slowly, till a pellicle appears, and then letting it stand undisturbed.

Experiment 2. Gallic acid may also be obtained, according to Scheele, in the following manner: reduce a pound of nut-galls to powder, and pour upon it six pounds of distilled water: leave this mixture to macerate for the space of 15 days at a temperature of 68° 77°. Then filter this liquor; and after the filtration, expose it in a vessel of glass or stone-ware, to evaporate slowly in the air. During this evaporation, which may be continued during two or three months, the gallic acid will be deposited in crystals, on the sides and bottom of the vessel, and on the inferior surface of a pellice which will have formed over the mixture. At the end of this period, pour off the liquor. Then dissolve whatever remains in the vessel in alcohol. This last solution, evaporated, will afford the gallic acid in crystals.

Experiment 3. Gallic acid may likewise be obtained by exposing powdered nut-galls in a retort to a moderate heat. The acid by this means sublimes; part condenses in small white crystals, and part is obtained in a fluid form, from its combination with a portion of water contained in the galls.

Remark. Gallic acid exists in the gall-nut, an ex-crescence growing on some species of oaks, in the husk of nuts, in oak bark, and in all those vegetables commonly called astringents.

Gallic acid appears in the form of minute, brilliant, colourless plates. Its taste is sour, and austere or astringent. It strongly reddens blue vegetable colours. It is soluble in about 10 parts of cold, and in three of boiling, water. It is likewise soluble in alcohol. It has a peculiar disagreeable odour when heated. It is not altered by exposure to air. Exposed gradually to a gentle heat, it sublimes without alteration; but if exposed to a strong heat, quickly applied, it becomes decomposed into carbonic acid, and carburetted hydrogen gas. It has a strong tendency to unite with metallic oxyds. With the red oxyd of iron it produces a deep black precipitate. This combination is the basis of ink

and black dyes. It precipitates gold, copper, and silver, brown; mercury, orange; bismuth, yellow; and lead, white. It has no action on the oxyds of platinum, tin, zinc, cobalt, manganese, and arsenic.

The use of the tincture of galls as a re-agent for the discovery of iron, is exhibited in the following experi

ments:

Experiment 4. Prepare a phial with pure water and a little tincture of galls; and another with a weak solution of sulphate of iron; then mix these transparent colourless fluids together, and they will instantly become black.

Experiment 5. Write with a weak solution of sulphate of iron; let it dry, and it will be invisible. By dipping a feather in tincture of galls and drawing the wet feather over the letters, the writing will be restored and appear black.

SECTION XXXVII.

OF LACTIC ACID.

Experiment 1. Take any quantity of whey, evaporate it very slowly to one eighth, filter it, and then add to it lime-water till no farther turbidness appears. Filter the mixture again, and let fall into the fluid, diluted with water, a solution of oxalic acid, till it produces no further cloudiness. Then filter it and evaporate the fluid to the consistence of syrup, and mingle with it alcohol. Having done this, separate the fluid again by filtration, add to it a small quantity of water, put it into a retort, and distil off the spirit; the remaining water then contains the lactic acid in a pure state.

Remark. This acid exists in the whey of milk; and, when separated, is capable of being changed into a solid form. It attracts moisture, and unites with salifiable bases.

SECTION XXXVIII.

OF LACCIC ACID.

This acid has been discovered by Dr. Pearson, in a wax-like substance called white lac, of Madras, formed: by certain insects of the coccus tribe.

Laccic acid naturally exists in the fluid state. It rises in distillation. It decomposes the carbonates of lime and soda. It renders the nitrate and muriate of barytes turbid. It assumes a green colour with lime water, and a purplish colour with sulphate of iron; and precipitates sulphuret of lime, white; tincture of galls, green; acetate of lead, reddish; nitrate of mercury, whitish, &c.

SECTION XXXIX.

OF BOMBIC ACID.

The phalana, or moth of the silk-worm, particularly when in the state of a crysalis, contains in a reservoir situated near the anus, an acid liquor, said to be of a peculiar nature.

Experiment 1. This acid is obtained by squeezing the juice of the crysalis through a cloth, and precipitating the mucilage by the admixture of alcohol.

Experiment 2. Or, it may be obtained by digesting the crysalis in alcohol, mixing the infusion with a little water, and subtracting the alcohol by a gentle heat. The acid thus obtained is of an amber colour. It affords prussic acid when distilled with nitric acid. Its other properties and combinations are hitherto little known. An acid analogous exists in several caterpil lars, in that of the willow, and in other insects.

SECTION XL.

OF AMNIOTIC ACID.

Vauquelin and Buniva have discovered a peculiar acid in the liquor of the amnios of the cow, to which they have given the name of amniotic acid.

Experiment 1. To obtain amniotic acid in a state of purity, evaporate the liquor of the amnios of the cow, to a syrupus consistence, which collects on the surface of the fluid. Transfer the concentrated Huid into alcohol, digest it by heat, decant the alcohol, and repeat this operation for several times. Having done this, mingle the alcohol solution with one quarter of water, and abstract the alcohol by heat. Crystals of amniotic acid will be deposited upon cooling. In this case the acid is not pure. It is necessary to re-dissolve it in boiling water or alcohol, or re-crystallize it repeatedly.

Whether this acid exists in the liquor of the amnios of other animals is not yet known.

Amniotic acid exists in the form of a white pulverulent powder. It is slightly acid, but sensibly reddens vegetable blues. It is difficultly soluble in cold, but readily soluble in boiling water, and in alcohol. When exposed to a strong heat it exhales an odour of ammonia and of prussic acid.

Amniotic acid does not decompose the alkaline carbonates at the usual temperature, but it does so when assisted by heat. These properties are sufficient to shew that it is different from every other acid. It approaches nearest to the mucous acid and the uric acid; but the first does not yield ammonia by heat, and the latter is not so soluble in hot water, does not crystallize in needles, and is insoluble in alcohol.

PART XVII.

OF SALTS.

Although the word salt was originally confined to muriate of soda or common salt, yet chemists have generalized the term, and applied it to all bodies which are sapid, easily melted, and soluble in water. The term was afterwards extended to all the com pounds, which the acids form with alkalies, earths and metallic oxyds; these compounds have been called alkaline, earthy, and metallic salts.

The following facts respecting saline bodies, may be proper to premise:

1. The salts are denominated from the acids they contain.

2. The alkali, earth, or metallic oxyd constitute the base of the salt.

3. An acid combined with two bases, forms a triple salt.

4. A salt formed with an excess, or an additional dose of acid, is called a supersalt.

5. A salt containing an excess of base is a subsalt 6. If the acid, in the salt, contains a maximum of oxygen, the termination is ate, as sulphate, nitrate & 7. Those which do not contain a maximum, end in ite, as sulphite.

8. Every particular species is distinguished by sub joining to its generic term the name of its base.

All salts, we have observed, are either alkaline earthy, or metallic, or they possess either an alka li, earth, or metal for their base, which is combined with an acid. The base may be in excess, which con stitutes a subsalt, or the acid may be in excess which forms a supersalt. Thus for instance, sulphuric acid united with soda forms sulphate of soda, or Glouber's salt, which is an alkaline salt, with lime it forms sul phate of lime or gypsum, an earthy salt; and with