Note

Finnemore and Williamson found that in 1-oz. aqeuous mixtures 10 minims of liq. strych. and 15 grains of sodii bicarb, remained clear for thirty days; 20 grains of potass, bicarb, and 20 minims of liquor crystallised in twenty days; 5 grains of ammon. carb. and 5 minims of liquor crystallised in ten days; 10 minims each of liq. ammon. and liq. strych. crystallised in one day; and 30 minims of spt. ammon. arom. and 8 minims of liq. strych. crystallised in seven days. Tincture and liquid extract of nux vomica of proportionate strychnine strength are more stable. ('Year-book;of Pharmacy,'1914.) Compare with p. 42.

The B.P. 1885 solution of strychnine was more acid than the 1914 one, and sometimes deposited on keeping. The 1914 solution also behaves differently with mercuric chloride:

This remains clear for several days if compounded with the 1914 solution, but with the 1885 one it gives almost immediately a crystalline precipitate.

No. III. is clear when compounded, but the strychnine hydrochloride reacts with potassium iodide, and strychnine hydriodide begins to separate in a few hours. No. IV. is a hopeless case of incompatibility, the alkaloid being precipitated as iodohydrargyrate.

Magnesii Sulphas

Although magnesium sulphate gives immediately with soluble alkaline carbonates a precipitate of magnesium carbonate, with many tap-waters no precipitate is formed. For example, the following gives an immediate precipitate if dispensed with distilled water, but none with London tap-water:

Fiat mistura.

Doubtless the presence of chlorides in the tap-water prevents precipitation of magnesium carbonate, for ammonium chloride is used for that purpose in distinguishing between magnesium and calcium salts. There is a limit to the prevention of a precipitate, and tap-waters vary in preventive power, but the point for dispensers and prescribers to note is that a chloride may with advantage be added to such a mixture as the above.

Although the sulphate dissolves easily in its own weight of water, it is practically insoluble in alcohol; hence, on adding the tinctures of No. I. to the solution of the salt, the mixture becomes almost solid in a few minutes from separation of the sulphate. No. II. mixture exhibits a characteristic common to magnesium salts-viz., that under certain conditions of hydration they form cements. Owing to the large amount of sulphate in this mixture, the suspended hydrocarbonate gradually aggregates, and in a few days forms a stone-like crust in the bottle.

Menthol is rarely prescribed for internal administration in mixture- in fact, the only prescription which we have met with is the following:

Fiat mistura.

The normal procedure in this case is to dissolve the menthol in the tincture, and add to the chloroform-water, the result being that the menthol is thrown out in indiffusible form. Acacia mucilage is useless for suspension in this case, but an ounce of rectified spirit provides a fairly good mixture. If the menthol is reduced to powder and triturated with an ounce of acacia mucilage, and water then added, a presentable mixture is obtained ; but the best plan of all for administering menthol and substances like it in aqueous mixture is to dissolve in four times their weight of olive oil, and emulsify with acacia.