PROPERTIES OF GELATINE AND GLUE 205 



the system lyophile, and is evidently connected also with the 

 solubility of the disperse phase, and may indeed be electro- 

 chemical forces tending to form a series of hydrates, or at 

 least to cause an orientation or definite arrangements of the 

 water molecules in the zone of compression. This idea 

 receives some support from the hydrate theory of solution, 

 and the zones of compression and orientation are the colloid 

 analogue of the hydrates supposed to exist in solutions of 

 electrolytes. The extension of such zones on cooling are 

 then analogous with the series of hydrates formed, for 

 instance, by manganese chloride with 2, 4, 6, n, or 12 

 molecules of water when crystallized at temperatures of 

 20, 15, 21, 30, and -48 C. respectively, the idea 

 being that the salts most hydrated in solution crystallize 

 with most water. 



As the compression is the result of two factors, one of 

 which depends upon the nature of the disperse phase, we 

 expect and find in other lyophile systems a considerable 

 variation in their power of gelation. Some indeed, though 

 very viscous, e.g. egg albumin, never quite set like gelatine, 

 and others (e.g. agar-agar) set to a stiff gel from a much 

 weaker sol than gelatine. When the zones of compression 

 are large, as in gelatine, the magnitude of the compressing 

 force on the outermost part of the zone is relatively small, 

 and it is not surprising that time is necessary for the victory 

 of this force over the kinetic energy of the water molecules. 

 Hence we find a 5 per cent, jelly sets readily on cooling, but 

 its elasticity increases steadily for many hours after it has set. 

 This phenomenon, known as hysteresis, we should expect 

 and find to be much more marked in a case where the zone 

 of compression is unusually large (e.g. an agar gel). We 

 should also expect and find that hysteresis is more marked 

 in a high-grade gelatine than in a low-grade gelatine where 

 both eventually form gels of equal elasticity. We should 

 expect too and we find that hysteresis is more prominent 

 in weak gels than in strong. These points are of obvious 

 importance in testing gelatine by its elasticity, e.g. the \vr11- 

 known " finger test." 



