THE COLLOIDAL CONDITION 209 



definite chemical hydration, however, as the same gel is formed 

 with all possible variations in the water content. 



Gels may be either rigid, as in the case of those of silicic acid, 

 etc., or elastic, as are those of gelatin, egg-albumin, agar-agar, etc. 

 The latter are the common type of gels among organic colloids. 

 They can be easily changed in shape, or form, without any change 

 in total volume. 



In gel-formation, the two phases of the system take on a dif- 

 ferent relationship to each other. The disperse, or solid, phase 

 becomes associated into a membrane-like, or film, structure, sur- 

 rounding the liquid phase in a cell-like arrangement. That is, the 

 whole mass takes on a structure similar to a honeycomb (except 

 that the cells are roughly dodecahedral in shape, instead of the 

 hexagonal cylinders in which the bees arrange their comb cells), 

 in which the original disperse phase constitutes the cell-walls and 

 the original liquid, or continuous phase, represents the cell-contents. 

 The cells of an elastic gel resemble closely the cells of a plant tissue 

 in many of their physical properties. They are roughly twelve- 

 sided in shape, as this is the form into which elastic spherical 

 bodies are shaped when they are compressed into the least pos- 

 sible space. 



Imbibition and Swelling of Gels. When substances which are 

 natural gels, such as gelatin, agar-agar, various gums, etc., are 

 submerged in water, they imbibe considerable quantities of the 

 liquid and the cells become distended so that the mass of the 

 material swells up very considerably. This swelling will take 

 place even against enormous pressures. For example, it has been 

 found that the dry gel from sea-weeds will swell to 330 per cent of 

 its dry volume, if immersed in water under ordinary atmospheric 

 pressure; but that it will increase by 16 per cent of its own volume 

 when moistened, if under a pressure of 42 atmospheres. 



During the swelling of gels by imbibition of water, the total 

 volume of the system (i.e., that of the original dry gel plus that 

 of the water absorbed) becomes less. For example, a mixture 

 of gelatin and water will, after the gelatin has swelled to 

 its utmost limit, occupy 2 per cent less space than the total 

 volume of the original gelatin and water. It has been computed 

 that a pressure equivalent to that of 400 atmospheres would be 

 necessary to compress the water to an extent representing this 

 shrinkage in volume. 



