AEROtOGY. 
ter; and its lofs of weight in water is as the lofs of 100 
grains of marble, minus the lofl'es of the pure calcareous 
earth and the water. 
too 
Lofs of too grs. of marble —-= 36-8 grs. 
Lofsof 55-92 grs. of calcareous- 
55'9s 
earth - - —-= 23-39 grs. 
2'39 
Lofs of 11-66 grs. of water = - 11 -66 
35'°5 
“Then the lofsof the fixed air 36-8 — 35-05 = 1-75; 
32-42 
eonfequently its fpeeific gravity is —-= 18-52.” 
i*75 
Fixed air differs conliderably front other airs. Its aci¬ 
dity is manifeft to tlie tarte, and (till more from its neu- 
traliling alkalies, fo as to form cryftals of a neutral or aci¬ 
dulous fait. It has a Confiderable power in checking the 
putrefaftion of animal fubffances; though in this cafe it 
acts only by abforbing the putrid effluvia already emitted 
from the body, and becomes itfelf offenfive while it fweet- 
ens the other. When taken into the lungs, it is equally 
poifonous with any other noxious air, and extinguilhes 
flame as effectually; but, when mixed with dephlogifti- 
cated air, it may be infpired without danger, and even in 
its pure flute may be copioufly fwallowed to effect the cure 
of difeafes, whence it has now become an article of the 
Materia Medica. As an acid, it (lands in the lowed rank, 
being expelled from alkalies by every other; though it 
feparates oils, fulphur, and the colouring matter of Pruf- 
fian blue, from the fubflances with which they are com¬ 
bined. 
The compound nature of fixed air, and the principles 
from which it is formed, were firft difeovered by Mr, 
Kirwan; but Dr. Prieftley was not convinced by the 
proofs he adduced, till after making fome experiments of 
his own. The firft was, by firing (havings'of iron in de- 
phlogilticated air; when he obferved a conliderable refi- 
duum of fixed air, though that in the receiver had been 
of the pureft dephlogifticated kind, and iron could only 
have yielded inflammable air. The hypotheiis of Mr. 
K. was ltill further confirmed by an experiment in which 
iron-filings, which could only have yielded inflammable 
air, w-ere mixed with red precipitate, which is known to 
yield only pure dephlogifticated air. On heating thefe in 
a glafs retort, they gave a great quantity of fixed air, in 
fome portions of which nineteen-twentieths were abforbed 
by lime-water, and the refiduum was inflammable; but 
when the red precipitate was mixed with powdered char¬ 
coal, which had been found to yield only inflammable 
air, the fixed air produced from it was fo pure that only 
one-tenth part remained unabforbed by water, which is 
as pure as that generally prepared from chalk and oil of 
vitriol. In (ome of thefe experiments it appeared, that 
three ounce-meafures of dephlogifticated air went to the 
compolition of two of fixed air : for one ounce of red 
precipitate gave (ixty ounce-meafures of dephlogifticated 
air ; and, when mixed with two ounces of iron-filings, it 
gave about forty ounce-meafures of fixed air that were ac¬ 
tually abforbed by water, belides a refiduum that was in¬ 
flammable. The fame proportion was obtained when half 
the quantity of materials was employed; but, on ufing an 
ounce of each, only twenty ounce-meafures of fixed air, 
including the refiduum, could be.got. 
In conlidering this fubjeft farther, it occurred to Dr. 
Prieftley, that his experiments, in which charcoal was 
ufed, lay open to an objection, that, fince dry wood and 
imperfedtly-made charcoal yield fixed air, it might belaid 
that all the elements of fixed air are contained in charcoal. 
