THE VOLATILE PART OF PLANTS. 65 



pcry feel. Place in separate test tubes a few drops of solution of cane- 

 sugar, a similar amount of the dextrin solution, obtained in Exp. 28; 

 of solution of dextrose, from raisins, or from Exp. 30; and of molasses ; 

 add to each a little of the copper solution, and place them in a vessel 

 of hot water. Observe that the saccharose and dextrin suffer little or 

 no alteration for a long time, while the dextrose and molasses shortly 

 cause the separation of cuprous oxide. 



EXP. 32. Heat to boiling a little white cane-sugar with 30 c. c. of 

 water, and 3 drops of strong sulphuric acid, in a glass or porcelain dish, 

 for 15 minutes, supplying the waste of water as needful, and test the 

 liquid as in the last Exp. This treatment transforms saccharose into 

 dextrose and levulose. 



The quantitative estimation of the sugars and of starch is commonly 

 based upon the reaction just described. For this purpose the alkaline 

 copper solution is made of a known strength by dissolving a given 

 weight of sulphate of copper, etc., in a given volume of water, and tha 

 dextrose or levulose, or a mixture of both, being likewise made to a 

 known volume of solution, the latter is allowed to flow slowly from a 

 graduated tube into a measured portion of warm copper solution, until 

 the blue color is discharged. Saccharose is first converted into dex- 

 trose and levulose, by heating with an acid, and then examined in the 

 same manner. 



Starch is transformed into dextrose by heating with hydrochloric 

 acid or warming with saliva. The quantity of sugar stands in definite 

 relation to the amount of copper separated, when the experiment is 

 carried out under certain conditions. See Allihn, Jour./ilr Pr. Chemie, 

 XXII, p. 52, 1880. 



Galactose, C 6 H 12 6 , is formed by treating right- 

 polarizing gum arabic, galactin, or milk-sugar with 

 dilute acids. It crystallizes, is sweet, melts at 289 and 

 with nitric acid yields mucic acid (distinction from ara- 

 binose, dextrose and levulose). 



Mannose (Seminose?) C 6 H 12 6 is a fermentable sugar 

 prepared artificially by oxidation of mannite (see p. 74), 

 and, according to E. Fischer, is probably identical with 

 the Seminose found by Reiss as a product of the action 

 of acids on a body existing in the seeds of coffee and in 

 palm nuts. (Serichte, XXII, p. 365). 



Arabinose, C 5 Hi 5 , obtained from arabin (of left- 

 polarizing gum arabic), and from cherry gum by action 

 of hot dilute acids, appears in rhombic crystals. It is 

 less sweet than cane sugar, and fuses at 320. 



c. The Sucroses, C^H^On, are sugars which, boiled with 

 dilute acids, undergo chemical change by taking up the 

 5 



