30 



in a former experiment, Thus it is 

 shown that the metallic covering of the 

 ball reflects the greatest portion of heat 

 falling upon it, and therefore that the 

 temperature of the instrument cannot 

 be much raised. 



A glass mirrgr being substituted for 

 the metallic one, and an uncovered ther- 

 mometer placed in its focus, when the 

 black surface of the cubical vessel was 

 presented, so little of the heat was re- 

 flected by the glass, that the thermo- 

 meter was affected in a very small de- 

 gree ; and when the mirror was covered 

 with a thin coating of China ink, no 

 perceptible effect was produced upon 

 the thermometer, all the heat being ab- 

 sorbed by the blackened glass. But 

 when the surface of the mirror was 

 covered with tin foil, the thermometer 

 was ten times as much affected as it 

 was by the heat reflected from the un- 

 covered glass mirror. 



It will appear from the foregoing ex- 

 periments, that bodies which radiate 

 heat most effectually, absorb it in the 

 same proportion ; and those which are 

 the best reflectors of heat have the least 

 radiating power. 



Mr. Leslie made an extensive series 

 of experiments on the powers of differ- 

 ent bodies to intercept radiant caloric. 

 A frame similar to that represented at 

 fig. 10, being adapted to receive various 

 substances, it is placed, when each sub- 

 stance is applied, in the manner of a 

 screen, between the tin vessel and the 

 reflector. When a sheet of tin foil was 

 attached to the frame, and placed at the 



distance of two inches from the black- 

 ened surface of the tin cannister, no 

 effect was produced on the thermometer 

 in the focus of the reflector ; nor was 

 the result at all affected by altering the 

 distance of the screen from the tin ves- 

 sel ; the whole of the heat being inter- 

 cepted at whatever place the screen 

 might be situated between the source of 

 heat and the thermometer. The same 

 effect was produced by gold leaf, al- 

 though it is 600 times thinner than the 

 tin foil. A plate of glass being inter- 

 posed at the distance of two inches from 

 the tin vessel, the thermometer shewed 

 an increase of temperature equal to 20, 

 eighty degrees of the effect, capable of 

 being produced by the blackened sur- 

 face, being intercepted. The rise of tem- 

 perature when paper was interposed was 

 about three degrees greater. 



Mr. Leslie explains the effect result- 

 ing from radiant heat, by an hypothesis 

 peculiar to himself. He believes air to 

 be the sole agent concerned in conveying 

 heat in these experiments. A portion 

 of air coming into contact with the heat- 

 ed surfaces is suddenly expanded, and 

 communicates the impulse which is thus 

 imparted to it, along with a portion of 

 its newly- acquired heat ; and the par- 

 ticles continuing to be thus acted upon 

 in succession, the heat is conveyed 

 through space, with the swiftness of 

 sound, by a series of undulations or 

 wavering motions so produced. These 

 undulations being extended to the mir- 

 ror, they are reflected and brought to a 

 focus. He contends that when glass or 



