2 72 POPULAR SCIENCE MONTHLY. 



cms solutions of silver nitrate and sodium chloride are mixed, an abun- 

 dant 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 impor- 

 tance, especially in relation to photography; for upon it is based the 

 preparation of the so-called emulsions of silver salts in gelatine, col- 

 lodion 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 micro- 

 scopic 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 gel- 

 atine 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 state- 

 ment is also made that the first appearance of the droplets is attended 

 by a great increase in viscosity, while the subsequent increase is a con- 

 tinuous 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. 



