PROPERTIES OF GELATINE AND GLUE 203 



zones of compression begin to come in contact, and the 

 system will then show a considerably increased viscosity. 

 With further increase in concentration the zones of com- 

 pression will overlap throughout the system, and when the 

 layers under considerable pressure are thus continuous, the 

 whole system will acquire a rigidity much greater than water 

 and approaching that of a solid body. This is a gelatine gel, 

 or " jelly." With increasing concentration the jelly becomes 

 increasingly rigid, and if it be eventually dried out under 

 suitable conditions it forms what is practically solid body 

 gelatine which, however, still contains from 12 to 18 per 

 cent, of water. 



It will be clear that, in the case of gelatine jellies (e.g. of 

 3-10 per cent, strength), an increase in temperature will 

 cause an increase in the kinetic energy of the particles and 

 effectively reduce the zones of compression. Indeed, they 

 may be reduced to such an extent that they are no longer 

 in contact, and the rigidity due to the continuous contact of 

 the layers of great compression will then disappear ; as we 

 say usually, the jelly melts. On cooling, the decreased 

 kinetic energy of the water molecules results in the return of 

 the state of compression, with rapidly increasing viscosity 

 and eventual gelation ; as we say usually, the jelly sets. 

 Neither of these changes takes place at a definite temperature 

 (like a melting-point), and in " melting " (solation) or in 

 " setting " (gelation) the temperature-viscosity curve is 

 quite continuous. By various arbitrary devices, however, 

 approximate melting and setting points may approximately 

 be determined. The results also vary somewhat with the 

 concentration of the gel or sol. Gels between 5 and 15 per 

 cent, strong melt about 26-30 C. and set at i8-26 C. 



On this view, we must regard a gelatine gel as a continuous 

 network of water under great compression, and in this net- 

 work are zones of still greater compression, which surround 

 the particles of the disperse phase the gelatine itself, and 

 zones of less compression which in a weak gel, at any rate, 

 have a compression equal to or much the same as the normal 

 state of compression in water. 



One consequence of this system is, that when a piece of 



