HEAT OR CALORIC. 44 



(g.) The panes of a common window do not become heated by 

 the passage of the sun's rays through them ; or at most, the. effect is 

 scarcely perceptible. 



(h.) Rays of caloric pass through glass with difficulty, if the tem- 

 perature be below that of boiling water, but they traverse it with a 

 facility always increasing with the temperature of the body emitting 

 the heat, as it approaches the point where bodies become luminous. 

 Hen. 



(i.) Calorific rays that have already passed through a glass screen 

 pass through another with much greater facility. Rays emitted by a 

 hot body differ in their power of passing through glass. 



" A thick glass, though as permeable to light as a thin glass of a 

 worse quality, or even more so, allows a much less quantity of radiant 

 heat to pass ; but the difference is so much the less as the tempera- 

 ture of the radiating source is more elevated,"* * 



9. RAYS OF CALORIC, EMITTED FROM HOT BUT NOT LUMINOUS 

 BODIES, CAN BE REFLECTED BY MIRRORS, AND BROUGHT TO A 



FOCUS. 



Hot water, hot mercury, and hot, but not luminous solid bod- 

 ies are good examples ; e. g. a cannon ball, a stone, &c. 



(a.) In making these experiments, either one mirror or two may 

 be employed. The mirrors should be of copper, plated with silver; 

 or, brass or tin will answer very well, if highly polished. 



(b.) If one mirror be employed, the hot body should be placed in 

 the axis of the mirror and the thermometer in the focus ; if two mir- 

 rors are employed, the thermometer should occupy one focus and 

 the hot body the other. 



10. RAYS ARE EMITTED BY THE SUN WHICH DO NOT PRODUCE 

 EITHER HEAT OR VISION, BUT EFFECT CERTAIN CHEMICAL DECOM- 

 POSITIONS OR COMBINATIONS. 



(a.) Muriate of silver is tarnished or blackened by the sun's rays 

 but in the prismatic spectrum, this effect is least in the red ray, and 

 increases constantly towards the violet ; the ratio of the blue and 

 red rays is inversely, as 1 5 to 20 that is, to produce a given effect 

 in fifteen minutes by the blue, requires twenty in the red. 



(b.) Beyond the violet ray, the same effect is still produced in the 

 dark. 



(c.) Berard,f by a lens, concentrated that part of the spectrum, 

 from the green to the violet, and by another the portion from the 

 green to the red. The focus of the last was a white point, scarcely 

 tolerable to the eye, but it did not alter the muriate of silver in two 

 hours: the other focus was much less bright and less hot, but 

 blackened the muriate in less than six minutes. 



* De la Roche, Annals of Phil. II, 100. t Ann. of Phil. II, 165. 



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