204 ' ANIMAL PROTEINS 



gelatine swells, there is a considerable enlargement in the 

 zones *of compression ; in other words, some, at least, of the 

 imbibed water is compressed. Now the compression of 

 water means that work is done, and when gelatine swells, 

 therefore, we expect and actually find that heat is 

 liberated (57 cal. per g. gel). Hence also by the I^e Chatelier 

 theorem, we expect and find that gelatine swells best in 

 cold water. Further, the compression of water involves a 

 decrease in volume, and we therefore expect and actually 

 find that the volume of the swollen jelly is appreciably less 

 than the volume of gelatine plus the volume of water imbibed. 



Another consequence of such a compressed system is 

 that a gelatine jelly, even in water, will have a surface tension 

 towards water just as the water itself has such a tension to 

 the water vapour above the liquid. This interfacial tension 

 of the jelly will of course have a contractile effect, and will 

 tend to resist swelling and to limit it as far as it possibly can. 

 This force, tending to contract the jelly and resist imbibition 

 is therefore one of the main influences at work in the swelling 

 of gelatine, and is one of the two principal factors which 

 determine the extent of the maximum swelling when equili- 

 brium is established. The force tending to resist swelling is, 

 in the ultimate, just surface tension. Its actual magnitude 

 depends, of course, mainly upon the extent of compression 

 in the dispersion medium of the gel, and will be a resultant 

 which is a function of this compression. The magnitude 

 will thus vary with the average compression in the con- 

 tinuous network of compressed water. It will be obvious 

 that as the jelly swells the power of resisting the swelling 

 will decrease, and the interfacial tension with the external 

 water will tend to disappear. If the force tending to swell 

 were great enough the swelling would continue until the 

 zones of compression were no longer in contact and the gel 

 would become sol. 



As suggested above, it is probable that the extent of the 

 zones of compression is determined by another factor in 

 addition to the great development of surface. That factor 

 is connected if not identical with that power which makes 



