II. Evolution of a Gas.

The most common instances are the evolution of carbon dioxide when a carbonate or bicarbonate and an acid or acid salt are brought together in solution. This action is often desirable either to give the patient a freshly formed salt or to have the agents dispensed in a liquid charged with carbonic acid.

Examples of intentional mixtures of this class are prescriptions for salicylic acid and sodium bicarbonate or the formulae for the preparation of the solution of potassium citrate, solution of ammonium acetate, solution of magnesium citrate, seidlitz powders, etc.

A common example of the unintentional prescription of this class is ordering ammonium carbonate and syrup of squills.

Gas may be liberated with explosive violence when strong oxidizing and easily oxidizable substances are brought together.

Potassium chlorate should not be prescribed with easily oxidizable substances if trituration is necessary.

It should not be prescribed with sulphuric acid or with hydrochloric acid or glycerin except in the presence of considerable water.

Spirit of nitrous ether should not be prescribed with substances containing tannic acid, as undesirable nitric oxide gas is evolved.

Nitric acid should not be prescribed with glycerin or other easily oxidizable substances.

Sulphuric acid should not be prescribed with chlorates.

Glycerin should not be rubbed with dry oxidizing agents, as potassium chlorate, potassium permanganate, etc.

Iodine should not be prescribed with oil of turpentine in concentration. It should not be ordered with ammonia water (to make the so-called colorless tincture of iodine), as explosions may occur after evaporation of the liquid.

Chromium trioxide is a strong, oxidizing agent. It is seldom used by the general practitioner. It should be used alone.

III. Separation of an Immiscible Liquid.

(A) The Result Of Chemical Action

The only common example of this is the separation of the oily chloral alcoholate when hydrated chloral is dissolved in a strongly alcoholic liquid.

(B) Due To Physical Incompatibility

Most oils are readily soluble in or miscible with alcohol, and only sparingly so with water. When, therefore, an alcoholic solution of an oil has water added to it in sufficient quantity, more or less of the oil separates, producing, first, a cloudy or milky appearance, then collecting as a separate strata.

Alcoholic solutions that are particularly liable to this actio» are aromatic spirit of ammonia, spirit of orange, spirit of lemon, spirit of peppermint, spirit of camphor, camphorated tincture of opium, etc.

IV. Formation of a Gelatinous Mass. The most common examples are: ordering phenol with collodion or albumin, or an aqueous solution of acacia with alcohol, ferric salts, or strong solutions of borates or lead subacetate.

V. Incomplete Solutions.

This constitutes one of the most common sources of trouble with which the pharmacist has to contend.

A physician may prescribe insoluble or sparingly soluble substances to be dissolved in liquid, as bismuth subnitrate in water.

Sometimes a moderately soluble substance is prescribed in excess, as in ordering for a 2- aqueous solution 1 3 of boric acid.

Sometimes too small an amount of the solvent is used, as wishing to give 2 grains of quinine hydrobromide in solution at a dose, to prescribe it 2 grains to the teaspoonful. The solvent should be increased so as to represent the two grains to two teaspoonfuls.

Often the wrong solvent is ordered, as prescribing alkaloidal or metallic salts in alcohol instead of aqueous liquids in which they are more soluble.

VI. Immiscibility.

The most common examples are prescribing oils with aqueous liquids.

Incompatibility in Solids.

I. Chemical Changes.-Dry medicinal agents are not so apt to undergo chemical changes and the possibility of such occurring need not often be a cause of apprehension. The following, however, may well be remembered:

Chemical changes may take place after the agents have been dissolved in the fluids of the intestinal tract; so drugs that in solution would form poisonous or inert compounds should not be prescribed together even in dry form. Examples are calomel and potassium bromide or silver nitrate and sodium chloride.

Agents apt to explode when rubbed together should not be prescribed, as potassium chlorate and tannic acid.

Certain salts when mixed with certain other agents liberate water of crystallization. The most common example is iron sulphate with alkaline carbonates.

II. Undesirable Pharmaceutical Mixtures.-Some drugs when mixed develop excessive moisture. The following should usually not be prescribed together in dry form:

Camphor, menthol, thymol, hydrated chloral, citric acid, sodium phosphate.

The above should not usually be prescribed in dry form with acetanilide, antipyrine, acetphenetidin, sodium salicylate, phenyl-salicylate, resorcin, betanaphthol, diuretin, sulphonal, trional.

There are, of course, some minor exceptions in the foregoing.

In ointments, aqueous liquids should not be ordered with oily or fatty bases, as tincture of opium or solution of lead subacetate with petrolatum.

Therapeutic Incompatibility.

This is the condition resulting where agents are prescribed together that have an antagonistic therapeutic effect. The consideration of this belongs to the realm of therapeutics. Therapeutic incompatibility is not always undesirable. Where an agent affects several organs, another agent may be employed with it that modifies or counteracts its effect upon one or more of these parts and leaves its action on the others more or less uninfluenced. When an agent has two or more different actions, another agent may be employed with it that will modify or counteract one or more of these effects without materially interfering with its other action.