Prof. Draper on the ■production of Light hy Heat. 347 



points of support, by reason of the conducting power of tlie 

 metals to wliich it is attached. 



Physiological considerations would also lead us to suspect 

 that the self-luminous temperature must vary with different 

 eyes. The experiments of Bouguer, hereafter to be reierred 

 to, indisputably show that some persons are much more sen- 

 sitive to the impressions of light than others. So far as my 

 limited investigation of this matter has gone, I have not how- 

 ever found appreciable differences in the estimate of the tem- 

 perature of incandescence. Different individuals, observing 

 the platinum, have uniformly perceived it at the same time. 



Against the number 977° it may also be objected, that an- 

 timony melts at a much lower temperature, and yet emits 

 light before it fuses. If this statement were true, it would 

 lead us to believe that all bodies have not the same point of 

 incandescence. But I think the experiments of Mr. Wedg- 

 wood on gold and earthenware are decisive of that question ; 

 and, moreover, 1 have reason to believe that the melting- 

 point of antimony is much higher than commonly supposed. 



With a view of determining directly whether different bodies 

 vary in their point of incandescence, I took a clean gun-barrel, 

 and having closed the touch-hole, exposed the following sub- 

 stances in it to the action of the fire: — platinum, chalk, marble, 

 fluor spar, brass, antimony, gas-carbon, lead; each specimen 

 was small; the platinum was in the form of a coil of stout 

 wire. 



When one of these bodies was placed in the gun-barrel 

 and the temperature raised, it is clear that any difference in 

 their point of incandescence would be detected by the eye. 

 Thus, if the ignition of platinum required a higher degree 

 than iron, on looking down the barrel the coil of wire should 

 be dark, when the barrel itself begins to shine ; or, if the pla- 

 tinum was incandescent first, the wire should be seen before 

 the barrel is visibly hot; and these results might be corro- 

 borated by observing the inverse phaenomena, when the barrel 

 is taken from the fire and suffered to cool. 



W^ith respect to platinum, brass, antimony, gas-carbon and 

 lead, they all became incandescent at the same time as the 

 iron barrel itself. I could not discover the slightest difference 

 between them, either in heating or cooling ; and it is worthy 

 of remark, that the lead was of course in the liquid condition. 

 But the chalk and marble were visible before the barrel was 

 red-hot, emitting a faint white light; and the fluor spar still 

 more strikingly so, its light being of a beautiful blue; and 

 even when the barrel had becouKt bright red I could still see 

 the spar, which had decrepitated to a coarse powder, by its 



