[ 63 ]■ 
fiUed with the water that has given place to the air. The - 
quantity of chalk to be thrown into the acid at one time,, 
mull be determined by the capacity of the uppermoft 
vefleL Should more air be extricated than is fufficient, , 
in the conduct of the procefs, to fill that vefiel, the wa- 
ter will run over the top of it, and will continue to run 
as long as- any air afcends in the middle vefiel, or till the 
furface of the water is below the extremity of the bent 
tube. Botlrthefe accidents are to be carefully avoided; 
as in one cafe, the whole would be wet and difagreeable ; 
and in the other, a quantity of fixed air would be unne- 
cefiarily loft. Half a dram of chalk will, in general, pro- 
duce air enough to fill the uppermoft veflTel with water;, 
and it muft be remembered, that the chalk employed to 
produce the effervefcence, fliould be finely powdered, as 
a felenitic cruft will otherwife form around it, and thus 
prevent the action of the acid on the interior part. To 
keep the neck of the glafs clean, through which the chalk 
is put, it will be necefiary to include the chalk loofely 
in paper ;.and this circumftance is by no means to be neg— 
ledted, as the accurate jundtion of the glalTes depends on 
it, and confequently the whole of the procefs. , When, 
the uppermoft vefieL is filled with water, and there is,, 
therefore, a confiderable quantity of fixed air in the mid- 
dle one, thefe two vefiels are to be feparated from the 
lowermoft, and the air and water are to be agitated toge- 
ther, to promote their union. If, during the agitation, a 
ftopper be put into the uppermoft glafs, the defcent of. 
the water in it will not Ihew the abforption of the fixed 
