LECTURE IX.] HISTORY OF CHEMISTRY. l6l 



which we do not observe in the compounds themselves ; in all 

 their relations they possess properties of the same kind." 



Liebig then returns to Davy's ideas, and in doing so he 

 states his views as follows : 23 



"Acids are, accordingly, certain compounds of hydrogen, 

 in which the hydrogen can be replaced by metals. 



" Neutral salts are those compounds of the same class, in 

 which the hydrogen is replaced by the equivalent of a metal. 

 Those substances which we at present call anhydrous acids, 

 acquire the property of forming salts with metallic oxides, for 

 the most part, only on the addition of water ; or they are com- 

 pounds which decompose the oxides at a high temperature. 



"On bringing together an acid and a metallic oxide, the 

 hydrogen is separated, in the majority of cases, in the form of 

 water. It is a matter of complete indifference for the constitu- 

 tion of the new compound, in what manner the formation of 

 this water is conceived : in many cases it is formed by the 

 reduction of the oxide ; in others it may be produced at the 

 expense of the elements of the acid we do not know which. 



" We only know that, without water, no salt can be pro- 

 duced at ordinary temperatures, and that the constitution of the 

 salts is analogous to that of the hydrogen compounds which we 

 call acids. The principle of the theory of Davy, which must 

 be especially kept in sight in criticising the theory, is that he 

 makes the capacity of saturation of an acid dependent upon the 

 hydrogen or upon a part of the hydrogen which it contains ; so 

 that, if the other elements of the acid, collectively, are called the 

 radical, the composition of the radical does not possess the most 

 remote influence upon this capacity." 



These statements are recognised, on the whole, as correct 

 even at present. Together with what I have stated above con- 

 cerning the polybasic acids, they constitute the basis of our 

 views regarding acids. No doubt the characters which dis- 

 tinguish polybasic from monobasic acids were considerably 

 extended by Gerhardt and Laurent, so that the conceptions and 



23 Annalen. 26, 181. 

 L 



