LECTURE XXXI. 



CHEMISTRY. 



COMBUSTIBLE SUBSTANCES* 



COMBUSTIBLES are substances capable of com- 

 bustion, which process has been explained in a 

 preceding lecture. You will remember, that 

 about ne-third of the air of our atmosphere is 

 oxygen gas, or air. This part of the air is al- 

 ways destroyed by the process of combustion, as 

 is proved by letting a candle burn under a re- 

 ceiver till it goes out. The assumed explanation 

 of the phenomenon is, that the combustible body, 

 being in part decomposed by the action of heat, 

 unites with the oxygen of the air, and together 

 they form a third substance (as hydrogen or in- 

 flammable gas and oxygen form water*), and by 

 this condensation of the oxygen, a large quantity 

 of caloric escapes, and becomes sensible, so as to 

 produce heat, and frequently flame. But, per- 

 haps, our knowledge will not at present allow 

 us to say more decidedly of combustion, than 

 that it is the effect of intense chemical action. 



* If a receiver, properly cooled, is placed over one of 

 Argand's lamps, a large quantity of water will be found 

 in it. 



