THE PROPERTIES OF COLLOIDS ,;,, 



Ramsden lias shown that this mechanical coagulation is a surface pheno 

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

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

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

 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 t hfl 

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

 as the older surfaces are withdrawn into the fluid, the solid pellicle on th.-m 

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

 tinue 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. The ' dissolved ' molecules now begin to 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 surface 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 

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

 This phenomenon, which was long ago studied by Chevreul and has lately 

 been the subject of a series of careful experiments by Overton, is exhib 

 all animal tissues and all colloids. Thus elastic tissue dried in vu 

 from a saturated solution of common salt 36-8 per cent, of water 

 dried colloids be suspended in a closed vessel over various i 

 will take up water in the form of vapour from the solution, anc 

 pressure of the solution in question will inform us as to the a 

 which would be necessary in order to separate the wate 

 colloids. 



