-Lesson XXVIIK] FIRE. 181 



of light j and having the angle of their reflection 

 equal to the angle of their incidence. This is 

 supposed to be the case, not merely with the 

 caloric immediately derived from the sun, but 

 with that which proceeds from our common fires, 

 candles, and even hot water, and iron heated not 

 so much as to become lucid. It appears, by the 

 experiments of Professor Pictet, that ihe radiation 

 of heat, and even its reflection, takes place inde- 

 pendent of light : thus a piece of iron heated, 

 but not so high as to emit any light, being 

 placed in the focus of a concave mirror, will very 

 sensibly affect a thermometer, placed in the focus 

 of another mirror opposed to the former. On the 

 same principle, if ice be employed instead of 

 heated iron, the thermometer will be affected in 

 a contrary direction. From this latter experiment 

 it has been conjectured that cold, as a body, is 

 emitted from the ice, and reflected by the mirror. 

 This opinion, however, appears to be entirely un- 

 founded. On a lighted candle being thus em- 

 ployed, and a plate of clear glass being placed be- 

 tween the mirrors, the caloric appeared to be in- 

 tercepted, although the light passed with its usual 

 facility ; the thermometer sinking 14, in nine 

 minutes, and rising 12, in seven minutes after its 

 removal. Caloric is transmitted through some 

 bodies with unabated rapidity, whilst its passage 

 through others is very considerably retarded ; and 

 hence bodies have been named either good or lad 

 cojiductors. The cause of this difference has never 

 yet been satisfactorily ascertained, All bodies 



appear 



