ON THE ACTION OF AIR AND WATER UPON IRON. 231 
absorbed gas, traces of hydrogen can be detected: at a subsequent 
stage, and especially if the mass and surface of iron be great in 
proportion to the volume of water, hydrogen and nitrogen are 
both evolved, and the white, greenish or black oxides before 
produced, become red. When this has arrived, the presence of 
air is no longer essential to carry on the decomposition of the 
water, the sesquioxide of iron, which acts as an acid to its own 
base, supplying its place. At a still later stage, ammonia is very 
frequently formed, especially where much carbon is disengaged 
from the iron in the state of plumbago. These reactions closely 
analogize with those presented by tin when acted on by nitric 
acid. This metal, when pure, will scarcely, if at all, decompose 
water cold; yet, when it decomposes nitric acid, it decomposes 
water at the same moment; hydrogen is given off, and ammonia 
is sometimes produced during the oxidation of the tin; but this 
rather digresses from the practical intention of the present re- 
ort. 
171. The powerful corrosive action of foul sea water (evi- 
denced in Table No. IV.) by water holding putrifying organic 
matter in solution and suspension, is due, in great part, to 
the quantity of hydrosulphuric acid (H+S) disengaged from 
the mud at the bottom, and with which the water is im- 
pregnated. The iron, acted on by the water in presence of 
its combined air and carbonic acid, forms hydrated oxides 
(Fe O+H O) and (Fe, O,+Fe O+H O), and carbonate of 
iron (Fe O+C O,), and probably, in some cases, basic salts of 
some organic acids. These, continually exposed to streams 
of hydrosulphurice acid, are in part converted into protosul- 
phuret of iron (Fe §), and in part into the bisulphuret of the 
protosulphuret or magnetic pyrites (6 Fe S+ Fe S,), both being 
_ formed in an amorphous state, or occasionally deposited in mi- 
~ 
croscopic crystals in the tissue of decaying organic substances. 
Both of these sulphurets are of most unstable constitution, and 
rapidly oxidize under the action of air and water, forming proto- 
- sulphate (Fe O+S O;) +6 HO, and the disulphate of the sesqui- 
oxide (2 Fe,O,+SO;) +6 H O, and frequently various other more 
_ basic sulphates. These, when soluble, are washed away, and 
_Yapidly expose fresh surfaces of the iron to oxidation. In every 
ase, the water charged with these salts has become a better 
conductor, and amore powerful agent in maintaining corrosion. 
_ But organic matter in a state of putrefaction is one of the 
most powerful deoxidizing agents known,-—so much ‘so, as to be 
capable even of reducing sulphate of lime in the state of gypsum ; 
hence these sulphates of iron are in their turn reduced in part 
