BOLOGNA STONE.
The Bologna stone, in consequence of its property of shining in the dark, which was observed by accident, has given rise to many laborious researches and experiments, and to writings almost without number, which have not so much enlarged our knowledge of light, as proved that all the hypotheses hitherto offered by philosophers for explaining it, if not entirely false, are at least insufficient and uncertain. The history of this stone, therefore, though not unknown, deserves to be here repeated, especially as many parts of it require to be rectified.
As a complete description of it would be superfluous to mineralogists, it may be sufficient to remark, that this kind of stone is found in plates or single pieces, which in general are more or less of a conical form, have a dirty white or semi-transparent water-colour, and a foliaceous structure, which is observed on its being broken, though the stone, considered in another direction, appears to be fibrous. The surface of single pieces is uneven. But what distinguishes this species from the gypseous spars, to which it bears the greatest resemblance, is its extraordinary weight; and this it has in common with all the varieties of heavy spar, to which, according to its component parts, it belongs.
This stone is found on different eminences around Bologna, and particularly on the hill of Paderno, which is situated at the distance of about a German mile from the city, loose, and scattered about between gypseous stones, in a marly earth, some of which is still seen adhering to pieces in my possession. It is found most readily after heavy rains, particularly in the streams which run down the sides of the hill; and it is there collected by persons who sell it at Bologna. In the year 1730, when Keysler was there, a pound of it could be purchased for a paolo[1042].
I shall take this opportunity of remarking, that the Bologna stone, according to its external characteristics, heaviness and hardness excepted, has a great similarity to those gypseous spars or selenites which were first described by Lehman[1043], and at the time perhaps by him alone; according to whose account, it is mentioned also by Vogel[1044] and by Wallerius[1045], under the name of Selenites globosus: on the other hand, it has not been mentioned by modern mineralogists under any particular appellation. In the county of Mansfeld it is found in detached masses or single pieces, more or less conical; and, to judge from the earth purposely left on the specimens in my possession, which were picked up in the neighbourhood of Sangershausen, in a yellowish-red sandy clay. The pieces, many of which are round balls, two or three inches in diameter, and others longish rolls, have, externally as well as internally, a grayish colour, appear foliated on the fracture, or seem to consist of cuneiform radii, which meet in the centre of the ball. Many are hollow in the inside; and in this case the ends of the cunei or needles, which have between them a granulated gypsum mixed with a little clay, project into the cavity. Lehman says that the leaves, when placed in a heated stove, emit a hesperus, that is, shine; and this circumstance made Wallerius doubtful whether this selenite did not belong to the fluor-spars; but it is undoubtedly a sulphate of baryta. When the crude stone is put into acids, a very faint effervescence is sometimes observed, arising from foreign matters; but when burnt pieces are employed, this effect is much stronger. It does not crack or break in the fire; but if exposed only a short time to a red heat, it becomes totally opake, whiter, and void of all lustre; it is also more friable, and crumbles to dust in water, exactly in the same manner as bastard lapis specularis. The luminous appearance in a warm stove I did not observe in the few pieces which I subjected to experiment. I was desirous to make this remark, because the mineralogists before-mentioned place globular selenite along with the Bologna stone, to which however it does not belong.
To render it capable of shining in the dark, a piece particularly heavy, foliaceous and pure, must be selected[1046]. After being made red-hot, it is pounded and reduced to a fine powder, which, by means of a solution of gum-tragacanth, is converted into a kind of paste, and formed into small cakes. When these are dried, they are brought to a state of ignition between coals, and then suffered to cool; after which they are preserved from the air and moisture in a close vessel. If one of these cakes be exposed a few minutes to the light, and then carried into a dark place, it will shine like a burning coal. It appears therefore to attract the light, or to be as it were a light-magnet. This power of emitting light becomes lost in the course of time; but it may be restored at first by heating, and afterwards by exposure again to ignition. I shall pass over the rules necessary to be observed in the numerous experiments made with this stone, as well as the consequences deduced from them. The former may be found in works on chemistry, and the latter in those on natural philosophy.
