MAGNESIA ALBA, a white earth procured from the mother-liquors of nitre, or sea-salt, by precipitation with a fixed alkali, and afterwards washing of the salt with water. See CHEMISTRY, n° 37.
Magnesia alba is a good absorbent; and undoubtedly
Magnesia. ly to be preferred to crab's eyes, on account of its purgative quality when united with an acid, which the other has not. It has been esteemed hurtful in bilious habits where there is a disposition in the stomach contrary to acidity. This, however, according to Mr Henry, is doubtful: and where putrid bile is to be corrected, he thinks good purposes may be answered by taking magnesia with an acid in a state of effervescence; as the fixed air, thus extricated, will correct the putridity of the contents of the intestines, while they are at the same time evacuated downwards. He is also of opinion, that in cutaneous diseases it may enter the circulation in form of a neutral salt, and, by acting as a diaphoretic and diuretic, prove an excellent alternative.
For some medical purposes, magnesia is used in a calcined state; in which case it is deprived of its fixed air, and then it proves nearly as aperient as a double quantity of magnesia in its uncalcined state. Mr Henry is of opinion, that it may be useful in distensions of the bowels arising from flatus; that it may be successfully employed as a cathartic with patients labouring under the stone, who are using the lixivium saponaecum; and that, joined with warm aromatics, it may be of service in correcting the great flatulency which so much afflicts people of a gouty disposition. From several experiments made by the same author, it also appears that magnesia has a considerable antiseptic power. The like virtue he ascribes to all kinds of testaceous powders: from whence he concludes, that medicines of this kind are by no means improper in fevers of a putrescent type; that where bile is suspected to be the cause of any putrid disease, those antiseptics should be prescribed which particularly impede its corruption; that, as calcined magnesia is a more powerful antiseptic than most other absorbents, it merits a preference to these; and that where an acid cacochymy prevails, magnesia or other absorbents, taken immediately before or after meal-time, may, by increasing the putrefactive fermentation of animal food, be of very great service. He hath also found, that magnesia hath a power of promoting the solution of resinous gums in water; and thus we have an elegant and easy method of preparing aqueous tinctures from these substances. Such tinctures, however, are calculated only for extemporaneous prescription, as most of them deposit a sediment when they have been kept a week or two.