I/AVO1S1EK*S THEORY OF COMBUSTION. 215 



and phlogiston, during a great part of the pre- 

 ceding century. 



After the discovery of latent heat by Dr. Black, 

 and of oxygen by Priestley, the theory of Stahl 

 was gradually superseded by that of Lavoisier. 

 This celebrated chemist maintained that oxygen 

 consisted of caloric and light, united with a 

 ponderable base, and that during every case of 

 combustion this base combined with the burning- 

 body, by which its volume was diminished; while 

 the caloric and light of the oxygen were given 

 out in the form of fire. 



It is scarcely necessary to add, that this theory 

 was also found to be exceedingly defective and 

 erroneous in nearly all its essential conditions ; 

 that atmospheric oxygen is not indispensable to 

 the process of combustion, nor the exclusive 

 source of caloric and light ; that all bodies contain 

 definite quantities of caloric in a latent or com- 

 bined state, and may be expanded into flame or 

 light by a sufficient quantity of the same prin- 

 ciple.* He was equally mistaken in supposing 



* When carbon, sulphur, phosphorus, boron, and the metals 

 are heated in vacuo, they become luminous without the agency 

 of oxygen. Another decisive proof that oxygen is not the only 

 source of light is, that its colour varies according to the nature 

 of the combustible. Hydrogen affords a bluish light ; potassium, 

 and many other bodies, burn with a red light. Hence it is, that 

 bituminous coal burns with a bluish light when only partially 

 ignited, because the hydrogen alone is volatilized, while its carbon 



