cylinder (Fig. 152). By means of such a centrifugal press 200 or 300 pounds of moist sugar can be centrifuged to almost dryness in the space of five or ten minutes. A modern application of the centrifugal machinery, totally different from expression, is the well-known centrifugal sedimenter, for the precipitation of blood and of urinary sediment (Fig. 153).

PERCOLATION

Percolation is the process of depriving a drug of its soluble constituents by passage of a solvent through the powder contained in a suitable vessel. The solvent in this case is called the menstruum, the vessel is called the percolator, and the solution of active principles emerging from the percolator is called the percolate.

There is no exaggeration in saying that percolation is a distinctive American process. Although the forerunner of percolation-lixiviation-was used in extracting lye from ashes long before America was discovered; although the first pharmaceutical application was made by Count Real, a Frenchman; to Americans is due the credit of bringing the process to its present stage of perfection. Introduced in the pharmacopoeia of 1840 in the preparation of a few tinctures, it has so grown into favor that it is now employed in the manufacture of almost all the tinctures and fluidextracts of the present pharmacopoeia, besides being employed in preparing wines, vinegars, oleoresins, and extracts. Foreign countries have not taken kindly to the process, and in the German Pharmacopoeia of to-day maceration is largely used in preference. However, percolation is beginning to attract the attention of the Germans, and it was amusing to note, a few years since, in a leading German pharmaceutic journal, a long account of what was called "a new process called percolation."

Principle of Operation.-Careful study of the definition of percolation will show that in the process advantage is taken of the attraction of gravitation, for it is hardly necessary to say that a liquid poured on top of a powder contained in a suitable utensil will gradually penetrate that powder in a downward direction, and if there is an orifice in the bottom of the vessel, the descending liquid will emerge from the vessel, being attracted by gravity toward the center of the earth. This downward force of gravitation is aided by another force, merely a modification of the former, namely, the weight of the column. of liquid above the powder. Mention of the latter is made here merely because the downward force varies according to the height of the column of liquid, whereas the force of gravitation remains unchanged. If this combined action of gravitation was the only force coming into. operation, the process would be a simple one. However, there is an opposing force which comes into play during the process of percolation, namely, the upward force of capillarity.

The term capillarity comes from the Latin term "capilla," meaning hair, because it was first observed that in every narrow, hair-like tube liquid was drawn up by the cohesive power of the walls of the tube, and, as already explained, to this same force is due the appearance of the meniscus (p. 46). All bodies possessing orifices more or less narrow have this property of drawing up liquids by their own cohesive force, and such is eminently the case with most vegetable

drugs, since, as is mentioned above, they consist of a mass of cells, each cell consisting of an orifice of more or less fineness, surrounded by a cell-wall. In other words, cells can be compared to long sealed tubes, and when the cells are broken by powdering, we have a vast mass of tubes, each possessing capillary attraction. This capillarity explains why a so-called porous substance, for example, a sponge, will absorb so much more water than a non-porous substance, such as sand.

To get back to percolation, besides the force of gravity and its modifications, the pressure of the column of liquid above, forcing the solvent downward through the drug, we have at the same time the capillarity of the cells of the drug attracting the menstruum in the opposite direction. The moment the solution commences to emerge from the vessel, the capillary attraction of the cells tends to keep the liquid within the substance; therefore, acting against the force of gravity. Experience shows that the capillary force of the drug is usually greater than the simple gravitating force of the earth, for unless the gravitating force of the earth is reinforced by the height of the column of liquid above the powder, the liquid ceases to drop from the percolator. As the liquid passes through the drug on its downward course it penetrates the interior of each cell (provided the substance is in fine enough powder) and removes from the same the soluble constituents. Passing from one series it penetrates the next tier of cells, and, taking up more active principle, such solution continues until the solvent is saturated with the soluble principles of the drug. When once a saturated solution, it mechanically passes through the rest of the drug, and finally emerges from the orifice at the bottom of the percolator, providing always that there is a sufficient column of the liquid to force it down. Its passage is followed by a fresh portion of the solvent, which passes through the same cells which it has formerly penetrated, removing the last vestiges of soluble matter, and finally emerging from the percolator also a saturated solution. In this way menstruum is continually poured on until the cells have been so completely exhausted of their soluble constituents that the final portion of the menstruum comes through devoid of color and taste, whereupon the drug is said to be exhausted.

The great advantage percolation possesses over maceration is in the fact that, by means of percolation, we are enabled completely to remove every trace of soluble matter from a drug, inasmuch as the saturated solvent does not remain in contact with the drug (if the menstruum column is maintained), but passes from it, being followed by a fresh portion of solvent, which removes the remainder of the soluble principle.

Apparatus.-The utensil in which the drug is packed is called a percolator, and consists of a suitable cylindric vessel of metal or glass, provided with an orifice at the bottom. In selecting a percolator those of glass should be chosen when possible, metal percolators having the double disadvantage of being more easily attacked by acid or other substance which the menstruum or drug may contain and also the fact that their opacity prevents watching the operation. Of course, however, there are certain limits to the size of a glass percolator, the writer's experience being that the use of a glass percolator larger than

two gallons is an expensive luxury, inasmuch as such are costly and are quite liable to break. Above two gallons, recourse was had to the galvanized iron percolator or the well tube percolator, in which the container is an earthenware crock.

Shape of Percolators.-The simplest form of percolator is the conic (Fig. 154), consisting of an ordinary chemical funnel. This, however, offers too great a width of surface for the menstruum, which, therefore, has a correspondingly smaller course to travel before emerging from the bottom of the percolator, and, therefore, the familiar tapering percolator came into use (Fig. 155). At the present time the Oldberg cylindric percolator (Fig. 156) is very popular, for by means of it we are able to exhaust a drug far more thoroughly than in either a tapering or a conic percolator, from the simple fact that each portion of menstruum has to travel through a larger amount of the drug before emerging from the percolator.

