PYROPHORUS, formed of pyr, phos, and phor, I bear, in chemistry, the name usually given to that substance called by some black phosphorus; a chemical preparation possessing the singular property of kindling spontaneously when exposed to the air. See CHIMISTRY, no 1414.
This substance was accidentally discovered by M. Homberg, who prepared it of alum and human feces. See PHOSPHORUS. It was apprehended, for a considerable time after the discovery, that human feces were essential to the operation, till the youngest son of the great Lemeri found that honey, sugar, flour, and indeed any animal or vegetable matter, might be substituted instead of the human feces; and since that time, M. De Sauvigny has shown that most vitriolic salts may be substituted for the alum; having added to the aluminous pyrophorus of Homberg two other classes of substances of this kind, viz. the metallic, or those made with the three vitriols of iron, copper, and zinc; and the neutral, or those composed of vitriolated tartar and Glauber's salt.
Mr Bewly prepares his pyrophorus in the following manner. "I fill (says he) half or three-fourths of the bowl of a tobacco-pipe with a mixture, consisting of two parts of alum, previously calcined in a red heat, and of powdered charcoal and salt of tartar each one part; pressing the matter down slightly, and filling the remainder of the bowl with fine sand. As soon as the powder becomes hot, the sand lying over it is put into a state of ebullition, which generally continues several minutes. This appearance seems to proceed partly from the vitriolic acid in the alum leaving its earth, and expelling fixed air from the alkali; while another part of it is possibly converted into vitriolic acid air. This phenomenon is succeeded by the appearance of a blue sulphureous flame, proceeding from the combination of the same acid with what was formerly called the phlogiston of the coal, and which continues about ten minutes or a quarter of an hour. After it ceases, no other remarkable appearance presents itself. The matter is now to be kept in a red heat 20 minutes or half an hour; or it may continue there two hours longer, if the operator pleases, without any injury to the pyrophorus. The pipe being taken out of the fire, the matter is knocked out of it as soon as it becomes cool, and generally pretty soon afterwards takes fire spontaneously."
In another experiment, having added successively various and increasing quantities of fixed alkali to the salt heated as above, till the vitriolic acid contained in the mixture might be considered merely as an evanescent quantity, a pyrophorus was still produced on calcining it with charcoal as before. He also mixed equal parts of salt of tartar and vegetable or animal coal, or sometimes three parts of the former with two of the latter, and calcined them in the usual manner: and this composition, on being exposed to the air, generally kindled in half a minute or a minute; though, as it contained no sulphur, it did not burn with so much vivacity as the vitriolic pyrophori. This, which Mr Bewly calls the alkaline pyrophorus, differs in no circumstance from M. De Sauvigny's neutral pyrophori, except in its not containing that principle to which he ascribes their accension. However, lest it might be suspected that the salt of tartar which he employed might accidentally contain vitriolated tartar, or vitriolic acid, he repeated the experiment with tartar calcined by himself, as well as with nitre fixed or alkali by deflagration with charcoal, and with iron filings; and in all these cases with the same result. By diversifying in a like manner M. De Sauvigny's experiments on the metallic pyrophori, Mr Bewly found that none of the three vitriols, heated with charcoal alone, in his usual method, could produce a pyrophorus. And thus he found that the addition of an alkaline salt to the composition, which was a part of M. De Sauvigny's process, was essential to its success.
Treating in the usual manner equal parts of calcined green vitriol and charcoal, the powder, which contained no sulphur nor hepatic sulphur, did not acquire any of the properties of a pyrophorus. The vitriolic acid seemed to have been entirely dissipated, having no base to detain it, when dislodged from the metallic earth. The charcoal and calx of iron left in this process were calcined again, together with some salt of tartar: and a pyrophorus was produced, which exhibited indications of its containing a scarce perceptible portion
portion of hepar sulphuris. Thirty grains of erocus martis astringens were calcined with 15 grains of charcoal, and the same quantity of salt of tartar; and the mixture burnt spontaneously, though it contained no hepar sulphuris or vitriolic acid. Having by these experiments evinced that metallic pyrophorus may be prepared without vitriolic acid, Mr Bewly proceeded to form an aluminous pyrophorus of the same kind. For this purpose he procured the earth of alum by a long and violent calcination; and examining a part of it, he found, by the usual tests, that it neither contained any sulphur, hepar sulphuris, nor alum undecomposed. This he considered as perfectly pure, though he afterwards found that it contained a small quantity of vitriolated tartar; and yet it repeatedly furnished a pyrophorus, as active as when alum itself is employed. From these and similar experiments, he infers, that the several kinds of pyrophorus are not kindled by moisture, attracted by the vitriolic acid, as M. De Sauvigny has maintained; and his conclusion is farther confirmed by some experiments of Dr Priestley; from which it appears, that they are kindled in dry, nitrous, and what he calls dephlogisticated air.
