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36 THE GASES OF THE ATMOSPHERE CHAP.

ing mixture ; the action and re-action of the aerial
and sulphureous particles is, in many fermenting
mixtures, so great as to excite a burning heat, and
in others a sudden flame ; and it is, we see, by the
like action and re-action of the same principles, in
fuel and the ambient air, that common culinary
fires are produced and maintained.

" Tho' the force of its elasticity is so great as
to be able to bear a prodigious pressure, without
losing that elasticity, yet we have, from the fore-
going Experiments, evident proof that its elasticity
is easily and in great abundance destroyed ; and is
thereby reduced to a fixt state by the strong attrac-
tion of the acid sulphureous particles which arise
either from fire or from fermentation ; and therefore
elasticity is not an essential immutable property of
air-particles ; but they are, we see, easily changed
from an elastick to a fixt state, by the strong
attraction of the acid, sulphureous, and saline
particles which abound in air. Whence it is reason-
able to conclude that our atmosphere is a Chaos,
consisting not only of elastick, but also of unelastick
air-particles, which in plenty float in it, as well as
the sulphureous saline, watry, and earthy particles,
which are no ways capable of being thrown off into



BOYLE, MAYOW, AND HALES 37



a permanently elastick state, like those particles
which constitute true permanent air. Since, then,
air is found so manifestly to abound in almost
all natural bodies ; since we find it so operative
and active a principle in every chymical operation ;
since its constituent parts are of so durable a
nature, that the most violent action of fire or
fermentation cannot induce such an alteration
of its texture as thereby to disqualify it from
resuming, either by the means of fire or fer-
mentation, its former elastick state: unless in
the case of vitrification, when, with the vegetable
Salt and Nitre in which it is incorporated, it may,
perhaps, some of it, with other chymical principles,
be immutably fixt, since then this is the case,
may we not with good reason adopt this now fixt,
now volatile Proteus among the chymical principles,
and that a very active one, as well as acid sulphur ;
notwithstanding it has hitherto been overlooked
and rejected by chymists, as no way entitled to
that denomination ? "

This quotation shows us how little Mayow's
shrewd reasoning and well-devised experiments had
impressed the thinkers of his age. While Hales
quotes frequently from Boyle's and Newton's works,



38 THE GASES OF THE ATMOSPHERE CHAP, i

his reference to Mayow is meagre ; nor does he
adopt any one of Mayow's conclusions. One would
have thought that, having prepared so many gases
by means of apparatus well adapted to their pur-
pose, and having observed that certain substances
introduced into air produced contraction, he would
have drawn the conclusion that such " airs " were
essentially different kinds of matter. But the
" Proteus " was too much for him ; and he left the
subject practically in the same state of " Chaos" in
which he found it.



CHAPTER II



"FIXED AIR" AND "MEPHITIC AIR" THEIR DIS-
COVERY BY BLACK AND BY RUTHERFORD



BEFORE relating the history of the discoveries of
Black, Rutherford, and Priestley, it will be appro-
priate to give an account of a theory which pro-
fessed to explain the phenomena of combustion, and
with it the conversion of metals into calces, and the
reduction of these calces to the reguline or metallic
state. Like other theories, it was slow in develop-
ing. Its germ is to be traced to the writings of
Johann Baptist van Helmont of Brabant, Seigneur
of Merode, Royenboch, Oorshot, and Pellines, who
was born in Brussels in 1577. He adopted a
fantastical creation of Paracelsus, the archaeus, a
kind of demon which, by means of fermentation,
draws together all the particles of matter. Be-
lieving that water was the true principle and origin
of everything (for he had succeeded in producing

39



40 THE GASES OF THE ATMOSPHERE CHAP.

a willow tree, weighing 164 Ibs., from water alone,
the earth in which it grew having neither gained
nor lost appreciably in weight), he conceived that
it was acted on by a ferment or principle pre-
existing in the seed developed by it, and exhaling
an odour by which the archaeus was attracted.
Water undergoing the action of this ferment de-
veloped a vapour, to which van Helmont gave the
name of "gas." A "gas" was a substance inter-
mediate between spirit and matter, and the word
was probably derived from Geist, the common
German word for spirit. Another word introduced
by him to denote the life-principle of the stars was
Bias, connected probably with blasen, to blow, and
our English word blast.

