1824.] Mr. Powell on Solar Light and Heat. 323 



It has been equally well established by Prof. Leslie, Count 

 Rumford, &c. that the heat emanating from a mass of non- 

 luminous hot matter, has no such relation to the colour, though 

 a very close one to the nature and texture of the surface. 



(4.) The experiments of Sir E. Home (Phil. Trans. 1821, 

 Part I.) are particularly deserving of attention, as exhibiting 

 what might at first sight be considered an exception to the 

 above remarks; a greater effect being produced in some instances 

 on a white than on a black surface. A more attentive examina- 

 tion, however, will show us that these experiments prove thus 

 much. The heat occasioned by the rays of the sun when 

 received directly, or when in some degree intercepted as by thin 

 white cloth, on the skin, is greater than that communicated by 

 conduction to the same skin, through a black cloth in contact 

 with it, which is itself, in the first instance, heated by absorbing 

 the rays. 



A white skin is scorched, and a negro's skin is not, in ten 

 minutes by the direct rays of the sun ; that is, as before, the 

 outer coat of the white skin allows some of the direct rays to 

 pass through and affect the sentient substance beneath ; 

 whereas in the case of the black skin, the rays are absorbed by 

 the black surface, and so affect the sentient parts only as heat 

 of temperature. 



II. (5.) As to the nature of this heating effect, the greatest 

 difference of opinion has long prevailed among the most distin- 

 guished philosophers ; one party maintaining the totally distinct 

 existence of light and radiant heat in the compound solar beam; 

 the other contending for the absolute identity of the two : the 

 same principle being merely displayed under two different modi- 

 fications. (See Sir W. Herschel, Phil. Trans. 1800, Part II. ; 

 Leslie on Heat, p. 162, Biot, Traite de Physique, vol. iv. p. 

 690, &c.) 



Without entering upon an examination of the merits of either 

 theory, we may proceed to remark that the first object in the 

 inductive examination of this subject is to ascertain distinctly 

 what peculiar properties of this heating emanation we can fix 

 upon by which its nature may be defined, and by the help of 

 which we may be enabled to compare it with other heating 

 emanations. 



(6.) Among the most obvious properties of the solar rays, we 

 perceive that before adverted to, viz. that they produce heat on 

 bodies in proportion to the darkness of their colour, and not in 

 regard to the absorptive qualities of the texture of their surfaces 

 for the heat from non-luminous bodies. This relation is univer- 

 sal, and without any exceptions: it is consequently one which 

 we can satisfactorily adopt as the foundation of a distinctive 

 description. 



(7.) We may from this advance to another test, which will 

 afford an additional characteristic. It has been distinctly shown 



