Intelligence and Miscellaneous Articles. 239 



&c.) I omit to describe, as these manipulations do not touch my 

 purpose*. 



My experiments on the luminous phenomena in question were 

 performed in bright daylight at noon, the temperature of the air 

 being about 14° R. f had only one hour for them, and could 

 only make use of the few sorts of stone that were at hand. 



The phenomena I observed were essentially of two kinds. As 

 soon as a stone of about the hardness of quartz was pressed against 

 the revolving grindstone, there was produced between them an in- 

 tense red light, which at the same time radiated round the object 

 and emitted numerous sparks. This appearance was the same with 

 all hard stones. 



The second phenomenon occurred simultaneously with the other, 

 but only with translucent and transparent stones, not with com- 

 pletely opaque ones. The stones of the former quality shine with 

 brilliant red light slightly yellow-tinged. They appear almost like 

 red-hot iron ; and it really looks as if the hands of the grinder must 

 be severely burned. All the stones tried became warm in grinding, 

 but not very hot ; feeling them with my hand, I estimated the in- 

 crease of temperature at from 10° to 12° E,. 



The stones on which I experimented in this way were the fol- 

 lowing : — 



Chalcedony of a yellowish grey colour, a little translucent, from 

 Uruguay, South America ; a piece 2 inches thick became magnifi- 

 cently fiery red, and therewith transparent. 



Chalcedony, white and opalescent, said to be oriental ; behaviour 

 as in the preceding experiment. 



Chalcedony of a reddish colour, translucent, from Idar ; gave the 

 same result. 



Chrysoprase from Silesia; a thick piece, pale green, very 

 slightly translucent at the edges, hardly useful as a jewel, gave a 

 little red light. 



Bock-crystal, colourless, perfectly transparent, from Brazil. The 

 luminous appearance was very magnificent ; but the fiery red, in 

 comparison with the above experiments, was more subdued, almost 

 rose-red. 



Rock-crystal, smoke-grey, transparent (the so-called smoky topaz), 

 from the Swiss Alps. Pretty nearly the same result, only some- 

 what less beautifuL 



Carnelian of a beautiful red colour, intensely translucent, from 

 India ; gave the most magnificent red light, as the colour of it was 

 saturated with the natural colour of the carnelian. 



Amethyst, crystals, intensely translucent, from Idar. The light 

 was pale violet, because here also the violet-blue colour of the stone 

 blended with the fiery red of the light, beautifully transparent. 



Of perfectly opaque stones were tried red agate jasper from Idar, 

 black lydite with thin white stripes of quartz, artificially black- 

 coloured chalcedony from Uruguay, opaque heliotrope from India, 

 * A very precise account, illustrated with figures, will be found in 

 Collini, Journal d'un Voyage qui contient differentes observations minera- 

 logiques, &c. Mannheim : 1776. 



