THE THEORY OF MOLECULES. 285 



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 approximations 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 na- 

 ture of a probable conjecture. These are the absolute mass of a mole- 

 cule,- 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 them- 

 selves. 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 during an encounter. This conjecture 

 has been put to the test by Lorenz Meyer, who has compared the den- 

 sities of different liquids with the calculated relative densities of the 

 molecules of their vapors, 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 com- 

 bined 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 con- 

 siderations 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 

 million of them in a row would occupy a millimetre, and a million 

 million million million of them would weigh between four and five 

 grammes ! 



In a cubic centimetre of any gas at standard pressure and tern- 



