248 REPORT—1840. 
All the experiments were carried on with water and air at 
about 62° Fahr. The cast iron is that of « 77, (Table No. I.), 
or hard gray, mixed, Welsh and Scotch iron. 
The wrought iron, No. 2, Welsh bar, and the steel () cast 
steel, of the Mersey Steel Company’s make. The depth of im- 
mersion in all the submerged experiments was uniformly twelve 
inches, in glass vessels. The zinc used was nearly pure. The 
volume of water employed in each experiment was fifty cubic 
inches. 
Of Cast Iron in simple contact with Zinc immersed in Fresh 
Water. 
207. If cast iron be perfectly free from any initial stains of 
rust, and quite homogeneous in texture, it is electro-chemically 
preserved by the contact of an equal surface of pure zinc, for an 
indefinite period, during which the zinc is oxidated, the oxide of 
zincistransferred to the surface of the iron, and forms mammillary 
concretions on it; after which the protective power of the zinc 
is greatly diminished, and at this stage the contact of any sub- 
stance, even a neutral one,—such as glass with the iron,—is 
sufficient to originate oxidation upon it, which, once established, 
gradually extends, without the zinc having power to arrest it. 
The oxide of iron produced has the composition (Fe O + Fe, Og) 
+HO*. 
208. If cast iron, having a polished surface, be suffered to 
contract any coating of rust, although the surface be afterwards 
perfectly polished to the eye, yet zinc, in simple contact, has 
lost nearly the whole of its power of protection ; the zinc and 
iron both oxidize from the moment of immersion. If the surface 
be removed by the file to some depth, however, the remaining 
metal is preserved. 
209. If wrought iron has been exposed to solvent action in 
contact with a powerfully electro-negative metal, as copper or 
mercury, for a considerable time, and its surface be then removed, 
even to the depth of 5th of an inch, or more, with the file, and 
immersed in contact with zinc, the latter is found to have lost 
nearly all protective power with respect toit. Cast iron so cir- 
cumstanced corrodes from the first moment, and the oxide is de- 
posited in tubercles. 
210. On the other hand, if wrought iron, a portion of whose 
* The formule used for these oxides have respect merely to composition, and 
not to proportion, which varies with the duration of exposure. 
