can have no effect. Befides, if rain was owing to the immedi ate product of evaporation, its fall must always be accompanied by a diminution of heat, but this is not the cafe in many inftances, particularly in high mountains, the region in which it is formed. Some authors have fuppofed that warmer winds, furcharged with vapour, will let fall the fuperabundant quanrity of water when in a colder atmosphere; and the colder winds, in turn, condense the vapour in the warmer air, without reflecting, that hot wind will warm the colder, on their own syftem, and make it a more powerful menftruum, or the contrary: nor will even the partial fog, which may in the first moments arife from the union of the hot and cold winds, long continue. Its occurrence depends on a fact which probably happens but rarely, and we must fuppofe that the wind is, in this inftance, faturated with humidity: on the other hand, our author endea vours to flow, that it is one of the effential laws of hydrology, that humidity never arifes to its maximum in an air composed only of transparent vapours, except when it is very near to con gelation; and that above this degree, the extreme humidity, the neceffary forerunner of the precipitation of water, is gradually farther and farther diftant. The whole of this difcuffion is illuftrated by fome curious obfervations which occurred to our author and fome other philofophers, in their different journies on the Alps. M. de Luc next examines Dr. Hutton's theory of rain, in the first volume of the Edinburgh Transactions: we noticed it in our LXVIth volume, p. 110, and we thought his principle erroneous. Our author examines it at greater length, and points out its fallacy.

M. de Luc next proceeds to his first examination of clouds;' and, after candidly confeffing his former mistakes, he shows that rain cannot arife from vapours existing in the air previous to clouds, and that no ftratum of the atmosphere can produce fo much water, fimply evaporated,' as to produce rain. Rain, therefore, must be owing to veficular vapours, which form clouds, and M. de Luc endeavours to prove alfo, that these clouds proceed from elaftic vapours, which rapidly pass through their maximum: in other words, the veficular vapours proceed from the first precipitation of water, combined with the air in that ftate, whatever it may be, which is the first confequence of evaporation. When water evaporates by the heat of the fun, the first effect is a veficular vapour, produced by a quick evaporation foon paffing its maximum. This fog, when fur veyed from a high mountain, rifes into fhort columns and tu bercles, feemingly from the lateral preffure of the veficles: another evaporation takes place, and invisible transparent elaftic vapours are formed, which do not affect the hygrometer, or hygrofcopic fubftances. It is, in appearance, pure dry air, perhaps it is fuch in reality. This feems to be the progress, and on this fyftem M. Sauffure builds his subsequent one: he fuppofes that these claftic vapours rife the colder regions,

6

and

and are there condenfed into veficular vapours from the cold alone, till, by their denfity, they obfcure the fun, which adds to the cold in the inferior ftrata of the atmofphere, and of course to the condensation of vapour, which farther condenfes into drops, and produces rain. M. de Luc fhows that this fyftem, though apparently fimple and accurate, is not confiftent with phænomena; and, independent of thofe facts and arguments, which he has adduced, many appearances occur to us which this theory will not explain. The real caufe certainly lies deeper.

Our author afterwards enquires into the nature of clouds, which are formed of hollow fpherules, evidently kept in that state by latent fire; fince on their destruction fome heat is detached: they refift the freezing point of cold, but are congealed by a few degrees of additional cold; perhaps by the continuance of cold at the freezing point, for their refiftance seems to depend on the difengagement of their fire.

On the fubject of the duration of clouds, M. de Luc repeats his first principle, that, in every fog, the vapour foon paffes its maximum, in other words, is produced fafter than it is dif fipated, and, confequently, is decompofed: to this he adds, that the continuance of every fog is owing to additional vapour. Clouds conftantly change, and are diffipated in fome parts while they increase in others, even when in appearance stationary. The general fource of these vapours is the air itself, and when it furnishes no more the clouds foon difappear: this fact is fupported by numerous obfervations, and feems to be strictly juft. When thefe veficular vapours are formed, rain enfues, either by water dropping to the lower part and giving it a difproportionate weight, or by their rapid increase fo as to prefs on each other. The chapter on the ftate of watery vapours in the atmosphere contains little more than a recapitulation of our author's former opinions, and his retracting his fyftem refpecting the cause of the fall of the barometer previous to rain. The vapours in the lower part of the atmosphere contain but little water when in the greateft quantity, and the upper parts are fill higher. It must follow then, that vapours reft in ftrata of air ftill higher, which is inconfiftent with the clearness of the air in the highest regions, or that thefe vapours form elastic invisible fluids. They certainly lofe the appearance of water, and no longer affect the hygrometer as water: there is a change from water to air; and during rain this new air again affumes its former appearances, and again becomes water. But the caufe of these changes is fill undiscovered.

