HEAT. 



laws of refraction. A remarkable dif- 

 ference however was discovered between 

 the radiant heat of the sun's rays and 

 that proceeding from a fire, the former 

 passing much more easily through a 

 glass than the latter. Two equally de- 

 licate thermometers were exposed to the 

 rays of the sun, one being uncovered, 

 and the other covered with very trans- 

 parent glass, having a bluish white 

 tinge ; in five minutes the temperature 

 of the uncovered thermometer was 

 raised from 67 to 73, and the covered 

 one from 67 to 7U; by which it ap- 

 pears that one fourth part of the ra- 

 diant heat falling upon the thermo- 

 meters, was intercepted by the glass 

 which covered one of them. When the 

 thermometer was covered with flint 

 glass, 2i tenths of an inch thick, the 

 uncovered thermometer rose 5i in 

 five minutes, and the covered one 5, 

 so that fewer of the rays were inter- 

 cepted by flint glass, than by the glass 

 used in the first experiment. The 

 thermometers were then exposed, under 

 the same circumstances, to the rays 

 of a lighted candle; in five minutes 

 the one which was covered with the 

 same bluish glass used in the first ex- 

 periment, rose from 59 to 60f, while 

 the uncovered one rose from 59| to 

 62| : in this last experiment more than 

 half the heating rays proceeding from 

 the candle were stopped by the glass 

 of the covered thermometer. A similar 

 effect was produced when the thermo- 

 meter was covered with flint glass. It 

 is shown by these experiments, that the 

 heating rays which accompany the sun's 

 light, are "able to pass through glass 

 with greater ease, than the heating rays 

 which proceed from a burning body. 



Lest the results of the foregoing ex- 

 periments should have been affected by 

 the light accompanying the heat, the 

 experiments were varied so as to render 

 the introduction of error in this way im- 

 possible. Covered and uncovered ther- 

 mometers, as before, were exposed to 

 the influence of the invisible heating 

 rays of the prismatic spectrum ; in five 

 minutes the covered one rose from 47 

 to 43f, and the uncovered one from 48 

 to 49f . When the flint glass was used, 

 the increase of temperature indicated by 

 both thermometers was nearly equal. 

 Experiments were also tried on heat 

 projected from bodies not luminous ; 

 and it was found that the results were 

 very nearly the same when light was ab- 

 sent as when it was present. 



It was further asc 

 schel, that the different 

 into which light is separat 

 prism, pass through diffei 

 stances with different degrees of facil 

 that the invisible calorific rays, when se- 

 parated from light, pass through more 

 readily than when accompanied by light ; 

 and that coloured glass intercepts 

 more of the rays than colourless glass. 



The differential thermometer invented 

 by Mr. Leslie, and which has already 

 been described and represented (Jig. 7,) 

 was of great use in his researches 

 on radiant heat. The apparatus used 

 by him in his experiments, consisted 

 of the above-named thermometer, a 

 highly-polished concave reflector of 

 tinned iron, and hollow cubes of tin, 

 from three to ten inches, for the pur- 

 pose of holding hot water. One of 

 these cubical vessels, filled with boiling 

 water, being placed at the distance of 

 a few feet from the reflector, and one 

 of the bulbs of the differential ther- 

 mometer in the focus of the mirror (fig. 

 1 1), an instantaneous rise of temperature 

 is indicated by the instrument. By this 

 method of conducting the experiment, 

 great facilities are afforded for trying 

 the radiating powers of different sur- 

 faces. Mr. Leslie covered one side of 

 a six-inch cubical vessel with lamp- 

 black, another side with writing-paper, 

 a third side with glass, and left the 

 fourth side without a covering. The 

 vessel being filled with boiling water, 

 and its black side being turned towards 

 the reflector, the fluid in the thermo- 

 meter, which was placed in the focus of 

 the reflector, indicated a rise of tem- 

 perature equal to 100: the papered 

 side being presented to the reflector oc- 

 casioned a rise of 98 ; the glass side 

 90; and the metallic side only 12: 

 the relative radiating powers of these 

 surfaces being as the numbers ex- 

 pressing the effects which they pro- 

 duced. 



The reflecting powers of different sur- 

 faces were also tried by Mr. Leslie. He 

 coated the ball of the thermometer 

 placed in the focus of the reflector with 

 tin foil, and then exposed it to the radiant 

 heat of the blackened surface ; the rise 

 of temperature produced, instead of 

 being 100, as it was before the ball was 

 coated with tin foil, amounted only to 

 20 ; and when the metallic, surface of 

 the tin vessel was turned to the re- 

 flector, the thermometer increased in 

 temperature only 2, instead of 12, as 



