while for continuous washing a combination of bottles with a funnel may be used.

Decantation is the pouring or drawing off a supernatant liquid into another vessel. If done by pouring, a guiding-rod for the liquid to run on is an effective adjuvant; if by drawing, the siphon in some form is usually employed.

Colation, or Straining is another very simple operation, so familiar to every one of ordinary experience as to be scarcely worth describing. The strainers are made of cotton flannel, fine muslin, gauze, woollen felt and other fabrics.

Filtration is a process of straining through a medium so fine as to deliver the filtrate in transparent condition. The filters are made of paper usually, though charcoal, asbestos, sand and other articles are sometimes employed, and are supported in a funnel of glass or other material held by the ring of a retort-stand. The best filtering-paper is made in Sweden by Munktell, and is white; but a good paper for ordinary use is the 'Prat Dumas White," which should always be employed for filtering alkaline or alkaloidal solutions. The gray French papers answer well enough for fluid extracts, tinctures or colored liquids, but should never be used for solutions containing free alkali.

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Filtering-paper is folded by doubling a sheet upon itself, and then folding it again directly in the middle. When opened four distinct sections appear, one of which is separated from the other three, and the filter thus formed is placed in a funnel. This arrangement is known as a plain filter, which by repeated creasing is converted into the plaited filter; the latter being the form generally used in pharmaceutical operations of small extent. In large laboratories special processes of filtration are employed, with apparatus of more or less complexity for hot filtration, rapid filtration, etc.

Clarification is the separation from liquids of solid matter, which prevents their being transparent, without using filters or strainers. It may be effected by heat (as in the case of Mel Despumatum), by adding a lighter liquid, by adding albumen, gelatin, milk, or paper-pulp, by fermentation, or by subsidence of the particles in the form of a sediment through long standing.

Decoloration, or the removal of coloring-matter from liquids or from solids in solution, is effected by the use of animal charcoal, which in small operations may be arranged in a funnel or a percolator, and the liquid placed thereon. It should not be forgotten that charcoal absorbs many other principles besides coloring-matter, especially alkaloids, bitters, and astringents, so

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that the process of decoloration may be one of serious injury to the efficiency of the preparation.

Separation of liquids which do not mix with each other is a simple mechanical process performed with pipettes of various forms, or with funnels having stop-cocks in their necks. Special forms of receivers are used for the separation of volatile oils from the water which may accompany them during distillation.

Precipitation is the process of separating solids from their solutions, and is usually effected by chemical reaction, though it may be accomplished by other methods, as by adding a second liquid in which the substance is insoluble, by heating albuminous solutions, or by exposing solutions of silver salts to the action of light. The most familiar example of chemical precipitation is the addition of a solution of Mercuric Chloride to one of Potassium Iodide, the result being a double decomposition of the salts and the formation of Mercuric Iodide, which falls to the bottom of the vessel as a brilliant, red, insoluble and crystalline powder. The precipitate is the separated substance, which is usually thrown down, but it remains suspended in some cases, and in others it rises to the top. The precipitant is the substance which is added to produce the precipitation. A magma is a thick, tenacious precipitant remaining behind after the supernatant liquid is removed by decantation or otherwise. Precipitates are termed flocculent, gelatinous, curdy, granular, crystalline, etc., according to the forms assumed. In small operations they are usually collected on plain filters, and washed by the repeated addition of

water.

