inohy fromellitic acid, forming hard tetragonal prisms, soluble in water, but insoluble in hydrochloric acid. Both hydromellitic acids on heating with sulphuric acid, yield trimesic, prehnitic, and melo phanic acids.

Constitution of the Tri- and Tetra-basic Acids.

1116. According to the benzene theory at present accepted the acids C,H,CO.OH), and C,H,/CO.OH), can each exist in three isomeric modifications, all of which are known.

In determining their constitution the formation of trimesic acid from mesitylene leads to the formula:

of which the (2)=1:2:3 probably belongs to hemimellitic acid, as it decomposes into carbonic anhydride, water, and phthalic anhydride on heating, leaving (3) 1:2:4 for trimellitic acid, which on heating yields only water and anhydromellitic acid.

With regard to the three tetrabasic acids the most important evidence is given by the formation of their anhydrides. As in the case of the dibasic acids, only the ortho compound, phthalic acid, gives such a body, it is probable that anhydride formation depends on the neighbouring position of the CO.OH groups. Two of the tetrabasic acids, namely

CO.OH

CO.OH

could yield, like pyromellitic acid, a double anhydride. The third possible isomer:

CO.OH

CO.OH

CO.OH

CO.OH

on the contrary, would only yield anhydro-acids, thus agreeing with prehnitic acid. The formula (1) or (2) cannot as yet be decidedly assigned to pyromellitic and mellophanic acids respectively, but the

symmetrical expression (1) probably corresponds to pyromellitic acid, as that is obtained directly from mellitic acid.

The constitution of mellitic acid admits of no doubt; the formation of two isomeric hexahydromellitic acids only necessitates the view

that the hydromellitic acid first formed, (C.CO.OH) 6, during its con

version into the iso-acid exchanges hydrogen atoms and CO.OH groups in such a way that some of the carbon atoms of the benzene ring are in union with two CO.OH groups, and as many others united to two hydrogen atoms.

FURTHER SIMPLE BENZENE DERIVATIVES.

1117. There are still some groups of compounds to be described in addition to the simple benzene derivatives already mentioned, which essentially stand in genetic relation to the latter, i.e. which are derived from a closed ring of six carbon atoms. The constitution of these bodies has, however, as yet not been made out with sufficient clearness.

TERPENES, C10H16

1118. The hydrocarbons of the formula C10H16, to which the general name of terpenes has been applied, occur either ready-formed in the essential oils of plants or are derived from these natural terpenes by chemical reactions. They are known in numerous isomerie modifications, in a large part of which the isomerism appears to be only physical (§ 47); polymeric modifications are also known. Those best investigated stand in near relation to cymene, C10H14 (§ 1023), into which they can be converted in various ways for instance, by heating with iodine, when hydrogen is eliminated. They

are therefore dihydro-cymenes, C¿H¿C ̧H,' .CH3 one of the double carbon

unions in the benzene nucleus having been converted into monovalent, the other two double unions still remaining unchanged. This theory of the constitution of the terpenes agrees with their union with halogens, hydro-acids, water, &c., to form additive products, from which either cymenes or the terpenes themselves can be again obtained.

The boiling points of the terpenes lie between 155° and 175°, their sp. gr. between 85 and 88. Many of them can be distinguished by their power of rotating a beam of polarised light in different directions, whilst others are quite inactive.

Oil of Turpentine.

1119. Turpentine, a viscous liquid which flows from incisions made in the bark of various coniferæ (especially from Pinus, Abies, and Larix), is a solution of a resin in oil of turpentine. On distilling turpentine with water, the oil passes over, whilst the resin remains behind and on fusion is obtained in the form of colophony.

The turpentine oils floating on the top of the aqueous distillate possess different optical rotary powers, according to the source from whence obtained. Most-for instance, that obtained from Pinus maritima (French oil of turpentine)—are lævorotary; that from Pinus australis (English turpentine oil) is dextrorotary.

Oil of turpentine occurring in commerce is invariably impure, as when exposed to light it readily absorbs atmospheric oxygen, being converted into resin. In order to purify it it is first shaken with solutions of alkaline carbonates, in order to remove free acids (especially formic and acetic acids), and is then purified by distillation in

vacuo.

The pure turpentine oils so obtained-terebenthene, from Pinus maritima, and australene, from Pinus australis-are mobile, colourless liquids of peculiar odour; they boil at 156°, have sp. gr. 864 at 15° and a vapour density of 4.698. They are nearly insoluble in water, but impart their odour when shaken with it; they mix in every proportion with alcohol, ether, and acetic acid. They burn with a strongly smoky luminous flame, and are good solvents for sulphur, phosphorus, fats, resins, and many other bodies insoluble in water. Repeated distillation at ordinary atmospheric pressure converts them into isomeric and polymeric modifications of higher boiling points. This change occurs more quickly on heating at 250° in closed vessels. One of the polymers, metaterebenthene, boils at 360° and has the formula C20H32.

