different fituations; and it feems to have been from this confideration that the first doubt on the subject arofe in his mind.

He found, that though the leaves, expofed in water to the action of light, actually do ceafe, in a few days, to furnish any air, yet, after a fhort interval, they regain that property; and that, after all the powers of vegetation are apparently deftroyed, they furnish (or rather caufe the water to furnish) more and better air than they did at firft.

In water faturated with pure air, fresh leaves acted in the same manner as in common water; whereas, according to the theory, they ought to have immediately died, as there is no inftance of any vegetable or animal being able to nourish itself with its own excrement.

Subftances in which no elaboration, or circulation of juices, can poffibly be fufpected to take place, caufed the water to yield dephlogisticated air, in like manner as recent vegetables; and even in much greater quantities, and purer in quality. Such particu larly were the dry down of the black poplar tree, and raw filk; which, with fresh portions of water, continued to furnish dephlogisticated air for feveral months fucceffively.

It is plain from thefe facts, that the production of the air in queftion cannot be afcribed to the agency of any vegetative powers. Sir Benjamin has not yet been able fatisfactorily to afcertain its real origin; but his experiments have thrown great light upon it, and we fhall prefent our Readers with an abstract of what appear to us the moft remarkable particulars obferved in them.

When raw filk, or the other bodies above mentioned, are exposed in water to the fun, for the first time, a little phlogisticated air is produced, prior to the pure air; but if they have previously been well washed with water, the air proves pure from the beginning. After a certain time, the production of air ceases, that is, no more is obtained from the fame water; but the fame fubftances, in fresh water, continue to furnish pure air as before. The air is purer, and more copiously produced, when the fun fhines bright, than when his rays are more feeble, or when they are frequently intercepted by Aying clouds; but with filk, or the poplar cotton, it is in all cafes better than common air, and better than the air which is in general produced by the fresh leaves of vegetables in the experiments of Dr. Ingenhoufz. The medium heat of the water, at the time that air was produced in greatest abundance, was about 90° of Fahrenheit: when the glafs globe was covered from light, but kept in the fame heat by means of a ftove, only a few detached bubbles appeared : when the globe was fet in the fun, but kept cool to about 50° by the repeated application of ice-water, air was produced, but not fo abundantly as when the glass was fuffered to become hot

by the fun's rays: ftrong light from candles, with a heat of 90°, had the fame effect as the fun, only in a fomewhat lower degree, probably from its lefs intenfity.

It feems as if water, in order to the production of air, re-, quired fomething to be communicated to it; and, whatever this fomething may be, that it is frequently contained in the water itself, and more abundantly in fome waters than in others. Pond water yielded more than twice as much air as fpring water did in the fame circumftances. The fine glafs threads, called fpun glafs, incapable of communicating any thing to water, furnished only an inconfiderable quantity of air, worse than that of the atmosphere: this was doubtless the air contained originally in the water, and we may hence conclude, that the air existing in water is worse than common air.

In all cafes where any confiderable quantity of pure air was feparated from water by the influence of light, the water loft part of its transparency, and acquired a greenish caft: at the fame time a quantity of whitish-yellowish earth precipitated, which was with difficulty got off from the glass.

It might be fuppofed, agreeably to Dr. Priestley's hypothefis, that this green matter is a vegetable fubftance, which attaches itfelf to the bodies expofed in the water, and grows, as a plantattached to its foil; and that the air yielded in the experiments is produced in confequence of the exertion of its vegetative powers. But, by a careful and attentive examination of the green water under a moft excellent microfcope, at the time when the water appeared moft difpofed to yield pure air in abundance, Sir Benjamin was convinced, that, at that period, it contains nothing which can poffibly be fuppofed to be of a vegetable nature. The colouring matter of the water was evidently of an animal origin, being nothing more than the affemblage of an infinite number of very fmall, active, oval-shaped animalcules; without any thing refembling that kind of green matter, or watermofs, which forms on the bottom and fides of the veffel when this water is suffered to remain in it for a confiderable time, and into which the animalcules above mentioned are supposed by Dr. Ingenhoufz to be actually transformed.

