SOURCES OF HEAT. 209 



dor, Iho heat which it produces is also, at the same time, rendered more 

 intense. 



467. When a body, naturally incapable of 



When is a . . , • i " i ^ . 



bodyincandes- emittinsr li2:ht, IS heated to a siimcient extent 



ceat or ignited? , '=' ^ '. . . • i - i • i 



to become luminous, it is said to bo incandes- 

 cent, or ignited. 



What is flame 468. Flame is ignited gas issuing from 

 ^ and fire? ^ bumiug bodj. Fire is the appearance of 

 heat and light in conjunction, produced by the combus- 

 tion of inflammable substances. 



The ancient philosophers used the term fire as a characteristic of the nature 

 of heat, and regarded it as one of the four elements of nature ; air, earth, and 

 water being the other three. 



Heat and the attraction of cohesion act constantly in opposition to each 

 other ; hence, the more a body is heated, the less will be the attractive force 

 between the particles of which it is composed. 



SECTIOX I. 



SOURCES OF KEAT. 



^nd ai^ '^° 469. Six great sources of heat are recognized, 

 sourcesofheat? They are — 1. The sun ; 2. The interior of the 

 earth ; 3. Electricity ; 4. Mechanical action ; 5. Chemical 

 action ; 6. Vital action. 



What is the 470. The greatest natural source of heat is 

 ?Ii^''sourcr*of the sun, as it is also the greatest natural source 

 ^•^'^ of light. 



Although the quantity of heat sent forth from the sun is immense, its rays, 

 falling naturally, are never hot enough, even in the torrid zone, to kindle 

 -, combustible substances. By means, however, of a 



burning-glass, the heat of the sun's rays can be con- 

 centrated, or bent toward one point, called a focua, 

 in sufficient quantity to set fire to substances sub- 

 mitted to their action. 



Fig. 196 represents the manner in which a burning- 

 glass concentrates or bends down the rays of heat 

 until they meet in a focus. 

 Two opinions, or theories, have been entertained in order to account for the 

 production of heat and light by the sun ; one supposes that the sun is an 

 intensely-heated mass, which throws off its light and heat like an intensely- 

 heated mass of iron : the other, based on the ground that heat is occasioned 



