2l6 PHILOSOPHICAL TRANSACTIONS. [aNNO 1792. 



but the differences between the pohshed and the blackened ones were not so 

 remarkable in these, as in the silver. By repeating this experiment many times, 

 he found, by observations with a stop-watch, that the blackened silver cylinder 

 began to shine, at a medium, in -^ of the time which the polished one required; 

 and that, after its removal from the crucible, it continued to shine only -§- of the 

 time that the other did. 



From this experiment it would seem, that a great part of the light emitted by 

 the cylinders was absorbed from the red-hot crucible, as the blackened one, 

 which absorbs most rays, not only became first red, but also shone brightest. 

 Tlie following experiment, however, affords a different conclusion. 



Exper. 2. An earthen-ware pipe, of a zig-zag form (fig. 7), was placed in a 

 crucible, which was filled up with sand, the 2 open ends of the pipe being left 

 uncovered ; one of them was of a proper form for receiving the nosle of a pair 

 of bellows, the other bent into angles of the form of the letter z : on this last 

 was fastened a globular vessel a, with a lateral bent pipe, to let out air but 

 exclude all external light, and with a neck in which was inserted a circular plate 

 of glass. The crucible, with the sand and the part of the pipe contained in it, 

 was then heated to redness. Having my eye fixed in the neck of the vessel a, 

 and observing it perfectly dark within, I directed an assistant to blow with the 

 bellows. The stream of air, sent through the red-hot tube, not being at all 

 luminous, I fixed a small strip of gold into the orifice of the tube at b, which, 

 after 2 or 3 blasts, became faintly red ; thus proving, that the air, though not 

 luminous, was equal in temperature to what is usually called red heat. I then 

 heated the crucible to a brighter redness : the stream of air, blown through the 

 bright red-hot tube, still came out perfectly dark, but the strip of gold, exposed 

 to it, shone both sooner and brighter tlian before. 



Hence it apf)ears, that the greater brightness of the blackened cylinder, in the 

 first experiment, was owing to its being of a higher temperature ; and that it 

 would have been equally bright had it been raised to the same temperature by 

 any other means than the absorption of light ; the metal being here brought to 

 a faint, and to a bright ignition, without the access of any visible light. But 

 perhaps another consequence may be fairly drawn from this experiment. As the 

 gold may be made to emit light for any length of time, by being supplied with 

 heat from the dark air of the temperature of red heat, neither the gold nor the 

 air suffering any chemical change whatever, is not the light emitted identical 

 with the heat received ? This identity appears to be confirmed by the following 

 observation : that if the solar rays be made to converge on one end of a black- 

 ened cylinder of metal, the other parts will become red-hot, and emit light ; or, 

 if the rays be converged on the tube blackened, and air passed through it, the 

 gold placed in the dark current will yield a constant light. 



