On the Diffusion of Moving Particles among one another. 21 



a mechanical explanation of the cause was to be had on some 

 such principles as the following : — Vapour of sodium must pos- 

 sess by its molecular structure a tendency to vibrate in the 

 periods corresponding to the degrees of refrangibility of the double 

 line D. Hence the presence of sodium in a source of light must 

 tend to originate light of that quality. On the other hand, 

 vapour of sodium in an atmosphere round a source, must have a 

 great tendency to retain in itself, i. e. to absorb and to have its 

 temperature raised by light from the source, of the precise qua- 

 lity in question. In the atmosphere around the sun, therefore, 

 there must be present vapour of sodium, which, according to the 

 mechanical explanation thus suggested, being particularly opake 

 for light of that quality, prevents such of it as is emitted from 

 the sun from penetrating to any considerable distance through 

 the surrounding atmosphere. The test of this theory must be 

 had in ascertaining whether or not vapour of sodium has the 

 special absorbing power anticipated. I have the impression that 

 some Frenchman did make this out by experiment, but I can 

 find no reference on the point. 



" I am not sure whether Professor Stokes's suggestion of a me- 

 chanical theory has ever appeared in print. I have given it in 

 my lectures regularly for many years, always pointing out along 

 with it that solar and stellar chemistry were to be studied by 

 investigating terrestrial substances giving bright lines in the 

 spectra of artificial flames corresponding to the dark lines of the 

 solar and stellar spectra." 



II. Illustrations of the Dynamical Theory of Gases. By J. C. 

 Maxwell, M.^., Professor of Natural Philosophy inMarischal 

 College and University of Aberdeen. 



[Concluded from vol. xix. p, 32.] 



Part II. On the Process of Diffusion of two or more kinds of 

 moving particles among one another. 



WE have shown, in the first part of this paper, that the 

 motions of a system of many small elastic particles are 

 of two kinds : one, a general motion of translation of the whole 

 system, which may be called the motion in mass ; and the other 

 a motion of agitation, or molecular motion, in virtue of which 

 velocities in all directions are distributed among the particles 

 according to a certain law. In the cases we are considering, the 

 collisions are so frequent that the law of distribution of the mole- 

 cular velocities, if disturbed in any way, will be re-established in 

 an inappreciably short time; so that the motion will always con- 



