58 Proceedings of Royal Society of Edinburgh. [sess. 
concentration of H* ions. We can thus diminish this concentration 
below the limiting value above referred to, and thus allow of the 
precipitation of the whole of the zinc as ZnS. 
Either of these explanations is sufficient to account for the 
phenomena. But we can arrange a crucial experiment to decide 
between the two theories. To a solution of ferrous acetate add 
enough acetic acid to prevent the precipitation of FeS on addition 
of sulphuretted hydrogen ; add sulphuretted hydrogen to this acid 
solution of ferrous acetate, of course no black precipitate appears ; 
now add solution of sodium acetate, and FeS is thrown down. As 
there is here no acid present but acetic acid, the first theory offers 
no explanation of the phenomena, but the second does. We can 
add to ferrous acetate enough acetic acid to have the concentration 
of H’ ions above the limiting value for FeS (which is much below 
that for ZnS), and so prevent the formation of FeS ; and we can 
add to this solution containing ferrous acetate, acetic acid, and 
sulphuretted hydrogen, enough sodium acetate to diminish the con- 
centration of the H' ions below the limiting value, and so allow of 
the formation of FeS. 