The second chapter is on ftorms, and the first of these is the fudden fquall (in the failor's language) of rain, which fometimes happens in the warmest and cleareft days: it is generally attended with fome uncommon, fome irregular motion of the air; but is not connected, fo far as our author found, with any previous remarkable moisture. Fail is perhaps not equally

Z 2

difficult

difficult to explain: the nucleus of every hail ftone is fnow, and fnow is not probably formed of water in that divided ftate which will admit of regular crystallization, in other words, of veficular vapours. In it fall, meeting with water, it acquires aditional buik. Sometimes, however, large hail-ftones fall from a comparatively fina 1 height; and, in every inftance, it is not eafy to account for the fudden change of temperature neceffary to produce large hail. Thunder, we know, arises from an unequal distribution of the electrical fluid; but M. de Luc fhows, that this is not the effect of evaporation, as M. M. Volta and Cavallo fuppofed from their experiment; nor, on the other hand, do phenomena fupport the other opinion of M. Sauflere, that electricity is connected with the formation of thefe clouds, which conlifts of velicular vapour. On the whole, fays our author, and it is one of the most important confiderations of me corol gy, if every rainy cloud is formed only of the vapours that exist in the place which it occupies, the electrical Auid, darted from thefe clouds, was not confined in the vapours that were previously there: its ingredients or itfelfexitted in tome other form, as well as the water which is manifested at the fame time. Our author fufpects, that not only the water of which rain confilis, but the electrical fluid, may be occafionally deft oyed and recompofed.

The clectrical fluid is one of the great affiftants of M. de Sauffore in his meteorological theories, and we own that it appears to us the great and principal agent, though we feel all, or at least the greater part of the difficulties which M. de Luc has fuggeted to the tytiem of the philofopher of Geneva. There is, we admit, a great defect in the links of our meteorological knowledge; but they will be bett fupplied by purfuing more clofely the fources of information which feem connected with our deficiencies. It must be confiffed that the electricity of the atmosphere and its in Buence are yet in their infancy; for we have only of late been able to meafore with accuracy very small degrees of electricity, and these enquiries have not frequently been applied to the fubject before us. The last fection of this chap ter is on the interval between evaporation and rain; an interval in which the water difappears, or at leaft is too intimately combined with the atmosphere to be fentible to the niceft hygrometer. It is certain, however, that evaporation goes on to an almost boundlefs extent; the fluids evaporated unite with the air fo intimately, as not to disturb its tranfparency, and yet are not fenfible to the nicest tefts; are not precipitated, or precipitated in a very flight and inadequate degree by cold; while the higher ftrata of the atmosphere are drier than the lower. The conclusion, however, that water is changed into air is too precipitare: our hygrometrical reits may not have fufficient affinity to water to be ble to feparate this intimate union; for we know by the phænomena of the harmattan, that air paffing over dry countries has its water extracted in fo great a degree, that

3

that it absorbs it again even from bodies of which it is a component part. It takes the water from the leaves of trees, from the oldeft timber, from the human skin. These circumftances fhould therefore teach us fome caution, and we must not conclude that air contains no water, in a flate of folurion, becaufe hair, whalebone, or ivory, extract none from it. Ong fact mult have occurred to M. de Luc, that the atmoiphere is never fo truly transparent as when loaded with water, almost in the moment of feparating from it.

