THE DYNAMICAL THEORY OF GASES. 27 



Now we know that in fluids the elasticity of form is evanescent, while 

 that of volume is considerable. Hence such theories will not apply to fluids. 

 In solid bodies the elasticity of form appears in many cases to be smaller in 

 proportion to that of volume than the theory gives*, so that we are forced 

 to give up the theory of molecules whose displacements are functions of their 

 co-ordinates when at rest, even in the case of 'solid bodies. 



The theory of moving molecules, on the other hand, is not open to these 

 objections. The mathematical difficulties in applying the theory are considerable, 

 and till they are surmounted we cannot fully decide on the applicability of the 

 theory. We are able, however, to explain a great variety of phenomena by the 

 dynamical theory which have not been hitherto explained otherwise. 



The dynamical theory supposes that the molecules of solid bodies oscillate 

 about their 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 shew 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 Volumes. It also 

 explains the diffusion of one gas through another, the internal friction of a gas s 

 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 which 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 



* In glass, according to Dr Everett's second series of experiments (1866), the ratio of the elasticity of 

 form to that of volume is greater than that given by the theory. In brass and steel it is less. 

 March 7, 1867. 



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