COWL' • ides of many of the acids are known. Oxalates may be formed from cyanogen compounds (see p. 554).

Series C„11„.._1(H0)(00211)2, Malic Series : - Tartronic or oxymalonic acid CH(H0)(CO211)2 Malic or oxysuccinic acid C2113(H0)(CO2H)2 Citrainalic and glutanic acids . C3II5(H0)(CO211)2 Oxyadipic acid 04H7(110)(CO5H)2 Oxysuberic acid 06Hii(H0)(CO2II)5 Malic acid is found free or combined in the juice of most fruits.

Series C„112„_2(H0)2(CO2H)2, Tartaric Series. - The following are known : - Mesoxalie acid (?) C(H0)2(CO211)2 Tartaric acid C2H2(H0)2(CO21I)2 Homo-, citra-, and ita-tartaric acids C3H4(H0)2(CO2-E)2 Dioxyadipic acid C4H6(H0)2(CO2H)2 Dioxysuberic C6Hio(H0)2(CO211)2 Tartaric, like malic and oxalic acids, is of frequent occurrence in the vegetable kingdom, being found free or combined in the juice of many fruits. It is generally obtained from argol or tartar (crude acid potassium tartrate), which is deposited from fermenting grape juice. It can be formed by the action of Ag20 and water on dibromsuccinic acid, C2112Br2(CO2H)2. Five modifications of tartaric acid, differing chiefly in their optical properties, are known, viz., dextrotartaric, levotartaric, racemic, mesotartaric, and meta-tartaric acids. Normal and acid tartrates are known. Tartar emetic is potassio-antimonious tartrate ; the acid potassium tartrate is known as cream of tartar.

Series C„1-12„_2(C0 21=1)2, Fumaric Series. - This consists of fumaric and maleic acids, 02112(0021I)2; citraconic, itaconic, and mesaconic acids, C,H,(CO,H),.

Series C„112,,....,(CO2H)2, Phthalic Series, comprises :- Phthalic (ortho-), isophthalic (meta-), and tereC6H4(CO311)2 phthalic (pare-) acids The phthalic acids are obtained by the oxidation of many aromatic hydrocarbons and their derivatives. Derived from these acids are hydro-phthalic and hydroterephthalic acids (0811804).

Tribasic Acids.

Meconic acid, C41-10(CO2H)3, is obtained from opium ; tricarballylic acid, 03112(00211)3, from tricyanopropane (see p. 568).

Citric acid, 03114(110) (CO211)3, exists in many fruits, and is generally obtained from lemon juice. It forms colourless crystals readily soluble in water. Being a tribasic acid, it forms with metals three classes of salts, typified by M'C611707, 1\12'C,11,07., 11.18'C611307, &c.

Aconitic acid, C31-13(CO2H)3, is derived from citric acid, and is found also in monkshood (Aconitum Napellus).

Trimellitic (para-), trimesic (meta), and hemimellitic (ortho-) acids, C6113(CO2H)3, are tricarboxyl derivatives of benzene.

Tetrabasic and Hexabasic Acids.

The following tetracarboxyl benzene derivatives are known : - promellitic (para), prehnitic (meta), and mellophanic (ortho-) acids, C6132(00213)4.

The hexabasic acids known are mellitic acid, 06(00211)6, and its derivative hydromellitic acid, C6116(00211)6.

It is to be remembered that the foregoing list is necessarily brief. Many acids of vegetable and animal origin, and others derived from them by artificial methods, are known, but their constitution is in many cases still undecided.

These compounds are the ethers of acid radicles (see p. 553), and may be prepared iu many cases by analogous reactions : - C2II30.01 + C21130.0Na = (021130)20 + NaC1 Acetyl chloride. Sodium acetate. Acetic anhydride.

C21130. Cl 041170.0Na = 021130.0.041170 + NaCl.

Acetyl chloride. Sodium butyrate. Aceto-butyric anhydride.

Many anhydrides are obtained by heating the corresponding acids : - (C31140)"(H0)2 – 0112 = (C81140)"0 Lactic acid. Lactic anhydride (lactide).

(C8H402)"(H0)2 - OH2 = (C811402)"0.

l'hthalic acid. Phthalic anhydride.

Anhydrides when acted on by water yield the corresponding acids :- (C21130)20 + 0112 = 2(021130)110 021130.0.041170 + 0112= (0211,0)110 + (041170)110.

Aceto-butyric anhydride. Acetic acid. Butyric acid.

Compound anhydrides when distilled are resolved into two simple anhydrides2C21130.0.C7HSO = (021130)20 + (C71150)20.

Accto-benzoic anhydride. Acetic anhydride. Benzoic anhydride.

Acetic anhydride is an oily liquid, boiling at 137°, and having a pungent odour. By the action of P2S5 it yields thiacetic anhydride, (C21130)„S, and by Ba02 acetic peroxide, (021130)202. It combines directly with aldehydes.

These may be regarded as the haloid ethers of acid radicles. They can be prepared by the action of haloid phosphorus compounds on the acids containing the corresponding radiclesAcetic acid. Acetyl chloride.

Succinic acid. Snccinyl chloride.

A similar reaction can be effected with the haloid substitution compounds of the acids. Thus, C2II2C10.C1 C2HC120.C1, and 020130.01 (mono-, di-, and tri-chloracetyl chlorides) can be obtained by the action of PC13 on mono-, di-, and tri-chloracetic acids.

The compounds of this family must be distinguished from the haloid salts of the acids which contain the halogen atom in the place of the carboxyl hydrogen ; for instance-