darwin and modern science-第194章
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about two centres。 The first sign of this change is that the spherical layers cease to be quite concentric and then the layers of equal density begin to assume a somewhat pear…shaped form analogous to that which we found to occur under rotation for an incompressible liquid。 Accordingly it appears that while a sphere of liquid is stable a sphere of gas may become unstable。 Thus the conditions of stability are different in these two simple cases； and it is likely that while certain forms of rotating liquid are unstable the analogous forms for gas may be stable。 This furnishes a reason why it is worth while to consider the unstable forms of rotating liquid。
There can I think be little doubt but that Jeans is right in looking to gravitational instability as the primary cause of fission； but when we consider that a binary system； with a mass larger than the sun's； is found to rotate in a few hours； there seems reason to look to rotation as a contributory cause scarcely less important than the primary one。
With the present extent of our knowledge it is only possible to reconstruct the processes of the evolution of stars by means of inferences drawn from several sources。 We have first to rely on the general principles of stability； according to which we are to look for a series of families of forms； each terminating in an unstable form； which itself becomes the starting…point of the next family of stable forms。 Secondly we have as a guide the analogy of the successive changes in the evolution of ideal liquid stars； and thirdly we already possess some slender knowledge as to the equilibrium of gaseous stars。
From these data it is possible to build up in outline the probable history of binary stars。 Originally the star must have been single； it must have been widely diffused； and must have been endowed with a slow rotation。 In this condition the strata of equal density must have been of the planetary form。 As it cooled and contracted the symmetry round the axis of rotation must have become unstable； through the effects of gravitation； assisted perhaps by the increasing speed of rotation。 （I learn from Professor Jeans that he now （December 1908） believes that he can prove that some small amount of rotation is necessary to induce instability in the symmetrical arrangement。） The strata of equal density must then become somewhat pear… shaped； and afterwards like an hour…glass； with the constriction more pronounced in the internal than in the external strata。 The constrictions of the successive strata then begin to rupture from the inside progressively outwards； and when at length all are ruptured we have the twin stars portrayed by Roberts and by others。
As we have seen； the study of the forms of equilibrium of rotating liquid is almost complete； and Jeans has made a good beginning in the investigation of the equilibrium of gaseous stars； but much more remains to be discovered。 The field for the mathematician is a wide one； and in proportion as the very arduous exploration of that field is attained so will our knowledge of the processes of cosmical evolution increase。
From the point of view of observation； improved methods in the use of the spectroscope and increase of accuracy in photometry will certainly lead to a great increase in our knowledge within the next few years。 Probably the observational advance will be more rapid than that of theory； for we know how extraordinary has been the success attained within the last few years； and the theory is one of extreme difficulty； but the two ought to proceed together hand in hand。 Human life is too short to permit us to watch the leisurely procedure of cosmical evolution； but the celestial museum contains so many exhibits that it may become possible； by the aid of theory； to piece together bit by bit the processes through which stars pass in the course of their evolution。
In the sketch which I have endeavoured to give of this fascinating subject； I have led my reader to the very confines of our present knowledge。 It is not much more than a quarter of a century since this class of observation has claimed the close attention of astronomers； something considerable has been discovered already and there seems scarcely a discernible limit to what will be known in this field a century from now。 Some of the results which I have set forth may then be shown to be false； but it seems profoundly improbable that we are being led astray by a Will…of…the…Wisp。
XXIX。 THE EVOLUTION OF MATTER。
By W。C。D。 WHETHAM； M。A。； F。R。S。 Trinity College； Cambridge。
The idea of evolution in the organic world； made intelligible by the work of Charles Darwin； has little in common with the recent conception of change in certain types of matter。 The discovery that a process of disintegration may take place in some at least of the chemical atoms； previously believed to be indestructible and unalterable； has modified our view of the physical universe； even as Darwin's scheme of the mode of evolution changed the trend of thought concerning the organic world。 Both conceptions have in common the idea of change throughout extended realms of space and time； and； therefore； it is perhaps not unfitting that some account of the most recent physical discoveries should be included in the present volume。
The earliest conception of the evolution of matter is found in the speculation of the Greeks。 Leucippus and Democritus imagined unchanging eternal atoms； Heracleitus held that all things were in a continual state of fluxPanta rei。
But no one in the Ancient Worldno one till quite modern timescould appreciate the strength of the position which the theory of the evolution of matter must carry before it wins the day。 Vague speculation； even by the acute minds of philosophers； is of little use in physical science before experimental facts are available。 The true problems at issue cannot even be formulated； much less solved； till the humble task of the observer and experimenter has given us a knowledge of the phenomena to be explained。
It was only through the atomic theory； at first apparently diametrically opposed to it； that the conception of evolution in the physical world was to gain an established place。 For a century the atomic theory； when put into a modern form by Dalton； led farther and farther away from the idea of change in matter。 The chemical elements seemed quite unalterable； and the atoms； of which each element in modern view is composed； bore to Clerk Maxwell； writing about 1870； 〃the stamp of manufactured articles〃 exactly similar in kind； unchanging； eternal。
Nevertheless throughout these years； on the whole so unfavourable to its existence； there persisted the idea of a common origin of the distinct kinds of matter known to chemists。 Indeed； this idea of unity in substance in nature seems to accord with some innate desire or intimate structure of the human mind。 As Mr Arthur Balfour well puts it； 〃There is no a priori reason that I know of for expecting that the material world should be a modification of a single medium； rather than a composite structure built out of sixty or seventy elementary substances； eternal and eternally different。 Why then should we feel content with the first hypothesis and not with the second？ Yet so it is。 Men of science have always been restive under the multiplication of entities。 They have eagerly watched for any sign that the different chemical elements own a common origin； and are all compounded out of some primordial substance。 Nor； for my part； do I think that such instincts should be ignored。。。that they exist is certain； that they modify the indifferent impartiality of pure empiricism can hardly be denied。〃 （〃Report of the 74th Meeting of the British Association〃 （Presidential Address； Cambridge 1904）； page 9； London； 1905。）
When Dalton's atomic theory had been in existence some half century； it was noted that certain numerical relations held good between the atomic weights of elements chemically similar to one another。 Thus the weight （88） of an atom of strontium compared with that of hydrogen as unity； is about the mean of those of calcium （40） and barium （137）。 Such relations； in this and other chemical groups； were il
