376 ON THE UNION OE AMMONIA NITRATE WITH AMMONIA. 
When the positive electrode is formed of platinum or iron, it is unacted upon during 
the electrolysis ; but when it is formed of silver , mercury, lead, copper, zinc, or mag- 
nesium, it is corroded, and no nitrogen is evolved. The silver, lead, and copper elec- 
trodes readily dissolve as nitrates, the copper one soon becoming coated with an azure- 
blue deposit. Mercury dissolves only slightly, but its surface becomes covered with a 
grey deposit. Zinc and magnesium dissolve as nitrates, but somewhat more slowly 
than the other metals mentioned, a fact probably to be accounted for by their tendency 
to direct or local action upon the nitrate. 
