The effect of rain depends upon the solvent action of the gases which it contains, and upon its mechanical effect in the wear of pattering drops and streams trickling down the face of the stone.
A gentle breeze dries out the moisture of a building stone and tends to preserve it; but a violent wind wears it away by dashing sand grains, street dust, ice particles, etc., against its face. The extreme of such action is illustrated by the vast erosion of the sand stone in the plateaus of Colorado, Arizona, etc., into tabular mesas, isolated pillars, and grotesquely shaped hills, by the erosive force of sand grains borne by the winds. The effect is similar to that of the sand blast as used in various processes of manufacture. A violent wind also forces the rain-water, with all the corrosive acids it con tains, into the pores of stones, and carries off the loosened grains, thus keeping a fresh surface of the stone exposed. Again, the swaying of tall edifices by the wind causes a continual motion, not only in the joints between the blocks, but among the grains of the stones themselves. Many stones have a certain degree of flexibility, it is true; and yet the play of the grains must gradually increase, and a tendency to disintegration results.
Experience in great fires in the cities shows that there is no stone which can withstand the fierce heat of a mass of burning buildings. oandstone seems to be the least affected by great heat, and granite most.
Friction affects sidewalks, pavements, etc., and may also affect bridge piers, sea walls, docks, etc.
The effect of pressure in destroying stone is of little importance, provided the load to be borne does not too nearly equal the crushing strength. The pressure to which stone is subjected does not generally exceed one tenth of the ultimate strength as determined by methods already described.
destroy stones. The sulphur acids and carbonic acid, which result from the combustion of gas, coal, etc., and sometimes from certain kinds of manufactories, have a very marked effect upon the durability of stone. The nitric acid in the rain and the atmosphere exerts a perceptible influence in destroying building stone.
The durability of a building stone de pends upon three conditions, viz.: the chemical and mineralogical nature of its constituents, its physical structure, and the character and position of its exposed surfaces.
A siliceous stone, other things being equal, is more durable than a limestone; but the durability of the former plainly depends upon the state of aggregation of the individual grains and their cementing bond, as well as on the chemical relation of the silica to the other chemical ingredients. A dolomitic limestone is more durable than a pure limestone.
A stone that absorbs moisture abundantly and rapidly is likely to be injured by alternate freezing and thawing; hence clayey con stituents are injurious. An argillaceous stone . is generally compact, and often has no pores visible to the eye; yet such will disintegrate rapidly either by freezing and thawing, or by corrosive vapors.
The presence of calcium carbonate, as in some forms of marble and in earthy limestones, renders a building material liable to rapid attack by acid vapors. In some sandstones the cementing material is the hydrated form of ferric oxide, which is soluble and easily removed. Sandstones in which the cementing material is siliceous are likely to be the most durable, although they are not so easily worked as the former. A stone that has a high percentage of alu mina (if it be also non-crystalline), or of organic matter, or of pro toxide of iron, will usually disintegrate rapidly. Such stones are generally of a bluish color.