Abbildungen der Seite
PDF
EPUB

but a very

Auid, the abundance of which in the atmosphere diminishes its pressure upon the barometer, was supposed to produce rain. Having published this system in the above mentioned work, it obtained much approbation among natural philosophers, because no satisfactory explanation had been yet given of the above connexion of phenomena ; however, I did not intend to give it a full assent, till I had succeeded in the construction of a true hygrometer ; judging already, that, without such an instrument, nothing could be determined with any certainty concerning the modifications of evaporated water in the atmosphere. This judgment was soon confirmed : for M. de Saussure, who had made a quicker progress than myself in hygrometrical experiments, discovered the fallacy of the above plausible system, which at first he had adopted with applause. He demonstrated by direct processes, that though the aqueous vapour is specifically lighter than air, the difference between its greatest and smallest quantity in the atmosphere at any time is so little, that it can explain

inconsiderable part of the variations of the barometer. I had not yet carried my own experiments so far, but I did not doubt the main result of his, as they bore all the characters of a true inquiry, and I abandoned my system, as applied to aqueous vapour.

But the general conclusion that I had deduced from the great reduction of anomalies in the measurement of heights by the barometer, which was principally owing to the introduction of an equation for the differences of the quantity of free fire in the air, still remained ; namely, that some expansible fluids as imponderable as the former, hitherto unknown to us, might also account for that remarkable correspondence between the changes of the weather, and the variations of the barometer. For, the same fuids, which, from their abundance at certain times, lessen the pressure of the atmospheric columns on a certain extent of country, by dilating the air and repelling it to other parts, may also prepare its decomposition for the production of rain, either alone, or accompanied with other meteors; and at other times they may be dissipated without producing any of these effects, occasioning only the fall of the barometer.

The above series of facts and their immediate consequences present the greatest assemblage of operations of physical causes on our globe ; and a general consequence certainly results from them, namely, that all these operations are so intimately connected with the nature of aeriform fluids, of water, light, fire, and electric fluid, that we cannot determine any thing with the smallest degree of certainty on the nature of any of these substances, without embracing the whole. When therefore we discover some new phenomenon of any of these Auids, at what distance soever this phenomenon may be from connecting itself with the operations which we observe in the atmos phere, it is not to be neglected; for we cannot arrive at any distant object, but by successive steps.

This is the consideration that has induced me to fix my attention on the electric phenomena manisested by the instrument, which I

have described under the name of aerial electroscope : as from the above atmospheric phenomena concerning lightning and thunder which cannot leave any doubt, that they are produced by a certain decomposition of the atmospheric air ; and from the correspondent circumstance of a formation of new electric fluid in the atmosphere, during the period of the day when, the greatest part of the aqueous vapour vanishing in it, there remains scarcely any ponderable Auid but atmospheric air; it is manifest, that the electric fluid is one of the substances most intimately concerned in the chemical processes which take place in the atmosphere, and on the nature of which it is the most important to acquire more knowledge.

It may be seen in the tables of my observations of this new instrument, that the changes in the frequency of the strikings of the little pendulum have no determined connexion with those of either heat or moisture in the room ; for though heat commonly increases in the course of each day, at the same time as the frequency of the strikings, nevertheless the former is not the cause of the latter; since with the same degrees of heat the frequency of the strikings is very different on different days. With respect to the correspondence of this phenomenon with the variations of the barometer, my observations have been too short for deciding any thing on this subject, though I felt much interested in it; and besides, the barometer had but small variations during this short time, being always rather high. Therefore, this is a course of correspondent observations which remains to be followed.

The observations contained in the last table create a new interest in this pursuit, as they may become a mean of discovering the changes in the comparative electric states of the ground and the air near it. The little pendulum, by its silver wire, being placed in connection with the zinc side in these experiments, was therefore positive ; and in this case (as well as when it is connected with the copper side) it must rise more rapidly towards the ball 18 in proportion as the electric state of the latter differs more from its own. We know that in the first case (that of my observations), when the ball 18 communicates with the copper extremity of the columns, it is negative, and thus differs from the pendulum as negative from positive ; the standard of which, according to the important determination of Sig. Volta, is the actual electric state of the ambient air. Now, the observations contained in the above table show that the frequency of the strikings is not always the greatest, when the ball 18 is undoubtedly negative, by communicating with the copper side of the columns; it being often equal, and sometimes even greater, when the ball communicates with the ground. This is a remarkable phenomenon, showing, that sometimes the ground contains less electric fluid than the air near it, and it may in future lead to some important discovery concerning the operations going on at the surface of the ground depending on the atmosphere.

I have employed much time and labour to arrive at the entrance

of this new road in the investigation of terrestrial phenomena : the entrance, I say, for I do not even consider it as completely open. With respect to the instrument itself, I may judge that it is susceptible of farther improvements, both in the composition of the column, and in the machinery added to it ; for in such a complication of new physical effects and mechanical dispositions of parts, it is not to be expected that every thing can be conceived by one individual. The very composition of the column might be improved with regard to the intensity of effect by some other metallic coating than that of copper on paper, which I have employed on account only of its being ready prepared by using Dutch gilt paper. In some trials, I have found more effect in using paper covered with real gold, and with silver; and I have also found some advantage in doubling the Dutch gilt paper, by pasting a thin paper over its own paper. Many such trials may be made with a proper condenser, before whole columus are composed. As for the arrangement of the machinery connected with the column, the instrument which I have described having been successively augmented upon its original base, I suspect that it is too much crowded, and that thus its parts may have on each other an influence prejudicial to the effects, which I have marked in other cases.