It seems, on the whole, as if the pure air, in the different experiments, was generated by means of these animalcules, for it evidently accompanied them; and that the leaves, filk, &c. did no more than affift in making its efcape, by affording a convenient furface to which it could attach itself, in order to its being collected together, and affuming its elaftic ftate. Defcription of a new Electrometer.

Bennet, M. A.

By the Rev. Abraham

Appendix to the Defcription of a new Electrometer. By the fame. These curious papers are accompanied with three plates; two of them exhibiting different views of the electrometer, and the

A a 2

third

third the application of it in different experiments. The aftonishing fenfibility of this inftrument, to low and unheeded degrees of electricity, induces us to gratify our philofophical Readers with a full defcription of it; and, from the fimplicity of its conftruction, we hope to render it intelligible without figures.

It confifts of two flips of gold leaf, about three inches long and a quarter of an inch broad, fufpended, clofe together, in the middle of an upright glafs; which glafs is open at both ends, and feems to be about an inch and a half wide and five inches high.

The glafs is fet in a wooden or metal foot; and has a flat metal cap on the top, about an inch more in diameter than the glafs. Round the outer edge of this cap is a rim, about three quarters of an inch deep, to keep off drops of rain or duft; and within this rim is another, about half the depth of the outer one, lined with filk or velvet, that it may fit tight on the glass, and be eafily taken off occafionally. From the centre of the cap, hangs a tin tube, a little longer than the depth of the inner rim; and to a small peg in the end of this tube the flips of gold leaf are faftened, with pafte, gum water, or varnish. That the gold may not be affected by any electricity communicated to the glafs, two long pieces of tin-foil are faftened with varnish on opposite fides of the internal furface of the glafs (where the leaf gold may be expected to ftrike) and continued down to the foot. The upper end of the glass is covered and lined with fealing-wax as low as the bottom of the outermoft rim, to make the infulation more perfect.

Mr. Bennet has given an account of many curious experiments made with this inftrument, but they are fo concisely drawn up as not to admit of abridgment. We can only mention a few of the general refults, to give our Readers fome idea of its extraordinary fenfibility.

Powdered chalk, wheat flour, and various other powders, blown on the cap from a pair of bellows or with the mouth, projected by means of a brush or wing, or by clapping the leaves of a book together, the duft ftirred up from the road with a stick, powders let fall from one plate upon another plate refting on the cap, in fhort every application of powdery fubftances, earthy, refinous, or metallic, produced electricity in the gold leaf, pofi tive in fome circumftances, and negative in others; and the fame circumftances which occafioned fome powders conftantly to produce the one electricity, occafioned others to produce conftantly the other.

The fenfibility is ftill further increased by placing a lighted candle on the cap. A cloud of chalk powder, that before would only have opened the gold leaves, will now caufe them to ftrike against the fides for a long time together. A cloud of chalk or flour being made in one room, and the electrometer with its

candle brought leifurely from another, the cloud will electrify it before it comes very near.

In clear weather, when no clouds were vifible, the infulated ftring of a kite, without metal, applied to the electrometer, has caufed the gold to ftrike the fides: in cloudy weather, with a wire in the ftring, the electricity was fenfible at the distance of ten yards, or more, from the ftring. Sometimes the electricity has been fenfible without a kite, though in a very unfavourable fituation, encompaffed by buildings, in a town furrounded with hills a thunder cloud paffing over occafioned the gold to ftrike the fides very quick, at every flash of lightning.

A tobacco pipe being heated at the fmaller end, a little water poured through it upon the cap, produces negative electricity, while the afcending vapour, received on another electrometer, electrifies it pofitively; phenomena which, as the Author obferves, may in fome measure illuftrate the electrification of fogs and rain.

