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In liquids the attraction of cohesion is so weak, that the particles glide over each other with the greatest ease, and instantly mould themselves to the form of the vessel in which they may be contained. Thus they would appear to have no cohesive force; but that they have some is evident by the coherence of every drop of water. In airs, gases, and vapours, however, this cohesive attraction is altogether absent. The gaseous particles not only have no cohesive attraction, but, on the contrary, a powerful repulsion, whereby they are constantly endeavouring to separate themselves as far as possible from each other. It is this repulsive force which constitutes the elasticity of aeriform bodies, of which we have now to speak.

A thin fragile vessel of any size is not crushed by the pressure of the atmosphere, on account of its perfect equability, the external pressure being exactly counterbalanced by the internal. We have only to remove one of these pressures, and we shall witness the energy with which either of these forces act when unrestrained by the other. If the neck of a square glass vessel be screwed into an exhausting syringe, and the internal air removed, it will be crushed into small fragments by the external atmospheric pressure. So, on the contrary, if a similar vessel be carefully closed at the neck and placed under the receiver of an airpump, on removing the external pressure the vessel will be blown to pieces by the pressure or elastic force of the air inclosed within the vessel.

We see, from this last experiment, that a portion of air cut off from all communication with the atmosphere still exerts a pressure in all directions against the sides of the vessel containing it, and it can be proved that this internal pressure is exactly equal to the pressure which an equal surface undergoes from the weight of

ELASTICITY OF AIR.

17

the external atmosphere. But this internal pressure cannot arise from the weight of the included air (for this is only a few grains); it must, therefore, arise from its elasticity, or expansive force; that is to say, the force with which it tends to expand to its natural bulk, or that bulk which it would occupy if subject to no pressure; if, for example, it were removed to the top of the atmosphere.

9. The elasticity of air and the law by which it is regulated can be very well illustrated by means of a long bent glass tube, Fig. 4, open at its longer extremity, and furnished with a stop-cock at the shorter.

The stop-cock being open, a quantity of mercury is poured into the open end. The surfaces of the mercury A a will of course stand at the same level in both legs. The two columns of air a c and a D sustain a pressure equal to the weight of a column of air continued from A and a to the top of the atmosphere. If we now close the stop-cock D, the effect of the weight of the whole atmosphere above that point is cut off, so that the surface a can sustain no pressure arising from the weight of the atmosphere. Still the level of the mercury remains the same, because the elasticity of the column of air a D is precisely equal to the weight of the whole column before this small length was cut off. The sur

B

Fig. 4.

-c

1-b

face a is still pressed by the whole atmospheric column, and thus we see that these two different properties of the atmosphere, its elasticity and its weight, exactly counterbalance each other.

Now we know that the atmospheric pressure under ordinary circumstances is equal to 14 lbs. on the square inch, or to a column of mercury 30 inches high. It is evident, therefore, that the atmospheric pressure acting on A is the same as would be produced by a column of mercury 30 inches high resting on the surface A. So, also, the force with which the air confined in a D presses by its elasticity on the surface a is also equal to a column of mercury 30 inches high. The pressure of the atmosphere acting on the surface a is transmitted by the mercury to the surface a, and balances the elastic force of the isolated column a D.

A

If we now pour an additional quantity of mercury into the open end of the tube at c, an increased pressure arising from the weight of the metal will be transmitted to the surface a, and will prevail over the elasticity of the confined air; the surface a will, therefore, rise towards D, compressing the air into smaller space. On continuing to pour in mercury until the surface a rise to b, or half way between a and D, that is, until the confined air is compressed into exactly half its former limits, it will be found, on drawing a horizontal line from the surface b to the opposite point b' in the longer limb, that the column of mercury b' в measures exactly 30 inches, the weight of which is equal to the atmospheric pressure. The force with which the surface b is pressed upwards towards D is, therefore, equal to two atmospheres, or double the force with which a was pressed upwards towards D. Hence it appears that the elasticity of the confined column of air, b D, is double its former elasticity when filling the space a D, so that when the air is compressed into half its volume its elasticity is doubled. If we again pour mercury into the tube at c until the air inclosed in the shorter limb be reduced to a third of its bulk, as

LAW OF MARIOTTE.

19

at c D, the compressing force will be equal to three times the atmospheric pressure. The height of the compressing column of mercury would reach to c, namely, 60 inches above the level c. If we still add mercury until the column rises to the height of 90 inches above its level in the short limb, the elastic force of the confined air would be four times greater than at first, and it would be compressed to the bulk of one-fourth of its original volume.

It appears, then, that the elastic force of air varies in exactly the same proportion as its density; and this simple and important law, which is called, after its discoverer, the law of Mariotte, applies not only to air, but to all gaseous bodies when subject to such variations of pressure as can be readily commanded. Air has been allowed to expand into more than 2000 times its usual bulk, and it would have expanded still more if a greater space had been allowed. Air has also been compressed into less than a thousandth of its usual bulk, so as to become denser than water; but its elasticity has not been exactly determined at these extreme degrees, either of condensation or rarefaction, so that we have no proof that the law of Mariotte applies so extensively. On the contrary, recent experiments on the compression of gases render it nearly certain that they all vary from this law when subject to very great pressure, their density being increased in a greater ratio than their elasticity; this variation, however, is less in air than in most other gaseous bodies, and the simple law is found to apply to it very accurately when condensed as much as 50 times, and also when allowed to expand to several times its usual bulk.

10. The principle of those useful instruments, the exhausting syringe and the air-pump, depends upon

Fig. 5.

the elasticity of the air. The exhausting syringe, Fig. 5, consists of a cylinder of brass or some other metal, with a piston or plug accurately fitting it. The lower part of the cylinder contains two valves or little doors, the one opening upwards into the cylinder, and the other at the side, out into the air. The vessel to be exhausted is screwed into a short tube projecting from the cylinder. This vessel must be furnished with a stopcock, to prevent the air from re-entering after the exhaustion is complete. Now, suppose the vessel to be screwed into the short tube, its stop-cock open and the piston at the bottom of the cylinder. Supposing we drew up the piston instantaneously, or in no time, a vacuum or empty space must evidently be left between the bottom of the cylinder and the piston. Consequently the air in the vessel, no longer being counterbalanced by the atmospheric pressure, expands by its elasticity, forces open the valve a, and fills the empty space below the piston. When the piston is drawn progressively to the top of the cylinder, no vacuum is formed, the air from the vessel expanding and following it all the way. After this the piston is forcibly driven down again, whereby the valve a is closed, and bis opened; the whole of the air in the cylinder is thus driven out through b, and when the piston is at the bottom of the cylinder matters are in the same state as at the commencement of the operation, except that the air in the vessel is much less dense and elastic than before. On drawing up the piston a second time,

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