A knowledge of the specific gravities of bodies is of frequent application. In mineralogy it aids in determining mineral species. The jeweller determines by its aid the precious stones. It enables us to find the weight of a body when we know its volume. Thus, a cubic foot of iron weighs 11.35 times as much as a cubic foot of water; but a cubic foot of water weighs 1000 ounces, hence a cubic foot of iron weighs 11,350 ounces, or about 709 lbs.

Beaume's Areometer.

96. BEAUME'S AREOMETER consists of a bulb of glass, ballasted at bottom by a second bulb containing mercury, and terminating at top in a cylinder of uniform diameter, as shown in Fig. 67.

When plunged into liquids, it sinks till the weight of the displaced fluid equals that of the areometer. In light fluids it therefore sinks deeper than in heavy

ones.

The plan of graduating BEAUME'S areometer is as follows. It is ballasted so that in distilled water it will sink to the point a, on the stem, which is marked 0. A mixture of salt and pure water is then formed, in the proportion of 15 of the former to 85 of the latter, into which the instrument is plunged. The upper surface then cuts the stem at some point, c, which is marked 15. The intermediate space between a and c, is divided into 15 equal parts, and the division is continued downwards on the stem. The divisions and numbers are on a slip of paper in the interior of the stem.

Fig. 67.

What are some of the applications of the specific gravity of bodies? (96.) Describe BEAUME's Areometer? How is it graduated?

The use of the instrument thus graduated is to ascertain the amount of salt in any solution of salt in water. It is plunged into the solution in question, and the number to which it sinks, denotes the per-centage of salt in the solution.

Instruments constructed on this principle have been devised for determining the strength of other solutions, whether of acids or salts. Also for determining the strength of saccharine solutions and the like.

The Alcoholometer.

97. The ALCOHOLOMETER is similar in its construction to the areometer just described. It is graduated so as to show the percentage of alcohol in any mixture of alcohol and

water.

The instrument is first ballasted so that when plunged in pure water it will float with nearly all of its stem above the water. The line of flotation is marked 0. Mixtures are then formed, containing 1, 2, 3, &c., per cent. of pure alcohol and water, and the instrument is plunged into them in succession. The lines of flotation are marked 1, 2, 3, &c., as in the instrument previously. In this case the numbers run upwards. It is necessary to graduate it throughout by trial, as the divisions are not uniform.

Fig. 68.

To use the instrument, it is plunged into the solution of alcohol in water to be tested, and the per-centage is read off on the paper scale within the tube, or else the scale is scratched upon the stem with a diamond.

What is its use? How used? What other instruments are constructed on the same principle? (97.) Describe the Alcoholometer. How is it graduated? How used?

The Lactometer.

98. The LACTOMETER is entirely analagous in principle to BEAUME's areometer, and is used to determine the purity of milk. The instrument, and

the method of using it, are shown in Fig. 69.

It is graduated by trial, using mixtures of milk and water. In the first trial pure water is used, then mixtures containing 10, 20, 30, 40, &c., per cent. of milk. The scale

Fig. 69.

is therefore divided into 10 parts, between pure water and pure milk.

(98.) What is a Lactometer? How graduated and used?

CHAPTER III.

MECHANICS OF GASES AND VAPORS.

I. THE ATMOSPHERE.

General Properties of Gases and Vapors.

99. GASES and VAPORS have been defined to be highly compressible fluids.

The distinction between a gas and a vapor, is not very clear. When a body in a gaseous form can, by moderate pressure, be reduced to a liquid form, it is usually called a vapor. For most of the purposes of Physics the distinction is unimportant.

Besides the property of compressibility, or rather as a consequence of it, gases and vapors continually tend to expand so as to occupy a greater space. The force which they exert in this way, is called their Tension, or their Elastic Force.

Thirty-four gases are known, thirty of which are compound, and four are simple. The four simple gases are, oxygen, hydrogen, nitrogen, and chlorine. Most of the gases are colorless, but some are

not so.

Of the thirty-four gases, all but five have been liquefied by pressure, and the application of cold. The five that have thus far resisted are, oxygen, hydrogen, nitrogen, deutoxyde of nitrogen, and carbonic oxyde.

(99.) What are Gases and Vapors? What is the difference between them? What is meant by Tension? How many known gases are there? Which are simple! Which have not been liquefied?

Description of the Atmosphere.

100. The air we breathe is a mixture of oxygen and nitrogen, with a slight quantity of carbonic acid, watery vapor, and some accidental impurities. The oxygen and nitrogen are mixed in the proportion of 21 to 79.

The oxygen of the air supports life and combustion; without it, neither could long exist. The nitrogen serves to dilute it. Were the air composed entirely of oxygen,bodies would burn with too much rapidity, even many of the metals would be consumed. Animal life, too, would soon be exhausted by overaction in such an atmosphere.

The atmosphere is transparent, without odor, and colorless, except when seen in masses. In masses, it assumes a blue tint, and it is this which causes the sky to take a blue color.

Without an atmosphere, the celestial vault would appear perfectly black; in ascending high mountains, the sky gradually loses its blueness, and approaches a hue of black; this is because the mass of air above the observer rapidly diminishes as we ascend.

The air, by virtue of its elasticity, serves as a medium for the transmission of sound; it also serves as a means of transporting the vapors of oceans and lakes to fall upon the land in the form of rain, snow, and the ́like.

Expansive Force of Air.

101. Air, like simple gases, always tends to assume a greater volume.

To show this property, take a bladder fitted with a stop-cock, as shown in Fig. 70. Having moistened the bladder to make it more flexible, open the cock, squeeze out most of the air, and then close it.

(100.) Describe the composition of the atmosphere. oxygen? Of the nitrogen? What is the color of air? celestial appearances? Mention some of the uses of the is the expansive force of air shown

What is the use of the What effect has the air on atmosphere. (101.) How