It is not easy to lay down any rule for the dispenser when he comes across an incompatibility. In such cases he would do well to put to himself, previous to compounding the prescription, such questions as the following: (1) Was this incompatibility foreseen and intended by the prescriber?

(2) Does it in any way endanger the health of the patient ?

(3) Is it necessary to trouble the prescriber (supposing he can be communicated with) regarding the incompatibility ? (4) Can the incompatibility be in any way mitigated or avoided ? The following examples are given as illustrations of these remarks :

Fiat mistura.

As sulphate of magnesium is already ordered in the prescription, it is improbable that the prescriber intended also the addition of this salt by extemporaneous preparation. Further, the prescriber probably adds the acid merely to assist the solution of the quinine, and an excess is added in this instance, as is generally the case with quinine mixtures. Lastly, there is the probability that the magnesium carbonate was intended to act as an antacid; so that, everything considered, there was no difficulty in substituting q.s. for 3iss. of acid to dissolve the quinine. On the whole, however, it is better to omit the acid altogether, because the soluble sulphate of quinine will be decomposed by the magnesium carbonate. It answers well to rub the quinine to fine powder and suspend in the mixture along with the carbonate.

Fiat mistura.

It was not easy to arrive at any satisfactory conclusion as to the intention of the prescriber in this instance; but as the ammonium phosphate formed by interaction of ammonium carbonate and phosphoric acid is altogether harmless, and as free phosphoric acid in the mixture would, on exposure to light, liberate iodine from the potassium iodide and precipitate morphine from the nepenthe, there was no hesitation in dispensing the prescription as it stood.

Fiat mistura.

This will probably appear a very innocent mixture when first dispensed, but if prepared strictly according to the letter it will bring the dispenser almost certainly into trouble. It will form a clear mixture when first dispensed, but after an interval, depending on the purity of the ingredients, it will turn first brown and then quite black, and become unfit for use. A prescription such as this should never be dispensed without an explanation to the patient of the changes which may be expected to take place, and a caution not to use the mixture after decomposition.

It is rather a neat dispensing feat to produce a black lotion from the following:

Fiat lotio.

Make the black wash first, then add the zinc chloride. In the course of a few hours the zinc chloride interacts with the black mercurous oxide, and the lotion becomes white. It is therefore just as well to add the zinc chloride to the lime-water in the first instance.

Ft. lot. To be applied to the scalp once a day.

Here the oxide of mercury is changed to black suboxide. The prescription is very interesting in more ways than one. There is for the pharmacist to consider the meaning of the writer; he might pause, and wonder if a fluid which has every appearance of ' black wash' is what was intended for a 'scalp' application. It is easy to imagine the dirty condition the head would be in after a few applications, although, in cases of ringworm, more unsightly and sticky applications are often used. The chemistry of the lotion will be best understood when it is considered that ammonia and oxide of mercury form mercuro-hydroxylamine, thus :

2HgO + NH4OH - NHg2OH + 2H2.O.

This compound is readily reduced in the presence of a substance capable of oxidation. This we have in chloral hydrate, which is oxidised to trichloracetic acid, the ammonia salt of which is formed in the presence of free ammonia, thus :

OH CC13.CH< + NHg2OH = Hg2O + CCl3.COONH4.

Many decompositions are intentional, as in the case of mist, ferri co., B.P., or the frequent combination of tincture of opium with solution of subacetate of lead for injections; also in the following :

M. Ft. lot. Modo dicto utend.

Here an abundant precipitate renders the lotion almost creamy, and necessitates mixing half the water with the extract, and the remainder with the liquor before mixing, else a disagreeable lumpy product results. Such combinations may be dispensed as written, and sent out with a 'shake'label.

Occasionally, however, the decompositions are of such a character that the chemist may feel pretty sure the writer of the prescription is unacquainted with the reaction or has overlooked it. For example:

In these cases metathesis takes place, the water of crystallisation of the sulphates is liberated, and the powders become wet. The use of an equivalent quantity of the dried salts removes the difficulty.

The following prescription has become famous in textbooks:

Fiat mistura.

This solution deposits in a few hours the greater part of the strychnine salt as an insoluble bromide in transparent crystals. A lady in England lost her life by taking a similar mixture: the precipitated strychnine collected at the bottom, and in taking the last dose she swallowed nearly all of it. Potassium iodide also precipitates strychnine.

One of the most remarkable cases of incompatibility is the following, which at first sight appears perfectly harmless, but at least one case of death is on record from the administration of a similar mixture:

Fiat mistura.