322 - Proceedings of the British Association. 
. magnesia, when from the slowness of their decomposition they 
continue sterile; and proposed in such cases to acceierate the dis- 
integration by pouring upon the sub-soil dilute sulphuric acid. 
r. Prideaux communicated the results of inquiries into the 
causes of the increased destructibility of modern copper sheathing. 
Experiments made on various kinds of copper sheathing immers- 
ed in sea water, showed, that in the laboratory, under parallel 
circumstances, they do not observe the same order of durability 
and waste, as they had done in use. The cause of comparative 
waste appears therefore to be, in part at least, due to external con- 
ditions ; and of these, two classes may be noticed ; one depending 
on the connexion with the ship, the other on the circumstances 
of her employment. Of the first class, two suggested them- 
selves,—the position on the ship’s side, and the nails by which the 
copper is fastened. The lower part of a ship’s copper seems to 
suffer much less than the upper, so long as she continues in deep 
water; but when she grounds at low water, if on black mud, 
this part suffers most from the action of sulphuretted hydrogen, 
peeling off in blue flakes. The influence of the nails offers rather 
more chemical interest. They are never of pure copper, and 
being very numerous, all in contact with the copper sheets, whilst 
their heads present also a considerable metallic surface to the salt 
water, they may produce very decided effects, either preservative 
or destructive, by a slight electro-chemical diforence: Various 
experiments were tried, which proved that most nails are destruc- 
tive. The conclusion resulting was this, that the nails might be 
rendered slightly electro-positive to rolled copper, by the addition 
of zinc, which would not injure their flexibility nor enhance their 
cost. The test»by the galvanometer would, after a little prac- 
tice, be easily applied, in making up the svete for casting them. 
Another mode of protection is offered, by coating the coppef 
when new, with fish oil, which in one instance has been of sig- 
nal ‘service. The preservative effect of coal-tar was also noticed. 
This tar had trickled down over the copper from the wood-work 
above, and had crossed the sheets just where most subject to the 
wash and friction; and whilst the naked metal had been quite 
worn away, ‘the coal-tarred streaks remained entire ; the surface 
of the copper, on melting off the tar, being as perfect as when 
fresh from the roll. Bat it ‘remains to be seen whether it will 
keepa clean surface fr y incrus- 
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