1285 



vvliicli have lirsl passed tlirougli it in opposite direclioii with siicli 



a small velocity that tliev conld not reach the -\- plane, hnl reversed 



of 

 their velocity in consequence of the force — - before- having reached 



oz 



it. When also these molecules are taken into account the total 



diffusion current from -\- to — is found hy integrating expiession 



(IJ) with respect to u and r hel ween — x ami +x and with 



respect to //'' t)elweeii and — x. 



The molecules (lowing- in opposite i.e. in the -|" direction through 



the plane are found by taking a group of molecides emitted tVom 



the - plane : 



' n — ^ -^ roe ^^ ^^^' » d d d 



l/n'V ÖZ 3 



and by integrating a and /• in this between — x and -|- x and iv' 

 between and -|- ^• 



Thus it is found that the plane .1 is passed per second and per 

 unit of .area in the — ■ direction by the following number of molecules : 



\ ói lys 1 d£ /l/3 



fi 



dn/l/3\ kTdz -i f dw/l/3\ /c7 dr .J 

 d^ 3 y \ bz ^ J \ 



?>\/3X \b2 dz fcTj 3|/rr \n dz dz kTj ^ ' 



In this we shall assume //,/ to be constant, though, strictly speaking, 

 this is only allowed for gases at small densities. In the case of 

 thermodynamic equilibrium this diti'usion current must be zero through 



very |)lane, so that then l{ri) -\- ^ = constant, whicli gives the 



kT 



known distribution of the molecules in S|)ace in that case. 



We shall make use of the value of the diffusion current in equa- 

 tion (12) in ordei- to calculate how much enters through the six 

 sides of a xolume element dvdi/dz. We find for this: 



« 1/3 \ e I 



• — 7il "-7' \l (n) f — > d.v dv dz , 



so that we find for the condition for a stationary slate in coiniection 

 with equation (10): 



a 1/ 'S [ F i on 



4-^-/7' yi^^) + ,r\ =". ... (13) 



o l/.T f • kJ ] o.r 



1 shall here leave out of account the question what corrections 



