than ten minutes the silk became covered 
with an iniinite number of air bubbles, 
gradually increasing in size, till at the end of 
two hours the silk was buoyed up, by their 
means, to the. top of the water. They sepa- 
rated themselves by degrees, and formed a 
collection of air in the upper part of the 
globe, which, when examined by the esta- 
blished test, appeared to be very pure. In 
three days he collected three and three- 
fourths of a cubic inch of pure air, into which 
a wax taper being introduced, that had just 
before been blown out, the wick only remain- 
ing red, it instantly took lire, and burned with 
a bright and enlarged dame. The water in 
the globe had acquired the smell of raw silk, 
it lost something of its transparency, and as- 
sumed a faint greenish cast. 
It has been observed, that when this expe- 
riment was made in the dark, only a few in- 
considerable bubbles were formed, which re- 
mained attached to the silk; nor was it other- 
wise when the glass globe was removed into a 
German stove. In the latter case, indeed, 
some single bubbles had detached themselves 
from the silk, and ascended to the top, but 
the air was in too small a quantity to be 
either measured or proved. 
In these experiments, the oxygen or pure 
air was extracted by an actual decomposition 
of a part of the water, by means of a capil- 
lary attraction, aided by 'the solar influence 
and in effect the same philosopher was en- 
abled to extract it, though in a smaller quan- 
tity", by means of a number of very minute 
glass tubes immersed in water, and exposed 
to the sun. 
The reason that pure air is the most essen- 
tial of all the fluids to the support of life is, 
probably, because a great quantity of heat is 
necessary for this purpose, and because this 
fluid contains it in great quantity, and parts 
with it very freely when it meets with any 
substance with which it has an affinity. But 
as its basis (oxygen) combines itself very easily 
with the basis of coal which is found in the 
bfood and lungs, and during this combination, 
loses part of its caloric, or heat, which goes to 
the support of life, the remainder of the ca- 
loric and oxygen, combined with the coal, 
form the acid carbonic gas or fixable air, 
which is always found to exist in a larger quan- 
tity in air which has been respired, than in at- 
mospherical air which has not been subser- 
vient to that function. Of this a very easy 
experiment affords sufficient proof; it is 
founded on the property which the carbonic 
gas has of rendering lime-water turbid. A 
crooked tube open at both ends is partly filled 
with lime-water ; a person applies his mouth 
to one end of the tube, and inspires, by draw- 
ing the air through the lime-water contained 
in it. By this the transparency of the lime- 
water is not affected ; but it becomes turbid 
as soon as the person expires, which is owing 
to the carbonic acid formed in the lungs. It 
is therefore the great attraction which exists 
between the matter of coal and the basis of 
pure air which renders this fluid so proper for 
breathing. The pure air which we breathe 
performs two functions equally necessary to 
our preservation; it carries off from the blood 
that matter of coal, the superabundance of 
which would be pernicious, and the heat 
which this combination deposits in the lungs 
repairs the continual loss of heat which we 
experience from surrounding bodies. Ac- 
A I R 
cording to Dr. Priestley, and others, the basis 
of oxygen gas is also absorbed by the blood. 
Since, therefore, a great quantity of heat 
is disengaged from pure air in respiration, it 
foi lows, that this fluid must be very pernicious 
to animals who breathe this air alone for a 
considerable time ; which is consonant with 
the observations of physicians, who have at- 
tempted to cure phthisis by the respiration of 
vital air. 
The basisof this emyprealorpure air, oroxy- 
gen gas, as the French chemists term it, is one 
of the constituent parts of, water, it lias been 
mentioned, that it is also the matter which 
gives the acid character to the acids ; sul- 
phur, for instance, is a very innoxious, insipid 
body, till by burning, that is by absorbing 
oxygen, it becomes vitriolic acid.' V\ bother 
the basis of this empyreal air is a simple or 
compound substance, we are unable to de- 
termine; in the present state, however, of 
philosophical knowledge, we are justified in 
considering it as a simple elementary body, 
for it has never yet been decomposed. 
Air Azotic, or Nitrogen gas, named also 
mephitis, is a substance" unlike the former, 
being incapable of supporting animal life. It 
extinguishes flame, and is only characterised 
by possessing none of the distinguishing qua- 
lities of. the other known airs, "it is lighter 
than common air, in the proportion of 985 to 
1 000. It may be considered indeed as the 
residue of common air, when vitiated by com- 
bustion. It is easily disengaged from animal 
matters, by a slight increase of temperature ; 
but different parts of animals afford different 
proportions ot it, and the concrescible fibrous 
matter the most. The portion of it they 
in general afford is proportioned to their quan- 
tity of volatile alkali. 
