How is this vapour formed into clouds?

After it has remained some time in the atmosphere, it becomes in a measure condensed by causes unknown to us; and the particles of water of which it is composed unite, and form small hollow vesicles, which accumulate together and produce clouds.

From this property of the air we derive many advantages. It has a tendency to preserve every thing on the face of the earth in a proper degree of moisture. In one season of the year, in the interior parts of Africa, a wind prevails called the Harmattan, which is so extremely dry that household furniture is destroyed by it; the pannels of wainscots split; boarded floors are laid open; and the scarf skin of the body peels off, during its continuance. Were it not for the property which atmospheric air has of holding water in solution, this would be the case every where.

It appears from the experiments of some aëronauts, that the air is much drier in the higher regions than it is near the surface of the earth. Phil. Mag vol. xix. 378.

a The formation of clouds was formerly attributed solely to the solution of water in atmospheric air, and the production of rain to the mixture of airs of different temperatures; but there are so many difficulties attending every hypothesis, that I believe chemists in general now consider these operations of nature inexplicable. The subject is well investigated by Dr. Thomson, in vol. iii. of his System of Chemistry; and he concludes from all the facts, that "the formation of clouds and rain cannot be accounted for by a single principle with which we are acquainted." It is, however, probable that electricity alone is the primary cause, and that all the phænomena of nature may originate from this source, See Additional Notes, No. 63.

Saussure conjectures that it is the electrical fluid which surrounds these vesicles, and prevents them from dissolving in the air. These vesicles are said to be from 1.380 to 1-190th of a line in diameter.

An elaborate essay, by Luke Howard, esq. on the various forms which clouds assume, may be seen in the 16th and 17th volumes of the Philosophical Magazine.

See Dr. Darwin's theory of rain and dew, in Notes to the Botanic Garden, 4to, part i. pages 114 and 169.

What further changes take place in this aqueous vapour?

By the operation of causes which are also in a great measure unknown, the clouds after a time become further condensed, and are converted into water a.

What is the consequence of this change of vapour into water?

When the vapour is condensed, it becomes too heavy for the air to support, and falls down in rain, hail, or snow.

a It has before been remarked, that a portion of the water which rises in vapour is held in solution by the atmospheric air. When two opposite currents of air meet, of different temperatures, the vapours are sometimes condensed thereby, and rain follows. All the known gases have also the property of taking up water and holding it in solution.

It may be remarked, that if the temperature of our atmosphere had been 212°, or upwards, rain could never have fallen upon the earth; for the water taken up by evaporation would have been converted into a permanently elastic fluid. It is impossible ever to contemplate the various ways in which the different operations of nature are made to correct and balance each other, without being struck with the infinite comprehension of the Divine mind, which could thus foresee the tendency of every law which it was about to establish. How many cases are there in which the slightest oversight would have produced the destruction of a world!

b It may be observed that rain not only affords a proper degree of moisture to the vegetable creation, but is of service in bringing the soils into a proper state to perform their office. Dry earth of itself has little effect; but when moistened it has the property of decomposing atmospheric air, and of conveying its oxygen to the roots of those plants which vegetate within it. We are indebted to Humboldt for the knowledge of this fact.

"The clouds consign their treasures to the fields,
And, softly shaking on the dimpled pool
Prelusive drops, let all the moisture flow
In large effusion o'er the freshen'd world."

What is the use of this constitution of nature? This principle of evaporation is of very general utility: it is subservient to many natural processes, and is perpetually of use to man in every occupation of life".

This principle of evaporation not only is the primary cause of all rain, mist, dew, &c., bu! it moderates the effects of the sun's heat by carrying off an immense quantity of caloric in combination with the aqueous vapours. Were it not for the cold produced by evaporation, we should faint under any great bodily exertion, or die by excessive heat. But Nature, always provident, has furnished man with a fluid, which, insensibly perspiring and becoming evaporated from the surface of the body, is the vehicle which carries off the superabundant heat as fast as it is generated. Cold-blooded animals, whose temperature is regulated by the medium in which they live, never perspire; but man, who was intended to live in a variety of climates, and designed for active exertion, is thus preserved from the effects of heat, which would otherwise be destructive. The blood of an inhabitant of the torrid zone is no warmer than that of an inhabitant of the mountains of Lapland; which may be proved by placing a thermometer upon the tongue or under the arm. The various means which have thus been adopted for the promotion of our convenience and comfort, are full of instruction and highly gratifying to every reflecting mind. The operation of this principle may be made apparent by the following experiment: Take a small tube with a little water in it, fold a little lint close round it, and having immersed it in ether till the lint is soaked through, hold it in the air for the ether to evaporate. The cold produced by this evaporation will cause the water in the tube to freeze. If the outside of the glass tube be scratched with a flint or a diamond previous to its being wet by the ether, the effect will more readily take place.

