Mr. W. K. Bridgman on the Theory of the Voltaic Pile. 379 



of the former commencing to undergo solution ; and after having 

 been suffered to remain undisturbed for twenty days, it presented 

 the appearance exhibited in the diagram, fig. 1. Fig. 1. 



The portion A which had been immersed in 

 the acid was partially corroded into pits and 

 furrows, gradually decreasing in extent down- 

 wards. 



The upper end, B, exposed to the atmosphere 

 had become coated with a layer of minute and 

 beautiful crystals of sulphate of copper, extending 

 from the top down to within about three-six- 

 teenths of an inch of the liquid. 



At the intermediate portion, C, a greater 

 amount of chemical action had been induced — cor- 

 roding the wire, as represented, about halfway 

 through and forming a neck tapering upwards. 



The solution containing the end A was only 

 slightly tinged in proportion to the amount of 

 copper dissolved, the crystallization having been 

 derived almost wholly from the metal above the 

 surface of the liquid. 



" It is at present generally admitted that, in the normal con- 

 dition, the atmosphere is charged with positive electricity .... 

 The terrestrial globe, on the contrary, is charged with negative 

 electricity, as is proved by a variety of observations, direct and 

 indirect ; it is, moreover, a consequence of the presence of posi- 

 tive electricity in the atmosphere ; for one of the electricities 

 cannot manifest itself in the free state without the appearance of 

 an equal quantity of the other kind "*. 



It is a fair inference to assume that it is in obedience to this 

 law that the exposed portion of the metal has been rendered 

 electro-negative, as its behaviour indicates it to be, while that 

 submitted to the acid has taken the opposite or electro-positive 

 state. 



That the action which arises between the metal and the acid 

 is due to polarization is evidenced by the following proceeding. 



A piece of copper wire wholly submerged in the acid so as to 

 entirely exclude any portion of it from coming into contact with 

 the air, has remained for many months without imparting the 

 slightest tinge to the liquid. Another portion having a piece of 

 platinum-foil connected with it has been attended with similar 

 results. A piece of ^^amalgamated zinc-foil has also been kept 

 in dilute acetic acid in the same way with equal effect. 



But on suffering the liquid to evaporate so as to bring the 



* Phil. Mag. S.4. vol. xxxiv. p. 322, " Note on the Electrical Condi- 

 tion of the Terrestrial Globe," bv A. De la Rive. 



' 2C2 