It is curious to notice how the idea of an
archaeus survived down to later times under the
name of a " life-principle " a conception that all
organic substances must necessarily owe their
origin to life itself, and not to the usual chemical
and physical transformations.

Van Helmont was acquainted with various
kinds of gases, as appears from his treatise " De
Flatibus." His gas sylvestre was evolved from
fermenting liquors, and he knew that it was formed



ii FIXED AIR AND MEPHITIC AIR 41

during the combustion of charcoal, and also that it
was present in the Grotto del Cane near Naples.
He was likewise acquainted with combustible gases,
which he named gas pingue, gas siccum, or gas
fuliginosum.

These principles of van Helmont's apparently
suggested to his successors, Becher and Stahl, the
notion of a principle inherent in every combustible
substance, which was lost during combustion. The
development of this the phlogistic theory is
almost wholly due to the latter chemist, and
indeed it is difficult to trace Becher's share in it.

George Ernest Stahl was born at Anspach in
1660 ; he studied and graduated in medicine at
Halle, and in 1694 he was appointed second pro-
fessor of medicine at that University, where he
continued to teach for twenty-two years. His
most important work was his Fundamenta chymiae
dogmaticae et experimentale. . His theoretical
views are contained in the last part of this work.
He there treats of zymotechnia, or fermentation ;
halotechnia, or the production of salts ; and pyro-
technia, or the doctrine of combustion. It is the
last of these sections which gives an account of the
doctrine of phlogiston.



42 THE GASES OF THE ATMOSPHERE CHAP.

The fundamental conception of this doctrine is
that all combustible bodies are compounds. During
combustion one of these constituents, common to
all, was dissipated and escaped, while the other,
sometimes an acid, sometimes an earthy powder or
calx, remained behind. Thus sulphur and phos-
phorus, when burnt, give acids; and the metals form
calces. Non-combustible substances, such as lime,
were imagined to be calces, and it was supposed
that if phlogiston were restored to them, they too
would be converted into metals. This combustible
principle was thought to be inherent in all com-
bustible bodies whatsoever ; it corresponds in kind
with the " sulphur " of more ancient writers, but
differs from the latter inasmuch as no very precise
ideas were entertained of the identity of the
"sulphur which conferred on the substances
containing it as a constituent, or possessing it
as a property, their power of combustion." It was
also made more definite by Stahl that substances
capable of burning or conversion into calces are
compounds containing phlogiston in combination
with other substances.

Stahl can hardly be credited with more than
the invention of the term " phlogiston," and with



ii FIXED AIR AND MEPHITIC AIR 43

bringing the subject in a clear and definite form
before his contemporaries. For Stahl wrote in
1720 ; and we find Mayow, in 1674, entering into
an elaborate argument to prove that sulphuric acid
is not contained in sulphur, but that it is produced
by the union of the sulphur with his fire-air
particles. But Stahl amplified the doctrine which
Mayow had controverted, in pointing out that if
such substances as phosphorus, sulphur, or metals
are heated, they burn, and are changed into phos-
phoric acid, sulphuric acid, or " calces " ; and
reciprocally, if phosphoric acid, sulphuric acid, or
a calx such as that of tin or lead, is heated with
matter rich in phlogiston, such as charcoal, pit-
coal, sugar, flour, etc., phlogiston is restored to the
burnt substance, and the original material, phos-
phorus, sulphur, tin, or lead, is reproduced. The
idea at once captivated the minds of the chemists
of that age, who received it with approbation, and
devised experiments designed to extend the appli-
cations of the theory and to confirm its truth.

Substances were not supposed always to be
completely deprived of phlogiston by combus-
tion. Indeed, if the phlogiston were removed
wholly, or nearly so, it was by no means easy to



44 THE GASES OF THE ATMOSPHERE CHAP.

restore it. Thus the calx of zinc, or of iron, which
was regarded as nearly devoid of phlogiston, is
difficult to reduce to the metallic state by ignition
with substances rich in phlogiston, such as coal or
charcoal. The addition of phlogiston alters the
appearance of the substance as regards colour or
metallic lustre, and these vary according to the
proportion of phlogiston present.

There existed no very definite idea regarding
the appearance or properties of phlogiston itself.
Becher's name for it was terra pinguis, and it was
represented by Becher and by Stahl as a dry
substance of an earthy nature, consisting of very
fine particles, which were capable of being set into
violent motion ; this idea was derived partly from
the fact that combustion is usually accompanied by
flame, which was supposed to be produced by the
motion of the particles of the body, communicated
to it by the phlogiston.

