The strength of spirit of wine is, by law, regulated by proof spirit (sp. gr. .920) as a standard; and accordingly as it is either stronger or weaker than the above, it is called so much per cent. above or below proof. The term _per cent._ is used in this instance in a rather peculiar sense. Thus, spirit of wine at 56 per cent. overproof, signifies that 100 gallons of it are equal to 156 gallons of proof spirit; while a spirit at 20 per cent. underproof, signifies that 100 gallons are equal to 80 gallons at proof. The rectified spirit of the Pharmacopoeia is 56 per cent. overproof, and may be reduced to proof by strictly adhering to the directions there given, viz., to mix five measures with three of water. The result, however, will not be eight measures of proof spirit; in consequence of the _contraction_ which ensues, there will be a deficiency of about [Symbol: oz.]iv in each gallon. This must be borne in mind in preparing tinctures. During a long series of experiments on the preparation of ethers, it appeared a desideratum to find a ready method of ascertaining how much spirit of any density would be equal to one chemical equivalent of absolute alcohol. By a modification of a rule employed by the Excise, this question may be easily solved. The Excise rule is as follows:-- To reduce from any given strength to any required strength, _add_ the _overproof_ per centage _to_ 100, or _subtract_ the _underproof_ per centage _from_ 100. Multiply the result by the quantity of spirit, and divide the product by the number obtained by _adding_ the _required_ per centage overproof, or _subtracting_ the _required_ per centage underproof, to or from 100, as the case may be. The result will give the measure of the spirit at the strength required. Thus, suppose you wished to reduce 10 gallons of spirit, at 54 overproof, down to proof, add 54 to 100 = 154; multiply by the quantity, 10 gallons (154 x 10) = 1540. The required strength being proof, of course there is nothing either to add to or take from 100; therefore, 1540 divided by 100 = 15.4 gallons at proof; showing that 10 gallons must be made to measure 15 gallons, 3 pints, 4 fl. oz., by the addition of water. To ascertain what quantity of spirit of any given strength will contain one equivalent of absolute alcohol. Add the overproof per centage of the given spirit to 100, as before; and with the number thus obtained divide 4062.183. The result gives in gallons the quantity equal to four equivalents (46 x 4). _Example._--How much spirit at 54 per cent. overproof is equal to 1 equivalent of absolute alcohol? Here, 54 + 100 = 154 and 4062.183 = 26.3778 galls., or 26 galls. 3 pts. -------- 154 which, divided by 4, gives 6 gallons, 4 pints, 15 oz. Suppose the spirit to be 60 overproof,-- 4062.183 {one-fourth of which is equal then ---------- = 25.388 gallons, {to 6 gallons, 2 pints, (100 + 60) {15-1/2 oz.