342. M. Saussure has given an instance of this periodic flux in electricity. On the 22nd of Feb. 1785, one of the coldest days ever remembered at Geneva, the hygrometer and thermometer were suspended in the open air on a terrace exposed to the south-west; the electrometer, from its situation, indicated an electricity equal to what it would have shown if it had been placed on an open plain. The height of the barometer was reduced to what it would have been if the mercury had been constantly at the temperature of 10° of Reaumur's thermometer. The place of observation was elevated sixty feet above the level of the lake. The observations of the day preceding and following this great cold were registered by him. There was a weak south-west wind during the whole three days; and it is rather remarkable, that most of the great colds, which have been observed at Geneva, were preceded by, or at least accompanied with, a little south-west breeze. From the first eighteen observations made during these three days, when the sky was quite serene, we learn that the electricity was pretty strong at nire o'clock A. M.; that from thence it gradually diminished till towards six o'clock P. M., which was its first minimum; after which it increased till eight o'clock, its second maximum; from whence it gradually declined till six the next morning, which was the time of its second minimum; after which, it again increased till ten in the morning, which was the first maximum of the following day; as this was cloudy, the periods were not so regular.

343. As we have hitherto but slightly noticed the method of exploring the atmosphere by the electrical kite, we shall here introduce an account of an experiment of this kind made by Mr. Cavallo, which is, in every respect, most gratifying and satisfactory; at least, it will be viewed in this light by every real lover of the science of electricity; and will, at the same time, read him a most useful lesson on the caution to be observed in making this boldest of all electrical experiments. It must be observed, that this experiment was made when there was no thunder; and that there had been none for three days before, nor did there happen to be any for three days after at the place where the scene occurred.

344. After,' says Mr. Cavallo, 'having rained a great deal in the morning and night before, the weather became a little clear in the afternoon, the clouds appearing separated, and pretty well defined. The wind was west, and rather strong, and the atmosphere in a temperate degree of heat. In these circumstances, at 3 P. M., I raised my electrical kite with 360 feet of string. After that the end of the string had been insulated, and a leather ball, covered with tin-foil, had been hung to it, I tried the power and quality of the electricity, which appeared to be positive and pretty strong.

345. In a short time, a small cloud passing over, the electricity increased a little; but the cloud being gone, it decreased again to its former degree. The string of the kite was now fastened by the silk lace to a post in the yard of the house in which I lived, which was situated near Islington, and I was repeatedly charging VOL. VIII

two coated phials, and giving shocks with them. While I was so doing, the electricity, which was still positive, began to decrease, and in two or three minutes' time it became so weak, that it could be hardly perceived with a very sensible cork-ball electrometer. Observing at the same time that a large and black cloud was approaching the zenith (which, no doubt, caused the decrease of the electricity), indicating imminent rain, I introduced the end of the string through a window, in a first-floor room, in which I fastened it by the silk lace to an old chair. The quadrant electrometer was set upon the same window, and was, by means of a wire, connected with the string of the kite.

346. It being now three-quarters of an hour after three o'clock, the electricity was absolutely unperceivable; however, in about three minutes time, it became again perceivable, but now upon trial was found to be negative; it is therefore plain, that its stopping was nothing more than a change from positive to negative, which was evidently occasioned by the approach of the cloud, part of which by this time had reached the zenith of the kite, and the rain also had begun to fall in large drops. The cloud came farther on, the rain increased, and, the electricity keeping pace with it, the electrometer soon arrived to 15°. Seeing now that the electricity was pretty strong, I began again to charge the two coated phials, and to give shocks with them; but the phials had not been charged above three or four times, before I perceived that the index of the electrometer was arrived at 35°, and was keeping still increasing. shocks now being very smart, I desisted from charging the phials any longer; and, considering the rapid advance of the electricity, thought to take off the insulation of the string, in case that if it should increase farther it might be silently conducted to the earth, without causing any accident, by being accumulated in the insulated string.

