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THE PRESIDENTIAL ADDRESS. 7 
gases was invariably equal to the charge carried by the hydrogen ion 
in the electrolysis of water, which we have seen was assumed, and 
assumed correctly, by Johnstone Stoney to be the fundamental unit 
of charge. Various methods were devised to measure the magnitude 
of this fundamental unit; the best known and most accurate is 
Millikan’s, which depends on comparing the pull of an electric field on 
a charged droplet of oil or mercury with the weight of the drop. His 
experiments gave a most convincing proof of the correctness of the 
electronic theory, and gave a measure of this unit, the most funda- 
mental of all physical units, with an accuracy of about one in a 
thousand. Knowing this value, we can by the aid of electrochemical 
data easily deduce the mass of the individual atoms and the number 
of molecules in a cubic centimetre of any gas with an accuracy of 
possibly one in a thousand, but certainly better than one in a hundred. 
When we consider the minuteness of the unit of electricity and of the 
mass of the atom this experimental achievement is one of the most 
notable even in an era of great advances. 
The idea of the atomic nature of electricity is very closely connected 
with the attack on the problem of the structure of the atom. If the 
atom is an electrical structure it can only contain an integral number 
of charged units, and, since it is ordinarily neutral, the number of units 
of positive charge must equal the number of negative. One of the 
main difficulties in this problem has been the uncertainty as to the 
relative part played by positive and negative electricity in the structure 
of the atom. We know that the electron has a negative charge of one 
fundamental unit, while the charged hydrogen atom, whether in elec- 
trolysis or in the electric discharge, has. a charge of one positive unit. 
But the mass of the electron is only 1/1840 of the mass of the hydrogen 
atom, and though an extensive search has been made, not the slightest 
evidence has been found of the existence of a positive electron of small 
mass like the negative. In no case has a positive charge been found 
associated with a mass less than that of the charged atom of hydrogen. 
This difference between positive and negative electricity is at first sight 
_ very surprising, but the deeper we pursue our inquiries the more this 
fundamental difference between the units of positive and negative 
electricity is emphasised. In fact, as we shall see later, the atoms are 
quite unsymmetrical structures with regard to the positive and negative 
units contained in them, and indeed it seems certain that if there were 
not this difference in mass between the two units, matter, as we know 
it, could not exist. 
It is natural to inquire what explanation can be given of this striking 
difference in mass of the two units. I think all scientific men are 
convinced that the small mass of the negative electron is to be entirely 
associated with the energy of its electrical structure, so that the electron 
