G. McGUIRE AND K. G. PALK 441 



are at hand. In the following experiments, in comparing different 

 gels, their consistency is taken to be the greater or smaller resistance 

 to deformation, the ability to retain the shape of the vessel in which 

 they were prepared, even when separated from it, etc. In any one 

 series a satisfactory comparison can be made, but in different series 

 carried out at different times, anything more than a rough qualitative 

 study is difficult. 



Since calcium salts were found to be involved in the gel formation 

 in the dialysis experiments, a number of series of experiments were 

 carried out in which solutions of calcium salts were added directly 

 to banana extracts under different conditions without dialyzing. 



The acidity of the mixture was the first determining factor. A 

 banana-water extract (4 parts of banana pulp with 1 part of water and 

 filtered in the usual way) gave no indication of gel formation at the 

 pH of the juice (about 5.0) with 0.29 mg. of calcium, added as cal- 

 cium oxide, or 0.22 mg. of calcium, added as calcium chloride, per 

 cc, in 48 hours at 5°-10°C. The same extract at pH 7.5 with 0.04 

 mg. of calcium as calcium oxide added per cc. formed a firm gel in 

 5 minutes which retained the shape of the container. These results 

 were confirmed repeatedly. It appeared to be impossible to obtain 

 gel formation with calcium salts with solutions more acid than about 

 pH 6.0. With solutions at pH 7.0 or more alkaline, gels were obtained 

 readily. The greater the concentration of calcium the more rapid 

 the formation of a firm gel, and vice versa. For example, with a cal- 

 cium concentration of 0.03 mg. per cc. and the juice mentioned above, 

 gel formation had only begun after 20 minutes, while a firm gel was 

 present in 7 hours. With 0.02 mg. of calcium per cc. at 5° in 18 hours, 

 definite gel formation had occurred, while with 0.01 mg., the gel for- 

 mation was doubtful. Low temperatures favored the formation of 

 the gel. While extracts obtained at different times differed slightly, 

 the general conclusion that with calcium salts no gel formation at 

 pH 6.0 and more acid, and gel formation at pH 7.5 and more alkaline, 

 was found to hold in every case. 



It is of interest to note that if the banana extract is boiled for a 

 few minutes, some solid shreds forming, a gel could not be obtained 

 either by dialysis against tap water or at pH 7.5 by the addition of 

 calcium salts. 



