MATTER, SPACE, AND TIME 207 



provisionally the idea of the molecular structure 

 of matter. The next point to consider was 

 whether it is possible to obtain any exact know- 

 ledge of the dimensions of this structure, that is, 

 of the number of molecules we must suppose to 

 exist in a cubic centimetre, and of the size of the 

 molecular individuals. 



It is clear that the molecules must be at least 

 as small as the most minute piece of matter we 

 can prepare and recognise, and in many ways it is 

 possible to obtain substances in a very fine state 

 of division. Gold leaf can be beaten out till its 

 thickness does not exceed the millionth part of 

 an inch, while the deep blue colour of thin smoke 

 coming from a wood fire shows that the particles 

 therein are able to distinguish selectively the 

 various waves making up a beam of white light, 

 and must therefore be comparable in minuteness 

 with the lengths of those waves. 



Such results as these, while fixing an upper 

 limit to the size of molecules, are powerless to 

 assist in the determination of a lower limit, smaller 

 than which the inter-molecular distances cannot 

 be. Such inferior limits can, however, be deter- 

 mined, and to one of the methods by which they 

 have been obtained — one due to Lord Kelvin — 

 ^e will now turn. 



A soap bubble always tends to contract and 

 diminish its area, and therefore, in order to 

 increase its size, it is necessary to do work against 

 the force of contraction to an amount which may 

 be calculated by measuring the surface tension of 

 the film. Adding the energy required to prevent 

 the film from cooling during its extension, we can 

 calculate the total work absorbed per unit increase 



