Molecular constitution of air 65 



95] What principally makes me think that this is the case, is that as 

 far as I can judge from some experiments I have made*, the quantity 

 of fluid in different bodies agrees very well with those propositions, on a 

 supposition that the electric repulsion is inversely as the square of the 

 distance. It should also seem from those experiments, that the quantity 

 of redundant or deficient fluid in two bodies bore very nearly the same 

 proportion to each other, whatever is the shape of the canal by which 

 they are joined, or in whatever direction they are situated in respect of 

 each other. 



96] Though the above propositions should be found not to hold good 

 when the bodies are joined by real canals, still it is evident, that in the 

 22nd proposition, if the plates AB and DF are very near together, the 

 quantity of redundant fluid in the plate AB will be many times greater 

 than that in the body H, supposing H to consist of a circular plate of the 

 same size as AB, and DF will be near as much undercharged as AB is 

 overcharged. 



97] Sir Isaac Newton supposes that air consists of particles which 

 repel each other with a force inversely as the distance: but it appears 

 plainly from the foregoing pages, that if the repulsion of the particles was 

 in this ratio, and extended indefinitely to all distances, they would compose 

 a fluid extremely different from common air. If the repulsion of the particles 

 was inversely as the distance, but extended only to a given very small 

 distance from their centers, they would compose a fluid of the same kind 

 as air, in respect of elasticity, except that its density would not be in pro- 

 portion to its compression : if the distance to which the repulsion extends, 

 though very small, is yet many times greater than the distance of the 

 particles from each other, it might be shewn, that the density of the fluid 

 would be nearly as the square root of the compression. If the repulsion 

 of the particles extended indefinitely, and was inversely as some higher 

 power of the distance than the cube, the density of the fluid would be as 

 some power of the compression less than f . The only law of repulsion, 

 I can think of, which will agree with experiment, is one which seems not 

 very likely; namely, that the particles repel each other with a force in- 

 versely as the distance ; but that, whether the density of the fluid is great 

 or small, the repulsion extends only to the nearest particles: or, what 

 comes to the same thing, that the distance to which the repulsion extends, 

 is very small, and also is not fixed, but varies in proportion to the distance 

 of the particles f. 



[* Exp. Ill, Art. 265.] [f Note 6, p. 370.] 



c.p, i. 