It must not be forgotten that at this time it
was perfectly well known that metals gain weight
on calcination. Jean Key was quite aware of this,
and Boyle relates an experiment to show that tin
gains weight when converted into calx ; and it will
be remembered that Mayow made experiments on



ii FIXED AIR AND MEPHITIC AIR 45

the ignition of antimony by the aid of a burning-
glass, and rightly conjectured that the substance
produced was the same as that formed by treat-
ing it with nitric acid, and subsequent ignition.
Boyle's view was that calx of tin was a compound
of tin and heat ; Mayow's more correct view was
that calx of antimony was a compound of antimony
and fire-air. But in spite of these well-proved
facts, the adherents of the theory of phlogiston
ignored them, and it does not appear to have
occurred to Becher or to Stahl that they were
inconsistent with their theories.

When this difficulty was stated, which was not
until a much later date, a lame explanation of a
metaphysical nature, and in itself contradictory,
was all that could be offered. It was that phlo-
giston is endowed with the contrary of gravity or
weight, i.e. levity or absolute lightness. This
means, of course, that it is repelled by the earth.
But if repelled by matter, how comes it that it
enters into combination with matter ? For it could
not remain united if its property were to repel and
not to attract. Notwithstanding this, however,
the idea satisfied some as to the gain in weight
which metals undergo in changing into calces.



46 THE GASES OF THE ATMOSPHERE CHAP.

It is indeed astonishing that men of such
great ability and acumen as Black and Cavendish
should have so long lain under the yoke of this
absurd theory. It is probable that, in the case of
these two great chemists, they stated their results
in terms of the theory, partly because they were
content to express the facts to which they wished
to call attention in this manner, partly because
they were not in a position to replace the theory
by a more rational one. It is not easy to revo-
lutionise a language, even though its vocabulary
be a restricted one. The object of writing is to
convey thoughts to others ; and it is certainly
more convenient to make use of terms understood
by others, even if they only imperfectly convey
the meaning which it is desired to express, than
to attempt a revolution which will probably be
unsuccessful, and even if successful, will at all
events take time. It is not so difficult to under-
stand Priestley's attitude, which we shall have to
consider later ; for Priestley was first of all an
experimentalist, and was captivated more by the
acquisition of a new fact than by assigning to that
fact its proper place in the cosmogony of nature.

The influence of the phlogistic theory on the



ii FIXED AIR AND MEPHITIC AIR 47

knowledge of the nature of air was of such a kind as
to retard its progress. For how could that know-
ledge be furthered, when the most active constituent
of air was represented by a negation ? It may be
said that it is easy to be wise after the event in
this case the discovery of oxygen ; but here was a
theory which was in contradiction to many known
facts, which furnished but a lame explanation of
phenomena, and which had been anticipated by
another theory, subsequently proved to be correct.
Its sole support was the authority of its inventors
or adapters, and the deeply-ingrained notions of
centuries. We may read from it a lesson that
it is wiser to seek out facts which test and prove
a theory rather than those which support it, and
we may learn for the hundredth time the folly of
relying on authority, however ancient and associ-
ated with famous namejs it may be. This was
happily expressed by Boyle when he wrote : * " For
I am wont to judge of opinions as of coins : I
consider much less in any one that I am to receive,
whose inscription it bears, than what metal 'tis
made of. Tis indifferent enough to me whether
'twas stamped many years or ages since, or came

1 A Free Inquiry into the Vulgar Notion of Nature ; Prefatory remarks.



48 THE GASES OF THE ATMOSPHERE CHAP.

but yesterday from the mint. Nor do I regard
how many or how few hands it has passed through,
provided I know by the touchstone whether or no
it be genuine, and does or does not deserve to
have been current. For if, on due proof, it appears
to be good, its having been long, and by many,
received for such will not tempt me to refuse it.
But if I find it counterfeit, neither the prince's
image nor superscription, nor the multitude of
hands it has passed through, will engage me to
receive it. And one disfavouring trial, well made,
will much more discredit it with me than all these
spurious things I have named can recommend it."

