132 PHILOSOPHICAL TRANSACTIONS. [aNNO 1792. 



transparent, from the East Indies (l). 17. Iceland spar (O). 18. White 

 enamel (2) ; tobacco-pipe (3). White mica (O). 19. Unglazed biscuit earthen- 

 ware, blackened by exposing it, buried in charcoal in a close crucible, to a 

 white heat (4). 20. * Black, vitreous mass, made by melting together 5 of 

 fluor, ] of lime, and some charcoal powder (4). 21. Fluor; aerated and 

 vitriolated barytes ; white and black Derbyshire marble ; calcareous spar ; crys- 

 tals of borax ; deep blue glass ; mother of pearl. 



Rock crystal, quartz, flint-glass, and many other hard bodies, during attri- 

 tion, emit now and then reddish sparks of a vivid light, which retain their 

 brightness in a passage of 1 , 2, and even 3 inches, through the air. 



A piece of opaque agate, applied to the circumference of a wheel of fine grit, 

 revolving at a moderate rate, becomes brightly red, even in day-light, at the 

 touching part ; if the wheel revolve at a quicker rate, the touching part emits a 

 pure white light. In both cases, glowing sparks are continually emitted, some 

 of which are not extinguished before they have passed 12 or 14 inches through 

 the air ; they explode gunpowder and inflammable air, and burn the skin ; their 

 brightness is not sensibly increased by passing into pure air. The corner of an 

 angular piece of window-glass being applied to the wheel in motion, a full 8th 

 of an inch of the glass above the point of contact becomes apparently red-hot 

 and retains the redness for a second or 2 of time after its removal from the 

 wheel ; during the attrition, large red sparks are continually emitted, and a mix- 

 ture of softened glass, and the sand of the stone wheel, is collected about the 

 touching point. Quartz, transparent agate, rock crystal, and window-glass, 

 give nearly the same flashing light, when rubbed against the stone wheel, or in 

 the ordinary manner, excepting the tinge of red in the former, which it receives 

 from the light of the grit: the transparent agate becomes red-hot for a little 

 way about the part in contact with the wheel, and is thus deprived of its trans- 

 parency, as it would be if made red-hot in a common fire ; porcelain is heated to 

 redness by the same treatment. The red sparks which are emitted by all these 

 bodies during their attrition, are heated particles about the magnitude of grains 

 of fine sand, broken off by the friction. 



Bodies give out their light the instant they are rubbed on each other, and 

 cease to be luminous when the attrition is discontinued. Colourless, transparent, 

 and semi-transparent bodies emit a flashing light, their whole masses being, for 

 a moment, illuminated ; opaque bodies give little more than a defined speck of 

 red light, and are not luminous below the part struck. The greatest apparent 



* Some of this mixture taken out of the crucible before it was perfectly fused, gave out, when 

 rubbed, a strong smell like phosphorus of urine ; and on throwing some of it pulverized on a plate 

 of iron, heated just below redness, it was very luminous, and presented every appearance of burning 

 phosphorus. — Orig. 



