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 perfection the actual path of the particles in traversing matter. The next photograph (Fig. 11) shows the effect of passing a pencil of
Röntgen 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 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
