PET
PET
in that ground; and this feems to he the mofl probable opi- nion as to the origin of the petrifactions, not only of" our fields,
hut of thofe found in many parts of the world among fand 01 gravel.
It is to be obferved alfo, that the finer the lapidific particles are, the more beautiful and natural the petrfact't n will ap pear. Of this kind there was once found a beautiful fpecimen onthefhores of t lie Lough Neagh. This v/as a root of the com- mon yellow flag-flower, with the bottom part wholly con- verted into (lone, but yet retaining all the fibres, and every part of the texture of the root; the pith in the middle was turned into a whiter ftony fubftance, and the upper fur ace had ail the traces of the ieveral tubercles, or firft moots of leaves, tho' a very hard (tone; and the outer covering of fome of thefe retained its vegetable nature frill fo far, as to be flexile in a fmall degree, while all within was folid (tone,
Some of the pieces of petrified wood, found on the fhores of the lake, have curious veins of a red and bluei/h colour, and others are beautifully variegated with black and white Una?. The woody part of them gives a red tincture to aqua fortis, and when taken out of that menftruum, {hews fome red fpots in its pores, which, perhaps, are its firft approaches towards petrifaclion, and are either of a mineral or ftony matter. It is common to find in the cracks of thefe ftones, nay, and of fuch parts of them as are yet wood and not turned into ftone, great numbers of very beautiful and regular, tho' fmall, cry- ftals: thefe are tranfparent and hexangular, and of the fame kind with the fmall cryftals found in the cracks of ferrugi- neous ftones, and within our common flints. When calcined, the powder yields to the magnet, which fhews that they have fome ferrugineous as well as ftony particles : and it is not un- frequent to, meet with pieces of unaltered wood in the very center of the largeft of thefe petrified mafles ; and the petrified parts of the mafles, when examined by the microfcope, ap- pear extremely beautiful, their pores being all vifibly filled with pure and very bright cryftal. The different degrees of petrifaction appear very various in the feveral pieces, fome are woody on the outfide, and altogether ftonv within ; and others outwardly ftony, and inwardly unaltered wood ; and others having the woody or the ftony part appearing only in thin lines and coats on different parts. The harder or more ftony parts of thefe petrifactions always give fire with ftcel. Phil. Tranf. N". 481.
Some of them fhew the grain of afh, fome of holly, and fome of fir, others of oak ; but thefe are more rare. They frequent- ly have the plain traces of knots and other irregularities, and fome arc found bent and half broken, and plainly have been worked by art before they were petrified.
It is a common opinion, that thefe ftones are what are ufed for hones in different parts of England ; but this is an error, the fubftance of which thefe are made being real ftone, and not found here, but at Drogheda. Thofe petrifac- tions of the lake, which have part wood, part (lone, ufually lofe the wood afterwards, by its cracking and falling to pieces as the ftone dries; but if examined while it remains on the ftone, it is very convincing, as to the reality of the origin of the whole from wood. Id. ibid. Petrification, in medicine. See Ancubitus, PETRIFIED Ivy. See the article Ivy.
PETRIFYING waters AlonfoBarba gives an account of fome petrifying waters of Peru, which greatly exceed all thofe we have in Europe in the quantity of ftony matter they contain. He tells us, that they foon choak up their own paiTage by the ftony matter they depofit there ; and that all the cattle that drink of them die. Ke adds another ffory, which he probably took a little too haftily upon credit, which is, that they have moulds of the fhape of our bricks, which they fill with this water, and that on being expofed to the fun a few days, the water is wholly converted into a ftone of the fame fhape ; and that they build their houfes and other buildings with thefe ftones. Aknfo Harba dc Metal. PETROCOSSYPHUS, in zoology, a name given by fome au- thors to the bird more ufually called from its colour thecaru- leus.
It lives among the rocks, in woody mountains, and fmgs very fweetlv. See C#:rui.E(<s. PETROL, or Petroleum— (Cycf.)
Mr. Boulduc made feveral experiments with the white petro- leum of Modena, an account of which he gave to the Paris academy.
It eafily took fire on being brought near a candle, and that without immediately touching the flame ; and when heated in any vefiel, it will attract the fiame of a candle, tho' placed at a great height above the vefiel, and the vapour it fends up taking fire, the flame will be communicated to the veffe! of heated liquor, and the whole will be continued. It burns in the water, and when mixed with any liquor fwims on the furface of it, even of the higheft rectified fpirit of wine, which is 7 heavier than pure petroleum. It readily mixes with all the eflential oils of vegetables, as oil of lavender, turpen- tine, and the reft, and feems very much of their nature: nor is this very ftrange, fince the alliance between thefe bodies is probably nearer than is imagined, as the effeutial oils of vegetables may have been originally mineral ones, and drawn
up out of the earth into the veflels of the plant. Mem. Aead. Par. 1715.
