phenomena are never manifested to us without matter; that there is no static phenomenon without a body coercing and preserving the electricity; that there is no dynamic phenomenon without a conducting body; that wherever there is an electric phenomenon, there is a ponderable body. The only question which leaves any doubt, is that of knowing whether the spark is or is not accompanied by terrestrial matter, and whether the electric radiation can be made without the transport of this matter. Not being able to quote actually all the experiments which have determined us to adopt this opinion, we will call to mind that the radiation between two bodies takes place the more easily, as they are more volatile; that balls of platina must be held nearer to one another in vacuum than balls of zinc, to have a constant current of light; that the radiation is still more easily made with mercury, and incomparably better with water. Moreover, the experiments we are about to give, and which depend only upon meteorology, will be sufficient to render it probable that in this phenomenon ponderable matter is invariably transported, and that the general phenomenon only exists in consequence.

36. The preceding experiments have proved that the earth is a body charged with resinous electricity, or to express it more logically, that it possesses the cause of the phenomena to which we have given this name for more than a century. The celestial space, uot being a material body, does not possess this power of coercion, it is not in the same state of resinous electricity; and it is this state of negatively resinous which has been named vitreous. It is not, as we have already said, a special, real state, caused by a peculiar substance or a peculiar modification; it is only the absence of the resinous state, or this state at a less degree. It is merely a difference and not a particular state. To avoid circumlocution, we have retained the word vitreous to express this difference in minus of resinous coercion. In a special work we will hereafter give the proofs which induce us to reject this nomenclature as defective, and likely to retard the progress of the science of electricity.

CHAPTER III.

Of the Presence of Vapours in the Atmosphere.

37. We have seen that the vapours produced solely by the influence of the air and of heat below 110° did not possess free electricity; we are now going to resume these experiments by placing ourselves in the circumstances which exist in nature in order to obtain the same effects. Two kinds of evaporation take place in the atmosphere; that which is effected at the surface of water and humid ground, and that which is effected when opaque clouds return into the state of elastic vapour. To imitate the first, I fill a platina capsule with distilled or common water, and I place it upon a thermoscopic tripod, insulated on cakes of resin, and I connect the tripod with an appropriate rheometer; I allow the spontaneous evaporation to continue some time, until the rheometer remains con

stant during the cooling process. I then separate the rheometer in order to insulate the tripod, and I keep the capsule during a few minutes in a rather strong resinous tension. Afterwards, on re-establishing the communication with the rheometer, the latter indicates a higher degree than that of the spontaneous evaporation; which proves that during the action of resinous electricity the evaporation has been more considerable. The needle afterwards gradually returns to the primitive degree, and thus completes the demonstration. The same fact may be verified in another manner. The capsule is placed on an insulating body, and a body charged with vitreous electricity is suspended above it; under the induction of this body the water of the capsule quickly becomes vitreous, which indicates that the vapour which rises from it is resinous, and that it is the material vehicle which carries to the vitreous body the contrary tension by which it is neutralized. The vitreous tension of the water is so great that it must not be examined without a proof plane, for if the electroscope were brought into immediate contact, the gold leaves might be destroyed by it.

38. I have recently made an electric hygrometer, founded on the ratio which exists between the loss of electricty and the quantity of vapours contained in the atmosphere: I shall elsewhere give a description of it. I will merely state, to render the experiment to which I am about to refer intelligible, that it is nothing else than my index electrometer surmounted by a metallic tuft, to which a constant charge of electricity is given. I exposed this hygrometer in the open air upon a very elevated place; Saussure's hygrometer indicated 75°, vapours disturbed the aspect of the sky, and it was necessary to raise the electrometer one meter to get a divergence, which was obtained by raising it two decimeters beneath a perfectly clear sky. The hygrometer was 30 minutes losing the quantity of electricity necessary to reduce it from 60° to 50°, although it was surmounted by a large tuft of very fine copper wire. On moistening this tuft, or by covering it with a wet cloth, it required only 10 minutes to allow the same quantity to pass off. Another time, the temperature and Saussure's hygrometer being at the same degrees as in the preceding experiment, but the electrometer indicating very resinous vapours in the air, the index of the instrument, instead of proceeding towards 0°, augmented its deviation by several degrees. Lastly, in another experiment, made in the midst of a strongly vitreous fog, the index rapidly descended to 0°.