The

347. To effect this, as I had no proper apparatus near me, I thought to remove the silk lace, and fasten the string itself to the chair; accordingly I disengaged the wire that connected the electrometer with the string, laid hold of the string, untied it from the silk lace, and fastened it to the chair; but while I effected this, which took up less than half a minute of time, I received about a dozen or fifteen very strong shocks, which I felt all along my arms, in my breast and legs, shaking me in such a manner, that I had hardly power enough to effect my purpose, and to warn the people in the room to keep their distance. As soon as I took my hands off the string, the electricity (in consequence of the chair being a bad conductor) began to snap between the string and the shutter of the window, which was the nearest body to it. The snappings, which were audible at a good distance out of the room, seemed first isochronous with the shocks which I had received, but in about a minute's time oftener; so that the people of the house compared their sound to the rattling noise of a jack going when the fly is off.

348. The cloud now was just over the kite; it was black, and well defined, of almost a cir

F

cular form, its diameter appearing to be about 40°; the rain was copious, but not remarkably heavy. As the cloud was going off, the electrical snapping began to weaken, and in a short time became inaudible. I went then near the string, and finding the electricity weak, but still negative, I insulated it again, thinking to keep the kite up some time longer; but observing that another larger and denser cloud was approaching apace towards the zenith, as I had then no proper apparatus at hand, to prevent every possible accident, I resolved to pull the kite in; accordingly a gentleman who was by me began pulling it in, while I was winding up the string. The cloud was now very nearly over the kite, and the gentleman, who was pulling in the string, told me that he had received one or two slight shocks in his arms, and that if he were to feel one more, he would certainly let the string go; upon which I laid hold of the string, and pulled the kite in as fast as I could, without any farther observation, being then ten minutes after four o'clock.'

349. Mr. Cavallo, from the numerous experiments which he made with electrical kites, lays down the following results, which will be found correct in the generality of cases. (1.) The air appears to be electrified at all times; its electricity is constantly positive, and much stronger in frosty than in warm weather; but it is by no means less in the night than in the day-time. (2.) The presence of the clouds generally lessens the elasticity of the kite; sometimes it has no effect upon it; and it is very seldom that it increases it a little. To this the above-mentioned instance is a most remarkable exception. (3.) When it rains, the electricity of the kite is generally negative, and very seldom positive. (4.) The aurora borealis seems not to affect the electricity of the kite. (5.) The electric spark taken from the string of the kite, or from any insulated conductor connected with it, especially when it does not rain, is very seldom longer than a quarter of an inch; but it is exceedingly pungent. When the index of the electrometer is not higher than 20° the person who takes the spark will feel the effect of it in his legs; it appearing more like the discharge of an electric jar, than the spark taken from the prime conductor of an electrical machine. (6.) The electricity of the kite is generally stronger or weaker, according as the string is longer or shorter; but it does not keep any exact proportion to it. The electricity, for instance, brought down by a string of 100 yards, may raise the index of the electrometer to twenty, when, with double that length of string, the index of the electrometer will not go higher than twenty-five. (7.) When the weather is damp, and the electricity is pretty strong, the index of the electrometer, after taking a spark from the string, or presenting the knob of a coated phial to it, rises surprisingly quick to its usual place; but in dry and warm weather it rises very slowly. (8.) The principal use of the electrical kite is to show the electricity of the atmosphere; and it is perhaps the only instrument that will do this at all times with certainty, though several others have been invented for that purpose. But another use to which electri

cai kites have been applied, is to bring down quantities of the electric fluid from the upper regions of the atmosphere, for the purpose of supplying that deficiency of electricity, which is supposed to be hurtful to vegetation."

350. From numerous experiments on the spontaneous electricity of the atmosphere made by Mr. Read, he has drawn the following conclusions:-(1.) That in moderate weather it is uniformly positive, and experiences an increase and a decrease in the degree of its intensity twice in twenty-four hours. (2.) That the electricity is strongest about two or three hours after sun-rise, and some time both before and after sun-set; and is in general in the weakest state between noon and four o'clock. (3.) And that this period cal electricity is obviously influenced by heat and cold.'

