THE earth which we inhabit is entirely enveloped, or surrounded, by a thin, transparent, and invisible fluid, called air. This air, together with the various gases, steams, vapors, and exhalations that are constantly thrown into it, and which form clouds, is called by the general name of the atmosphere. Consequently, atmospheric air is of a very mixed nature; but when pure, it is found, by chemical examination, to consist of two permanently elastic gases, or airs, called nitrogen

and oxygen, as we shall hereafter show in our chapter on Chemistry.

Air, though invisible, is a material substance, and partakes of all the properties which belong in common to other matter; for it occupies space, attracts and is attracted, and, consequently, has weight. It likewise partakes of the nature of fluids, for it adapts itself to the form of the vessel which contains it; and it presses equally in all directions; consequently, it must be considered as a material fluid. But, inasmuch as it is highly elastic, a property which is common to all gases, steams, and vapors, while the more visible and tangible fluids, such as water, oil, spirits, &c., possess this character in a very slight degree, if at all, so they require a separate examination.

The various airs, or gases, are called permanently elastic, because, under all changes which can be wrought upon them, they maintain their characters of fluidity and elasticity, and will not admit of being congealed, or rendered solid. With steams and vapors, the case is very different; for they arise from inelastic fluids by the application of heat, and they are highly elastic so long as they retain their form of vapor; but on being cooled, they return again to their original state of inelastic fluid, and therefore differ very materially from air, and cannot be said to be permanently elastic. Water affords a very good instance, for this is inelastic; but its steam is elastic in the highest degree; whenever this steam becomes cooled, it reverts back into its original state of water, and of course resumes all its former characters.

Since air has weight, and every thing upon the earth XIII.-11

is surrounded by it, it follows that all things must be subject to the pressure which will be exerted, not only upon them, but upon itself; and since air is elastic, or capable of yielding to pressure, so, of course, the lower part of the atmosphere will be more dense, or in a state of greater compression, than that which is above. Suppose, for example, that the whole height of the atmosphere is divided into 100 equal parts, and that each of these may weigh an ounce, or may be equivalent to the production of that pressure; then the earth, and all things upon its surface, will be pressed with the whole 100 ounces; the lowest stratum of air will be pressed by the 99 ounces above it, the next by 98, and so on till we arrive at the 99th stratum from the bottom, which will, of course, be subject to no more than one ounce of pressure.

Springs of metal, or wood, expand or contract, until they arrive at a state of equilibrium with the force that is acting upon them. The air acts in the same way; for, being of an elastic nature, it will, of course, yield to any force that may be impressed upon it, until its spring becomes a balance to that force. It is on this account alone that we are insensible of the air's pressure; for, notwithstanding the body of a man of ordinary stature is calculated to sustain no less a pressure of air than 32,400 pounds, yet the spring of the air contained within the body exactly balances, or counteracts, the pressure from without, and makes him insensible of the existence of any pressure at all. The spring and pressure of air will thus balance each other in all cases, except when the communication is cut off, and the natural equilibrium is destroyed by some disturbing cause.

The air-pump is the instrument that is generally used for the destruction of this equilibrium; for, by means of this machine, the air may be taken from the interior of vessels which are put upon its plate, and then the effects of the external and undisturbed air immediately begin to show themselves. Thus, for example, if a small glass receiver, which is open both above and below, be placed upon the plate of an airpump, and the palm of the hand be put upon it, so as to cover it completely, without leaving any orifice for the admission of the external air,- as soon as the pump is set in motion, the hand will be forcibly held down to the receiver, and cannot be released without difficulty; for the air within the glass being rarefied or diminished in quantity, that without will preponderate by its weight, which keeps the hand down, while the spring of that air which is contained in the hand will cause its lower side to swell, and enter the glass to a considerable depth. This shows the necessity of having all glasses, to be used upon the air-pump, with hemispherical or rounded tops, that they may present a dome, or arched form, to the pressure of the external air; and all such glasses are called by the general name of receivers. If an open-topped receiver be covered with a piece of flat glass, the pressure from without will break it.

If a small portion of the shell of an egg be broken away at the small end, and it is then placed under a receiver, and the air is exhausted, the bubble of air that is always contained in the large end will expand, and force out the contents of the egg. A withered apple, placed under a receiver, will expand, and appear fresh, provided its skin be not broken. That air is

contained in water appears plain from the following experiment: Place a tumbler of clear water, in which not a single bubble of air is visible, under a receiver, and then exhaust it; the water will instantly appear full of bubbles, which become large, and rise to the top; but as soon as the air is returned into the receiver, they are all instantly compressed, and disappear.

The ascent of water in a common pump is caused by atmospheric pressure; for the water in the pump being raised by the action of the upper pump-box, a vacuum is created below, which is immediately filled by the pressure of the air from without, which forces the water in the bottom of the well upward, to supply that vacuum. But, as equal weights must, of course, exactly balance each other, and as the weight of the atmosphere is limited, it is evident that only a column of water of a certain height can be raised by that weight. Accordingly, it has been found that water cannot be raised in a pump, by the mere pressure of the external air, higher than 32 or 33 feet; whence the inference is plain, that a column of water of this height is exactly equal in weight to that of the atmosphere on the same surface. The diameter of the column of water, in this case, is of no consequence; because, whatever it may be, an equal-sized column of air always acts against it.

This balance of power between a perpendicular column of water and atmospheric pressure was first observed by Galileo, in erecting a pump for the grand duke of Tuscany; but he appears not to have been aware of its cause. This was first investigated by Torricelli, who made use of quicksilver, a fluid 14