found to contain iron in a metallic ftate. On the whole-for our noble observer does not appear to be a profound chemistwe may infer, that the folid contents of the rain in queftion were the effects of a volcanic eruption.

Article 6. An Account of fome Scoria from Iron Works, which refemble the vitrified Filaments defcribed by Sir William Hamilton : "By Samuel More, Efq.

! In the 70th volume of the Philofophical Tranfactions, Sir William Hamilton, treating of an eruption of Mount Vesuvius, gives an account of certain long filaments of vitrified matter, like fpun glafs, which were mixed with and fell with the afhes. The origin and nature of these curious filaments are explained by the Author of this Article; who has prefented the Society with a fpecimen of fome flag, or vitrified cinder, taken from one of the largeft works in England for fmelting iron; and which, by means of the ftrong blaft of air from the bellows, has been drawn out, while in its fluid ftate, into cobweb-like threads (fome of them 10 or 12 feet in length) which being driven upwards by the blaft, fix themselves to the beams and other parts of the bellows room. They are fo extremely flender, as to refemble cotton in appearance; but, being examined with a microfcope, are found in all refpects fimilar to thofe defcribed by Sir William Hamilton; which are undoubtedly formed of the melted lava, ejected from the mouth of the crater, and probably drawn out into threads, by the force of thofe violent torrents of air which must be required to fupport so intense a body of fire as that of the volcano.

ELECTRICITY. Article 16. Del modo di render, &c. Of the Method of rendering very fenfible the weakest natural or artificial Electricity: By M. Alexander Volta, Profeffor of Experimental Philofophy in Como, &c. &c.

There are few philofophers who have contributed more largely to the improvement of electricity, and indeed of fome other branches of philofophical knowledge, than the very ingenious Author of this Article; who here gives us fome electrical obfervations of a curious and fingular nature: particularly the defcription of a fimple apparatus, by means of which, the fmalleft, and otherwife imperceptible, degrees either of natural or artificial electricity are rendered fenfible. What the microfcope effects, in bringing to our view bodies otherwife totally invifible, is performed by this electrical magnifier, with refpect to electricity; by its rendering fenfible fuch small quantities of that fluid, as would otherwife wholly efcape the notice of our fenfes. Nor is this a mere matter of barren curiofity only; but may-it will foon appear that it has already-let us e of the fecrets of nature's operations, particularly re

teorology.

The

The apparatus, by which thefe effects are produced, is no other than the Author's well-known Electrophorus; the refinous coat of which ought to be exceedingly thin (not perhaps above the 50th part of an inch in thicknefs); and its furface, as well as that of the metal plate adapted to it, fhould be as plain and as fmooth as poffible.

When the fky is perfectly clear, and free from electrical clouds, fo that an infulated conductor, fitted up to obferve the electricity of the atmosphere, does not exhibit the leaft fign of electricity, by even attracting the fineft thread; if a temporary metallic communication be made by means of a wire between the atmospherical conductor and the metal plate, lying on the refinous furface; and this communication be fuffered to fubfift for a certain time: on removing the wire, the metal plate will, on being lifted up, exhibit evident figns of electricity, by attracting light bodies, and giving fparks. In this cafe it is fometimes found neceffary to prelerve the communication of the electrophorus with the atmospherical conductor even 8 or 10 minutes. But if the atmospherical conductor alone be capable of barely attracting a light thread; the communication above mentioned need to laft a few feconds only in which time the metal plate will receive, and, as it were, condenfe fuch a quantity of electricity, as to dart even a strong spark.

The effects produced by this apparatus appear as extraordinary in difcovering the prefence of artificial electricity, when it is fo weak as to be fcarce or at all perceptible by any other means. These appearances too are connected with a hitherto unobferved property of what may be deemed a new clafs of bodies, fuch as marble, dry wood, &c. and which may be called femi-conductors, or half-conductors. In this cafe, the refinous plate is not wanted :—but the relation of one experiment will beft explain our meaning.

