

Matter and Force in Theoretical Physics. 469 



Newton, who has Queries about it at the end of his ' Optics/ 

 and mentions it expressly in the very last paragraph of the 

 Principia. The physical fact which perhaps more than any 

 other suggests this hypothesis, is the ascertained velocity of 

 light, which is so great as to be scarcely compatible with the 

 transmission of matter, and may reasonably be ascribed to trans- 

 mission of motion, like the propagation through the air of waves 

 which produce sound. The aether is accordingly assumed to 

 be a very elastic fluid, universally diffused, and filling all space 

 not occupied by the atoms of visible and tangible bodies. It 

 must be a fluid medium, and extremely rare, because it allows 

 the large masses of the planets to move through its substance 

 without perceptible impediment. As the aether is the supposed 

 medium by which all physical force is exerted, it cannot be 

 acted upon by any of the different kinds of physical force, as 

 gravity, heat, magnetism, &c; and consequently in its undis- 

 turbed state its pressure and density are everywhere the same. 

 The fact of transmission of waves of the aether, like waves of 

 air, implies that it is susceptible of variations of density and 

 pressure ; and these variations may be supposed to have been 

 originally produced by mutual action between it and the solid 

 atoms. Respecting the relation between the density and the 

 pressure, I make the hypothesis, suggested by what is known of 

 air of given temperature, that the pressure is always exactly pro- 

 portional to the density. Here it might be objected that this 

 is not an allowable hypothesis, because it is the expression of a 

 quantitative law, and ought therefore, according to a rule of 

 philosophy before adduced, to be derivable by mathematical 

 reasoning from antecedent premises. My answer to this ob- 

 jection is, that on the hypothesis of the dynamical action of an 

 aether, the law, so far as it regards air of given temperature, is, 

 in fact, deducible by a priori reasoning, such as that which I 

 have employed in an investigation relative to this] law con- 

 tained in the Philosophical Magazine for June 1859. Hence, in 

 assuming that the law holds good for the aether, we attribute to 

 the aether a quality which is suggested and made intelligible by 

 reasoning as well as experiment, and the hypothesis is on 

 that ground proper for forming, with others, the foundation 

 of a general physical theory. It is consequently not ne- 

 cessary to inquire what may be the reason that the aether con- 

 forms to this particular law ; but if the inquiry should be in- 

 sisted upon, it is obvious to reply that we can conceive of the 

 existence of another order of aether having the same relation to 

 the first as that has to the air, and so on ad libitum', for " nature 

 knows no limit." Possibly, in accounting for the law in this 

 way, we reach the ultimate conception that we can form of phy- 

 PhiL Mag. S. 4. Vol. 31. No. 211. June 1866. 2 I 