M. Proust, cited by Mr Bewly, describes a variety of new pyrophorus, which neither contain vitriolic acid, nor seem likely to owe their accension to the attraction of humidity from the air. These principally consist of a coaly matter simply divided by metallic or other earths; such are the sediment left on the filter in preparing Goulard's extract, various combinations of tartar or its acid, or the acetic acid, with metals, calcareous earth, &c.
Mr Bewly, having evinced the insufficiency of M. De Sauvigny's theory, and discovered that the pyrophorus are not kindled by moisture, attracted (merely) by the vitriolic acid, directed his attention to the nitrous acid, which Dr Priestley has shown to be a constituent part of atmospheric air, as the probable agent in the production of this phenomenon. The strong affinity which this acid has with phlogiston, and the heat, and even flame, which it is known to produce with certain inflammable matters, manifested that it was equal to the effect; and having excluded the vitriolic acid from having any essential concern in this operation, he suggests, either that the pyrophorus is kindled by moisture attracted by some of the other ingredients which compose it; or that it has the power of decomposing atmospheric air, by suddenly attracting its nitrous acid, and thereby generating a heat sufficient to kindle the phlogistic matter contained in it. This idea appeared plausible, when he farther considered that Dr Priestley produced the purest respirable air with this same acid combined with other principles; and that this as well as common air is diminished, and probably in part decomposed, in a variety of phlogistic processes. This ingenious writer concludes, upon the whole, from the experiments he hath made, that the pyrophorus seems to owe its singular property to its being a combination of earth or alkali with phlogiston; the vitriolic acid, when present, only occasionally increasing or diminishing the effect, according to circumstances. In the process of calcination, the earth or alkaline principle is not merely mixed, but actually, though loosely, combined with the phlogistic principle of the coal; so that the pyrophorus,
considering it in its most simple state, is only a perfectly dry phlogisticated alkali or earth. On these data, the phenomena may be explained in the two following methods; with respect particularly to the influence of moisture and heat upon the pyrophorus. Supposing either the alkaline or earthy principle to have a greater affinity to water than to the phlogiston with which either of them is united, they may, on being exposed to a moist atmosphere, attract the humidity, and thereby set the phlogistic principle at liberty; which may, in its turn, attract, and be ignited by, the supposed aerial acid; its strong affinity to which is well known:—or, if this hypothesis be rejected, the inflammable matter may be kindled, merely in consequence of the heat produced by the combination of the alkali, &c. with moisture.
Mr Keir gives the following description of a process for preparing a new pyrophorus which he has lately discovered: "I filled about five-sixths of the contents of a copper cylindrical box, which had a lid fitted to it, and which was three inches in diameter and two inches in depth, with saw-dust, which I pressed down; and I laid upon the saw-dust as much well-washed plumbum corneum as entirely filled the box, which I then covered with its lid. I placed the box on the coals of a chamber-fire, so that its bottom only should be in contact with the fuel, and I kept it on the fire till no more vapour seemed to issue at the joining of the lid. I then removed it from the fire; and while it was hot, I closed up the joining of the lid with sealing-wax, by which means the external air was excluded. After it had stood in the cold about ten hours, I opened the box; and the corneous lead, which was very white before the operation, was now rendered black by the vapour which had arisen from the saw-dust, and which was obliged to pass through the lead before it could escape. This black metallic mass was no sooner exposed to the air, than ignited sparks appeared, which spread more and more, while the lead was seen to revive in the form of minute globules, and the part which did not revive was changed into a yellow powder, or calx of lead. It is to be observed, that before I opened the box, I placed it at the side of the fire, in order to melt the sealing-wax, to enable me to separate the lid. It is possible that this small degree of heat may be necessary, or conducive, to the accension. I ought also to acquaint you, that the preparation of this pyrophorus requires nicer attention than that of any which I am acquainted with. For a small excess of heat will revive the lead, which will spoil the experiment. Also, if any air be admitted through the joints of the vessels employed, the kindling property will be prevented by the absorption of the air; which in this case is generally too gradual to produce inflammation. The metallic substance in this state of impregnation with inflammable matter, although not a pyrophorus, is an exceeding quick tinder. For when touched, however slightly, by an ignited body, it will instantly kindle, and the fire will spread over the whole piece, reviving the lead wherever it goes, and exhibiting a very beautiful example of metallic reduction, not unlike the familiar experiment of reviving the lead of a wafer containing minium at the flame of a candle; but with this difference, that the fire in the wafer requires to be kept up by flame; whereas in this metallic tinder it spreads and creeps spontaneously along without flame over the mass.