melting
+point below the temperature of the surrounding ice; melting;
+absorption of heat; and cooling to the new melting point, _i.e._
+to that proper to the pressure beneath the blade. The skate now
+passes on. Then follow: Relief of pressure; re-solidification of
+the water; restoration of the borrowed heat from the congealing
+water and reversion of the ice to the original temperature.
+
+If we reflect for a moment on all this, we see that we do not
+skate on ice but on water. We could not skate on ice any more
+than we could skate on glass. We saw that with light weights and
+when the pressure
+
+276
+
+{Diagram}
+
+Diagram showing successive states obtaining in ice, before,
+during, and after the passage of the skate. The temperatures and
+pressures selected for illustration are such as might occur under
+ordinary conditions. The edge of the skate is shown in magnified
+cross-section.
+
+277
+
+Was not sufficient to melt the ice, the friction was much the
+same as that of metal on glass. Ice is not slippery. It is an
+error to say that it is. The learned professor was very much
+astray when he said that you could skate on ice because it is so
+smooth. The smoothness of the ice has nothing to do with the
+matter. In short, owing to the action of gravity upon your body,
+you escape the normal resistance of solid on solid, and glide
+about with feet winged like the messenger of the Gods; but on
+water.
+
+A second condition essential to the art of skating is also
+involved in the melting of the ice. The sinking of the skate
+gives the skater "bite." This it is which enables him to urge
+himself forward. So long as skates consisted of the rounded bones
+of animals, the skater had to use a pointed staff to propel
+himself. In creating bite, the skater again unconsciously appeals
+to the peculiar physical properties of ice. The pressure required
+for the propulsion of the skater is spread all along the length
+of the groove he has cut in the ice, and obliquely downwards. The
+skate will not slip away laterally, for the horizontal component
+of the pressure is not enough to melt the ice. He thus gets the
+resistance he requires.
+
+You see what a very perfect contrivance the skate is; and what a
+similitude of intelligence there is in its evolution. Blind
+intelligence, because it is certain the true physics of skating
+was never held in view by
+
+278
+
+the makers of skates. The evolution of the skate has been truly
+organic. The skater selected the fittest skate, and hence the fit
+skate survived.
+
+In a word, the possibility of skating depends on the dynamical
+melting of ice under pressure. And observe the whole matter turns
+upon the apparently unrelated fact that the freezing of water
+results in a solid more bulky than the water which gives rise to
+it. If ice was less bulky than the water from which it was
+derived, pressure would not melt it; it would be all the more
+solid for the pressure, as it were. The melting point would rise
+instead of falling. Most substances behave in this manner, and
+hence we cannot skate upon them. Only quite a few substances
+expand on freezing, and it happens that their particular melting
+temperatures or other properties render them unsuitable to
+skating. The most abundant fluid substance on the earth, and the
+most abundant substance of any one kind on its surface, thus
+possesses the ideally correct and suitable properties for the art
+of skating.
+
+I have pointed out that the pressure must be such as to bring the
+temperature of melting below that prevailing in the ice at the
+time. We have seen also, that one atmosphere lowers the melting
+point of ice by the 1/140 of a degree Centigrade; more exactly by
+0.0075 deg.. Let us now assume that the skate is so far sunken in the
+ice as to bear for a length of two inches, and for a width of
+one-hundredth of an inch. The skater weighs,
+
+279
+
+let us say--150 pounds. If this weight was borne on one square
+inch, the pressure would be ten atmospheres. But the skater rests
+his weight, in fact, upon an area of one-fiftieth of an inch. The
+pressure is, therefore, fifty times as great. The ice is
+subjected to a pressure of 500 atmospheres. This lowers the
+melting point to -3.75 deg. C. Hence, on a day when the ice is at
+this temperature, the skate will sink in the ice till the weight
+of the skater is concentrated as we have assumed. His skate can
+sink no further, for any lesser concentration of the pressure
+will not bring the melting point below the prevailing
+temperature. We can calculate the theoretical bite for any state
+of the ice. If the ice is colder the bite will not be so deep. If
+the temperature was twice as far below zero, then the area over
+which the skater's weight will be distributed, when the skate has
+penetrated its maximum depth, will be only half the former area,
+and the pressure will be one thousand atmospheres.
+
+An important consideration arises from the fact that under the
+very extreme edge of the skate the pressure is indefinitely
+great. For this involves that there will always be some bite,
+however cold the ice may be. That is, the narrow strip of ice
+which first receives the skater's weight must partially liquefy
+however cold the ice.
+
+It must have happened to many here to be on ice which was too
+cold to skate on with comfort. The
+
+280
+
+skater in this case speaks of the ice as too hard. In the
+Engadine, the ice on the large lakes gets so cold that skaters
+complain of this. On the rinks, which are chiefly used there, the
+ice is frequently renewed by flooding with water at the close of
+the day. It thus never gets so very cold as on the lakes. I have
+been on ice in North France, which, in the early morning, was too
+hard to afford sufficient bite for comfort. The cause of this is
+easily understood from what we have been considering.
+
+We may now return to the experimental results which we obtained
+early in the lecture. The heavy weights slip off the ice at a low
+angle because just at the points of contact with the ice the
+latter melts, and they, in fact, slip not on ice but on water.
+The light weights on cold, dry ice do not lower the melting point
+below the temperature of the ice, _i.e._ below -10 deg. C., and so
+they slip on dry ice. They therefore give us the true coefficient
+of friction of metal on ice.
+
+This subject has, more recently been investigated by H. Morphy,
+of Trinity College, Dublin. The refinement of a closed vessel at
+uniform temperature, in which the ice is formed and the
+experiment carried out, is introduced. Thermocouples give the
+temperatures, not only of the ice but of the aluminium sleigh
+which slips upon it under various loads. In this way we may be
+certain that the metal runners are truly at the temperature of
+the ice. I now quote from Morphy's paper
+
+281
+
+"The angle of friction was found to remain constant until a
+certain stage of the loading, when it suddenly fell to about half
+of its original value. It then remained constant for further
+increases in the load.
+
+"These results, which confirmed those obtained previously with
+less satisfactory apparatus, are shown in the table below. In the
+first column is shown the load, _i.e._ the weight of sleigh +
+weight of shot added. In the second and third columns are shown,
+respectively, the coefficient and angle of friction, whilst the
+fourth gives the temperature of the ice as determined from the
+galvanometer deflexions.
+
+Load.        Tan y.          y.        Temp.
+
+5.68 grams.  0.36+-.01     20 deg.+-30'    -5.65 deg. C.
+10.39                                -5.65 deg.
+11.96                                -5.75 deg.
+12.74                                -5.60 deg.
+13.53                                -5.65 deg.
+14.31                                -5.65 deg.
