496 POPULAR SCIENCE MONTHLY. 



atoms known. Thus the atomic weight of uranium is 240, that of 

 thorium 233, that of radium 225 or, according to a recent spectroscopic 

 test, by Eunge, 256. There is no other property in which these three 

 substances are at all alike. In their chemical characteristics they are ex- 

 tremely different. Now, according to our modern mechanical theory of 

 heat, the atoms of all substances are in extremely rapid rotation. It ap- 

 pears, therefore, that these rapidly rotating systems of heavy atoms, such 

 as characterize radio-active substances, not infrequently become unstable 

 and project off a part of their mass. These particles which are first 

 projected were found to be the alpha particles, and this process of 

 projecting the alpha particles is the first stage of radio-activity. The 

 mass which is left behind, namely, the uranium X, the thorium X, 

 or the emanation, according as the original atom was uranium, thorium 

 or radium, is itself unstable, and projects still other particles. The 

 remainder, at least in the case of thorium and radium, is still unstable, 

 and another particle is projected. Thus we were able to follow the 

 disintegration of the atoms through at least four (according to Euther- 

 ford, five) successive stages. How many more stages there may be 

 no one can tell, but as soon as the stable condition is reached and no 

 more particles are projected the product is of course no longer radio- 

 active, and its presence can no longer be detected by the delicate test of 

 radio-activity. It is then only after it has accumulated in sufficient 

 quantity to be capable of detection by the ordinary methods, namely, 

 by spectroscopic or chemical analysis, that it could be expected to be 



found. 



The Birth of Helium. 



More than two years ago Eutherford, with this picture of the 

 mechanism of radio-activity in mind, made a prediction which has 

 recently been most remarkably verified. The history of science 

 scarcely affords a more striking instance of the fulfilment of scientific 

 prophecy. Since 'helium' (the element which was first discovered in 

 the sun, by means of a line in the solar spectrum which did not agree 

 with the lines of any of our known elements, and which was dis- 

 covered on the earth only a few years ago by Lord Eayleigh and Pro- 

 fessor Eamsay) is found in nature only in connection with radio-active 

 minerals, i. e., in connection with those minerals which contain 

 uranium, thorium or radium, Eutherford predicted that helium would 

 one day be found to be one of the ultimate products of the disintegra- 

 tion of the radio-active elements. A year later, it may be remembered, 

 Eutherford himself found that the alpha particle, which is certainly 

 one of the products of radio-active change, had about the same mass as 

 the helium atom. This pointed still more strongly to the confirmation 

 of his original prophesy. Last July Professor Eamsay and Mr. Soddy 

 actually saw the spectrum of helium grow out of the emanation of 

 radium. They collected the emanation from fifty milligrams of radium 



