50 
ME. CLEEK MAXWELL ON THE DYNAMICAL THEOEY OE GASES. 
positions of equilibrium, but do not travel from one position to another in the body. 
In fluids the molecules are supposed to be constantly moving into new relative positions, 
so that the same molecule may travel from one part of the fluid to any other part. In 
liquids the molecules are supposed to be always under the action of the forces due to 
neighbouring molecules throughout their course, but in gases the greater part of the 
path of each molecule is supposed to be sensibly rectilinear and beyond the sphere of 
sensible action of the neighbouring molecules. 
I propose in this paper to apply this theory to the explanation of various properties 
of gases, and to show that, besides accounting 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 equivalent weight, commonly called the 
Law of Equivalent V olumes. 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 apparently at rest are due 
to the action of invisible molecules in rapid motion is to be found in Lucretius. In the 
exposition wdiich he gives of the theories of Democritus as modified by Epicurus, 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 occasional 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 sunbeam. The language of Lucretius must of course be 
interpreted according to the physical ideas of his age, but we need not wonder that it 
suggested to Le Sage the fundamental conception of his theory of gases, as well as his 
doctrine of ultramundane corpuscles. 
Professor Clausius, to whom we owe the most extensive developments 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 Physique 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 corpuscles also set in 
motion the particles of light and various aethereal media, which in their turn act on the 
molecules of gases and keep up them 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 second memoir, by Prevost, contains new applications of 
the principles of Le Sage to gases and to light. A more extensive application of the 
theory of moving molecules^ was made by HERAPATiif. His theory of the collisions of 
* Poggendorff’s ‘ Annalen,’ Jan. 1862. Translated by G. C. Poster, B.A., Phil. Mag. June 1862. 
t Mathematical Physics, &c., by John Herapath, Esq. 2 yols. London : Whittaker & Co., and Herapath’s 
Eailway Journal Office, 1847. 