+15.10 grams. 0.17+-.01    9 deg..30'+-30'  -5.60 deg.
+16.67                                -5.55 deg.
+19.81                                -5.60 deg.
+24.52                                -5.60 deg.
+5.68 grams. 0.36+-.01      20 deg.+-30'    -5.60 deg.
+
+"These experiments were repeated on another occasion with the same
+result and similar results had been obtained with different
+apparatus.
+
+"As a result of the investigation the following points are
+clearly shown:--
+
+282
+
+"(1) The coefficient of friction for ice at constant temperature
+may have either of two constant values according to the pressure
+per unit surface of contact.
+
+"(2) For small pressures, and up to a certain well defined limit
+of pressure, the coefficient is fairly large, having the value
+0.36+-.01 in the case investigated.
+
+"(3) For pressures greater than the above limit the coefficient
+is relatively small, having the value 0.17+-.01 in the case
+investigated."
+
+It will be seen that Morphy's results are similar to those
+arrived at in the first experimental consideration of our
+subject; but from the manner in which the experiments have been
+carried out, they are more accurate and reliable.
+
+A great deal more might be said about skating, and the allied
+sports of tobogganing, sleighing, curling, ice yachting, and
+last, but by no means least, sliding--that unpretentious pastime
+of the million. Happy the boy who has nails in his boots when
+Jack-Frost appears in his white garment, and congeals the
+neighbouring pond. But I must turn away at the threshold of the
+humorous aspect of my subject (for the victim of the street
+"slide" owes his injured dignity to the abstruse laws we have
+been discussing) and pass to other and graver subjects intimately
+connected with skating.
+
+James Thomson pointed out that if we apply compressional stress
+to an ice crystal contained in a vessel
+
+283
+
+which also contains other ice crystals, and water at 0 deg. C., then
+the stressed crystal will melt and become water, but its
+counterpart or equivalent quantity of ice will reappear elsewhere
+in the vessel. This is, obviously, but a deduction from the
+principles we have been examining. The phenomenon is commonly
+called "regelation." I have already made the usual regelation
+experiment before you when I compressed broken ice in this mould.
+The result was a clear, hard and almost flawless lens of ice. Now
+in this operation we must figure to ourselves the pieces of ice
+when pressed against one another melting away where compressed,
+and the water produced escaping into the spaces between the
+fragments, and there solidifying in virtue of its temperature
+being below the freezing point of unstressed water. The final
+result is the uniform lens of ice. The same process goes on in a
+less perfect manner when you make--or shall I better say--when you
+made snowballs.
+
+We now come to theories of glacier motion; of which there are
+two. The one refers it mainly to regelation; the other to a real
+viscosity of the ice.
+
+The late J. C. M'Connel established the fact that ice possesses
+viscosity; that is, it will slowly yield and change its shape
+under long continued stresses. His observations, indeed, raise a
+difficulty in applying this viscosity to explain glacier motion,
+for he showed that an ice crystal is only viscous in a certain
+structural
+
+284
+
+direction. A complex mixture of crystals such, as we know glacier
+ice to be, ought, we would imagine, to display a nett or
+resultant rigidity. A mass of glacier ice when distorted by
+application of a force must, however, undergo precisely the
+transformations which took place in forming the lens from the
+fragments of ice. In fact, regelation will confer upon it all the
+appearance of viscosity.
+
+Let us picture to ourselves a glacier pressing its enormous mass
+down a Swiss valley. At any point suppose it to be hindered in
+its downward path by a rocky obstacle. At that point the ice
+turns to water just as it does beneath the skate. The cold water
+escapes and solidifies elsewhere. But note this, only where there
+is freedom from pressure. In escaping, it carries away its latent
+heat of liquefaction, and this we must assume, is lost to the
+region of ice lately under pressure. This region will, however,
+again warm up by conduction of heat from the surrounding ice, or
+by the circulation of water from the suxface. Meanwhile, the
+pressure at that point has been relieved. The mechanical
+resistance is transferred elsewhere. At this new point there is
+again melting and relief of pressure. In this manner the glacier
+may be supposed to move down. There is continual flux of
+conducted heat and converted latent heat, hither and thither, to
+and from the points of resistance. The final motion of the whole
+mass is necessarily slow; a few feet in the day or, in winter,
+
+285
+
+even only a few inches. And as we might expect, perfect silence
+attends the downward slipping of the gigantic mass. The motion
+is, I believe, sufficiently explained as a skating motion. The
+skate is, however, fixed, the ice moves. The great Aletsch
+Glacier collects its snows among the highest summits of the
+Oberland. Thence, the consolidated ice makes its way into the
+Rhone Valley, travelling a distance of some 20 miles. The ice now
+melting into the youthful Rhone fell upon the Monch, the Jungfrau
+or the Eiger in the days when Elizabeth ruled in England and
+Shakespeare lived.
+
+The ice-fall is a common sight on the glacier. In great lumps and
+broken pinnacles it topples over some rocky obstacle and falls
+shattered on to the glacier below. But a little further down the
+wound is healed again, and regelation has restored the smooth
+surface of the glacier. All such phenomena are explained on James
+Thomson's exposition of the behaviour of a substance which
+expands on passing from the liquid to the solid state.
+
+We thus have arrived at very far-reaching considerations arising
+out of skating and its science. The tendency for snow to
+accumulate on the highest regions of the Earth depends on
+principles which we cannot stop to consider. We know it collects
+above a certain level even at the Equator. We may consider, then,
+that but for the operation of the laws which James Thomson
+brought to light, and which his illustrious brother,
+
+286
+
+Lord Kelvin, made manifest, the uplands of the Earth could not
+have freed themselves of the burthen of ice. The geological
+history of the Earth must have been profoundly modified. The
+higher levels must have been depressed; the general level of the
+ocean relatively to the land thereby raised, and, it is even
+possible, that such a mean level might have been attained as
+would result in general submergence.
+
+During the last great glacial period, we may say the fate of the
+world hung on the operation of those laws which have concerned us
+throughout this lecture. It is believed the ice was piled up to a
+height of some 6,000 feet over the region of Scandinavia. Under
+the influence of the pressure and fusion at points of resistance,
+the accumulation was stayed, and it flowed southwards the
+accumulation was stayed, and it flowed southwards over Northern
+Europe. The Highlands of Scotland were covered with, perhaps,
+three or four thousand feet of ice. Ireland was covered from
+north to south, and mighty ice-bergs floated from our western and
+southern shores.
+
+The transported or erratic stones, often of great size, which are
+found in many parts of Ireland, are records of these long past
+events: events which happened before Man, as a rational being,
+appeared upon the Earth.
+
+287
+
+A SPECULATION AS TO A PREMATERIAL UNIVERSE [1]
+
+"And therefore...these things likewise had a birth; for things
+which are of mortal body could not for an infinite time back...
