. 



aeeoracY, and. from a knowledge of the mean path, we can calculate their 

 relative diameter* approximately. From these we can deduce the relative 

 deneittea of different kinds of molecules. The relative densities so calculated 

 bar* been flump*** 1 by Lorenz Meyer with the observed densities of the liquids 

 into which the gases may be condensed, and he finds a remarkable correspond- 

 ence between them. There is considerable doubt, however, as to the relation 

 between the molecules of a liquid and those of its vapour, so that till a 

 larger number of comparisons have been made, we must not place too much 

 reliance on the calculated densities of molecules. Another, and perhaps a more 

 refined, method is that adopted by M. Van der Waals, who deduces the 

 molecular volume from the deviations of the pressure from Boyle's law as the 

 gas is compressed. 



The first numerical estimate of the diameter of a molecule was that made 

 by Losclunidt in 1865 from the mean path and the molecular volume. Inde- 

 pendently 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 onlv in this way, but from considerations derived from the thickness of 

 soap bubbles, and from the electric action between zinc and copper. 



The diameter and the mass of a molecule, as estimated by these methods, 

 are, of course, very small, but by no means infinitely so. About two millions 

 of molecules of hydrogen in a row would occupy a millimetre, and about two 

 hundred million million million of them would weigh a milligramme. These 

 numbers must be considered as exceedingly rough guesses ; they must be 

 corrected by more extensive and accurate experiments as science advances ; 

 but the, main result, which appears to be well established, is that the deter- 

 mination of the mass of a molecule is a legitimate object of scientific research, 

 and that this mass is by no means immeasurably small. 



Loschmidt illustrates these molecular measurements by a comparison with 

 the smallest magnitudes visible by means of a microscope. Nobert, he tells us, 

 can draw 4000 lines in the breadth of a millimetre. The intervals between 

 these lines can be observed with a good microscope. A cube, whose side is 

 the 4000th of a millimetre, may be taken as the minimum visibile for observers 

 of the present day. Such a cube would contain from GO to 100 million 

 molecules of oxygen or of nitrogen; but since the molecules of organised 

 substances contain on an average about 50 of the more elementary atoms, we 

 may assume that the smallest organised particle visible under the microscope 



