[Pg 160] All determinations of from radioactive data involve one or the other of these two counts, namely, that of Rutherford and Geiger or that of Regener. Thus, Boltwood and Rutherford[98] measured the total weight of helium produced in a second by a known weight of radium. Dividing this by the number of -particles (helium atoms) obtained from Rutherford and Geiger’s count, they obtain the mass of one atom of helium from which the number in a given weight, or volume since the gas density is known, is at once obtained. They published for the number of molecules in a gas per cubic centimeter at 0°76 cm., , which corresponds to This last method, like that of the Brownian movements, is actually a determination of , rather than of , since is obtained from it only through the relation . Indeed, this is true of all methods of estimating , so far as I am aware, except the oil-drop method and the Rutherford-Geiger-Regener method, and of these two the latter represents the measurement of the mean charge on an immense number of -particles. Thus a person who wished to contend that the unit charge appearing in electrolysis is only a mean charge which may be made up of individual charges which vary widely among themselves, in much the same way in which the atomic weight assigned to neon has recently been shown to be a mean of the weights of at least two different elements inseparable chemically, could not be gainsaid, save on the basis of the evidence contained in the oil-drop [Pg 161] experiments; for these constitute the only method which has been found of measuring directly the charge on each individual ion. It is of interest and significance for the present discussion, however, that the mean charge on an -particle has been directly measured and that it comes out, within the limits of error of the measurement, at exactly two electrons—as it should according to the evidence furnished by measurements on the -particles. II. THE EVIDENCE FOR THE EXISTENCE OF A SUB-ELECTRON Now, the foregoing contention has actually been made, and evidence has been presented which purports to show that electric charges exist which are much smaller than the electron. Since this raises what may properly be called the most fundamental question of modern physics, the evidence needs very careful consideration. This evidence can best be appreciated through a brief historical review of its origin.