BODIES SMALLEK THAN ATOMS. 233 



of those particles amount ingto one ten-thousandth of a milligram, while 

 to carry the samt 1 charge by hydrogen atoms would require a mass of 

 one-tenth of a milligram. 8 



Thus to carrv a given Gharge of electricity by hydrogen atoms re- 

 quires a mass a thousand times greater than to carry it by the nega- 

 tively electrified particles which constitute the cathode rays, and it 

 is very significant that, while the mass of atoms required to carry a 

 given charge through a liquid electrolyte depends upon the kind of 

 atom, being, for example, eight times greater for oxygen than for 

 hydrogen atoms, the mass of cathode ray particles required to carry a 

 given charge is quite independent of the gas through which the rays 

 travel and of the nature of the electrode from which they start. 



The exceedingly small mass of these particles for a given charge 

 compared with that of the hydrogen atoms might he due either to the 

 mass of each of these particles being very small compared with that of 

 a hydrogen atom or else to the charge carried by each particle being 

 large compared with that carried by the atom of hydrogen. It is there- 

 fore essential that we should determine the electric charge carried by 

 one of these particles. The problem is as follows: Suppose in an in- 

 closed space we have a number of electrified particles each carrying 

 the same charge, it is required to find the charge on each particle. It is 

 easy by electrical methods to determine the total quantity of electricity 

 on the collection of particles, and knowing this we can find the charge 

 on each particle if we can count the number of particles. To count 

 these particles the first step is to make them visible. We can do this 

 by availing ourselves of a discovery made by C T. R. Wilson, working 

 in the Cavendish Laboratory. Wilson has shown that when positively 

 and negatively electrified particles are present in moist dust-free air 

 a cloud is produced when the air is closed by a sudden expansion, 

 though this amount of expansion would be quit* 1 insufficient to produce 

 condensation when no electrified particles are present: the water con- 

 denses round the electrified particles, and. if these are not too numer- 

 ous, each particle becomes the nucleus of a little drop of water. Now 

 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 



"Professor Schuster in 1889 was tin- first to apply the method of the magnetic 

 ilctlection of the discharge togeta determination of the value < if mje. He found rather 

 widely separated limiting values for this quantity, and came to the conclusion that it 

 was of the same order as in electrolytic solutions. The result of the method mentioned 

 above, as well as those of Wiechert, Kaufmann, and Lenard, make it verv much 

 smaller. 