I have made many other remarks on this instrument, but my present purpose is more to engage other experimental philosophers in this pursuit, than to forward it myself: for with respect to these observations, I consider them as newly born. It will first require some time for the understanding what may be called the language of the instrument; i.e. its meaning as to the indication of the electric state of the ambient air, by its influence on the motions of the pendulum. This study has been opened too late for me, though I was engaged in it by considerations resulting from long meteorological observations, which, as they are of the greatest importance to natural philosophy, must be the incitement to this pursuit. Wishing therefore, that such observations may become a more general object of attention among natural philosophers, I have here endeavoured to show, by an abstract view of their present results, what knowledge, in following them, may be still obtained concerning the atmospheric operations. It is true, that observations of this kind require the neighbourhood of mountains (unless those who ascend in balloons should carry proper meteorological instruments, and apply themselves to these observations), but in general no real knowledge of the nature of the atmosphere can be obtained without in some manner, ascending in it; and it is no less certain, that without this knowledge no chemical theory can possess any certainty.

Systems are useful for promoting science, provided they be founded on all the knowledge already acquired respecting their object; but even then, as long as they contain hypotheses, they must be only considered as leading to new researches on determined points. With this view, I shall here conclude by an abstract of a meteorological system which I have fully explained in my former works, and especially in that under the title of Introduction à la Physique terrestre par les Fluides expansibles.

İ. During the time that the sun's rays pervade the atmosphere, the aqueous vapour ascending in it by the evaporation which continually takes place on the surface of the earth, is transformed into atmospheric air by some combination of this vapour with the electric fluid, which, during the same time is formed in the atmosphere. A formation of electric fluid at that time is shown by M. de Saussure's observations already mentioned; but that the quantity thus manifested is not the whole, and that a great part of this new fluid is employed in the above transformation, is proved, as will be seen hereafter, by the production of lightning and thunder, which cannot have any other source.

II. Thus, but by a particular operation, is formed that subtle fluid which I have called vector, possessing many of the properties of light, but with the characteristic differences which I have determined. This fluid pervades instantly all bodies, is constantly present in the atmosphere, and has probably a great share in its phenomena; but its only function yet determined is, to unite with the electric matter composed at the same time; and, being thus the cause of the expansibility of the electric fluid, it produces the phenomena known under the name of electric influences, as I have explained.

III. In clear weather dew is produced at sunset, because that formation of electric fluid then ceasing in the atmosphere, the aqueous vapour, which continues to ascend in it, remains unchanged, and its quantity increasing too much in the air comparatively to the decreasing heat, it precipitates in visible particles of water : when heat decreases very rapidly in the air after sunset, the vapour is seen condensed as a mist over meadows; and at last in autumn it produces fogs.

IV. The return of atmospheric air into aqueous vapour, whence result clouds, and afterwards rain, is produced by some subtle fluid ascending from the base of the atmosphere, the affinities of which with the ingredients whereby the aqueous vapour has been transformed into atmospheric air decompose the latter. Thus, particles of aqueous vapour being substituted for particles of air in some stratum of the atmosphere, and becoming much too abundant to subsist in the same space, they first precipitate in the vesiculæ which form clouds; and if the decomposition of the air continues some time in the same stratum, these vesiculæ collapse into drops, and form rain.

V. This is one of the causes of the variation of the barometer, not as a prognostic, but as a consequence. The absolute mass of the atmosphere is constantly changing by these inverse operations. When there is a long duration of fine weather over a great extent of country, the absolute quantity of air increases in the atmosphere, by the aqueous vapour which ascends in it continuing to be transformed into air during the day; and the barometer ascends, even in parts at some distance where it rains ; when on the contrary there prevails over a great extent of country a long continuance of decomposition of air into rain, the mass of the atmosphere decreases, and the barometer falls, even in adjacent countries where there is fine weather. It is not therefore to be expected, that rain and fine weather should be positively connected with certain absolute heights of the barometer ; its small motions, when it is more or less high, have the surest correspondence with the local weather; the fall indicating the presence of that subtle fluid which tends to decompose the air, and the ascent the cessation of this influence.

VI. If, during the decomposition of atmospheric air, the fluid operating this effect so unites with the ingredients of the electric fluid which had entered into the composition of that air, as to form a new compound in which the electric fluid does not possess its characteristic properties, rain only is produced, with little or no electric symptom : and this is the most common case. But when, from the nature of the new fluids which come to be spread in that stratum of the atmosphere, the decomposition of atmospheric air is such as to permit electric fluid to be produced by the precise ingredients (i.e. neither more nor less) necessary to its characteristic properties, it darts suddenly into the air in lightning : but this is only a first effect, and not yet thunder, a most astonishing phenomenon, consisting undoubtedly in successive detonations, such as the report of cannons fired in a rapid succession ; and the former detonations must have with the latter this analogy of cause, that they are explosions of a particular expansible fluid, produced by that kind of sudden decomposition of atmospheric air, as happens by firing gunpowder and other processes.

VII. A direct proof of these sudden decompositions of some substances in such clouds, and simultaneous compositions of other substances, is the production of hail. This effect shows, that in a certain combination of circumstances, such a quantity of free fire enters suddenly into some combination, that the freezing point is much surpassed in the upper part of the clouds : hence the formation of grains of sleet so cold, that in falling through the clouds, their size is increased in the form of icicles, by the watery vesiculæ freezing upon them: of which formation the hail-stones bear all the characters, especially by having in the centre that opaque grain of sleet.

The foregoing are the most conspicuous of the operations produced, at certain times, in some strata of the atmosphere, but not all those which an attentive observer may perceive : they are here, as must be the case in the first steps concerning all invisible processes producing visible effects, explained only by general analogies with known causes in our chemical processes; and if we cannot yet approach nearer to specific causes, it is because we are still very backward in the knowledge of the subtle fluids, which, at different times,

« ZurückWeiter »