Coefficients of Slip and Momentum Transfer in Hydrogen, Helium, Air and Oxygen

Coefficients of slip for polished silver oxide surfaces in hydrogen, helium, air and oxygen.—The rotating cylinder method was used, and besides taking precautions to insure spectroscopically pure gases, a continuous circulation of gas was maintained to minimize the effect of the release of occluded gas from the walls of the apparatus. The mean values reduced to 23°C and 76 cm Hg, are: 121(${\mathrm{H}}_2$); 190(He); 68.5(Air); 72.1(${\mathrm{O}}_2$), all times ${10}^{-7\mathrm{}}$ and accurate to about one per cent. Maxwell's coefficient of diffuse reflection f comes out 1.00, 1.00,.98, and.99 for the four gases. To check these results, measurements of the variation of the deflection with pressure were made at low pressures,.0003 to.0015 mm, where the mean free path is considerably greater than the distance between the cylinders. From a comparison of these results with the slopes computed from kinetic theory assuming complete diffuse reflection, $f$ comes out 0.99, 0.99, 0.97 and 0.96 for the four gases, in good agreement with the values given by the slip measurements. The intercepts for zero pressure indicate the presence of some vapor, but this should not affect the slopes appreciably. It is concluded (1) that for these gases practically all molecules are diffusely reflected from the surfaces used, the percentages specularly reflected being close to zero for ${\mathrm{H}}_2$ and He, and about 2 or 3 for air and ${\mathrm{O}}_2$; also (2) that values of $f$ deduced from Maxwell's equation of slip are reliable.