92 HISTORICAL INTRODUCTION TO CHEMISTRY CHAP. 



It is readily soluble in water, but is expelled by boiling. A 

 candle burns in it with an enlarged flame, as in oxygen. The 

 pure gas can be breathed with impunity (Davy), and has been 

 used as an anaesthetic in dentistry since 1800. 



The formation of soluble brown nitrous fumes (nitrogen 

 dioxide, NO 2 )on mixing nitrous air with common air had been 

 noticed by Hales. In presence of water the action is more 

 complex, both nitric and nitrous acid being produced (see 

 Chapter X), thus : 



Without water 2NO + O 2 = 2NO 2 



(Nitrogen dioxide.) 



With water 4N O + 3O 2 + 2 H 2 O = 4 H N O 3 



(Nitric acid.) 



(Nitrous acid.) 



When common air is mixed over water with an excess of 

 nitrous air (nitric oxide) all the oxygen is absorbed, together 

 with as much of the nitrous air as has combined with it ; 

 Priestley used the contraction produced by mixing equal volumes 

 of the two gases in a eudiometer as a measure of the "good- 

 ness" of air. 



The brown nitrous fumes were prepared in an impure state 

 by Priestley by the action of bismuth on nitric acid 



Bi + 6HNO 3 - 



(Nitric (Bismuth (Nitrogen 



acid.) nitrate.) dioxide.) 



But this gas is better prepared by heating lead nitrate 

 2Pb(NO ) 2 -> 2PbO + 4NO 2 + O 2 . 



(Lead nitrate.) (Litharge.) (Nitrogen 

 dioxide.) 



It can be purified by condensing it to a liquid. Priestley 

 noticed that the gas darkened in colour when heated, but 

 became light when cooled again. The loss of colour is due 

 to the formation of a more complex compound (dinitrogen 

 tetroxide) 



2NO 2 = N 2 O 4 , 



(Brown.) (Colourless.) 



which can be frozen out as a colourless ice (Chapter XX, 

 p. 525), but begins to decompose as soon as it is melted. The 

 mixture of these two oxides is called "nitrogen peroxide." 



