194 ESSAYS anv OBSERVATIONS 
bundance when joined toa pure acid. In 
the prefent cafe, the alkali is really joined 
to an acid, but without any vifible emif= 
_fion of air ; and yet the air is not retain- 
ed in it: For the neutral falt, into which 
it is converted, is the fame in quantity, 
and in every other refpect, as if the acid 
employed had not been previoufly fatura- 
ted with magnefia, but offered to the alka- 
Hin its pure ftate, and had driven the air 
out of it in their conffi@. It feemsthere= 
fore evident, that the air was forced from 
the alkali by the acid, and lodged itfelf 
in the magnefia. 
THESE confiderations led me totry a 
few experiments, whereby I might know 
what quantity of air is expelled from an 
alkali, or from magnefia, by acids. 
Two drams of a pure fixed alkaline 
filt, and an ounce of water, were put in- 
to a Florentine flatk, which, together with 
its contents, weighed two ounces and two 
drams. Some oil of vitriol diluted with 
water was dropt in, until the falt was ex- 
actly faturated ; which it was found to be, 
when two sisi two icruples, and three 
grains 
