It is usual for dispensers to keep solutions of salts often required in prescriptions. These are not only convenient, but frequently impart to a mixture a bright appearance which otherwise would be wanting. No one would recognise a mixture made with infus. rosae acid, and a solution of Epsom salt as the same preparation as one made by dissolving the salt in the infusion of roses. The same is true, in greater or less degree, of very many other previously made solutions; to a large extent this accounts for the physical differences observed in mixtures dispensed at different establishments.

The following are the solutions which are in most common use. It should be noted that solids for any quantity in ounces should be taken by apothecaries' weight- that is, 480 grains; for example, sol. ammon. brom. 1 in 4 means 480 grains ammon. brom. in 4 fluid ounces of the solution. The solutions should be made from the unpowdered substances:

Solutions containing 10 grains of the solid in each fluid drachm are on the whole most convenient, provided they keep, for in many instances concentrated solutions keep much better than weaker solutions. Quinine and iron citrate is a notable example, a solution of 1 part in 2 keeping for weeks, while a weaker solution quickly becomes bad. It is advisable to keep glycerin dissolved in its own volume of distilled water, which ensures more accurate measurement.

The solutions of alkali bicarbonates are liable to change, with formation of carbonate and consequent introduction of all sorts of unexpected reactions, not to mention the distinctly greater alkalinity of the carbonate. It is better not to keep the bicarbonates in solution. It is a good plan to keep all these dispensing-solutions in a dark cupboard: many of them are affected by light. In weak solution chloral hydrate soon decomposes and becomes acid, acquiring an odour not unlike a mixture of tolu and benzene. We prefer not to keep this chemical in solution, but in powder form, so that it dissolves readily. A 1-in-1 solution may, however, be kept. It is made by dissolving 100 grains of chloral hydrate in sufficient water to make the solution measure 100 minims. In cold weather sodium-salicylate solution (1 in 1) occasionally crystallises, a hydrated (6H20) salt separating.

Normal saline solution, prescribed for intravenous injection, washing wounds, etc, is a solution of 11 grains of sodium chloride in 4 ounces of sterilised distilled water.

Solutions should be made either by stirring the solid with the solvent in a measure, or by shaking the two together in a bottle. It is a mistake to use a glass mortar, or any mortar. Very frequently glass mortars are disrupted with explosive force when solids are rubbed or dissolved in them. This is especially the case with hypophosphites. Wedgwood mortars should not be used for making solutions until they have been carefully washed out in hot water and a little alkali (liquor potassae). The most useful mortar for solutions is one of the high-glazed porcelain sort.

Solution, it may be noted, is, so far as dispensing is concerned, the mixture of a solute (gas, liquid, or solid) with a solvent so as to form a clear product termed a solution. The quantity of a solute which dissolves in a solvent is constant for each substance under particular conditions. The solubilities of chemicals in water are usually determined at 15.5° C. (60° F), and represent the weight of the substance dissolved by a weight of water. In this case the weight and volume of the solvent are synonymous, but in the cases of alcohol, glycerin, and other solvents whose densities are less or more than unity, text-book figures as to solubility may be doubtful, because continental workers and many in this country invariably mean the solvent by weight. Figures employed in this book represent liquids by volume. Some knowledge of the phenomena and principles of solution is of great value to dispensers, who may frequently by the application of that knowledge avoid or overcome difficulties otherwise unexplainable.

The more important theory of solution is the hydrate theory, in which it is assumed that in the course of solution of substances in water the substances undergo hydration, and, in fact, chemically combine with the water. This is well exemplified in solutions of alcohol in water, of caustic potash in water, and of sulphuric acid in water, much heat being evolved during solution. This proof of chemical action has had corroboration in the isolation of hydrates of the dissolved substances; thus the alcohol in proof spirit does not exist as C2H6O, but as C2H6O.xH2O, and so also with sulphuric acid and caustic potash, each of which in aqueous solution is chemically associated with several molecules of water.

Whatever the theory of solution may be, there is at least agreement amongst the theorists in regard to the promotion of solution by certain forces. Fine division of the solute has a great influence in promoting solution; powdered chemicals are more easily dissolved than the crystals, and although, as already mentioned, some powdered chemicals do not yield clear aqueous solutions, that is because the chemicals have been soiled, as it were, by the mills in the course of grinding. The preparation of aromatic waters, upon which we have just touched, affords an admirable example of the advantages of fine division in promoting solution. If we rub up an essential oil with talc or calcium phosphate, we simply coat the particles of the solid with a film of the essential oil, so that when water is added a very large surface of the oil is offered to the water and solution is rapid. So also if the oil is dissolved in rectified spirit and poured into the water; here the oil separates in minute globules, hence is more quickly taken into solution.

It is obvious, however, that the powder method ensures the larger surface, hence quicker and better solution.