The only reagent for detecting oil of turpentine in oil of lemon was hitherto copper nitroprusside, and this is only really serviceable when the adulteration is very considerable, the admixture of a small quantity only of oil of turpentine requiring the comparison of the reaction of the sample tested with that of perfectly pure oil of lemon.

Dr. G. Heppe claims to have found an excellent reagent in copper butyrate, the employment of which does not necessitate the comparison with a sample of the pure oil.

A small quantity of the oil to be tested is heated slowly to 170° C. to 180° C. over a sand bath with a piece of copper butyrate about the size of a pin's head, previously dried and powdered. If the oil of lemon is pure the copper salt dissolves completely, coloring the oil green. When even a trace, however, of oil of turpentine is present, the mixture turns muddy and a reddish yellow precipitate of cuprous oxide is formed. Care must be taken that the temperature does not exceed 180° C.

Other tests for turpentine in essential oils are:

Draggendorff's: - Add an equal volume of 80° alcohol to some of the suspected oil. If oil of turpentine be present the solution appears turbid.

Mero's: Shake with an equal volume of poppy-seed oil. Milky if pure; but clear with oil of turpentine.

Vogel's: Add five drops of the suspected oil to one drop sulphuric acid. If color alters it indicates the presence of oil of turpentine.