The difference between the act of ionization when produced by a -ray and when produced by an ether wave seems, then, to consist wholly in the difference in the energy with which the two agencies hurl the electron from its mother atom. Wilson’s photographs show that -rays do not eject electrons from atoms with appreciable speeds, while ether waves may eject them with tremendous [Pg 139] energy. Some of Wilson’s photographs showing the effect of passing X-rays through air are shown in the most interesting plate opposite p. 190. The original X-rays have ejected electrons with great speeds from a certain few of the atoms of the gas, and it is the tracks of these electrons as they shoot through the atoms of the gas, ionizing here and there as they go, which constitute the wiggly lines shown in the photograph. Most of the ionization, then, which is produced by X-rays is a secondary effect due to the negative electrons, i.e., the -rays which the X-rays eject. If these -rays could in turn eject electrons with ionizing speeds, each of the dots in one of these -ray tracks would be the starting-point of a new wiggly line like the original one. But such is not the case. We may think, then, of the -rays as simply shaking loose electronic dust from some of the atoms through which they pass while we think of the X-rays as taking hold in some way of the negative electrons within an atom and hurling them out with enormous energy. V. IONIZATION BY -RAYS But what happens to the electronic constituents of an atom when an -particle, that is, a helium atom, shoots through it? Some of Bragg’s experiments and Wilson’s photographs show that the -particles shoot in straight lines through from 3 to 7 cm. of air before they are brought to rest. We must conclude, then, that an atom has so loose a structure that another atom, if endowed with enough speed, can shoot straight through it without doing anything more than, in some instances, to shake off from that atom an [Pg 140] electron or two. The tracks shown in Figs. 14 and 15, facing p. 190, are Wilson’s photographs of the tracks of the -particles of radium. They ionize so many of the atoms through which they pass that the individual droplets of water which form about the ions produced along the path of the ray, and which are the objects really photographed, are not distinguishable as individuals. The sharp changes in the direction of the ray toward the end of the path are convincing evidence that the -particle actually goes through the atoms instead of pushing them aside as does a bullet. For if one solar system, for example, endowed with a stupendous speed, were to shoot straight through another similar system, but without an actual impact of their central bodies, the deflection from its straight path which the first system experienced might be negligibly small if its speed were high enough, and that for the simple reason that the two systems would not be in each other’s vicinity long enough to produce