2. Blacks the Product of the fire:-under this head come, charcoal blacks, foot blacks, black metallic calces.

3. Blacks obtained by mixture:-of which kind are, black from iron, black from filver, and black from lead and fulphur.

Our Author's experiments, obfervations, and conclufions, relative to the above particulars, are curious, accurate, and "ufeful. To enter into a detail of these, would fwell this article to an undue bulk; we fhall confine ourselves therefore to what he says

Of Black produced from Iron.

The infufions of certain vegetable aftringents, mixed with green vitriol, which is a folution of iron in the vitriolic acid, produce a deep black liquor, of most extensive ufe for dying and ftaining black.The aftringent fubftances chiefly employed are the excrefcences of the oak-tree, called galls, and of thefe the Aleppo galls are deemed the beft; all the parts of the oaktree, the leaves, acorns, and more particularly the bark and wood; other vegetable fubftances likewife, the leaves, fmall branches, and flowery clusters of the fumach-tree; balaustine flowers, pomgranate peel, alder bark, biftort root; and in general all thofe which are auftere, aftringent, or corrugating to the taste, are poffeffed of the fame virtue with galls: the power by which thefe fubftances ftrike black with vitriol, and their aftringency, are proportional to one another, and feem to depend upon one and the fame principle. Of the other properties of this aftringent and colouring matter, little more is known, than that it is diffolved and extracted both by water and spirit of wine, and that it does not exhale on the evaporation of the menftruum.

When a decoction, fays Dr. Lewis, or infufion of the galls, is dropt into a folution of the vitriol largely diluted with water, the first drops produce bluish or purplish red clouds, which foon mingling with the liquor, tinge it uniformly of their own bluish or reddish colour. It feems to be on the quality of the water that this difference in the colour depends. With diftilled water, or the common fpring waters, the mixture is always blue. If we previously diffolve in the water the moft minute quantity of any alkaline falt, too fmall to be discoverable by any of the common means by which waters are examined, or if the water is in the leaft degree putrid, the colour of the mixture proves purple or reddifh. Rain-water caught as it falls from the clouds. in an open field, in clean veffels, gives a blue, but fuch as is collected from the tops of houses, grows purple with the vitriol and galls; from whence it may be prefumed, that this last has sontracted a putrid tendency, or received an alkaline impreg

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nation, though so flight as not to be fenfible on other ways of trial.

Both the blue and the purple liquors, on adding more of the aftringent infufion, deepen to a black, more or less intense according to the degree of dilution: if the mixture proves of a deep opake blacknefs, it again becomes bluish or purplish when further diluted. If fuffered to ftand in this dilute ftate for two or three days, the colouring matter fettles to the bottom, in form of a fine black mud, which, by flightly fhaking the veffel, is diffufed again through the liquor, and tinges it of its former colour. When the mixture is of a full blacknefs, this feparation does not happen, or in a far lefs degree; for though a part of the black matter precipitates in ftanding, yet fo much remains diffolved, that the liquor continues black. This fufpenfion of the colouring fubstance in the black liquid may be attributed in part to the gummy matter of the aftringent infufion increafing the confiftence of the watery fluid, for the feparation is retarded in the diluted mixture by a small addition of gum arabic; though another principle appears alfo to concur for part of the effect.

If the mixture, either in its black or diluted ftate, be poured into a filter, the liquor pafles through coloured, only a part of the black matter remaining on the paper. The filtered liquor, to the eye perfectly homogene, on 1tanding for fome time, becomes turbid and full of fine black flakes: being freed from thefe by a second filtration, it again contracts the fame appearance, and this repeatedly, till all the colouring parts are feparated, and the liquor has become colourlefs. It fhould feem therefore, there happens a gradual and flow concretion of the black corpuscles, into particles large enough to fubfide by their own weight, or to be retained on a filter; and that this concretion is greatly influenced by dilution with water. Perhaps it is affected alfo by the action of the air; for having once fet fome of the diluted mixture to settle in a close stopt glass, the feparation of the black matter was remarkably more flow than in the other experiments, in which the veffel was open.

The colouring matter, thus feparated from the liquor, being drained on a filter and dried, appeared of a deep black, which did not feem to have fuffered any change on lying expofed to the air for upwards of four months. Made red hot, it glowed and burnt, though without flaming, and became a rusty brown powder, which was readily attracted by a magnetic bar; though in its black flate, the magnet had no action on it. The vitriolic acid, diluted with water and digefted on the black powder, diffolved the greateft part of it, leaving only a very little quantity of whitish matter. Solution of pure fixt alkaline falt diffolved very little of it: the liquor received a reddish brown co

lour,

lour, and the powder became blackish brown. This refiduum was attracted by the magnet after being made red-hot, though not before the alkaline tincture, paffed through a filter, and mixed with folution of green vitriol, ftruck a deep brownishblack colour, nearly the fame with that which refults from mixing with the vitriolic folution an alkaline tincture of galls.

From these experiments it feems to follow, that the colouring matter in the black mixtures is iron, extricated from its acid folvent in a highly attenuated or divided state, and combined with a peculiar fpecies of matter contained in aftringent vegetables; which matter, after the watery fluid that the compound floats in has been feparated, is in part extracted from the iron by alkaline liquors, and may thence be again transferred into fresh diffolved iron.

