86 Mr. Poicell on Solar Light and Heat', [Aug. 



portion of heating to illuminating effect in respect to light of 

 different intensities. Another point of inquiry which appeared 

 to me not uninteresting in relation to the same subject, is the 

 similar question with respect to the proportions of heating effect 

 developed by differences of light in respect of the light or dark 

 colour of surfaces : and whether the same proportion which is 

 observed in the heat produced on a black and on a white sur- 

 face at ordinary intensities is preserved or not at higher degrees 

 of concentration in the rays. 



The heating effect of light is commonly said to be produced 

 by the absorption of the rays, and is supposed to be proportional 

 to the degree of that absorption. In order to advance towards 

 a clear and systematic knowledge of the subject, it would be 

 necessary that this should be proved, especially as we may thus 

 become better acquainted with the nature of the heating effect 

 developed or excited when light impinges on surfaces of different 

 colour. 



We have not, perhaps, any very precise idea as to the mode 

 by which light exerts its heating power ; nor can it be assumed 

 that any exact proportion is followed by the absorbing power of 

 surfaces, with the degree of heat produced. It is obvious that 

 a variety of laws may be supposed to obtain. 



The heating effect may not be in proportion to the quantity of 

 light absorbed, or the quantity absorbed may not be in the pro- 

 portion of that impinging, or both may take place jointly. 



It, therefore, becomes necessary to inquire, first, whether on 

 the black and the white surface the heating effects are in the 

 same ratio as that of the intensities of light acting upon them; 

 secondly, whether, in the case of the diffuse and of the concen- 

 trated rays, the black and white surfaces receive heating effects 

 in the same ratio as that of the light which acts upon them in 

 respect to their colour. 



(34.) In order to follow up these inquiries, the following expe- 

 riments were tried. 



I employed two thermometers, one having its bulb coated 

 with Indian ink, and the other with a thin paste of chalk and 

 water. They were both fixed on one mounting, so that it might 

 be safely assumed, that they were both equally exposed to the 

 heating power to which they were subjected ; and the bulbs 

 were completely free from any contact with the mounting, more 

 than one-eighth of an inch intervening on all sides. 



By measurement, the following data were obtained : 



Diameters of the bulbs d— -55 in. d, = -45 



Whence the surfaces s=*950 s l = *636 



Diameter ofthe focal disk= -25, 



whence its area a = -049. ., --049 --049 



•901=«-a -587-s^a 

 ■ And a = tt, 



