Ammonia is a gas formed by the decomposition of nitrogenous substances, but chiefly obtained, on a large scale, from the so-called “gas liquor” of gas works. By itself it develops a very disagreeable odor and stimulates the lachrymal glands to secretion—a fact which can be verified in any stable. A solution of the gas (water of ammonia; liquor ammoniæ) possesses the same properties. In perfumery ammonia is never used alone, but only in combination with other odors, namely, in the manufacture of smelling salts (French: sels volatils; German: Riechsalze), which are much in favor in England and in this country. For the purposes of the perfumer, the greater part of the commercial ammonia is unsuitable owing to its tarry odor. Pure ammonia is best prepared by heating equal parts of quicklime and powdered sal-ammoniac in a retort, and conducting the generated gas into water which dissolves it with avidity, one quart of water dissolving more than seven hundred quarts of ammonia gas. CARBONATE OF AMMONIA, a combination of ammonia with carbonic acid, occurs in commerce in large transparent lumps, often covered with a white dust of bicarbonate of ammonia, which in the air continually develop ammonia and therefore always smell of it. This commercial product is, as a rule, sufficiently pure to be used in perfumery; as to its application the same remarks apply as were made under the head of ammonia. OIL OF BITTER ALMONDS (OLEUM AMYGDALÆ AMARÆ). This is made from bitter almonds, previously deprived of fatty oil by pressure, which are mixed with an equal weight of water and set in a warm place. The amygdalin undergoes decomposition into sugar, hydrogen cyanide, and benzoyl hydride or oil of bitter almonds. After one or two days the mass is distilled; the distillate being a colorless liquid, containing, besides oil of bitter almonds, hydrogen cyanide or prussic acid, one of the most virulent poisons, from which it must be freed. This is done by shaking the liquid repeatedly with dilute solution of potassa, followed by agitation with water. Pure oil of bitter almonds is not poisonous, but has a very strong narcotic odor of bitter almonds, which, however, becomes most marked when largely diluted with water. BENZOIC ACID (ACIDUM BENZOICUM). This acid, contained in benzoin, is made also synthetically from other materials, in chemical laboratories. When pure it forms needle-shaped crystals having a silky gloss; they have a peculiar acrid taste, but no odor. Synthetic benzoic acid is worthless to the perfumer; in his art he can use only a benzoic acid made from gum benzoin by sublimation, because it contains a very aromatic essential oil for which the acid is merely the vehicle and which can also be employed alone. As this sublimed benzoic acid is often adulterated with the artificial, we advise the manufacturer of perfumery to make his own benzoic acid according to the following directions. _The Manufacture of Sublimed Benzoic Acid._