298 PROPERTIES CONFERRED BY COLLOIDAL CONSTITUENTS 



so that, in the course of time, the motion of the disc is not merely 

 opposed by the friction of immediately adjacent molecules, but by the 

 inertia of all of the protein molecules of the fluid which are attached 

 indirectly, through a continuous meshwork, to the oscillating disc. 



JELLIES AND GELATINIZATION. 



The structure which confers upon protein solutions their peculiar 

 type of Viscosity leads in many cases when the solutions are sufficiently 

 concentrated to their acquiring certain of the properties of solids. 

 Such solutions are what we term Jellies, and they resemble solids in 

 presenting pronounced resistance to deformation which, however, 

 yields to the slightest force if its action be sufficiently prolonged. 

 Where forces of an instantaneous character are concerned, therefore, 

 the jellies are solids, but where forces of prolonged action are concerned 

 they are fluids. The distinction between a solid and a jelly is, in fact, 

 largely a matter of degree. A solid will flow under a sufficiently great 

 pressure applied for a relatively brief period of time, but a sharp impact 

 affects it as it affects an elastic solid, causing oscillation and recoil, 

 but not deformation. Intermediate states of matter are afforded by 

 such materials as Sealing-wax which, even at ordinary temperatures, 

 will flow under slight pressure applied for very prolonged periods, 

 but which under even considerable forces acting sufficiently suddenly 

 exhibits all the brittleness of a solid. 



Under certain conditions, when the meshes are sufficiently coarse, 

 various jellies or "gels" clearly display a network or spongy structure. 

 If an insoluble gel, such as White of Egg coagulated by fixatives, the gel 

 of Colloidion produced by the action of chloroform upon an ether 

 solution, common black India-rubber, or the hydrogel of Silica be 

 examined under high magnification they can all be seen to possess a 

 fine sponge-like structure. When, for example, a thirteen per cent, 

 solution of egg-white is fixed with sublimate, sections are found to 

 show a sponge- structure, or, what corresponds to a sponge in two 

 dimensions, a network-structure. W. B. Hardy, who has especially 

 investigated this gel, failed to obtain with acid or basic dyes any 

 staining of the substance within the meshes of the net, and pressure 

 applied to the gel resulted in the squeezing of fluid out of its interstices. 

 The structure of the gel is, therefore, that of an open sponge-work of 

 solid, containing fluid within its meshes. Direct experimentation with 

 A gar jellies has shown that in a gel containing one r>er cent, of agar, the 

 solid framework is a solution of water in agar, while the fluid in the 

 interstices is a dilute solution of agar in water. Upon heating the 

 solution the two components become miscible in each other and we 

 obtain what appears to be a homogeneous solution. Upon the basis 

 of these facts Hardy draws a far-reaching analogy between the jellies 

 which liquefy when heated, and gel when cooled, and the system 

 Phenol-water, which, if it contains more than 71 per cent, or less than 



