Prof. Tyndall on Calorescence. 



387 



rendered graphically in fig. 2. Here the area ABCD represents 

 the invisible, while CDE represents the visible radiation. 



Fig. 2. 



A T> E 



Spectrum of Sun (Muller). 



With regard to terrestrial sources of heat, it may be stated 

 that all such sources hitherto examined emit those obscure rays. 

 Melloni found that 90 per cent, of the emission from an oil- 

 flame, 98 per cent, of the emission from incandescent platinum, 

 and 99 per cent, of the emission from an alcohol-flame consists 

 of obscure rays*. The visible radiation from a hydrogen-flame 

 is, according to my own experiments, too small to admit of mea- 

 surement. With regard to solid bodies, it may be stated gene- 

 rally that, when they are raised from a state of obscurity to vivid 

 incandescence, the invisible rays emitted in the first instance 

 continue to be emitted with augmented power when the body 

 glows. For example, with a current of feeble power the carbons 

 of the electric lamp may be warmed and caused to emit invi- 

 sible rays. But the intensity of these same rays may be aug- 

 mented a thousandfold by raising the carbons to the tempera- 

 ture necessary for the electric light. Here, in fact, the lumi- 

 nous and non-luminous emission augment together, the maxi- 

 mum of brightness of the visible rays occurring simultaneously 

 with the maximum calorific power of the invisible ones"j\ 



At frequent intervals during the past ten or twelve years I 

 have had occasion to experiment on the invisible rays of the 

 electric light, and have finally made them the subject of special 

 investigation. The present paper contains a brief account of 

 the inquiry. I endeavour, in the first place, to compare the 

 luminous with the non-luminous radiation of the electric light, 

 and to determine their relative energy ; I point out a method 

 of detaching the luminous from the non-luminous rays; and 

 afterwards describe various experiments illustrative of the calo- 

 rific power of the invisible rays, and of the transmutations of 

 which they are capable. 



§ 2. The instrument employed by Professor Muller in the in- 

 vestigation above alluded to, was a form of the thermo-electric 



* La Thermochrose, p. 304. 



t On this point see the Rede Lecture for 1865, p. 33 (Longmans). 



