216 VEGETABLE ENZYMES. XVIII 



other investigators,^ who, however, worked with animal liquids, and 

 not with filtered vegetable juices. 



Upon repeating these experiments, it was found that, in accordance 

 with the results of Doby, the supernatant liquids in many cases 

 gave no test for starch, but on testing the precipitates with iodine, 

 blue colors or compounds were obtained, showing the presence of 

 starch. The presence of starch could also be shown by testing the 

 whole mixture with iodine and shaking. A blue or purple color would 

 be formed throughout the mixture, which would settle with the 

 precipitate, leaving the supernatant liquid colorless. Wherever, 

 therefore, a precipitate was formed, the results of the test as given by 

 Doby must be looked upon as inconclusive. He found that sodium 

 fluoride accelerated the amylase action. This observation can 

 evidently be interpreted as the forrrfation of a precipitate of the 

 starch with the amylase, protein, or other material present which was 

 produced more readily in the presence of this salt. With larger 

 amounts of potato juice, hydrolysis of the starch would be complete, 

 but the whole mixture must be tested in every case. These condi- 

 tions were not realized by Doby in many of his experiments. 



A different difficulty was encountered in attempting to use the 

 Wohlgemuth* method for following amylase actions. In most of the 

 experiments, in testing the final mixture or even the initial solution 

 with iodine, an intense blue or purple color was obtained, which, 

 however, faded rapidly, sometimes in a few seconds, leaving the 

 mixture colorless. The rapidity of fading depended upon the quan- 

 tity of potato juice present. It was necessary to add considerable 

 iodine solution before a color, constant even for 3 or 4 minutes, could 

 be obtained. Also, the end-points of these solutions were uncertain. 

 Even when constant colors were obtained, these ranged from yeliow- 

 gray through gray to blue so that it was difficult to decide which tube 

 represented the complete or final decomposition of the starch. Be- 

 cause of the presence of the chemically unknown, iodine-combining 

 components of potato juice and the uncertainty of determining the 



^ Starkenstein, E., Biochem. Z., 1910, xxiv, 191. Schirokauer, H., and Wilenko, 

 G. G., Biochem. Z., 1911, xxxiii, 275. 



" Wohlgemuth, J., Biochem. Z., 1908, ix, 1. 



