CALORIC. 27 



held close to the heated matter ; and this will be 

 the case, even if the heated body is not luminous or 

 incandescent, as hot water, for instance; so that the 

 invisible rays of heat also are reflected like those of 

 light. The chief part of the heat received from a 

 common fire is in the form of radiant heat ; and 

 whatever kind of construction will most promote 

 the reflection of radiant heat into the room, will be 

 the most advantageous form of the chimney. It is 

 upon this principle that the grates introduced into 

 common use by Count Rumford are so much pre* 

 ferable to all others. 



The effect of the solar rays upon bodies differs 

 much according to their colour ; black and dark 

 coloured bodies are more heated than white ones ; 

 the latter throwing off the rays, while the former 

 absorb them. For this reason, black clothes are 

 more heated by the sun than white ones. Polished 

 surfaces, also, which reflect best, do not absorb so 

 much heat as rough surfaces. 



The boiling or ebullition of liquids is a phenome- 

 non which depends upon the liquid being converted 

 into vapour by a certain degree of temperature j 

 consequently, those liquids which assume the va- 

 porous or aeriform state at the lowest temperature 

 are most easily made to boil. The ebullition, or 

 the noise and motion of the liquid in boiling, is 

 occasioned by small quantities of vapour being 

 formed at the bottom of the vessel, which rise by 

 their lightness in a globular form, and break at the 

 surface. The ebullition of liquids is easier in pro- 

 portion as the pressure to which they are subjected 

 is less ; thus water, which boils only at 212 Fahr. 

 in the air, will boil with a much less degree of heat 

 in an exhausted receiver of the air pump ; and it 



