204 ESSENTIALS OF CHEMICAL PHYSIOLOGY 



and so the volumes of the two masses of water will remain unchanged. If 

 now we imagine the membrane m is not permeable except to water, and the 

 compartment a contains water, and the compartment b contains a solution 

 of salt or sugar ; under these circumstances water will pass through into b, 

 and the volume of b will increase in proportion to the osmotic pressure of 

 the sugar or salt in solution in b, but no molecules of sugar or salt can get 

 through into a from b, so the volume of fluid in a will continue to decrease, 

 until at last a limit is reached. The determination of this limit, as measured 

 by the height of a column of fluid or mercury which it will support, will give 

 us a measurement of the osmotic pressure. Membranes of this nature are 

 called semi-permeable. One of the best kinds of semi-permeable membrane is 

 ferrocyanide of copper. This may be made by taking a ceU of porous 

 earthenware and washing it out first with copper sulphate and then with 

 potassium ferrocyanide. An insoluble precipitate of copper ferrocyanide is 

 thus deposited in the pores of the earthenware. If such a cell is filled with 

 a 1-per-cent. solution of sodium chloride, water diflEuses in till the pressure 

 registered by a manometer connected to it registers the enormous height of 

 5,000 millimetres of mercury. Theoretically it is possible to measure osmotic 

 pressure by a manometer in this way, but practically it is never 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 crj-stalloid. 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 explanation 

 is perhaps the best, and may be rendered most intelligible by an illustration. 

 Suppose we have a solution of sugar separated by a semi-permeable mem- 

 brane from water, that is, the membrane 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 proportional 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 it it occupied the same space in the form of a gas (Van't Hoffs 

 hypothesis). The nature of the substance makes no difference ; it is only the 



