An uncrystallisable substance of doubtful nature, which has been described as a constituent of muscle juice.

(b) Cryptophanic acid C10H18N2O10 an amorphous acid said to exist in small quantities in human urine (Thudichum).'

(c) Sulpho-cyanic acid CNHS united to potassium or sodium to form a sulpho-cyanide (CNKS) is found in saliva, and occasionally also in urine, milk, and blood. It gives a red colour with ferric chloride, due to the formation of sulpho-cyanide of iron.

(d) Cynurenic acid C20H14N2O6 is found in dog's urine. It is a decomposition product of proteids, but is apparently not derived from the putrefaction, which occurs in those substances in the alimentary canal (Baumann).3 On heating its crystals, which contain two molecules of water of crystallisation, to 250° a base called cynurin CH1NO, is obtained. By means of certain reagents Kretschy 5 obtained chinolin C,H,N from it.

18 14 2

(e) Urocanic acid C6H6N,O2+2H2O was found in the urine of a dog in which the urea was diminished. At 212° C. it breaks up into carbonic acid, water, and a base urocanin C11H0N40 (Jaffe).6

1 Journ. Chem. Soc. (2) viii. 132.

2 Hofmeister, Zeit. physiol. Chem. v. 67.

3 Zeit. physiol. Chem. x. 123.

4 Schmiedeburg and Schultzen, Ann. Chem. Pharm. clxiv. p. 155. Ber. d. deutsch. chem. Gesellsch. xii. 1673.

6 Ibid. viii. p. 811.

CHAPTER IX

THE CARBOHYDRATES

THE carbohydrates form a most important group of organic substances. They are found chiefly in vegetable tissues; a few are found in the animal organism; many of the vegetable carbohydrates are used as food for animals, and so they are of importance in a consideration of animal physiology.

The carbohydrates may be conveniently defined as compounds of carbon, hydrogen, and oxygen, the two last named elements being in the proportion in which they occur in water.

They may be for the greater part arranged into three groups, according to their empirical formulæ. The names and formula of these groups, and the most important members of each, are as follows :

The + and sign in the above list indicate that the substances to which they are prefixed are dextro- and lævo-rotatory respectively, as regards polarised light. The formule given above are merely empirical; and there is no doubt that the quantity in the starch group is variable and often large. Investigations relating to the molecular weights of the different carbohydrates have yielded very unsatisfactory results. The most recent work in this direction is that of Brown and Morris." The method these observers adopted was devised by Raoult,3 and is the outcome of his elaborate investigation into the laws governing the freezing-point of very dilute solutions. Briefly

1 Musculus and Meyer (Bull Soc. Chim. (2) xxxv. 370) attempted to determine the relative size of the molecules by observing their rate of diffusion.

2 Trans. Chem. Soc. 1888, p. 610.

3 Ann. Chem. Phys. 1883, 1884, 1885, 1886. Comptes rend. 94, 1517; 101, 1056; 102,

1307.

stated, the law is as follows:-When certain quantities of the same substance are successively dissolved in a solvent, on which it has no chemical action, there is a progressive lowering of the freezing-point of the solution, which is proportional to the weight of the substance dissolved in a constant weight of the solvent. It is unnecessary here to describe the actual methods employed, and will be sufficient to quote the principal results obtained by Brown and Morris.

The carbohydrates are not however simple compounds of carbon with water; their reactions and derivation products show that their molecular structure is more complicated: they are in fact derivatives of the hexatomic alcohol, mannite. The glucoses may be regarded as the aldehydes of mannite; they contain in their rational formula the characteristic aldehyde group COH. Thus:

The sucroses are condensed glucoses, i.e. they are formed by the combination of two molecules of a glucose with the loss of one molecule of water (C6H12O6+C6H12O6 - H2O=C12HO). The amyloses may be regarded as the anhydrides of the glucoses (C6H12O6—H2O =C,H1005).

1 This confirms Kiliani's observation that dextrose and levulose yield hydroxy-acids containing seven atoms of carbon.

2 Cane sugar and maltose are thus isomeric, not polymeric. The difference in their properties must therefore be due to difference of the arrangement of the atoms in their molecules.

By oxidation of the sugars by means of nitric acid, an acid is obtained; that is, the H, removed from the alcohol (mannite) to form the aldehyde (glucose) is replaced by O. The formula of the acid so obtained is therefore CH1207; this is monobasic, and called mannitic acid. On repeating the process, that is, replacing another H2 by O, we obtain an acid with the formula C6H10O; this is dibasic; of this there are two isomerides, one called mucic acid, which is slightly soluble in water, the other saccharic acid, which is readily soluble in water. On oxidation some sugars yield one, some the other, acid; by further oxidation, tartaric acid, then oxalic acid, and finally carbonic acid and water, are obtained. The most important carbohydrates may now be described one by one.

