NOTE E, p. 342.

As M. PELTIER deduces from his experiments and observations interpretations very different from those of other philosophers, we deem it useful to present here the summary of the researches that he has published up to the present time, and those which he has recently communicated to us.

An old experiment of DE SAUSSURE and ERMANN, which remained without any result, is the starting point of the series of facts which induce him to regard the aqueous and igneous phenomena on the atmosphere in an entirely new point of view. The following is this fundamental experiment, as it has been modified by M. PELTIER.*

A person stands on a perfectly open place, above all the surrounding objects: he takes an electrometer, armed with a rod of about four decimetres in length, surmounted by a ball of polished metal of from three to four centimetres of radius, in order to increase the effects of induction, and to avoid the escape of the electricity which may be repelled in the upper part. The instrument is held in one hand and adjusted with the other, by putting the rod in connexion with the platinum. All the reactions being equal on all sides, the gold leaves of the electrometer fall straight, and mark zero. In this state of equilibrium the instrument may be left in contact with the free air for a whole day, under a clear sky, without its manifesting the least signs of electricity: we may even move it and agitate the air; from the instant it is held at the same height, it will remain entirely still. But if, instead of leaving it in the same stratum of air, we raise it from four to five decimetres, we immediately see the gold leaves diverge and indicate a vitreous tension. (M. PELTIER prefers the words vitreous and resinous to positive and negative, as being less significant and not savouring of any theory.) If we restore the instrument to the original place, the leaves fall again exactly to zero; if we cause it to descend below this point of equilibrium, the leaves diverge again, but then they are charged with resinous electricity. On raising it again to the original point, the instrument recovers its zero, and preserves nothing of the free electricities which it shewed an instant before. Since no free electricity remains in the instrument, the air, therefore, has not communicated any thing to it, and the signs which it gave were only the result of a new distribution in the electricity, which the rod possessed at the point of equilibrium; we have merely to replace the instrument at the same point to make them disappear. They were, in short, only signs of induced electricity in a body which approaches or recedes from another charged with free electricity--a phenomenon

Vide his Memoirs in the Annales de Chimie et de Physique, t. iv. 3d series; Mémoires de l'Acad. de Bruxelles, t. xv. 2d part; his Traité des Trombes; the article Atmosphère of the Supplement to the Dictionnaire des Sciences Naturelles; and the Comptes rendus de l'Acad. des Sciences of Paris, from 1838 to 1842.

which may be reproduced in a room by standing on a resinous or under a vitreous surface.

If, instead of a polished ball, we place one or several points, or a lighted match, as VOLTA did, the phenomenon ceases to be simple, and no longer permits us to distinguish whether the primitive effect was a new distribution of electricity, or whether it is electricity taken from the atmosphere. In fact, when we raise the instrument, the resinous electricity coerced by the vitreous induction of celestial space to the extremity of the rod, instead of maintaining itself there, escapes by the points or by the flame; when we lower the instrument it is deficient of all the dispersed electricity, and the former equilibrium cannot be re-established. Then there remains permanent vitreous electricity, which is wrongly attributed to the contact of the air; it is in reality only the portion separated from that of the contrary name which has vanished by the points, and which can no longer be neutralised when we replace the instrument at the original point.

This experiment, proving that neither the air, nor the vapour that it contains, possesses free vitreous electricity, invalidated the consequences that VOLTA, LAVOISIER, and LAPLACE had drawn from their experiments. On repeating and analysing these latter,* M. PELTIER has endeavoured to prove that vapour produced at a temperature below 110° centigrade never carries off free electricity; that there is no electricity but that formed at a temperature higher than 110. This temperature not being that of the surface of the globe, the electric vapours which rise cannot, therefore, proceed from the simple evaporation of saline or pure waters.†

The electricity of clouds and fogs cannot be denied: he inquired whence it proceeded. As soon as he had established that the terrestrial globe is a body charged with resinous electricity, it was easy to demonstrate, by experiment, that the vapour which rises is resinous like itself, that this electric state of the globe is a powerful cause of evaporation, and that this latter may be quintupled and sextupled by a high tension. The vapour which rises from the earth being resinous like itself, its tension must react downwards against that of the globe, and successively reduce all its effects. This is what takes place; and it demonstrates the decrease of terrestrial induction on the electrometers in proportion as vapour is formed during the heat of the day. These instruments do not give the measure of the whole of the electricity, but only the difference of the quantities which act upon the coatings on the one hand, and upon the rod that sustains the gold leaves on the other. It follows, that they may be placed in the centre of a mass of vapours, charged with a great quantity of electricity, without giving the least sign of it. Thus, their decreasing manifestation with the formation of vapours is a proof that these latter are charged with resinous electricity like the globe, and that they react from above downwards against its action, which is from below upwards.

