Page:Cyclopaedia, Chambers - Supplement, Volume 2.djvu/287

 POT

and lying over the bolder; and the giving orders, that the;' (hould be watched, and not fuffered to deep in that manner for the future, proved with moft of them fufHcient for the cure, without any further adiftance, and that in many cafes even in maladies which had been of lo long {landing as to be- come in a manner habitual.

How frequently does it happen, that the not regarding things of this kind, which has been the caufe of difeafes, occa- fions the mod unhappy events in the courfe of their cure ? nay, fbmetimes irrecoverable misfortunes happen, without the phyficians being able to guefs why; and this fometimes after a feemingly perfect cure. A very remarkable inftance of a thing of this kind Mr. Window gives, in the cafe of a woman to whom he was called to examine the cure of a fracture of the thigh. The perfon was lame after the cure, notwith- ftanding that there were all the common proofs of the frac- ture's having been perfectly well reduced, and that the bone had united in its proper fhape and dimenfions, and no way differed from that of the other thigh. He caufed the perfon to be laid flat down, and when fhe was in that pofition, placing the knees, legs, and feet perfeflly even, the thigh which had been fet appeared perfeflly like the other ; but obferving, that in a moment afterwards, the leg on the fraftured fide thruft itfelf up, as if of its own accord, out of the level of the other, and appearing then fhorter than the other, he examined the hips; and perceiving that they were perfectly even while the leg was thus a little elevated, he again deprefl'ed it to the level of the other, and then the hips became uneven and oblique. It appeared very plain from this, that the thigh-bone had loft fomewhat of its original length from the irregular coalefcence of the fraflure ; and that the furgeon who had fet it, for want of examining the attitude of the hips at the fame time with that of the thighs, had deceived himfelf, by trailing the com- mon method of examination, into a belief that the thigh was of the proper length : what adds greatly to the occafion of error in the common way of examining thefe reduced fractures, by comparing the length of the two legs is, that the patient, dreading pain, always gives into the deceit, and naturally de- prefl'es the hip, without confidering why, till the leg, tho' really much fhorter, appears of the fame length with the other. Mem de l'Acad. Scienc. Par. 17+0.

POT (Cycl.)^ — Pct, in our old writers, a head-piece of war. It is mentioned flat. 1 3 Car. II. c. 6. Blount.

G/a/r-PoTS. See GLAss-porr.

PoT-a/J (Cyr/.)— The art of reducing vegetables to this ftate is a very valuable one ; the foap-maker, fuller, fcowerer, dyer, and glafs-man, as well as the chemift and apothecary, depend- ing greatly on it in many particulars.

The authors who are farther to be confulted in this cafe, are Glauber, in his profperity of Germany; but the chemical foundation of the whole appears in Boerhaave's chemiftry, and the papers of Stahl on this fubject. This fait has been made in New England in confiderable quantities from rotten wood. What gave the occafion of knowing it, was, that a white oak in that province decayed, and a third part of its fubftance became rotten. This rotten part was tried to burn, and was found to turn a'moft wholly into a fine alkali fait, or pot-afh, much ftronger than that which is made in the ufual way. What was moft obfervable in the making of this fait was, that while the wood was burn- ing, it would melt of itfelf, and run down into hard lumps of fait; and this, none of the wood of the fame tree, which was found, would do, but only the rotten part : and what was moft rotten of all, yielded the greateft quantity of fait, and that the moft readily ; whereas all the common alkali falts of wood, made by incineration, are blackifh at firft ; and a lixivium made of them, altho' often filtered, will ftill be of a brown colour, occafioned by fome of the charcoal of the wood fo clofefy united to the fait as not to be eafily fepa- rated from it. This alkali made from rotten wood was very white, even before folution; and the lixivium made with it was not at all tinged with brown, but clear like pure water, only a very fmall quantity of afhes fubfided to the bottom. The lixivium was eafily decanted from this, without the trouble of filtration ; and when evaporated to a drinefs, left the fait perfectly fine and pure. In the making this fait, as the fire grows more intenfe, the wood is feen to run together into great lumps, and bubble with a hiding noife. The weight of the pot-ajh prepared in the common way is very inconfiderable, in proportion to the wood ufed ; but in this, the fait nearly equals the whole weight of the wood. In the common way of making the pot-ajh, the fait is never diftinguifhed by the eye among the afhes, nor even caufes them to run into lumps ; but in this cafe the fait always runs into large lumps, and is as eafily diftinguifhed from the reft of the afhes, tho' white, as fait of tartar of the pureft kind would be.

It is very certain, that rotten wood in many places has been found, on trial, to yield much lefs fait than found wood ; but the found wood of this tree being tried, was found to yield no more fait than other wood, and confequently much lefs than the rotten part of it. On examining this' tree on the. foot, it was thought by goodjudtr.es, that it had been at fome diftance of time (truck with lightening, it being evidently torn buppr.. Vol. II.

