RADIANT HEAT. 323 



is not, in 10 minutes, by the direct rays of the sun; that is, as before, 

 the outer coat of the skin allows some of the direct rays to pass through 

 and affect the sentient substance beneath; whereas, in the case of the 

 black, the rays are absorbed and converted into heat of temperature, 

 which diffuses itself equally, and does not produce the effect of 

 scorching. 



5.) The most singular facts connected with the absorption of the 

 sun's rays, are those exhibited by the substances called "phosphori" 

 or "pyrophori." — (Thomson's Chem»i, 17.) 



The general fact is, that after exposure to the sun, on being 

 removed into the dark they give out light, but it is after a time 

 exhausted; it is given out more copiously and exhausted sooner if 

 heat be applied. Many solar phosphori will always emit light of one 

 color only, to whatever colored ray they may have been exposed. 

 In a short notice given by Dr. Young, in his valuable Catalogue of 

 Authors, it appears that M. Grosser found that such phosphori as 

 emitted red light only were made to shine most by exposure to blue 

 light.— (Hozier, xx, 270.) 



Beccari, in a memoir " de Phosphoris, " extracted in the Phil. 

 Trans., 174G, p. 81, gives as one of his results, that the light emitted 

 was brightest when the surface of the mass was of a rough texture; 

 those which were smooth and polished retained little or none, but 

 (supposing the color the same) a rougher surface would evidently 

 absorb more light than a smooth one, and therefore might emit more. 



Mr. T. Wedgewood compared two pieces of phosphorescent marble, 

 one naked, the other painted black; on applying uniform heat the 

 coated marble gave out no light, though the other did. — (Phil. Trans., 

 1792.) 



But the coating increased the radiating power, and it therefore 

 probably did not retain heat enough to cause the extrication of light. 



Mr. Morgan, {Phil. Trans., 1785,) after examining many of the 

 phenomena of phosphorescence, generalizes his views by maintaining 

 that all phosphori emit light, proceeding in order from violet to red, 

 in proportion as the process is effected by the application of an in- 

 creasing degree of heat. 



This is a very curious subject, as connected with the whole theory 

 of the relations of light and heat. Some valuable information might 

 probably be obtained as to the degree of heat necessary, and whether 

 there is any loss of heat when light is evolved, compared with cases 

 when no light is evolved; as there should be, on the hypothesis of 

 conversion of heat into light, or on that of heat becoming latent in 

 the light. 



In Mr. Wedgewood's paper, above cited, is an account of the prin- 

 cipal researches on the subject. 



e.) Effect of screens. 



1 .) That no diminution of the effect of the sun's rays on a blackened 

 thermometer is occasioned by a transparent screen was remarked by 

 several experimenters, particularly De la Roche. — (Biot, iv, 611.) 



