[ 66 ] 
contaimng fubllances in the adl of effervefcence. In that 
cafe, Ae extricated air will afcend through the perfora- 
tions in the fuperior and inferior pieces, fhe middle one 
vSd m fh ^ -0-" 
rtip • r ^'trrent of air rulhing upwards; but when 
. e air ceafes to afcend, and the prelTure of the water 
above takes place, the middle piece will prevent the wa- 
ter from defcending, its plane furface being then applied 
to the plane furface of the piece below it. ThusfsiR, 
this glafs valve will anfwer in every cafe where the ivory 
will undoubtedly prove preferable, particularly when 
corrofive fubftances are fubjefted to experimenri 
V. Account 
