This section is from the book "A Treatise On Beverages or The Complete Practical Bottler", by Charles Herman Sulz. Also available from Amazon: A Treatise On Beverages.

The highest grade of distilled alcohol is called Cologne spirits, used largely in the preparation of perfumes, etc., and is said to be more absolute in its purity than ordinary alcohol. It should be so pure that it is absolutely colorless and odorless, Diluted Alcohol or Proof Spirit. - Diluted alcohol (alcohol dilu-tum), proof spirit, is spirit containing fifty per cent, by volume of absolute alcohol and water, and having the specific gravity 0.936 at 15.55° 0. (60° F.), and this strength has been adopted as the standard proof spirit of the United States custom house and internal revenue service.

In the United States the term proof spirit has a somewhat different signification. According to law, "proof spirit shall be held and taken to be that alcoholic liquid which contains one-half its volume of alcohol of a specific gravity of seven thousand nine hundred and thirty-nine ten thousandths (0.7939) at 60 degrees Fahrenheit," referred to water at its maximum density. Therefore, proof spirit has, at 60° F., a specific gravity of 0.93353, 100 parts by volume containing 50 parts of absolute alcohol (by volume) and 53.71 parts of water. [The apparent excess in volume of the water is due to the fact that the mixture shrinks, and will then form exactly 100 volumes.] Now, the hydrometers used by government are so graduated as to indicate the number of parts by measure or number of volumes of proof spirit contained in 100 volumes of the spirit tested, at the temperature of 60° F. That is, in pure water the hydrometer will stand at 0 degrees, in absolute alcohol at 200 degrees, and in proof spirit at 100 degrees. Absolute alcohol is, therefore, 100 degrees over or above proof; a spirit of 10 degrees (or per cent.) over proof, or as it is more commonly called, one of "110 proof," would contain 55 per cent, of absolute alcohol.

The British Proof Spirit has the specific gravity 0.920 (0.9198), and contains 49.24 per cent, by weight of absolute alcohol. Spirits stronger than this standard are lighter, and are said to be over proof; they are 20 over proof if 100 measures require to be diluted with water to 120 measures to become reduced to proof strength. 100 measures of rectified spirit, sp. gr. 0.838, when mixed with 60 measures of water, yield 156 measures of proof spirit; rectified spirit is therefore said to be 56 per cent, over proof. A spirit which is weaker than the standard is heavier, and it is said to be under proof; it is 20 nnder proof if 100 measures require the addition of alcohol, sp. gr. 0.825 (the strongest obtainable by simple distillation), to make 120 measures of proof spirit.

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