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free from "air" but combined with water.

In order to show that the " air" which exists in
combination with lime or alkalies is not the air
which is contained in solution in water, lime-water
was placed under an air-pump, along with an equal
quantity of pure water; on making a vacuum,
an approximately equal amount of air was evolved
from each. " Quicklime, therefore, does not attract
air when in its most ordinary form, but is cap-
able of being joined to one particular species
only, which is dispersed through the atmosphere,
either in the shape of an exceedingly subtile
powder, or more probably in that of an elastic
fluid. To this I have given the name of fixed air,
and perhaps very improperly ; but I thought it
better to use a word already familiar in philosophy
than to invent a new name, before we be more fully
acquainted with the nature and properties of this



ii FIXED AIR AND MEPHITIC AIR 57

substance, which will probably be the subject of
my further inquiry."

The next proceeding was to render " mild
alkali " caustic by means of lime, and to determine
that nearly the same amount of acid is required to
saturate the caustic alkali as to saturate the mild
alkali from which the caustic alkali had been pre-
pared. On exposure to air for a fortnight, the
caustic alkali again became mild, owing to its
absorption of fixed air. Careful experiments were
made to prove that such caustic alkali contains no
lime, and does not therefore owe its causticity and
corrosive properties to the presence of that in-
gredient. The volatile alkali (ammonium carbon-
ate) was also rendered caustic, and Black " obtained
an exceedingly volatile and acrid spirit, which
neither effervesced with acids nor altered in the
least the transparency of lime-water ; and although
very strong was lighter than water, and floated
upon it like spirit of wine."

After a description of some unsuccessful at-
tempts to render mild alkalies caustic by heat alone
(i.e. to expel carbon dioxide from potassium car-
bonate), Black examines the action of the "seda-
tive salt" or boracic acid on mild alkalies, by rubbing



58 THE GASES OF THE ATMOSPHERE CHAP.

them together in presence of some water. At first
there is no effervescence, but on adding successive
quantities of boracic acid, brisk effervescence finally
takes place, borax being formed. "This pheno-
menon may be explained by considering the fixed
alkalis as not perfectly saturated with air ... if
they expel a small quantity of air from some of the
salt, this air is at the same time absorbed by such
of the contiguous particles as are destitute of it."
And on "exposing a small quantity of a pure
vegetable fixed alkali (carbonate of potash) to the
air, in a broad and shallow vessel, for the space of
two months," crystals were obtained, which possessed
a milder taste than that of ordinary salt of tartar,
which effervesced with acids more violently than
usual, and which could not be mixed with the
smallest portion of boracic acid without emitting a
sensible quantity of air (hydrogen potassium car-
bonate). It therefore follows that such alkaline
substances have an attraction for fixed air; and
this was proved by mixing magnesia alba in fine
powder with caustic alkali, and shaking for some
time. The magnesia was converted into the variety
which did not effervesce with acids, and the alkali
was rendered mild, like a solution of salt of



ii FIXED AIR AND MEPHITIC AIR 59

tartar. These are the principal results of Black's
researches, and he concludes with a table of affinity
of acids for fixed alkali, calcareous earth, volatile
alkali, and magnesia, contrasting it with the affinity
possessed by fixed air for the same bases.

It was the habit of the Scottish students to
pass down notes taken during the lectures of their
professors from one generation to another. As the
lectures were generally read, and not delivered
extempore, the process resulted in an almost
verbatim report of the actual words of the lecturer.
One of these copies of lectures, bearing the date
1778, gives an account of the experiments which
have been described, in words almost identical with
those used in the thesis of 1755. Black appears to
have shown his class this air, made, however,
according to Hales' plan, by heating magnesium
carbonate in a bent gun-barrel, and collected over
water in the usual way. He demonstrated its
weight by pouring it from one vessel to another,
and showed that it extinguished the flame of a
candle. He mentions also that in 1752 he dis-
covered that this air is the same as choke-damp,
and that it is fatal to animal life. He speaks of
the Grotto del Cane, and observes that fixed air



60 THE GASES OF THE ATMOSPHERE CHAP.

is produced by fermentation, and by the burning
of charcoal, and showed to his class experiments
in which air from each source is shaken with
lime-water, giving a turbidity of carbonate. The
well - known experiment of inspiring air through
lime-water, which, owing to the small amount of
carbonic anhydride it contains, does not produce a
turbidity, and expiring through lime-water, show-
ing the formation of carbon dioxide in the lungs,
is described and performed. He next describes
Cavendish's experiments on the solubility of fixed
air and its density, and researches by Dr. Brownrigg
and Dr. Gahn of Sweden on its occurrence in
mineral waters. He also explains how calcareous
petrifactions are produced by the escape of fixed
air from water, which then deposits its dissolved
calcium carbonate, present in solution as bicarbon-
ate. The deposit of iron from chalybeate waters
is ascribed to the same cause, and the explanation
is attributed to Mr. Lane.

