OSMOSIS 257 



2. That the liquids be miscible with each other and be differently 

 constituted. Although it is found that the currents are most active 

 when liquids are of different densities, this condition is not indispen- 

 sable ; for currents will take place between solutions of different sub- 

 stances, such as salt, sugar or albumin, even when they have the same 

 density. 



Diffusion of liquids takes place in accordance with the law of 

 diffusion of gases, formulated by Boyle. The molecules of gases are 

 in constant movement and would pass into space unless restrained or 

 confined by the walls of a containing vessel or by the force of gravity. 

 The pressure of a gas is equal to the pressure required to keep it at 

 a certain degree of concentration. At the surface of the earth, this is 

 measured by the barometer and is equal to about 30 inches (i 

 meter) of mercury. One atmosphere is equal to a pressure of 14.7 

 pounds to the square inch (about I kilogram to the square centimeter). 

 The pressure of a gas increases with elevation in temperature, by ^^ 

 of the pressure at 32 Fahr. (o C.) for each rise of i C. From this 

 the hypothetical, or absolute, zero has been calculated to be 273 C. 

 Van't Hoff and others have shown that osmotic pressure obeys this 

 law within certain limits. 



Osmotic pressure is the force exerted by various solutions such as 

 a salt or a cane-sugar solution which draws pure water through a 

 permeable layer, such as an animal membrane or unglazed porcelain. 

 If both liquids are capable of penetrating the membrane, currents pass 

 in both directions until diffusion is complete and both solutions are of 

 equal density. 



In order to measure osmotic pressure, it is necessary to use a mem- 

 brane that will allow a current in one direction only. This is called a 

 semipermeable membrane. If unglazed porcelain, is impregnated with 

 copper ferrocyanide, it is rendered impermeable to certain solutions, 

 such as common salt and cane-sugar, while it admits the passage of 

 water. A good example of a semipermeable membrane is the mem- 

 brane of a hen's egg. This is impermeable to albumin but not to 

 water. Exposing this membrane by. removing part of the shell, an 

 egg-endosmometer may be constructed that will measure the osmotic 

 pressure of albumin. 



It has been ascertained that a one per cent solution of sodium 

 chloride has an osmotic pressure of five meters (about two hundred 

 inches) of mercury, which is equal to six and a half atmospheres. A 

 two per cent solution has double this pressure. While* the pressure is 

 in direct ratio to the concentration of the solution, it has not been 

 possible, thus far, to measure the pressure of solutions higher than six 



