THE CHANGEABLENESS OF THE ELEMENTS 93 



stances steadily and continuously decayed. But the 

 thorium from which they had been removed and 

 which was thereby rendered nearly non-radioactive, 

 gradually recovered its original activity again. 

 Investigation proved that the thorium was in fact 

 continually growing a fresh crop of these radioactive 

 constituents. As fast as it was purified from them 

 by a chemical process, more began to form. The 

 quantities of material involved in these processes are 

 so minute that they are far beyond the limit of 

 detection by the balance or the spectroscope. 

 Indeed, it is estimated that geological epochs of time 

 would have to elapse in the case of thorium before 

 a weighable quantity of the new materials was formed. 

 Nevertheless the characteristic radioactivity they 

 produce enables them to be followed and dealt with 

 as easily, or perhaps more easily, than ordinary 

 substances in weighable amount. Moreover, in 

 certain cases the radioactive products are gases 

 called the radioactive emanations and in these cases 

 no chemical separation is needed, as they diffuse 

 away by themselves from the radioactive substance 

 into the surrounding air and are the cause of many 

 striking phenomena. 



The doctrine of the unchangeableness of the 

 chemical elements is no fixed article of belief in the 

 chemists' creed, but is simply the expression of the 

 facts known before the discovery of radioactivity, 

 that in all material changes known the chemical 

 elements do not essentially change. When a chemical 

 element or one of its compounds is purified, it remains 

 pure unless it is again mixed with other substances. 

 The discovery that the radioactive substances are 

 continually producing from themselves entirely 

 different chemical elements overthrew the doctrine 

 of the unchangeableness of the elements so far as 

 those that are radioactive are concerned. Since 



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