351. The latest experiments that have been made on this subject, on a large scale, are those by Andrew Crosse, Esq. of Broomfield, near Taunton. They were made by means of an apparatus the most extensive ever constructed. It consisted of a copper wire, one-sixteenth of an inch in thickness, stretched and insulated between two strong upright poles measuring from 100 to 110 feet in height.

352. No pains was spared to render this apparatus the most extensive and perfect that has been constructed. The insulated wire was one mile and a quarter in length, but having been exposed to various depredations, and liable to injury from other causes, it was shortened to 1800 feet. Every contrivance was tried to insulate this wire, but Mr. Crosse could not succeed in preserving the insulation during a dense fog, or a driving snow. A contrivance was adopted to lower the insulators, for the purpose of freeing them from spiders' webs; and it was necessary to fix the wire very securely, in order that it might be able to resist the weight of the great numbers of swallows that often perched upon it, and of wood-pigeons and owls that flew against it with considerable force. This apparatus has been in use for some years, and has enabled Mr. Crosse to draw the following conclusions, which confirm the observations of preceding authors.

353. (1.) The electricity is invariably positive during the usual state of the atmosphere, and subject to a regular increase and decrease, as stated in some of the preceding observations.

354. (2.) Fogs, rain, snow, hail, and sleet, produce changes in the electrical state of the wire. The electricity is negative when they first appear. It frequently changes to positive, increasing gradually in strength; and then decreasing in a similar manner, and changing from positive to negative every three or four minutes. Those phenomena have been so constantly observed, that, whenever the wire appears negatively electrified, it is considered as a certain indication, that either rain, snow, hail, mist, or a thunder-cloud, is approaching.

355. (3.) The approach of a charged cloud at first is sometimes found to produce positive, and sometimes negative electricity; but, whatever be the kind of the electricity which first appears, its intensity increases to a certain degree, then diminishes, and finally disappears, and is suc

ceeded by the opposite electricity; which increases to a higher degree than the first had done; it then diminishes, and vanishes, and is again succeeded by the electricity which first appeared. These alternations of positive and negative electricity are often exceedingly numerous, and on different occasions succeed one another with different degrees of rapidity. The electricity, in general, becomes more intense at every repetition, till a copious and dense stream of sparks issues from the atmospherical conductor to the receiving-ball, stopping at intervals, and returning with redoubled force.

356. (4.) In this state of things, a strong current of air flows from the wire, and the apparatus with which it is connected. At every flash of lightning an explosive stream, attended by a very peculiar noise, passes between the balls and the apparatus, and brilliantly illuminates the surrounding objects, while the effects on the spectator are heightened by the successive peals of thunder, and the consciousness of being so near their cause.

357. When the electricity becomes too powerful to allow the observer to operate in safety, he connects the insulating wire with the ground; along this the whole passes off silently and harmlessly to the ground.

358. (5.) During a driving fog, or a smart rain, the wire is electrified almost as powerfully as during a thunder-storm, and the electricity exhibits similar changes.

359. (6.) A weak positive electricity generally prevails in cloudy weather. It often changes to negative when rain falls; but the positive electricity re-appears when the rain has ceased to fall. 360. (7.) The electricity is always stronger in clear frosty weather than during a fine summer's day.'

361. The following table, drawn up by Mr. Crosse, exhibits the intensity of the electrical appearances of the atmosphere in different states, commencing with those in which it is most powerful :

(1.) During the occurrence of regular thunder clouds.

(2.) A driving fog, accompanied by small rain. (3.) A fall of snow, or a brisk hail-storm. (4.) A smart shower, especially in a hot day. (5.) Hot weather succeeding a series of wet days.

(6.) Wet weather following a series of dry days. (7.) Clear frosty weather, either in the night or day.

