TABLE 695.— MEAN FREE PATHS OF MOLECULES 541 



Let L = mean free path, 5 = molecular diameter. Then 



and 7) = 0A99pvcL (2) 



when tj z=z coefficient of viscosity 



p = density of gas at given pressure and temperature 

 Unit of 7) is the poise = g cm" 1 sec" 1 



Hence L = 1.1451 X 10 4 -J- \l - cm (3) 



=8 - 589 p^Vl cm (4) 



and ■■- 2714X10- Jht. 



?= " nAW J MT cm' (5) 



V, as a function of 7, is given by the relation 



where y<, = value at T„, 77 =r value at T and C" is known as the Sutherland constant. For 

 short ranges of temperature, the exponential relation is used, of the form 



( VT /r, ) = (T/T.)' (7) 



In Tables 691 and 696, which give values of L, S and related data for a number of gases 

 and vapors, 



tj 15 = coefficient of viscosity at 15°C 



770= " " " " 0°C 



and i) iS = " 25°C 



x = value of exponent in equation (7) 



L 1 = value of mean free path (in cm) at 0°C and 

 1 mmHg 



L o 760 = value of mean free path (in cm) at 0°C and 

 760 mmHg 



L25 1 = value of mean free path (in cm) at 25°C and 

 1 mmHg 



Z.25 780 = value of mean free path (in cm) at 25 °C and 

 760 mmHg 



5 ;= value of molecular diameter (in cm) at 0°C 



N, = 1.154/5 2 = of molecules per cm 2 to form a mono- 

 layer (assuming that the spacing is that of 

 close-packed or face-centered lattice) 



u> = collision-frequencv at 25°C and 760 mmHg 



= Va/ Lis 



For the vapors of H 2 and Hg (Table 691), P = vapor pressure in mmHg at the tem- 

 perature /, and L t and 5, denote the values of the mean free path and diameter, respec- 

 tively, at this temperature. For H 2 vapor, C = 650 and 7715 = 9.26 X 10" s . For Hg, C = 

 942.2 and value of 77 at t = 219.4°C was used. The values of tj and S„ /or Hg at 0°C are 

 those given by Jeans. 



SMITHSONIAN PHYSICAL TABLES 