It has been necessary to enter at some length
into the nature of the phlogistic theory, because the
discoveries of the time were expressed in its language.
The fire-air or vital air of Mayow was termed
dephlogisticated air; i.e. air wholly deprived of the
power of burning, or air more capable of supporting
combustion than ordinary air ; while airs capable of
burning were supposed to be more or less highly
charged with phlogiston ; indeed, at one time,
it was imagined that hydrogen was phlogiston
itself.

It is to Joseph Black that the discovery of




JOSEPH BLACK



ii FIXED AIR AND MEPHITIC AIR 49

carbon dioxide, that constituent of air first to be
definitely recognised, if we except Mayow's early
work, is generally ascribed. But we must remember
that it had been prepared by Becher and by Hales,
and had been doubtless obtained in an impure state
by many others. It will be seen that Black's work
was so complete, and established the identity of
this gas in so definite a manner, that his right
to be named as its true discoverer can hardly be
questioned.

Black was born near Bordeaux in 1728. His
father, a wine-merchant, was originally a native
of Belfast, being descended from a Scottish family
which had been settled there for some time.
When twelve years of age, Black returned to
Belfast, and received his education in the local
grammar-school, afterwards proceeding to the
University of Glasgow in 1746, at the age of
eighteen. He was a pupil of Dr. Cullen, then
Lecturer on Chemistry at the College there, who
is mentioned by Professor Thomas Thomson in his
History of Chemistry as an excellent and instruc-
tive lecturer. Black intended to choose the career
of medicine, and he indeed practised occasionally as

a medical man during the greater part of his life.

E



50 THE GASES OF THE ATMOSPHERE CHAP.

He began his medical studies in Edinburgh in the
year 1751, and in 1755 he published, as his thesis for
the degree of M.D., the work which has rendered his
name famous. It appears that as early as 1752 he
had been occupied with investigations on quicklime,
which was then attracting attention as a remedy
for urinary calculi. Opinion was divided regard-
ing its virtue. In a manuscript copy of notes of
Black's lectures, which the author is so fortunate
as to possess, he mentions that his attention was
directed to the subject through the rival views of
Drs. Alston and Whytt. It was not long before he
proved that, in opposition to the commonly received
notion, quicklime had gained nothing from the fire
in which it was made, but that the limestone used
for its preparation had lost nearly half its weight
in becoming caustic. He also attempted success,
fully to trap the escaping gas, and again placed it in
presence of lime, confining it over water. Instead
of any escape of material when the lime became
mild, " nothing escapes the cup rises considerably
by absorbing air." And in his notes, a few pages
farther on, he compares the loss of weight under-
gone by limestone on being calcined with its loss
on being dissolved in muriatic acid. These experi-



ii FIXED AIR AND MEPHITIC AIR 51

ments appear from his journal to have been made
before November 1752.

His thesis was not published, however, until
1755. Immediately after, in 1756, he succeeded
Dr. Cullen as Professor in Glasgow, where he re-
mained until 1766. During these ten years he
began and made great progress with his well-known
researches on the heat of fusion of ice, and the heat
of vaporisation of water, or, as he termed them, the
" latent heats " of water and of steam. In 1766 Dr.
Cullen was appointed Professor of Medicine in the
University of Edinburgh, and Black again succeeded
him as Professor of Chemistry. There he lectured
until 1797, when he retired from public life ; he died
as peacefully as he had lived, in 1799, in the seventy-
first year of his age. / Thomson, who relates these
particulars, was one of his last students ; he writes :
" I never listened to any lectures with so much
pleasure as to his ; and it was the elegant simplicity
of his manner, the perfect clearness of his state-
ments, and the vast quantity of information which
he contrived in this way to communicate, that
delighted me. . . . His illustrations were just
sufficient to answer completely the object in view,
and no more." /



52 THE GASES OF THE ATMOSPHERE CHAP.

Black's original thesis for his degree was entitled
Experiments upon Magnesia Alba, Quicklime,
and other Alkaline Substances. It was published
in 1755, and several times reprinted. It is now to
be had in convenient form as one of the " Alembic
Club Keprints."

