26 



PHYSIOLOGY 



back to A. As long as the number of particles is greater 

 in A than in B, a greater number from A than B hit the 

 membrane in a unit of time, and the passage from A to 

 B is more rapid than from B to A (Fig. 12). In course 



A B 



Fig. 11.— Case 1. A gas confined by a membrane. 



of time the number of particles in B equals the number 

 in A. The chance of hitting the membrane and of pass- 

 ing through is then the same in both spaces (Fig. 13). 

 In other words, when the pressure of a gas is greater 



A B 



• • • • 



• • • • 



• • • • 



• • • • 



Fig. 12.— Case 1, after an interval. 



on one side of a membrane than on the other, the passage 

 takes place in both directions, but from the side of 

 greater pressure it overbalances that from the side of 

 less pressure until the pressure on the two sides is equal, 

 when an equilibrium is established. Equilibrium does 

 not mean that passage stops. It means that in any 



