These properties, however, belong only to the commercial stronger or so-called “druggists’ alcohol,” and more particularly to a very pure quality of it, as free as possible from fusel-oil compounds, known as cologne spirit. As absolute alcohol is also necessary for the purposes of perfumery, we shall briefly describe its preparation. In order to make absolute alcohol, sulphate of copper is heated in a retort until it has changed into a white powder. After the powder has cooled in the covered retort, it is at once introduced into a large glass bottle; over it is poured the strongest obtainable alcohol (96% Tralles) which must be free from fusel oil; then the bottle is closed air-tight and repeatedly shaken. The sulphate of copper which has lost its water of crystallization by the heat reabsorbs it from the alcohol and again becomes blue and crystalline. Generally four pounds of sulphate of copper are used for ten quarts of alcohol; when white burnt sulphate of copper after long contact with alcohol still remains white, the alcohol is proved to be practically anhydrous (it may still contain about two per cent of water). Larger quantities of absolute alcohol are made in a copper still containing fused anhydrous chloride of calcium in small pieces. The apparatus is closed and alcohol of 94 to 95% is poured in through a tubulure. The mixture often grows so warm that the alcohol begins to pass over, so that but little heat need be applied to make the absolute alcohol distil over. Absolute alcohol obtained in this way—for by repeated distillation we get at most an alcohol of 96%—abstracts water from the air with avidity; hence it must be preserved in air-tight vessels which should contain a small amount of anhydrous sulphate of copper. [Illustration: FIG. 2.] Strong commercial alcohol contains varying amounts of water—from four to twenty parts by volume (96 to 80% alcohol); at the present time, however, it is always customary for dealers in this country to supply the officinal alcohol of 94%, when “strong alcohol” is called for. Its strength is measured by an areometer which sinks in proportion to the purity of the alcohol; the alcoholometer of Tralles or volumeter shows at once on its scale how many parts by volume of absolute alcohol (volume per cent) are contained in 100 volumes of alcohol. The adjoining figure (Fig. 2) shows Tralles’ alcoholometer, with the vessel in which the test is made. The readings of the instrument, however, are correct only at a temperature of 15·6° C. (60° F.), the so-called normal temperature; at a higher or lower point they must be corrected according to the tables appended. At temperatures below the normal, the amount of alcohol is greater than the areometer indicates, hence a percentage must be added; at higher temperatures a percentage must be deducted.