ON THE DURABILITY OF STONE BUILDINGS.
On the Choice of a Stone for Building Purposes.
“Everything belonging to the earth, whether in its primitive state,
or modified by human hands, is submitted to certain and innumerable
laws of destruction, as permanent and universal as those which produce
the planetary motions. The operations of nature, when slow, are no
less sure; however man may for a time usurp dominion over her, she is
certain of recovering her empire. He converts her rocks, her stones,
her trees, into forms of palaces, houses, and ships; he employs the
metals found in the bosom of the earth as instruments of power, and the
sands and clays which constitute its surface as ornaments and resources
of luxury; he imprisons air by water, and tortures water by fire to
change, to modify, or destroy the natural forms of things. But in some
lustrums his works begin to change, and in a few centuries they decay
and are in ruins; and his mighty temples, framed, as it were, for
divine purposes, and his bridges formed of granite, and ribbed with
iron, and his walls for defence, and the splendid monuments by which he
has endeavoured to give eternity even to its perishable remains, are
gradually destroyed; and these structures which have resisted the waves
of the ocean, the tempest of the sky, and the stroke of the lightning,
shall yield to the operation of the dews of heaven, of frost, rain,
vapour, and imperceptible atmospheric influences; and as the worm
devours the lineaments of his mortal beauty, so the lichens and the
moss, and the most insignificant plants, shall feed upon his columns
and his pyramids, and the most humble and insignificant insect shall
undermine and sap the foundations of his colossal works, and make their
habitations amongst the ruins of his palaces, and the falling seats of
his earthly glory.”[1]
Although it is true that all human works must decay, yet it is a point
of great importance to ourselves and our successors whether that decay
be slow or speedy. The causes enumerated in the above eloquent passage,
though sure, are exceedingly slow in their action, and provided the
building materials have been selected with reference as well to their
durability as to their beauty, the resulting structure may defy the
corroding tooth of time for many ages, and we may thus transmit to a
long posterity, lasting memorials of our wisdom and science, as well as
of our piety. Modern science has, to a very great extent, enabled the
architect and builder to determine beforehand what is the durability of
any given stone; and it is with great pleasure that we now notice the
extensive inquiry made at the suggestion of Mr. Barry, the architect
of the new Houses of Parliament, under the Commission issued by Her
Majesty’s Government, to investigate the qualities of stone in various
parts of the kingdom, in order to select that which should best
ensure perpetuity to this grand national monument. This commission,
consisting of Mr. Barry, Sir H. T. De la Beche, Dr. W. Smith, and Mr.
C. H. Smith, visited one hundred and five quarries, and examined one
hundred and seventy-five edifices; and their collected specimens were
then submitted to tests, both mechanical and chemical, by Professors
Daniell and Wheatstone, of King’s College, London. In order to leave
a permanent record of their labours, the Commissioners published a
Report, and deposited in the Museum of Economic Geology, a variety of
specimens of the stones which they had collected. From this Report, we
select such details as are calculated to serve the purposes of popular
instruction. The Commissioners did not consider it necessary to extend
their inquiries to granites, porphyries, and other stones of similar
character, on account of the enormous expense of converting them to
building purposes in decorated edifices, and from a conviction that an
equally durable, and in other respects more eligible material, could be
obtained for the object in view from among the limestones or sandstones
of the kingdom.
The Commissioners soon had striking proofs of the necessity and
importance of this inquiry in the lamentable effects of decomposition
observable in the greater part of the limestone employed at Oxford; in
the magnesian limestones of the Minster, churches, and other public
edifices at York; and in the sandstones of which the churches and
other public buildings at Derby and Newcastle are constructed; and
numerous other examples. The unequal state of preservation of many
buildings, often produced by the varied quality of the stone employed
in them, although it may have been taken from the same quarry, showed
the propriety of a minute examination of the quarries themselves, in
order to gain a proper knowledge of the particular beds from whence the
different varieties have been obtained. An inspection of quarries was
also desirable for the purpose of ascertaining their power of supply,
and other important matters; for it frequently happens, that the
best stone in quarries is often neglected, or only partially worked,
in consequence of the cost of laying bare, and removing those beds
with which it may be associated; whence it happens, that the inferior
material is in such cases supplied.
