272 Mr. T. Wedgwood's Experiments and Observations 
ends of the pipe being left uncovered ; one of them was of a 
proper form for receiving the nozzle 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 
two or three 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 than 
before. 
Hence it appears, 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 ac- 
cess 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 
