ELECTRICAL STRUCTURE OP MATTER RUTHERFORD 167 



of the charges carried by electrons, a particles, and the ions produced 

 in gases by X rays and the rays from radioactive matter. It was 

 first shown by Townsend that the positive or negative charge carried 

 by an ion in 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 that used by Millikan, 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 convinc- 

 ing proof of the correctness* of the electronic theory, and gave a 

 measure of this unit, the most fundamental 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 centi- 

 meter of any gas with an accuracy of possibly one in a thousand, 

 but certainly better than one in a hundred. ^Vlien 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 plaj'ed 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 electrolysis or in the electric discharge, has a charge ol 

 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 emphasized. 

 In fact, as we shall see later, the atoms are quite unsymmetrical struc- 

 tures 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. 



