or thirteen minutes at longest. Not all diamonds show this quality, and nothing in their form or appearance serves to determine their possession of it. However, Mons. du Fay observed that the yellow diamonds, of which he tried a considerable number, were luminous. A single emerald, out of twenty that were tested, proved to be luminous.50
Boyle's experiments led to the discovery that some diamonds, when rubbed against wood or other hard substances, and even against cloth or silk, will emit a ray of light which seems to follow them; this is what is called triboluminescence.
The power of absorbing sunlight or artificial light and then giving it off in the dark is only possessed by certain diamonds. These are Brazilian stones, slightly milky in tint, or blue-white as they are often termed, and it is an included substance and not the diamond itself that possesses the power of storing up light and then giving it out. Willemite, kunzite, sphalerite (sulphide of zinc) and some other minerals possess the same power. Their peculiar property may be due to the presence of a slight quantity of manganese or to that of some of the uranium salts. That it is only the ultra-violet rays that are thus absorbed by these diamonds is proved by the fact that the phenomenon is not observable when a thin plate of glass is interposed between the sunlight or artificial light and the diamond, as glass is not traversed by these rays. The still undetermined substance to whose presence in diamonds of this type the special class of phenomena must be due, was named by the author
w " Journal des Scavane," 1739, pp. 438, 439, of Amsterdam edition, citing "Hist, de l'Acad. Roy. des Sciences," 1735 (vol. xxxviii).
