ON THE ACTION OF AIR AND WATER UPON IRON. 265 
259. The ratio of the surface of the cast-iron parallelopipeds 
used in these two sets of experiments to that of similar slips 
of the alloys, is given: both iron and alloys were filed to an ex- 
act gauge, so as to present strictly equal surfaces, in pairs, and 
the former made all of equal weight. The amount of loss or 
the ‘ index of corrosion ’’ of cast iron in contact with any given 
alloy in the Tables, will of course vary according to some un- 
ascertained function of the surface of the alloy and of the iron 
exposed to chemical action. The law regulating this remains 
to be developed ; it is a subject of considerable complexity and 
experimental difficulty. The ratio, however, of surface of the 
electro-negative to that of the electro-positive metal, may vary 
to a considerable extent without very materially affecting the 
results given in the present Tables, as respects their primary 
object. 
260. These experiments were necessarily made ina limited 
" quantity of sea water (12 cubic inches each), and all in separate 
glass vessels ; and as this water was soon exhausted of much of 
its previously combined air, the eleventh and twelfth columns 
in each Table give the ratio of the actual corrosion in a limited 
quantity of water to that which would take place in an unlimited 
one, or in the open sea, which is deduced from the result of 
(# 77) in Table I. first series, being on the same sort of cast 
iron as the present. 
_ 261. There are several collateral points of scientific interest 
this branch of our inquiry presents, which I pass, as out of the 
“subject of inquiry. I would remark, however, that the results 
confirm fully what had been previously stated respecting the 
impossibility of protecting iron by brass in the ordinary sense of 
the word. They also point out the inutility, not to say the ab- 
_surdity, of some inventions for the so-called prevention of ox- 
_idation, for which patents have recently been obtained; for ex- 
ample, more than one patent, for so preserving iron by dipping 
it into melted copper *, or coating it with copper or brass in 
‘various other wayst. 
_ 262. It has recently been proposed to substitute zinc for 
lead in the operation of “ cramping,”’ or running a fluid fusible 
metal into the joints of iron-works to secure them together, or to 
_ * Repertory of Arts for 1839-40. 
__+ lobserve with pleasure that M. Karsten of Berlin has recently published 
Se lnetitut No. 275, April 1839) some experiments on the electro-chemical re- 
ations of alloys of copper and zinc, &c., to solutions of their own metals. It 
is to be regretted that his experiments do not seem to have been made on 
alloys of atomic constitution; but while unknown to each other travelling 
(though with different objects) on neighbouring roads, it is pleasant to find 
that our results have so far brought us to the same resting-place. 
