528 SCIENCE PROGRESS 



contact with the brass and aluminium. The charged aluminium 

 leaf is repelled through a considerable angle. Its movement 

 is observed by a telescope through windows in the brass or 

 tin vessel. The telescope carries a scale ; observations are 

 made of the time taken by the falling leaf to traverse a 

 certain fixed portion of that scale. The bottom of the vessel 

 is usually removable, the substances to be tested for radio- 

 activity being thus introduced. In testing gases an airtight 

 apparatus is used and the gases are introduced through metal 

 stopcocks. 



Under ordinary conditions, in the absence of radioactive 

 matter the time taken by the falling leaf to pass over the 

 fixed portion of the scale is termed the natural leak of the 

 electroscope. This natural leak is quite considerable, and 

 the time may even amount to several hours. The presence 

 inside the case of a comparatively weak radioactive substance 

 reduces the leak to a few minutes, or even seconds. It is 

 desirable to give some account of the reasons underlying this 

 behaviour. 



An electroscope can be discharged readily through the 

 action of several different kinds of energy. Thus ra3^s of 

 ultraviolet light allowed to fall on the outer disc of an ordinary 

 electroscope, or the rays given out by a lighted candle placed 

 near it, cause the leaves to fall together rapidly. X-rays behave 

 similarly, and through a detailed study of their behaviour 

 J. J. Thomson and Rutherford have put forward a theory capable 

 of explaining the whole series of phenomena ; so far it has stood 

 the test of experience. The different rays are supposed to give 

 rise to particles termed ions carrying either positive or negative 

 charges of electricity. The initial action is a collision with an 

 atom of the gas which the ray is traversing ; an electron^ or 

 negative particle of electricity, is set free, the atom minus this 

 electron exhibiting a positive charge. Both, by virtue of their 

 charges, attract to themselves a cluster of uncharged atoms. 

 The number in the case of the electron is estimated at about 

 thirty; the positive cluster is smaller. Both clusters behave 

 like ordinary charged bodies, being attracted by unlike, repelled 

 by like, charges of electricity. Consequently in the absence of 

 any strong electric charge in their neighbourhood they ultimately 

 collide with each other, and their charges become neutralised. 

 But if such ions are liberated in the neighbourhood of a charged 



