-particles (Figs. [14] and [15], opposite [p. 190]) is even more illuminating as to the structure of the atom. For the
-particle, being an atom of helium eight thousand times more massive than a negative electron, could no more be deflected by one of the latter in an atom through which it passes than a cannon ball could be deflected by a pea. Yet Figs. [14] and [15] show that toward the end of its path the
-particle does in general suffer several sudden deflections. Such deflections could be produced only by a very powerful center of force within the atom whose mass is at least comparable with the mass of the helium atom.
Fig. 18—Photographs of the tracks of
-particles ejected by x-rays from molecules of air