12 



Mr. G. J. Stoney on the Physical 



[Recess, 



of molecules moving about actively and irregularly in all directions, the 

 path of any one being for the most part rectilinear, or, in other words, 

 most of its motion being executed at a sufficient distance from the neigh- 

 bouring molecules to be beyond the reach of sensible influence from them. 

 Every now and then, however, each molecule comes sufficiently near some 

 other molecule to have its course bent, on which occasions it darts off in a 

 new direction. Moreover many facts in physics and chemistry lead irre- 

 sistibly to the conclusion that the molecules are resolvable into simpler 

 elements ; and the probability distinctly is that each in most gases is a 

 highly complex system. When a body so constituted is enclosed, the 

 molecules by flinging themselves against the walls of the containing vessel 

 produce the pressure of the gas. If the enclosure be at the same tempe- 

 rature as the gas, they do so without gain or loss of vis viva. But if the 

 wall be at a higher temperature, the activity of those molecules which strike 

 it is increased, and vice versa. The altered activity is shared with the rest 

 of the gas by conduction and convection — or more slowly by conduction 

 only, if the circumstances do not admit of convection ; and so the tempe- 

 rature of the whole becomes changed. 



22. When we compare different gases, we find that their molecules differ 

 both in mass and in the motions that prevail within * them. That the 

 internal motions differ is abundantly testified by the amazing variety in 

 the grouping of the spectral lines to which the various gases give rise f . 

 Again, the number of molecules per cubic millimetre is known to be the 

 same in all perfect gases, when taken at the same temperature and pressure. 

 Hence the masses of the molecules are in most simple gases proportional 

 to what chemists have called their atomic weights ; and in those instances 

 in which this is not the case they stand in the same simple relation to these 

 atomic weights as the densities of the gases nearly do. Thus the masses 

 of the gaseous molecules of hydrogen, nitrogen, oxygen, chlorine^ selenium, 

 bromine, iodine, and tellurium bear to one another the ratios of the num- 

 bers 1, 14, 16, 35-5, 79*5, 80, 127, 129 — which are the atomic weights of 

 these substances, and nearly in the ratios of their vapour-densities. But 

 to represent the mass of molecules of phosphorus on the same scale, we 

 must double the number used as its atomic weight, and take 62 instead of 



gases, it follows that no gas consists of distinct particles at rest. The same proof ap- 

 plies, by the principles of the differential calculus, to the hypothesis of a continuous and 

 homogeneous substance. For this proof given more at large, see Proceedings of the 

 Eoyal Irish Academy, vol. vii. (1858), p. 37. 



* The molecular motions of a gas consist of two very distinct parts — the motions of 

 the molecules among one another, and the motions in the interior of each molecule. 



•j* [An inquiry into the numerical relations between the motions of gases and waves 

 of light forms a collateral inquiry introduced here into the MS. of the present paper 

 as sent to the Eoyal Society. It has, however, been separated and published inde- 

 pendently in the Philosophical Magazine for August 1868, in order to shorten what 

 is here printed as far as possible by confining the collateral inquiries to those which 

 are indispensable. — September 1868.] 



