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MECONATES-COMENIC ACID.

Meconates.-Meconic acid is tribasic, the formula of its normal salts being M',E,HO,. The meconates of the alkali metals are soluble. The normal meconates of calcium, barium, lead, copper, and silver are nearly insoluble in water, but are soluble in acetic acid. Meconate of silver when heated is decomposed with explosion.

It is sometimes important for medico-legal purposes to be able to ascertain the presence of meconic acid in cases where opium is suspected to have been administered as a poison. With this view the following process may be adopted :-The solution is to be mixed with acetate of lead, an impure meconate of lead is thus precipitated if meconic acid be present; the precipitate after it has been washed is decomposed by sulphuretted hydrogen, and the meconic acid thus set at liberty is concentrated by evaporation at a temperature not exceeding 158° (70° C.); a very dilute solution of ferric chloride is then added to the concentrated liquid. If meconic acid be present it strikes a deep blood-red colour, which may be distinguished from that due to potassic sulphocyanide by the effect of the addition of a few drops of sulphuric acid with a fragment of zinc; if the colour be due to the sulphocyanide the red tint will disappear, and sulphuretted hydrogen will be extricated; but if the compound contain meconic acid the colour will not be affected: a solution of trichloride of gold or of corrosive sublimate also discharges the colour of the ferric sulphocyanide, but is without effect upon the

meconate.

(1329) Comenic Acid (H,H), is dibasic. It is sparingly soluble in water, and crystallizes in hard, warty nodules. Nitric acid decomposes it rapidly into oxalic, carbonic, and hydrocyanic acids. This acid, like the preceding one, gives a bloodred solution with the ferric salts, and a white precipitate with acetate of lead, but it gives no precipitate with salts of barium.

If either the meconic or the comenic acid be distilled, a new monobasic acid, pyromeconic acid (HЄH ̧ ̧) is sublimed :—

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This acid is isomeric with the pyromucic, but is distinguished from it by yielding with the ferric salts a solution of a blood-red colour, and by giving no precipitate with a solution of basic acetate of lead.

5. Different Varieties of Tannic Acid.

(1330) The term tannin was formerly applied to the various forms of the astringent principle which were employed in the process of tanning hides. Most of these substances are now known to possess an acid reaction; but it has been ascertained that there are several distinct compounds which resemble each other in properties, though they possess a different chemical composition. These astringent principles are very extensively diffused throughout the vegetable kingdom. The bark and leaves of most forest trees, such as the oak, the elm, the willow, the horse-chestnut, and the pine, and of many fruit trees, such as the pear and plum, contain tannin in notable quantity. The wood and bark of many shrubs, such as the sumach and the whortleberry, and the roots of the tormentilla and bistort, are also powerfully astringent, owing to the presence of one of the forms of tannin. Coffee and tea, as well as Paraguay tea, likewise contain a modification of this astringent principle. All these bodies, except coffee, precipitate the ferric salts of a bluishblack colour, or if a free acid be present, the solution assumes a dark-green colour. Many vegetables contain an astringent principle which precipitates the salts of iron of a dark-green instead of a blue colour: catechu and kino offer good instances of this variety of tannin. Some few astringent plants yield an infusion which precipitates ferric salts of a grey colour, among which are matricaria, rhatany (Krameria triandra), and the common nettle (Urtica urens). Tannic acid in all its forms absorbs oxygen rapidly when moist.

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The most important of these acids, and the one which has been most accurately studied, is the tannic acid of the gall nut, or gallotannic acid; that of the oak or quercitannic acid is also a distinct species (Stenhouse), as well as that of the fustic (Morus tinctoria): moritannic acid, as the latter variety is termed, is a yellow crystalline solid, which, according to the experiments of Wagner, may be represented by the formula (CH1010). Quinotannic acid, or the tannin of the cinchona, according to Hlasiwetz, is represented by the formula (€8H3817?); and there is no doubt that the mimotannic acid or tannin of the catechu is a distinct variety, although, from its proneness to change, the true composition of this substance is uncertain. Kino, valonia (the acorncups of the Quercus ægilops), and divi-divi (the pod of the legume of Casalpinia coriaria), all contain powerfully astringent com

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GALLOTANNIC ACID-GALLOTANNATES.

pounds which, according to Stenhouse, are distinct from gallotannic acid.

(1331) GALLOTANNIC ACID ; Tannic acid (27H2217; Strecker). -This acid is obtained in a state of purity from the gall nut, an excrescence produced upon the leaves of a species of oak (Quercus infectoria), by the puncture of a small hymenopterous insect. The gall nut contains as much as two-thirds of its weight of tannic acid, and about two per cent. of gallic acid (Guibourt). In order to extract the tannic acid, Pelouze directs the gall nuts to be reduced to powder, and digested with about an equal weight of washed ether; the decanted liquid separates, on standing, into two portions, the denser of which is of a yellow colour, and consists of a concentrated aqueous solution of tannic acid; the other portion is of a greenish colour, and is composed of ether holding gallic acid and colouring matter in solution. The yellow solution, when evaporated to dryness, yields a porous, pale buffcoloured residue of amorphous gallotannic acid. According to Stenhouse the tannic acid of sumach is identical with that of the gall nut, but it is not readily obtained in a pure form. Gallotannic acid is freely soluble in water; it reddens litmus paper, and dissolves the carbonates with effervescence. It has a purely and intensely astringent taste; it is soluble in dilute alcohol, but sparingly so in ether. Its diluted aqueous solution absorbs oxygen from the air slowly, and is converted into gallic acid; this process is favoured by the presence of a particular ferment which is contained in the gall nut. A solution of tannic acid in water is coagulated by chloride of potassium, and by many other salts. Sulphuric, hydrochloric, phosphoric, and some other acids also diminish its solubility, and precipitate it from a concentrated aqueous solution. Nitric acid converts it into oxalic and saccharic acids. If boiled with dilute sulphuric, or with hydrochloric acid, it unites with the elements of water and is converted into sugar and gallic acid (Strecker); the latter crystallizes on cooling, whilst the glucose remains in solution :

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A boiling concentrated solution of potash also decomposes tannic acid, and occasions the formation of gallic acid. If air be admitted to the mixture, the acid last named is in turn decomposed into a black ulmin-like substance.

