On the Distribution of Heat in the Visible Spectrum. 203 



of the atomic theory. A modified form of Berthollet's asser- 

 tion that the affinity of an acid is greatest for that base with 

 which it combines in smallest quantity, seems to be suggested 

 by Thomsen's results (confirmed by Ostwald's volumetric 

 method) that large affinity-value is accompanied by low heat 

 of neutralization. 



Ostwald furnishes chemistry with a new method for solving 

 some of her most difficult problems ; and Guldberg and Waage 

 lead the way in the application of mathematical reasoning to 

 the facts of chemical science. 



XXIII. The Distribution of Heat in the Visible Spectrum. 

 By Sir John Conroy, Bart., M.A.* 



IN a paper " On the Distribution of Heat in the Spectrum/' 

 originally published in the Philosophical Magazine for 

 August 1872, and since reprinted in a volume of ' Scientific 

 Memoirs/ Dr. J. W. Draper states the theoretical reasons for 

 supposing that all the rays in the spectrum have the same heat- 

 ing effect, in the following words : — " A given series of waves 

 of red light impinging upon an extinguishing surface will pro- 

 duce a definite amount of heat ; and a similar series of violet 

 waves should produce the same amount; for though an undu- 

 lation of the latter may have only half the length of one of the 

 former, and therefore only half its vis viva, yet in consequence 

 of the equal velocity of waves of every colour, the impacts or 

 impulses of the violet series will be twice as frequent as those 

 of the red. The same principle applies to any intermediate 

 colour ; and hence it follows that every colour ought to have 

 the same heating effect." 



Dr. Draper gives an account of some experiments he has 

 made on the distribution of heat in the visible spectrum of 

 sunlight. He finds that if the visible spectrum between. A 

 and H 2 be divided into two equal portions, and all the light of 

 wave-lengths between 7604 and 5768 be collected together, 

 and also all that of wave-lengths between 5768 and 3933, the 

 heat-intensity of these two series of undulations as determined 

 by the thermopile are equal. 



The distribution of heat in the spectrum of sunlight had 

 been previously experimentally investigated by Sir W. Her- 

 schel (Phil. Trans. 1800, p. 255), J. Mtiller (Pogg. Ann. cv. 



* Communicated by the Physical Society, having been read at the 

 Meeting held on June 28. 



