THE 



POPULAR SCIENCE 



MONTHLY. 



APEIL, 1904. 



EECENT DISCOVEEIES IN" EADIATION AND THEIR 



SIGNIFICANCE. 



By Professor R. A. MILLIKAN, 



UNIVERSITY OF CHICAGO. 



n^HERE are times when the atmosphere seems to be fairly saturated 

 -*- with the spirit of scientific discovery. Such a time existed 

 during the opening years of the nineteenth century when John Dalton 

 was putting the atomic theory of matter upon an experimental rather 

 than upon the purely speculative foundation upon which it had 

 previously rested; when Count Rumford, an American by birth, was 

 laying the corner-stone of the modern mechanical theory of heat, in 

 accordance with which heat consists in the vibratory motion of the 

 particles of which matter is composed; when Thomas Young was 

 forging the final links in the chain of proof that light consists in the 

 wave motion of some all-pervading medium, the ether. 



It is not a little interesting that the opening years of the twentieth 

 century have also been marked in no less a degree than those of its 

 predecessor by epoch-making discoveries in physics. Most of this new 

 activity has been grouped about the general subject of radiation, dis- 

 coveries of new rays having followed one another in such rapid suc- 

 cession that it is difficult even for a physicist to keep posted about 

 them all. As a result of these discoveries important progress has been 

 made toward the solution of one of the most fundamental questions 

 with which science has to deal, viz., the question as to the nature and the 

 constitution of matter. 



The Discovery of X-rays. 

 The discovery of X-rays is to be regarded as the starting point of 

 this epoch of investigation upon radiation. It was in the Christmas 



VOL. LXIV. — 31. 



