150 CHEMICAL DISCOVERY AND INVENTION 



very complex, and unlike the cathode rays, some are not affected 

 by a magnetic or electric field and they differ in the extent to 

 which they may be stopped by metallic or other screens inter- 

 posed in their path. 



According to Professor Rutherford 1 radio-active substances 

 afford three types of radiation which he distinguishes by the 

 Greek letters, alpha a, beta /3, and gamma y. 



The a-rays are readily stopped by tinfoil or a sheet of writing- 

 paper, and travel only a short distance even through air, but are 

 little influenced in direction by a magnet. 



The /5-rays are similar in character to the cathode rays pro- 

 duced in a vacuum tube (see p. 116). 



The y-rays resemble Rontgen or X-rays. 



At this point it will be well to explain briefly the principles 

 of the methods which are employed in studying these radio- 

 active substances. Three general methods have been used, and 

 reference to them has already been made in the preceding 

 account of the discovery of radium. The first depends on the 

 action of the radiation on a photographic plate. The second on 

 the luminosity produced when the rays strike the surface of 

 certain substances, such as zinc sulphide (blende) or certain 

 platinocyanides. The third process is the most important, as 

 it lends itself to the purposes of exact measurement more readily 

 than the other two. This is electrical and rests on the property 

 possessed by the radiations of ionizing the gas or gases through 

 which they pass. lonization means the production of positively 

 and negatively electrified particles, which act as carriers of 

 electricity and remove the charge on gold leaves or other charged 

 surfaces exposed to contact with them. 



An electroscope which has been much used by Professor 

 Rutherford is shown in the accompanying figure. Within a 

 brass case, provided with a window W, is suspended a brass 

 plate B connected by a rod D with the gold leaf C. It is sup- 

 ported by the plug of sulphur S, which is a very perfect insulator. 

 One side of the lower box opens on a hinge, and access is thus 

 gained to the lower plate A, which is connected through the case 

 to earth. The gold leaf is charged to a suitable potential to give 

 it a deflection of about 40, and the cap E is then placed over 



1 For the information contained in this much condensed account of the 

 radiations from radio-active bodies the author is indebted chiefly to Butherford's 

 "Radio-active Substances and their Radiation" (Cambridge Univ. Press, 1913), 

 to which work the reader is referred for further detail. 



