28 THE DYNAMICAL THEORY OF OASES. 



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 Mfoanique, 

 publics 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 ethereal 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 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 Herapathf. His theory of the collisions of 

 perfectly hard bodies, such as he supposes the molecules to be, is faulty, 

 inasmuch as it makes the result of impact depend on the absolute motion of 

 the bodies, so that by experiments on such hard bodies (if we could get them) 

 we might determine the absolute direction and velocity of the motion of the 

 earth J. This author, however, has applied his theory to the numerical results 

 of experiment in many cases, and his speculations are always ingenious, and 

 often throw much real light on the questions treated. In particular, the theory 

 of temperature and pressure in gases and the theory of diffusion are clearly 

 pointed out. 



* PoggendorflTs Annalen, Jan. 1862. Translated by G. C. Foster, B.A., Phil. Mag. June, 1862. 



t Mathematical Physics, <fea, by John Herapath, Esq. 2 vols. London: Whittaker and Co., and 

 Herapath'g Railway Journal Office, 1847. 



* Mathematical Physics, ttc., p. 134. 



