THE KINETIC THEORY OF MATTER. 



147 



the square root of the absolute temperature, but more rapidly, apparently 

 indicating that at a fixed density the free path also increases with the 

 temperature (Meyer, I.e., p. 216). 



In the equation ry = ^ , substituting for V from the equation 

 8 



1 = 3p, we obtain 



L = TK / 



whence L can be found when rj is known. 



Since the number of collisions per second, or the collision frequency, 

 is equal to mean speed/mean free path, we can also calculate the collision 

 frequency, i.e., the number of collisions per second, 



Below we give the values obtained for several gases at 760 mm. 

 pressure from our approximate numbers, which are sufficiently near the 

 truth to show the order of the magnitudes involved. We give the value 

 for water vapour on the supposition chat it could be compressed to I 

 atmosphere at C. Air is regarded as a simple gas. 



Had we used the exact formulae we should have obtained values for 

 the M.F.P. about ^ greater, and of course the collision frequencies would 

 be proportionately reduced. 



Conduction Of Heat in Gases. The conduction of heat in gases 

 can be explained in a manner similar to that in which viscosity has been 

 explained. If there is a temperature slope in a gas, there is a continual 

 passage of more energetic molecules from the hot side across any given 

 plane, and a continual passage of less energetic molecules from the 

 cold side, with the net result that there is a transfer of energy down 

 the slope. The following investigation, though very incomplete, gives 

 an estimate of the amount of conduction. 



Let EF, Fig. 78, be 1 square cm. in a layer of which the tempera- 

 ture is 0, and let the slope of temperature perpendicular to EF be -j-- 



Y 



Let us take mass p as passing through EF per second from the 



upper sicle and as having on the average the temperature of the layer AB 

 a distance from EF equal to L, the M.F.P. i.e. a temperature + L-r-. 



