Aldehyde, a liquid obtained from alcohol. Liebig was the first to study the products formed by abstracting hydrogen from alcohol, and to give the name of alcohol dehydrogenated to the first of the series. If vinic alcohol be burned at a low temperature and with a limited supply of air, the vapors emitted have a peculiar irritating effect on the eyes and nose, due to the production of a remarkable body named aldehyde. Similar compounds are furnished by the imperfect combustion of the other alcohols, so that there are as many aldehydes as there are alcohols. There are numerous ways of preparing ordinary aldehyde, among which may be mentioned the oxidation of alcohol by platinum black, chromic acid, nitric acid, chlorine water, and a mixture of sulphuric acid and black oxide of manganese. The method usually employed is to distil 2 parts of 80 per cent, alcohol, 3 parts oil of vitriol, and 2 parts of water into a well cooled receiver. After about 3 parts have passed over, the distillate is mixed with an equal weight of chloride of calcium and further distilled until 1 1/8 part has passed over. It is further rectified by mixing 1 volume with 2 volumes of ether surrounded by cold water, and passing ammonia through it to saturation.

Crystals of aldehyde-ammonia separate, which are washed with absolute ether and dried; by subsequently distilling these crystals with sulphuric acid and a little water, pure anhydrous aldehyde is obtained. Aldehyde as commonly known is a thin, transparent liquid, with a strong, suffocating odor. It boils at 69.5° F. It dissolves sulphur, iodine, and phosphorus; absorbs dry sulphurous acid; forms definite compounds with the acid sulphites of the alkali metals; and reduces salts of silver. Upon this last property is founded the manufacture of silver mirrors by the reduction of nitrate of silver and the deposition of the metal upon glass.