80 



MOLECULAR MOTION AND ITS ENERGY 



37 



easily answered if we are content with an approximation 

 which must be admissible when the excess of pressure and, 

 therefore, the speed of efflux are small. With this assump- 

 tion we can make the calculation just as for a gas at rest. 

 We need only determine the number of molecules which 

 are forced per unit time out of the interior of the vessel into 

 the orifice. To simplify the calculation we shall further 

 provisionally assume that all the particles move with the 

 same mean speed ft. 



Before we determine the number of particles which 

 arrive at the orifice, let us find the number of those which 

 reach it in a given direction. In the accompanying diagram 



E A B F 



FIG. 3 



let EF denote the wall of the containing chamber, and AB 

 the orifice in the wall. Let CA be the direction correspond- 

 ing to the number of particles we are considering, and let it 

 make angle s with the normal AG. We see at once from 

 the diagram that all particles which can meet the surface 

 AB in the given direction must come from a volume CABD, 

 the edges CA, DB of which are parallel to each other. If 

 we wish to determine the number of particles that pass 

 through AB in unit of time, we have to limit the space 

 CABD by taking the lengths CA, DB to measure the speed 

 ft. For it is obvious that the surface AB can be reached in 

 unit time only by such particles as were initially distant 

 from AB by a less length than the path ft traversed in unit 

 of time. The volume CABD, from which the molecules 

 come, is equal to 



GA.AB = Fl cos s, 



