444 



NA TURE 



[August 31, 1905 



output gives a quotient of about 20 millions. Hence it 

 seemed to be imperatively necessary that the whole history 

 of the solar system should be comprised within some 20 

 millions of years. 



This argument, which is due to Helmholtz, appeared 

 to be absolutely crushing, and for the last forty years the 

 physicists have been accustomed to tell the geologists that 

 they must moderate their claims. But for myself I have 

 always believed that the geologists were more nearly 

 correct than the physicists, notwithstanding the fact that 

 appearances were so strongly against them. 



And now, at length, relief has come to the strained 

 relations between the two parties, for the recent marvellous 

 discoveries in physics show that concentration of matter 

 is not the only source from which the sun may draw its 

 heat. 



Radium is a substance which is perhaps millions of 

 times more powerful than dynamite. Thus it is estimated 

 that an ounce of radium would contain enough power to 

 raise 10,000 tons a mile above the earth's surface. Another 

 way of stating the same estimate is this : the energy 

 needed to tow a ship of 12,000 tons a distance of six 

 thousand sea miles at 15 knots is contained in 22 ounces 

 of radium. The Saxon probably burns five or si.\ thousand 

 tons of coal on a voyage of approximately the same length. 

 Again, M. and Mme. Curie have proved that radium 

 actually gives out heat,' and it has been calculated that 

 a small proportion of radium in the sun would suffice to 

 explain its present radiation. Other lines of argument 

 tend in the same direction." 



Novy we know that the earth contains radio-active 

 materials, and it is safe to assume that it forms in some 

 degree a sample of the materials of the solar system. 

 Hence it is almost certain that the sun is radio-active also ; 

 and besides it is not improbable that an element with so 

 heavy an atom as radium would gravitate more abundantly 

 to the central condensation than to the outlying planets. 

 In this case the sun should contain a larger proportion 

 of radio-active material than the earth. 



This branch of science is as yet but in its infancy, but 

 we already see how unsafe it is to dogmatise on the 

 potentialities of matter. 



It appears, then, that the physical argument is not 

 susceptible of a greater degree of certainty than that of 

 the geologists, and the scale of geological time remains 

 in great measure unknown. 



I have now ended my discussion of the solar system, 

 and must pass on to the wider fields of the stellar universe. 



Only a few thousand stars are visible with the unaided 

 eye, but photography has revealed an inconceivably vast 

 multitude of stars and nebuls, and every improvement in 

 that art seems to disclose yet more and more. About 

 twenty years ago the number of photographic objects in 

 the heavens was roughly estimated at about 170 millions, 

 and some ten years later it had increased to about 400 

 millions. Although Newcomb, in his recent book on " The 

 Stars," refrains even from conjecturing anv definite 

 number, yet I suppose that the enormous number of 400 

 million must now be far below the mark, and photography 

 still grows better year by year. It seems useless to con- 

 sider whether the number of stars has any limit, for 

 infinite number, space, and time transcend our powers of 

 comprehension. We must then make a virtue of necessity, 

 and confine our attention to such more limited views as 

 seem within our powers. 



.\ celestial photograph looks at first like a dark sheet 

 of paper splashed with whitewash, but further examination 

 shows that there is some degree of method in the arrange- 

 ment of the white spots. It may be observed that the 

 stars in many places are arranged in lines and sweeping 

 trains, and chains of stars, arranged in roughly parallel 

 curves, seem to be drawn round some centre. A surface 

 splashed at hazard might present apparent evidence of 

 system in a few instances, but the frequency of the occur- 

 rence in the heavens renders the hypothesis of mere 

 chance altogether incredible. 



' Lord Kelvin has estimated ihe age of the earth from the rate of increase 

 of temperature underground. But the force of his argument seems to be 

 entirely destroyed by this result. 



^ See W. E. Wilson, Nature, July 9, 1903 ; and G. H. Darwin, Nature 

 September 24, igoi. 