A little consideration would convince any one of the importance of the principle of evaporation. Innumerable instances of its use m ght be adduced: suffice it to say, that without it neither grass nor corn could be sufficiently dried to lay up for use. Our clothes when washed could not be dried; neither could a variety of the most common operations be carried on, which conduce much to our comfort and convenience.

CHAPTER II.

OF ATMOSPHERIC AIR.

a

WHAT are the obvious properties of atmospheric

air?

Fluidity, elasticity, expansibility, and gravity. What do you mean by the elasticity of the air? If atmospheric air be compressed into a small

a The habitable and cultivated parts of the earth are lavishly adorned with every thing to gratify the eye; the diversified appearance communicates pleasure, and prevents satiety; while the whole is surrounded by an atmosphere which preserves vegetable and animal life. It may be remarked, that to the transparency of this atmosphere we owe all the pleasure we receive from the variegated prospects which the earth affords. b The fluidity of the air was considered in the last chapter. See pages 29, 33.

If a bladder be tied up with a very small quantity of air within it, and put under the receiver of an air pump, it will be seen to inflate gradually as the pump is exhausted, till it becomes of its full size; owing to the elasticity of the small quantity of air within the bladder, which dilates in this manner, as the atmospheric pressure is removed. A wrinkled apple placed under the receiver of an air pump becomes plump and smooth, from the same cause. The elasticity of the air is such, that Mr. Boyle caused it to dilate by means of an air pump till it occupied nearly fourteen thousand times the space that it usually does.

The elasticity of the air is proved by carrying a bladder half full of air to the top of a high mountain; for the air will be perceived to expand gradually as it approaches the summit.

d Atmospheric air may be compressed into about the 128th of its usual volume; though if an apparatus could be procured of sufficient strength, it might doubtless be compressed to a much greater degree: but owing to its elasticity it would regain its original bulk the instant the pressure was removed. Saussure made use of an instrument to show the elasticity of the air, which he called a manometer. It was a barometer inclosed in a well luted globe. Being thus inclosed, it was only sensible to the elasticity of the air within the globe. Berthollet's Che→ mical Statics.

compass, it has the property of recovering its former state, as soon as the pressure is removed; which is called its elasticity a.

What is meant by the expansibility of the air?

The expansibility of the air is its property of being rarefied by heat, so as to occupy a larger space than it otherwise would b.

It is now generally supposed that the air owes its elasticity to the caloric which it contains; and that, if it could be deprived entirely of its caloric, it would lose its clastic form.

The rebounding of a common foot-ball would be a familiar instance to explain the elasticity of the air to a child.

Bubbles of air rising from the bottom of a glass of water will be seen to dilate as they rise to the surface; owing to the pressure of the liquor becoming less and less.

The air gun and the forcing pump are constructed on this principle. It is by this property of air that fishes are enabled to rise and sink in the water; nature having furnished them with an air-bladder, which they have the power of contracting or dilating at pleasure. When the animal compresses this bladder, its whole volume becomes less, and it sinks in the water: when the pressure is removed, the air within the bladder instantly expands, and the creature is enabled to rise.

Mr. Gregory has remarked that, if the air were not elastic, and so fluid as to be easily put in motion, there would be an end to all the melody and harmony which now so much delight us. How admirably is every thing contrived, that it may not only administer to our wants, but be subservient to our pleasures also! That music was designed by the Deity to produce particular effects on man, might be shown very satisfactorily; for who can doubt but

"That Heaven is pleased, when this bright power

Dispels the clouds of earth, too apt to lower
On every human mind, in life's precarious hour?"
HAYLEY.

If the neck of a bladder, containing a small quantity of air, be closely tied up and held to the fire, the swelling of the bladder, by the rarefaction of the air within it, will afford an idea. of the expansibility of the air.

Mr. Robins has calculated that the air which is disengaged in the firing of gunpowder is rarefied by the heat, so as to