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Maxwell's next contribution to the Kinetic Theory of Gases was the 

 Bakerian Lecture to the Royal Society in 1866, " On the Viscosity or 

 Internal Friction of Air and other Gases." This is an account of 

 experiments to determine the coefficient of gaseous friction. He 

 employed for this purpose the oscillations of three disks placed be- 

 tween fixed disks, and capable of motion about a common axis, the 

 quantity observed being the successive times of oscillation of the 

 movable disks. The chief results of this paper are, that the coefficient 

 of friction is (1) independent of the density of the gas, (2) propor- 

 tional to its absolute temperature. 



The second of these laws involves the rejection of the hypothesis 

 of the impact of hard elastic particles, leading as it does to propor- 

 tionality of the square root of the temperature. Accordingly, Max- 

 well's next memoir " On the Dynamical Theory of Gases," following 

 closely on the Bakerian Lecture, propounds a theory in closer agree- 

 ment with his experiments. Instead of hard elastic spheres, he now 

 substitutes " small bodies or groups of smaller molecules, repelling one 

 another with a force whose direction passes very nearly through the 

 centres of gravity of the molecules, and whose magnitude is repre- 

 sented very nearly by some function of the distance of the centres of 

 gravity." The necessity of supposing groups of molecules arises from 

 the fact that, in a body of invariable form, the motions of its parts 

 relatively to the centre of gravity consists entirely of rotations which 

 the supposed force is incapable of generating, whereas a group of 

 loosely-connected bodies may have oscillations among themselves as 

 well as rotations. That the energy of the system cannot be entirely 

 translational had been previously pointed out by Clausius. 



In this memoir the statistical method is restated more fully, clearly, 

 and systematically than before, but the only law of force between the 

 molecules which leads to simple results is the inverse fifth power 

 of the distance. This law being adopted, the general equations re- 

 sulting are applied to a variety of problems : in (a), proving Dal ton's 

 law for the pressure of a mixture of gases; (&), explaining Graham's 

 experiments on the diffusion of carbonic acid and air; (c), proving 

 Graham's law for the inter diffusion of gases contained in different 

 vessels connected by a very small hole; (cl), finding the equilibrium 

 condition as regards temperature for a mixture of gases, and again 

 establishing Gay-Lussac's law ; (e), finding expressions for the specific 

 heats of gases on Clausius' assumption that the whole energy bears a 

 constant ratio to that of translation; (/), proving the laws of vis- 

 cosity, the same as those stated above; (g), finding the viscosity of a 

 mixture of gases, which, if not in close agreement with the results of 

 Graham's experiments, at least exhibits the same general peculiarities ; 

 (/&), proving that in a column of air the temperature is independent 

 of gravity ; (i), deducing an expression for the conductivity. 



