before, its heat is more diffused over an equal space, and the thermometer will again denote cold; and by thus proceeding, repeated calls are made upon the air in the receiver to supply the repeated losses of that from the barrel, until at length it is exhausted, and the thermometer will fall several degrees.

It will now be impossible to lift the receiver from the plate, because it is firmly held on by the pressure of the external atmosphere, which amounts to about fifteen pounds upon each square inch of surface; this pressure manifests the endeavor of the external atmosphere to enter and to restore the equilibrium that has been disturbed.

By removing the small screw that is placed under the valve chamber, the external air will immediately enter, the equilibrium will be restored, the density of the air in the receiver will be the same as it was, and the thermometer will slowly rise to its original degree.

During the ascents and descents of the piston in this experiment, another phenomenon will be observed, namely, the interior of the receiver appears as though it were filled with mist; this is really the case, and for the following reason.

Air invariably contains the vapor of water, and to raise such vapor, and to retain it, a certain amount of heat is necessary. The air included by the receiver contained the vapor of water, invisibly maintained throughout its volume by heat; but upon being suddenly compelled to expand, the heat is as suddenly diffused throughout such expanded volume: the vapor thus chilled or robbed of heat, is converted into minute drops of water, and forms an artificial mist.

This experiment accordingly furnishes the chemist with an explanation of the formation of a natural mist, the invisible heated watery vapor present in the air becoming visible, minute drops of water, when its temperature falls, either by sudden local rarefaction of the air, or by coming in contact with cold surfaces of the earth and waters.

To impress these curious and important matters upon the mind of the young student of Nature, take the following example: -Suppose a cubic foot of air contain a certain amount of heat, equally diffused throughout its elementary and compound constituents, and capable of affecting the thermometer to a given degree; if this volume of air be compressed to one-tenth of a cubic foot, of course there will be ten times as much heat concentrated into that tenth as there was, and the thermometer would indicate a rise of temperature.

On the other hand, suppose the cubic foot of air to be expanded to ten cubic feet, the heat would be so diffused throughout such volume, that the thermometer would indicate depression of temperature, or, in other words, the air would feel cold.

These experiments of the laboratory furnish the chemist with a key to the explanation of natural phenomena; an enormous store of heat is contained in the atmosphere; but this medium is not of uniform density from the earth to its own confines; those portions that are directly incumbent upon the earth are most dense, because they bear the weight of superincumbent portions; as, to use a very humble simile, the hay at the lower part of a rick bears the weight of that above, and is therefore more compact and dense; or it may be conveyed by the accompanying engraving, the gradual shading of which

denotes the gradual diminution in the density of the atmosphere in proportion to its altitude.

Fig. 28.

The compression of the lower, by the upper portion of the atmosphere, alters its capacity for heat, as it was altered by the compression of the piston, in the experiment so recently described, and accordingly heat escapes from the atmosphere in quantity suited to time and season for the welfare of the animated creation.

If we ascend a lofty mountain, and thus approach many hundred feet nearer to the sun than we were in a valley, we might expect to find the temperature considerably warmer; but the contrary is the case. When we left the valley, it was glowing under the heat of the summer sun, we can see it still glowing, and though we are nearer the sun on the mountain summit, the air is intensely cold; what reason can be assigned for this? The chemist submits the following:-It has been already stated that the sun does not heat the transparent air; and at the elevation just mentioned, it is greatly rarefied or comparatively free from pressure; the heat that it has derived from contact with the warm earth is widely diffused, and in a given volume of air on the mountain summit, there is not the actual heat that exists in a similar volume in the valley.

Again; warm air in ascending from the valley expands,

and remotion of heat, or, in other words, cold is produced, and frequently to such an extent that the watery vapor of the air condenses in clouds, which, when surcharged, deliver rain, or it congeals as snow, and clothes the mountain summit.

Air, in rising from the level of the sea, becomes nearly one degree colder for the first two hundred feet of ascent, and altogether about fifty degrees colder in rising fifteen thousand feet; thus, water would freeze at this elevation even near the equator, where the temperature of the low plains is at least eighty degrees; this is the reason why the summits of lofty mountains are covered with perpetual snow, and the height at which it occurs is called the "snow line," or "line of perpetual congelation."

The Andes present a magnificent illustration of these phenomena attendant upon the varying capacity of air for heat; placed nearly under the equator, they tower from parching sands; about their middle height the climate is temperate; their summits are crowned with unchanging snow, and these ranges of temperature are always distinct; the hot winds from below, if they ascend, become gradually cooled by expansion, and if by any force of the blast the cold air be driven downwards, it is condensed, and rendered warmer in its fall.

How grand, how beautiful, how harmonious, are these adjustments and balancings of the Almighty Hand, who "made the weight for the winds, and weighed out the waters by measure!" how eminently calculated they are to inspire us with emotions of admiration and gratitude towards the Author of our being!

He has ordained the level earth to be the chief dwelling-place of mankind, and commanded the air directly

incumbent upon it to be of the just weight or density, and to contain the due amount of heat which vitality requires for the proper maintenance of its manifold recondite functions; whilst on greater elevations, which are but thinly peopled, or on mountain summits, which are but rarely visited, these wondrous agents, air and heat, are rare and feeble, because they are not required in abundance and power for the support of life!

Exploring and investigating throughout the treasures of the Creation, we become more and more impressed that it is founded upon the most miraculous scale of beneficence; that bounteous gifts are not indiscriminately scattered where they would be useless to organic beings, but are abundantly presented where required for their manifold wants.

Great, inexpressibly great, is the delight in thus being permitted to follow the marvelous works of the Creator, to trace the unbounded power and transcendent goodness which are displayed alike in the mightiest and the lowliest parts of the terrestrial world.

"In the heavens hath He set a tabernacle for the sun, which is as a bridegroom coming out of his chamber, and rejoiceth as a strong man to run a race; his going forth is from the end of heaven, and his circuit unto the ends of it, and there is nothing hid from the heat thereof."

In the splendid poetical images of the above passage, are set forth the beauty, power, glory, and genial effects of solar light and heat during an unclouded day; let us trace the Hand by which these are all directed to work together for our good.

During the brilliant zenith of summer, even in torrid climes, although the heat of the sun impinges upon the