REFLECTION. 41 



to the body more of the heat which comes to it asso- 

 ciated with intense light, as is the case with the heat 

 of the sun, and therefore its advantage as an article of 

 winter clothing. 

 Ti r , . . 81. TRANSMISSION OF HEAT. The heat 



What is 



said of the of the sun passes with its light through 



transmission ,, , -r i / 



of heat thro' a U transparent substances. But heat from 

 bodies ? j ess m t ense sources is absorbed, and in large 



part stopped by many substances which allow light to 

 pass : such are water, and alum, and glass to a less extent. 

 A glass plate held between one's face and the sun will not 

 protect it, but held before the fire will intercept a large 

 part of the heat. So a glass lens or burning-glass will 

 stop the heat of a fire, instead of transmitting and con- 

 centrating its raysj as it does those of the sun. It is a 

 singular fact, on the other hand, that many substances 

 which allow heat to pass, effectually stop the light. 

 Such are black glass and smoked quartz crystal. Rock 

 salt allows heat to pass so perfectly that it has been 

 called the glass of heat. 



82. REFLECTION OF HEAT. Polished 



What bodies 



are the best re- metallic surfaces are the best reflectors. 



S53***gJ Coffee takes lon s er to boil in a bri s ht cof- 



subject. fee-pot, because the heat is reflected from 



the bright surface and does not enter the liquid. If it 

 were desired to heat a liquid as rapidly as possible, and 

 keep it hot as long as possible in the same vessel, it 

 would be wise to take a dingy one for the rapid heat- 

 ing of the liquid, and then to polish it in order to fasten 

 the heat in. Glass mirrors do not reflect heat so well 

 as those of uncovered metal, because of the absorbing 



