1825.] Mr. Powell OH Solar Light and Heat. 201 



Article VI. 

 Remarks on Sular Light and Heat. By B. Powell, MA. FRS. 



(Continued from vol. viii. p. 293. 



(57.) In a former part of these remarks (16), I adverted to the 

 experiments on the heating power accompanying or belonging 

 to the different prismatic rays, this being one of the principal 

 modifications to which the solar light has been subjected, and 

 from which conclusions respecting the nature of its heatingpower 

 have been deduced. On this part of the subject I propose now 

 to make a few further observations with a view to ascertaining 

 how far such conclusions may be substantiated, and will assist 

 in forming a correct idea of the nature of the heating effects. 



(58.) It is well known that the heating power belongs to the 

 differently coloured rays m very different proportions. Among 

 the results of different experimenters, there exists considerable 

 discrepancy. The first person to whom we owe the idea of such 

 investigations was the Abbe Rochon. — (See Phil. Mag. June, 

 1815, and Biot, Traitc de Physique, tome iv. p. 600.) He 

 found the maximum in the yellow orange rays. There is a much 

 closer agreement however between subsequent observers, if we 

 except the disagreement respecting the effect beyond the red 

 rays. 



(59.) The causes of these differences are to be in some mea- 

 sure sought in the different nature of the surfaces of the ther- 

 mometers employed, or in the colour of the substances with 

 which they were filled : as well as in the varying circumstances 

 of the prism, &c. 



Two coatings equally described as black may be very different 

 in the shade of colour which they exhibit. If the tint incline to 

 red for example, a less effect will be produced by the red rays. 



(60.) The Abbe Uochon's result agrees with one which I have 

 constantly obtained when the bulb was painted red. From the 

 account of his experiments, Phil. Mag. June, l''-il5, it does not 

 clearly appear what the nature of his thermometer was; but if, 

 as I understand, it was filled with spirits, and they, as we may 

 presume, were tinged red, his result is fully accounted for. My 

 experiments were as follows : 



Judical inits of Differential Thermometer. 



Sept. 9. — 9 a. m. Bulb coated with lake and vermilion. 



Away 5° 



In the orange yellow 28 



In the red 25 



Half inch beyond 12 



