.57.' MOLECULES. 



during an encounter. This conjecture has been put to the test by Lorenz Mover. 

 who baa compared the densities of different liquids with the calculated relative 

 densities of the molecules of their vapours, and has found a remarkable cor- 

 respondence between them. 



Now Loachmidt has deduced from the dynamical theory the following 

 remarkable proportion: As the volume of a gas is to the combined volume of 

 all the molecules contained in 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-bul. 

 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 millimetre, and a million million million millio: 

 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 piv 

 conjectural In order to warrant us in putting any confidence in nun. 

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

 of independent data than we have as yet obtained, and to shew that they 

 lead to consistent results. 



Thus far we have been considering molecular science as an inquiry 

 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. It leaves him in possession of methods which nothing 

 but scientific work could have led him to invent; and it places him in a 

 position from which many regions of nature, besides that which he has been 

 studying, appear under a new aspect. 



The study of molecules has developed a method of its own, and it 

 also opened up new views of nature. 



