IMBIBITION 201 



imbibition pressure involved in the swelling of seeds causes 

 bursting of the seed coat, thus allowing the embryo to emerge. 



Gels may hold an extraordinary amount of water by imbibi- 

 tion. Some colloidal solutions (of cadmium and of calcium 

 germanate) may set to a firm gel even though the concentration 

 of solid matter is but 0.2 per cent, and the water content, there- 

 fore, 99.8 per cent. This is almost equaled by the jellyfish, 

 which contains 99 per cent water. 



The shrinking of gels from loss of water produces a tension, or 

 pull, as great as imbibition pressure. This is illustrated by 

 gelatin, which, when shrinking, pulls with sufficient force to 

 chip the glass to which it is attached. 



The physical nature of imbibition is little understood. The 

 problem is best handled by approaching it along those paths 

 which will give the surest footing; then the venture may be 

 taken a little way into the field of uncertainty. Capillary forces 

 (surface tension and adhesion) are responsible for the absorption 

 of water by porous gels, such as the inorganic silica gel. These 

 nonextensile gels contain myriads of pores or capillaries. Such a 

 structure of the silica gel and the nature of the force filling it with 

 water are questioned by none. The sizes of the pores have been 

 accurately determined, with a minimum of 5 m/u, which gives to 

 the gel an ultramicroscopic spongelike structure (Fig. 90). 

 Pumice stone is a similar example; water enters its pores under 

 the pull of surface forces acting in tiny spaces. Pumice stone 

 and silica gel are of the nonswelling type. We do not, therefore, 

 have to do with imbibition in them. 



The turgescent gels, or jellies, are chiefly organic, gelatin and 

 albumin being typical examples. Some are organic salts; 

 among these are carbohydrates, such as starch, cellulose, and 

 agar, and certain soaps and dyes, which form perfect jellies. 

 Agar is a trisaccharide obtained from seaweed and forms a 

 typical jelly much like gelatin in its physical properties, though 

 its chemical nature is quite different. Concerning the structure 

 of these jellies almost nothing is known, with the exception of 

 cellulose. 



The mechanics of swelling is apparently not due to capillary 

 forces, which one might assume to be operative in the case of 

 porous material such as wood (cellulose) ; capillarity is not a dis- 

 tending force; it is a tension. The pull of a surface film which 



