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softer or more soluble substances may be separated, andpure lignin is left. In the process for preparing flax,the stems of the flax plant are allowed to remain inwater for some time; the green, soft parts decay, and atlast nothing but the lignin or woody fibre is left.
225. Pure lignin is a white tough fibrous substance,composed of an infinite number of very fine threads orfibres perfectly insoluble in water, and not at all alteredby keeping in dry air. When heated in the fire it soonturns brown, being partially decomposed; if it be stillfurther heated, it takes fire and burns with a brightflame, the results of its combustion being water andcarbonic acid gas.
226. Starch is almost always found in considerablequantity in all parts of plants. When pure, it is a whitepowder, insoluble in cold water, but readily dissolving inthat fluid when boiling hot. There are many differentvarieties of starch, distinguished from one another bysome peculiar property, and which have received variousnames, according to the plant from which they areobtained. Common wheat-starch, which exists in suchlarge quantities in the seed of wheat, is a good exampleof the general character of this substance.
227. When grated potatoes are placed on a sieve,under a stream of water, a very large quantity of starchmay be washed out. The starch will soon settle to thebottom of the water, the soluble matters of the potatowill be dissolved, and at last there will remain on thesieve little else besides the lignin or fibre which thepotatoes contained.