218 PHILOSOPHICAL TKANSACTiONS. [aNNO 1792. 



Exper. 8. Observing that colourless transparent glass had a paler hue, when 

 red-hot, than most other bodies, I conceived that it might not be luminous at 

 so low a temperature. I therefore took a circular piece of glass, about v^ of an 

 inch thick, and having gilt one side of it, exposed the ungilt side to a stream of 

 air passed through a red-hot tube ; but did not perceive that the gold shone at 

 all before the glass. This experiment however, is not decisive ; glass being so 

 slow a conductor of heat, that its exterior surface might have been heated some time 

 before the interior, and thus have deceived the eye. I could not meet with any 

 glass sufficiently thin for this purpose, nor think of any other possible mode 

 of trial. 



Exper. Q. Having often remarked, that the surfaces of red-hot metals had 

 an appearance different from what they present by reflected light when cold, I 

 had an idea that this peculiar appearance inight be derived from a transmission of 

 the light through the superficial parts of the ignited body. To ascertain whether 

 they acquired any degree of transparency by heat, I fixed a circular plate of 

 fine gold, about Vt of an inch thick, on the end of a tube, which was perfectly 

 closed by it ; then having heated it to redness, and looking down into the tube, 

 I pressed the outer surface of the gold against single grains of gunpowder : the 

 red light of the gold looked whiter on every flash. To be satisfied that no light 

 found admission through the sides of the tube, which were of thick earthen- 

 ware, I covered the exterior surface of the gold plate with a thick coat of clay 

 luting, and again making it red-hot, fired gunpowder with it as before, but no 

 increase of light was now perceptible from the flash ; which proves, that the 

 sides of the tube were impervious to the light. When this gold was cold, I 

 stuck a few grains of gunpowder on its surface, and looking within the tube, 

 fired them by pressing them against a hot iron, but the light of the explosions 

 was not then sensible. Plates of silver, and of iron, gave the same results. 



Exper. 10. A lump of the most luminous marble, and an equal lump of the 

 same marble blackened over, were placed together on a mass of iron heated just 

 under redness : the former gave out much light, the latter none. On a second 

 exposure, the lump not blackened gave a faint light ; the blackened one, as 

 before, none at all. Then wiping off the black, and placing them together 

 on the heater, I found the one which had been blackened to emit as little light 

 as the other : thus the phosphorescent property was nearly destroyed, without 

 any visible light leaving the body. 



Exper. 11. If a piece of glass, or glazed or unglazed earthen-ware, with any 

 enamel, painting, gilding, or writing in ink on it, be made red-hot, the coloured 

 parts appear considerably more red than the others, and continue longer visible. 

 Iron wire, within a red-hot glass tube, looks much more red than the glass. 

 Black matter, on a large polished mass of fine gold, did not remain any longer 

 red than the gold. 



