OUTLINE OF RADIOACTIVITY 117 



the original activity is reduced to one-half and the time of reduction to 

 this "half value" is conveniently used to define and identify the several 

 substances/" For radium the half-value period is about 1,300 years, but 

 for uranium it is computed at no less than 1,000 million years, and one 

 of the products of the disintegration of radium (radium A) has a half 

 value period of only 3 minutes. 



The law of decay also leads to the conclusion that in the spontaneous 

 decay of uranium a condition of radioactive equilibrium must eventually 

 be reached in which each component radioactive substance decays just as 

 fast as it is replenished. Considering helium and lead as the stable end 

 products of the process of degeneration, Mr Eutherford points out that 

 this should afford a means of determining the age of minerals. Helium 

 indeed might escape in some cases, but he thinks lead would accumulate 

 and afford an accurate measure of time. Mr Boltwood has more recently 

 discussed a number of analyses on this basis, showing that the ratio of 

 metallic lead to metallic uranium in a mineral multiplied by 10 million 

 should give the age in years approximately. This hypothesis of course 

 assumes that the law of decay is a true law of nature, not an approxima- 

 tion; that no chemical or physical conditions modify it, and that the 

 mineral has been subject to no external attack by which either lead or 

 uranium could be removed. It makes the further assumption that no 

 lead compounds were soluble in the solutions from which uranium min- 

 erals were deposited. I shall be obliged to recur to this subject. 



Besides the uranium series, there are two other series of radioactive 

 minerals the properties of which are less fully investigated. Thorium 

 compounds, as Mrs Curie discovered, are radioactive and emit a particles 

 which appear to have the same mass as the helium atoms expelled by 

 radium.^^ The other end product of thorium degeneration is not cer- 

 tainly known, but the difference between the atomic weights of thorium 

 and bismuth is just 24, which suggests that Bi + He* = Th. If so, 

 liowever, there is one undiscovered step in the process. Actinium also 

 yields prodiTcts which emit a particles, but the atomic weight of actinium 

 is unlcaown. It is thought that it may be a sort of collateral descendant 

 of uranium. 



" The average life of a radioactive product is 



oo 



X 



■ \t 



dt = 'll\ = 1.443 X half-value period. 



/O 



^ Mr R. J. Strutt has recently described a mineral from Greenland which contains 

 much thorium, but only a trace of radium, and gives off large quantities of helium. 

 Chemical News, November 29, 1907, 



