The ligneous matter forming the substance of trees. It is, in most cases, possessed of colour, taste, and smell, from the presence of extractive matter, mucilage, resin, or essential oil; and it is only when these have been extracted by water and alcohol, that wood can, as a chemical principle, be regarded as pure. In this state, it is insoluble in water: it is equally insoluble in alcohol, and hence it forms the residuum, when any of the solid parts of plants have been acted on by these fluids. From the action of the air, if dry, it does not appear to suffer any change; but, when humid, it is gradually decomposed, and passes through many intermediate states, to that of a black mould, consisting principally of carbon. The oxygen of the atmospheric air is, during this change, absorbed, and carbonic acid formed with a portion of water; and the latter, being derived from the combination of the oxygen, leaves carbon predominant. When the air is entirely excluded, wood decomposes with extreme slowness, even though humid; as, for example, when it is buried in the earth, the alkalies act on wood, and stain it of a dark colour: with the assistance of heat, they soften, and partly dissolve and decompose it. The stronger acids act on it.

Sulphuric acid carbonizes it, rendering it speedily black and soft. Nitric acid gives it a yellow tinge, and, when acted on in large quantity, disengages nitrogen gas, and converts it into oxalic acid, with small quantities of malic and acetic acids.

Wood suffers decomposition from heat; a large quantity of an acid liquor distils over, with a portion of empyreumatic oil. Carburetted hydrogen and carbonic acid gases are disengaged, and a portion of ammonia is produced, which is neutralized by the acid. A charcoal remains, which retains the figure and even texture of the wood. The acid procured in this process, was observed to be similar to vinegar, and was afterwards regarded as a peculiar one, and named pyro-ligneous acid; but the researches of Fourcroy and Vauquelin proved that it is only acetic acid, with an impregnation of empyreumatic oil.

When air is admitted, and the heat raised to ignition, wood burns. Its combustion at first, gives much light, from the formation and extrication of carburetted hydrogen: this soon ceases, and the charcoal remains, which burns with its usual red light. The products of the combustion are principally carbonic acid and water. Nitrogen appears to be a constituent principle of wood; ammonia, therefore, is also evolved, and accordingly, an ammoniacal salt is found in the soot of wood.

The colouring of wood is effected by a variety of processes. Stains do not he, like paints, upon the surface of wood, but sink more or less into its substance. Hence, the material which has been stained, exhibits its natural grain and hardness: and it must be remembered that, if the wood be not white, the colour taken will be a compound of that of the wood and the stain. The dyeing woods employed, are in small chips or raspings.

The woods which have been stained are afterwards rubbed up with rushes, then with a cloth, dipped in a solution of bees' wax in spirits of turpentine; and afterwards rubbed with a woollen cloth alone. When the stain is intended to be very deep, the pieces should be boiled in the staining liquor, and not merely brushed over. To stain wood red, take two ounces of Brazil wood, and two ounces of potash; mix them with a quart of water, and let the composition stand in a warm place for several days, stirring it occasionally. With this liquor, made boiling hot, brush over the wood till the desired depth of colour is obtained: then with another brush, brush over the wood while yet wet, with a solution of alum, in the proportion of two ounces of alum to a quart of water. For a pink or rose red, use double the quantity of potash. For a less bright red, dissolve an ounce of dragon's blood in a pint of spirits of wine, and brush over the wood with the tincture till the stain appear to be as strong as is desired; but this is, in fact, rather lacquering than staining.

For a pink or rose red, add to a gallon of the above infusion of Brazil wood two additional ounces of the pearl-ashes, and use it as was before directed; but it is necessary, in this case, to brush the wood over with the alum-water. By increasing the proportion of pearl-ashes, the red may be rendered yet paler; but it is proper, when more than this quantity is added, to make the alum-water stronger. To stain wood green, dissolve verdigris in vinegar, or crystals of verdigris in water, and brush over the wood with the hot solution. To stain wood blue, dissolve copper in diluted nitric acid, and brush it while hot several times over the wood; then make a solution of pearl-ashes, in the proportion of two ounces to a pint of water, and brush over the stain made with the solution by copper till the colour be per-fectly blue. The green stain, made as above with verdigris, may be changed to a blue, by the solution of pearl-ashes. The sulphate of indigo, which may be had, ready prepared, of the dyers, will, when diluted with water, make a blue stain. To stain wood black, brush the wood several times with a hot decoction of logwood, then several times with common ink.

To make a very fine black, brush over the wood with a solution of copper in nitric aid as for blue, and afterwards with logwood, till all the greenness of the copper solution is gone. To stain wood purple, take one ounce of logwood and two drachms of Brazil wood; boil them together in a quart of water, over a moderate fire. When one half of the fluid is evaporated, strain the decoction, and brush it several times over the wood. After the wood is dry, brush it over with a solution of a drachm of pearl-ashea in a pint of water.