230 REPORT—1840. 
sea salt, deprived of air, can scarcely be made to absorb air at 
all; and that in dissolving most salts, expel the combined air from 
the water of the solution, united with the circumstance, that the 
voltaic conducting power of the water is greater in proportion to 
the amount of its saline contents, indicate that fresh water may 
hold so much combined air (not to speak of carbonic acid) as to 
act more rapidly on iron than sea water; that, on the other 
hand, with much less combined air, the superior conducting 
power of the saline solution may place its corrosive power on a 
level with or above that of the former; and that, by the variable 
combination of these two elements, within their respective limits 
of saturation, any assignable ratio may exist of the corrosive 
power of aérated fresh and sea water. 
This might seem to render our enlarged experiments in the 
open sea nugatory; but it will be recollected, that the compo- 
sition of sea water, both as to solid and gasiform contents, is 
very nearly constant. : 
The experiments from which these conclusions* have been 
deduced, were made on equal parallelopipeds of the cast iron, 
as in class No. 1. « 77, exposed during equal times, and the 
oxides produced washed off, filtered and weighed; hence they 
were all made in vessels of limited size. 
The part which the combined air plays in these reactions is 
very remarkable ; it seems to belong to the same class of phe- 
nomena as those known both in inorganic and organic chemistry, 
where the presence of a third body is required to commence or 
sustain the reactions of the three, any two of which alone are 
quiescent. Thus gallic acid, lignin and other analogous sub- 
stances, suffer no change, either in air or in water; but add to 
the water a minute portion of an alkali, or alkaline earth, and the 
process of oxidation commences at once. Alcohol stands in the 
like predicament. 
170. It seems probable that the air, in the case of the reaction 
on iron, is not decomposed directly by the iron at all, but by its 
protoxide, previously formed by decomposition of the water, 
catalytically, or due to the presence of the combined air. If so, 
we should expect to find hydrogen evolved from the first mo- 
ment, as well as nitrogen, produced by the decomposition of 
the air. But water, although combined with air, absorbs about 
one and a half per cent. of hydrogen, while, unless the air be 
previously expelled, it absorbs no nitrogen ; hence decomposition 
proceeds for some time before hydrogen is evolved; but on stop- 
ping the reaction at an early stage, and expelling the air and 
* Report, § 158 to 169. 
