Derivation.—Magnesium sulphate is a constituent of sea water and of some saline springs. It also occurs native, either crystallized in slender, prismatic, adhering crystals, or as an efflorescence on certain rocks and soils which contain magnesia and a sulphate or sulphide. In the United States it is found in the great caves so numerous to the west of the Alleghany Mountains.

Properties.—Small, colorless, rhombic prisms, or acicular crystals, without color and having a cooling, saline and bitter taste; slowly efflorescent in dry air; .85 part of water; insoluble in alcohol.

Actions.—Magnesium sulphate is a hydragogue and cholagogue cathartic; alterative and febrifuge and is also feebly diuretic and diaphoretic. As a cathartic it resembles common and glauber salts, and is more active than potassium bitartrate or sodium phosphate. When magnesium sulphate is administered it causes outpouring of secretion from the walls of the small intestines, most quickly and abundantly when the bowels have been partially emptied by several hours’ fasting. Neither pancreatic fluid nor bile is materially increased. But magnesium sulphate has a low diffusing power. It is slowly absorbed, and moreover, retards diffusion and absorption of fluid present in the bowels. In this twofold action by increased secretion and retarded absorption the fluid contents of the bowels are increased, producing more or less mechanical distension and provoking, like other salines, slight peristalsis. The retarded removal of accumulating liquid is apt to produce the formation of gases in the bowels, which is relieved by conjoining carminatives, as ginger or capsicum, while effectual removal of the intestinal fluids is attained by using with magnesium sulphate aloes, calomel or oil. It acts in from twelve to sixteen hours; in small doses it stimulates the secretions of the kidneys and skin. In febrile diseases it is used in small repeated doses. It is valuable in treating animals suffering from reflex skin irritation, combined with large doses of bicarbonate of sodium, generally gives relief, especially in urticaria of the horse. It is quite commonly used as a cathartic for horses, but most often for cattle and sheep.

Doses.—When repeated two or three times as a laxative and alterative horses take 2 to 4 ounces, cattle 3 to 6 ounces, sheep and pigs, 1 to 2 drachms. As a cathartic cattle take 1 to 2 pounds, calves two to three months old 3 to 4 ounces, sheep 4 to 6 ounces, dogs 1 to 4 drachms. Aloes is a much better cathartic for horses, and castor or linseed oil acts much better as a cathartic in pigs.