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when hot, and fhorfer and fmalier when cold : A thousand experiments prove this, and the books of experimental phi- lolbphy have fuiKciently expatiated upon it ; on the contrary, it is found to be the property of Wood, that it is longeft in cold weather and fhorieft in hot ; this change is owing to the remains of the fap yet in the Wood, which being con- denied by cold is enlarged in its furface, as all liquors are when frozen into ice ; and fhrinks into a leis fpace or bulk again, when liquated by heat.
It follows from this, that all Wood muft change its furface more or lefs, according as it contains more or lefs fap, and this may be made a teft of great ufe for the determining what kinds of Wood have moff, and what leaft fap. This would be a very valuable piece of knowledge, fince there are many ufes for which that fort of Wood muft always ferve belt, which has the imaHeft quantity of fap remaining in it. See the article Hygroscope, Cycl. and SuppL Thus, in the great article of preferving of flour, no barrels are at prefent ufed, butthofe of feafoncd dry oak; the whole advantage of this Wood is, that it contains lefs fap than ©thers ; for the fap in the Wood makes the flour damp, and it then becomes rancid, and breeds worms. So that if any other Wood can by this means be found out to contain lefs fap when dried in the common way than oak does, it will be fo much the better for this purpofe ; or if a cheaper Wood fhould be found only to contain as little fap as the oak, it would do as well, and the price of oak would be feved in thefe veffels. A proper way of trying when the fap was fufiiciently ex- haled out of trees, might alio be found by this experiment, and much benefit would accrue from it ; for our mips, when made of timber not fufficiently dried, prove injurious to the healths of the people on board ; and it has been remarked both by the French and ourfelves, that many more men in general die in the firft voyage of a new fhip, than in the fame time in an old one; and indeed the h" rft fix months are ufually obferved in this cafe to be moft fatal. The ex- halation of the fap from the Wood of the veffel, is certainly the occafion of this, and if it could be contrived to have this fap properly exhaled before the timber was ufed, it would not only prevent this mortality among the men, but the vef- lel itfelf would be the founder., and the better for it. Def- iands, Trait, de Phyf.
Wood ufed for fuel is required of various kinds, in regard to the various works to be performed by it. Neri every where commends oak for the Wood to be burnt in the glafs-houfes, as the propereft Wood for making a itrong and durable fire with a good flame. Imperato, on the contrary, commends am on the fame oc- cafion ; becaufe, as he fays, it gives a fubifantial, rather than a great flame :' And Camerarius defervedly commends juni- per Wood, as affording a lafting ftrong and fweet fire, could plenty of it be had. Among the antients, Pliny commends light dry Wood; and Plutarch, the tamerifk in particular, for making the glafs-houfe fires ; but glafs-making requires £o great a fire, as cannot be eafily made from fuch Wood. Nor can afh be proper, becaufe though it gives a good fire, it foon decays. Merret's Notes on Neri, p. 275. Foffilc Wood. FoJJile Wood, or whole trees or parts of them, are very frequently found buried in the earth, and that in different ftrata ; fometimes in ftone, hut more ufually in earth ; and fometimes in fmall pieces loofe among gravel. Thefe, according to the time they have lain in the earth, or the matter they have Iain among and in the way of, are found differently altered from their original ftate ; fome of them having fuffered very little change, and others being fo highly impregnated with cryftalline, fparry, pyritical, or other extraneous matter, as to appear mere manes of ftone or lumps of the common matter of the pyrites, fcfV. of the dimenfions, and more or lefs of the internal figure of the vegetable bodies into the pores of which they have made their way.
The fofftle Wood, which we find at this day, may, according to thefe differences, be arranged into three kinds : 1. The iefs altered. 2. The pyritical. And, 3. the petrified. Of the trees or parts of them lefs altered from their original ftate, the greateft ftore is found in digging to fmall depths in bogs, and among what is called peat or turf-earth, a itib- ftance ufed in many parts of the kingdom for fuel. In dig- ging among this, ufually very near the furface, they find im- menfe quantities of vegetable matter buried, and that of va- rious kinds : In fome places there are whole trees fcarce al- tered, except in colour ; the oaks in particular, being ufual- ly turned to a jetty black ; the pines and firs, which are alfo very frequent, are lefs altered, and are as inflammable as ever, and often contain between the bark and wood a plain refin. Large parts of trees have alfo been not unfre- quently met with unaltered in beds of another kind, and at much greater depths, as in the ftrata of clay and loam, 1- mong gravel, and fometimes even in folid ftone. Hill's Hift. of Foff. p. 638.
