Prof. Draper oJi tJic prodiiclioii of Light by Heat. 357 



platinum 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, eacii 

 number being the mean of several successive trials; and the 

 fourth the mean of 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 de- 

 viations of the needle as expressing the force of the thermo- 

 electric current, or in other words, as being proportional to 

 the temperatures. This hypothesis, it is known, is admissible. 



It therefore appears that the quantity of heat radiated by 

 incandescent platinum at 980° being taken as unity, it will 

 have increased at 1440° to 25 ; at 1900° to 7 8 ; and at 2360° 

 to 17'8, nearly: the rate of increase is therefore very rapid. 

 Further, it may be remarked, as illustrative of the same fact, 

 that the increased quantity of heat radiated by a mass of pla- 

 tinum in passing from 1000° to 1300°, is nearly equal to the 

 amount it gives out in passing from common temperatures up 

 to 1000°. 



I cannot here express myself with too much emphasis on 

 the remarkable analogy between light and heat which these 

 experiments reveal. The march of the phaenomena in all 

 their leading points is the same in both cases. The rapid 

 increase of effect as the temperature rises is common to both. 



It is not to be forgotten, however, that in the case of light 

 we necessarily measure its effects by an apparatus which pos- 

 sesses special peculiarities. The eye is insensible to rays 

 which are not comprehended within certain limits of refrangi- 

 bility. In these experiments, it is requisite to raise the tem- 

 perature of the platinum almost to 1000° before we can dis- 

 cover the first traces of light. Measures obtained under such 

 circumstances are dependent on the physiological action of 

 the visual organ itself, and hence their analogy with those ob- 

 tained by the thermometer becomes more striking, because 

 we should scarcely have anticipated that it could be so com- 

 plete. 



Description of the apparatus employed in the foregoing 

 experiments. 



The source of light is in all instances a slip of platinum 

 foil 1"35 inch long, and ..'j^th of an inch broad, ignited by the 

 passage of a voltaic current, and j)laced in such a position 

 that its dilatation could be measured by the movements of an 

 index over a graduated scale. 



