COLLOIDS 41 



more soluble by heat. The change of colloids into insoluble forms, 

 the 'Specious'' condition of (irahani, re(|uir('.s the presence of water, 

 for the dry colloids ni;i\ lie iieated to relatively hiprh temperatures 

 witliout losino- their s()lul)ilit y. On the other hand, deliydration of 

 colloids while in solution will result in their precipitation and coagu- 

 lation, as occurs, in protein solutions when alcohol is added. 



If solutions of two oppositely char<ied colloids are broujrht together 

 they nuiy ])recipitate, but if either is present in excess the precii)ita- 

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

 of precipitation is of particular interest because it so closely resem- 

 bles the phenomenon observed in the precipitin reaction, whereby an 

 excess of the antigenic protein will ])revent precipitation. Also cer- 

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

 trolytes, which fact is tlie 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 differ- 

 ent 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 

 precipitate immediately if minute amounts of electrolytes are intro- 

 duced. As the former type resembles in many details the true solu- 

 tions, while the latter approaches more closely to the suspensions, it 

 has been proposed to distinguish them by the terms "colloidal so- 

 lution" and "colloidal suspension." -- Of the two types, the colloidal 



21 American Journal of Phvsiolofry. 190.5 (14), 203. 

 22^0768, Amorican Chemical Journal, 100.5 (27), 85. 



