DURABILITY. The ability of a stone to resist deterioration by changes in weather and tem perature, by chemical agencies in the atmos phere and rain, by growing org,anisms, and by ordinary wear and tear—or, in other words. the durability of a obviously an important factor in its value for construction purposes. Granites are liable to disintegration chiefly from the constant contraction and expan sion caused by natural temperature ehanges. They absorb very little water, and thus suffer very little from the expansion of the contained water when frozen, and they are not much affect ed by any of the chemicals ordinarily carried in the air and rain. Limestones suffer less than granite by mere expansion and contraction, but make up for this by greater susceptibility to the chemicals in the air and rain. They do not munch water, and are therefore but little distufied by its freezing. Sandstones, being porous, absorb more water than limestone or granite. and are particularly subject to dis integration by freezing. The following figures from the United States census of 1590 show the life of various kinds of building-stone in New York City: Speaking generally, the durability of any stone depends on its relation toward atmospheric agencies. If a stone is dense, there is little pore space for moisture to soak in and exert a prying action when it expands in freezing. Many stones contain small quantities of moisture naturally, which is known as quarry-water, and the freez ing of such water, if the stone is quarried in cold weather, may aid in the disintegration of the rock. if the rock is quarried in warm weather, the quarry-water is evaporated, and any mineral matter which it holds in solution is deposited around the grains of the stone. cementing them together. and this is why some stones harden after quarrying. The porosity of huilding-stone. ()r its absorption. is variable. being from I to 2 per cent. in granites, 2 to 4 per cent. in lime and 2 to 8 per cent. in sandstones. The hardness and crushing strength of a stonedepends on the firmness with which the particles arc bound together. In igneous and metamorphic rocks the particles are interlocked as the result of crystallization. in sedimentary rocks the
individual grains are together by cement ing material, such as silica, iron oxide, or lime carbonate, which has been deposited between them as described above. In trying climates a stone must be hard and dense to insure dura bility, but in mild climates Nery soft. porous stones can often be employed. The resistance which it stone offers to frost action is often tested by soaking it in water, and then freezing it, the process being repeated a number of times, and note taken of any disintegration that occurs. A second but inaccurate method consists in boil ing the stone in a sulphate of soda solution and then allowing it to dry; the swelling of the soda-crystals in forming as the water evaporates exerts a far greater pressure than frost action. The liability of building-stone to decay is some times increased by the presence of injurious min erals. Pyrite grains decompose. yielding rust stains. and mica at times behaves in a similar manner. Marbles may contain both pyrite and aetinolite, the decomposition of the latter yield ing little bits of greenish clay that absorb mois ture.
The crushing strength of a stone is often very great, indeed far greater than is really necessary. (Aranite and syenites are among the strongest, having a crushing strength of 25.000 to :10,000 ',minds per square inch sandstones range from 10.000 to 15,000 pounds; and limestones and marbles 8000 to 12,000 pounds per square inch. The lower limit even is probably sufficient in most eases.
Various kinds of solutions have been tried for preserving the life of stone. but none of them have been found to lie perfeetly satisfactory. For the strength of various kinds of building stones, see STRENGTH OF MATERIALS. See also BATIISTONE; BROWNSTONE; CAEN STONE: FREESTONE; GRAN ITE; LIMESTONE; MARBLE; SANDSTONE; SLATE; STONE,ARTIFICIAI; BUILDING; and MASONRY. For comprehensive popular treatises on building stones, consult: Treattse on the Building and Ornamental Stones of Oreat Britain (Lon don, 1872) ; Merrill, Stones for Building and Decoration (New York, 1891) ; Johnson, The Materials of Construction (New York, 1899).