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AIR.

(From the Museum of Science and Art,' by Dr. Lardner.)

os-cil-la'-tion, a moving backward and in-teg'-u-ment, anything that covers or

forward, vibration

ve-lo'-ci-ty, swiftness, prop-a-ga'-ted, spread

speed

stra'-tum, plu. stra'-ta (Lat.), a bed, a layer

at-ten'-u-ate, to make thin or slender

envelops another

de-vas-ta'-tion, ruin, desolation
mo'-bile, that may be easily moved
vas'-cu-lar, consisting of vessels
di-late', to enlarge, to widen

Of all common things, air is the most common. No space or place is accessible to us that is not filled with it. It is of all material wants that which is most incessantly indispensable to our existence. Food is an occasional want; an intermitting supply is all that is needed. Clothing may in certain cases be dispensed with, and habit may inure us to a deficiency of it. The want of warmth must be extreme to become fatal. But the privation of air, even for a brief interval, is attended with instant and certain death. Unlike other natural wants, our consumption of air is not voluntary. The action of the lungs is like the oscillations of a pendulum. It is incessant: sleeping or waking, in sickness or in health; sitting, standing, or moving, it is maintained with a regularity and continuity quite independent of the will. Its suspension is the suspension of life. Must we not then be prompted by a natural and irresistible curiosity to obtain some acquaintance with a physical agent so universal, so omnipresent, and so indispensable to our vitality? Air is the transparent, colourless, invisible, light, and attenuated fluid with which we are always surrounded. It is drawn into our lungs by the action called suction, and after remaining a moment there, is forced out through the mouth and nose by the muscular compression of the chest. This alternate action, by which the air enters and leaves the lungs, is called respiration. During the moment it remains in the lungs, it undergoes a certain change, which we shall presently explain, in consequence of which, when expired, it is not the same as that which was inspired. The effect produced on the blood by this change is essential to the maintenance of life.

The air which, thus changed, is expired, is unfit for respiration. If, therefore, the same air be taken several times successively into the lungs, death must ensue.

The air around us, therefore, requires to be continually changed, that which we expire being carried away and replaced by fresh and pure air. The apparent lightness of air, the freedom with which we move through it, and its invisibility,

led the ancients to imagine that it was unsubstantial and immaterial, and hence the disembodied souls of the dead came to be called spirits, from the word spiritus, which signifies air.

It is a great mistake, however, to imagine that air is destitute of weight, that quality which is inseparable from whatever is material. Light it undoubtedly is, but only by comparison. Bulk for bulk it is lighter than stone, earth, or water, or any other substance in the solid or liquid state. But light as it is, it has a certain definite weight, and a quantity of it can be assigned which will weigh many tons. The pressure produced by its weight is under certain assignable circumstances quite enormous, and when it is moved with a certain velocity its force is so irresistible that trees are torn by it from their roots, the most solid buildings overturned and reduced to ruins, and devastation spread over vast tracts of country.

When it is considered that the mass of air which taken collectively is called the atmosphere, extends above us to the height of more than fifty miles, it will easily be imagined that the weight with which it presses on the surface of every object exposed to it must be very considerable. If, for example, we take a square inch of level surface, it is clear that that square inch must bear the weight of a column of air extending from the surface to the top of the atmosphere. It has been ascertained by experiments, susceptible of the greatest precision, that this pressure or weight amounts to about 15 lbs., and that it is subject, from time to time, to a variation not exceeding threequarters of a pound.

It is a well-known property of fluids, that any pressure which they exert acts equally in all possible directions. Thus, if any body be let down into the sea, the weight of the water which is above it will press equally on its top, bottom, and sides. It is very easy to demonstrate this by a simple experi

ment.

Let several empty bottles be carefully corked, and being loaded with weights so as to sink in the water, the neck of one being presented upwards, that of another downwards, another horizontal, and the others oblique in various degrees, it will be found that when they have sunk to a certain depth, the corks will be all forced into the bottles by the pressure of the surrounding water, with which the bottles will be immediately filled, and this will take place equally, and at the same time, with all the bottles, in whatever direction the corks may be presented to the water.

It is evident, therefore, that the pressure produced by the weight of the incumbent column of water at any given depth is equally propagated in all directions, and that a body, a fish

for example, or body of a diver, sustains that pressure, not downwards only, or on the upper surface of the body, as might be at first imagined, but equally on the under surface, the sides, and, in a word, on every part of the body in contact with the water.

Now, this equal transmission or propagation of pressure in all directions is not an exclusive property of water, but is common to all substances whatever in the fluid state. Air possesses fluidity in even a greater degree, if possible, than water, being more freely mobile, and air accordingly transmits freely and without diminution in all directions whatever any pressure which it receives. The stratum of air in which we live is under the pressure, as it has just been stated, of the incumbent column of air extending upwards to the limits of the atmosphere, this pressure amounting to 15 lbs. on each square inch. A body, therefore, exposed to the contact of this air is subject at all parts of its surface, upper, under, and lateral, to this pressure; and the total amount of the pressure by which it is affected will be expressed in pounds weight by the number obtained by multiplying the number of square inches in its entire surface by 15.

