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PNEUMATICS.

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1. OUR knowledge of the atmosphere, as revealed to us, by the united aid of Chemistry and Pneumatics, is one of the greatest triumphs of modern science. was not until the close of the last century that air was removed from the list of ancient elements, and was proved to consist of two gases of essentially different chemical properties. Its physical properties were discovered earlier; but our present extensive knowledge of the atmosphere, considered chemically, physically, and meteorologically, is chiefly the fruit of modern investigation, the result of improved instrumental aid and methods of research. And it is natural to suppose that the working parts of so vast and complicated a machine as the atmosphere, which can be scarcely said to appeal to any one of our senses, should remain much longer unknown than the properties of solids. and liquids, which are so much more obvious. Even the heavenly bodies, although appealing to only one of the senses, and removed from us by vast distances, are calculated by their beauty and the regularity of their motions to attract attention, much more than an invisible attendant, which, although constantly present with us, performs most of its varied offices in silence and in secret.

The atmosphere or sphere of gases (Tu) is the

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general name applied to the whole gaseous portion of this planet; as the term ocean is applied to its liquid, and land to its solid portions. Being much lighter than either land or water, it necessarily floats or rests upon them; but unlike the ocean, which is confined to the depressions of the solid surface, and covers only about three-fourths of it, the atmosphere is in sufficient quantity to cover the highest mountains, and to rise to nine or ten times their height above the sea-level, so as to form a layer over the whole surface, averaging probably between forty and fifty miles in thickness; which is about as thick in proportion to the globe as the liquid layer adhering to an orange after it has been dipped in water. The accompanying figure (Fig. 1) will convey an idea of the proportion which the highest mountains bear to the curvature of the earth and the thickness of the atmosphere. The concentric lines divide the atmosphere into six layers, containing equal quantities of air, showing the great compression of the lower layers by the weight of those above them.

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3. The atmosphere consists essentially of two gases, called oxygen and nitrogen. In its pure state oxygen is remarkable for the energy with which it promotes combustion and respiration and other chemical changes. An iron or steel wire, heated to redness at one extremity, and plunged into a vessel full of this gas, will take fire and burn most brilliantly. If an animal be

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placed in pure oxygen, its pulses will throb with increased rapidity, and it will die from excess of vital action. The properties of nitrogen, on the contrary, appear to be of a negative character. It supports neither combustion nor respiration, and on the latter account it was formerly called azote*. It does not appear, however, to have any poisonous properties, like some other gases; an animal cannot live in it, simply from the absence of oxygen, not because nitrogen is in itself prejudicial: on the contrary, its uses in the atmosphere appear to be simply those of a dilutent; it subdues and modifies the activity of the oxygen, and in order to do this most effectually it exists in the atmosphere in much greater abundance than the oxy-gen. Every atom or particle of oxygen is accompanied by four atoms or particles of nitrogen; that is, if any measure or volume of air be separated or decomposed into its component parts, the nitrogen will occupy four times as much space as the oxygen. It is, however, an important property of gases to mingle or become diffused together so intimately, that, although oxygen is somewhat heavier than nitrogen, these two gases always exist in the atmosphere in the proportion of 1 to 4, and this without any regard to the localities from which samples of the air may be taken. In the crowded courts of our metropolis, as well as in the breezy downs of the country, in the arid deserts of Arabia, in the open ocean, in the polar regions, at heights accessible only to the balloon, in the fever hospital, and in the flower garden, the proportion of these two essential ingredients of the atmosphere remain constantly the same.

And this fact will appear the more wonderful when we consider the innumerable sources of vitiation tend* From a wn, privative of life.

ing to destroy the useful properties of the oxygen by depriving it of its powers to support life and combustion. Indeed these two very actions are the chief means of vitiating it, for during the combustion of our lamps, and candles, and fuel, and during the respiration of animals, a quantity of carbon is liberated, every six parts of which (by weight) unite with 16 parts by weight (or about 3600 times their own bulk) of oxygen to form a compound gas called Carbonic Acid, which resembles nitrogen in not supporting animal life or combustion; but it differs from nitrogen in being soluble in water, having an acid reaction, and some other characteristic properties which readily distinguish it from that inert element. Carbonic acid is always present in the atmosphere in small but varying quantities; and it is because the quantity is liable to change that this gas is not reckoned as one of the essential constituents of the atmosphere. In 10,000 volumes or measures of atmospheric air the mean proportion of carbonic acid is only five volumes; the proportion, however, is subject to constant variation, from 6.2 as a maximum to 3.7 as a minimum. Near the surface of the earth the proportion of carbonic acid is greater in summer than in winter, and during night than during day. It is also rather more abundant in elevated situations, as on the summits of high mountains, than in the plains; and although this gas is considerably heavier than its own bulk of pure atmospheric air (its specific gravity being about 1.52), yet it appears to be diffused through the whole mass.

When we consider that the innumerable animals which inhabit both the land and the water all depend more or less upon oxygen for their very existence; that combustion of various kinds, as carried on in our daily operations, that fermentation and other processes

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all consume immense quantities of oxygen, i. e. convert it into steam and carbonic acid, chiefly the latter, it does indeed appear wonderful that the quantity of carbonic acid in the atmosphere should be so small. By a beautiful arrangement, however, the carbonic acid thus formed is made to be the food of the vegetable world. The green parts of plants, under the influence of light, absorb carbonic acid, retain and assimilate the carbon, and restore the pure oxygen to the air. During the night, however, plants absorb oxygen and give out carbonic acid; but in the course of the twentyfour hours they give out considerably more oxygen than they consume, and in this way a compensation is made for the loss of oxygen occasioned by respiration and combustion.

The atmosphere also contains a variable quantity of aqueous vapour, arising partly from combustion as above mentioned, but chiefly from contact with the surface of the sea, lakes, rivers, and moist soil. In 100 parts by weight of atmospheric air the mean quantity of watery vapour is nearly one part and a half. The amount, however, varies according to the temperature. At 50° (the mean temperature of England) the air can contain th of its weight of water in an invisible state, without forming cloud, mist, or rain. It does not always contain so much, but the quantity cannot exceed this, without a portion being precipitated in a visible form. At a higher temperature, however, more vapour could remain invisible; thus at 82° (the mean temperature of the equator) the air may contain as much as 3rd of its weight of invisible steam; and air that contained onlyth would be injuriously dry, though the same air cooled down to 50° would be at its maximum of humidity.

There are other accidental ingredients in the atmo

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