66 HISTORICAL INTRODUCTION TO CHEMISTRY CHAP. 



handle it. It was therefore left to Cavendish to make the 

 first careful examination of those gases whose existence had 

 been vaguely recognised during the preceding one hundred 

 and fifty years. Cavendish writes : 



" By factitious air, I mean in general any kind of air 

 which is contained in other bodies in an unelastic state, 

 and is produced from thence by art." 



" By fixed air, I mean that particular species of factitious 

 air, which is separated from alkaline substances by solution 

 in acids or by calcination ; and to which Dr. Black has 

 given that name in his treatise on quick-lime (Phil. Trans., 

 1766, 56, MI). 



Describing next the action of acids on metals he writes : 



" I know of only three metallic substances, namely, zinc, 

 iron and tin, that generate inflammable air x by solution in 

 acids ; and those only by solution in the diluted vitriolic 

 acid, or spirit of salt." 



" Zinc dissolves with great rapidity in both these acids ; 

 and, unless they are very much diluted, generates consider- 

 able heat. One ounce of zinc produces about 356 ounce 

 measures of air 2 : the quantity seems just the same whichever 

 of these acids it is dissolved in. Iron dissolves readily in 

 the diluted vitriolic acid, but not near so readily as 

 zinc. One ounce of iron wire produces about 412 

 ounce measures of air: the quantity was just the 

 same, whether the oil of vitriol was diluted with ij, or 

 7 times its weight of water : so that the quantity of air 

 seems not at all to depend on the strength of the acid " 

 (ibid., p. 144). 



Inflammable air was also obtained by the action of spirit 

 of salt on iron, but the quantity was not measured. One 

 ounce of tinfoil dissolved in strong spirit of salt yielded 

 202 ounce measures of inflammable air ; the same gas 

 was also produced slowly by the action of vitriol on tin. 



1 The name " inflammable air " is here used for the first time. 



2 The "ounce-measure" as used by Cavendish was the volume 

 occupied by an ounce of water. 