+have been able to set at naught the puissant strength of
+immeasurable age."--LUCRETIUS, _De Rerum Natura._
+
+"O fearful meditation! Where, alack! Shall Time's best jewel
+from Time's chest lie hid?" --SHAKESPEARE.
+
+IN the material universe we find presented to our senses a
+physical development continually progressing, extending to all,
+even the most minute, material configurations. Some fundamental
+distinctions existing between this development as apparent in the
+organic and the inorganic systems of the present day are referred
+to elsewhere in this volume.[2] In the present essay, these
+systems as having a common origin and common ending, are merged
+in the same consideration as to the nature of the origin of
+material systems in general. This present essay is occupied by
+the consideration of the necessity of limiting material
+interactions in past time. The speculation originated in the
+difficulties which present themselves when we ascribe to these
+interactions infinite duration in the past. These difficulties
+first claim our consideration.
+
+[1] Proc. Royal Dublin Soc., vol. vii., Part V, 1892.
+
+[2] _The Abundance of Life._
+
+288
+
+Accepting the hypothesis of Kant and Laplace in its widest
+extension, we are referred to a primitive condition of wide
+material diffusion, and necessarily too of material instability.
+The hypothesis is, in fact, based upon this material instability.
+We may pursue the sequence of events assumed in this hypothesis
+into the future, and into the past.
+
+In the future we find finality to progress clearly indicated. The
+hypothesis points to a time when there will be no more
+progressive change but a mere sequence of unfruitful events, such
+as the eternal uniform motion of a mass of matter no longer
+gaining or losing heat in an ether possessed of a uniform
+distribution of energy in all its parts. Or, again, if the ether
+absorb the energy of material motion, this vast and dark
+aggregation eternally poised and at rest within it. The action is
+transferred to the subtle parts of the ether which suffer none of
+the energy to degrade. This is, physically, a thinkable future.
+Our minds suggest no change, and demand none. More than this,
+change is unthinkable according to our present ideas of energy.
+Of progress there is an end.
+
+This finality _a parte post_ is instructive. Abstract
+considerations, based on geometrical or analytical illustrations,
+question the finiteness of some physical developments. Thus our
+sun may require eternal time to attain the temperature of the
+ether around it, the approach to this condition being assumed to
+be asymptotic in
+
+289
+
+character. But consider the legitimate _reductio ad absurdum_ of
+an ember raked from a fire 1000 years ago. Is it not yet cooled
+down to the constant temperature of its surroundings? And we may
+evidently increase the time a million-fold if we please. It
+appears as if we must regard eternity as outliving every
+progressive change, For there is no convergence or enfeeblement
+of time. The ever-flowing present moves no differently for the
+occurrence of the mightiest or the most insignificant events. And
+even if we say that time is only the attendant upon events, yet
+this attendant waits patiently for the end, however long
+deferred.
+
+Does the essentially material hypothesis of Kant and Laplace
+account for an infinite past as thinkably as it accounts for the
+infinite future? As this hypothesis is based upon material
+instability the question resolves itself into this:-- Is the
+assumption of an infinitely prolonged past instability a probable
+or possible account of the past? There are, it appears to me,
+great difficulties involved in accepting the hypothesis of
+infinitely prolonged material instability. I will refer here to
+three principal objections. The first may be called a
+metaphysical objection; the second is partly metaphysical and
+partly physical, the third may be considered a physical
+objection, as it is involved directly in the phenomena presented
+by our universe.
+
+The metaphysical objection must have presented itself to every
+one who has considered the question. It may
+
+290
+
+be put thus:--If present events are merely one stage in an
+infinite progress, why is not the present stage long ago passed
+over? We are evidently at liberty to push back any stage of
+progress to as remote a period as we like by putting back first
+the one before this and next the stage preceding this, and so on,
+for, by hypothesis, there is no beginning to the progress.
+
+Thus, the sum of passing events constituting the present universe
+should long ago have been accomplished and passed away. If we
+consider alternative hypotheses not involving this difficulty, we
+are at once struck by the fact that the future of material
+development is free of the objection. For the eternity of
+unprogressive events involved in the future on Kant's hypothesis,
+is not only thinkable, but any change is, as observed,
+irreconcilable with our ideas of energy. As in the future so in
+the past we look to a cessation to progress. But as we believe
+the activity of the present universe must in some form have
+existed all along, the only refuge in the past is to imagine an
+active but unprogressive eternity, the unprogressive activity at
+some period becoming a progressive activity--that progressive
+activity of which we are spectators. To the unprogressive
+activity there was no beginning; in fact, beginning is as
+unthinkable and uncalled for to the unprogressive activity of the
+past as ending is to the unprogressive activity of the future,
+when all developmental actions shall have ceased. There is no
+beginning or ending to the activity of the universe.
+
+291
+
+There is beginning and ending to present progressive activity.
+Looking through the realm of nature we seek beginning and ending,
+but "passing through nature to eternity" we find neither. Both
+are justified; the questioning of the ancient poet regarding the
+past, and of the modern regarding the future, quoted at the head
+of this essay.
+
+The next objection, which is in part metaphysical, is founded on
+the difficulty of ascribing any ultimate reality or potency to
+forces diminishing through eternal time. Thus, against the
+assumption that our universe is the result of material
+aggregation progressing over eternal time, which involves the
+primitive infinite separation of the particles, we may ask, what
+force can have acted between particles sundered by infinite
+distance? The gravitational force falling off as the square of
+the distance, must vanish at infinity if we mean what we say when
+we ascribe infinite separation to them. Their condition is then
+one of neutral stability, a finite movement of the particles
+neither increasing nor diminishing interaction. They had then
+remained eternally in their separated condition, there being no
+cause to render such condition finite. The difficulty involved
+here appears to me of the same nature as the difficulty of
+ascribing any residual heat to the sun after eternal time has
+elapsed. In both cases we are bound to prolong the time, from our
+very idea of time, till progress is no more, when in the one case
+we can imagine no mutual approximation of the
+
+292
+
+particles, in the other no further cooling of the body. However,
+I will riot dwell further upon this objection, as it does not, I
+believe, present itself with equal force to every mind. A reason
+less open to dispute, as being less subjective, against the
+aggregation of infinitely remote particles as the origin of our
+universe, is contained in the physical objection.
+
+In this objection we consider that the appearance presented by
+our universe negatives the hypothesis of infinitely prolonged
+aggregation. We base this negation upon the appearance of
+simultaneity ~ presented by the heavens, contending that this
+simultaneity is contrary to what we would expect to find in the
+case of particles gathered from infinitely remote distances.
