Tho hypothesis, however, receives some degree of Support train the alleged fact, that the same quantity of watery vapour is contained in equal volumes of dif ferent gases saturated with moisture. Saussure ascer tained this to be the case with common air, carbonic acid, and hydrogen gas, or rather that these gases exhi bited, in the same circumstances, the same hygroscopi cal condition. Clement and Desormes were also led, by their experiments, to infer that all gases contain, under similar circumstances, equal portions of water. It was even stated by Saussure, and the assertion was after wards repeated by De Luc and Dalton, that the quan tity of watery vapour contained in any gas is the same as that which would be contained in a vacuum of equal extent with the space occupied by the gas. These facts, if admitted, seem to favour the opinion that water is converted into vapour, and retained in that state, not by an affinity subsisting between the water and the gas, (for it is not likely that this affinity should be the same with all the gases.) but merely by the watery vapour being raised by the action of caloric, and afterwards diffusing itself mechanically by its own elasticity.
On the other hand, it may be said, that the accuracy of these facts is, from their extreme delicacy, very ques tionable; particularly as they have rather been dedu ced by inference, than proved by direct experiment. A portion of water is allowed to be converted into va pour in vacuo, and the depression which it produces upon a column of mercury, is assumed as the measure of its elasticity : a quantity of dry air is also introduced into a receiver of equal capacity with that in which the vacuum was formed, and an equal portion of water is introduced, the temperature in both cases being the same. The acquired elasticity of the vapour, indicated by the effects of its pressure upon the mercury in an inclosed barometer, is the same as that In the vacuum, and produces an elevation of half an inch, at the tem perature of eo°. It is therefore concluded, that an equal quantity of water has in all these cases been con verted into vapour. But, in opposition to this conclu sion, it has been said that the elasticity exerted by the vapour under these different circumstances, is not an exact test of its quantity, unless it be admitted that the vapour is not all attracted by the gases; and this is the very point to be established. If the vapour be combi ned with the gas, its elasticity will be counteracted in a degree proportional to the attraction exerted ; and it is easy to conceive, that, at the point of saturation, the elasticity may be so modified by the power of affinity, as to indicate the same degree of intensity, though the quantities of vapour which are present be very differ ent.
But it has also been urged, in support of this theo ry, that the quantity of moisture deposited by equal volumes of different gases in a saturated state, is, in all cases, the same when they are exposed to substances which have a strong attraction for water. Clement and Dcsormes established this fact by experiment with atmospheric air, oxygen, hydrogen, nitrogen, and car bonic•acid gas; the differences in result being so small as might naturally be expected from the mode of per forming the experiments. But though the argument afforded by these experiments, in support of the opi nion that the vapour exerts no action with the gases, must be admitted to be of some weight, it cannot be regarded as perfectly conclusive ; since it still remains fo be determined, whether the quantities of water, which cannot be taken from the different gases by de siccation, are exactly equal; and there appears to be no means of ascertaining this by direct experiment.
It cannot be doubted, that the gases do differ front one another, with respect to their affinity for water ; since unequal quantities of them are absorbed by that fluid under equal degrees of pressure; and though this affinity may be diminished when the water exists in a vaporous state, it is probably never entirely subverted. Upon the whole, then, it may be concluded, that the facts which have been stated are not incompatible with the opinion, that spontaneous evaporation is produced by a mutual affinity subsisting between air and water. This opinion, which is the one generally received, was first suggested by Dr Halley ; it was afterwards adopt ed by 1)1. Franklin, and has been ably supported by Al. Le Rol, Dr Hamilton, Dr Murray, &c. That a chemical attraction does exist between water and at mospheric air, io proved by the fact, that water ab sorbs a certain portion of its component gases, and re tains them in combination with considerable force ; and every kind of attraction being mutual, air must also be capable of dissolving water in a degree depend ing upon this affinity, though modified, no doubt, by the elasticity of the vapour which is formed. Mr Dal ton maintains, that the combination is entirely mecha nical; and that the absorbed gas received into the vacant spaces, between the particles of the water, is retained there without the aid of chemical attraction. This opi nion is somewhat countenanced by the discoveryof Dr Henry, respecting the law by which the absorption of gases is regulated. This very able and accurate chemist found, that the temperature being the same, the quan tity of any gas absorbed by water, was exactly pro portional to the pressure to which it was subjected, or, to use his own words, that " under equal circumstan ces of temperature, water takes up, in all cases, the same volume of condensed gas, as of gas under ordi nary pressure." But admitting the law, as Dr Mur ray has stated with his usual ingenuity, the conclusion does not follow which has been drawn from it. "In the absorption of gas by water, two powers operate, or may be conceived to operate, independent of pressure,—the affinity between the gas and water tending to combine them, and the elasticity of the gas counteracting this, and placing limits to the combination,—precisely in the same manner as in the solution of a solid, there are two forces operating, the chemical affinity, and the cohesion of the solid. In the absorption of a gas, whatever fa
