574 
H,Cr,0,, KCIO,, (NH,),5,O, ete. and of the reducing agents likeurea, 
sodium sulphite ete. is certainly due to the destruction of the nitrous 
acid as soon as it is formed. When the experiment was performed 
in such a condition so as to cover the copper wire with solid urea, 
the reaction became very slow, but it did not stop altogether. 
It is very difficult to explain the difference in the behaviour of 
the nitrates on the oxidation of copper by nitric acid. Lithium nitrate 
is an accelerator, whilst sodium and potassium nitrates are retarders; 
from analogy 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. chem. Soc. 1911, 99, 1035) have found 
that the accelerating or retarding effects of the nitrates of K, Rb, 
Cs were functions of the temperature and of the concentration of 
the acid. 
Hienry (Amer. chem. Jour. 17, 18 (1895)) has shown that both 
NO, and N,O, are the products of the reaction between copper and 
nitric acid. Evidently in the solution, we should consider the following 
equilibria : 
SHINO} == NO USEINO SoHE Ont eee ee een 
NO; HO HNO“ ENO, ee ENE) 
SNOR EKO =O NOM INOl. = ane) 
Lewis and Epear (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 autocataly tic. 
As for example, the actions of nitric acid on metals like Copper, 
Silver, Bismuth, Mercury ete., on starch, on sugar, on arsenious 
oxide, on hydrogen iodide (Ecksräpr, Zeit. anorg. Chem. 1901, 29, 
51), on nitric oxide (Lewis and Epaar, 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 
