That the diamond and garnet have been long observed to possess this property, is probable from the occasional mention of them that is made in the Oriental tales. The dark subterranean caverns of the genii and enchanters are often lighted by carbuncles ; and the diamond which Saad took out of the fish that lie had caught, served his household for a lamp. So general a belief must have had a foundation ; and, indeed, if we examine all the phe nomena described in these tales, which are not absolutely extramundane and supernatural, we shall find them all founded on facts, exaggerated and inverted.
The minerals which phosphoresce on friction are far less numerous than those which exhibit the same property when heated. We have already mentioned the diamond and the garnet, although we have ourselves succeeded with neither. But the phosphorescence of quartz, on friction or collision, is very remarkable and lively ; while it is also attended with a peculiar smell, as if some combustion took place. It was once imagined that it produced black glo bules on being thus treated ; but that ultimately proved to be a fallacy. We already mentioned the occasional phos phorescence of tremolite, and in some cases, it is such as to be excited even by a feather ; while, in many more, it can not be produced at all. In the harder crystalline minerals, the friction or the abrasion produced by a file is necessary to excite the light ; while, in some, it is only discovered by pounding them in a mortar. We have observed it to take place in the carbonate of lime, provided the surfaces are rough, and the force considerable; but, in making ex periments with rocks, it is necessary to take care that we are not misled by the presence of particles of quartz.
The phosphorescence of minerals by heat is well known, with regard to fluor spar at least, as it is the substance commonly exhibited for this purpose at popular lectures. Mr. Wedgewood was among the first to enlarge the list of minerals in which it existed. His experiments were per formed by reducing the substance under trial to powder, and strewing it on a plate of iron heated just below the visible redness, and removed into a dark place. In this way he found that this property was possessed by the fol lowing substances, namely :— But Dr. Brewster's experiments have materially enlarg ed this list; so that the property of phosphorescence, used by Natty as one of the distinguishing characters for minerals, has almost ceased to be of any use in this res pect. If Dr. Brewster has thus deprived mineralogists of one of their convenient empirical marks, he has conferred a greater benefit on them, by preventing them from deceiv ing themselves by trusting to it. In some of these experi
ments, he followed nearly the same plan, namely, that of placing a fragment of the mineral in question on a hot iron, and then carrying it into a dark room. When the light was not perceptible in this manner, the fragment was placed in a pistol barrel, which was then heated to a point short of redness, and the phosphorescence observed by looking down the bore. On other occasions, the barrel was first heated, and when the red heat was gone, the mineral was introduced into it, and examined in the same manner.
We extract from his communications on this subject, the following table, where the results are exhibited in a condensed form.
It was asserted by Mr. Wedgewood, that minerals could not be deprived of this property by many beatings, nor by any degree of heat. Dr. Brewster found, on the contrary, that a specimen of green fluor, which was highly phospho rescent, having been wrapped in a leaf of platina, and ex posed for an hour to the heat of a common fire, lost not only its green colour, but its power of phosphorescence, although it had not flown to pieces, as it usually does on the iron plate. Neither the ordinary rays of the sun; ap plied for several days, nor even the focus of a burning-glass, restored this property ; and it is farther worthy of remark, that when placed on the hot iron, it no longer flew in pieces, as it always does while it possesses the phospho rescent property. We have tried the same experiment with other varieties of fluor, and with corresponding re sults.
We may now acid to these experiments of Dr. Brewster, that we have farther observed that the phosphorescence was, in some minerals, excited by a high degree of heat when inferior ones produced no such effects.• Not having had this object in view in these researches, we neglected to record the substances, having been at that time pursu ing a different train of investigations. We also noticed that the phosphorescence, in some minerals, was excited with peculiar facility in melted nitre, or upon its surface when far short of a red heat. Among others, which, for the same reason, have escaped our memory, we noticed some specimens of crystalline primary limestone, of tre molite, of chlorite, and of chalcedony. In some of these cases the light was as bright as the sun, and seemed to per vade the whole of very large fragments.