338 BOLTWOOD— RADIOACTIVITY. [April 22, 



that the disintegration of the radio-active substances is of a wholly 

 different character from the ordinary chemical changes. This is 

 exactly what would be expected if the radioactive processes occur 

 within the atoms themselves, for, in accordance with our general 

 theories, chemical forces appear to be restricted in their action to the 

 exterior of the atomic systems only. 



We have thus far considered only the laws which govern the 

 transformation of radioactive matter and the radiations which ac- 

 company the disintegration of the atoms ; let us now turn our atten- 

 tion to the substances themselves. Investigation has brought to light 

 three main groups of radioactive elements — the uranium series, the 

 thorium series and the alkali metals. Of the last mentioned our 

 knowledge is not very extensive. A type of beta radiation appears 

 to be emitted by the salts of potassium and rubidium but their title 

 to be considered as true radio-elements is not as yet entirely clear. 



The uranium series, in addition to the parent substance, contains 

 ten products which may be properly considered as in the main line 

 of descent. These are uranium X, ionium, radium, radium emana- 

 tion, radium A, radium B, radium C, radium D, radium E and radium 

 F. Each of these products exhibits a characteristic chemical beha- 

 vior which is different from that of the parent element uranium. The 

 half value period of uranium has already been mentioned as exceed- 

 ing five billions of years and the disintegration of its atoms is accom- 

 panied by the expulsion of alpha particles. Uranium X, ionium and 

 radium are solids, the two first having chemical properties similar to 

 thorium, while radium has those of an alkali earth and particularly 

 resembles barium. Uranium X has a half value period of about 

 twenty-four days, and it emits only beta and gamma rays. The rate 

 of disintegration of ionium is not as yet know^n with any degree of 

 accuracy but it is certainly a relatively stable product and is trans- 

 formed but slowly. Its half value period is probably of the order of 

 ten thousand years. It emits alpha rays only. The half value period 

 of radium is approximately two thousand years. Rutherford and 

 Geiger have deduced a somewhat lower value, namely 1,760 years, 

 as a result of their experiments, but this value is probably an under 

 estimate, as w'ill be explained later in this paper. Radium emits 

 alpha rays and probably very low velocity beta rays also. The 



