192 PHYSICAL SCIENCE 



stance had approximately the same velocity, but 

 that velocity depended on the substance used> 

 Radium emanation, for instance, emits a particles 

 with a velocity of 1.62 x lo^ while those from 

 radium C move with a speed of 1.92 x 10^ centi- 

 metres per second. 



On the other hand, for all a particles, whatever 

 their source, the ratio e/m of the charge to the 

 mass is constant, and equal to about 4820 electro- 

 magnetic units. The value of e/m for the hydrogen 

 ion in liquid electrolytes is 9649 in the same units. 

 If, therefore, the a particle carries the same unit 

 electric charge as the hydrogen ion, its mass must 

 be twice as great ; but if it carries a double charge 

 its mass will be four times that of the hydrogen 

 atom and equal to that of helium. 



Clearly, then, the determination of the charge 

 carried by the a particle was a problem of funda- 

 mental importance, and this problem was attacked 

 by Rutherford and Geiger. 



We have already described the scintillations 

 produced on a screen of zinc sulphide by the impact 

 of the a rays from a minute speck of a radium com- 

 pound. Rutherford and Geiger proved that each 

 particle causes a visible scintillation, and then 

 counted the particles emitted by a measured 

 amount of a radium product. They also used 

 another method. A quantity of gas at a low 

 pressure exposed to an electric field of force just 

 not strong enough to give a spark, is in a very 

 sensitive state. An a particle shot through it 

 produces a large number of ions by collision with 

 its molecules, and each of these ions is set in 

 motion by the electric force, producing other 

 ions in its turn. Each a particle in this way. 



