the peroxide is brown and soluble. Citrate of zinc is sparingly soluble.

Citrate of Lead.-The tribasic salt, Ci,3PbO,aq. is a white powder, resolved, by washing with water, into the soluble bibasic salt, and an insoluble salt with excess of base. The bibasic salt, Ci,2PbO,HO, +2aq. forms crystalline grains. The basic citrate of lead occurs in two forms; the first is Ci,6PbO: the second Ci,3PbO,PbO

HO

The only citrate of copper yet described is a green crystalline powder, Ci,3CuO,CuO = Ci,4CuO. It contains, therefore, an excess of base.

Citrate of Silver forms a brilliant white powder, which, if washed and dried below 60°, is Ci,3AgO,aq. When gently heated, it gives off water, and becomes Ci,3AgO.

Potash-citrate of Antimony.-This salt, which has properties analogous to tartar-emetic, forms dazzling white prisms. It is composed of Ci,Sb2O3 + Ti,3KO + 5aq. (Thaulow.) The water is expelled at 375°.

ACTION OF HEAT ON CITRIC ACID.

This subject has occupied the attention of Lassaigne, Dumas, Baup, Robiquet, Dahlström, and Berzelius; but the products vary so much with the circumstances of the experiment, that their results appear contradictory and irreconcileable. The very recent researches, however, of Crasso (Liebig's Annalen, April 1840) have cleared up the whole subject, and reconciled the contradictions of his predecessors. We have only space for his

results.

According to Crasso, then, crystallized citric acid, when exposed to heat, exhibits four stages of decomposition. During the first, the water of crystallization alone is given off, and the residue contains unaltered citric acid. The second stage is characterized by white vapours, and the production of acetone, carbonic oxide, and carbonic acid, while the residue consists of hydrated aconitic acid, which is therefore the true pyrocitric acid. In the third stage, the aconitic acid, not being volatile, is itself decomposed, yielding carbonic acid, and an oily liquid, which soon crystallizes. This is the pyroaconitic acid, the citricic acid of Baup, for which Crasso pro

poses the name of Itaconic acid. In the fourth period, empyreumatic oil is produced, and a voluminous coal remains behind. Crasso has further shown that the aconitic acid, heated by itself, is converted into itaconic acid and carbonic acid; and that the itaconic acid, when heated, yields a volatile acid isomeric with itself, which is identical with the citribic acid of Baup, which he proposes to call Citraconic acid, and which is always more or less. formed along with the itaconic acid, owing to the easy decomposition of the latter. The action of heat on citric acid, therefore, gives rise to three distinct acids in regular sequence; 1st, aconitic or pyrocitric acid; 2nd, itaconic or pyroaconitic acid; 3rd, citraconic or pyro-itaconic acid.

ACONITIC ACID.

Formula, CHO, +aq. Symb. At+aq.

This acid was discovered by Peschier, in the aconitum napellus, and analyzed by L. A. Buchner, jun. The acid observed by Braconnot in equisetum fluviatile, and analyzed by Regnault, seems to be identical with it; as does also the acid obtained from citric acid by Berzelius and Dahlstrom, as above described. It is easily extracted from the plants, by combining it with oxide of lead, decomposing the salt by sulphuretted hydrogen, and purifying the acid by solution in ether.

It forms a white crystalline mass, soluble in water, of a pleasant acid taste. It is decomposed by heat, yielding two new acids. Regnault supposed it to be identical with maleic acid; but as the latter is converted by heat into fumaric acid, a change not observed in aconitic acid, this identity is doubtful. It ought to be mentioned also, that the acid from equisetum fluviatile may be sublimed without change, although in all other particulars identical with the acid from aconitum napellus.

Its production is easily explained, for 1 at. anhydrous citric acid, C12H5O11, yields

ACONITATES.

The alkaline aconitates are soluble in water, the others insoluble or sparingly soluble. Their properties are not very interesting. The general formula for these salts is At,MO. They sometimes contain water of crystallization.

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There is some reason to believe that this acid may prove to be tribasic. It will then have the formula C12H30, +3HO; and the neutral aconitates will have the formula C12H,O,+3MO. Future investigations must determine this point.

ITACONIC ACID.

Syn. Pyrocitric Acid; Citricic Acid. (Baup.) Formula, C¿H2O+HO. Symb. It +aq.

3

This acid is obtained, as above mentioned, in the third stage of the distillation of citric acid, or by the action of heat on aconitic acid. It is generally accompanied by some of the acid next to be described, citraconic acid. It may be obtained from the oily mass which distils over, and which commonly crystallizes after a short time, by solution in absolute alcohol, and spontaneous evaporation. The crystals thus obtained, when dissolved in water, yield by slow evaporation large rhomboidal crystals, or octahedrons, soluble in 17 parts of water at 60°.

