42 THE RADIATIONS OF IGNITED BODIES. [MEMOIR I. 



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 the object thus proposed was mainly to illustrate 

 the remarkable analogy between light and heat, the ex- 

 periments now to be related were arranged so as to re- 

 semble 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 deter- 

 mined their value by photometric methods, so in this 

 case I received the rays of heat upon a screen placed at 

 an invariable distance, and measured their intensity by 

 therrnometric methods. In this instance the screen em- 

 ployed was, in fact, the blackened surface of a thermo- 

 electric pile. It was arranged at a distance of about one 

 inch from the strip of ignited platinum, a distance suffi- 

 cient to keep it from any disturbance from the stream 

 of hot air arising from the metal ; care was also taken 

 that the multiplier itself was placed so far from the rest 

 of the apparatus that its astatic needles could not be af- 

 fected by the voltaic current igniting the platinum, or 

 the electro-magnetic action of the wires or rheostat used 

 to modify the degrees of heat. 



In Fig. 6, a 1) is the ignited platinum strip, c the ther- 

 mo-electric pile, d d the multiplier. 



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 correspond- 

 ing temperature, and radiated its heat to the face of the 

 pile. The instant this current passed, the needles of the 

 multiplier moved, and kept steadily advancing on the 

 scale. At the close of one minute the deviation of the 

 needle and the temperature of the platinum were si- 



