ON THE ESSENTIAL PROPERTIES OF MATTER. 6l$ 



Mr. Dalton has proposed a singular theory respecting the constitution artd 

 mutual repulsion of elastic fluids; he imagines that when any two gases of 

 different kinds are mixed, the particles of each gas repel only the similar 

 particles of the same gas, without exerting any action on those of the other 

 gas, except when the ultimate solid atoms chance to interfere. The idea is 

 ingenious and original, and may perhaps be of use in connecting some facts 

 together, or in leading to some other less improbable suppositions; but it 

 may easily be shown, that Mr. Dalton's hypothesis cannot possibly be true 

 in all its extent, since it would follow from it, that two portions of gases, of 

 different kinds, could not exist, for a sensible time, in the same vessel, without 

 being uniformly diffused throughout it, while the fact is clearly otherwise ; 

 for hydrogen gas remains, when left completely at rest, a very considerable 

 time above, and carbonic acid gas below, a ffortion of common air with 

 which it is in contact; nor is there any circumstance, attending the mixture 

 of gases, which may not be explained without adopting so paradoxical an 

 opinion. Mr. Dalton thinks that, from the laws of hydrostatics, no two gases, 

 not chemically united, could remain mixed, if their particles acted mutually on 

 each other: but the laws of hydrostatics do not apply to the mixture of 

 single particles of fluids of different kinds; since they are only derived from 

 the supposition of a collection of particles of the same kind. 



In liquids and m solids, this repulsive force appears at first sight to be want- 

 ing; but when we consider that the particles both of liquids and of solids are 

 actuated by the attractive force of cohesion, we shall see the necessity of the 

 presence of a repulsive force, in order to balance it ; it is, therefore, probable 

 that the particles of aeriform fluids still retain their original repulsive powers, 

 when they are reduced to a state of liquidity or of solidity, by being subjected 

 to the action of a second force, which causes them to cohere. 



The mutual repulsion of the particles of matter is a reciprocal force, actino- 

 equally, in opposite directions, on each of the bodies concerned. It scarcely 

 requires either experiment or argument to show, that if two bodies repel each 

 other, neither of them will remain at rest, but both of them will move, with 

 equal quantities of motion. Thusj if a portion of condensed air be made to 

 act upon the bullet of an air gun, it will force the gun backwards with as 

 much momentum as it impels the bullet forwards. 



VOL. I. 4g 



