REPORT ON RADIANT HEAT. 
289 
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 colour only, to whatever coloured 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. ( Rosier , xx. 270.) 
Beccari, in a memoir “ de Phosphoris” extracted in the Phil . 
Trans. 1746, 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 colour 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 uni¬ 
form heat, the coated marble gave out no light, though the 
other did. {Phil. Trans. 1792.) 
But the coating increased the radiating power, and it there¬ 
fore probably did not retain heat enough to cause the extrica¬ 
tion of light. 
Mr. Morgan {Phil. Trans. 1785,) after examining many of the 
phaenomena of phosphorescence, generalizes his views by main¬ 
taining that all phosphori emit light proceeding in order from 
violet to red, in proportion as the process is effected by the 
application of an increasing degree of heat. 
This is a very curious subject, as connected with the whole 
theory of the relations of light and heat. Some valuable infor¬ 
mation 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 
principal 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.) 
2. ) I tried the point by two thermometers, (as in the case of 
terrestrial heat,) and found no 'perceptible differencevo the ratio, 
T 
