ETHER AND PONDERABLE MATTER 



one - one - hundred - millionth of a centimetre for the 

 minimum. Such figures convey no particular meaning 

 to our blunt senses, but Lord Kelvin has given a 

 tangible illustration that aids the imagination to at 

 least a vague comprehension of the unthinkable small- 

 ness of the molecule. He estimates that if a ball, say 

 of water or glass, about " as large as a football, were to 

 be magnified up to the size of the earth, each constitu- 

 ent molecule being magnified in the same proportion, 

 the magnified structure would be more coarse-grained 

 than a heap of shot, but probably less coarse-grained 

 than a heap of footballs." 



Several other methods have been employed to esti- 

 mate the size of molecules One of these is based upon 

 the phenomena of contact electricity ; another upon the 

 wave- theory of light; and another upon capillary at- 

 traction, as shown in the tense film of a soap-bubble! 

 No one of these methods gives results more definite 

 than that due to the kinetic theory of gases, just out- 

 lined ; but the important thing is that the results ob- 

 tained by these different methods (all of them due to 

 Lord Kelvin) agree with one another in fixing the di- 

 mensions of the molecule at somewhere about the 

 limits already mentioned. We may feel very sure in- 

 deed, therefore, that the molecules of matter are not the 

 unextended, formless points which Boscovich and his 

 followers of the eighteenth century thought them. But 

 all this, it must be borne in mind, refers to the molecule, 

 not to the ultimate particle of matter, about wlm h \\c 

 shall have more to say in another connection. Curi- 

 ously enough, we shall find that the latest theories as 

 to the final term of i cs are not so very far afield 



299 



