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and the specific gravity of air, the weight of the contained air can be computed; subtract this from the previous weight, and we shall have the true weight of the globe alone; determine in succession the weights of the globe filled with water and with the gas in vacuum, and from each subtract the weight of the globe; divide the latter result by the former; the quotient will be the specific gravity required.

Hydrometers.

166. A hydrometer is a floating body, used for the purpose of determining specific gravities. Its construction depends upon the principle of floatation. Hydrometers are of two kinds. 1. Those in which the submerged volume is constant. 2. Those in which the weight of the instrument remains constant.

167.

Nicholson's Hydrometer.

This instrument consists of a hollow brass cylinder A, at the lower extremity of which is fastened a basket B, and at the upper extremity a wire, bearing a scale-pan C. At the bottom of the basket is a ball of glass E, containing mercury, the object of which is, to cause the instrument to float in an upright position. By means of this ballast, the instrument is adjusted so that a weight of 500 grains, placed in the pan C, will sink it in distilled water to a notch D, filed in the neck.

Fig. 147.

To determine the specific gravity of a solid which weighs less than 500 grains. Place the body in the pan C, and add weights till the instrument sinks, in distilled water, to the notch D. The added weights, substracted from 500 grains, will give the weight of the body in air. Place the body in the basket B, which generally has a reticulated cover, to prevent the body from floating away, and add other weights to the pan, until the instrument again sinks to the notch D. The weights last added give the weight of the water displaced by the body.

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Divide the first of these weights by the second, and the quotient will be the specific gravity required. To find the specific gravity of a liquid. weighed the instrument, place it in the weights to the scale-pan till it sinks to D. The weight of the instrument, plus the sum of the weights added, will be the weight of the liquid displaced by the instrument. Next, place the instrument in distilled water, and add weights till it sinks to D. The weight of the instrument, plus the added weights, gives the weight of the displaced water. Divide the first result by the second, and the quotient will be the specific gravity required. The reason for this rule is evident.

A modification of this instrument, in which the basket B, is omitted, is sometimes constructed for determining specific gravities of liquids only. This kind of hydrometer is generally made of glass, that it may not be acted upon chemically, by the liquids into which it is plunged. The hydrometer just described, is generally known as Fahrenheit's hydrometer, or Fahrenheit's areometer.

Scale Areometer.

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168. The scale areometer is a hydrometer whose weight remains constant; the specific gravity of a liquid is made known by the depth to which it sinks in it. The instrument consists of a hollow glass cylinder A, with a stem C, of uniform diameter. At the bottom of the cylinder is a bulb B, containing mercury, to make the instrument float upright. By introducing a suitable quantity of mercury, the instrument may be adjusted so as to float at any desired point of the stem. When it is designed to determine the specific gravities of liquids, both heavier and lighter than water, it is ballasted so that in distilled water, it will sink to the middle of the stem. This point is marked on the stem with a file, and since the specific gravity of water is 1, it is numbered 1 on the scale. A liquid is then formed by dissolving common salt in water whose specific gravity is

B

Fig. 148.

1.1, and the instrument is allowed to float freely in it; the point E, to which it then sinks, is marked on the stem, and the intermediate part of the scale, HE, is divided into 10 equal parts, and the graduation continued above and below throughout the stem. The scale thus constructed is marked on a piece of paper placed within the hollow stem. To use this hydrometer, we have simply to put it into the liquid and allow it to come to rest; the division of the scale which corresponds to the surface of floatation, makes known the specific gravity of the liquid. The hypothesis on which this instrument is graduated, is, that the increments of specific gravity are proportional to the increments of the submerged portion of the stem. This hypothesis is only approximately true, but it approaches more nearly to the truth as the diameter of the stem diminishes.

When it is only desired to use the instrument for liquids heavier than water, the instrument is ballasted so that the division 1 shall come near the top of the stem. If it is to be used for liquids lighter than water, it is ballasted so that the division 1 shall fall near the bottom of the stem. In this case we determine the point 0.9 by using a mixture of alcohol and water, the principle of graduation being the same as in the first instance.

Volumeter.

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169. The volumeter is a modification of the scale areometer, differing from it only in the method of graduation. The graduation is effected as follows: The instrument is placed in distilled water, and allowed to come to a state of rest, and the point on the stem where the surface cuts it, is marked with a file. The submerged volume is then accurately determined, and the stem is graduated in such a manner that each division indicates a volume equal to a hundredth part of the volume originally submerged. The divisions are then numbered from the first mark in both directions, as indicated in the figure. To use the instrument, place it in the iquid, and note the division to which it sinks;

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Fig. 149.

divide 100 by the number indicated, and the quotient will be the specific gravity sought. The principle employed is, that the specific gravities of liquids are inversely as the vol umes of equal weights. Suppose that the instrument indicates parts; then the weight of the instrument displaces x parts of the liquid, whilst it displaces 100 parts of water. Denoting the specific gravity of the liquid by S, and that of water by 1, we have,

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A table may be computed to save the necessity of performing the division.

Densimeter.

170. The densimeter is a modification of the volumeter, and admits of use when only a small portion of the liquid can be had, as is often the case in examining animal secretions, such as bile, chyle, &c. The construction of the densimeter differs from that of the volumeter, last described, in having a small cup at the upper extremity of the stem, destined to receive the fluid whose specific gravity is to be determined.

The instrument is ballasted so that when the cup is empty, the densimeter will. sink in distilled water to a point B, near the bottom of the stem. This point is the 0 of the instrument. The cup is then filled with distilled water, and the point C, to which it sinks, is marked; the space BC, is divided into any number of equal parts, say 10, and the graduation is continued to the top of the tube.

Fig. 150

To use the instrument, place it in distilled water, and fill the cup with the liquid in question, and note the division to which it sinks. Divide 10 by the number of this division, and the quotient will be the specific gravity required. The principle of the densimeter is the same as that of the volu

meter.

Centesimal Alcoholometer of Gay Lussac.

171. This instrument is the same in construction as the scale areometer; the graduation is, however, made on a different principle. Its object is, to determine the percentage of alcohol in a mixture of alcohol and water. The graduation is made as follows: the instrument is first placed in absolute alcohol, and ballasted so that it will sink nearly to the top of the stem. This point is marked 100. Next, a mixture of 95 parts of alcohol and 5 of water, is made, and the point to which the instrument sinks, is marked 95. The intermediate space is divided into 5 equal parts. Next, a mixture of 90 parts of alcohol and 10 of water is made; the point to which the instrument sinks, is marked 90, and the space between this and 95, is divided into 5 equal parts. In this manner, the entire stem is graduated by successive operations. The spaces on the scale are not equal at different points, but, for a space of five parts, they may be regarded as equal, without sensible error.

To use the instrument, place it in the mixture of alcohol and water, and read the division to which it sinks; this will indicate the percentage of alcohol in the mixture.

In all of the instruments, the temperature has to be taken into account; this is usually effected by means of corrections, which are tabulated to accompany the different instruments.

On the principle of the alcoholometer, are constructed a great variety of areometers, for the purpose of determining the degrees of saturation of wines, syrups, and other liquids employed in the arts.

In some nicely constructed hydrometers, the mercury used as ballast serves also to fill the bulb of a delicate thermometer, whose stem rises into the cylinder of the instrument, and thus enables us to note the temperature of the fluid in which it is immersed.

EXAMPLES.

1. A cubic foot of water weighs 1000 ounces. Required

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