HYDROPHILIC SOLS AND GELS 141 



not of coagula. The pressure thereby exerted may be tre- 

 mendous. Peas packed into a bottle, then covered with water 

 and tightly corked break the bottle on swelling. The swelling 

 of wood may split rock, as done by roots swelling in rock crevices 

 and artificially by wooden wedges used by the ancient Egyptians 

 for quarrying stone. Starch in water may exert an imbibition 

 pressure of over 2,000 atmospheres (the pressure in the average 

 locomotive boiler is 10 atmospheres). Gels on shrinking also 

 exert a pull. Dehydrating (drying) gelatin pulls with sufficient 

 force to chip the glass to which it is attached. 



Heat of imbibition, or heat of swelling, or heat of wetting holds 

 the same important position in the thermal chemistry of colloids 

 and the theory of colloidal systems as do heat effects in ionic 

 systems when these go into solutions. Every freshman in 

 chemistry knows how hot a beaker of water will become when 

 sulphuric acid is added to it. Swelling jellies get hot in the 

 same way, though not so much so. Heat of imbibition is 

 expressed in calories per gram of substance. It may give rise 

 to an increase of 0.2 to 0.9°C. when carbon, starch, cotton, or 

 animal membranes are wet. 



Heat of wetting has been used by Bouyoucos to define soil 

 colloids. The question whether the colloidal state shall be 

 distinguished on the basis of structure (size of particle) or of 

 activity (energy manifestations), such as heat of imbibition, is 

 here answered in favor of the latter dynamic viewpoint. Again 

 we can say, we inquire of colloids not what they are but what 

 they do. The inevitable difficulty arises, as in the case of all 

 criteria of the colloidal state, that there is no sharp line of demar- 

 cation. Heat of swelling is not wholly present or wholly absent. 

 Its apparent presence or absence depends on the sensitiveness of 

 our method for measuring it. However, it can be said that the 

 greater the dispersion (i.e., the smaller the particles) in soil the 

 greater is the heat of wetting. To this extent is heat of imbi- 

 bition a measure of the colloidal state in general. 



Adsorption. — Adsorption (Chap. X) is the concentration of 

 one substance at the surface of another. 



Silica gel, when thoroughly dry, is hard, glassy, and very 

 finely porous (Fig. 90). It functions as an ideal adsorbent of 

 many things. Formerly, charcoal played this role in the com- 

 mercial world, but through the development of better methods 



