20 A TEXTBOOK OF PHYSIOLOGY 



Diffusion of Gas through a Liquid Film. The solubility of a gas 

 is important in determining the passage of a gas through a watery 

 film. It is found that the velocity of its diffusion is directly pro- 

 portional to the absorption coefficient of a gas in water. It is 

 also found that, other things being equal, the amount of gas passing 

 from the place of high pressure through a watery film to the place of low 

 pressure is proportional to the difference in pressure of the gas on the 

 two sides of the film. The importance of the first factor may IK- 

 demonstrated as follows: A piece of pig's bladder is tied over one end 

 of a short wide tube. The other end of the tube is closed by a rubber 

 cork through which is passed a narrow glass tube, which passes to a 

 manometer containing coloured water or to some other mechanism 

 for recording change in pressure. The membrane is now impregnated 

 with water; it is important to note that membranes dried' in air are 

 almost if not quite impermeable to such gases as carbon dioxide 

 and oxygen. A beaker containing a gas is then inverted over the 

 tube carrying the membrane. If the gas be hydrogen, no movement 

 of fluid is recorded by the manometer. On the other hand, with a very 

 soluble gas such as ammonia, a rise of pressure is shown in a short time. 

 The exchange of ox}*gen and carbon dioxide in aquatic plants 

 depends upon the power of the water to dissolve these gases, and on 

 the diffusion of the dissolved gases through the membrane of the 

 plant, which is impregnated with water. If the medium in which 

 the plant lives be freed from air, the plant dies. The process of tin- 

 diffusion through the walls of submerged plants has been shown to 

 follow the laws cited above; the gaseous interchange is therefore a 

 slow process. On this account the oxygen and carbon dioxide liber- 

 ated in the assimilatory and respiratory processes of the plant are 

 stored in intercellular spaces and kept for future use. This is particu- 

 larly the case in the parts of aquatic plants which are embedded in the 

 mud at the bottom of the water. Further, since oxygen and carbon 

 dioxide are more soluble in water at low temperatures, the facilities 

 for gaseous interchange are greater at these temperatures, and it is 

 known that marine plants such as alga? flourish more abundantly in 

 the Arctic than in warmer waters. 



In dealing with the gaseous interchange in the process of respiration, 

 we shall have to discuss whether this takes place according to the 

 principles regulating the diffusion of gases across a liquid film. 



Solubility of Gases in Salt Solutions. In general, the more con- 

 centrated a salt solution, the less soluble a gas in it. Thus, while 

 1 c.c. of pure water at 25 C. dissolves 0308 c.c. of oxygen, 1 c.c. of 

 a * solution of NaCl dissolves but 0-0262 c.c. of this gas, a N 

 solution* 0-0223 c.c., a 2 N solution 0-0158 c.c. We shall see 

 that the absorption of oxygen by the blood, which contains 

 salts in solution, is not a physical process; for, instead of taking 



* A normal solution (N solution) is made by dissolving the molecular weight in 

 grammes of NaCl in 1 litre of water. ( denotes a half normal solution, 2 N a twice 

 normal solution, and so on. 



