RADIOACTIVITY. 127 



the experiments of Rutherford, in this direction. Consequently 

 we liave uranium, the element with the highest atomic Aveight, con- 

 sisting of the greatest number of electrons and which, in accord- 

 ance wdth the electron hypothesis, we accept as the mother substance. 

 A something which so far we have been unable to seize hold of and 

 to analyze isolates itself from the mother substance and we have 

 radium, out of which, as already noted, the radium emanation arises. 

 We know the constituents into which this separates— namely, helium 

 and the so-called '' radium A." Then radium B appears and out of 

 it radium C, and so on until we reach radium G. These intermediate 

 j^roducts, called '^ metabolons," are so far known to us only through 

 their radioactive properties. No one has yet seen, weighed, or meas- 

 ured them ; but in spite of this we are able to distinguish them by the 

 rays which they emit, as well as by their period of duration. 



Let us take, for example, the radium emanation B, whose presence 

 we note through the luminosity of the air, and it is easy to show 

 that this emits only the slightly persistent rays which, deflected 

 by a magnet, shoAv the characteristics of a positive electric charge. 

 If we leave a body for a day or two in this emanation, it begins to 

 show itself radioactive. It might be said that this is only from the 

 traces of the emanation left on its surface, just as a platinum wire 

 would show traces of hydrogen gas in which it had been left. It 

 IS true that something which I can not see with the finest optical 

 instruments, but which I can remove with muriatic acid, or by 

 polishing with sandpaper, has attached itself to the surface of the 

 body. This product is certainly not an emanation whose rays, as 

 well known, will not go through a sheet of paper, while with the 

 rays emitted by the piece of w^ood, glass, or metal, which I have left 

 in the emanation I can influence a photographic plate through a 

 screen. The rays emitted by the body thus treated, whatever it 

 ma}?^ be, Avhether a needle, pencil, cork, or morsel of bread, are hardly 

 less penetrating than the Rontgen ray. And not only by the nature 

 of the ray, but by its duration can it be shown that it is now^ a ques- 

 tion of a new substance. The measure of the duration of activity, 

 or the half-value, as that time is called during which the activity of 

 these substances is diminished one half, has been one of the most 

 serviceable means of identifying them. A radioactive body is one in 

 j)rocess of transmutation, and experience has shown that this action 

 always proceeds at the same rate in a given substance. The radium 

 emanation loses one-half of its activity in four days; radium C, the 

 more penetrating rays of w^hich I have just referred to, does the 

 same in twenty-eight minutes. The half-value of the thorium ema- 

 nation is fifty-four seconds ; on the other hand, the half-value of the 

 excited thorium activity corresponding to radium C, is eleven hours. 

 Given an unknown radioactive substance to identify, I estimate the 