The blacknefs is generally attributed to the iron being barely revived from the vitriol in its metallic ftate; the black matter being fuppofed to be of the fame nature with the impalpable black powder, into which fine iron filings are changed by lying for many months under water. But this black matter differs from that of our mixtures in two very material properties. It is attracted in its black ftate by the magnet; and, when moistened and exposed to the air, it changes speedily into ruft. The refiftance of ours to the magnet and to the air proceeds doubtless from the combination of the other matter with the iron; and there appears fome analogy, in regard to the manner of production, between this black fubftance and Pruffian blue; one being a precipitation and coalition of diffolved iron with one fpecies of matter, and the other with another: the principal difference is, that the fubftance combined with the iron in the Pruffian blue defends the metal from the action of acids, which that in the black compound is unable to do.'-It appears likewife, from the experiments of our Author on the folutions, and different foluble preparations of iron made with the nitrous, marine, and vegetable acids; that all thefe preparations ftrike a black colour with the infufions of the aftringent vegetables; that the experiments from which a contrary conclufion has been drawn, were made with folutions in which the acid was not perfectly faturated, and hold equally true of the vitriolic folution when the faturation is not compleat :-that this colouring matter once produced, is again deftroyed by the addition of any of the acids, as the acid re-diffolves the ferrugeneous matter; hence the ufe of acids for difcharging the ftains of ink, or other black mixtures of this clafs-that alkalies deftroy the colour on a different principle; that they diffolve the aftringent matter, and precipitate the iron nearly in the fame ochery ftate, as in the fimple and acid folutions of this metal :-that the black colour discharged by an alkali, is reftored by the addition of any acid

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in fuch quantity as to faturate the alkali; and that, on the other hand, this colour difcharged by an acid, is in like manner reftored by the addition of an alkali.

After thus giving an experimental hiftory of the feveral claffes of materials, which are practifed with in order to obtain and fix black colours, Dr. Lewis proceeds to apply these general principles to the particular arts.-In the fifth fection he treats of black paint with oil, black paint with water, compofitions. for marking fheep, compofitions for preferving wood, &c. compofitions for blacking leather, fpirit varnish, amber varnishes for papier maché, &c. varnish for metals, fealing-wax, printing ink, rolling-prefs ink.- -We fhall give our Readers extracts from

two or three of thefe articles.

Of Black Paint with Water, and of the valuable Black called

Indian Ink.

An opake deep black for water-colours is made by grinding ivory-black with gum-water, or with the liquid which fettles from whites of eggs, after they have been beaten up and fuffered to ftand a little. Some ufe gum-water and the white of eggs together; and report, that a small addition of the latter makes the mixture flow more freely from the pencil, and improves its gloffiness.

It may be obferved, that though ivory-black makes the deepest colour in water as well as in oil painting, yet it is not always, on this account, to be preferred, in either kind, to the other black pigments. A deep jet-black colour is feldom wanted in painting; and in the lighter fhades, whether obtained by diluting the black with white bodies, or by applying it thin on a white ground, the particular beauty of ivory-black is in a great meafure loft: the fame intentions may be answered by pigments of le's price, and more easily procurable.

A valuable black for water-colours is brought from China and the Eaft-Indies, fometimes in large rolls, more commonly in fmall quadrangular cakes, generally marked with Chinese characters. By dipping the end of one of the cakes in a little water, and rubbing it about on the bottom or fides of the veffel, a part of its fubftance is taken up by the water, which may thus be readily tinged to any fhade of black or grey, from such as will juft colour paper, to a full black. The compofition of this Indian-ink has not hitherto, fo far as I can learn, been revealed; and I therefore made fome experiments with a view to dif

cover it.

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Though the Indian-ink is readily diffufed through water, it is not truly diffolved: when the liquid is fuffered to stand for fome time, the black matter fettles to the bottom in a muddy form, leaving the water on the top colourless; in the fame

manner

manner as the common black pigments fettle from diluted gumwater. The ink, kept moift, in warm weather, becomes in a few days putrid, like the fluid or foft parts of animals; as does likewife the clear water, after the black matter has fettled and separated from it. The Indian-ink appears therefore to contain an animal fubftance foluble in water; and to confift of a black powder mixed with fome animal glue. For the greater certainty, in regard to this conglutinating ingredient, I boiled one of the China cakes in feveral fresh portions of water, that all its foJuble parts might be extracted, and having filtered the liquors through paper, fet them to evaporate in a stone bafon: they fmelt like glue, and left a very confiderable quantity of a tenacious fubftance, which could not be perceived to differ in any respect from common glue.

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Being thus convinced of the compofition of the mafs, I tried to imitate it, by mixing fome of the lamp-black, which I had myself prepared from oil, with as much melted glue as gave it fufficient tenacity for being formed into cakes. The cakes, when dry, anfwered fully as well as the genuine Indian-ink, in regard both to the colour, and the freedom and smoothness of working. Ivory black and other charcoal blacks, levigated to a great degree of fineness, which requires no finall pains, had the fame effect with the lamp-black; but in the ftate in which ivory-black is commonly fold, it proved much too gritty, and feparated too haftily from the water.'

The conclufions from thefe experiments we find confirmed by Du Halde, in his Hiftory of China. He gives three receipts for the preparation of Indian-ink, two from Chinese books, and the third communicated by a native to one of the miffionaries. The colouring-matter in all these receipts is lamp-black, and in one of them there is added a quantity of horse-chefnut, burnt till the smoke ceafes: the conglutinating ingredient, in one, is a thin fize of neats leather; in another, a folution of gum tragacanth; and in the third, a mixture of fize with a decoction of certain vegetables to us unknown.-In the appendix to this vol. Dr. Lewis obferves, that the gum tragacanth, here mentioned, is not the conglutinating matter in any of the famples of Indian-ink which he has examined; that the vegetable decoctions can be of no ufe where fize is employed, unle's to scent the compofition; and that the receipt, of lamp-black and a thin fize of neats leather, is the very compofition pointed out by his experiments.

Of Compofitions for marking Sheep.

Great quantities of wool are annually made unferviceable by the pitch and tar,, with which the farmer marks his fheep: thefe, as they confiderably increafe the weight of the fleece at a trifling

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