In case a particular gas is required, the funnel is then provided with a double-bored cork. Through one opening the gas is introduced, and it passes out by the other, a connection being made with the filter flask as before.
(c) An apparatus for filtering and drying very oxidisable precipitates is shown in Fig. 30, and is constructed of glass with the exception of the cover C, which is of brass. The tube G, which is connected with the brass tube M by the cork F, is bent over and unites with a small flask containing the precipitate. This small flask has a cork with three holes, one for the tube G, a second for a glass funnel, and the third for a tube connecting with a gas generating apparatus. When all connections are found to be air-tight the tube C is connected with a water pump, and the cock £ is opened; the precipitate is drawn over by lowering the tube G into the precipitate in the flask as soon as the apparatus is filled with the indifferent gas. The precipitate collects on the funnel A, and distilled water can be drawn over to wash the precipitate there, by pouring down the funnel in the small flask. By opening the tap D instead of E, the filtration can proceed more slowly, and this tap is also useful with very muddy precipitates.
To dry the precipitate, the cork E is replaced by another without an opening, the glass tube G is fused off, and the upper part B of the apparatus is removed from the lower part A, and transferred afterwards to a drying oven.
(d) A filter support, which is an improvement on the arrangement for rapid filtration described by Fessenden in (f) below, is shown in Figs.31-3. It is made from platinum wire, copper wire, or any other suitable material, and bent in the shape shown in Fig. 33. A paper, folded as described by Fessenden, is pushed in between the wires, a and b, Fig. 31, which serves the same purpose as the glass rod, that is, to support the inner folds of the filter; whereas the ring, c f, supports the outer folds, giving the whole an appearance of a paper formed with two compartments. This may now be placed in a glass funnel, or used alone simply by suspending it over the beaker as in Fig. 32, the liquid following the wire and dripping from the point, g. As a means of drying precipitates on the filter, it is far superior to the old way of placing the glass funnel with its filter in the drying oven, as the air has access to the paper from all sides, whereby it dries much more rapidly and thoroughly. For a 7$-in. filter, a support of 1/32in. platinum wire, with the ring, c f, 2 1/2 in. diameter, and the wires, a and b 3 in. long, gave excellent service.
(e) An automatic rapid filter is shown in Fig. 34, which indicates the manner of its operation from a common table, being entirely independent of the customary retort-stand. It uses very small circular filters 3 7/8 in. diameter, and yet filters many' times faster than the largest heretofore used, doing its work thoroughly, and absolutely without attention, no matter how large the amount to be filtered. It is got ready quicker than the funnel, never breaks the filter-papers, and has no metal contacts. One of its strongest points is in filtering very small quantities as well as large. The former are run through in a moment's time without tedious dropping, as by the funnel.
(/) The use of filter pumps, as every chemist is aware, does not, in a very great number of cases, facilitate filtration: first, because a dense layer of the precipitate forms next the paper, which continually requires to be removed, and second, if any considerable pressure is used, particles of the precipitate will pass through. To increase the surface seems to be the better plan. Plaited filters partially effect this, but the precipitate cannot be easily detached from them, and they are troublesome to prepare. Ribbed funnels, while also an improvement, have only one side of the filter for use, I be other side being covered with three thicknesses of filter paper. The following method (Fig. 35) enables titrations to be made very rapidly, and in such a manner that the precipitate can be readily removed: - The filter paper is folded three times; folds Nos. 1 and 2 are toward the reader. No. 3 from him. The filter is then gathered (Fig. B), and a piece of glass rod, bent at a very acute angle, is inserted in the cleft of the filter (Figs. C and D) thus giving a filtration surface of nearly four times the usual one. The filtration being complete, the glass rod is grasped by the projecting ends and lifted from the funnel, hearing the filter upon it.
One end of the filter paper is then bent down, and the precipitate is easily washed off (Fig. E). An improvement on this is to use, Instead of the glass rod, a plate of glass (Fig. P) ribbed on both sides. This renders the filtration Very rapid Indeed, and if it were made by the manufacturers of chemical apparatus, would no doubt be used. (R. A. Fessenden.)
(g) The production of a partial vacuum within the vessel receiving the filtrate has long been employed in chemical manipulations for the separation of dense precipitates, and also to save time in the ordinary processes of filtration requited in the practice of chemical analysis. There is no reason why, in the absence of a centrifugal machine, the filtering of gelatine emulsions should not be hastened in a similar an efficient, filter-pump can be manufactured.