439 



APPENDIX IV 

 VISCOSITY OF GASES 



44*. General Remarks on Viscosity 



THE viscosity or internal friction of gases is, according to this 

 theory, nothing else than the transference from one place to 

 another of the momentum of translation or flow of the medium by 

 means of the heat-motions of its particles. In order to find the 

 magnitude of the force exerted by one layer upon another by 

 reason of viscosity, we have to determine the amount of momentum 

 which is carried over in unit time by the molecules as they move 

 backwards and forwards across the separating plane or surface of 

 friction. 



Starting from this conception of the action, which has 

 been explained more at length in 73, I published, in a memoir l 

 that appeared in 1865, a theory of viscosity which I will 

 here first reproduce. However, since for easiness of calculation 

 I then made the not strictly accurate assumption that all the 

 molecules move with the same speed, I shall follow up this cal- 

 culation, made in accordance with Clausius' assumption, by 

 another which I shall found on Maxwell's law of the distribu- 

 tion of speeds. 



46*. Theory of Viscosity on the Assumption of 

 Equal Speeds for all Molecules 



While I now ascribe at first to all molecules of the gas equal 

 molecular or heat motions that occur equally in all directions, 

 I assume, further, a forward movement of given magnitude and 

 direction ; of this I assume the direction to be the same at all 



1 ' Ueber die innere Eeibung der Gase,' erste Abhandlung. ' Ueber 

 den Einfluss der Luft auf Pendelschwingungen,' Pogg. Ann. 1865, cxxv. 

 p. 586. 



