tacks. A strainer hung in this way will hold more liquid and do much better work than one which is tacked all around the frame. This method of straining is particularly useful in collecting precipitates which require washing. Colation in Smaller Operations.-When solid particles are to be separated from liquids in the operations of the dispensing counter, several methods may be used. One of the most convenient is to insert a plug of absorbent cotton in the neck of a funnel and then pass the liquid through; a funnel with a circle of brass-wire gauze soldered in it twothirds of the way down is sometimes used, although not recommended, because of the difficulty of cleaning it. A better small strainer is made by using the hard-rubber sieve, VH (see Fig. 238). The muslin gauze, C, is easily replaced, and the sieve may be placed in a funnel. The cotton cloth used by the makers of cheese, called cheese-cloth, is admirably adapted for many kinds of straining. When coarse muslin strainers are used for the first time, they are prepared by soaking them in hot water and placing them in a funnel carefully so as to line the inside. Care should also be taken to see that the strainer is not too large, for if the wet strainer projects over the edge of the funnel, a syphon action may be set up and more of the liquid delivered outside of the bottle than inside. Fig. 239 shows the action of such a strainer. QUESTIONS ON CHAPTER X. SEPARATION OF FLUIDS FROM SOLIDS. What is meant by lotion or displacement washing? How may it be effected? What is a spritz bottle, and what is its use? What is continuous washing? How may it be effected? What is decantation? What is a guiding-rod, and how is it used? What is a syphon? What is the principle of its action? Where a poisonous, caustic, or disagreeably tasting liquid is to be drawn off by a syphon, how may it conveniently be started? What is colation? What materials are used for the purpose? In what cases are strainers used? What are felt strainers? How are woollen strainers used? What advantages have cotton flannel strainers over those made of woollen? CHAPTER XI. FILTRATION. Filtration is the process of separating liquids from solids with the view of obtaining the liquids in a transparent condition. The intervention of porous substances, called filters, to intercept the solid particles, is necessary in performing this process. These are usually made from paper, paper pulp, sand, asbestos, ground glass, charcoal, porous stone, etc. The liquid which has passed through the filter is called the filtrate. Paper Filters are the most useful of all kinds for the pharmacist, and they are employed in all the finer operations requiring filtration. The solid particles are much more completely separated by filtration through good paper filters than through strainers, owing to the pores of the paper being smaller and more numerous. The paper used for this purpose is especially prepared, and is called filtering-paper: it is made now upon a large scale, and can be had of excellent quality. Unlike a strainer, it is never used more than once; its cost is so trifling, and it is so easily ruptured when wet, that it is not worth while to attempt to save filters for subsequent use. Filtering-paper is found in commerce in two forms,-in large, nearly square sheets, and in circular sheets. The former is used for large filters, and has some advantages, if the waste pieces can be put to use; but it is usually more convenient for the pharmacist to rely for constant use upon the circular sheets of different sizes: the difference in price between the two is now so trifling that the latter is almost always preferred. Two kinds of square-sheet filtering-paper are commonly found,-German white, a rather thin, but good paper, and heavy French, the latter very thick and porous, having a rough surface; it is the more expensive paper, but is better for special purposes. Of the round filters, the French, Swedish, German, English, and Scotch are among those best known in America. For pharmaceutical purposes the French filters are almost universally used, the "Prat Dumas" brand being the most common, the gray paper being made from a mixture of cotton, flax, wool, etc.; this paper answers sufficiently well for filtering colored liquids, fluid extracts, or tinctures, but, owing to the coloring matter it contains, it should never be used for any solution containing free alkali. It is safer to form the habit of never using it for liquids that are intended to be colorless when filtered; of course it is entirely unfitted for analytical work. The "Prat Dumas White" is of good quality, and it or some other good quality of white paper should always be on hand for special purposes, for filtering alkaline or alkaloidal solutions, and for the nicer operations. Very good filtering-paper of English and Scotch manufacture may occasionally be procured; that made in Sweden, however, by Munktell, is preferred for the processes of ignition and analytical work; it yields the smallest amount of ash, and is practically free from soluble salts and impurities: at least a small stock should be kept by pharmacists for especially accurate work. Methods of Folding Filtering-Paper.