By precipitation is understood the sudden destruction of the soluble form of a substance which is held in solution; this may be effected by the addition of another substance to the solution, or by some external agency. The substance thus thrown out of solution is termed the precipitate, and the substance or force causing the separation, the precipitant. Precipitation is employed in pharmacy as a method of pulverization and purification, and as a convenient means for obtaining many insoluble substances.

The first of these comes under the head of what may be termed simple or physical precipitation, usually brought about by the addition to the solution of some substance in which the dissolved body is insoluble; as in the precipitation of ferrous sulphate or of tartar emetic from aqueous solution by means of alcohol. Other examples of physical precipitation are the separation of iodine or camphor from alcoholic solution by the addition of water, the precipitation of solution of acacia by alcohol, the precipitation of lime-water by boiling, and the preparation of the official resin of jalap.

The process of precipitation when intended as a means of purification, or of the preparation of insoluble compounds, almost invariably involves chemical action, as in the purification of metals by electrolysis, the manufacture of mercuric iodide, etc.; in the former case simple decomposition of a salt is effected, while in the latter case mutual decomposition between two salts is as a rule necessary.

Some insoluble compounds are precipitated by simple decomposition of a substance by means of water, as bismuth subnitrate, yellow mercuric subsulphate, etc.; in the former case an acid solution is freely diluted with water, in the latter case white mercuric sulphate is thrown into boiling water.

Mercuric oxide can be obtained in a much finer state of division by precipitation than by any other method, but it must be brought about by chemical action. If a solution of mercuric chloride be poured into a solution of sodium or potassium hydroxide two new compounds, yellow mercuric oxide and sodium chloride, are formed, the latter remaining in solution, while the former separates as an impalpable powder, being insoluble in all neutral liquids. Lead iodide, magnesium carbonate, ammoniated mercury, and precipitated chalk are familiar examples of compounds prepared by chemical precipitation.

The character of the precipitate depends largely upon the conditions under which its formation is effected; thus, concentrated solutions are apt to yield dense precipitates, particularly if heat be employed, whereas cold dilute solutions, as a rule, produce light bulky precipitates. In the preparation of new chemical compounds by precipitation it is important that the proportion in which the precipitant is to be employed should be determined by calculation, as a deficiency or an excess may result in loss from imperfect precipitation or re-solution of the precipitate. Mutual decomposition between two salts always takes place in definite molecular propor

tions, and the necessary quantities may be readily ascertained by writing out an equation showing the decomposition; thus the formation of yellow mercuric oxide is demonstrated by the equation HgCl2+2NaOH = HgO + 2NaCl + H2O, which shows that 1 molecule or 270.54 parts of mercuric chloride requires 2 molecules or 79.92 parts of sodium hydroxide for complete precipitation. In this case an excess of sodium hydroxide is not hurtful, but a deficiency would result in the production of mercuric oxychloride of brownish color instead of a pure yellow oxide. The equation HgCl2+2KI = HgI2+ 2KCl shows that in the formation of red mercuric iodide 2 molecules or 331.12 parts of potassium iodide are necessary for the complete precipitation of 1 molecule or 270.54 parts of mercuric chloride; these proportions must be strictly observed, otherwise a loss will result, as red mercuric iodide is soluble in both potassium iodide and mercuric chloride solutions. When precipitation by mutual decomposition between two salts is proposed, the salts are mixed in the form of separate solutions, and perfect blending is accomplished by stirring the mixture.

The most convenient style of vessel for precipitation is a glass or stoneware jar considerably broader at the base than at the top, and provided with a lip; this greatly facilitates the subsidence of the precipitate, and the subsequent removal of the clear liquid remaining above the precipitate, known as supernatant liquid.

The purification of precipitates is effected by a process of washing, which consists either in mixing them repeatedly with fresh portions of water in a suitable jar, and decanting the supernatant liquid after it has become perfectly clear, or in continued affusions of water on the precipitate contained in a cloth strainer or paper filter; each portion of water should be well mixed with the precipitate and the washing continued until the complete removal of the soluble byproduct has been ascertained by appropriate tests. When a precipitate tenaciously retains liquid, forming a thin paste, the mixture is termed a magma, and forcible expression must frequently be resorted to in order to remove the liquid, as in the case of washing ferric hydroxide, freshly precipitated calcium phosphate, etc.

The official reduced iron is an instance of a metal obtained in a finely divided state by reduction; ferric oxide being heated to redness in an atmosphere of hydrogen, in suitable tubes, and allowed to cool without contact of air. This method of producing metallic iron in fine powder yields better results than any other known.

Granulation is a process by which certain substances soluble in water are obtained in the form of coarse powder by simple evaporation of their solution, with constant stirring, until all moisture is dissipated. It is employed either for deliquescent and difficultly crystallizable substances, as potassium citrate and carbonate, or in cases where the solution, if allowed to evaporate very slowly, would yield larger crystalline masses, as ammonium chloride, lead acetate Granulated powders, as the name indicates, never represent a

etc.

fine state of division, but offer a very convenient form for dispensing purposes. Zinc and tin may be readily granulated in the metallic state by heating them to a temperature a little below their meltingpoint, when they become very brittle, and can then be rubbed into coarse powder in a mortar.

