THE GREAT LONDON CHEMISTS 37 



a gas could be contained in a solid state, as for instance 

 in carbonate of linie or of magnesia, or in what were then 

 known as the ' mild alkalies ' ; and that it could possess 

 weight. He termed carbonic anhydride 'fixed air.' 

 Cavendish's first published paper deals with 'Factitious 

 Air'; it appeared in 1766, seven years after the publica- 

 tion of Black's memoir on ' Magnesia alba, Quick-lime, and 

 other Alkaline Substances.' ' Factitious air ' was defined 

 by Cavendish as ' any kind of air which is contained in 

 other bodies in an unelastic state, and is produced from 

 thence by art.' He first treats of hydrogen, next of 

 carbon dioxide, and lastly of gases evolved during fermen- 

 tation and putrefaction. Although not the first to prepare 

 hydrogen (for it must have been known for centuries that 

 an inflammable gas was evolved on bringing metals into 

 contact with certain dilute acids), yet he was the first to 

 characterise hydrogen as a definite substance, and not a 

 mere variety of common air. He prepared this gas from 

 zinc, iron, or tin, and weak sulphuric or hydrochloric acid. 

 He found that the substance was identical in each case, 

 by weighing a known volume ; which he did with no great 

 accuracy in a bladder, but with considerable exactitude 

 by weighing a flask containing, for example, zinc and 

 acid, unmixed ; and after mixture, weighing again ; a 

 further experiment served to determine the volume of gas 

 obtainable from a known weight of zinc. Another method 

 of establishing their identity, curious to our notions, was 

 to mix the sample with a known volume of air, and 

 estimate the loudness of the explosion which took place 

 on applying a flame. Cavendish also prepared ' the 

 volatile sulphurous acid,' by substituting concentrated 

 sulphuric acid for dilute; and a non-inflammable air 

 (nitric oxide), by the action of nitric acid. 



Cavendish did not suppose that the 'air' came from 

 the acid, but from the metal. It must be remembered 



