CH. vi THE COMPOSITION OF FIXED AIR 95 



obtained by burning charcoal in air ; but Cavendish showed 

 that "a quantity of common air was reduced from 180 to 

 1 60 ounce measures, by passing through a red-hot iron 

 tube filled with the dust of charcoal " and " observed, that 

 there had been a generation of fixed air in this process, but 

 that it was absorbed by soap leys " (quoted by Priestley, 

 Experiments on Air, I. 129). 



Fixed air is formed during the burning of charcoal 

 (Priestley, 1772) and the reduction of metallic calces 

 (Lavoisier, 1774). Priestley, in 1772, by means of a 

 burning mirror, or lens, heated fragments of charcoal in air 

 confined over water. He found that the volume of the air 

 was diminished by one-fifth, that the residue extinguished 

 flame, was noxious to animals, and suffered no further 

 diminution of volume when exposed to the action of iron 

 filings and sulphur, or when mixed with nitrous air ; the air 

 had therefore been deprived of all its ^goodness" (the 

 discovery of oxygen was not made until two years later) 

 whilst the production of fixed air was shown by the 

 cloudiness which was produced when the gas was confined 

 over lime-water. When mercury was used instead of water, 

 the volume of the air was not diminished by the burning 

 charcoal until lime-water was added, when one-fifth of the 

 air was absorbed (" Of Air infected with the Fumes of 

 Burning Charcoal," Experiments on Air, 1774, I. 129-132). 



About the same time, Lavoisier (Physical and Chemical 

 Essays, 1774, Chapter V; Works I, 598-613) found that 

 a large volume of " air " was produced when minium or red 

 lead was reduced by heating with charcoal in an iron retort 

 (Fig. 21.) A very large quantity of gas was set free, 560 

 cubic inches being liberated in the production of about 

 f cubic inch of lead, although the charcoal heated alone 

 gave only eight cubic inches of gas in the course of two 

 days. The gas dissolved in lime-water, and extinguished a 

 lighted candle; a rat died almost instantly in it. The 



