CARBOHYDRATES. 511 



252. The Carbohydrates. 



These substances, which occur in plants and animals, have received their name, 

 because in addition to C (at least 6 atoms), they contain H and O, in the propor- 

 tion in which these occur in water. They are all solid, chemically indifferent, 

 and without odour. They have either a sweet taste (sugars), or can be 

 readily changed into sugars by the action of dilute acids; they rotate the ray of 

 polarised light either to the right or left; as far as their constitution is concerned, 

 they may be regarded as fatty bodies, as hexatomic alcohols, in which 2H are 

 wanting. 



They are divided into the following group: 



I. Division. Glucoses (C 6 H 12 6 ) (i) Grape-sugar (glucose, dextrose, or 



diabetic sugar) occurs in minute quantities in the blood, chyle, muscle (? liver), 

 urine, and in large amount in the urine in diabetes mellitus (p. 352). It 

 is formed by the action of diastatic ferments upon other carbohydrates, during 

 digestion. In the vegetable kingdom, it is extensively distributed in the 

 sweet juices of many fruits and flowers (and thus it gets into honey). It 

 is formed from cane-sugar, maltose, dextrin, glycogen, and starch, by boiling 

 with dilute acids. It crystallises in warty masses with one molecule of water of 

 crystallisation; unites with bases, salts, acids, and alcohols, but is easily decom- 

 posed by bases; it reduces many metallic oxides (p. 297). Fresh solutions have a 

 rotatory power of + 10G. By fermentation with yeast, it splits up into alcohol and 

 C0 2 (p. 298) ; with decomposing proteids, it splits into two molecules of lactic 

 acid (p. 373) ; the lactic acid splits up under the same conditions in alkaline 

 solutions, into butyric acid, C0 2 and H. For the qualitative and quantitative 

 estimation of glucose, see 149 and 150. In alcoholic solution, it forms very 

 insoluble compounds with chalk, barium, or potassium, and it also forms a 

 crystalline compound with common salt. 



(2.) Galactose, obtained by boiling milk-sugar (lactose) with dilute mineral 

 acids; it crystallises readily, is very fermentable, and gives all the reactions of 

 glucose. When oxidised with nitric acid it becomes transformed into mucic acid. 

 Its specific rotatory power = + 88 '08. 



(3.) Laevulose (left-fruit-, invert- or mucin-sugar) occurs as a colourless syrup in 

 the acid-juices of some fruits and in honey; is non-crystallisable, and insoluble in 

 alcohol ; specific rotatory power = 106. It is formed normally in the intestine 

 (p. 370), and occurs rarely as a pathological product in urine. 



II. Division contains carbohydrates with the formula Ci 2 H 22 On, and which 

 may be regarded as anhydrides of the first division (1) Milk-SUgar or lactose 

 occurs only in milk, crystallises in cakes (with one molecule of water) from the 

 syrupy concentrated whey; it rotates polarised light to the right =+ 59 '3, and is 

 much less soluble in water and alcohol than grape-sugar. When boiled with dilute 

 mineral acids it passes into galactose, and can be directly transformed into lactic 

 acid only by fermentation; the galactose, however, is capable of undergoing the 

 alcoholic fermentation with yeast (Koumis preparation, p. 468). For its quantita- 

 tive estimation, see Milk. 



(2.) Maltose (Ci 2 H 22 On) + H 2 (O'Sullivan) has one molecule of water less 

 than grape-sugar (Ci 2 H 24 12 ), is formed during the action of a diastatic ferment, 

 such as saliva upon starch (p. 294); is soluble in alcohol, right rotatory power = 

 150, it is crystalline, while its reducing power is only two-thirds that of dextrose. 



(3. Saccharose (cane-sugar) occurs in sugar-cane and some plants, it does 

 not reduce solutions of copper, is insoluble in alcohol, is right rotatory, and not 

 capable of fermentation. When boiled with dilute acids, it becomes changed 

 into a mixture of easily fermentable glucose (right-rotatory) and laevulose 

 (invert-sugar) which ferments with difficulty and is left-rotatory (p. 370). When 

 oxidised with nitric acid, it passes into gl';cic acid and oxalic acid.) 