Let a Leyden vial be charged, and then difcharged; fo that it will scarce affect a light thread: or-as we have varied the experiment, in order to obviate certain objections-let an uncharged Leyden vial be brought to the conductor of an electrical machine; fo as to receive from it only two or three moderate fparks. If the metal plate only of the electrophorus (or condenfer) be placed on a dry marble flab, a table, or dry piece of wood, or any other imperfectly conducting fubftance; and the knob of the vial be made to touch the metal plate, or, in some cases, to pals over its furface; the latter, on being lifted up, by its infulating handle, will be found to be highly electrified, fo as to give very strong fparks: and this it will do repeatedly for fome time, on alternately applying the knob of the vial to the metal plate, and then lifting up the latter from the flab, and examining it.

Cc 4

This

This method of bufbanding and condenfing fmall quantities of electric matter, fo as to procure ftrong fparks by means of a vial which is not fufficiently charged to give a single perceptible fpark, is peculiarly applicable to the firing and lighting the Author's inflammable air-piftol and lamp: especially if the operator be provided with one of Mr. Cavallo's electrical pocket vials. These vials will retain a fenfible charge for several days; and an infenfible one for weeks, or months. Even in this laft cafe, when the electricity of the vial appears to be extinct; the Author, with his condenfer, is enabled to procure fparks from it, fufficient to fire the inflammable-air pistol.

Mr. Cavallo, reafoning on the phenomena of the Author's condenfer, has carried this matter ftill further; fo as greatly ta increase the magnifying power of the inftrument, by employing another in addition to it. When the electricity, even of the condenfer itself, is fo fmall, as not to affect an extremely fen fible electrometer, he produces a fenfible degree of electricity, by applying it to another, but fmaller metal plate, or condenjer, placed likewife on marble, &c. and not exceeding the size of a hilling. M. Volta does not think that he should exaggerate, were he to affirm, that, by means of both these condenjers, the intensity of the original flock of electricity is increated 1000

times.

We have not room to explain the theory by which M. Volta accounts for thefe phenomena. It is founded on fome curious obfervations and experiments of his, on the capacity of fimple conductors, compared with that of a Leyden vial (or a nonconducting coated furface) which he publifhed in a philofophical Journal at Milan, in the year 17,8. He there fhews, that the capacity of 16 fquare inches of coated furface (in a Leyden vial, or glass plate coated), is equal to the capacity of a conductor made of filvered, wooden, cylindrical rods, and nearly 100 feet long; the capacity of which is to great, that its park occafions a fhock confiderably ftrong. It will be fufficient for us to obferve, that the phenomena of the condenfer depend on this circumftance-that a metal plate, or other conducting fubftance, poffeffes a much greater capacity for acquiring and preferving electricity, when fupported by marble or other imperfectly conducting bodies, than when it is perfectly infulated.

But one of the most curious difcoveries, in our opinion, which the Author has made with this delicate electrical teft, is his having fucceeded in exhibiting, by its means, electrical phenomena, and even fparks, produced by the evaporation of liquids, combuftion, effervescence, &c. To account for the manner in which the clouds acquire their electricity, many experiments had been made by others, as well as the Author, but without fuccefs; notwithstanding Dr. Franklin's luminous experiment

of

of the filver can and chain [See his Experiments, &c. Letter 12, p. 121.], by which he attempted to explain (it now appears, on very just grounds) by what means the electricity of the clouds was produced. It is now evident, that their failure in this inquiry was occafioned, not by an erroneous theory, but, through the want of an inftrument fufficiently delicate to detect small or evanefcent quantities of electricity. Without reciting the various circumftances neceffary to be attended to, in the conduct of these delicate experiments, we fhall only relate the principal refults.

