300 PROPERTIES CONFERRED BY COLLOIDAL CONSTITUENTS 



rich phase is concave; in other words the water-proof phase is the 

 Internal Phase of the gel, and the water-rich material constitutes the 

 External Phase of the gel. If, however, to a ternary mixture of gelatin, 

 alcohol and water which forms such a gel as that described above, 

 more gelatin be added, the character of the gel changes entirely and 

 its structure becomes inverted. The water-poor phase becomes con- 

 cave and the water-rich phase, instead of being, as formerly, concave, 

 becomes convex to it. On cooling such a mixture to a temperature 

 below that at which it forms an optically homogeneous solution, 

 droplets separate out which are poor in gelatin, while the interstitial 

 portion of the system, which is rich in gelatin, solidifies. Thus the 

 gel comes to possess a Honeycomb-structure the droplets being poor in 

 gelatin and rich in water. This is very clearly shown by the following 

 analyses made by Hardy : 



TEMPERATURE OF THE MIXTURE, 15 C. (EQUAL PARTS OF WATER AND 



ALCOHOL). 



From these analyses it is also clear that the two phases in a protein 

 gel are not of constant composition, but may, under different conditions 

 of total concentration, etc., vary widely in their relative and absolute 

 gelatin and water-content. This system differs, therefore, from the 

 system phenol-water, not only in the extent of the surface which 

 separates the phase, but also in the variability of the composition of 

 its phases, in this respect resembling rather the system hydrated 

 silica-water. 



The reason for this inversion of structure which occurs in concen- 

 trated gelatin Jellies is the same as that which is the origin of the 

 inversion of structure in olive-oil-water Emulsions when the proportion 

 of oil to water is increased beyond a certain limit. The spreading- or 

 covering-power of water is not unlimited and therefore the amount of 

 oil or gelatin which it can surround is correspondingly restricted. 



The question has been raised whether the jelly which is formed by 

 gelatin dissolved in water (instead of alcohol-water mixtures) really 

 possesses a structure analogous to that observed by Hardy in ternary 

 systems. It has been urged that this structure is an artefact arising out 

 of partial Coagulation of the protein, since it is not directly visible in 

 binary systems. The action of coagulants such as alcohol or sublimate 

 upon jellies which already possess a structure of this type, however, is 

 not to otherwise alter, but merely to coarsen their structure. This is due 

 to loss of water on the part of the colloid-rich droplets with a consequent 

 diminution of the volume of the colloid-rich phase and an increase in 

 the volume of the more fluid interstices. This can be shown, not only 



