A modification of the hydrometer, called the alcoholometer, has been constructed by which the percentage strength by volume or by weight may be directly ascertained. As the density increases with a diminution of temperature, and decreases in about the same ratio with an elevation of temperature, alcoholometers are constructed with reference to a standard temperature, which is either 15° C. or 60° F.; every degree of variation indicates a difference of 0.36 for the former and 0.25 for the latter scale, which must be added if the alcohol tested was below, or subtracted if it was above, the standard temperature. In the United States and Germany the scale of Tralles, and in France that of Gay-Lussac, are in use. Both instruments are practically identical; they sink in distilled water to 0 and in absolute alcohol to 100, each intervening division indicating a volumetric per-centum of absolute alcohol. Richter's alcoholometer is essentially that of Tralles, combined with a scale indicating also the percentage by weight of absolute alcohol. The alcoholometer employed in the British revenue system is that of Sykes.

In the tables of the U. S. P., are given the percentage of alcohol by volume and weight for each division of Traile's scale, together with the specific gravity; also the density indicated by Gay-Lussac's alcoholometer at the corresponding degree. It will be observed that the absolute alcohol of this scale really contains 99.9 per cent, of absolute alcohol - a difference which is too insignificant for all practical purposes; but it should be remembered that the specific gravity for Tralle's scale is given for 15.55° C. (60° F.), compared with water at 3.9° C. (39° F.), while that of Gay-Lussac's scale applies to 15° C. (59° F.) for both alcohol and water. The table, by Fownes, gives the percentage of absolute alcohol by weight for alcohol of different specific gravities, temperature 15.55° C. (60° F.); upon it is based the table of Hehner, which is published in the U. 8, Pharmacopoeia. A very useful table for the reduction of alcohol (U. S. P.) was published by Dr. A. B. Lyons in 1882. It gives the measures of officinal alcohol and water required for obtaining 100 measures of the mixture at 15.55° C. (60 °F.) and the specific gravity of this mixture at the same temperature; incidentally, the table gives also the amount of condensation.

Fig. 422.   Alcoholometer and Jar with Thermometer

Fig. 422. - Alcoholometer and Jar with Thermometer.