+Whether these particles were endowed with relative motions or not
+is unimportant to the consideration. In what respects do the
+phenomena of our universe present the appearance of simultaneous
+phenomena? We must remember that the suns in space are as fires
+which brighten only for a moment and are then extinguished. It is
+in this sense we must regard the longest burning of the stars.
+Whether just lit or just expiring counts little in eternity. The
+light and heat of the star is being absorbed by the ether of
+space as effectually and rapidly as the ocean swallows the ripple
+from the wings of an expiring insect. Sir William Herschel says
+of the galaxy of the milky way:-- "We do not know the rate of
+progress of this mysterious chronometer, but it is nevertheless
+certain that it cannot
+
+293
+
+last for ever, and its past duration cannot be infinite." We do
+not know, indeed, the rate of progress of the chronometer, but if
+the dial be one divided into eternal durations the consummation
+of any finite physical change represents such a movement of the
+hand as is accomplished in a single vibration of the balance
+wheel.
+
+Hence we must regard the hosts of glittering stars as a
+conflagration that has been simultaneously lighted up in the
+heavens. The enormous (to our ideas) thermal energy of the stars
+resembles the scintillation of iron dust in a jar of oxygen when
+a pinch of the dust is thrown in. Although some particles be
+burnt up before others become alight, and some linger yet a
+little longer than the others, in our day's work the
+scintillation of the iron dust is the work of a single instant,
+and so in the long night of eternity the scintillation of the
+mightiest suns of space is over in a moment. A little longer,
+indeed, in duration than the life which stirs a moment in
+response to the diffusion of the energy, but only very little. So
+must an Eternal Being regard the scintillation of the stars and
+the periodic vibration of life in our geological time and the
+most enduring efforts of thought. The latter indeed are no more
+lasting than
+
+"... the labour of ants In the light of a million million of
+suns."
+
+But the myriad suns themselves, with their generations, are the
+momentary gleam of lights for ever after extinguished.
+
+294
+
+Again, science suggests that the present process of material
+aggregation is not finished, and possibly will only be when it
+prevails universally. Hence the very distribution of the stars,
+as we observe them, as isolated aggregations, indicates a
+development which in the infinite duration must be regarded as
+equally advanced in all parts of stellar space and essentially a
+simultaneous phenomenon. For were we spectators of a system in
+which any very great difference of age prevailed, this very great
+difference would be attended by some such appearance as the
+following:--
+
+The aupearance of but one star, other generations being long
+extinct or no others yet come into being; or, perhaps, a faint
+nebulous wreath of aggregating matter somewhere solitary in the
+heavens; or no sign of matter beyond our system, either because
+ungathered or long passed away into darkness.[1]
+
+Some such appearances were to be expected had the aggregation of
+matter depended solely on chance encounters of particles
+scattered through infinite space.
+
+For as, by hypothesis, the aggregation occupies an infinite time
+in consummation it is nearly a certainty that each particle
+encountered after immeasurable time, and then for the first time
+endowed with actual gravitational potential energy, would have
+long expended this energy
+
+[1] It is interesting to reflect upon the effect which an entire
+absence of luminaries outside our solar system would have had
+upon the views of our philosophers and upon our outlook on life.
+
+295
+
+before another particle was gathered. But the fact that so many
+fires which we know to be of brief duration are scattered through
+a region of space, and the fact of a configuration which we
+believe to be a transitory ore, suggest their simultaneous
+aggregation here and there. And in the nebulous wreaths situated
+amidst the stars there is evidence that these actually originated
+where they now are, for in such no relative motion, I believe,
+has as yet been detected by the spectroscope. All this, too, is
+in keeping with the nebular hypothesis of Kant and Laplace so
+long as this does not assume a primitive infinite dispersion of
+matter, but the gathering of matter from finite distances first
+into nebulous patches which aggregating with each other have
+given rise to our system of stars. But if we extend this
+hypothesis throughout an infinite past by the supposition of
+aggregation of infinitely remote particles we replace the
+simultaneous approach required in order to accotnt for the
+simultaneous phenomena visible in the heavens, by a succession of
+aggregative events, by hypothesis at intervals of nearly infinite
+duration, when the events of the universe had consisted of fitful
+gleams lighted after eternities of time and extinguished for yet
+other eternities.
+
+Finally, if we seek to replace the eternal instability involved
+in Kant's hypothesis when extended over an infinite past, by any
+hypothesis of material stability, we at once find ourselves in
+the difficulty that from the known properties of matter such
+stability must have been
+
+296
+
+permanent if ever existent, which is contrary to fact. Thus the
+kinetic inertia expressed in Newton's first law of motion might
+well be supposed to secure equilibrium with material attraction,
+but if primevally diffused matter had ever thus been held in
+equilibrium it must have remained so, or it was maintained so
+imperfectly, which brings us back to endless evolution.
+
+On these grounds I contend that the present gravitational
+properties of matter cannot be supposed to have acted for all
+past duration. Universal equilibrium of gravitating particles
+would have been indestructible by internal causes. Perpetual
+instability or evolution is alike unthinkable and contrary to the
+phenomena of the universe of which we are cognisant. We therefore
+turn from gravitating matter as affording no rational account of
+the past. We do so of necessity, however much we feel our
+ignorance of the nature of the unknown actions to which we have
+recourse.
+
+A prematerial condition of the universe was, we assume, a
+condition in which uniformity as regards the average distribution
+of energy in space prevailed, but neterogeneity and instability
+were possible. The realization of that possibility was the
+beginning we seek, and we today are witnesses of the train of
+events involved in the breakdown of an eternal past equilibrium.
+We are witnesses on this hypothesis, of a catastrophe possibly
+confined to certain regions of space, but which is, to the
+motions and configurations concerned, absolutely unique,
+reversible to
+
+297
+
+its former condition of potential by no process of which we can
+have any conception.
+
+Our speculation is that we, as spectators of evolution, are
+witnessing the interaction of forces which have not always been
+acting. A prematerial state of the universe was one of unfruitful
+motions, that is, motions unattended by progressing changes, in
+our region of the ether. How extended we cannot say; the nature
+of the motions we know not; but the kinetic entities differed
+from matter in the one important particular of not possessing
+gravitational attraction. Such kinetic configurations we cannot
+consider to be matter. It was _possible_ to construct matter by
+their summation or linkage as the configuration of the crystal is
+possible in the clear supersaturated liquid.
+
+Duration in an ether filled with such motions would pass in a
+succession of mere unfruitful events; as duration, we may
+imagine, even now passes in parts of the ether similar to our
+own. An endless (it may be) succession of unprogressive,
+fruitless events. But at one moment in the infinite duration the
+requisite configuration of the elementary motions is attained;
+solely by the one chance disposition the stability of all must
+go, spreading from the fateful point.
