REPORT ON RADIANT HEAT. 
265 
d.) Effect of surface on the absorption of heat from non- 
luminous hot bodies . 
1. ) De Saussure and Pictet, with the apparatus before de¬ 
scribed, found that the thermometer rose in two minutes, 
Plain.4J° Fahr. 
Blackened . 3 J 
2. ) By the same apparatus as before described, Prof. Leslie 
found that on coating the bulb of the thermometer with 
the different substances, the absorptive power was very nearly 
in the same proportion as the radiative ; and by making the 
same modifications in the surface of the reflector, he found 
that reflective power is inversely as the radiative or absorptive . 
{Inquiry, pp. 19, 81, 98.) He also gives a very precise set of 
experiments on the effect of coatings of jelly of increasing 
thicknesses, (p. 100.) 
3. ) Dr. Ritchie has devised a very elegant mode of showing 
that the absorptive power of surfaces is precisely proportional 
to their radiating power. ( Royal Inst . Journ. vol. v. p. 305.) 
The instrument consists of a large differential thermometer, 
whose bulbs are chambers of considerable size, presenting large 
and equal plane surfaces on the sides which are towards each 
other: of these one is plain or polished, the other coated. 
Midway between them is placed a canister having equal plane 
surfaces facing each of the former respectively, and one po¬ 
lished, the other coated with the same pigment as before ; this 
canister is filled with hot water, and is capable of turning on a 
vertical axis ; thus the coated surface of the canister can be 
turned to the coated bulb or to the polished: in the former 
case a great effect is produced on the coated bulb, and a very 
small effect on the plain: in the second case the better radia¬ 
ting surface is directed to the worse absorptive one, and the 
worse radiating to the more absorptive, and the liquid in the 
tube remains perfectly stationary : the exact equality, therefore, 
of the absorptive and radiating powers is established. The 
whole is on a large scale, and can be exhibited to a Class. 
4. ) The most recent and curious researches on this part of 
the subject (and extending, as we shall see, to other parts 
also,) are those of MM. Nobiliand Melloni. (Annales de Chimie , 
October, 1831 ; Recherches sur plusieurs Phenomenes calori- 
fiques, &c.) 
The authors commence by describing their thermo-multiplier , 
by the aid of which their researches were carried on. This 
