PROTEINS AND THEIR CLASSIFICATION. 1017 



on the two sides will be mixtures having a uniform composition. Not only 

 water molecules, but the molecules of many substances in solution, such 

 as sugar, may pass to and fro through membranes, so that two liquids sepa- 

 rated from each other by an intervening membrane and originally unlike 

 m composition may finally, by the act of diffusion, come to have the same 

 composition. Diffusion of this kind through a membrane is frequently 

 spoken of as dialysis or osmosis. In the body we deal with aqueous solu- 

 tions of various substances that are separated from each other by living 

 membranes, such as the walls of the blood capillaries or of the alimentary 

 canal, and the laws of diffusion through membranes are of immediate im- 

 portance in explaining the passage of water and dissolved substances through 

 these living septa. In aqueous solutions such as we have in the body we must 

 take into account the movements of the molecules of the solvent, water, 

 as well as of the substances dissolved. These latter may have different de- 

 grees of diffusibility as compared with one another or with the water mole- 

 cules, and it frequently happens that a membrane that is permeable to 

 water molecules is less permeable or even impermeable to the molecules of 

 the substances in solution. For this reason the diffusion stream of water 

 and of the dissolved substances may be differentiated, as it were, to a greater 

 or less extent. The energy or force responsible for the diffusion of molecules 

 in solution is an outcome of the intrinsic energy of the molecules which keeps 

 them in movement. This energy is designated as osmotic pressure. It can 

 be measured accurately, although its exact nature is not understood. 



Osmotic Pressure. If we imagine two masses of water separated by 

 a permeable membrane, we can readily understand that as many water mole- 

 cules will pass through from one side as from the other; the two streams, 

 in fact, will neutralize each other, and the volumes of the two masses of 

 water will remain unchanged. The movement of the water molecules in 

 this case is not actually observed, but it is assumed to take place on the 

 theory that the liquid molecules are continually in motion and that the 

 membrane, being permeable, offers no obstacle to their movements. If, 

 now, on one side of the membrane we place a solution of some crystalloid 

 substance, such as common salt, and on the other side pure water, then it 

 will be found that an excess of water will pass from the water side to the 

 side containing the solution. In the older terminology it was said that the 

 salt attracted this water,' but in the newer theories the same fact is expressed 

 by saying that the salt hi solution exerts a certain osmotic pressure, in conse- 

 quence of which more water flows from the water side to the side of the 

 solution than in the reverse direction. As a matter of experiment it is found 

 that the osmotic pressure varies with the amount of the substance in solu- 

 tion. If in experiments of this kind a semipermeable membrane is chosen 

 that is, a membrane that is permeable to the water molecules, but not to 

 the molecules of the substance in solution the stream of w r ater to the side 

 of the crystalloid will continue until the hydrostatic pressure on this side 

 reaches a certain point, and the hydrostatic pressure thus caused may be 

 taken as a measure of the osmotic pressure exerted by the substance in solu- 

 tion. Under these conditions it can be shown that the osmotic pressure 

 is proportional to the concentration of the solution, or, in other words, to 

 the number of molecules (and ions) of the crystalloid in solution. As a 

 matter of fact, most of the membranes that we have to deal with in the 

 body are only approximately semipermeable that is, while they are readily 

 permeable to w y ater molecules, they are 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 mem- 

 brane, by diffusion, to the other side. In course of time, therefore, the 

 dissolved crystalloid will be equally distributed on the two sides of the mem- 

 brane, the osmotic pressure on both sides will become equal, and osmosis 

 of the water will cease to be apparent, since it is equal in the two directions. 

 All substances in true solution are capable of exerting osmotic pressure, 

 and the important discovery has been made that the osmotic pressure, meas- 

 ured in terms of atmospheres or the pressure of a column of water or mer- 

 cury, is equal to the gas pressure that would be exerted by a number of 



