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the principal refults, or the conclufions that he draws from these experiments.

It was formerly almost universally supposed, that bodies received and loft heat more flowly or readily, in proportion as they were more or less denfe. Mercury however furnished a remarkable exception to this hypothefis, when it was found that this fluid which is about fourteen times denfer than water, acquired and loft heat much more quickly than water, or even the lightest known fluids. From the Author's experiments on metals it appears, that their respective susceptibilities, with regard to the receiving and lofing heat, are fo far from depending on their denfities, that, except in one inftance, they very nearly follow a directly contrary ratio.

The metal which is the fubject of this exception is tin, which is the lightest of all the metals, and at the same time acquires and lofes heat the quickeft: but with respect to the rest, the order of their denfities, or fpecific gravities, beginning with the lightest, is iron, copper, filver, lead, gold. Now gold, the laft or denfeft of thefe bodies, which is 2 times fpecifically heavier than iron, or the first and lightest of them, lofes and acquires heat more quickly than the last mentioned metal, or by one fixth of the time. The fame may nearly be affirmed with respect to lead, filver, and copper, in the foregoing feries, which are all denier than iron, and which, like gold, grow hot, and lofe the heat they have acquired, more quickly than this lighter metal.

The quality on which the Author difcovered that the speedy acquifition and lofs of heat depended, in metals, as well as in femi-metals, and other metallic mineral fubftances, is fufibility, or their greater facility of being melted, or reduced to a fluid ftate. Another general principle deduced from his experiments is, that the most denfe fluid body is more quickly heated or cooled, than even the rareft folid body. With respect to the metals, if we rank them according to their fufibility, beginning with the most fufible, they will ftand in the following order,tin, lead, filver, gold, copper, iron; and this is the very order in which, according to the Author's many experiments here recited, they receive and tranfmit beat.

Calcareous and vitrefcible earths furnifh exceptions to the foregoing rule; the progress of heat in thefe fubftances not be ing in proportion to their refpective fufibilities, but to their denfities. The Author endeavours to account for thefe exceptions, and to establish a general rule with refpect to calcinable and vitrefcible earths; according to which he infers, that when an intense heat is requifite for the calcination or fufion of these fubftances, and when equal difficulty attends thefe proceffes in M m 2

any

any of them, the progrefs of heat generally follows the order of their denfities.

The third memoir contains fome curious obfervations relating to that fingular metal, or metallic fubftance, Platina; the true nature of which still seems not to be thoroughly known. The Author's Experiments on this fubject furnish results which differ in fome very obfervable particulars from those of the first chemifts of this age, who have endeavoured to investigate the na ure of this fingular production. It comes to us generally in fmall whitish grains, the corners of which are somewhat rounded, intermixed with a ferrugineous coarfe powder, and with particles of fpar, quartz, &c. That which has been seen in lumps, or formed into toys, is not pure, but has evidently been mixed with metallic fubftances, added to it to promote its fufion. The Author fays, however, that a perfon of credit had affured him, that platina is fometimes found in large maffes; and that he had seen a lump of it weighing no lefs than twenty pounds, which had not been melted, but had been taken in that state out of the mine.

Almoft all the chemifts who have inquired into the nature of this substance agree in calling it a perfect metal. They affirm, that its specific gravity is nearly equal to that of gold; and fome even have declared that it is greater. Like the two perfect metals, it is incapable of acquiring ruft, and is perfectly indeftructible in the fire. Like gold it has been found to refift the action of every one of the three mineral acids fingly. When it is diffolved in aqua regia, æther, according to M. Macquer, feparates it in the fame manner as it does gold, from the foluFinally, the pure grains of platina are capable, as M. de Buffon himself acknowledges, of being extended to a certain degree under the hammer; and though not perhaps perfectly fufible in our furnaces, Meffrs. Macquer and Baun é have fucceeded in melting small portions of it, by means of the folar heat collected in the focus of a concave mirror. We could add many more circumftances from Margraaf, Scheffer, Macquer, Baumé, and Dr. Lewis, which feem to prove that pure platina is a true, fimple, and perfect metal, fui generis, or that it differs from every other body of that clafs.

