Gelatine has been crystallized by von Weimarn by evaporating a dilute solution in aqueous alcohol whilst in a desiccator containing potassium carbonate, the temperature being maintained at 60°-70° C. The carbonate takes up water only, and the concentration of the alcohol therefore slowly increases until the gelatine is no longer soluble. Gelatine is usually found and known in the colloid state, however, and its behaviour in this state only is of practical importance. The fundamental idea of modern colloid chemistry is that colloids are heterogeneous systems, usually two-phased, in which one phase is liquid and the other phase either liquid or solid. The latter phase, which is divided into small separate volumes, is known as the "disperse phase," whilst the other is the "continuous phase" or "dispersion medium." The "dispersity" is the degree to which the reduction of the dimensions of the disperse phase has been carried, and is best expressed numerically in terms of "specific surface," _i.e._ surface area divided by volume, but it is also often expressed as the thickness or diameter of a film or particle. When the dispersity is not high, we have ordinary "suspensions" and "emulsions," which with increasing dispersity merge into the typical colloids. By analogy, colloids have been divided into "suspensoids" and "emulsoids," when the disperse phase is solid and liquid respectively. The classification, however, has not been found satisfactory, for some systems in which the disperse phase is undoubtedly liquid, exhibit characteristic properties of suspensoids, and _vice versâ_. A more satisfactory division, therefore, is found in the presence or absence of affinity between the two phases, the systems being termed "lyophile" and "lyophobe" respectively. If water be the continuous phase the terms "hydrophile" and "hydrophobe" are often used. Broadly speaking, the lyophile colloids correspond to the emulsoids, and the lyophobe colloids to the suspensoids. Gelatine is a typical hydrophile colloid.