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these matters at once, and the air is all the clearer afterwards.

It will be necessary, in a future page, to advert to the uses of the Trade Winds in carrying forward and preserving the purity of the atmosphere on the whole. They also subserve another and scarcely less important function—that of preserving in a sort of equilibrium the temperature of the atmosphere, and to some extent of the regions over which their influence is felt. The tropics are thus the perpetual sources of enormous floods of warm air, which, rising up and flowing over, proceed ultimately to the poles, and in all probability exert a very considerable modifying influence over the severity of the arctic regions, as well as over those which they traverse on their way thither.

How wonderful is the unity and dependence of creation! The movement even of a breath of air is not without its purpose and its end. Let this soft and scarcely sensible current, which as we talk of these things here salutes our cheek, when we are reminded of all the marvels of its origin, and of the intentions for which it has been put in motion, waft our praises to Him whose divine power and love reveals itself even in a summer wind!

CHAPTER VI.

THE ATMOSPHERE AND ANIMALS.

CONSIDERING that all animate and inanimate bodies alike are immersed in a sea of gaseous fluids, which possess affinities of a powerful kind for the different elements of the organic and inorganic kingdoms, it will not surprise the reader to learn that important chemical functions are perpetually discharged by the balmy and apparently inert air which fans his brow or cools his cheek. To these chemical relations of the air we are now to draw attention; and in so doing we shall first direct the consideration to the chemistry of one of the most important functions of the animal framenamely, the function of breathing, or respiration.

When we expire the air we had previously taken into the lungs-no matter whether from the mountain-ridge, or in the less healthful atmosphere of a crowded town-an important alteration in its chemical composition has taken place. We are not conscious of this fact; but it is one which may be readily proved by the

simplest means. If a vessel is filled with water in which some fresh-burnt lime has been slaked, and the water decanted off clear-which is now what is commonly called lime-water—and if then, taking a glass tube, we breathe some of the air we are expiring from the lungs through it, we shall have rapid evidence that a change of some kind has taken place in this air in the altered

Air,

appearance of the pre-
viously clear and pellucid
fluid. It now becomes
quickly turbid and milky,
and eventually deposits a
whitish sediment.
in its ordinary condition,
would not produce this
decomposition, whatever
it may be; for the li-
quid remains unclouded,
though a large volume of
air be passed through it
Therefore

by a bellows.

the air we take into the lungs has this striking difference from that we expire from them, that while it produces no alteration in the colour or composition of limewater, the latter decomposes it and renders it turbid. It will be interesting now to inquire -What is the nature of this difference?

CHEMISTRY OF BREATHING.

335

The white precipitate is carbonate of lime, an earth formed by the union of carbonic acid gas with lime. From analyses already given of the atmospheric composition, carbonic acid has been found to be invariably present in air, and therefore it might be said this precipitate indicates nothing more than what might have been expected. And it is true that lime-water, after being exposed for any length of time to the air, is decomposed, and carbonate of lime falls to the bottom of the vessel. But in the simple experiments above-mentioned this difficulty disappears, for it will be found that the bellows must be moved all day long to get a sensible precipitation, whereas a single expiration of air from the lungs will instantly render the fluid turbid. Although, therefore, it is certain that a minute portion of carbonic acid exists in all air, it is, on the other hand, equally certain that there is an enormous disproportion in the quantities contained in ordinary and in expired air. In the one the amount is merely fractional; in the other it is present to a large per centage. The air which has done duty in the lungs receives a large amount of carbonic acid gas as it leaves them.

Let us now enter upon another range of thought. In cases of disease it is very often considered necessary to open a vein, and to

withdraw variable quantities of the blood circulating in the system. The fluid thus derived is always of a dark colour, and sometimes is almost black. But occasionally disease calls for the opening of an artery, and then the most striking difference is perceptible in the appearance of the blood; for it is of a vivid bright scarlet hue. If the dark venous blood is exposed for some little time to the fresh air, it loses its dark colour, and assumes the lighter aspect of arterial blood: but it still differs from arterial blood in many important particulars. This change is directly attributable to the influence of air, for it would not take place in a vacuum. If a moist piece of bladder were laid over the fluid, it would not prevent the change from dark to red; and it is known to physiologists, that when dark blood becomes circulated in an organised living structure over a large surface, upon which alternate currents of fresh air play, the mere circumstance that air is not brought into direct contact with blood does not interfere with its chemical effects on that fluid. Direct contact with air is not therefore necessary to effect the change, since it will take place very readily through the medium of an interposed animal membrane. This is, in part, due to the laws of the interpenetration or diffusion of gases, and in part to the remarkable forces called

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