398 Scientific Proceedings, Royal Dublin Society. 



whence, finally, we may take this number to be about — : in other 



words, this molecule of the wall is struck on the average at inter- 

 vals of about 40 times r. If the colour of the wall be green, the 

 molecular motions which occasion this colour are repeated in the 

 molecule 2000 each r, and therefore something like 80,000 in the 

 intervals between the shots to which the molecule is subjected by 

 the aerial artillery. This serves to explain why the incessant 

 bombardment by the air does not alter the colour of the wall. 

 Between the encounters long intervals elapse : intervals so long 

 compared with the motions of light, that any small (m) disturbance 

 in the periodic time which may be caused by the encounter probably 

 lasts for but a very trivial part of the long intervals of respite. 



Thus in both liquids and gases, what are called the dynamical 

 properties of the medium do not exist when we come to close 

 quarters ; and, accordingly, investigations based on these dynami- 

 cal properties, and carried out by integrations, will yield results 

 that are valuable only when the integration (J7J" {x, y, z) dxdydz) 

 furnishes a result nearly identical with that which would be fur- 

 nished by a summation (S0(#, y, z) A#AyAs), where each of the 



Again, proceeding as in equation (2) of footnote (/), we find that the number N" 

 which would reach a sphere with radius a, 



N" = k . 7ra 2 . 2ird cos 



r.f, 



Jo 



= 2-K i a i k. (7) 



Comparing (6) and (7), we find that 



N" = 2N'. 



(m) Probably but small : since the periodic time seems to depend much more on the 

 relation which subsists (and acts without intermission) between ponderable matter and 

 the luminiferous ether, than upon the occasional events which occur in the grappling 

 of molecules with one another. 



This view is borne out by observations made by the author on the absorption spec- 

 trum of the vapour of chlorochromic anhydride (CrO^C^), the lines of which were 

 found to have sensibly the same appearance whether air was or was not present with 

 the vapour. It was expected that the spectrum might exhibit an appreciable difference 

 in these two cases, since when air is present the molecules of the vapour are subjected 

 to a largely multiplied number of encounters — notwithstanding which no alteration in 

 the appearance of the lines of the spectrum could be detected. 



For an account of the very remarkable spectrum of this vapour, see a paper by 

 Professor Emerson Eeynolds, F. R. S,, and the Author, in the Philosophical Magazine 

 for July, 1871, p. 41. 



