ON THE ACTION OF AIR AND WATER UPON IRON, 253 
Surface of Zinc. Surface of Iron. 
UA OOYM hes or : sth 
Met tha, or EO OS eet -Fatsy 5 anit 
Bile Ay gm cratic tap 010 Jed are Re : Baiiaph tare 
SUPER SAAN eam “lat : Siena: howe 
=the 0 DS ea be . ere eaers 
eS Watery te A OROGS, te een’ ae ts 
St ee SOROUST 2S Ser. = Py desig 
eee SOONG Zhe ; SR 
Sue. OOO786 0%, : hh ig sk 
1 4 ona OS = & 
ell ee El ee 
the zinc in all is corroded, and the amount of oxide formed in 
equal times, is proportionate to the surface of zinc ; the other, 
or e—, elements being equal. The oxide of zinc is deposited on 
the zinc pole in mammillary concretions. The iron remains bright 
and free from oxide, with every proportion of zinc, down to the 
ratio of 0°103125 of zinc :: 1 of iron; with this and below it, 
tubercular oxidation takes place on the iron surface to an extent 
in equal times proportional to some unascertained function of 
the surface of zinc. 
228. Hence it is proved that at the depth of immersion of all 
these experiments, viz. twelve inches, the limit of protective 
power of zinc in metallic contact with iron in fresh water lies 
"between =1,th and #,nd of the surface of iron; or that it re- 
quires about four times the amount of zinc surface to put in 
motion the same quantity of electricity, and thus to protect 
“wrought i iron in fresh water by its aid that will effect this result 
‘in sea water. But after the lapse of a considerable period in fresh 
water, all the other parallelopipeds began to show signs of rust 
or of tarnish in the inverse order of their respective surfaces of 
‘zinc ; hence time alone seems requisite in fresh water to cause 
the protective power of any amount of surface of zine for iron 
‘to cease, which is confirmed by the following fact. 
_ 229. A plate of iron, whose entire surface was covered 
‘with zinc in metallic contact, was immersed for twenty-five 
months in fresh water. On examination, much flocculent oxide 
of zinc had been formed, and lay in the bottom of the glass 
yessel, which was in some places stained with red oxide of iron. 
The zinc surface was found in irregularly scattered patches, 
wholly removed down to the iron, which was covered with per- 
oxide. Hence about two years appear to be the limit of pre- 
Servative power of zinc to iron in fresh water, applied in fusion 
over its wholesurface by the ordinary method. Itis to be noticed, 
that the zinc surface was removed by solution, unequally or in 
patches, indicating local action ab initio; and it has been before 
