COL'LOIDS (from Gk. K6XXa, holds, glue + floor, cidus, form). A name applied by Graham to a group of substances, including ferric oxide, alumina, silicic acid. starch, dextrin, gum, albu min, gelatin, tannin, caramel, agar-agar, and others. These substances, though not by any means belonging to the same class chemically, behave alike in certain respects when obtained in solution in water or in some other solvent. In the first place, they diffuse, when dissolved, very much more slowly than most other sub stances ordinarily met with. In the second place, their presence in solution has scarcely any effect on the freezing-point or ,on the vapor tension of the solvent, while most other sub stances have the effect Of notably lowering both the freezing-point and the vapor-tension. Again, colloids often spontaneously deposit from their solutions in the form of gelatinous masses that cannot, in many cases, be re-dissolved and that usually retain mechanically a large amount of water. Such 'gelatinized solutions' are now used for a variety of purposes in the arts. advan tage being taken of the mass being in a semi solid condition, while the liquid retained by it may be used for the same purposes as when in the free state: such masses are used in photog raphy by the 'dry' process. in making 'dry' eleetrie batteries, in the manufacture of certain valuable explosives. etc. In scientific researches gelatinized solutions are now used for the pur pose of studying the relative rates at which various substances diffuse in water. For this
purpose it is important that the solutions should remain absolutely undisturbed for a considerable length of time, and this is accomplished best by adding to them a certain amount of agar-agar or some other colloid, and causing them to 'gel atinize,' the 'dry' solutions thus obtained show ing precisely the same rates of diffusion as ordinary aqueous solutions.
Another important property of colloids is their incapacity of traversing parch meat paper and animal membranes. This permits of the separation of colloids from non-eollidds (called 'crystalloids') without any difficulty; the process of separation being known as dialysis. Thus, to dialyze a solution containing common salt (a crystalloid) and silicie acid (a colloid), the solution may be placed in a bag of parch ment paper and immersed in pure water: the salt will then readily pass through the paper, sili•ic acid will remain behind.
The properties of colloids are undoubtedly due to the comparatively very large size of their molecules. Thus, while the molecular weight of water is only 18, and that of most organic sub stances only a few hundred, the molecular weight of starch has been shown to be about 25,000, and that of silicie acid is at least 50,000.