and—
| gallons | of alc. × 16·121 | = | lbs. weight of pf. spt. |
| ” | pf. spt. × 4·53 | = | content in lbs. weight of alc. |
In these cases a knowledge of the first four rules of decimal fractions is necessary, or, at least, advantageous; as the Excise officers carry their calculations to two figures of decimals, or 1⁄100ths. Their plan is to reject the third decimal figure when less than 5; but to carry 1 to the next figure on the left hand, when it exceeds 5. Thus, 5·432 is set down as only 5·43; but 5·437 is written 5·44. In the delicate chemical processes of the laboratory, even greater accuracy is observed.
Formerly, spirit was said to be 1 to 3, 1 to 4, &c., over-proof, by which it was meant that 1 gal. of water added to 3 or 4 gals. of such spirit would reduce it to ‘proof.’ On the other hand, 1 in 5, or 1 in 8, under-proof, meant that the 5 or 8 gals., as the case might be, contained 1 gal. of water, and the remainder represented the quantity of ‘proof spirit.’ This method of calculation has now long given way to the ‘centigrade system,’ which not only admits of greater accuracy, but is quite as simple. It should be adopted by every spirit-dealer in England, from being that which is employed by the Revenue officers, whose ‘surveys’ it is absolutely necessary that the trader should understand, in order that his own estimation of his stock and his business calculations should correspond with theirs.
Several other methods of alcoholometry, besides those already noticed, have been adopted at various times, but the majority of them possess so little accuracy as to be quite inapplicable to the purposes of trade, and of the laboratory. Thus, the strength was at one time estimated by what was called the ‘proof.’ A little of the spirit was poured upon a small quantity of gunpowder, contained in a spoon or saucer, so as just to moisten it, and was then inflamed. If at the end of the combustion the gunpowder took fire, the spirit was held to be ‘above proof,’ if it only languidly fizzed away, or slowly burnt, the spirit was said to be ‘proof,’ but if the gunpowder failed to ignite, the spirit was esteemed ‘below proof.’ Hence arose the terms ‘proof’ and ‘proof spirit,’ which have since been adopted by Act of Parliament. Another method was that of dropping oil into the spirit; if the oil floated, the spirit was considered to be ‘under proof,’ if it sunk, it was rated as ‘proof’ or ‘over-proof.’ The ‘gunpowder test’ is quite fallacious; for, if a certain quantity of a spirit is capable of firing the gunpowder, a little excess of a spirit 20% or 25% stronger will often fail to do so, so much water being formed as to prevent the ignition. The ‘Preuve d’Holland’ test, of the French, or the ‘BEAD,’ is still frequently employed by persons unacquainted with the use of the hydrometer. It consists in shaking the spirit in a phial, and observing the size, number, and duration of the bubbles or beads, as they are called. The larger and more numerous these are, and the more rapidly they break and disappear, the stronger the spirit is presumed to be. This method is unreliable, as the presence of sugar or acid, even in minute quantities, will sometimes give to a weak sample the appearance of one many degrees stronger. Lovi’s beads are also often employed to ascertain approximately
the strength of spirit, when a hydrometer is not at hand.
The insufficiency of most of the methods of alcoholometry here referred to, throws us back on the Revenue System (Sykes’ hydrometer), or on the specific gravity for unsweetened spirits. For sweetened spirits, as cordials, wines, beers, &c., there are none of the tests which give such accurate results as the distillation test, previously described as the Revenue Method.
The spirituous liquors of commerce being sold by measure, and not by weight, the methods of alcoholometry which give the results, per cent., by volume, are those we have chiefly explained. In the laboratory, the method by weight is that most generally employed in delicate processes and in analyses. By weight, the per-centage of alcohol remains the same for all temperatures, for the same sample; whilst by volume, the per-centage varies with the temperature of the liquid. This variation explains the cause of many of the sudden apparent decreases and increases, which occur in large stocks of spirits. Persons purchasing spirits during very warm weather, and paying for them according to their apparent quantity and strength, lose considerably by selling the same spirit when the weather becomes colder, without being conscious of such loss from the hydrometer. The reason of this is obvious, for, whilst the relative proportions of the alcohol to the water continue the same, the sp. gr. and the volume alter with the temperature; the latter being increased by warmth, and decreased by cold, in exact opposition to the former. Accuracy requires, in all cases, that a spirituous liquor should be tested for its strength at the temperature at which it was measured; and measured at the same temperature at which its strength was determined.
A consideration of these facts has led some of the great houses to introduce the system of weighing their spirits, instead of measuring them, the weight of an imperial gallon at 60° Fahr. being taken as the standard gallon. This is the method adopted by the Inland Revenue, at all distilleries, for assessing the duty, and will be readily understood by the following example:—
| Cwts. | qrs. | lbs. | ||
| Gross weight of full cask | = | 13 | 2 | 27 |
| Tare | = | 2 | 2 | 5 |
| —— | —— | —— | ||
| Net weight of spirit | = | 11 | 0 | 22 |