186 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1915. 



erties with those of known elements, although they differ from them 

 in their atomic weight and radioactivity. For example, radium B 

 appears to be identical in ordinary chemical and physical proper- 

 ties with lead, although its atomic weight, 214, is quite distinct from 

 lead, 207. The probable explanation of this, at first sight, remark- 

 able 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 tho- 

 rium 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 prop- 

 erty which allows us to study the mode and rate of transformation 

 of their atoms. 



It may be asked wdiat is the essential difference between radio- 

 active 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 disinte- 

 gration 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 motion of the swift alpha and beta particles into 

 heat, and is thus in a sense a secondary effect of the radiations. The 

 enormous emission of energy is most simply illustrated by consider- 

 ing the case of the radium emanation, together with its swiftly 

 changing products radium A, radium B, and radium C. The heat- 

 ing effect of a given volume or weight of this gas has been accu- 

 rately determined. From the data it can be calculated that 1 pound 

 weight of the emanation would emit heat energy initially at the 



