COLLOIDS 37 



peculiar, in that sonic varieties, as silicic acid, aluininiuni hydrate, and 

 many proteins are rendered so insoluble that they cannot again be 

 dissolved in any fluid without first being modified in some way; where- 

 as colloids of the type of gelatin and agar are made more soluble by 

 heat. The change of colloids into insoluble forms, the "pedous" 

 condition of Graham, requires the presence of water, for the dry col- 

 loids may be heated to relativelj'' high temperatures without losing 

 their solubility. On the other hand, dehydration of colloids while in 

 solution will result in their precipitation and coagulation, as occurs in 

 protein solutions when alcohol is added. 



If solutions of two oppositely charged colloids are brought together 

 they may precipitate, but if either is present in excess the precipita- 

 tion may be incomplete, or even completely absent. This inhibition 

 of precipitation is of particular interest because it so closely resembles 

 the phenomenon observed in the precipitin reaction, whereby an 

 excess of the antigenic protein will prevent precipitation. Also cer- 

 tain colloids will prevent the precipitation of other colloids by elec- 

 trolytes, which fact is the basis of the Lange reaction of spinal fluid 

 with colloidal gold. 



Colloids are precipitated by many electrolytes, apparently through 

 the formation of true ion compounds, one or both of the ions of the 

 electrolytes uniting with the colloid ion; although some writers, as 

 Spiro, believe that the combination is merely an additive one between 

 entire molecules. Mathews^^ has advanced the theory that the solu- 

 tion tension of the ions is an important factor in determining the pre- 

 cipitation of colloids by electrolytes. In general, precipitation of 

 colloids results from the reduction of the surface in proportion to the 

 mass, because of an aggregation of the particles; this may be brought 

 about by changing the surface electrical conditions, by uniting the 

 molecules chemically, or by reducing the amount of the solvent. 



The Structure of Colloids and of Protoplasm. ^^ — Two very 

 different sorts of substances are usually included under the term colloid, 

 because they show the essential features of colloids in most respects; 

 but as in many other respects they are quite unlike each other, it may 

 be well to distinguish between them in some way. As a type of one 

 class we may take gelatin; of the other, such a substance as colloidal 

 arsenious sulphide. Gelatin solutions form gels upon cooling or evap- 

 oration, and redissolve when heated or when more solvent is added. 

 Arsenious sulphide does not form gels upon cooling, and when solidified 

 in any way, does not redissolve. In addition, the gelatin type is very 

 viscous, and is not coagulated by the presence of salts unless these are 

 added in large amounts; while the other type does not render the fluid 

 in which it is dissolved appreciably more viscid, and it forms a precipi- 

 tate immediately if minute amounts of electrolytes are introduced. 



^* American Journal of Physiology, 1905 (14), 203. 



" Review by Harper, Amer. Jour. Botany, 1919 (6), 273. 



