398 Prof. Draper on the Production of Light by Heat 



the temperature rises. At 2590° the brilliancy is more than thir- 

 ty-sir times as great as it is at 1900°. 



Thus, therefore, the theoretical anticipation which we founded 

 on the analogy of light and heat is completely verified ; and we 

 discover that as the temperature of a self-luminous solid rises, it 

 emits light in a greater proportion than would correspond to the 

 mere difference of temperature. To place that analogy in a still 



more striking point of view, I will here introduce some experi- 



ments I have made in relation to radiant heat. No chemist, so 

 far as I am aware, has hitherto published results for high temper- 

 atures, or endeavored to establish, through an extensive scale, the 

 principle of Delaroche, that " the quantity of heat which a hot 

 body gives off in a given time by way of radiation to a cold 

 body, situated at a distance, increases, other things being equal, 

 in a progression more rapid than the excess of the temperature 

 of the first above that of the second." 



As my object on the present occasion is chiefly to illustrate the 

 remarkable analogy between light and heat, the experiments now 

 to be related were arranged so as to resemble the foregoing ; that 

 is to say, as in determining the intensities of light emitted by a 

 shining body at different temperatures, I had received the rays up- 

 on a screen placed at an invariable distance, and then determined 

 their value by photometric methods ; so, in this case, I received 

 the rays of heat upon a screen placed at an invariable distance, 

 and determined their intensity by thermometric methods. In 

 this instance the screen employed was in fact the blackened sur- 

 face of the thermo-electric pile. It was placed at a distance o 

 about one inch from the slip of incandescent platinum, a distance 



sufficient to k 



of hot 



keep it from any disturbance from the stream oi uu* 

 air arising from the metal ; care also was taken that the multip ie 

 itself was placed so far from the rest of the apparatus, that lis 

 astatic needles could not be affected by the voltaic current ign - 

 ting the platinum, or the electro-magnetic action of the wir 

 osed to modify the degrees of heat. « f 



The experiments were conducted as follows :— The need e 

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

 taic current was passed through the platinum, which ^ mec rJ th ' e 

 rose to the corresponding temperature, and radiated its ^^ ]p 

 face of the pile, 



the multiplier mo\cu, »m *cpi svcauny «w*«*"w- D - x , 



At the close of one minute, the deviation of the needle an ^ 

 temperature of the platinum were simultaneously noted, an 



the voltaic current was stopped. Itiplie r 



Sufficient time was now given for the needle of the mu ^r^ 

 to come back to zero. This time varied in the differ^ i ^ 

 according to the intensity of the heat to which the pile n ^ 

 exposed : in no instance, however, did it exceed six minu ; 



oncung temperature, ana raanueu i» «~~ ^ 

 The instant the current passed, the needie 

 red, and kept steadily advancing upon "* - the 



