198. MAGNESIA FAMILY.
This is a family of minerals which comprehends all the combinations of magnesia with acids.
When freed from extraneous matters, magnesia is a powdery substance of limpid white colour.
199. EPSOM SALTS, or SULPHAT OF MAGNESIA, consist of magnesia in conjunction with sulphuric acid ([24]).
It is said that Epsom salts have been found in the Alps, and in Switzerland, under a powdery form, and sometimes even in masses, or a state of incrustation on stones and rocks. They are, however, chiefly found dissolved in mineral waters, and particularly in those at Epsom in Surrey, and Sedlitz in Bohemia. Their taste is bitter and unpleasant. So little are they affected by exposure to the air, that the Abbé Haüy kept some by him for more than twelve years without any sensible alteration.
These salts are much used in medicine, and are sometimes manufactured from the waters of Epsom ([290]) and Sedlitz ([289]), but more frequently, and in much greater abundance, from sea-water.
The magnesia of the shops is prepared by dissolving Epsom salts in water, and adding to the solution half their weight of potash ([205]). The substance that sinks to the bottom is magnesia; and this, washed with a sufficient quantity of water and dried, has the appearance of a light, soft, and white powder, of insipid taste.
Magnesia is used in medicine, both in a simple state and when calcined or burned. It is also employed in some chemical processes; and is in considerable request in the manufacture of enamel and porcelain. If putrid water be agitated with a small quantity of magnesia, it will lose a considerable portion of its bad taste and smell.
200. SODA FAMILY.
Soda, like potash ([205]), is an extremely caustic alkali ([42]). It has a greyish white colour, and agrees exactly with potash ([205]) in taste, smell, and corrosive quality, but it is not so heavy.