vours the exertion of the elasticity will lessen the quantity absorbed; whatever represses it, will promote the absorption. These effects are produced by varia tions of mechanical pressure; the diminution of pres sure favouring the exertion of elasticity, and vice versa. But from this no just argument can be drawn against the conclusion, that a chemical affinity exists, and is the primary cause of the combination; and the law stated by Dr Henry agrees as well with the one theory as the other." The principal objection which has been offered against the theory, that evaporation is owing to a chemical so lution of water by air, is, that according to that sup position, the process ought to go on most briskly when the air is in a condensed state; but the very reverse is the case. But this objection may be answered by say ing, that though the quantity of air, which is Ow; brought in contact with the water, is increased, its pressure is increased in an equal degree ; and the effect of quantity may be more than counteracted by the in creased difficulty which the water now has to overcome in assuming the vapourous state. On the whole, there fore, we arc disposed to think, that no argument has been urged against this theory, of sufficient Force to in validate its truth; and since a variety of facts, both of a direct and analogical kind, conspire to give it support, we consider it as affording a more satisfactory explana tion of the process of spontaneous evaporation than any which has been proposed. We shall, therefore, now proceed to illustrate the nature of the operations, by which the aqueous vapour is supposed to be elevated into the higher regions of the atmosphere. The cause of solution being the stronger attraction of water for air, than for its own particles, the part already dissolved will, in consequence of the superior affinity of the less saturated portion of air immediately above it, gradually diffuse itself, and thus allow the lower strata, in contact with the fluid, to dissolve an additional quantity. The process, like the solution of a salt in the bottom of a ves sel of water, will go on very slowly when the solvent is not exposed to any agitation; and, accordingly, it is well known, that evaporation is greatly promoted by wind, which brings a continual current of fresh air, while it carries off the portion of it already saturated with mois ture. The water being thus converted into vapour, and gradually elevated into the higher and colder regions of the atmosphere, is partially condensed, and gives rise to all the diversified appearances which we call clouds, mists, fogs, &c. These appearances take place at dif ferent heights in the atmosphere, according to the tem perature of the air, and the quantity of water which it holds in solution. Hence we can perceive the reason why clouds are higher in summer than in winter ; and why a lowering atmosphere generally announces the ap proach of rain, as it indicates the air to be so copiously loaded with moisture, that the reduction of temperature produced by a very small elevation, is sufficient to con dense the excess of water, which the air is unable to dissolve. When different strata of air, saturated with moisture, but at different temperatures, are mixed to gether, by winds or any other cause, a precipitation of water must be the consequence ; for it has been ascer tained, that the solving power of air increases in a faster ratio than the temperature. This is well illustrated by the great quantity of rain which falls in mountainous districts, compared with that in flat and extensive plains. If the strata of air which are mixed together be in a state of complete saturation, and differ considerably in temperature, the quantity of water precipitated will be very copious: this may be expected to hold particular ly when the strata are in opposite states of electricity, and thus mix rapidly and thoroughly together; hence the reason of the very heavy showers which usually ac company thunder storms. If the temperature of the atmosphere where the condensation takes place be suf ficient to freeze the watery particles as they are formed, snow is produced; but if they have time to collect into drops before they are frozen, they become hailstones. Southerly winds are commonly followed by rain, because being warm, and saturated with aqueous vapour, they are cooled by coming into a colder climate ; while northerly winds, by being exactly in an opposite condition, are dry and parching, and usually attended with fair weather. When the atmosphere is completely saturated with moisture, and passes from a denser to a rarer state, it suffers a diminution of temperature by its increased ca pacity for caloric, and being unable to hold the same quantity of water in solution, it deposits a portion of it. It is owing to the same cause that a mist or cloud is formed upon exhausting a receiver with the air pump ; and in like manner may be explained the reason why fogs usually cover the tops of hills when a current of air, loaded with moisture, is driven over them. The air, which is perfectly transparent at the bottom of the vallies, because its temperature is sufficient to make it dissolve completely the moisture with which it is charg ed, becomes opaque in the more elevated regions, on account of the reduction of temperature which it un dergoes, partly from change of position in the atmos phere, and partly from rarefaction in consequence cf that change. On the contrary, when the atmosphere passes from a rarer to a more condensed state, instead of depo siting moisture, its solving power is increased ; and it dissolves either wholly or partially, the clouds which may have been previously formed. This is one reason why the rising and falling of the mercury in the baro meter become indications of the state of the weather.