142 PHYSIOLOGY 



up, but the solution which it absorbs will be more concentrated than the 

 solution in which it is immersed, so that the proportion of salt in the latter 

 will be diminished. When, however, equilibrium is established between a 

 gel and the surrounding fluid, it is found to present no appreciable resistance 

 to the passage of dissolved crystalloids. Thus salt or sugar diffuses through 

 a column of solid gelatin as if the latter were pure water. On the other 

 hand, gels are practically impermeable to other colloids in solution. This 

 impermeability is made use of in the separation of crystalloids from colloids 

 by dialysis, membranes used in this process being generally irreversible 

 and heterogeneous gels (i.e. vegetable parchment, animal membranes). 

 Other gels, such as tannate of gelatin or copper ferrocyanide, are not only 

 impermeable to colloids, but also to many crystalloid substances. These 

 membranes, therefore, were used by Pfeffer for the determination of the 

 osmotic pressure of such crystalloids as cane sugar. 



PROPERTIES OF HYDROSOLS. Substances such as dextrin or egg- 

 albumin may be dissolved in water in almost any concentration. If a 

 solution of egg-albumin be concentrated at a low temperature, it becomes 

 more and more viscous and finally solid. But there is no distinct point 

 at which the fluid passes into the solid condition. Such solutions are known 

 as hydrosols. Much discussion has arisen whether they are to be regarded 

 as true solutions or as pseudo-solutions or suspensions. The chief criterion 

 of a true solution is its homogeneity. In a solution the molecules of the 

 solute are equally diffused throughout the molecules of the solvent, and 

 it is impossible, without the application of energy, to separate one from 

 the other. Thus filtration, gravitation leave the composition of the solution 

 unchanged. It is true that, by the employment of certain kinds of mem- 

 brane, e.g. the semi-permeable copper ferrocyanide membrane, it is possible 

 to separate solute from solvent, but in this case the force required to effect 

 the filtration is enormous and grows with every increase in the strength 

 of the solution. The measure of the force required is the osmotic pressure 

 of the solution, and it becomes natural therefore to regard the possession 

 of an osmotic pressure as a distinguishing criterion of a true solution. 

 Is there any evidence that colloid solutions also display an osmotic 

 pressure ? 



I have shown that it is possible to determine the osmotic pressure of 

 colloidal solutions directly, taking advantage of the fact that colloidal 

 membranes, while permitting the passage of water and salts, are im- 

 permeable to colloids in solution. 



The method originally adopted was as follows : In order to determine the osmotic 

 pressure of the colloidal constituents of blood-serum, 150 c.c. of clear filtered serum are 

 filtered under a pressure of 30-40 atmospheres through a porous cell which has been 

 previously soaked with gelatin. The first 10-20 c.c. of nitrate, which contain the 

 water squeezed out of the meshes of the gelatin and have also lost salt in consequence 

 of absorption by the gelatin, are rejected. The filtration is allowed to go on for another 

 twenty -four hours, when about 75 c.c. of a clear colourless filtrate is obtained, perfectly 

 free from all traces of protein, but possessing practically the same freezing-point as the 

 original serum. (Although the colloids, if they possess an osmotic pressure, must 



