Prof. Draper on the production of Light by Heat. 355 



The intensity of the light of the platinum is of course in- 

 versely proportional to the square of the distance of the Ar- 

 gand lamp at the moment of the obliteration of the shadow. 



In this table the first column gives the temperatures under 

 examination in Fahrenheit degrees; the second and third the 

 distances of the Argand lamp from the screen, in English in- 

 ches, in two different sets of experiments ; the fourth the mean 

 of the two; and the fifth the corresponding intensity of the 

 light. 



From this it is at once perceived, that the increases in the 

 intensity of the light, though slow at first, become very rapid 

 as the temperature rises. At 2590^ the brilliancy is more than 

 thirty-six times as great as it is at 1 900°. 



Thus, therefore, the theoretical anticipation which we 

 founded on the analogy of light and heat is completely veri- 

 fied ; and we discover that as the temperature of a self-lumi- 

 nous 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 experiments I have made in relation to 

 radiant heat. No chemist, so far as I am aware, has hitherto 

 published results for high temperatures, or endeavoured to 

 establish, through an extensive scale, the principle of Dela- 

 roche, 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 progres- 

 sion 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 experi- 

 ments 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 upon a screen placed at an invariable 

 distance, and then determined their value by j)hotometric 

 methods; so, in this case, I received the rays of heat upon a 

 screen placed at an invariable distance, and determined their 

 intensity by therniometric methods. In this instance the screen 

 employed was in fact the blackened surface of the thermo- 

 electric pile. It was placed at a distance of about one inch 

 from the slip of incandescent platinum, a distance sufficient to 

 keep it from any disturbance from the stream of hot air arising 

 from the metal ; care also was taken that the multiplier itself 

 was placed so far from the rest of the apparatus, that its 

 astatic needles could not be affected by the voltaic current 

 2 B 2 



