« PreviousContinue »
Mr. Bennet's Doubler is the subject of a more minute and more important investigation. The principle of it is this. By M. Volta's femi-infulated plates, diffufed electricity is attracted, and condensed into a small compafs. When one plate has received electricity, pofitive for inftance, another is laid upon it; and the atmosphere of the firft, repelling the electric fluid from the fecond (which is touched with a conducting fubftance, to carry off the fluid), produces in this fecond an equal negative electricity, without lofing any of its own: the fecond is, in like manner, made to produce an equal pofitive electricity in a third: the first and third, now both positive, being placed fide by fide, act with double force, and the fecond, laid upon them, acquires an electricity nearly double to the firft; and by repeating this fimple procedure, the minuteft degrees of electricity are continually doubled, till they become fenfible. Now it is plain, that if any electricity exifts in the plates them felves, this electricity of their own will be multiplied as well as that which we want to difcover by them; and Mr. Cavallo has fhewn, from a variety of experiments, that they always do contain electricity, which becomes fenfible after a certain number of doublings. He fhews alfo, that this is not peculiar to the femi-infulated plates, but that, Atrictly speaking, every fubftance is always electrified, viz that every fubftance, and even the various parts of the fame body, contain at all times more or lefs electric fluid than that quantity of it which it ought to contain, in order to be in an electrical equilibrium with the bodies that furround it.' The fluctuating electric ftate of the air, the paffage of electrified clouds, the evaporation and condenfation of fluids, and the friction arifing from various caufes, are perpetually acting on the electric fluid of all bodies, fo as to increase or diminish it, or difturb its equilibrium; and very fmall differences in its diftribution, fuch as do not affect our inftruments, may be fufficient for several interefting operations of nature. The excitation of electricity by our machines appears, from the author's curious obfervations, to be no other than a rapid doubling or multiplication, on principles analogous to the above mentioned, of the natural furplus of electricity exifting in the glass or rubber. Obfervations on the Manner in which Glafs is charged with the Electric Fluid, and difcharged. By Edward Whitaker Gray, M. D. F. R. S.
It is commonly fuppofed, that the natural quantity of electric fluid in glafs cannot be increased or diminished; and that it is impoffible to add any to one furface of a plate or jar, unless an equal quantity be, at the fame time, given out from the other furface. Dr. Gray, on the contrary, confiders it as one of the fundamental laws of electricity, that glafs, and every other
known fubftance, may have their natural quantity of electric fluid either increased or diminished to a certain limited degree; which degree (cæteris paribus) is in proportion to the extent of furface. An infulated jar will receive, by its knob, a certain quantity of electric Auid on the inner furface, and nearly an equal quantity may then be drawn off by the finger from the outer furface; but this departure of the fluid from the outfide cannot be (as it has been fuppofed) the cause which permits the addition of fluid to the infide, but merely the confequence of the action of that furplus quantity which was thrown in, and which may be taken out again by touching the knob instead of the coating. When this firft quantity has been taken from the outfide, another like quantity may be added to the infide, and fo on fucceffively till the jar is completely charged. The Doctor supposes the discharge to happen on the fame principle, and to be no other than an inconceivably rapid fucceffion of fuch fmail. quantities of the fluid, as may be fent off without caufing a deftruction of the equilibrium. For if the whole charge left the jar at once, there would be a point of time, in which the jar could have no electric fluid either on one fide or the other: nay more, when a large jar or battery is discharged by means of a few inches of thin wire, there would be a point of time, at which the whole quantity of the charge must be contained in a piece of wire weighing only a few grains.
Frigorific Experiments on the mechanical Expansion of Air, explaining the Caufe of the great Degree of Cold on the Summit of high Mountains, the fudden Condenfation of aereal Vapour, and of the perpetual Mutability of atmospheric Heat. By Erafmus Darwin, M. D. F.R. S.
