206 On the Conduction of Heat by Rarefied Gases. 



or /, and in consequence also 7, different for different solid 

 surfaces, but it is remarkable that its value depends so much 

 also on the nature of the gas. In hydrogen, at least, the 

 term depending on /3 must be several times greater than the 

 first term independent of it, whereas it is comparatively small 

 for air. 



I believe an explanation may be afforded by the following- 

 reasoning : — The molecules of the gas, striking against the 

 particles (molecules?) of the solid body, will be different 

 generally from them in respect to size or mass. Now the 

 impact between two bodies generally tends towards producing 

 an equalization of their vis viva, but it is easy to show that 

 this equalizing effect is so much the smaller, the greater the 

 difference is between the masses of the colliding bodies. 

 Therefore /3 will be great, and y too, if the molecules of the 

 gas have a much smaller mass than those of the solid body, 

 which certainly is the case in the above example for hydrogen 

 in contact with glass. 



It seems to be possible to arrive by similar arguments at 

 conclusions about the mass of the particles of the solid, the 

 motion of which constitutes the heat of the body, and about 

 which we do not know anything at present ; but as this 

 requires a great deal more experimental data, I am first going 

 to carry on further such experimental investigations. 



It would be very interesting, too, to verify some other con- 

 clusions of the kinetic theory of gases, easily arrived at, con- 

 cerning the conduction of heat betw r een solid walls the 

 distance of which is much less than the mean length of free 

 path (for instance with high exhaustions) ; in this case the 

 quantity of heat carried over by the molecules of the gas 

 ought to be the same as if — with unchanging k, and y put 



equal zero — the plates were at the distance 2\- — ^ (of 



course, apart from radiation) ; and this quantity ought to be 

 independent of the distance of the plates, provided this is 

 very small in comparison with \, 



