Mr. Cavennisy’s Experiments on Air. ise 
fixed air; nor has rain-water been ever found to contain any, 
which it certainly fhould on the fame hypothefs; even Mr. 
CavenpisH himfelf could find no fixed air in the refiduum or 
produéts of about 1040 ounce meafures of common air, which 
he burnt with inflammable air. 
It is true, Dr. PrizstLey fuppofed common air to. cone 
tain 2. of its bulk of fixed air; but he drew this conclufion 
not from any direct experiment, but from the quantity of fixed 
air produced by breathing, which he at that time believed to 
have been barely precipitated, and not generated, an opinion 
which he has found reafon to alter from his own experiments. 
I think I may therefore conclude, that the quantity of fixed 
air contained in the atmofphere is abfolutely inappreciable. 
Secondly, fuppofing the atmofphere to contain a very fmalk 
proportion of fixed air, yet Ido not think it can be inferred 
that metals, during their calcination, extract any, becaufe I 
find that lime expofed to red heat ever fo long extras none, 
though it is formed by a calcination in open air, which lafts 
at leaft as long as that of any metal; neither does precipitate 
per fe attra&t any, though its calcination lafts feveral months ; 
“nor does this proceed from the want of affinity, for if a fatu- 
rate folution of mercury in any of the acids be precipitated by 
a mild vegetable alkali, very little effervefcence is perceived, 
and the precipitate weighs much more than the, quantity of 
mercury employed, and that this increafe, of weight arifes in 
part from the fixed air abforbed will prefently be feen. 
Since then metals may be calcined in clofe veflels, fince they. 
then abforb one fourth part of the common air to which they — 
are expofed, fince all metallic calces (except thofe of! mercury, 
which I fhall prefently mention) yield fixed air, fince common 
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