Non-existence of Density in the Elemental Oilier. 473 



air does not alter the colour of the wall. Between the en- 

 counters long intervals elapse : intervals so long compared 

 with the motions of light, that any small * 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 dyna- 

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

 close quarters ] and, accordingly, investigations based on these 

 dynamical properties, and carried out by integrations, will 

 yield results that are valuable only when the integration 

 ( (T \<£ (x,y, z) dxdydz) furnishes a result nearly identical 

 with that which would be furnished by a summation 

 (£</>(#, y, z) Ax Ay Az\ where each of the blocks Ax Ay Az is 

 sufficiently large to include an enormous number of the indi- 

 vidual operations that are in reality what actually go on in the 

 medium. 



We come upon the same result when we make a similar 

 inquiry with regard to solids. But I forbear going into 

 numerical details in this branch of our subject until I can 

 publish investigations on which I was engaged some years 

 ago, by which it appears that the form and dynamical pro- 

 perties of many crystals can be connected with their chemical 

 constitution. When this subject is gone into, it becomes plain 

 that the dynamical properties of solids also, such as their 

 power of propagating shearing-stresses, are, like those of 

 liquids and gases, due to events of an utterly different kind 

 that occur between parts so close, and in periods of time so 

 brief, that enormous shoals of these events occur in a very 

 small fraction of a second, within elements of volume many 

 times smaller than the most tiny speck the microscope can 

 show. Accordingly, what we regard as dynamical properties 



* Probably but small ; since the periodic time seems to depend much 

 more on the relation which subsists (and acts without intermission) be- 

 tween ponderable matter and the luminiferous aether, 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-spectrum of the vapour of chlorochromic anhydride 

 (CrO 2 01 2 ), the lines of which were found to have sensibly the same ap- 

 pearance 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 Beynolds, F.K.S., and the Author, in the 

 Philosophical Magazine for July 1871, p. 41. 



