408 SCIENCE PROGRESS 



different, and that the 8 per cent, gel did not become Hke the 

 i6, 24, or 32 per cent, gel while being dried." Similar experi- 

 ments have been recorded by Procter (Jonrn. Chem. Soc, 

 1 9 14, 105, 313) and have been confirmed by Sheppard and 

 Elliott (Journ. Amer. Chem. Soc, 1922, 44, 373). In con- 

 nection with such experiments, reference is often made to a 

 swelling maximum, but the existence of such a maximum has 

 never been proved. Brotman {Journ. Soc. Leather Trades 

 Chemists, 192 1, 5, 226) has shown that the imbibition of water 

 by gelatin can be increased by dispersion of the gelatin. Pure, 

 air-dry gelatin swells to a so-called maximum amount in 

 about twenty-four hours, the maximum varying according 

 to the variety of gelatin used. If the equilibrium gel is then 

 dispersed by placing the test tube containing it in hot water 

 (at about 80°) for two minutes, and then allowed to set again, 

 further swelling will take place on immersion in the previous 

 swelling water. 



If the results obtained with gelatin in pure water are so 

 complicated, it is no wonder that they are still more so in the 

 presence of acids, bases, and salts. This m.ay be illustrated by 

 comparatively simple experiments, which are cited from Wo. 

 Ostwald's book on Theoretical and Applied Colloid Chemistry. 

 A 2i- per cent.' solution of gelatin, set to a jelly in a test tube, 

 can be detached from the sides of the tube and broken into 

 pieces by hard shaking. If, however, several per cent, of 

 magnesium sulphate are added to the sol before setting, the 

 resulting gel can no longer be broken up by shaking. On the 

 other hand, if sufficient potassium iodide is added to the sol 

 it will not set and remains fluid. Sulphates, citrates, and 

 phosphates act similarly to magnesium sulphate, that is, they 

 increase the viscosity of gelatin, and to a much greater degree 

 than they do that of pure water. Bromides and cyanides act 

 similarly to iodides. Organic substances also act differently, 

 for example, chloral hydrate and urea decrease the viscosity, 

 whilst alcohol, in small amounts, increases it. 



Similar results would be obtained if, instead of using a 

 gel, the viscosity of the sol were measured in a viscometer. 

 Such measurements would show, however, that further com- 

 plications arise, in that there may be either an increase or a 

 decrease in the viscosity, depending on the concentration of 

 the added salt. For example, sodium chloride in medium 

 concentration (about N/4) produces a viscosity which exceeds 

 that of pure gelatin, whereas at higher and lower concentrations 

 the viscosity is less than that of pure gelatin. 



^ The concentration necessary to give these results will depend on the 

 kind of gelatin used. 



