September 10, 1920] 



SCIENCE 



237 



If the contraction theory were proposed to- 

 day as a novel hypothesis I do not think it 

 would stand the smallest chance of accept- 

 ance. From all sides — biology, geology, phys- 

 ics, astronomy— it would be objected that the 

 suggested source of energy was hopelessly in- 

 adequate to provide the heat spent during the 

 necessary time of evolution; and, so far as 

 it is possible to interpret observational evi- 

 dence confidently, the theory would be held to 

 be definitely negative. Only the inertia of 

 tradition keeps the contraction hypothesis 

 alive — or rather, not alive, but an unburied 

 corpse. But if we decide to inter the corpse, 

 let us frankly recognize the position in which 

 we are left. A star is drawing on some vast 

 reservoir of energy by means unknown to us. 

 This reservoir can scarcely be other than the 

 sub-atomic energy which, it is known, exists 

 abundantly in all matter ; we sometimes dream 

 that man will one day learn how to release 

 it and use is for his service. The store is 

 well-nigh inexhaustible, if only it could be 

 tapped. There is sufficient in the sun to 

 maintain its output of heat for 15 billion 

 years. 



Certain physical investigations in the past 

 year, which I hope we may hear about at this 

 meeting, make it probable to my mind that 

 some portion of this 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 4 hydrogen 

 atoms bound with 2 electrons. But Aston 

 has further shown conclusively that the mass 

 of the helium atom is less than the sum of 

 the masses of the 4 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 1 

 part in 120, the atomic weight of hydrogen 

 being 1.008 and that of helium just 4. I will 

 not dwell on his beautiful proof of this, as 

 you will no doubt be able to hear it from him- 

 self. Now mass can not be annihilated, and 

 the deficit can only represent the mass of the 



electrical energy set free in the transmuta- 

 tion. 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 

 liberated will more than suffice for our de- 

 mands, 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 Rutherford 

 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 diffi- 

 cult 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, imlike the 

 generation of helium from hydrogen, is endo- 

 thermic. If combinations requiring the ad- 

 dition of energy can occur in the stars, com- 

 binations which liberate energy ought not to 

 be impossible. 



We need not bind ourselves to the forma- 

 tion 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 absorption, of 

 energy. It is a question of accurate measiu-e- 

 ment of the deviations of atomic weights from 

 integers, and up to the present hydrogen is 

 the only element for which Mx. Aston has 

 been able to detect the deviation. ITo doubt 

 we shall learn more about the possibilities in 

 due time. The position may be summarized 

 in these terms: the atoms of all elements are 

 built of hydrogen atoms bound together, and 

 presumably have at one time been formed 



