.ft- TRUBB IN RARIFIED OASES 



point of each sphere which lies furthest from the solid the pole of the sphere, 



greater proportion of molecules will strike any one of the outer layer of 

 spheres near its pole than near ita equator, and the greater the obliquity of 



adence of the molecule, the greater will be the probability that it will strike 



sphere near its pole. 



The direction of the rebounding molecule will no longer be with equal 

 probability in all directions, but there will be a greater probability of the 

 tangential part of its velocity being in the direction of the motion before 

 impact, and of its normal part being opposite to the normal part before 



impact. 



The condition of the molecules which leave the surface will therefore be 

 intermediate between that of evaporated gas and that of reflected gas, approach- 

 ing most nearly to evaporated gas at normal incidence and most nearly to 

 reflected gas at grazing incidence. 



If the spheres, instead of being hard elastic bodies, are supposed to act 

 on the molecules at finite, though small distances, and if they are so close 

 together that their spheres of action intersect, then the gas which leaves the 

 surface will be still more like reflected gas, and less like evaporated gas. 



We might also consider a surface on which there are a great number of 

 minute asperities of any given form, but since in this case there is consider- 

 able difficulty in calculating the effect when the direction of rebound from the 

 first impact is such as to lead to a second or third impact, I have preferred 

 to treat the surface as something intermediate between a perfectly reflecting 

 and a perfectly absorbing surface, and, in particular, to suppose that of every 

 unit of area a portion f absorbs all the incident molecules, and afterwards 

 allows them to evaporate with velocities corresponding to those in still gas at 

 the temperature of the solid, while a portion 1 / perfectly reflects all the 

 molecules incident upon it. 



We shall begin by supposing that the surface is the plane yz, and that 

 the gas is on that side of it for which x is positive. 



The incident molecules are those which, close to the surface, have their 

 normal component of velocity negative. We shall distinguish these molecules 

 by the suffix (,). For these, and these only, , is negative. 



The rebounding molecules are those which have positive. We shall 

 distinguish them by the suffix (,). Those which are evaporated will be further 

 distinguished by an accent. 



