For at least twenty years we have had direct experimental proof[139] that the fastest of the -particles, or helium atoms, [Pg 190] which are ejected by radium, shoot in practically straight lines through as much as 7 cm. of air at atmospheric pressure before being brought to rest. This distance is then called the “range” of these -rays. Figs. 14 and 15 show actual photographs of the tracks of such particles. We know too, for the reasons given on p. 139, that these -particles do not penetrate the air after the manner of a bullet, namely, by pushing the molecules of air aside, but rather that they actually shoot through all the molecules of air which they encounter. The number of such passages through molecules which an -particle would have to make in traversing seven centimeters of air would be about a hundred and thirty thousand. Further, the very rapid -particles, or negative electrons, which are shot out by radium have been known for a still longer time to shoot in straight lines through much greater distances in air than 7 cm., and even to pass practically undeflected through appreciable thicknesses of glass or metal. We saw in chap. VI that the tracks of both the - and the -particles through air could be photographed because they ionize some of the molecules through which they pass. These ions then have the property of condensing water vapor about themselves, so that water droplets are formed which can be photographed by virtue of the light which they reflect. Fig. 17 shows the track of a very high-speed -ray. A little to the right of the middle of the photograph a straight line can be drawn from bottom to top which will pass through a dozen or so of pairs of specks. These specks are the water droplets formed about the ions which were produced at these points. Fig. 14—Photographs of the Tracks of -Particles Shooting through Air Fig. 15—Photographs of the Tracks of -Particles Shooting through Air Fig. 16—Photographs of the Tracks of -Particles Shooting through Air Fig. 17—Photographs of the Tracks of -Particles Shooting through Air [Pg 191] Since we know the size of a molecule and the number of molecules per cubic centimeter, we can compute, as in the case of the -particle, the number of molecules through which a -particle must pass in going a given distance. The extraordinary situation revealed by this photograph is that this particular particle shot through on an average as many as 10,000 atoms before it came near enough to an electronic constituent of any one of these atoms to detach it from its system and form an ion. This shows conclusively that the electronic or other constituents of atoms can occupy but an exceedingly small fraction of the space inclosed within the atomic system. Practically the whole of this space must be empty to an electron going with this speed.