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results ; and the methods which Priestley had
borrowed from Mayow have attested their superi-
ority by their survival.

The earliest date at which Priestley began to
experiment with gases was the beginning of the
year 1766, led, no doubt, by the lectures he had
heard at Warrington. In 1767, at Leeds, he had
made some experiments on the conductivity of
"airs" for electricity, using for this purpose common,
inflammable, and fixed airs. He appears to have
abandoned the study of gases in 1768, and to have



in DISLOVERY OF DEPHLOGISTICATED AIR 77

resumed it in 1772. The first gas which he then
investigated was "nitrous gas," or, as it is now
named, nitric oxide. It had previously been pre-
pared by Mayow (see p. 25) by the action of nitric
acid on iron ; and Mayow had made the important
observation that when it was introduced into
ordinary air confined over water, the volume of
the air was decreased, and a rise of temperature
occurred. But Mayow did not apply his discovery
to the analysis of air, though he rightly conjectured
that the reason of the decrease in volume of the
latter was due to combination between the nitric
oxide and his " fire-air particles." It was left for
Priestley to rediscover this fact, and to apply it to
the analysis of air, or, as he expressed it, to the
determination of its " goodness."

Priestley's use of a mercurial trough enabled
him to collect and investigate various kinds of
airs, among others " marine acid air " or gaseous
hydrogen chloride, a gas differing entirely in pro-
perties from ordinary air. This made his mind
familiar with the thought that different kinds of air
exist, not necessarily modifications of atmospheric
air. He had previously from his experiments
come to the conclusion that "atmospheric air is



78 THE GASES OF THE ATMOSPHERE CHAP.

not an unalterable thing, for that the phlogiston
with which it becomes loaded from bodies burning
in it, and animals breathing it, and various other
chemical processes, so far alters and depraves it as
to render it altogether unfit for inflammation,
respiration, and other purposes to which it is sub-
servient ; and I had discovered that agitation in
water, the process of vegetation, and probably
other natural processes, by taking out the super-
fluous phlogiston, restore it to its natural purity.
But I own I had no idea of the possibility of going
any further in this way, and thereby procuring air
purer than the best common air."

On the 1st of August 1774, Priestley heated by
means of a burning-glass red oxide of mercury.
This was produced by heating mercury until it oxi-
dised, and therefore had been untouched by acids,
or by any substance which could have " imparted
phlogiston " to atmospheric air. The resulting air
was insoluble in water, and supported combustion
better than common air, for a candle burned more
brightly, and a piece of red-hot wood sparkled in
it. This air he also produced from " red precipi-
tate," the product of heating nitrate of mercury ;
and at the same time from red-lead, or minium.



in DISCOVERY OF DEPHLOGISTICATED AIR 79

It differed from " modified nitrous air," in which a
candle also burns brightly, inasmuch as shaking with
water the gases produced after a candle had burned
for some time in it did not deprive it of its power
of supporting combustion ; nor did it diminish the
bulk of common air, as the nitrous air does in some
degree. Priestley here refers to a mixture ob-
tained by distilling nitrates, which is essentially a
mixture of nitric peroxide with oxygen. A candle
burns in such a mixture, depriving the nitric per-
oxide of part of its oxygen, and converting it into
nitric oxide mixed with nitrogen. Nitric oxide,
deprived of the excess of peroxide by shaking with
water, with which the peroxide reacts and is ab-
sorbed, is no longer capable of supporting the
combustion of a candle ; and when added to
ordinary air it combines with its oxygen, again
forming nitric peroxide, which in its turn is absorbed
by water.

Priestley's experiments were performed at inter-
vals from August 1774 till March 1779, and at
that date it occurred to him to mix with his de-
phlogisticated air some nitric oxide over water ;
absorption took place, and he concluded that he
might assume his new air to be respirable. And



8o THE GASES OF THE ATMOSPHERE CHAP.

what surprised him especially was, that even after
addition of nitric oxide and agitation with water,
the residue still supported the combustion of a
candle. A mouse, too, lived half an hour in the
new air, and revived after being removed ; whereas
similar experiments with an equal volume of
common air had shown that, after respiring it for a
quarter of an hour, a mouse was indisputably dead.
Even after the mouse had breathed it for so long a
time, it was still capable of supporting the com-
bustion of a candle ; and this induced him to add
more nitric oxide to the respired air, when he
found that a further contraction occurred. He
reintroduced the same unfortunate mouse into the
remainder of the air a portion to which nitric oxide
had not been added when it lived for another half-
hour, and was quite vigorous when withdrawn.

