«The Source of the Electrical Charges on Colloids.»—Different views are held as to the source of electrification of colloidal suspensions. Only the two views of most direct interest in analysis can be considered here. In the first place, it is possible that ‹partial ionization› of the colloidal suspension produces the electrical charge. It is a significant fact that ‹basic colloids› (metal hydroxides, some basic dyes) receive a ‹positive› charge, such as would be left, if they were slightly ionized as bases (or salts of bases) and insoluble (suspended) [p133] ‹positive ions were retained by the particles›. That would be the case, for instance, if particles of colloidal ferric hydroxide, [Fe(OH)_{3}]_{‹x›}, sent a few hydroxide ions into solution and the suspended particles included positive insoluble ions. ‹Acid› colloids, on the other hand, assume a ‹negative› charge, as would be expected from this point of view. Then, the behavior of silicic acid is particularly suggestive; in alkaline or ‹slightly› acid liquids, in which its ‹ionization as a weak acid› is favored or predominant (Chapter X), it carries a ‹negative› charge, the charge that would be left on it, if it ionized, in part, as an acid or its salt. Silicic acid shows some slight tendency to ionize also as a base (see Chapter X) and the basic form of ionization would be favored by the presence of a strong acid (Chapter X). Colloidal silicic acid, as stated above, ‹changes its charge› from negative to positive as the solution passes from an alkaline to a (strong) acid reaction,[265] just as if, in the acid liquid, it ionized slightly as a base (salt of a base) and ‹retained a difficultly soluble positive ion›. Albumen, which has the property of being both a base and an acid,[266] shows the same change of charge[267] in the colloidal condition and the change has been ascribed[268] to the change of basic and acid functions. According to the other of the two theories, here considered, the electrification of the colloid may result from what is known as contact or surface electricity.[269] At the surface of two different substances there is always a potential difference.[270] In the case of finely divided suspensions, like the colloids, the contact surfaces are enormous, as compared with the surfaces involved in ordinary contact. Whether metals (Au, Pt, Ag) owe their charges to simple contact effects or to their (minimal) tendency to ionize (Chapters XIV and XV) is not known. In fact, no exact knowledge of the source of electrification of any colloid has yet been obtained.