422 Prof. Clausius on the Conduction of Heat by Gases. 



we call conduction of heat, and which we have to consider in 

 the sequel*, 



II. Behaviour of the molecules emitted from an infinitely thin 

 stratum. 



§ 3. We will begin by considering somewhat more closely the 

 nature of the motions of the individual molecules. 



We will suppose two parallel planes to be placed perpendicu- 

 larly to the axis of x and infinitely near to each other, so as to 

 enclose an infinitely thin stratum. Since molecules are continu- 

 ally flying through this stratum in all directions, it must some- 

 times happen that two molecules strike each other within it and 

 then rebound again. For the sake of shortness we will call these 

 molecules, which, after having lost their previous motions by the 

 impact, leave the stratum again with different motions, the mo- 

 lecules emitted from the stratum] and we will now fix our atten- 

 tion npon their motions. 



These motions differ very much from each other ; and we must 

 distinguish between variations of two kinds, occasioned by two 

 mutually independent causes, and therefore susceptible of being 

 separately considered. The one kind consists of those irregular 

 variations which always prevail in the molecular motions called 

 heat, and which would therefore also occur if the gas were of 

 uniform temperature and density throughout. They arise from 

 various accidental inequalities accompanying the individual im- 

 pacts : we will designate them accidental variations. The other 

 kind of variations is caused by the circumstance of the gas not 

 having an equal temperature and density throughout. These 

 variations depend in a definite manner upon the laws which 

 govern the differences of temperature and density existing in 

 different parts of the gas : we will call them normal variations. 



It is the latter which have especially to be considered in the 

 conduction of heat, and we will therefore direct our attention 

 first of all to them. 



* According to what is said above, we take account only, in considering 

 conduction, of the heat which is inherent in the molecules themselves, and 

 is communicated by one molecule to another solely by their impact. But 

 besides this, each molecule radiates heat, which is transmitted by the aether, 

 and is partially absorbed by other molecules on its way ; so that there is 

 thus also a transmission of heat from one molecule to another. The com- 

 munication of heat in this way, in the case of bodies of such low radiating 

 and absorbing powers as the gases, can, however, scarcely be reckoned as 

 conduction, since the great distances which the rays of heat may traverse 

 without being absorbed gives it an entirely different character. In any case, 

 however, it is allowable to consider separately each of these two ways in 

 which heat moves ; and we shall accordingly in the sequel always speak of 

 the conduction of heat in this sense. 



