296 BELL SYSTEM TECHNICAL JOURNAL 



The first achievement of Rutherford in this field was to identify 

 the rays emitted by radioactive bodies. He identified three kinds, and 

 gave them the names which they still bear and assuredly always will: 

 alpha, beta and gamma rays. The last-named (which are of the nature 

 of light) are the ones whereby radioactivity was first detected, for they 

 are the ones which fog the plates and produce the phosphorescence 

 outside of a rather narrow space just around the radioactive substance 

 itself. They are also the ones responsible for the great work already 

 done in medicine by radioactive bodies, and on which (I am told) 

 there is great reliance for the future. If this were a medical lecture 

 the gamma-rays would require most of its content; but as it is not, I 

 leave them with this brief allusion, and turn to the others. The beta- 

 rays are electrons, which may be of either sign (Rutherford, like the 

 rest of the world, was acquainted only with the negative ones until 

 five or six years ago). The alpha-rays are also charged particles, much 

 heavier than electrons; I shall be defining them more exactly before 

 long. The beta-rays and the alpha-rays, and for that matter the 

 gamma-rays as well, are detected by very ingenious devices of which 

 most types are electrical, though the particular type which supplies 

 the photographs seen in this lecture is not. 



Now at last I exhibit the list of the radioactive elements. 



This list (Fig. 1) is none other than the veritable Table of the 

 Elements itself! Of all the known elements there now remains just one 

 of which no radioactive form has yet been discovered or invented. Hydro- 

 gen is the exception (the listener may say "Of course!" but it is not 

 excluded that some day a radioactive type of hydrogen may be pro- 

 duced). At the end of the list stands uranium, the first of the radio- 

 active bodies to be discovered. It has stood there ever since Mendel eieff 

 set up the table in this fashion, but actually it now must yield its 

 pride of place, for physicists have lately created radioactive elements 

 which lie beyond it. I have though gone ahead too rapidly in speaking 

 of these already. Once more let me state the marvelous fact that of 

 all the known elements, every one but hydrogen exists in a radioactive 

 form, or perhaps in more than one. 



In speaking of "forms" I have been alluding to something which the 

 Table as it stands in Fig. 1 does not make clear. Everyone recognizes 

 the "chemical atomic weights" there appended to the symbol of each 

 element. If an element has only one kind of atom, this figure is the 

 actual mass of the atom, expressed in terms of a unit which I will 

 presently define. There are such elements; beryllium and fluorine, 

 sodium and aluminium are examples. Most elements, however, have 

 two or more kinds of atoms dififering in mass. Thus, in Fig. 1, the 



