THE PROPERTIES OF COLLOIDS 149 



centrated solutions of protein, heat coagulation results in the formation of a 

 .gel, i.e. a network of insoluble protein, containing water or a very dilute 

 solution of protein in its meshes. In dilute solutions the result is the 

 production of a flocculent precipitate. 



Another method is the so-called mechanical coagulation. If a solution 

 of globulin or albumin be introduced into a bottle, which is then violently 

 shaken, a shreddy precipitate makes its appearance in the solution, and this 

 precipitate increases, so that by prolonged shaking it is possible to throw 

 down 80 or 90 per cent, of the dissolved protein in the coagulated form. 

 Ramsden has shown that this mechanical coagulation is a surface pheno- 

 menon. It depends on the fact that a large number of substances in solution 

 (viz. any which lower the surface tension of their solutions) undergo concen- 

 tration at the free surface of the fluid. Such substances are proteins, bile- 

 salts, quinine, saponin, &c. In the case of proteins the concentration reaches 

 such an extent, and the molecules at the surface are so closely packed 

 together, that they form an actual solid pellicle, which hinders the movement 

 of any object, such as a compass needle, suspended in the surface. When the 

 solution is violently shaken, new surfaces are constantly being formed, and 

 as the older surfaces are withdrawn into the fluid, the solid pellicle on them 

 is rolled up into a fine shred of coagulated protein, and this process will 

 continue until there is no protein left to form a pellicle. 



We must conclude that colloidal solutions, although differing so widely 

 from true solutions in many of their properties, are connected with these by 

 all possible grades. In a solution of an ordinary crystalloid or electrolyte 

 the molecules of the dissolved substance are distributed equally and homo- 

 geneously among the molecules of the solvent. In the various grades of 

 solution a colloid solution or hydrosol may be assumed to begin when the 

 size of the molecule is increased out of all proportion to that of the molecules 

 of the solvent, r* The * dissolved ' ^molecules now have the properties of 

 matter in mass and to present surfaces with all their attendant attributes. 

 The same sort of solution may be formed with smaller molecules, such as 

 Si0 2 , when these are aggregated together with adsorbed water into huge 

 molecular complexes or, as in metallic sols, by the division of the solid metal 

 into ultra-microscopic particles. The distinguishing features of a colloidal 

 solution are due to this lack of homogeneity, and to the fact that in every 

 solution there are two phases^ a fluid phase, and a second phase which is 

 either solid or a concentrated or supersaturated solution of the colloid. The 

 huge size of the molecules and the development of svttf ace not only determine 

 the formation of adsorption combinations but, on account of the inertia of 

 the system, cause a delay in changes of state, and tend to the formation of 

 false equilibria dependent on the past history of the system. 





IMBIBITION 



All colloids, even those such as starch or gelatin, which are insoluble 

 in cold water, exhibit a phenomenon, viz. * Quellung ' or imbibition, which 

 many cases it is impossible to distinguish from the process of solution. 





