thirty pounds of iron borings, thirty pounds of their i remarkable powdered zinc, and thirty-five pounds of i quick lime; the whole is stirred up from time to time 1 for twenty-four hours, when it is ready for use. If the r bath is not considered sufficiently strong, a little more i lime and zinc are introduced. t

The Chemical theory of the process is, that the zinc, i under the influence of the lime, décomposés water, com- g bining with its oxygen, and the hydrogen thus liberated, removes oxygen from the indigo, which then dissolves 1 in the lime. i

To dye cotton yarn in the above vats, it is simply f necessary to dip it for a few minutes in the dye bath, I and expose it to the atmosphère, when the green hue it € has acquired passes rapidly into blue. This operation is ] repeated until the yarn has attained the required depth i of shade, when it is passed into weak vitriol, washed, 1 and is ready for market. 1

To dye calicos, the pièces are hooked on frames, passed ] through a bath of weak milk of lime, and then dipped . into the reduced indigo vat. After fifteen minutes, the : frame is taken out and the cloth exposed to the air for i about the same length of time. It is again dipped, the ; process being repeated until the required depth of tint is 1 attained. It is then passed through weak vitriol, and washed. The cold vats are especially used when it is wished to obtain white and yellow designs on a blue ground ; but when the object in view is to produce a self-colour, a more rapid process is adopted. This consista in passing the pièces through a dye-beck, then through an acid liquor, and lastly in water, by means of rollers fixed in the vat. The bath is composed of lime, sulphate of iron, and indigo, but is kept hot, instead of cold as in the former case.

There is still another process, which is now used to a limited extent only, but was at one time very extensively employed. It produces on the cloth a pale blue, which has a great similarity of tint to that seen on the china porcelain, from which it dérivés its name of china-blue. To produce it the pièces are printed with a mixture con-taining very finely-powdered indigo, and a little acetate of iron, and are made to pass through six successive vats. The first two contain lime, the third sulphate of iron, the fourth a solution of caustic soda, the fifth a dilute solution of sulphuric acid, and the sixth water. When the design has acquired the required depth of blue, the pièces are washed, passed once more through weak sulphuric acid, and again washed. The Chemical reactions are exactly similar to those in the cold vat process.

For dying wool, a modification of the oldwoadvat is employed. The use of woad being nowalmost entirelydiscon-tinued, I shall merely call your attention to the process in which indigo has been substituted for woad. Itbears the name of Indian vat, doubtless from the process having been practiced in India and imported from thence. It is as follows :—Eight pounds of powdered indigo is added to a bath containing 32 pounds of bran, 32 pounds of madder, and 12 pounds of potash, which is maintaincd for several hours at a température of 200° Fahr. It is then allowed to cool to 100° Fahr., when fermentation ensues. After about forty-eight hours, the indigo is rendered soluble, being reduced, by the décomposition of the sugar and other products contained in the bran and the madder-root during the process of fermentation. The bath should have a greenish yellow appearance, having a frothy scum of a blue coppery hue.

Of late years, improvements have been made in this class of vats, by which the expense of using madder is avoided. They are now prepared by adding to water, at a température of 200° Fahr., twenty buckets of bran, twenty-six pounds of soda crystals, twelve pounds of indigo, and five pounds of slacked lime. After five hours, the bath is allowed to coolto 100° Fahr., when fermentation ensues, and the indigo is dissolved in the alkali. The management of these vats require great experience and care, for if the fermentation is too slow the indigo is not pro-perly reduced, whilst if too active large quantifies of indigo

may be lost. The researches of Dr. Schunck, already referred to, not only show the method of avoiding this loss, but explain why it occurs. The remarks which I made as to the causes of failure in the manufacture of indigo are applicable here, namely, that if the fermentation becomes alcoholic and acetic, the non-oxidisable indigo compounds described by Dr. Schunck are generated.

I cannot leave the subject of indigo without bringing before you a most curious source of its production, namely, the human body. Medical men had observed from time to time that urine, secreted under certain pathological conditions, became brown, and sometimes even blue, when exposed to the atmosphère. The late Dr. Hassel discovered that in some instances the colour-ing matter was indigo, but here, again, we are indebted to Dr. Schunck for much information on the subject. In three papers presented to the Royal Society, he has proved that urine, in cases similar to those examined by Dr. Hassel, contained the glucoside of indigo, or indican. He also observed that indican was a very frequent constituent of urine secreted by persons in a healthy state, and, in fact, that it is produced generally when persons do not take sufficient exercise, and he bas on several occasions succeeded in producing it by taking in his food a rather large excess of sugar.

Orchil, Cudbear, Litmus.—I shall now call your attention to a colour which was discovered in 1300, by an Italian named Federigo, who, during his travels in the East, observed the tinctorial powers of a certain class of weeds called lichens, and he introduced the colour into Florence under the name of orchil. By this discovery he and his family made a very large fortune.

Lichens are small plants which live either in the stems or leaves of trees, or on rocks, or damp soils. To this class belongs all the végétation found in the Arctic circle, but the species growing there are not employed to produce the colouring matter orchil, the varieties used for this purpose being found in warmer, and especially in tropical climates. These latter can be divided into two classes ; the first and most abundant, which grow on rocks near the soa-side, includes the species Roccella tinctoria and Roccella fuciformia. They are obtained from the Canary Islande, Cape Vere, and Sardinia, but principally from Madagascar, Zanzibar, Angola, and South America. The second class grows [inland, and includes the species Variolaria orcina, found especially in the Pyrénées.

Lichens do not contain any colouring matter already formed, but contain colourless acids, which, under the influence of ammonia and the oxygen of the atmosphère, give rise to the orchil. As the lichens imported into this country vary considerably in the amount of orchil which they yield, Dr. Stenhouse has rendered a great service ; to the manufacturers in furnishing them with a simple and accuratc process of ascertaining their commercial ? value. The folio wing is an outline of his method :— . One hundred grains of the lichen are macerated with a ; dilute solution of caustic soda, two treatments being l sufficient to extract the whole of the colouring matter. i A solution of hypochlorite of sodium of known strength l is added to the soda solution from a graduated. alkali-r meter ; the moment the bleaching liquor cornes in con-. tact with the soda solution of the lichen a blood red , colour is produced, which disappears in a minute or two, and the liquid has only a deep yellow colour. A new 3 quantity of bleaching liquid should then be poured into 3 the soda solution, and the mixture carefully stirred. t The operation should be repeated as long as the addition , of the hypochlorite of sodium causes the production of f the red colour, for this shows that the soda solution still , contains unoxidised colouring principle. Towards the - end of the process the bleaching solution should be e added by only a few drops at a time, the mixture being l carefully stirred between each addition. By noting how - many measures of the bleaching liquor have been o required to destroy the colouring matter in solution,

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