THE CARBOHYDRATES.

The carbohydrates are classified into the monosaccharids, disaccharids, trisaccharids, and polysaccharids.

Monosaccharids.—The principal members of this class are dextroglucose (ordinary glucose or grapesugar) and fructose (fruit-sugar).

Disaccharids.-Saccharose (cane-sugar), lactose (milk-sugar), and maltose (malt-sugar).

Trisaccharids.-Raffinose.

Polysaccharids.-Starch, the dextrins, glycogen,

and cellulose.

All these compounds are chain compounds, the carbon atoms not being arranged in a ring. Inosite, which at one time was supposed to belong to this class, is, however, a cyclic compound, and cannot therefore be looked upon as a carbohydrate. All the above substances are alcohols, containing the hydroxyl group - OH. In addition, they either have a ketone group, CO, or the aldehyd group, - COH.

Their action toward oxidizing agents is quite in accord with this constitution, and they therefore "reduce" them readily, and are themselves oxidized in so doing.

It is to this property that many of the tests for carbohydrates are due, and it must not be forgotten that these tests respond equally well with other reducing agents, and that a response to Fehling's test, for example, is by no means an indication that the substance which is acting on Fehling's solution

is glucose. As, however, in the case of urine the only reducing substance likely to be present in that fluid is glucose, the reduction of an alkaline copper solution is usually set down to glucose.

Glucose. This substance is not found in the animal body in any amount under physiological conditions, but with pathological change it sometimes occurs in exceedingly large quantities, and may then readily be detected in the blood and tissues, but especially in the urine.

Glucose is formed from the more complex carbohydrates, such as starch and cane-sugar, by a process of hydrolysis, in which the sugar or starch takes up a molecule of water and breaks down into the simpler monosaccharid. This may be accomplished by means of inorganic acids, or, in fact, by any substance which on solution dissociates, giving hydrogen ions. It is also formed by certain organic ferments acting on the higher carbohydrates.

It has the property of undergoing a peculiar change when its solutions are treated with the micro-organism saccharomyces cerevisiæ. Under these circumstances, if the solution be not too concentrated, the glucose breaks up according to the following formula:

C6H12O6 = 2CO2 + 2C¿H¿O,

giving alcohol and carbon dioxid. This change is known as fermentation, and all the ethylic alcohol used in commerce is prepared in this way.

I. Heat a small quantity of glucose on platinum Note that the substance melts and turns deep

foil.

brown. Do not heat sufficiently to char the glucose. Dissolve the residue in I c.c. of water. Note the deep-brown color of the solution. The substance formed by heating glucose is caramel.

2. Make a solution by dissolving 2 grams of the dry glucose in 100 c. c. of water. Taste the solution, and compare the sweetness of the solution. with that of a similar solution of sugar. reaction of the solution with litmus-paper.

Test the

3. Heat a small quantity of the solution with an equal amount of potassium hydroxid solution. Note the change in color, which passes through yellow to deep brown (Heller's test).

The following tests are due to the reducing power of the glucose on mild oxidizing reagents. In these reactions the aldehyd group - COH is oxidized to the carboxyl group - COOH.

4. To about 5 c.c. of the glucose solution add 1 c. c. of an ammoniacal solution of silver nitrate, made by adding dilute ammonia to silver nitrate till the brown precipitate which at first forms is dissolved. Care must be taken not to add too great an excess of the ammonia. Warm the mixture of the ammoniacal silver nitrate and the glucose. Note the formation of a mirror on the sides of the test-tube, and the precipitate of a gray color, which is due to metallic silver. This reaction is made use of in the preparation of mirrors. What is the substance. formed by the oxidization of the glucose?

5. To a very dilute solution of indigo-sulfonic acid, prepared by dissolving indigo in concentrated sulfuric acid, and diluting, add a drop of glucose

solution. Then add a drop of sodium carbonate solution till the reaction of the mixture is alkaline. Warm the mixture. The indigo-blue color is discharged. Indigo-white is formed, which is colorless. On shaking the mixture with air the blue color returns. This is due to the indigo-white formed in the first instance being reoxidized by the oxygen of the air to indigo-blue.

6. Prepare some Fehling's solution by mixing equal quantities of solutions 1 and 2, and diluting with 4 parts of water. Boil the solution and add a couple of drops of the glucose solution. Note the change which takes place on boiling. Allow the tube to stand, and examine the sediment which is formed. Add a further quantity of glucose solution and boil again. The red precipitate which is formed consists of cuprous oxid, while the yellow compound which is seen on first boiling the mixture is the hydrated cuprous oxid, which loses water and is transformed into the red oxid:

7. Böttger's Test.-This belongs to the same class of reaction as the above. Take I c. c. of glucose solution and 4 c. c. of water. Add 1 c.c. of potassium hydroxid solution, and a small quantity of dry bismuth subnitrate. Boil the mixture. A black precipitate is produced of metallic bismuth.

8. Molisch's Test. This test is a general one for carbohydrates, and depends on the formation of furfurol when a carbohydrate is treated with concentrated sulfuric acid. Furfurol gives a red

color with a solution of a-naphtol. As this test is exceedingly delicate, it is necessary to dilute the 2 per cent. glucose solution used for the foregoing tests with 10 times its volume of water. TO IO C. c. of the diluted solution, in a test-tube, add a drop of an alcoholic solution of a-naphtol. By means of a pipette allow a couple of c. c. of concentrated sul

FIG. 1. Fermentation apparatus.

furic acid to flow down the side of the test-tube. At the point of junction of the sulfuric acid and the glucose solution a red contact-ring will form.

9. The Fermentation of Glucose.-Take a flask holding about 250 c.c. and place in it 150 c.c. of a 2 per cent. solution of glucose, and add half a cake of compressed yeast.

Fit the neck of the flask with a doubly bent tube,