appellation of the woad-plant, contains one atom more of oxygen than indigo blue: it may be obtained by heating indigo in a dilute solution of potassic dichromate and sulphuric acid, or by the following process:-Powdered commercial indigo, of good quality, is to be mixed with a sufficient quantity of water to reduce it to a thin cream, and the mixture is to be heated till it boils: undiluted nitric acid must then be added in small quantities at a time until the blue colour has disappeared; 10 parts of indigo require 6 or 7 parts of acid. The mass containing isatin mixed with a large quantity of a brown resinous matter must next be largely diluted with water, boiled, and filtered whilst boiling hot; the undissolved residue must be again boiled with water, and the filtered solution added to the first portions: crystals are deposited as the liquid cools; they must be washed with water containing a little ammonia, then dissolved in alcohol and recrystallized. When considerable quantities of isatin are required, Hofmann purifies the crude product of the action of nitric acid upon indigo by dissolving it in a dilute solution of potash; hydrochloric acid is next added to the deep brown liquid, until the precipitate, which is at first brown, assumes a clear red tint, the liquid is then filtered and precipitated by an excess of hydrochloric acid; this precipitate consists of isatin, which, after washing with cold water, is chemically pure.

Isatin crystallizes in brilliant, transparent, orange-coloured, six-sided, or rhombic prisms; it is sparingly soluble in cold water, but is more readily dissolved by boiling water, and still more so by hot alcohol. Ether dissolves it less readily. On the application of heat to the crystals, they melt and are volatilized, emitting acrid vapours of a yellow colour. When isatin is distilled in closed vessels a considerable portion of it undergoes decomposition. Isatin is capable of exchanging an equivalent of hydrogen for one of a metal; thus, if an alcoholic solution of isatin be mixed with one of nitrate of silver, a wine-red amorphous precipitate of argentisatin (E,H,AgNO) is formed. Solution of potash dissolves isatin freely, and forms a purple liquid, which on boiling becomes yellow. When examined after this change of colour has occurred, the elements of the isatin are found to have combined with those of water, thus producing isatic acid (II,II.NO), which remains in combination as potassic isatate.

8

If isatin be heated with solid caustic potash it is decomposed, hydrogen is liberated, potassic carbonate is formed, whilst aniline distils over:—

688

DERIVATIVES OF ISATIN.

Concentrated nitric acid decomposes isatin; when heated with it copious ruddy fumes are evolved, a brown resinous body containing nitroxyl (NO) is formed, and if the action be carried further oxalic acid is found in the solution. No carbazotic acid is formed. Chlorine, if transmitted through water in which isatin is suspended, displaces one atom of hydrogen and forms chlorisatin.

8

(1519) The action of chlorine upon indigo is peculiar: gaseous chlorine has no action upon dry indigo, but if a current of chlorine gas be transmitted through water containing indigo in suspension, the blue colour is gradually destroyed, and a rusty brown compound is formed: this substance, in addition to formic acid, ammonia hydrochlorate, and a brown resinous substance, contains four distinct chlorinated compounds. Two of these, viz., chlorisatin HCINO, and dichlorisatin E.H,CI,NO, are yellow, sparingly soluble, crystallizable substances formed upon the type of isatin. A third chlorinated compound is trichloraniline (E ̧H ̧Cl ̧N), and it is usually accompanied by a certain quantity of trichlorophenic acid (HЄ,H,Cl). Chlorisatin and dichlorisatin are soluble in alcohol; when heated with a solution of potash they yield a salt of a corresponding acid, the chlorisatic (HЄ.H.CIN→ ̧), and dichlorisalic acid (HE HCl,NO), corresponding with isatic acid; and, if distilled with caustic potash, they give chlorinated bases of a composition corresponding to that of aniline,—

6 2 3

The reaction is precisely analogous to that with pure isatin, thus :

CH_CINO,+4 KHO=CH[C]N+2K,€€g+Hg.

2

6

3

The action of bromine upon indigo is analogous to that of chlorine, though it is less violent.

(1520) Ammoniacal Derivatives of Isatin.-When isatin is treated with ammonia it gives rise to a number of amidated products, the composition of which varies with the degree of concentration of the ammonia: they have been particularly studied by Laurent (Ann. de Chimie, III. iii. 483). The following table exhibits the reactions which attend their formation :—

Chlorisatin and dichlorisatin when heated with ammonia yield corresponding compounds.

8

(1521) Isathyd (Є.H.NO). This substance, as its name implies (isatin hydrogenized), stands in the same relation to isatin that white indigo does to blue indigo; that is to say, its molecule contains one atom more of hydrogen than isatin. In order to procure isathyd, an alcoholic solution of isatin is mixed with one of ammonium hydrosulphide (H ̧NHS), and set aside for a week in a closed bottle; crystals of sulphur are gradually deposited, mixed with prismatic needles of isathyd. The sulphur must be removed by digestion with carbonic disulphide. A better method, however, consists in placing powdered isatin in a flask with a considerable quantity of water acidulated with sulphuric acid, then introducing a plate of pure zinc, and heating the whole to the boiling point. As the isatin becomes dissolved, it combines with the nascent hydrogen, and is converted into isathyd, which is deposited in the form of a crystalline powder. Isathyd is a greyish white, tasteless substance, which is insoluble in water, and but sparingly soluble in alcohol and in ether even at a boiling temperature: it is deposited from these solutions as they cool in minute oblique prisms. If heated, it softens and becomes decomposed, and acquires a violet-brown tint. Boiling nitric acid colours it violet, and dissolves it. Chlorisatin and dichlorisatin, when treated with ammonium hydrosulphide, yield chlorinated bodies corresponding with isathyd, termed chlorisathyd, ЄH,CINO, and dichlorisathyd, EH ̧CINO. By analogous methods corresponding compounds containing bromine may be obtained.

