72 HISTORICAL INTRODUCTION TO CHEMISTRY CHAP. 



considered a certain portion of water as absolutely essential 

 to them; and consequently all the experiments that have 

 hitherto been made on the affinities of the acids, and alkalis 

 are, in fact, nothing more than the affinities of compound 

 substances, consisting of adds or alkali, and water. I have 

 been so particular in stating these historical facts, for the 

 sake of those chymists who can see nothing new in my 

 experiments on the several acids and alkali, divested of 

 water, and exhibited in the form of air " (Experiments on 

 Air, 1777, Vol. III. pp. 325 et seq.). 



Priestley (1772) prepares and examines nitrous air. 



The gas which Mayow prepared by the action of iron on nitric 

 acid was dismissed by Cavendish with the remark that iron, 

 zinc and tin, " dissolve readily in the acid and generate air ; 

 but the air is not at all inflammable." Priestley, however, 

 after examining fixed air and inflammable air, proceeded to 

 make a detailed study of this action. By dissolving various 

 metals (amongst them brass, iron, copper, tin, silver, mercury) 

 in nitric acid, Priestley collected over water a colourless gas, 

 which he named NITROUS AIR. (Experiments on Air, 1774, 

 I. 109). The gas was noxious to animals, and extinguished a 

 lighted taper. But it differed from fixed air in that it did 

 not precipitate lime-water and was only slightly soluble in 

 common water; water, he says, "absorbs one-tenth of its 

 bulk of nitrous air." The gas is, however, freely soluble in 

 a solution of green vitriol in water, which can be made to 

 "absorb more than ten times its bulk of nitrous air, without 

 any sensible approach to saturation." (Experiments and 

 Observations, 1779, IV. 48) ; this fact was afterwards used 

 by Humboldt and by Davy to test the purity of different 

 samples. The solution in which the gas has been absorbed 

 " becomes of a very dark colour ; but becomes green again 

 on being exposed to the open air " (Experiments on Air, 

 1777,111. Preface p. xxxiii). Davy showed that the green 

 colour could be restored and pure nitrous air recovered 

 by gently heating the solution ( Works, III. 99). 



