It is sometimes referred to 4° C. = 39·2° F. There is no advantage in this temperature, that of water at its maximum density; corrections for temperature will always have to be made in exact determinations, while, in the approximate determinations of trade, the indoor standard of 62° F. requires no correction.
The specific gravity of gases being in the ratio of their molecular weight, which is referred to that of hydrogen, this gas is taken as the standard.
The density of solutions of salts, sugar, acids, &c., is referred to that of water or stated on a trade-scale usually indicating the percentage in solution.
The density of spirits is referred to that of water, but their alcoholic strength to:
(a) A scale indicating the volume of alcohol per cent. (Gay-Lussac and Tralles).
(b) An arbitrary scale (Cartier and Baumé).
(c) An excise-scale on the basis of proof-spirit (Sykes).
Proof-spirit meant originally a spirit sufficiently strong to take light, and which, if poured on gunpowder and lighted, would cause the powder to explode. This was the ‘Holland-proof,’ By a statute of 1816 it was defined as of specific gravity 12/13 that of water at 51° F., which is = 0·923; but at the present standard temperature of 60° = 15·5° C. it is 0·920.
With Sykes’s hydrometer, used in England:
Under Proof (U.P.)—each degree means 1 per cent. of water, the rest being proof spirit.