v THE STUDY OF GASES 91 



Zinc on Vitriolic Acid, Zn + H 2 S O 4 ->Zn S O 4 + H ; 

 Zinc on Spirit of Salt, Zn+ 2HCl->ZnCl 2 + H 2 ; 

 Iron on Vitriolic Acid, Fe + H 2 SO 4 ->FeSO 4 +H 2 ; 

 Tin on Spirit of Salt, Sn + 2HCl->SnCl 2 + H 2 . 



By these measurements he proved the identity of the 

 different samples, and established the method of investigating 

 gases by the exact measurement of their physical properties. 

 He found that water dissolved an equal volume of fixed air, 

 which escaped when the water was boiled or exposed to the 

 air ; fixed air could, however, be stored permanently in a bottle 

 inverted over mercury. 



Joseph- Priestley, Experiments on Air, Vol. I. 1774 ; Vol. 

 II. 1775; Vol. III. 1777, prepared a large number of gases 

 and studied their chemical properties very thoroughly. He 

 examined fixed air and inflammable air, prepared three gases 

 by the action of nitric acid on metals, discovered oxygen, and 

 collected over mercury a series of gases which were too soluble 

 to be collected over water. 



B. GASES DERIVED FROM NITRIC ACID 



Nitrous air (nitric oxide, NO), was prepared by van Hel- 

 mont, by Mayow, and by Hales, by the action of nitric acid 

 on metals, but was first recognised as a distinct substance by 

 Priestley (1772), who named it nitrous air. 



It dissolves in ten volumes of water, but is freely soluble in 

 solutions of green vitriol (ferrous sulphate), forming a dark 

 brown liquid from which the gas can.be recovered by gentle 

 heating. The preparation of the gas from copper and nitric 

 acid is often represented by the equation 



3Cu + 8HN0 3 -> 3Cu(NO 3 ) 2 + 2NO + 4H 2 O, 



but many other products are formed at the same time. . 



Diminished nitrous air, or " laughing gas " (nitrous oxide, 

 N 2 O), was prepared by Priestley (1772) by the action of iron 

 filings on nitrous air. 



Berthollet (1785) prepared it by heating ammonium nitrate. 

 (NH 4 )NO 3 -> N 2 O + 2H 2 O. 