Some substances obstinately resist pulverization by any of the methods mentioned, and require a different treatment; for instance, camphor cannot be reduced to a fine powder without being first brought to a state of partial or perfect solution by means of alcohol; a smooth paste being first formed of camphor and alcohol in a mortar, which is then triturated until perfectly dry and in the form of an impalpable powder-excessive pressure should be avoided during the trituration. Powdered camphor thus prepared is apt to return gradually to a crystalline condition, no matter how carefully it is preserved, but this can be prevented by precipitating the camphor in the presence of some powder with which it will become intimately mixed. Such a process was first published in Parrish's Treatise on Pharmacy, and is as follows: Four ounces of camphor dissolved in 8 fluid ounces of alcohol are poured slowly, with constant stirring, into a smooth mixture of 15 grains of calcined magnesia and 2 pints of water; the precipitated camphor, enveloping the magnesia, soon rises to the surface, and is recovered by pouring the whole mixture on a paper filter, where it is allowed to drain. To facilitate drying of the mass, it is cut with a spatula into small particles, and is finally preserved in bottles. Although retaining a very small amount of moisture, this precipitated camphor keeps excellently, and may be used for all purposes requiring camphor, except cases of solution. Iodoform and boric acid can also be quickly reduced to an impalpable powder by trituration with alcohol, whereby partial solution is effected, and a dry powder is obtained upon evaporation of the alcohol. Friable substances, which are not held together by strong cohesive force, but the particles of which are apt to cake when submitted to pressure, may be powdered by simple friction over a perforated surface; no better method is known for obtaining magnesium carbonate in an impalpable condition than by rubbing the cakes over the surface of an inverted bolting-cloth sieve.

CHAPTER VII.

SOLUTION.

WHEN a solid body is brought into contact with a fluid in such an intimate manner that it loses its original form and assumes that of the fluid, producing a clear and uniform liquid, the process is termed solution, as is also the newly-formed homogeneous liquid; but solution is by no means restricted to the liquefaction of solids by fluids, as gaseous and liquid substances can also be brought to the condition of perfect molecular blending characteristic of solution. The fluid used to produce solution is called a solvent or menstruum. The hypotheses at present engaging the minds of scientists regarding the electrochemical decomposition of bodies in a state of solution need not be considered here; by some the process of solution is looked upon as one of great force and activity, and this view may in the course of time clear up many hitherto unexplained phenomena.

Two kinds of solution are recognized, namely, simple and complex solution; in the former the solvent produces no change in the sensible characteristics of the dissolved body, simply altering its physical condition, while in the latter, where solution takes place as the result of chemical action, the properties of both the solvent and the dissolved body become modified by the loss of old or the acquisition of new properties. In the case of a simple solution, the taste, odor, color, and chemical properties of the dissolved body remain intact and are imparted to the solution; as, for instance, solutions of sugar, table-salt, or potassium permanganate in water. In simple solutions the dissolved body can be recovered in its original condition by evaporation of the solvent. Complex solutions should not be confounded with compound solutions; the latter term indicates a mixture of solutions, which may all be simple in character, while complex solutions are understood to be the result of chemical action and are accompanied by one or more of the following phenomena: heat, effervescence, change of color, odor, and taste; as, for example, the solution of a Seidlitz powder or the solution of red mercuric oxide in nitric acid. The products obtained by evaporation of a complex solution will be found to have new properties, not possessed originally by the solvent or the dissolved body.

The greater the extent of surface exposed by the solid body to the liquefying action of the solvent, the more rapidly will solution be effected; hence mechanical division facilitates solution, because the latter process is in direct opposition to cohesion. A simple solution of solid substances may be considered as a fluid produced by the

intimate union of the solvent and the dissolved body in a state of minute division, the union and division being so complete that the forces of cohesion and gravity are suspended, otherwise a mixture only is produced, and the solid substance will again separate. The agitation of a mixture of a solid substance and solvent also causes more rapid solution, by constantly bringing fresh portions of the fluid into contact with the solid; if equal weights of acacia or sugar, in lumps or in fine powder, be placed in separate vessels with a sufficient quantity of water, the one being actively stirred while the other is allowed to remain at rest, solution will be completed in the former vessel long before it occurs in the latter; this is due to the fact that in the second vessel a dense solution will form immediately around the solid particles, and thus prevent the remainder of the fluid from exerting its solvent action.

The term "solubility," when no solvent is mentioned, always refers to the behavior of the substance toward water at the ordinary temperature, about 15.6° C. (60° F.); thus the statements that sugar is soluble and bismuth subnitrate is insoluble refer solely to the liquefying effect which water will have upon the two substances. Different degrees of solubility are expressed by such terms as sparingly soluble, soluble, and very soluble; these varying degrees of solubility do not determine the rapidity of solution, for some substances are known to dissolve slowly but to a greater extent than others which enter into solution more rapidly but in less proportion. Substances differ greatly in their solubility in water; as extremes may be mentioned zine chloride, soluble in one-third of its weight of water, and barium sulphate, which requires about eight hundred thousand times its weight of water for solution. Substances but slightly soluble in water may be very soluble in other liquids; as camphor, which requires about 1000 parts of water for solution but is readily soluble in one-third of its weight of chloroform.

Heat, as a rule, favors the solution of solids and diminishes the solubility of gases, but there are no substances totally insoluble in the cold which become soluble by the aid of increased temperature. The effect of the application of heat is the establishment of currents in the liquid which will facilitate solution just as agitation of the vessel favors the same result; and moreover, since heat intensifies molecular motion in both the menstruum and the solid, not only will an increased quantity of the latter assume the fluid state, but solution will also be effected in less time, on account of the energetic intramolecular activity. There are some exceptions to the general rule that heat increases the solubility of substances; for instance, common salt is about as soluble at ordinary temperatures as at the boilingpoint of water; sodium sulphate or Glauber's salt increases in solubility rapidly from 15° C (59° F.) to 34° C. (93.2° F.), at which point water takes up four times its weight of the salt, but beyond this temperature its solubility again decreases until 100° C. (212° F.) is reached, when water takes up about 2.13 times its weight of the salt;