STONE FOIL BUILDING. There are few practical questions belonging to the building arts of more importance than those connected with the nature of the stones used in them ; for not only does the character of the architecture adopted materially depend upon the faci lities the stones present for artistic expression, but the durability of the buildings, and the retention of their original effect must be regu lated entirely by the powers of resistance of those materials to the causes of decay which are always at work upon them. Where stone of large dimensions, and of great transverse strength, is used habitually, architecture assumes an imposing palatial effect, with a tendency to prefer the horizontal to the vertical line : where stone of small dimen sions is used, vaulted construction, heavy piers, and the minute style of ornamentation (which always accompanies the use of the rendering coats, almost necessarily used on such stones,) may be as distinctly traced as the external characteristics of the architecture adopted ; and the horizontal line habitually disappears in favour of the arch, or of the vertical line : where stones are easily worked, florid ornamentation is adopted ; where they are hard, and comparatively unmanageable, massive, and sometimes clumsy forme are adhered to. Tho durability of the stones, and their capacity of supporting fine sculpture, moreover, must materially affect the decision of the architect with respect to the amount of decoration to be employed on them, and to the choice of the mode in which they are to be treated ; for it must evidently to absurd to spend time and money on the decoration of materials which are susceptible of rapid or of unequal decay. The resistances of stones to crushing forces have likewise a marked influence upon the style of building they are capable of receiving; for that style is often light, or massive, in proportion to the resistance, provided always that the labour in the conversion of the stone should offer little resistance to what may be called its plastic treatment. The colour of stones has also an influence upon the ssthetical effect of a building ; and it, therefore, becomes in its turn a matter of importance to the architect, both on account of the original. oolour, and of its possible modi
fications.
A good building stone must possess, then, great powers of resistance to crushing, and breaking, weights; it must be even in its grain, and able to furnish large blocks; it must not contain 'elements which are susceptible of decomposition by reason of the atmospheric action upon them, or of their mutual reactions upon one another ; it must be non-absorbent of water, or of vapour, to a certain extent (but only to a certain extent, as will be shown in the sequel); and, as far as possible, be of a homogeneous nature. As a general rule, the denser stones are more durable than the lighter ones ; but this law only holds good when their structure is of a permanent and of a crystalline nature. The stones which are of a massive character from top to bottom of the quarry-bed are usually more durable than the foliated, or fissile materials of the same description ; and the crystalline stones are almost always superior to the amorphous ones, both in appearance and durability. The granites, quartz rocks, and transition marbles furnish the most lasting building stones, though even they are occasionally subject to destructive actions of peculiar natures ; the sandstone conglomerates, and the sandstones partially metamorphosed by the vicinity of plutonic rocks, are usually more durable than the limestones; whilst of the latter, the stones presenting a regular crystallisation are far superior to the saccharoid marbles, or to the amorphous earthy-textured stones. The clay slates, which perhaps can hardly be considered to be building stones, present singular powers of resistance to atmospheric agents, after they have been exposed to the action of plutonic rocks; and the magnesian limestones, when selected from the crystalline varieties, are of great practical value. When stones are mainly composed of organic remains united by a more or less crystalline cement, or when they are composed of materials having different rates of expansion, there are great probabilities of the occurrence of rapid decay, and materials of this description should be avoided for external uses.