ON BODIES SMALLER THAN ATOMS. 



By Prof. J. J. Thomson, 

 Cambridge University. 



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

 about thirty years ago by methods due to Loschmidt, Johnstone, 

 Stone} T , 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 con- 

 siderations. The results of these investigations might be thought not 

 to leave much room for the existence of anything smaller than ordinary 

 atoms, for they showed that in a cubic centimeter of gas at atmospheric 

 pressure and at C C. there are about 20 million, million, million 

 (2 X 10 19 ) molecules of gas. 



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

 question, the result itself has been confirmed by considerations of 

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

 ber of molecules per cubic centimeter 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 cany 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 to carry 

 the unit charge of electricitj^ 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 Irydrogen we 

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



a Reprinted, by permission, from Popular Science Monthly, August, 1901. 



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