THORIUM PRODUCTS 113 



thorium can be separated, and as often as they are 

 separated they are regenerated at a perfectly definite 

 and regular rate. One of these constituents, the 

 emanation, is gaseous, and it can be separated from 

 the thorium by no more elaborate means than by a 

 puff of air. Certainly the actual quantity of thorium 

 emanation is infinitesimal, but this did not hinder its 

 complete chemical characterisation, for it was found 

 to pass unabsorbed through every reagent tried, one 

 or other of which would have absorbed every known 

 gas with the exception of the gases of the argon 

 family. The conclusion that the thorium emanation 

 was a gas of the argon family produced by thorium, 

 later extended to the similar gaseous products of 

 radium and actinium, was a purely experimental 

 conclusion reached before any theory whatever as to 

 the nature of radioactivity had been advanced. 



Another constituent responsible for part of the 

 radioactivity we called thorium-A". It is left in the 

 filtrate when a solution of thorium is precipitated 

 with ammonia, although not when the thorium is 

 precipitated by other reagents, such as sodium 

 carbonate or phosphate. After this removal, how- 

 ever, thorium- J re-forms in the thorium. Moreover, 

 it is thorium--^, not thorium, that produces the 

 emanation. The latter in turn produces the non- 

 volatile active deposit, in which the successive 

 products, called thorium-^, -B, -C, and -D, are now 

 recognised. The false interpretation of a similar 

 phenomenon in the case of radium, before the radium 

 emanation had been recognised, led to the view that 

 inactive matter could be rendered temporarily radio- 

 active by "induction," through contact with or 

 association with radioactive matter. In the case of 

 thorium, the discovery of the chemical character of 

 the thorium emanation rendered the nature of the 

 phenomenon clear almost from the first. 



