positive current is obviously the converse of the flow of water through a porous clay diaphragm with the current. It does not, however, give an obvious explanation of the facts that the basic colloidal particles become positively charged and the acidic and neutral ones negatively charged, or of the peculiar behavior of albumen. The other assump- tion accounts for these facts. According to it the phenomenon is a simple case of ionization, the character of which may be best illustrated by specific examples. Thus, each aggregate of ferric hydroxide mole- cules may dissociate into one or more ordinary hydroxyl ions and a residual, positively charged colloidal particle, and each aggregate of silicic or stannic acid molecules into one or more hydrogen ions and a residual negatively charged colloidal particle. Albumen, which is known to be capable of forming salts with both acids and bases, would, acting as a salt, dissociate into an ordinary positive ion and a colloidal negative one in alkaline solution, and into an ordinary negative ion and a colloidal positive one in acid solution. To explain the behavior of neutral substances like gold or quartz by this hypothesis, it is neces- sary to supplement it by the assumption that in these cases it is the water or other electrolyte combined with or adsorbed by the colloidal particles which undergoes ionization. It seems not improbable that there may be truth in each of these hypotheses, contact electrification occurring in the case of the coarser suspensions, and ionization in the case of those which approximate more nearly to colloidal solutions. It should be noticed that these hypotheses do not differ as to the charge on the colloidal particle itself, the existence of which is in fact experi- mentally demonstrated, but only as to the location of the accompany- ing charge of opposite sign, namely, as to whether it is on the water itself or on ordinary ions dissolved in it. This matter is not essential to our further considerations.
Let us turn now to the extremely important phenomenon of the coagulation of colloidal suspensions. It will be seen that this phe- nomenon is certainly closely related to the electric charges on the col- loidal particles. Indeed, it seems highly probable that they remain in suspension because of their electrification. Thus it has been found that egg-albumen, whose particles are shown by their migration to be positively charged in acid solution and negatively in alkaline solution, immediately coagulates when the solution is made neutral. Attention may also be called to another interesting fact having, apparently, the same significance; namely, to the fact that when two colloidal suspen- sions, whose particles have an electric charge of the same sign, are mixed, they have no influence upon each other, but when two suspen- sions, with particles oppositely electrified, are brought together, the two colloids combine with each other, and with proper proportions a complete coagulation results. Thus, upon mixing suspensions of col-
