ATOMIC WEIGHTS ASTON. 195 



be ascribed to packing or not, we may consider it absolutely certain 

 that if hydrogen is transformed into helium a certain quantity of 

 mass must be annihilated in the process. The cosmical importance 

 of this conclusion is profound and the possibilities it opens for the 

 future very remarkable, greater in fact than any suggested before by 

 science in the whole history of the human race. 



We know from Einstein's Theory of Relativity that mass and 

 energy are interchangeable, 28 and that in C. G. S. units a mass m at 

 rest may be expressed as a quantity of energy mc 2 , where c is the 

 velocity of light. Even in the case of the smallest mass this energy 

 is enormous. The loss of mass when a single helium nucleus is formed 

 from free protons and electrons amounts in energy to that acquired by 

 a charge e falling through a potential of nearly thirty million volts. 

 If instead of considering single atoms we deal with quantities of 

 matter in ordinary experience the figures for the energy become pro- 

 digious. 



Take the case of one gram atom of hydrogen; that is to say, the 

 quantity of hydrogen in 9 c. c. of water. If this is entirely trans- 

 formed into helium the energy liberated will be 



.0077 X 9 X 10 20 = 6.93 X 10 18 ergs. 



Expressed in terms of heat this is 1.66 X 10 11 calories or in terms of 

 work 200,000 kilowatt hours. We have here at last a source of energy 

 sufficient to account for the heat of the sun. 29 In this connection 

 Eddington remarks that if only 10 per cent of the total hydrogen on 

 the sun were transformed into helium enough energy would be liber- 

 ated to maintain its present radiation for a thousand million years. 



Should the research worker of the future discover some means of 

 releasing this energy in a form which could be employed, the human 

 race will have at its command powers beyond the dreams of scientific 

 fiction ; but the remote possibility must always be considered that the 

 energy once liberated will be completely uncontrollable and by its 

 intense violence detonate all neighboring substances. In this event 

 the whole of the hydrogen on the earth might be transformed at once 

 and the success of the experiment published at large to the universe 

 as a new star. 



In considering the spectra of isotopes there is every reason to sup- 

 pose that the light emitted by an atom will depend upon the move- 

 ments of its planetary electrons, and therefore upon the force con- 

 trolling these — that is, the nuclear charge. We therefore expect 

 that the difference between the spectra of two isotopes will be ex- 

 tremely small, since the nuclear charges are identical. This expecta- 



28 Eddington : " Time, Space and Gravitation," p. 146, Cambridge, 1920. 



29 Eddington : Brit. Assoc, address, 1920 ; Perrin : Scientia, Nov., 1921. 



