THE PHLOGISTON THEORY 



an acid, an earth or ash-like substance, which was 

 supposed to be the really pure substance. When the 

 latter was united with phlogiston it formed the metal, 

 the sulphur, or other combustible. Sulphur, for in- 

 stance, being the compound of sulphuric acid and 

 phlogiston, etc. The phlogiston, when escaping into 

 the air, was absorbed by plants and animals, either 

 directly or indirectly, and became the source of heat 

 and life to man. It was soon found that a metal 

 when burned, Mercury, for instance, yielded a calx 

 (an oxide) that weighed more than the metal and 

 phlogiston did originally. This was accounted for 

 by attributing negative gravity, or a principle of 

 lightness or levity, to the phlogiston, and found sup- 

 port in the fact that the flame from burning matter 

 ascended in the air. Absurd as this theory now seems 

 to us, it commanded general assent for many years, 

 and retained its adherents even after the discovery 

 of oxygen and the true phenomena of combustion. 



In 1755 Joseph Black (1728-1799), of Edinburgh, 

 described the preparation of " fixed air," or carbon 

 dioxide. Until this time all gases were looked upon 

 as being identical with air. No difference in nature 

 or properties were known to exist. Black showed 

 that the gas obtained by heating Carbonate of Mag- 

 nesia was the same as that produced in combustion, 

 in breathing, and in the fermentation of beer, and 



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