CHAPTER VI 



THE COMPOSITION OF FIXED AIR, CARBON, CARBONIC ACID, 

 AND THE CARBONATES 



A. THE COMPOSITION OF FIXED AIR. 



Fixed air a product of combustion. Van Helmont 



derived the name of his poisonous " gas sylvestre " from its 

 presence in the fumes of a fire of wood-charcoal (Latin 

 sy/va, a wood or forest). But as the only test which he 

 applied was that of a lighted candle, no importance attaches 

 to the fact that he gave the same name to the gases 

 liberated in fermentation, and by the action of acids upon 

 chalk. 



The recognition by Black, in 1755, of fixed air as a 

 constituent of chalk provided for the first time a test by 

 which a gas differing from common air could be detected and 

 identified with certainty. By its power of reconverting lime 

 into chalk, he proved the presence of fixed air in substances 

 such as magnesia, sal-volatile, and the mild alkalis, as well 

 as in natural waters and in common air. Cavendish, in 

 1766, added to these tests the method of measuring the 

 physical constants (density and solubility) of the gas, and 

 proved that these were the same for fixed air prepared from 

 alkaline substances and by the fermentation of sugar. 

 These physical tests could not be applied to the crude gas 



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