PHYSICAL ano LITERARY. 173. 
trous earths, by the procefs commonly u- 
fed in cryftallizing falts. In this procefs 
the brine is gradually diminifhed, and at 
length reduced to a {mall quantity of an 
unctuous bitter faline liquor, affording no 
more falt-petre by evaporation; but, if 
urged with a brifk fire, drying up intoa 
confufed mafs which attracts water ftrong: 
ly, and becomes fluid again when expofed 
- to the open air. | | 
To this liquor the workmen have gi- 
ven the name of the mother of nitre; and 
Hoffman, finding it compofed of the mag- 
_ nefia united to an acid, obtained a fepara- 
tion of thefe, either by expofing the com- 
pound to a ftrong fire in which the acid 
was diflipated and the m:juefta remained 
behind, or by the addition of an-alkali 
_ which attraéted the acid to itfelf: And 
this laft method he recommends as the 
beft. He likewife makes an inquiry into 
the nature and virtues of the powder thus 
prepared ; and obferves, that it is an aul 
forbent earth which joins readily with all 
acids, and mutft neceffarily deftroy any a- 
cidity it meets in the ftomach; but thae 
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