SIMPLE VOLTAIC CIRCLE. 209 



face of zinc only to the acid. If the mercury were exposed to 

 the acid, that metal would completely derange the result, acting 

 locally like a copper plate, as will afterwards be explained. 

 The previous combination of the zinc (with mercury,) likewise 

 prevents that metal from yielding easily to the chlorine of hy- 

 drochloric acid ; and the zinc of the amalgam is, therefore, not 

 dissolved, till the affinities are enhanced by the introduction of 

 a copper plate into the acid, and the formation of a voltaic 

 circle. 



It would thus appear that zinc, associated with copper, dis- 

 solves more readily in the acid than when alone, because the 

 attraction or affinity of the zinc, for chlorine is increased by the 

 completion of a circle of similarly polarized particles, in the 

 same manner as the magnetic intensity at one of the poles of 

 a magnet is increased on completing the circle of similarly po- 

 larized molecules, by connecting that pole by means of soft 

 iron with the other pole (Fig. 6., page 203). 



Although the terms of the electrical hypothesis are at present 

 avoided, still it will be convenient to denominate the zinc, 

 being the metal which dissolves in the acid, the active or 

 positive metal, and the copper, which does not dissolve, the 

 inactive or negative metal of the voltaic circle. 



Looking to the condition of the two connected metals in 

 the acid, it will be observed that the surface of the zinc pre- 

 sented to the acid has zincous affinity, or is zinco-polar, 

 but the surface of the copper presented to the acid has, on 

 the contrary, chlorous affinity, or is chloro-polar. Such a 

 condition of the copper is necessary to the propagation of the 

 induction ; and the advantage of copper or platinum as the ne- 

 gative metal in a voltaic arrangement depends upon there being 

 little or no impediment to either of these metals assuming the 

 chlorous condition, that can arise from the peculiar affinity of 

 the metals named for the chlorine of the acid ; an affinity which 

 tends to cause them to be superficially zincous instead of chlo- 

 rous. If the second metal were zinc, the surface of it would 

 be disposed to dissolve in the acid, and becoming on that 

 account zincous, would induce a polarization in the intermediate 

 acid, in an opposite sense, from that induced by the first plate 

 of zinc ; which counter polarizing actions would mutually neu- 

 tralize each other. The acid between the two zinc plates would 



