226 TEXT-BOOK OF PHYSIOLOGY 



the molecules of the substance will at once begin to separate themselves and 

 to diffuse throughout the water until the solution becomes homogeneous, and 

 notwithstanding the fact that the dissolved substance possesses weight, the 

 solution remains homogeneous. The force of gravity is overcome by the force 

 of diffusion. 



The velocity with which the molecules of a substance will diffuse through 

 a solvent like water, varies considerably. The experiments of Graham show 

 that if the molecules of a given weight of hydrochloric acid diffuse completely 

 in a unit of time, the molecules in the same weight of sodium chlorid, cane-sugar, 

 albumin and caramel, will require for their diffusion 2.33, 7, 48, and 98 units of 

 time respectively. 



Osmosis. Osmosis may be defined as the passage of the molecules of water 

 through an intervening membrane. If the water on one side of the membrane, 

 parchment for example, contains in solution substances such inorganic salts, 

 their molecules will also pass through the membrane though the time required 

 for this to take place may be much longer than in the case of the water molecules. 

 The passage of the dissolved substance through the membrane though usually 

 included under the term osmosis is more properly termed dialysis. 



If the two volumes of water on opposite sides of the membrane are the same 

 in amount, and if the one volume contains a salt in solution, the salt molecules 

 will continue to pass through the membrane until the water on both sides contains 

 the same number of molecules, or, in other words, until it is homogeneous in com- 

 position. The time required for their passage being longer than the time required 

 for the passage of the water molecules, there will be (owing to factors which will 

 be explained later), a temporary increase in the volume of the water originally con- 

 taining the salt, but in time the two volumes will again become equal. Certain 

 other substances which may be in solution, such as albumin, starch, etc., will not 

 pass across a membrane, because of the large size of their molecules. Graham 

 termed all those substances which by virtue of the small size of their molecules 

 pass through membranes, crystalloids, and all those which by virtue of the large size 

 of their molecules do not pass through membranes or to a very slight extent, colloids. 



It was stated in the foregoing paragraph that if two equal volumes of water 

 are separated by a parchment septum, one of which contains in solution an 

 inorganic salt, the molecules of the salt-free water will osmose through the septum 

 into salt-containing water, more rapidly than they will in the opposite direction, 

 and as a result, there will be a temporary increase in the volume of the water 

 containing the salt. If the membrane were impermeable to the salt molecules, 

 the difference in the two volumes of the water would be far more permanent and 

 striking. The reason assigned for this is that the molecules of the salt exert a 

 pressure against the outer layer of the water molecules and these in turn against 

 the membrane, in consequence of which there is a more rapid osmosis of the 

 water molecules towards the salt than in the reverse direction. To this pressure 

 is applied the term 



Osmotic Pressure. Osmotic pressure may be denned as the pressure exerted 

 by the molecules of the substance in solution against the outer layer of the mole- 

 cules of the solvent. If the solvent is enclosed by an elastic membrane it is 

 expanded and in consequence there is an osmosis of a surrounding solvent towards 

 and through it. The reason for this pressure lies in the fact that, when the mole- 

 cules of a substance are separated a certain distance, as they are when in solution, 

 they repel one another as do the molecules of a gas and in their flight strike 

 against the outer layer of the solvent. The pressure of the molecules of a substance 

 in solution is therefore comparable to the pressure of the molecules of a gas. 



Three methods may be employed for measuring the force of the osmotic 

 pressure of different substances, viz.: i. Physical. 2. The determination of the 

 freezing point. 3. By calculation. 



