Terpenes. A number of compounds are inluded in the Terpene group, all of which are intimately related chemically, and resemble one another very closely. They are chiefly obtained from the exudations from trees of the order Coniferae, from the resins, etc., of which they are derived by distillation. The distillate known as oil of turpentine contains a number of the terpenes, all of which possess the composition represented by C10H16. It is a mobile liquid, colourless or slightly yellow, which is insoluble in water, and possesses a specific grayity of about .85 to .89. That derived from pine consists chiefly of pinene, and bols at about 158°. Other terpenes distinguished as limonene, citrene, etc., occur in the turpentine of other trees, the most noticeable physical difference between the various products being in their action on polarised light. They all have agreeable odours and are good solvents for resins, sulphur, caout-chouc, etc. They are on this account largely employed for the manufacture of varnishes, oil-colours, etc. Their constitution has not yet been completely determined, but most appear to be derived from the componnd cymene, a paramethylpropyl benzine.