THE DYNAMICAL THEORY OF GASES. 71 



This result is confirmed by the experiments of Mr Graham on the Tran- 

 spiration 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 Dynamical Theory of Gases, whatever be the law of force between the 

 molecules. It was deduced by myself J from the hypothesis of hard elastic 

 molecules, and M. 0. 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 temperature of 62* Fahrenheit, and in dry air, 



/* = 0-0936. 



If the pressure is 30 inches of mercury, we find, using the same units, 



p = 477360000. 



Since pT=fi, we find that the modulus of the time of relaxation of 

 rigidity in air of this pressure and temperature is 



f 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 ||. 



* Philosophical Transactions, 1846 and 1849. 



t Proceedings of the Royal Society, February 8, 1866 ; Philosophical Transactions, 1866, p. 249. 

 J Philosophical Magazine, January 1860. [Vol. I. xx.] 

 Poggendorff'a Annalen, 1865. 



|| " On the effect of the Internal Friction of Fluids on the motion of Pendulums," Cambridge Transac- 

 tions, Vol. ix. (1850), art. 79. 