M. de Buffon, on the other hand, is led by his experiments, to deny that it is a metal, as it wants, he affirms, two of the effential qualities of a metal, ductility and fulibility. His opirion is, that it is only a mixture of gold and iron, combined

* We should obferve, however, that Dr. Lewis, in his fcientific and accurate analyfis of this fubftance, affirms, that platina is not takea from its folution in aqua regia by æther.

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together by nature in the bowels of the earth, in a manner which cannot be imitated by art; and that the peculiar qualities of the fuppofed compound refult from the intimate combination of the particles of these two metals. Out of more than eight ounces of platina, confifting of grains as ufual, he affirms, that near seven ounces were gradually drawn away from the heap by a strong magnet; and that he does not doubt but that the whole of the remainder might likewife have been taken up by the magnet, had he had patience to profecute the experiment to the end. It is evident, therefore, he obferves, that a large quantity of iron is contained in this fubftance; not fimply intermixed with it, as a foreign matter, but moft intimately united with it, and conftituting a part of its proper fubftance: unless we recur to this violent fuppofition, that there exifts in nature another fubftance, befides iron, that poffeffes the property of being attracted by the loadstone.

From fome chemical experiments, made by the Count de Milly, which are related in this memoir, it is likewise inferred, that iron is contained in platina; and it is affirmed, that the nitrous acid alone really acts upon this metallic fubftance, though without any fenfible effervefcence. That gentleman took a small quantity of platina, from which all the particles attracted by the magnet had been separated, and added to it fome strong fpirit of nitre. No effervescence could be perceived, even with a magnifying lens; but on adding fome diftilled water, and afterwards a few drops of phlogisticated alcali, or alcali faturated with the colouring matter of Pruffian blue, a precipitate was formed, which was found to be a true Pruffian blue; of which fubftance iron is known to be a neceffarily conftituent part.

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M. Margraaf, we believe, had before made the fame obfervation; but perhaps the experiments of thefe two gentlemen, in ftrictness only proves that their platina was impure. It appears too, from a fubfequent courfe of experiments made by M. de Morveau, in September 1773, on fome of the abovementioned platina given him by the Author, that on treating it in the manner above defcribed, no Pruffian blue was formed on the addition of phlogisticated alcali, if the experiment was made with those particles of the platina that were infenfible to the magnet. We fhould add, however, the refult of another experiment of M. Morveau's, that favours the Author's opinion.

This gentleman feems, by dint of repeated cupellations, to have fucceeded in working off, to a fingle grain, all the lead that he had mixed with a fmall quantity of platina; which appeared to have been at length brought to a ftate of perfect fufion and purity. The entire little bead was compact, and of a closer and finer grain than that of the best steel, after it has been Mm 3

highly

highly tempered. It bore a certain degree of extenfion under the hammer, but at length cracked. On being beaten into pieces, not one of them thewed the leaft fenfibility on the application of a magnet. Neverthelefs, we are told that thofe very pieces of perfectly cupelled, and feemingly pure platina, being reduced to a ftill finer powder in an agate mortar, fome of the lighteft particles thewed fymptoms that the button had ftill contained iron, by their being attraded by a magnetic bar applied to them. And left it fhould be fuppofed that this effect might be owing to adhefion, or fome other caufe foreign to magnetifm, it is obferved, that on bringing a bar of the fame kind, but not magnetical, into contact with the powder, not a fingle particle was taken up by it.

