AERO 
tre with a ftrong heat; by which means it may be obtain¬ 
ed in very conliderable quantity, and in as great purity as 
by the more expenfive precedes. It appears however, 
that the mode of applying heat lias alfo a very confident - 
ble efl'eit on the quantity of air produced. Thus, Dr. 
Prieftley remarks, that “ from equal quantities- of red 
lead, without any mixture of fpirit of nitre, and tiling the 
fame apparatus for dillilling it, he obtained, by means of 
heat applied fuddenly, more air than when (lowly applied 
in the proportion of ten to (ix. The proportion of fixed 
air was the fame in both cafes, and the remainder equally 
dephlogifticated.” 
By heat alone, the dodlor found, that fedative fait, rrran- 
ganefe, lapis calaminaris, and the mineral called lapis pon- 
derofus, wolfram, or tungficn, would yield dephlcgillicated 
air; the firtl indeed, in very fmall quantity, and fometimes 
even of a quality very little fuperior to common air. In 
thefe experiments, he made ufe of fmall-bellied retorts of 
green glafs, which can (land the fire bell, containing about 
an ounce of water, and having narrow necks eighteen or 
tw enty inches long. The fubltance to be examined was 
put into a retort of this kind, and then expofed to a red 
heat, either in (and or over a naked fire, while the neck 
of the veil'd was plunged in water or mercury. 
Having dilTolved fix pennyweights of very clean iron in 
oil of vitriol, and thendiftilled the folution to drynefs in a 
long-necked retort, he received the common air a little 
phlogifticated, fome fixed air, much vitriolic acid air, and 
lafily eighteen ounce-meafures of dephlogifticated air. 
The iron that remained undifl'olved weighed twenty-three 
grains, fo that the air was yielded by five pennyweights 
onegrainof iron. The ochre weighed feven pennyweights 
thirteen grains: fo that, fays he, there probably remained 
a quantity of oil of vitriol in it; and confequently, had the 
heat been greater, more air would have been obtained. 
In his experiments with the nitrous acid, as it had con- 
flantlv been found, that by pouring frefii nitrous acid on 
the refiduum, and repeating the operation, more dephlo- 
gifticated air might be obtained, the doftor determined to 
try whether the fame would not hold good with vitriolic 
acid alfo. For this purpofe, he added more oil of vitriol 
to the refiduum of the lad-mentioned experiment. When 
in a red heat wfith a glafs retort, it yielded a quantity of 
vitriolic acid air, no fixed air, but about twenty four 
ounce-meafures of dephlogifticated air ; when, the retort 
being melted, a good deal of the air was necelfarily loft; 
but, on refuming the procefs in a gun-barrel, he procured 
as much air as had been got before. 
The refultof thefe and innumerable other experiments 
made by philofophers in different countries, was, that de¬ 
phlogifticated air may be obtained from a vaft variety of 
mineral and metallic fubftances by means of the vitriolic 
and nitrous acids. It now remained only to difeover in 
what manner this fluid, fo eflentially neceflary to the fup- 
port of animal life, is naturally produced in quantities fuf- 
ficient for the great expence of it throughout the whole 
world, by the breathing of animals, the fupport of fires, 
&c. This difeovery, indeed, had been made before even 
the exiftence of dephlogifticated air itfelf was known. 
