1811.] 
stone become harder, by exposure to the 
atmosphere. An inquiry into the causes 
of decay in different kinds of stone, 
“and also in’ stones considered of the 
same kind, may not be undeserving the 
attention of the public, 
To ascertain these causes with pre- 
‘eision, it will be necessary to attend, first 
to the external character of stones, and 
the qualities and proportions of the 
earths of which they are composed ; and, 
‘secondly, to the decomposing or disin- 
tegrating effects of the agents to which 
they are to be exposed. 
The four earths which form the prin- 
cipal part of all building-stones are 
silex, or the earth of flint, clay, lime, 
and magnesia. The substances which 
sometimes enter into their composition, 
and alter their qualitv, are oxyd of iron, 
water, and carbonic acid: the other 
‘earths, or metals, are generally in too 
small quantities to deserve the attention 
of the architect. ‘The qualities which 
these four earths communicate I shall 
afterwards state. The most important ex- 
ternal characteristics uf -tone for building 
are, compactness of texture, hardness, 
degrees of frangibility, and specific gra- 
vity. Compact texture, or closeness of 
grain, is always an advantage in stones 
of the same kind; for it is evident, that a 
porous stone will be more exposed to 
the action of air, or moisture, than a 
uenser one of the same kind: but com- 
pactness of texture is no test of excel- 
dence in stones of different kinds, for 
chalk is frequently more compact than 
many kinds of durable sand-stone, Hard- 
ness is also an important character in 
comparing varieties of the same species 
of stone, but it will not serve as a test 
of durability in stones of a different ge- 
nus, qj) account of the different effects 
which different agents have upon them. 
The hardness of natural or artificial sub- 
stances is no direct proof of their 
strength; glass, which is harder than 
iron, is more frangible than sbft lime- 
stone. Incompound stones, which have 
a crystallized texture, we frequently find 
the parts extremely hard, but the adhe- 
sion of the parts to each other very 
slight, as in some kinds of sand-Stone. 
Great specific gravity, or weight, isa 
proof of excellence in stones of the same 
kind, unless it arise from a combination 
with water, or the presence of iron, which 
18 a circumstance deserving great atten- 
tion; as iron, when in combination with 
stones, is acted upon by air and water, 
which. oecasions their decay, Some 
to the Selection of Stones for durable Architecture. 
403 
-basalts, which are extremely compact, 
ponderous, and hard, striking fire with 
steel, contain more than 25 per cent. of 
iron, and are soon decomposed at the 
surface, when exposed to the atmosphere, 
In general, specific gravity, where it does 
not arise from the absorption of water, 
and the presence of iron, or other me- 
tallic earths, may be considered as a 
quality indicating excellence for purposes 
of architecture. No stones, except those 
which contain the earth of strontian, or 
barytes, weigh three times as heavy as 
an equal bulk of water, unless they are 
combined with some metallic substance, 
which is generally iron. The specific 
gravity of few stones, except foreign mar- 
bles, exceeds 2.80, unless some iron be 
present. 
The quality of the three earths, silex, 
clay, and lime, is essentially distinct; 
but there are scarcely any stones that 
can be employed in architecture, in 
which they are not more or less combined 
together; communicating their character 
according to the proportion in which they 
combine. | Hardness is a_ distinctive 
character of stones, into which silex 
enters in a very large proportion. Silex 
being insoluble in water, and all the 
acids, except one of rare occuirence, 
silicious stones are, of all others, the 
most durable, and best suited for the 
foundation of bridges, piers, and docks, 
Silicious stones are frequently com. 
pounds, containing two, three, or more, 
substances, united together ina granular 
or crystallized form. 
Granite is composed of quartz, felspar, 
and mica. The quartz contains more 
than ninety parts of silex, the felspar 
sixty, and the mica, which is generally 
in the smallest proportion, about forty, 
The grains, or crystals, though distinct, 
are as firmly imbedded and united to 
each other, as if they had been melted 
together, Granite is not only extremely 
hard, but is also very. infrangible, re- 
sisting the effects of violent percussion, 
Some kinds of porphyry, which contain 
crystals of felspar, imbedded ina silicious 
base, are as hard, and still less frangible 
than granite. Many of the edye-stones 
of the foot pavements in London. are of 
porphyry, which appears to be of a very 
durable kind, and might probably be 
used to great advantage in forming the 
foundation and base of the arches of the 
two. new bridges, to be erected over tha 
Thames. Granite is found ghiefly on the 
western side of our island; there is, how. 
aver, a range of low granite mountains, 
at 
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