REPORT ON RADIANT HEAT. 
275 
also candles, &c. (p. 447), a distinction pointed out between 
this and the solar rays, (p. 83, 54.) 
The light from putrescent substances does not appear to be 
accompanied with any appreciable degree of heat, according to 
Dr. Hulme. (Thomson’s Chem. i. 414, 4th edit.) But the effect, 
if any, must be so small that we cannot positively assert there 
is none. 
The same remark may apply to many other very faint lights, 
b.) Reflexion of heat. 
1. ) Mariotte collected the heat of a fire in the focus of a re¬ 
flector. {Mem. Acad, of Sciences, 1682.) 
Lambert, with burning charcoal in the focus of conjugate 
reflectors, found a combustible body kindled in the other focus. 
(Lambert, Pyrometrie ; Saussure, Voyage, iv. 119.) 
Scheele (On Air and Fire, p. 67—71,) observes that a glass 
mirror, though it reflects the light of a fire, does not reflect the 
heat (it is not stated by what means the heat was estimated); 
but the mirror becomes heated. A polished metallic mirror 
reflected both the light and heat, and did not become much 
heated itself; if blackened, it was soon hot. 
Pictet extended the experiments with conjugate reflectors to 
this case, by placing a candle in one focus. The thermometer 
rose nearly 10° in 6 minutes (. Essais de Phys. p. 63.) 
Sir W. Herschel (Phil. Trans. 1800, p. 297,) placed a candle 
at 29 inches from a concave metallic reflector ; the focal ther¬ 
mometer in 5 minutes rose 3^°; another out of the focus was 
not affected. 
The same took place with a fire, and with red hot steel. 
2. ) Polarization by reflexion. 
Berard (Memoir before cited,) tried the polarization of heat 
from luminous sources, and found a considerable diminution in 
the position when the light ceases to be reflected. 
There was of course here no distinction drawn between the 
heat accompanying the light, and the simple heat: of the latter 
nothing is proved; the former may be merely an effect of the 
absorption of light, and if so, the term polarization is applied 
to the heat without any proof. 
I repeated these experiments, and, after all precautions, 
thought there was a small perceptible effect, (when the simple 
heat was cut off by a glass screen,) which w T as diminished in the 
position of non-reflexion for the light; when the whole heat was 
admitted, no proportional diminution took place. ( Edinb. Journ. 
of Science, vi. 303.) 
