i26 THE POPULAR SCIENCE MONTHLY 



their distinct physical and chemical properties. The extraordinary 

 differences in j)roperties which sometimes exist between a product and 

 its parent substance are well illustrated by the comparison of radium and 

 its product, the emanation. Radium is a solid element of atomic weight 

 226, which has chemical properties allied to barium but is capable of 

 separation from it. The emanation is a heavy monatomic gas of atomic 

 weight 222, which by its absence of chemical properties is allied to the 

 well known group of rare gases, helium, argon, neon, xenon and 

 krypton. In some cases, the elements show almost identical physical 

 and chemical properties with those of known elements, although they 

 differ from them in their atomic weight and radioactivity. For ex- 

 ample, radium B appears to be identical in ordinary chemical and 

 physical properties with lead although its atomic weight, 214, is quite 

 distinct from lead, 207. The probable explanation of this, at first 

 sight, remarkable identity will be discussed later. 



It is of interest to note that in the majority of cases a radioactive 

 element breaks up in only one way which is characteristic for all the 

 atoms of that element, and gives rise to only one new product. The 

 work of Fajans and Marsden, however, has clearly shown that in the 

 case of radium C and the corresponding products in the thorium and 

 actinium series, the atoms break up in two distinct ways and give rise 

 to two distinct radioactive elements. It has already been pointed out 

 that actinium is in reality one of these side or branch products. It is 

 supposed that uranium X breaks up in two distinct ways, the smaller 

 fraction giving rise to actinium. The evidence, however, on this point, 

 is not yet complete. 



The radioactive elements are in some respects more interesting and 

 important than stable elements, for, in addition to the ordinary physical 

 and chemical properties, they possess the radioactive property which 

 allows us to study the mode and rate of transformation of their atoms. 



It may be asked what is the essential difference between radioactive 

 changes and ordinary chemical changes. In the radioactive changes we 

 are not dealing with the dissociation of molecules into atoms but an 

 actual disruption of the chemical atom. The disintegration of any 

 given element appears to be a spontaneous and uncontrollable process 

 which, unlike ordinary chemical changes, is quite unaffected by the 

 most drastic changes in temperature or by any other known physical or 

 chemical agency. 



The radioactive changes differ entirely from chemical changes not 

 only in the peculiar character of the emitted radiations but also in the 

 enormous emission of energy. It can be simply shown that the energy 

 emitted from a radioactive substance which expels alpha particles is 

 several million times greater than the energy emitted from an equal 

 weight of matter in any known chemical reaction. This emission of 

 energy is mainly to be ascribed to the conversion of the energy of 