The fourth chapter contains the foundation of the opinion, that water is contained in the atmosphere itfelf; and the firit fection is on the difcovers of water in the form of air; the fecond, on fome phenomena, in which air appears to produce water. This chapter contains the various experiments on the compofition of water; but, fince the publication of this volume, there is much reafon to hesitate, before we conclude that water is really compofed of different airs, and capable of being produced and decompounded at will. We fhould befitate fil more refpecting the application of thefe experiments to meteorological facts. Our readers are fufficiently acquainted with this fubject, and we fhall only enlarge on a remark or two in the second section. M. de Luc feems to think, that with much heat-a fufficient degree to feparate the latent heat of the airs--water is produced; with a lefs heat, fixed air; a conclufion influenced in fome degree by the various additions to inflammable air, when procured from peculiar fources. In Argand's lamp, the water may arife, in his opinion, from the more complete ignition of the foot, which contains the ingredients of water: in reality, it contains water ready formed.

The next chapter is on atmofpheric air; and M. de Luc begins with a general idea of meteorological caufes.' Rain cannot, in his opinion, be owing to the mixture of pure and inflamable airs, as water is fuppofed (by fome) to be produced by their union in our experiments. But the changes muft, he thinks, be produced by fome mixture, not indeed of aerial fluids, but of fomething more fubtile. As the atmosphere, however, is not compofed of two diftinct fluids; but as thefe, by their intimate anion, form one homogeneous fluid, the fame arguments, M. de Luc fuppofes, will how that, if rain is not owing to the immediate product of evaporation, the intermediate state of the water in the air must be that of air; and, on the other hand, when rain is produced, it must arife from a decompofition of a part of the air, exactly like the reft: a conclufion which we think neither chemitry nor meteorology will, in their prefent ftates, fupport, and which our author, neither in the former part of his work, nor in the latter, has clearly established. The compofition of water is not yet afcertained beyond doubt, and an active caufe to again combine the feparated particles of water is wanting, even if we should allow, as M. de Luc contends, in his fection on the relation of atmo

fpheric air with water,' that the atmosphere contains the two in gredients of water. To obtain this active power, and to prove this laft pofition, he introduces the nitrous acid, formed in the experiments of Mr. Cavendish, as the fource of the inflammable air; and, after remarking that perhaps the vital air only contributes to the fluidity of the acid, fuggefts that the real acid, a fmall part perhaps of the concrete fluid, may be that fubftance by whofe intervention air becomes different from watery va pours. We now know that vital air is neceffary to the acidifi cation, not to the fluidity; that the inflammable air in the acid is in very small proportion, and that there is no diftinctive bafe,' which can produce fuch powerful effects.

M. de Luc proceeds to fome meteorological confiderations refpecting light; and first to the connection of the progress of light with that of watery vapours. To this he was led by a remarkable fact: when the hygrometer, in the night, had been at extreme moisture, he found, on the fun rifing, that it turned towards drynefs, but with a proportionally greater celerity than the thermometer rofe; and towards the fetting of the fun, the progrefs was more rapid than fome hours before. As light therefore is, in the opinion of many philofophers, one of the ingredients of heat, this circumftance feemed to fuggeft, that the other ingredient might be connected with the change of water into air. The relation of light to the heat of the atmofphere is therefore the next object of enquiry: in this fection we find our author modifying in fome meafure his former opinion; and, instead of light being one of the ingredients of heat, he fuppofes it acts by giving a greater expanfive force to the fire already farmed. M. de Luc then proceeds to reply to M. Sauffure's objections to his fyftem, refpe&ting the state of fire in the atmosphere; the opinion that the rays of the fun are not, by themselves, the caufe of heat; and that, from hence, the 'comparatively greater cold of the fuperior ftrata of the air is owing. To examine the difpute at length would lead us too far: we may remark that, on the whole, M. de Luc's opinions feem to be just.

It is of more importance to trace this fyftem in its applica tion to the transformation of vapours in the atmosphere, or, in the language of our author, in the meteorological conjectures on the modification of the rays of the fun in the air. He thinks, with much reafon, that the rays of the fun are powerful agents in this operation; they feem to furnish the fire neceffary for the vapours to affume the form of air, and, in this ftate, to produce a compound, which the mere privation of heat will not destroy. But when he goes farther, and confiders light as producing, at the fame time, the radical bafe of the nitrous acid, he is not fufficiently fupported by modern difcoveries; and when he thinks that the differently refrangible rays have different chemical properties, which may be exerted in meteorological changes, he feems to fpeculate too refinedly. But he purfues

this