WHAT ARE COLLOIDS 9 



Gels behave differently; in fact there are a number of colloids 

 which separate from their solution almost water-free. If sols of 

 gold, silver, platinum, arsenious sulphid or antimony sulphid hydro- 

 sol, prepared .according to the method of BREDIG or the method of 

 SVEDBERG, precipitate, that is, separate from their solutions in the 

 form of flocks, they are almost free from water. Many inorganic sols 

 (i.e., the artificial ones), and according to my knowledge nearly all 

 natural organic sols, retain a large quantity of water upon separation. 



Gelatin is the most characteristic gel; its aqueous solutions (con- 

 taining only 1 per cent of water-free gelatin) gelatinize at ice-box 

 temperature. Furthermore, other sols, such as egg albumin, starches, 

 silicic acid, iron oxid, etc., on separation in gel form retain many 

 times their own weight of water, and form jelly-like masses in which 

 the proportion between the solid substance and the retained solvent 

 is by no means constant. According to the circumstances attending 

 the separation, the amount of water held fast in the gel has wide 

 limits of variation. This is a cardinal distinction. In accord with 

 it, I have adopted the happily chosen nomenclature of J. PERRIN 

 and call such colloids as throw down a practically water-free hydro- 

 gel , hydrophobe and those which produce a hydrogel swollen and rich 

 in water, hydrophile colloids. 1 



The gels of hydrosols (cf. p. 11) stabilized by protective colloids 

 are somewhat hydrophile, because very minute quantities of pro- 

 tective colloid are sufficient to give the inorganic sol the properties 

 of the protective colloid. 



The Structure of Jellies. 



Jellies are formed from their respective solutions by such physical 

 and chemical changes as would cause the separation of crystals in 

 the solution of a crystalloid, e.g., by cooling, by removal of water 

 either by a chemical change or by forming an insoluble substance (e.g., 

 by boiling or by acidifying an albumin sol) . It is thus apparent that 

 jellies are to be considered two-phased structures. 



Two phases are much more obvious in coagulated egg albumen 

 whose opacity and white color suggest a non-homogeneity of struc- 

 ture. Recently BACHMANN * has demonstrated ultramicroscopically 

 the two-phased structure of transparent jellies such as gelatin and 

 silicic acid. In gelatinizing, it is evident that granular flocculent 



1 I mentioned above that as far as I knew all natural organic sols are hydro- 

 phile. It might be objected that epidermis, hair, feathers, bark and numerous 

 other vegetable structures are deposited from natural sols and become very 

 poor in water. This is met by the assertion that when they are deposited they 

 contain much water and that the loss of water or drying out occurs subsequently. 



