September 10, 1920] 



SCIENCE 



235 



phere. It is the star as a storehouse of heat 

 which especially engages our attention. In the 

 hot bodies familiar to us the heat consists in 

 the energy of motion of the ultimate particles, 

 flying at great speeds hither and thither. So 

 too in the stars a great store of heat exists in 

 this form; hut a new feature arises. A large 

 proportion, sometimes more than half the total 

 |heat, consists of imprisoned radiant energy — 

 ether-waves travelling in all directions trying 

 to break through the material which encages 

 them. The star is like a sieve, which can only 

 retain them temporarily; they are turned 

 aside, scattered, absorbed for a moment, and 

 jflung out again in a new direction. An ele- 

 ment of energy may thread the maze for hun- 

 dreds of years before it attains the freedom of 

 outer space. Nevertheless the sieve leaks, and 

 a steady stream permeates outwards, supplying 

 the light and heat which the star radiates all 

 xound. 



; That some ethereal heat as well as material 

 (heat exists in any hot body would naturally 

 be admitted; but the point on which we have 

 here to lay stress is that in the stars, particu- 

 larly in the giant stars, the ethereal portion 

 (rises to an importance which quite transcends 

 pur ordinary experience, so that we are con- 

 fronted with a new type of problem. In a red- 

 hot mass of iron the ethereal energy consti- 

 tutes less than a billionth part of the whole; 

 but in the tussle between matter and ether the 

 ether gains a larger and larger proportion of 

 the energy as the temperature rises. This 

 change in proportion is rapid, the ethereal 

 energy increasing rigorously as the fourth 

 jpower of the temperature, and the material , 

 energy roughly as the first power. But even 

 lat the temperature of some millions of de- 

 grees attained inside the stars there would still 

 remain a great disproportion ; and it is the low 

 (density of material, and accordingly reduced 

 material energy per unit voliune in the giant 

 stars, which wipes out the last few powers of 

 ^0. In all the giant stars known to us, widely 

 as they differ from one another, the conditions 

 are just reached at which these two varieties of 

 heat-energy have attained a rough equality; 

 at any rate one can not be neglected compared 



with the other. Theoretically there could be 

 conditions in which the disproportion was re- 

 versed and the ethereal far out-weighed the 

 material energy; but we do not find them in 

 the stars. It is as though the stars had been 

 pleasured out — that their sizes had been de- 

 termined — ^with a view to this balance of 

 power; and one can not refrain from at- 

 tributing to this condition a deep significance 

 in the evolution of the cosmos into separate 

 stars. 



' Study of the radiation and internal condi- 

 tions of a star brings forward very pressingly 

 a problem often debated in this section : What 

 is the source of the heat which the sun and 

 stars are continually squandering? The an- 

 swer given is almost unanimous — that it is 

 obtained from the gravitational energy con- 

 verted as the star steadily contracts. But al- 

 most as unanimously this answer is ignored in 

 its practical consequences. Lord Kelvin 

 showed that this hypothesis, due to Helmholtz, 

 necessarily dates the birth of the sun about 

 20,000,000 years ago; and he made strenuous 

 efforts to induce geologists and biologists to 

 accommodate their demands to this time- 

 scale. I do not think they proved altogether 

 tractable. But it is among his own colleagues, 

 physicists and astronomers, that the most out- 

 rageous violations of this limit have prevailed. 

 I need only refer to Sir George Darwin's 

 theory of the earth-moon system, to the present 

 Lord Rayleigh's determination of the age of 

 terrestrial rocks from occluded helium, and to 

 all modern discussions of the statistical equi- 

 librium of the stellar system. N'o one seems 

 to have any hesitation, if it suits him, in 

 carrying back the history of the earth long be- 

 fore the supposed date of formation of the 

 solar system; and in some cases at least this 

 appears to be justified by experimental evi- 

 dence which it is difficult to dispute. Lord 

 Kelvin's date of the creation of the sun is 

 treated with no more respect than Archbishop 

 Ussher's. 



The serious consequences of this contraction 

 hypothesis are particularly prominent in the 

 case of giant stars, for the giants are prodigal 



