Ill THE BURNING OF METALS 31 



balance " appeared again in a modified form in the THEORY 

 OF PHLOGISTON which dominated the science of chemistry 

 during the next hundred years, but was finally overthrown 

 by the work of Lavoisier from 1770 to 1787. This theory 

 was elaborated by two German philosophers, John Joachim 

 Becher (1635-1682), and George Ernest Stahl (1660- 

 1734), who sought to explain the phenomena of combustion 

 as due to a fire-element, or principle of inflammability, to 

 which Stahl gave the name PHLOGISTON. It was supposed 

 that phlogiston was present in all combustible substances ; 

 the largest proportion was contained in soot, which was 

 thought to be almost pure phlogiston, since it left only the 

 smallest residue of ash when burnt. In the smelting of 

 metals the phlogiston of the fuel combined with the ore to 

 produce the metal ; when the metal was burnt it parted with 

 the phlogiston it had taken from the fuel and was converted 

 into an incombustible calx, or ash. It will be noticed that 

 whilst Boyle regarded the calx as a compound of the metal 

 with igneous particles from the fire, thus 



Metal '+ Fire = Calx, 



Stahl regarded the calx as a simple substance, and the 

 metal as a compound of the calx with phlogiston, thus, 



Calx + Phlogiston = Metal. 



Metal- Phlogiston = Calx. 



As the calx is heavier than the metal from which it is 

 derived it was clear that the phlogiston which escaped 

 during calcination must have a negative weight ; this curious 

 conclusion, although not much considered at first, led 

 ultimately to the destruction of the theory. 



In Stahl's opinion, air was required in combustion merely 

 to absorb the phlogiston set free by the burning substance ; 

 air which had become exhausted by combustion was 

 thought to be saturated with phlogiston, and was called 



PHLOGISTICATED AIR. 