" Upon the whole," these manuscript notes
relate, "this sort of air is quite distinct from
common air, though it is commonly mixed with
it in small quantity." "With regard to its origin,
when treating of inflammable substances and metals



ii FIXED AIR AND MEPHITIC AIR 61

I shall consider this more completely. I shall now
only hint that it is a vital air changed by some
matter, seemingly the principle of inflammability.
This appears from several phenomena when an
animal or burning body is enclosed with a certain
quantity of this air, until it is changed as much
as possible. The air is diminished in volume by
the breathing of the animal or by the burning of
the candle. And Dr. Priestley has found that
' growing vegetables had the power of restoring
this sort of air to common or vital air again, which
must be by their taking away some matter which
it had received from the burning body or animal/ "
Black's account of fixed air and its properties
is the first example we possess of a clear and well-
reasoned series of experimental researches, where
nothing was taken on trust, but everything was
made the subject of careful quantitative measure-
ment. It was not long since Hales had pro-
nounced air to be a chaotic mixture of effluvia.
Black showed that common air contains a small
amount of fixed air, and that fixed air must be
considered as a fluid differing in many of its
properties from common air, especially in its being
absorbed by quicklime and by alkalies. It must



62 THE GASES OF THE ATMOSPHERE CHAP.

be remembered that at that time carbon was not
recognised as an element ; and hence, though Black
knew that fixed air was a product of the combus-
tion of charcoal, he did not attribute it to the
union of carbon with oxygen, although the sen-
tence quoted above closely approaches to the truth.

The discovery of nitrogen was next in the order
of time. It was made by Daniel Kutherford, a
pupil of Black's, and at his instigation, and its
description formed a thesis for his degree of Doctor
of Medicine.

Daniel Kutherford was born at Edinburgh
on November 3rd, 1749. He was the son of a
medical man, Dr. John Rutherford, one of the
founders of the Medical School in that city. He
was educated at Edinburgh University, and after
graduating in Arts, became a medical student,
taking his degree of M.D. in 1772. His diploma
was obtained on 12th September. He then travelled
for three years in England, France, and Italy, and
in 1775 he returned to his native town, where he
practised his profession. In 1786 he succeeded
Dr. John Hope in the Chair of Botany in his
University, but he did not on that account resign
his practice. He was president of the Royal College




DANIEL RUTHERFORD.



ii FIXED AIR AND MEPHITIC AIR 63

of Physicians of Edinburgh from 1796 to 1798.
During the greater part of his life he suffered from
gout; he died in 1819, at the age of seventy.

Kutherford does not seem to have pursued
the study of chemistry further : his duties led him
into other fields. His genial, pleasant face, seen in
the portrait by Raeburn, shows him to have pos-
sessed a happy disposition ; and he is said to have
maintained until his death his friendship with
Black, and his interest in the progress which
science was then rapidly making.

The title of Rutherford's dissertation, of which
I have been able to find a copy only in the British
Museum, is Dissertatio Inauguralis de aere fixo
dicto, aut mephitico. It was published at Edin-
burgh in 1772, seventeen years after Black's
memorable dissertation on Fixed Air. As will be
seen shortly, Priestley had nearly anticipated
Rutherford ; and, indeed, he speculated on the
nature of the residual gas, left after combustion
and absorption of the fixed air produced. Evi-
dently Black had noticed that a residue was left
after the combustion of carbonaceous bodies in air,
and absorption of the fixed air produced by the
combustion, and had suggested to Rutherford, then



64 THE GASES OF THE ATMOSPHERE CHAP.

a student of his, the advantage of further investi-
gating the matter, and ascertaining the properties
of the residual gas.

Rutherford begins his essay with an apt
quotation from Lucretius :

Denique res omnes debent in corpore habere
Aera, quandoquidem rara sunt corpora et aer
Omnibus est rebus circumdatus appositusque.