Befide thefe harder parts of trees, there are frequently found alfo in the peat-earth vaft quantities of the leaves and fruits and catkins of the hazel, and the like trees ; thefe are ufu-
ally intermixed among the fedge and roots of graft, and are fcarce at all altered from their ufual texture. The moft: common of thefe are hazel nuts ; but there are frequently found alfo the twigs and leaves of the white poplar ; and a little deeper ufually there lies a cracked and mattered Wood, the crevices of which are full of a bituminous black matter ; and among this the ftones of plumbs and other ftone-fruits arc fometimes found, but that more rarely. In places where the fir and pine kinds are buried, the fmalier twigs, and the cones or fruit are frequently met with near the furface, the bodies of the trees being lodged deeper. The oaks are in many places fo found, that the Wood is little inferior in hardnefs to that cut out of the moft folid frefh tree. The foJlile trees in our peat-earth and moors are often found entire, with their roots remaining fixed to them ; but more frequently the bodies of the trees are broken oft" near the roots, and that and the tops of the branches with the fruit, which are often feparated alfo, are found at fmall dif- tances in the fame moor : Many of the larger branches
are alfo found fevered from thefe trees, and r
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at fome
diftance ; when the roots are torn up and lie in irregular di- rections, the extremities of thefe are alfo broken off. But very frequently the roots of trees remain in their natural pofitton, and their larger roots and tap-root run ftrait down into the clay, or other folid ftratum which is the bottom of the bog ; the ftump remaining above the furface, and the tree lying horizontally at a fmall diftance, buried at a greater or fmalier depth.
It is idle to imagine, that thefe have been thus buried either at the creation,- or, as many are fond of believing, at the univerfal deluge : at the firft of thefe times the ftrata mult have been formed before trees were yet in being, and the peat Wood is fo far from being of antediluvian date, that much of it is well known to have been growing within thefe three hundred years, in the very places where it is now found buried.
In this ftate, that is little altered from their original con- dition, it is, that the fruits, and larger parts of trees are ufu- ally found ; what we find of them more altered, are fome- times large and long, fometimes fmalier and ihorter branches of trees ; fometimes fmall fragments of branches, and more frequently fmall fhapelefs pieces of Wood. The larger and longer branches arc ufually found bedded in the ftrata of ftone, and are more or lefs altered into the nature of the ftratum they lie in : The fhorter and fmalier branches are found in vaft variety in the ftrata of blue clay, ufed for mak- ing tiles in the neighbourhood of London ; thefe are pro- digioufly plentiful in all the clay-pits of this kind, and ufu- ally carry the whole external refemblance of what they once were, but nothing of the inner ftructure ; their pores being- wholly filled, and undiftinguifhably clofed by the matter of the common vitriolic pyrites, fo as to appear mere fimple mafi.es of that matter. Thefe fall to pieces, on being long cxpofed to a moifture, and are fo pregnant in vitriol that they are what are principally ufed for making the green vi- triol or copperas, at Deptford, and other places. The irregular manes or fragments of Wood, are principally of oak, and are moft ufually found among gravel ; though fometimes in other ftrata. Thefe are variously altered by- the iniinuation of cryftalline and ftony particles, and make a very beautiful figure when cut and polifhed, as they ufu- ally keep the regular grain of the Wood, and fhew exactly the feveral circles which mark the different years growth. Thefe, according to the different matter which has filled their pores, affume various colours, and the appearance of the various foluls that have impregnated their ; fome are perfectly white, and but moderately hard ; others of a brown- ifh black, or perfectly black, and much harder ; others of a reddifh black, others yellowifh, and others greyifh, and fome of a ferrugineous colour. They are of different weights alfo and hardnefl.es, according to the nature and quantity of the ftony particles they contain : Of thefe fome pieces have been found with every pore filled with pure pellucid cryftal ; and others in large manes part of which is wholly petrified, and feems mere ftone, while the reft is crumbly and is un- altered Wood.
That this alteration is made in Wood even at this time, is alfo abundantly proved by the inftances of Wood being put into the hollows of mines, as props and fupports to the roofs, which is found after a number of years as truly petri- fied as that which is dug up from the natural ftrata of the earth. In the pieces of petrified Wood found in Germany, there are frequently veins of fpar or of pure cryftal, fome- times of earthy fubftances, and often of the matter of the common pebbles : Thefe fragments of Wood fometimes have the appearance of parts of branches of trees in their natural ftate ; but more frequently they refemble pieces of broken boards ; thefe are ufually capable of a high and elegant polifii.
Many fubftances, it is certain, have been preferved in the cabinets of collectors, under the title of petrified Wood, which have very little right to that name. But where the whole outer figure of the Wood, the exact lineaments of the bark, or the ftbrofe and fiftular texture of the ftrite, and the
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