140 PROTOPLASM 



Lyophobic Gels. — We have so far considered two main types 

 of gels — the organic lyophiHc ones which take up water, swell, 

 and form elastic jellies, such as gelatin, albumin, agar, vegetable 

 gums, and cellulose; and the inorganic lyophilic ones, such as the 

 silica gel, which also take up water but do not swell and are not 

 extensible. There are exceptions like rubber, which is of organic 

 material and highly elastic but not lyophilic; i.e., it does not take 

 up water. There is still another class of gels — those of the 

 oxides or hydroxides of metals. These gels are of inorganic 

 material and lyophobic; they do not absorb water. The fact 

 that they are oxides of metals tends to put them with the suspen- 

 sion colloids, but their gel-forming capacities class them as 

 lyophilic colloidal systems; they are thus border-line systems. 



MetaUic oxides that form gels are Fe203, CuO, V2O5, AI2O3, 

 SeaOa, ZrOz, and Sn02. Best known among these lyophobic 

 inorganic gels is that of iron oxide. When crystals of ferric 

 chloride are thrown into hot water, an insoluble and colloidally 

 dispersed oxide or hydroxide of iron is formed. Such a sus- 

 pension is indiscriminately called an oxide or hydroxide, because 

 it is not known with certainty whether the colloidal particles in 

 suspension are Fe(0H)3, Fe203 (FeOCl)^, or some other form. 

 The vanadium pentoxide, V2O5, sol will probably always go by 

 that name even though the particles may be a hydroxide. 

 Boehm studied the iron oxide sol and beheved it to be mostly 

 of basic iron chloride, Fe(0H)Cl2, but also in part hydrated iron 

 oxide, FeO(OH). This can be interpreted to mean that the 

 core of the colloidal particle is iron oxide, and the surface a shell 

 of adsorbed water and HCl. Heller added to and supported 

 this viewpoint but with emphasis on the hydroxide ("Goethit," 

 one of the most common hydroxides of iron in nature), with the 

 basic salt as incidental. It seems probable that most of the 

 metallic oxide solutions are hydrated. Choosing ferric hydroxide 

 as representative of the group, the metallic colloidal hydroxides 

 must, if not classed separately, be put with the lyophobic sys- 

 tems, not only because the precipitated basic salt shows little 

 affinity for water but also, and perhaps primarily, because the 

 sol is very sensitive to electrolytes. 



Imbibition.— The taking up of water when accompanied by 

 swelling is imbibition (Chap. XH) ; the resulting condition is 

 known as turgescence. Imbibition is characteristic of jellies but 



