

Laivs of Molecular Force. 255 



As it is subsequently to be shown that in liquid ethyl oxide 

 and in all regular compound liquids the molecules are paired, 

 and that each pair acts on the others as if it were a single 

 molecule, we may estimate it as likely that the mean distance 

 apart of the pairs in ethyl oxide is between 1 and 10 micro- 

 millimetres (1/10 6 mm.). This result, though 100 times as 

 large as Sir William Thomson's limits for the distance apart 

 of molecules in liquids, namely 2 X 10 -9 cm. and 7 x 10~ 9 cm., 

 is yet in better agreement with the estimate of molecular dis- 

 tances arrived at by Riicker (Journ. Chem. Soc. 1888) as the 

 most probable result obtainable from the most important 

 attempts yet made to measure the range of molecular forces. 

 The most suo-gestive of these is Remold and Rucker's dis- 

 covery, that the equilibrium of a soap-solution film becomes 

 unstable when its thickness is reduced to between 96 and 45 

 micromillimetres, but again becomes stable when the thickness 

 is still further reduced to 12 micromillimetres. At the latter 

 thickness the film shows black in reflected light. If the 

 intermolecular distances are nearly the same in soap-solutions 

 as in liquid ethyl oxide, then the black film must be regarded 

 as consisting of a single layer of molecules or groups of mole- 

 cules (in the case of water the molecules will subsequently be 

 shown to go in double pairs). This is an intelligible result, 

 and gives the simplest explanation of Reinold and Rucker's 

 beautiful discovery of a stable thickness supervening on the 

 unstable, for we recognize a single layer of molecules as a 

 stable configuration. Of course it is to be understood that 

 what we mean by the thickness of a single layer of molecules 

 is the one nth part of the thickness of n layers ; and if the 

 black film is really only a single layer, it is in this sense that 

 Reinold and Rucker's estimate of 12 micromms. is to be taken, 

 for they did not measure an actual distance from the front to 

 the back of a black film, but only estimated from accurate and 

 accordant measurements, made in entirely different manners, 

 that the number of layers in the black film is to the number 

 in a thickness of 1 centim. as 12 micromms. is to 1 centim. 



If the black film consists really of only a single layer of 

 molecules, it is surely a hopeful sign for molecular physics 

 that measurements should have been possible on it, though 

 only visible through its invisibility. 



If, encouraged by this experimental support, we say that in 

 round numbers the mean distance between the pairs of mole 

 cules in liquid ethyl oxide is 10 micromms., then one gramme 

 contains 2v L x 10 18 molecules, or the mass of a single molecule 

 is 



1/2*88 xl0 18 grm. =3'5/10 18 , 