39. The vapours produced at the surface of the globe, under a clear sky, are necessarily resinous, since they are formed at the surface of a body which possesses a considerable resinous tension, and in presence of the celestial space which does not possess it. These vapours preserve this tension during the whole time that they float in the atmostphere, and are insulated from bodies less resinous than themselves, or from conductors to the ground. They also rise to a height above that which they would attain by their gravity, and they do not stop until the point where their specific lightness, added

to the electric repulsion of the globe, is in equilibrium with gravitation. The resinous vapours are maintained at a higher elevation than the neutral vapours, and still more than those which are in the vitreous state. The mutual repulsion which the particles of resinous vapour exert upon each other as insulated electric bodies, being augmented by that of the terrestrial globe, they are caused to recede further from each other; they are, in fact, more dilated than consists with the mere state of elastic vapour; they weigh less upon other bodies, and we shall see, in another memoir, that it is to these electric differences of the vapours that the horary and accidental oscillations of the barometer are owing.

40. The first effect of such an atmospheric state is to place us in the centre of an homogeneous induction, resinous towards the earth, and also resinous towards the vapours which are above us; and our instruments, which only indicate differences, being immersed in a uniform inclosure, obey in a less degree the resinous tension of the earth, as the atmosphere contains more of this primitive vapour. The indication of the electroscope will then diminish in the course of the day proportionally to the quantity of vapours which have been disengaged under the opposite tensions of the earth and of space; and the variations of the instrument, if other causes did not concur, would serve to indicate hygrometic changes. Baron von Humboldt says, in the Tableau Physique des Régions Equinoxiales, in 4to, 1807, p. 100, "In the lower equinoctial regions, from the sea to 200 meters, the lower strata of the air are slightly charged with electricity it is difficult to find signs after ten o'clock in the morning, even with Bennet's electrometer. All the fluid appears to be accumulated in the clouds, which causes frequent electric explosions, which are periodical, generally two hours after the culmination of the sun, at the maximum of heat, and when the barometric tides are near their minimum. In the valleys of the great rivers, for example, in those of the Magdalen, the Rio Negro and the Cassiquiaré, the storms are constantly towards midnight. In the Andes, the height at which the electric explosions are stronger and make more noise is between 1800 and 2000 meters. The valleys of Caloto and Popayan are known by the frightful frequency of these phenomena." We quote this passage to show that in the regions where the electric phenomena are more considerable and more numerous, the electrometer indicates nothing; but it will now be understood that this absence of sign arises from its being always placed, in these regions, at the centre of a sphere of resinous vapours, produced every day by the high temperature.

41. When the atmosphere is thus charged with resinous vapours, it is evident that in raising oneself above the ground the instrument is disengaged from these vapours, and that the opposition of tension must re-appear; the inferior induction again takes its resinous superiority, and the superior induction loses it, and becomes vitreous by opposition. In atmospheric experiments, the number of decimeters or of meters which it is necessary to raise an electrometer in

order to have an equal divergence, is the measure of the resinous vapours existing in the upper atmosphere; and if in very dry weather, elevating the electrometer a decimeter is sufficient to obtain a divergence of five degrees, and should it be necessary, another day to raise the same instrument sixteen decimeters to obtain the same divergence, the upper atmosphere contains four times more of this elastic vapour. The vitreous induction of space decreases as the square of the resinous inductions which are interposed between it and our instruments.

