DIFFUSION OF GASES 



227 



nitrogen; the former can pass through while the latter cannot. 

 When this system is permitted to stand for some time, what 

 changes will take place in the original equilibrium? Experiment 

 shows that the hydrogen will diffuse through the palladium into 

 the nitrogen until the hydrogen is evenly distributed throughout 

 the entire system At the end there will be just as much hy- 

 drogen in the inner box as in the outer one. The nitrogen, 

 unable to pass through the palladium, remains inside. Conse- 

 quently after diffusion has been complete and equilibrium has 



N 

 H 



H 



B 



Fig. 11.— Illustrating diffusion. A, at the beginning of the experiment; B, 

 at the end after equilibrium is reached. The hydrogen is evenly distributed 

 throughout the system, having passed through the palladium wall. 



been reached, the manometer on the outer box will register only 

 a half atmosphere of pressure, while the one on the inside will 

 register one and a half atmospheres. In other words, each gas exerts 

 its own partial pressure and each acts independently of the other. 



A parchment membrane soaked with water, separating carbon 

 dioxide and oxygen, with the oxygen on the inside, would show 

 a similar result. The water membrane is much more permeable 

 to carbon dioxide than to oxygen because of the greater solu- 

 bility of the carbon dioxide in water. It would therefore pass in 

 more freely than the oxygen would pass out. 



These examples explain why there is a continuous stream of 

 carbon dioxide passing into the leaf and into the cells. As the 

 carbon dioxide is used up inside the cell, its partial pressure is 

 less inside than outside and consequently more comes in to take 

 its place. The oxygen which is present in greater quantity per 

 unit of volume on the inside than on the outside will diffuse out- 

 ward for the same reason. Each gas moves independently of the 

 others from the place where its partial pressure is the greatest to the 

 place where it is the least. 