POT

from the top to the bottom : oh that fide where the decayed part feparated from the found, there had been a channel of about five inches wide all the way down; but ibis was doled over by fucceeding bark, and (hewed no trace of the accident till on examining underneath, where the tree was found black for fuch a fpace, and the bark not of a piece with the reft. From this it may be conjeaured, that the wood having been for fo long a time, as till the growth of this bark, expodd to the wind and weather, naturally became' rotten from the wet it received ; and that the lightening having penetrated the wood, had fo altered and difpofed the parts and pores of it, that they attracted and retained the nitrous falts of the air in great abundance, even as fait of tartar, and other the like falts, do ; which, as Glauber obrerves, will be reduced, in continuance of time, to an abfolutely nitrous fait, and the quantity alfo confiderably increafed.

The parts of this rotten wood were of a very different ftrac- turefrom thofe of ordinary wood in this condition; and tho' the lightening had not calcined it into a fait, yet it had," as appears, diffidently altered it to give that tendency to imbibe, and a power to retain the nitrous particles from the air, as well as the alkalies of vegetables perfectly calcined will do. If it (hould be objected, that nitre alone will not calcine into an alkali, it is eafily anfwered, that nitre with charcoal will ; . and the remaining wood might very well ferve as charcoal in this procefs. Phil. Tranf. N'\ 361. p. 121. Some fort of pet ajlm have been found to contain a large quantity of fea-falt, which is probably, mixed with them by the makers; not only as it is much cheaper, but as it makes them whiter, and more eafily run into large mafles. Med. Eff. Edinb. abr Vol. I. p. 165. Not.

The French have two kinds of /.tt-ajb, the produce of their own manufactures ; the method of making which M. du Fay, of the academy at Paris, defcribes in the following manner. The fird which is the common or coarfer kind, is made of the beech or elm ; the firft of which trees they always find tp yield the largeft quantity.

They cut down the largeft and moft full grown trees, and cutting them into pieces of ten or twelve foot long, they Jay them together in form of a pile, and burn them to afties in the open air. They gather up the afiies, and make a very ftrong lixivium of them in water; into this liquor they throw pieces of old rotten and fpungy wood, which is, from its tex- ture, capable of imbibing a great quantity of humidity : they ufe fo many of thefe pieces as are diffident to foak up the whole quantity of their lixivium. When this is done, they dig a fquare pit in the earth ; over this they lay fome iron bars, to fuftain a quantity of dry wood, to ferve as fuel. When they have laid together a diffident quantity of this, they pile upon it the rotten billets which have imbibed the lixivium ; and when the dry wood underneath is fet on fire, and heats the top of the pile, the lixivium lofes its water by the heat, and the fait contained in it is fctn running down through the fire, in form of a (bower : this is received in the pit made on purpofe for it. When all is melted out of the billets at firft laid on, they bring others, which the fire acts upon in the fame manner ; and fo on, till the whole pit is filled with pot-ajh.

When this is done, they remove the fire as quickly as they can ; and with rakes clear the furface of the pot ajh as well as they can from pieces of charcoal and afhes which have fallen into it: but with all their care, they are not able to prevent a great deal, both of coals and afhes, remaining in it; as- is eafy to conceive from the manner of the procefs. The fait after this forms one folid cake or mafs, and this they dig up out of the pit while it is yet hot ; and barrel it up for fear of its receiving damage from the humidity of the air, which it. is very ready to imbibe. This is the French pot-ajh of the com- mon or coarfe kind.

The finer or purer kind is made of the fame wood with the other, and the procefs is conducted in the fame manner, fo far as to the making of the lixivium; but, when this is pre- pared, inftead of foaking it up with rotten wood, they pour it off into an iron caldron of a large fize, and evaporate it, ftill adding more lixivium as the former boils away, till they have employed all they have made ; they then continue the fire till the whole humidity is evaporated : they then take the dry mafs of fait out of the bottom of the velfel, and put it into a furnace made with a clofe arched top ; by means of which the flame is reverberated from all parts upon the fait. They here give it fo much fire as ferves to calcine it to a white- nefs, and then the whole operation is finiihed. It is feldom, however, quite white ; but ufually retains fome- what of the colour it had before its calcination; This the workmen fay is owing to the nature of the wood the afhes were made from. They fay that thofe trees which grow on the tops of high hills, yield a pot-afn of a pale blue colour; and thofe which grow in wet marihy places, yield a reddifh fait. Trees which have ftood in other foils and fituations, they fay, yield a white pot-ajh; but they obferve, that none yield it in fo large a quantity as thofe which grow on the tops of mountains. Mem. Acad. Par. 1J.J. POTAMOGEITON, pndwud, in the Linniean fyftem of bo- tany, a genus of plants, the characters of which are thefe - 2 Y y th e