DIFFUSION PRESSURE 87 



level of the liquid in the glass tube, until it falls to that of the water in the 

 beaker. 



The explanation of this sequence of events is as follows. For reasons 

 which are discussed later the rate of diffusion of hydrogen is greater than 

 that of nitrogen or oxygen under comparable conditions. When the porous 

 clay cup is first enclosed within the bottle of hydrogen, rapid diffusion of 

 hydrogen gas occurs through the pores of the cup. This raises the total gas 

 pressure within the cup, since outward diffusion of oxygen and nitrogen occurs 

 at a much slower rate. Hydrogen gas diffuses into the cup in spite of the 

 fact that this results in a greater total pressure inside the cup than in the 

 surrounding atmosphere. The direction of the diffusion of the hj^drogen gas 

 is controlled entirely by its own differences in concentration, and is unaffected 

 by the presence of other gases. The greater gas pressure inside the cup results 

 in the outward bubbling of a mixture of all three gases at the lower end of 

 the vertical glass tube. When the bottle is removed from around the porous 

 clay cup, the direction of the diffusion of hydrogen is reversed, since the con- 

 centration of hydrogen gas is now greater inside of the cup. Since, while 

 enclosed in the bottle of hydrogen, some of the nitrogen and oxygen diffused 

 out of the cup and some was lost by the bubbling of gases from the lower 

 end of the glass tube, this rapid outward diffusion of hydrogen results tem- 

 porarily in a lower total gas pressure inside the cup than that originally 

 present. Hence the solution rises rapidly in the glass tube. Finally, nitrogen 

 and oxygen diffuse slowly inward, because of the reduced concentration of 

 these gases inside of the cup, and the liquid slowly falls in the glass tube to 

 its original level. 



That the process of diffusion may result in the development of pressure 

 also can be shown very strikingly by means of another simple experiment. If 

 a pure rubber balloon containing only a little air be suspended in a closed 

 bottle of carbon dioxide gas it will gradually become distended. Rubber 

 membranes are quite readily permeable to the molecules of carbon dioxide but 

 virtually impermeable to those of oxygen, nitrogen, or other gases of the 

 atmosphere. Since, temperature remaining constant, the pressure exerted by 

 any gas is directly proportional to its concentration (number of molecules 

 per unit volume), it follows that the diffusion of the molecules of gases may 

 be interpreted in terms of the differences in the partial pressure (Chap. II) 

 exerted by that gas in different parts of a system. Because of the greater 

 partial pressure of carbon dioxide in the surrounding atmosphere, diffusion 

 of this gas through the walls of the balloon continues until the partial pressure 

 of the carbon dioxide is the same on the two sides of the rubber membrane. 

 Since the initial partial pressure of carbon dioxide inside the balloon was 



