ATOMIC DISINTEGRATION MILLIKAN 279 



have apparently ceased to inquire whence it comes. We are dis- 

 posed to assume, however, that it is not now being formed on earth. 

 Indeed, we have good reason to believe that the whole radioactive 

 process is confined to a very few, very heavy elements which are now 

 giving up the energy which was once stored up in them — we know 

 not how — so that radioactivity', though it seemed at first to be point- 

 ing away from the heat-death, has not at all, in the end, done so. 

 Indeed, it seems to be merely one mechanism by which stored-up 

 energy is being frittered away into apparently unreclaimable radiant 

 heat — another case of humpty-dumpty. 



The fifth significant discovery was the enormous lifetime of the 

 earth — partly through radioactivity itself, which assigns at least a 

 billion and a half years — and the still greater lifetime of the sun 

 and stars — thousands of times longer than the periods through which 

 they could possibly exist as suns if they were simply hot bodies 

 cooling off. This meant that new and heretofore unknown sources 

 of heat energy had to be found to keep the stars pouring out such 

 enormous quantities of radiation for such ages upon ages. 



The sixth discovery, and in many ways the most important of all, 

 was the development of evidence for the interconvertibility of mass 

 and energy. This came about in three ways. In 1901, Kaufman 

 showed experimentally that the mass of an electron could be increased 

 by increasing sufficiently its velocity; that is, energy could be 

 definitely converted into mass. About the same time tlie pressure 

 of radiation was experimentally established by Nichols and Hull at 

 Dartmouth College, New Hampshire, and Lebedew, at Moscow. This 

 meant that radiation possesses the only distinguishing property of 

 mass, the property by which we define it, namely, inertia. The 

 fundamental distinction between radiation and matter thus disap- 

 peared. These were direct, experimental discoveries. Next, in 1905, 

 Einstein developed the interconvertibility of mass and energy as a 

 necessary consequence of the special theory of relativity. If, then, 

 the mass of the sun could in any way be converted into radiant heat, 

 there would be an abundant source of energy to keep the sun going 

 so long as necessary, and all our difficulties about the lifetimes of the 

 sun and stars would have disappeared. But what could be the mech- 

 anism of this transformation? 



Then came the seventh discovery, which constituted a very clear 

 finger-post, pointing to the possibility of the existence of an integrat- 

 ing or building-up process among the physical elements, as well as 

 in biological forms, in the discovery that the elements are all defi- 

 nitely built up out of hydrogen; for they — the 92 different atoms — 

 were all found, beginning about 1913, by the new method of so-called 

 positive ray analysis, to be exact multiples of the weight of hydrogen 



