v THE STUDY OF GASES 75 



a slight inferiority in the air of his study as compared 

 with that outside, and in a sample of air from York as com- 

 pared with that of Leeds. 



Two years later he discovered the gas, richer than com- 

 mon air, to which Lavoisier gave the name oxygen. On 

 adding nitrous air to the gas prepared from red precipitate 

 or from mercury calx, he found that it was about five times 

 " as good as the best common air " he had ever tested. 



It was soon recognised that the diminution of volume of 

 common air was due to the absorption of oxygen, but the 

 variations which Priestley thought he had found in the 

 behaviour of different samples were proved to be due to 

 experimental errors. The careful experiments made by 

 Cavendish in 1783 showed that the composition of air 

 is remarkably constant, the value found for 100 volumes 

 of air being : 



Diminution by nitrous air (oxygen) 20*84 volumes. 

 Residue (azote) 79' 1 6 volumes. 



The graduated glass tube which Priestley used for 

 measuring the goodness of air is still known as a 

 EUDIOMETER (Greek cv, good, /uerpov, a measure: see 

 Priestley, Experiments on Air, 1777, III. 379 and 380). 



Priestley (1777) describes the properties of nitrous 

 fumes. On account of their solubility and corrosive pro- 

 perties, the brown nitrous fumes produced by combination 

 of nitrous air with oxygen, or by the action of strong nitric 

 acid on metals, could not be collected by the methods which 

 Priestley employed for other gases. He was, therefore, 

 obliged to collect the gas by displacing the air from narrow- 

 necked flasks provided with glass stoppers. This gas, as 

 Priestley prepared it by the action of nitric acid upon 

 bismuth, was obviously contaminated both with nitrous 

 air and with common air. He writes : 



" Being disappointed, as has been seen, in my expecta- 



