156 HEAT. 



Artificial cold is used in the preparation of some arti- 

 cles of luxury for the palate, and is often employed in 

 philosophical experiments, as in freezing mercury, &-c. 



OP THE SOURCES OF HEAT. 



The two principal sources of heat are, the sun, and 

 combustion ; there are others, but we shall confine our 

 remarks to these. Heat is constantly radiating from the 

 sun, and evolved or given out during the combustion or 

 burning of bodies. 



The vital part of our solar system is the sun, from this 

 source we receive all the heat necessary for producing 

 the fruits and flowers of our earth, which are matured 

 and perfected by the light from the same body. We may 

 behold the wisdom of that Power, that balanced the sun 

 and planets in the heavens, displayed in an equal degree 

 in the distribution of the animals and plants of our little 

 planet, according to their respective natures, as far as 

 respects heat and light. Those animals which are placed 

 in the arctic regions, are all protected from the cold by a 

 covering of fine fur, exactly adapted to keeping out the 

 cold ; while those of the tropics have a covering adapted 

 to their climate like the elephant, who has scarcely any 

 covering. 



It is not our purpose at present, to inquire into the 

 cause of the immense quantity of heat and light which 

 we constantly receive from the sun. Dr Herschel sup- 

 poses it to be owing to luminous clouds, which float in 

 the atmosphere of the sun, and as these clouds are subject 

 to various changes, both in quantity and lustre, he ac- 

 counts for the difference in the heat of different years. 



When a piece of glass is exposed to the rays of the 

 sun, it is not soon heated ; but if a piece of iron, of the 

 same thickness be exposed during the same time, it will 

 soon become heated ; in the same manner, all transparent 

 bodies stop but few of the solar heating rays, while opaque 

 bodies intercept more or less of them ; and the darker the 

 color of the opaque body, the more heat is intercepted. 

 Hence, arises Dr Franklin's rules for the color of cloth- 

 ing. Black or dark colors during winter, and white or 

 light colors for summer. But it is to be questioned 

 whether these conclusions are correct. By a reference to 



