126 HEAT OR CALORIC. 



VIII. COMBUSTION. 



(a.) In common language it means the same as burning ; that is, 

 in most cases, the apparent consumption* of a body, and an entire 

 change in its properties, with the emission of heat and light. 



(b.) In what was catted the new or French theory, combustion was 

 synonymous with a combination of oxygen with a combustible body, 

 attended by augmentation of its weight, and change of its nature, heat 

 and light being at the same time emitted. Now, Chlorine is added as 

 another agent possessed of similar powers with oxygen ; also, by 

 some, Iodine ; and many even regard every case of intense chemical 

 action, with the emission of heat and light, as combustion. 



" Whenever the chemical forces that determine either combina- 

 tion or decomposition, are energetically exercised, the phenomena of 

 combustion, or incandescence, with a change of properties, are dis- 

 played."! 



In general we shall use the word combustion in its common and 

 more restricted sense, taking due notice, however, of the other cases 

 as we come to them. 



(c.) It would be premature to consider combustion fully at pre- 

 sent ; for its theory and phenomena are best developed progressively 

 as we proceed. 



We mention combustion in this place merely to complete our list 

 of the effects of heat; for, as commonly seen, it sustains a very close 

 connexion with heat, since an exalted temperature is usually neces- 

 sary to its existence. Heat is, however, often the consequence, as 

 well as the cause of combustion. 



(d.) Phlogiston is a name formerly given to a principle of com- 

 bustion, supposed to reside in all inflammable bodies ; dissipated, as 

 was imagined, in the form of heat and light, during combustion ; the 

 body being thereby rendered uninflammable, and its inflammability 

 being again restored by recombining with phlogiston, as when red 

 lead is heated with charcoal which causes the incombustible metallic 

 oxide to become again combustible in the form of metallic lead. 



This theoryf is now obselete, but in its time, it rendered important, 

 service to the science of Chemistry, and was in vogue for a century. 

 Phlogiston comes very near to the modern idea of combined and free 

 caloric. If we substitute a combination of oxygen for the extrication 

 of phlogiston, and the extrication of oxygen for the combination of 

 phlogiston, we translate, very nearly, all the common cases of com- 

 bustion, from one theory into the other. 



* Sometimes the body remains, but in an incombustible state. 



i lire's Chem. Die. Invented by Becher, and more fully illustrated by StaML 



