Injections may be divided into five classes:—Hypodermic, vaginal, urethral, rectal, and intramuscular. Hypodermic injections are prepared by dissolving the required medicament in recently boiled and cooled distilled water, and the strength is indicated in parts per 100 parts by volume. The alkaloidal salts used are those which are most soluble and give the most stable products. In cases where the quantity of dissolved substance is small, a nearly true percentage solution is obtained, but to be strictly accurate it should be described as weight in volume percentage, or w/v per cent., and corresponds to grammes per 100 mils; thus a 1 per cent. w/v solution contains 1 gramme per 100 mils (4.375 grains per fluid ounce). Hypodermic solutions ale best preserved in ampoules; being hermetically sealed, they retain their activity in a sterile condition almost indefinitely. Vaginal injections are usually concentrated solutions, which are diluted to a suitable strength immediately before use, 1 to 2 pints being injected from a douche can, or from a Higginson's or other suitable form of syringe. Urethral injections are generally aqueous solutions of substances which have an astringent and antiseptic action, 2 to 4 fluid drachms being used at a time. Rectal injections may be either purgative or nutrient (vide Enemata). Intramuscular injections are of two kinds, those prepared with soluble salts of mercury, and those containing mercury and the insoluble mercurials in a very fine state of subdivision suspended in an oily liquid, or as a thick cream. Solutions of the soluble salts should be put in ampoules containing from 6 to 10 minims each. The insoluble mercurials being thick bodies should be supplied in small sterilised glass tubes, 1 ½ inches long, and having a diameter of about ½ inch, corked and sealed with paraffin, the tube is gently warmed before use to render the contents just sufficiently liquid to be drawn into the syringe, which for this purpose must be fitted with a stout needle, having a larger bore, and about twice the length of those used hypodermically.