Chemical Composition—The turpentines are mixtures of resin and essential oil. The latter, which amounts to from 15 to 30 per cent., consists for the greater part of various hydrocarbons, corresponding to the formula C₁₀H₁₆. Many of the crude turpentine oils, and some of them even after rectification, are energetically acted on by metallic sodium. This reaction proves the presence of a certain quantity of oxygenated oils, not one of which has thus far been isolated.
The turpentine oils, although agreeing in composition, exhibit a series of physical differences according to their origin. One and the same tree, indeed, yields from its several organs oils of different properties. The boiling point varies between 152° and 172° C. The sp. gr. at mean temperatures ranges from 0·856 to 0·870. Greater differences are exhibited in the optical properties, some varieties of the oil turning the plane of polarization to the right, others to the left. This rotatory power differs in many cases from that of the turpentine from which the oil was derived.[2278] The odour of oil of turpentine varies with the species from which it has been obtained.
When crude turpentine is distilled with water, nearly the whole of the oil passes over, while the resin remains. This resin is called Colophony or Rosin. When it still contains a little water, it is distinguished in English trade as Yellow Rosin; when fully deprived of water, it becomes what is called Transparent Rosin. That of deeper colour acquired by a still longer application of heat, bears the name of Black Rosin.
Colophony softens at 80° C., and melts completely at 100° into a clear liquid. At about 150° it forms a somewhat darker liquid, but without undergoing a loss in weight; at higher temperatures, it gradually decomposes. Pure colophony has a sp. gr. of 1·07, and is homogeneous, transparent, amorphous, and very brittle. At temperatures between 15° and 20° C., it requires for solution 8 parts of dilute alcohol (0·883). On addition of a caustic alkali, it dissolves in spirit much more freely. It is plentifully soluble in acetone or benzol.
The composition of colophony agrees with the formula C₄₄H₆₂O₄. By shaking coarsely powdered colophony with warm dilute alcohol, it is converted into a crystalline body, Abietic Acid, C₄₄H₆₄O₅,—a result due simply to hydration. Under such treatment, colophony yields 80 to 90 per cent. of abietic acid,[2279] and therefore consists chiefly of the anhydride of that acid. This is probably the case with the resins of other conifers. The living tree contains only the anhydride, for the fresh resinous juice is clear and amorphous after the expulsion of the oil; and when exposed to the air it loses oil, takes up water and solidifies as the crystalline acid,—a change which may easily be traced by the aid of the microscope, in drops taken direct from the trunk. Amorphous colophony retains its transparency even in a moist atmosphere, and appears to be capable of passing into the state of abietic acid, only when the assumption of the needful molecule of water is aided, in nature by the presence of the essential oil, or artificially by that of alcohol.
Colophony when boiled with alkaline solutions forms greasy salts of abietic acid, the so-ccalled resin-soaps, which are used as additions to other soaps.
Siewert’s Silvic Acid is regarded by Maly (1864) as a product of the decomposition of abietic acid; and the Pimaric, Pinic and Silvic Acids of former investigators, as impure abietic acid. Pimaric acid however, which is the chief constituent of Galipot, appears to be decidedly different, so far as we can judge from the experiments of Duvernoy (1865) and of one of ourselves (F.).
Abietic acid, as well as the unaltered coniferous resins, deviate the ray of polarized light, whereas American colophony, dissolved in acetone, is devoid of optical power.
Commerce—The supplies of turpentine are chiefly derived from the United States, but the trade has undergone a great change, as shown by the following figures, which represent the quantities imported in the several years:—
| 1869 | 1870 | 1871 | 1872 |
| 60,468 cwt. | 51,257 cwt. | 2,231 cwt. | 1,000 cwt. |