Sols in which the disperse phase is a solid are known as "suspensoids"; while those in which it is a liquid are "emulsoids." Thus, sols of most inorganic compounds, of dextrin, gelatin, and (probably) of casein, etc., are suspensoids; while sols of egg-albumin, of oils, etc., are emulsoids. The classification of these substances into suspensoids and emulsoids is, however, more a matter of convenience than of real difference in composition, since it is practically impossible to say whether many of the organic substances which normally exist in colloidal form are themselves liquids or solids, when in the non-dispersed form.

CONDITIONS NECESSARY TO THE FORMATION OF SOLS

Suspensoids differ from mechanical suspension of solids in a liquid in that in the latter the solid particles settle toward the bottom of the mixture, because of the effect of the attraction of gravity upon them. The rate at which such particles settle depends upon the size and density of the particle and the viscosity of the liquid, and can be roughly calculated from the formula for Stokes' law for the rate of falling of a spherical body in a liquid. This formula is

For example, if this formula be applied to determine the rate at which the particles of gold of the size of those in a red gold sol would settle, if they were in mechanical suspension in water (r = 10µµ, or one-ten-thousandth of a millimeter; s = 19.3; s´ = 1; g = 980, and n = 0.01), it will be found that such particles will settle at the rate of approximately 0.0146 millimeter per hour, or a little over 10 mm. (0.4 inch) per month. Hence, the settling of such particles, if in mechanical suspension, would be measurable, although very slow. Shaking up the suspension would cause the particles to rise through the liquid again. But in a gold sol, or suspensoid, which contains particles of gold of the size used in this calculation, the gold particles do not settle, even at the slow rate as calculated above. They remain uniformly distributed throughout the liquid for an indefinite period or time. The reason for this phenomenon undoubtedly lies in the fact that these minute particles carry an electric charge, which, is of the same sign for all of the particles and results in a repellent action which keeps the particles in constant motion. This constant motion may easily be conceived to keep the particles uniformly distributed throughout the liquid, just as constant shaking would keep those of a mechanical suspension uniformly distributed through the mixture.

The sign of the electric charge on the particles of a sol may be either negative or positive, depending upon the chemical nature and dielectric constants of the two phases of the system. The proportion of the total electric charge of the system which is of the opposite sign to that borne by the dispersed particles is, of course, borne by the liquid which constitutes the other phase. The origin of this electric charge on the colloidal particles is, as yet, not known with certainty; but it seems probable that it is due to a partial ionization of these small particles, similar to, but not so complete as, that which takes place when compounds which are soluble go into true solution in water, or other solvents which bring about the dissociation of dissolved substances.

The conditions necessary to bring a solid substance into a colloidal mixture with some liquid, or, in other words, to produce a suspensoid sol, require that the proportion of liquid to solid shall be large and some means of disintegrating the material which is to be dispersed into very fine particles. Many common chemical reactions, if carried out in very dilute solutions, result in the production of sols, especially if a small amount of some emulsoid is present in the reacting mixture; sols produced in this way are very stable, and the emulsoid which is used in stabilizing the sol is known as a "protective colloid." Direct methods of disintegration; such as reduction by chemical agents, discharge of a strong electrical current through the substance which is to be dispersed while it is submerged in the liquid, alternate treatment of finely ground material with alkali and acid so as to frequently change the electric charge, etc., are utilized for bringing inorganic compounds into the colloidal state.

Suspensoids usually contain less than 1 per cent of the solid dispersed through the liquid. In fact, extreme dilution is one of the necessary conditions for suspensoid-formation.

Emulsoids are much more easily produced than are suspensoids. The property of forming an emulsoid seems to be much more definitely a characteristic of the substance in question than does the formation of sols from solids which, under other conditions, may form true solutions. This difference may be due to the fact that the liquids which easily form emulsoids (usually those of organic origin) have very large molecules, so that the transfer from molecular to colloidal condition involves much less change in such cases than it does in the case of solid (inorganic) substances of relatively low molecular weight. This view of the matter is further borne out by the fact that solids which have very large molecules (generally of organic origin) take on the colloidal form much more readily than do those of small molecular size.