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Boyle's Law.

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Such an enormous

body is 37,560 pounds, or upwards of 16 tons. pressure might seem impossible to be borne; but it must be remembered that in all directions there are equal and contrary pressures which counterbalance one another. It might also be supposed that the effect of this force, acting in all directions, would be to press the body together and crush it. But the solid parts of the skeleton could resist a far greater pressure; and as to the liquids contained in the organs and vessels, from what has been said about liquids, (74) it is clear that they are virtually incompressible. The gases, too, are compressed by the weight of the atmosphere; but they resist it in virtue of their elasticity. They are, in short, like a bottle full of air. The sides of the latter are pressed in by the weight of the atmosphere; but they can stand this, however thin their walls, for the pressure of the gas from within quite counterbalances that which presses externally.

The following experiment (fig. 105) illustrates the effect of atmospheric pressure on the human body. A glass vessel, open at both ends, being placed on the plate of the machine, the upper end of the cylinder is closed by the hand, and a vacuum is made. The hand Fig. 105. then becomes pressed by the weight of the atmosphere, and can only be taken away by a great effort. And as the elasticity of the gas contained in the organs is not counterbalanced by the weight of the atmosphere, the palm of the hand swells, and blood tends to escape from the pores.

The operation of cupping in medicine is an application of the effect of removing the atmospheric pressure from the human body.

CHAPTER II.

MEASUREMENT OF THE ELASTIC FORCE OF GASES.

132. Boyle's law. The law of the compressibility of gases was discovered by Boyle, and subsequently, though independently, by Mariotte. In consequence it is in England commonly called

Boyle's law, and, on the Continent, Mariotte's law. It is as follows: 'The temperature remaining the same, the volume of a given quantity of gas is inversely as the pressure which it bears.'

This law is verified by means of an apparatus called Mariotte's tube (fig. 106). It consists of a long glass tube fixed to a vertical

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support it is open at the upper part;

Fig. 107.

and the other end, which is bent into a short vertical leg, is closed. On the shorter leg there is a scale, which indicates equal capacities; the scale against the long leg gives the heights. The zero in both scales is in the same horizontal line.

A small quantity of mercury is poured into the tube, so that its level

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Boyle's Law.

119 in both branches is at zero, which is effected without much difficulty. The air in the short leg is thus under the ordinary atmospheric pressure. If mercury is then poured into the longer tube the volume of the air in the smaller tube is gradually reduced. If this be continued until it is only one-half, that is, until it is reduced from Jo to 5, as shown in figure 107, and if the height of the mercurial column, CA, be now measured, it will be found exactly equal to the height of the barometer at the time of the experiment. The pressure of the column CA is therefore equal to an atmosphere, which, with the atmospheric pressure act

ing on the surface of the column at C, makes two atmospheres. Accordingly, by doubling the pressure, the volume of the gas has been diminished to one-half.

If mercury be poured into the longer branch until the volume of the air is reduced to one-third its original volume, it will be found that the distance between the level of the two tubes is equal to two barometric columns. The pressure is now three atmospheres, while the volume is reduced to one-third. Dulong and Petit have verified the law for air up to 27 atmospheres, by means of an apparatus analogous to that which has been described.

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The law also holds good in the case of pressures of less than one atmosphere. To establish this, mercury is poured into a graduated tube, until it is about two-thirds full, the rest being air. It is then inverted in a deep trough containing mercury (fig. 108), and lowered until the levels of the mercury inside and outside the tube are the same, and the volume AB, which is then under a pressure of one atmosphere, is noted. The tube is then raised, as represented in fig. 109, until the volume of the air, AC, is doubled. The height of the

Fig. 108.

Fig. 109.

mercury in the tube, above the mercury in the trough, is then found to be exactly half the height of the barometric column. Accordingly, for half the pressure the volume has been doubled.

In the experiment with Mariotte's tube, as the quantity of air remains the same, its density must obviously increase as its volume diminishes, and vice versâ. The law may thus be enunciated : 'For the same temperature the density of a gas is proportional to its pressure. Hence, as water is 770 times as heavy as air, under a pressure of 770 atmospheres, air would be as dense as water.

Until within the last few years Boyle's law was supposed to be absolutely true for all gases at all pressures; but several physicists have since observed that the gas is not rigorously exact, especially in the case of those gases which can be liquefied. They are more compressed than is required by the law. For air, Dulong and Arago investigated the pressure up to 27 atmospheres, and observed that the volume of air always diminished a little more than is required by Boyle and Mariotte's law. But, as these differences were very small, they attributed them to errors of observation, and concluded that the law was perfectly exact, at any rate up to 27 atmospheres.

For ordinary pressures Boyle's law may be assumed to be sufficiently near for all gases.

133. Manometers.—Manometers are instruments for measuring the elastic force of gases or vapours. In all manometers the unit chosen is the pressure of one atmosphere, or 30 inches of mercury at the standard temperature, which, as we have seen, is nearly 15lbs. to the square inch. The open air manometer is represented in fig. 110 fixed against a board fastened to a wall, and connected with a steam boiler. It consists of a glass tube about 20 feet in height, open at the top, and fixed at the other end to a glass bath C, containing mercury. A long tube connects this with the boiler.

When the elastic force of the vapour in the boiler is equal to the pressure of the atmosphere, it will counterpoise the weight of the atmosphere which is transmitted through the tube, and the level of the mercury is then the same in the tube and in the bath. At this level the number I is marked on the board. Then since a column of mercury 30 inches in height represents a pressure of an atmosphere, the number 2 is marked at this height above I ; at a height of 30 inches above this the number 3 is marked, and so on, each interval of 30 inches representing an atmosphere. Thus, for instance, if the mercury had been forced up to 33, as represented in

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Manometers.

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the drawing, that would indicate that the tension of the vapour in the boiler is 3 atmospheres; so that, on each square inch of the internal surface of the boiler, there is a pressure of 3 x 15 pounds, or 52 pounds.

The manometer with compressed air is founded on Mariotte's law;

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it consists of a glass tube closed at the top (fig. 111), and filled with dry air. It is firmly cemented in a small bath containing mercury. By a tubulure, this bath is connected with the closed

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