ME. CLEEK MAXWELL ON THE DYNAMICAL THEOEY OE OASES. 
The coefficient of instantaneous rigidity of a gas is therefore p. 1 
The modulus of the time of relaxation is T. > . . . (136) 
The coefficient of viscosity is [h—pV. J 
Now p varies as the density and temperature conjointly, while T varies inversely as 
the density. 
Hence p varies as the absolute temperature, and is independent of the density. 
This result is confirmed by the experiments of Mr. Graham on the Transpiration of 
Gases *, and by my own experiments on the Viscosity or Internal Friction of Air and 
other Gases f. 
The result, that the viscosity is independent of the density, follows from the Dyna- 
mical Theory of Gases, whatever he the law of force between the molecules. It was; 
deduced by myselfj from the hypothesis of hard elastic molecules, and M. O. E. Meyer ^ 
has given a more complete investigation on the same hypothesis. 
The experimental result, that the viscosity is proportional to the absolute temperature, 
requires us to abandon this hypothesis, which would make it vary as the square root of 
the absolute temperature, and to adopt the hypothesis of a repulsive force inversely as 
the fifth power of the distance between the molecules, which is the only law of force 
which gives the observed result. 
Using the foot, the grain, and the second as units, my experiments give for the tem- 
perature of 62° Fahrenheit, and in dry air, 
^=00936. 
If the pressure is 30 inches of mercury, we find, using the same units, 
^=477360000. 
Since pT=(A, we find that the modulus of the time of relaxation of rigidity in air of 
this pressure and temperature is 
5099100000 a second * 
This time is exceedingly small, even when compared with the period of vibration of 
the most acute audible sounds ; so that even in the theory of sound we may consider the 
motion as steady during this very short time, and use the equations we have already 
found, as has been done by Professor Stokes ||. 
Viscosity of a Mixture of Gases. 
In a complete mixture of gases, in which there is no diffusion going on, the velocity 
at any point is the same for all the gases. 
* Philosophical Transactions, 1846 and 1849. 
t Proceedings of the Eoyal Society, Eebruary 8, 1866 ; Philosophical Transactions, 1866, p. 249. 
X Philosophical Magazine, January 1860. § Poggekdobet’s ‘Annalen,’ 1865. 
|| “ On the effect of the Internal Eriction of Eluids on the motion of Pendulums,” Cambridge Transactions, 
vol. ix. (1850), art. 79. 
M 2 
