202 Dr. M. Smoluchowski de Smolan 



on 



radiation being eliminated altogether, in the above-described 

 manner. 



The very simplest way of explanation, however, is afforded 

 by the kinetic theory of gases, which, in quite a similar way 

 also explains the slipping of a gas moving along the surface 

 of a solid, as has been shown by Kundt and Warburg (and 

 afterwards by Maxwell too) . 



Suppose two plane parallel plates, at different temperatures, 

 separated by a layer of gas, the thickness of which may be 

 great in comparison with the mean length of free path of the 

 molecules. 



The temperature at any point of the gas is the mean value 

 of the vis viva of the molecules travelling from the colder to 

 the hotter plate and in the opposite direction. 



Now consider the state of things near the surface of the 

 cold one PP'. The molecules going towards it are endowed 

 with a greater energy than that which would correspond to 

 the temperature of the plate, since they are coining from 

 hotter regions ; those going out from it, after rebounding, 

 have only its exact temperature, if there is a complete 

 equalization of temperature (resp. energy) during the act 

 of impact on the plate ; therefore the mean value of both 

 must be greater than the temperature of the plate itself; 

 there must be a finite break in the distribution of tempera- 

 ture*. 



In reality this will be still greater than would follow from 

 this reasoning, since it is not probable — and is disproved by 

 the experiments, as will be shown afterwards — that the mole- 

 cules of the gas assume, at one impact only, the exact 

 temperature of the body. 



I have tried to make an approximate calculation of these 

 effects after both theories of molecular action developed until 

 now, Clausius' and Maxwell's, and the results are quite similar, 

 only differing in the numerical value of the coefficients. 



10. The first one, the theory considering molecules as 

 elastic balls, requires several simplifying suppositions in order 

 to allow of an easy reckoning, which let the result appear 

 only as a rough approximation. 



Then the condition that the flow of heat be stationary 

 = const, can be expressed by the equation 



* As I notice now, something similar has been pointed out by 

 Dr. Johnstone Stoney in his very suggestive paper " On the Penetration 

 of Heat across La)ers of Gas ' (Phil. Mag. vol. iv. p. 424, v. p. 457), 

 the understanding of which is rendered difficult, however, in consequence 

 of wrong reasonings about the conduction of heat. 



