A I it 
such arc the vapours which arise in distillation, 
and which are condensed as soon as the -super- 
fluous caloric, or heat, is withdrawn (See Dis- 
tillation), while others retain their aerial 
form in all the varieties of our atmosphere, 
hence arose a distinction between elastic 
fluids and permanently elastic fluids. These 
latter, for a reason that will be presently as- 
signed, were called gases. Some of these 
will be found to be simple , that is, when caloric 
is united with some simple elementary sub- 
stance ; others will be found of a compound 
nature, when two elementary substances are 
held suspended in a mixed state by the matter 
of caloric ; and such is our common atmo- 
spheric air, which consists of two elementary 
substances ; oxi/gen in the proportion of about 
one-fourth, and azote about three-fourths, sus- 
pended iu a mass of caloric. See Oxygen, 
Azote, &c. 
Hence it is evident that the aeriform fluids 
differ from each other, according to the na- 
ture of the elementary principles which form 
their respective bases. This was, however, a 
matter which did not strike the early che- 
mists. Though they perceived that those 
fluids which were occasionally raised in their 
processes, were permanently elastic, and dif- 
ferent in their nature from common air, they 
did not enquire into their different properties, 
contenting themselves with giving them a 
name which meant nothing, denominating 
them, in general, spiritus sijlvestris. 
Van Helmont distinguished them by the 
name o i'gas, which lie defined to be a spirit, 
or incoercible vapour, as the word gas, or ra- 
t hev-ghoa-st, in the Dutch language, signifies. 
He supposes the gas to have beeu retained by 
the substances from which it is extracted, in 
a fixed or concrete form. lie asserts, that 
sixty-two pounds of charcoal contain sixty-one 
of gas, and only one of earth, and attributes 
the fatal effects which workmen experience 
occasionally in mines to the emancipation of 
this spirit. On the same principle he accounts 
for the eructations from the stomachand bowels, 
and for the floating of drowned bodies ; and 
he concludes by determining, that this gas is a 
fluid of a nature quite different from that of 
our common air. 
The existence of two different kinds of va- 
pour or elastic fluids, had been previously ob- 
served in mines and coal-works : the one was 
observed to affect animals with a sense of suf- 
focation, and to extinguish life, and. it therefore 
obtained the name of the c hook-damp ; the 
other, from the dangerous property of catch- 
ing fire when a caudle or any ignited body 
was brought in contact with it, was termed tiie 
fire-damp. 
A specimen of the fire-damp, or inflam- 
mable air, was collected from a coal-mine of 
sir James Lowtber, in Cumberland, and 
brought up in bladders to lie exhibited to the 
Royal Society at London, in the year 1733 ; 
and in the year 1736 Mr. John Maud procured, 
from the solution of iron in oil of vitriol, a 
quantity of the very same species of inflam- 
mable air, and demonstrated that the same 
might be procured from most of the metals in 
certain circumstances. 
The experiments of Van Helmont were 
greatly improved upon by the sagacious Boyle. 
He changed the name ot gas to that of artifi- 
cial air ; he demonstrated, that this artificial 
air was not always tiie same ; for instance, 
that the air produced by fermentation is es- 
sentially different from that which is formed 
from the explosion of gunpowder. He was 
the first who perceived that the volume of air 
was diminished by the combustion of certain 
substances. 
This last observation of Mr. Boyle seems 
particularly to have attracted the attention of 
Dr. I tales, and he invented instruments for 
determining the quantities both of the air, 
which was on some occasion produced, and on 
other occasions absorbed, by different sub- 
stances. These experiments deserve the at- 
tention of every philosopher, and for accu- 
racy or ingenuity have never been exceeded. 
Among other circumstances, which were 
particularly remarked by Dr. Hales, was the 
great quantity of air contained in the acidu- 
lated mineral waters, and to this air he sus- 
pected they were indebted for their sparkling 
and brightness, and some other of their pecu- 
liar qualities. in observing the absorption of 
air by bodies in combustion, he saw that this 
absorption had its limits : he remarked also, in 
some cast's, the alternate production and ab- 
sorption of air, as for instance in respect to 
the air which he produced from the burning 
of nitre, which air, he observed, was very soon 
diminished in bulk, though he did not per- 
ceive that the absorption w as owing to the 
water, which he always used in his experi- 
ments. The production of an air capable of 
inflammation from the distillation of certain 
substances did not escape his observation ; 
and he Iras advanced, that the augmentation 
of weight in the metallic calces was in some 
degree owing to the air which thev imbibed. 
That the phosphorus of Homberg diminishes 
the air in which it is burned ; that nitre cannot 
explode in vacuo ; and that air is in general 
necessary to the crystallization of salts ; are 
among the facts, which are noticed by this 
philosopher. 
