GELATIN 407 



suspensoid colloids may consist of the elements themselves, e.g., 

 colloidal gold and silver. Suspensoid sols are practically as 

 mobile as water itself, that is, the colloidal particles hardly 

 alter the viscosity of the dispersion medium, but emulsoid 

 sols, of which gelatin is a typical example, show large vis- 

 cosity values, even in comparatively low concentrations, and 

 with increase in concentration the viscosity increases enor- 

 mously, whereas there is only a small increase in the viscosity 

 with suspensoid sols under similar conditions. Also when a 

 gelatin sol is cooled down it is well known that it sets to a 

 jelly, whereas no such change takes place with a suspensoid 

 sol, such as that of gold, when treated in the same way. 

 Moreover, the change from sol to gel is reversible, since on 

 warming the gel it liquefies readily to a sol. Again, if the gel 

 is dried and then immersed in water it imbibes water and 

 swells to a greater or lesser extent. 



Naturally it was such physical properties as those just 

 mentioned, namely, viscosity and swelling, which were the 

 earliest investigated, and it was soon found that the relations 

 were very complicated, depending on previous history, even in 

 systems made up from gelatin and pure water alone. For 

 example, shaking, or repeated passage through a viscometer, 

 will decrease the viscosity of a gelatin sol ; at ordinary tem- 

 peratures the viscosity of a freshly made sol gradually increases, 

 whilst that of a freshly diluted sol gradually decreases ; in a 

 freshly made gel or sol the intensity of the Tyndall cone 

 gradually increases ; and so on. All such phenomena are 

 indicative of the formation of a structure and of the attain- 

 ment of an equilibrium of some kind, and complicate the 

 investigation of gelatin very considerably. 



As an example of the complicated nature of swelling the 

 following quotation may be given from Bancroft's book on 

 Applied Colloid Chemistry : " If dried gelatin is placed in cold 

 water it swells considerably, and ma}'" take up ten times its 

 weight of water ; but there are no experiments to show that 

 it would ever go up say to an 8 per cent. gel. On the other 

 hand, it is possible to start with an 8 per cent, gel and dry 

 it to a 96 percent, gel, after which it will take up water rapidly 

 to an 8 per cent. gel. This means that the structure of the 

 gelatin plays an important part in the rate of swelling. This 

 is confirmed by some unpublished preliminary results of Mr. 

 Cartledge. Gelatin gels were made up containing 8, 16, 24 

 and 32 per cent, of gelatin. These were all dried at room 

 temperature to about 96 per cent, concentration. When 

 water was added each swelled rapidly to the original concen- 

 tration and then took up water slowly. If these results are 

 accurate, it means that the four 96 per cent, gels were all 