+
+Possibly the material segregation was confined to one part of
+space, the elementary motions condensing upon transformation, and
+so impoverishing the ether around till the action ceased. Again
+in the same sense as the
+
+298
+
+stars are simultaneous, so also they may be regarded as uniform
+in size, for the difference in magnitude might have been anything
+we please to imagine, if at the same time we ascribe sufficient
+distance sundering great and small. So, too;, will a dilute
+solution of acetate of soda build a crystal at one point, and the
+impoverishment of the medium checking the growth in this region,
+another centre will begin at the furthest extremities of the
+first crystal till the liquid is filled with loose feathery
+aggregations comparable in size with one another. In a similar
+way the crystallizing out of matter may have given rise, not to a
+uniform nebula in space, but to detached nebula, approximately of
+equal mass, from which ultimately were formed the stars.
+
+That an all-knowing Being might have foretold the ultimate event
+at any preceding period by observing the motions of the parts
+then occurring, and reasoning as to the train of consequences
+arising from these nations, is supposable. But considerations
+arising from this involve no difficulty in ascribing to this
+prematerial train of events infinite duration. For progress there
+is none, and we can quite as easily conceive of some part of
+space where the same Infinite Intelligence, contemplating a
+similar train of unfruitful motions, finds that at no time in the
+future will the equilibrium be disturbed. But where evolution is
+progressing this is no longer conceivable, as being contradictory
+to the very idea of progressive development. In this case
+Infinite Intelligence
+
+299
+
+_necessarily_ finds, as the result of his contemplation, the
+aggregation of matter, and the consequences arising therefrom.
+
+The negation of so primary a material property as gravitation to
+these primitive motions of (or in) the ether, probably involves
+the negation of many properties we find associated with matter.
+Possibly the quality of inertia, equally primary, is involved
+with that of gravitation, and we may suppose that these two
+properties so intimately associated in determining the motions of
+bodies in space were conferred upon the primitive motions as
+crystallographic attraction and rigidity are first conferred upon
+the solid growing from the supersaturated liquid. But in some
+degree less speculative is the supposition that the new order of
+motions involved the transformation of much energy into the form
+of heat vibrations; so that the newly generated matter, like the
+newly formed crystal, began its existence in a medium richly fed
+with thermal radiant energy. We may consider that the thermal
+conditions were such as would account for a primitive
+dissociation of the elements. And, again, we recall how the
+physicist finds his estimate of the energy involved in mere
+gravitational aggregation inadequate to afford explanation of
+past solar heat. It is supposable, on such a hypothesis as we
+have been dwelling on, that the entire subsequent gravitational
+condensation and conversion of material potential energy, dating
+from the first formation of matter to the stage of star
+formation
+
+300
+
+may be insignificant in amount compared with the conversion of
+etherial energy attending the crystallizing out of matter from
+the primitive motions. And thus possibly the conditions then
+obtaining involved a progressively increasing complexity of
+material structure the genesis of the elements, from an
+infra-hydrogen possessing the simplest material configuration,
+resulting ultimately in such self-luminous nebula as we yet see
+in the heavens.
+
+The late James Croll, in his _Stellar Evolution_, finds objections
+to an eternal evolution, one of which is similar to the
+"metaphysical" objection urged in this paper. His way out of the
+difficulty is in the speculation that our stellar system
+originated by the collision of two masses endowed with relative
+motion, eternal in past duration, their meeting ushering in the
+dawn of evolution. However, the state of aggregation here
+assumed, from the known laws of matter and from analogy, calls
+for explanation as probably the result of prior diffusion, when,
+of course, the difficulty is only put back, not set at rest. Nor
+do I think the primitive collision in harmony with the number of
+relatively stationary nebula visible in space.
+
+The metaphysical objection is, I find, also urged by George
+Salmon, late Provost of Trinity College, in favour of the
+creation of the universe.--(_Sermons on Agnosticism_.)
+
+A. Winchell, in _World Life_, says: "We have not
+
+301
+
+the slightest scientific grounds for assuming that matter existed
+in a certain condition from all eternity. The essential activity
+of the powers ascribed to it forbids the thought; for all that we
+know, and, indeed, as the _conclusion_ from all that we know,
+primal matter began its progressive changes on the morning of its
+existence."
+
+Finally, in reference to the hypothesis of a unique determination
+of matter after eternal duration in the past, it may not be out
+of place to remind the reader of the complexity which modern
+research ascribes to the structure of the atom.
+
+302
+
+INDEX
+
+A.
+
+Abney, Sir Wm., on sensitisers, 210.
+
+Abundance of life, numerical, 98-100.
+
+Adaptation and aggressiveness of the organism, 80.
+
+Additive law, the, with reference to alpha rays, 220.
+
+Age of Earth, comparison of denudative and radioactive methods of
+finding, 23-29.
+
+Aletsch glacier, 286.
+
+Allen, Grant, on colour of Alpine plants, 104.
+
+Allen, H. Stanley, on photo-electricity, 203.
+
+Alpha rays, nature of, 214; velocity of, 214; effects of, on
+gases, 214; range of, in air, 215; visualised, 218; ionisation
+curve of, 216; number of, from one gram of radium, 237; number of
+ions made by, 237.
+
+Alpine flowers, intensity of colour of, 102.
+
+Alps, history of, 141; Tertiary denudation of, 148; depth of
+sedimentary covering of, 148; evidence of high pressures and
+temperatures in, 149; recent theories of formation of, 150 _et
+seq._; upheaval of, 147; age of, 147; volcanic phenomena
+attending elevation of, 147.
+
+Andes, trough parallel to, 123; not volcanic in origin, 118.
+
+Angle of friction on ice, 261-265, 281-283; on glass, 261-265.
+
+Animate systems, dynamic conditions of, 67; and transfer of
+energy, 71; and old age, 72; mechanical imitation of, 76, 77.
+
+Animate and inanimate systems compared, 73-75.
+
+Appalachian range, formation of, 120.
+
+Arrhenius, on elevation of continents, 17.
+
+Aryan Era of India, 136.
+
+Asteroids, probable origin of, 175; discovery of, 175; dimensions
+of, 176; orbits of, 176; Mars' moons derived from, 177.
+
+B.
+
+Babbage and Herschel, theory of mountain building, 123.
+
+Babes (and Cornil), size of spores, 98.
+
+Becker, G. F., age of Earth by sodium collection, 14; age of
+minerals by lead ratio, 20.
+
+Berthelot, law of maximum work, 62.
+
+Bertrand, Marcel, section of Mont Blanc Massif, 154.
+
+Beta rays, nature of, 246; accompanied by gamma rays, 247;
+production of, by gamma rays, 247; as ionising agents, 249.
+
+Biotite, containing haloes, 223; pleochroism of, 235; intensified
+pleochroism in halo, 235.
