Page:Popular Science Monthly Volume 59.djvu/336

326 Sir George Stokes has shown how we can calculate the rate at which a drop of water falls through air if we know the size of the drop, and conversely we can determine the size of the drop by measuring the rate at which it falls through the air, hence by measuring the speed with which the cloud falls we can determine the volume of each little drop; the whole volume of water deposited by cooling the air can easily be calculated, and dividing the whole volume of water by the volume of one of the drops we get the number of drops, and hence the number of the electrified particles. We saw, however, that if we knew the number of particles we could get the electric charge on each particle; proceeding in this way I found that the charge carried by each particle was about 6.5 $$+$$ 10-10 electrostatic units of electricity or 2.17 $$\times$$ 10-20 electro-magnetic units. According to the kinetic theory of gases, there are 2 $$x$$ 1019 molecules in a cubic centimeter of gas at atmospheric pressure and at the temperature 0° C; as a cubic centimeter of hydrogen weighs about 1/11 of a milligram each molecule of hydrogen weighs about 1/(22 $$\times$$ 1019) milligrams and each atom therefore about 1/(44 $$\times$$ 1019) milligrams and as we have seen that in the electrolysis of solutions one-tenth of a milligram carries unit charge, the atom of hydrogen will carry a charge equal to 10/(44 $$\times$$ 1019) $$=$$ 2.27 $$\times$$ 10-20 electro-magnetic units. The charge on the particles in a gas we have seen is equal to 2.17 $$\times$$ 10-20 units, these numbers are so nearly equal that, considering the difficulties of the experiments, we may feel sure that the charge on one of these gaseous particles is the same as that on an atom of hydrogen in electrolysis. This result has been verified in a different way by Professor Townsend, who used a method by which he found, not the absolute value of the electric charge on a particle, but the ratio of this charge to the charge on an atom of hydrogen and he found that the two charges were equal.

As the charges on the particle and the hydrogen atom are the same, the fact that the mass of these particles required to carry a given charge of electricity is only one-thousandth part of the mass of the hydrogen atoms shows that the mass of each of these particles is only about 1/1000 of that of a hydrogen atom. These particles occurred in the cathode rays inside a discharge tube, so that we have obtained from the matter inside such a tube particles having a much smaller mass than that of the atom of hydrogen, the smallest mass hitherto recognized. These negatively electrified particles, which I have called corpuscles, have the same electric charge and the same mass whatever be the nature of the gas inside the tube or whatever the nature of the electrodes; the charge and mass are invariable. They therefore form an invariable constituent of the atoms or molecules of all gases and presumably of all liquids and solids.

Nor are the corpuscles confined to the somewhat inaccessible