Density (in grams per c.c.) of mixtures of ethyl alcohol and water. . cont.

For examining very dilute alcohol solutions in the tropics, the following table of the "density" at 32.5° C. (= 90.5° F.) of solutions containing 0 to 1.6 per cent. of alcohol by weight, may be found useful. It is due to A. F. Joseph and W. N. Rae.1

There are three methods in general use for expressing the percentage of alcohol in a liquid - (1) percentage of alcohol by weight, (2) percentage of alcohol by volume, and (3) percentage of proof spirit by volume. The first method is the best for scientific purposes, and in general for miscellaneous analytical work, where often alcohol is only one of several ingredients of a mixture the composition of which is required. But the second method is by far the most convenient when alcoholic beverages or spirituous medicines are concerned, and both in this country and on the Continent, when "percentage of alcohol" is stated in connection with wines, spirits, etc., percentage by volume is generally understood, unless the contrary is stated or obviously implied. To obtain percentage by weight it is necessary to take the specific gravity of the liquid examined, or to weigh the liquid out; and when obtained, the percentage by weight may be misleading unless used carefully. For example, two specimens of wine may contain identical amounts of alcohol per litre, but expressed as percentage by weight the quantities may be appreciably different one from the other, because one wine may be sweetened and the other not. The sweetened wine being heavier than the other, contains a smaller percentage of alcohol by weight, though a litre of each contains the same amount.

1 J. Soc. Chem. Ind., 1914, 33, 991.

Beverages and spirituous medicines being almost invariably used, measured, compared, and referred to in terms of their volumes, and being of widely different densities, a truer idea of their relative alcoholic contents is given by expressing the latter as percentage by volume rather than by weight.

For commercial and fiscal purposes in this country, however, "proof spirit" is the unit of volume for alcohol. That this is so is, from some points of view, a pity. The use of the term, and of its adjuncts "over proof" and "under proof," is confusing and cumbersome. But it has been so long in employment, and is so . much interwoven with trade practices, that a change to the simpler method of expressing alcoholic strength by means of alcohol percentage would undoubtedly cause serious inconvenience. It must be carefully borne in mind, however, that although "proof spirit" terminology is cumbersome, there is no loss of scientific precision in adopting a diluted alcohol as unit, instead of absolute alcohol, so long as the unit is properly defined. On the contrary, there is more accuracy, because the diluted alcohol can be defined by means of its specific gravity to any degree of precision required, whilst the exact specific gravity of absolute alcohol is a matter of some doubt. In this connection it is of interest to quote from a report made in 1833 by a Committee of the Royal Society, who had been asked by the Treasury to advise them on the subject of spirit valuation for fiscal purposes: -

"With regard to the substance alcohol, upon which the excise duty is to be levied, there appears to be no reason, either philosophical or practical, why it should be considered as absolute. A definite mixture of alcohol and water is as invariable in its value as absolute alcohol can be. It is also invariable in its nature, and can be more readily and with equal accuracy identified by that only quality or condition to which recourse can be had in practice, namely, specific gravity. A diluted alcohol is, therefore, that which is recommended by us as the only excisable substance."

The legal definition of " proof spirit " is given a little later on.