326 POPULAR SCIENCE MONTHLY. 



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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 X 10~^° electrostatic units of electricity or 2.17 X lO""-" 

 electro-magnetic units. According to the kinetic theory of gases, 

 there are 3 X lO^'' molecules in a cubic centimeter of gas at atmos- 

 pheric 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 X 10^^) milligrams and each atom therefore 

 about 1/(44 X 10^") milligrams and as w^e have seen that in the elec- 

 trolysis of solutions one-tenth of a milligram carries unit charge, the 

 atom of hydrogen will carry a charge equal to 10/(44 X 10^^) = 

 2.27 X 10~-° electro-magnetic units. The charge on the particles in a 

 gas Ave have seen is equal to 2.17 X 10"-** 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, Avho 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 j^art 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 recog- 

 nized. These negatively electrified particles, which I have called cor- 

 puscles, 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 



