THE 



POPULAR SCIENCE 



MONTHLY. 



AUGUST, 1901. 



OX BODIES SMALLER THAN ATOMS. 



By professor J. J. THOMSON, 



CAMBRIDGE UNIVERSITY. 



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

 -■- about thirty 3'ears 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 not to leave much 

 room for the existence of anvthing smaller than ordinarv atoms, for 

 they showed that in a cubic centimeter of gas at atmospheric pressure 

 and at 0° C. there are aboiit 20 million, million, million (2 X lO^'-*) 

 molecules of 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 diiferent kind. Thus Lord Eayleigh 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 carry the same 

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

 tity of electricity is proportional to the quantity carried. We know 



