STONE FOR BUILDING. Stone fit for building must in general be freestone firm enough to sustain great pressure, and yet so aggregated as to admit of being worked with facility by ordinary tools, and of receiving correct surfaces in any direction. It must in general yield masses of great dimensions. In the great variety of limestones and sandstones which are adapted for building purposes, we remark, by the aid of the microscope, three principal modes of molecular aggregation : mixtures of grains ; segregated concretions of grains; and compacted crystallisations. Mix tures of very unequal or very dissimilar parts, as millstone-grit ; concretions which have earthy textures in their interstices, as some oolitic limestones ; crystallisations which do not produce compactness, as in some magne. Sian limestones,—are not in general durable.
The commissioners (Mr. Barry, Sir H. de la Beche, and Dr. W. Smith) who reported on the choice of stone for the construction of the New Houses of Parliament, made valuable ex periments as to the Hardness, closeness, den sity, colour, and durability of many varieties of stone. The strength of several sorts of stone, as measured by the weight necessary to be applied for lireaking and crushing them,appears below: Of sixteen specimens selected, the stone most absorbent of water was proved to be the Bath oolite from Box ; that most injured by Brard's artificial process of disintegration was the Barnack stone : and that which was most easily crushed was the Bath oolite from Box, Generally speaking, sandstones were least absorbent, magnesian limestones least disin tegrating ; sandstones appeared to be strongest, though choice magnesian limestones (as that of Bolsover, finally recommended by the Com missioners) were fully equal in this respect, and were almost as little absorbent.
Stones of uniform texture commonly decay by disintegration at the surface, losing grain by grain in proportion to time and exposure. Stones composed of parts unequally mixed suffer unequal waste in different parts. Shells, corals, concretions, and crystallised masses, thus appear prominent from earthy limestones, and indicate the general fact that, in propor tion to the force of molecular aggregation in the stone, is the resistance which it offers to decay. It is not the amount but the kind of exposure which governs the decay of stone : the southern and western parts of our cathe drals give way more than the northern and eastern.