•;64 RICHARDSON— DYNAMICAL EFFECTS OF [April 22, 



3. The Number of Electrons in an Atom. 



The behavior of very rapidly moving electrons in their passage 

 through matter is a very interesting subject of investigation. Thanks 

 to the discovery of the radio-active substances we are able to experi- 

 ment, if we wish, with electrons whose speed is almost equal to that 

 of light (3 X io^° centimeters per second). These rapidly moving 

 electrons are able to shoot right through the atoms of bodies, but 

 when in their flight they pass very close to one of the constituent 

 electrons their paths are deviated. When a group of them passes 

 through a considerable thickness of matter these deviations tend to 

 accumulate ; so that a group of electrons, all of which were moving 

 parallel to one another to start with, becomes divergent. Sir J. J. 

 Thomson showed how the average deviation arising from a single 

 impact could be calculated and also how the average divergence of 

 the beam, which was caused by its passage through a given amount 

 of matter, would depend on the number of electrons present in the 

 matter traversed. 



Experiments made by Barkla showed that when matter, which 

 was made up of elements of low atomic weight, was traversed by 

 Roentgen rays it was caused to emit so-called secondary Roentgen 

 rays, which were precisely similar in character to the primary 

 Roentgen rays which excited them. A careful study of these second- 

 ary rays showed that they were primary rays which had been scat- 

 tered. The phenomenon is in fact very analogous to that which 

 gives rise to the blue color of the sky, which was shown by Lord 

 Rayleigh to arise from light scattered by innumerable small particles 

 present in the atmosphere. The amount of such scattering depends 

 on the number of particles which are engaged in doing it. In the 

 case of the Roentgen rays these particles are the electrons present 

 in the matter, each one of which is set into violent motion by the 

 Roentgen ray pulse. The exact way in which the amount of the 

 scattering should depend on the number of electrons engaged in the 

 operation was figured out by Thomson who showed from Barkla's 

 experimental results that the number of electrons reckoned per atom 

 of the material was comparable with the atomic weight. 



We have seen already that information of a like nature may be 



