1823.] Mr. Pomll on Solar Light and Heat. 203 



Dr. Seebeck, of Beikn. These very important researches tend 

 to estabhsh the conclusion that the position of the maximum 

 point of heat varies in the spectrum according to the nature of 

 the dispersing medium. With some prisms it is situated in the 

 yellow or orange, in those of crown glass in the centre of the red, 

 and of flint glass beyond the red. 



These experiments well explain the discrepancies between 

 different observers, though other causes before adverted to may 

 have had some share in producing those differences. 



In viewing these results in reference to the nature of theheat- 

 ino effects accompanying the rays of light, it becomes desirable 

 to inquire whether such changes in the heating power at differ- 

 ent parts of the spectrum are accompanied by corresponding 

 variations in the intensity of light : whether the greater heat be 

 owing to a greater number of calorific and illuminating rays 

 thrown into the same space, owing to the different law of dis- 

 persion followed by the different refracting media. It is very 

 doubtful however whether there are any means of ascertaining 

 this with certainty and accuracy by means of the illuminating 

 powers, so as to arrive at any such conclusion. But if it were 

 shown that the bght is dispersed in different proportions to the 

 same part of the spectrum by different prisms, and that such 

 difference corresponded to the difference of heating power, Dr. 

 , Seebeck's results would in this case present no objection to the 

 ' idea of the heating effect being inherent in the light, or resulting 

 merely from light so modified as to become caloric. 



The elaborate experiments of M. Frauenhofer on the refractive 

 and dispersive powers of different substances (Edin. Phil. Journ. 

 No. 18, Art. 16), exhibit instances of a considerable alteration 

 in the relative dispersion of the rays by different media. This 

 was ascertained with great precision by means of the well- 

 defined bright and dark lines which he observed crossing the 

 spectrum, "it would be extremely desirable to ascertain what 

 effect these lines have on the heating powers of the different 

 rays. 



'if this view of the subject were not established, it might seem 

 a natural inference that these results favour the idea of the heat 

 beiug due to a separate set of rays ; for if the heating power in 

 the different parts of the spectrum can be made to vary, and the 

 maximum can be thrown at pleasure into different coloured rays, 

 it might be argued that the effect must depend upon some inde- 

 pendent agent or set of rays distinct from the luminous rays. 



Such a conclusion however is, perhaps, more than the facts 

 will safely warrant. Those who have rejected the idea of sepa- 

 rate rays of heat have usually gone to tlie opposite extreme, and 

 supposed the heat to be identical with the light ; and that the 

 heating effect is merely the display of the same agent, light, in 

 another form. But is this the necessary alternative i Is there 