Among the other reafons that determine the Author, to be lieve that platina is an intimate natural combination of gold with iron, are the following. He affirms, that the specific gravity of this metallic body is lefs than has been fuppofed; that according to experiments made by M. Tillet and himself, it is Specifically lighter than gold by at least an eleventh or twelfth part; and that its denfity is a mean between that of gold and iron; or fuch as would refult from a combination of thefe two metals, united together in the proportion of three-fourths of the former to one-fourth of the latter. He acknowledges, however, that its magnetical quality is compatible with the fuppofition that it contains a much smaller proportion of iron than this: as he has feen, in the poffeffion of M. Baumé, a button formed of a mixture of these two metals, weighing fixty fix grains, that con tained only fix grains (that is one eleventh part) of iron, which button was however easily taken up by a good magnet; fo that platina, notwithstanding its magnetic quality, may contain only one-eleventh part of iron. He inclines nevertheless to the opinion that one-fourth of it is iron; as gold, allayed with an eleventh part of that metal, ftill partly retains the colour of gold, and is much yellower than even the highest coloured platina; whereas, when it is allayed with one-fourth part of iron, the compound perfectly refembles platina in colour.

We cannot pafs over another obfervation of M. de Buffon's, which appears to be new, and may perhaps incline fome to favour his opinion concerning the nature of this anomalous mineral. He affirms, that the ferrugineous fand, which is fo abundant in all the fpecimens of platina, is not peculiar to that fubftance, or to the mines from which it is taken; for that he has found a matter of the fame nature, though in very small quantities, in feveral of his own iron mines. Thefe particles are fomewhat rounded at the edges, and fhine like fresh iron hlings. They perfectly refemble the ferrugineous fand of pìatina, are as magnetical and difficult of fufion, and equally re

fift the action of acids, as well as that of humidity; being equally incapable of acquiring ruft. He attributes the forma tion of this substance (many specimens of which he has fince found in the Royal Cabinet of Natural Hiftory, which have been fent from different parts of the world,) to the action of fire; by which iron has been reduced to fcoria, that have been afterwards decompounded; and thefe particles of pure iron, ot fubject to ruft or any other alteration, have been detached from it, and carried by the rains into the earth, to the depth of fome feet.

The Author adds, that on pulverifing the fcoria of iron, that have been exposed to an intense heat, a fmall quantity of this pure iron may be found amongst it, and which having refifted the action of the fire, equally refifts that of folvents, and is not liable to ruft;' and that there is not any reason to doubt of its perfect identity with the ferrugineous powder fo abundant in platina, from which it appears to be infeparable after repeated cupellations, and to conftitute an effential part of its fubftance.

These are the principal circumstances to be collected from the Author's inquiry into the nature and probable formation of the Diabolus metallorum, as this refractory fubftance has been properly enough denominated. To thefe obfervations we fhall only add the following anecdote concerning it. The Baron de Sickengen, Minister of the Elector Palatine, informed the Count de Milly that he had in his poffeffion two memoirs prefented to him by M. Kellner, chemift and metallurgift, in the fervice of the Prince de Berkenfeld at Manheim; wherein he makes an offer to the court of Spain to deliver nearly an equal weight of gold, in return for any quantity of platina that they should put into his hands.

In the fourth memoir are contained many obfervations on the tenacity and decompofition of iron,' founded on the large experience which the Author feems to have had in fmelting and other operations performed on that metal, This is followed by a memoir containing an account of fome experiments made in large furnaces, on the effects of what the Author calls

Chaleur obfcure, or a clofe and fmotbering heat, on ftones and other bodies exposed to it; in oppofition to those produced by the action of a violent and open fire. Thefe experiments are too numerous and complicated to admit of an abridgment.

The fixth memoir is divided into three parts or articles, in which the Author treats of Burning Mirrors and Lenfes; and contains an account of numerous and diverfified experiments made by him on this curious fubject, which could only be executed by a man of fortune, fpirit, and ingenuity. The first of thefe articles only was formerly printed in the Memoirs of the Royal Academy of Sciences, and contained M. de Buffon's acMm 4

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