Prof. J. C. Maxwell on Molecules. 463 



as yet of the nature of a probable conjecture. These are : — the ab- 

 solute 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 de- 

 duce the relative densities of the molecules themselves. So far 

 we are on firm ground. 



The great resistance of liquids to compression makes it pro- 

 bable that their molecules must be at about the same distance 

 from each other as that at which two molecules of the same sub- 

 stance in the gaseous form act on each other during an encounter. 

 This conjecture has been put to the test by Lorenz Meyer, who 

 has compared the densities of different liquids with the calcu- 

 lated relative densities of the molecules of their vapours, and has 

 found a remarkable correspondence between them. 



Now Loschmidt has deduced from the dynamical theory the fol- 

 lowing remarkable proportion : — As the volume of a gas is to the 

 combined volume of all the molecules containedin it, so is the mean 

 path of a molecule to one eighth of the diameter of a molecule. 



Assuming that the volume of the substance, when reduced to 

 the liquid form, is not much greater than the combined volume 

 of the molecules, we obtain from this proportion the diameter of 

 a molecule. In this way Loschmidt, in 1865, made the first 

 estimate of the diameter of a molecule. Independently of him 

 and of each other, Mr. Stoney in 1868, and Sir W. Thomson in 

 1870, published results of a similar kind, those of Thomson 

 being deduced not only in this way, but from considerations 

 derived from the thickness of soap-bubbles and from the electric 

 properties of metals. 



According to the Table, which I have calculated from 

 Loschmidt' s data, the size of the molecules of hydrogen is such 

 that about two millions of them in a row would occupy a milli- 

 metre, and a million million million millions of them would 

 weigh between four and five grammes. 



In a cubic centimetre of any gas at standard pressure and 

 temperature there are about nineteen million million million 

 molecules. All these numbers of the third rank are, I need not 

 tell you, to be regarded as at present conjectural. In order to 

 warrant us in putting any confidence in numbers obtained in 

 this way, we should have to compare together a greater number 

 of independent data than we have as yet obtained, and to show 

 that they lead to consistent results. 



Thus far we have been considering molecular science as an 

 inquiry into natural phenomena. But though the professed 

 aim of all scientific work is to unravel the secrets of nature, it 

 has another effect, not less valuable, on the mind of the worker. 