Stone buildings decay more rapidly in towns than in the open country,
where dense smoke, fogs, and vapours, which act injuriously on
buildings, do not exist. There is also another curious cause which
contributes to the durability of stone buildings situated in the
country. In the course of time, the stone becomes covered with minute
lichens, which, though in themselves decomposing agents, act with
extreme slowness, and when once firmly established over the entire
surface of the stone, seem to exercise a protective influence, by
defending the surface from the more violent destructive agents;
whereas, in populous smoky towns, these lichens are prevented from
forming, and thus the stone is exposed to severer trials than stone of
the same kind situated in the country.
As a remarkable illustration of the difference in the degree of
durability in the same material, subjected to the effects of the air
in town and country, the appearance is noticed of several frusta of
columns, and other blocks of stone, that were quarried at the time
of the erection of St. Paul’s Cathedral, London, and which are now
lying in the Isle of Portland, near the quarries from whence they were
obtained. These blocks are invariably found to be covered with lichens,
and, although they have been exposed to all the vicissitudes of a
marine atmosphere for more than one hundred and fifty years, they still
exhibit beneath the lichens their original form, even to the marks of
the chisel employed upon them; whilst the stone which was taken from
the same quarries, (selected no doubt with equal, if not greater care,
than the blocks alluded to,) and placed in the Cathedral itself, is, in
those parts which are exposed to the south and south-west winds, found,
in some instances, to be fast mouldering away.
Colour is more important in the selection of a building-stone to be
situated in a populous and smoky town, than for one to be placed in
the open country, where all edifices become covered with lichens; for,
although in such towns, those fronts which are not exposed to the
prevailing winds and rains, will soon become blackened, the remainder
of the building will constantly exhibit a tint depending upon the
natural colour of the stone.
The chemical action of the atmosphere produces a change in the entire
matter of the limestones, and in the cementing substance of sandstones,
according to the amount of surface exposed to it. The particles of the
stone first loosened by the action of frost are removed by powerful
winds and driving rains. The buildings in this climate were generally
found to suffer the greatest amount of decomposition on their south,
south-west, and west fronts, arising doubtless from the prevalence of
winds and rains from those quarters.
Those buildings which are highly decorated, such as the churches of
the Norman and pointed styles of architecture, generally afford a more
severe test of the durability of a building-stone, than the more simple
and less decorated castles of the fourteenth and fifteenth centuries;
because, in the former class of buildings, the stone is worked into
more disadvantageous forms than in the latter, as regards exposure to
the effects of the weather. Buildings in a state of ruin, from being
deprived of their ordinary protection of roofing, glazing of windows,
&c., afford an equally severe test of the durability of the stone
employed in them.
The durability of various building-stones in particular localities was
estimated by examining the condition of the neighbouring buildings
constructed of them. Among sandstone buildings was noticed the remains
of Ecclestone Abbey, of the thirteenth century, near Barnard Castle,
constructed of a stone closely resembling that of the Stenton quarry,
in the vicinity, in which the mouldings and other decorations were in
excellent condition. The circular keep of Barnard Castle, apparently
also built of the same material, is in fine preservation. Tintern
Abbey is noticed as a sandstone edifice, that has to a considerable
extent resisted decomposition. Some portions of Whitby Abbey are fast
yielding to the effects of the atmosphere. The older portions of Ripon
Cathedral; Rievaulx Abbey; and the Norman keep of Richmond Castle, in
Yorkshire, are all examples of sandstone buildings, in tolerably fair
preservation.
Of sandstone edifices in an advanced state of decomposition, are
enumerated Durham Cathedral, the churches at Newcastle-upon-Tyne,
Carlisle Cathedral, Kirkstall Abbey, and Fountain’s Abbey. The
sandstone churches of Derby are also extremely decomposed; and the
church of St. Peter, at Shaftsbury, is in such a state of decay, that
some portions of the building are only prevented from falling by means
of iron ties.
The choir of Southwell Church, of the twelfth century, affords an
instance of the durability of a magnesio-calciferous sandstone after
long exposure to the influences of the atmosphere. The Norman portions
of this church are also constructed of magnesian limestone, similar to
that of Bolsover Moor, and which are throughout in a perfect state, the
mouldings and carved enrichments being as sharp as when first executed.
The following buildings, also of magnesian limestone, are either in
perfect preservation, or exhibit only slight traces of decay: the keep
of Koningsburgh Castle; the church at Hemingborough, of the fifteenth
century; Tickhill Church, of the same date; Huddlestone Hall, of the
sixteenth century; Roche Abbey, of the thirteenth century.
The magnesian limestone buildings which were found in a more advanced
state of decay, were the churches at York, and a large portion of the
Minster, Howden Church, Doncaster Old Church, and buildings in other
parts of the county, many of which are so much decomposed, that the
mouldings, carvings, &c., are often entirely effaced.
