Alum, being a cheap substance, is most commonly ufed. It confifts of vitriolic acid, pure clay, or argillaceous earth, and water. According to Bergman, one hundred parts of crystalised alum contain thirty-eight of vitriolic acid, eighteen of clay, and forty-four of water. The clay is generally fuperfaturated with acid, which is proved by the phenomena produced on the addition of mild, vegetable, fixed, alkali. On' the first portions of alkali being added a finall portion of the earth precipitates from thofe parts of the alum with which the alkali comes in con tact; and as pure clay has an attraction for aërial acid, the effervefcence produced is at firft small; but presently the remaining free acid, attacking this precipitate, rediffolves it, an effervefcence appears, occafioned by the difcharge of the aerial acid from the clay. This precipitation, follow-' ed by rediffolution, and a difcharge of gas, continues, till the acid be perfectly faturated; the precipitation then goes on regularly and the earth is no longer diffolved a new, except the alkali be continued to be added, after the precipitation is fully accomplished; nor does any effervefcence follow, when fresh portions of alkali are added.

Thefe are the appearances when mild, or aerated, vegetable alkali is ufed; but if the pure or cauftic

*The conftituent parts of common clay are argillaceous, mixed with filiceous earth, in various proportions. Parel argillaceous earth is only obtainable from alum.

alkali be employed, the precipitation takes place more flowly; and if the pure alkali continue to be added, after the precipitation is effected, the earth will be rediffolved.

This earth has a ftrong attraction for colouring matter; particularly for fuch as forms the red and yellow colours. Infomuch, that if a folution of alum be poured into water, deeply tinged with madder or weld, the earth will quit its acid to unite with the colouring particles of these substances, and form with them a precipitate or laque; from which it cannot be separated, either by the action of water, or spirit of wine, and which is not even totally destroyed by that of fire.

Marine falt of tin, and that formed by aqua regia have, for their bafis, the white earth of that metal, which has also a strong attraction for colouring matter, and is, in fome cafes, preferred to that of alum. When united to the colouring matter of cochineal, it forms a beautiful pigment, well known by the name of carmine. If the addition of these faline fubftances to the coloured liquors, be fufficiently long continued and under proper circumftances, the whole of the colouring matter will be precipitated, and the water be left colourless*.

Tin is not the only metal which affords bafes for colouring fubftances. Lead, bifmuth and zinc alfo afford earths or calces, which attract Macquer Dictionaire de Chymie.

colouring

colouring matter; but the two first have defects which render them less eligible." The calx of zinc may perhaps be usefully employed. But should first be thoroughly purified from the iron which it generally contains.

These all form bases for the more brilliant reds and yellows. For the laft, the calx of copper is alfo employed, having a ftrong attraction for the colouring matter of weld. Salts containing iron furnish a bafis, which, with the aftringent matter of vegetables, produces a black dye."

When alum is used to fupply a basis for the dying of wool, it is the practice to join with it, either crude tartar or its purified crystals, in the proportion of five ounces of alum, to one of tartar. This laft fubftance we know confifts of an alkaline vegetable falt, fuperfaturated with a peculiar acid, which bears its name. The fuperabundant vitriolic acid of the alum, will decompofe a part of the tartar, by attaching itself to the alkali; and thus the quantity of free tartarious acid is increased; which has no properties injurious to the cloth.

In the aluming of filk, no tartar is employed; for the filk, in the previous preparation, being impregnated with alkaline or soapy matter, the fuperabundant acid will be neutralifed by it. In this process, when the tubs have been long ufed, a very confiderable incrustation is formed on their fides; which, the dyers, finding no injury. from,

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from, fuffer to accumulate. Some of the soap, ufed for the ouring, adheres to the filk, notwithstanding the washing it undergoes; and the alkali of the foap, uniting with the acid of the alum, fome of its earth, is precipitated, joins. with the detached oil, and forms, the incrufting fubftance; the undecompofed alum and the vitriolated, tartar perhaps alfo, in part, entering into the combination*.

Cotton requires, a ftill different treatment-as the vitriolic acid is injurious to cotton, and it is neceffary that the aluminous folutions fhould be well dried on it, before it be washed; the acid, being concentrated, by the evaporation of the water, would corrode the cotton, It is therefore proper to faturate the fuperabundant acid, previous, to the aluming of the cotton; and, for this purpose or of pearl afhes is to be added.,

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But the aluminous liquor, ufed by the printers, is prepared in a different manner. three pounds of alum diffolved in, a gallon of hot water, a pound and half of fugar of lead is added; the mixture is ftirred, together for a confiderable time, and the agitation repeated often, during two or three days; when a few ounces of whiting are to be added, gradually, as a strong effervefcence enfues. On adding the fugar, of lead to the alum, a double elective attraction produces two decompofitions, and two new compounds. The, vitriolic acid, forfakes the

the earth of alum to unite with the calx of lead: of the faccharum faturni ;mands this new fale, poffeffing very little folubility,falls to the bottom of the veffel in form of a white precipi tate. The earth of alum, being left at liberty, and in fo minutely divided a fate, is attacked, in the act of precipitation, and diffolved by the acetous acid; which, having quitted the lead, is ready to form this new union; and thus a very foluble falt, being the product, it remains diffolved in the water and, when thickened with, gum, is applied by means of blocks to the cloth. The piece being afterwards dried in a hot stove; the vinegar, which as the cloth dries, becomes highly concentrated and very volatile * ; not having a strong attachment to the aluminous earth, flies off and leaves the earth, upon thes cloth, ready to receive the colouring matter." And herein confifts the advantage of the change: of the vitriolic for the acetous acidi.

Thus we fee that the printer's liquor, for the red and yellow, colours, is not, as those artists. generally imagine, a mixture of alum and sugar of lead, but merely an acetated argill, or alumi<»; nous earth, combined with vinegar. The additión of whiting is intended only to neutralife: ther

The vinegar, by its union to the calx of lead, feems to have acquired fome new properties. For on feparating it from the lead, by diftillation, it always contains fome portion of other. When the cloth, has become dry in the hot stove, Mr. Charles Taylor has obferved flashes of electric light, darting from its furface.

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