G. MCGUIRE AND K. G. FALK 217 



end-point of the actions, it was decided not to use this method in this 

 work. 



The Lintner^ method, which was used with the vegetable juices 

 for which the results were given in the former paper,^ could not be 

 used because the cuprous oxide was formed in such a finely divided 

 state that it was impossible to teU whether the remaining Fehling's 

 solution was still blue or had become colorless. 



The method developed by Sherman, Kendall, and Clark,^ pre- 

 cipitating and weighing the cuprous oxide from an excess of Fehling's 

 solution under definite conditions, was finally adopted. 



The general method of experimentation was as follows: The pota- 

 toes were thinly peeled, weighed, ground in a food chopper, squeezed 

 through mushn, then the juice was filtered through fresh musHn, and 

 finally filtered through asbestos on a Buchner funnel. The insoluble 

 starch was removed as completely as possible in this way, but the 

 solutions still contained a certain amount of soluble starch. The 

 actions of the enzymes on the soluble starch and simpler carbo- 

 hydrates present in the solutions, as well as on added starch and 

 other substances, were followed. The tests were set up with 25 cc. of 

 potato juice and 50 cc. of 1 per cent Lintner soluble starch solution, 

 50 cc. of water, or 50 cc. of 1 per cent cane sugar or maltose solution, 

 as desired. Toluene was added as a preservative. Immediately 

 after mixing, 5 to 15 cc. were pipetted out for the determination of 

 the reducing action for blanks. The remainder was incubated for 

 18 to 24 hours at 37-40°C., and portions were then removed for 

 tests. For the determination of the reducing action, Sherman's 

 method was followed closely. The portion taken, 5 to 15 cc, was 

 diluted in an Erlenmeyer flask with boiling water to 50 cc, 50 cc. of 

 Fehling's solution were added, heated in a boiling water bath for 15 

 minutes, filtered on an asbestos mat in a porcelain Gooch crucible, 

 washed with boifing water, alcohol, and ether, dried at 100°, and 

 weighed. The cuprous oxide was then dissolved and the crucibles 

 reweighed to check up possible mechanical losses of the mats. The 

 starch solution was prepared by adding a paste of the Lintner soluble 



^Lintner, C. J., /. prakf. Chem., 1886, xxxiv, 378. 



s Sherman, H. C, KendaU, E. C, and Clark, E. D., /. Am. Ghent. Soc, 1910, 

 xxxii, 1083. 



