Whenever substances rich in oxygen or easily deoxidised are ordered to be mixed with other ingredients, the dispenser should always carefully consider the order of mixing. Such substances should never be rubbed with easily oxidisable bodies.

Substances which easily part with their oxygen are picric acid, and chlorates, iodates, bichromates, permanganates, nitrates, and picrates, and oxide of silver. In compounding these substances each should separately be first rubbed to a powder in a mortar, then be mixed with the safe ingredients, and lightly blended on paper with a bone spatula. The more common oxidisable substances are charcoal, organic powders, iodine, sulphur, sulphides, reduced iron, iodide of iron, hypophosphites, camphor, essential oils, and ammonia salts. The following are specimens of explosive compounds:

The potassium chlorate should first be rubbed to a fine powder; the other ingredients should be separately mixed ; lastly, the chlorate should be combined with the other powders by mixing on paper with a bone spatula. The pressure of a pestle may induce a dangerous explosion- indeed, chlorates are amongst the most explosive compounds known, and should always be handled carefully. The same applies to hypophosphites- always rub them gently, and be careful how you apply heat to them, either when dry or in solution, especially with glycerin.

Explosion took place in the following when the oil of peppermint was added :

Fiat pulvis.

Even without the oil the powders explode if rubbed hard in a mortar.

Oxide of Silver, if to be combined with any organic substance, should be first damped with water. If creosote is compounded with oxide of silver in a pill, it will explode. Pills containing oxide of silver are liable to inflame if they become warm. They have taken fire in a person's pocket, causing severe burns. (See also page 101.)

Nitrogen Compounds

Tincture of iodine and ammonia are often prescribed together, and iodide of nitrogen is produced under certain conditions. An explosion has resulted from the preparation of the following prescription, iodide of nitrogen being evidently the cause:

A concentrated solution of iodine and iodide of ammonium was filtered through paper. The next day the filter was touched with a view to being removed, when the paper and funnel were shivered into atoms with a loud explosion.

Reference has been made to the incompatibility of iodine with essential oils. The reaction may be so rapid as to cause explosion. This happened with the following:

Fiat solutio.

Erythrol Tetranitrate should be handled with great care. In consequence of a fatal accident to a young chemist at Dartford in 1897 (he was mixing the tetranitrate with sugar of milk in a mortar when the whole thing exploded) the Chief Inspector of Explosives advises dispensers that the tetranitrate 'is more sensitive to percussion than dynamite or guncotton.' A medical man threw a sample bottle of the drug into his waste-paper basket. Next morning the basket was emptied into a dustpan containing hot ashes. An explosion ensued, and the housemaid was partly stunned, and received about two dozen small wounds on the hands, arms, and face. Mannitol hexanitrate explodes violently on being struck with a hammer, or when suddenly heated.

Therapeutical Incompatibility is much too wide a subject to discuss in this volume, nor is it one which a pharmacist can adequately treat. Prescribers rarely sin in this respect, and it is noteworthy that many of the apparent therapeutical incompatibles are not so in reality, for the physiological actions of some substances are exerted or completed before others begin, so that such things may be given together quite appropriately. Doctors are very fond of giving ammonia with salicylates : they must have free ammonia to act as a heart-stimulant, because the salicylate is somewhat depressing. The mixture is chemically incompatible, and becomes brown in a few days, but it acts well nevertheless.

As a rule it is inadvisable to prescribe glucosidal bodies in aqueous mixture, especially in presence of acids, because the bodies sooner or later hydrolyse, and thus the therapeutic action may be obliterated or untoward results occur. This is observed in the case of tincture of strophanthus, which in aqueous mixture, kept for a week or two, produces disagreeable purgation and little effect on the heart. Pharmacists may be able by observation and application of their peculiar knowledge to explain such things to prescribers, but it is scarcely their province to interfere in supposed instances of therapeutical incompatibility.