Subsequent experiments with nitric oxide
showed that air from red precipitate or from
" mercurius calcinatus " (red oxide of mercury in
each case, although prepared in different ways) was
" between four and five times as good as common
air." He proceeds : * " Being now satisfied with
respect to the nature of this new species of air, viz.

1 LOG. cit. p. 46.



in DISCOVERY OF DEPHLOGISTICATED AIR 81

that being capable of taking more phlogiston from
nitrous air, it therefore originally contains less of this
principle, my next inquiry was, by what means it
comes to be so pure, or, philosophically speaking, to
be so much dephlogisticated." He therefore went on
to heat the various oxides of lead, but without any
special results worth chronicling. On moistening
red-lead with nitric acid, however, and distilling the
mixture, he obtained, in successive operations, air
which was " five times as good " as common air.
This process formed lead nitrate, which on distilla-
tion yielded nitric peroxide and oxygen ; the gas
was, of course, collected over water, which absorbed
the peroxide, allowing pure oxygen to pass. He
found that red-lead was not the only "earth"
which produced this effect ; but that " flowers of
zinc" (zinc oxide), chalk, slaked lime, and other
substances also gave a gas, when distilled with
nitric acid, which was " better " than common air.
In some cases he broke up nitric acid by heat into
water, nitric peroxide, and oxygen ; in others he
heated nitrates. His conclusion is : "Atmospherical
air, or the thing we breathe, consists of the nitrous
acid and earth, with so much phlogiston as is
necessary to its elasticity ; and likewise so much

G



82 THE GASES OF THE ATMOSPHERE CHAP.

more as is required to bring it from its state of
perfect purity to the mean condition in which we
find it." 1

When such experiments were made by heating
nitrates in a gun-barrel, " phlogisticated air " was
obtained. This was nitrogen, for the iron had re-
duced the oxides of the latter, and combining with
their oxygen, had formed nitrogen ; moreover, it
had absorbed to a greater or less extent the oxygen
simultaneously produced.

Having concluded that respirable air was a
compound of nitrous acid, phlogiston, and earth,
Priestley endeavoured to ascertain what was the
nature of this earth. He concludes " that the
metallic earths, if free from phlogiston, are the
most proper, and next to them the calcareous
earths"

" Dephlogisticated air may be procured from any
kind of earth with which the spirit or nitre will
unite." A few quantitative experiments would
surely have refuted this erroneous conclusion.
Those which he attempted to make were very
crude. A bladder (of which he does not give the
capacity) was filled with

1 Loc. cit. p. 55.



in DISCOVERY OF DEPHLOGISTICATED AIR 83

Phlogisticated air, and weighed 7 dwts. 15 grs.
Nitrous air 7 16

Common air 7 17

Dephlogisticatedair,, 7 19

He concludes (taking into consideration that in-
flammable air is very light) "that the less phlogiston
any kind of air contains, the heavier it is ; and the
more phlogiston it contains, the lighter it is." 1
Strange that this should not have led to the
rejection of the phlogistic hypothesis !

Priestley had the curiosity to breathe his
" good " air. He says : " My reader will not
wonder that, after having ascertained the superior
goodness of dephlogisticated air by mice living in
it, and the other tests above mentioned, I should
have the curiosity to taste it myself. I have
gratified that curiosity by breathing it, drawing it
through a glass syphon, and by this means I
reduced a large jar full of it to the standard of
common air. The feeling of it to my lungs was not
sensibly different from that of common air, but I
fancied that my breast felt peculiarly light and easy
for some time afterwards. Who can tell but that in
time this pure air may become a fashionable article

1 LOG. cit. p. 94.



84 THE GASES OF THE ATMOSPHERE CHAP.

in luxury ? Hitherto only two mice and myself
have had the privilege of breathing it." l

It will be seen from this account that Priestley's
work was to some extent that of an amateur. He
performed experiments, often without any definite
object ; and he was not always successful in
devising theories. As before remarked, his chemi-
cal pursuits were to him a recreation, and were
undertaken during the intervals of his necessary
work. His mind was therefore not given over to
them alone ; and this is to be seen from the
character of his writings. His style is a delight-
fully familiar one : he exposes his inmost thoughts
with perfect frankness, and his writings are there-
fore very readable. We have now to compare his
work with that of his contemporary, Scheele, whose
mission in life was that of a chemist ; and the
reader will be interested in noting the different
points of view which these two eminent discoverers
adopted.

Carl Wilhelm Scheele was born on the 9th of
December 1742 in Stralsund, the capital of Swedish
Pomerania, where his father was a merchant and a
burgess. He was the seventh of eleven children.

