Mr. J. C. Maxwell on the Dynamical Theory of Gases. 211 



in which the molecules move in straight lines without acting on 

 each other at all, deserves notice from those who speculate on 

 theories of physics. 



The fluidity of our medium is therefore due to the mutual 

 action of the molecules, causing them to be deflected from their 

 paths. 



The coefficient of instantaneous rigidity of a gas isl 



therefore p. I ,, ~. 



The modulus of the time of relaxation is T. I 



The coefficient of viscosity is fj,=pT. J 



Nowp varies as the density and temperature conjointly, while 

 T varies inversely as the density. 



Hence fju varies as the absolute temperature, and is indepen- 

 dent 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 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 tem- 

 perature, 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 experi- 

 ments give, for the temperature of 62° Fahr., and in dry air, 

 p, = 0-0936. 



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

 same units, 



^ = 477360000. 



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

 of rigidity in air of this pressure and temperature is 



of a second. 



5 9 9 10 



This time is exceedingly small, even when compared with the 



* Philosophical Transactions, 1846 and 1849. 



t Proceedings of the Royal Societv, February 8, 1 866 j Philosophical 

 Transactions, 1866, p. 249. 



X Philosophical Magazine, January 1860. 

 § Poggendorff's Annalen, 1865. 



P2 



