The more complex prescriptions consist of -

(1) The Basis, or principal active ingredient curare".

(2) The Adjuvans, or that which assists its action cito.

(3) The Corrigens, or that which corrects its operation tuto.

(4) The Constituens, vehicle, or excipient, which imparts an agreeable form Jucunde.

Thus the object of every prescription is to cure quickly, safely and pleasantly. For example in Pilula Rhei Composita the rhubarb is the basis, the aloes and myrrh form the adjuvans, and the oil of peppermint is the corrigens to prevent the griping. In Mistura Cretae the cinnamon water is the vehicle. Many drugs do not require anything to assist their action or correct their operation. The scientific physician usually prefers to administer the remedies separately, in order to more accurately observe their effect, and as well to discontinue, or change the dose of, any one which may be necessary.

Incompatibility of ingredients should be particularly-avoided in prescriptions. There are three kinds of incompatibility:

(a) Chemical Incompatibility; e.g., Glucosides should not be ordered with free acids, which decompose them; nor Alkaloids or Alkaloidal Salts with alkalies, alkaline salts, tannic acid, iodides, or bromides, for they precipitate them.

Examples of chemical incompatibility are the prescribing of (1) tannic acid or substances containing it with alkaloids or metallic salts, especially those of iron; (2) vinegars or syrups containing acetic acid prescribed with carbonates lead to the evolution of carbon dioxide; (3) strychnine sulphate is decomposed by potassium bromide, and strychnine is precipitated; (4) chloral hydrate and alkalies form chloroform; (5) quinine sulphate and potassium acetate together cause a voluminous precipitate of quinine acetate; (6) lime water with mercury salts (this incompatibility is intentional in Lotio Nigra and Lotio Flava), precipitates mercuric oxides; it decomposes carbonates and bicarbonates of alkalies; it precipitates solutions of quinine and morphine salts; (7) corrosive mercuric chloride is incompatible with most substances.

The following table, drawn up by Potter Materia Medica, Pharmacy and Therapeutics, 7th Ed., p. 540 shows the most important instances of solutions which mutually precipitate each other. The letter " P" means " forms a precipitate with."

Solutions of

Alkaloidal Solutions (generally).

Metallic Solutions (generally).

Solutions of Lead or Silver Salts.

Solutions of Calcium Salts.

Solutions of Magnesium Salts.

Solutions of Albumin or Gelatin.

Alkalies..............................

P

P

P

P

P

Tannic acid.......................

P

P

P

,

P

Carbonic acid and Carbonates .

P

P

P

P

P

Sulphuric acid and Sulphates...................

P

P

. .

Phosphoric acid and Phosphates .

P

P

P

P

P

Boric acid and Borates ....

P

P

P

Hydrochloric acid and Chlorides .

P

Hydrobromic acid and Bromides .

P

Hydriodic acid and Iodides .

P

.

P

Sulphides......................

P

P

Arsenical Preparations.....................

P

P

Albumin.................................

P

P

With the following drugs it is particularly difficult to avoid chemical incompatibility.

Antipyrin.

Chlorine in solution.

Liquid preparations of Iron.

Lead salts.

Zinc salts.

Silver salts.

Corrosive Mercuric Chloride

(especially). Iodine and the Iodides. All Bromides.

Potassium Permanganate.

Potassium Acetate.

Nitrites.

Tannic Acid.

Gallic Acid.

Acidum Hydrocyanicum Dilutum.

Mineral Acids.

Liquor Potassae.

Quinine Sulphate.

Tincture of Guaiacum.

Substances rich in oxygen, as chlorates, iodates, permanganates, picrates, nitrates and bichromates should not be mixed with readily oxidizable substances, such as charcoal, sulphur, iodine, carbolic acid, glycerin, turpentine, and organic compounds generally, for explosive compounds are very liable to be formed.

Poisonous compounds may be formed by the admixture of substances in solution: e.g., potassium chlorate and the syrup of ferrous iodide liberate iodine; diluted hydrocyanic acid and calomel form mercuric cyanide; potassium chlorate and potassium iodide form, at the temperature of the body, a poisonous compound, probably potassium iodate. Death has occurred owing to patients having taken some of these careless prescriptions.

If, in a mixture, incompatibles are inevitable, they should both be diluted with the vehicle before they are added to each other. The careful prescriber will avoid combining any of the above incompatible substances.