woo
w o o
( 4.93 )
fame time coygred with a tenacious froth.When this is thoroughly wafhed off, and thewood dried, the furface is of a deep charcoalblack, and hard enough to take a good polifh;but it is not eafy to get rid of every particle ofthe acid. A much finer and better black isgiven by firft foaking the wood in any I'olutionof lead, and afterwards for a day or two in afolution of arfenical liver of fulphur, made byboiling in water a mixture of two parts ofquicklime and one of orpiment. Prpbably thefimple fulphuret of lime would anfwer as well.All the other metallic folutions that naturallyblacken with tlie fulphurets will anfwer thefame purpofe, fuch as thole of fiiver, bifmuth,and iron, but lead fucceeds the bell.
With the woods that contain no gallic acida yellow-brown dye, varying to brown-red, isgiven by fmearing them over a few times with afolution of iron in dilute nitric acid. 0 Thisdye is very permanent, and does not change bythe aCtion of light or air.
A greenifh hue may be given by the folutionsof copper, but it is dilhcult to make wood takeit equally, or to give any depth of colour bythis metallic folution.
Different hues may be given by ftrong de-coftions of the colouring woods and other dyesmixed with alum, which need not be here enu-merated.
In all the above proceffes, and particularlywhere any faline or metallic folution is em-ployed, the natural glofs of the wood is muchimpaired, which requires that it fhould be fi-nifhed with linfeed, or amber, or any fimilarvarnilh, which has the effeCt both of renderingthe dye more permanent, and of much increafingits beauty.
Wood in ordinary circumftances, whetherupon the furface of the earth or buried underit, is by degrees, by the concurrent aCtion ofair and water decompofed ; its texture is brokendown, the connexion between the ieveral vege-table principles of which it confifts is diffolved,its ingredients enter into new combinations,and no veitige remains either of its organiza-tion or chemical properties. Sometimes how-ever it happens that the external figure andinternal arrangement are prelerved while thechemical properties have undergone verynotable alteration, in confequence of which thenatural decay is prodigioully retarded, andfpmelimes even wholly fufpended. Wood that
has undergone this change is faid to be petre-fied, or mineralized.
There are three fubftances by which thischange is brought about, namely pyrites, oxydof iron, and filiceous earth in the form of agateor hornilone. Thefe confiitute fo many fpe-cies of petrefied wood.
The pyritous fojfrf wood occurs principally inthe independent coal formation; it is compofedentirely of common pyrites, often in a ftate offemi-decompofition : it appears to contain noligneous particles, but retains with confiderableexactnels the external figure, and in fomedegree the internal organization of wood.The reafon why this fpecies prefents a leisftriking refemblance to wood than the othersdo, is probably the ftrong- cryftalline polaritywhich pyrites poflefies; in fplitting longitudi-nally a piece of pyritized wood it not unfre-quently happens that the fibres reprefenting theconcentric layers of wood are compofed ofminute cubes laterally aggregated to each other.
The ferruginousfojjil wood is found in hema-tite, and efpecially in argillaceous iron ore.Externally it prefents the appearance of trunksand branches, and its internal texture has aclofe refemblance to that of wood. In its che-mical compofition it does not appear to differmaterially from common argillaceous iron ore.
But agatized wood is that which has beenthe mod examined. It has been made a dirtinCtmineral fpecies by Werner, who has given itthe name of Holzstein or Woodftone, of whichthe following are the characters.
Its colour is afh-grey, palling into greyifh-black, yellowifh, brownifh, and blood-red: thecolours run into each other forming clouds anditripes in a longitudinal direction. It occursin the form of trunks, branches and roots, andprefents in the utmofl perfection the internalorganization of wood, not only the longitudinalfibres and concentric layers being vifible, buteven the knots and medullary procefles. Jts iinternal luftre is various, being between gliften*ing and dull. Its crofs fraCture is imperfeCtlyconchoidal, its longitudinal fraCture is fplinteryand fibrous. It is moderately tranflucent; isharder than glafs, and gives fire with fteel, butis eafily frangible.
It occurs in land and fandftone in variouscountries, efpecially in the hill St. Symphorien,near Etampes in France, in Saxony , Bohemia ■,and Hungary; near Loch Neagh in Ireland , inthe beds of fandftone that lie above the fuller’s
c Repertory, Vol. xiv.