ON THE ESSENTIAL PROPERTIES OF MATTER. 469 



other, or somewhat nearer, they support each other's weight in the same 

 manner as if they were in actual contact, and that some additional force is 

 required in order to make them approach still nearer ; nor does it appear 

 probable that the contact is ever perfect, otherwise they might be expected 

 to cohere in such a manner as to become one mass. Professor Robison* 

 has ascertained by experiment the force necessary to produce the greatest 

 possible degree of contact, and finds it equivalent to a pressure of about a 

 thousand pounds for every square inch of glass. It is therefore obvious 

 that in all common cases of the contact of two distinct bodies, it must be 

 this repulsive force that retains them in their situation. I have found that 

 glass, placed on a surface of metal, exhibits this force nearly in the same 

 degree as if placed on another piece of glass ; it is also independent of the 

 presence of air ; but under water it disappears. 



The existence of a repulsive force, extending beyond the actual surface 

 of a material substance, being proved, it has been conjectured by some 

 that such a force, unconnected with any central atom, may be sufficient for 

 producing all the phenomena of matter. This representation may be 

 admitted without much difficulty, provided that it be allowed that the 

 force becomes infinite at or near the centre ; but it has been sometimes 

 supposed that it is every where less than infinite, and consequently that 

 matter is not absolutely impenetrable ; such a supposition appears however 

 to lead to the necessity of believing that the particles of matter must some- 

 times be annihilated, which is not a very probable opinion. 



The magnitude of the repulsive force by which the particles of any single 

 body are enabled to resist compression, increases nearly in proportion to 

 the degree of compression, or to the decrease of the distances between the 

 particles. This is almost a necessary consequence of any primary law 

 that can be imagined, for the immediate actions of the particles : for 

 instance, if the repulsion increased either as the square or as the cube of 

 the distance diminished, the effect of a double change of dimensions would 

 always be nearly a double change of the repulsive force ; that is, if an 

 elastic substance were compressed one thousandth part of its bulk, it would 

 in either case resist twice as much as if it were only compressed one two 

 thousandth. 



It is obvious that if the particles of matter are possessed of a repulsive 

 force decreasing in any regular proportion with the increase of distance, 

 they can never remain at rest without the operation of some external 

 pressure, but will always retain a tendency to expand. This is the case of 

 all elastic fluids, the density of which is found to vary exactly as the com- 

 pressing force, whence it may be demonstrated, that the primary repulsive 

 force of the particles must increase in the same proportion as the distance 

 decreases. It follows also that this force can only be exerted between such 

 particles as are either actually or very nearly in contact with each other; 

 since it requires no greater pressure, acting on a given surface, to retain a 

 gallon of air in the space of half a gallon, than to retain a pint in the space 



* Robison's Mech. Phil. vol. i. Corpuscular Action, art. 241. See also Huy- 

 gens, Ph. Tr. No. 86. Hauksbce, ibid. 1709, p. 306. 



