There should be no difficulty in recognising acetic acid; the odour alone is, in most cases, strong and unmistakable. Traces are detected by distilling, neutralising the distillate by soda, evaporating to dryness, and treating the residue as follows:--A portion warmed with alcohol and sulphuric acid gives a smell of acetic ether. Another portion is heated in a small tube of hard glass with arsenious acid; if acetic acid is present, or an acetate, a smell of kakodyl is produced. V.--Ammonia. § 91. Ammonia, (NH₃), is met with either as a vapour or gas, or as a solution of the pure gas in water. =Properties.=--Pure ammonia gas is colourless, with a strong, irritating, pungent odour, forming white fumes of ammonic chloride, if exposed to hydric chloride vapour, and turning red moist litmus-paper strongly blue. By intense cold, or by a pressure of 6½ atmospheres at the ordinary temperature, the gas is readily liquefied; the liquid ammonia boils at 38°; its observed specific gravity is ·731; it freezes at -57·1°. Ammonia is readily absorbed by water; at 0° water will take up 1000 times its own volume, and at ordinary temperatures about 600 times its volume. Alcohol also absorbs about 10 per cent. Ammonia is a strong base, and forms a number of salts. Ammonia is one of the constant products of the putrefaction of nitrogenous substances; it exists in the atmosphere in small proportions, and in everything that contains water. Indeed, water is the only compound equal to it in its universality of diffusion. The minute quantities of ammonia thus diffused throughout nature are probably never in the free state, but combinations of ammonia with hydric nitrate, carbon dioxide, &c. § 92. =Uses.=[115]--A solution of ammonia in water has many applications in the arts and industries; it is used in medicine, and is an indispensable laboratory reagent. [115] Sir B. W. Richardson has shown that ammonia possesses powerful antiseptic properties.--_Brit. Med. Journal_, 1862. The officinal caustic preparations of ammonia are--_ammoniæ liquor fortior_ (_strong solution of ammonia_), which should contain 32·5 per cent. of ammonia, and have a specific gravity of ·891. _Liquor ammoniæ_ (_solution of ammonia_), specific gravity ·959, and containing 10 per cent. of ammonia. There is also a _liniment of ammonia_, composed of olive oil, 3 parts, and ammonia, 1 part. _Spiritus Ammoniæ Fœtidus_ (_fœtid spirit of ammonia_).--A solution of assafœtida in rectified spirit and ammonia solution, 100 parts by measure, contains 10 of strong solution of ammonia. Strong solution of ammonia is an important ingredient in the “_linimentum camphoræ composita_” (_compound liniment of camphor_), the composition of which is as follows:--camphor, 2·5 parts; oil of lavender, ·125; strong solution of ammonia, 5·0; and rectified spirit, 15 parts. Its content of strong solution of ammonia is then about 22·6 per cent. (equivalent to 7·3 of NH₃).[116]