394 HISTORICAL INTRODUCTION TO CHEMISTRY CH. xvn 



could not easily be freed from moisture and from carbonic 

 anhydride. The black oxide of copper was first used by Gay- 

 Lussac, in 1815, for the combustion of mercuric cyanide 

 (Ann. de Chimie, 1815, 95, 184), and in the hands of 

 Liebig (Pogg. Ann. der Physik, 1831, 21, 1-43) soon became 

 the standard oxidising agent in organic analysis (see Fig. 51). 



B. THE STRUCTURE OF SALTS. 



Lavoisier's oxygen-theory of the structure of salts. 

 The idea that compounds possess a definite structure 

 originated from observations on the interactions of acids 

 and bases to form salts. Mayow, in considering this action, 

 concluded (A.C.R. XVII. 160) that "although [acids] and 

 alkalis pass into a neutral substance when they meet, yet 

 they do not, as is generally supposed, entirely destroy each 

 other." Each grain of salt must then contain, intimately 

 united within it, a certain portion of acid and a certain 

 portion of alkali. Lavoisier elaborated this idea by regard- 

 ing the acid and base as themselves compounded of a non- 

 metal or a metal united with oxygen, so that the structure 

 of the salt might be represented as : 



SALT = ACID -f BASE 



= {NON-METAL + OXYGEN} + {METAL + OXYGEN} 



" The acidifiable substances, when combined with oxygen 

 and converted into acids, acquire a great tendency to 

 combination ; they become capable of uniting with earthy 

 and metallic substances, and it is by this union that 

 neutral salts are produced" (Treatise, 1789; Works, I. 

 "5). 



Berzelius's dualistic theory (181&). Lavoisier's theory 

 broke down when it was discovered that acids (such as 

 muriatic acid) and bases (such as ammonia) existed, in 

 which no oxygen was present, but its essential features were 

 revived by Berzelius in his "Theory of Chemical Propor- 



