EXISTING IN PLANTS.

87

stances are found in the ashes of plants, combined withcarbonic, sulphuric, phosphoric, and some other acids.

261. The ashes of plants very frequently contain aconsiderable quantity of carbonate of potash (127);this salt does not exist in the growing plants, but isformed during their combustion. Potash is found in avery great number of plants, sometimes in combinationwith sulphuric acid or silica, but more commonly unitedto tartaric, oxalic, or some other organic acid, consti-tuting a tartrate, oxalate, or other salt of potash. Whenplants are burned, all the organic acids are burned, aswell as the lignin and similar substances which theycontain; consequently, although in living plants thepotash is combined with organic acids to form neutralsalts, yet, as these acids are destroyed when plants areburned, caustic potash is left (128), which, of course,combines with carbonic acid to form carbonate ofpotash (244).

262. In the same way, carbonate of soda is frequentlyformed by burning plants which contain salts of sodaunited to various organic acids.

263. The ashes of many plants contain silica (191),generally in combination with either potash or soda.

264. Lime is almost invariably found in the ashes ofall plants, and frequently constitutes a large proportionof the earthy substances present in them. It sometimesexists in combination with sulphuric acid and sulphateof lime (166), but far more commonly united to phos-phoric acid, or in the state of phosphate of lime (170).