Conduction of Heat hi/ Rarefied Gases. 199 



in a different way in the two vessels, being for instance 

 in air at the pressure p = 0"0I mm. in vessel I., K = 0'23, and 

 in vessel II., K = 0*56. 



(4) It is explained, however, by introducing a discon- 

 tinuity of temperature according to formula (1), at the surface 

 between the gas and the solid ; the values of the coefficient 

 7, calculated on this supposition, are in fact very nearly the 

 same for both vessels ; they are inversely proportional to the 

 pressure, therefore proportional to the free path of molecules 

 of the gas — exactly the same law which has been found by 

 the before-named experimenters for the coefficient of slipping. 

 The mean value, derived from a great number of observations, 

 is for air in contact with glass 



7=0-0000171 cm. — , 

 P 



for hydrogen 



7 = 0-000129 cm. — , 

 P 



or by using the values of the mean free path calculated 

 by O. E. Meyer : 



7=l-70\, 7=6-96\. 



Considering the wide range of pressures experimented 

 upon, which correspond in some cases to a reduction of the 

 apparent conductivity to less than T -Jy- of its normal value, 

 the agreement between observations and calculations, as 

 shown by the constancy of the coefficient y/X, must be con- 

 sidered very satisfactory. 



Comparison with Mr. C. F. Brush's Experiments. 



8. Mr. Brush's experiments were not undertaken with 

 the same express intention as these, but as they are made 

 evidently with great carefulness, and extend over a great 

 range of pressures, it is very interesting to look into them 

 from the theoretical point of view, and it is very satisfactory 

 to find the best agreement with the accepted theories, and 

 also with the conclusions drawn in the above from my experi- 

 ments. 



According to w r hat has been said in the beginning of this 

 paper, and to Mr. Brush's own interpretation of his results, 

 the " sether line " in his diagrams gives the effect of pure 

 radiation; the remaining part of the ordinates is due to con- 

 vection-currents and conduction. 



The effect of the first ones is very considerable in the larger 

 bulb, much less in the smaller one ; it was not perceptible at 

 all in my experiments w T ith the vessel I. 



