September 2, ig2o] 



NATURE 



19 



sub-atomic energy is actually being set free in the 

 stars. F. W. Aston 's experiments seem to leave no 

 room for doubt that all the elements are constituted 

 out of hydrogen atoms bound together with negative 

 electrons. The nucleus of the helium atom, for 

 example, consists of four hydrogen atoms bound with 

 two electrons. But .\ston has further shown con- 

 clusively that the mass of the helium atom is less 

 than the sum of the masses of the four hydrogen 

 atoms which enter into it ; and in this, at any rate, 

 the chemists agree with him. There is a loss of mass 

 in the synthesis amounting to about i part in 120, 

 the atomic weight of hydrogen being looS and that of 

 helium just 4. 1 will not dwell on his beautiful proof 

 of this, as you will, no doubt, be able to hear it from 

 himself. Now mass cannot be annihilated, and the 

 deficit can only represent the mass of the electrical 

 energy set free in the transmutation. We can there- 

 fore at once calculate the quantity of energy liberated 

 when helium is made out of hydrogen. If 5 per cent, 

 of a star's mass consists initially of hydrogen atoms, 

 which are gradually being combined to form more 

 complex elements, the total heat liberated will more 

 than suffice for our demands, and we need look no 

 further for the source of a star's energy. 



But is it possible to admit that such a transmuta- 

 tion is occurring? It is difficult to assert, but perhaps 

 more difficult to deny, that this is going on. Sir 

 Ernest Rutlierford has recently been breaking down 

 the atoms of oxygen and nitrogen, driving out an 

 isotope of helium from them ; and what is possible in 

 the Cavendish Laboratory may not be too difficult in 

 the sun. 1 think that the suspicion has been gener- 

 ally entertained that the stars are the crucibles in 

 which the lighter atoms which abound in the nebulae 

 are compounded into more complex elements. In the 

 stars matter has its preliminary brewing to prepare 

 the greater variety of elements which are needed for a 

 world of life. The radio-active elements must have 

 been formed at no very distant date; and their syn- 

 thesis, unlike the generation of helium from hydrogen, 

 is endothermic. If combinations requiring the 

 addition of energy can occur in the stars, com- 

 binations which liberate energy ought not to be 

 impossible. 



We need not bind ourselves to the formation of 

 helium from hydrogen as the sole reaction which sup- 

 plies the energy, although it would seem that the 

 further stages in building up the elements involve 

 much less liberation, and sometimes even absorption, 

 • nergy. It is a question of accurate measurement 

 ;he deviations of atomic weights from integers, and 

 ;i|i to the present hydrogen is the only element for 

 • !iirh Dr. .'Vston has been able to detect the deviation, 

 doubt we shall learn more about the possibilities in 

 time. The position may be summarised in these 

 ns : the atoms of all elements are built of hydrogen 

 rns bound together, and presumably have at one 

 time been formed from hydrogen; the interior cf 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 uiiaccounted for. You may draw a conclu- 

 •ii'm if you like. 



If, indeed, the sub-atomic energy in the stars is 

 :ig freely used to maintain their crcat furnaces, it 

 rns to bring a little nearer to fulfilment our dream 

 ' ontrnlling this Intent power for the well-being of 

 hum.Tn race— or for its suicide. 

 ~>o f.nr as the immediate n<vd» of astronomy are 

 ■irerned, it is not of any great ronsequenre wliethor 

 this suggestion we have nrfually laid a fineer on the 

 !<■ source of ffie heat. It is sufficient if the dis- 



NO. 265^, VOL. 106] 



cussion opens our eyes to the wider possibilities. We 

 can get rid of the obsession that there is no other con- 

 ceivable supply besides contraction, but we need not 

 again cramp ourselves by adopting prematurely what 

 is perfiaps a still wilder guess. Rather we should 

 admit that the source is not certainly known, and seek 

 for any possible astronomical evidence which may help 

 to denne its necessary character. One piece of evi- 

 dence 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 (Pubns. Ast. Soc. t-acific, 

 August, 1919). If so, the supply may come mainly 

 from the hottest region at the centre. I have already » 

 stated that the general uniformity 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 reasonable 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 contemplated that the energy is supplied 

 entirely in a restricted region round the centre. Now, 

 the more concentrated the supply, the lower is the 

 opacity requisite to account for the observed radiation. 

 I have not made any detailed calculations, but it 

 seems possible that for a sufficiently concentrated 

 source the deduced and the theoretical coefficients 

 could be made to agree, and there does not seem to 

 be any other way of accomplishing this. Conversely, 

 we might perhaps argue that the present discrepancy 

 of the coeftfcients shows that the energy supply is not 

 spread out in the way required by the contraction 

 hypothesis, but belongs to some new source available 

 only at the hottest, central fiart of the star. 



I should not be surprised if it is whispiered that this 

 address has at times verged on being a little bit 

 speculative ; perhaps some outspoken friend may 

 bluntly say that it has been highly speculative from 

 beginning to end. I wonder what is the touchstone 

 by which we may test the legitimate development 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 speculation. Moreover, 

 distrust of speculation often serves as a cover for 

 loose thinking ; wild ideas take anchorage in our 

 minds and influence our outlook ; whilst it is consi- 

 dered too speculative to subject them to the scientific 

 scrutiny which would exorcise them. 



If we are not content with the dull accumulation of 

 experimental facts, if we make any deductions or 

 generalisations, if we seek for any theory to guide us, 

 some degree of speculation cannot be avoided. Soine 

 will prefer to take the interpretation which s<>ems to 

 be indirato<l most immediately and at once a<lopt that 

 OS an hypothesis ; others will rather seek to explore 

 and classify the widest possibilities which are not 

 definitely inconsistent with the facts. Either choice 

 has it dangers : the first may be too narrow n view 

 and lead progress into a cul^-»ac; the second may 

 be so broad that it is useless as a guide, and diverges 

 indefinitely from experimental knowledge. When this 

 last rase happens, it must be concluded that the 

 knowledge is not yet ripe for theoretical treatment and 