He next proceeds to define the atmosphere as a
pellucid thin fluid, in which clouds float and
vapours rise. Its necessity for animal and vege-
table life is acknowledged by all. It possesses
weight and elasticity. It can be fixed by other
bodies ; but the air obtained from them by dis-
tillation differs from ordinary vital, salubrious air,
and is often termed mephitic or poisonous.

After acknowledging his debt to his illustrious
preceptor Black, he proceeds to quote from the
latter to the effect that mephitic or fixed air is the
air which proves fatal to animals and extinguishes
fire ; which is easily absorbed by quicklime and
by alkaline salts ; which occurs in the Grotto
del Cane, and in mineral waters ; and which is
produced during exhalation from the lungs, by
combustion, and during certain kinds of ferment-



ii FIXED AIR AND MEPHITIC AIR 65

ation. Its density, compared with that of ordinary
air, is as 15 J or 16 to 9 ; hence it can be kept for
some time in an open glass, and if a jar of it
be inverted over a lighted candle, the candle
is extinguished. It has an agreeable taste and
smell; and it changes the colour of syrup of
violets from blue to purple. It prevents putre-
faction, but putrefied bodies are not made fresh
by it. It possesses the power of combining with
lime, which acquires new properties as the result
of its action. Eutherford then recalls Black's
experiments on lime and on magnesia, pointing
out how these bases absorb fixed air, and how
it can be recovered from them and from its
compounds with alkalies, sometimes by heat, and
always by the action of acids.

Eutherford next describes experiments which
show that a mouse, placed in atmospheric air,
and left till dead, diminishes the volume of the
air by one -tenth; and that the residual air, on
treatment with alkali, loses one -eleventh of its
volume. The residue extinguishes the flame of
a candle; but tinder continues to smoulder in
it for a short time. It is thus proved that after
the whole of the fixed air has been withdrawn by



66 THE GASES OF THE ATMOSPHERE CHAP.

alkalies, the residue is still incapable of supporting
life and combustion.

Some burning bodies deprive air of its " salu-
brity" more easily than others. The phosphorus
of urine continues to glow in air in which a candle
has ceased to burn, or in which charcoal has
burned until it is extinguished. Even after
the absorption of all fixed air by alkalies, phos-
phorus burns, emitting clouds of the dry acid
of phosphorus, which can be absorbed by lime-
water.

" It therefore appears that pure air is not con-
verted into mephitic air by force of combustion,
but that this air rather takes its rise or is thrown
out from the body thus resolved. And from this
it is permissible to draw the conclusion that that
unwholesome air is composed of atmospheric air
in union with, and, so to say, saturated with,
phlogiston. And this conjecture is confirmed by
the fact that air which has served for the cal-
cination of metals is similar, and has clearly
taken away from them their phlogiston." Such
air differs from the air evolved from metals by the
action of acids, which is more thoroughly im-
pregnated with phlogiston ; and also from that



ii FIXED AIR AND MEPHITIC AIR 67

from decaying flesh, which is a mixture of mephitic
air and combustible air.

He proceeds : " I had intended to add some-
thing regarding the composition of mephitic air,
and to seek for a reason for its unwholesome effects,
but I have not been able to find out anything
with certainty. Certain experiments appear to
show, however, that it consists of atmospheric
air in union with phlogistic material : for it is
never produced except from bodies which abound
in inflammable parts ; the phlogiston ever appears
to be taken up by other bodies, and is hence of
value in reducing the calces of metals. I say from
phlogistic material, because, as already mentioned,
pure phlogiston, in combination with common
air, can be seen to yield another kind of air
[viz. hydrogen]. ... I have lately heard that
Priestley believes that vegetables growing in
mephitic air dispel its noxious ingredients, or,
as it were, extract them, and restore its original
wholesomeness ; and that mephitic air, added to
air from putrid flesh, partly mitigates its unwhole-
some character. But I have been unable to try
such experiments."

