HO PHYSIOLOGY 



of wood ; water was poured on the wood, and the swelling of the wedges 

 split the rock in the desired direction.* 



On account of the extent of surface it is practically impossible to wash 

 out the inorganic constituents from a gel. The diminution of the osmotic 

 pressure of many dissolved substances at surfaces causes the concentration 

 at the surface of the solid phase to be greater than that in the surrounding 

 medium. Thus if dry gelatin be immersed in a salt solution it will swell 

 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 become^ 

 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- 

 branes, 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 colloidal 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 mem* 



* According to Rodewald, the maximal pressure with which dry starch attracts 

 water amounts to 2073 kilo, per sq. cm. 



