ABSORPTION. 223 



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

 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. 



i. Physical Method. For the purpose of measuring osmotic pressure by 

 physical methods, it is customary to make use of an apparatus similar to that 

 represented in Fig. 95, which consists of an earthenware vessel (a), into the 

 upper open end of which a tall vertical glass tube has been hermetically sealed. 

 The pores of the earthenware vessel have been filled by a membrane made by 

 precipitating ferrocyanid of copper within them. This membrane is freely 

 permeable to water, but impermeable to certain substances in solution, e.g., 

 cane-sugar. Such a membrane, which permits the passage of the molecules of 

 the solvent but not the molecules of the dissolved substance, is termed a semi- 

 permeable membrane, and its use is absolutely necessitated when it is desired 

 to obtain the actual pressure exerted by any given substance in solution. An 

 apparatus of this character is termed an osmometer. 



When, therefore, the osmometer containing a solution of cane-sugar is placed 

 in the vessel () containing water, the following phenomena occur, viz.: an ascent 

 of the cane-sugar solution in the vertical glass tube, and a descent of the level 

 of the water in the vessel b. These phenomena continue until the level of the 

 fluid in the glass tube reaches a certain height, when it becomes stationary, and 

 no further effect takes place. 



In explanation of the foregoing phenomena it may be said that the molecules 



