Aqueous solutions are filtered practically through long bags, made of twilled cotton cloth (Canton flannel). These bags are usually made about twelve or fifteen inches in diameter, and from four to eight feet long (see Fig. 371), and are enclosed in bottomless casings, or bags of coarse canvas, about five to six or eight inches in diameter, for the purpose of condensing a great extent of filtering surface into the smallest possible space. A number of these double bags (from one to fifty or sixty) are connected with corresponding holes in the bottom of a block-tin or tinned-copper cistern, into which the liquid to be filtered is poured. The mode in which these bags are fastened to the cistern is of the utmost importance, as on the joint being close and secure depends the integrity of the apparatus. Three methods of doing this are figured in the engraving, which, with the references, will explain themselves, the same letters referring to the same parts of each.

Fig. 370.   Oil Filter

Fig. 370. - Oil Filter.

Fig. 371.   .4, Filtering Bag of Cotton Cloth; B, Cot ton Filtering Bag,

Fig. 371. - .4, Filtering Bag of Cotton Cloth; B, Cot-ton Filtering Bag," Creased," or Enclosed in its Canvas Envelope, ready for Fixing.

The second of the above arrangements is the least expensive, and certainly the most convenient in practice; and when the cylinder I fits the hole closely (allowing for the bag), is as safe, or safer, than an ordinary screw.

The bags are surrounded by a wooden screen fitted up with doors for the purpose of keeping off the dust; and the bottom of the apartment is furnished with large steam-pipes, by which a proper temperature may be kept up in cold weather. This is, for instance, the kind of filtering arrangement in sugar-refineries to filter the "liquor;" a few dozens of such filter bags are enclosed in a solid iron box fitted with open steam, and do very practical work. In the carbonator's laboratory the aqueous solutions and syrup may be filtered with them.

When cotton cloth bags are employed without being enclosed in others, they should not be longer than about three or four feet, and not a, Bottom of cistern; b, Filtering-bag; c, Screw of the conical nozzle fitting into the cistern d, Binding cord connecting bag and nozzle; e, Binding cord connecting bag and lower nozzle; f, Bayonet-catch, connecting the lower portion of the nozzle fastened to the bag with the upper and fixed part g; i, The thick hem at the top of the bag purposely made large by enclosing a piece of thick cord therein, resting on the boulders, k; J, A metallic cylinder, loosely fitting the hole in the cistern, and over which the top of the bag is drawn, before being put into its place; when fitted, as in the engraving, it retains the hem i securely in its place above the shoulder k.

Fig. 872.   Mode of Fastening Filtering Bags to Cistern

Fig. 872. - Mode of Fastening Filtering Bags to Cistern.

Wider than about five or six inches when filled. When larger they are dangerous.