The report speaks in high terms of the preservation of buildings
constructed of oolitic and other limestones; such are Byland Abbey, of
the twelfth century; Sandysfoot Castle, near Weymouth, constructed of
Portland oolite in the time of Henry the Eighth; Bow-and-Arrow Castle,
and the neighbouring ruins of a church of the fourteenth century, in
the island of Portland.
The oolite in the vicinity of Bath does not seem to wear well.
The excellent condition of the parts which remain of Glastonbury Abbey
shows the value of a shelly limestone similar to that of Doulting;
whilst the stone employed in Wells Cathedral, apparently of the same
kind, and not selected with equal care, is in parts decomposed. In
Salisbury Cathedral, built of stone from Chilmark, we have evidence of
the general durability of a siliciferous limestone; for, although the
west front has somewhat yielded to the effects of the atmosphere, the
excellent condition of the building generally is most striking.
The materials employed in the public buildings of Oxford, afford a
marked instance both of decomposition and durability; for whilst a
shelly oolite, similar to that of Taynton, which is employed in the
exposed parts of the more ancient parts of the Cathedral, in Morton
College Chapel, &c., is generally in a good state of preservation, a
calcareous stone from Heddington, employed in nearly all the colleges,
churches, and other public buildings, is in such a deplorable state of
decay as, in some instances, to have caused all traces of architectural
decoration to disappear, and the ashler itself to be, in many places,
deeply disintegrated.
In Spofforth Castle, two materials, a magnesian limestone and a
sandstone, have been employed, the former in the decorated parts, and
the latter for the ashler, and although both have been equally exposed,
the magnesian limestone has remained as perfect in form as when first
employed, while the sandstone has suffered considerably from the
effects of decomposition. In Chepstow Castle a magnesian limestone is
in fine preservation, and a red sandstone rapidly decaying. A similar
result was observed in Bristol Cathedral, which afforded a curious
instance of the effects of using different materials; for a yellow
limestone and a red sandstone have been indiscriminately employed both
for the plain and the decorated parts of the building; not only is the
appearance unsightly, but the architectural effect of the edifice is
also much impaired by the unequal decomposition of the two materials.
After enumerating these and other examples, the Report gives the
preference to the limestones, on account of their more general
uniformity of tint, their comparatively homogeneous structure, and the
facility and economy of their conversion to building purposes; and, of
this class, preference is given to those which are most crystalline.
Professor Daniell is of opinion that the nearer the magnesian
limestones approach to equivalent proportions of carbonate of lime and
carbonate of magnesia, the more crystalline and better they are in
every respect.
It was considered that this crystalline character, together with
durability, as instanced in Southwell Church, &c.; uniformity in
structure; facility and economy in conversion; and advantage in
colour, were all comprised in the magnesian limestone, or dolomite
of Bolsover[2] Moor and its neighbourhood, and was accordingly
recommended as the most fit and proper material to be employed in
the New Houses of Parliament.[3] This opinion was not arrived at,
nor this recommendation made, until after a very extensive series of
experiments had been completed by Professors Daniell and Wheatstone
upon specimens of the stones of the various quarries visited by the
Commissioners. The specimens, as delivered to these gentlemen, were
in the form of two-inch cubes. These experiments were of a most
comprehensive kind. The composition of the stones was determined by
chemical analysis:--their specific gravities; their weights after
having been perfectly dried by exposure in heated air for several days;
then their weights after having been immersed in water for several
days so as to become saturated; the object being to ascertain the
absorbent powers of the stones, which was further tested by placing
them in water under the exhausted receiver of an air-pump. The stones
were also subjected to the process of disintegration, invented by
M. Brard, the object of which is to determine, by easy experiments,
whether a building-stone will or will not resist the action of frost.
Lastly, the cohesive strength of each specimen, or its resistance to
pressure, was tested by the weight required to crush it. This weight
was furnished by a hydrostatic press, the pump of which was one inch in
diameter: one pound at the end of the pump lever produced a pressure
on the surface of the cube equal to 2·53 cwt., or to 71·06 lbs. on the
square inch. These trials were made with caution; the weight on the
lever was successively increased by a single pound; and, in order to
ensure a gradual action, a minute was allowed to elapse previous to the
application of each additional weight. It was noted for each specimen
the pressure at which the stone began to crack, and also the pressure
at which it was crushed.
