THE ATOMIC THEORY. 259 



This word ".subatomic" is one of ominous import. It implies atomic 

 complexity, and it also suogests something more. The property of 

 radio-activity is most strikingly exhibited b}' the metals radium, tho- 

 rium, and uranium; and these have the highest atomic weights of any 

 elements known. If the elements are complex, these are the most 

 complex, and therefore, presumably, the most unstable. Are they in 

 the act of breaking down? Is there a degradation of matter compar- 

 able with the dissipation of energy ? We can ask these questions, l)ut 

 we may have to wait long for a reply. There is, however, another 

 side to the shield, and the universe gives us glimpses of a generative 

 process, an elementary evolution. 



The truth or falsity of the nebular h3^pothesis is still an open ques- 

 tion. It is a plausible hypothesis, however, and commands many 

 strong arguments in its favor. We can see the nebuhe and prove 

 them to be clouds, of incandescent gas; we can trace a progressive 

 development of suns and systems, and at the end of the series we have 

 the hal)itable planet upon which we dwell. The nebular hypothesis 

 accounts for the observed condition of things, and is therefore by 

 most men regarded as satisfactory. But this is not all of the story. 

 Chendcally sj^eaking, the nebuh>3 are exceedingly simple in composition; 

 the whiter and hotter stars are a little more complex; then come stars 

 like our sun, and finally the finished planets, with their many chemical 

 elements and their myriads of compounds. Here again we have evi- 

 dence bearing upon our prol)lem, evidence which led me," more than 

 thirty years ago, to suggest that the evolution of planets from nebulse 

 had been accompanied by an evolution of the elements themselves. 

 This thought, stated in a reversed form, has since been developed and 

 amplified by Lockyer, and it is doubtless familiar to you all. In the 

 development of the heavenly bodies we seem to see the growth of the 

 elements; do we, in the phenomena of radio-activity, witness their 

 decay ? This is a startling, possibly a rash, speculation, but it rests 

 upon evidence which must be considered and weighed. 



We have, then, various lines of convergent testimony, and there are 

 more which I might have cited, all pointing to the conclusion that the 

 chemical atoms are complex, and that elemental matter, in the last 

 analj'sis, is not of many kinds. That there is but one fundamental 

 substance is not proved; and 3'et the probability in favor of such an 

 assumption must be conceded. Assuming it to be true, what, then, is 

 the nature of the Daltonian atom? 



To the chemist the simplest answer to this question is that fur- 

 nished by the researches of J. J. Thomson, to which reference has 

 already been made. A cluster of smaller particles or corpuscles satis- 

 fies the conditions that chemistry imposes on the problem, their ulti- 

 mate nature being left out of account. For chemical purposes we 



« " Evolution and the spectroscope," Popular Science Monthly, January, 1873. 



