ORGANIC ACIDS.

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and likewise albumen contain nitrogen ; and all organicsubstances containing nitrogen have a great tendency toputrefy.

241. Every one knows that plants possess veryvarious properties: some are noted for their fragrantsmell, others for the brilliant colours they produce ;some yield oils, others resins, and a great many arevalued for their peculiar medicinal qualities : all thesevarious properties are caused by the presence of a certainquantity of some organic substance, some peculiar com-pound of oxygen, hydrogen, carbon, and nitrogen, whichis formed by the plant.

242. The number of these organic substances whichhave been discovered in plants is very great ; it is quiteunnecessary to describe or even enumerate them all ; itwill be sufficient to mention very briefly the most im-portant of them, arranged under three or four heads.

243. Amongst the substances formed by plants is anumerous series of organic acids, substances composedof oxygen, hydrogen, and carbon, which possess acidpowers, and combine eagerly with bases to form neutralsalts ; in general they do not exist in plants in the freestate, but are combined with various bases derived fromthe soil; sometimes they are found in the state of per-fectly neutral salts, but more frequently they form acidsalts, that is to say, there is more acid than the base isable to neutralize ; such plants have a sour taste.

244. When the salts of organic acids are burned orstrongly heated, the acids are decomposed; in these cases