Mr. J. C. Maxwell on the Dynamical Theory of Gases. 131 



sensibly rectilinear and beyond the sphere of sensible action of 

 the neighbouring molecules. 



I propose in this paper to apply this theory to the explana- 

 tion of various properties of gases, and to show that, besides ac- 

 counting for the relations of pressure, density, and temperature 

 in a single gas, it affords a mechanical explanation of the known 

 chemical relation between the density of a gas and its equiva- 

 lent weight, commonly called the Law of Equivalent Volumes. 

 It also explains the diffusion of one gas through another, the 

 internal friction of a gas, and the conduction of heat through 

 gases. 



The opinion that the observed properties of visible bodies ap- 

 parently at rest are due to the action of invisible molecules in 

 rapid motion is to be found in Lucretius. In the exposition 

 which he gives of the theories of Democritus as modified by Epi- 

 curus, he describes the invisible atoms as all moving downwards 

 with equal velocities, which, at quite uncertain times and places, 

 suffer an imperceptible change, just enough to allow of occa- 

 sional collisions taking place between the atoms. These atoms 

 he supposes to set small bodies in motion by an action of which 

 we may form some conception by looking at the motes in a sun- 

 beam. The language of Lucretius must of course be interpreted 

 according to the physical ideas of his age; but we need not won- 

 der that it suggested to Le Sage the fundamental conception of 

 his theory of gases, as well as his doctrine of ultramundane cor- 

 puscles. 



Professor Clausius, to whom we owe the most extensive de- 

 velopments of the dynamical theory of gases, has given* a list 

 of authors who have adopted or given countenance to any theory 

 of invisible particles in motion. Of these, Daniel Bernoulli, in 

 the tenth section of his ' Hydrodynamics/ distinctly explains 

 the pressure of air by the impact of its particles on the sides of 

 the vessel containing it. 



Clausius also mentions a book entitled "Deux Traites de Phy- 

 sique Mecanique, publies par Pierre Prevost, comme simple Editeur 

 du premier et comme Auteur du second," Geneve et Paris, 1818. 

 The first memoir is by G. Le Sage, who explains gravity by the 

 impact of "ultramundane corpuscles'" on bodies. These cor- 

 puscles also set in motion the particles of light and various 

 sethereal media, which in their turn act on the molecules of 

 gases and keep up their motions. His theory of impact is faulty, 

 but his explanation of the expansive force of gases is essentially 

 the same as in the dynamical theory as it now stands. The se- 

 cond memoir, by Prevost, contains new applications of the prin- 



* Poggendorff's Annalen, January 1862. Translated by G. C. Foster, 

 B.A., Phil. Mag. June 1862. 



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