380 Mr. W. K. Bridgman on the Theory of the Voltaic Pile. 



upper end of the metal near to its surface, the instant the slight- 

 est portion becomes exposed chemical action immediately begins. 



The first perceptible indication of this polarization is in the 

 partial dewing of the copper immediately above the surface of 

 the liquid. This gradually increases in extent until the whole 

 exposed portion becomes wet with the solution, after which mi- 

 nute crystals soon make their appearance and in time cover the 

 exposed part, as shown in fig. 1 . The determination of fluids to the 

 negative portion causes the acid to rise and spread itself over the 

 surface of the metal ; and this, becoming saturated in its ascent, 

 furnishes the material from which the crystallization is derived. 



Two equal portions of wire were similarly placed in acid, only 

 that one was fully exposed to the atmosphere in an open tube, 

 while the other was placed in a phial, the acid occupying half 

 its height, and was kept closely corked for several weeks — after 

 which the fully exposed metal had lost in weight two-fifths more 

 than the one which had been excluded from contact with fresh 

 portions of air, showing that contact with the atmosphere in bulk 

 is necessary to the fullest action. 



A piece of copper wire 3 inches long was immersed one-third 

 in dilute acetic acid and exposed to the atmosphere in an open 

 tube. In a very short time a dull coating of amorphous acetate 

 of copper had been formed on the surface as far as the vapour of 

 the acid had reached ; but by degrees this dry formation became 

 moistened, and as this occurred it was at once converted into 

 minute and beautiful dark-green crystals. 



In each of these instances it is thus indisputably shown that, 

 in the position in which the plates of the battery are placed (that is, 

 one portion immersed in the exciting liquid and the other exposed 

 to the air), chemical action does invariably occur, and is in fact an 

 inevitable consequence of such partial immersion; and taking place 

 where there is no sufficient normal affinity existing between the 

 metal and the acid to effect the decompo- 

 sition of water, but arising from the metal 

 being first polarized by the atmosphere, 

 there is hence an additional element in- 

 troduced that assumes a very significant 

 character when applied to the composition 

 of the battery. 



Let A B, fig. 2, represent the zinc ele- 

 ment of the battery immersed half its 

 length in the acid. The condition it im- 

 mediately assumes will correspond to that 

 shown in fig. 1 — that is, the upper end 

 negative, and the immersed end positive. 



It will now appear that there are two 



-N 



