In combining remedies, the subject of incompatibilities must never be lost sight of. These are two in number - physiological and chemical. . The first of these it would require large space to discuss fully, but the intelligent practitioner can readily make all necessary deduction. It will suffice here, to point out certain principles, and a few special reactions.

Soluble Salts which can by mutual decomposition form an insoluble compound, will undergo such decomposition when they meet in solution, and will precipitate, unless in some very rare instances, in which a double salt is formed. Soluble Salts which are not capable of forming an insoluble salt, never precipitate, and rarely undergo decomposition when they meet in solution.

Mineral Acids decompose salts of the weaker (carbonic, acetic, etc.,) acids, and form ethers with alcohol and alcoholic preparations.

Alkalies precipitate the alkaloids and the soluble non-alkaline metallic salts.

Glucosides. such as santonin and colocynthin, should not be prescribed with free acids or emulsion.

Tannic Acid, and all substances containing it, are incompatible with alkaloids and drugs con-containing them, with albumen and gelatin, and with most soluble metallic salts used in medicine.

Iodine and Iodides are incompatible with the alkaloids and the substances containing them, as well as with most soluble metallic salts. The iodide of potassium should always be prescribed . alone, or only in combination with corrosive sublimate (with which it forms a double salt), or with iodine itself.

Tinctures, and other Alcoholic Preparations containing resin, precipitate the latter when water is added.

Nitrate of Silver should always be prescribed alone, or in company with opium, or extract of hyoscyamus only. Most vegetable extracts decompose it, and with creasote, it is said to make an explosive compound.

Corrosive Sublimate is incompatible with almost everything, and should be given in simple syrup; even the compound syrup of sarsaparilla is said to decompose it.

Syrup of Squill, containing acetic acid, is incompatible with carbonate of ammonium, but not with the chloride.

Acetate and subacetate of lead are incompatible with almost every thing, but are, nevertheless, frequently used in lotion with opium, the insoluble compound formed being therapeutically active.

Vegetable infusions are generally incompatible with metallic salts. *

Poisonous compounds may be formed by the admixture of many substances in solution, such as Potassium chlorate with potassium iodide in solution together do not react at ordinary temperature, but in the system they evolve a poisonous agent, probably the iodate of potassium.

*Prof. H. C. Wood.

Potassium chlorate with syrup of iodide of iron, liberates iodine from the iodide in the warm stomach, causing severe gastric irritation, perhaps gastritis of a dangerous degree.

Dilute hydrocyanic acid, or potassium cyanide, with calomel, forms the bichloride and bicyanide of mercury, both virulent poisons; - with metallic hydrates, carbonates, sub-nitrates or sub-chlorides, cyanides of the metals are formed which are even more poisonous than the acid itself in its usual diluted form.*