Homberg discovered this substance, in the year 1680. Hence it is sometimes called Homberg's pyrophorus. He was operating upon a mixture of human excrement and alum; and, when he examined the contents of his vessel, in three or four days after, he was surprised to see it take fire spontaneously, when brought to the air. Soon after Lemery, the younger, discovered, that honey, sugar, flour, or almost any animal or vegetable matter, could be used in lieu of human fæces; and, as Macquer informs us, M. Lejoy de Suvigny showed, that other salts, containing sulphuric acid, may be substituted for alum. Mr. Scheele (Treatise on fire, &c.) found by experiment, that, when alum was deprived of potassa, it was incapable of forming pyrophorus, and that vitriolated tartar (sulphate of potassa) may be used in the place of alum. The experiments of Mr. Proust prove, that a number of neutral salts, composed of vegetable acids and earths, when submitted to heat, leave a residuum that inflames spontaneously. This statement agrees with the experiments of M. Chenevix. From the experiments and observations of sir H. Davy, and Dr. J. R. Coxe, late professor of chemistry, but now of materia medica, &c. in the University of Pennsylvania, it is rendered very probable, that pyrophorus owes its property of inflaming spontaneously to a small portion of potassium, which is formed in the process.
The preparation of pyrophorus is explained on the principle, that the vegetable matter is first decomposed; that the hydrogen and a part of the carbon decompose the sulphuric acid of the alum, by uniting with its oxygen; that water, carbonic oxide, and carburetted hydrogen are disengaged, along with a part of the sulphur; and that, while the excess of charcoal remains intimately mixed or divided with the alumina, the sulphur and the sulphuret of potassa, form together a compound, which has the property of inflaming spontaneously in the open air. Some suppose, as alum is a triple salt, having potassa, as well as alumina, for its base, that the potassa is decomposed in the process, and potassium, as we remarked, produced; to the presence of which they ascribe the singular property of inflaming in the open air.
The spontaneous combustion of charcoal, in several instances, is supposed by some to have been owing to the presence of pyrophorus, by others to phosphorus, and by others again to nascent hydrogen. To the presence of this substance, is attributed the explosion of gunpowder mills. (See [Gunpowder.])
Several different mixtures, and torrefied substances, form a kind of imperfect pyrophori, and have more than once occasioned fires, from no suspicion of their properties being entertained.
Besides pyrophorus, other compositions, which, in like manner, take fire on exposure to the open air, have been by degrees made known to us: 1. The scoria of the martial regulus of antimony, or antimony freed from sulphur by the intervention of iron and nitre, as well crude as also after being dissolved, have been observed to take fire spontaneously, when laid upon a hot stone, or in the sun. Of the truth of the latter case, Wiegleb says, he is assured by his own experience. 2. The residuum of the acetate of copper is another pyrophorus. 3. Some assert, that they have observed an inflammation ensue from honey and flour, calcined according to the rules laid down. 4. According to Geoffroy, a calcined mass of three parts of black soap, and one of diaphoretic antimony, has been known to take fire spontaneously. 5. Meuder has observed, that a pyrophorus is obtained, when equal parts of orpiment and iron-filings are sublimed together, and ten parts of this sublimate are triturated in a mortar along with twelve of nitrate of silver. 6. A pyrophorus is produced, according to Penzky, when two drachms of white sand, three of common salt, one of sulphur, two of sulphuric acid, and half an ounce of muriatic, are mixed together and distilled in a glass retort. In this operation, a sublimate is said to be obtained, which bursts out in flames, as soon as it comes into contact with the air. 7. The spontaneous precipitate of osteocolla, from a solution of it in sulphuric acid, after having been separated by means of a filter, and dried, took fire in a warm place. S. Pott observed the same phenomenon in the earth of the residuum, after the distillation of urine, that had been putrid for a considerable time. 9. To these may also be referred, a mass composed of equal parts of sulphur and iron-filings; which, when thoroughly moistened with water, after some time, grows hot, swells, and at last breaks out into vapour, smoke, and flame. (See [Artificial Volcano.])
Cadet's fuming liquor, prepared by distilling equal parts of acetate of potassa, and arsenious acid, emits a very dense, heavy, fetid, noxious vapour, which inflames spontaneously in the open air. Black wadd, an ore of manganese, when dried by the fire, and mixed with linseed oil, gradually becomes hot, swells, and then bursts into flame.
M. Chenevix (Annales de Chimie, tom. LXIX,) remarks that almost all the metallic residuums, which are formed by the distillation of acetates per se, are pyrophoric, after cooling; which Mr. C. attributes to the presence of finely divided charcoal, mixed with the metallic part. He experimented on several acetates, with the view of ascertaining the quantity of pyroacetic spirit they would yield, and found, in every instance, that charcoal existed in the residue, sometimes with reduced metal, and at other times with metallic oxide. A table of these experiments may be seen in Ure's Chemical Dictionary. The residuum of acetate of copper has long been known to possess pyrophoric properties.
Sect. XXXIII. Of Sal Ammoniac.
This salt enters into the composition of fire-works, to give, more particularly, a peculiar colour to flame, which is that of green, or yellowish-green. Sal ammoniac is a salt, composed of muriatic acid and ammonia, and, when pure, is white, and capable of being sublimed without decomposition. Its purity may be known by its complete volatilization. It is readily pulverized.