A HISTORY OF SCIENCE 



in straight lines, striking against the sides of the con- 

 taining vessel and thus producing pressure. It is not 

 necessary to suppose each particle to travel to any 

 great distance in the same straight line; for the effect 

 in producing pressure will be the same if the particles 

 strike against each other; so that the straight line 

 described may be very short. M. Clausius has deter- 

 mined the mean length of path in terms of the average 

 of the particles, and the distance between the centres 

 of two particles when the collision takes place. We 

 have at present no means of ascertaining either of these 

 distances ; but certain phenomena, such as the internal 

 friction of gases, the conduction of heat through a gas, 

 and the diffusion of one gas through another, seem to 

 indicate the possibility of determining accurately the 

 mean length of path which a particle describes be- 

 tween two successive collisions. In order to lay the 

 foundation of such investigations on strict mechanical 

 principles, I shall demonstrate the laws of motion of 

 an indefinite number of small, hard, and perfectly 

 elastic spheres acting on one another only during im- 

 pact. If the properties of such a system of bodies are 

 found to correspond to those of gases, an important 

 physical analogy will be established, which may lead 

 to more accurate knowledge of the properties of matter. 

 If experiments on gases are inconsistent with the hy- 

 pothesis of these propositions, then our theory, though 

 consistent with itself, is proved to be incapable of ex- 

 plaining the phenomena of gases. In either case it is 

 necessary to follow out these consequences of the hy- 

 pothesis. 



"Instead of saying that the particles are hard, 



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