346 RADIANT HEAT. 



equally affected by the heat of the mirror; when the canister dis- 

 played a cooling effect, the varnished side being the most efficacious. 



3. Again, with the same conditions, except that the canister was 

 heated 10° or 12° above the air, it was placed at different distances; 

 at near distances it showed a cooling effect; at a certain point this 

 ceased, and beyond it it began to produce a slight heating effect. 



4. Some attempts were made to try the effects while the bulb was 

 kept cool by evaporation; the canister being also cooled below the 

 air, the cooling of the bulb was increased beyond what took place 

 when the canister was at the temperature of the air. These experi- 

 ments were confessedly imperfect, from the difficulty of regulating 

 the evaporation. 



The author considers them as favorable to the theory of the radia- 

 tion of cold; he also refers to them as in some degree confirmatory of 

 Leslie's view of pulsation. 



The most remarkable result is that of case 2; it seems to prove that 

 a mirror, when heated, will still reflect rays of heat, thrown upon it 

 from a source of much lower temperature. 



The results are viewed by the author as supporting the theory of 

 the radiation of cold. I believe the doctrines of that theory may in 

 all cases be equally well expressed in other language, in conformity 

 with the view to which I referred in my former report, [p. 300.] 



Dr. Hudson has speculated with much ingenuity on another point 

 of great interest, the different radiating powers of different surfaces. 

 Understanding by the surface a certain physical thickness, ho con- 

 ceives the radiating power to depend on the capacity for heat of the sub- 

 stance of the lamina, which seems perfectly conformable to the gen- 

 eral law of the equilibrium of temperature. 



Influence of Surface and Color on Radiation: Stark and Bache. 



• 



The influence of the color of a surface on its powers for absorbing 

 and radiating heat is a question which has long attracted notice, and 

 has often been involved in no small confusion from false analogies. 

 The sun's rays, and, in general, what is called luminous heat, are 

 absorbed by surfaces (cceteris paribus) in proportion to the darkness of 

 their colors ; but it has been too hastily assumed that the same would 

 hold good with non-luminous heat, and still more groundlessly, that 

 the color would influence the radiating power of the surface ; the 

 texture of the surface, however, is known to exert a powerful influ- 

 ence. These distinctions are fully insisted on in my former report. 



Since that period, however, the subject has been taken up by Dr. 

 Stark, who, in an elaborate paper in the Phil. Trans, for 1833, details 

 a number of ingenious experiments, which he conceives support the 

 doctrine of the influence of color, not only on the absorption of dark 

 heat, but even on odors, miasma, &c. 



The object of the present report is not controversial ; I will there- 

 fore merely state that I discussed in detail Dr. Stark's reasonings, in 

 a paper published in the Edinburgh New Philosophical Journal, Oc- 

 tober, 1834, where, though allowing the value and accuracy of the 



