356 Prof. Draper on the production of Light by Heat. 



igniting the platinum, or the electro-magnetic action of the 

 wires used to modify the degrees of heat. 



The experiments were conducted as follows : — The needles 

 of the thermo-multiplier standing at the zero of their scale, 

 the voltaic current was passed through the platinum, which 

 immediately rose to the corresponding temperature, and ra- 

 diated its heat to the face of the pile. The instant the current 

 passed, the needles of the multiplier moved, and kept steadily 

 advancing upon the scale. At the close of one minute, the 

 deviation of the needle and the temperature of the platinum 

 were simultaneously noted, and then the voltaic current was 

 stopped. 



Sufficient time was now given for the needle of the multi- 

 plier to come back to zero. This time varied in the different 

 cases, according to the intensity of the heat to which the pile 

 had been exposed : in no instance, however, did it exceed 

 six minutes, and in most cases was much less. A little con- 

 sideration will show that the usual artifice employed to drive 

 the needles back to zero, by warming the opposite face of the 

 pile, was not admissible in these experiments. 



The needles having regained their zero, the platinum was 

 brought again to a given temperature, and the experiment 

 conducted as before. The following table exhibits a series of 

 these results. 



Table of the Intensity of Radiant Heat emitted by Platinum 

 at different Temperatures. 



In this table the first column gives the temperatures of the 



