Dr. Hall on the Agency of Carbonic Acid. 263 



were noticed by M.Thenard in the eleventh volume of the ' An- 

 nates de Chimie et Physique ;' at least that paper is particu- 

 larly quoted, and its conclusions adopted, by M. Thenard in the 

 later editions of the ^ Trait6 de Chimie'.* The accuracy of 

 my experiments, and the justness of my conclusions from 

 them, are admitted ; whilst the opposite result obtained by 

 M. Guibourt, is attempted to be explained, not upon the 

 principle of a greater relative quantity of the metal compared 

 with that of the water, but by the agency of galvanism, supposed 

 to be excited by a circle formed by the iron, a portion of oxide, 

 and the water. 



Such was the state of the question, when I undertook a 

 fresh series of experiments upon this subject, two or three years 

 ago. These experiments appear to me to have finally removed 

 the obscurity in which this action of iron and water was in- 

 volved, to have confirmed my former conclusions, and to have 

 brought to light some new and interesting facts. 



The first of these facts is, that in every case in which water 

 is decomposed by the contact of iron, there is necessarily the 

 superadded agency of carbonic acid ; the second fact, which is 

 a consequence of the former one, is, that in every such instance 

 of the decomposition of water, the process is immediately 

 arrested, by the addition of some substance which shall 

 effectually absorb and remove that acid. These facts bear 

 upon two chemical questions : the first, the analogy which 

 exists between the diluted carbonic acid and some other diluted 

 acids, in their chemical action upon iron ; the second, the 

 question of the action of water and iron in ordinary circum- 

 stances, already noticed. I purpose, therefore, to divide this 

 Memoir into two sections, relating to these points respectively. 



The first object of the present Memoir, is to show, by the 

 detail of a series of experiments, that the carbonic acid accords 

 with that general law which is known to obtain in regard to 

 the sulphuric, the muriatic, and some others, in determining 

 the decomposition of water which is placed in contact with iron. 



In order to resolve this interesting question, it was neces- 

 sary, in the first place, to ascertain whether iron in con- 

 tact with pure water induces any decomposition of that fluid. 

 * See ed. 4e, torn. ii. p. 19. 



