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and wringing. But where the folid refidue isthe only valuable part (as in filtering the citratof lime formed by chalk and lemon juice) flannelanfwers as well and is more fpeedy. In fmallquantities, and where accuracy is required, thematerial of the filter is generally a fquare thinunfized paper, firft doubled from corner tocorner into a triangle, and the latter againdoubled into half the fize, making another tri-angle. This when opened forms a paper cone,into which the liquor is poured, and which muftbe fupported by being put into a glafs funnel.There is always fome little lofs of materials in-curred by filtration, particularly of the fluidwhich paffes through, for the paper muft ofcourfe abforb a quantity of it, and though afterthe firft filtration, the filter is repeatedly filledwith warm water, it is impoflible to wafh outevery particle of the faline folution with whichit was at firft impregnated. This is one reafonwhy in all analyfes of minerals in the moift way,the fum of products obtained by the mod accu-rate experiments always falls fhort a little of theoriginal weight of the mineral employed;fo that if they fhould prove exaCtly equal,there is reafon to fufpect fome little error in thecomputation, or a want of deficcation in theproducts equal to that of the fubftance ufed.

To collect at the bottom of the filter everyparticle of the refidue, it is very ufeful to havea fmall glafs tube open at both ends, and drawnout into a very fine capillary point at one of them.When this is filled with diftilled water and thelarger end put into tlye mouth, the force of thebreath can dire£t a fmall ftrong ftream of waterround the fides of the paper funnel, which willwafti down to the bottom every minute particleof folid fubftance. When the filtration isfinifhed and the refidue properly edulcorated,it muft be dried in part before it can be takenclean off the paper. Sometimes the quantityof refidue is fo minute, and fticks fo much tothe paper, that it cannot be fcraped off when’dry, at lead not with fufficient accuracy ; as, forexample, where the quantity of fulphurettedhydrogen of a mineral water is to be eftimatedby precipitating and weighing the fulphur. Infuch cafe the molt accurate way is to filter asufual, but inftead of attempting to remove therefidue, to weigh it againft another piece offiltering paper previoufly cut exactly of the famefize as the filter, wetted, and dried at the fametemperature, and the difference in the weightswill indicate the quantity required.

The fubftances that cannot be filtered inthe common way, are thofe which aft tooVOL. II.

powerfully upon the paper arid other materialsemployed. The concentrated acids and alkalinefolutions are of this kind. Where it is requiredto filter them, which is not often the cafe, theymay be poured through a ftratum cf coarfelypowdered glafs, or well waflied white fand, ina glafs funnel. See Funnel.

A convenient ftand for filtration with a drawerat bottom to hold filtering paper, &c. is {hewnin PI. VI. Fig. 65.

FLASK.

This hardly differs from the Matrnfs.

The flaiks of green glafs ingenioufly coveredwith rufhes, in which the fineft oil is importedfrom Italy , form very ufeful chemical vefi'elsand at a lew price. They are extremely thin,and will bear heat very well, and being made of ahard glafs without litharge, they will bear a lowred heat without materially foftening or lofingtheir fhape. On this account too they are thebeft veffels for performing fublimation in, withfmall quantities of materials, and if neceflarythey may be coated for this purpofe by diffufingclay in water to the confidence of thick cream,and dipping the fiafk in up to the neck, repeatingthe operation fucceflively as foon as each coatingdries.

The flafks muft firft be cleaned by a littlefand, and any refufe folution of alkali, fome-what cauftic, boiled in it, and fhaken abouttill all the oil within is diffolved. Very ufe-ful cup-fhaped vefi'els for evaporation andother purpofes, may be made by cutting off' oneof thefe flafks round the bottom of the neck orthe middle. This is done with eafe in the fol-lowing way : if the flalk is found, heat a pieceof the thickeft iron wire fully red-hot, and lay iton the part you wifh to divide, moving it back-wards and forwards flowly in the intended direc-tion of the crack. This will generally fucceedin beginning the crack, after which it may beled along with extreme eafe, {imply by bearinggently with the hot iron in the courfe whichthe crack is to purfue, till the divifion is com-pleat. But among a number of flafks there aregenerally fome that have a little crack or flawin them, and this may be led over any part ofthe vefi'el by the fame means, and will cut it tothe required fhape. It fhould be rememberedhowever, that though it requires a very hotiron to fplit a found flalk, yet when the leadcrack is made, it may be led along by a muchlefs heat, and the cooler the iron is, the lefslikely the crack is to deviate from the intendeddirection. If the flafle is fplit in the contrary di-rection, that is, perpendicularly through both the3 «