C

acetic acid, yielding a compound which does not permit the indigo it contains to be oxidised on exposure to the air. This explains the loss of indigo in its manufacture. Dr. Schunck represents the change by the fol-ing formulæ :—

Indigo. Alcohol. Acetic acid.

CH,NO + 8(C,H.0) + 3(C.H,02) + co2

= C31H39N04 + 13H2O.

île produced this compound more easily by treating pure indigo blue with alcohol, to which was added an alkaline solution of protoxide of tin, until the indigo was dissolved, acetate of soda was then added, and the whole digested at a moderate température. The indigo blue after some time ceased to be deposited on exposure to air, having entirely disappeared, having been converted into the new compound.

Pure indigo, or indigo tine, can be prepared from indican, by the process already described, or by reducing indigo to powder, placing it in a small dish on which is a cover, and applying a beat of about 300° or 400° Fahr., when beautiful prismatic needles of indigotine are sublimed, which are removed mechanically. It is insoluble in water, alcohol, or ether, or in weak acids, or alkalis, but is slightly soluble in créosote, carbolic acid, and anhydrous acetic acid, to which a very small quantity of sulphuric acid bas been added. From this latter solution, the indigotine may be precipitated by the addition ( i water. It is the only solution of blue indigo that can be applied directly on a fabric.

Indigotine, when heated, gives of beautiful violet va-pours, having a characteristic odour. Under the action of oxidising agents, such as nitric acid, it yields three distinct Products, isatin, C8 H. N.O,; indigotic or nitrosalycilic acid C7 H. (NO2) 03, andpicric acid, C6 H3 3 (NO2) 0. The first is formed by simple oxidation, whilst in the second and third, part of the carbon is oxidised, and hyponitric acid is substituted for part of the hydrogen. These Chemical reactions lead me to call your attention to some very interesting scientific déductions, showing indigo to bear a very close relation to aniline and carbolic acid, both products derived from coal-tar. If indigotine is heated with potash in a small retort, aniline is produced, and distils over. When indigo is heated for some time withcaustic potash solution of spécifie gravity, 1'35, it yields anthranilic acid, which may be considered as a phenyle-carbamic acid. Anthranilic acid again, being heated to 570° Fahr., gives salycilic acid, which may be considered as a compound of carbonic with carbolic acid. Lastly, indigotine, like carbolic acid, yields on treat-ment with excess of nitric acid, picric, or trinitrophenic acid.

These facts have produced on the minds of chemists a conviction that indigotine will one day be artificially prepared from carbolic acid, and recently MM. Emmer-ling and Engler have accomplished the scientific arti-ficial production of indigotine from a compound acetone, discovered in 1857 by M. Friedel, to which these gentlemen have given the name of acetophenone. Their pro-cess consists in acting on this compound with fuming nitric acid, when two nitro-compounds are produced, one crystalline, the other syrupy. To the latter, after its evaporation to the state of a résinons mass, they add ten parts of reduced zinc and one part of soda-lime. The mixture is heated in small tubes, when a little water evaporates and a dark coloured substance sublimes, which contains indigotine in small quantifies. This resuit is certainly a triumph of scientific chemistry, and brings us another stage nearer the commercial artificial production of a most valuable dye.

A great variety of substances,by adding one équivalent of hydrogen, couvert blue indigo into white indigo, which is a colourless substance, without taste or odour, insoluble in water, but soluble in alcohol and ether, and in solutions of the alkalis and alkaline earths. On this point I may observe that, many years ago, I devised a process, by which many pounds worth of indigo were

recovered from the refuse bottoms of the blue dip vats, the préparation of which I shall explain further on. I found that indigo was susceptible of forming insoluble compounds with the lime and protoxide of iron. White indigo dissolves freely in strong sulphuric acid, giving a dark purple blue liquor.

Many substances are employed commercially to couvert blue indigo into the white indigo, soluble in alkalies. Thus, I may mention lime and protoxide of iron, caustic alkali and protoxide of tin, or sulphuret of tin or arsenic, and zinc and caustic alkali ; also organic substances, such as grape sugar and bran, which enter easily into fermentation. The following formulæ show the différence of composition of the two indigos :—

Indigotine. Hydrogen. White Indigo.

2(C8H5NO) + 2H = 016 H12 N 04

I shall now have the pleasure of proceeding to de-scribe some of the methods employed by chemists to déterminé the relative commercial value of samples of this expensive dye. The best qualifies of indigo from Bengal, Java, and Guatamala, are light, have a uni-form texture, and a fine coppery hue, which is increased by friction. The following analysis, made by M. Chevreul, shows the composition of a fair sample of commercial indigo :—

Indigotine ....................... 45

Matters soluble in alcohol.......... 30

Matters soluble in ether............ 12

Resin soluble in hydrochloric acid.... 6

Minéral matters ..................... 7

100

Commercial indigos are often adulterated with minerai matters of varions kinds. This fraud is easily detected by calcining a known weight of the sample, which ought not to leave a residue of more than ten per cent. The most common adultération, however, is the addition of starch. This can be detected by boiling some of the pulverised indigo with a weak solution of hydrochloric acid. The insoluble starch is thus converted into soluble dextrinc, which yields a beautiful purple colour with iodine.

There are several processes employed to déterminé the amount of indigotine in commercial indigos. I shall here only give the outline of three. The first consista in dissolving one gramme of the dry pulverised indigo in twelve grammes of concentrated sulphuric acid, and heating the whole at a température not exceeding 120° Fahr., when the indigo combines with the sulphuric acid, and becomes perfectly soluble in water. . It is then diluted with water, so that the whole occupies one litre. The operation is repeated with one gramme of pure indigotine, which serves as a standard of comparison. A solution of bleaching powder, or bichromate of potash—the first proposed by M. Chevreul, the latter by Dr. Penny—is prepared of such a strength that one hundred volumes of the solution will completely destroy the whole of the colour produced by the gramme of indigotine. Part of the same liquor is then applied to the solution of the commercial indigo, and the number of volumes required to destroy the colour représenta the pcr-centage of indigo. Thus, if 60 divisions are required, it is assumed that there is 60 per cent, if 70 divisions, 70 per cent. This method of estimating the indigotine is not to be relied on, as it always gives a much higher pcr-centage of colour-giving principle than exists in the indigo, owing to the hypochlorous acid of the bleaching powder, and the chromic acid of the bichromate not only decomposing the indigotine, but acting on several colouring matters which are in-cluded intho portion soluble in alcohol in M. Chevreul’s analysis.

The second process gives better rcsults ; it is due to Professer Fritzsche, and consists in introducing one part of indigo, finely pulverised, and one part of grape sugar,

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