The most unmanageable mixtures are those in which alkaloidal precipitants are present. The more common of these associated with quinine in prescriptions are the alkaline carbonates and hydrates, iodides and iodine, perchloride of mercury, and infusions or tinctures containing tannin. In all circumstances these substances precipitate quinine as insoluble compounds, which in most cases are adhesive. The alkaline hydrates and carbonates precipitate quinine as hydrate, and there is no means of avoiding the precipitation. Prescribers appear to be fonder of ordering the alkalies- generally in the form of aromatic spirit of ammonia- with citrate of iron and quinine than with the plain salts of quinine, probably under the impression that the double salt is not affected by the alkali. Ammoniated tincture of quinine (in which the alkaloid exists as hydrate dissolved in alcohol) is sometimes prescribed along with water, the result being that quinine hydrate is precipitated. Examples of such cases are given in the following pages, and it will be seen that the addition of mucilage is recommended, this being all that is necessary to diffuse the precipitated quinine permanently in the mixture instead of letting it adhere to the bottle.

Iodide of potassium forms different compounds with quinine salts, the difference depending upon the other ingredients of the mixture. Neutral solutions of quinine sulphate and potassium iodide do not react chemically, but the presence of free acid invariably induces a chemical change, and this is accentuated if there is any substance in the mixture which liberates iodine, such as nitric acid or spirit of nitrous ether. Once an alkaloid in solution comes into contact with free iodine all hope of a satisfactory mixture is gone. In such cases the dispenser should, if possible, communicate with the prescriber, so as to suggest the exclusion of the oxidising body; failing that, the reaction between the alkaline iodide and the oxidising body should be carried out with the minimum quantity of iodide, the rest of the iodide being mixed with the quinine and a little mucilage before the iodine solution is added to it. Double iodides, such as liquor arsenii et hydrargyri iodidi (Donovan's solution), precipitate alkaloids at once; the same is the case with perchloride of mercury, the precipitates being heavy and poisonous, therefore dangerous. Glycerin has a wonderful inhibitive influence in such cases.

A few years ago it was shown, as the result of a long research, that many chemical reactions are retarded, and in some cases prevented, by glycerin, sugar, and gum. Long before that (viz., in 1881) the late Mr. William Martindale had proved before the Pharmaceutical Society the inhibitive influence of glycerin in the following case:

The first two ingredients form potassio-mercuric iodide, the well-known alkaloidal precipitant, so the mixture would not 'go' at all. After many trials, Mr. Martindale produced a solution by using 1 1/2 ounce of glycerin in place of as much water. The quinine sulphate was rubbed with the glycerin, the iodide added and dissolved, then the tincture and spirit, and lastly the perchloride dissolved in 1/2 ounce of water. The precipitate formed on the last addition was immediately redissolved, and the mixture was turned out slightly opaque, owing to the resinous matter in the tincture of cinchona. Galenical preparations containing tannin, especially the acid infusion of roses, are troublesome when prescribed along with quinine salts, and require special treatment. Many of the difficulties with quinine mixtures are amenable to the two rules which should be observed, viz.:

1. Chemical reaction should be effected in the most dilute solutions; and...

2. A means for the proper apportioning of the dose should be adopted. For this purpose mucilage of acacia is not only generally suitable, but it has been shown to retard or modify chemical action. We now give a number of prescriptions which have actually been met with at the dispensing-counter.

With Acetates quinine salts yield a precipitate of quinine hydrate in feathery crystals if the proportion of the alkaloidal salt is greater than 1 grain in 3 ounces of aqueous mixture.

With Carbonates or Hydrates quinine frequently forms a troublesome precipitate which requires careful management. The best result is usually obtained by diluting, as far as possible, the incompatible solutions before mixing. This method answers with the following:

But if the tincture be poured into a strong solution of the carbonate, the alkaloid separates in flocks, which adhere to the sides of the bottle.

Mixed in whatever manner, the precipitated quinine adheres in lumps. A good mixture was, however, obtained by using a proportionate quantity of pure quinine instead of sulphate; this was diffused through a portion of the water, and the diluted sal volatile added.

The plan to follow in this case is to rub the quinine to fine powder in a mortar, and mix it with 7 ounces of water; add the sal volatile, shake well, then add the rest of the ingredients and sufficient water to make up to 8 ounces.