The specific gravity of beer is determined by first removing the excess of carbonic acid by repeatedly agitating the sample in a capacious glass flask, or by pouring it from one beaker into another several times, and then filling a specific gravity bottle with the liquid and allowing it to stand at rest until all air or gas bubbles have escaped; the weight of the bottle and its contents is now taken at 15°. In order to determine the proportion of alcohol present, 100 c.c. of the beer are introduced in a suitable flask which is connected with a Liebig’s condenser and subjected to distillation until about one-half of the quantity taken has passed over. The distillate is then made up to its original volume by the addition of water, and its density ascertained by means of the specific gravity bottle, from which the percentage of alcohol present (by weight and by volume) is readily obtained upon referring to the alcoholometric table on p. 144. The frothing of beer and the volatilisation of the free acids present are best obviated by the addition of a little tannic acid and baryta-water to the sample before the distillation. An indirect method for the determination of alcohol in beer is also frequently employed. It is accomplished by first ascertaining the density of the liquor, next removing the alcohol present by evaporation over the water-bath, subsequently adding sufficient water to restore the original volume and again taking the specific gravity of the product. The density of spirit of equal strength to the beer taken (X) is obtained by the formula, D/D´ = X, in which D is the original gravity of the sample, and D´ the gravity of the de-alcoholised liquor when made up to its first volume. The following table (see p. 144) from ‘Watts’ Dictionary of Chemistry’ gives the percentages of alcohol by volume and weight, corresponding to different densities at 15°.