186 ESSAYS anp OBSERVATIONS ~ 
Dissotve a dram of any calcarious 
fubftance in the acid of nitre or of com- 
non falt, taking care that the folution be 
rendered perfectly neutral, or that no fu- 
perfluous acid be added. Mix with this 
folution a dram of magnefia in fine powder, 
and digett, it in the heat of boiling water 
about twenty four hours; then dilute the 
mixture with double its quantity of water, 
and filtrate.” The greateft part of the 
earth now left in the filtre is calcarious, 
and the liquor which pafled through, if 
mixed with a diffolved alkali, yields a 
white powder, the largeft portion of which , 
is a true magnefia, 
From this experiment it appears, that 
an acid quits a calcarious earth to join it- 
felf to magnefia; but the exchange being 
performed flowly, fome of the magnefia is 
fill undiffolved, and part of the calcarious 
earth remains yet joined to the acid. 
WHEN a fmall quantity of magnefia is 
thrown into a folution of the corrofive 
fublimate of mercury, it foon feparates 
part of the mercury in the form of a dark 
fed powder, and is itfelf diffolved. 
IMa* 
