CARBOHYDRATES. 349 



water and is less sweet than cane-sugar, the sweetness of glucose com- 

 pared to that of cane-sugar being about 3 to 5 ; when heated to 170 

 C. (338 F.) it loses water, and is converted into glucosan, C 6 H 10 O 5 ; 

 by stronger heating it loses more water and forms caramel, a mixture 

 of various substances ; it turns the plane of polarized light to the 

 right. 



Grape-sugar combines with various metallic oxides (alkalies, alka- 

 line earths, etc.), and also with a number of other substances, form- 

 ing a series of compounds known as glucosides. 



Grape-sugar may be recognized analytically : 



1. By causing a bright-red precipitate of cuprous oxide, when 

 boiled with a solution of cupric sulphate in sodium hydroxide, to 

 which tartaric acid has been added. (A solution containing these 

 three substances in definite proportions is known as Fehling's solu- 

 tion. See index.) 



2. By precipitating metallic silver, bismuth, and mercury, when 

 compounds of these metals are heated with it in the presence of 

 caustic alkalies. 



3. By easily fermenting when yeast is added to the solution, 

 alcohol and carbon dioxide being formed : 



C 6 H 12 6 : : 2C 2 H 5 OH + 2C0 2 . 



Fruit-sugar, C 6 H 12 O 6 (Levulose), occurs with glucose in sweet 

 fruits and honey ; it resembles glucose in most chemical and physical 

 properties, but does not crystallize from an aqueous solution ; it may, 

 however, be obtained in white, silky needles from an alcoholic solu- 

 tion ; it is met with generally as a thick syrup, is about as sweet as 

 cane-sugar, and turns the plane of polarized light to the left; it is 

 formed by the action of dilute mineral acids or ferments on cane- 

 sugar, which latter takes up water and breaks up thus : 



C 12 H 22 O n + H 2 : . C 6 H 12 6 + C 6 H 12 6 . 

 Cane-sugar. Dextrose. Levulose. 



Mannitose, C 6 H 12 O 6 . Obtained by the oxidation of mannite; it 

 does not crystallize and resembles grape-sugar. 



Galactose, C 6 H 12 O 6 , is formed together with dextrose when either 

 milk-sugar or gum-arabic is boiled with dilute sulphuric acid. Galac- 

 tose crystallizes, reduces an alkaline copper solution, but does not 

 ferment with yeast. 



Inosite, C b H 12 O 6 (Muscle-sugar), occurs in various muscular tissues, 

 in the lungs, kidneys, liver, spleen, brain, and blood. Although 



