Prof. -Draper on the production of Light by Heat. 353 



lours of light, occur also for heat; a fact conveniently desig- 

 nated by the phrase "ideal coloration of heat:" and further, 

 that the colour of the heat emitted depends upon the tem- 

 perature of the radiating source. It is one thing to inves- 

 tigate the phgenomena of the exchanges of heat-rays of the 

 same colour, and another when the colours are different. A 

 perfect theory of the exchanges of heat must include the prin- 

 ciple ot ideal coloration, and, of course, so too must a law of 

 cooling applicable to any temperature. 



There is another fact to some extent considered by Dulong 

 and Petit, but not of such weight in their investigations, where 

 the range of temperature was small, as in ours, where it rises 

 as high as nearly 3000= F. ; 1 mean the difference of specific 

 heat of the same body at different temperatures. At the high 

 temperatures considered in this memoir, there cannot be a 

 doubt that the capacity of platinum for heat is far greater than 

 that at a low point. This therefore must control its rate of 

 calorific emission, and probably that for light also. 



From these and similar considerations, we should be pre- 

 pared to discover that as the temperature of an incandescent 

 .solid rises, the intensity of the light emitted increases very 

 rapidly. 



I pass now to the experimental proofs which substantiate 

 the foregoing; reasoning. 



The apparatus employed as the source of the light and 

 measure of the temperature was the same as in the preceding 

 experiments, — a strip of platinum, brought to a known tem- 

 perature by the passage of a voltaic current of the proper force, 

 and connected with an index which measured its expansion. 



The principle upon which I have determined the intensities 

 of the light is that first described by Bouguer, and recently 

 introduced by M. Masson. After many experiments I have 

 been led to conclude that this is the most accurate method 

 known. 



Any one who will endeavour to determine the intensities of 

 lights by Rumford's method of contrasting shadows, or by 

 that of equally illuminated surfaces, will find, when every 

 precaution has been used, that the results of repeated experi- 

 ments do not accord. There is moreover the great detect, 

 that where the lights differ in colour it is impossible to obtain 

 reliable measures, except by resorting to such contrivances as 

 that described by me*. 



Bouguer's principle is far more exact; and where the lights 

 differ in colour, that difference actually tends to make the 



• Phil. Mag., August 1844. 

 Phil. Mag. S. 3. Vol. 50. No. 202. May 1847. 2 B 



