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ZiiY by the water, as the external atmorphcrical air will 
•enter below, at the valve, to fill the Ipacc which tlie ab- 
Ibrbed fixed air would otherwile leave void. But, on the 
contrary, if the uppermoft vefiTel be open, during tlte agi- 
tation, the prelTure of the atmofphere on the furface of 
the water in that veflbl, will force the water down into 
the middle one, as fafi; as the abforption of the fixed air 
below will allow it room. This latter method mav be 
}>urfued, when a perfon wifiies to know the quantity of 
fixed air that the water can abforb; but in common ufe, 
It will be better to itop the uppermoft veliel, as the air 
and water may be then more forcibly agitated without 
inconvenience, and of courfe, the impregnation more ex- 
peditioufly efTe6ted. During the effervefcence, the up- 
permoft glafs is tc remain open, and it is only to be ftop- 
ped w'hen the agitation is performed. It is not to be 
■expecled, that the impregnation will be confiderable at 
firfi; it will indeed be neceflliry to repeat the procefs, 
wdth the fame water, four or five times, before it will be 
highly imi3regnated. After an agitation, therefore, when 
a ffronger impregnation is Avilhed for, the uppermoft vef- 
fel is to be opened, and raifed from the middle one, to al- 
low the water to defcend, that was before driven up. 
When the middle glafs is again full, a frefli quantity of 
chalk is to be put into the lowermoft velfel, and the agi- 
tation to be repeated, as'foon as the effervelcence ceafes. 
It is feldom neceifary to repeat the procefs more tlian 
four times, to produce a very flrong impregnation; but 
fhould it be thought proper, to have the tvater as highly 
^ faturated 
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