This electrometer may likewife be applied to Mr. Volta's condenfers, both large and fmall; and Mr. B. defcribes a fimple and convenient method of conneding them together. Magnetical Experiments and Obfervations. Cavallo, F. R. S.

By Mr. Tiberius

The doubts which we expreffed in our account of the preceding Paper on this fubject, appear now to have been well founded; for the Author has tacitly given up the opinion which he there laboured to establish, of fome kinds of brafs being poffeffed of a power of attraction to the magnet, independent of any iron in them.

Though the needle, which he had contrived, be more fenfible than thofe in common ufe, it is certainly much inferior, for exploring very low degrees of magnetifm, to Profeffor Brugman's method, viz. placing the body to be examined, on the furface of mercury (or, in fome cafes, of water) in a veffel fix or eight inches wide, and prefenting to it a frong magnet; for befide that the needle has less power than the magnet, and that it cannot move fo freely, how fine foever the point be on which it turns, as a body does on the furface of a fluid, its own tendency to the magnetic meridian neceffarily counteracts or confumes a part of the magnetifm to be explored, fo that no needle can give us any intimation of very low degrees of magnetifm, that is, of fuch as are not more than fufficient to overcome that tendency as well as the friction, for it is only this furplus that is discoverable by a needle..

With this nicer teft of magnetism, Mr. Cavallo repeated fome of his former experiments, and was thereby convinced, that the prefence of iron is much more general than he had imagined;

See Monthly Review for March last.

that the brafs, which he thought had been rendered magnetic by hammering, was really so before the operation; that the magnetifm was fometimes confined to particular parts of the mafs; and that when he thought he had perfectly incorporated iron with brass by fufion, the iron was rather concealed in fome part or other of the brafs, than diffused equably through its fubftance.

In the course of thefe experiments, he obferved an unexpected phenomenon, in regard to the furface of the mercury, viz. that though fubstances float on it with wonderful nimbleness when the mercury is firft poured into the open veffel, they will by no means float with equal facility after expofure to the air for an hour or two; fo that pieces of brass, which at first were evidently attracted by the magnet, were, about an hour after, not in the leaft attracted by it. There feems to be formed a kind of cruft on the furface, owing (doubtless) chiefly to the imperfect metals from which quickfilver is feldom free, and partly (perhaps) to moisture or duft adhering, as Mr. C. has obferved it, in fome measure, in the pureft quickfilver that can be procured. When the mercury has thus become fluggish, he finds it to be effectually purified by paffing it through a paper funnel, that is, a piece of clean writing-paper, rolled up conically, with an aperture at its apex of about a fiftieth part of an inch in diameter.

Mr. Cavallo proceeds to another fubject of enquiry, the attraction of ferruginfous fubftances to the magnet in their different fates of exiflence.-As foft fteel, or iron, is known to receive magnetifm very eafily, and to lofe it again with equal facility, while hard fteel receives the power with difficulty, but retains it obftinately; it was natural to imagine, that a piece of steel, placed red hot (that is, in the ftate of greateft foftnefs) between two magnetic bars, and hardened in that fituation by fuddenly pouring cold water on it, might receive and retain a greater degree of power than can be communicated in the ordinary way. From feveral trials it appeared, that a confiderable power, but by no means extraordinary, is producible by this method, especially in fmall bars; and confequently that it will be of fome advantage, in making artificial magnets, to harden them in the pofition above mentioned; after which they may be polished, and further impregnated in the usual manner. We ourselves made fome experiments of the fame kind many years ago; but the power we obtained was very inconfiderable, perhaps from our bars having been too large for the power that we had to act upon them; we were led to this enquiry by the known fact of an iron cross on a fteeple at Delft having become in part ftrongly magnetic, and the magnetic part fo hard as not to be touched by a file; whence we fuppofed, that the temporary or tranfient magnetifm, which long bars of iron acquire naturally in certain pofitions, had in this cafe been rendered fixed by the converfion of the iron into hard

fteel,