Crystallization is the process which bodies undergo in passing from the liquid or the gaseous state to the geometrical forms called crystals. Six systems of crystals are recognized by crystallography, which has assumed the dignity of a separate science. Bodies which are not capable of crystallization are termed amorphous. Every crystallizable body assumes its own peculiar form, or some other form directly derived from or related to it. The process of crystallization is effected (1) by fusion and partial cooling, as in the cases of some metals and Sulphur; (2) by sublimation, as Benzoic Acid, Mercuric Chloride, etc.; (3) by deposition from hot saturated solutions while cooling; (4) by deposition from a solution during evaporation; (5) by deposition caused by passing a galvanic current through the solution; (6) by precipitation, as in the case of the Mercuric Iodide; (7) by the addition to the solution of a substance having a strong affinity for

water, as in the adding of Calcium Chloride to an aqueous solution of Sodium Chloride, or Alcohol to a solution of Potassium Nitrate, or to an aqueous syrup. In a few cases amorphous solids may crystallize without undergoing liquefaction, as Sulphur, Barley-sugar, Iron or Brass wire. The methods most frequently employed are those by deposition from supersaturated solutions, and by deposition during evaporation. The more slowly the process is carried on the larger and more regular will be the crystals. The process is facilitated by use of foreign bodies as nuclei around which the crystals are deposited; a familiar instance being the thread in the centre of a mass of rockcandy.

The Water of Crystallization is the H2O with which most substances combine in the act of crystallization, and the number of molecules thereof differs for each body and for the same body frequently under different conditions. Exsiccation is the driving off of this combined water by heat, the crystals assuming thereby the form of a dry powder. Efflorescence is a similar process occurring spontaneously on exposure of the crystals to the air, the effloresced portion appearing as a dry powder on the surface of the crystals. Deliquescence, on the other hand, is the act of absorbing water from the atmosphere, a property possessed by some substances which are therefore said to be hygroscopic.

Granulation is a process of reducing a coarsely crystalline substance to a granular powder by dissolving it in water and evaporating the solution with constant stirring until the product becomes perfectly dry. Many salts are thus treated for convenience in dispensing, as the Bromide, the Iodide, the Carbonate and the Citrate of Potassium. Sulphate of Iron, though generally dispensed in the exsiccated powder, may be granulated into minute crystals by filtering an aqueous solution of it into alcohol.

Dialysis is a process by which crystallizable substances are separated from non-crystallizable ones, by suspending a solution containing both upon a porous diaphragm having its under surface in contact with water. The crystalloids pass through the diaphragm, while the non-crystalline remain above it, and are termed colloids. Examples of the latter class are gelatin, gum, glue, starch, dextrin, albumen and extractive matters, which are generally the inert and valueless constituents of vegetable drugs. Parchment-paper and bladders are used for the diaphragm; the whole apparatus being termed the dialyzer, while the water into which the crystalloids pass is called the diffusate.

The unofficial preparation known as Dialyzed Iron or Ferrum Dialysatum is a colloidal substance obtained by treating Ferric Chloride in solution with

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Ammonia whereby Ferric Hydrate is precipitated and then dissolved by agitation. The mixture being placed on a dialyzer, the crystalloids formed (Ammonium Chloride and Ferric Chloride), together with any free acid present, pass into the diffusate, leaving the neutral colloidal liquid (solution of Ferric Oxychloride) above on the septum.

Maceration is one of the processes of extracting the soluble principles from drugs, and consists in steeping or soaking the comminuted substance in a suitable liquid called the menstruum, generally alcohol, for a period varying from 2 to 14 days, during which it is occasionally agitated. The liquid is then poured off, the residue is expressed, and the mixed liquors are filtered. Several of the official tinctures are prepared by this method, and in many others are subjected to maceration first and percolation afterwards.

Expression is the forcible separation of liquids from solids, by subjecting them to pressure. Hand-pressure through straining-cloths may be employed, but mechanical presses are more efficient and are coming into general use. Oils obtained in this manner are called expressed or fixed oils, to distinguish them from the volatile oils obtained by distillation.

