PROOF SPIRIT. The term applied to standard mixtures of alcohol and water which form the basis of charge of Customs and Excise duties in Britain and elsewhere.
The Spirits Act of 1816 (Great Britain) in legalising Sikes' hydrometer for revenue purposes gave for the first time a legal definition of proof spirit as follows :—"that which at the tempera ture of 51° F weighs exactly 44 of an equal measure of dis tilled water." The act did not state the temperature of the water and considerable discussion has taken place on this point. Docu mentary evidence has been discovered recently which places be yond all doubt that the temperature of the water was intended to be 51° F. The most recent investigation has established the fact that, at 6o° F, proof spirit has a specific gravity of 0.91976, compared with water at the same temperature, and that it con tains 49.28 per cent by weight or 57.10 per cent by volume of alcohol.
The British Proof system is based upon volume percentage at 50° F. Owing to the difference in the rate of expansion of alcohol and water the volume composition of any mixture of these two substances varies with the temperature. The British system ig nores this change and allocates to a mixture the volume percentage and therefore the proof strength appropriate to it at 50° F. Al
though not strictly correct at other temperatures, the system has the merit of being highly convenient to both the revenue au thorities and the spirit trade. Spirits, at strengths other than "proof," may be described as containing a percentage of proof spirit or as being "over proof" or "under proof." The percentage of proof spirit is that number of volumes of Proof Spirit which can be obtained from one hundred volumes of the mixture. Thus at 5o° F zoo volumes of absolute alcohol if diluted with water to proof strength give 175.35 volumes. It is therefore said to con tain 175.35% proof spirit or to have a strength of 75.35 over proof. In order to ascertain the number of gallons of proof spirit equivalent to the alcohol contained in a mixture of known strength and volume the number of gallons of the mixture is multiplied by the percentage of proof spirit and divided by ioo.
American proof spirit contains 5o% by volume of Tralles' alcohol at 15.6° C (6o° F). This alcohol is not quite anhydrous having a specific gravity, at 15.6° C, of 0.7946, as compared with for absolute alcohol. In the Dutch system Proof Spirit contains 5o% of anhydrous alcohol at 15° C F.)