ARISING FROM INEQUALITIES OF TEMPERATURE. 685 



be placed within the finite portion of gas which, as we have said, is already 

 in equilibrium. The equilibrium will not be disturbed. We may introduce 

 any number of spheres at different temperatures into the portion of gas, so as 

 to form a body of any shape, heated in any manner, and when the flow of 

 heat has become steady the whole system will be in equilibrium. 



12. How, then, are we to account for the observed fact that forces act 

 between solid bodies immersed in rarified gases, and this, apparently, as long 

 as inequalities of temperature are maintained ? 



I think we must look for an explanation in the phenomenon discovered in 

 the case of liquids by Helmholtz and Piotrowski *, and for gases by Kundt and 

 Warburg t, that the fluid in contact with the surface of a solid must slide 

 over it with a finite velocity in order to produce a finite tangential stress. 



The theoretical treatment of the boundary conditions between a gas and 

 a solid is difficult, and it becomes more difficult if we consider that the gas 

 close to the surface is probably in an unknown state of condensation. We 

 shall therefore accept the results obtained by Kundt and Warburg on their 

 experimental evidence. 



They have found that the velocity of sliding of the gas over the surface 

 due to a given tangential stress varies inversely as the pressure. 



Q 



The coefficient of sliding for air on glass was found to be G = - 



centimetres, where p is the pressure in millionths of an atmosphere. Hence 

 at ordinary pressures G is insensible, but in the vessels exhausted by Mr Crookes 

 it may be considerable. 



Hence, if close to the surface of a solid there is a tangential stress S, 

 acting on a surface parallel to that of the body in a direction h parallel 

 to that surface, there will also be a sliding of the gas in contact with the 



solid over its surface in the direction h with a finite velocity =--. 



r* 



13. I have not attempted to enter on the calculation of the effect of 

 this sliding motion, but it is easy to see that if we begin with the case in 

 which there is no sliding, the instantaneous effect of permission being given 



* Wiener Sitzb., xl. 1860, p. 607. 

 t Pogg. Ann., civ. 1875, p. 337. 



