86 CJiemishy. [Lecture 26. 



body is raised to a certain temperature, it begins 

 to be decomposed, and to combine with the oxy- 

 gen of the atmosphere, and this oxygen during 

 its combination lets go the caloric with which in 

 the state of air or gas it was combined. Hence 

 combustion consists of two tilings, a decom- 

 position, and a combination ; and the substance 

 which has undergone combustion is essentially 

 altered; it is, in fact, a compound of the body 

 which has been subjected to combustion and 

 oxygen. It is of course incombustible, because 

 its base being already saturated with oxygen, 

 cannot combine with any more. 



Upon the same principles, if by any process 

 the oxygen is taken away, the substance will 

 again be rendered combustible. To illustrate 

 this whole theory, take a familiar instance. Sul- 

 phur, we have just seen, is reduced to a corrosive 

 acid by burning in the open air ; but by charcoal 

 applied in a particular manner with the assist- 

 ance of fire, this acid may be again converted 

 into inflammable sulphur. It is the same thing 

 whether we use charcoal made of blood, flesh, or 

 bone, or whether the charcoal from any vege- 

 table matter. Nor is it necessary to have re- 

 course to charcoal, as pit-coal and other com- 

 bustible bodies have the same effect. The fact 

 is easily explained upon the principles just laid 

 down. The sulphur, when burned, extracts the 

 oxygen, or acidifying principle, from the air, 



