THE ALPHA-, BETA- AND GAMMA- RAYS. 107 



it strikes the zinc sulphide target is at once made visible 

 to the eye. The appearance is that of a swamp full of fire- 

 flies, or the scintillating stars on a clear night. And when 

 one remembers that these flashes of light are caused by a 

 rain of projectiles, each impact being marked by a flash of 

 light, just as sparks fly off from iron when it is struck by a 

 hammer, and, moreover, that this rain of projectiles is in- 

 cessant, day in and day out, year in and year out, the 

 wonder becomes most impressive that the radium should 

 not soon dissipate itself by this continuous projection of 

 matter. The number of atoms,' however, in the small mass 

 of radium is so enormous that the process will probably 

 continue several hundred years before an appreciable part 

 of the radium has disappeared. Becquerel has recently 

 brought forward evidence to show that the light-flashes ob- 

 served are due to cleavages produced in the zinc sulphide 

 crystals by the impact of the alpha particles. 



THE BETA-RAYS. 



These have been the most widely studied of the rays 

 from radium, owing to their great penetrating power and 

 their action in exciting phosphorescence in bodies which 

 they strike. It is hardly necessary to tell the reader, form- 

 ally, what they are. Suppose we construct a table of com- 

 parison between the beta-rays from radium and the corpus- 

 cles from candle flames and hot metals. Let us make X 

 equal the possession of any one specific property possessed 

 by both in equal decree. 



