ON THE ACTION OF AIR AND WATER UPON IRON. 255 
of thirty times the surface of zinc to that of iron, complete 
electro-chemical protective power could not be procured even 
for a few hours. With equal surfaces of zinc and iron the 
latter became red-rusty in twelve hours, in tubercular masses, 
the oxide formed having the composition (Fe O + Fe, O;) 
+ HO. 
232. From the foregoing series of experiments on the re- 
actions of fresh and sea water on iron and zinc, besides the 
- immediate facts obtained, we are in a condition to make some 
general deductions. It has been shown that the oxidizing 
_ effect of fresh water, holding one volume in eight of air and 
carbonic acid, is much greater than that of sea water, holding 
i one volume ms seventy of air and carbonic acid, on cast iron, 
_ wrought iron, and steel, in voltaic contact with zine, all other 
_ things being the same ; this arises from two circumstances—the 
: difficulty with which a saline solution absorbs air when once 
robbed of it, but still more from the fact, that the oxide of zinc 
f formed plays a very different part in sea water to what it does 
in fresh water. 
Mi, 233. In sea water the oxidation of the zinc produced oxide 
(Zn O), and at length a coat of suboxide, which forms a dis- 
tinct ort gray layer on the zine surface, and may be detached 
_ on bending the metal so as to obtain the suboxide in a state of 
complete insulation from admixed metal, but none of the first 
adheres to the metallic surface ; the oe of the oxide of zinc 
_ formed is either washed away in a pulverulent form, or is dis- 
_ ‘solved by decomposition of the sulphates, bromides, and chlo- 
t rides of the sea water, forming sulphate, bromide, and chloride 
_ of zinc, while the lime and magnesia form, with absorbed car- 
_ bonic acid, insoluble carbonates ; or if the iron be peroxidized, 
_ the oxide of zinc forms in part a saline double oxide with the 
: sesquioxide of iron; but the zinc surface is preserved clean 
and uniform to the last, either in the metallic state or as a sub- 
oxide, except when the reaction has been very slow and the 
electrical current very feeble ; in which case, after the lapse of 
a long period, crystals of calc spar form on both metals. 
934. Not so, however, in fresh water; here the oxide of 
Zine undissolved forms local concretions of oxide on the sur- 
face of the metal already covered with a coat of suboxide. 
‘Now the precise condition constituting a suboxide, as Berze- 
Tius has well remarked, is to be decomposed, under the play of 
very slight affinities, into protoxide and metal, as in the ana- 
logous cases of the suboxides of copper, Dianaith: arsenic, &c., 
at each local centre of deposition, then of protoxide of zinc, 
the suboxide is so decomposed in fresh water, attended with 
Pps 
