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LIME.—MAGNESIA.—IKON.

but it is evident that plants, sucli as grasses, winchcontain silica, must obtain it from the soil, by graduallyabsorbing it in combination with alkali dissolved inwater.

306. Lime and magnesia, both which have a powerfulaffinity for acids, are never present in the soil except incombination with some acid, and this is most commonlythe carbonic ; the former substance is also not unfre-quently found combined with sulphuric acid, constitutinggypsum, or sulphate of lime.

307. The oxides of iron in the soil are usually uncom-bined, as they have not sufficient attraction for carbonicacid to combine with that gas, which is always present inthe air. They in great measure occasion the variations ofcolour observed amongst soils ; for according as the ironis in the state of protoxide or peroxide, it gives to thesoil a black or brownish-red colour.

308. It must not be supposed, however, that thecolour of soils is wholly dependent on the iron whichthey contain, or that the blackness of any particularsoil is indicative of the presence of protoxide of iron.The decomposing vegetable substances, which all soilscontain in greater or less quantity, are usually of abrown or black colour, and therefore not unfrequentlygive a very dark colour to soil which does not containany iron.

309. - Sulphate of iron is also sometimes present insoils in very small quantity, being formed by the gra-dual oxidation of sulpliuret of iron in the manner pre-