PHYSICAL PROCESSES IN CELLS. 



ho 



Thus, dry meml.irane will show a higher osmotic equivalent than 

 fresh membranes or dried membranes moistened, from the fact that the 

 membrane retains more water by imbibition, while the passage of the 

 salt is facilitated. 



If an animal membrane separates water and alcohol, the water will 

 pass in much greater amount, for membrane absorbs water much more 

 readily than alcohol or a mixture of water and alcohol. 



Rubber or collodium membranes, on the other hand, allow alcohol 

 to pass with greatest readiness, as such membranes absorb alcohol more 

 readily than water. 



The general phenomena of osmosis may be well illus- 

 trated by the egg -osmometer (Fig. 40). This is prepared 

 by picking off a little of the shell from one end of an egg, 

 taking care to leave the shell-membrane intact, while a 

 glass tube is cemented around a small hole pierced through 

 both shell and shell-membrane at the opposite end. The 

 end at which the shell has been removed and the membrane 

 left undisturbed is then immersed in distilled water. After 

 a time it will be found that water has passed from the out- 

 side to the interior of the egg, as shown by the increased 

 volume, the white of the egg being forced up into the tube 

 cemented on the open end of the egg. At the same time 

 the addition of nitrate of silver to the water in which the 

 egg was immersed will show, by the white precipitate 

 formed, that the chlorides have passed from the inside to 

 the outside of the egg. No trace of albumen, however, is 

 to be seen in the distilled water. The salts of the egg, or 

 its crystalloids, have' thus passed by osmosis through the 

 egg-membrane, water has also passed, while the egg-albu- 

 men, a colloid, has been retained. 



These facts, already alluded to, that crystalloids in solu- 

 tion will pass through an animal membrane, while colloids 

 will not, has been made use of in a process which is fre- 

 quently employed by the chemist to separate bodies of these 

 two classes. Thus, albumen, or any other colloid, may be. 

 entirely freed from crystalloids, such as salt or sugar, by 

 placing it on one side of a membrane with a large quantity 

 of distilled water, which is frequently renewed, on the other. 

 The salts pass through the membrane to the water, their 

 place being taken by water, while the albumen, with the 

 exception of becoming more diluted, remains unchanged. 

 This process is termed dialysis. 



Osmotic phenomena, consequently, may be referred to the following 

 causes (Wundt) : — 



1. The affinity which the two liquids exert on one another. 



2. The relative affinity which the membrane exerts on the two 

 liquids. 



3. On the narrowness of the pores through which the liquids diffuse. 



4. On the overcoming of the adhesion of the liquids to the walls of 

 the pores through increase of temperature. 



The importance of this process in the action of the animal organism is 

 very evident. Nearly all animal tissues are, during their entire life, in 



Fig. 40.— Egg Pke- 

 pauk1j so as to 

 Illustrate Os- 

 motic Action. 

 (Flint.) 



