360 ELEMENTS OF MATERIA MEDICA.

Composition. —The composition of subsesquiphosphate of lime is as

follows:—

Berzelius . Fuchs. Vauquelin.Eq. Eq.Wt. Per Cent. (Artificial.) (Artificial.) (Apatite.)

Lime.1J . . . 42 . . . 5.T85 . . . 5L68 . . . 54'74 . . . 54'28

Phosphoric Acid.1 . . . 36 . . . 4(i‘15 . . . 48'32 . . . 45'2G . . . 45'72

Subsesquiphosphate of Lime 1 . . . 78 . . . lOO’OO . . . lOO’OO . . . lOO’OO . . . lOO’OO

Bone-ash obtained from the bones of the ox consists of subsesquiphos-phate of lime, carbonate of lime, phosphate of magnesia, and a trace offluoride of calcium.

Physiological Effects. —Its effects are not very obvious. “ As phos-phate of lime is very difficultly soluble,” observes Wibmer {die Wirkung,&c. ii. 9), “ it is absorbed in small quantity only, and then acts more orless like lime, as a slight astringent on the tissues and the secretions, andincreases, incontestibly, the presence of calcareous salts in the bones,the blood, and the urine. Large doses disorder the stomach and diges-tion by their difficult solubility.”

Uses.—-I t has been administered in rickets, with the view of promotingthe deposition of bone-earth in the bones. The sesquioxide of iron maybe advantageously conjoined with it.

Administration.—-T he dose of subsesquiphosphate of lime is fromten grains to half a drachm. For internal use the preparation of theDublin College is to be preferred.

Order 14. Compounds of Magnesium.

Magne'sia. — Magnesia.

History.—I t was first chemically distinguished from lime in 1755, byDr. Black, who also shewed the difference between magnesia and itscarbonate. From the mode of procuring it, it is frequently termed cal-cined or burnt magnesia ( magnesia calcinata seu usta.) It is sometimescalled talc earth (Talkerde ), or bitter earth [Bittersalzerde.)

Natural History.—I t occurs in both kingdoms of. nature.

(a.) In the inorganized kingdom .—Magnesia is found native, in thesolid state or in solution, in sea or some mineral waters, in combinationwith water and various acids (carbonic, sulphuric, boracic, silicic, andnitric.) Chloride of magnesium exists in sea water, as also in somesprings.

(b.) In the organized kingdom .—Combined with acids it is found insome vegetables (as Salsola Kali and Fucus vesiculosus), and animals (asin the urine and some urinary calculi of man.)

Preparation.—M agnesia is obtained by exposing the subcarbonateto a full red heat for two hours in a crucible, so as to drive off' the car-bonic acid.

Properties.—-I t is a light, fine, white, colourless, odourless, andtasteless powder, having a sp. gr. 23. When moistened with water itreacts as an alkali on test papers. It is very slightly soluble in water,and like lime is more soluble in cold than in hot water. Dr. Fyff'e statesthat it requires 5142 parts of cold, and 36000 parts of hot water to dis-solve it. Unlike lime it evolves scarcely any heat when mixed withwater. By the combined voltaic and oxy-hydrogen flames it has beenfused by Mr. Braude {Manual of Chemistry.) It absorbs carbonic acidslowly from the atmosphere.