75 VISCOSITY OF GASES 175 



called internal friction, 1 which has already been given in 

 general terms. 



If, according to that analysis, the internal friction con- 

 sists in a transfer of the motion of flow from one layer to 

 another, and in a transformation of the motion so transferred 

 into heat-motion, then no gas can be free from internal 

 friction. For the molecules carry over velocity from one 

 place to another by their very hither-and-thither motions, 

 and by their frequent encounters the directions of their 

 motions undergo continual change, so that the flow of the 

 gas which existed at the beginning, and in which all the 

 molecules moved in the same direction, must gradually 

 be changed into a motion that proceeds in all possible direc- 

 tions, that is, into heat. The phenomenon of internal 

 friction is therefore explained by the kinetic theory in an 

 unforced manner. 



Our theory, however, gives more than this explanation 

 on general lines ; since the friction is caused by the exceed- 

 ingly rapid motion of the molecules, the theory justifies us 

 in concluding, in agreement with experiment, that gases 

 possess no little viscosity, and that this viscosity will in- 

 crease with the temperature, since the speed increases with 

 the temperature. 



We obtain the amount of the friction brought into play 

 by summing the momentum which is carried over from one 

 layer into another by reason of the heat-motion of the mole- 

 cules, for on our explanation this momentum and the friction 

 are identical. To form directly the measure of the viscosity 

 given above, i.e. the coefficient of viscosity, I start for the 

 calculation of this sum from the illustration given in 74, 

 wherein a gas flows over a horizontal surface in the imme- 

 diate neighbourhood of which it is at rest, elsewhere flowing 



1 Maxwell, Phil Mag. 1860 [4] xix. p. 31. 0. E. Meyer, Pogg. Ann. 

 1865, cxxv. p. 586. Maxwell, Phil. Mag. 1868 [4] xxxv. p. 209. V. von 

 Lang, Wiener Sitzungsber. 1871, Ixiv. Abth. 2, p. 485; Pogg. Ann. 1871, 

 cxlv. p. 290 ; Einleitung in die theor. Physik. 1867, p. 526. Stefan, Wiener 

 Sitzungsber. 1872, Ixv. Abth. 2, p. 360. Boltzmann, ibid. 1872, Ixvi. Abth. 

 2, p. 324 ; 1880, Ixxxi. Abth. 2, p. 117 ; 1881, Ixxxiv. Abth. 2, pp. 40, 1230 ; 

 1887, xcvi. Abth. 2, p. 891. 



