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come in contaft with the contained gas, andthe bottle from time to time immerfed againunder water, and the fide ftopper a littleloofened that the water may ruffi in to fupplythe partial vacuum produced by the absorption.This will indeed fomewhat dilute the eudiome-trical liquor, but not fo as to prevent it fromacting. When no more abforption takes placethe procefs is finifhed, (which only requires afew minutes at longeft) and the abforption isnoted at once by the graduated fcale ; or if thetube is not graduated, the refidual gas may betransferred to another that is.

Another eudiometer which is intended merelyfor the analyfis of the atmofpheric air of any fitu-ation, is that of Mr. Davy, PI. IV. Fig. 56. whichconfifts fimply of a portion of glafs barometertube, bent into two legs of equal length, both ofwhich are furnifhed with glafs ftoppers, and oneof them graduated. It is ufed in the followingway. The tube is of courfe filled with the airof the plSce which it occupies, or for greatercertainty, it maybe firft entirelyfilled with water,and then inverted, by which the adjoining airwill come in as the water falls out. Thenholding it with the bend lowermoft, and boththe ftoppers out, drop down through the plainor ungraduated fide a little water, till it reachesthe beginning of the graduation, and clofe thegraduated fide with the ftopper, fo that the airoccupies exa£tly the whole of the graduatedfpace. Then pour down the plain fide uponthe water the eudiometrical liquor (fulphat ofiron holding nitrous gas in folution) till thisfide is quite full, and clofe this ftopper alfo.The whole cavity will then contain a knownportion of air in contaft with the proper liquor,which by agitation will abforb all the oxygenof the air in a few feconds, and by loofeningthe ftopper of the plain fide under water, therefiduary gas on the other fide will fhrink toits proper volume, and may immediately beeftimated.

Volta's eudiometer for the detonation of theinclofed air, with meafured portions of hydro-gen, has been already mentioned under thearticle Detonating Jar.

EVAPORATING VESSEL.

Evaporation is one of the commoneft opera-tions in chemiftry, and in the fmall way may beperformed with fufficient cafe in almoft anyopen veffel. The evaporating vefl'els made forthis exprefs purpofe are fometimes of metal,but generally either of glafs or earthenware.The glafs vefl'els ufed in this country are feg-ments of fpheres blown thin enough to bear a

lamp heat, and fold in aflortments of differentfizes, from the capacity of nearly a pint to thatof about an ounce. A broken matrafs or Flo-rence flalk, cut round, will anfwer the purpofeextremely well. The globular fhape of all thefevefl'els however renders them difficult to heat,but it is difficult to get glafs much flatter atbottom and retaining that uniform thinnefswhich is neceffary to make it bear the appli-cation of heat.

Very convenient evaporating vefl'els nearlyflat-bottomed, are made by Mefl'rs. Wedgwoods,and fold in nefts or aflortments, the largeft ofwhich will hold 8 or ie pints. They are fome-times plain, fometimes with a lip in the upperedge, that the liquor they contain may pourbetter. The ffiape of a plain evaporating veffelis feen in PI. I. Fig. 9. They are made eitherwith or without glazing. Even when unglazedthey hold any liquor without foon imbibing it,as the texture is a very denfe, hard, bifeuitporcelain ; though in great length of time falinefolutions will make their way through, and fhewthemfelves in efflorefcences on the outfide.

Thefe evaporating vefl'els will bear to beheated to boiling, over a dear hot fire, fuch asthat of charcoal or cinders, but they are liableto crack when expofed to jets of flame, or un-lefs heated with fome caution at firft. Noliquor ffiould be boiled to drynefs in thefe vefl'els,as they generally crack whilft the laft portionof fluid is expelled unlefs the heat is muchlowered.

FILTER. Filtration.

Filtration is conftantly required in chemicalproceffes, in order to feparate fluids from fub-ftances fufpended in them. In moft inftancesit is performed by throwing the whole upon aconical bag made of any porous materials, fuchas flannel, linen, or paper, which detains the folidpart, and allows the fluid to pafs through clear.Where the quantity of materials is large, and thefolid is of a nature to be not eafily fufpendedin water, a flannel or linen bag is preferable, asit is much more expeditious than paper. Thusfor example, where in the firft ftep of makingphofphorus, bonc-afh is decompofed by fulphu-ric acid, aud the whole diluted with much water,the liquor is obtained very clear by throwingthe whole on a conical linen bag ; and thoughthe firft few ounces may be turbid, the runningsfoon become perfectly limpid. Where theobject is, as in this cafe, to preferve the liquorthat runs through, linen is preferable to flannel,as the latter foaks up a larger quantity of liquor,which mull be got out by fubfequent walking