‘a12 ESSAYS AND OBSERVATIONS | 
V WE have fhewn in the former ex- 
periments, that abforbent earths lofe their 
air when they are joined to an acid; but 
recover it, if feparated again from, that 
acid, by means of an ordinary alkali: The 
air pafling from the alkali to the earth, 
at the fame time that the acid pafles from 
the earth to the alkali. 
Ir the cauftic alkali therefore be defti- 
tute of air, it will feparate magnefia from 
an acid under the. form of a magicfia 
free of air, or which will not effervefce 
with acids; and the fame cauftic alkali - 
wili alfo feparate a calcarious earth from 
acids under the form of a calcarious earth 
deftitute of air, but faturated with ‘wa- 
ter, or under the form of flaked lime. 
TuHeEse were all neceflary conclufions 
from the above fuppofitions. Many of 
them appeared too improbable to deferve 
any further attention: Some, however, I 
found upon reflection were already fe- 
conded by experience. Thus Hoffman 
has obferved, that quick-lime does not ef- 
férvefce with f{pirit of vitriol *; and itis 
well 
* Hoff. Op T, iv: p. 480: 
