316 A CENTURY OF SCIENCE 



radioactive elements has produced a profound effect 

 upon chemical theory. It was found that the two ele- 

 ments of the highest atomic weights, uranium and 

 thorium, are always spontaneously decomposing into 

 other elements at a fixed rate of speed which can be con- 

 trolled by no artificial means, and that the elements 

 resulting from these decompositions likewise undergo 

 spontaneous changes into still other elements at greatly 

 varying rates of speed, forming in each case a remark- 

 able series of temporary elements. These transforma- 

 tions are accompanied by the emission at enormous 

 velocities of three kinds of rays, one variety of which has 

 been shown to consist of helium atoms. The greater 

 number of the elements formed in these transformations 

 have not as yet been obtained in a pure condition, and 

 they are known only in connection with their radio- 

 activity, volatility, etc.; but radium and niton, two of 

 these products, have been obtained in a pure condition, 

 so that their atomic weights and their places in the 

 periodic system have been fixed. 



We owe much of our knowledge of the radioactive 

 transformations to the researches of Rutherford and of 

 Soddy, and of their co-workers, but one of the important 

 products of the transformation of uranium, an element 

 which he called ionium, was characterized by Boltwood of 

 Yale (25, 365, 1908). 



Radium and niton, apart from their radioactive prop- 

 erties, resemble barium and the inert gases of the atmos- 

 phere, respectively. The rates at which their progeni- 

 tors produce them, and the rates at which they themselves 

 decompose, bring about a state of equilibrium after a 

 time. Therefore a given amount of uranium, which 

 decomposes exceedingly slowly, can yield even after 

 thousands of years only a very small proportional 

 quantity of undecomposed radium, one-half of which 

 disappears in about 2500 years, because the amount 

 decomposed must eventually be equal to the amount pro- 

 duced. The first conclusive evidence that radium is a 

 product of the decomposition of uranium was given by 

 Boltwood in the Journal (18, 97, 1904). He found that 

 all uranium minerals contain radium; and the amount 

 of radium present is always proportional to the amount 



