Magnesia Levis. Light Magnesia.

Prep. Carbonate of magnesia, four ounces, burnt in a Cornish or Hessian crucible closed loosely by a lid, and exposed to a red heat, as long as a little of the powder taken from the centre, when cooled and dropped into dilute sulphuric acid, gives rise to effervescence. In preparing the light magnesia, the light carbonate is used. In this process the carbonate is converted into the oxide by the heat driving off the carbonic acid.

The light magnesia differs from magnesia only in its greater levity, the volumes corresponding to the same weight being in the ratio of three and a half to one.

Prop. & Comp. A white powder with scarcely any taste; almost insoluble in water, but when moistened gives a slight alkaline reaction to turmeric paper, turning it brown. It dissolves in hydrochloric acid without effervescence; and the solution when neutralized by a mixed solution of ammonia and hydrochlorate of ammonia gives a copious crystalline deposit when phosphate of soda is added (the ammonio-magnesian phosphate). Dissolved in nitric acid and neutralized with a mixture of ammonia and hydro-chlorate of ammonia it does not give any precipitate with oxalate of ammonia or chloride of barium, showing the absence of any sulphate of lime, or carbonate of magnesia or lime. Composition (Mg O).

Off. Prep. Of magnesia levis.

Pulvis Rhei Compositus. Compound Powder of Rhubarb. (Rhubarb, in powder, two ounces; light magnesia, six ounces; ginger, in powder, one ounce.) This is also known as Gregory's Powder.

Therapeutics. In small doses magnesia acts first as an antacid upon the alimentary canal; then becomes absorbed, and renders the urine alkaline; the salts formed by the presence of any acid in the stomach, have a purgative tendency: in large doses it produces distinct purgative effects. It is used in acidity of the stomach and heartburn, and in affections connected with an increased excretion of uric acid, or urates: also as a purgative in acid conditions of the alimentary canal, especially in children. It should be given cautiously, as it is rather apt to form concretions in the intestines when taken for a lengthened period.

Dose. Of magnesia or magnesia levis: as an antacid, 10 gr. to 20 gr.; as a purgative or adjunct, 20 gr. to 60 gr.; of pulvis rhei compositus, 5 gr. to 10 gr. for young children; 60 gr. to 120 gr. for adults.

Adulteration. It is apt to contain a little sulphate, as the carbonate is prepared from the sulphate; also lime, the sulphate being usually obtained from dolomite, a magnesian limestone; lastly, some carbonate, from imperfect calcination: these can be all detected by the above tests.