Herschel's ^Ethereal Physics. 257 



£ 1? d Y =- elasticity, density, of air ; 

 e 2 , d 2 = elasticity, density, of aether ; 

 v.=V^X=916ft.; 



v K = 186,000 miles: 



% : e ± : : v] » : cj : : 1 : 1,148,000,000,000. . . (1) 



Accordingly, he says : — " Let us suppose now that an 

 amount of our aetherial medium equal in quantity of matter to 

 that which is contained in a cubic inch of air (which weighs 

 about one third of a grain) were enclosed in a cube of an inch 

 in the side. The bur sting -power of air so enclosed we know to 

 be 15 lb. on each side of the cube. That of the imprisoned 

 aether, then, would be 15 times the above immense number 

 (or upwards of 17 billions) of pounds. Do what we will, adopt 

 what hypotheses we please, there is no escape, in dealing of 

 the phenomena of light, from these gigantic numbers ; or 

 from the conception of enormous physical force in perpetual 

 exertion at every point, through all the immensity of space." 



The italics in the above extract, which are HerschePs, show 

 that the " bursting-power " referred to is not that of simple 

 aetherial elasticity, assumed to be a the same as that of air at 

 sea-level/'' but is that which is represented by the ratio of the 

 elasticity to the density, or that which would be exerted if 

 the air and the aether were reduced to the same density. 



The identity of Herschel's and Wood's methods is shown 

 by the following extract* : — " It may be asked, Can the 

 kinetic theory, which is applicable to gases in which waves 

 are propagated by a to-and-fro motion of the particles, be 

 applicable to a medium in which the particles have a trans- 

 verse movement, whether rectilinear, circular, elliptical, or 

 irregular ? In favour of such an application it may be stated 

 that the general formulae of analysis by which wave-motion 

 in general, and refraction, reflection, and polarization in par- 

 ticular, are discussed, are fundamentally the same ; and in 

 the establishment of the equations the only hypothesis in 

 regard to the path of a particle is — It will move along the 

 path of least resistance. The expression V 2 oce-r-S is gene- 

 rally true for all elastic media, regardless of the path of the 

 individual molecules. Indeed, granting the molecular consti- 

 tution of the aether, is it not probable that the Kinetic theory 

 applies more rigidly to the aether than to the most perfect of 

 the known gases ? " The identity of results is shown by Prof. 

 Wood's statement (/. c. p. 416) that " In a pound of the aether 

 there is some 100,000,000,000 times the Kinetic energy of a 

 * Phil. Mag. 1. c. p. 392. 