+
+Body and mind, as manifestations of progressiveness of the
+organism, 86.
+
+Boltwood, age of minerals by lead ratio, 20.
+
+Bose, theory of latent image, 203.
+
+Bragg and Kleeman, on path of the alpha ray, 215; stopping power,
+219; laws affecting ionisation by alpha rays, 220; curve of
+ionisation and structure of the halo, 232.
+
+Brecciendecke, sheet of the, 154.
+
+Brdche, sheet of the, 154.
+
+Burrard and Hayden on the Himalaya, 138; sections of the
+Himalaya, 139.
+
+C.
+
+Canals and "canali," 166; curvature of, and path of a satellite,
+188 _et seq._; double and triple accounted for, 186, 187;
+doubling of, 195; disappearance and reappearance of, 196-198;
+photography of, 198; not due to cracks, 167; not due to rivers,
+167; of Mars, double nature of, 166, 170; crossing dark regions
+of planet's surface, 168; of Mars, Lowell's views on, 168 _et
+seq._; shown on Lowell's map, investigation of, 192 _et seq._;
+radiating, explanation of, 193, 194; number of, 194; developed by
+secondary disturbances, 194; nodal development of, due to raised
+surface features, 195.
+
+Chamberlin and Salisbury, the Laramide range, 121.
+
+Clarke, F. W., estimate of mass of sediments, 9; age of Earth by
+sodium collection, 14; average composition of sedimentary and
+igneous rocks, 42; on average composition of the crust, 126;
+solvent denudation of the continents, 17, 40.
+
+Claus, protoplasm the test of the cell, 67; abortion of useless
+organs, 69.
+
+Coefficient of friction, definition of, 262; deduction of, from
+angle of friction, 263; abnormal values on ice, 261-265, 282; for
+various substances, 265.
+
+Continental areas, movements of, 144.
+
+Cornil and Babes, size of spores, 98.
+
+Croll, James, dawn of evolution, 301.
+
+Crust of the Earth, average composition of, 126; depth of
+softening in, 128.
+
+Curie, definition of the, 256.
+
+D.
+
+Dana, on mountain building, 120.
+
+Dawson, reduction of surface represented by Laramide range, 123.
+
+Deccan traps, 137
+
+_deferlement_, theory of, 155; explanation of, 155 _et seq._;
+temperature involved in, 156.
+
+Deimos, dimensions of, 177; orbit of, 577.
+
+De Lapparent, exotic nature of the Prealpes, 150.
+
+De Montessus and the association of earthquakes with
+geosynclines, 142.
+
+Denudation as affected by continental elevation, 17; factors
+promoting, 30 _et seg._; relative activity in mountains and on
+plains, 35-40; solvent, by the sea, 40; the sodium index of,
+46-50; thickness of rock-layer removed from the land, 51.
+
+De Quincy, System of the Heavens, 200.
+
+Dewar, Sir James, latent image formed at low temperatures, 202.
+
+Dixon, H. H., and AGnadance of Life, 60.
+
+Double canals, formation by attraction of a satellite, 585-187.
+
+Douglass, A. E., observations on Mars, 167.
+
+Dravidian Era of India, 135.
+
+E.
+
+Earth, early history of, 3, 4; dimensions of, relative to surface
+features, 117.
+
+Earth's age determined by thickness of sediments, 5; determined
+by mass of the sediments, 7; determined by sodium in the ocean,
+12; determined by radioactive transformations, 19; significance
+of, 2.
+
+Earthquakes associated with geosynclincs, 142.
+
+Efficiency, tendency to maximum, in organisms, 113, 114.
+
+Elements, probable wide diffusion of rare, 230; rarity of
+radioactive, 241.
+
+Elster and Geitel, photo-electric activity and absorption, 207;
+photo-electric properties of gelatin, 212; Emanation of radium,
+therapeutic use of, 256-259; advantages of, in medicine, 256;
+volume of, 257; how obtained, 257; use of, in needles, 258.
+
+Equilibrium amount, meaning of, 254, 255.
+
+Evolution and acceleration of activity, 79; of the universe not
+eternal a pane ante, 298.
+
+F.
+
+Faraday and ionisation, 57.
+
+Finality of progress a part, post, 289.
+
+Flahault, experiments on colour of flowers, 108.
+
+Fletcher, A. L., proportionality of thorium and uranium, 26,
+
+G.
+
+Galileo, discovery of Jupiter's moons, 162.
+
+Gamma rays, nature of, 247: production of, by beta rays, 247; as
+ionising agents, 249.
+
+Geddes and Thomson, hunger and living matter, 71.
+
+Geiger, range of alpha rays in air, 215; ionisation affected by
+alpha rays in air, 216; on "scattering," 217; scattering and the
+structure of the halo, 232.
+
+Geikie, Sir A., uniformity in geological history, 15.
+
+Geosynclines, 119; association with earthquakes and volcanoes,
+142; of the tethys, 142; radioactive heat in, due to sediments,
+130; temperature effects due to lateral compression of, 131.
+
+Glacial epoch, phenomena of, 287.
+
+Glacier motion, cause of. 285.
+
+Glossopteris and Gangamopteris flora, 136.
+
+Gondwanaland, 136.
+
+Gradient of temperature in Earth's surface crust, 126.
+
+H.
+
+Haimanta period of India, 135.
+
+Halley, Edmund, finding age by saltness of ocean, 13.
+
+Hallwachs, photo-electric activity and absorption, 207.
+
+Haloes, pleochroic, finding age of rocks by, 21; due to uranium
+and thorium families, 227; radii of, 227; over-exposed and
+underexposed, 228; intimate structure of, 229 _et seq._;
+artificial, 229; tubular, in mica, 230; extreme age of, 231;
+effect of nucleus on structure of, 232; inference from spherical
+form of, in crystals, 233; structure of, unaffected by cleavage,
+235; origin of the name "pleochroic,"235; colouration due to
+iron, 235; colouration not due to helium, 236; age Of, 236; slow
+formation of, 237, 238; number of rays required to build, 237;
+and age of the Earth, 238-241.
+
+Hayden, H.H., geology of the Himalaya, 134, 138, 139.
+
+Heat-tendency of the universe, 62.
+
+Heat emission from the Earth's surface, 126; from average igneous
+rock due to radioactivity, 126.
+
+Helium and the alpha ray, 214, 222; colouration of halo not due
+to, 236.
+
+Hering, E., and physiological or unconscious memory, 111.
+
+Herschel and Babbage theory of mountain building, 123.
+
+Herschel, Sir W., on galaxy of milky way, 293.
+
+Hertz, negative electrification discharged by light, 204.
+
+Himalaya, geological history of, 134-139.
+
+Hobbs, on association of earthquakes and geosynclines, 143.
