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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 semi-insulated plates, diffused electricity is attracted, and condensed into a small compass. When one plate has received electricity, positive for instance, another is laid upon it; and the atmosphere of the firft, repelling the electric Auid from the second (which is touched with a conducting substance, to carry off the Auid), produces in this second an equal negative electricity, without losing any of its own : the second is, in like manner, made to produce an equal positive electricity in a third : the first and third, now both positive, being placed side by side, act with double force, and the second, laid upon them, acquires an electricity nearly double to the first; and by repeating this fimple procedure, the minutest degrees of electricity are continually doubled, till they become sensible. Now it is plain, that if any electricity exists in the plates themselves, this electricity of their own will be multiplied as well as that which we want to discover by them; and Mr. Cavallo has shewn, from a variety of experiments, that they always do contain electricity, which becomes sensible after a certain number of doublings. He shews also, that this is not peculiar to the semi-insulated plates, but that, ftrictly speaking, every substance is always electrified, viz. that every substance, and even the various parts of the same body, contain at all times more or less electric Auid than that quantity of it which it ought to contain, in order to be in an electrical equilibrium with the bodies that surround it. The Auctuating electric state of the air, the passage of electrified clouds, the evaporation and condensation of fluids, and the fri&tion arising from various causes, are perpetually acting on the electric fluid of all bodies, so as to increase or diminish it, or disturb its equilibrium ; and very small differences in its distribution, such as do not affect our inftruments, may be sufficient for several interesting operations of nature. The excitation of electricity by our machines appears, from the author's curious observations, to be no other than a rapid doubling or multiplication, on principles analogous to the above mentioned, of the natural surplus of electricity existing in the glass or rubber. Observations on the Manner in which Glass is charged with the
Electric Fluid, and discharged. By Edward Whitaker Gray, M.D. F. R. S.
It is commonly supposed, that the natural quantity of electric fuid in glass cannot be increased or diminished; and that it is impossible to add any to one surface of a plate or jar, unless an equal quantity be, at the same time, given out from the other surface. Dr. Gray, on the contrary, confiders it as one of the fundamental laws of electricity, that glass, and every other known substance, 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 surface; but this departure of the Auid from the outfide cannot be (as it has been supposed) the cause which permits the addition of Auid to the inside, but merely the consequence of the action of that surplus quantity which was thrown in, and which may be taken out again by touching the knob instead of the coating. When this first quantity has been taken from the outside, another like quantity may be added to the inside, and so on successively till the jar is completely charged. The Doctor supposes the discharge to happen on che same principle, and to be no other than an inconceivably rapid fucceffion of such fmall quantities of the Auid, as may be sent off without causing 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 side 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 Cause of the great Degree of Cold on the Summit of high Mountains, the sudden Condensation of aereal Vapour, and of the perpetual Mutability of atmospheric Heat. By Erasmus Darwin, M.D.F.R.S.
From the great degrees of cold produced by the evaporation of Auids, and the great quantity of heat necessary for expanding them into vapour, Dr. Darwin was led to suspect, that by mere mechanical expansion the capacity for heat is enlarged; or in other words, that elastic fluids, while they expand, absorb heat from bodies in their vicinity, so as to produce cold in them; and conversely, that while they are condensed, the Auid heat is squeezed out, and communicated to the contiguous bodies. The experiments here stated confirm this ingenious conjecture, and afford a new and moft interesting explanation of atmospheric heat, vapour, and rain.
The condensed air in an air-gun, and in the air-vessel of a water-engine, expanding in its discharge from them, was found to fink chermometers exposed to the blast; though, previously, of the same temperature with them. In condensing the air into the receiver of the air-gun, a heat was produced, sensible to the hand; independently of what might be attributed to friation in the syringe. In the receiver of an air.pump, cold was produced, Rev. Oa. 1788.
boch both when the included air was expanded by rapid exhauftion, and when the external air was exponded on its re-admission inte the vacuum. In all these cases of expanfion, the watery vapour contained in the air was condensed or rendered visible; in con. fequence of the beat, that kept it in folution, being absorbed from it by the expanding air. The Doctor takes notice of a curious phenomenon of this kind, which is observed in the fountain of Hiero, conltructed on a large scale in one of the Hungarian mines : the air is compreff d by a column of water 260 feet high; and in consequence of this great condensation, on opening a stop cock, it expands and absorbs heat with lo much vehemence, that i he moisture it contains is not only precipitated, but falls in a shower of snow, with icicles adhering to the nosel of the cock.
Now, as the air which surrounds our globe is in perpetual circulation, its different parts must be perpetually varying in denfity and heat. Rising up to the summits of mountains, it is expanded, and absord's heat from them : descending into vallies, it is compreffed, and gives out ibai beat to the bodies with which it comes in contact : by its great expansion in the higher regions, what watery vapour it contains is fo far deprived of heat, as to be precipitated in loow or hail.
