ON THE ACTION OF AIR AND WATER UPON IRON. 17 
_ 836. As the bulls of iron ships cannot, be ordinarily got at to keep them 
» uniformly covered with any common paint or varnish (which have however 
“alone but a limited palliative effect in preventing corrosion), such vessels 
‘should in all respects be viewed with reference to corrosion, as if the iron 
was always quite bare; and if so, Table XV., before given, affords data for 
determining their duration if wholly unprotected, but as we shall see hereafter, 
iron vessels may be so treated, that in regard to corrosion it is difficult to 
assign a limit to their durability, which it is generally admitted depends 
simply on the question of corrosion. 
$37. The plates of an iron ship are likely in general to be corroded most 
round the rivet-heads, both outside and inside, and adjacent to any spots 
where the plates have been hardened by hammering or bending, or in any 
other way have had their homogeneity destroyed, and least round the bows, &c., 
where the oxide formed is swept off by the ship’s motion through the water. 
338. The contact of oak timber especially, and generally of all timbers 
which contain tannic or gallie acids, is extremely injurious to iron, and for 
keelsons, &c.. or other timbers in contact with iron and water, teak should 
always be used in preference, which does not act at all, or but very slightly, 
upon iron. The bolts and nails of a gate of the fort at Canara, East Indies, 
after having been exposed to the weather for half a century, were found as 
sound as when put in: the gate was of teak. In the “Chiffone” frigate 
certain teak planks had been bolted to her sides; on subsequent removal the 
iron was sound and uncorroded in the teak, but eaten through in the oak. 
339. This injurious effect of oak timber as applied to iron ship-building, 
might however probably be completely obviated by steeping the timber, 
prior to insertion, in a solution of sulphate of iron, which would engage the 
whole of the organic acids which act so injuriously upon iron. ‘The oak 
would become black from the gallate and tannate of iron formed in its pores; 
its durability would most probably be increased fully as much as by steeping 
in sulphate’ of copper, for which, as a mode of preventing dry-rot, a patent 
has been obtained, and there is no reason to suppose that the timber would 
suffer any deterioration in toughness, while it would certainly become harder. 
340. Kyanized timber of all sorts is destructive to iron in sea water to a 
prodigious extent; a portion of the corrosive sublimate (whether more or 
less changed) contained in the pores of the wood, is decomposed by the con- 
tact of the iron, and the quicksilver reduced to the metallic state, which, by 
its powerful electro-negative relation to iron, promotes the corrosion of the 
latter. The actual amount of corrosion on best Staffordshire iron, by my 
experiments, when in sea water and in contact with kyanized oak, amounted 
_ in two years to a depth of 0-122 of an inch of iron removed all over the 
surface, while the same iron freely exposed to the sea water alone, lost not 
half so much in the same time. Indeed the utility of kyanizing timber which - 
is to be immersed in sea water appears very dubious, even if it were in this 
respect harmless ; for M. Lassaigne has shown that whatever be the nature of 
the combination which the corrosive sublimate forms with the albumen of the 
wood, it is soon washed out, being soluble in salt water; and I have already 
stated that, in timber freely exposed in sea water, kyanizing is no protection 
ainst marine boring animals, kyanized oak being eaten through, two inches 
hick, in about two years, by the Limnoria terebrans, in Kingstown Harbour. 
_ 341. Of course the contact of a metal electro-negative to iron, as lead or 
copper, with iron ships, either exteriorly or interiorly, should be avoided, and 
when itis inevitable, increased scantling should be given to the plates, &c. at 
and around the spot. | 
og contact also of brass should as much as possible be avoided ; but the 
, c 
