M. Dessaignes has also remarked, that many minerals which shine at 2000, (centigrade,) refuse to give light when heated to a higher point ; and, of course, he concludes that it is not the consequence of incandescence, as we had done from our own trials with melted nitre. He has also ob served, like ourselves, that different substances require dif ferent temperatures to enable them to give out light. Thus, fluor-spar, apatite, and adularia, shine at 100° or 112°, (centigrade,) while quartz and many of the harder crystalline minerals, as well as glass and porcelain, do not give out light till they are raised to 375°. He also asserts, that, at a mean heat, or about 256°, a point easily marked by the melting of bismuth, all these phosphorescent sub stances become luminous.
Farther, it is remarked by the same observer, that the light yielded is in a direct ratio to the degree of tempera ture, but that its duration is in an inverse one. Farther; if any mineral, such as fluate of lime, which shines at a low temperature, is exposed to a higher heat, it will no longer emit light at a lower one, but will do so at a temperature still higher than the first. Dr. Brewster's experiment agrees with the first of these, and does not contradict the second. The same substance, also, being subjected seve ral times in succession to a temperature of 300°, gave a bright light, which, at the first trial, lasted 30 seconds, at the second 15, and at the third 10 Thus the power of giving light seemed For a time gradually to diminish ; but still it was not destroyed, as in fifteen more successive trials it continued of the same strength.
Farther," vitreous bodies," including certain of the crys talline minerals, lose their phosphorescent properties with great difficulty, and not without heating them strongly for half an hour, or an hour ; while the metallic substances and their salts, together with their phosphorescent oxides, lose their luminous quality immediately after they are strongly heated. Lime, barytes, strontian, alumine, mag nesia, and silex, cannot be deprived of it at all. Also, if these are first heated to 100° or 125°, they do not emit any light, if they are afterwards placed on a hot body heated to 250°; whereas, if thrown on it cold, they shine without dif ficulty. The carbonates of lime, barytes, and strontian, lose their power of phosphorescing when moderately cal cined ; but if after that they are heated to whiteness for half an hour, they resume this property ; apparently in con sequence of the loss of their carbonic acid.
The power of absorbing light is not a property of all the minerals which give it out on heating. Yet it belongs to
some of them. We have already observed, that it belongs to some diamonds and some garnets. It has also been ob served in blende, in the cat's-eye, in apatite, in hyaline quartz, in some emeralds, in lapis lazuli, according to Brug natelli, and, according to Beccaria, in almost all substances. The power of phosphorescing on heating appears to be most common in the imperfectly transparent minerals, and in the coloured ones ; but the colour of the light has no re lation to that of the mineral. It is not necessarily connect ed with the power of giving light by friction ; and sub stances which have been deprived of the property of phos phorescing by heat will still yield it by attrition.
But some farther remarks on the phosphorescence pro duced in minerals by exposure to light are requisite, and we shall quote those of Mr. Heinrich, made in the manner already mentioned.
He observed, as others had done before him, that some diamonds were phosphorescent after exposure to light, and others not ; although there was nothing in their external appearance to indicate, a priori, what effects were to be ex pected. In some of these also, it was found that the light continued for a short time, while in others it was very du rable. These differences were so great, as to vary from five seconds to an hour. On exposing farther the phos phorescent diamonds to the red rays of the prism, it was found that they acquired no luminous power at all ; while, when exposed to the blue, a durable phosphorescence was induced.
After remarking generally, that the fluates and carbo nates of lime became phosphorescent in this manner, it was observed that the luminous appearances, in all the sa line combinations of lime, varied with the acid in the salt. The fluates were the most phosphorescent, and their light continued for an hour on some occasions. The carbonates were the next in point of luminous power, in intensity at least, if not in duration. The light which these emitted was, in some instances, so clear and white, that it was pos sible to read by it ; but it did not last more than from thirty to forty-five seconds. The sulphates were found to shine but for a short time, and very faintly-, while the phosphates proved to be the most feeble of the whole. The siliceous, argillaceous, and magnesian earths were not found to phos phoresce under these circumstances; but some of their na tural combinations were observed to possess this in a very slight degree.