From the great degrees of cold produced by the evaporation of fluids, and the great quantity of heat neceffary for expanding them into vapour, Dr. Darwin was led to fufpect, that by mere mechanical expansion the capacity for heat is enlarged; or in other words, that elaftic fluids, while they expand, abforb heat from bodies in their vicinity, fo as to produce cold in them; and converfely, that while they are condenfed, the fluid heat is squeezed out, and communicated to the contiguous bodies. The experiments here ftated confirm this ingenious conjecture, and afford a new and most interesting explanation of atmospheric heat, vapour, and rain.
The condensed air in an air-gun, and in the air-veffel of a water-engine, expanding in its difcharge from them, was found to fink thermometers exposed to the blaft; though, previously, of the fame temperature with them. In condenfing the air into the receiver of the air-gun, a heat was produced, fenfible to the hand; independently of what might be attributed to friction in ,the fyringe. In the receiver of an air-pump, cold was produced, REV. Oa. 1788. Y
both when the included air was expanded by rapid exhauftion, and when the external air was expended on its re-admiffion inte the vacuum. In all these cafes of expanfion, the watery vapour contained in the air was condenfed or rendered vifible; in confequence of the heat, that kept it in tolution, being abforbed from it by the expanding air. The Doctor takes notice of a curious phenomenon of this kind, which is obferved in the fountain of Hiero, conftructed on a large fcale in one of the Hungarian mines: the air is compreffd by a column of water 260 feet high; and in confequence of this great condenfation, on opening a ftop cock, it expands and abforbs heat with fo much vehemence, that the moisture it contains is not only precipitated, but fails in a fhower of fnow, with icicles adhering to the nofel of the cock.
Now, as the air which furrounds our globe is in perpetual circulation, its different parts must be perpetually varying in denfity and heat. Rifing up to the fummits of mountains, it is expanded, and abfords heat from them: defcending into vallies, it is compreffed, and gives out that heat to the bodies with which it comes in contact: by its great expanfion in the higher regions, what watery vapour it contains is fo far deprived of heat, as to be precipitated in fnow or hail.
Some fudden changes of heat, correfponding with the height of the barometer, are accounted for on the fame principle. The Doctor has frequently obferved, that when the barometer rofe (the wind continuing in the fame quarter) the air became many degrees warmer; and Muffchenbroek relates that, in winter, when the barometer finks, the cold increases.
The precipitation or condenfation of vapour the Doctor calls devaporation; a term new in our language, but fo useful and expreffive, that we make no doubt of its general reception. As vapour is formed by heat, the abftraction of heat devaporates it. Now, when the barometer finks (from whatever cause not yet understood), the preffure being diminished, the lower air muft expand in expanfion, it abforbs heat, and its moifture is devaporated. The air, thus freed from its vapour, becomes more compreffible, and occupies lefs fpace: the contiguous parts of the atmosphere have therefore room to expand alfo and thus the expanfion, abforption of heat, and devaporation, are propagated fucceffively through a large extent, fo that the original expanfion of a fmall province of atmosphere may produce ultimately an immenfe quantity of rain.
This theory of the fucceffive and fudden propagation of devaporation is admirably illuftrated by a circumftance which takes place in the steam engine. In the excellent one of Meff. Watt and Boulton (which, from the happy combination of chemical and mechanical power, the Doctor efteems, and we believe juftly,
the first machine of human invention), when the cylinder is filled with fteam, a communication is opened between it and a fmall cell, which is kept cold and free from air: a fmall corner of the fteam in the cylinder, next to this vacuum, rushes into it, and the whole of the fteam is thus fuddenly expanded, and inftantly devaporated (whence the very quick reciprocations of the piston), though the cylinder itfelf is always kept as hot as boiling water, that is, as hot as the fteam was previous to its devaporation.
Something very fimilar to this, he obferves, is often seen at the commencement of thunder forms: a fmall black cloud at firft appears, in a few minutes the whole heaven is covered with condenfing vapour, and the accumulation or escape of electric matter feems to be rather the confequence, than the cause, of this fudden and general devaporation.
