MEASUKEMENTS BASED ON RADIOACTIVITY 843 



down. From the degrees of radioactivity and the rate of decay of the 

 radioactivity of each suhstance the relative instabilities may be measured 

 and these may be translated into the length of time when half of the 

 given substance will have been transformed. This is known as the half- 

 value period. For example^ let a certain weight of radium be designated 

 as 64: A. In 1,660 years it will have been half transformed into descen- 

 dent elements and 32 A will remain as radium. In 1,660 years more, or 

 3,320 years from the beginning, half of this will have been transformed 

 and the quantity of radium will be 16 A. In another 1,660 years 8 A will 

 remain. Thus in 9,960 years an initial quantity of 64 A will be reduced 

 to A. It is clear, then, that the parent element, uranium, exists in the 

 earth's crust only because its half -value period is enormously long — long 

 even in comparison with the geologic eras. 



There are at present listed fifteen members and a couple of branch 

 variations in the uranium series, beginning with uranium and ending 

 with lead. Several of these are not chemically separable from certain other 

 elements and are known as isotopes. The half-value periods of repre- 

 sentative members in the order of their genesis are given by Eutherford^^ 

 as follows : 



Uranium 6 X 10^ years 



Ionium greater than 20,000 years 



Radium 2,000 years 



Radium emanation 3 . 75 days 



Radium D 16.5 years 



Radium F 136 days 



Lead stable 



Eecent determinations by Ellen Grleditsch of a high order of precision 

 have shown the half-value period of radium to be about 1,660 years.^^ 

 Professor Boltwood, in whose laboratory this work was done, regards the 

 error of this mean as probably not more than 2 per cent. The half -value 

 period of uranium is decreased correspondingly. 



In a uranium mineral all of the descendent elements are present, and 

 after about a million years a state of equilibrium is reached, after which 

 time each decays at the same rate at which it is generated. It follows 

 that after this a constant ratio is maintained between each, the relative 

 amounts being determined by the rates of decay. Thus, in a uranium 

 mineral, radium occurs to the amount of about 1 part to 3,125,000 parts 

 of uranium. These radioactive substances exist consequently in such small 

 amounts that they are insensible in gravimetric analysis, but through 



8«E. Rutherford: Radioactive substances and their transformations, 1913, p. 24. 

 s"? Ellen Gleditsch : The life of radium. Am. Jour. Scl., vol. 41, 1916, pp. 112-124. 



