290 ADVANCED ELECTRICITY AND MAGNETISM. 



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 these 

 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 T-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 thickness 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 four times as 

 massive as a hydrogen atom. These ions are projected from the 

 radioactive 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-par- 

 ticles is reduced to so low a value as to render them no longer per- 

 ceptible by their ionizing effects. 



The /3-rays consist of electrons (negative ions) each about 1/800 

 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 pene- 

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

 property of ionizing a gas. 



The a-rays and the 0-rays are deflected by the magnetic field 

 and by the electric field. The direction of the deflection of the 



