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 _V_ = 2_r_^2(_s_ - _s_´)_g_ / 9_n_; _V_ = velocity of the falling body, in millimeters per second; _r_ = radius of the particle, in millimeters; _s_ = specific gravity of the solid; _s_´ = specific gravity of the liquid; _g_ = the attraction of gravity, in dynes; _n_ = the viscosity of the liquid. 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.