Imagens das páginas
PDF
ePub

vessel containing all the gas or vapour whose tension is to be measured.

In the graduation of this manometer, the quantity of air contained in the tube is such, that when the aperture communicates freely with the atmosphere, the level of the mercury is the same in the tube and in the bath. Consequently, at this level, the number I is marked on the scale to which the tube is affixed. As the pressure acting through the tubulure A increases, the mercury rises in the tube, until its weight added to the tension of the compressed

[merged small][graphic][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][merged small][merged small]

air, is equal to the external pressure. It would consequently be incorrect to mark two atmospheres in the middle of the tube; for since the volume of the air is reduced to one-half, its tension is equal to two atmospheres, and, together with the weight of the mercury raised in the tube, is therefore more than two atmospheres. The position of the number is a little below the middle, at such a height that the elastic force of the compressed air, together with the weight of the mercury in the tube, is equal to two atmospheres. The exact position of the numbers 2, 3, 4, etc., on the manometer scale can only be determined by calculation.

-135]

Aneroid Barometer.

123

134. Aneroid barometer. This instrument derives its name from the circumstance that no liquid is used in its construction (à, without, vŋpòs, moist). Fig. 112 represents one of the forms of these instruments, constructed by Mr. Casella; it consists of a cylindrical metal box, exhausted of air, the top of which is made of thin corrugated metal, so elastic that it readily yields to alterations in the pressure of the atmosphere.

When the pressure increases, the top is pressed inwards; when on the contrary it decreases, the elasticity of the lid, aided by a spring, tends to move it in the opposite direction. These motions are transmitted by delicate multiplying levers to an index which moves on a scale. The instrument is graduated empirically by comparing its indications under different pressures with those of an ordinary mercurial barometer.

The aneroid has the advantage of being portable, and can be constructed of such delicacy as to indicate the difference in pressure between the height of an ordinary table and the ground. It is hence much used in determining heights in mountain ascents. But it is liable to get out of repair, especially when it has been subjected to great variations of pressure; and its indications must from time to time be compared by means of a standard barometer.

MIXTURE AND SOLUTION OF GASES.

135. Laws of the mixture of gases. We have seen that liquids, when they do not act chemically, tend continually to separate, and to become superposed in the order of their densities. This is not the case with gases; being under a continual tendency to expand, when they mix, their mixture is found to be subject to the following laws:

I. Whatever their densities, gases mix in equal proportions in all parts of the vessel in which they are contained.

II. The elastic force of the mixture is equal to the sum of the elastic forces of the constituents.

The first law was shown experimentally by Berthollet, by means of an apparatus represented in fig. 113. It consisted of two glass globes provided with stopcocks, which could be screwed one on the other. The upper globe was filled with hydrogen, and the lower one with carbonic acid, which has 22 times the density of hydrogen. The globes having been fixed together were placed in the cellars of the Paris Observatory, and the stopcocks then

Ex

opened, the globe containing hydrogen being uppermost. Berthollet found, after some time, that the pressure had not changed, and that, in spite of the difference in density, the two gases had become uniformly mixed in the two globes. periments made in the same manner with other gases gave the same results, and it was found that the diffusion was more rapid in proportion as the difference between the densities was greater.

Fig. 113.

In accordance with this law, air being a mixture of nitrogen and oxygen, which are different in density, its composition should be the same in all parts of the atmosphere, which in fact is what has been observed.

Gaseous mixtures follow Boyle and Mariotte's law, like simple gases, as has been proved for air (132), which is a mixture of nitrogen and oxygen.

136. Mixture of gases and liquids. Absorption.-Water and many liquids possess the property of absorbing gases. Under the same conditions of pressure and temperature a liquid does not absorb equal quantities of different gases. At the ordinary temperature and pressure water dissolves its volume of nitrogen, 46 its volume of oxygen, its own volume of carbonic acid, and 430 times its volume of ammoniacal gas.

25

1000

The general laws of gas-absorption are the following:

I. For the same gas, the same liquid, and the same temperature, the weight of gas absorbed is proportional to the pressure. This may also be expressed by saying that at all pressures the volume dissolved is the same; or that the density of the gas absorbed is in a constant relation with that of the external gas which is not absorbed.

Accordingly, when the pressure diminishes, the quantity of dissolved gas decreases. If a solution of a gas be placed under the airpump and a vacuum created, the gas obeys its expansive force and escapes with effervescence.

The manufacture of aerated water is a practical application of this law. By means of force-pumps an excess of carbonic acid is

-137]

Air-pump.

125

dissolved in the water, and the solution is then preserved in carefully closed vessels.

It is the carbonic acid dissolved in beer, in champagne, and in all effervescing liquids, which, rapidly escaping when the bottles are uncorked, produces the well-known report, and carries with it a greater or less quantity of the liquid.

II. The quantity of gas absorbed is greater when the temperature is lower; that is to say, when the elastic force of the gas is less. III. The quantity of gas which a liquid can dissolve is independent of the nature and of the quantity of other gases which it may already hold in solution.

CHAPTER III.

APPARATUS FOUNDED ON THE PROPERTIES OF AIR.

137. Air-pump.-The air-pump is an instrument by which a vacuum can be produced in a given space, or rather by which air can be greatly rarefied, for an absolute vacuum cannot be produced by its means. It was invented by Otto von Guericke in 1650, a few years after the invention of the barometer.

Fig. 114 gives a perspective view of the pump, fig. 115 gives a detailed longitudinal section, and fig. 116 gives a cross section.

The pump consists of two stout glass barrels in which two pistons, P and Q, made of leather well soaked with oil, move up and down, and close the barrels air-tight. The pistons are fixed to two racks, A and B, working with a pinion (K, fig. 116), which is moved by a handle MN, so that, when one piston rises, the other descends.

The two barrels are firmly cemented on the base, H, which is of brass; on this plate is a column, I, terminated by a plate, G. On this plate is a glass bell jar which is called the receiver. In the interior of the column is a conduit, which is prolonged below the base to between the two barrels. It there branches in the shape of a T, terminating in two apertures, a and b, in the bottom of the cylinders. These apertures are conical, and are closed by two small conical valves; these latter are fixed to metal rods which work air-tight, but with gentle friction in the pistons. In the pistons is a cylindrical cavity communicating with the lower part of the pump by two apertures, s and t (fig. 116). These apertures are closed by small clack

valves, kept in position by springs which surround the rods themselves. The four valves, a, b, s, t, it may be remarked, open upwards. These details being known, the working of the machine is readily understood. It is sufficient to consider what takes place in a single

[graphic][subsumed][subsumed][subsumed][merged small]

piston (fig. 114). The piston P being first at the bottom of its stroke, on rising it raises the rod which traverses it, and therewith the valve a, which remains open during the ascent. The valve, t, which is in the piston, remains closed by the action of the spring and the pressure of the atmosphere, which acts in the barrel

« AnteriorContinuar »