12H4 



ciiles al aii,v poiiil iiuiv 1)0 found in llie füllowiiig nvuv. We sliall. 

 riamel}', iissiime Ihe velocities of Ihe inolectiles wliicii liave collided 

 ill a cei'tain layer to coiisisi of two coiiipoiieiils : J. llie velocity ot 

 ciirieiil ill the layer in which they have collided, and 2 the heat 

 motion, of which latter it will Ue assnnied that it is distributed over 

 the (litferent molecules according to Maxwkj.i/s law. Undoubtedly 

 we make an error when supposing these things, but we may expect 

 that this will only l>e an error in a numerical coefficient, and that 

 the natni-e of the |)lienomenon and also the order of magnitude 

 will be correctly represented Ity the tormnlae derived by the aid 

 of these suppositions. 



In order to examine the dilfusion current ihroiigh a |)lane .1, we 

 shall consider two planes lying on either side of the plane A at a 



1/ 3 



distance ^ /.(/== mean length ot path ot the molecules). And we 



shall considei' the molecules passing through the plane ^1 as "emitted ' 

 from one of these two planes, by which we understand that they 

 liav(! had their last collision there. Let lis first consider the molecules 

 that collide in the -|- plane'), and which possess a component of 

 ^elocity normal to the plane .1 lietween //• and h' -\- dir. Arrived 

 in the plane .1 these molecules have obtained a normal velocity //'' 

 determined by the etpiation : 



dt iy'ó 



The number of molecules of this group passing per second and 



per unit of area through plane A, is when ii represents the density 



dn /l/'3 

 of the molecules in .4, and /i -|- ^ — that in the -|- plane: 



,—An + ^-^-]ir\' ^^ ^kTö. , a^d-d- . . (11) 



l/rr^\ dz Z J a (( f( 



When we pay only attention to the molecules emitted from the 



, . of l[/S 

 -j- plane. /// must alwavs have a value tor which V» '?'"^* is^ ^ v " • 



oz 8 



In the dii-ection from -f~ ^<^' — however, there go, also molecules 



') i.e. the plane parallel to A at a distance ';</> 3 on the side where Die 



potential energy c of the molecules is greater than in A. The plane lying on the 



other side will be called the — plane. The r-axis will be normal to A in the 



df 

 direction horn the — plane to the + plane, so that ^ > 0. 



^02 



