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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.
Since the density of a liquid varies as its temperature changes, the scale is adjusted to a certain temperature, usually about 15 degrees C., at which determinations must be made.
When the hydrometer is placed in a liquid, it sinks sufficiently to displace a volume of the liquid equal in weight to the weight of the instrument, and floats in an upright position. Should the hydrometer sink so deeply into the liquid that the scale is entirely below the surface, the density is less than the spindle is intended to measure, and one having lower * numerical readings should be used. If, on the contrary, the spindle does not sink deep enough to bring the scale into the liquid, an instrument having higher numerical scale readings is necessary.
Three systems of hydrometer scales are in common use, besides a great number of special scales intended to give one particular factor in the density of a liquid; e.g. the per cent of alcohol in a mixture of alcohol and water, or the amount of sugar in a syrup, etc.

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.
Twaddell readings are converted into specific gravity as follows: Multiply the reading by .005, and add 1.000 to the product. Thus 15 Twaddell becomes 1.075 sp. gr. (1.000 +[15 X .005J = 1.075.)

Baumé's hydrometer is a very unscientific instrument, but is
largely used in technical. work. Its readings bear no very direct
relation to true specific gravity. Baumé dissolved 15 parts of pure salt in 85 parts of pure water at 12.5° C. The point to which his instrument sank in this solution was marked 15; the point to which .it sank in pure water was marked O. The distance between these points was divided into fifteen equal parts, and the entire stem marked off in divisions of this width. This produced an instrument for liquids heavier than water.
For liquids lighter than water, the point to which the instrument sank in a 10 per cent solution of salt was marked 0, and that to which it sank in distilled water was marked 10, the distance between these points was divided into 10 equal parts, and this gradation continued the entire length of the spindle. The 0 thus being placed at the bottom of the stem, the lighter the gravity of the liquid tested, the greater numerically is the reading of the scale. For instance, a liquid reading 70 Bé. is of less density than one of 50 Bé., which in turn is lighter than water at 10 Bé.
To further complicate matters, the instrument makers appear to have become confused, and produced instruments with erroneous scales. A test made a few years ago disclosed thirty-four different scales, none of which were correct! *
The conversion of Baumé readings to specific gravity involves some calculation and is usually accomplished by reference to tables.
The formula for this conversion are as follows:-

The pyknometer is not very often used in technical work, but a brief description of it may not be out of place here. It consists of a small bottle, having ground into its neck a capillary tube enlarged at its upper end, to form a reservoir which is closed by a stopper. The tube is removed and the bottle filled with the liquid to be tested; the tube is then inserted tightly, the liquid displaced rising through the capillary to the enlarged part of the tube. The stopper is then loosely inserted and the bottle placed in a bath at the temperature at which the density is to be taken. When the bottle and contents have reached this temperature the stopper is taken out and the liquid in the reservoir removed by means of absorbent paper, until the level of the liquid recedes within the capillary to a mark thereon. 'fhe stopper is then tightly inserted and the bottle removed from the bath, and after cleaning and drying its outside, allowed to stand until it reaches the normal temperature of the room. It is then weighed, and the density of the liquid is calculated from its known volume, previously determined by calibration of the bottle. (For determining the density of solids by means of the pyknometer, see T. E. Thorpe's Dictionary of Applied Chemistry, Vol. III., p. 528.)

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
liquid it displaces. The characteristic feature of the instrument is the decimal graduation of the beam, with the use of riders of 0.1, 0.01, and 0.001 part of the weight of the water displaced by the plummet. This permits the actual specific gravity to be at once read off on the beam, as soon as the latter has been brought to equilibrium with the plummet suspended in the liquid in question.

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