on thefpecific Gravities , &c. of Saline Subfiances . 215 
that Dr. Priestley, in a fimilar experiment, did not obferve 
this turbidity ; but he accounts for this circumftance very 
juftly, by fuppofing, that the calx of lead abforbed the fixed 
air preferably to the lime. And this fuppofition is not gra- 
tuitous ; for metallic calces, and particularly thofe of lead, are 
known to attract fixed air as ftrongly as quick lime, or rather 
more ftrongly* : and what lets this matter beyond all doubt, 
' the calces of lead all yield fixed air by heat, and the grey calx 
of lead, in particular, which was that produced by Dr. priest- 
ley, in the experiment to which I allude, affords by heat 
fixed air only. Other calces of lead after fixed air afford alfo 
dephlogifticated air ; but this 1 fhall fhew alfo- to have been 
originally fixed air. If filings of iron be mixed with water in 
dole veflels, they will be converted into ruft, and the incum- 
bent air diminilhed one-fourth, as Mr. lavoisier attefts f ; 
but Dr. PRIESTLEY has fhewn, that ruft of iron yields fcarce 
any other than fixed air, which may be expelled' out of it by 
mere heat J. Nay,, iron alone, expofed to, common air over a 
veflel of water for three months, reduced this air one-fifth; 
and being expofed to dephlogifticated air, over a veflel of mer- 
cury, it reduced it one-tenth in nine months §. In all thefe 
cafes the fixed air could furely come from nothing elfe but the 
incumbent refpirable air and the phlogifton of the metal. 
Secondly, It is well known, that if nitrous air be decom- 
pofed by refpirable air over lime-water, the lime will be preci- 
pitated ||. In this cafe alfo, the fixed air muff; proceed from the 
* voget,, § 599. 2 N, A£. Upfal. 240. IX Mem. Scav. Etrang. 544. 
■f 1 lavois. 192. 
J 2 PR. I I 2. 
§ 2 PR. 182. 4 pr. 25 3. 
II I PR. 114. 3 FR. 30. r PR. ,138.. 
refpirable 
