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_.