CONSTITUTION OF MATTER — EUTHERFOKD. 183 



scattering of the particle by collisions with the atoms of matter in 

 its path. Although the trail is clearly defined, the density corre- 

 sponding 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 

 form,ed around 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 (pi. 4, fig. 2) shows the effect of passing a pencil of 

 Rontgen 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 traverse 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 transformation of the atom, for no evidence has yet been 

 obtained that the passage of an electric current through a gas or 

 metal is accompanied by a permanent alteration of the atoms of 

 matter through which the current passes, although there is little 

 doubt the current is carried in part at least by the electrons liberated 

 from the atoms. 



The first definite evidence of the transformation of matter was 

 obtained from a study of the processes occurring in radioactive sub- 

 stances. The writer and Mr. Soddy in 1903 put forward the theory 

 that the radiations from active matter accompanied a veritable 

 transformation of the atoms themselves. The correctness of this 

 theory as an explanation of radioactive phenomena is now gen- 

 erally accepted. As an illustration of these processes, consider the 

 transformation of the radioactive element uranium. The series of 

 substances which arise from the transformation of uranium are 

 shown clearly in the diagram (fig. 4). The best known of these 

 elements is radium, which will be taken as a typical example of a 



