RADIANT HEAT. 3G9 



one position of refraction, which will have a bounding or limiting 

 position, beyond which no ray of however great wave-length can be- 

 refracted. This will be different for each medium, but will in 

 general correspond to a refractive index not greatly below the index 

 for the extreme red ray, and which is calculated, in my treatise just 

 referred to, for various media. For rock salt it is a little lower than 

 Professor Forbes' s index for dark heat. 



The data will be best seen as collected in the following table.* 



Rock salt. 



Value of p. 



31. Knoblauch's Researches. 



Among the most important of recent researches on the subject of 

 radiant heat, are those of M. Knoblauch, Professor of Natural Philoso- 

 phy in the University of Marburg, which are not to be surpassed for 

 elaborate extent and accuracy of detail. They are given in Poggen- 

 dorff's "Annalen," January and March, 1847, and translated in 

 Taylor's "Foreign Scientific Memoirs," Parts XVIII and XIX, 1848. 



The memoir is of great extent, and is divided into six sections. 



Section I is entitled "On the Passage of Radiant Heat through 

 Diathermanous Bodies, with especial regard to the Temperature of 

 the Source of Heat." 



The author commences with a summary of the results previously 

 obtained, in which he cites the results of Delaroche and others, with- 

 out reference to the different interpretation which must be put upon 

 them if the experiments and conclusions just referred to be admitted. 



He observes that, from the experiments of Melloni, rock salt appears 

 equally permeable by heating rays of all hinds; from those of Forbes, 

 prepared rock salt would seem penetrated by heating rays in a greater 

 degree when the source was at a lower temperature. Here I would 

 observe that the temperature of the source, as such, manifestly bears 

 no direct proportion to the degree of luminosity, it being perfectly 

 well known that the temperature of luminosity is very different for 

 different bodies; and these are also of very different illuminating 

 powers. Now, as in all cases there are several different species of 



« On this point some notices were submitted to Section A, in 1840 and 1841. See Report, 

 1840, Sect. Proa, p. 14, and 1841, p. 25. 



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