.158 ESSAYS AND OBSERVATIONS 



affording no more falt-petre by evaporation ; 

 but, if urged with a brifk fire, drying up into 

 aconfufed mafs which attrads water flrongly, 

 and becomes fluid again when expofed to the 

 open air. 



'to this liquor the workmen have given 

 the name of the mother of nitre ; and Hoffman^ 

 finding it compofed of the magnejia united to 

 an acid, obtained a feparation of thefe, either 

 by cxpofing the compound to a flrong fire in 

 which the acid was difiipated and the 7nagne~ 

 j^a remained behind, or by the addition of an 

 alkali which attracted the acid to itfelf : and 

 this laft method he recommends as the befl. 

 He like wife makes an inquiry into the nature 

 and virtues of the powder thus prepared j and 

 obferves, that it is an abforbent earth which 

 jclns readily with all acids, and mufl ne- 

 ceffarily deftroy any acidity it meets in the 

 ftomach j but that its purgative power is un- 

 certain, for fometimes it has not the leafl 

 effecfl of that kind. As it is a mere infipid 

 earth, he rationally concludes it to be pur- 

 gative only when converted into a fort of 

 neutral fait by an acid in the^ flomach, and 

 that its effedl is therefore proportional to the 

 quantity of this acid. 



Altko' 



