ciples of arts.-The artist is hence enabled to supply his de. fects, to multiply his refources, to fimplify and forward many complex proceffes, to enrich one branch with the practices, materials, and fometimes even the refuse matters of another, and thus to form a happy intercourse, and Philofophical Commerce of Arts,

The French Academy, with the advantage of experienced artists in the different departments, and honoured with the encouragement of the fovereign, have long been engaged in compiling materials for a hiftory of arts: fome of thefe materials have lately been digefted, and published in the following mannereach hiftory forms a feparate and independent work, containing a minute detail of the whole feries of operations relative to one art, with defcriptions and plates of all the inftru ments made use of in fuch art. - A work executed upon this plan, is very obviously different from that of Dr. Lewis; and the hiftorian of the academy, in giving notice of their publication, bears a fenfible and honourable teftimony to the advantages which are peculiar to the Plan of the Commercium PhilofophicoTechnicum. An inconvenience to be feared, fays he, is the want of that knowlege, and of thofe general principles, which bind arts as it were together, and establish between them a reciprocal communication of light. All the arts, for example, that employ iron, have common principles, but it would be in vain to expect the knowlege thereof from those who exercise thefe arts, each of whom knows only the application of those principles to his own art. The farrier, the locksmith, the cutler, know how to work iron; but each of them knows only the manner of working which he has learnt, and is perfectly igno rant that the art of working iron has general principles, which would be infinitely useful to him in a great number of unforeseen cafes, to which his common practice cannot be applied.-It is only by bringing the arts as it were to approach one another, that we can make advances towards their perfection: we shall thus put them in a condition of mutually illuftrating each other, and perhaps of producing a great number of useful discoveries it is only by this means that we can know effectually their true principles, and enable them to receive affiftance from theory.'

In profecution of this excellent defign, Dr. Lewis now enters upon the Hiftory of Colours-Black is the fubject of his present inquiries; and, after fome general obfervations on black colours, he proceeds to the chemical hiftory of thote fubjects, which are fitted to produce this effect in the different arts. The order is as follows:

1. Native black colours :-thefe are, black chalk, pitcoal, black fands, black vegetable juices, cuttle-fish ink. 2. Blacks

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 thefe, would fwell this ar ticle to an undue bulk; we shall confine ourselves therefore to what he says

Of Black produced from Iran,

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 extenfive use for dying and ftaining black.The aftringent fubftances chiefly employed are the excrefcences of the oak-tree, called galls, and of these the Aleppo galls are deemed the beft; all the parts of the oak tree, the leaves, acorns, and more particularly the bark and wood; other vegetable fubftances likewife, the leaves, fmall branches, and flowery clufters of the fumach-tree; balauftine flowers, pomgranate peel, alder bark, biftort root; and in ge neral all those which are auftere, aftringent, or corrugating to the tafte, 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 firft drops produce bluish or purplifh 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 most minute quantity of any alkaline falt, too small 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 reddish. 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 Contracted a putrid tendency, or received an alkaline impreg

nation, though fo 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 diffused again through the liquor, and tinges it of its former colour. When the mixture is of a full blackness, 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 substance 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 paffes through coloured, only a part of the black matter remaining on the paper. The filtered liquor, to the eye perfectly homogene, on ftanding for fome time, becomes turbid and full of fine black flakes: being freed from thefe by a fecond filtration, it again contracts the fame appearance, and this repeatedly, till all the colouring parts are separated, and the liquor has become colourlefs. It fhould feem therefore, there happens a gradual and flow concretion of the black corpufcles, 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 fettle 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 separated 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 exposed to the air for upwards of four months. Made red hot, it glowed and burnt, though without flaming, ard became a rusty brown powder, which was readily attracted by a magnetic bar; though in its black ftate, 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 quartity of whitish matter. Solution of pure fixt alkaline falt diftolved very little of it: the liquor received a reddish brown co

lour, and the powder became blackifh 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 results from mixing with the vitriolic folution an alkaline tincture of galls.

From these experiments it seems 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 blackness 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 state by the magnet; and, when moistened and exposed to the air, it changes fpeedily 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 these preparations strike 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 use 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 :-tirat the black colour discharged by an alkali, is restored by the addition of any acid

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

After thus giving an experimental hiftory of the several 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 sheep, 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 these 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 measure loft: the fame intentions may be answered by pigments of lefs price, and more eafily procurable.

A valuable black for water-colours is brought from China and the Eaft-Indies, fometimes in large rolls, more commonly in small quadrangular cakes, generally marked with Chinefe 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 fuch 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.

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

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