549 
KOIO3, (NHJjSjOg etc. and of the i'ediicing ageuts like urea, 
sodiiim sulphite etc. is certainly due to the destriiction ofthenitrous 
acid as soon as it is foraied. Wlien tlie experiment was performed 
iji such a condition so as to cover the eopper wire witli solid urea, 
the reaction became very slow, but it did not stop altogether. 
It is very ditïicult to explain the diflference in the behaviour of 
the nitrates on the oxidation of eopper by nitric acid. Lithium nitrate 
is an accelerator, whilst sodium and potassium nitrates are retarders; 
from anology we should expect calcium nitrate to have an accele- 
rating effect, but as a matter of fact, both calcium and strontium 
nitrates are retarders, whilst barium nitrate is an accelerator. 
Rennie and Cook (Trans. ehem. Soc. 1911, 99 , 1035) have found 
that the accelerating or relarding effects of the nitrates of K, Rb, 
Cs were functions of the temperature and of the concentration of 
the acid. 
Higley (Amer. chem. Jour. 17, 18 (1895)) has shown that both 
NO.J and NjO, are the products of the reaction between eopper and 
nitric acid. Evidently in the solution, we should consider the following 
equilibria ; 
3 HNO, 2 NO + KNO, + H,0 .... ( 1 ) 
N,0, + H.O^HNO, + HNO, (2) 
3 NO, 4- H,0 ^ 2 HNO 3 + NO (3) 
Lewis and Edgah (J. Amer. chem. Soc. 1911, 33, 292) have shown 
that in equilibrium ( 1 ) there is a change in the equilibrium constant 
with the concentration of nitric acid. It seems probable that nitrates 
may affect one or more of these equilibria and change the concen- 
tration of nitrous acid, which being the activating agent. 
In this connection, it is interesting to observe that several reac- 
tions, in which nitric acid is the oxidizing agent, are autocatalytic. 
As for example, the actions of nitric acid on metals like Copper, 
Silver, Bismuth, Mercury etc., on starch, on sngar, on arsenious 
oxide, on hydrogen iodide (Eckstadt, Zeit. anorg. Chem. 1901, 29 , 
51), on nitric oxide (Lewis and Edgar, loc. cit.) etc. become more 
pronounced as the Chemical change proceeds. 
The explanation is not far to seek. The nitrous acid is the active 
substance and its concentration and hence the reaction velocity 
increase with the progress of the Chemical change. In all these cases 
I have found that the Chemical change becomes more rapid when 
a nitrite is added at the commencement of the reaction. 
It has been observed that the Chemical change between nitric acid 
and copper may be practically stopped by agitating vigorously the 