The results of all these experiments (which are stated for each
stone) gave a decided preference to the Bolsover magnesian limestone,
which was noticed as being remarkable for its peculiarly beautiful
crystalline structure, while it was the heaviest and strongest of
all the specimens, and absorbed least water. Its composition was 50
per cent. of carbonate of lime, and 40 of carbonate of magnesia; the
remaining ten parts consisting chiefly of silica and alumina.
An easy Method of determining whether a Stone will resist the Action of
Frost.
In the choice of a stone for building purposes, it is of the utmost
importance to be able to determine, by a few prompt and easy
experiments, whether the proposed stone is capable of resisting the
destructive action of moisture and frost. The means of ascertaining
this were difficult and uncertain, until M. Brard, several years ago,
communicated his method to the Royal Academy of Sciences at Paris.
This learned body having appointed a Committee of their own members to
inquire into the merits of M. Brard’s process, and to make a report
thereon, the united testimony of engineers, architects, masons, and
builders from different parts of France, was received, and proved
so favourable as to its merits and simplicity, that the Committee
recommended the plan to public notice and general adoption. From their
Report we select a few details, which hitherto, we believe, have not
appeared in English.
When water is converted into ice an increase in bulk suddenly
takes place with such amazing force that it appears to be almost
irresistible. This is the force which cracks our water-bottles and
ewers; splits asunder the trees of our forests; and destroys some of
the stones of our buildings. But the action of frost upon stone is
very gradual; it is confined to the surface, and when we see a layer
of stone separated from the rock or the building, we see the result of
the action of the frost during several successive winters, whereby the
fragment is gradually thrust out of its perpendicular position, and at
length falls. This natural process is repeated in our buildings: we
rarely see squared stones split into large fragments by the action of
frost except there be a cavity of some considerable size, in which a
quantity of water can be collected. The usual action of the frost is at
the surface, which is destroyed by the chipping off of small fragments
in consequence of the adhesion of the materials of the stone being
partially destroyed.
All stones absorb water in greater or less quantities, and there is no
rock that does not contain some humidity. The great difference between
stones which is now to be considered is in their power of resisting
frost. Stones of the same kind, nay, stones from different parts of
the same quarry, are acted upon very differently by frost; for, while
one stone soon begins to show the destructive effects of its action,
another remains uninjured during many centuries. It will, therefore,
be convenient to call those stones, of whatever kind, which withstand
the action of frost, _resistant_, and those which yield to its action,
_non-resistant_.
M. Brard’s first idea, in order to test these resistant properties
in building-stones, was, to saturate the stone with water, and then
expose it to cold artificially produced; but this was found to be
impracticable on a large scale, and the freezing mixtures and other
means of producing cold were liable to act chemically upon the stone,
and thus produce other effects than those of cold.
M. Brard was then led to compare water with those numerous solutions of
the chemist, which, under certain modes of treatment, crystallize. The
expansive force of salts in crystallizing is very great, and he saw no
reason why water should not be regarded as a crystalline salt similar
in its nature to those saline bodies which effloresce at the surfaces
of stones, and in time destroy them and even reduce them to powder.
He therefore tried, in a very large number of experiments, the action
upon building-stones of solutions of nitre, of common salt, of Epsom
salts, of carbonate and sulphate of soda, of alum and of sulphate
of iron, and found that the stones cracked and chipped, and in many
cases behaved precisely in the same way as when under the influence
of freezing water. In the course of these trials, sulphate of soda
(Glauber’s salts) was found to be the most energetic and active, and to
be the best exponent of the action of freezing water.
In order, therefore, to determine promptly if a stone be resistant or
non-resistant, the following process was adopted. A saturated solution
of sulphate of soda was made in cold water; the solution being put into
a convenient vessel, the stone was immersed, and the solution boiled
during half an hour: the stone was then taken out, and placed in a
plate containing a little of the solution. It was then left in a cool
apartment, in order to facilitate the efflorescence of the salt with
which the stone was now impregnated. At the end of about twenty-four
hours the stone was covered with a snowy efflorescence, and the liquid
had disappeared either by evaporation or by absorption. The stone was
then sprinkled gently with cold water until all the saline particles
disappeared from the surface. After this first washing the surfaces
of the stone were covered with detached grains, scales, and angular
fragments, and the stone being one that was easily attacked by frost,
the splitting of the surfaces was very marked. But the experiment was
not yet terminated: the efflorescence was allowed to form, and the
washing was repeated many times during five or six days, at the end of
which time the bad qualities of the stone became fully established. The
stone was finally washed in pure water; all the detached parts were
collected, and by these the ultimate action of the frost upon the stone
was estimated.
