26a Dr. Marshall Hall on the 



of galvanic agency''^. It is plain, from the whole of the ac- 

 count given of this subject by M. Thenard, who appears to 

 have considered it with far more attention than any other wri- 

 ter on chemistry, that it remained involved in the utmost 

 obscurity. Many circumstances had led to the opinion, that 

 water is altogether indecomposable by iron ; others appeared to 

 denote that this is not absolutely the case, but that, under pe- 

 culiar circumstances, a decomposition of the water is effected 

 by the contact of this metal. Not only light and the galvanic 

 agency, but, as I have already stated, the relative quantities of 

 the metal and of the fluid have been supposed to influence the 

 result. No one appears to have suspected the necessity for the 

 superadded agency of the carbonic acid. 



That this phenomenon is not dependent upon the agency of 

 light, is proved by the fact of its being totally prevented by the 

 addition of a small quantity of lime water, or of calcined mag- 

 nesia. And that it is not an effect of galvanism, is proved by 

 its being prevented or immediately arrested by the same means. 

 It is equally certain, from the same facts, that the relatively 

 large quantity of the metal has no influence upon the result: 

 this is further quite obvious, from the fact that the water is 

 decomposed, however small the quantity of the iron, if the 

 agency of carbonic acid be conjoined with that of this metal. 



The decomposition of water, by the contact of iron, has in- 

 deed, in every instance, depended upon the concealed agency 

 of carbonic acid contained in the water, or united to a portion 

 of the oxide of the metal. This decomposition has been ef- 

 fected more slowly or more rapidly, according as the quantity 

 of the acid has been smaller or greater. There is a most 

 marked difference in the rapidity with which the water is de- 

 composed, between a portion of distilled water which has been 

 boiled for a short time, and another portion which has been 

 simply charged with air expired from the lungs, into each of 

 which precisely the same quantity of iron has been put. And 

 in every case the decomposition of the water has been pre- 

 vented or arrested by withdrawing the influence of the carbonic 

 acid altogether. 



* Trait^ de Chimiej torn. 1* p. 336^ notej and particularly torn, 2. pp, 17 — 19* J 



