252 Free Path Phenomena [CH. xn 



1. Calculated by Meyer from Graham's Transpiration Experiments. 

 Meyer, Kinetic Theory, p. 192. 



2. From transpiration experiments by von Obermayer. Meyer, Kinetic 

 Theory, p. 192, or Weinstein, Thermodynamik und Kinetik der Korper, 

 i. p. 332. 



3. From oscillation experiments by Kundt and Warburg. The experi- 

 ments were conducted at 15 C., but the coefficients of viscosity are corrected, 

 according to the experimentally observed variation with the temperature, to 

 C. Meyer, Kinetic Theory, p. 192. 



4. Lord Rayleigh, Proc. Roy. Soc. LIX. p. 108. 



5. The value for air is the mean value of a great number of experiments, 

 taken from Landolt and Bornstein's tables. Meyer, also considering a great 

 number of experiments, gives the value '000172 (Kinetic Theory, p. 197). 



The mean values in the fourth column are not always the mean of the 

 observations recorded in the second column. Where possible I have used 

 the mean values given in Landolt and Bornstein's tables, which are derived 

 from the consideration of a greater number of experimental data than those 

 recorded in the second column. These values are marked by an asterisk. 



The agreement of the values of \cr given in the last column, with those 

 already calculated from the deviations from Boyle's law (p. 141), is seen to be 

 tolerably good. 



Variation of K with Density. 



307. Since p = mv, equation (581) can be put in the form 



* = -44-.!S- (582), 



V27T<7 a 



shewing that theoretically K is independent of the density of the gas. 



Indeed, whatever structure we assume for the molecules of the gas, it is 

 clear that I will, to a first approximation, vary inversely as the number of 

 molecules per unit volume of the gas. Hence equation (573) gives a value 

 of K which is independent of v, and we obtain Maxwell's law : 



The coefficient of viscosity of a gas is independent of its density. 



In spite of its apparent improbability, this law was predicted by Maxwell 

 on purely theoretical grounds, and its subsequent experimental confirmation 

 has constituted one of the most striking triumphs of the Kinetic Theory. 



