324 Mr. Powell on Solar Light and Heat. [May, 



that all heating emanations from terrestrial bodies whether lumi- 

 nous or not, are more or less stopped, or even in some cases 

 totally intercepted, by the interposition of a glass screen. Similar 

 experiments may easily be tried on the solar rays. 



That little or no diminution of effect is produced on a black- 

 ened thermometer exposed to the sun, by the interposition of 

 glass, has been shown by several experiments. As it is 

 remarked by De la Roche (Biot, Traite de Phys. vol. iv. p. 61 1), 

 I have also frequently observed the same thing, taking notice of 

 the temperature of the glass, as will be subsequently seen. But 

 there is another part of the question which still appears to me to 

 want further examination. The sun's rays produce some heating 

 effect on surfaces of a light colour. I have, therefore, tried 

 whether also in this case, and when the texture of the surface 

 was very absorptive for simple radiant heat, a glass screen has 

 any power to diminish the effect. Two thermometers were 

 exposed together to the direct and screened rays, one having its 

 bulb coated with indian ink ; the other with a thin paste of chalk 

 and water; the bulbs were free from contact. If there existed 

 in the solar beam any rays of such a nature that they were 

 affected by the texture rather than the colour of surfaces, and 

 were not capable of passing through glass, they would be affected 

 by a surface of chalk more than one of indian ink. If they 

 formed only a small proportion of the whole, the diminution, 

 when glass was interposed before the inked thermometer, might 

 be so small as to be imperceptible ; but with the whitened sur- 

 face, it would be much more conspicuous. 



(8.) The following are the results of two sets of experiments 

 conducted on this principle : 