Petroleum, when fhaken, yields a few bubbles ; but they fooner fubfide than in almoft any other liquor, and the liquor re- fumes its clear ftate again almoft immediately. This feems owing to the air in this fluid being very equally diftributed to all its parts, and the liquor being compoied of panicles very evenly and nicely arranged.
1 he extenfibility of this oil is alfo amazing. A drop of it will fpread over feveral feet of water, and in this condition it gives a ^reat variety of colours, that is, the feveral parts of which this thin film is compofed, act as fo many prifms. The moft fevere froft never congeals petroleum into ice, and paper wetted with it becomes tranfparent, as when wetted with oil ; but it does not continue fo, the paper becoming, opake again in a few minutes, as the oil dries away. Spirit of wine, which is the great diffolvent of fulphur, has no effect upon petr-leum, not even with ever fo long a digeftion. It will not take fire with the depblegmatcd acid fpirits, as oil of cloves, and other of the vegetable efiential oils do: and in diftillation, either by balneum mari;e, or in fand, it will nei- ther yield phlegm nor acid fpirit; but the oil itfelf rifes in its own form, leaving in the retort only a little matter, thick as honey, and of a brownifh colour. Whoever, therefore, would ufe this oil in medicine, muft take it as nature has pre- pared it, art having no power to make any alteration in it. Mem. Acad. Paris 17(5.
Alonfo Barba, in his book of metals, gives a very melancholy inftance of the power of petroleum of taking fire at a diftance : he tells us, that a certain well, yielding petnkum on the fur- face of its water, being to be repaired, the workman took down into the well with him a lanthurn and a candle in it; there were fome holes in the lanthorn, through which the petro'eum, at a confiderable diftance, fucked out the flame of the cand'e, and taking fire, burft up with the noifc of a can- non, and tore the man to pieces. Alcnfi Barha deMet. The pe pie of mount Ciaro, in Italy, have fome years fince found out a much eafier way of finding petroleum, than that they formerly had been ufed to.
This mountain abounds with a fort of greyifh fait, which lies in large horizontal beds, mingled with ftrata of clay, and large quantities of a fpar of that kind called by the Ger- man s felem'tes ; which is the common fort, that ferments with acids, and readily diflblves in them, and calcines in a fmall fire. They pierce thefe flates in a perpendicular direction till they find water, and the petroleum which had been difperfed among the cracks of thofe flates, is then warned out by the water, and brought from all the neighbouring places to the hole or well which they have dug, on the furface of the water of which it fwims after eight or ten days. When there is enough of it got together, they lade it from the top of the water with brafs bafom, and it is then eafily feparated from what little water is taken up with it., It is remarkable, that all the petroleum got by this means is white, whereas that of Modena is yellow, and that of Parma brown. Thefe wells or holes continue to furnifh the oil in different quantities for a confidtrable time, and when they will yield no more, they pierce the flates in fome other place. Mem, Acad. Scien. Par. 1736. See the article Naphtha. ■ Petroleum is never ufed among us ; but the French give it in- ternally in hyfteric complaints, and to their children for worms:' fome alfo give it from ten to fifteen drops in wine, for fup- preflions of the menfes. This, however, is rather the prac- tice of the common people than of the faculty.
PETROMrtRULA, in botany, a name given by feveral au- thors to the pyramidal rapunculus of the ifland of Crete, called by Mr. Tournefort, rapunculus creticus feu pyramidalis alter. See Rapunculus.
PETROMYZON, the /one-fucker, in the Linnsan fyftem of zoology, the name of a genus of fifties of the chondropterygii order, comprehending the lamprey, &c. The characters of this are, that the apertures of the branchiae are feven on each ftde. Linnai Syftem. Natur. p. 52
The name Is originally Greek, and is derived from the words vtyy, a ftone, and (av&u, to fuck ; this fifli being ufually found in rivers adhering to the ftones by fucking, and fo keeping its place.
The characters of this genus, according to Artedi, are thefe: the foramina, or apertures of the branchiae, or lungs, are fe- ven on each fide; they are placed longitudinally, and befide thefe, there is one fingle aperture in the head, placed imme- diately between the eyes. The body is long, and nearly of a cylindric figure; it is fmooth, and has only two fins, and thofe are both placed on the back.
The fpecies of this genus, enumerated by Artedi, are thefe: 1 . The petromyzon, with only one row of minute teeth in the verge of the mouth, except the large lower ones. This is the IcrHpetra fuviaiilis miner of authors, or the lamprey eel: and the la?npct> a; medium genus, called by the G ermans the brki, is only a variety of this, not adiftinct fpecies. 2. The fpotted^- tromyzoU) with about twenty rows of teeth. This is the lampctra major of authors, and is called in Englifh the lamprey. 3. The petromyzon with an annulated body, and with two appendages on each fide of the mouth. Thi.s is the enwophthalmui ccecm of 2 au-
