ECONOMIC GEOLOGY 
crushing strength, and indicates durability and resistance to 
frost. 
The specific gravity of a building stone shows its weight 
and its porosity, which is an important factor in durability. 
Porosity is often expressed by the weight of water absorbed 
by a stone divided by the dry weight of the stone. The test 
of soakage in water for 24 hours usually gives too low a poros- 
ity, as many of the pores are not filled. Soaking for a longer 
time or under pressure gives more reliable results. The 
nature of the porosity must also be considered ; for a stone 
with open pores loses water readily and thus is less likely 
to be injured by frost. The former test of weakening by 
freezing was to boil a sample in a saturated solution of 
sodium sulphate, and hang it up to dry. The crystalliza- 
tion of the salt breaks off fragments of the stone which are 
washed, dried, and weighed. In Hirschwald’s method a piece 
is thoroughly soaked with water and frozen and thawed 
twenty-five times during a month; the tensile strength of 
the stone is measured before and after this treatment and 
the difference is the weakening by freezing. Some rocks 
are seriously weakened by saturation with water, especially 
those with a clay cement. This quality may be tested by 
the ratio of the crushing strengths before and after 28 days’ 
soaking. This ratio may vary in sandstones from ‘I to ‘0, 
and no stone in which the ratio is lower than -6 should be 
used in a damp frosty locality. 
The tests of building stones have been condemned as 
valueless because they are little used in British practice, 
where architects and builders have the experience of centuries 
to guide them. That much value is attached to the crushing 
strength may be inferred from its frequent statement in 
trade advertisements ; and tests are advisable for new stones 
or new conditions. Building stones which appear of good 
quality sometimes fail through some unsuspected weakness. 
The most important practical tests are the crushing 
strength, the weakening of the cement by water, and the 
porosity. The weakening by water may be judged roughly 
by placing a piece of the stone in water for 24 hours, then 
stirring the water and observing the extent of the muddy 
streaks which arise from the stone. The porosity can be 
tested by placing a squared sample on wet blotting paper 
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