The first trials in which the Author was fuccefsful were made at Paris laft year, in company with M. Lavoifier, and M. de la Place. Chafing-difhes, containing burning charcoal, were placed on a large infulated metal plate; and a communication was then formed between it and the condenfer, laid on a piece of marble. After a proper interval of time, the metal plate of the condenser being lifted up, was found, on prefenting it to Mr. Cavallo's electrometer, to have acquired negative electricity. Instead of the chafing-difhes, four veffels containing iron filings were placed on the infulated plate. Diluted vitriolic acid being poured into them, and a strong effervefcence excited, the condenfer not only charged the electrometer with negative electricity, but gave a fenfible Spark. Electrical appearances likewife occurred, by means of the effervefcences in which fixed air and nitrous air are produced.

At this time, the Author's experiments on the evaporation of water were not perfectly fatisfactory; but he afterwards fucceeded in London, on ufing the expedient of throwing water on the lighted coals contained in an infulated chafing-difh. After this he met with equal fuccefs, on throwing only a spoonful of water on three or four fmall coals burning in an infulated crucible. In thefe cafes, the fudden evaporation of the water never failed to electrify the chafing-difh negatively, fo as to render the electricity fenfible, even by means of the fimple electrometer, without having recourfe to the condenfer; though on ufing that inftrument the effects were greater, to as that parks were obtained. We fhould imagine that the electrical appearances would be greatly increafed, fo as to produce a ftrong spark, by giving water an extraordinary degree of heat, in a Papin's digefter, and then fuddenly giving vent to the vapour: as, in the case of the common, flow evaporation, the flight degree of electricity which is generated is diffipated nearly as faft as it is produced, in confequence of the unavoidable imperfection of the infulation.

MISCELLANEOUS ARTICLES. Art. 10. Account of an improved Thermometer: By Mr. James Six. It is fcarce poffible, without the affiftance of a figure, to give a 7

clear

clear idea of this inftrument: though perhaps we may fucceed in explaining the ingenious manner in which the principal effct, for which it is conftrued, is produced. The intention is to ascertain the greatest degree of heat and cold that has happened in the courfe of 24 hours, in the abfence of the obferver. It is properly a fpirit thermometer; though mercury is employed in it, for the purpose of fupporting a certain index. The tube in which the mercury moves is of the form of a U, and contains an index in the cavity of each fide or leg. This index confifts of a fhort piece of glafs tube, ufed as a float, and which is adapted to the bore of the curve thermometrical tube, fo as to move treely in it. This float contains a piece of fleel wire in its cavity; the ufe of which will foon appear. From its upper end rifes a fpring of glafs, of the fineness of a hair; which, being fet a little oblique, preffes lightly against the inner furface of the thermometrical tube.

Suppofing the index, on one fide, to float on the furface of the mercury, the latter, when it rifes, carries it up along with it. When the mercury, after having attained its greateft height, defcends, it leaves the index behind it: becaufe its fituation in the tube is preferved, by means of the preffure of its glafs fpring; the extremity of which points out the grea eft beat, on a scale placed parrallel to the tube. On the contrary, when the mercury has defcended to its loweft ftation on the fame fide, during the intervals of obfervation, the index, or float, on the other fide, is proportionably elevated, and fhews the obferver the greatest degree of co'd that has exifted in that interval.

It is evident, that this inftrument, from its construction, muft require rectification daily. This is eatily effected by the following ingenious contrivance. The Author only applies a fmall magnet to that part of the tube against which the index refts; by the action of which, the included piece of fteel wire (and confequently the index) is eafily brought down to the furface of the mercury. When this has been done, the inftrument is rectified for the next day's obfervation; without heating, cooling, feparating, or at all disturbing the mercury, or moving the

inftrument.

The conftruction of this inftrument appears to be difficult ; but it has been executed by the Author, who informs us in a note, that with a thermometer of this fort, he obferved the greatest heat and cold that happened every day and night Throughout the year 1781-Other, and fomewhat fimilar conructions are likewife defcribed; in which the above mentioned thermometer is, as it were, divided into two feparate parts; one of which only fhews the greatest degree of heat, and the other the greatet degree of cold.

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