THE PROPERTIES OF COLLOIDS 167 



of network, the result varying according to the nature of the colloid 

 and its concentration. Thus gelatin changes from the condition of 

 hydrosol to hydrogel with fall of temperature. The same is true of 

 agar. On the other hand, by adding calcium chloride to an alkaline 

 solution of casein, we obtain a mixture which sets to a jelly on warming, 

 but becomes fluid again on cooling. Other agencies may lead to the 

 production of changes which are irreversible. Thus a strong solution 

 of colloidal silica sets to a solid jelly on the addition of a trace of neutral 

 salt, and it is not possible to reform the hydrosol, however long the 

 jelly is ..ubmitted to dialysis. 



Two methods of bringing about coagulation of hydrosols deserve 

 special mention. The first of these is heat- coagulation. If a solution 

 of egg-albumin or globulin be heated in neutral or slightly acid medium 

 and in the presence of neutral salt, the whole of it is thrown down 

 in an insoluble form. This coagulated protein is insoluble in dilute 

 acids or alkalies. The same coagulative effect of heating is observed 

 in the metallic sols. With concentrated solutions of protein, heat 

 coagulation results in the formation of a gel, i.e. a network of insoluble 

 protein, containing water or a very dilute solution of protein in its 

 meshes. In dilute solutions the result is the production of a floccu- 

 lent precipitate. 



Another method is the so-called mechanical coagulation. If a 

 solution of globulin or albumin be introduced into a bottle, which is 

 then violently shaken, a shreddy precipitate makes its appearance in 

 the solution, and this precipitate increases, so that by prolonged 

 shaking it is possible to throw down 80 or 90 per cent, of the dissolved 

 protein in the coagulated form. Rarnsden has shown that this 

 mechanical coagulation is a surface phenomenon. It depends on the 

 fact that a large number of substances in solution (viz. any which 

 lower the surface tension of their solutions) undergo concentration 

 at the free surface of the fluid. Such substances are proteins, bile- 

 salts, quinine, saponin, &c. In the case of proteins the concentra- 

 tion reaches such an extent, and the molecules at the surface are 

 so closely packed together, that they form an actual solid pellicle, 

 which hinders the movement of any object, such as a compass needle, 

 suspended in the surface. When the solution is violently shaken, new 

 surfaces are constantly being formed, and as the older surfaces are 

 withdrawn into the fluid, the solid pellicle on them is rolled up into 

 a fine shred of coagulated protein, and this process will continue until 

 there is no protein left to form a pellicle. 



We must conclude that colloidal solutions, although differing 

 so widely from true solutions in many of their properties, are con- 

 nected with these by all possible grades. In a solution of an ordinary 

 crystalloid or electrolyte the molecules of the dissolved substance are 



