Some fabrics, such as swansdown, close-textured twill calico, etc, filter as brightly as paper does, and may be used for that purpose as distinct from ordinary straining, provided the solid particles separate from the liquid in which they are suspended with ease, but when this is not the case, they are of much less value; indeed, with paper as a medium, slimy deposits present considerable difficulty. Pepsin wine, prepared from the fresh, undried pepsin, might be regarded as typical of this class of liquids, the tendency being to choke up the pores of the filter almost immediately the operation commences. In such cases, some kind of coarse straining material placed within the paper cone helps materially to obviate the difficulty. Hair cloth, and thin coarse flannel answer well for this purpose; they operate by collecting on their rough projecting surfaces the larger proportion of the undissolved slimy matter, without becoming sufficiently choked up to materially impede the progress of the operation.
Succus taraxaci, as expressed from the root and mixed with spirit according to the B. P. instructions, is typical of a class containing a large quantity of starchy matter and where subsidence in a closed vessel previous to filtration is of great service. The liquor from poppy capsules, in the process of preparing syrupus papaveris alb., furnishes us with an example of a liquid containing a large quantity of albuminous matter and mucilage which, when coagulated by spirit, has to be filtered off, and here again subsidence in a closed vessel helps the separation materially. The greater portion of the liquor can, after a time, be poured almost bright into the filter, and the remaining soft mass can, with care, be slowly pressed almost dry, the chief difficulty in the Latter operation being to press sufficiently slowly to separate the liquid from the solid, and yet not to expose it to the air long enough to lose much spirit by evaporation, as in that case some of the solid portion would be again taken up in imperfect solution.
The Druggists9 Circular recommends chamois skin, free from thin places, cut of the desired size, washed in a weak solution of any alkali, to remove the grease, and rinsed thoroughly in cold water before using. Tinctures, elixirs, sirups, and even mucilages are filtered rapidly. A pint of the thickest sirup will run through in 4-5 minutes. By washing thoroughly after each time of using, it will last a long time.
For removing suspended particles from strong acids, spun glass, known as "glass wool," answers best, but this might be regarded as straining rather than filtration. With ordinary liquids, when there is but little insoluble matter, absorbent cotton not only strains, but by fairly tight packing, filters brightly. In cases where it is desired to save the deposit, and possibly to dry or incinerate it, asbestos paper can be recommended; the liquid passes through it slowly, but it is very strong, and it is indestructible by heat. Paper lint, as introduced from America some few years ago, answered well as a filtering medium, being both strong and absorbent.
So far we have considered filtration as conducted only in funnels or funnelshaped arrangements, as the various forms in which atmospheric pressure is commonly employed are described in works which treat of such matters. They are chiefly those in which a long column of liquid is carried above the point of filtration, as in Proctor's arrangement, where exhaustion is obtained by means of a syringe underneath, or suction by means of a bent tube, as described by Schacht at the meeting of the Conference at Birmingham, in 1865.
Symes considers that upward filtration is the direction from which we may expect the best results.
Some years ago, William R. Warner, of Philadelphia, invented an oil filter on this principle, consisting of 2 vessels in superposition, measuring altogether about 40 in. in height by 10 in. in diameter, and which is said to be capable of filtering a barrel of oil per day. This, of course, would depend on the nature of the oil and the temperature at which it is used.
Recently Symes devised a form of upward filter in one vessel only, and added to it a suction tube. It occupies comparatively little space, is simple in construction, efficient in action, and can be made by any tinman at little cost. It is shown in Fig. 165, and consists of a plain tin cylindrical vessel a, with a tap-hole b l 1/2 in. from the bottom; it is 22 in. high, and 8 in. in diameter. A tin tray c, 7 in. in diameter, with a vertical rim 1-1 1/4 in. deep, has a hole d in the rim, this and the hole near the bottom of the cylinder being fitted with a short female screw of the same pitch of thread. Over the tray, the filtering material e (flannel, calico, paper supported by muslin, or any other material that may be suited to the liquid to be operated on) is tied securely; it is then inverted and placed in the cylinder so that the holes b dare exactly opposite one another. A tap /, with a bend at a right angle, is screwed in so that it holds the two together, and assists a short leg g in supporting the tray in position. To the end of the tap is attached a rubber tube turned on itself h, or a long glass tube of similar construction (in fact, take a large safety funnel deprived of the thistle head), which can be attached by a short piece of rubber tube.
It will be obvious that any communication between the tap and the contents of the vessel must be made through the filtering medium which covers the inverted tray, and that any deposition which takes place must be on the bottom of the vessel itself, or on the opposite side of the tray, but not on the filtering surface, and herein lie the special advantages of the filter. The use of a long delivery tube is not new; it formed part of an oil filter patented by Britten, of Liverpool, some years before Schacht's application of it to his filter. Neither is upward filtration new, as already stated; but the combination of the two, and in this particular form, will probably commend itself to any one who will give it a trial. The dimensions given furnish a filter of about 3 gal. capacity, at a cost of some 10-12s. (Pharm. Jour.)