In order, therefore, to expel all the fixed air from char¬ 
coal, he made a quantity of it from dry oak, and, pound¬ 
ing it while hot, inftantly mixed four meafiures of it with 
iSt 
one of red precipitate, and, putting them into an earthen 
retort, got, with a heat no greater than what was futficient 
to revive the mercury, a large quantity of air, half of 
which was fixed. Afterwards the proportion of fixed air 
was lefs, and at laft no fixed air at all was obtained ; but, 
as the refiduum was worfe than the common atmofphere, 
he is thence inclined to believe, notwithftanding Mr. Ca- 
vendifh’s experiments, that phlogifticated air may be com- 
pofedof phlogiltonand dephlogifticated air. Inanotherex- 
periment he found a better proportion of charcoal and red 
precipitate. This was.by mixing one ounce of precipi¬ 
tate with the fame quantity of perfect charcoal hot from 
the retort in which it was made. Putting thefe into a 
coated retort, he expelled from them, by a ftrong heat, 
about thirty ounce-meafures of air, the whole of which 
was the pureft fixed air, leaving only about one-fortieth 
part unabforbed by water, and this almoft perfectly phlo¬ 
gifticated. 
H aving recollected that formerly he had obtained fixed 
air from nitrous acid and charcoal, he repeated the ex¬ 
periment with fome of the fame charcoal which had then 
been ufed ; when fixed air was obtained, in the quantity 
fometimes only of one-fifth, and fometimes of one-half; 
to the formation of which he fuppofed the dephlogiftica¬ 
ted air produced by heating the nitrous acid mutt have 
contributed. On account of the objections, however, 
which might be made to the ufe of charcoal, he next em¬ 
ployed iron, which was liable to nothing of this kind ; and 
on mixing an ounce of it with an ounce of charcoal, and 
then heating them in a.glafs retort, he obtained twenty 
ounce-meafures of. air, of which one-feventh remained 
unabforbed by water. The refiduum was of the ftandard 
of 1-52, but (lightly inflammable. Repeating the expe¬ 
riment with half an ounce of. iron-filings, he got twenty- 
lix ounce-meafures of air, of which the firft part was 
pretty pure, but afterwards one-tenth remained unabforb¬ 
ed by water; but, on mixing one ounce of precipitate 
with two ounces of filings, he got about forty ounce-mea¬ 
fures of air, of the firft portions of which only one-twen¬ 
tieth was unabforbed by water, though towards the con- 
clufion the refiduum was greater. In this procefs he 
got in all thirty-fix ounce-meafures of pure fixed air,, 
completely abforbed by water, befides about four other 
ounce-meafures, which, hel’uppofes, might have been ab¬ 
forbed in receiving the air, and transferring it into other 
veflels. 
Fixed air was alfo produced from red precipitate mixed 
with brafs-filings, with zinc, from turbith-mineral with 
iron-filings, and from the black powder into which mer¬ 
cury mixed with lead is eafily converted. In this laft cafe 
the dodtor fuppofes, that the fixed air was produced from 
the dephlogifticated kind abforbed by the metals and the 
phlogifton of the lead; and this is confirmed by an obfer- 
vation, that the fixed air always comes firft in the procefs, 
when the phlogifton is molt readily feparated, but after-- 
wards the produce becomes quite pure and dephlogiftica¬ 
ted. In attempting, however,, to increafe the quantity of 
fixed air by heating this black powder in dephlogifticated 
air, he found only an augmentation of the quantity of. 
dephlogifticated air, and that of the pureft kind. 
“ Perhaps (fays he) as decilive a proof as any of the 
real production of fixed air from phlogifton and dcphlo- 
gifticated air, may be drawn from the experiments in which 
I always found a quantity of it when I burned lulphur in 
dephlogifticated air. In one of thefe experiments, to 
which I gave particular attention, fix ounce-meafures and 
a half of the dephlogifticated air were reduced to about 
two ounce-meafures, and one-fifth of this was fixed, air. 
When both the vitriolic acid and fixed air produced by 
this operation were abforbed by water, the remainder was 
very pure dephlogifticated air. 
“ I-had always concluded, that no fixed air could be 
procured by the decompbfition of inflammable air which 
had been produced by mineral acids, becaufe 1 had nor 
been able to do it with'that which, I had got by means of 
vitriolic 
