730 Prof. Harkins and Mr. Wilson on Energy Relations 



built up entirely of helium nuclei, and hence would give 

 only helium and no hydrogen on disintegration. Later it 

 will be shown that there is another reason for helium only 

 being given off, which does not depend in any way on any 

 theory as to the exact structure of the higher atomic weight 

 elements. 



The conception of mass as a function of energy leads to 

 some very important and interesting conclusions. If tbe 

 ma^s of a body is a function of the total energy, any body 

 weighs more when hot than when cold, more when carrying 

 an electric charge than when uncharged, and more when in 

 motion than when at rest. In any chemical change where 

 there is heat evolved, there is a decrease in mass. To cal- 

 culate the magnitudes of these effects it is necessary to know 

 the form of the function, and this will be considered biter. 

 The law of the conservation of mass, even on this newer 

 conception, does not fail when applied to the entire universe. 

 The energy liberated in any change of mass goes to increase 

 the energy, and hence the mass, of some other body. It is 

 only when a restricted system is considered that there are 

 actual losses of mass. It will be seen later that for 

 practically all of the ordinary chemical and physical pro- 

 cesses with which we have to deal, this loss is negligible. 



From the theory of relativity, Einstein * in 1905 and 

 Lewis fin 1908 derived the following equation expressing 

 mass as a function of the total energy : 



M-5: 



where M is the mass, E the energy, and e the velocity of 

 light. Also Comstock, from electromagnetic considerations, 

 has derived a similar equation which, however, differs from 

 the one given here by a constant. Comstock attempted to 

 apply this equation t to the problem of atomic weights, and 

 recognized that if mass is a function of energy, the elements 

 may be built up of hydrogen even though the atomic weights 

 are not exact multiples of the atomic weight of hydrogen. 

 He considered, however, that the energy could change only 

 through changes in velocity. His calculations are rendered 

 worthless, because in the application to actual cases he made 



* Ann. Phys. xviii. p. 639. 



t Phil. Mag. xvi. p. 705 (1908). 



% J. Amer. Chem. Soc. xxx. p. 683. 



