COLLOIDAL MIXTURES. 269 



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 desig- 

 nated 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 re- 

 searches 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 prop- 

 erties 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 addi- 

 tion 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, gel- 

 atinizing, 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 solu- 

 tions — 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 



