General rules break down now and then, so that the dispenser should have some acquaintance with the behaviour of certain substances which must be treated according to their characteristics. The chapter on Special Drugs gives useful information for general guidance, and in the paragraphs which here follow various peculiarities are illustrated by actual prescriptions.

Acetanilidum

The sparing solubility of this drug in water may tempt the dispenser, as in the following instance, to treat it as an insoluble substance when prescribed in mixture :

Ft. mist.

The prescriber appears to have considered the antifebrin insoluble, and therefore ordered the gum to suspend it, but the glycerin suffices to dissolve and keep it in solution. Dissolve the acetanilide and quinine hydrochloride in 1 ounce of boiling water ; add the glycerin and nearly all the rest of the water, then an equivalent of acacia mucilage, and make up.

Acidum Benzoicum

This is readily soluble in alcohol of various dilutions and in glycerin. If, therefore, any tincture is prescribed along with it, the acid should be dissolved in the tincture and the water added. If the acid is in excess of the alcohol solubility power, it must be suspended, but it is absolutely requisite to rub it down to fine powder first, owing to its light feathery nature, which would prevent it diffusing, as in this example:

Acidum Carbolicum

Rarely prescribed in mixtures for internal use, but when it is, the acid should be sent out in complete solution, as undissolved acid acts as a caustic. The use of hot water ensures safety and rapidity.

Acidum Gallicum

Cold water dissolves about 1 per cent.; hot water, one-third of its weight. It is usually prescribed in excess of its solubility in cold water. It should be rubbed down to fine powder, which is distributed by shaking.