THE UNDERLYING FACTS OF SCIENCE 569 



of particles, until recently considered negative electrons, but, accord- 

 ing 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 y3 rays are extremely complex and 

 interesting and their power of penetration is about ten times that of the 

 a rays. The y rays, which are themselves about ten times more pene- 

 trating than the /S 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 y rays are very similar 

 in degree of penetration and in some other properties to the X rays. 

 In addition to the y 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 de- 

 composed into radiation and emanation. The most significant prod- 

 uct of the disintegration of the radio-elements, however, is helium, 

 wliich 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 origi- 

 nal 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 com- 

 pound of helium and of some other element. It has even been sug- 

 gested 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 hope- 

 less. Eadium 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. 



VOL. LXXVIII. — 40 



