CONTEMPORARY ADVANCES IN PHYSICS 57 



everything that was known about their actions. This condition pre- 

 vails no longer. It is established that alpha-rays are atoms of helium 

 each bearing a charge -\-2e; that beta-rays are electrons, that gamma- 

 rays are composed of electromagnetic radiation. Information about 

 the first two belongs to the vast body of doctrine concerning the 

 properties of fast-flying electrified particles; information about the 

 last belongs to the science of the properties of radiation. I do not 

 mean to imply that the information is redundant. One can produce 

 in the laboratory fast-flying electrons, but none so fast as the fastest 

 beta-rays; swift positively-charged atoms, but none nearly so swift as 

 the alpha-particles; electromagnetic waves of many wavelengths, but 

 none nearly so short as the shortest to be found among gamma- rays. 

 The knowledge acquired from studying the properties of the rays is 

 exceedingly important, and if the radioelements had not been discov- 

 ered, most of it would not been acquired so early, and some of it 

 would still be unattainable; but it is not knowledge of radioactivity. 

 What then is knowledge of radioactivity? So far as now appears, 

 we know all that can be known about the radioactivity of a radio- 

 element if we know what are the speeds of the alpha-particles emitted 

 from it, if any; what are the speeds of the electrons emitted from it, 

 if any; what are the wavelengths of the electromagnetic waves which 

 it emits, if any; how many of each kind of particle (for we may speak 

 of the waves as particles also, meaning by "particle" a quantum) are 

 emitted from a given number of atoms in a given time; and what 

 element or elements result from these processes. Apparently, if we 

 could know all of these things for a particular radioelement, we should 

 know everything which determines its peculiar actions upon the out- 

 side world. This unfortunately is not the same thing as being able to 

 solve the problems of predicting all of these actions or understanding 

 them ; but these problems are now transferred out of the field of radio- 

 activity into the field of the science of fast-flying charged particles and 

 short-wave radiation. Let us leave them there, and restrict the field 

 of radioactivity to the speeds of the particles and the frequencies of the 

 waves which issue from each radioelement, and the rates at which 

 they come forth, and the condition of the atoms they leave behind.^ 



^ The specific statements made in this article are derived chiefly from three recent 

 synopses of the data of radioactivity: the National Research Council bulletin Radio- 

 activity, by A. F. Kovarik and L. W. McKeehan; the Manual of Radioactivity, by 

 G. v. Hevesy and F. Paneth; and the relevant articles by St. Meyer, L. Meitner, W. 

 Bothe and 0. Hahn in volume 22 of the new Geiger-Scheel Handbuch der Physik. 

 As these are all well supplied with bibliographies (and so likewise, I presume, is the 

 new edition of Meyer and von Schweidler's Radioaktivitiit) I have omitted references 

 to individual papers except a few published since 1923. At several places I venture 

 to refer under the name " Introduction " to my Introduction to Contemporary Physics 

 for topics not falling within the field of radioactivity as here defined. 



During the composition of this article I have had the advantage of frequent con- 

 sultation with my colleague Dr. L. VV. McKeehan. 



