Prof. Draper on the Production of Light by Heat. 



399 



The needles having regained their 



in most eases was much less. A little consideration 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. 



zero, the platinum was 



brought again to a given temperature, and the experiment con- 

 ducted as before. The following table exhibits a series of these 

 results. 



Table of the Intensity of Radiant Heat emitted hj Platinum at differ- 



ent Temperatures, 



D 



Temperature of the ! 

 platinum. 



980° 



i Inten; 



1 Kxperimeut 



•75 



1095 



100 



1210 



M0 



1325 



1-60 



1440 



2-20 



1555 



2 75 



1670 



3-65 



1785 



5-00 



1900 



6-70 



2015 



8-60 



2130 



1000 



2245 



]2-50 



2360 



15-50 



Intensity of heat emitted. 



Experiment 2. 



Mean. 



100 



1*20 



1-60 



200 



2-20 



2-85 



3-75 



5-00 



690 



8-60 



1000 



12-50 



15-50 



•b7 



M0 



1-50 



1-80 



2-20 



2 80 



3-70 



500 



680 



8-60 



1000 



12-50 



15-50 



In this table the first column gives the temperatures of the pla- 

 tinum in Fahrenheit degrees ; the second and third two sets of 

 experiments, expressing the arc passed over by the needle at the 

 close of a radiation lasting for one minute, each number being 

 the mean of several successive trials ; and the fourth the mean ot 

 the two. It therefore gives the radiant effect of the incandescent 

 platinum upon the thermo-multiplier for the different temperatures. 



Of course it is understood that I here take the angular devia- 

 tions of the needle as expressing the force of the thermo-electric 

 current, or in other words, as being proportional to the tempera- 

 tures. This hypothesis, it is known, is admissible. 



It therefore appears that the quantity of heat radiated by in- 

 candescent platinum at 980° being taken as unity, ^ihha.e in- 

 creased at 1440° to 25 ■ at 1900° to 7-8 ; and at 2360° to 17 8 

 -. i . „ . 7 • ^i c^r, xtov\t mniri. Mirtner, it 



very 



may be remarked, as illustrative of the same fact, that the increa- 

 sed quantity of heat radiated by a mass of platinum n Passing 

 from 1000° to 1300°, is nearly equal to the amount it gives out 

 «i passing from common temperatures up to 100U . 



I cannot here express myself with too much em phasis on the 

 remarkable analogy between light and heat which these experi- 

 ments reveal. The march of the phenomena in all their leading 

 Points is the same in both cases. The rapid increase of effect as 



temperature 



