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ciples of arts.—The artist is hence enabled to supply his de. fects, to multiply his resources, to simplify and forward many complex proceffes, to enrich one branch with the practices, materials, and sometimes even the refuse matters of another, and thus to form a happy intercourse, and Philosophical Commerce of Arts,
The French Academy, with the advantage of experienced artists in the different departments, and honoured with the encouragement of the sovereign, have long been engaged in compiling materials for a hifiory of arts : some of these materials have lately been digested, and published in the following manner:each history forms a separate and independent work, containing a minute detail of the whole series of operations relative to one art, with descriptions and plates of all the inftru ments made use of in such art.- A work executed upon this plan, is very obviously different from that of Dr. Lewis ; and the historian of the academy, in giving notice of their publication, bears a sensible and honourable testimony to the advantages which are peculiar to the Plan of the Commercium PhilosophicoTechnicum. An inconvenience to be feared, says he is the want of that knowlege, and of those 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 these 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 ignorant that the art of working iron has general principles, which would be infinitely useful to him in a great number of unforeseen cases, 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 sha!! thus put them in a condition of mutually illustrating each other, and perhaps of p'oducing 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 affistance from theory.'
In prosecution of this excellent design, Dr. Lewis now enters upon the History of Colours.--Black is the subject of his present inquiries; and, after some general observations on black colours, he proceeds to the chemical history of thote subjects, which are fitted to produce this effect in the different arts. The order is as follow's :
1. Native black colours :-these are, black chalk, pitcoal, black sands, black vegetable juices, cuttle-fish ink.
2. Blacks nation,
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, observations, and conclusions, relative to the above particulars, are curious, accurate, and useful.---To enter into a detail of these, would swell this ara ticle to an undue bulk; we shall confine ourselves therefore to what he says
Of Black produced from Iran, The infusions of certain vegetable aftringents, mixed with green-vitriol, which is a solution of iron in the vitriolic acid, produce a deep black liquor, of most extensive use for dying and Itaining black. The aftringent substances chiefly employed are the excrescences of the oak-tree, called galls, and of these the Aleppo galls are deemed the best ; all the parts of the oak, tree, the leaves, acorns, and more particularly the bark and wood; other vegetable substances likewise, the leaves, small branches, and Aowery clusters of the sumach-tree; balaustine flowers, pomgranate peel, alder bark, bistort root; and in ge. neral all those which are austere, astringent, or corrugating to the taste, are possessed of the same virtue with galls : the power by which these substances strike black with vitriol, and their aftringency, are proportional to one another, and seem to de. pend upon one and the same principle. Of the other properties of this aftringent and colouring matter, little more is known, than that it is dissolved and extracted both by water and spirit of wine, and that it does not exhale on the evaporation of the menftruum,
. When a decoétion, says Dr. Lewis, or infusion of the galls, is dropt into a solution 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 seems to be on the quality of the water that this difference in the colour depends. With distilled water, or the common spring waters, the mixture is always blue. If we previously diffolve in the water the most minute quantity of any alkaline salt, too small to be discoverable by any of the common mcans by which waters are examined, or if the water is in the least degree putrid, the colour of the mixture proves purple of reddith. Rain-water caught as it falls from the clouds in an open field, in clean vessels, gives a blue, but such as is collected from the tops of houses, grows purple with the vitriol and galls; from whence it may be presumed, that this last has contracted a putrid tendency, or received an alkaline impreg
nation, though so Night as not to be sensible on other ways of trial.
• Both the blue and the purple liquors, on adding more of the astringent infusion, deepen to a black, more or less intense according to the degree of dilution : if the mixture proves of a deep opake blackness, it again becomes bluish or purplish when further diluted. If suffered to stand in this dilute state for two or three days, the colouring matter settles to the bottom, in form of a fine black mud, which, by flightly shaking the vessel, 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 less degree; for though a part of the black matter precipitates in standing, yet so much remains diffolved, that the liquor continues black. This suspension of the colouring substance in the black liquid may be attributed in part to the gummy matter of the aftringent infusion increasing the consistence of the watery Auid, for the separation is retarded in the diluted mixture by a small addition of gum arabic; though another principle appears also to concur for part of the effect.
