of particles, until recently considered negative electrons, but, according to Professor Soddy, by no means finally proved as such for all cases. These particles of the a radiation are probably of relatively large size and are beautifully exhibited by the scintillation of the zinc sulphide in the spinthariscope. The rays are extremely complex and interesting and their power of penetration is about ten times that of the rays. The rays, which are themselves about ten times more penetrating than the rays, but do not affect the photographic plate to an equal extent, seem to be produced by the explosive disturbance which takes place at the formation of the rays, just as X rays are produced by the impact of cathodic rays; in fact, these rays are very similar in degree of penetration and in some other properties to the X rays. In addition to the rays, the explosive disturbance referred to produces an emanation, a veritable spray of the radio-active element. The emanation is, according to present standards, a form of matter, whereas the radiations can not be positively defined as such if judged by the same standards. The emanation, which is therefore a spray of the radio-element, a vapor, renders any object bathed in it radio-active and the action does not cease until the dust deposited on it has decomposed into radiation and emanation. The most significant product of the disintegration of the radio-elements, however, is helium, which has been mentioned in connection with the a ray; it is a distinct element with a distinct spectrum, perfectly stable chemically and therefore quite unlike the other products of disintegration.
Helium seems to be the state in which the unstable atom of the original uranium at last finds rest. It has been suggested that the helium may be merely occluded, but valid arguments have been brought to bear against the idea, and, if anything, radium would be a true compound of helium and of some other element. It has even been suggested that all chemical elements may be helium compounds. This is a return to Front's theory, but with helium in place of hydrogen. Contrary to general belief, helium is not exclusively a product of the radio-elements; Strutt has recently succeeded in obtaining a very fair percentage of it from New Hampshire beryl which did not exhibit any measurable radio-activity; it may, however, have done so in the past, the helium remaining occluded.
The speed of decomposition of the radio-elements, or rather of their salts—the bromides and chlorides being the most generally used—is so rapid that the use of chemical methods of analysis is almost hopeless. Radium is comparatively manageable, but actinium, which is said to be at least one thousand million times as active as radium, has a life period of less than eight seconds. It has been suggested that actinium is an intermediate product between uranium and radium.