What is the specific gravity of atmospheric air? A pint measure of atmospheric air weighs only eight grains; the same measure of pure water weighs one pound, avoirdupois*. If the specific gravity of water be so much greater than that of atmospheric air, horv is water retained in the atmosphere? The water which is taken up by the atmosphere is not in an aqueous state, but is converted into vapourt, by the matter of heart. How is it that heat converts water into vapour? A large portion of the matter of heat combining chymically with water renders it specifically lighter; which is the cause of its rising in the air, and floating theres. close ove principle, the oxygen gas which is secreted by vegetables, is detained in the lower regions of the air, that it may combine with the nitrogen gas produced by animal respiration, to preserve the purity of the atmosphere;-that, owing ble to this, the carbonic acid gas, which is still heaver, and which is formed also by animal respiration and combustion, is confined within the reach of vegetables, which absorb it with avidity, and separate it into its original elements; --and that to this principle it is also attributable that the mephitic air arising from the crowded haunts of men, does not produce pestilence, but mounts above the clouds to await fresh combinations, by means which nature has appointed to render it again useful and salubrious. One great use of lightning may perhaps be to promote the union of this nitrogen with the oxygen of the water which is held in solution in atmospheric air. That the Deity has established some way of effecting this is evident, for without some such contrivance the purity of the atmosphere could not be preserved. Winds also may probably promote this union. Oce Is this effected in any of the great operations of nature? Yes: a great part of the rain which falls upon the earth becomes changed at length into vapour*, and rises into the air in that state to form clouds. What is the cause of the waters of the earth being thus vapourized? As the rays of the sun warm the ground+, a portion of the matter of heat combines with a portion of the water of the earth, and converts it into vapour‡. But what causes the vapour to rise in the air? If a cork be placed at the bottom of a bason of water, it rises immediately to the top, because it is specifically lighter than the according to the dryness or moisture of the air. Slips of whale bone, strings of cat-gut, and the beard of the wild oat, are the substances most commonly employed in fabricating these instruments. Bp. Watson found that, even when there had been no rain for a considerable time, and the earth was dried by the parching heat of summer, it still gave out a considerable quantity of water. By inverting a large drinking glass on a close mown grass-plat, and collecting the vapour which attached itself to the inside of the glass, he found that an acre of ground dispersed into the air above 1600 gallons of water in the space of 12 hours of a summer's day. -Watson's Chymical Essays, vol. iii. The ocean loses many millions of gallons of water hourly by evaporation. The Mediterranean alone is said to lose more by evaporation than it receives from the Nile, the Tiber, the Rhone, the Po, and all the other rivers that fall into it. This water is conveyed by the winds to every part of the continents: -these it fertilizes in the form of rain, and afterwards supplies the rivers, which flow again into the sea. This is one of those continual circulations whereby all inatter is made to subserve various purposes, which have been devised by the Creator for the promotion of his beneficent designs. "The bounteous sun Lifts the light clouds sublime, and spreads them thin, THOMSON.. In our climate evaporation is found to be about four times as much from the vernal to the autumnal equinox, as from the autumnal to the vernal. Heat facilitates all solutions; and the greater the difference between the temperature of the air and the evaporating surface, the greater will be the evaporation.-Walker's Lectures, vol. i. 237. † In winter the earth, at eighteen inches depth, is warmer than the air; in summer, the air is warmer than the earth at that depth: these effects are owing to the earth being a bad conductor of heat. The action of the sun upon the earth in producing vapours, &c. is easily explained on chymical principles. Nicholson, or any of the other modern writers on natural philosophy, may be consulted. The shelter which is afforded to the earth by clouds, mists, &c. is due to the operation of this cause. To this all powerful principle we owe The copious mists which shield this world below; And roll unscorch'd beneath the torrid blaze, water*; so vapour rises in the air, because it is specifically lighter than the airt. What becomes of the water which thus evaporates from the earth? It occupies the lower regions of the atmosphere‡, and is preserved there partly dissolved§ in air]], and partly in the state of elastic vapour. * I have introduced this and other very familiar examples, because I think it of consequence that those who have the care of very young people should encourage them to inquire the cause of every thing which they see around them. A preceptor, who is well qualified for his employment, will not think any thing too trivial which can furnish him with an opportunity of imparting useful knowledge. It is evident that water exists in the atmosphere in abundance in the driest seasons and under the clearest sky. There are substances which have the power of absorbing water from the air at all times; such as the fixed alkalies, and sulphuric acid; the latter of which will soon absorb more than its own weight of water from the air when exposed to it. There can be no doubt but that in general the vapours occupy the lower strata of the atmosphere: that they sometimes ascend very high, is also true; for it is well known that clouds are seen forming above the tops of the highest mountains. Indeed, the clouds begin to form always at some considerable height. Dr. Thomson, vol. iii. 373. Persons who have been in the habit of making observations on the clouds may have sometimes noticed a cloud, which appeared to be just in the act of precipitating, suddenly arrested by a warm current of air, and entirely dissolved thereby, so as to become in a few seconds invisible. This is the same kind of solution as takes place whenever a warm breeze passes over a river, or the ocean. Whenever the air is ponderous, and susceptible of holding the aqueous vapour in solution, the mercury in the barometer rises up to 30 or upwards; when lighter, and liable to suffer the vapours to coalesce, its pressure is less on the base of the barometer, and the mercury falls. See a further account of the barometer in the next chapter. By the experiments of Saussure it appears that a cubic foot of atmospheric air will hold eleven grains of water in solution. 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. 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. 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. 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 raing, hail, or snow. The formation of clouds was formerly attributed 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 chymists in general now consider these operations of nature inexplicable. The subject is well investigated by Dr. Thomson, in vol. iii. of his System of Chymistry; 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 + 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 elouds assume, may be seen in the 16th and 17th volumes of the Philosophical Magazine. See Dr. Darwin's account of his theory of rain and dew, in Notes to the Botanic Garden, part i. pages 114 and 169. 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 current's 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! § 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 piants which vegetate within it. We are indebted to Humboldt for the knowledge of this fact. "The clouds consign their treasures to the fields, THOMSON 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 lifet. What is the ultimate use of this principle? The Almighty has contrived that moisture should continually rise from the earth, and from the various bodies upon its surface, to shield this world of ours from the intense heat of the sun, and to return in rain to water the ground, causing grass to grow for the cattle, and corn and herbs for the service of mant. * This principle of evaporation not only is the primary cause of all rain, mt, dew, &c. but it moderates the effects of the sun's heat, by carrying off an unmense 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, the product of an animal secretion, 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 is 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. How various are the means which have been adopted for the promotion of our convenience and comfort! 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. † Á little reflection will convince any one of the importance of the principle of evaporation. Innumerable instances of its use might be adduced: suffice it to say, that withou 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. It is a characteristic of the works of creation that they will bear examination, and that the more they are examined the more pleasure they afford. The simplicity of the means which NATURE has chosen for performing the necessary operation of watering the earth, must strike every one who has any reli-h for what is grand and excellent. And surely the consideration of such subjects is not without its use; for, "who can med tate on the order observable in NATURE, and not endeavour to reduce his conduct to a similar stoc f regularity?" |