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EARTHY SUBSTANCES

257. The various vegetable substances, though sodifferent in properties, are very similar in chemicalcomposition, and may for the most part be readily con-verted or changed into each other by simple means.They may, when pure, be preserved unchanged for anunlimited time, if quite dry; but when exposed to airand moisture they sooner or later begin to decompose.Those which consist of carbon, hydrogen, and oxygenalone, are far less prone to decompose than those whichcontain in addition nitrogen; and these latter, whendecaying, possess the singular property of causingsubstances which do not contain nitrogen to decomposeor change likewise.

258. When vegetable substances, such as lignin orsugar, are burnt in the air, water and carbonic acid areproduced; precisely the same substances are formed bydecay as by combustion. When gluten and albumenare burnt, water, carbonic acid, and ammonia areformed.

259. All plants contain a small quantity of inorganicmatter. Besides the lignin, gum, sugar, starch, gluten,and other similar substances which consist of carbon,hydrogen, oxygen, and nitrogen, and which burn in theair when heated, there are always found substanceswhich cannot burn, but which are of an earthy nature;these substances remain as ashes, when plants are burnt.

260. The inorganic substances found in plants areby no means numerous; the most common are salts oflime, potash, soda, magnesia, and silica. These sub-