ther it consists of particles infinitely divisible or not. On the latter hypothesis, it must really be finite, and even if its particles be infinitely divisible, it is known, by experience, to be of extreme tenuity at very small heights. The barometer rises in proportion to the superincumbent pressure. At the level of the sea, in the latitude of 45°, and at the temperature of melting ice, the mean height of the barometer being 29-922 inches, the density of air is to the density of a similar volume of mercury, as 1 to 10477-9, consequently the height of the atmosphere, supposed to be of uniform density, would be about 4-95 miles; but as the density decreases upwards in geometrical progression, it is considerably higher, probably about fifty miles. The air, even on the mountain-tops, is sufficiently rare to diminish the intensity of sound, to affect respiration, and to occasion a loss of muscular strength. The blood burst from the lips and ears of M. de Humboldt as he ascended the Andes, and he experienced the same difficulty in kindling and maintaining a fire at great heights that Marco Polo, the Venetian, did on the mountains of Central Asia. At the height of thirty-seven miles, the atmosphere is still dense enough to reflect the rays of the sun when eighteen degrees below the horizon; and although at the height of fifty miles, the bursting of the meteor of 1783 was heard on earth like the

report of a cannon, it only proves the immensity of the explosion of a mass, half a mile in diameter, which could produce a sound capable of penetrating air three thousand times more rare than that we breathe; but even these heights are extremely small when compared with the radius of the earth.

The action of the sun and moon disturbs the equilibrium of the atmosphere, producing oscillations similar to those in the ocean, which ought to occasion periodic variations in the heights of the barometer. These, however, are so extremely small, that their existence in latitudes far removed from the equator is doubtful. M. Arago has lately been even led to conclude that the barometrical variations corresponding to the phases of the moon are the effects of some special cause, totally different from attraction, of which the nature and mode of action are unknown. La Place seems to think that the flux and reflux distinguishable at Paris may be occasioned by the rise and fall of the ocean, which forms a variable base to so great a portion of the atmosphere.

The attraction of the sun and moon has no sensible effect on the trade winds; the heat of the sun occasions these aërial currents, by rarefying the air at the equator, which causes the cooler and more dense part of the atmosphere to rush along the surface of the earth to the equator,

to be no limit to the ascent of the liquid. The phenomena arising from the force of cohesion are innumerable: the spherical form of rain-drops and shot, the rise of liquids between plane surfaces, the difficulty of detaching a plate of glass from the surface of water, the force with which two plane surfaces adhere when pressed together,—are all effects of cohesion, entirely independent of atmospheric pressure, and are included in the same analytical formulæ, which express all the circumstances accurately, although the law according to which the forces of cohesion and repulsion vary is unknown, except that they only extend to insensible distances.

The difference between the forces of cohesion and repulsion is called molecular force, and, when modified by the electrical state of the particles, is the general cause of chemical affinities, which only take place between particles of different kinds of matter, though not under all circumstances. Two substances may indeed be mixed, but they will not combine to form a third substance different from both, unless their component particles unite in definite proportions. That is to say-one volume of one of the substances will unite with one volume of the other, or with two volumes, or with three, &c., so as to form a new substance, but in any other proportions it will only form a mixture of the two. For example, one volume of hydrogen gas will combine with eight volumes of oxygen, and form water; or

it will unite with sixteen volumes of oxygen, and form deutoxide of hydrogen; but added to any other volume of oxygen, it will merely be a mixture of the two gases. This law of definite proportion, established by Dalton of Manchester, being universal, is one of the most important discoveries in physical science, and furnishes unhoped-for information with regard to the minute and secret operations of nature in the ultimate particles of matter, whose relative weights are thus made known. It would appear also that matter is not infinitely divisible, and Dr. Wollaston has shown that, in all probability, the atmospheres of the sun and planets, as well as of the earth, consist of ultimate atoms, no longer divisible, and if so, that our atmosphere will only extend to that point where the terrestrial attraction is balanced by the elasticity of the air.

All substances may be compressed by a sufficient force, and are said to be more or less elastic according to the facility with which they regain. their volume when the pressure is removed, a property which depends upon the repulsive force of their particles. But the pressure may be so great as to bring the particles near enough to one another to come within the sphere of their cohesive force, and then an aëriform fluid may become a liquid, and a liquid a solid. Mr. Faraday has

K

reduced some of the gases to a liquid state by very great compression; but, although atmospheric air is capable of a great diminution of volume, it always retains its gaseous properties, which resume their primitive volume the instant the pressure is removed, in consequence of the elasticity occasioned by the mutual repulsion of its particles.

SECTION XVI.

THE atmosphere is not homogeneous; it appears from analysis that, of 100 parts, 79 are azotic gas, and 21 oxygen, the great source of combustion and animal heat. Besides these, there are three or four parts of carbonic acid gas in 1000 parts of atmospheric air. These proportions are found to be the same at all heights hitherto attained by man. The air is an elastic fluid, resisting pressure in every direction, and is subject to the power of gravitation: for, as the space in the top of the tube of a barometer is a vacuum, the column of mercury suspended by the pressure of the atmosphere on the surface of the cistern is a measure of its weight; consequently, every variation in the density occasions a corresponding rise or fall in the barometrical column. The pressure of the atmosphere is about fifteen pounds on every square inch, so that the surface of the whole globe sus