When a province of air, by being deprived of its vapour, is compreffed into less space, the vacuity must be supplied by winds rufhing in on all fides. When this happens to the north of our climate, a fouth-west wind, he obferves, will be produced here, which is otherwise very difficult to understand.
Experiments on local Heat. By James Six, Efq.
Thefe experiments are a continuation of thofe of which we gave an account in the 72d volume of our Review, p. 256. They confirm the former obfervations respecting a remarkable refrigeration, which, in clear weather, takes place near the earth in the night time; for, although the furface of the earth is moft heated by the fun during the day (in fummer at least), yet in the night the air near the ground is found to be colder than at any elevation in the atmosphere within the limits of thefe experiments (viz. a height of 220 feet); fometimes two degrees colder at the height of one inch than at nine. This refrigeration appears to be a conftant and regular operation of nature, taking place at all feafons of the year, but never in any confiderable degree, except when the air is ftill, and the sky perfectly unclouded. Moift vapours, fuch as dews and fogs, did not feem to impede, but rather to increase it. In very fevere frofts, when the air frequently depofites a great quantity of frozen vapour, it was generally found to be the greateft.
Experiments on the cooling of Water below its freezing Point. By Charles Blagden, M. D. Sec. R S. &c.
Thefe experiments were made, to investigate the cause, and afcertain the modifications, of that well-known property of water, of bearing to be cooled, under different circumstances, confiderably below the freezing point, without congealing. Though the fubject ftill remains involved in great oblurity, the experiments have elucidated feveral points refpecting it, and corrected fome erroneous opinions.
The water was generally put in a glafs tumbler, and cooled very gradually by a freezing mixture. The greateft cold which the Doctor could make it bear was 12° below the freezing point, that is 20° of Fahrenheit, though there are faid to be inftances of its remaining fluid at 14 or 15°.-The water continues to expand during the whole progrefs of the cooling, and the expanfion feemed to proceed in an increafing ratio, being much greater on the laft degrees of cooling than it was on the firft.
Water freed from its air, as much as it could be by boiling, did not freeze fo foon as the fame water unboiled; hard pump water fooner than pure; and turbid water, whatever kind of fubftance was mixed with it to produce the turbidness, could not be cooled at all below 32 without congealing. It is probably this circumftance, as the Doctor obferves, which gave rife to the opinion that boiled water freezes fooner than unboiled: for if the water contain calcareous earth, held in folution by means of fixed air, as is the cafe with most kinds of spring water, this earth will be precipitated by the boiling, and the water will confequently lofe its tranfparency; which, if exposed to the cold in that ftate, will be liable to freeze fooner than the fame kind of water unboiled and tranfparent.
Saline folutions bore generally to be cooled below their refpective freezing points, nearly as much as water did below 32; and in thefe, as in water itself, the moft tranfparent were those which admitted of being cooled with the greateft eafe and certainty.
It is commonly fuppofed, that the cooling of water below its freezing point depends on reft, and that agitation is the general caufe by which it is brought to fhoot into ice. In fome of the Doctor's experiments, agitation feemed to have no effect, even when the cold was brought within 1 degree of the greatest that the water could bear. In others, it occafioned inftant.congelation, though the water was not cooled fo low by feveral degrees, fo that the effect must have depended on fome further circum. ftances than mere want of reft; one of which he suspects to be a fort of tremulation, rather agitating fmall portions of the water feparately, than moving the whole body together.
The contact of the leaft particle of ice is known to make the water freeze inftantly, and many of the circumftances attending the procefs are fhewn to depend on this principle. Thus, frozen particles, almost always floating in the air in frosty weather, produce congelation in water expofed to them, while fuch as is heltered, in air equally cold, continues fluid: oil poured on the furface, which in the latter fituation has no effect, ferves as a defence in the former, and impedes the congelation.
The Doctor endeavours to account for these phenomena, by fuppfing the particles of water to be possessed of a kind of pola