EMANATION X AND THE BIRTH OF HELIUM. 123 



tion or in the radium from which the emanation had been 

 removed. The emanation X emits alpha-rays as well, and 

 it is therefore probable that it, in its turn, breaks down into 

 two other forms giving rise to the beta- and gamma-rays 

 respectively, but we have now reached the last link in the 

 chain of consequences which result from the decay of ra- 

 dium. The instruments of the physicist, refined though 

 they are, refuse to take him farther. The radio-activity 

 of radium may be explained to this extent. The solid 

 radium compound is continuously giving off energy, in the 

 form of alpha-, beta- and gamma-rays. Of this energy 25 

 per cent, belongs to the radium itself, consists of alpha- 

 rays, and may not be removed. Of the remaining 75 per 

 cent. 18 per cent, belongs to the emanation proper and con- 

 sists also of alpha-rays; while the residue of the energy, 57 

 per cent., belongs to the emanation X, and the final products 

 and consists of all three types of rays. To the emanation X 

 the solid active body produced by the emanation and capable 

 of settling on other bodies the major part of the energy from 

 radium must thus be ascribed. In a solid radium compound, 

 therefore, all three bodies, the regenerating radium, the de- 

 caying emanation gas and the solid emanation X exist to- 

 gether; and the total activity of the radium is due at any 

 one instant to a balance between the qualities and activi- 

 ties of the three bodies therein contained. A question 

 must now suggest itself to the reader. What is the end of 

 this chain of consequences? What is the final product 

 formed in the decay of radium? Surely some evidence of 

 it must exist. Now, some twenty-five years ago, Sir Nor- 

 man Lockyer discovered that there exists in enormous 

 quantities in the atmosphere of the sun a certain element 

 which, because it was not then known on earth, was named 

 after its habitat helium, or the sun element. Twenty 



