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OILS AND RESINS.

the base, which was previously combined with theorganic acid, is found after its destruction combined withcarbonic acid in the state of a carbonate.

245. The most important of the organic acids arevinegar, citric, tartaric, malic, and oxalic acids. Thereare few plants which do not contain a small portion ofone or other of these acids, either in the free state, or elsecombined with a base. Unripe fruit contains a consi-derable quantity of malic and tartaric acids; and theleaves and stems of many plants, such as sorrel and rhu-barb, have a strong sour taste from the presence ofoxalic and malic acids.

246. The oily substances formed by plants are like-wise very numerous ; they are of two kinds, fixed andvolatile, those which when exposed to the air remainwithout diminishing in bulk, and those which when soexposed evaporate. Olive oil, castor oil, and cocoa-nutoil, belong to the former class ; whilst oil of lavender, oilof cloves, and oil of lemons, are volatile oils.

247. All vegetable oils, whether fixed or volatile,contain oxygen, hydrogen, and carbon ; they are allcombustible, and when burning form water and carbonicacid gas. Oils are found in all parts of plants, butrather more abundantly in the seeds and fruits than inother parts. The seeds of some plants, such as thepoppy, linseed, rape, and mustard, contain frequentlynearly half their weight of oil.

248. The resins, which are not less numerous thanthe oils, resemble them in chemical composition and