THE MONTHLY REVIEW, For NOVEMBER, 1787. ART. I. Philofophical Tranfactions. Part I. For the Year 1787. Experiments of the Production of dephlogifticated Air from Water W HEN the fresh leaves of healthy vegetables are expofed, in water, to the action of the fun's rays, a quantity of dephlogiflicated or pure air is produced. This fact, difcovered by Dr. Ingenhoufz, is generally confidered as an inftance of the purification of the atmosphere by the vegetable kingdom, and even alleged as an argument in fupport of that beautiful theory. It is fuppofed that phlogifticated or fixed air is imbibed by the leaves, and decompofed by the powers of vegetation; that a part of thofe airs, which conftitutes their impurity with regard to animal life, is retained,as nourishment to the vegetable; while the pure air, fo effential o animals, is thrown out, as being, to the vegetable, excrementitious. Among many facts brought to prove that the air in queftion is really thus elaborated in the veffels of the plant, particular stress is laid on the production of the air continuing only for a fort time, till the leaves change their colour, for after that period no more air has been obtained. This is conceived to be owing to the powers of vegetation being then destroyed, or, in other words, to the death of the plant; and hence it is inferred, not only that the leaves actually retained their vegetative powers for fome time after they were separated from the ftock, but that it was in confequence of the exertion of these powers that the air, yielded in the experiments, was produced. Plaufible as this account appears, Sir Benjamin has proved, by a great number of experiments related in this paper, that it is erroneous. Indeed the circumftances of the leaves of a plant, accuftomed to grow in air, being feparated from the ftem and confined in water, are, as he obferves, fo unnatural, that we can hardly conceive the fame functions to be performed in fuch VOL. LXXVII, different A a different fituations; and it feems to have been from this confderation that the firft doubt on the fubject arose 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 cause the water to furnish) more and better air than they did at firft. In water faturated with pure air, frefh leaves acted in the fame 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. Substances 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 particularly were the dry down of the black poplar tree, and raw filk; which, with frefh portions of water, continued to furnish dephlogisticated air for several 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 abftra&t of what appear to us the moft remarkable particulars obferved in them. When raw filk, or the other bodies above mentioned, are expofed in water to the fun, for the firft 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 ceafes, that is, no more is obtained from the fame water; but the fame subRances, in fresh water, continue to furnish pure air as before. The air is purer, and more copiously produced, when the sun 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 glafs 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 seems as if water, in order to the production of air, required 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 thread's, 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 exifting 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 plant attached 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 microscope, at the time when the water appeared most disposed 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 fuffered to remain in it for a confiderable time, and into which the animalcules above mentioned are supposed by Dr. Ingenhousz 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 conve nient surface to which it could attach itself, in order to its being collected together, and affuming its elastic state. Defcription of a new Electrometer. By the Rev. Abraham Bennet, M. A. Appendix to the Defcription of a new Electrometer. By the fame. Thefe curious papers are accompanied with three plates; two of them exhibiting different views of the electrometer, and the A a 2 thir third the application of it in different experiments. The aftonifha ing 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 construction, 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 glass is open at both ends, and seems 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 glass. 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 påfte, gum water, or varnish. That the gold may not be affected by any electricity communicated to the glass, two long pieces of tin-foil are faftened with varnish on oppofite 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 drawh up as not to admit of abridgment. We can only mention a few of the general results, 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 leave's 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, pofitive in fome circumftances, and negative in others; and the fame circumstances which occafioned fome powders conftantly to produce the one electricity, occafioned others to produce conftantly the other. The fenfibility is ftill further increafed by placing a lighted candle on the cap. A cloud of chalk powder, that before would only have opened the gold leaves, will now cause them to strike against the fides for a long time together. A cloud of chalk or flour being made in one room, and the electrometer with its 1 andle 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 diftance 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. describes a fimple and convenient method of connecting them together. Magnetical Experiments and Obfervations. By Mr. Tiberius Cavallo, F. R. S. 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 ftrong 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 magnetism to be explored, fo that no needle can give us any intimation of very low degrees of magnetism, 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 difcoverable 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, A a 3 that |