Imagini ale paginilor
PDF
ePub

as many of his friends had complained, perhaps not without reafon, that they found fome difficulty in comprehending the principles on which that hypothesis is founded. This difficulty Mr. Magellan very properly afcribes to Dr. Crawford's not having entered into a fufficient detail, or explained the bafis of his theory in a manner fufficiently familiar. This effay is not only well adapted to ferve as an explanatory commentary on Dr. Crawford's work, but it likewife contains much new matter, and particularly the refults of feveral experiments made by Mr. Kirwan, on the fpecific heat, as it is called, of various bodies. For the fake of fuch of our philofophical readers as are yet unacquainted with this theory, or as an example of the doctrine, we fhall give the fubftance, or rather an amplification of one of Mr. Magellan's propofitions, as being well adapted to convey a general idea of the changes that have been obferved in the temperature of certain bodies, when they have been mixed together, which are not supposed to have any proper chemical action on each other.

It has long been known, that if a pound of water, of the temperature, for inftance, of 162° F, be mixed with an equal quantity of water of the temperature of 32°, or just on the point of freezing, but fill fluid; the temperature of the mixture will be 97°, that is, juft one half of the sum of the two numbers denoting the two temperatures; the heat being equally diffused throughout the mafs*. But if one of these portions of water be converted into ice, fo as to affume a folid form, though its temperature be fearce fenfibly changed +, the heat of the other portion being 162°, as before, the refult, on their admixture with each other, will be very different. The following propofition of the Author's alludes to this laft case.

Propofition 3d. The difference between the specific heat,'or, as Dr. Black has termed it, the latent heat,- of a fluid body, and that of the fame body in a folid ftate (that is to fay, in a state of cryftallization, fixity, or hardness), is very confider

able.'

• Demonftration. Take one pound of water, at the temperature of 162°; mix it with 1 pound of powdered ice, the temperature of which is 32°; agitate the mixture continually, fo that the ice may be diffolved or melted; and the temperature of the mixture' (inftead of being 97°, as in the preceding cafe)

* See M. Review, November 1779, pag. 384. Note A.

It may be proper to obferve, for the fake of those who may not be much conversant in observations of this kind, that there is a smali latitude refpecting the freezing point. Water, at 32°, may ftill preferve its fluidity; and ice, at the fame temperature, may ftill retain its folid form.

will only be 32°. Therefore the specific heat of water, in a ftate of fluidity, is 130° (for 162-32=130) more than that of the fame water, in the form of ice, or in a solid state.'

The Author afterwards makes an ingenious fuppofition, which may be confidered as the converfe of the foregoing propofition. He thinks it may be reasonably concluded, from the refult of this last mentioned experiment, that if a pound of ice, 130° below the freezing point (that is, as many degrees below it as the water in the preceding experiment was above it) could be intimately mixed with, and diffolved in a pound of water at 32°, but ftill fluid, the temperature of the mixture would, in this cafe likewife, be 32° or that the fluid water (at 32") would in this cafe lofe all that portion of its (fpecific) heat (amounting to 130°) to which alone it owed its state of fluidity; and which it would communicate to the pound of ice, fo as juft to melt it; the two pounds or masses of matter acquiring, on their admixture, exactly the temperature of 32 degrees. This experiment is perhaps impracticable; but a fimilar trial might be made with equal portions of ice and water, not varying fo very greatly in temperature from each other as in this inftance; or in other words, with ice not fo cold, and water proportionably warmer.

These inftances will be fufficient to give fuch of our philofophical readers, as may be unacquainted with this new theory of fire, fome idea of the nature of the facts, or experiments from which it is deduced. We learn with pleafure, from a Poftfcript at the end of this effay, that Dr. Black has at length determined fhortly to publish the obfervations which he has made relative to this curious fubject. We fhould not neglect to obferve, that Mr. Magellan defcribes a method of constructing thermometers peculiarly adapted to inquiries of this nature. Nor fhould we omit taking notice of a short intimation, that a correfpondent of the Author's, the ingenious M. Achard of Berlin, has lately invented a thermometer, adapted to indicate, with exactnefs, degrees of heat much fuperior to whatever can be fuftained by those of glass. The bulbs and tubes of these new thermometers are made of a transparent kind of porcelain; and contain, inftead of mercury, a metallic composition, confifting of two parts of bifmuth, one of lead, and one of tin. As this mixture melts in a heat equal to that of boiling water, and as the mercurial thermometers will indicate about 600° of Fahrenheit, it is easy to obferve that these new thermometers may be rendered comparable to thofe of mercury, and the scale be extended upwards in a regular manner.

The laft tract contains a kind of defcriptive catalogue, or inventory, of every member belonging to the aftronomical, furveying. or, in fhort, philofophical apparatus, fent to the court of Spain. In this are comprehended directions, not only refpecting

Kk3

fpecting the aftronomical quadrants, and other inftruments before treated of, but likewife accounts of aftronomical clocks, and pocket watches (including a fhort description of a wooden pendulum invented by the Author, which is not affected by heat or cold, nor fenfibly by moisture or drynefs), telescopes, mariner's compaffes, magnetic bars, theodolites, and various other fubjects that intereft the aftronomical obferver, or philofophical traveller.

Though this performance appears to owe its publication to a particular circumftance-which, by the bye, reflects the greateft credit on the artifts of this kingdom-thofe who are interested in the subjects difcuffed in thefe tracts will here meet with many ufeful particulars not to be found in the lateft publications in which aftronomical and other philofophical inftruments are defcribed. On whatever fubject the Author treats, he every where fpeaks the language of information; and fhews a defirewe ought rather to fay a zeal-not only to be useful to the two courts who have had the good fortune to accept his fervices, and avail themselves of his talents, on this occafion, but to the public in general.