+
+Holmes, A., original lead in minerals, 20; age of Devonian, 21.
+
+Horst concerned in Alpine _deferlement_, objections to, 156.
+
+Hyperion, dimensions of, 177.
+
+I.
+
+Ice, melting of, by pressure, 267 _et seq._; expansion of water
+in becoming, 267; lowering of melting-point by pressure, 267;
+fall of temperature under pressure, 268 _et seq._; viscosity of,
+284.
+
+Igneous rocks, average composition of, 43.
+
+Inanimate actions, dynamic conditions of, 61.
+
+Inanimate systems, secondary effects in, 63-65; transfer of
+energy into, 66.
+
+Indian geology, equivalent nomenclature of, 139.
+
+Initial recombination of ions due to alpha rays, 221, 222, 231;
+and structure of the halo, 231.
+
+Insect life in the higher Alps, 104, 105; destruction of, on the
+Alpine snows, 106.
+
+Ionisation by alpha ray, density of, 221; importance in chemical
+actions, 250; in living cell, 250.
+
+Ions, number of, produced by an alpha ray, 237.
+
+Isostasy, 53; and preservation of continents, 53.
+
+Ivy, inconspicuous blossoms of, 107; delay in ripening seed,
+107.
+
+K.
+
+Kant and Laplace, material hypothesis of, does not account for
+the past, 290.
+
+Kelvin, Lord, experiment on effects of pressure on ice, 268-270.
+
+Kleeman and Bragg. See Bragg.
+
+Klopstock introduces skating into Germany, 273.
+
+L.
+
+Lakes, cause of blue colour of, 55.
+
+Land, movements of the, 53, 54.
+
+Laukester, Ray, the soma and reproductive cells, 85.
+
+Lapworth, structure of the Scottish Highlauds, 153.
+
+Latent heat of water, 266.
+
+Latent image, formed at low temperatures, 202; Bose's theory of,
+203; photo-electric theory of, 204, 209 _et seq._
+
+Least action, law of, 66.
+
+Lembert and Richards, atomic weight of lead, 27.
+
+Length of life dependent on conditions of structural development,
+93; dependent on rate of reproduction, 94.
+
+Life-curves of organisms having different activities, 92.
+
+Life, length of, 91.
+
+Life waves of a cerial, 95; of Ausaeba, 87; of a species, 90.
+
+Light, effects of, in discharging negative electrification, 204;
+chemical effects of, 205; experiment showing effect of, in
+discharging electrified body, 205.
+
+Lindemann, Dr., duration of solar heat, 29.
+
+Lowell, Percival, observations on Mars, 167 _et seq._; map of
+Mars, reliability of, 198.
+
+Lucretius, birth-time of the world, 1.
+
+Lugeon, formation of the Prealpes, 171; sections in the Alps,
+154.
+
+Lyell, uniformity in geological history, 15.
+
+M.
+
+Magee, relative areas of deposition and denudation, 16.
+
+Mars, climate of, 170; position in solar system, 174, 175;
+dimensions of satellites of, 177; snow on, 169; water on, 169;
+clouds on, 169; atmosphere of, 170; melting of snow on, 170;
+dimensions of canals, 171; signal on, 172; times of opposition,
+164; orbit of, 165; distance from the Earth, 165; eccentricity of
+his orbit, 165; observations of, by Schiaparelli, 165, 166;
+Lowell's observations on, 167 _et seq._
+
+Maxwell, Clerk, changes made under constraints, 65; on
+conservation of energy, 61.
+
+M'Connel, J. C., viscosity and rigidity of ice, 284.
+
+Memory, physiological, 111, 112.
+
+Metamorphism, thermal, in Alpine rocks, 132, 149
+
+Millicurie, definition of, 256.
+
+Molasse, accumulations of, 148.
+
+Morin, coefficients of friction, 265.
+
+Morphy, H., experiments on coefficient of friction of ice, 281.
+
+Mountain-building and the geosynclines, 119-121; conditioned by
+radioactive energy, 125; energy for, due to gravitation, 122;
+reduction of surface attending, 123; depression attending, 123;
+instability due to thermal effects of compression, 132; igneous
+phenomena attending, 132; rhythmic character of, accounted for,
+133; movements confined to upper crust, 122; movements due to
+compressive stresses in crust, 122; movements, rhythmic character
+of, 121.
+
+Mountain ranges built of sedimentary materials, 118.
+
+Mueller, J., coefficient of friction of skate on ice, 265, 274.
+
+Muth deposits of India, 135.
+
+N.
+
+Newton, Professor, of Yale, on origin of Mars' satellites, 177.
+
+Nucleus, dimensions of, 237; amount of radium in, 238.
+
+Nummulitic beds of Himalaya, 138.
+
+O.
+
+Ocean, amount of rock salt in, 50; cause of black colour of, 55;
+estimated mass of sediments in, 48; increase of bulk due to
+solvent denudation, 52; its saltness due to denudation, 41.
+
+Old age and death, 82-85; not at variance with progressive
+activity, 83.
+
+Organic systems, origin of, 78.
+
+Organic vibrations, 86 _et seq._
+
+Organism and accelerative absorption of energy, 79; and economy,
+109-111; and periodic rigour of the environment, 94,95.
+
+Organism and sleep, 95; ultimate explanation of rythmic events
+in, 96, 97; law of action of, 68 _et seq._; periodicity of; and
+law of progressive activity, 82 _et seq._
+
+P.
+
+Penjal traps, 135.
+
+Pepys and skating, 273.
+
+Perry, coefficient of friction of greased surfaces, 265.
+
+Phobos, dimensions of, 177; orbit of, 177.
+
+Photoelectric activity and absorption, 207; persists at low
+temperatures, 208, 209; not affected by solution, 213.
+
+Photo-electric experiment, 205; sensitiveness of the hands, 207;
+theory of latent image, 204, 209 _et seq._
+
+Photographic reversal, experiments on, by Wood, 211; theory of,
+210.
+
+Piazzi, discovery of first Asteroid, 175.
+
+Pickering, W. H., observations on Mars, 167.
+
+Planet, slowing of axial rotation of, 189.
+
+Plant, expectant attitude of, 109.
+
+Pleochroic haloes, measurements of, 224; theory of, 224 _et
+seq._; true form of, 226; radius of, and the additive law, 225;
+absence of actinium haloes, 225; see _also_ Haloes; mode of
+occurrence of, 223 _et seq._
+
+Poole, J. H. J., proportionality of thorium and uranium, 26.
+
+Poulton, uniformity of past climate, 17.
+
+Pratt, Archdeacon, and isostasy, 53.
+
+Prealpes, exotic nature of, 150, 151.
+
+Prematerial universe, nature of a, 297, 300.
