action of gravity, however long continued, nor by filtration through paper, but which are so separated when the liquid is forced through animal membranes, the substance then remaining behind being designated the colloid. This distinguishes them, on the one hand, from suspensions of fine visible particles, and, on the other, from ordinary solutions, and it implies that the colloidal particles are intermediate in size between the particles of such suspensions and the molecules which are present in ordinary solutions.
It is obvious, however, that this definition is not based upon a really fundamental distinction either in the properties exhibited by the various mixtures or in the character of their particles. It would, therefore, not be surprising to discover that the so-defined group of colloids include substances having very different properties in other respects than that just considered. And the first result of the researches upon colloids which should be emphasized is that there are in fact at least two kinds of dissolved or suspended substances retained by animal membranes, which differ so radically in their other properties that their inclusion in the same class is sure to lead to serious confusion, unless special pains be taken to discriminate between them. As types of these two classes of colloidal mixtures may be taken an aqueous solution of gelatine and one of colloidal arsenious sulphide. The former possesses a much greater viscosity than that of water; the latter does not appreciably differ from it in this respect. The former gelatinizes upon cooling or upon evaporation, and passes again into solution upon heating or addition of the solvent; the latter does not gelatinize upon cooling, and if gelatinized by other means it does not redissolve upon heating. The former is not coagulated by the addition of salts (unless in excessive amount); the latter immediately gives an abundant precipitate. This difference may be readily shown by adding to a tube containing a one per cent, gelatine solution and to one containing a colloidal suspension of arsenious sulphide a little strong magnesium chloride solution, when no effect will be observed in the first tube; while a voluminous yellow precipitate will result in the second. We have, therefore, to distinguish the viscous, gelatinizing, colloidal mixtures, not coagulated by salts, from the non-viscous, non-gelatinizing, but readily coagulable, mixtures. The former class may be designated colloidal solutions, the latter, colloidal suspensions. This nomenclature is based upon the belief that a more fundamental distinction between the two classes of mixtures is the possession by the former of the characteristic properties of true solutions—osmotic pressure, diffusibility, and usually a limited solubility of the colloid at some temperature, and the absence of these properties in the members of the latter class and the manifestation by them of many similarities to macroscopic and microscopic suspensions. Even though
