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By density or specific gravity of a liquid is meant its relative weight compared with the weight of an equal volume of pure water at a definite temperature. The determination of density is one of the most frequent operations in chemical work. This may be done with a pyknometer when very exact results are required, but in technical operations, sufficient accuracy for all practical purposes may be attained by the hydrometer. This is usually a glass instrument, consisting of a cylindrical bulb, weighted at the lower end, and drawn out at the upper end to a long, slender tube, carrying a scale. The gradations of the scale begin at the top and read downward, the numerically greater reading being at the bottom, except in one instance, - that of Baumé's scale for liquids lighter than water.
The direct specific gravity hydrometer is so constructed thatthe reading on its scale shows the density of the liquid directly as compared with pure water at the same temperature (15 degrees C.). Its scale is adapted to liquids heavier or lighter than water. The point to which it sinks in pure water at 15 degrees C. is marked 1.000. As usually furnished, a set of these hydrometers consists of four spindles, the scale being thus divided into four sections. The first spindle, with gradations from 0.700 to 1.000, is for liquids lighter than water, and the others are for those heavier than water. The scale is usually divided about as follows: 1.000 to 1.300 on the second spindle, 1.300 to 1.600 on the third, and 1.600 to 2.000 on the fourth. The gradations at the top of each spindle are further apart than those at the bottom of the stem,* rendering the reading somewhat more difficult in dense liquids than in those of lighter gravity.
Twaddell's hydrometer is also a direct reading instrument. The system consists of a series of spindles (usually six in number) carrying gradations from 0 to 174. The reading in pure water, at 15.5° C., is taken as 0, and each subsequent rise of 0.00;) sp. gr. is recorded on the scale as one additional division. Thus 10 Twaddell becomes 1.050 sp. gr. The gradations on this scale are also closer together as the density increases, but as its total length is divided among six spindles, the readings are not so difficult even at the highest densities. The instruments arc small, the gradations on each stem occupying about three lineal' inches, so that it may easily be used in an ordinary 100 cc. measuring cylinder. For the reasons that it is easy to read, requires but a small quantity of liquid to he tested, and permits a ready conversion of its readings into specific gravity by a very simple calculation, this is the most convenient hydrometer for ordinary factory or laboratory use. It is, however, not adapted to liquids lighter than water.
Baumé's hydrometer is a very unscientific instrument, but is
Westphal's balance is a special form of balance for determining the density of liquids. A glass plummet of known weight and volume is suspended from the beam by a fine platinum wire, and is submerged in the liquid to be tested. The weight which the plummet loses by this submersion is the weight of the volume of
Organic Chemistry for the industry
Inorganic Chemistry for the industry