214 



Table 1 82 



viscosity of gases 

 Variation of Viscosity with Pressure and Temperature 



According to the kinetic theory of gases the coefficient of viscosity p = §(pcl), p being the 

 density, c the average velocity of the molecules, / the average path. Since / varies inversely 

 as the number of molecules per unit volume, pi is a constant and p. should be independent of the 

 density and pressure of a gas (Maxwell's law). This has been found true for ordinary pressures; 

 below ?V atmosphere it may fail, and for certain gases it has been proved untrue for high pres- 

 sures, e.g., C0 3 at 33° and above 5c atm. See Jeans, " Dynamical Theory of Gases." 



If B is the amount of momentum transferred from a plane moving with velocity U and parallel 

 to a stationary plane distant d, and s is a quantity (coefficient of slip) to allow for the slipping 

 of the gas molecules over the plane, then n = (B/U) (d + 25); s is of the same magnitude as I, 

 probably between .7 (Timiriazeff) and .9 (Knudsen) of it; at low pressures d becomes negligible 

 compared with 25 and the viscosity should vary inversely as the pressure. 



7 depends only on the temperature and the molecular weight, "varies as the v"7\ but fi 

 has been found to increase much more rapidly. Meyer's formula, pt = po(i + at), where a 

 is a constant and no the viscosity at o° C, is a convenient approximate relation. Sutherland's 

 formula (Phil. Mag. 31, 1893), 



is the most accurate formula in use, taking in account the effect of molecular .forces. It holds 

 for temperatures above the critical and for pressures following approximately Boyle's law. It 

 may be thrown into the form T = KT^/p, — C which is linear in terms of T and T^/p, with a 

 slope equal to K and the ordinate inter ?pt equal to -C. See Fisher, Phys. Rev. 24, 1907, 

 from which most of the following table is taken. Onnes (see Jeans) shows that this formula does 

 not represent Helium at low temperatures with anything like the accuracy of the simpler formula 

 p = Mo(r/273.i) n . 



The following table contains the constants for the above three formulae, T being always the 

 absolute temperature, Centigrade scale. 



.273/ 



Gas. 



Air 



Argon . . . 

 Carbon mo- 

 noxide . 

 Carbon dioxide 

 Chloroform 

 Ethylene. . 

 Helium. . . . 

 Helium .... 



Gas. 



Hydrogen. . . . 



Krypton 



Neon 



Nitrogen 



Nitrous oxide, 



N 2 



Oxygen 



Xenon 



72 

 1 88 



3*3 



131 



K 



X 10 7 



66 



143 



172 

 176 



.00269 

 •00345 



.69 



•74 



•93 



•79 



* The authorities for n are: Air, Rayleigh; Ar, Mean, Rayleigh, Schultze; CO, CO2, N 2 , 

 N 2 0, von Obermayer; Helium, Mean, Rayleigh, Schultze; 2d value, low temperature work of 

 Onnes; H>, 2 , Mean, Rayleigh, von Obermayer. 



Smithsonian Tables. 



