70 THE KINGDOM OF MAN 



from one another by the spectrum, the sign-manual 

 of an element given by the light emitted in each case 

 by the gas when in an incandescent condition. A fifth 

 inert gaseous element was discovered by Ramsay as a 

 constituent of certain minerals which was proved by 

 its spectrum to be identical with an element discovered 

 twenty-five years ago by Sir Norman Lockyer in the 

 atmosphere of the sun, where it exists in enormous 

 quantities. Lockyer had given the name ' helium ' to 

 this new solar element, and Ramsay thus found it 

 locked up in certain rare minerals in the crust of the 

 earth. 



But by helium we are led back to radium, for it 

 has been found only two years ago by Ramsay and 

 Soddy that helium is actually formed by a gaseous 

 emanation from radium. Astounding as the statement 

 seems, yet that is one of the many unprecedented facts 

 which recent study has brought to light. The alchemist's 

 dream is, if not realised, at any rate justified. One 

 element is actually under our eyes converted into 

 another; the element radium decays into a gas which 

 changes into another element, namely helium. 



Radium, this wonder of wonders, was discovered 

 owing to the study of the remarkable phosphorescence, 

 as it is called the glowing without heat of glass 

 vacuum-tubes through which electric currents are made 

 to pass. Crookes, Lenard, and Rontgen each played 

 an important part in this study, showing that peculiar 

 rays or linear streams of at least three distinct kinds 

 are set up in such tubes rays which are themselves in- 

 visible, but have the property of making glass or other 

 bodies which they strike glow with phosphorescent light. 

 The celebrated Rontgen rays make ordinary glass give 

 out a bright green light ; but they pass through it, and 



