276 MODERN SCIENCE READER 



We thus obtain families of elements allied by a relation- 

 ship which connects them in a common but distinct origin. 

 Such are : the family of radium, which comprises polonium 

 also ; the family of uranium ; of thorium ; of actinium. 



Radium itself is not a primary substance, but probably 

 derives from uranium. We are confronted, in fact, by the 

 existence of about thirty radioactive elements, of which 

 many, in truth, will never be characterized as such, because 

 they have too brief an existence. 



In fact only those radioactive elements can accumulate 

 in appreciable quantities, of which there is a continuous 

 production, and in which the rapidity of destruction of 

 the quantity produced is not too great. 



On the other hand, the intensity of radioactive phe- 

 nomena is proportional to the rapidity of destruction ; and 

 if we compare bodies of analogous ray-emission and in sim- 

 ilar quantities, the bodies most strongly radioactive are 

 those which have the greatest rapidity of destruction. 

 Hence, the most strongly radioactive substances are those 

 which we should expect to find in nature in smallest pro- 

 portions, and this is borne out by experience. 



Among the products of destruction of radioactive bodies 

 is one which is particularly interesting: the gas helium, 

 which is produced constantly by radium, actinium, polon- 

 ium, uranium, and thorium. 



Experience has proved that the atoms of helium emitted 

 should be considered as particles which have lost their 

 electric charge. 



On the other hand, the a rays of the various radioac- 

 tive bodies seem constituted of the same material particles. 



It results from this that the atom of helium forms, in all 

 probability, one of the constituents of all, or nearly all, 

 radioactive atoms, and perhaps a constituent of atomic 

 structures in general. 



The discovery of the production of helium by radium is 

 due to Ramsay and Soddy and constitutes one of the most 

 important facts in the history of radioactivity. 