The behaviour of various non-resistant stones under this process was
remarkable. Some were found to have deteriorated in the course of
the third day; others to have entirely fallen to pieces; those of
which the power of resistance was somewhat greater, held out till the
fifth or sixth day; but few stones, except the hard granites, compact
limestones, and white marbles, were able to stand the trial during
thirty consecutive days. For all useful purposes, however, eight days
suffice to test the resistant qualities of any building-stone.
The explanation of this process is very easy. The boiling solution
dilates the stone and penetrates it to a certain depth, nearly in
the same way that rain water by long-continued action introduces
itself into stones exposed to the severity of our changeable climate.
Pure water when frozen occupies a greater bulk than when fluid, and
the pores or cellules of the stone not being able to accommodate
themselves to the increased bulk of the water, great pressure is
exerted between and among them, whereby a portion of the water is
driven to the surface, and in doing so rends and detaches small
portions of the stone. The same action takes place with the saline
solution; it is introduced into the stone in a fluid state, from which
passing into the solid it occupies a greater bulk, and a portion of
it appears at the surface. The repeated washings have no other object
than to allow the salt to exert its greatest amount of destructive
action upon the stone. There is a striking analogy between the effect
of congealed water and that of the efflorescence of salts, in the
disintegration of non-resistant stones; namely, that pure water acts on
the stones destructively only in a state of snowy efflorescence, which
evidently proceeds from the interior to the exterior like the saline
efflorescence; whilst water at the surface of the stones may freeze
into hard ice without injuring them, just in the same way as salts,
which may crystallize upon stones without exerting any injurious action.
The experience of several engineers, extending as it does over several
years, fully proves, of a large variety of stones whose qualities
were well known, that the action of M. Brard’s process and that of
long-continued frost exactly coincide.
It is not the least interesting part of the inquiry to know that this
process may be applied with perfect success to ascertain the solidity
and resistant power of bricks, tiles, slates, and even mortar. From a
mass of minute detail, we will select a few general results.
During one winter season M. Vicat composed seventy-five varieties of
mortar, the difference between any two consisting in the proportion of
sand and the method of slaking the lime. In the following June these
mortars were exposed to the disintegrating process. Most of them were
attacked in twenty-four hours; almost all of them in forty-eight hours;
and all except two in three days. This gentleman also found that a
mortar made ten years previously, of one hundred parts lime, which had
been left exposed to the air, under cover, during a whole year, and
then mixed up into a paste with fifty parts of common sand, withstood
the trial admirably during seventeen days, while the best stones of
the neighbourhood speedily gave way. In this case the solution was
saturated while hot, which is so powerful in its effects that stones
which have resisted the action of the frost for ages, soon gave way
when exposed to it.
M. Vicat calculates that the effect of the sulphate of soda upon a
non-resistant stone after the second day of trial equals a force
somewhat greater than that exerted by a temperature of about 21°
Fahrenheit, on a stone saturated with water.
The action of the process upon bricks proved that, whatever their
qualities in other respects, if imperfectly burnt, they are speedily
acted on. The sharp edges of the brick, and then the angles, are first
rounded, and finally the brick is reduced to powder. Such is precisely
the action of frost often repeated. Well-baked bricks, on the contrary,
retain their colour, form, and solidity by this process, as well as
under the influence of frost. Ancient Roman bricks, tiles, and mortar,
and hard well-baked pottery resisted the process perfectly; as did also
white statuary marble of the finest quality, while common white marble
was soon attacked. In Paris, portions of buildings which had been
exposed to the air during twenty years without undergoing the least
alteration, were submitted to this ordeal, and the experiment agreed
with observation. In one extensive series of experiments on stones from
different quarries of France, the action of the salt was continued
for seven days, and the results noted down; it was then continued for
fourteen days, and the results compared with the preceding ones; which
only served to confirm the judgment first given, for those stones which
were noted as of bad quality crumbled to dust or split into fragments,
while those noted for their good qualities had experienced no sensible
alteration.
One of the great advantages of this process is the power it gives to
the architect of choosing a hard, durable stone for those parts of the
building most exposed to the action of the weather, when the funds
are insufficient to admit of the whole building being so constructed.
Thus the cornices, the columns, and their capitals, are struck in all
directions by rain, and hail, and damp air, and are consequently far
more exposed to their destructive action than the flat surface of a
wall, which offers but one plane to the air.