NO. 1870, VOL. 72] 



Thus there is order of some sort in the heavens, and, 

 although no reason can be assigned for the observed 

 arrangement in any particular case, yet it is possible to 

 obtain general ideas as to the succession of events in 

 stellar evolution. 



Besides the stars there are numerous streaks, wisps, 

 and agglomerations of nebulosity, the light of which we 

 know to emanate from gas. Spots of intenser light are 

 observed in less brilliant regions ; clusters of stars are 

 sometimes imbedded in nebulosity, while in other cases 

 each individual star of a cluster stands out clear by itself. 

 These and other observations force on us the conviction 

 that the wispy clouds represent the earliest stage of develop- 

 ment, the more condensed nebuls a later stage, and the 

 stars themselves the last stage. This view is in agree- 

 ment with the nebular hypothesis of Laplace, and we may 

 fairly conjecture that the chains and lines of stars repre- 

 sent pre-existing streaks of nebulosity. 



As a star cools it must change, and the changes which 

 it undergoes constitute its life-history, hence the history 

 of a star presents an analogy with the life of an individual 

 animal. Now, the object which I have had in view has 

 been to trace types or species in the physical world through 

 their transformations into other types. Accordingly it falls 

 somewhat outside the scope of this address to consider 

 the constitution and history of an individual star, interest- 

 ing although those questions are. I may, however, 

 mention that the constitution of gaseous stars was first 

 discussed from the theoretical side by Lane, and sub- 

 sequently more completely by Ritter. On the observational 

 side the spectroscope has proved to be a powerful instru- 

 ment in analysing the constitutions of the stars, and in 

 assigning to them their respective stages of developinent. 



If we are correct in believing that stars are conden- 

 sations of inatter originally more widely spread, a certain 

 space surrounding each star must have been cleared of 

 nebulosity in the course of its formation. Much thought 

 has been devoted to the determination of the distribution 

 of the stars in space, and although the results are lack- 

 ing in precision, yet it has been found possible to arrive 

 at a rough determination of the average distance from star 

 to star. It has been concluded, from investigations into 

 which I cannot enter, that if we draw a sphere round the 

 sun with a radius of twenty million millions of miles,' it will 

 contain no other star; if the radius were twice as great 

 the sphere might perhaps contain one other star ; a sphere 

 with a radius of sixty million millions of miles will contain 

 about four stars. This serves to give some idea of the 

 extraordinary sparseness of the average stellar population ; 

 but there are probably in the heavens urban and rural 

 districts, as on earth, where the stars may be either more 

 or less crowded. The stars are moving relatively to one 

 another with speeds which are enormous, as estimated by 

 terrestrial standards, but the distances which separate us 

 from them are so immense that it needs refined observ- 

 ation to detect and measure the moveinents. 



Change is obviously in progress everywhere, as well in 

 each individual nebula and star as in the positions of these 

 bodies relatively to one another. But we are unable even 

 to form conjectures as to the tendency of the evolution 

 which is going on. This being so, we cannot expect, by 

 considering the distribution of stars and nebula?, to find 

 many illustrations of the general laws of evolution which 

 I have attempted to explain ; accordingly I must confine 

 myself to the few cases where we at least fancy ourselves 

 able to form ideas as to the stages by which the present 

 conditions have been reached. 



Up to a few years ago there was no evidence that the 

 law of gravitation extended to the stars, and even now 

 there is nothing to prove the transmission of gravity from 

 star to star. But in the neighbourhood of many stars the 

 existence of gravity is now as clearly demonstrated as 

 within the solar system itself. The telescope has disclosed 

 the double character of a large number of stars, and the 

 relative motions of the pairs of companions have been 

 observed with the same assiduity as that of the planets. 

 When the relative orbit of a pair of binary or double stars is 

 examined, it is found that the motion conforms exactly to- 

 those laws of Kepler which prove that the planets circle 



1 This is the distance at which the earth's distance from the sun would 

 appear to be i". 



