ous solutions of silver nitrate and sodium chloride are mixed, an abundant curdy precipitate is produced, but, if a little gelatine solution be first added to one of the salt solutions, only an opalescence results, and the silver chloride formed by the metathesis remains indefinitely in the state of a colloidal suspension. Glycerine, sugar and even ether in some cases have a similar influence. This result may arise from the fact that in the presence of the gelatine the particles of silver chloride after attaining a certain size are not capable of diffusing, and hence of coming into contact with one another. It is probable, however, that, at any rate in many cases, the gelatine prevents the coagulation by forming an envelope around the solid particle. Whatever may be the explanation of the phenomenon, it is one of great technical importance, especially in relation to photography; for upon it is based the preparation of the so-called emulsions of silver salts in gelatine, collodion or albumen with which dry plates, films and printing-out paper are coated.
Recent investigations have proved that the gelatinization of these colloidal solutions arises from the separation of a portion of the colloid in the solid state in more or less continuous masses. The resulting jelly, or gel, as it is technically called, has been shown to have an irregular sponge-like structure, the web consisting of a solid mixture of the two substances and the interstices being filled with a liquid solution of them. This has been proved in some cases by direct microscopic observation, and in others by separating the liquid from the solid portion by pressure and by analyzing these portions, which were thus shown to have a very different composition with respect to the proportions of the two constituents. Thus one of the investigators of this subject, Hardy, states that when a solution of 13.5 grams of gelatine in a mixture of 50 c.c. of alcohol and 50 c.c. of water is gradually cooled, it remains homogeneous until a temperature of 17° centigrade is reached. Then it separates into two liquid phases, and is seen to consist of small microscopic droplets suspended in a fluid matrix. As the temperature falls, these droplets cohere to one another and at 12° they have become solid, forming a framework built of little spherical masses. The mixture as a whole has then become a jelly. At 14° the droplets were separated and found to contain 18 per cent, of gelatine while the matrix contained only 5.5 per cent. The important statement is also made that the first appearance of the droplets is attended by a great increase in viscosity, while the subsequent increase is a continuous one. The abnormal viscosity of such colloidal mixtures is, therefore, probably always due to a physical heterogeneity of this kind. The investigations made with other gelatinizing colloids, such as agar, albumen, starch and even silicic acid, have led to a similar conclusion in regard to the structure of the jelly.
