Prof. Draper on the Production of Light by Heat. 389 



to believe in the existence of cardinal distinctions, not only be- 

 tween these but also other imponderable agents; and I may 

 therefore state, that when this investigation was first undertaken, 

 it was in the expectation that it would lead to results very differ- 

 ent from those which have actually arisen. 

 The following are the points on which I propose to treat : — 



1. To determine the point of incandescence of platinum, and 

 to prove that different bodies become red-hot at the same tem- 

 perature. 



2. To determine the color of the rays emitted by self-luminous 

 bodies at different temperatures. This is done by the only relia- 

 ble method — analysis by the prism. 



From these experiments it will appear, that as the temperature 

 rises the light increases in refrangibility ; and making a due al- 

 lowance for the physiological imperfection of the eye, the true 

 order of the colors is red, orange, yellow, green, blue, indigo, 



violet. 



3. To determine the relation between the brilliancy of the light 



emitted by a shining body and its temperature. 



Here we shall find that the intensity of the light increases 

 far more rapidly than the temperature. For example, platinum 

 at 2600° emits almost forty times as much light as it does at 

 1900°. 



As I prefer to give a complete description of the apparatus em- 

 ployed in these investigations after the general results are stated, 

 it is sufficient here to understand that the source of light is in all 

 instances a very thin strip of platinum 1-35 inch long and ,' ? th 

 of an inch wide, brought to the temperature under investigation 

 by a voltaic current. Platinum was selected from its indisposi- 



temperature 



without fusion. 



The slip of platinum, thus to be brought to different tempera- 

 tures by an electric current of the proper force, was fastened at 

 one end to an inflexible support, and at the other was connected 

 with a delicate lever-index, which enabled me to determine its 

 expansion and thereby its temperature For this purpose I have 

 used the coefficient of dilatation of Dulong and Petit. The tem- 

 peratures here given are upon the hypothesis of the J«nd% 

 of that coefficient at all thermometric degrees j they are therefore 



to some extent in error. 



B^LTd of reSg wires of different >^ •***£* 

 I was able to vary the force of the electric current in the platinum, 

 and thereby visits temperature. My first attempts were to d» 

 cox r the point at winch the metal begins to emit h^it. 



The platinum and the voltaic battery were placed n a dark 



room, the temperature of which was GQO; ml after I had re- 

 mained there a sufficient length of time to enable my eyes to be- 



