The Lactometer.

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

Place the nearly empty bladder under the receiver of an air-pump, and exhaust the air. As the air becomes rarer in the receiver, the bladder will be seen to expand, showing that the air within it is expansible. In the same way, it may be shown that any gas is expansible.

102. Air, like other bodies, has weight.

To show this, take a hollow globe of glass, fitted with a stop-cock, as shown in Fig. 71. Having attached it to one scale pan of a delicate balance, counterpoise it by weights placed in the other. Then by means of the air-pump exhaust the air from the globe; the opposite scale pan will descend, and some weights will have to be added

(102.) How is it shown that air has weight.

to the first scale pan to restore the equilibrium will indicate the weight of the exhausted air.

The weights added

Composition of the Atmosphere.

103. It has been stated that our atmosphere is composed principally of oxygen and nitrogen, with small quantities of carbonic acid and watery vapor.

The amount of watery vapor depends upon the place, the season, the temperature, and the direction of the wind; under all circumstances it forms but a small per-centage of the entire atmosphere.

The carbonic acid in the atmosphere arises in a great measure from respiration and combustion. A continual supply of this gas is afforded by volcanoes. On the other hand, it is being continually taken up in the process of vegetation. Plants continually absorb it, appropriating the carbon, and giving off the oxygen which it contains. Another cause of diminution in the amount of carbonic acid in the air, is absorption by the water of our streams. Water absorbs large quantities of it, which thus becomes the means of dissolving earthy matters, and eventually of causing calcareous deposits.

It is the result of observation, that the supply and loss are very nearly balanced, so that the per-centage of carbonic acid in the atmosphere remains nearly constant. It amounts to about a thousandth part of the entire atmosphere.

Atmospheric Pressure.

104. The atmosphere, by virtue of its weight, exerts a force of pressure upon the surface of the earth as well as upon every object with which it is in contact. This force is called the Atmospheric Pressure.

(103.) Upon what circumstances does the watery vapor in the air depend? Whence is carbonic acid supplied? What becomes of the excess of carbonic acid? How do the supply and loss compare? What is the amount in the atmosphere? 104.) What is the Atmospheric Pressure?