All the Italian writers who first describe this remarkable phænomenon give the following account of the discovery. At the beginning of the seventeenth century there was at Bologna a shoemaker, who, having quitted his trade, applied himself to chemical labours, and particularly to the art of gold-making. I do not know whether those who have made the very just remark, that many shoemakers go beyond their last into the province of other arts or sciences, have mentioned among the already numerous instances this shoemaker of Bologna, whose name was Vincentius Casciorolus; but he certainly deserves a niche in the temple of fame, because it may with truth be said of him, that he kindled up a light to the learned; whereas the shoemaker of Görlitz, Jacob Behmen, darkened or extinguished the existing light to the learned as well as the unlearned, so that the minds of many are still left in obscurity.
In the year 1602 Casciorolus came to Scipio Begatello of Bologna, who at that time was particularly well known by his attachment to the art of gold-making, and showed him this stone, under the mystical name of lapis solaris, which, on account of its weight and the sulphur it contained, as well as of its attracting the golden light of the sun, seemed to be fit for converting the more ignoble metals into gold, the sol of the alchemists. He showed it also to J. A. Maginus, the professor of mathematics; and the latter, who in all probability was no adept, sent both the natural and prepared stones to princes and learned men, and perhaps contributed more than any other person to make known this singular discovery[1047].
It however appears as if the Italian chemists concealed the preparation of this stone, or were not all acquainted with it. It was always said to be a secret known only to a few individuals in Bologna. Misson, who was there in the year 1690, asserts that Bartholomew Zanichelli was the only person at that time in possession of it. In 1666 it was announced in the Philosophical Transactions[1048], that a clergyman, who exclusively possessed the art, had died, without communicating it to any one. Niceron[1049], Lemery[1050], and many others say, that Homberg, during his residence at Bologna, had again discovered it, after many experiments; and that Lemery learned it from him and made it publicly known.
This however cannot be altogether true; for in the year 1622, P. Potier, or Poterius, a French chemist, who lived at Bologna, taught the preparation of it in his work already quoted, as did Kircher[1051] in 1641, and the jesuit Casati[1052] in 1686; though the process then employed was indeed not the best or most convenient; the proper method being first found out, after many accurate experiments, by the German chemist Marggraf, who showed also how similar light-magnets or luminous stones can be prepared from most of the heavy spars and fluor-spars[1053].
But even at present, those who prepare this stone for sale at Bologna talk in such a manner as if the secret were known to them alone. This was the case, in 1771, with the director of the institute in that city[1054]. Keysler purchased a piece, as large as a dried fig pressed flat, for about two or three paoli.
I shall embrace this opportunity of bringing to recollection, from De Thou’s history of his own times, a relation which indeed contains many things incredible, and in all probability exaggerated, yet seems to be too well confirmed to be altogether rejected as false. If this be admitted, it may then be conjectured that, about the year 1550, either the Bologna stone, or what at present is called phosphorus and pyrophorus, was known to a few individuals. In the above year, when Henry II. king of France made his solemn entrance into the town of Boulogne, on its restoration by the English, a stone from India, which was not hard, which had a luminous appearance like fire, and which could not be touched without danger, was presented to him by a stranger. For the truth of this account De Thou refers to the testimony of J. Pipin, in a letter to Ant. Mizaud, who asserts that he himself saw the stone. Morhof, who seems inclined to consider this stone as that of the philosophers[1055], remarks that this passage is found in the first Paris edition in octavo, and in the Frankfort re-impressions, both in folio and octavo; but not in the other editions. He quotes also the words from the letter to Mizaud, which must be printed somewhere, but in what work I do not know. It appears that the historian inserted it almost without any change.
[We may take this opportunity of describing one or two other pyrophori: thus Canton’s pyrophorus is prepared by heating a mixture of three parts of sifted calcined oyster-shells with one part of flowers of sulphur to an intense heat for one hour; Homberg’s, by mixing equal weights of alum and brown sugar, and stirring the mixture over the fire in an iron ladle until quite dry; it is then put into an earthenware or coated glass bottle and heated red-hot as long as a flame appears at the mouth; it is then removed, carefully stopped and suffered to cool. The black powder which it contains becomes glowing hot when exposed for a few minutes to the air.]