ticles and the lower for alpha particles. I am indebted to Dr. Geiger for this photograph. It is seen that the effect of a beta particle is just as marked and as definite as for an alpha particle with the old form of detector. We are thus in a position not only to count single atoms of matter but also to detect the presence of a single electron in swift motion, although the mass of the latter is exceedingly small compared with that of the lightest atom.
I would now very briefly direct your attention to some results, which to my mind not only completely prove the hypothesis of the atomic structure of matter but allow us at once to calculate the number of atoms in a given weight of matter with the minimum amount of assumption. We have seen that by direct counting it has been found that alpha particles are expelled per second from one gram of radium in equilibrium with its rapidly changing products. Now it has been definitely shown, by methods I need not discuss here, that each alpha particle consists of a helium atom carrying two unit positive charges. Since the alpha particle, when it has lost its charge, becomes a neutral helium atom, we should expect to find that helium would be produced by radium at a definite rate. This is found to be the case, and it is not difficult to determine by actual measurement the volume of helium formed by a known quantity of radium in a given time. It has been found that one gram of radium in equilibrium produces each year 156 cubic millimeters of helium at standard pressure and temperature. Now the number of alpha particles expelled per year per gram is , giving rise to 156 cubic millimeters of helium; each of these alpha particles is an atom of helium, and consequently the number of atoms of helium in one cubic centimeter of that gas at normal pressure and temperature is .
It appears to me that no more direct and convincing proof could be obtained of the atomic structure of matter or of the number of atoms forming a given weight or volume of helium; for the number of separate constituents are counted and the volume of the resulting gas is measured. The value so obtained is in good accord with measurements based on entirely different data of various kinds.
It is somewhat remarkable that while the study of radioactive phenomena has clearly indicated that the atom is not always permanent and indestructible, it has at the same time supplied the most convincing proof of the actual reality of atoms, and has provided some of the most direct methods of determining the values of atomic magnitudes.
Tracks of Swift Atoms and Electrons
We have seen how it is possible to detect single alpha and beta particles and to count their number. We will next consider a most remarkable experimental method not only for detecting such particles but of following in detail the effects produced by them in their passage