We see, then, that Kutherford's claims to the



68 THE GASES OF THE ATMOSPHERE CHAP, n

discovery of nitrogen amount to this : He removed
the oxygen from ordinary air by combustibles such
as charcoal, phosphorus, or a candle ; and having got
rid of the carbon dioxide, in those cases when it was
formed, by alkali or lime, he obtained a residue,
now known as nitrogen. His view of the nature
of this gas, in the phlogistic language of the time,
was that the burning bodies had given up some
of their "phlogistic material" to the air, which
was thus altered. Nitrogen was " phlogisticated
air," even though incombustible ; hydrogen, too,
was phlogisticated air, but air produced by the
union of pure phlogiston with atmospheric air.
The step taken by Eutherford, under Black's
guidance, was an advance, though not a great
one, in the development of the theory of the true
nature of air ; and he may be well credited with
the discovery of nitrogen.



CHAPTER III

THE DISCOVERY OF " DEPHLOGISTICATED AIR " BY
PRIESTLEY AND BY SCHEELE THE OVERTHROW
OF THE PHLOGISTIC THEORY BY LAVOISIER

WE have seen that Stephen Hales must have
prepared oxygen, among the numerous gases and
mixtures of gases which he extracted from various
substances ; for, among the many materials which
he heated, one was minium, or red-lead. The red-
lead of that day, however, must have contained
carbonate, because, as we shall see, Priestley always
obtained a mixture of oxygen and carbon dioxide
from that source. In the account of his researches
Hales only incidentally mentions the collection of
gas from minium ; and he appears to have made
no experiments with the object of ascertaining its
properties.

The discovery of oxygen was made nearly

simultaneously by Priestley and Scheele, though it

69



70 THE GASES OF THE ATMOSPHERE CHAP.

appears from the recent publication of Scheele's
laboratory notes by Baron Nordenskjold that
Scheele had in reality anticipated Priestley by
about two years. His researches, however, were
not published until a year after Priestley had
given to the world an account of his experiments.
Priestley had no theory to defend ; his experi-
ments were undertaken in an almost haphazard
manner, probably as a relaxation. " For my
own part," he says, 1 " I will frankly acknowledge
that, at the commencement of the experiments
recited in this section, I was so far from having
formed any hypothesis that led to the discoveries
made in pursuing them, that they would have
appeared very improbable to me had I been told of
them ; and when the decisive facts did at length
obtrude themselves upon my notice, it was very
slowly, and with great hesitation, that I yielded to
the evidence of my senses." On the other hand,
Scheele was engaged in forming a theory of the
nature of fire. He writes: 2 "I perceived the
necessity of a knowledge of fire, because without
this it is impossible to make any experiment ; and

1 Experiments and Observations on Different Kinds of Air, vol. ii.
By Joseph Priestley, LL.D., F.R.S. Second edition (1776), p. 29.

2 Chemical Treatise on Air and Fire (1777), 3.



in DISCOVERY OF DEPHLOGISTICATED AIR 71

without fire or heat it is impossible to utilise
the action of any solvent. I began, therefore, to
dismiss from my mind all explanations of fire, and
undertook a series of experiments in order to gain
as full knowledge as possible of these lovely phe-
nomena. I ere long found, however, that it was not
possible to form any correct opinion concerning the
appearances which fire exhibits, without a know-
ledge of the air. After a series of experiments, I
saw that air really is concerned in the mixture
termed fire, and that it is a constituent of flame and
sparks. I learned, moreover, that such a treatise on
fire as this could not be compiled with thoroughness
without also taking air into consideration."

Scheele's views concerning fire need not be
mentioned here ; but his researches on air are so
methodical and so complete as to command our
entire admiration. They remind us of those of
Mayow, and had the latter lived a little longer,
they would not improbably have been carried out
by him. Since, however, Priestley had the advan-
tage of priority of publication, we shall commence
with an account of his researches.



Joseph Priestley was born in 1733 at Field-



72 THE GASES OF THE ATMOSPHERE CHAP.

head, about six miles from Leeds. His father, a
maker, and dresser of woollen cloth, lost his wife
when his son Joseph was about six years of age ;
and being poor, his sister, Mrs. Keighley, offered
to bring up the boy. The early associations of
the lad were closely connected with dissent; and
after some time spent at a public school in the
neighbourhood, he was sent, in 1752, to the
Academy at Daventry, in which he was trained
for the ministry. There he gained some know-
ledge of mechanics and metaphysics, and also
acquired some acquaintance with Chaldee, Syriac,
and Arabic, besides being a competent French and
German scholar. After leaving the Academy, he
settled at Needham in Suffolk, as assistant in a small
meeting-house, where his income was not over 30
a year. His views were, however, too liberal for his
hearers ; and after some years he moved to Nant-
wich in Cheshire, where he preached and also taught
a school. Here his income was improved, though
still miserably small ; yet he managed to buy some
books, a small air-pump, and an electrical machine.
He removed to Warrington in September 1761,
being employed there in teaching and in literary
work ; there he began to pay some attention to