From tire uncertainty, however, of Dr. 
Hales and his predecessors, with regard to se- 
veral material circumstances, of w'hich they 
appear to have had some casual glimpses, and 
from their total ignorance of others, the doc- 
trine of the aerial fluids was but in a state of 
infancy, till the decisive experiments of Dr. 
Black, Mr. Cavendish, and Dr. Priestley, fur- 
nished us with a new system in this important 
department of natural history. 
The first of these philosophers observed, 
that lime and magnesia, in their mild state, 
consist of an union of a certain aerial fluid with 
the earthy base ; that this aerial matter is actu- 
ally extracted by the operation of burning, 
which reduces ordinary calcareous earth to the 
state of quick-lime ; and that it is afterwards 
re-absorbed by the quick-lime when exposed 
to the air. On this principle he was able, not 
only to account for the loss of weight by the 
burning of lime-stone, but to estimate to the 
greatest nicety the additional weight which it 
could acquire from the atmosphere. He ex- 
tracted the gas, to which he gave the name of 
iixed or fixable air, also by another process, 
namely, by dissolving the calcareous earth in 
acids v he found that the causticity of lime de- 
pended upon its violently attracting from ve- 
getable and. animal matter a portion of that 
air of w'hich- k had been deprived, and that 
upon this principle lie was enabled to render 
caustic the alkaline salts. 
To Mr. Cavendish the second place in the 
order of this history belongs. He pursued the 
experiments of Dr. Black, and ascertained the 
29 
quantity of fixed air which could be retained 
by the fixed and volatile alkalies. He ac- 
counted for the nature of acidulated waters 
by the livable air which they contained. He 
procured a species of inflammable air from 
solutions of iron and zinc in vitriolic acid; and 
he was the first who remarked, that a solution 
of copper in spirit of salt, instead of yielding 
inflammable air, like that of iron, or zinc, 
afforded a particular species of air, which lost 
its elasticity by coming in contact with water. 
Dr. Priestley commenced his philosophical . 
career by some experiments upon fixable air: 
and the first of his communications to the 
public related to the impregnating of water 
with this air, by means of chalk and oil of vi- 
triol; a method first hinted by Dr. Brown- 
rigg of Whitehaven, and now commonly 
practised in the imitations of the acidulated 
mineral waters. The doctor tried the power 
of fixable air upon animal and vegetable life, 
and found it fatal to both. 
The indefatigable mind of Dr. Priestley 
w r as not, however, to be satisfied with the in- 
vestigation of a single object. He next turned 
his attention to the nature of atmospheric air. 
He observed, after Dr. Hales, its diminution 
by different processes, as by combustion, &c. 
but differs as to the cause. Dr. 1 laics supposed 
the specific gravity of the air to be increased ; 
but Dr. Priestley judged, that the denser part 
of the air is precipitated, and that the re- 
mainder is actually made lighter. The dis^ 
covery that the atmospheric air is purilied by 
vegetation is also Dr. Priestley’s. 
On pursuing the experiments of Mr. Ca- 
vendish on inflammable air, the doctor found 
that it was not only producible from iron, and 
zinc, but from every inflammable substance 
whatever. 
Dr. Priestley discovered the cause that 
air, which has been respired, is fatal to animal 
life, to be, that it becomes impregnated w ith 
something stimulating to the lungs, for they 
are affected in the same manner as when ex- 
posed to any other kind of noxious r.ir. 
But his most important discovery was, that 
the nitrons air whi<*h he procured from the so- 
lution of metals in the nitrous acid, had the 
property of destroying the purest part of tiie 
common air. On pursuing Ins experiments. 
Dr. Priestley found, on extracting airs from 
different substances* that the air produced 
from calcined mercury was of a purer nature,, 
and more favourable to animal life and com- 
bustion, than common atmospheric air : it was,, 
in truth, the same with the pure part of the 
air which was destroyed by the nitrous gas, as- 
well as by breathing and' combustion. He 
therefore denominated it pure or vital air, 
and this was afterwards found to be what is 
now called oxygen air or gas, the basis of 
which is oxygen, already noticed under the 
article Acid. 
Dr. Priestley continued his experiments on 
inflammable air, and found that all the metals 
which yield it when dissolved in acids, yielded 
it by means of heat alone ; his mode of ex- 
tracting it was by subjecting the filings of the 
different metals in vacuo to the action of a 
burning-glass. 
The next remarkable, andperhaps the most * 
important discovery, was that of Mr. Caven- 
dish, which .has explained to us the nature 
and composition of water. Mr. Cavendish 
was led to this great discoverv by an expe- 
riment of Mr. Warltire,. related By Dr. 