DEXTROSE OR GRAPE SUGAR

This carbohydrate exists in fruits, honey, and in small quantities in the blood, and in numerous tissues, organs, and fluids of the body. It is the form of sugar found in the urine in the disease known as diabetes. It is formed by the hydration of members of the amylose and saccharose group; such as is brought about by boiling with dilute sulphuric acid.

Dextrose is soluble in hot and cold water, and in alcohol. It is not so sweet as cane sugar. It crystallises from an aqueous solution in white spheroidal masses, and from alcohol in transparent anhydrous prisms. solutions rotate the ray of polarised light to the right; (a)ь=+56° (Hoppe-Seyler).1

FIG. 41.-Dextrose crystals.

Its

Heated with alkali, dextrose gives a brown or yellow colour due to the formation of glucic and melassic acids.2

Nitric acid oxidises dextrose to saccharic acid.

In alkaline solutions, dextrose reduces salts of silver, bismuth, mercury and

copper; in the case of the first three, the metal is precipitated; cupric are reduced to cuprous compounds, with the separation of cuprous oxide. In the presence of ammonia, dextrose is precipitated by neutral or basic lead acetate.

1 Tollens gives (a)p=+531°. Fresh watery solutions may indicate 104°.

2 These acids are of doubtful composition. In Watts' Dictionary the formula for glucic acid is given as C24H18018, and for melassic acid as C12H1005. In Beilstein's Dictionary the acids formed are stated to be glycic acid (C12H22O12) and saccharumic acid (C14H18011).

Dextrose forms compounds with certain acids and bases (e.g. potash, lime): these are called glucosates.

Under the influence of yeast, dextrose is converted into alcohol and carbonic acid (C6H12O6=2C2H¿O+2CO2).' It may also undergo the lactic acid fermentation, under the influence of certain bacterial growths.

When sugar is heated to 200°, a brown substance called caramel is formed. This has been separated into three bodies of complex formulæ and doubtful nature (see Watts' Dict.').

Sugar is also turned brown by the action of sulphuric or hydrochloric acid. This is partly due to charring. A number of other substances, called humous substances by Hoppe-Seyler, have also been separated. These brown products are similar to many produced in vegetable growths naturally, in peat, &c., and are of complex nature and doubtful composition. They have received various names (humin, ulmin, ulmic acid, phlobaphene, tannin-red, hymatomelanic acid, &c.). Among the decomposition products of these substances are formic acid, pyrocatechin, and protocatechuic acid.2

The origin, rôle, and destination of dextrose in the body, and other physiological problems connected with its presence, will be more conveniently described with the various tissues and fluids in which it occurs (see liver, muscle, blood, urine, diabetes, food, digestion).

Tests for Dextrose.-1. Trommer's test.3—Add a few drops of dilute cupric sulphate solution to a solution of dextrose and caustic potash or soda in excess. The result is a deep blue solution; the precipitate of cupric hydrate which is formed being soluble in the presence of dextrose. Heat the solution; a little below the boiling-point a red precipitate of cuprous oxide, or a yellow precipitate of cuprous hydrate, is formed. This reduction is due to the formation of glucic and melassic acids which, having a strong affinity for oxygen, take it from the cupric compound.

2. Fehling's test.-The principle of this test is the same as that of the preceding. Fehling's solution is thus prepared :-Solution A. Dissolve 36-64 grammes of copper sulphate in 500 c.c. of water, Solution B. Dissolve 173 grammes of sodio-potassium tartrate (Rochelle salts) in 100 c.c. of a solution of caustic soda, having a specific gravity

1 Small quantities of glycerine and succinic acid are also formed, and were regarded by Pasteur as derived from the sugar on which the yeast acts (Ann. Chim. Phys. (3) xviii. p. 323). v. Udranszky (Zeit. physiol. Chem. xiii. p. 539), however, states that the glycerine certainly, and probably the succinic acid, is derived from the substance of the yeast itself, probably from the lecithin it contains.

2 A full description of humous substances will be found in a paper by Hoppe-Seyler, Zeit. physiol. Chem. xiii. 66. 3 Ann. Chem. Pharm. xxxix. (1841) p. 360.