On repeatedly distilling terebenthene with small quantities of concentrated sulphuric acid until a distillate is obtained which is completely optically inactive, terebene is obtained; it is a liquid of thymelike odour, boils at 156°, and unites with two atoms of bromine, and on then boiling with alcoholic potassic hydrate yields cymene :

C10H16Br2+ 2KOH= 2KBr + 2H2O + C10H14.

2

By oxidation with warm nitric acid terebenthene is converted into terebic acid (§ 867), and, on passing its vapours over soda lime heated to 400°, into terebentilic acid, CH10O2 = C6H6 { COOH (?); this

latter crystallises in small white needles, melts at 90°, and boils at 250°.

1120. Compounds with One Molecule of Hydrochloric Acid.-The turpentine oils absorb large quantities of hydrochloric acid gas, forming with it two isomeric compounds of the formula C10H1C1. If the temperature is kept as low as possible, a crystalline body is chiefly formed; at higher temperatures a liquid addition product is mainly obtained. They are separated by solution in hot alcohol. On cooling the first separates in colourless needles of camphoraceous odour (artificial camphor). It is insoluble in water, melts at 115° (or in an atmosphere of hydrochloric acid gas, with special precautions to prevent decomposition, at 131°-132°), and can be sublimed on careful heating.

1121. C'amphenes. Both the isomeric hydrochlorides, on heating with basic hydrates or alkali salts of weak acids, are re-converted into hydrocarbons of the original formula, but which show altered properties. The crystalline hydrochlorides, on heating with dry soap at 220°, give solid camphenes, melting at 45° and boiling at 160° (terecamphene being lævorotary and austracamphene dextrorotary). By decomposition with sodic benzoate both hydrochlorides yield optically inactive camphene, which also melts at 45° and boils at 160°. All three camphenes unite directly with a molecule of hydrochloric

acid, forming solid hydrochlorides. By decomposition with lime heated to 200° these yield a liquid inactive terpene, camphilene, boiling at 160°, which also yields a solid compound, C10H17Cl, with hydrochloric acid.

The original liquid hydrochlorides, on the other hand, are converted by heated lime into a liquid, terebilene, which is optically inactive, and forms a liquid hydrochloride, C10H17Cl.

Terebene (§ 1119) yields a solid hydrochloride, which melts in an atmosphere of hydrochloric acid at 125° and, unlike its isomers, is quickly decomposed by water into hydrochloric acid and solid camphene.

1122. Compounds with Two Molecules of Hydrochloric Acid.—On long contact of turpentine oils with concentrated bydrochloric acid, or by saturation of their solutions in alcohol, ether, and acetic acid with hydrochloric acid gas, rhombic crystals are formed of the dihydrochlorides, C10H18Cl2; they are insoluble in water, but on long boiling with it yield terpinol, C20H34O.

1123. Terpine.-Colourless crystals of terpine hydrate separate after a time from oil of turpentine containing water:

C10H22O3 = C10H 2002,H2O.

These are obtained in quantity when eight parts of oil of turpentine, two parts of dilute nitric acid, and one part of alcohol are allowed to react on one another for some time, the mixture being frequently shaken. Terpine hydrate forms large, brilliant, colourless, short rhombic prisms, and can be crystallised from alcohol, ether, and hot water. In an exsiccator it loses a molecule of water and is converted into terpine, C10H2002, which melts at 103° and sublimes at higher temperatures in long needles. Hydrochloric acid and phosphoric chloride convert it into the above-mentioned dihydrochloride :

C10H18C12.

On long heating of its aqueous solution with some hydrochloric and sulphuric acids it yields terpinol, C20H340. This latter is a liquid of hyacinth-like odour; it boils at 168° and has sp. gr. 852. Hydrochloric acid reconverts it into C10H18C12.

The relations of these bodies to one another are shown in the following formula :

.CH2
CH6 C3H7

Terpene.

Cl.C6H7C3H7
.CH3

Hydrochloride.

(HO)2.C6H8 C3H7
.CH3

Other Terpenes.

2

Terpine.

1124. Citrene is the ethereal oil obtained from lemon peel. It boils at 173° and has sp. gr. 85 at 15°. Hydrochloric acid readily converts it into citrene dihydrochloride, C10H18Cl, melting at 44°;