TUE 

 LONDON, EDINBURGH and DUBLIN 



PHILOSOPHICAL MAGAZINE 



AND 



JOURNAL OF SCIENCE. 



[FOURTH SERIES.] 



APRIL 1859. 



XXXVI. Investigations on the Thermal Effects of the Solar 

 Spectrum. By Dr. J. Mulleb*. 



Introduction. 



AFTER the extension of the solar spectrum beyond the violet 

 had become more fully recognized by means of fluoi-escence 

 and photography, additional interest was attached to the exten- 

 sion of the spectrum in the opposite direction, that is, to the 

 part which stretches beyond the red of the visible spectrum, and 

 which can only be recognized by its thermal eflfects. 



R. Franz (see Poggendorff's Annalen, vol. ci. p. 59) has lately 

 published a summary of all the investigations which have been 

 hitherto performed upon the thermal rays in the solar spectrum. 

 It is therefore quite unnecessary for me to refer to the literature 

 of the subject. 



The researches of Melloni formed a new epoch in the doctrine 

 of radiant heat. Numerous as were the discoveries which resulted 

 from the application of the thermo-battery, and the introduction 

 of the multiplier as thermoscope, nevertheless our acquaintance 

 with the thermal relations of the solar spectrum was not greatly 

 advanced, although Melloni had, in rock-salt, discovered a body 

 which admitted the passage of all heat-rays alike. 



Melloni's experiments on the warmth of the solar spectrum 

 were almost exclusively confined to the determination of the 

 position of the thermal maximum. In experimenting with a 

 prism of rock-salt, he found that the maximum heating eflfect 

 occurred at a point as far beyond the red boundary of the spec- 

 trum as the latter is distant from the point of transition of the 



* Translated by Dr. F. Guthrie, from Poggendorff's Annalen, vol. cv. 

 p. .'W?. 



Phil. Mag. S. 4. Vol. 17. No. 114. Apnl 1859. R 



