28 THE RADIATIONS OF IGNITED BODIES. [MEMOIR I. 



ly found that the index to which the platinum was at- 

 tached stood at the eighth division when this took place. 

 The metal had therefore dilated ^ f ^s length ; the 

 elevation of its temperature was about 917, which, add- 

 ed to the existing height of the thermometer, 60, gave 

 for the temperature of incandescence 977 Fahr. 



To the correctness of this number it might be objected 

 that, owing to the narrowness of the metallic strip, it was 

 not well calculated to make an impression on the eye 

 when the light emitted was feeble, and that we ought 

 not to take the dilatations given by the index as repre- 

 senting the uniform temperature of the whole platinum, 

 which must necessarily be colder near its points of sup- 

 port, on account of the conducting power of the metals 

 to which it was attached. 



Physiological considerations might also lead to a suspi- 

 cion that the self-luminous temperature must vary as es- 

 timated by different eyes. The experiments of Bouguer, 

 hereafter to be referred to, indisputably show that some 

 persons are much more sensitive to the impressions of 

 light than others. So far as my limited investigation of 

 this matter has gone, I have not, however, found appreci- 

 able differences in the estimation of the temperature of 

 incandescence. Different individuals observing the plat- 

 inum have uniformly perceived 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 I think that the ex- 

 periments of Mr. Wedgwood on gold and earthenware 

 are decisive in this particular; and, moreover, I have 

 reason to believe that the melting-point of antimony is 

 much higher than commonly supposed. 



With a view of determining directly whether different 



