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310 PHYSICAL AND CHEMICAL CHARACTER, ETC. upon the application of a small quantity of acid to the surface; and if the quantity of lime is very large, the effervescence will be proportionably brisk. The presence of carbon—often in dark coloured marbles or limestones—may be ascertained simply by heating. If the mineral is present, it will then be driven off, and the stone will become of a lighter hue. 1 Having thus presented the reader with a view of the physical and chemical characters of the more ordinary rocks, and the mode of determining them for general purposes, I shall pass on, in the next chapter, to make some observations on the selection of building materials with reference to climate and the nature of the atmosphere. 1 This, however, may not always be a test of the presence of carbon, as there are other substances which are capable of giving a dark shade to rocks, and which would become pale when heated.
