HEAT. 



light of a fire to pass through, inter- 

 cepts the heat ; that a glass mirror 

 reflects the light without the heat, ab- 

 sorbing the latter; while a metallic 

 mirror reflects both heat and light, so 

 that it is not quickly warmed, unless 

 its surface be blackened, which occa- 

 sions it to absorb the heat. 



The experiments of Lambert were 

 repeated, and varied by Saussure and 

 Pictet. They used two concave reflec- 



tors, of polished tin plate, one foot in 

 diameter, with a focal length of four 

 inches and a half; they placed these 

 twelve feet two inches distant, and 

 exactly opposite to each other. In the 

 focus of one reflector they placed a ball 

 of iron, two inches in diameter, heated 

 so as not to appear luminous in the dark, 

 and in the focus of the other they placed 

 the ball of a mercurial thermometer, 

 (fig. 10.) The temperature of the ther- 



mometer began to increase as soon 

 as the ball was put in its place, and 

 continued rising from 4 of Reaumur's 

 scale to 14, which it did in six minutes. 

 Another thermometer, at the same dis- 

 tance from the heated ball, but out of 

 the focus of the reflector rose only from 

 4 to 61. 



The heated ball a, (Jig. 10,) 'in the 

 focus of one reflector, b, projects heat 

 from every part of its surface ; those 

 rays that proceed towards the reflector 

 are intercepted by it, and in consequence 

 of its shape, they are again projected 

 into space, in straight lines, towards 

 the other reflector c, by which they are 

 reflected, and brought to a focus at the 

 point where the thermometer d is 

 placed: the heat, thus accumulated, 

 affects the thermometer d y and makes 

 it rise. 



To prove that the thermometer is 

 not affected by heat proceeding directly 

 from the ball, a plate of glass may be 

 held between it and the reflector b, 

 which will prevent any effect from 

 being produced upon the thermometer ; 



or the plate of glass may be held be- 

 tween the reflector c, and the thermo- 

 meter d : in either case the instrument 

 will remain unaffected. An air, or diffe- 

 rential thermometer, (fig. 5. or^-. 7.) 

 answers much better for this experiment. 



Whatever hot substance may be used 

 as the source of heat, the effect is the 

 same, and is always in proportion to 

 the temperature of the body used. 



Pictet made other experiments, some 

 of which approached near to the dis- 

 covery of the different radiating powers 

 of different surfaces. When he used a 

 glass concave mirror behind the ball, 

 instead of a metallic one, very little 

 effect was produced upon the thermo- 

 meter. A glass plate covered with 

 amalgam on one side, having its coated 

 part presented to the hot body, and the 

 uncoated glass surface to the thermo- 

 meter, produced the effect of 3.5 ; 

 the glass surface being turned to the 

 hot body, and the metallic surface to- 

 wards the thermometer, the effect was 

 only as 0.5. When the metallic side 

 was blackened and presented to the 



