272 HISTORY OF CHEMISTRY. 



CHAPTER V. 



CHEMISTRY OF GASES. BLACK. CAVENDISH. 



rPHE study of the properties of aeriform substances, or Pneumatic 

 -L 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 

 carne 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 

 compound was equal to the sum of its elements. The latter propo- 

 sition, indeed, cannot be looked upon as a discovery, for it had been 

 frequently acknowledged, 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 clone, it 

 produced a revolution in chemistry. 



[2nd Ed.] [Though the view of the mode in which gaseous ele- 

 ments become fixed in bodies and determine their properties, had great 

 additional light thrown upon it by Dr. Black's discoveries, as we shall 

 see, the notion that solid bodies involve such gaseous elements was 

 not new at that period. Mr. Vernon Harcourt has shown ' that New- 

 ton and Boyle admitted into their speculations airs of various kinds, 

 capable of fixation in bodies. I have, in the succeeding chapter (chap, 

 vi.), spoken of the views of Rey, Hooke, and Mayow, connected with 

 the function of airs in chemistry, and forming a prelude to the Oxygen 

 Theory.] 



Notwithstanding these preludes, 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. 8 He found that the difference 

 between caustic lime and common limestone arose from this, that the 

 latter substance consists of the former, combined with a certain air, 

 which, being thus fixed in the solid body, he called fixed air (carbonic 



Phil. Mag. 1846. a Thomson's Hist. Chem. i. 317. 



