JOSEPH BLACK 91 



assured himself, by experiments, that the product from both was 

 exactly the same. 



My curiosity led me, some time ago, to inquire more particularly 

 into the nature of magnesia, and especially to compare its properties 

 with those of the other absorbent earths, of which there plainly ap- 

 peared to me to be very different kinds, although commonly con- 

 founded together under one name. I was indeed led to this ex- 

 amination of the absorbent earths, partly by the hope of discovering 

 a new sort of lime and lime-water, which might possibly be a more 

 powerful solvent of the stone, than that commonly used; but was 

 disappointed in my expectations. 



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 Hquor 

 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 crystaUization, 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 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 con- 

 tains 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, sep- 

 arately, 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 bot- 

 tom, 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 



