224 
corrodes, appearing red in the water, and 
grass-green where it is in contact with the 
air. _ Carbonate of soda gradually forms 
upon this grass-green’ matter, and’ these 
changes~continue ‘until the )water becomes 
much less-saline»o The;green precipitate, 
when examined, by the, action ‘of the solu- 
tion of ammonia_and_ other. tests, appears 
to consist. -of. an) insoluble. compound of 
copper (which may be called a hydrated 
sub-muriate) and hydrate of magnesia. 
According ‘to the views which T deve- 
loped fourteen years ago, of the nature of 
the compounds of ¢hiorine, and which are 
now genérally adopted, it is evident that 
soda and magnesia cannot appear in sea- 
water by=the action of a metal, unless in 
consequence of an absorption or transfer of 
_ oxygen. It was therefore necessary, in 
order to produce these changes, that water 
should be decomposed, or that oxygen 
should be absorbed from the atmosphere. 
I found that no hydrogen was disengaged, 
and consequently no water was decomposed : 
the oxygen of the air must therefore ‘have 
been the agent concerned, as appeared sub- 
sequently by numerous experiments. 
Copper placed in. sea-water; deprived of 
air by -boiling, or exhaustion, and exposed 
in an exhausted receiver, or in an atmo- 
sphere of, hydrogen gas, underwent. no 
change whatever. But an absorption of 
atmospheric ait was shewn, when copper 
and sea-water were exposed to its agency 
in close vessels. 
Sir Humphrey, after referring to the 
principles of chemical and electrical 
agency, which he developed twelve or 
fourteen years ago by his beautiful ex- 
periments on the alkalis, farther ob- 
serves : 
Copper is a metal only weakly positive 
in the eleetro-chemical scale, and, accord- 
ing to my ideas, it could only act upon 
sea-water when in a positive: state, and 
consequently, if it could be rendered slightly 
negative, the corroding action of sea-water 
would be null; and whatever might be the 
differences of the kinds of copper sheathing 
and their electrical action on each other, 
still every effect of chemical action must 
be prevented, if the whole surface were ren- 
dered negative......+-+--I began with an 
extreme case. I rendered sea-water slightly 
acidulous by sulphuric acid, and plunged 
into it a polished piece of copper, to which 
a piece of tin was soldered equal to about 
one-twentieth of the copper. Examined 
after three days, the copper remained per- 
fectly clean, whilst the tin was rapidly cor- 
roded. No blueness. appeared in the 
liquor: though in a comparative expe- 
riment, when copper alone and the same 
fluid mixture were used, there was a con- 
siderable corrosion of the copper and a 
distinct blue tint in the liquor. If one- 
twentieth part of the surface of tin pre- 
yented the action of sea-water, rendered 
On the Preservation of the Copper-sheathing of Ships. 
[Oct. 1, 
slightly acidulous-by sulphuric acid, I had 
no doubt a much smaller quantity would 
neutrdlize the action of sea-water, when 
depending only on the oxygen contained in 
common air, And on trial; I found that 
one two-hundreth part,of tin in proportion 
to the copper was sufficient to preyent the 
corrosion of the Jatter.,.......In pur- 
suing these experiments, and applying them 
in every possible form and connection, the 
results were of the most satisfactory kind. 
A piece of zinc as large as a pea, or the 
point of a small iron nail, was’ found fully 
adequate to preserve forty or ‘fifty square 
inches of copper;.and the result) was 
equally satisfactory, -inewhatever part of the 
sheet of copper the other metal was placed, 
And even when the jconnection between 
different sheets of copper. was completed 
by wires or thin filaments of the fiftieth of 
an inch diameter, the effect was the same ; 
every side, every surface or particle of the 
copper remained perfectly bright after being 
placed in sea-water for matty weeks ; while 
the iron or zine was slightly corroded. 
A piece of thick sheet-copper was cut in 
such a manner as to form seven divisions, 
connected,only by the smallest. filaments 
that could be left; and a slip of zine, one~ 
fifth of an inch wide, was soldered to the 
upper edge. The whole, after being im- 
mersed for a month in sea-water, left the 
copper in a bright polished stale, as at first. 
The same experiment succeeded with a slip 
of iron, soldered to the copper; whilst si- 
milar pieces of copper, undefended, were 
considerably corroded by the salt water. 
The importance of this discovery in 
the preservation of our shipping can at 
present scarcely be -appreciated; for 
there appears to be not a shade of 
doubt as to its complete efficacy when 
reduced to practice. Sir Humphrey is 
still pursuing his researches on a large 
scale; but his observations on a com= 
parative experiment, made for the pur- 
pose of demonstrating its practical ef- 
fects, is all I shall venture to extract 
from his late communication to the 
Royal Society.— 
As the ocean may be considered, in its 
relation to the quantity of copper in a ship, 
as an infinitely extended conductor, I en- 
deavoured to ascertain whetber this cireum- 
stance would influence the results. By 
placing two very fine copper wires, one wn- 
defended, the other defended by a particle 
of zinc, in a very large vessel of sea-water, 
which water may be considered, as haying 
the same relation to so minute a,portion of 
metal, as the, sea to the copper-sheathing 
of aship. The result of this experiment 
was equally satisfactory with that of all the 
preceding. The defended copper under- 
went no change whatever ; whilst the un- 
defended wire tarnished, corroded, and de- 
posited a green powder. P 
These 
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