Vapour being in a trifling degree conducteous, does not long retain the equal distribution of its resinous tension; the incessant action of the globe repels the resinous electricity towards the upper strata, and thus renders the lower strata vitreous. The new distribution of

Researches on the Cause of the Electric Phenomena of the Atmosphere, Annales de Chimie et de Physique, t. 4, 3d series.

↑ Vide note l, Appendix No. II.

electricity is made the more easily in proportion as the density increases. This is why the electrometer, which had almost ceased to give electric signs in the middle of the day, gradually recovers extent in its indications when the condensation of the evening is felt the lower vapours become vitreous by induction, and the upper vapours become more resinous. During the night, the lower vapours being deposited in dew, the quantity of the vitreous vapours has diminished, the upper vapours then react more freely, and, towards the morning, the electrometer gives a less amount of indication than it did on the previous evening.

The first effect of the rising sun is to cause the vapours, that are condensed during the night, to return into the state of elastic vapour, whether they are or are not in the vesicular state.

These vapours being placed between the resinous earth and the vitreous celestial space, the first, that pass into the state of elastic fluid, carry off, on rising, a higher resinous tension, which they obtain by weakening such of the vapours as they leave behind, and which, having thus become less resinous than the globe, are vitreous in comparison to it and our instruments. During this first moment of the re-evaporation of the upper vapours, the strata left behind having become vitreous, are attracted by the earth, act more on our instruments by their proximity, and often produce a second dew, until at length the sun, by darting direct rays upon the earth itself, warms it and reproduces resinous vapours, which are diffused in the atmosphere, and react from above downwards on the instrument, as on the previous evening, and reduce anew the effect of the globe.

This play of electric induction is shewn on a very grand scale, and several times a-day, around the tops of high mountains. Since M. PELTIER has established that all grey and slate-coloured clouds are charged with resinous electricity, and that all the white, rose, or orange-coloured clouds are charged with vitreous electricity, it was easy for him to follow at a distance this order of phenomena, without being obliged to go and measure their tension with the electrometer. This is the extract of his observations. When a white cloud hangs over the summit of a mountain, its powerful vitreous tension rouses and hastens the evaporation of its damp sides; the quantity of vapours produced get beyond the point of saturation of these cold regions, they immediately pass into the state of vesicular vapour, and appear under the form of ashy-grey flakes, of a tint which is deeper as the upper cloud is of a more dazzling white. The grey tint does long remain uniformly distributed; the vitreous attraction of the white cloud renders the bordering of the grey cloud more resinous, which then takes a deeper tint and forms a narrow riband at its upper part. This extreme stratum is divided into sinuous and trembling striæ, which shake, rise, and disappear, by passing again into the state of elastic vapour. These first vapours being dispersed, are replaced by others, which experience the same transformation, and so on. The greatest resinous tension of the upper riband, as manifested by its more slate-coloured tint, cannot take place but by taking from the former vapours the resinous electricity which they have carried off from the earth; by the decrease of their resinous tension, these latter lose by degrees their grey tint, and finally become almost as white as the upper cloud. This last itself has lost its original lustre, in proportion as its own vapours were neutralized by those which radiated from the grey band. The phenomenon then

stops, and the mountain ceases to smoke, not to commence again until the winds shall have cleared it from those clouds which have become similar.

The presence of a grey cloud above the summit of the mountain produces an analogous effect, but with inverse electric signs. The cloud which issues from its sides is white; it is charged with vitreous electricity; its upper riband is more dazzling than the centre; it repasses into the state of electric fluid, and the rest loses by degrees its lustre and becomes grey. In fact, these same phenomena are again produced under a clear sky, but with less energy; the vitreous tension of celestial space is sufficient to carry the evaporation beyond the point of saturation of that stratum of air. The electrometer also frequently indicates that the invisible elastic vapour is powerfully charged with electricity, which is sometimes vitreous, at other times resinous. Under this new influence, the smoking of the mountains considerably increases; this abundance of vapours proceeding from the side of mountains is itself an indication of the presence of upper vapours, still in a transparent state. It also shews that their near condensation, in proportion as their electricity is neutralised, will give abundant rains.

