BUILDING STONES AND ROAD METALS 177 
equal to the blow of a ten-ton hammer per square inch of 
surface. Repeated variations of temperature in a rock of 
which the constituents expand unequally when heated 
produce cracks parallel to the surface; this ** spalling” 
in the tropics breaks granite into thin slabs, and they peel 
off leaving dome-shaped hummocks. Spalling is in some 
countries used in quarrying ; as the sudden chilling by water 
of a hot rock surface breaks it into slabs; their thickness 
varies with the treatment. 
Stones are also attacked by organic agencies. Bacteria 
contribute largely to the decay of rock debris into soil, and 
they doubtless also affect building stones. A block of stone 
may suddenly decay at one point; if unchecked the process 
will spread like an infection, but it may be stopped if the 
decayed material be removed and the part sterilized. 
Tests oF BuiLping StoNe—The test of the durability 
of building stone most often used is the crushing strength; 
rectangular blocks 2 inches square by 3 inches high, are 
crushed by a machine which records the breaking pressure. 
Weaker stones are tested in 4-inch or 6-inch cubes. The 
value of this test is in the main indirect, for practically all 
stones withstand much higher pressure than they are subject 
to in buildings. The crushing strength of granites is from 
850 to 1300 tons per square foot; of sandstone from 200- 
1000 tons; of limestone from 100-1000 tons, and dolomites 
from 300-600 tons. In an ordinary building no stone is sub- 
ject to a greater load than about 10-12 tons per square foot; 
the greatest load in the Washington Monument, which is 
555 feet high, is only 22-26 tons per square foot. Stones 
that best resist crushing generally best resist weathering. 
The value of this test is lessened by its variability; the 
crushing load may vary 30 to 40 per cent. in material from 
the same quarry, and great variation may occur in samples 
cut from the same block. 
The resistance to shearing, which is especially important 
in building material, is tested by a rod pressing against 
samples of a standard size of 6 inches by 4 inches by 2 inches. 
Granite has a resistance to shearing of from 65 to 200 tons 
per square foot, sandstone from 65-85 tons per sq. foot, and 
marble about 100 tons per square foot. 
The tensile strength is usually about one-fifth of the 
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