According to M. Biot, the camphor formed by the oil of lemons does not exercise any action on polarized light, whilst the oil of lemons itself rotates a ray to the right.  The camphor from oil of turpentine, on the contrary, does exercise on the polarized ray the same power as the oil possessed while in its isolated state, of rotating to the left.  These molecular properties establish an essential difference between the oils of turpentine and lemons, and may serve to detect adulteration and fraud.  It is also a curious fact, that from the decomposition of these artificial camphors by lime, volatile oils may be obtained by distillation, isomeric with the original oils from which the camphors were formed; but in neither case has the new product any action on polarized light.

In conclusion, I would recommend that this oil, as well as all other essential oils, be kept in a cool, dark place, where no very great changes of temperature occur.

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BENZOIC ACID, AND TESTS FOR ITS PURITY.

BY W. BASTICK.

Dr. Mohr’s process for obtaining benzoic acid, which is adopted by the Prussian Pharmacopoeia, unquestionably has the reputation of being the best.  According to this process, coarsely-powdered gum benzoin is to be strewed on the flat bottom of a round iron pot which has a diameter of nine inches, and a height of about two inches.  On the surface of the pot is spread a piece of filtering paper, which is fastened to its rim by starch paste.  A cylinder of very thick paper is attached by means of a string to the top of the iron pot.  Heat is then applied by placing the pot on a plate covered with sand, over the mouth of a furnace.  It must remain exposed to a gentle fire from four to six hours.  Mohr usually obtains about an ounce and a half of benzoic acid from twelve ounces of gum benzoin by the first sublimation.  As the gum is not exhausted by the first operation, it may be bruised when cold and again submitted to the action of heat, when a fresh portion of benzoic acid will sublime from it.  This acid thus obtained, is not perfectly pure and white, and Mohr states that it is a question, in a medicinal and perfumery point of view, whether it is so valuable when perfectly pure, as when it contains a small portion of a fragrant volatile oil, which rises with it from the gum in the process of sublimation.

The London Pharmacopoeia directs that it shall be prepared by sublimation, and does not prescribe that it shall be free from this oil, to which it principally owes its agreeable odor.

By the second sublimation the whole of the benzoic acid is not volatilized.  What remains in the resin may be separated by boiling it with caustic lime, and precipitating the acid from the resulting benzoate of lime with hydrochloric acid.  Benzoic acid can be obtained also in the wet way, and the resin yields a greater product in this process than in the former; yet it has a less perfumery value, because