PHYSICAL anv LITERARY. 199 
their air by being joined to acids, but 
_ fhew evident figns of their having reco-= 
vered it, when feparated from them by 
alkalis, received it from thefe alkalis which 
loft it in the inftant of their joining with 
the acid. 
THE following are a few experiments 
apon three of the abforbent earths, made 
in order to compare them with one an- 
other, and with maguefia. 
Sufpecting that magnefia might poflibly 
be no other than a common calcarious 
earth, which had changed its nature, by 
having been previously combined with an 
_ acid, I faturateda fmall quantity of chalk 
with the muriatic acid, feparated the acid 
from it again by means of a fixed alkali, 
and carfully wafhed away the whole of the 
~ falt. 
The chalk when dried was not found 
to have fuffered any alteration; for it ef- | 
fervefced with the vitriolic acid, but did 
not diffolve in it; and, when expofed to 
a violent fire, was converted into a quick-. 
lime, in all refpects fimilar to: that obtain- 
ed from common chalk, 
a Ix 
