66 l.V AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



the water side to the Bide containing the solution. In the older terminology it 

 was said thai the sail attracted this water, but in the newer theories the same fact is 

 expressed by saying that the salt in solution exerts a certain osmotic pressure, in eonse- 

 quence of which more water flows from the water side to the side of the solution than in 

 the reverse direction. A- a matter of experiment it is found that the osmotic pressure 

 varies with the amount of the substance in solution. If in experiments of this kind a 

 semi-permeable membrane is chosen — that is, a membrane that is permeable to the water 

 molecules, hut not to the molecules of the substance in solution— the stream of water to 

 i lie Bide of the crystalloid will continue until the hydrostatic pressure on this side 

 reaches a certain point, and the hydrostatic pressure thus caused may he taken as a 

 measure of the osmotic pressure exerted by the substance in solution. Under these con- 

 dition- it can he shown that the osmotic pressure is proportional to the concentration of 

 the solution, or, in other words, to the Dumber of molecules and ions of the crystalloid in 

 solution. As a matter of fact it is difficult, if not impossible, to construct membranes 

 that are truly semi-permeable; most of the membranes that we have to use in practice 

 are only approximately semi-permeable— that is, while they are readily permeable to 

 water molecule-, they arc also permeable, although with more or less difficulty, to the 

 substances in solution. In such cases we get an osmotic stream of water to the side of the 

 dissolved crystalloid, but at the same time the molecules of the latter pass to some extent 

 through the membrane, by diffusion, to the water side. In course of time, therefore, the 

 dissolved crystalloid will he equally distributed on the two sides of the membrane, the 

 osmotic pressure on both sides will become equal, and osmosis of the water will cease 

 to be apparent, since it will be equal in the two directions. All substances iu solution are 

 capable of exerting osmotic pressure, and the important discovery has been made that 

 the osmotic pressure, measured in terms of atmospheres or the pressure of a column of 

 water or mercury, is equal to the gas pressure that would be exerted by a number of 

 molecules of gas equal to that of the crystalloid in solution, if confined within the same 

 -pace ami kept at the same temperature. A perfectly satisfactory explanation of the 

 nature of osmotic pressure has not been furnished. We must be content to use the term 

 to express the fact described. A comparatively simple explanation, however, has been 

 suggested, which has the great merit of referring the whole phenomenon to the molec- 

 ular movements of the solvent and of the substance dissolved — that is, to the same 

 ultimate cause that brings about the entire process of diffusion in liquids. The nature 

 of this explanation may he understood from a simple illustration. Suppose that we 

 have a solution of cane-sugar separated from a mass of water by a semi-permeable mem- 

 brant — that is, in this case a membrane permeable to the water molecules but not to the 

 sugar molecules. Under these conditions the stream of water from the two sides 

 will lie unequal, because on the one side we have water molecules moving against 

 the membrane in what we may call normal numbers, while on the other side both water 

 and BUgar molecules may be considered as striking against the membrane. On this side 

 the sugar molecules screen the membrane, as it were, from contact with a certain num- 

 ber of water molecules, and the result follows that in a given unit of time fewer mole- 

 cule- of water will penetrate the membrane from this side than from the other; or, to 

 put it in another way, the osmotic stream of water from the unscreened water side to the 

 sugar side will be greater than in the reverse direction. Upon this hypothesis one 

 can readily see why the osmotic pressure should be proportional to the number of mole- 

 cules of the crystalloid in the solution — that is, to the concentration of the solution. It 

 i- a matter of great importance to measure the osmotic pressures of various solutions. 

 A- was -tated above, this mea-uremeiit could lie made easily for any solution provided 

 a really -emi-permeable membrane could be constructed. As a matter of experience, 

 however, it is possible to make stub membranes in only a few cases, and in these cases 

 perhaps the semi-permeability is only approximately complete. In actual experiments 



other methods must 1 mployed, and a brief statement of a theoretical and a practical 



method of arriving at the value of osmotic pressures may be of service in further illus- 