Some sudden changes of heat, corresponding with the height of the barometer, are accounted for on the fame principle. The Doctor has frequently observed, that when the barometer rose (the wind continuing in the same quarter) the air became many degrees warmer ; and Musschenbroek relates that, in winter, when the barometer fonks, the cold increases.
The precipitation or condensation of vapour the Doctor calls devaporation; a rerm new in our language, but so 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 pressure being diminilhed, the lower air muft expand : in expansion, it absorbs heat, and its mofture is devaporated. The air, thus freed from its vapour, becomes more compredible, and occupies less space: the contiguous parts of the atmosphere have therefore room to expand allo: and thus the expansion, absorption of beat, and devaporation, are propagated successively through a large extent, so that the original expanfion of a small province of atmosphere may produce ultimately an immense quantity of rain. • This theory of the fucceffive and sudden propagation of devaporation is admirably illustrated by a circumstance which takes place in the steam engine. In the excellent one of Meff. Wate and Boulton (which, from the happy combination of chemical and mechanical power, the Doctor efteens, and we believe juftly,
the first machine of buman invention), when the cylinder is filled with steam, a communication is opened between it and a small cell, which is kept cold and free from air : a small corner of the fteam in the cylinder, next to this vacuum, sushes into it, and the whole of the Ateam is thus suddenly expanded, and instantly devaporated (whence the very quick reciprocations of the piston), though the cylinder itself is always kept as hot as boiling water, that is, as hot as the fteam was previous to its devaporation.
Something very similar to this, he observes, is often seen at the commencement of thunder Atorms: a small black cloud at first appears, in a few minutes the whole heaven is covered with condenfing vapour, and the accumulation or escape of electric matter seems to be rather the consequence, than the cause, of this sudden and general devaporation.
When a province of air, by being deprived of its vapour, is compressed into less space, the vacuity muft be supplied by winds rushing in on all sides. When this happens to the north of our climate, a south-west wind, he observes, will be produced here, which is otherwise very difficult to understand.
Experiments on local Heat. By James Six, Esq. These experiments are a continuation of those of which we gave an account in the 720 volume of our Review, p. 256. They confirm the former observations respecting a remarkable refrigeration, which, in clear weather, takes place near the earth in the night time ; for, although the surface of the earth is most heated by the sun during the day (in summer 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 these experiments (viz. a beight of 220 feet); sometimes two degrees colder at the height of one inch than at nine. This refrigeration appears to be a constant and regular operation of nature, taking place at all seasons of the year, but never in any confiderable degree, except when the air is ftill, and the sky perfectly unclouded. Moist vapours, such as dews and fogs, did not seem to impede, but rather to increase it. In very severe frosts, when the air frequently depofires 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. These experiments were made, to investigate the cause, and ascertain 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 subject ftill remains involved in great oblurity, the experi'ments have elucidated several points respecting it, and corrected fome erroneous opinions.
The water was generally put in a glass tumbler, and cooled very gradually by a freezing mixture. The greatest cold which the Doctor could make it bear was 12° below the freezing point, that is 20° of Fabrenheit, though there are said to be instances of its remaining Auid at 14 or 159:- The water continues to expand during the whole progress of the cooling, and the expanfion seemed to proceed in an increafing ratio, being much greater on the last degrees of cooling than it was on the first.
Water freed from its air, as much as it could be by boiling, did not freeze so soon as the fame water unboiled; hard pump water sooner 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 circumstance, as the Doctor observes, which gave rife to the opinion that boiled water freezes sooner than unboiled : for if the water contain calcareous earth, held in folution by means of fixed air, as is the case with most kinds of spring water, this earth will be precipitated by the boiling, and the water will consequently lose its transparency; which, if exposed to the cold in that state, will be liable to freeze sooner than the same kind of water unboiled and transparent.
Saline solutions bore generally to be cooled below their refpective freezing points, nearly as much as water did below 32; and in there, as in water itself, the most transparent were those which admitted of being cooled with the greateft ease and certainty.
It is commonly supposed, that the cooling of water below its freezing point depends on rest, and that agitation is the general cause by which it is brought to shoot into ice. In some of the Doctor's experiments, agitation seemed to have no effect, even when the cold was brought within i degree of the greateft that the water could bear. In others, it occasioned instant. congelation, though the water was not cooled so low by several degrees, so that the effect must have depended on some further circum. stances than mere want of reft ; one of which he suspects to be a sort of tremulation, rather agitating small portions of the wa. ter separately, than moving the whole body together.
The contact of the least particle of ice is known to make the water freeze instantly, and many of the circumftances attending the process are shewn to depend on this principle. Thus, frozen particles, almost always floating in the air in frosty weather, produce congelation in water exposed to them, while such as is Theltered, in air equally cold, continues Auid : oil poured on the furface, which in the latter situation has no effect, serves as a defence in the former, and impedes the congelation.
The Doctor endeavours to account for these phenomena, by supp ling the particles of water to be possessed of a kind of pola