390 Prof. Draper on the Production of Light by Heat 



come sensible to feeble impressions of light, I caused the current 

 to pass, gradually increasing its force, until the platinum was vis- 

 ible. In several repetitions of this experiment it was uniformly 

 found that the index to which the platinum was attached, stood 

 at the eighth division when this took place. The metal had 

 therefore dilated t fa of its length ; the elevation of its tempera- 

 ture was about 917°, which added to the existing height of the 

 thermometer, 60°, gives for the temperature of incandescence 

 977° F. 



To the correctness of the number it may be objected, that 

 owing to the narrowness of the metallic strip it is not well calcu- 

 lated to make an impression on the eye when the light it emits is 

 so feeble ; nor can we take the dilatations given by the index, as 

 representing the uniform temperature of the whole platinum, 

 which must necessarily be colder near its points of support, by 

 reason of the conducting power of the metals to which 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 referred to, indisputably 

 show that some persons are much more sensitive to the impres- 

 sions of light than others. So far as my limited investigation of 

 this matter has gone, I have not however found appreciable difier- 

 ences in the estimate of the temperature of incandescence. Dif- 

 ferent individuals, observing the platinum, have uniformly per- 

 ceived it at the same time. 



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

 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 

 think the experiments of Mr. Wedgwood on gold and earthenware 

 are decisive of that question ; and, moreover, I have reason to be- 

 lieve 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-barre , 

 and having closed the touch-hole, exposed the following substai i - 

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

 spar, brass, antimony, gas-carbon, lead ; each specimen was sma > 

 the platinum was in the form of a coil of stout wire. , 



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

 the temperature raised, it is clear that any difference in their p 

 of incandescence would be detected by the eye. Thus, i 

 ignition of platinum required a higher degree than iron, on 

 ing down the barrel the coil of wire should be dark, wne " de9 . 

 barrel itself begins to shine ; or, if the platinum was inca " ibly 



cent first, the wire should be seen before the barrel is vi 



