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CHAPTER V. 



CHEMISTRY OF GASES. BLACK. CAVENDISH. 



THE study of the properties of aeriform sub- 

 stances, or Pneumatic Chemistry, as it was 

 called, occupied the chemists of the eighteenth 

 century, and was the main occasion of the great 

 advances which the science made at that period. 

 The most material general truths which came into 

 view in the course of these researches, were, that 

 gases were to be numbered among the constituent 

 elements of solid and fluid bodies; and that, in 

 these, as in all other cases of composition, the com- 

 pound was equal to the sum of its elements. The 

 latter proposition, indeed, cannot be looked upon as 

 a discovery, for it had been frequently acknow- 

 ledged, though little applied ; in fact, it could not 

 be referred to with any advantage, till the aeriform 

 elements, as well as others, were taken into the 

 account. As soon as this was done, it produced a 

 revolution in chemistry (F). 



The credit of the first great step in pneumatic 

 chemistry is, with justice, assigned to Dr. Black, 

 afterwards professor at Edinburgh, but a young 

 man of the age of twenty-four at the time when he 

 made his discovery 1 . He found that the difference 

 between caustic lime and common limestone arose 



1 Thomson's Hist. Chem, i. 317- 



