573 
It is quite possible that some nitrous acid is produced by the direct 
reduction of nitric acid by ferrous ions. The formation of nitrous 
acid either by the direct reduction of nitric acid by ferrous salts 
or by the indirect reduction through the intervention of nitric oxide 
is proved by the following experiment. If nitric acid of the strength 
used in this research be taken in a test tube and a crystal of ferrous 
ammonium sulphate or ferrous sulphate be added to it, almost 
immediately the crystal is covered with the deep brown FeNO°° ion 
and a little nitric oxide also escapes. If urea crystals are now added, 
they are immediately oxidized with the evolution of carbon dioxide 
and nitrogen, indicating the presence of nitrous acid. So in presence 
of ferrous salts, nitrous acid, which is the active substance in the 
action of nitric acid on copper, is formed when we have an excess 
of nitrie acid. This explains the accelerating influence of ferrous salts 
in the complete dissolution of copper in 20°/, nitric acid. 
As a matter of fact, the accelerating effect of ferrous salts is slightly 
greater than the accelerating effect of sodium nitrite on the dissolution 
of copper in nitric acid. The greater the concentration of the ferrous 
salt, the greater is the acceleration. 
Ferric sulphate, ferric nitrate and ferric chloride exert a marked 
accelerating effect, though their activity is slightly less than that of 
sodium nitrite and the accelerating effect is proportional to the 
concentration of the ferrie salt. It would appear that the acid nucleus 
in this ease also, plays no part. The explanation of this activation 
seems to lie in the reduction of ferric salts by the nitric oxide which 
is a product of the chemical change between nitric acid and copper. 
The ferrous salt, which may thus be formed, will reduce a part of 
the nitrie acid into nitrous acid, which activates the action of nitric 
acid on copper. It seems plausible that a part of the ferric salt would 
be reduced to the ferrous state by the metallic copper. It is well 
known that when a solution of a ferric salt is shaken with metallic 
copper, the ferric salt is partly reduced to the ferrous state and the 
copper is oxidized to the cupric salt and an equilibrium is set up: — 
2 FeCl, + Cu Z 2 FeCl, + CuCl, 
The ferrous salt thus formed reduces the nitric acid to nitrous 
acid, which accelerates the action of nitric acid on copper. 
In a similar way the accelerating effect of arsenious oxide, strych- 
nine sulphate, phthalic anhydride ete. may be explained on the 
basis of the formation of nitrous acid by the action of these reducing 
agents on the nitric acid. 
The retarding effect of the oxidizing agents like, H,O,, KMuO,, 
38 
Proceedings Royal Acad. Amsterdam Vol. XXIL. 
