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and Milpliurif acids ari' cninixxinds (if siiipliui- witli oxygt'ii, and that "'fixed ;iir" 

 is also formed \>y tlie coinhiistion of candles and of otiier oryanic matter. These 

 experiments led him to not only clear and correct views of tlie phenomena of 

 comhnstion and oxidation, bnt they also gave rise to a radically new coiK'eption 

 of the nature of acids and salts. The ojjinions whicii he develo|)ed were after- 

 Avards found to he imperfect, but they were a very gi'eat advance on anythin<>- which 

 had preceded, and were of incalculable value in the ilevelopn)ent of chemical 

 science. After linding from his own experiments and those of others that oxygen 

 is a common constituent of carbonic, phosphoric, sulphurous, sulphuric and 

 nitric acids he made the generalization that oxygen is the source of all acid 

 properties, and called it by its jiresent name, which means "acid former." To 

 Jiim an acid was simply a comjjound of carbon, sulphur, or some other element 

 with oxygen, and a salt was a compound of such an oxide with an oxide of a 

 metal. This view held practical sway in chemistry foi- sixty years, and is at the 

 basis of many expressions which chemists still use. While doubtless less perfect 

 than the view which considers acids as compounds of hydrogen, it nevertheless 

 •expressed clearly some truths wliich iiur modern chemistry does not (juite so 

 (dearlv express, for oxygen is still, as always, a great acid-forming principle, ami 

 salts contain metals as well as non-metals in an oxidized form. I^avoisier con- 

 sidered that the comldnation of a metal with an acid may take place in two ways. 

 Either the metal combines with a part of the oxygen of its acid forming an oxide 

 which then combines with the acid, or as we should say, with the aidiydrideof the 

 acid ; or the metal, by tiie aid of the acid, decomposes the water present, com- 

 bining with its oxygen and liberating its hydrogen, and the oxide formed there 

 combines with the acid. The first view may still be considered as essentially cor- 

 rect as an explanation of such cases as the action of concentrated sulphurit- acid 

 on copper; here coppei- oxide is undoubtedly formed, for some of it escapes condji- 

 nation with the acid, and suli)hur tri-oxide is present as an independent compound 

 at the temperature of the reaction, and very proliably t'oinbines with the co})])er 

 oxide as it is formed, to produce 'copper sulphate. As regards the second view, 

 which applies to such cases as the solution of zinc in dilute sulphuric acid, there 

 is still some diversity of opinion and some uncertainty in the minds of chemists. 

 The common statement of our text-books is that the action consists in a substitu- 

 .tii>n of the metal for the hydrogen of the acid, and this is undoubtedly correct, 

 as' a superficial view of the matter. The explanation whicii has been more re- 

 centlv jn-oposed, however, and which has already gained many adherents, is that 

 direct substitution takes place in such cases with very great difficulty, if at all, 

 and that action takes place readily only when the acid has been dissociated into 



