Conduction of Heat by Rarefied Gases. 193 



gradient of temperature, which he supposed to be distributed 

 everywhere in a continuous manner. 



Poisson, however, constructing his theory on the sup- 

 position of a special mechanism of conduction (defined, in a 

 somewhat vague way, as " molecular radiation "), inferred 

 from it that there must be a discontinuity of temperature- 

 distribution at the surface of separation between two bodies 

 of different conductivities when there is going on an exchange 

 of heat between them. 



The difference of temperature ^ — 2 (ordinarily very 

 small) at both sides of this surface should be proportional 

 to the flow of heat passing through it, and therefore also 

 to the slope of temperature in either of them. This is 

 expressed by the equation 



',-*-*!£ •••;••; (1) 



where y may be called the coefficient of discontinuity of 

 temperature. 



But until now there has been no experimental evidence for the 

 existence of such a discontinuity, and the coefficient 7 has been 

 supposed commonly to be zero, so much the more as Poisson's 

 theory of " molecular radiation " has lost all credit and the 

 kinetic nature of the conduction of heat is generally 

 accepted. 



Kundt and Warburg, however, who discovered the slipping 

 of rarefied gases moving along solid surfaces, thought it 

 probable that something analogous — viz. a discontinuity of 

 temperature — may arise in such gases when conduction 

 of heat is going on. 



To decide whether this is the case or not, was the scope of 

 the present work. 



Praxis and Theory of Experiments. 



3. The chief difficulty in examining the conduction of heat 

 by gases consists in separating the pure conductive effect 

 from the effects of the convection-currents and of the direct 

 radiation, which always are present to some degree where 

 conduction is going on. 



The convective currents can be avoided to a great extent 

 by a proper shape of the vessel containing the gas, so as 

 to leave the least possible free space for their development. 

 Besides, their effect can be still diminished, and made prac- 

 tically negligible, by rarefying the gas ; since its viscosity 

 remains the same, whereas the disturbing forces decrease 

 proportionally with the density. (See Kundt and Warburg, 

 Pogg. Ann. civ. p. 156.) 



