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yet determined. The beginning and ending of twilight indeed show, that the height at which the atmosphere begins to refract the rays of the sun is about forty-five miles. But this, perhaps, may be only the height to which the aqueous vapours are carried: for it cannot be thought an unreasonable supposition, that light is refracted only by means of the aqueous vapour contained in the atmosphere; and that, where this ceases, it is still capable of supporting the electric fire at least, as bright and as strong as at the surface. That it does extend much higher, is evident from the meteors already mentioned; for these are undoubtedly carried along with the atmosphere; otherwise that of 1783, which was seen for about a minute, must have been left 1000 miles to the westward, by the earth flying out below it, in its annual course round the sun.

The preceding account is perfectly compatible with the chemical theories which prevailed at the time the Contemplative Philosopher' was originally published: but there has been a wonderful revolution in this department of science in the course of the last twenty years; and it therefore becomes necessary to advert, though very briefly, to the opinions and principles which now occupy the place of those which are developed in this paper.

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The phlogistic theory is now almost entirely abandoned; and not only a new theory, but a new and very ingenious nomenclature adopted. But we must here confine ourselves to the composition of atmospheric air, or, of that transparent, colourless fluid, which invests the terraqueous globe, and possesses permanent elasticity and gravity. This fluid is composed of 78 parts of nitrogen and

74 nitrogen and 26 oxygen; and is soluble in about 30 times its bulk in water: 100 cubic inches weigh 31 grains. On the surface of the earth it is compressed by the weight of the superincumbent atmosphere; its density therefore diminishes according to its height above the earth. It is dilatable by heat; so that at 60° of temperature, its bulk is increased about part.

The constituent principles of atmospheric air are rendered evident by the following experiment : Quicksilver being inclosed in a proper vessel of atmospheric air, on heat being applied, the air will be diminished, and the quicksilver lose its splendour, and gradually change to a reddish powder; acquiring, at the same time, an augmentation of weight. When neither the air nor the quicksilver suffers any farther change, the separation of the principles has taken place: the one, the gas remaining in the receiver, is now unfit for supporting flame, or maintaining respiration, and is nitrogen gas; the other is absorbed by the quicksilver, while reducing to the state of an oxide, and may be extricated from it on the application of heat: when the powder, to which the quicksilver is reduced, will be restored to its metallic state, but will have lost the weight it had gained during its oxidation; this deficiency being exactly equal to the weight of the evolved gas, which is oxygen gas.

These separated gases, thus differing in their properties from each other and from atmospheric air, being again mixed, form atmospheric air of the ordinary degree of purity. It must, however, be acknowledged, that in thus forming respirable air, an aëriform fluid is obtained, differing in some trifling respects from the ordinary air of the atmosphere. M. Humboldt is of opinion, that the composition of atmospheric air may so vary,

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that the oxygen may exist in it in the proportion of from 0,23, even to 0,29.

He also supposes, that our not being able to form an aëriform fluid, perfectly similar to that of the atmosphere, does not proceed from our ignorance of the quantity or quality of the gaseous bases, but from a difference in their union: that in the atmosphere they may be considered as in a state of chemical combination, but, in the artificial, merely as a mixture.

Respiration and combustion depending on the presence of oxygen, these processes will always be affected by the proportion in which the oxygen gas exists in the air in which they are performed. The atmosphere also contains foreign matters, such as other gaseous bodies, water which it holds in solution, minute detached particles of bodies, &c. It commonly contains about 0,01; or, according to Humboldt, from 0,005 to 0,01 of carbonic acid gas.

Sir H. Davy states, that the atmospheric air differs very little in the proportion of its ingredients in different parts of the world, that of Europe, Asia, Africa, and America, being all found to contain 0,22 of oxygen in volume.

Mr. Dalton considers the general atmosphere as composed of four fluids principally, or four particular atmospheres: nitrogen gas, oxygen gas, aqueous vapour, and carbonic acid gas, mechanically mixed. These, he supposes, to be totally unconnected with each other; the particles of the one not acting on the particles of the other: but this opinion is not generally adopted.

Observation has long shown, that the air contains water; but philosophers have been divided, respecting whether the water is held in the air in a state of solution, or in the state of steam or va

on this subject, appear to be almost decisive in favour of this latter opinion. Water, in an exhausted receiver, is diminished even faster than in open air, its disappearance being here proved to proceed from its conversion into vapour; hence it is probable that it is converted into a state of vapour, and exists so in the open air. But the strongest proof is, that as Mr. Dalton has shown that water in the atmosphere possesses the same degree of elasticity which it does when in a state of vapour, in a state of vacuum, at the same temperature, we may conclude, safely, from these observations, that almost all the water contained in the atmosphere is in a state of vapour; and, most probably, mechanically mixed with it. A certain portion of moisture seems, indeed, to be essentially necessary to a due constitution of the air. The atmosphere, therefore, seems to consist of three elastic fluids, in the following proportion: air, 98; carbonic acid, 1; water, 1.

No. XIV.

ON THE MECHANICAL PROPERTIES AND
EFFECTS OF THE AIR,

And the beneficial Ends to which it is subservient.

Mundi pars est aër, et quidem necessaria: hic est enim qui coelum terramque connectit.

SENECA.

Ipse aër nobiscum videt, nobiscum audit, nobiscum sonat, nihil enim eorum sine eo fieri potest.

CICERO.

IN my former paper, I took a cursory review of the principal modern discoveries respecting the

different kinds of air: I stated the most generally received opinions, in consequence of the component parts of our atmosphere; and entered into some discussion of the height to which philosophers have supposed it to extend. The mechanical properties and effects of this wonderful fluid, with the variety of beneficial ends to which it is subservient, are subjects no less worthy of consideration.

The most considerable properties of air are its fluidity, weight, and elasticity.

That the air is a fluid is evident from the easy passage through it which it affords to bodies; as in the propagation of smells and other effluvia, and the easy conveyance it affords to sounds. These and similar effects prove it to be a body, whose parts give way to any force impressed, and, in yielding, are easily moved among themselves; which is the definition of a fluid. Besides, it is certain that no degree of cold has ever yet been been produced, natural or artificial, strong enough to deprive it of its fluidity. The cause of this fluidity of the air is attributed, by some late philosophers, to the fire intermixed with it; without which, they imagine, the atmosphere would harden into a solid, impenetrable mass.

The weight, or gravity, of the air was first discovered by Galileo; but the pressure of the atmosphere was observed by his disciple Torricelli, and the variations of it, depending on different heights, by M. Pascal. That the air is heavy, follows from its being a body; weight being an essential property of matter. But we have many arguments to the same purpose from sense and experiment: thus, the hand, applied to the orifice of a vessel empty of air, soon feels the load of the

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