Mr. Cavenpisu’s Experiments on Airs = at 
do fo when prepared by methods in which they are not in con- 
ta with it. 
Dr. PriesTLey alfo obferved, that ginclbdnds fouled by the 
addition of lead or tin, depofits a powder by agitation and expo- 
fure to the air, which confifts in great meafure of the calx of 
the imperfect metal. He found too fome powder of this kind 
to contain fixed air* 3 but it is by no means clear that this air 
was produced by the phlogiftication of the air in which the 
quickfilver was fhaken; as the powder was not prepared on 
purpofe, but was procured from quickfilver fouled by having 
been ufed in various experiments, and may therefore have con- 
tained other impurities befidesthe metallic calces. 
I never heard of any fixed air being produced by’ the burning 
of fulphur or phofphorus; but it has been afferted, and com- 
monly believed, that lime water is rendered cloudy by a mix- 
ture of common and nitrous air; which, if true, would be a 
convincing proof that on mixing thofe two fubftances fome 
fixed air is either generated or feparated; I therefore examined 
this carefully. Now it muft be obferved, that-as common air 
ufually contains a little fixed air, which is no effential part of 
it, but is eafily feparated by lime-water 3 and as nitrous air may 
alfo contain fixed air, either if the metal from which it is pro- 
cured be rufty, or 1f the water of the veflel in’ which it is 
caught contain calcareous earth, fufpended by fixedair,- as moft 
waters do, it is proper firft-to free both airs from it by préeviouf 
Jy wafhing them with lime water +. Now 1 found, by repeat- 
| | , ed 
ok 
Exper. in Nat. Phil. vol. I. p. 144. 
¢ Though fixed air is abforbed in confiderable quantity by w ater, « as I fhewed 
in Phil. Tranf. vol, LYI. yet it is not eafy to deprive common air of all the fixed 
Vor, LXXIV, R air 
