ITE, SLATE, etc.), the following general remarks may be added here. The desirable properties in a building-stone are, that it should be compact, insoluble in water, not easily altered by the atmosphere, and not liable to take on a vegetable coating. These qualities depend upon its chemical composition and on its mech:inical structure. Build ing stones may be divided into three classes—siliceous, calcareous, and composite. Sili ceous stones (including granite, porphyry, gneiss, greenstone. basalt, sandstone, slate, serpentine, etc., and containing from 45 to 99 per cent of silica) are, as a general rule, the most durable for building. Their durability is affected by certain of their ingre dients, as by the felspar in granite, and salts of iron in sandstone. Calcarous stones (sim ple limestone, travertin, marble, etc.), are slightly soluble in pure water, and more so in carbonic acid water; they are liable to splinter by water freezing in their pores, are acted on by acid gases (e.g., the sulphurous acid gas produced by the combustion of
most kinds of coal), and are somewhat liable to be stained by minute plants. Still, some of them are lasthig enough in a country atmosphere. The failure of the magnesian lime stone selected for the British houses of parliament is a good instance of a stone lasting for centuries in a country church, and yet quite unable to withstand the wasting action of the. atmosphere of a great city. Composite stones, in which neither the silica nor the lime greatly predominates, are unimportant.
The most exhaustive account of the building stones of the British islands is given in the parliamentary blue book embodying the report of the commissioners appointed to select a stone for the houses of parliament, published in 1S39. Much scientific informa tion regarding all kinds of stone will be found in the catalogue of the reek specimens of the museum of practical geology, London.