156 CHEMICAL DISCOVERY AND INVENTION 



Another point to remember is that the a particles, when they 

 give up the two unit positive charges carried by each leave 

 an atom of helium of which the atomic weight is 4. Hence the 

 loss of an atom of helium corresponds to a diminution in the 

 atomic weight of the residue to the extent of 4 units. Thus the 

 change from Ra 226 (approx.) to one a particle and one atom 

 of niton leaves the atomic weight of the latter 222 (approx.). 



The number of radio-active substances is not limited to the 

 uranium-radium series. The minerals containing thorium 

 were early found to exhibit properties similar to those of 

 uranium. 



An examination of Ceylon thorianite, by Dr. 0. Hahn in Sir 

 William Ramsay's laboratory in 1905, led to the discovery of a 

 substance which evolves the same emanation as thorium, and a 

 year or two later two products, meso-thorium and radio-thorium, 

 were separated. 



Meso-thorium has properties similar to those of radium, and 

 is prepared commercially in Germany as a substitute for that 

 substance. 



It will be sufficient here to show in the following tables the 

 successive products which result from the disintegration of 

 thorium and actinium, from which it will be seen that the 

 phenomena are similar to those observed in the case of radium, 

 the various stages representing very different degrees of stability. 



