THE PROPERTIES OF COLLOIDS 157 



tenacity. In consequence of its structure, it presents an enormous 

 extent of surface on which adsorption can take place. At this surface 

 the vapour-tension of fluids is diminished, as well as the osmotic 

 pressure of dissolved substances. On this account gelatin must be 

 heated for many hours at a temperature of 120 C. in order to be 

 thoroughly dried. When dry, it, as well as other solid colloids, can 

 exert a considerable amount of energy when caused to swell up by 

 moistening. This energy was made use of by the ancient Egyptians 

 in the quarrying of their stone blocks by the insertion of wedges of 

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

 split the rock in the desired direction.* 



It is on account of the extent of surface that it is practically impos- 

 sible to wash out the inorganic constituents from a gel. The diminu- 

 tion of the osmotic pressure of many dissolved substances at surfaces 

 causes the concentration at the surface of a gel 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 ferro- 

 cyanide, 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 crystal- 

 loids 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 applica- 



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

 attracts water amounts to 2073 kilo, per sq. cm. 



