compared with the Motions of Waves of Light. 137 



one system of rays, and above that temperature to a wholly dif- 

 ferent system, as, for example, in nitrogen*. 



6. When we compare different gases, we find that their mole- 

 cules 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. And as the number of mole- 

 cules per cubic millimetre is known to be the same in all perfect 

 gases when taken at the same temperature and pressure, it fol- 

 lows that the masses of the molecules are in most simple gases 

 proportional to what chemists call their atomic weights, and, in 

 those instances in which this is not the case, that they stand in 

 the same simple relation to these atomic weights as the densities 

 of the gases nearly do. 



7. Very accurate measures have been effected of several of the 

 minute magnitudes concerned in the phenomena of light ; but 

 these are the only determinations yet made in molecular phy- 

 sics upon which we can fully rely. Nevertheless w T e are not left 

 to mere conjecture in regard to the other quantities with which 

 we have to deal ; for by many indirect methods it is possible to 

 estimate their dimensions, not indeed with precision, but yet so 

 as to indicate unmistakeably whereabouts they lie, and, as it 

 were, to introduce us into their neighbourhood. Such loose de- 

 terminations are of great use, from their enabling us to form 

 just conceptions of the kind of magnitudes of which we talk, and 

 so guarding us against many errors. 



8. The molecules of a gas are darting about with very differ- 

 ent velocities ; but at each temperature there is a certain mean 

 velocity towards which the collisions that prevail within the gas 

 tend to bring velocities which deviate fromitf, and round which 

 accordingly the innumerable velocities existing at any moment 

 group themselves. This mean velocity Clausius succeeded in 

 estimating at about 



485 a / 7^77 — metres per second {, 



where T is the absolute temperature estimated in Centigrade de- 

 grees, and p the relative specific gravity of the gas compared 



* The phenomena of nitrogen very explicitly declare that temperature is 

 but one condition, and that it may he overruled by other circumstances. 

 The lines withdrawn from the solar spectrum by the cold nitrogen of our 



atmosphere all (so far as they are known) belong to the nitrogen spectrum 

 "of the second order," as it is called by Plucker — that spectrum which, in 

 his experiments, was only obtained at the higher temperatures. 



t The mean velocity so defined is not the arithmetic mean of the values 

 of v, but the square root of the arithmetic mean of the values of v 2 . 



\ Phil. Mag. 1857, vol. xiv. p. 124. 



