The hydrometers most frequently used in the United States and other countries are those constructed upon the plan of Baume.

Fig. 355.   Baume's Hydrometer

Fig. 355. - Baume's Hydrometer.

Fig. 356.   Hydrometer Jar

Fig. 356. - Hydrometer Jar.

For liquids heavier than water the point to which the instrument sinks in pure water is 0, and the point to which it sinks in a solution of 15 parts of dry table salt in 85 parts of water is marked 15, the distance between the two points being divided into 15 equal parts, and the scale continued with divisions of the same size. For liquids lighter than water the instrument is floated in a solution of 10 parts of dry table salt in 90 parts of water, and afterward in pure water, the distance being divided into 10 equal parts, and the scale continued in like manner; the point indicating the density of water is marked 10. The hydrometers were originally constructed at a medium temperature. In the United States they are made for the temperature of 60° F. (15.55° C), and the scales as originally published by Henry Pemberton (1852) are recognized. They agree closely with the determinations made by Schober and Pescher, for liquids heaver than water, and differ but little for liquids lighter than water.

In France Baume's hydrometer is usually employed to indicate the density of liquids heavier than water; but for those lighter than water, it has recourse to the instrument of Oartier, which is made for the temperature of 17.5° C. It has the same point for the zero of its scale as Baume's, but its degrees are rather smaller, 30° Baume being equal to 32° Cartier.

In Germany Beck's hydrometer is used, which is made for the temperature of 12.5° C; also Baume's hydrometer is employed, which is made for the same temperature, in which it differs from the United States Baunu'-'s scale, which is made for a higher temperature (15.55° C = 60° F.); there is therefore some difference in the indications.

In England Twaddel's hydrometer is applied for liquids heavier than water; for lighter liquids (alcohol) Sykes' hydrometer. (See "Alcohol".)

Twaddel's hydrometer is so graduated that the real sp. gr. can be deduced by an extremely simple method from the degree of the hydrometer, namely, by multiplying the latter by 5 and adding 1,000; the sum is the sp. gr., water being 1,000. Thus 10° Twaddel indicates a sp. gr. of 1050, or 1.05; 90° Twaddel, 1450, or 1.45; however, we append a comparison table on page 452.

The instruments called the acidometer, alcoholometer, (specially described under "Alcohol"), and saccharometer, (described under "Plain Syrups"), are all modifications of the hydrometer. While the "Baume hydrometer" is the universal instrument, those three are modified for special purposes, viz., to ascertain the density of acids, alcohol, or syrup, and may be used instead.