Stress and Strain on the Properties of Matter. 489 



ratio of the fifth power of the distance. Now, Clerk Maxwell 

 in his valuable researches on the viscosity of gases arrived at the 

 important result that the viscosity of a gas is independent of its 

 density, and proportional to the temperature measured from the 

 absolute zero of the air-thermometer.* - Maxwell- points out that the 

 constancy of the viscosity of a gas for all changes of density when 

 the temperature is constant is a result of the " Dynamical Theory of 

 Gases," whatever hypothesis we adopt as to the mode of action 

 between the molecules when they come near one another ; but that if 

 the viscosity be as the first power of the absolute temperature, then 

 in the dynamical theory, which is framed to explain the facts, we 

 must assume that the force between two molecules is proportional 

 inversely to the fifth power of the distance between them.f It is true 

 that we cannot regard the distance between two consecutive molecules 

 of a solid as large compared with the diameter of the molecules ; 

 indeed, we must, on the contrary, assume the molecules to be nearly 

 touching each other; consequently Poisson's formula would require 

 modification before it could be strictly adapted to the case in point. 

 It is equally true that the equation e X a 7 =a constant cannot possibly 

 hold good for all temperatures, inasmuch as the value of e decreases 

 more with rise of temperature than a 7 increases, and that conse- 

 quently the function fr must contain the temperature. Nevertheless, 

 the coincidence of the results as regards the law of force between 

 two molecules obtained by Wertheim and Maxwell in quite different 

 ways, seemed to warrant me in making more extended inquiries in 

 this direction, and with this object I set about making a most careful 

 determination of the thermal capacity of each of the different metals 

 which had been used by me in the earlier portion of this inquiry. 

 My object in doing so will be seen from what follows : — 



If we denote the atomic mass by A, the density by A, the thermal 

 capacity per unit mass by C m , and the thermal capacity per unit 

 volume by C v , we have the following relations : — 



* "Phil. Trans.," 1866, vol. 126, Part I. 



t I find that Maxwell in a later memoir (" Phil. Trans.," 1879, Part I) expresses 

 an opinion that the balance of evidence is against this law. On the contrary, the 

 results obtained by Heen (" Bulletins de l'Academie "Royale de Belgique," 3e Serie, 

 t. iv, 1882), seem to prove most conclusively that for liquids and solids of a definite 

 constitution, as shown by their thermal expansibility at different temperatures, the 



C w xA=a constant ; 



=A X C m ; 



e X a 7 =a constant ; 



