DETERMINATION OF SUGARS AND DEXTRINS 157 



maltoses) in the mixture. Of pure dextrose 0.5 gm. in I per-cent. 

 solution reduces 101.1 cc. of Fehling's solution diluted with four 

 volumes of water. But as we are not dealing with a pure dextrose 

 the results can only be approximate. To a known quantity of 

 dilute Fehling's solution boiling, add from a burette the sugar solu- 

 tion drop by drop. When just sufficient has been added to cause 

 the last trace of blue color to disappear from the boiling mixture 

 the reduction may be considered complete. If maltose was pres- 

 ent in the solution it also acted upon the Fehling's solution and 

 it would be impossible to calculate directly the amount of dextrose. 

 But if experiment 4 revealed no maltose the dextrose may be esti- 

 mated from the data given above. 



2. To the other half of the solution add enough hydrochloric 

 acid to make a 2 per-cent. solution. Heat on the water-bath for 

 two or three hours, cool and neutralize. Determine the reducing 

 power of this solution. The difference between this and the fore- 

 going determination will give the amount of invert sugar formed. 1 



224. Starch. Starch is capable of being separated from the tis- 

 sues in which it occurs by grinding the tissue to a pulp and wash- 

 ing on a coarse cloth. The starch is carried through with the 

 water, and if this is allowed to flow into a tall jar or cylinder 

 it will settle to the bottom, and may be washed and separated 

 by repeated accession and decanting of water. The following 

 tests will be found the most useful. 



1 . Rub a gram of starch with cold water in a mortar, and stir 

 the paste into 50 cc. of boiling water. An opalescent, imperfect 

 solution is obtained. Starch is only slightly soluble even in hot 

 water . 



2. To a little of the starch solution add iodine solution. A 

 deep blue color appears, which disappears on heating, and reap- 

 pears on cooling, if not boiled too long. 



1 Maquenne, L. Les Sucres et leurs Principaux Derives. Paris. 1900. 

 Stirling, Wm. Practical Physiology. Philadelphia. 1898. 

 Verworn, Max. General Physiology. London. 1899. 

 Darwin and Acton. Physiology of Plants. Cambridge. 1894. 

 Rijn, J. J. L. van. Die Glykoside. Berlin. 1900. 



