250 REPORT—1840. 
Yet, after the lapse of a considerable time, the whole surface 
of the zinc becomes covered with a thin, black, hard crust of 
sub-oxide, on which are deposited minute crystals of cale spar, 
produced by decomposition of the salts of lime in the sea water. 
When this has taken place, the protective powers of the zinc are 
greatly diminished, or nearly destroyed. 
Of Cast. Iron in simple contact with Zine at an indefinitely 
small depth in Fresh Water. 
214. Cast iron, free from initial rust, so exposed in contact 
with an equal surface of zinc, is oxidized from the first moment 
of exposure. The zinc is oxidized also, and the oxide forms 
concretions at the point of contact of the metals, and increases 
the oxidation of both metals ; so that of two equal surfaces of 
cast iron, exposed during equal times to an indefinitely small 
depth of fresh water, the one alone, and the other in simple con- 
tact with an equal surface of zinc, the latter will lose the greater 
amount by oxidation. 
215. When cast iron, free from initial rust, is exposed to an 
indefinitely small depth of sea water, in simple contact with 
an equal surface of zinc, its oxidation is retarded, but not pre- 
vented, and after a time takes place, as in the last case. 
Of Wrought Iron in simple contact with Zine immersed in 
Fresh Water. 
216. Wrought iron, free from initial rust, exposed in contact 
with an equal surface of zinc, is preserved from oxidation until 
a large amount of oxide of zinc has concreted at the point of 
junction of the metals, when the iron gradually begins to form 
tubercular points of oxide on its upper side. The oxide has the 
composition (Fe O0+Fe, O;)+H O. Carbon is deposited in 
microscopic crystals on the zine. 
217. Wrought iron, under the same circumstances as above, 
but immersed in sea water, is preserved for atime. But although 
the oxide of zinc deposits on the iron with greater difficulty in 
sea than in fresh water, yet it does so at length, along with cry. 
stals of cale spar; after which the protection of the zinc becomes 
uncertain, and is disturbed by the contact of any neutral solid. | 
Of Wrought Iron exposed in simple contact with Zinc at an 
indefinitely small depth in Fresh Water. 
218. When wrought iron, free from initial rust, is exposed 
thus, in simple contact with an equal surface of zinc, oxidation 
commences at once, and proceeds rapidly. The zinc is oxidized 
also, and the oxide of zinc adheres to the points of contact of 
