THE CONSTITUTION OF MATTER 123 



is clearly defined, the density corresponding to the number of drops 

 per centimeter is much smaller than for the alpha particle. In fact 

 by magnifying still further small portions of the track, the individual 

 ions, or rather the drop formed round each ion produced by the beta 

 particle, are clearly visible. In this way, it is obviously possible to 

 count directly the number of ions produced in any length of the path. 



These beautiful photographs thus not only bring out clearly that 

 alpha and beta particles are definite entities but show with great per- 

 fection the actual path of the particles in traversing matter. The 

 next photograph (Fig. 11) shows the effect of passing a pencil of 



Fig. 11. Beta Particles produced by Passage of X-rays through Air (Wilson). 



Eontgen rays through the expansion chamber. It is believed that these 

 rays do not ionize the gas directly but indirectly through the slow- 

 speed electrons which are liberated by some of the atoms acted on by 

 the radiation. These electrons are not nearly so swift as some of those 

 emitted by radium, for they are only able to transverse a few millimeters 

 of air before being stopped. "The photograph brings out clearly these 

 effects, and shows the tortuous path of a beta particle resulting from 

 collisions with the atoms. Such scattering effects become more marked 

 the slower the velocity of ejection of the beta particle. 



Transformation of Matter 

 While the discovery of the independent existence of the electron as a 

 constituent of the structure of atoms gave a great impetus to the study 

 of atomic structure, it was soon found that the removal or addition of 

 an electron from an atom did not appear to cause a permanent trans- 

 formation of the atom ; for no evidence has yet been obtained that the 

 passage of an electric current through a gas or metal is accompanied 