It was the custom in those days to administer
alkalies as a remedy for urinary calculi ; and about
the year 1750 lime-water was tried as a substitute.
Opinion was divided as regarded its efficacy ; and it
was with the view of preparing a better remedy
that Black undertook researches on magnesia alba.
Black prepared magnesia from "bittern/ 1 which
remains in the pans after the crystallisation of salt
from sea- water, and also from Epsom salts, " which
is evidently composed of magnesia and the vitriolic
acid." The magnesia is thrown down from the
sulphate as carbonate, by the addition of pearl ashes,
at the temperature of ebullition, the soluble product
being " vitriolated tartar," or potassium sulphate.
He describes how " magnesia is quickly dissolved
with effervescence or explosion of air, by the acids
of vitriol, nitre, and of common salt, and by distilled
vinegar," and gives an account of the properties of
the sulphate, nitrate, chloride, and acetate. He sub-



ii FIXED AIR AND MEPHITIC AIR 53

sequently heated this magnesia, and found that it
lost " a remarkable proportion of its weight in the
fire," and his " attempts were directed to the in-
vestigation of this volatile part." The residue in
the retort did not effervesce on the addition of acids ;
hence the volatile part had been driven away by
the heat. " Chemists have often observed, in their
distillations, that part of the body has vanished
from their senses, notwithstanding the utmost care
to retain it ; and they have always found, upon
further inquiry, that subtile part to be air, which,
having been imprisoned in the body, under a solid
form, was set free, and rendered fluid and elastic by
the fire. We may safely conclude that the volatile
matter lost in the calcination of magnesia is mostly
air ; and hence the calcined magnesia does not emit
air, or make an effervescence when mixed with acids."
Magnesia, thus freed from "air" by ignition,
was dissolved in "spirit of vitriol" and thrown
down with an alkali. Its weight was nearly equal
to that which it possessed before calcination, and
it again effervesced with acids. " The air seems to
have been furnished by the alkali, from which it
was separated by the acid ; for Dr. Hales has clearly
proved that alkaline salts contain a large quantity



54 THE GASES OF THE ATMOSPHERE CHAP.

of fixed air, which they emit in great abundance
when joined to a pure acid. In the present case
the alkali is really joined to an acid, but without
any visible emission of air : and yet the air is not
retained in it ; for the neutral salt, into which it is
converted, is the same in quantity, and in every
other respect, as if the acid employed had not been
previously saturated with magnesia, but offered to
the alkali in its pure state, and had driven the air
out of it in their conflict. It seems, therefore, evident
that the air was forced from the alkali by the acid,
and lodged itself in the magnesia"

After an account of some experiments showing
that magnesia is not identical with lime or with
alumina, he proceeds : " It is sufficiently clear that
the calcareous earths in their native state, and that
the alkalis and magnesia in their ordinary condi-
tion, contain a large quantity of fixed air ; and this
air certainly adheres to them with considerable
force, since a strong fire is necessary to separate
it from magnesia, and the strongest is not suffi-
cient to expel it entirely from fixed alkalis, or
take away their power of effervescing with acid
salts.

"These considerations led me to conclude that



ii FIXED AIR AND MEPHITIC AIR 55

the relation between fixed air and alkaline sub-
stances was somewhat similar to the relation
between these and acids ; that as the calcareous
earths and alkalis attract acids strongly, and can
be saturated with them, so they also attract fixed
air, and are, in their ordinary state, saturated with
it ; and when we mix an acid with an alkali, or
with an absorbent earth, that the air is then set at
liberty, and breaks out with violence ; because the
alkaline body attracts it more weakly than it does
the acid, and because the acid and air cannot both
be joined to the same body at the same time. . . .
Crude lime was therefore considered as a peculiar
acrid earth, rendered mild by its union with fixed
air ; and quicklime as the same earth, in which, by
having separated the air, we discover that acri-
mony or attraction for water, for animal, vegetable,
and for inflammable substances."

The solubility of slaked lime in water is next
discussed. If a solution of lime "be exposed to
the open air, the particles of quicklime which are
nearest the surface gradually attract the particles
of fixed air which float in the atmosphere."

Black next points out that, on mixing magnesia
alba with lime-water, the air leaves the magnesia



56 THE GASES OF THE ATMOSPHERE CHAP.

and joins itself to the lime ; and as both magnesia
and calcium carbonate are insoluble in water, the
water is left pure. Similarly quicklime deprives
alkalies of their air and renders them caustic. And
it. follows that if caustic alkali be added to a salt of
magnesia or of lime, it will separate the magnesia
or the calcareous earth from the acid, in a condition


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