202 PROTOPLASM 



will draw a free droplet of water into a sphere is not going to 

 force the structural framework of cellulose apart. 



There are other difficulties in interpreting the swelling of 

 jeUies on the basis of capillary forces. To what is due the fur- 

 ther swelling of a jelly which, having swollen to its maximum 

 capacity in pure water, swells more when acid is added? There 

 are no capillary spaces left to account for the additional swelling, 

 and those already full have become larger through swelling, 

 which lessens capillary attraction. When wood swells, the filling 

 of tiny spaces with water by capillarity (surface tension) is the 

 first force which comes into play, but the further swelling of the 

 wood is due to forces other than capillary ones. Another objec- 

 tion to capillarity as the mechanism underlying swelling in 

 jellies lies in the fact that there is no conclusive evidence of a 

 porous structure in jellies {e.g., gelatin). No capillaries are 

 visible in a gelatin jelly, and there is doubt as to the likelihood 

 of there being ultramicroscopic capillaries. If the structural 

 unit of gelatin is a micelle, then the jelly structure is that of a 

 sponge, and the spaces are capillaries; but if the structural unit 

 is a molecule, as some believe, then capillaries, as ordinarily 

 thought of, do not exist. With molecular dimensions come other 

 forces. Among these is adsorption. 



Water molecules orient themselves at the surface of some sub- 

 stances very readily. Such adsorbed water is known as water of 

 hydration. The older and still prevalent idea of the nature of 

 water of hydration (or crystallization) is that the bond between 

 the water and the crystal molecules is one of secondary valence 

 or adsorption, which implies that the union between the water 

 and the crystal molecules is a loose one, as the two are readily 

 separable, e.g., by heat. The newer viewpoint sees no special, 

 clear identity of the water molecule. The crystal with the water 

 is a new compound altogether, with a new arrangement of the 



atoms. 



Imbibition vs. Osmosis.— The swelling of gelatin has been 

 interpreted as an osmotic phenomenon, the claim being made 

 that imbibition and osmosis are identical in so far as the forces 

 involved are concerned. Thus, J. Loeb said, "the reason that 

 osmotic pressure, the viscosity of protein solutions, and the 

 swelling of protein gels are all influenced in a similar way by 

 electrolytes is that all three properties are in the last analysis 