There is some reason to suppose that it may be a bibasic acid, in which case its formula will be C10H4O6+2HO. In the production of this acid from aconitic acid, 3 at. hydrated aconitic acid, 3(CHO2+ HO) = C12H6O12, yield 2 at. carbonic acid, 2CO2, and 2 at. itaconic acid, 2(C¿H2O, + HO).

3

ITACONATES.

The neutral itaconates have the general formula C,H2O, + MO, or C10H4O6+2MO. The acid salts are either 2(C2H2O3)+MO

+ HO, or C10H406 +

MO но

}. They have been studied by Baup

and Crasso, to whose memoirs the reader is referred for details.

CITRACONIC ACID.

Syn. Pyrocitric Acid; Citribic Acid. (Baup.) Formula, C2H2O,+HO. Symb. Ct+aq.

This acid is formed by the action of heat on itaconic acid, with which it is isomeric. It is volatile, and thus escapes the further It is soluble in less than half its weight of It is very

action of the heat. water at 60°, and very soluble in alcohol and ether. probably also a bibasic acid, having the formula C10H4O6 +2HO; but this must be determined by future researches.

CITRACONATES.

Citraconic acid forms neutral and acid salts, which have been examined by Crasso. The neutral salts have the general formula CH2O+MO, or C10H4O6+2MO. The acid salts are either

3

2(C¿H2O3) + HO + MO; or С10H4O6 +

10

но

B}

MO

It forms with ammoniacal gas a peculiar amorphous compound, which does not contain the water usually present in ammoniacal salts. The formula of this compound is 2(CH2O3)+NH ̧. When this substance is dissolved in water, and the solution evaporated, it yields crystals of the acid citraconate of ammonia, 2(C,H2O ̧) + NH2O + HO. (Crasso.)

2

TARTARIC ACID.

A bibasic acid. Formula of the crystallized acid, CH4O10+ aq.; of the acid in the bibasic salts, C,H4010=T. When tartaremetic dried at 212° is exposed to a stronger heat, it loses water, and there remains in the salt an acid, the formula of which must be CH2O; but when this salt is redissolved in water, the original acid is reproduced. (Liebig.)

8

8

Tartaric acid occurs chiefly in the juice of the grape, but also in many other fruits and parts of vegetables. It was discovered by Scheele, in 1770. It is extracted from the tartar of the grape, which is monobasic tartrate of potash, T,KO,HO, by a process similar to that employed for citric acid, namely, converting it into tartrate of lime, and decomposing this salt by sulphuric acid. It

is prepared in large quantity for the calico printers, and for medical use, and is sold in a state of purity at a very low price. Its medical use is the same as that of citric acid. It cannot be easily prepared in the laboratory on a small scale; but a process is given in the Pharmacopoeia for making it. Tartaric acid forms large transparent, oblique, rhombic prisms, which have a pleasant sour taste. The aqueous solution becomes mouldy by keeping: the solution in alcohol, when heated, is converted into monobasic tartrate of oxide of ethule. When heated, it melts, and yields a variety of new and interesting products.

It contains the elements of 1 at. hydrated acetic acid, and 2 at. hydrated oxalic acid; or of 2 at. hydrated formic acid, and 1 at. anhydrous malic acid. When heated with caustic alkalies, it is resolved into acetic and oxalic acids. (Gay-Lussac.) With hyperoxides it yields formic acid, carbonic acid, and tartrate of protoxide with diluted sulphuric acid it yields a large quantity of formic acid.

A solution of tartaric acid causes with solutions of caustic lime, baryta, and strontia, white precipitates soluble in excess of acid. It does not precipitate the salts of these earths. It is used as a test for potash and its salts, in which it produces the sparingly soluble monobasic tartrate of potash (cream of tartar). The presence of tartaric acid in the solutions of many metallic oxides prevents their precipitation by alkalies.

TARTRATES.

Tartaric acid forms with bases monobasic and bibasic salts. The former are acid, and have the formula T,MO,HO: the latter are neutral, and their formula is T,2MO. These two eq. of metallic oxides may be different: hence a series of salts with two bases, MO With potash and oxide of antimony, tartaric acid

T, mo

1

forms an important series of compounds. Tartar-emetic contains, for 1 eq. tartaric acid, 1 at. =3 eq. sesquioxide of antimony, and eq. of potash; it must therefore be viewed as a salt with excess of base. The neutral salt has been lately described. As already mentioned, tartar-emetic at 392°, without blackening, gives off 2 at. water, formed either at the expense of the acid, or by part of the oxygen of the sesquioxide, with hydrogen from the acid. In the