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



bodies. Indeed the properties of a body supposed to be a uni- 

 form plenum may be affirmed dogmatically, but cannot be ex- 

 plained mathematically. 



Molecular theories suppose that all bodies, even when they 

 appear to our senses homogeneous, consist of a multitude of 

 particles or sniall parts, the mechanical relations of which con- 

 stitute the properties of the bodies. Those theories which sup- 

 pose that the molecules are at rest relatively to the body may be 

 called statical theories, and those which suppose the molecules 

 to be in motion, even while the body is apparently at rest, may 

 be called dynamical theories. 



If we adopt a statical theory, and suppose the molecules of a 

 body kept at rest in their positions of equilibrium by the action 

 of forces in the directions of the lines joining their centres, we 

 may determine the mechanical properties of a body so constructed, 

 if distorted so that the displacement of each molecule is a func- 

 tion of its coordinates when in equilibrium. It appears from 

 the mathematical theory of bodies of this kind, that the forces 

 called into play by a small change of form must always bear a 

 fixed proportion to those excited by a small change of volume. 



Now we know that in fluids the elasticity of form is evanes- 

 cent, 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 coordinates 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 ap- 

 plying 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 rela- 

 tive 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 



* [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 3 1867.] 