A

Fig. 154.-Conic percolator. Fig. 155.-Tapering Fig. 156.-Oldberg

percolator.

percolator.

variety of percolator used exclusively in chemistry is the Soxhlet condenser (Fig. 157).

The saturated solution, when emerging from the percolator, is called the percolate, and, of course, this must be collected in an appropriate vessel. For this purpose a wide-mouthed bottle, the receiving bottle, is usually employed. Elaborate forms of receiving bottles are on the market in which the graduations are engraved upon the glass. These, while presenting an elegant appearance, are scarcely necessary, as the pharmacist has many wide-mouthed bottles at his disposal, and he can usually graduate same by pouring in definite quantities of liquid from an accurate graduate, and marking amount by cutting with a file or on a strip of paper pasted along the length of the bottle (marking same with ink). The percolator must be supported on an appropriate stand, and for this purpose nothing can be better than the retort stand pictured in Fig. 77, it being always understood, however, that the glass percolator must never

come in contant with the iron ring, being protected from the same by covering the ring with paper or cloth (Fig. 158).

When large amounts of percolation are being carried on, special percolating stands have been devised; such, for example, is the

Fig. 158.-Correet percolation.

Shinn percolating closet and the Remington percolating shelf ("Am. Jour. Pharm.," 1877, p. 589).

Operation. The process of percolation may be divided under the following headings: Comminution of the drug; moistening the same; packing in percolator (with maceration either before or after packing); pouring on the menstruum and collecting percolate until exhausted.

Comminution.-Before percolating the drug it is essential that it be reduced to particles of more or less fineness, for the same reason that we comminute drugs before solution. As mentioned above, in percolation it is essential that the drug be ground sufficiently fine to rupture all the cells of which it is composed, and this degree of fineness is dependent upon the structure of the drug, those drugs having small cells calling for a finer degree of comminution than those in which the cells are large. Thus, nux vomica must be powdered very finely, whereas in gentian the cells are large and a coarse powder can be utilized. This explains why the pharmacopoeia directs various degrees of fineness for the powders used in percolation. Another point regarding comminution of the drug is the rapidity or completeness of exhaustion. Thus, in percolating fluidextracts it is usually desirable to exhaust the drug with less menstruum than in preparing tinctures from the same drug, and in order to do this the drug used in percolating a fluidextract is generally directed to be in finer powder than that used in making the tincture. The powdering of the drug is accomplished by those methods of comminution described in Chapter VI. best serving the particular purpose, the main requirement being that the drug used in percolation be of uniform fineness.

Moistening of the Drug. Before packing the drug in the percolator it is directed to be moistened with menstruum, this being done. because, when the drug comes in contact with the menstruum, the compressed dried cells are swollen to their normal size, and if this swelling occurred within the percolator, it would cause sufficient expansion to cause a stoppage of the operation.

This moistening is accomplished by placing the drug in a suitable container, preferably of porcelain or stoneware, adding the required quantity of menstruum, and mixing same thoroughly with the drug by stirring with a wooden paddle. After being moistened it should be allowed to remain for half an hour in order to complete the swelling, thereupon the pharmacopoeia directs the moist drug to be passed through a coarse sieve, thus making the product into a coarser granular powder, ready for packing.

Packing the Drug.-To receive this granular powder the clean and

absolutely dry percolator is prepared by placing in its neck a plug of absorbent cotton, followed by a layer of clean dry sand. This latter precaution is directed by the pharmacopoeia, both for the purpose of filtration and also to hold the plug of cotton in place. Many careful pharmacists, however, omit the sand as an unnecessary detail. Upon this sand the granulated powder is poured in comparatively small quantities, the usual rule being to place in the percolator about onefourth at a time, and each fourth is carefully and firmly packed before the next portion is added. The packing is accomplished by means of a wooden plunger, and for this purpose a piece of wood will suffice, there being nothing better than a wooden potato-masher. The packing of the drug is a matter of skill, and in order to do it successfully it is necessary to know the character of the drug and the menstruum. Drugs in finer powder are not, as a rule, packed so tightly as those of coarser powders, but the supreme test of the correct packing is the rapidity of the flow of the percolate. If the drug is packed too tightly, the percolate drops too slowly, and if too loosely, the percolate runs so rapidly as not fairly to exhaust the drug; therefore, packing must be put down as chiefly a matter of experience. The nature of the menstruum also has an influence on the packing of the percolator. If the menstruum is pure alcohol, the drug can generally be packed more tightly than if it consists of water, and if glycerin is present in the menstruum, care should be taken not to pack the drug too tightly. It is hardly necessary to say that in packing the drug the pressure should be equal on all sides of the powder. If the drug on one side of the percolator is packed more loosely than on the other, it stands to reason that the menstruum will choose the easiest possible

way and will run down the looser side, and, therefore, the drug will be incompletely exhausted.

All the drug being packed in the percolator, there is carefully placed above it a sheet of filter-paper which is held in place by a glass weight, any unused glass stopper answering the purpose. The object of the paper is to prevent the disturbance of the powder when the menstruum is poured in, the force of the fall of the liquid being apt to cause the rising of the powder to the surface of the menstruum.

Instead of filter-paper, the writer found very convenient disks of ordinary clean Manila card-board, made from one of the many rejected paste-board boxes found in every drug-store. These disks, perforated by the point of a file, when just the size to fit snugly above the drug within the percolator, required no weight to keep them down.