THE PHENOMENA OF ELECTROSTATICS. 235 



radium-covered plate is held near to the metal plate of the elec- 

 troscope. Uranium and thorium have the same effect but the 

 discharge which they produce is not so rapid unless a large 

 quantity of material is employed. This property of these metals 

 and of their compounds is called radio-activity, a name which 

 originated because of the peculiar radiations which are given off 

 by radio-active substances and to which the discharging action is 

 due. These radiations are of three distinct kinds, which are 

 called the a-rays, the /3-rays, and the 7-rays, respectively. The 

 7-rays penetrate through a foot or more of solid metal or through 

 many feet of air ; the /3-rays penetrate through a moderate thick- 

 ness of a light metal, such as aluminum ; whereas the a-rays are 

 stopped by a very thin layer of aluminum or by a layer of air 

 two or three inches in thickness. 



The a-rays consist of positive ions each about twice as massive 

 as a hydrogen atom. These ions are projected from the radio- 

 active substance at a velocity of about 20,000 miles per second, 

 and each of them ionizes about 100,000 air molecules before it is 

 brought to rest "by repeated collision. After traveling two or 

 three inches through the air, the velocity of these a-particles is 

 reduced to so low a value as to render them no longer perceptible 

 by their ionizing effects. 



The /3-rays consist of electrons (negative ions) each about -g-^-jj- 

 as massive as a hydrogen atom. These electrons are projected 

 from the radio-active substance at a velocity which in some cases 

 is nearly as great as the velocity of light (186,000 miles per 

 second). The /3-particles also have the property of ionizing 

 the gas through which they pass but not to so great an extent 

 as the a-particles, and they travel several feet through the air 

 before their velocity is reduced to so low a value as to render 

 them no longer perceptible by their ionizing effects. 



The 7-rays are extremely abrupt waves in the ether essentially 

 the same in character as Roentgen rays, but much more penetrat- 

 ing than ordinary Roentgen rays. The 7-rays also have the 

 property of ionizing a gas. 



