574 Professor J. J. Thomson [April 19, 



WEEKLY EVENING MEETING, 



Friday, April 19, 1901. 



Hrs Grace The Duke of Northumberland, E.G. D.C.L. F.R.S., 

 President, in the Chair. 



Professor J. J. Thomson, M.A. Sc.D. F.R.S. 



The Existence of Bodies Smaller than Atoms. 



The masses of the atoms of the various gases were first investigated 

 about thirty years ago by methods due to Loschmidt, Johnstone 

 Stoney and Lord Kelvin. These physicists, using the principles of 

 the kinetic theory of gases, and making certain assumptions (which it 

 must be admitted are not entirely satisfactory) as to the shape of the 

 atom, determined the mass of an atom of a gas ; and when once the 

 mass of an atom of one substance is known the masses of the atoms 

 of all other substances are easily deduced by well-known chemical 

 considerations. The results of these investigations might be thought 

 to leave not much room for the existence of anything smaller than 

 ordinary atoms, for they showed that in a cubic centimetre of gas at 

 atmospheric pressure and at 0° C. there are about 20 million, million, 

 million (2 X 10 19 ) molecules of the gas. 



Though some of the arguments used to get this result are open 

 to question, the result itself has been confirmed by considerations of 

 quite a different kind. Thus, Lord Rayleigh has shown that this 

 number of molecules per cubic centimetre gives about the right value 

 for the optical opacity of the air ; while a method which I will now 

 describe, by which we can directly measure the number of molecules 

 in a gas, leads to a result almost identical with that of Loschmidt. 

 This method is founded on Faraday's laws of electrolysis ; we deduce 

 from these laws that the current through an electrolyte is carried by 

 the atoms of the electrolyte, and that all these atoms carry the same 

 charge, so that the weight of the atoms required to carry a given 

 quantity of electricity is proportional to the quantity carried. We 

 know too, by the results of experiments on electrolysis, that co carry 

 the unit charge of electricity requires a collection of atoms of hydro- 

 gen which together weigh about one-tenth of a milligram ; hence, if 

 we can measure the charge of electricity on an atom of hydrogen, we 

 see that one-tenth of this charge will be the weight in milligrams of 

 the atom of hydrogen. This result is for the case when electricity 

 passes through a liquid electrolyte. I will now explain how we can 

 measure the mass of the carriers of electricity required to convey a 



