DIFFUSION AND OSMOSIS. 65 



Some Preliminary Considerations upon the Processes of Diffusion and 

 Osmosis, and their Importance in the Nutritive Exchanges of the 

 Body. 



In recent years the physical conceptions of the nature of the processes of diffusion and 

 osmosis have changed considerably. As these newer conceptions are entering largely 

 into current medical literature, it seems advisable to give a brief description of them 

 for the use of those students of physiology who may be unacquainted with the modern 

 nomenclature. The very limited space that can be devoted to the subject forbids any- 

 thing more than a condensed elementary presentation. For fuller information reference 

 must be made to special treatises. 1 



Diffusion, Dialysis, and Osmosis. — When two gases are brought into contact a homo- 

 geneous mixture of the two is soon obtained. This interpenetration of the gases is 

 spoken of as diffusion, and it is due to the continual movements of the gaseous 

 molecules to and fro within the limits of the confining space. So also when two mis- 

 cible liquids or solutions are brought into contact a diffusion occurs for the same reason, 

 the movements of the molecules finally effecting a homogeneous mixture. If the two 

 liquids happen to be separated by a membrane, diffusion will still occur, provided the 

 membrane is permeable to the liquid molecules, and in time the liquids on the two sides 

 will be mixtures having a uniform composition. Not only water molecules, but the mole- 

 cules of many substances in solution, such as sugar, may pass to and fro through mem- 

 branes, so that two liquids separated from each other by an intervening membrane and 

 originally unlike in 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 solutions 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 importance 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 degrees of diffusibility as compared 

 with one another or with the water molecules, and it frequently happens that a membrane 

 that is permeable to water molecules is less permeable or even impermeable to the mole- 

 cules of the substances in solution. F'or this reason the diffusion stream of water and of 

 the dissolved substances may be differentiated, as it were, to a greater or less extent. In 

 recent years it seems to have become customary to limit the term osmosis to the stream 

 of water molecules passing through a membrane, while the term dialysis, or diffusion, is 

 applied to the passage of the molecules of the substances in solution. The osmotic 

 stream of water under varying conditions is especially important, and in connection 

 with this process it is necessary to define the term osmotic pressure as applied to 

 solutions. 



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

 membrane, we can readily understand that as many water molecules 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 thai 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 he found that an excess of water will pass from 



1 Consult: II. C. Jones, The Theory of Electrolytic Dissociation, 1900; " Diffusion, Osmosis, and 



Filtration," by E. W. Reid, in Schlifer's Test-book of Physiology, 1898; Solution and Electrolysis, 

 by W. C I). Whetham, Cambridge Natural Science Manuals. 1895. 

 Vol. I.— 5 



