that the distinction between colloidal solutions and suspensions is not one of quality, but only one of degree.
Properties related to the Electrification of the Particles.—A quite distinct class of properties may be next considered, which depend not on the size of the colloidal particles, but apparently upon the presence of electric charges upon them.
The most direct evidence of this electrification is furnished by the migration of the colloidal particles through the liquid under the influence of an applied electromotive force. This effect may be well illustrated with colloidal suspensions of arsenious sulphide and ferric hydroxide contained in two U-tubes. The tops of the U-tubes are covered with goldbeaters' skin and are surrounded by wider tubes containing pure water in which platinum electrodes are placed, so that the products of electrolysis collecting around them may not influence the colloid. These tubes are then connected in parallel with the terminals of a 110-volt circuit in such a way that the current will flow through each of them in the direction from left to right. It is some minutes before any result is observed. Then it is seen that the ferric hydroxide has moved down with a sharp surface of demarkation on the side where the current enters, leaving a clear layer of water above, and that the arsenious sulphide has done the same, but on the opposite side of the tube. In other words, the ferric hydroxide particles are moving with the positive current towards the cathode, the arsenious sulphide with the negative current towards the anode. The former are, therefore, positively, and the latter negatively, charged. These results are typical ones: such movement, or migration, as it is commonly called, is exhibited by all colloidal suspensions, and, it may be added, also by fine microscopic suspensions, like those of quartz, kaolin and lampblack. Other basic hydroxides, like those of aluminum, chromium and thorium, and certain dyestuffs, migrate to the cathode just as does the hydroxide of iron. The suspended particles of almost all other substances, whether colloidal or microscopic, migrate to the anode. This is true, for example, of silicic acid, stannic acid, metallic sulphides, salts like silver iodide and Prussian blue, and metals like gold and platinum. Of special interest with reference to the explanation of the phenomenon is the recently discovered fact that an egg-albumen suspension migrates towards the cathode in an acid liquid and towards the anode in an alkaline one.
In regard to the cause and character of the electrification two assumptions deserve consideration: one is that it is simply an example of contact electricity, the colloid particle assuming a charge of one sign and the surrounding water that of the other. This correlates this phenomenon of migration with that of electrical endosmose; for the motion of suspended kaolin, for example, through water against the
