BLACK. 7 



The air that was driven through the liquid again pre- 

 cipitated the lime in the form of chalk. Finally, he 

 ascertained by breathing through a syphon filled with 

 lime-water, and finding the lime again precipitated, that 

 animals, by breathing, evolve air of this description. 



The great step was now made, therefore, that the air 

 of the atmosphere is not the only permanently elastic 

 body, but that others exist, having perfectly different 

 qualities from the atmospheric air, and capable of los- 

 ing their elasticity by entering into chemical union 

 with solid or with liquid substances, from which being 

 afterwards separated, they regain the elastic or aeriform 

 state. He gave to this body the name of fixed air, to 

 denote only that it was found fixed in bodies, as well 

 as elastic and separate. He used the term "air" only 

 to denote its mechanical resemblance to the atmos- 

 pheric air, and not at all to imply that it was of the 

 same nature. No one ever could confound the two 

 substances together ; and accordingly M. Morveau, in 

 explaining some years afterwards the reluctance of 

 chemists to adopt the new theory of causticity, gives 

 as their excuse, that although this doctrine " admirably 

 tallies with all the phenomena, yet it ascribes to fixed 

 air properties which really make it a new body or ex- 

 istence" ("forment reellement un nouvel etre").* 



In order to estimate the importance of this discovery, 

 and at the same time to show how entirely it altered 

 the whole face of chemical science, and how completely 

 the doctrine was original, we must now examine the 

 state of knowledge which philosophers had previously 

 attained upon the subject. 



It has often been remarked that no great discovery 

 was ever made at once, except perhaps that of loga- 

 rithms ; all have been preceded by steps which con- 

 ducted the discoverer's predecessors nearly, though not 

 quite, to the same point. Some may possibly think 



* Supplement to the ' Encyclopedic,' vol. ii., p. 274, published in 1777. 



