MOLECULES. 371 



of the opposite kind, it enters again into a more or less permanent union with 

 it till it is again dissociated by another shock. Electrolysis, therefore, is a 

 kind of diffusion assisted by electromotive force. 



Another branch of molecular science is that which relates to the exchange 

 of molecules between a liquid and a gas. It includes the theory of evaporation 

 and condensation, in which the gas in question is the vapour of the liquid, and 

 also the theory of the absorption of a gas by a liquid of a different substance. 

 The researches of Dr Andrews on the relations between the liquid and the 

 gaseous state have shewn us that though the statements in our elementary 

 text-books may be so neatly expressed as to appear almost self-evident, their 

 true interpretation may involve some principle so profound that, till the right 

 man has laid hold of it, no one ever suspects that any thing is left to be 

 discovered. 



These, then, are some of the fields from which the data of molecular 

 science are gathered. We may divide the ultimate results into three ranks, 

 according to the completeness of our knowledge of them. 



To the first rank belong the relative masses of the molecules of different 

 gases, and their velocities in metres per second. These data are obtained from 

 experiments on the pressure and density of gases, and are known to a high 

 degree of precision. 



In the second rank we must place the relative size of the molecules of 

 different gases, the length of their mean paths, and the number of collisions 

 in a second. These quantities are deduced from experiments on the three 

 kinds of diffusion. Their received values must be regarded as rough approxi- 

 mations till the methods of experimenting are greatly improved. 



There is another set of quantities which we must place in the third rank, 

 because our knowledge of them is neither precise, as in the first rank, nor 

 approximate, as in the second, but is only as yet of the nature of a probable 

 conjecture. These are : The absolute mass of a molecule, its absolute diameter, 

 and the number of molecules in a cubic centimetre. We know the relative 

 masses of different molecules with great accuracy, and we know their relative 

 diameters approximately. From these we can deduce the relative densities of 

 the molecules themselves. So far we are on firm ground. 



The great resistance of liquids to compression makes it probable that their 

 molecules must be at about the same distance from each other as that at which 

 two molecules of the same substance in the gaseous form act on each other 



472 



