Filters of unsized paper are well suited for all liquids that are not of a corrosive or viscid nature, and are universally employed for filtering small quantities of liquids in the laboratory. A piece of the paper is taken of a size proportionate to the quantity of the liquid to be filtered, and is first doubled from corner to corner into a triangle (see Fig. 359, A), which is again doubled into a smaller triangle B, and the angular portion of the margin being rounded off with a pair of scissors, C, it constitutes a paper cone, which is placed on a funnel of proportionate capacity, and is then nearly filled with the liquid. A piece of paper so cut, when laid flat upon the table, should be nearly circular.

Filtering paper is now sold ready cut, in circles of various sizes, which simply require doubling for use. Another method of forming a paper filter, preferred by some persons, is to double the paper once, as above, and then to fold it in a similar way to a tan, observing so to open it and lay it on the funnel that a sufficient interval be left between the two to permit of the free passage of the filtered liquid on its descent towards the receiver. The "plaited filter," as thus formed, is exceedingly useful for general purposes; it exposes the entire surface of the paper to the liquid, and allows filtration to proceed more rapidly than a "plain filter" does. (See Fig. 360).

Fig. 358.   Glass Funnel

Fig. 358. - Glass Funnel.

Fig. 359.   Paper Filters

Fig. 359. - Paper Filters.

One takes objection to the ordinary plain paper filter employed in the laboratory, because of the superfluous fold which in two thicknesses lies under one half of the extended surface of the filter, and says the interposition of these two extra layers compels the liquid to pass through three thicknesses of paper on the half side of the extended filter, whilst the other half side presents only a single thickness. It is evident that the two hidden layers are a very appreciable impediment to the current, aside from the more important fact that the liquid will traverse this side less rapidly than the other, and thus occasion an imperfect washing of the precipitate, or at least prolong the operation beyond reasonable limits, Recognizing these objections another paper filter has been proposed.

To make the new filter: - Cut the circular disk of filtering paper in • two through the line of its diameter, take either half disk, and fold it across the line of the radius, then turn down the double edge of the cut side, and fold it over several times; finally, run a hard smooth surface along the seam thus produced, to compress it, and spread the finished filter into an appropriate funnel, first moistening it with water before the liquid to be filtered is poured in.