Percolation or Displacement is a process of obtaining the soluble constituents of a substance in powder by the descent of a solvent through it. Though an ancient process for the making of lye from wood-ashes (lixiviation), it has only within the last forty years been adopted as an official process in pharmacy, and it is gradually taking the place of maceration as a means of extracting the soluble principles of drugs. The vessel used to hold the powdered drug is called the percolator, of which there are many forms employed by the manufacturers. The liquid used as a solvent is called the menstruum, and when coming from the percolator it is termed the percolate. The U. S. Pharmacopoeia gives the following directions concerning this process:

"The process of percolation, or displacement, directed in this Pharmacopoeia, consists in subjecting a substance, or substances, in powder, contained in a vessel called a percolator, to the solvent action of successive portions of menstruum in such a manner that the liquid, as it traverses the powder in its descent to the recipient, shall be charged with the soluble portion of it, and pass from the percolator free from insoluble matter.

"When the process is successfully conducted, the first portion of the liquid, or percolate, passing through the percolator will be nearly saturated with the soluble constituents of the substance treated; and if the quantity of menstruum be sufficient for its exhaustion, the last portion of the percolate will be destitute of color, odor, and taste, other than that possessed by the menstruum itself.

"The percolator most suitable for the quantities contemplated by this Pharmacopoeia should be nearly cylindrical, or slightly conical, with a funnelshaped termination at the smaller end. The neck of this funnel-end should be rather short, and should gradually and regularly become narrower toward the orifice, so that a perforated cork, bearing a short glass tube, may be tightly wedged into it from within until the end of the cork is flush with its outer edge. The glass tube, which must not protrude above the inner surface of the cork, should extend from one and one-eighth to one and one-half inch (3 to 4 centimeters) beyond the outer surface of the cork, and should be provided with a closely fitting rubber tube, at least one-fourth longer than the percolator itself, and ending in another short glass tube, whereby the rubber tube may be so suspended that its orifice shall be above the surface of the menstruum in the percolator, a rubber band holding it in position.

"The dimensions of such a percolator, conveniently holding five hundred grammes of powdered material, are preferably the following: Length of body, fourteen inches (36 centimeters); length of neck, two inches (5 centimeters); internal diameter at top, four inches (10 centimeters); internal diameter at beginning of funnel-shaped end, two and one-half inches (6.5 centimeters); internal diameter of the neck, one-half inch (12 millimeters), gradually reduced at the end to two-fifths of an inch (10 millimeters). It is best constructed of glass, but, unless so directed, may be constructed of a different material.

"The percolator is prepared for percolation by gently pressing a small tuft of cotton into the space of the neck above the cork, and a small layer of clean and dry sand is then poured upon the surface of the cotton to hold it in place.

"The powdered substance to be percolated (which must be uniformly of the fineness directed in the formula, and should be perfectly air-dry before it is weighed) is put into a basin, the specified quantity of menstruum is poured on, and it is thoroughly stirred with a spatula, or other suitable instrument, until it appears uniformly moistened. The moist powder is then passed through a coarse sieve-No. 40 powders, and those which are finer, requiring a No. 20 sieve, whilst No. 30 powders require a No. 15 sieve for this purpose. Powders of a less degree of fineness usually do not require this additional treatment after the moistening. The moist powder is now transferred to a sheet of thick paper and the whole quantity poured from it into the percolator. It is then shaken down lightly and allowed to remain in that condition for a period varying from fifteen minutes to several hours, unless otherwise directed; after which the powder is pressed, by the aid of a plunger of suitable dimensions, more or less firmly, in proportion to the character of the powdered substance and the alcoholic strength of the menstruum; strongly alcoholic menstrua, as a rule, permitting firmer packing of the powder than the weaker. The percolator is now placed in position for percolation, and the rubber tube having been fastened at a suitable height, the surface of the powder is covered by an accurately fitting disk of filtering paper, or other suitable material, and a sufficient quantity of menstruum poured on through a funnel reaching nearly to the surface of the paper. If these conditions are accurately observed, the menstruum will penetrate the powder equally until it has passed into the rubber tube and has reached, in this, the height corresponding to its level in the percolator, which is now closely covered to prevent evaporation, and the apparatus allowed to stand at rest for the time specified in the formula.

"To begin percolation, the rubber tube is lowered and its glass end introduced into the neck of a bottle previously marked for the quantity of liquid