JOSEPH PRIESTLEY.



in DISCOVERY OF DEPHLOGISTICATED AIR 73

chemistry by attending a course of lectures delivered
by Dr. Turner of Liverpool. While at Warring-
ton he wrote a History of Electricity, which first
brought him into notice, and which procured for him
the degree of LL.D. of Edinburgh, thus giving him
a right to the title of Doctor, by which he was always
afterwards known. At Warrington, too, he married
a daughter of Mr. Wilkinson, an ironmaster of
Wrexham. We next find him being asked in 1767
to take the pastorship of Mill Hill Chapel at Leeds,
a call which he accepted. The house in which he
took up his abode before the " minister's house"
had been completed was in Meadow Eoad, next
door to the brewery of Jakes and Nell, and this
circumstance first induced him to take up the
subject of the chemistry of gases, which has made
his name famous. Here too he publishe^. his History
of Discoveries relative to Light and Colours.
After six years spent at Leeds he became librarian
to the Earl of Shelburne (afterwards Marquis of
Lansdowne), and travelled with him on the Conti-
nent. While with Lord Shelburne he published
the first three volumes of Experiments on Air, and
carried out investigations which were recorded in a
fourth volume, published after his removal to Bir-



74 THE GASES OF THE ATMOSPHERE CHAP.

mingham. After some years spent in this way he was
pensioned off, and settled as minister of a meeting-
house in Birmingham, where he employed his time
partly in theological controversy, and partly in
prosecuting researches in chemistry. He published
during this period other three volumes giving a
description of his experiments on air, and com-
municated several papers to the Philosophical
Transactions of the Royal Society, of which he had
been made a Fellow. Towards the year 1 790 he was
so unfortunate as to attack Burke's book on the
French Revolution ; and this had the effect of
rousing popular opinion against him, more especi-
ally that of the local clergy, whose political views
he had frequently opposed. During the riots which
took place at Birmingham in 1791 his house was
burned, and he was obliged to escape to London
under an assumed name. After some years spent
in the charge of a meeting-house at Hackney, he
left England for America. His opinions, though by
no means uncommon at the present day, were so
antagonistic to those of his English contemporaries
that he was cut by his Fellows of the Royal Society,
and he therefore resigned his Fellowship. And this
feeling was in no way lessened by the action of the



in DISCOVERY OF DEPHLOGISTICATED AIR 75

French Government of the time, which made him a
Citizen of the Eepublic, and even chose him as a
member of their Legislative Assembly. Arriving in
America in 1795, he was well received, and settled
at Northumberland, not far from Philadelphia.
There he died in 1804.

In Priestley's work on gases he employed the
form of apparatus which had been used by Mayow a
century before. Such apparatus is indeed generally
used now : the flasks with bent delivery-tubes, the
Woulfe's bottles with two necks, and the pneumatic
trough filled with water or mercury were his chief
utensils. By means of such apparatus, gases can
be collected in a state of comparative purity : they
can be easily transferred from one vessel to another,
and substances which it is desired to submit to
their action can be readily introduced. Scheele, on
the other hand, employed less convenient methods :
his gases were generally collected in bladders, and
their transference to bottles must have been
attended with the introduction of atmospheric
air. Scheele's method was to allow a certain
amount of gas to escape from the generating flask
in order to expel air ; an empty bladder was then
tied over the neck, and the gas entered the



;6 THE GASES OF THE ATMOSPHERE CHAP.

bladder. When he wished to transfer the gas to
a bottle, the bladder was tied at some distance
from the neck, and its loose open end was secured
by a string round the neck of a bottle full of
water. The string confining the gas was then
untied, and the bottle was inverted ; the water
ran into the bladder and was replaced by gas. A
cork was also enclosed in the bladder, and it was
possible to push this cork into the neck of the
bottle and re-tie the string which confined the gas ;
and then, by loosing the string which secured the
bottle to the bladder, the full bottle could be
conveyed away. This process is obviously a clumsy
one, although in Scheele's hands it yielded splendid


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