40 . SECTIONAL ADDRESSES. 



atomic weight of hydrogen being I'OOS and that of hehum just 4. I 

 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 therefore 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 hberated 

 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 transmutation is occurring? 

 It is difficult to assert, but perhaps more difficult to deny, that this is 

 going on. Sir Ernest Eutherford has recently been breaking down the 

 atoms of oxygen and nitrogen, driving out an isotope of hehum from 

 them ; and what is possible in the Cavendish laboratory may not be 

 too difficult in the Sun. I think that the suspicion has been generally 

 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 synthesis, unlike the generation of helium from 

 hydrogen, is endothermic. If combinations requiring the addition of 

 energy can occur in the stars, cMnbinations 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 supplies the energy, although it 

 would seem that the further stages in building up the elements involve 

 much less liberation, and sometimes even absolution, of energy. It is 

 a question of accurate measurement of the deviations of atomic weights 

 from integers, and up to the present hydrogen is the only element for 

 which Mr. Aston has been able to detect the deviation. No doubt we 

 shall learn more about the possibilities in due time. The position may 

 be summarised in these terms : the atoms of all elements are built of 

 hydrogen atoms bound together, and presumably have at one time been 

 formed 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. You 

 may drav^^ 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 immediate needs of astronomy 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 sufl&cient 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 con- 

 traction, but we need not again cramp ourselves by adopting prematurely 



