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XIL Researches in Achno - Chemistry , — Memoir First. On the 

 Vistrihution of Heat in the Spectrum. By John William 

 Draper, M.D.j LL.D., President of the Faculties of Science 

 and of Medicine in the University of New York^. 

 [With a Plate.] 



MANY experimenters at various times have occupied them- 

 selves with the problem of the Distribution of Heat in 

 the spectrum. At first it was supposed that there was a coinci- 

 dence between the luminous and the calorific radiations,, and that 

 the maximum of intensitjdn both occurred at the same point — that 

 is, in the yellow space. This view was abandoned on the publi- 

 cation of the well-known experiments of Sir W. Herschel, who 

 showed that in certain cases the maximum is below the red. 

 Subsequently, Melloni having discovered the singular heat- 

 transparency of rock-salt, proved that when a prism of that 

 substance is used the maximum in question is as far below the 

 red as the red is below the yellow — but that if the hght has 

 passed through flint-glass the maximum approaches the red — if 

 through crown glass, it passes into the red — if through water or 

 alcohol, it enters the yellow. 



In the case of the sun^s spectrum the distribution of heat was 

 more closely examined by Professor Miiller, whose results con- 

 firmed in a general manner the view then held, that the invi- 

 sible radiation below the red greatly exceeds that in the visible 

 spectrum; and still more recently Dr. Tyndall, examining the 

 spectrum of the electric light through rock-salt, showed that the 

 curve indicating the distribution ^^in the region of the dark rays 

 beneath the red shoots suddenly upwards in a steep and massive 

 peak — a kind of Matterhorn of heat, which quite dwarfs by its 

 magnitude the portion of the diagram representing the visible 

 radiation.-'^ These investigations were made under unexception- 

 able circumstances: the beam of electric light had practically 

 undergone no atmospheric absorption; and the optical refracting 

 train was of rock-salt. 



Sir J. Herschel had shown in 1840 that, when the sun^s rays 

 are dispersed by a flint-glass prism, the distribution of the heat 

 toward the less-refrangible regions is not continuous, but there 

 are three maximum points. These points, as shown by Dr. 

 Tyndall, do not exist in the spectrum of electric light, the de- 

 cline of which is perfectly continuous ; they are therefore to be 

 attributed to the absorptive disturbance which the suu^s rays 

 have undergone. Quite recently M. Lamansky has succeeded 

 in identifying these interruptions by the aid of the thermo- 

 multiplier. In his memoir he states that, with the exception of 

 * Communicated bv the Author. 



