470 LECTURE XLIX. 



of half a pint ; which could not be, if the particles exercised a mutual 

 repulsion at all possible distances. 



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

 and 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 connect- 

 ing some facts together, or in leading to some other less improbable suppo- 

 sitions ; 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 

 portion of common air w r ith which it is in contact ; nor is there any cir- 

 cumstance 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 in solids, this repulsive force appears at first sight to be 

 wanting ; 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, 

 acting 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. Thus, 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. 



Inertia is that property of matter, by which it retains its state of rest or 

 of uniform motion, with regard to a quiescent space, as long as no foreign 

 cause occurs to change that state. This property depends on the intimate 

 constitution of matter ; it is generally exhibited by means of the force of 

 repulsion, which enables a body in motion to displace another, in order to 

 continue its motion, or by means of some attractive force, which causes two 

 bodies to approach their common centre of inertia with equal momenta. 



* Manchester Memoirs, vol. v. See also Graham, Edin. Tr. 1831 ; Thomson, 

 Phil. Mag. 3rd Ser. vol. iv. p. 321, by whom the hypothesis of Dalton is established. 