Azotic or nitrogen gas may be procured by 
mixing equal weights of iron filings and sul- 
phur into a paste with water, and placing the 
mixture in a proper vessel over water. Then 
invert over it a jar full of common air, and 
allow" it to stand exposed to the mixture a day 
or two. 1 he air contained in the jar will gra- 
dually diminish, as will appear from the ascent 
of the water within the jar, till at last only 
about three-fourths of its original bulk re- 
main. 
This gas is not absorbed by water : it im- 
mediately extinguishes a lighted candle, and 
other burning substances : it is fatal to animals 
confined in it : plants, however, live and flou- 
rish in it : by it delicate blue colours are slight- 
ly reddened; and when mixed with oxygen 
gas, in the proportion of three or four parts of 
azotic gas to one of oxygen, it composes a 
mixture resembling atmospheric air. Its basis 
is azote, an elementary substance, which is 
also the basis of nitrous acid or aquafortis. Its 
combination with oxygen gives it the acid 
character. In this state it is combined with 
that substance. In the state of atmospheric 
air it is only mixed. 
Air, Inflammable, or Hydrogen gas. This 
air, named also fire-damp, is peculiarly dis- 
tinguished by its great levity and inflamma- 
bility. It is the lightest substance whose 
weight we are able to estimate. When pure, 
it is thirteen times lighter than atmospheric 
air, and it immediately explodes on mixture 
with oxygen. It is formed by the union of 
hydrogen and caloric. Its lightness is parti- 
cularly evinced by its use in balloons. Plants 
grow in this fluid without impairing its in- 
3l 
flammability. Water imbibes about one- 
thirteenth part of it ; and when again expelled, 
it is as inflammable as ever. By this addition 
both its bulk and specific gravity are increas- 
ed ; for it occupies one-eighth more space by 
its combination, and its weight is increased, 
so as to be only one-tenth lighter than com- 
mon air. 
_ Hydrogen gas is thus procured : let sulphu- 
ric acid, diluted with live or six times its 
weight of water, be poured on iron filings in a 
glass bottle or small retort. An effervescenc e 
wiil ensue, and the gas which escapes is in- 
flammable air or hydrogen gas. This effect 
is produced by the decomposition of the wa- 
tery parts of the acid ; the oxygen is absorbed 
by the metal, and with the 'sulphur forms a 
solid saline mass (martial vitriol or copperas); 
'I he hydrogen which enters into the compo- 
sition of the water is disengaged in the form oi 
hydrogen gas. 
According to M. Lavoisier, water is com- 
posed ot eighty-five parts of oxygen and fifteen 
parts of hydrogen. This philosopher has in- 
structed us in the following method of obtain- 
ing this gas by heat only. 
Let water pass drop by drop through the 
barrel of a gun, while it remains red-hot amidst 
burning coals; let a crooked tube be placed at 
the end of this iron, and bent, so that it may- 
be passed into a glass vessel full of water in- 
verted in the pneumatic apparatus. There 
will then pass into the glass vessel an aeriform 
fluid, which is inflammable air cr hydrogen 
gas. In this process the water suffers a de- 
composition, and while the hydrogen passes 
into the glass receiver, the ox\ gen unites with 
the substance of the gun barrel, and oxydates 
or rusts its internal surface. 
. The electric spark also, taken in any species 
of oil, produces hydrogen or inflammable air, 
this substance being a constituent part of all. 
the oils. The same may be said of aether, and 
alcohol or spirit of wine, which contain a great 
proportion of hydrogen. 
Mr. Cavallo informs us, that he has pro- 
cured this kind of air from the ponds about 
London, in the following manner : fill a wide- 
mouthed bottle with pond-water, and keep it 
inverted in it ; then with a stick stir the mud 
at the bottom of the pond just under the in- 
verted bottle, so as to permit the bubbles of 
air which rise to be received in the inverted 
bottle ; and this air will be found to be inflam- 
mable. 
The ignes fatui are supposed to proceed 
from the inflammable air which abounds in 
marshy grounds, and to be set on fire by elec- 
tric sparks. 
The most remarkable properties of this gas 
are, 1st. its great inflammability, which arises 
from its propensity to unite with oxygen and 
form water. 2dly. Its extraordinary levity, as 
already noticed. 3dly. Metals are very ea- 
sily revived or reduced from a calyx or oxyd 
to the metallic state when heated in a receiver 
filled with this air. This also arises from its 
attraction for oxygen, which in this case is ex- 
pelled from the calx ; and, uniting with the 
hydrogen in the receiver to form water, leaves 
the metal pure, and in its natural state. 
4thly. Plants vegetate in this fluid without 
impairing its inflammability . 5thly. Water 
will imbibe about 1-I3th of its bulk of this 
gas. 
This gas remains permanent over water ; 
it is inflammable. Fill a small jar or common 