(8.) Clear warm summer weather. (9.) A sky obscured by clouds. (10.) A mackerel back, or mottled sky. (11.) Sultry weather, the sky covered with light hazy clouds.

(12.) A cold damp night.

362. For common purposes, and occasional observations, Mr. Singer says, very simple contrivances may be employed. A common jointed fishing-rod, having a glass stick covered with sealing-wax substituted for the smallest joint, may be occasionally projected from the upper window of a house. A pair of pith-balls must be attached to a cork, in which the end of the glass stick is thrust; and this part of the appa

ratus is to be occasionally uninsulated, by placing a pin in the cork, connected with a thin wire held in the hand. In this uninsulated state, the fishing-rod and its attached electrometer are to be held for a few seconds projecting from the window, and, whilst in this position, the pin is to be withdrawn by pulling the thin wire; this insulates the electrometer, which may be then drawn in and examined. Its electricity will be found to be contrary to that of the atmosphere.

363. The last circumstance which we shall notice respecting the electricity of the atmosphere, is its effects on the vegetable kingdom. Much has been said relative to the merits of the question, whether the electricity of the atmosphere has or has not an influence on the process of vegetation. It is remarkable that this should ever for one moment have been matter of doubt with any one, since it is well known that light and heat, and, in one word, the free access of the air of our atmosphere, are all essential to the general process of vegetation. Where difficulties have occurred in the course of experiment on the subject, we are firmly persuaded that they relate entirely to the management of the apparatus employed; we can readily manage the regulations of warming, watering, and airing a small house, in which we wish to accelerate the growth of vegetable substances; but when we attempt to take, as it were, the management of the atmosphere in our own hands, and to regulate the general process of vegetation in our fields or gardens, we must expect to meet that disappointment which is ever attendant on quitting the sphere of action allotted to mortals. No series of experiments, how nicely soever conducted, can ever be expected to equal the silent, the invisible, but unerring operations of nature.

364. The first electrician who seems to have attended to this subject was Mr. Maimbray of Edinburgh, who, in the year 1746, electrified two myrtles, during the whole month of October, for some hours every day. The consequence was, that, in the following summer, these electrified myrtles put forth buds and blossoms sooner than those which had been left to nature.

365. Mr. Maimbray was followed by the abbé Nollet, who made some comparative experiments on the germination of seeds under similar circumstances, except that one plot was electrified three or four hours every day during the space of fifteen days. His experiments were attended with results similar to those obtained by Mr. Maimbray.

366. Experiments of a similar nature were repeated by others, and, as was to be expected, were in general followed by the like results. Hence the effect of electricity in promoting vegetation became universally acknowledged, till a series of well-conducted experiments, made by Dr. Ingenhousz, staggered the faith of philosophers in general on the subject

367. Several electricians, however, labored hard to support the credit of the abbé Nollet's system, although but with little success. Among these, the chief was the abbé Bertholon, who wrote a work which was entirely confined to the subject. The reasoning of this author, however incorrect it may appear to many, has certainly

is a calcareous substance, generally used in the form of a powder, which has the property of absorbing light when exposed to it, and afterwards appearing lucid when brought into the dark. Take some of this powder, and, by means of spirits of wine or ether, stick it all over the inside of a clear glass phial, and stop it with a glass stopper, or a cork and sealing-wax. If kept in a room perfectly darkened, it will give no light; but let two or three sparks be drawn from the prime conductor, when the phial is kept at about two inches from the sparks, so that it may be exposed to that light, and this phial will receive that light, and afterwards will appear illuminated for a considerable time. The powder may be stuck upon a board by the white of an egg, so as to represent figures of planets, letters, or any thing else; and these may be illuminated in the dark, in the same manner as the phial. A beautiful method to express geometrical figures with the above phosphorus, is to bend small glass tubes of about the tenth part of an inch diameter, in the shape and figure desired, and then fill them with the phosphoric powder. These may be illuminated in the manner described, and they are not so subject to be spoiled as the figures represented upon the board frequently are. The best method of illuminating this phosphorus, and which Mr. W. Canton generally used, is to discharge a small electric jar

near it.

