iv CHALK, LIME, AND THE ALKALIS 59 



pans after the evaporation of sea- water," but he " afterwards 

 made use of a salt called EPSOM SALT, which is separated 

 from bittern by crystallisation, and is evidently composed of 

 magnesia and the vitriolic acid " (A. C. R. I. 7). 



The magnesia prepared with the help of an alkali 

 effervesced with acids in just the same way as chalk, but 

 gave rise to a totally different series of salts ; in particular it 

 was dissolved by oil of vitriol, reproducing the Epsom salt 

 from which it had been prepared, instead of giving a 

 sparingly soluble residue of gypsum. 



Like chalk, it was decomposed by heat, its weight 

 decreasing to a remarkable extent. The product resembled 

 lime in dissolving without effervescence in acids, but differed 

 from it in that it could not be slaked, and was not soluble 

 in water. 



" An ounce [480 grains] 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 [200 grains], or had lost 7/12 of its former 

 weight" 



" I repeated, with the magnesia prepared in this manner, 

 most of those experiments I had already made upon it 

 before calcination, and the result was as follows : 



" It dissolves in all acids, and with these composes salts 

 exactly similar to those described in the first set of 

 experiments : but what is particularly to be remarked, it is 

 dissolved without any the least degree of effervescence " 

 (A. C. R. I. 14). 



This decomposition could be effected in a glass retort, 

 but although the magnesia " lost more than the half of its 

 weight," only 5 drams of water could be collected from 

 24 drams of the earth. 



" We may, therefore, safely conclude, that the volatile 

 matter lost in the calcination of magnesia, is mostly air; 

 and hence the calcined magnesia does not emit air, or make 

 an effervescence, when mixed with acids." 