+
+Prestwich and thickness of rigid crust, 128; history of the
+Pyrenees, 140.
+
+Primitive organisms, interference of, 89; life-curves of, 88.
+
+Proctor and orbits of Asteroids, 176.
+
+Protoplasm, encystment of, 68.
+
+Purana Era of India, 134.
+
+Pyrenees, history of, 140.
+
+R.
+
+Radioactive elements concerned in mountain building, 125.
+
+Radioactive layer, failure to account for deep-seated
+temperatures, 127; assumed thickness of, 128; temperature at base
+of, due to radioactivity, 129; in the upper crust of the Earth,
+125; thickness of, 126-128.
+
+Radioactive treatment, physical basis of, 251.
+
+Radioactivity and heat emission from average igneous rock, 126;
+rarity of, established by haloes, 241, 243.
+
+Radium, chemical nature and transmutation of, 244-245; emanation
+of, 245; rays from, 253, 254; table of family of, 253; period of,
+253; small therapeutic value of, 254.
+
+Radium C, therapeutic value of, 254; rays from. 254; generation
+of, 254.
+
+Rationality, conditions for development of, 163.
+
+Rays, similarity in nature of gamma, X, and light rays, 248;
+effects on living cell, 251; penetration of, 251.
+
+Reade, T. Mellard, finding age of ocean by calcium sulphate, 13.
+
+Recumbent folds, formation of, 155 _et seq._
+
+Regelation, 284; affecting glacier motion, 285.
+
+Reversal, photographic, explanation of, 211.
+
+Richards and Lembert, atomic weight of lead, 27.
+
+Richter, Jean Paul, Dream of the Universe, 200.
+
+Rock salt in the ocean, amount of, 13.
+
+Rocks, average composition of, 43; radioactive heat from, 126;
+rate of solution of, 36.
+
+Russell, I. C., river supply of sediments, 10.
+
+Rutherford, Sir E., determination of age of minerals, 19, 20; age
+of rocks by haloes, 22; derivation of actinium, 226; artificial
+halo, 229; number of alpha rays from one gram of radium, 237.
+
+S.
+
+Salt range deposits of India, 134. 135.
+
+Saltness of the ocean due to denudation, 41-46.
+
+Salisbury (and Chamberlin), the Larimide range, 121.
+
+Salmon, Rev. George, on creation, 301.
+
+Satellite, velocity of, in its orbit, 191; method of finding path
+of, over a rotating primary, 189 _et seq._; direct and
+retrograde, 178; ultimate end of, 178; path of, when falling into
+primary, 179; effect of Mars' atmosphere on infalling satellite,
+179; stability of close to primary, 180; effects of, on crust of
+primary, 180 _et seq._
+
+Schiaparelli, observations on Mars, 165 166.
+
+Schmidt, C., original depth of Alpine layer, 131-148; structure
+of the Alps, 152.
+
+Schmidt, G. C., on photo-electricity, 207, 208; effect of
+solution on photo-electric activity, 213.
+
+Schuchert, C., average area of N. America during geological time,
+16.
+
+Sedimentary rocks, average composition of, 43; mass of,
+determined by sodium index, 47.
+
+Sedimentation a convection of energy, 133.
+
+Sediments, average river supply of, 11; on ocean floor, mass of,
+48; average thickness of, 49; precipitation of, by dissolved
+salts, 56-58; radioactivity of 130; radioactive heat of,
+influential in mountain building, 130, 131; rate of collecting,
+7; determination of mass of, 8; river supply of, 10; total
+thickness of, 6.
+
+Semper, energy absorption of vegetable and animal systems, 78.
+
+Sensitisers, effects of low temperature on, 210.
+
+Simplon, radioactive temperature in rocks of, before denudation,
+132.
+
+Skates, early forms of, 273; principles of construction of, 273
+_et seq._; action of, on ice, 276; bite of, 278-280.
+
+Skating not dependent on smoothness of ice, 260; history of,
+273.
+
+Skating only possible on very few substances, 279.
+
+Soddy, F., on isotopes, 24.
+
+Sodium, deficiency of, in sediments, 44; discharge of rivers,
+14.
+
+Soils, formation of, 37-39; surface area exposed in, 39.
+
+Sollas, W. J., age of Earth by sodium in ocean, 14; thickness of
+sediments, 6.
+
+Spencer, on division of protoplasm, 67.
+
+Spores, number of molecules in, 97.
+
+Stevenson, Dr. Walter C., and technique of radioactive treatment,
+259.
+
+Stoletow, photo-electric activity anal absorption, 207.
+
+Stopping power of substances with reference to alpha rays, 219.
+
+Struggle for existence, dynamic basis of, 80.
+
+Strutt, Prof. the Hon. R. J., age of geological periods, 20;
+radioactivity of zircon, 223.
+
+Sub-Apennine series of Italy, 148.
+
+Suess, nature of earthquakes. 143.
+
+Survival of the fittest and the organic law, 80.
+
+T.
+
+Talchir boulder-bed, 136.
+
+Temperature gradient in Earth's crust, 126.
+
+Termier, section of the Pelvoux Massif, 254.
+
+Tethys, early extent of, 135-137; geosynclines of, 142.
+
+Thermal metamorphism in Alpine rocks, 132, 149.
+
+Thomson, James, prediction of melting of ice by pressure, 267.
+
+Thorium and uranium, proportionality of, in older rocks, 26.
+
+Triple canals, formation of, by attraction of a satellite, 187.
+
+Tyndall, colour of ocean water, 55.
+
+U.
+
+Uniformitarian view of geological history, 15-18.
+
+Universe, simultaneity of the, 293-295.
+
+Uranium-radium family of elements, table of, 253.
+
+V.
+
+Val d'Herens, earth pillars of, 33.
+
+Van Tillo, nature of continental rock covering, 9.
+
+Vegetable and animal systems, relative absorption of energy of,
+78.
+
+Vegetative organs, struggle between, 105, 106.
+
+Volcanoes and mountain ranges, 118; associated with geosynclines,
+142; Oligocene and Miocene of Europe, 147.
+
+W.
+
+Weinschenk and thermal metamorphism, 132,
+
+149.
+
+Weismaun, encystment of protoplasm, 68; length of life and
+somatic cells, 96; origin of death, 83; tendency to early
+reproductiveness, 98.
+
+Wilson, C. T. R., visualised alpha rays, 218.
+
+Winchell, progressive changes of matter not eternal, 302.
+
+Wood, R. W., on photographic reversal, 211.
+
+Z.
+
+Zircon, radioactivity of, 223; as nucleus of halo, 223.
+
+
+
+
+
+End of the Project Gutenberg EBook of The Birth-Time of the World and Other
+Scientific Essays, by J. (John) Joly
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