5

When a current of sulphuretted hydrogen is transmitted through a boiling concentrated alcoholic solution of isatin, the isatin is converted into isathyd, and this, at the moment of its formation, becomes decomposed, and sulphisathyd, a sulphuretted compound corresponding in composition to isathyd, but containing an atom of sulphur in place of one of oxygen, is formed:

When the liquid has become clear by the deposition of the sulphur, the sulphisathyd may be precipitated by the addition of water. It is a yellowish grey, tasteless powder, which softens at

690

INDIN-COLOURING MATTERS OF THE LICHENS.

212°; alcohol and ether dissolve it, but do not yield it in a crystalline form on evaporation.

Various other sulphuretted derivatives may be obtained from isatin (Laurent, loc. cit.).

2

(1522) Indin (HN2O).-This substance appears to be polymeric with indigo blue. It is of a beautiful rose-colour, is insoluble in water, and but sparingly soluble in alcohol and in ether, even at a boiling temperature. In order to prepare indin, sulphisathyd is triturated in a mortar for some minutes with a concentrated solution of potash, adding a few drops of the alkaline solution from time to time, until a rose-colour begins to show itself. Alcohol is then to be added in small quantities at a time, continuing to triturate the mass until the mixture has assumed a deep rose-colour. It is then to be diluted with alcohol, and the insoluble portion collected on a filter and washed, first with alcohol, then with water. If this residue be dissolved in a tepid concentrated solution of potash, black needles of potassic indinate (ЄH,KN,,) are deposited. They must be washed with absolute alcohol, and decomposed with a weak solution of hydrochloric acid. Pure pulverulent indin is left on washing the insoluble residue. Nitrindin H ̧(NO) N ̧л, chlorindin (H.CI,N,O), and dichlorindin (EH CIN,O) are substitution-compounds obtainable from indin.

8

16 9

2

2

16 8

2

Hydrindin 2 (EHN2O2),H2O.-This substance appears to stand to indin in a relation similar to that of reduced indigo to blue indigo. When the black solution of potassic indinate is heated with an excess of an alcoholic solution of the alkali, the colour gradually changes into a pale yellow, and the liquid on cooling deposits crystals of potassic hydrindinate. Water decomposes them, and leaves hydrindin in the form of a white powder, insoluble in water, and sparingly soluble in hot alcohol.

A number of other compounds have been obtained from indigo; they are described by Laurent, Ann. de Chimie, III. iii. 463, and by Erdmann (Journ. für prakt. Chemie, vols. xix., xxii., and xxiv.)

(1523) 2. COLOURING MATTERS OF THE LICHENS.-Many of the lichens afford colouring matters which are of considerable importance. A yellow lichen common on old walls (Parmelia parietina) contains a colouring matter identical with that of the rhubarb root (1501), and the different varieties of Usnea and Evernia furnish an analogous, straw-yellow, crystalline substance, termed usnic acid (HЄ19H15,?), which is easily dissolved by ether; it yields red compounds with the alkalies. When boiled

7

COLOURING MATTERS OF THE LICHENS-CUDBEAR.

8

691

with an excess of alkali it furnishes a substance termed by Stenhouse beta-orcin (E,H10?). The same compound is obtained by the distillation of usnic acid. These acids are unimportant as colouring matters, and need not be further noticed here.

(1524) The red, violet, and blue colours which are known in commerce under the names of archil, cudbear, and litmus, are supplied by different species of Roccella, Variolaria, and Lecanora. The Roccella tribe grow upon rocks on the sea-coast, in the Canary Islands, in Sardinia, and in Corsica, at the Cape of Good Hope, on the western coast of South America, and in various other parts of the world. The Variolaria abounds in certain parts of the Pyrenees, and of the Alps; and the Lecanora tartarea is abundant on the Swedish mountains.

14 7

In no case do these lichens furnish the colouring matter ready formed; but they contain from 7 to 12 per cent. of various colourless acids, such as the erythric (20H22010), evernic (Є17H167?), and lecanoric acids (Є16H1,,?); these bodies under the influence of alkalies are readily decomposed into new acids (which are characterized by the production of a violet colour with chloride of lime), and into a colourless neutral substance, termed orcin (H.). Orcin is the true colour-producing body; under the influence of ammonia it absorbs oxygen, and is converted into orcein (Є,H,NO), an azotised compound possessed of high tinctorial power.

In the preparation of Cudbear, the lichens (chiefly Lecanora tartarea) are first freed as far as may be from earthy impurities, and are made up into a paste with a solution of ammonia of sp. gr. about o'990; formerly stale urine to which a quantity of lime had been added, for the purpose of liberating its ammonia, was used; the mixture is then exposed for some weeks to the air, moistening it from time to time with fresh quantities of the ammoniacal liquid. A paste is thus obtained of an offensive odour and of an intense violet-red colour; when dried and reduced to powder it constitutes the cudbear of commerce. The colouring matter of this dye-stuff consists mainly of orcein. Cudbear is employed chiefly for woollen and silk goods, to which it imparts a brilliant but not very permanent dye, of various shades of violet, lilac, and purple. Archil is prepared by digesting the lichens in a hot solution of ammonia, allowing it to stand for a few hours, and exposing the clear solution which is drawn off from the lichen in deep jars to the air for about 3 weeks; the solution when concentrated by evaporation forms the archil liquor of commerce.