Gallotannates.-The composition of these salts is but imperfectly known, and there is considerable uncertainty whether the

GALLOTANNATES-WRITING-INK.

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acid should be regarded as dibasic or as tribasic. A solution of gallotannic acid occasions a violet-black precipitate in solutions of the ferric salts. It also causes precipitates in the solutions of the salts of morphia, quinia, brucia, and many other vegetable bases; these precipitates consist of gallotannates of the bases, and are readily soluble in acetic acid. The gallotannates of lead and antimony are white and insoluble. Those of the alkali metals are soluble; their solutions have an astringent taste, and do not precipitate gelatin until some acid is added to liberate the gallotannic acid from the base. If the solutions of these salts be exposed to the air, especially if the base be in excess, they rapidly become brown, and are decomposed with absorption of oxygen. None of the gallotannates can be obtained in crystals.

The colouring matter of ordinary writing ink is gallotannate of iron. Various recipes are given for its preparation : the following furnishes a very good ink :-Digest three quarters of a pound of bruised nut galls in a gallon of cold water, then add six ounces of ferrous sulphate with an equal weight of gum arabic, and four or five drops of kreasote to prevent the ink from becoming mouldy: let this mixture digest at ordinary temperatures for two or three weeks with occasional agitation; then allow it to settle, and decant for use. Oxalic acid and chlorine water readily discharge the colour of writing ink, and they may be employed to remove ink stains from linen. An ink-spot when washed with an alkaline. soap becomes yellow, forming what is familiarly known as an iron-mould. This yellow stain is due to ferric oxide, which constitutes the basis of the ink, and which has attached itself to the cloth, whilst the alkali of the soap has removed the tannic acid.

The most remarkable compound of tannic acid, however, is that which it forms with gelatin, which constitutes the basis of leather. The principles of the manufacture of this important substance will be considered (1659) after the properties of gelatin have been described. When a solution of gelatin or isinglass is added to an aqueous infusion of any vegetable containing tannic acid, a copious gelatinous precipitate occurs, which is soluble in excess of gelatin if the liquid be boiled. An excess of tannic acid prevents the resolution of the compound. If a piece of raw hide freed from hair, be immersed in a solution of tannic acid, the gelatinous tissue gradually combines with the acid, and retains it in the form of leather, the supernatant liquid being ultimately completely freed from all traces of tannin, if the piece of skin be of sufficient size.

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MIMOTANNIC ACID- -CATECHIN.

When gallotannic acid is subjected to a temperature of about 620° (327° C.), it is decomposed, and yields pyrogallic and metagallic acids (1333, 1334), whilst water and carbonic anhydride are expelled. The formation of pyrogallic acid appears to be characteristic of gallotannic acid, since the other varieties of tannin do not yield this compound (Stenhouse).

(1332) Mimotannic Acid.-The different species of catechu and of kino, although derived from trees belonging to several different natural families, appear to contain the same modification of the astringent principle, which, as already stated, differs in some essential particulars from the gallotannic acid. Formerly, kino and catechu were seldom employed in this country except for medicinal purposes; but of late years they have been extensively used by the dyer and the tanner, and have thus acquired considerable importance, both commercially and chemically.

The essential constituents of catechu are mimotannic acid and catechin, which are mixed with a brown substance, resulting from the oxidation of both these bodies.

Mimotannic acid is sparingly soluble in dilute sulphuric acid, although freely soluble in water. If a concentrated aqueous infusion of catechu be mixed with a small quantity of dilute sulphuric acid, a precipitate occurs, at first consisting chiefly of colouring matter; this may be removed, and on then gradually adding concentrated sulphuric acid so long as it occasions a precipitate, mimotannic acid is thrown down: this precipitate may be collected on a filter, washed with dilute sulphuric acid, pressed between folds of blotting-paper, and dissolved in water. The excess of sulphuric acid is then removed by digestion with carbonate of lead, and the filtered liquid, when evaporated in vacuo, furnishes mimotannic acid nearly pure.

This substance is distinguished from gallotannic acid, by yielding a greenish grey precipitate with the ferric salts; and by not precipitating a solution of tartar emetic: when subjected to heat it does not furnish pyrogallic acid. Its solution absorbs oxygen and becomes of a dark red colour, leaving, when evaporated, a substance no longer completely soluble in water. Under these circumstances, according to Delffs, a certain quantity of catechin is formed, and remains in a crystalline condition.

Catechin (20H18,H,Ð? Zwenger) is contained in the portion of catechu which is not soluble in cold water; it is soluble in three or four parts of boiling water, or of boiling alcohol; and when pure is deposited in minute colourless crystals. The caustic alkalies immediately attack it, causing it to absorb oxygen, when

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