[76 ECONOMIC GEOLOGY 
is not protected from the rise of water by a damp-course. 
The solvent effect of rain-water is mainly due to acids de- 
rived from the atmosphere. The most important is carbonic 
acid (CO,) ; it converts the insoluble unicarbonate of lime 
(CaO, CO,) into the bicarbonate (HyCaCyO4, ie. CaO, 
2C0,, H,0), which is removed in solution, causing the crumb- 
ling of rock with a calcareous cement. The silicates of lime 
and of the alkalis are also converted into carbonates and 
removed in solution. The air of industrial cities includes 
sulphuric acid, as coal generally contains in pyrites more 
than -5 per cent. of sulphur, and combustion of that amount 
produces 35 Ib. of sulphuric acid per ton of coal. Building 
stones are also attacked by sulphuric acid derived from 
sulphates in the mortar which are due-to pyrites in the fuel 
used to burn the lime. The insertion of new cement between 
old stones may ruin them, as carbonate of lime may be 
deposited in the pores and have a disruptive effect. The 
air receives nitric and hydrochloric acids from factories, 
nitric acid by the oxidation of nitrogen by lightning, and 
chlorides from the sea. 
Common salt, sodium chloride, is introduced into the air 
by the evaporation of sea spray and the minute winged 
crystals are blown far inland. The salt is dissolved and soaks 
into porous stone, which it weakens as its crystallization 
forces the grains apart, while it keeps the surface damp by 
absorption of moisture from the air. 
Building stones are also attacked by agencies which act 
mechanically. The wind often has a speed of 30 to 60 miles 
an hour and sometimes of over 100 miles (as in Glasgow, 
28 January, 1927), and like a sand-blast it hurls against 
buildings sharp fragments of quartz from the road stones 
and jagged shreds of iron from cart wheels. Soft particles 
at a high speed have a cutting effect, for soft wheaten flour 
eats into quartz or topaz when flung against it bv a sand- 
blast. 
Sudden changes of temperature fracture rocks. When 
water freezes in the pores of a rock its expansion forces off 
“ frost-flakes.” The chilling of rock in a tropical desert 
after sunset throws off similar flakes, and sudden exposure 
to the sun produces insolation flakes. The freezing of water 
in a stone has been estimated as having a destructive effect