EVAPORATION AND CONDENSATION 299 



both helium and nitrogen there is a slight tendency for a to decrease with 

 rising temperature. 



The probable relationship between the accommodation coefficient and 

 the reflectivity of molecules from surfaces will be discussed below, together 

 with other experimental data. 



THE "SLIP" IN GASES AT LOW PRESSURES 



The viscosity effect analogous to the temperature discontinuity at a 

 surface has been called the "slip." This efifect, first observed by Kundt 

 and Warburg, has been the subject of much recent study. Knudsen ^ 

 calculates the coefficient of slip on the assumption that the molecules strik- 

 ing any surface are emitted from it in directions which are absolutely 

 independent of the original directions of incidence. By elaborate experi- 

 mental investigations Knudsen obtains confirmation of his theoretical con- 

 clusions and thus justifies his assumptions. Smoluchowski ^ Timiriazeff,^** 

 Gaede,^^ and Baule,^^ however, conclude, on experimental and theoretical 

 grounds, that the directions of the emitted molecules are not entirely in- 

 dependent of the directions of the incident molecules. 



It is of interest to note that Gaede, as a result of extremely careful ex- 

 perimental work, concludes that the amount of specular reflectivity is 

 negative. That is, he finds at pressures above .001 mm., and up to 20 mm., 

 that the amount of gas which flows through a tube is less than that calcu- 

 lated by Knudsen on the assumption of completely irregular reflection. 

 Gaede draws the conclusion that a certain fraction (sometimes as large as 

 50 per cent.) of the incident molecules tend to return after collision in the 

 direction from which they came. The other observers, however, all find that 

 the amount of gas flowing through tubes is greater than that calculated by 

 Knudsen's formula. 



Knudsen, on the other hand,^^ carried out further measurements with 

 hydrogen at low pressures and found that there cannot be a specular re- 

 flection of hydrogen molecules amounting to more than about one per cent. 



Millikan ^'^ has calculated the coefficient of slip from his measurements 

 on the fall of small spheres and has concluded that the slip in air is about 

 10 per cent, and in hydrogen about 8 per cent, greater than would be ex- 

 pected, according to Knudsen's assumption regarding the absence of 

 specular reflection. 



The experimental evidence on the slip of gases at low pressures does 



^ Ami. Phys., 28, 75, 1908. ^ Plrl Mag., 46, 199, 1898. 



*° Ann. Phys., 40, 971, 1913. " Ann. Phys., 41, 289, 1913. 



^* Ann. Phys., 44, 145, 1914. ^^ Ann. Phys., 35, 389, 191 1. 



** Discussion before the American Phys. Soc, Chicago, Nov. 30, 1915. 