On attentively following all these transformations on the mountains or in the middle of plains, we see that every day produces very nearly the same series of facts. There are vapours produced either by temperature alone, or by temperature seconded by electric attraction; then towards evening, or during the night, their condensation takes place, and, consequently, a new distribution of electricity under the influence of the globe. At sunrise, it is the re-evaporation of opaque vapours, or a fresh dilatation of those which are yet elastic; both are produced under this same influence, resinous below, vitreous above the first vapours that rise are the most resinous, the last are less so, and are then vitreous in comparison to the first; they thus form opaque clouds of different tensions, when cooling condenses them. The daily vapours. by thus rising in the atmosphere, soon experience the effect of another electric induction, which powerfully reacts from above downwards: it is that of the upper current of the atmosphere, which carries away to the polar regions the resinous vapours of the tropical regions. The height of this current and the energy of its resinous tension varying with the seasons, produce reactions more or less distant from the surface of the earth. There is, again, between these two forces a resultant of the difference depending on the proximity of one or other of these forces, and the concomitant actions of temperature and winds. We cannot enter into further details, it would be trespassing beyond the limits of a note; it is enough to have pointed out the new path that M. PELTIER has followed in his works; each one will be able to obtain evidence for or against these results by new observations, and thus to decide, by the aid of time, which is the road that conducts most directly to the knowledge of the true cause of the meteors.

Note F. p. 409.

The succession of tints by which the sky coloured during twiTight offers several peculiarities which are not very easy to explain, in the imperfect state in which the optics of gases are still found. As a complete explanation of the phenomenon may serve as a basis to every theory that would explain these facts, I will here communicate the results of the observations made by M. BRAVAIS on the summit of the Faulhorn, 2683 metres above the level of the sea, and which embrace no less than thirty perfectly serene twilights. He had at his disposal the most exact means of appreciating both the position of the sun above or below the horizon, and the apparent distance of the coloured zones from the zenith of the beholder. We are about to follow the sun in each two degrees of its course, in proportion as it approaches the horizon, towards morning twilight. The following zenithal distances refer to the centre of the sun, not displaced by the effect of refraction; zenithal distances greater than 90° indicate that the sun has not yet risen.

1st. Zenithal distance of the sun 102°. In the east a reddish or orange-coloured band, whose height is nearly equal to 0°. We do not yet distinguish any other tint above this orange-coloured band; the height of the crepuscular curve is 7°. The spindle comprised between these two arcs is of a whitish blue, clearer than the rest of the sky.

2d. Zenithal distance of the sun 100°. The height of the orange zone is 1°. Above, yellow begins to appear, and its height attains 2° 30′ in the sun's vertical. No green is yet to be seen. The height of the crepuscular curve is 12°.

3d. Zenithal distance of the sun 98°. The part tinged with red extends from the horizon to 1° 15'. Above, yellow tint as far as 3° 10′. Green begins to appear upon the yellow; the greenish band scarcely gets beyond the height of 5°. Above, a weaker bluish shade, as far as 25°, where the limit of the twilight is formed.

4th. Zenithal distance of the sun 96°. The elevation of the orange-coloured and yellow zone has not changed; the greenish tint prevails to a height of 7°. The crepuscular curve rapidly reaches the zenith; its height is 70°. The western sky does not yet present any trace of light.

5th. Zenithal distance of the sun 94°. The yellow and orangecoloured bands preserve the same elevation above the horizon. The greenish zone attains to 12°. Above it a purplish tint begins to be manifested, at least if circumstances be favourable. M. BRAVAIS has never seen it begin till the zenithal distance of the sun is equal to 95°, nor remain after that distance has become less than 93°. It forms a little after the passage of the crepuscular curve beyond the zenith, and its existence does not last many minutes. It is towards the height of 25° that this rose-coloured tint presents its maximum of intensity, and it does not get beyond the height of 45°. We do not observe the yellowish fringe which separates it from the greenish region situated below. The tint of the zenith is blue, sometimes perhaps slightly tinged with green.