238 



SCIENCE 



[N. S. Vol. LII. No. 1341 



from hydrogen; the interior of a star seems 

 as likely a place as any for the evolution to 

 have occurred; whenever it did occur a great 

 amount of energy must have been set free; in 

 a star a vast quantity of energy is being set 

 free which is hitherto unaccounted for. Tou 

 may draw a conclusion if you like. 



If, indeed, the sub-atomic energy in the 

 stars is being freely used to maintain their 

 great furnaces, it seems to bring a little 

 nearer to fulfilment our dream of controlling 

 this latent power for the well-being of the 

 human race — or for its suicide. 



So far as the inunediate needs of astron- 

 omy are concerned, it is not of any great 

 consequence whether in this suggestion we 

 have actually laid a finger on the true source 

 of the heat. It is sufficient if the discussion 

 opens our eyes to the wider possibilities. We 

 can get rid of the obsession that there is no 

 other conceivable supply besides contraction, 

 but we need not again cramp ourselves by 

 adopting prematurely what is perhaps a stiU 

 wilder guess. Rather we shoiild admit that 

 the source is not certainly known, and seek 

 for any possible astronomical evidence which 

 may help to define its necessary character. 

 One piece of evidence of this kind may be 

 worth mentioning. It seems clear that it 

 must be the high temperature inside the stars 

 which determines the liberation of energy, as 

 H. N". Russell has pointed out. If so the 

 supply may come mainly from the hottest 

 region at the center. I have already stated 

 that the general un.iformity of the opacity of 

 the stars is much more easily intelligible if 

 it depends on scattering rather than on true 

 absorption; but it did not seem possible to 

 reconcile the deduced stellar opacity with the 

 theoretical scattering coefficient. Within rea- 

 sonable limits it makes no great difference in 

 our calculations at what parts of the star the 

 heat energy is supplied, and it was assumed 

 that it comes more or less evenly from all 

 parts, as would be the case on the contraction 

 theory. The possibility was scarcely contem- 

 plated that the energy is supplied entirely in 

 a restricted region roimd the center. Now, 

 the more concentrated the supply, the lower 



is the opacity requisite to account for the 

 observed radiation. I have not made any de- 

 tailed calculations, but it seems possible that 

 for a sufficiently concentrated source the de- 

 duced and the theoretical coefficients could be 

 made to agree, and there does not seem to be 

 any other way of accomplishing this. Con- 

 versely, we might perhaps argue that the pres- 

 ent discrepancy of the coefficients shows that 

 the energy supply is not spread out in the 

 way required by the contraction hypothesis, 

 but belongs to some new source only avail- 

 able at the hottest, central part of the star. 



I should not be surprised if it is whispered 

 that this address has at times verged on being 

 a little bit speculative; perhaps some out- 

 spoken friend may blimtly say that it has 

 been highly speculative from beginning to 

 end. I wonder what is the touchstone by 

 which we may test the legitimate develop- 

 ment of scientific theory and reject the idly 

 speculative. We all know of theories which 

 the scientific mind instinctively rejects as 

 fruitless guesses; but it is difficult to specify 

 their exact defect or to supply a rule which 

 will show us when we ourselves do err. It is 

 often supposed that to speculate and to make 

 hypotheses are the same thing; but more 

 often they are opposed. It is when we let 

 our thoughts stray outside venerable, but 

 sometimes insecure, hypotheses that we are 

 said to speculate. Hypothesis limits specula- 

 tion. Moreover, distrust of speculation often 

 serves as a cover for loose thinking; wild 

 ideas take anchorage in our minds and in- 

 fluence our outlook; whilst it is considered 

 too speculative to subject them to the scien- 

 tific scrutiny which would exercise them. 



If we are not content with the dull accumu- 

 lation of experimental facts, if we make any 

 deductions or generalizations, if we seek for 

 any theory to guide us, some degree of specu- 

 lation can not be avoided. Some will prefer 

 to take the interpretation which seems to be 

 most immediately indicated and at once adopt 

 that as an hypothesis; others will rather seek 

 to explore and classify the widest possibilities 

 which are not definitely inconsistent with the 

 factS; Either choice has its dangers; the first 



