About fifty years ago two eminent French chemists visited London, and
rather “astonished the natives” by a curious feature of their dress.
They wore on their hats large patches of colored paper. Coming, as they
did, from Paris, many supposed that this was one of the latest Paris
fashions, and the dandies of the period narrowly escaped the compulsion
to follow it. They probably would have done so had the Frenchmen shown
any attempt at decorative shaping of the paper. They neglected this
because it was litmus paper, and their object in attaching it to their
hats was to test the impurities of the London atmosphere.
Blue litmus paper, as everybody knows now-a-days, turns red
when exposed to an acid. The French chemists found that their
hat-decorations changed color, and indicated the presence of acid in
the air of London; but when they left the metropolis and wandered in
the open fields their blue litmus paper retained its original color. By
using alkaline paper they contrived to collect enough of the acid to
test its composition. They found it to be the acid which is formed by
the burning of sulphur, and attributed its existence to the sulphur of
our coal. At this time the domestic use of coal was scarcely known in
Paris.
Subsequent experiments have proved that they were right; that the
air of London contains a very practical quantity of sulphurous and
sulphuric acids, which are due to the combustion of that yellow shining
material more or less visible in most kinds of coal, and has been
occasionally supposed to be gold. It is iron pyrites, a compound of
iron and sulphur. When heated the sulphur is separated and burns,
producing sulphurous acid, which, exposed to moist air, gradually
takes up more oxygen and becomes sulphuric acid, which in concentrated
solution is oil of vitriol. In the air it is very much diluted by
diffusion, but is still strong enough to do mischief to some kinds of
building materials.
In manufacturing towns, such as Birmingham and Sheffield, the quantity
of this acid in the air is much greater than in London, and there
its mischief is consequently more distinctly visible. The church of
St. Philip, which stands nearly in the middle of Birmingham, and is
surrounded by an old churchyard, was so corroded by this acid that the
stone peeled away on all sides, and its condition was most deplorable.
The tombstones were similarly disintegrated on their surfaces, and
inscriptions quite obliterated. It became so bad that a few years ago
restoration was necessary, and it was newly faced accordingly.
Some of the old tombstones that are preserved may still be seen against
the church wall, and their peculiar structure is well worthy of study.
They display a lamination or peeling away due to unequal corrosion,
certain layers of the material of the stone having been evidently eaten
away more rapidly than others. Anybody visiting Birmingham may easily
examine these, as St. Philip’s churchyard is situated between the two
railway stations of New Street and Snow Hill, and is but two minutes’
walk from either.
Other stone buildings in the town have suffered, but in very different
degrees, and some have quite escaped, proving the necessity of careful
selection of material wherever coal fires abound. In Birmingham the
action of coal fires is assisted by other sources of acid vapor.
The process of “pickling” brass castings, _i.e._, brightening their
surface, by dipping first in common nitric acid (“pickle acky”) and
then in water, is attended with considerable evolution of acid fumes.
Besides this very widespread use of acid, there are several chemical
manufactories that throw still more acid into the air immediately
surrounding them.
As an example of the action of the atmospheric acids of London upon
building stones, I have but to name the Houses of Parliament, which
have only been rescued from superficial ruin by the patchwork replacing
of certain blocks of stone, and various devices of siliceous and other
washings that have been carried out at great cost to the nation. That
such an unsuitable material should have been used is disgraceful to
all concerned. The ruin commenced before the building was finished. At
the time when its erection commenced there were abundant evidences of
the ruinous action of London atmosphere on some kinds of stone and the
capability of others to resist it, for while many modern buildings are
peeling and crumbling, some of the oldest in the midst of the city show
scarcely any signs of corrosion.
The Birmingham and Midland Institute was established and in practical
operation a few years before the present noble building was erected. I
was the first teacher there and conducted the Science classes in the
temporary premises in Cannon street. Having observed with some interest
the disintegration of St. Philip’s Church and other buildings, I was
anxious for the safety of the new Institute buildings, and accordingly
made some experiments upon the material proposed to be used by the
architect. My method of testing was very simple, and as the practical
result has verified my anticipations I think it might be adopted by
others.
First, I immersed some lumps of the stone in moderately strong
solutions of sulphuric and hydrochloric acids successively, and
observed whether any visible action occurred after some days. There
was none. I then roughly tested the crushing pressure of small samples
in their natural state, and subjected similar sized pieces to the same
test after they had been immersed in the acids. I found thus that there
were no evidences of internal disintegration even after several days’
immersion, and therefore inferred that the stone would stand the acid
vapors of the Birmingham atmosphere. This has been the case with that
portion of the building that was built of the material I tested. As
I know nothing of the stone which is used for the extension of the
building under the present architect, Mr. Chamberlain, I am unable to
make any forecast of its probable durability.
The experiments I made at the time named with this and other building
materials justified the conclusion that the worst of all material for
exposure to acid atmospheres is a sandstone, the particles of which
are held together by limestone, or are otherwise surrounded by or
intermingled with limestone; and that the best of _ordinary_ material
is a pure sandstone quite free from lime. I do not here consider such
luxurious material as granite or porphyries.
Compact limestone, such as good homogeneous marble, stands fairly
well, although it is slowly corroded. The corrosion, however, in this
case, is purely superficial and tolerably uniform. It is a very slow
washing away of the surface, without any disintegration such as occurs
where a small quantity of limestone acts as binding material to hold
together a large quantity of siliceous or sandy material, and where
the agglomeration is porous, and the stone is so laid that a downward
infiltration of water can take place; for it must be remembered that
although the acid originally exists as vapor in the air, it is taken up
by the falling rain, and the mischief is directly done to the stone by
the acidified water. This, of course, is very weak acid indeed. That
which I used for testing the stone was many thousand times stronger,
but then I exposed the stone for only a few days instead of many
thousand days.
As above stated, my experiments were but rude, but I think it
would be quite worth while to construct crushing apparatus capable
of registering accurately the pressure used, and to operate with
standard solutions of acid upon carefully squared blocks of standard
size, and thus to make comparative tests of various samples of stone
when competitions for building materials are offered. In the case
of the Birmingham and Midland Institute building there was no such
competition, the choice was left entirely to the architect, and my
examination was unofficially conducted upon the material already chosen
with the intent of protesting if it failed. As it stood the test I
merely reported the results informally to the architect, the late Sir
Edward Barry, no further action being demanded.