ON THE ACTION OF AIR AND WATER UPON IRON. 251 
the metals in mammillary concretions. The oxide of iron formed 
has the composition (Fe 0+ Fe, O,)+ (Fe 0+C O,)+HO. 
219. When wrought tron is exposed under the same circum- 
_ Stances as above, but to sea water, the same phenomena as in 
_ the last case present themselves, but much more slowly, much 
_ of the oxide of zinc being dissolved in the sea water. 
: Of Cast Steel exposed in simple contact with Zinc, immersed 
; in Fresh Water. 
| 220. When cast steel, free from initial rust, is exposed in 
simple contact with an equal surface of zinc, the general surface 
_ of the metal remains bright, but tubercular oxidation gradually 
Sh place at the points of junction of the steel and zinc ; the 
tter oxidizes less as this proceeds, and finally ceases to protect 
the steel at all. Carbon is transferred from the steel to the 
surface of the zinc. The oxide formed has the composition 
(FeO+F,0,)+H O. 
_ Of Cast Steel in simple contact with Zinc immersed in Sea 
Water. 
__ 221. In this case, the same phenomena take place as in the 
last, but much more slowly. 
definitely small depth of Fresh Water. 
_ 222. Cast steel, free from initial rust, thus exposed in sim- 
ple contact with an equal surface of zinc, soon begins to rust in 
regular patches. ‘The zinc also oxidizes, and the oxide forms 
Concretions at the points of contact ; after which the steel ox- 
es still faster, so that in equal time it loses rather more by 
oxidation than an equal surface of cast steel, exposed as above, 
simple contact with an equal surface of zinc to an indefinitely 
mall depth of sea water, the same phenomena, as in the last 
present themselves, but much more slowly. 
0) Wrought Iron in metallic contact with Zinc, or Zinked 
Iron.— Of Zinked [ron immersed in Sea Water. 
224. A plate of zinked iron was immersed for twenty-five 
iths in sea water; its whole surface was zinked. On ex- 
tion, the surface was covered with a hard black coat of 
sub-oxide, over which was a thin coating of crystalline car- 
Donate of lime, but no symptoms of oxide of iron were to be 