In the course of this inquiry a very curious case arose. During the
erection of a church in Paris, the architect required a good durable
stone for the Corinthian capitals; and many circumstances disposed him
to select it from the neighbouring quarry of the Abbaye du Val. But, on
seeking the opinion of two brother architects, he was surprised to find
their estimations of the stone to be totally at variance, for while one
declared that he had employed it with the greatest success, another
said that he had seen it yield speedily to the effects of frost. On
visiting the quarry it was found that two beds of stone were being
worked, an upper and a lower bed; specimens of the stone were taken
from each, and on submitting them to a hot saturated solution, it was
ascertained almost immediately that the upper layer furnished excellent
stone, while the lower one supplied that of which the architect had
so much reason to complain. But it is remarkable that the stones from
the two beds had precisely the same appearance in grain, colour, and
texture; so much so, that when brought into the mason’s yard it was
impossible by ordinary tests to distinguish the good from the bad stone.
At the conclusion of the inquiry of the Committee, the Royal Academy
of Sciences proved the high estimation in which they held this
contribution of science to the useful arts, by directing to be
published the following practical directions for repeating the process,
for the use of architects, builders, master masons, land proprietors,
and all persons engaged in building.
1. The specimens of stone are to be chosen from those parts of the
quarry, where from certain observed differences in the colour, grain,
and general appearance of the stone, its quality is doubtful.
2. The specimens are to be formed into two-inch cubes, carefully cut,
so that the edges may be sharp.
3. Each stone is to be marked or numbered with Indian ink or scratched
with a steel point; and corresponding with such mark or number a
written account is to be kept as to the situation of the quarry,
the exact spot whence the stone was detached, and other notes and
information relating to the specimen.
4. Continue to add a quantity of sulphate of soda to rain or distilled
water, until it will dissolve no more. You may be quite sure that the
solution is saturated, if, after repeatedly stirring it, a little of
the salt remains undissolved at the bottom of the vessel an hour or two
after it has been put in.
5. This solution may be heated in almost any kind of vessel usually put
on the fire, but perhaps an earthen pipkin may be most convenient. When
the solution boils, put in the specimens of stone, one by one, so that
all may be completely sunk in it.
6. Continue the boiling for thirty minutes. Be careful in observing
this direction.
7. Take out the cubes one at a time, and hang them up by threads in
such a way that they may touch nothing. Place under each specimen a
vessel containing a portion of the liquid in which the stones were
boiled, having first strained it to remove all dirt, dust, &c.
8. If the weather be not very damp or cold the surfaces of each stone
will, in the course of twenty-four hours, become covered with little
white saline needles. Plunge each stone into the vessel below it, so as
to wash off these little crystals, and repeat this two or three times a
day.
9. If the stone be one that will resist the action of frost, the
crystals will abstract nothing from the stone, and there will be found
at the bottom of the vessel neither grains, nor scales, nor fragments
of stone. Be careful, in dipping the stone, not to displace the vessel.
If, on the contrary, the stone is one that will not resist the action
of frost, this will be discovered as soon as the salt appears on the
surface, for the salt will chip off little particles of the stone,
which will be found in the vessel beneath; the cube will soon lose
its sharp edges and angles; and by about the fifth day from the first
appearance of the salt, the experiment may be considered at an end.
As soon as the salt begins to appear at the surface its deposit is
assisted by dipping the stone five or six times a day into the solution.
10. In order to compare the resisting powers of two stones which are
acted upon by the frost in different degrees, all that is necessary is,
to collect all the fragments detached from the six faces of the cube,
dry them and weigh them, and the greatest weight will indicate the
stone of least resistance to the frost. Thus, if a cube of twenty-four
inches of surface loses 180 grains, and a similar cube only 90 grains,
the latter is evidently better adapted than the former to the purposes
of building.
FOOTNOTES:
[1] SIR HUMPHRY DAVY.
[2] Bolsover is a small market town in Derbyshire, on the borders of
the county of Nottingham, and about 145 miles from London.
[3] The various quarries visited by the commissioners are noticed in
the fullest and fairest manner. They have stated for each quarry its
name and situation; the names and addresses of the freeholder, of his
agent, and of the quarryman; the name of the stone; its composition;
colour; weight per cubic foot; entire depth of workable stone;
description of the beds; size of blocks that can be procured; prices,
per cubic foot, of block stone at the quarry; description and cost of
carriage to London; cost, per cubic foot, of the stone delivered in
London; cost, per foot of surface, of plain rubbed work, as compared
with Portland stone; and, finally, where known or reported to have been
employed in building.