The hydrometer, divided into 24 qiráts, was well known in the East, even before the time of the Caliphates.
The density or specific gravity of solids and fluids is usually referred to that of distilled water at 62° F. = 16·6° C.
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.
_Over Proof_ (O.P.)—each degree means 1 per cent. of water required to be added to bring the spirit down to proof.
The scale of Tralles’s alcoholometer only differs from Gay-Lussac’s by water being taken at 39·2° and pure alcohol as of sp. gr. 7939 at 60°.
Alcohol Sykes° Baumé° Sp. Gr. Vol. per cent. (Excise) (French)
1·000 0 100 U.P. 10 0·920 57·05 Proof 22 0·848 85·2 49·6 O.P. 36 Fr. Rect. Spirit Troix-six. 0·838 88·8 55·8 „ 38·2 Rect. Spirit, Brit. Pharm. 0·821 93·75 64·3 „ 41·8 Strongest Rect. Spirit. 0·794 100 —— 48 Absolute Alcohol.
The approximate relation of the five usual scales for proof and French rectified spirit are as follows: