BUILDING STONE. The chemical composition of B. S. varies. The majority of kinds are more or less silicious, and are designated sandstones. These consist of particles of sand, united together by the force of cohesion, and by a small proportion of a natural cement; in soma cases, ferruginous, consisting of a compound of iron; and in other instances, calcareous, composed of carbonate of lime. Igneous rocks furnish very durable B. S., though iu genera] the hardness of the materials renders theta so difficult to work, that they are seldom resorted to where softer stones can be procured. Thus, granite is largely employed in the construction of the houses in Aberdeen, in the erec tion of bridges, in the paving of streets, and wherever great durability is required. Greenstone and basalt are also occasionally used. The B. S. employed in the new houses of parliament is a magnesian limestone, or a double carbonate of lime and magnesia, which is very close and compact in texture, and is soft enough to be easily cut with the chisel; unhappily it has not proved firm enough to resist weathering, having already shown signs of decay. The other forms of carbonate of lime exhibit considerable dura bility. Near Bath and in the isle of Portland, an oolitic limestone is quarried, which is easily cut, and stands well.
A. very convenient and accurate way of determining the durability of a B. S.—in other words, its power of resisting the effects of frost and other atmospheric agencies— is to place a small block in a cold saturated solution of sulphate of soda; raise to the boiling-point, so as to expel air from cavities in the stone, which then become filled with the solution; then allow to cool, and suspend the block of B. S. in air. Every now and
then, it is dipped into the solution, and subsequently air-dried. The result is, that the sulphate of soda crystallizes on the outside and partially in the interior of the block, and in this respect acts as water does when it is frozen during winter; and if the B. S. he porous, and liable to decay by natural agencies, it gradually breaks up, and particles scale off. The amount of this corrosion can be determined by weighing the detached portions. Some building stones contain iron pyrites in little nodules diffused here and there throughout the mass, and such become discolored from the pyrites being decom posed by atmospheric influence, and the brownish-red oxide of iron (rust) is left as a st tin on the surface of the block. The liability to decay or to discoloration in a B. S. may be arrested to a great extent by coating the outer surface with boiled linseed-oil, which communicates a dark appearance to the stone, but prevents oxygen or moisture from gaining access to the block. Ordinary oil-paint is employed for the same purpose. For the preservation of B. S. from decay by means of various solutions, see STONE,