The body of a man of average size has a surface of about 2,000 square inches. The total pressure which it sustains from the surrounding air is therefore 15×2,000, or 30,000 lbs., or nearly fourteen tons!

It may seem wonderful that a force so enormous, acting on all parts of the surface of the body, should not crush it and actually destroy its delicately constructed organs. This, however, is prevented by the perfect equilibrium of pressure outwards and inwards, produced by the property of fluids just explained, in virtue of which they transmit freely, and undiminished, the pressure in all directions. The fluids which fill the entire vascular system are exposed, as well as the surface of the body, to the pressure of the atmosphere, which enters the lungs and all the cavities and open parts of the organs; these fluids transmit that pressure to all the inner parts of the body, so that the skin and integuments are pressed by them outwards by a force exactly equal to that with which the air presses the external surface of the skin inwards. These outward and inward pressures are necessarily always equal, because, in fact, they are one and the same pressure, i.e., that of the air, the pressure on the external surface acting inwards, being the immediate action of the air, and the pressure of the internal fluids acting outwards, being the same pressure of the air transmitted by those fluids to the inside of the skin and integuments.

That this outward pressure, transmitted by the fluids which fill the organs under the skin, is really at all times in operation,

and that it is only counteracted by the immediate pressure of the external air upon the skin, is rendered conspicuously manifest in the well-known surgical operation of cupping. In that process the open mouth of the cupping-glass being pressed upon the skin so as to exclude all communication with the external air, the air within the cup is withdrawn, or partially withdrawn, by means of a syringe attached to the glass. The moment the skin within the glass is relieved from even a small part of the pressure of the external air by this means, the outward pressure of the fluids under the skin begins to take effect, being no longer resisted; it swells up the skin within the glass, and when the skin thus dilated is punctured with the lancet, the blood is propelled from it by the force of the pressure of the fluids under the skin acting outwards.

Air has another characteristic and highly important quality called elasticity, which, like its compressibility, is unlimited

and uniform.

The capability of swelling without limit into augmented dimensions when relieved from the conditions which confine it is called elasticity. Like compressibility, it is a characteristic property in which no other form of matter participates. Liquids are for all practical purposes inelastic. Some solid bodies possess a certain elasticity, but not at all identical in its character or laws with the elasticity of air above described.

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Air in its natural and usual state has an elastic force of 15 lbs. per square inch, so that when it is shut up in any vessel or other envelope, and cut off from all communication with the external air, it will press on every square inch of the inner surface of such envelope with a force of 15 lbs. The stratum of air which rests on the surface of the earth, and in which the organised tribes that inhabit the earth. live, derives its pressure, elasticity, and density from the weight of the whole mass of the atmosphere which rests upon it. It must, therefore, be evident that if we ascend to greater elevations, leaving below us a certain stratum of the atmosphere, and having above us a proportionally less quantity of air, the weight of the incumbent air being less, the pressure, elasticity, and density of the stratum. which surrounds us will be proportionally less. And we find this actually to be the case. At great heights on mountain chains, such as the Pyrenees or the Alps, the air is very sensibly rarefied. It is lighter, and exercises a much less pressure. In like manner, persons who ascend to great elevations in balloons find much inconvenience from the thinness of the air. The fluids confined within the body are much less resisted, certain organs

become dilated, and the effect of a cupping-glass is occasionally produced, attended with bleeding at the nose and singing in the ears.

The ancients imagined that air was a simple substance which entered more or less into the composition of bodies in general, and hence they called it one of the elements - - the others being, in their theory of physics, water, earth, and fire. Better informed now, we know that neither air, water, nor earth, are simple or elementary substances, and that fire is not a substance at all, but a physical effect due to the sudden and large production of heat which attends the chemical combination of certain substances. Thus the ancient elements are not elements at all.

Air-meaning by that term the air of the atmosphere, the air we breathe, the air through which we behold the firmament, the air whose currents carry our commerce over the ocean from land to land-is a compound or mixture made up of two extremely different kinds of air.

As there are many sorts of air having extremely different qualities and properties, although they are alike in appearance, being all invisible, transparent, colourless, light, compressible, and elastic, it has been found convenient to call them by the general name gas (derived from the Saxon word gast), and to limit the application of the term 'air' to that particular compound or mixture of gases which constitutes the atmosphere.

The erroneous notion that air was a simple and elementary substance prevailed until the close of the last century, when Lavoisier, the celebrated French philosopher, who was one of the most illustrious of the founders of modern chemistry, showed that it was a mixture of two different gases in definite proportions, called oxygen, and azote or nitrogen. A hundred cubic inches of air is a mixture consisting of 80 cubic inches of azote, and 20 of oxygen. The result of the most exact analyses differs from this proportion by a minute fraction, which, though not unimportant in certain respects, need not here embarrass the reader, who will do well to fix in his memory this proportion of 80 to 20.

COMPOUND INTEREST.

(1) What is the compound interest on £97 for 31 years at 4 per cent. half-yearly?

(2) What is the compound interest on £450 for 3 years at 6 per

cent. ?

(3) What will £500 amount to in 5 years at 5 per cent., compound

interest?

(4) What is the compound interest on £100 for 6 years at 5 per

cent.?

(5) What will £54 amount to in 4 years at £2 10s. per cent. per half-year?

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