Mr. J. C. Maxwell on the Dynamical Theory of Gases. 135 



longer than that of the outer parts. This phenomenon was ob- 

 served by Weber in silk fibres, by Kohlrausch in glass fibres, 

 and by Liiyself in steel wires. 



In the case of a collection of moving molecules such as we 

 suppose a gas to be, there is also a resistance to change of form 

 constituting what may be called the linear elasticity, or iC rigi- 

 dity w of the gas, but this resistance gives way and diminishes 

 at a rate depending on the amount of the force and on the 

 nature of the gas. 



Suppose the molecules to be confined in a rectangular vessel 

 with perfectly elastic sides, and that they have no action on 

 one another, so that they never strike one another, or cause each 

 other to deviate from their rectilinear paths. Then it can easily 

 be shown that the pressures on the sides of the vessel due to the 

 impacts of the molecules are perfectly independent of each other, 

 so that the mass of moving molecules will behave, not like a 

 fluid, but like an elastic solid. Now suppose the pressures at 

 first equal in the three directions perpendicular to the sides, 

 and let the dimensions a, b, c of the vessel be altered by small 

 quantities, 8a, 8b, 8c. 



Then if the original pressure in the direction of a was p, it 

 will become 



/.. 8a 8b 8c\ 



or if there is no change of volume, 



p a 



showing that in this case there is a " longitudinal " elasticity of 

 form of which the coefficient is 2p. The coefficient of " rigi- 

 dity" is therefore =p. 



This rigidity, however, cannot be directly observed, because 

 the molecules continually deflect each other from their rectili- 

 near courses, and so equalize the pressure in all directions. 

 The rate at which this equalization takes place is great, but not 

 infinite; and therefore there remains a certain inequality of 

 pressure which constitutes the phenomenon of viscosity. 



I have found by experiment that the coefficient of viscosity in 

 a given gas is independent of the density, and proportional to 



the absolute temperature, so that if ET be the viscosity, ETx -. 



But E —p ■ therefore T, the time of relaxation, varies inversely 

 as the density and is independent of the temperature. Hence 

 the number of collisions producing a given deflection which 

 take place in unit of time is independent of the temperature, 



