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into forty-eight or fifty parts of boiling alcohol, to which is added two parts of a concentrated solution of caustic soda. The whole is put into a bottle exactly large enough to hold it, and left to cool. By exposing the colourless liquid to the atmosphère, the reduced indigo which is in solution absorbs oxygen, and the indigotine is precipitated under the form of beautiful prismatic crystals.

. The third process, devised by myself, I have found to give satisfactory results. It consists in introducing into a flask one part of finely pulverised indigo, two parts of green copperas, and two hundred parts of water con-taining ten per cent, of caustic soda. The whole is kept at the boil for a short time, and allowed to cool. The clear liquor is exposed in shallow vessels to the atmosphère, when the soluble indigo is oxidised, and précipitâtes as pure indigotine. The residue in the flask is submitted to the treatment three times. The whole of the indigotine thus obtained is collected on a filter, dried, and weighed.

A commercial process, founded on the above method, is carried on by Messrs. Haas and Co., who sell indigo so purified under the name of refined indigo. An inferior quality is also prepared by heating indigo, at a moderate température, with weak muriatic acid, which dissolves lime and other minerai matters, as well as any starch it may contain ; it is slightly washed and boiled with weak caustic soda, to dissolve the chlorophyl and other resinous impurities.

I shall now call your attention to three commercial préparations of indigo, obtained by the action of sul-phuric acid on it. The first is called sulpho-purpuric acid or phenicine, which is made by adding one part of indigo to four parts of highly concentrated sulphuric acid, heating for a short time, varying from half an hour to an hour, or until a small quantity of it mixed with a large quantity of water gives a deep purple colour. Great care must be bestowed on this part of the operation, so as to avoid the formation of a compound, to which I shall have again to call your attention, sulpho-indigotic acid. The acid mass produced is thrown into about 40 or 50 parts of water, when a beautiful purple precipitate is produced, which is collected on a filter, and slightly washed with weak muriatic acid. To dye wool with this sulpho-purpuric acid it is necessary to add to the bath a little muriatic acid, when it yields to the wool a fine dark purple-blue, that can be converted into various shades of purple by passing the wool so dyed in a bath containing a small quantity of carbonate or acetate of soda, which removes a small quantity of sulpho-indigotic acid that may be présent, and gives rise to sidpho-purpurate of soda, which is a faster dye than the acid itself.

Sulpho-indigotic Acid is manufactured by dissolving one part of indigo in ten or twelve parts of concentrated sulphuric acid, and heating the whole at a température of 120° F. very carefully for some hours. The operation is completed when a small quantity dissolves entirely in cold water.

The acids above described, when obtained perfectly pure,havethefollowing formulæ :—Sulpho-purpuric acid, C16 H1oN2 02,803; sulpho-indigotic acid, Cs H. N 0, S Og.

Berzelius admits a third compound, called hypo-sidpho-indigotic acid. These acids are transformed into neutral salts of soda, and sold under the names of neutral paste and carminé of indigo. They are prepared by neutralis-ing the sulpho-acids with carbonate of soda, and the paste so formed is thrown on a woollen filter to remove the sulphate of soda which it contains, as well as a green colouring matter, which is doubtless modified chlorophyl. The paste is then washed with a solution of chloride of sodium. It is a curious fact that carminés of indigo, which are perfectly soluble in pure water, are altogether insoluble in water containing either sulphate of soda or chloride of sodium.

Whilst speaking of the sulpho-indigotates, it may be

useful to notice that the sulpho-indigotates of potash and soda are soluble in 100 to 150 parts of water, the sulpho-indigotates of lime, magnesia, and alumina are freely soluble, whilst those of baryta and lead are insoluble.

The carminés of indigo are especially used by silk-dyers, in conséquence of the removal of the green colouring matter above referred to, and which, if allowed to remain, would spoil the blue or purple which they wish to obtain. The method practically adopted to ascertain if the sample bas been well washed, consists in rubbing a small quantity of it on a piece of glazed paper, which, when the colour dries, gives a colour varying from a pale blue to a rich copper purple, accord-ing to the mode of manufacture ; and if any green colouring matter is left in, it shows itself as a green ring round the blue circle.

The following may be taken as the composition of a sample of carminé of indigo of fair quality :—

Water ................................. 85

Indigo ............................... 10-2

Saline residue......................... 4'8

100-0

The sulpho-indigotic acids are especially used by woollen-dyers, who add to the dye-beck a little alum and cream of tartar, which helps thefixing of the indigo on the wool. The green colouring matter is in this case not objectionable, having no affinity for the fibre of wool.

The carminés of indigo, as well as the sulpho-acids, are easily decolourised by reducing agents, such as hydrogen and sulphuretted hydrogen, but they gradually reassume their original colour when exposed to the atmosphère, through the absorption of oxygen.

The above compounds, not being suitable for dyeing cotton, and not giving colours on silk and wool that may be considered fast, I shall now proceed to describe a few of the methods followed to obtain fast indigo blues. They are all based on the principle of the réduction of blue indigo into white indigo. The latter compound is held in solution by an alkali, which enables the dyer or printer to introduce it into the fibre of the fabric, where, on exposure to the atmosphère, the alkali combines with carbonic acid, and the white indigo thus liberated absorbs oxygen, and becomes insoluble blue indigo. The principal class of goods to which this Chemical reaction is applied is to the vegetable fibres, linen, and especially cotton. As far as dyeing is concerned, the processes can be classed under two heads, hot and cold. The hot process is principally applied to wool, the cold to vegetable fibres, especially cotton.

The oldest and still most generally employed method of preparing cold vats consists of putting into a vat containing about two thousand gallons of water, sixty pounds of indigo, very finely powdered, one hundred and eighty pounds of slacked lime, and one hundred and twenty pounds of sulphate of protoxide of iron or green vitrol (frec from any trace of copper sait), the two latter substances being added from time to time. The greater part of the lime used unités with the sulphuric acid of the iron sait, to produce sulphate of lime or gypsum, and the liberated protoxide of iron removes the oxygen from the indigo, becoming converted into saline oxide, whilst the reduced indigo dissolves in the excess of lime employed.

Messrs. R. Schloesser and Co., of Manchester, have introduced within the last year or two a marked im-provement in the préparation of cold vats, which removes the great objections of the bulky precipitate of sulphate of lime, the formation of an oxide of iron, and the loss of indigo by its combination with the oxide of iron referred to in the previous part of the lecture. The bath remaining much more fluid, the pièces are less apt to be spotted, and a better class of work is produced. To carry ont their process, they add to the ordinary two thousand gallon vat twenty pounds of ground indigo,

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