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.]