86 THE KINGDOM OF MAN 



rise to other atoms which we recognise as other 

 elements ? 



I cannot venture as an expositor into this field. It 

 belongs to that wonderful group of men, the modern 

 physicists, who with an almost weird power of visual 

 imagination combine the great instrument of exact 

 statement and mental manipulation called mathematics, 

 and possess an ingenuity and delicacy in appropriate 

 experiment which must fill all who even partially follow 

 their triumphant handling of Nature with reverence and 

 admiration. Such men now or recently among us are 

 Kelvin, Clerk Maxwell, Crookes, Rayleigh, and J. J. 

 Thomson. 



Becquerel showed early in his study of the rays 

 emitted by radium that some of them could be bent 

 out of their straight path by making them pass between 

 the poles of a powerful electro-magnet. In this way 

 have finally been distinguished three classes of rays 

 given off by radium : (i) the alpha rays, which are only 

 slightly bent, and have little penetrative power ; (2) the 

 beta rays, easily bent in a direction opposite to that in 

 which the alpha rays bend, and of considerable pene- 

 trative power ; (3) the gamma rays, which are absolutely 

 unbendable by the strongest magnetic force, and have 

 an extraordinary penetrative power, producing a photo- 

 graphic effect through a foot thickness of solid iron. 



The alpha rays are shown to be streams of tiny 

 bodies positively electrified, such as are given off by 

 gas flames and red-hot metals. The particles have 

 about twice the mass of a hydrogen atom, and they 

 fly off with a velocity of 20,000 miles a second ; that 

 is, 40,000 times greater than that of a rifle bullet. 

 The heat produced by radium is ascribed to the impact 

 of these particles of the alpha rays. 



