I have had no opportunity of seeing Hoffman's first magnesia or the liquor from which it is prepared, and have therefore been obliged to make my experiments upon the second.
In order to prepare it, I at first employed the bitter saline liquor called bittern, which remains in the pans after the evaporation of sea water. But as that liquor is not always easily procured, I afterwards made use of a salt called epsom-salt, which is separated from the bittern by crystallization, and is evidently composed of magnesia and the vitriolic acid.
There is likewise a spurious kind of Glauber salt, which yields plenty of magnesia, and seems to be no other than the epsom salt of sea water reduced to crystals of a larger size. And common salt also affords a small quantity of this powder; because being separated from the bittern by one hasty crystallization only, it necessarily contains a portion of that liquor.
Those who would prepare a magnesia from epsom-salt, may use the following process.
Dissolve equal quantities of epsom-salt, and of pearl ashes separately in a sufficient quantity of water; purify each solution from its dregs, and mix them accurately together by violent agitation: then make them just to boil over a brisk fire.
Add now to the mixture three or four times its quantity of hot water; after a little agitation, allow the magnesia to settle to the bottom, and decant off as much of the water as possible. Pour on the same quantity of cold water; and, after settling, decant it off in the same manner. Repeat this washing with the cold water ten or twelve times: or even oftner, if the magnesia be required perfectly pure for chemical experiments.
When it is sufficiently washed, the water may be strained and squeezed from it in a linen cloth; for very little of the magnesia passes thro'.
The alkali in the mixture uniting with the acid, separates it from the magnesia; which not being of itself soluble in water, must consequently appear immediately under a solid form. But the powder which thus appears is not intirely magnesia; part of it is the neutral salt, formed from the union of the acid and alkali. This neutral salt is found, upon examination, to agree in all respects with vitriolated tartar, and requires a large quantity of hot water to dissolve it. As much of it is therefore dissolved as the water can take up; the rest is dispersed thro' the mixture in the form of a powder. Hence the necessity of washing the magnesia with so much trouble; for the first affusion of hot water is intended to dissolve the whole of the salt, and the subsequent additions of cold water to wash away this solution.
The caution given of boiling the mixture is not unnecessary; if it be neglected, the whole of the magnesia is not accurately separated at once; and by allowing it to rest for some time, that powder concretes into minute grains, which, when viewed with the microscope, appear to be assemblages of needles diverging from a point. This happens more especially when the solutions of the epsom-salt and of the alkali are diluted with too much water before they are mixed together. Thus, if a dram of epsom-salt and of salt of tartar be dissolved each in four ounces of water, and be mixed, and then allowed to rest three or four days, the whole of the magnesia will be formed into these grains. Or if we filtrate the mixture soon after it is made, and heat the clear liquor which passes thro'; it will become turbid, and deposite a magnesia.