SECOND REPORT— 1832 . 
276 
c. ) Effect of surfaces on emission of heat. 
Nothing ascertained under this head, unless we except some 
remarks in the Edinh. Journ. of Science , No. ii. p. 302. 
d. ) Effect of surfaces on absorption of heat. 
All experimenters have usually blackened their thermome¬ 
ter. (Cavallo, Phil. Trans. 1780.) 
Prof. Robison exposed a thermometer on charred oah under 
a glass cover to the rays of a fire, when it rose to 212° Fahr. 
(Black’s Lect. i. 547; Thomson, i. 127.) 
e. ) Effect of screens. 
1. ) Mariotte interposed a glass screen between the fire and 
concave mirror, and found the heat no longer sensible at the 
focus. (Biot, iv. 606; Mem. Paris> i. 344.) 
Scheele interposed a glass screen in the experiment before 
mentioned, and found the heat of a fire so much intercepted 
as to be no longer sensible to the hand : not even sensible in the 
focus of a reflector. 
Pictet with the conjugate reflectors interposed a glass screen. 
The focal thermometer, which had risen 10°, fell 7° in 9 min.; 
on removing the screen it rose again. ( Essais de Phys. p. 63.) 
2. ) Sir W. Herschel tried experiments on this point. [Phil. 
Trans. 1800.) Two moveable objects illuminated by a lamp 
were viewed by the eye, one through an open hole, the other 
through a hole covered successively by different transparent 
media. One object was moved to greater or less distance, till 
they appeared equally bright; the interceptive power was esti¬ 
mated directly as the illumination required to produce the equali¬ 
zation, that is, inversely as the square of the distance. 
Two equal thermometers inclosed in a box, with apertures 
over the bulbs (which were plain), one open, the other covered 
successively by the different transparent media, were exposed 
to different sources of heat, and the interceptive effects com¬ 
pared together and with those of the same media for light. 
Thus among the results were the following:— 
Common Fire. Candle. 
r~ -- (— -•'N-.> 
Light. Heat. Light. Heat. 
Coach glass ... 0 . . . 750 . 86 . . . 625* 
Dark red glass . . 999 . . . 573 . 999 . . . 526 
* Out of 1000. 