• If the mixture, either in its black or diluted state, be poured into a filter, the liquor passes through coloured, only a part of the black matter remaining on the paper. The filtered liquor, to the eye perfectly homogene, on standing for some time, becomes turbid and full of fine black fakes : being freed from these by a second filtration, it again contracts the same appearance, and this repeatedly, till all the colouring parts are separated, and the liquor has become colourless. It should seem therefore, there happens a gradual and slow 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 also' by the action of the air ; for having once set: some of the diluted mixture to settle in a close stopt glass, the separation of the black matter was remarkably more now than in the other experiments, in which the vessel 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 seem to have suffered 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 digested on the black powder, dissolved the greatest part of it, leaving only a very little quarr. tity of whitih matter. Solution of pure fixt alkaline salt difColved very little of it: the liquor received a reddith brown co
lour, and the powder became blackish brown. This residuum was attracted by the magnet after being made red-hot, though not before : the alkaline tincture, passed through a filter, and mixed with solution of green vitriol, struck a deep brownishblack colour, nearly the same with that which results from mixing with the vitriolic solution 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 solvent in a highly attenuated or divided state, and combined with a peculiar species of matter contained in astringent vegetables ; which matter, after the watery fluid that the compound floats in has been separated, is in part extracted from the iron by alkaline liquors, and may thence be again transferred into fresh dissolved iron..
· The blackness is generally attributed to the iron being barely revived from the vitriol in its metallic state; the black matter being supposed 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 ipeedily into rust. The resistance of ours to the magnet and to the air proceeds doubtless from the combination of the other matter with the iron; and there appears some analogy, in regard to the manner of production, between this black substance and Pruffian blue; one being a precipitation and coalition of diffolved iron with one species of matter, and the other with another : the principal difference is, that the substance combined with the iron in the Prussian blue defends the metal from the action of acids, which that in the black compound is unable to do.' It appears likewise, from the experiments of our Author on the solutions, and different soluble preparations of iron made with the nitrous, marine, and vegetable acids ; that all these preparations strike a black colour with the infusions of the astringent vegetables ; that the experiments from which a contrary conclufion has been drawn, were made with solutions in which the acid was not perfectly saturated, and hold equally true of the visriolic solution when the saturation is not compleat :-that this colouring matter once produced, is again destroyed by the addition of any of the acids, as the acid re-dissolves the ferrugeneous matter; hence the use of acids for discharging the stains of ink, or other black mixtures of this class :—that alkalies destroy the colour on a different principle ; that they dissolve the astringent matter, and precipitate the iron nearly in the same ochery state, as in the fimple and acid solutions of this metal :-tirat the black colour dilcharged by an alkali, is restored by the addition of any acid in such quantity as to saturate the alkali; and that, on the other hand, this colour discharged by an acid, is in like manner restored by the addition of an alkali.
After thus giving an experimental history of the several classes of materials, which are practised with in order to obtain and fix black colours; Dr. Lewis proceeds to apply these general principles to the particular arts. - In the fifth section he treats of black paint with oil, black paint with water, compofitions for marking sheep, compositions for preserving wood, &c. compositions for blacking leather, fpirit varnish, amber varnilhes for papier maché, &c. varnish for metals, sealing-wax, printing ink,
rolling-press ink. We shall 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 settles from whites of eggs, after they have been beaten up and suffered to stand a little. Some use 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 glossiness.
• It may be observed, 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 seldom wanted in painting; and in the lighter shades, 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 same intentions may be answered by pigments of less price, and more easily procurable.
A valuable black for water-colours is brought from China and the East-Indies, sometimes in large rolls, more commonly in small 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 sides of the vessel, a part of its substance is taken up by the water, which may thus be readily tinged to any shade of black or grey, from such as will just colour paper, to a full black. The composition of this Indian-ink has not hitherto, so far as I can learn, been revealed ; and I therefore made fome experiments with a view to discover it.
« Though the Indian-ink is readily diffused through waters it is not truly dissolved : when the liquid is suffered to stand for some time, the black matter setules to the bottom in a muddy form, leaving the water on the top colourless ; in the fame