For filtering a larger quantity of a liquid than can be conveniently managed with a funnel, and also for sub-stances that are either too viscid or too much loaded with feculence to allow them to pass freely through paper, conical bags made of flannel, felt, twilled cotton cloth or Canton flannel, linen or calico, and suspended to iron hooks by rings or tapes, are commonly employed. The first two of the above substances are preferable for saccharine, mucilaginous, and acidulous liquors; the third for oily ones; and the remainder for tinctures, weak alkaline lyes, and similar solutions. These bags have the disadvantage of sucking up a considerable quantity of the fluid poured into them, and are, therefore, objectionable, except for large quantities, or when they are to be continued in actual use as filters for some time. On the large scale, a number of them are usually worked together, and are generally enclosed in cases to prevent evaporation, and to exclude dirt from the filtered liquor that trickles down their sides. These arrangements will be noticed further on.

Fig. 363.   Felt Filtering Bag

Fig. 363. - Felt Filtering Bag.

Fig. 364.   Flannel Filtering

Fig. 364. - Flannel Filtering.

Fig. 365.  Filtering Rack

Fig. 365.--Filtering Rack.

A Simple Method

A simple mode of filtering aqueous fluids, which are not injured by exposure to the air, is to draw them off from one vessel to another, by means of a number of threads of loosely twisted cotton or worsted, arranged in the form of a syphon. (See Fig. 367.) The little cotton rope at once performs the operations of decantation and filtration. This method is often convenient for sucking off the water from a small quantity of a precipitate.