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

tains a weight of 11449000000 hundreds of millions of pounds. Shell-fish, which have the power of producing a vacuum, adhere to the rocks by a pressure of fifteen pounds upon every square inch of contact.

Since the atmosphere is both elastic and heavy, its density necessarily diminishes in ascending above the surface of the earth, for each stratum of air is compressed only by the weight above it; therefore the upper strata are less dense, because they are less compressed than those below them. Whence it is easy to show, supposing the temperature to be constant, that, if the heights above the earth be taken in increasing arithmetical progression, that is, if they increase by equal quantities, as by a foot or a mile, the densities of the strata of air, or the heights of the barometer, which are proportional to them, will decrease in geometrical progression. For example, at the level of the sea, if the mean height of the barometer be 29.922 inches, at the height of 18000 feet it will be 14.961 inches, or one-half as great; at the height of 36000 feet it will be one-fourth as great; at 54000 feet it will be one-eighth, and so on, which affords a method of measuring the heights of mountains with considerable accuracy, and would be very simple if the decrease in the density of the air were exactly according to the preceding

law; but it is modified by several circumstances, and chiefly by the changes of temperature, because heat dilates the air and cold contracts it, the variation being for every degree of Fahrenheit's thermometer. Experience shows that the heat of the air decreases as the height above the surface of the earth increases; and it appears, from recent investigations, that the mean temperature of space is 58° below the freezing point of Fahrenheit, which would probably be the temperature of the surface of the earth also, were it not for the nonconducting power of the air, whence it is enabled to retain the heat of the sun's rays, which the earth imbibes and radiates in all directions. The decrease in heat is very irregular, but from the mean of many observations, it appears to be about 14° or 15° for every 9843 feet, which is the cause of the severe cold and eternal snows on the summits of the Alpine chains. The expansion of the atmosphere from the heat of the sun occasions diurnal variations in the height of the barometer. Of the various methods of computing heights from barometrical measurements, that of Ivory has the advantage of combining accuracy with the greatest simplicity. The most remarkable result of barometrical measurement was recently obtained by Baron Von Humboldt, showing that about eighteen thousand square leagues of the north

west of Asia, including the Caspian Sea and the Lake of Aral, are more than three hundred and twenty feet below the level of the surface of the ocean in a state of mean equilibrium. This enormous basin is similar to some of those large cavities on the surface of the moon, and is attributed, by Humboldt, to the upheaving of the surrounding mountain-chains of the Himalaya, of Kuen-Lun, of Thian-Chan, to those of Armenia, of Erzerum, and of Caucasus, which, by undermining the country to so great an extent, caused it to settle below the usual level of the sea. The very contemplation of the destruction that would ensue from the bursting of any of those barriers which now shut out the sea is fearful. In consequence of the diminished pressure of the atmosphere, water boils at a lower temperature on the mountain-tops than in the valleys, which induced Fahrenheit to propose this mode of observation as a method of ascertaining their heights; but although an instrument was constructed for that purpose by Archdeacon Wollaston, it does not appear to have been much employed.

The atmosphere, when in equilibrio, is an ellipsoid flattened at the poles from its rotation. with the earth: in that state its strata are of uniform density at equal heights above the level of the sea, and it is sensibly of finite extent, whe