Dr. Prieftley, after having tried various methods of puri¬ 
fying contaminated air unfuccefsfully, found at laft, that 
fome kinds of vegetables, but particularly mint, anfwered 
this purpofe very effectually. Having rendered a quanti¬ 
ty of air thoroughly noxious, by mice breathing and dying 
in it, he divided it into two receivers inverted in water, in¬ 
troducing a fprig of mint into one of them, and keeping 
the other receiver unaltered. About eight or nine days 
after, he found that the air of the receiver into which he 
had introduced the fprig had become refpirable ; for a 
moufe lived very well in this, whereas it died the moment 
it was put into the other. , 
From thefe experiments he at firft concluded, that in all 
cafes the air was meliorated by the vegetation of plants : 
but fome experiments of this kind did not anfwer fo well 
towards the end of the year as they had done in the hot lea- 
L O O Y. 143 
for.; and a fecond courfe feemed to bealrtioftentirely con¬ 
trary to the former. At the time of making thefe experi¬ 
ments, he fuppofed that the air tVas meliorated merely by 
the abforption or phlogifton from that which had been 
tainted ; but the experiments of Dr. Ingenhoufz, in 1779, 
(hewed that this was accompliftied, not only by the ab- 
lorption juft mentioned, but alfo by the emijfion of dephlm- 
gifticated air. He obferved in general, that plants hanr* 
power of correcting bad air, and even of improving com¬ 
mon air in a few hours, when expofed to the light of the 
fun ; but, in the night time, or when they are not influenced 
by tiie folarrays, they contaminate the air. This proper¬ 
ty, however, does not belong in an equal degree to all 
kinds of plants : nor is it poffible to difeover, by the exter¬ 
nal properties of a plant, whether it be fit for this purpofe 
or not. Air phlogifticated by breathing, and in which a 
candle could not burn, after being expofed to the fun fot 
three hours with a fprig of peppermint in it, w'asfo far cor¬ 
rected, as to be again capable of fupporting flame. The 
following experiment, made with a murtard plant, maybe 
looked upon as deciftve : A plant of this kind was put into 
a glafs receiver containing common air, and its (tern cut 
oft even with the mouth of the receiver. The velfel was 
then inverted in an earthern pan, containing fome water to 
keep the plant alive, and the whole apparatus was fet over¬ 
night in a room. Next morning the air was found fo much 
contaminated, that it extinguilhed the flame of a wax-ta¬ 
per. On expofing the apparatus to the fun for a quarter 
of an hour, the air was found to be foinewhat corrected ; 
and after an hour and an half it w T as fo far unproved, that 
by the teft of nitrous air it appeared confiderably better 
than common air. 
Among the difeoveries to which thefe experiments led, 
was that of procuring pure air from pump water, which 
had ftood till a green matter had extended itfelf over the 
bottom and fides of the velfel containing it. Water, in 
thisfituation, emits pure air very copioufly, and that even 
when the green matterexhibits iigns of decay, and becomes 
yellowifli; but the light of the fun, it appears, is effential 
to this procefs; very little pure air being emitted in the 
dark. An improvement on this idea was foon afterwards 
fuggefted by Dr. Ingenhoufz, who procured pure air very 
readily from the leaves of different vegetables, fome of 
them of the poifonous kind; but more efpecially from aqua¬ 
tic plants, the turpentine trees, and the green matter col¬ 
lected from a (tone trough. 
The purity of this dephlogifticated air was equal, if not 
fuperior, to that procured by the bed chemical precedes; 
as it fometimes required eight times its own quantity of ni¬ 
trous air to faturate it. All parts of the plants were not 
found equally proper for the production of it; the full- 
grown leaves yielded it in the greateft quantity and purity, 
efpecially from their under furface. It was alfo procured 
from the green (talks.—One hundred leaves of Nafturtiuni 
Indicum, put into a jar holding a gallon, filled with ordi¬ 
nary pump-water, and expofed to the fun from ten to 
twelve o’clock, yielded as much air as filled a cylindrical 
jar four inches and an half in length, and one and three 
quartersin breadth. On removingthisquantity of air, and 
expofing them again to the fun till feven o’clock, about 
half as much was produced, of a quality (till fuperior to 
the former; and next morning by eleven o’clock they yield¬ 
ed as much more of an equal quality. The rootsof plants 
he fays, when kept out of ground, generally yield bad air, 
and at all times contaminate common air, a lew only ex¬ 
cepted. Flowers and fruits, in general, yield a very fmall 
quantity of noxious air, and contaminate a great quantity 
of common air at all times, efpecially in the night, and 
when kept in the dark. Two dozen of young and fmall 
French beans, kept in a quart-jar of common air for a 
(ingle night, contaminated the air to fuch a degree, that a 
very lively chicken died by being confined in it lefs than 
half a minute. 
That the vegetation of plants is one of the great means 
employed by nature to purify the atmofphere, fo as to 
counteract. 
