310 On the Action of Nitric Acid upon Bismuth^ %c, 



contact the tin will be found to be inactive. This metal may 

 also be rendered inactive, by being made the positive pole of 

 a battery of a certain strength. It resists better the solvent 

 action of a current of higher intensity, and opposes a greater 

 obstacle to its passage than iron or bismuth. 



To the facts before stated in reference to copper, it may 

 now be added, that nitric acid of sp. gr. I '5 develops in this 

 metal also the peculiar state. In this state, it is slowly solu- 

 ble. When alone in acid of sp. gr. 1*47, there is at first vio- 

 lent action, then the copper acquires the peculiar state : when 

 connected with platina it acquires at once that state; and so 

 long as the contact with the platina is maintained, the copper 

 retains a bright lustre ; but when it is broken, the surface of 

 the metal becomes covered with a black film, which can only 

 be partially dissolved by the acid or by renewing the connec- 

 tion with the platina. 



A permanently peculiar state cannot be produced in zinc ; 

 but by connecting it with platina, or making it the positive 

 pole of a pile, its solution may be greatly retarded, so long 

 as the current continues to pass. 



In taking a general view of this subject, it is necessary to 

 distinguish the modification which the action of the acid un- 

 dergoes, while the metal forms part of a voltaic arrangement, 

 from the permanent modification which continues after its 

 connection with the battery has been destroyed. Having ex 

 tended my inquiries to other oxy-acids, I find that the che- 

 mical action of those acids in a concentrated state upon the 

 metals is diminished by voltaic associations. The effect of 

 galvanic combinations upon chemical action may be thus ge- 

 nerally stated. 



The contact of an electro-negative metal increases the ordi" 

 na7y action of an oxy^acid upon an electro-positive metal, if the 

 acid is so dilute that the latter becomes oxidized from the de- 

 composition of the water; and retards or ari-ests that action, if 

 the acid is so concentrated that the metal is oxidized from the 

 decomposition of the acid itself^. 



Thus, in the case of the sulphuric acid, if hydrogen gas is 

 disengaged at the surface of the platina, in a voltaic combina- 

 tion of zinc and platina, the ordinary solution of the zinc will 

 be greatly accelerated by contact with the platina ; but if sul- 

 phurous acid is set free at the platina surface, then the solu- 

 tion of the zinc will be, on the contrary, greatly retarded. 



* To the first part of this law an exception, which is perhaps rather ap- 

 parent than real, occurs in the action of some dilute acids upon iron 

 under certain circumstances ; to the second part, which I believe has not 

 been before observed, I have yet met with no exception. 



