SOLID ELEMENTS OF THE VOLTAIC CIRCLE. 219 



and therefore wholly inactive in an acid fluid. A negative metal 

 may thus be protected from the solvent action of saline and 

 acid liquids, by association with a more positive metal ; iron, for 

 instance, by zinc, as in articles of galvanized iron, which are 

 coated with the former metal ; and copper by either zinc or 

 iron, as was remarkably illustrated in the attempt made by Sir 

 H. Davy to defend the copper sheathing of ships from corrosion 

 in sea-water, by means of his protectors. These were small 

 masses of iron or zinc fixed upon the ship's copper, at different 

 points under the water line. They completely answered the 

 purpose of protecting the copper, but unfortunately gave rise to 

 a deposition of earthy matter upon that metal to which barna- 

 cles and sea- weeds attached themselves, and thereby diminished 

 the facility of the ship's motion through the water. 



A weak galvanic circle may even be formed of a single positive 

 metal in an acid, as the zinc A B (Fig. 20), provided the 

 ,-, surfaces of the metal exposed to the acid at 



A and B are in different conditions as to 

 purity or mechanical structure, and there- 

 fore unequally acted upon by the acid; 

 whereupon the part least disposed to dis- 

 solve becomes negative to the other. A zinc 

 plate may also be unequally acted on and 

 thrown into a polar state, from the liquid in 

 which it is immersed varying in composition 

 and activity, at different points of the me- 

 tallic surface. A circle may thus be formed of one metal A B 

 with two liquids A E and E B, which merge into each other, 

 and form together one polar element A B. 



The two metals in a circle have generally been exhibited in 

 metallic contact, and forming together one polar element, but 

 they may be separated, as are the zinc and copper plates A D 

 and C B in the diagram (Fig. 21), by two polarizable fluids, 

 p IG 21 provided these fluids are such as a strong 



acid at A B, and as iodide of potassium 

 at D C, the first of which acts very pow- 

 erfully on zinc, while the other acts very 

 f ee bty u P on t^at meta l (unless associated 

 with copper) ; so that of the consequent 

 _____ __ opposing inductions, that originating at A 



greatly exceeds and overpowers thatfromD. 



