When different substances, whether liquid or solid, are combined or associated and undergo a more or less complete change, they are said to be incompatible, the incompatibility consisting of two kinds : chemical and pharmaceutical. (Drugs that are opposed in their physiological action are preferably spoken of as antagonists.)

The incompatibles and antagonists of the different substances are fully mentioned under the respective drugs. The principles governing incompatibility, however, may well be considered here.

Chemical incompatibility is of the most importance.

The commonest forms of chemical incompatibility occur under the following conditions:

1. When a new and insoluble salt is formed, resulting from a mixture of solutions of soluble salts. Example (1): mixing solutions of lead acetate and zinc sulphate, both soluble salts, but producing by chemical decomposition a new and insoluble salt, the sulphate of lead; which is precipitated.

2. By the addition of a strong acid to solutions of salts of weak or volatile acids, such as carbonates and bicarbonates, with resulting decomposition. Example (2): ammonium carbonate, the salt of a weak acid radical, added to syrup of squills, containing acetic acid, causes decomposition to take place, with effervescence and the liberation of carbonic-acid gas.

3. Salts of a feeble or volatile base are decomposed by the 1 Therapeutics, 7 th ed., pp. 108 et sea.

addition of a strong alkali. Example (3): the evolution of ammonia when a strong alkali is added to ammonia alum, and when chloral hydrate is decomposed by alkalies, such as aromatic spirit of ammonia, lime solution, etc.

4. Alkaloids, or their salts, are thrown out of solution or precipitated from their solutions by the addition of alkalies or alkaline salts. Example (4): sulphate of strychnine in solution is precipitated as the insoluble bromide of strychnine by the addition of a larger proportion of potassium bromide. Quinine sulphate is precipitated as insoluble quinine acetate when mixed with a solution of potassium acetate.

5. Tannic and gallic acids and preparations containing them, as well as many other vegetable acids, produce discoloration or precipitation of iron and many of its compounds. Example (5): ink is the best illustration of this incompatibility. Writing fluids are usually combinations of tannic or gallic acid with some preparation of iron. Add the tincture of ferric chloride to tincture of cinchona, and notice the discoloration.

There are certain preparations of iron, like the compounds with ammonium or sodium citrate (see Tincture Ferri Citro-chloride, N. F., tasteless tincture of iron) which produce little discoloration with vegetable astringents, and none at all with vegetable preparations containing no tannic or gallic acid.

Pharmaceutical incompatibility is the production of a sediment by change of solvent without chemical action. Examples: vegetable tinctures of resinous drugs with water, such as tincture of guaiac and water; copaiba and oils with aqueous preparations; spirit of camphor with water; spirit of nitrous ether with mucilage of acacia, etc. The separation or precipitation may frequently be prevented by the intervention of some viscid substance, such as syrup, glucose, glycerin, mucilage of acacia, etc.

The following is a reference-list of the common incompatibles of individual drugs:

Those marked with a * are sometimes directed to be compounded tor the purpose of effecting some special change or producing new compounds.

Among the above, potassium permanganate forms an explosive mixture with glycerin; so does chromic acid. Chlorates of potassium, etc., explode when triturated with sulphur, tannic acid, or even particles of cork. The strong acids, nitric and sulphuric acids, and especially mixtures of these, react so strongly with volatile oils (hydrocarbons) as to cause explosion. Iodine affects these oils in the same way - fulminates.

It not infrequently happens that the physician intentionally writes a chemically incompatible prescription. "Black wash" (calomel and lime-water), and " yellow wash "(mercuric bichloride and lime-water) are examples. Other instances are such pharma-copceial preparations as liquor ammonii acetatis, mistura ferri com-posita, liquor magnesii citratis, and Blaud's pills.