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are, a Retort or arty other veffel in which thematerials are heated ; a Receiver to detain thatpart of the product which is condenfible bymere cooling ; and a bent Tube proceeding fromthe receiver to the bottom of a bottle placed byits fide, and full, or nearly fo, of water, or anyother liquid. If more than one bottle is em-ployed, thefe are ranged fide by fide, and con-nected with each other by bent tubes, each ofwhich proceeds from the top of the bottle im-mediately preceding, and plunges to the bottomcf the liquid of the bottle next in order. Everypart of the apparatus is air-tight except the endthe furthell from the retort, fo that every par-ticle of vapour or gas has to traverfe the whole •feries of veffels, and to pafs through the liquidin every one of the bottles before it efcapes intothe air; and thus if at all condenfible by theliquid ufed, it can hardly efcape condenfation.

Another part however is neeeffary to the per-fection of this apparatus. When the diftillationis nearly over, and the production of gas orvapour flackens, or ceafes altogether, a degreeof vacuum is always made in the retort, owingto the cooling of the elaftic vapour which itcontains. This circumltance caufes a fimilarvacuum in the receiver, to fill which, a ftrongabforption or fuCtion of the liquid takes placefrom the firft bottle back into the receiverthrough the bent tube that dips into the bottle.This again caufes an equal vacuum in the firftbottle, which is filled by abforption of fluidfrom the fecond bottle through the communi-cating tube, and fo on to the laft ; fo that themoment that abforption begins, a retrogrademotion takes place through the whole appara-tus. The inconvenience of this is the mixtureof the products that takes place •, for if, forexample, the diftillation of nitric acid is theprocefs, after the materials in the retort havegiven out every thing, the receiver, which wasat firft left empty, contains the concentratedacid, and the bottles contain water ftronglyimpregnated with nitrous vapour. But themoment that abforption begins, the diluteliquor of the firft bottle is poured into thereceiver, which makes it impoffible to colleftany concentrated acid, unlefs the operator ison the watch to loofen the luting between theretort and receiver the moment he perceivesthe liquor to rife in the bent tube between thet receiver and the firft bottle. To remedy thisdefect feveral contrivances have been adopted.The inventor, Mr. Woulfe, who was fullyaware of it, employed an intermediate veffelbetween the receiver fend the firft water-bottle

which was left empty, and into which the benttube from the receiver was immerfed only to afhort depth, in confequence of which when theabforption took place the liquor from the firftbottle was poured into this intermediate bottleand went no further. This contrivance is veryufeful where only a fingle water-bottle is em-ployed, but where more than one is ufed, itis obvious "that the liquors contained in themmuff; mingle, though they all keep feparatefrom the contents of the receiver; for whenany of the liquor of the firft bottle paffes intothe intermediate bottle, an equivalent portionof the fecond muft pour into the firft bottle, ofthe third, into the fecond, and fo on. Anothercontrivance (and which is now generallyadopted) is to prevent the abforption altogetherby letting in the external air to fill the partialvacuum. This is done either by inferringftraight capillary tubes into the different veffels,or by a curved tube of particular conftruftioninvented by Welter, and called Welter’s Tubes,or Tubes of Safety . A fingle example of theftraight tubes is fhewn in PI. IV. Fig. 58. ais a bottle with three openings, half filled withwater, which receives the gas or vapour fromthe preceding bottle, or from, the receiver ; thtexcefs of which after paffing through the waterof a pafles by the tube d into the bottle e, andwhich is only loofely flopped ; c is an uprightglafs tube with a very narrow bore which juftdips- into the water of a. Confequently, whenan abforption takes place in the receiver andwater is fucked up by b, it immediately finksthe level of fluid in a below the bottom of thetube c , which then gives the bottle a a freecommunication with the air, and prevents theliquor of e from paffing into it. This plan ofupright tubes may be adopted for every bottleof the apparatus, and it is convenient from itsfimplicity, and any perfon may fit this appara-tus for himfelf limply by cutting a piece ofglafs tube of the proper length, running itthrough a perforated cork, and luting all thejoinings. It has however the great inconveni-ence that it is inapplicable to the receiver itfelf(which is precifely the place where it is moftrequifite to prevent the abforption) as thisveffel is quite empty at firft, and the quantityof liquid is conftantly increafing ; and indeedthe increafing bulk of the fluid in the bottles isan objection to this method, as the end of thetube becomes progreflively lower beneath thelevel of the fluid, and may eafily be too low toaft properly.

The Tube of Safety. Fig- 60. is a fingle glaf*