-Two kinds of paper filters are used, the plain and the plaited. FIG. 240. FIG. 241. 1. The Plain Filter.-This filter is used habitually by the analytical chemist, and is preferred by the pharmacist where precipitates are to be collected, and in some other operations (see Fig. 240); it is made by exactly doubling a circular sheet of filtering-paper upon itself, and then folding this directly in the middle, so that, when opened, four equal divisions or sectors appear; the filter is used by separating one of the sectors from the other three, and placing the cone formed, into a funnel; the liquid is then poured upon the filter, and the process of separating the solid from the liquid commences. The advantages of the plain filter are, 1. Simplicity and rapidity in folding, no skill being required to make one. 2. In collecting precipitates, but one-half of the surface of the filter (two sectors) is in contact with the moist precipitate, which is often closely adherent, and therefore but one-half of the surface has to be cleaned. In some cases there may be a disadvantage in the use of the plain filter, owing to the un Plain filter. Double plain filter. equal rate of flow,-the tendency of the three folds being to attract the liquid to the side of the funnel upon which they rest, and thus the precipitate may be unequally washed. A stronger and more rapid filter may be made by placing one plain filter inside of another, so that the three sides of the upper one shall be in contact with one side of the lower one, and vice versa (see Fig. 241). If the sides of the funnel have an angle of 60°, the plain filter made as described will fit the funnel properly; but it frequently happens that the angles of funnels vary, and if an ordinary plain filter is placed in a funnel not having an angle of 60°, a portion of the filter is left unsupported, and the weight of the liquid is apt to rupture the moist paper. This difficulty may be overcome by making a fresh crease in the outside fold of the plain filter; if this is made to the right of the original crease of the 60° filter, and the inside fold pushed around a corresponding distance, FIG. 243. a filter having a smaller angle is produced, whilst if the fresh crease is made to the left of the original crease a larger-angled filter may be made. Rother's method of making a plain filter has the advantage of giving two filters from the same sheet that is usually required for one filter. To make it, the circular disk of filtering-paper is cut through in the line of its diameter, and half of the disk is folded into two equal parts; the double edge of the cut sides is turned down and folded over on itself narrowly several times (see Fig. 242), and with the blade FIG. 242 Rother's filter (first step). Rother's filter. of a spatula the fold is compressed so that it will retain its shape (see Fig. 243). This filter may be used in collecting precipitates. The Plaited Filter may well be called the "pharmacist's filter," for it is the form almost exclusively used in ordinary filtering operations. Figs. 244 to 254 show the progressive steps in FIG. 244. Filter. A upon HD, and HD upon ED, and it will be noticed that the folded semicircle has been creased into eight equal spaces, and that the direction 1 It will be observed that in the first folding of French filters, "Prat Dumas," the disks are not perfect circles: this causes one edge to project (see Fig. 245), and facilitates the opening of the filter. of each crease is the same, so that if the paper is lifted it will appear as shown in Fig. 251. The next step is to fold each one of these spaces back on itself (Fig. 252). BD is laid upon FD, and then BD is turned upward and back FIG. 245. A Folding plaited filter. until it is laid upon I D. This makes the crease QD, which is the first fold in the opposite direction. Taking both folds between the forefingers and thumbs of both hands, the edges BD and ID are folded upward and back upon FD, and the crease PD is formed; then these three edges, BD, ID, and FD, are taken all together and folded back upon KD, and the crease OD is formed, and so on, each space in turn being folded back in the opposite direction, until the last one is reached. The folded filter is then held at the apex with the left hand upon a table or flat counter, and pressed and smoothed out with the right hand in order to emphasize the folds it should then be placed in the funnel, whilst still unopened, to see whether FIG. 248. B D D Folding plaited filter. it needs trimming; if the rough edges of the filter project above the top of the funnel, the filter must be removed, and they must be cut off neatly with a sharp knife or a pair of scissors so that the whole of the filter may be placed inside of the funnel. Having satisfied this re |