348 



nication from Prof. Kamerlingh Onnes the gas probably still con- 

 tained a trace of nitrogen. For this reason I purified it once 

 more by Gehihoff's method "'). 



Whicliever of the known experimental methods''') one may choose 

 for the investigation of the heat conductivity of gases, there are 

 always two sources of error which will have to be specially con- 

 sidered: the difference of temperature between the surface and the 

 gas in contact with it and the convection. With diminution or 

 pressure of the gas the influence of the convection becomes sinaller, 

 that of the temperature-drop greater. For the latter, similarly to the 

 analogous quantity in the internal friction, the slipping alojig the 

 wall "j, depends on the ratio of the mean free path to the dimensions 

 of the apparatus. 



Whereas it has been found impossible to calculate the influence 

 of convection *) on the heat conduction, M. Knudsen ') and M. von 

 Smoluchowski ") have been able to bring the theoretical investigation 

 of the temperature-drop to a successful issue. 



In accordance with Kundt and Warburg ') the temperature-drop 

 A© at the solid wall is defined as follows: 



d& 



an 



where 7i i-epresenls the direction of the normal and C') the tetnjie- 

 rature. Kundt and Warburg by their experiments established the 

 fact, that 7 is proportional to the mean free path ).. 



Von Smotaichovvski based his first investigation on the kinetic theory 

 as developed by Clausius and was led to the following approximate 

 formula, in which I have introduced the accommodation-coefficient as 

 defined by Knudsen ^). 



■//;, = 0,70 -f -V-- in 



oa 

 Later on Smoluchowski made a new calculation of y, in this 



1) Gehlhoff, Verli. d. D. Pliysik. Ges. 13. (l'.)ll) p. 271. 



2) Gomp. A. VViNKELMANN, Hanclbuch der Fhysik ill, 1906 p. 525. 



3) H. Kamerlingh Onnes, G. Dorsman and S. Weber. These Proc. XV (2) 

 p. 1386 



4) A. Oberbe'jk. Wied. Ann. VII, 1879, p. 291 and L. Lorenz, Wied. Ann. 

 XIII. 1881, p. 582. 



6) M. Knudsen. Ann. d. Ph. (4) 3 4, (1911), p. 655. 



«) M. VON Smoluchowski, R. v. Smolan. Wien. Sitz. Ber. [2al 107, (1898), p. 

 304; 108, (1S99), p. 5. 



7) A. Kundt and E. Warburg. Pogg. Ann. 156, (1875), p. 177. 

 '') M. Knudsen. loc. cit. p. 608. 



