bonat; and thus the tube speedily increases to a stalactite. The atmosphere contains about onehundredth part of carbonic acid gas diffused in it.

There are yet several acids referable to the vegetable kingdom, some of which are native, others artificial, products. These are the acetic, tartaric, citric, gallic (acid of nutgalls), oxalic (acid of sorrel), malic (of apples), benzoic (of gum benzoin). I shall notice in particular the four first.

The acetic acid, commonly called vinegar, is (as its name expresses, vin aigre, sour wine) any proper vinous liquor rendered acid by oxygen absorbed from the atmosphere. In this country the infusion of malt after fermentation is made into an impure vinegar, which being distilled becomes a clear colourless liquid, moderately acid. By freezing, and by chemical processes, this may be deprived of its water to such a degree as to constitute a very strong acid of a pungent smell, and crystallizable at 22° of the thermometer.

arts.

This acid is a production of most extensive utility, not only in diet and medicine, but in the Combined with copper it forms verdigris, and with lead the salt improperly called sugar of lead. There are acetats of copper and some other metals.

If this acid is exposed to a red heat, there are soon no remains of it to be seen, unless it is united to a fixed alkali, in order to repress its volatility. If the neutral salt also produced by

this mixture is exposed to a very strong heat, the acid is either burnt or destroyed, and there arises a vapour of a very disagreeable smell, in which we can discover something of an inflammable nature, but nothing like the acid. If we examine the remains of the compound, we find it a little heavier, in consequence of the substances with which it is united.

The acetic acid is to be destroyed by heat when mixed with alkali, and by itself when the heat is properly applied. There are several substances which are converted into vapour before we can give them a sufficient heat for inflammation: this is the case with spirit of wine, of which the vapour only takes fire. Now if we manage the acetic acid in the same manner, and heat it till a copious vapour arises, the vapour will take fire as other inflammable substances. No acid is to be obtained from the inflamed vapour, and nothing is left behind.

Acetic acid is a compound of oxygen, hydrogen, and carbon. 100 parts contain—

46.82 oxygen.

6.35 hydrogen.

46.83 carbon.

Tartaric acid.-After the fermentation of wine is over, there separates a quantity of a substance called tartar, which encrusts the casks. In collecting it from the cask, it is necessary to throw in a quantity of hot water, which dissolves the salt, and when the water is evaporated the salts crystallize. Rhenish wine affords tar

tar in the greatest quantity: when old Rhenish wine is thus depurated it is called old hock. Tartar undergoes an operation to make it fit for medical purposes. It was customary in our laboratories to purify it by continuing the evaporation till the tartar concreted in a thin saline crust at the top, which was skimmed off as fast as it appeared, and was called cream of tartar. Lees of wine contain tartar: they are deposited in the form of a muddy liquor, and are separated by decantation. They yield, by being dried, &c., a large proportion of cendres gravellées, as it is called in France: it is used in the preparation of ink for copper-plates, under the name of Frankfort black.

Cream of tartar, or acidulous tartrat of potass, is a compound salt, from which, by a more powerful acid, the alkali may be separated, leaving the tartaric acid free and crystallizable by itself.

Tartaric acid consists of oxygen, carbon, and hydrogen, in the following proportions:

70.5 parts oxygen.

19. carbon.

10.5 hydrogen.

100'

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Citric acid mixed occasionally with tartaric constitues the native acid of most of our fruits. Great quantities of it are now obtained in a crystallized state from the juice of lemons and limes, which, together with the tartaric, are ap

plied to use in calico-printing; for, being mixed in paste and stamped on cotton stuffs in various patterns, they either prevent the colour with which the ground is formed from fixing itself in that part, or dislodge it, after dyeing, at pleasure, thus leaving the imprinted part in its original whiteness. This effect results from their attraction for the metallic basis of the colour, while, unlike the mineral acids, they leave the fibre of the cloth untouched.

The component parts of the citric acid are oxygen, hydrogen, and carbon.

The malic and oxalic acids nearly resemble those which we have now specified.

Gallic acid exists ready formed in galls. It produces a black dye, with solutions of iron, by uniting with that metal into an insoluble compound called gallat of iron. Hence, besides the utility of galls in writing ink and dyeing, the infusion of galls in water or spirit is one of the best tests for the presence of iron in solutions where it is suspected, but not otherwise appa

rent.

Prussic or hydrocyanic acid is also employed by the chemists for this purpose.

This, which may be regarded as an animal acid, is wholly a creature of art; and the discovery of Prussian blue (whose colouring basis is prussiat or hydrocyanat of iron) was made long before we knew any thing of this acid. For the preparation of Prussian blue, a quantity of potass is calcined with dried blood till it ceases to

smoke: in this process the acid in question is formed out of the animal matter employed (which is not confined to this material, blood), and immediately seizes the alkali. We have then a prussiate of potass, which being dissolved in water, and mixed with sulphat of iron (green vitriol), a double decomposition ensues, hydrocyanat of iron falls down in a precipitate, and the alkali is attached to the sulphuric acid, forming sulphat of potass, which may be washed off from the colour.

Phosphoric acid is the direct product of the combustion or oxygenation of phosphorus, which consequently affords more than its own weight of it. This acid, deprived of all its water by heat, is solid, colourless, and of a glassy appearance. It has a very sour taste, without any smell; is not corrosive to the skin, and readily deliquesces.

It dissolves in water, giving out much heat, like the sulphuric.

It forms with lime an insoluble compound, phosphat of lime, which has the appearance and properties of earths, and is the solid part or base of animal bones, in which it is united to gelatine, or, in plain language, glue. Hence calcined bones, by proper treatment, yield phosphorus, the oxygen being abstracted by the charcoal employed in the distillation, which is conducted in a strong heat. It also enters into the composi-. tion of many mineral substances, whole rocks being sometimes formed of this substance.