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THE POPULAR SCIENCE MONTHLY 



by a permanent alteration of the atoms of matter through which the 

 current passes, although there is little doubt the current is carried in 

 part at least by the electrons liberated from the atoms. 



The first definite evidence of the transformation of matter was 

 obtained from a study of the processes occurring in radioactive sub- 

 stances. The writer and Mr. Soddy in 1903 put forward the theory 

 that the radiations from active matter accompanied a veritable trans- 

 formation of the atoms themselves. The correctness of this theory as an 

 explanation of radioactive phenomena is now generally accepted. As 

 an illustration of these processes, consider the transformation of the 

 radioactive element uranium. The series of substances which arise from 

 the transformation of uranium are shown clearly in the diagram (Fig. 

 12). The best known of these elements is radium, which will be 



Fig. 12. Successive Substances produced by the Transformation of the 



Uranium Atom. 



taken as a typical example of a radioactive substance. Radium differs 

 from an ordinary element in its power of spontaneously expelling alpha 

 particles with very great speed. This property is ascribed to an in- 

 herent instability which is not manifest in the atoms of ordinary ele- 

 ments. A small fraction of the radium atoms — about one in 100,000 

 million — break up each second with explosive violence expelling a frag- 

 ment of the atom — the alpha particle — with very great speed. The 

 residue of the atom is lighter than before and becomes the atom of an 

 entirely new substance, which is called the radium emanation. The 

 atoms of the latter are far more unstable than those of radium, for half 

 of them break up in 3.85 days, while half of the radium atoms break up 

 in about 2,000 years. After the loss of an alpha particle, an atom of 

 the emanation changes into an atom of a new substance radium A, which 

 behaves as a solid. Radium A is very unstable, half of it breaking up in 

 3 minutes with the emission of an alpha particle, and gives rise to 

 radium B. The latter differs from the substances already mentioned in 

 the nature of its radiation, for it emits only beta rays but no alpha rays. 

 Notwithstanding this fact, it is transformed according to the same law 

 as an alpha ray substance, and gives rise to an entirely distinct element, 



