191 1] BOLTWOOD— RADIOACTIVITY. 341 



Its first product, mesothorium i. is another example of a rayless 

 change Hke radium D and actinium. Owing to the fact that large 

 quantities of monazite are commercially treated for the extraction 

 of thorium used in the manufacture of incandescent gas mantles and 

 that the technical separation and isolation of mesothorium appears 

 to be an economic possibility, there is some prospect that mesothorium 

 may become a competitor of radium for scientific and therapeutic 

 uses. Its life compared with radium is relatively short, however, 

 its half value period being 5^ years, but this in itself is not neces- 

 sarily a serious disadvantage. The chemical properties of meso- 

 thorium are similar to those of radium and barium. 



The fifth product in the thorium series is known as the thorium 

 emanation and is a chemically inert gas like the raditmi and actinium 

 emanations. The remaining four products constitute the tJwriiiiii 

 actk'c deposit. 



The combined uranium and thorium series includes 28 radio- 

 elements, of which only the two parent elements were known before 

 the development of radioactive methods. Radioactivity has there- 

 fore added a considerable quota to the known types of matter. 



An interesting relation which is met in the study of radioactive 

 change is the so-called radioacfiz'c equilibrium. If a relatively long- 

 lived radio-element A is the parent of a less stable product B, and 

 if A is initially entirely freed from B, then a certain definite fraction 

 of the atoms of A will undergo transformation each second to form 

 atoms of the product B. The number of atoms of B produced from 

 A in this manner each second will be essentially constant and the 

 amount of B will increase. But the atoms of B also undergo trans- 

 formation at a constant rate and, as the C|uantity of B increases, a 

 continually increasing number of its atoms will be transformed in 

 the unit of time. A point will finally be reached where the number 

 of atoms of B which disintegrate in any given time will be exactly 

 equal to the number of atoms of B formed from A in the same 

 interval. The relative amounts of A and B will then remain con- 

 stant and the conditions can be expressed by the equation 



where P is the number of atoms of A and A^ its constant of change, 



