Page:Transactions of the Geological Society, 1st series, vol. 2.djvu/19

Rh If turpentine, as it flows from the fir in a liquid state, be exposed for a considerable time to the action of the atmosphere, it becomes brittle, and is converted into resin, in consequence as it is supposed of the absorption of oxygen. If the same turpentine be exposed to the action of the fire, a colourless volatile oil is separated, and resin remains in the retort. This however is not a mere case of the separation of a more volatile from a fixed substance, for a decomposition takes place, and acetic acid is generated. Nor can turpentine be again reproduced by mixing together the essential oil and the resin—it then forms a varnish. The essential oil is in fact a new compound, produced from the vegetable elements by the action of fire; and although properly enough classed with those essential oils which are vegetable secretions, differs from them in some of its chemical properties. It is, for example, soluble in alcohol, but on exposure to air it becomes thick and yellow, and is then easy of solution in the same substance.

If the resin, which is the residuum of this distillation be still further heated, it gives over a thick and high coloured oil, gradually increasing in weight, till it equals, and at length exceeds the specific gravity of water. The residuum becomes ultimately black, and very brittle, remaining soluble in ether and in lixivium of potash, but refusing to dissolve in alcohol.

Common tar differs from turpentine in containing a portion of the vegetable tar now under review, mixed with common turpentine and with the acetic acid which is formed in the distillation to which the wood is subjected for the purpose of obtaining it. Evaporation converts this into pitch, by decomposing it.

In this process, an essential oil, compounded of the oil of turpentine and the oil of wood, together with a portion of acetic acid, is separated, and the residuum or common pitch, is a compound of

B