Conditions of Chemical Change. 181 



myself"* for the last three, that these several metals do not 

 dissolve in nitric acid, but if the reduction-product of nitr 

 acid, be it nitrous acid or nitrogen peroxide, is present, then 

 these metals dissolve. To put the facts in a logical form : — 

 (i.) These metals do not dissolve in nitric acid ; (ii.) 

 these metals do dissolve in nitric acid containing nitrous 

 acid f. Unfortunately at present there seems to be 

 no means of proving the third term, namely, that these 

 metals do not dissolve in nitrous acid, though it is clearly 

 demonstrated that the amount of metal dissolved varies 

 not with the amount of nitric acid within certain limits, but 

 with the amount of nitrous acid. It must therefore be con- 

 cluded that the presence of the nitrous acid is the cause, or a 

 necessary part of the cause, of the phenomenon of the solution; 

 and it would follow that, whatever be the correct interpretation 

 of the chemical change, nitric acid is not the actual solvent 

 of the metal. Indeed my own experiments have shown that, 

 within limits, the presence of an excess of nitric acid not only 

 does not increase, but actually retards the solution of the three 

 metals copper, mercury, and bismuth. But Armstrong con- 

 ceives that " the hydrogen of the acid is virtually directly 

 displaced by the metal with the assistance, however, of the 

 current-energy derived from its own oxidation by the nitrous 

 compound" J. There are, therefore, at least two interpreta- 

 tions of the problem : First, it is supposed that the hydrogen 

 of the acid is displaced, and this in the nascent condition, or 

 somehow or another, reduces the nitric to nitrous acid, thus:— ; 



2HN0 3 + M = M(N0 3 ) 2 + H 2 , 

 HN0 3 + H 2 = HN0 2 + H 2 : 



or 2HN0 3 + H 3 = N 2 4 + 2H 2 0. 



Secondly, it may be supposed that the nitrous acid or nitrogen 

 peroxide is formed from the nitric acid per se } and then the 

 metal can be dissolved thus : — 



(i.) HN0 3 = HN0 2 + 0, 



(ii.) 2HN0 2 + M = M (N0 2 ) 2 + 2H 2 + 2NO ; 



(i.) 2HN0 3 =N 2 4 + H 2 + 0, 

 (ii.) M + N 2 4 = M(N0 2 ) 2 . 



* Phil. Trans. 1891 [A], pp. 279-317. 



t I use the term " nitrous acid " rather for the sake of brevity, and 

 without wishing to exclude the possibility that the active material 

 is really nitrogen peroxide. 



X Armstrong, Proc. Ohem. Soc. 1892 ("On the Nature of Depolarisers "j 



Phil. Mag. S. 5. Vol. 37. No, 225. Feb. 1894 



