92 CLASSICS OF MODERN SCIENCE 

 times, or even oftener, 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 

 through. 



The alkali in the mixture, uniting with the acid, separates it from 

 the magnesia ; which, not being of itself soluble in water, must conse- 

 quently appear immediately under a solid form. But the powder 

 which thus appears is not entirely 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 through the mixture, in the form of a powder. Hence 

 the necessity of washing the magnesia with so much trouble ; for the 

 first effusion 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 con- 

 cretes 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 al- 

 lowed 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 through, it will 

 become turbid, and deposit a magnesia. 



An ounce of magnesia was exposed in a crucible, for about an hour, 

 to such a heat as is sufficient to melt copper. When taken out, it 

 weighed three drams and one scruple, or had lost 7-12 of its former 

 weight. 



I repeated, with the magnesia prepared in this manner, most of 



