«Electrical Conditions of Colloids.»—One of the most important discoveries made on colloids is the observation that the suspended particles of a large class of colloids carry electrical charges, so that there is a potential difference between the particles and the liquid in which the suspension exists.[258] Thus, in the colloidal suspension of arsenious sulphide (prepared as described on p. 125) the ‹sulphide particles› carry ‹negative› charges, and the solution bathing them is ‹positive›. Conversely, colloidal ‹ferric hydroxide› (p. 127), in water, is charged with ‹positive›, the water with ‹negative›, electricity. The existence and character of these charges may be readily demonstrated by the passage of a current of electricity through the colloidal suspension in U tubes (‹exp.›).[259] Colloidal arsenious sulphide is found to migrate toward the positive pole, ferric hydroxide [p132] to the negative pole, the movement being easily followed by the color of the suspended particles. The following colloids, which are of interest in analytical chemistry, are found to carry a ‹negative› charge in pure water: Colloidal ‹acids› (silicic, stannic), ‹sulphides› (As_{2}S_{3}, As_{2}S_{5}, CdS, etc.), ‹salts› (AgI, AgCl) and ‹metals› (Au, Pt, Ag). It is noteworthy that the suspensions of finely divided clay, kaolin, quartz, carbon, carry the same charge as these colloidal suspensions. On the other hand, ‹metal hydroxides› (ferric, aluminium, chromic), and ‹basic› substances in general, carry ‹positive› charges. On the other hand, the colloids of one important group are found to be almost without any electric charges;[260] the passage of an electric current through their suspensions has little or no effect on them. Perfectly neutral albumen and gelatine are common representatives of this group. The charge on a colloid seems to be liable to variation with the nature of the liquid in which it is suspended. Colloidal platinum in water is negative, in a mixture of water and alcohol, positive.[2] Of peculiar interest and importance is the fact that some colloids ‹change› the ‹character› of their ‹charge› when the liquid, in which they are suspended, is made to pass from an ‹acid› to an ‹alkaline› condition, and ‹vice versa›.[261] For instance, albumen[262], colloidal silicic acid[263] and colloidal stannic acid[264] are negative in alkaline liquids, positive in acid. The relation of these facts to the chemical nature of the substances will be discussed presently.