324 



LYMPH AND LYMPHATIC GLANDS 



[CH. XXII 



is increased beyond this artificially, water will be pressed through the semi-perme- 

 able walls of the cell and the solution will become more concentrated. 



In other words, in order to make a solution of sodium chloride of greater con- 

 centration than 1 per cent. , a pressure greater than 5000 mm. of mercury must be 

 employed. The osmotic pressure exerted by a 2 per cent, solution would be twice 

 as great. 



Though it is theoretically possible to measure osmotic pressure by a manometer 

 in this direct way, practically it is hardly ever done, and some of the indirect 

 methods of measurement described later are used instead. The reason for this is 

 that it has been found impossible to construct a membrane which is absolutely 

 semi-permeable ; they are all permeable in some degree to the molecules of the 

 dissolved crystalloid. In course of time, therefore, 

 the dissolved crystalloid will be equally distributed 

 on both sides of the membrane, and osmosis of water 

 will cease to be apparent, since it will be equal in 

 both directions. 



Many explanations of the nature of osmotic 

 pressure have been brought forward, but none is 

 perfectly satisfactory. The following simple ex- 

 planation is perhaps the best, and may be rendered 

 most intelligible by an illustration. Suppose we 

 have a solution of sugar separated by a semi-per- 

 meable membrane from water; that is, the mem- 

 brane is permeable to water molecules, but not to 

 sugar molecules. The streams of water from the two 

 sides will then be unequal; on one side we have 

 water molecules striking against the membrane in 

 what we may call normal numbers, while on the 

 other side both water molecules and sugar molecules 

 are striking against it. On this side, therefore, the 

 sugar molecules take up a certain amount of room, 

 and do not allow the water molecules to get to the 

 membrane ; the membrane is, as it were, screened 

 against the water by the sugar, therefore fewer 

 water molecules will get through from the screened 

 to the unscreened side than vice versa. This comes 

 to the same thing as saying that the osmotic stream 

 of water is greater from the unscreened water side to 

 the screened sugar side than it is in the reverse 

 direction. The more sugar molecules that are 

 present, the greater will be their screening action, 

 and thus we see that the osmotic pressure is pro- 

 portional to the number of sugar molecules in the 

 solution, that is, to the concentration of the solution. 



Osmotic pressure is, in fact, equal to that which 

 the dissolved substance would exert if it occupied the 

 same space in the form of a gas (Van't HofFs hypo- 

 thesis). The nature of the substance makes no differ- 

 ence ; it is only the number of molecules which causes osmotic pressure to vary. 

 The osmotic pressure, however, of substances like sodium chloride, which are elec- 

 trolytes, is greater than what one would expect from the number of molecules present. 

 This is because the molecules in solution are split into their constituent ions, and 

 an ion plays the same part as a molecule, in questions of osmotic pressure. In dilute 

 solutions of sodium chloride ionisation is more complete, and as the total number of 

 ions is then nearly double the number of original molecules, the osmotic pressure is 

 nearly double what would have been calculated from the number of molecules. 



The analogy between osmotic pressure and the pressure of gases is very com- 

 plete, as may be seen from the following statements : 



1. At a constant temperature osmotic pressure is proportional to the concentra- 

 tion of the solution (Boyle-Mariotte's law for gases). 



B 



FIG. 308. A, outer vessel, con- 

 taining distilled water; B, 

 inner semi-permeable vessel, 

 containing 1 per cent, salt 

 solution ; M, mercurial 

 manometer. (After Star- 

 ling.) 