42. The indication of the electrometers depending on the point of their neutralization and on the purity of the atmosphere, it would be necessary to fix the place of this point of neutralization, and to decide once for all on the moment which ought to be assumed for the maximum of resinous tension of the globe. To have the absolute measure of the terrestrial tension a complete absence of resinous vapour would be required; that is to say, it would be necessary to operate at the pole itself, when all the vapours have disappeared, and to measure, at the winter solstice, the divergence of a standard electrometer which has been raised one decimeter. If we possessed an experiment thus made, all other electrometers might be regulated by this standard electrometer, as barometers are regulated by a standard barometer. This means not being practicable, that must be chosen which is the most so for each climate. Thus in Russia, at Kasan, for example, a frost of -25° C., which has lasted twenty days, might be taken; at Berlin a frost of the same duration of 15° C. might be taken. At Paris it would be necessary to profit by a winter which would give during ten successive days a cold of 10° C. with an easterly wind. Severe and long frosts are too rare at Paris to serve as starting-points; it is better to adopt a less degree of cold, but which more frequently occurs, unless for the purpose of comparing the electrometers with those of Berlin, St. Petersburg or Kasan. It is obvious that the lower the temperature is, the less interposed vapour there will exist, and the nearer we shall be to the absolute tension of the earth.

43. During all these regular and successive changes, the fixed apparatus indicate nothing; they have always time to put themselves in equilibrium. These instruments are inactive unless their length be considerable, unless the electricity of induction, radiating from an extended surface of great curvature, as is the case with metallic wires, leaves in the instrument only the repelled electricity. When the exploring wire is very long, the radiation is sufficient to produce a continued current and cause the rheoscopes to act. These instruments like static electrometers, indicate differences only; the current will also diminish in proportion to the resinous vapours which the atmosphere, by which the instrument is surrounded, may contain, and it is necessary, by a greater elevation, to cause it to extend beyond this electrically homogeneous envelope. There is another cause of electric indication, which does not seem to have been sufficiently guarded against, viz. the chemical action of vapours on the

oxidizable wire or rods of which the apparatus consists. An iron bar, like that of a lightning conductor, gives a continued electric current; a moist copper wire, twenty to thirty meters long, likewise gives one. These currents are always negative from top to bottom, and they increase in proportion to the humidity which has been deposited. Another cause of error is that which arises from the metals which enter into the construction of the building. When the walls and the supports are rather damp and they have become conductors, the oxidation which these metals undergo furnish a chemical current to the neighbouring conductor which carries it to the ground, and it is then difficult to say what the direction of the current will be it will depend on the moisture of the vicinity to which the supports are attached. Of the electric apparatus which I possess, there is one which constantly gives an electric current, because it is formed of a bar of iron and of a zinc vane painted in oil colour. In damp weather the needle of rheometer ascends to 80°, without there being any electric action of the atmosphere. We must therefore take care not to register such results as atmospheric products. For more security we must only expose to the air a tuft of platina wire, and cover the conducting wire with silk, covered over with an oily varnish, and insulate it in the best manner possible. With these precautions, the height of our buildings is hardly ever sufficient to enable us to obtain an electric current beneath a serene sky, a current which in these circumstances is always resinous from bottom to top.

44. When by cooling, these resinous vapours are condensed and have formed opaque clouds, they preserve the quantity of electricity which they possessed; but this electricity is no longer distributed with the primitive uniformity which it had in elastic vapour below its point of saturation. The molecules gathered then into small spheres or vesicles no longer retain their entire electric tension; in this new state they conduct better, and the electricity is differently distributed among them; their agglomeration into small masses or cloudy flakes gives them the envelope of a body, as well as preserving the individuality of the constituent parts. There are then two perfectly distinct distributions in opaque vapours: the electricity which proceeds to the surface of the masses, as it does to the surface of bodies: and that which is retained around these vesicular particles, sufficiently insulated from each other to maintain a portion of their primitive electric tension. The result of this better conductibility is, that by the resinous induction of the earth, the lower side of the cloud will be less charged with this same electricity, the upper side will be more charged, and our instruments will give better indications beneath an opaque cloud than when they were inclosed in the elastic vapour which served to form it. On account of its importance we insist upon, and we return to the first modification of the elastic vapours: at first all are equally resinous, and the electric manifestations of our instruments immersed within them,

diminish more and more. In consequence of their transformation