156 Dr. Andrews on the Properties of Voltaic Circles. 



of hydrogen gas, on immersion in the cold acid, occurred only with the metals, 

 zinc, iron, and tin. The phenomena exhibited in this case by the platina and 

 zinc couple afford a remarkable example of the cessation of a chemical action 

 from the surface of the zinc acquiring a peculiar or polarized state. For although 

 a large mass of bubbles appeared at first, yet by heating gently the liquid these 

 not only were separated, but not the least trace of gas afterwards appeared at 

 either surface. There is a striking analogy in this, to the action of nitric acid of 

 certain strengths upon some of the metals. 



In the cases of iron and zinc, the disengagement of gas was wholly trans- 

 ferred from the surface of these metals to that of the platina in connexion with 

 them ; in those of bismuth, antimony, and tin, there was slight extrication of 

 gas at the surface of the dissolving metal ; in that of silver, the quantity of gas 

 was nearly the same from the platina and silver ; while in the cases of arsenic 

 and mercury, scarcely any gas was given off from the platina, the action of the 

 acid on the two latter metals not being perceptibly diminished by contact with 

 platina. 



If these results are compared with those which have been already obtained 

 with nitric acid, it will be evident that in the case of the concentrated acids the 

 formation of a voltaic circle has in general the tendency to diminish chemical 

 action. The following law will be found to be generally, although not univer- 

 sally true, the exceptions to it being probably, however, rather apparent than 

 real. 



The ordinary chemical action of an oxy-acid upon the metals soluble in it, 

 is, in general, diminished tvhen the acid is concentrated, by voltaically asso- 

 ciating them with certain electro-positive metals ; but on the contrary, is 

 increased when the acid is dilute. 



In the preceding experiments the diminution of chemical action occurred in 

 cases in which the acid itself suffered decomposition ; while in common voltaic 

 circles, where it is well known that the action is increased, the elements of water 

 alone are eliminated ; but how far this coincidence may be universal, must be 

 determined by future investigations. 



