JELLIES AND GELATINIZATION 301 



by direct observation, but also by the relative ease with which water 

 can be expressed from the jelly before and after "fixation." From 

 Poiseuilles' Law for the outflow of liquid from capillary tubes, it follows 

 that the pressure required to express the fluid must vary approximately 

 as the inverse fourth power of the diameter of the meshes, although, 

 of course, the variable viscosity of the expressed fluid will be a factor 

 introducing departures from this simple law. Now a hydrogel con- 

 taining 13 per cent, of Gelatin at a temperature of 15 C. will endure a 

 pressure of 400 pounds to the square inch without expression of water; 

 after fixation with formalin or corrosive sublimate, however, the fluid 

 can be expressed from the gel like water from a sponge, with simple 

 hand-pressure. 



Since more complete coagulation does not alter the type of structure 

 possessed by jellies of partially coagulated protein, but merely coarsens 

 it, it is a fair inference that jellies which have undergone no measure 

 of coagulation also possess the type of structure outlined by Hardy, 

 but that owing to its fineness the details of this structure are not 

 visible. 



The existence of a structure in jellies formed by the solution of gelatin 

 in water is also objectively demonstrated by the observation of Liese- 

 gang, that when silver nitrate diffuses into gelatin which is impregnated 

 with potassium bichromate, the precipitation of insoluble Silver 

 Bichromate does not occur indifferently in all parts of the area of diffu- 

 sion, but in concentric circles. It has also been shown by Rohonyi 

 that when thin films of gelatin are frozen the ice-crystals are formed 

 in concentric rings. It is difficult to clearly conceive any mechanism 

 which would permit this in a perfectly homogeneous medium. The 

 theory that crystallization is inhibited by the gelatin until a certain 

 degree of supersaturation is attained might account for failure of 

 precipitation or crystallization at certain points, but, provided the 

 jelly were strictly homogeneous and structureless, it fails to account for 

 its appearance at other points. 



The experiments of Hardy show that on adding water to the system 

 alcohol-water-gelatin the gelatin-rich phase progressively imbibes 

 water until it passes by a series of insensible transitions into a Solution 

 of gelatin. We have seen that solutions of protein show evidence, in 

 the peculiar type of resistance to deformation which they display, of 

 possessing a structure which is most easily conceived as a spongework 

 of protein molecules with intercommunicating meshes filled with water. 

 The Structure of the solution is therefore that of an attenuated jelly 

 and there is no distinction of kind, but only of degree, between a 

 protein solution and a protein jelly. As a matter of fact, if the Viscosity 

 of a solution of gelatin sufficiently concentrated to gelatinize at room- 

 temperature be measured at intervals while it is cooling, no sharp 

 change of viscosity is found to occur at gelation, the viscosity of the 

 solution just prior to that point being so great as to afford clear indica- 

 tion of the forthcoming semi-solidification. 



The structure of protoplasm, therefore, consisting as it does of an 



