DISCOVERY AND NATURE OF RADIOACTIVITY 9 



an hour. They cause most of the ionization observed near an un- 

 covered radioactive substance." The a particle travels 3.5 cm. 

 through air before it is stopped, breaking up, or ionizing, in this path 

 about 100,000 molecules. 



2. A stream of negatively charged electrons, moving with a 

 velocity of from \ to -^-^ the velocity of light, having a mass yiro ^^ 

 mass of a hydrogen atom, very penetrating to substances opaque to 

 ordinary light, and giving rise, when stopped, to a ray analogous to, 

 if not identical with, the X ray. Streams of these particles constitute 

 the /9 rays. 



3. An electromagnetic pulse in the ether, exceedingly pene- 

 trating to opaque bodies, and similar in all respects to the Rontgen or 

 Xray. These pulses are called j' rays. Not all radioactive substances 

 give off all three kinds of rays. Only a rays, for example, are 

 emitted by polonium. 



In October, 1907, Bragg ^^ put forward the hypothesis that, in 

 addition to positive and negative particles, atomic disintegration may 

 give rise to the emission of neutral particles, " such as, for example, 

 a pair consisting of one a. or positive particle and one /? or negative 

 particle." It is not impossible, he says, that the y rays, instead of 

 being ether pulses, may consist of streams of these neutral pairs, and 

 all the known properties of the 7- rays, as well as of X rays, are satis- 

 fied on this hypothesis. In a later paper he"" states that ether pulses 

 are a component of both X and ^rays, but do not compose the entire 

 phenomenon of the ray. Cooksey,"^ from his experiments, was un- 

 able to accept Bragg's view, and Kleeman's ''" experiments led him to 

 adhere to the older theory that y and X rays are, in general, alike, 

 both consisting of electro-magnetic pulses produced by the accelera- 

 tion of electric charges. 



4. In addition to the three types of rays, there is given off a very 

 dense, chemically inert, radioactive gas, which slowly diffuses from 

 radium (and also indirectly from thorium and actinium). The atom 

 of the emanation gives off, in its diisntegration, only a particles.* 



* Experiments of Rutherford "' indicate that the emanations of radium, thorium and 

 actinium differ from the other inert gases of the argon family in the fact that, in the 

 small amounts in which it is available, it is absorbed by charcoal. 



By comparing the rate of diffusion of radium emanation with that of mercury 

 vapor, Perkins*^ determined the molecular weight of the emanation to be 235. This 

 excess over the atomic weight of radium is explained by Perkins as due to experimental 

 errors. On the basis of the disintegration theory of Rutherford, and considering the 

 emanation as a monatomic gas, its molecular weight should be nearly that of the 

 atomic weie^ht of radium. 



