In a vacuum, exposed to a high-tension current of electricity, diamonds phosphoresce of different colours, most South African diamonds shining with a bluish light. Diamonds from other localities emit bright blue, apricot, pale blue, red, yellowish-green, orange, and pale green light. The most phosphorescent diamonds are those which are fluorescent in the sun. One beautiful green diamond in my collection, when phosphorescing in a good vacuum, gives almost as much light as a candle, and you can easily read by its rays. But the time has hardly come when diamonds can be used as domestic illuminants!
By permission of Mrs. Kunz, wife of the well-known New York mineralogist, I will show you perhaps the most remarkable of all phosphorescing diamonds. This prodigy diamond will phosphoresce in the dark for some minutes after being exposed to a small pocket electric light, and if rubbed on a piece of cloth a long streak of phosphorescence appears.
The luminescence produced by heat is wonderfully marked in the case of chlorophane, a variety of fluorite. A Siberian specimen of a pale violet color emitted a white light merely from the heat of the hand; boiling water caused it to give out a green light, which was so greatly intensified when the specimen rested on a live coal that the radiance could be discerned from a considerable distance. Similar phenomena were observable in the case of chlorophane from Amelia Court House, Va., and the writer found that specimens from this source also exhibited strong triboluminescence, resulting either from contact with one another, or with any hard substance.[261]
As the terms fluorescence and phosphorescence are sometimes rather carelessly employed, it may be well to note here that while both terms are used to denote the luminescence of a non-luminous body resulting from the action of light rays, of the electric current, or of radiant energy of any kind, as well as from heat, fluorescence signifies a luminosity which only continues so long as the exciting cause is present, while phosphorescence means a luminosity persisting for a longer or shorter period after the exciting cause has ceased to operate directly. The latter term therefore denotes a luminous energy stored up in the formerly non-luminous body and emitted by it for a certain time, at the expiration of which it again becomes non-luminous. Other special designations of induced luminosity in minerals are triboluminescence, the emission of light as a result of friction and thermoluminescence, a term used to denote light-emission excited by moderate heating, even by the warmth of the hand.
An old treatise in Greek, said in its title to come from “the sanctuary of the temple,” and containing material, partly of Egyptian origin, may help us to understand something of the processes employed by a temple priest to impress the common people by the sight of luminous gems. The writer of the treatise declares that for the production of “the carbuncle that shines in the night” use was made of certain parts (he says “the bile”) of marine animals whose entrails, scales and bones exhibited the phenomenon of phosphorescence. If properly treated, precious stones (preferably carbuncles) would glow so brightly at night “that anyone owning such a stone could read or write by its light as well as he could by daylight.”[262]
In the Annales de Chimie et Physique, the great French chemist, M. Berthelot, discusses this matter and expresses the following opinion[263]
“The texts leave no room for doubt as to the employment by the ancients of precious stones rendered phosphorescent in the dark by the superficial application of tinctures composed of materials whose phosphorescent quality is known to us. Although this luminescence, due to an application of organic oxidizable materials, could not well be durable, still it might be made to last several hours, perhaps several days, and it could always be renewed by repeating the application.”
The use of jewelled ornaments to heighten by their luminosity in obscurity or in darkness the effect produced by a sacred image, and to stimulate religious awe in the beholder, is testified to by the ultra-Protestant traveller, Fynes Moryson, Gent., who went to Italy in 1594. Of his visit to the Santa Casa in Loreto, he says that he himself and two Dutchmen, his companions, were permitted to enter the inner chapel of the sanctuary, “where,” he proceeds, “we did see the Virgin’s picture, adorned with pretious Jewels, and the place (to increase religious horror) being darke, yet the Jewels shined by the light of wax candles.” Although there is no question here of naturally luminous gems, this might have been the impression produced upon a more sympathetic pilgrim.[264]
Writing of the traditions in regard to luminous stones, Sir Richard F. Burton says, “There may be a basis of fact to this fancy, the abnormal effect of precious stones upon mesmeric sensitives.”[265] However, while some instances are recorded of psychic impression produced by precious stones on the minds of persons possessing a highly sensitive nervous system, it seems likely that some legends of luminous stones had their origin in the refractive powers of cut gems, by means of which a dim and distant light would be reflected from the surface of the stones and would seem to spring from them. Quite possibly, in other instances, there was a disposition to cater to the popular belief by placing a light so that the hidden beams traversed the stone and appeared to emanate from it.