146 



light, having the angle of incidence equal to the angle 

 of reflection; that is, if heat strike a surface, it flies off 

 from it at the same inclination with which it struck it. 



Experiment. Take two common metallic lamp reflec- 

 tors, place them a short distance apart, with a hot iron 

 ball in the focus of one, and a thermometer in the focus 

 of the other ; the thermometer will immediately begin to 

 rise, in consequence of the heat from the ball being 

 radiated to its reflector, from whence it is thrown to the 

 other reflector, and from thence it is reflected to its focus, 

 in which the thermometer is placed. 



If a pane of glass be held between the reflectors, the 

 thermometer does not rise ; this proves that the heat 

 does not proceed directly from the ball to the thermometer, 

 which is further shown by removing the reflector nearest 

 to the ball, as the thermometer will then fall. 



From numerous experiments, it has been shown, 

 that those surfaces which radiate heat best, are the 

 worst reflectors ; while those which radiate least, are 

 the best reflectors ; metals, consequently, are the best 

 reflectors, and when polished better than when tar- 

 nished. This is the reason that brass andirons, which 

 are very near the fire, are but little heated ; because brass 

 is one of the poorest radiators, and one of the best reflec- 

 tors. Advantage is taken of this property, in the culinary 

 operation of roasting with a tin kitchen, where the heat is 

 reflected upon the spit. 



Radiation and reflection do not take place under 

 water. Radiation is considerably retarded by interposing 

 a screen before the hot body; a piece of pine board, one 

 inch thick, is not so effectual in stopping radiating heat 

 as apiece of gold leaf -^fam part of an inch thick. Those 

 substances which radiate best, intercept the least of 

 it, and vice versa. Hence, those substances which 

 absorb the least heat, are the best interceptors of it. 

 Therefore, ladies' screens should not be made of paper, or 

 similar substances, but of some bright metallic substance. 

 Fine wire, polished, and woven fine, would stop heat 

 more effectually than paper. 



Conduction. If we take several pieces of different 

 substances, such as silver, copper, iron, lead, and glass, 

 <jfjthe same size, and place a coat of wax upon one end 

 ofeach, then place the other ends in the same heat, it 



