254 ' REPoORT—1840. 
shown, that as soon as oxidation takes place at any point upon 
the iron surface, the protective power of the zinc is at once 
diminished, or rendered null. 
Of Iron in metallic contact with Zinc exposed to an indefinitely 
small depth of Sea Water. 
230. When several equal parallelopipeds of iron, having the 
following ratios of zinked surface to those of iron, viz. 
Surface of Zinc. Surface of Iron. 
a 4:00 3 evauas) geaeael 
b P00) iWon ae ; - abel 
c SOOT, ear. Fane : ae Tei es 
d a EO OS ey ee z 2 He | 
e Sy a I eet arg” Ag : alba ll ell 
af 2 OOo s | le : eee Oh a | 
g 0-13125) 2. : Aaa fe La dl | 
h 0°015625 F 4 Pade sak 4) | 
z 0O;00786 . . : ] 
are exposed to an indefinitely small depth of sea water, the iron 
remained bright and free from oxide, down to the ratio of 0°065 
of zinc to 1 of iron; but in all below this the iron suffered ox-- 
idation tubercularly. The oxide of zinc formed did not adhere 
to either the iron or the zinc, and was partly precipitated in a 
flocculent form, and partly dissolved in the sea water. Within 
the period of experiment the limit of protective power of zinc 
in metallic contact with iron, under the present condition, lies 
between nd and ;4,th of the surface of the latter. 
Of Iron in metallic contact with Zine, exposed to an in- 
definitely small depth of Fresh Water. 
231. When several parallelopipeds of iron having the follow- 
ing ratios of zinked gurfaces to those of iron, viz. 
Surface of Zinc. Surface of Iron. 
td a peepee perp: rie Ni 
| ap, EES he RREE Sa 6. 
Me eae se wists ols LUO 
LS CY FRE A TAO) 
“ENGELS SS IT SA, 25! are 
FR aa ies emo Lo) Riis se 
were exposed to an indefinitely small depth of fresh water, 
oxide of zinc was formed from the moment of exposure, on all, 
which formed hard mammillary concretions on the surface of the 
zine, and also in isolated centres on that of the iron; but from 
the ratio of equal surfaces of zinc to iron, up to the proportion 
ell peel eel oe Il ate 
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