xan. 7 
quiries for the moft part imitated, I 
cannot but think more favourably 
of the importance of this difcovery 
than the author himfelf does. I 
even think with the Abbé, that, by 
ufing convenient and accurate in- 
ftruments, and by obferving to the 
greateft nicety all the manoeuvres of- 
the operation conftantly in the fame 
way, we may with as much preci-. 
fion judge of the degree of purity 
of common air, as we now are able 
to judge of its degree of heat and 
cold by a good thermometer. 
Indeed, by this method, even ma! 
the changes which the conftitut 
of the senof phere undergoes daily, 
in the fame place, are obferved with 
fo much accuracy, that, by making 
ten obfervations with the fame air, 
es the difference will f{carcely amount 
to 