ART. V.

Francifci de Wafferberg Inftitutiones Chemia, &c.-Chemical Inftitutions for the Ufe of Students. By Francis de Wafferberg. Three Vols. 8vo. 15 s. fewed. Vienna. 1778, and 1779. Imported by T. Lowndes, London.

CH

HEMISTRY is an art of the most unquestionable utility, and of moft extenfive application. It has, of late more particularly, affumed not only an unforbidding, but even an engaging form, by the facility with which many of its moft important researches are now conducted, and by the extenfion of them to numerous objects which had not before been attended to by the old fpagyrifts. Every well executed attempt to explain the principles of this art is of courfe intitled to a favourable reception from the public.

In the prefent publication, the Author does not pretend to give his readers much original matter; though he occafionally, and indeed frequently, makes pertinent remarks on the different fubjects of which he treats. But his principal intention is to explain the elements of chemistry, or rather to give the chemical hiftory of bodies, for the ufe of thofe particularly who wish to be initiated in the principles of that fcience. His work, however, cannot fail of being useful, in fome degree, to those even who are already well verfed in it; as he has collected and arranged, under each diftinct head, and in a language familiar to fcholars at leaft, the lateft obfervations and difcoveries which

have been published, relating to each fubject, as they lie difperfed in academic memoirs, literary journals, or separate and detached publications, written in various languages; feveral of which are neceffarily unknown even to many of those who have long cultivated this art.

In the three volumes now before us, the Author has executed only a part of his plan; the prefent publication comprehending only the chemical history of all the metals and femimetals. The bulk of the performance is increased, in confequence of the method which the Author has frequently adopted, of quoting the very words of the refpective authors who have best treated of particular fubjects. He has followed this plan, because his work is principally and profeffedly drawn up for the ufe of Tyros, who have not an opportunity of having recourfe to the originals. He wishes it, therefore, to be confidered in the light of a Chemical Library; the parts of which are digefted in a fyftematical order. In the first volume he has comprehended all the metals, except platina, the history of which he proposes to publifh hereafter fingly; and in the fecond and third he has given the chemical history of the femimetals the fecond volume being wholly employed in treating of mercury and antimony; and the third containing the history of zinc, bifmuth, nickel, arfenic, and cobalt.

As aurum fulminans is one of the most curious productions of chemistry, we shall extract from this work fome of the latest obfervations that have been made, and which the Author has here collected, concerning it. From the experiments of Mr. Bergman in particular, as related in this work, it appears to be established beyond a doubt, that the prefence of a volatile alcaline falt, either in the aqua regia, or the menftruum, in which the gold is diffolved, or the use of it as a precipitant, is abfolutely neceffary towards giving the golden precipitate the quality of fulmination. A calx of gold, precipitated with fixed alcali, and which will not detonate, immediately acquires a detonating quality, on being fimply digefted in volatile alcaline fpirit, and then edulcorated and dried. Nay a fimilar inert calx, which had been digested during twenty-four hours with vitriolic acid (with a view to expel any part of the aqua regia that might be fufpected to adhere to it), acquired a detonating quality, on being digefted either with a weak folution of volatile alcaline falt in water or in fpirit, or with cauftic volatile alcali; or even with a folution of common fal ammoniac, or with nitrous, or laftly vitriolic ammoniac.

This volatile alcali, which is fo readily expelled from other fubftances with which it is combined, either by means of a fixed alcali, or by the mineral acids, refifts every attempt (independent

Kk 4

dependent of heat) that has been made to expel it from the calk of this metal.

It is true, that it has been affirmed by our ingenious countryman Dr. Lewis, and by others, that aurum fulminans has been deprived of its fulminating property, by digefting it with fixed alcaline lixivia, or with oil of vitriol. Mr. Bergman, however, triturated for a long time fix parts of salt of tartar with one part of aurum fulminans, and a few drops of diftilled water. Adding more water, which was expelled in a heat of digeftion, no finell of volatile alcali was perceived during the procefs. The refiduum properly edulcorated and dried, not only detonated, but more ftrongly than ufual. Again, he expofed, during two hours, to a boiling heat, one part of fulminating gold, recently prepared, together with two hundred parts of cauftic alcaline lixivium: but in this cafe likewife, the detonating property of the powder was rather increased than diminished.'

The fact appears to be, that in the cafes above mentioned, in which the aurum fulminans loft its fulminating quality; that lofs was occafioned, not by the chemical action of the alcaline falt upon it, but by the increased heat, which a strong alcaline lixivium is capable of acquiring, when exposed to the fire: for it has been obferved, that heat alone, properly regulated, fo as to be juft inferior to that which will produce an explosion, will, if applied a fufficient time, deprive the aurum fulminans of its detonating property. Accordingly, Mr. Bergman, using a diluted alcaline folution, which could not acquire a heat much greater than that of fimple water, found, that the aurum fulminans was not injured by being boiled in it.

The Author alleges, that falt of tartar, and many other subftances, may deprive aurum fulminans of its peculiar property, merely by the mechanical interpofition of their particles, without any chemical action upon it. He took fome fulminating gold, which had apparently loft its detonating quality, by being treated with an alcaline falt. Having well edulcorated it, and boiled it in water, fo as to separate from it all the interpofed alcali, he found, that its fulminating property was again restored to it.

When aurum fulminans is boiled in oil of vitriol, it certainly lofes its fulminating quality; and, on the expulfion of the acid, the gold reaffumes its metallic ftate. But here too Mr. Bergman's experiments, as related by the Author, seem to fhew, that the effect depends entirely on the heat of the concentrated boiling acid: for, on diluting the acid, fo as to render it incapable of acquiring fo great a heat, the powder boiled in it did not lofe its fulminating property. Left objections

fhould

« ÎnapoiContinuă »