Agitation is another important solution-promoting factor, but all that agitation does is to bring the solute into contact with fresh portions of solvent. It stands to reason that if a salt lies at the bottom of the solvent it will take a long time before it is dissolved. One might keep an ounce of potassium chlorate at the bottom of 24 ounces of water for many days without complete solution being effected, because a layer of the water next to the salt would become saturated, therefore more dense than the solvent above, and we should have to rely entirely upon the slow process of diffusion to bring fresh molecules of water into contact with the undissolved chlorate. From this the value of agitation added to fine division may be judged. Any method which brings fresh solvent into contact with the undissolved substance answers the purpose, as, for example, in percolation ; or suspending the solute or solid in the solvent- which is the course adopted in making infusions, whereby the solution as it is formed sinks to the bottom of the vessel, and thus starts convection-currents in the whole fluid which amount practically to automatic agitation.

Heat is one of the most potent factors in promoting solution, and one of the most dangerous for dispensers to use, because, with very few exceptions, substances are more soluble in hot liquids than in cold, so that when hot solutions cool they deposit the excess of solute which they contain. Hence if the dispenser wishes to expedite matters by using hot water, he should assure himself by reference to the Pharmacopoeia solubilities that the amount of substance taken will remain in solution at 60° F.

Many examples of solution-phenomena are described throughout this work. It will be especially noted that the relations of alcohol to water in the solution of substances require all the dispenser's intelligence, especially with the view to ensuring that the patient gets the full dose of the active ingredient of an alcoholic medicine which has to be taken in water. The most familiar example of this group is tinct. quininae ammon., which is virtually a solution of quinine hydrate in alcohol. Quinine hydrate, like all other alkaloidal hydrates, is but feebly soluble in water, consequently on diluting this tincture with water the alkaloid is precipitated ; but if the dose be added to the water, the precipitate is in finer division and remains sufficiently long in suspension to admit of the dose being swallowed, which is not the case when the water is added to the tincture. The same order holds good with resinous tinctures or fluid extracts. It may also be observed that some solvents change the chemical characters of the solutes. This is virtually the case in tinct. quininae ammon., and when we add a dilute acid to quinine sulphate in mixture we promote solution of the salt by changing it into the much more soluble acid sulphate.

Some changes in solution are apt to be overlooked ; for example, stock solution of butyl-chloral hydrate made with glycerin and rectified spirit is very convenient, as when first made it mixes quite clear with water; but ere long on adding the solution to water, there is a precipitate of oily drops which are most intractable. These drops are really butyl-chloral alcoholate, formed by the alcohol molecule displacing the water molecule in the hydrate; and the alcoholate is much less soluble in water than the hydrate. A general idea of the substances which are less soluble in alcohol than in water is a good acquisition to the dispenser. This is the case with many colloidal substances; pepsin, for example, is precipitated from its solutions by 25 per cent, of alcohol.

Precipitation is almost as important as solution, for knowledge of the conditions controlling it is of distinct value in enabling the dispenser to judge when he may or may not filter mixtures. All the conditions which determine precipitation cannot be dealt with here, but it may be noted that saline substances tend to throw extractive matters out of solution. For example, if one were to saturate a fresh and cold vegetable infusion with magnesium sulphate, a muddiness would be produced which in few cases would be due to precipitation of active principle; on the other hand, the alkaline sulphates are employed for precipitating pepsin from its aqueous acid solution. Acids, alum, and some other substances are employed to secure more complete precipitation of resinoids. Alkalies are the best precipitants of most alkaloids, but they have a wonderful effect in clearing many aqueous mixtures, especially those containing traces of resinous and glucosidal bodies.

Applying such facts as these to dispensing, there should be some hesitation in filtering mixtures in which a precipitate forms in the course of compounding; but one would not hesitate to filter a tincture of nux vomica and acid mixture, because the precipitate cannot be the active principle, but traces of resin and fat which are soluble in the alcohol of the tincture and insoluble in the aqueous and acid liquid. Other instances of trivial and potential precipitations which call for the dispenser's attention will be noticed later.

Scale Preparations can with care be readily and easily dissolved in the bottle in which they are to be dispensed. A little of the water or aqueous vehicle should be put into the bottle first, being careful not to wet the neck; or, if this be done, dry the glass with a cloth, else the scale preparation will adhere and block the neck of the bottle. A solution is readily formed if the salt falls upon the water and is quickly agitated- not allowed to 'cake' at the bottom.

There are exceptions to every rule, however, and the slow solubility and extreme frothiness on agitation of sulphate of beberine with water mark it out as a decided exception to the other scale preparations. The better plan with this salt is to rub it down into fine powder in the mortar, then add water with constant stirring so as to prevent it forming an adhesive mass on either mortar or pestle. If this be done properly, it will dissolve quickly and without the least trouble ; if any other plan be tried, it will certainly cake and cause no end of trouble. A few drops of diluted sulphuric acid is- or was (for beberine is almost extinct)- sometimes added.

Ferrum tartaratum dissolves with difficulty in cold, but very readily in hot, water. The most satisfactory method of manipulation is to put it in a dry mortar and pour hot water over it, when it goes down with the least possible trouble. With distilled water it gives a perfectly clear solution, but with tap-water the solution never becomes clear.

A Supply of Hot Distilled Water is a great convenience at the dispensing-counter. Where space permits, a gas-stove and boiling-vessel (preferably with a tap) of suitable size should be set apart for this purpose, and the heat adjusted so as to keep the water nearly boiling. A small vessel may be fitted over the sealing gas-jet, thereby utilising its waste heat. An extremely useful arrangement is a circular ring tripod, 8 inches in diameter, fitted with a set of water-bath rings; the legs of the tripod should be of such length that the top of the gas-flame, when full on, reaches a little higher than the rings. The exact quantity of water required should be placed in an evaporating-basin, resting on the tripod (using a ring of smaller diameter than that of the surface of the water) and set over a gas-stove ; then, by the time the labels are written and the solid ingredients weighed, the hot water is ready.

Concentrated Mixtures, Etc. Where much dispensing is done daily, and there is an occasional 'rush,' the need for keeping certain articles ready prepared is universally recognised. It is even more necessary in cases where dispensing is spasmodic, but the difficulty of keeping certain articles fresh is considerable. This is especially the case with aqueous mixtures, which begin to ferment after a few days. Some of these keep well in a concentrated condition, and other articles may also be kept ready for diluting or massing as the case may require. The following hints may be useful to some who have not seen the ideas carried out in practice. It is a great help to expeditious dispensing to have ready prepared mixtures which a frequently ordered by local physicians, and generally it is possible to make these of the 1-to-7 type so that they may be diluted with an aqueous vehicle as required.