A simple and rapid method of filtering gelatinous mixtures will doubtless be acceptable to many photographers. The plans usually recommended are, the use of a funnel plugged with tow or cotton-wool, or a piece of cambric or other material spread over a jar, on which the solution is poured and allowed to percolate through. These plans are altogether unsuitable where large quantities of liquid have to be filtered, and even for mall quantities the process is slow. The gelatine has to be kept warm till the operation is complete, and a rather open material must be employed, or the solution will only fall through drop by drop. The plan here described will be found very expeditious, there is no waste, a filtering material of the closest texture may be used, and the warm mixture is filtered before it has time to thicken by cooling. It has been used successfully for filtering gelatino-bromide emulsions and the gelatinous mixtures employed in the preparation of carbon tissue. The arrangement referred to is shown in Fig, 169: a is a wooden stand 18 in. high, having a hole in the top 4 in. in diameter; J is a ring made of bent cane or whalebone, slightly larger than the hole in the stand.
The filtering material, which must be of the closest texture, should be cut in a circular form about O 2 22 in. in diameter; when secured to the ring with stout thread it forms a bag c, the ring b preventing it from falling through the opening in the stand.
To use the apparatus, the operator poors sufficient of the mixture into the bag to half fill it; he then seizes the bag, above the liquid, with his fingers, and presses the filtrate through into a receptacle placed below to receive it. Further portions of the mixture are poured in till the whole quantity has been filtered. With the measurements given above, quantities varying from 4 to 40 oz. may be readily operated upon. (Photog. Seat.)
Fig. 170 shows a rapid-acting filter by Vollmar, for liquids liable to change by exposure to the air. The filter is hermetically closed while working. It is lined inside with filtering paper, and the filtration takes place so that the turbid liquid enters the filter below, passes through the paper, and is discharged clear at the top, where a pipe conveys it into a receptacle. This arrangement is -of special service for filtering wines or other delicate liquids which should not be long exposed to air. A siphon inserted into the cask containing the turbid liquid, which stands on an elevated place, conveys the liquid to the filter and from there it flows into the new receptacle. If the liquid is very sensitive to air, and a layer of oil cannot affect its flavour, some pure olive oil may be poured into each cask, and the delivery tube leading from the filter be pushed down to the bottom of the receiving cask. In this way the liquid is absolutely protected from contact with.
These are usually drained in a filter consisting of a half boiler, cut longitudinally, loosely paved with bricks, which are covered with layers of coke to a depth of 9 in. or so; the bottom layer is composed of good-sized lumps, the top of small pieces, surmounted by a covering of coarse sand or cinders. Over the filter-bed are laid perforated iron plates or grids, upon which the mud is placed.
The filtration of syrups and saccharine fluids is largely performed in what are familiarly known as " bag" or "Taylor" filters. The construction and arrangement of these are shown in Figs. 171, 172. The filter consists of a wrought-iron case a, with openings at b, and an internal flange at top to carry a cast-iron box c, having holes in the bottom, for the reception of gun-metal bells d, to which are attached cotton-twill filter-bags. Fig. 172 shows an enlarged section of the gun-metal bell d. The hags e fastened to these bells are 3-6 ft. in circumference and 6-10 ft. long, woven without a seam. They are crumpled up inside "sheaths" of strong open webbing, about 18 in. in circumference, which restrict their expansion. They are arranged in series of 100 or more.
In sugar refineries use is largely made of animal charcoal, packed in huge cylinders.