328 



Hence the absorption of heat from the same light depends on the color of 

 the absorbing bodies. Those of a dark color absorb more heat than those of a 

 light color, because the former reflect the least calorific rays, while the latter 

 reflect the most. 



There are several substances which, when heated to a certain temperature, C 

 acquire a strong affinity for oxygen gas ; and when the elevation of tempera- > 

 ture takes place in an atmosphere of oxygen, or in ordinary atmospheric air, < 

 the oxygen rapidly combines with the heated body, and in the combination so J 

 great a quantity of heat is evolved that light and flame are produced. This ( 

 process is called combustion. Combustion is, therefore, a sudden chemical com- \ 

 bination of some substance with oxygen, attended by the evolution of heat and j 

 light. 



The flame of a candle or lamp is an instance of this. The substance in the J 



wick, having its temperature raised in the first instance by the application of / 



heat, forms a rapid combination with the oxygen of the atmosphere, and this ( 



combination is attended with the evolution of heat, which sustains the process \ 



.of combustion. 



Flame is, therefore, gaseous matter, rendered so hot as to be luminous. \ 

 There are a few other substances besides oxygen by combination with which 

 light and heat may be evolved, and which may therefore produce combustion. 

 These are the substances called, in chemistry, chlorine, iodine, and bromine ; 

 but, as they are not of common occurrence, the phenomenon of combustion at- 

 tending them may be regarded rather as a subject of scientific inquiry than of 

 practical occurrence. All ordinary cases of combustion are examples of the 

 combination of oxygen with a combustible. 



I have thus, in a succinct and clear manner, laid before you the principal 

 phenomena, and explained the most ordinary terms, which I shall have occa- 

 sion to use in the discourses I intend to deliver on the subject of heat. These 

 explanations will, I trust, greatly facilitate the comprehension of the la\vs and 

 the narrative of the discoveries which I shall unfold to you. 



