THE PROPERTIES OF BUILDING STONES, ETC. 185 
In the case of building stone the chemical decomposition of 
the minerals is so exceedingly slow that it seldom affects the 
strength or life of the stone after it has been placed in a building. 
Only in the case of limestone, dolomite, or marble, or where iron 
sulphide or iron carbonate occur in other rocks, is any material 
deterioration noticeable. 
Sulphurous acids—In the case of decomposition of iron sul- 
phide, in the presence of moisture, the formation of iron oxide is 
the most conspicuous, although not the only result. The decom- 
position of the sulphide produces sulphurous and sulphuric acids 
which, in the case of dolomite, act upon the magnesium carbon- 
ate, producing magnesium sulphate, which is often brought 
to the surface and deposited as an efflorescence or incrust- 
ation. 
The sulphurous and sulphuric acid gases are mainly present 
in the atmosphere of large cities where there is a large consump- 
tion of bituminous coal. The action of these acids is largely 
increased if the atmosphere contains a considerable amount of 
moisture. In London, where fogs predominate and the con- 
sumption of soft coal is very large, there seems to be little ques- 
tion but that the effect of these gases is worthy of careful con- 
sideration. But in the United States, with the exception of a few 
of the larger cities, the influence of these agents is comparatively 
small and needs but a passing mention. 
Carbon dioxide —Wherever water heavily charged with car- 
bonic acid gas is passed through calciferous rocks, more or less 
of the calcium carbonate is dissolved, lessening the adhesion 
between the different particles and weakening the rock. In 
nature the results of this process are very great, but the carbon 
dioxide has scarcely any appreciable affect on the durability of 
stone in the walls of a building. 
Organic acids.—The influence of organic acids resulting from 
decaying organisms on the life and strength of a rock, especially 
in the walls of buildings, is so slight as to barely warrant men- 
tion. 
12, IX, ISOC. 