378. The tourmalin.-The tourmalin, or the lapis electricus of Linnæus, is a hard semi-pellucid fossil, and was first observed to exhibit electrical phenomena, on being heated and cooled. This stone is found in abundance in the East Indies, and is named the electrical stone from its possessing many singular electrical properties. The properties of this stone seem to have been known to the ancients: Theophrastus mentions a stone which he terms the lyncurium, and describes it as being very hard; susceptible of a high polish, very useful for making seals, &c.; and possessing the property of attracting light substances. Hence there can be little doubt as to its being the tourmalin of the moderns. The Dutch seem to have discovered the properties of the tourmalin by observing that when placed in the fire it attracted the ashes; hence they gave it the name of aschen-trikker.

379. But by increasing its heat it becomes electrical, and still more so by diminishing it. Its electricity appears, not over its whole surface, but only on two opposite sides, which have been styled its poles, as they are in a line with its centre, and in the same direction with its strata; in which direction it is opaque, though semitransparent in the other.

380. During the process of heating, the tourmalin has one of its sides electrified positively, and the other negatively; but, while cooling, the former becomes negative and the latter positive. If heated and allowed to cool without either side being touched, the former will be positive and the latter negative, all the time it is heating or cooling. If excited by rubbing, each of its sides, or both at once may be rendered positive. I heated or cooled upon an insulated substance that substance will become possessed of the elec

tricity contrary to that of the side of the tourmalin, which was laid on it. The electricity of both sides, or of either, may be reversed by heating or cooling the stone, in contact with other bodies.

381. If a tourmalin be cut in pieces, each piece will have its positive and negative poles, as well as the whole stone. All the above properties are observable in vacuo. If this stone be covered over with wax, oil, or any similar electric, it will exhibit the same electric signs as without the covering.

382. In making experiments with the tourmalin, Mr. Canton observed a vivid light upon it, while heating in the dark, by which he could determine which end of the tourmalin was positive or negative. When excited, it emits very strong flashes in the dark, from the positive to the negative end. Mr. Canton has also observed the Brasilian emerald em.t light while heating in the dark. Mr. Cavallo imagines that every other precious stone will show it if its electric power be sufficiently strong; since the light is a consequence of the passage of a sufficient quantity of electricity through the air, or other partly resisting medium.'

383. The electrical power of the tourmalin is sometimes improved, sometimes injured, sometimes not in the least affected by a strong fire. Most of the above properties, which were supposed to be peculiar to the tourmalin, are possessed by several hard precious stones, which are capable of becoming electrical by heating and cooling, and have their positive and negative sides lying in the direction of their strata, &c.

384. Evaporation produces electricity.—This appears to have been a discovery of Signor Volta, who observed that the evaporation of water and some other fluids, as well as certain effervescences, generated electricity. His experiments seem to prove that fluids, or other bodies, reduced to vapor, become electrified positively, and leave the bodies from which they evaporated, electrified negatively; and that on the other hand, when the vapors are condensed into a fluid, they become electrified negatively, and leave the bodies with which they were last in contact electrified positively.

385. The common method of illustrating this is the following:-Place on the cap of a goldleaf electrometer a metallic dish containing a few lighted coals, and project on them a few drops of water, whilst an insulated funnel is held about a foot or eighteen inches above. Under these circumstances the electrometer is found to be negatively electrified, and the insulated funnel positively. On this experiment Mr. Walker observes, that the vapor carries off the latent electricity from the electrometer; the leaves diverge, and fly to the slips of tin-foil to supply their loss. He adds, perhaps the vapor derives its volatility from its union with electricity, for it is observable, that if insulated pith-balls be suspended in a fog or mist, they separate spontaneously with positive electricity.

386. To electrify the air of a room.-This is an experiment which may be easily performed with a powerful machine. The room ought to be very clean, and as free as possible from