148 POPULAR LECTURES AND ADDRESSES. 



smallness, or in our views and appreciation and 

 numerical expression of the magnitude. The 

 general results of the four lines of reasoning to 

 which I have referred, founded respectively on 

 the undulatory theory of light, on the phenomena 

 of contact electricity, on capillary attraction, 

 and on the kinetic theory of gases, agree in 

 showing that the atoms or molecules of ordinary 

 matter must be something like the i/io,ooo,oooth 

 or from the i/ 10,000, oooth to the i/ioo,ooo,oooth 

 of a centimetre in diameter. I speak somewhat 

 vaguely, and I do so not inadvertently, when 

 I speak of atoms and molecules. I must ask 

 the chemists to forgive me if I even abuse the 

 words and apply a misnomer occasionally. The 

 chemists do not know what is to be the atom ; 

 for instance, whether hydrogen gas is to consist 

 of two pieces of matter in union constituting 

 one molecule, and these molecules flying about ; 

 or whether single molecules, each indivisible, or 

 at all events undivided in chemical action, 

 constitute the structure. I shall not go into any 

 such questions at all, but merely take the broad 