1 Loc. cit. p. 102.




CARL WILHELM SCHEELE
1742 1786.



in DISCOVERY OF DEPHLOGISTICATED AIR 85

After receiving his education, partly in a private
school, partly in the public school (gymnasium) at
Stralsund, he was apprenticed at the age of four-
teen to the apothecary Bauch in Gothenburg. In
those days an apothecary was in large measure a
manufacturer as well as a retailer of drugs. He had
to prepare his medicines in a pure state from very
impure materials, as well as to mix them in order
to carry out prescriptions ; and indeed he himself
often, as sometimes happens still, ventured to pre-
scribe in mild cases. Scheele's master taught him
such methods, and in addition instructed him in the
use of the chemical symbols in vogue at that date ;
these he afterwards freely employed in his manu-
scripts, and this renders them exceedingly difficult
to decipher. There still exists a catalogue of the
drugs his master kept ; many of them are of a
fantastic nature, such as " ointment of vipers,"
" human brain prepared without heat," etc. ; but
among them were many of the well-known salts of
metals, and the commoner acids, besides phosphorus,
sulphur, rock-crystal, some ores, and some carbon
compounds ; for example, benzoic acid and camphor.
There was a fair chemical library, which included the
works of Boerhaave and Lemery, and his master



86 THE GASES OF THE ATMOSPHERE CHAP.

devoted much pains to his instruction. In a letter
to Scheele's father, however, he expressed a fear
that too great devotion to study and experimental
work would undermine the health of a growing lad.
In 1765 the business 'was sold, and Scheele
obtained a situation in Malmo with an apothecary
named Kjellstrom. His master testified that he had
extraordinary application and ability, and related
that he was in the habit of criticising all that he read,
saying of one statement, " This may be the case " ; of
another, " This is wrong " ; of a third, " I shall look
into this." His memory was prodigious : he is said
never to have forgotten anything which he had
read relating to his favourite subject. He took little
interest in anything else, and both his employers
appear to have encouraged him to the utmost
in his favourite pursuit. In 1768 he left Malmo
for Stockholm ; but here the exigencies of his duties
interfered with his leisure for experimentation.
While there, in conjunction with his friend Betzius,
he discovered tartaric acid, which up till then had
never been separated from tartar, its potassium
salt. Here too he made investigations on the acid
of fluor-spar (hydrofluoric acid) ; but finding his
time too greatly occupied with routine work, he



in DISCOVERY OF DEPHLOGISTICATED AIR 87

took a situation at Upsala, the seat of the largest
university of Sweden, in 1770. At that time
Bergman was Professor of Chemistry there, and
Linnaeus occupied the Chair of Botany ; both had
then achieved a wide reputation. With Bergman
he soon established close relations, and Eetzius
wrote that it was difficult to say which was pupil
and which teacher. While at Upsala he wrote his
great work on Fire and Air, which we shall shortly
have to consider. From his laboratory notes it
appears that before 1773 he had obtained oxygen
by the ignition of silver carbonate, red mercuric
oxide, nitre, magnesium nitrate, and from a mixture
of arsenic acid and manganese dioxide. Here too
he discovered chlorine, and made researches on
manganese, arsenic, and baryta. In 1775 he was
elected a member of the Eoyal Swedish Academy of
Sciences, an honour which much improved his social
status. In the same year he became manager of a
business at Koping, where he passed the rest of his
days, in spite of urgent appeals to engage in more
remunerative work ; indeed, he was strongly pressed
to go to Berlin, and also, it is said, to London, for
his publications had led to his recognition as one of
the greatest chemists of the age. His book on Fire



88 THE GASES OF THE ATMOSPHERE CHAP.

and Air was not published for some years after the
manuscript had been in the printer's hands. We
learn from his letters that he was much afraid of
being anticipated in his discoveries, as indeed
events showed that he had reason to be.

From his letters and from the verdict of his
contemporaries, Scheele is depicted as an amiable
and honourable man, singularly free from vanity
and selfishness. His last memoir on the action
of sunlight on nitric acid was published in 1786 ;
he died suddenly at the early age of forty-three
in May of that year, two days after his marriage
to Sara Margaretha Pohl. His devotion to science
had told on his health, and his death was caused
by a complication of diseases. Yet he was during
his life, as after his death, regarded as one of the
greatest of chemists : his great knowledge, extra-
ordinary aptitude in experimenting, and high
intellectual powers place him among the foremost
men of science of his day.

Near the beginning of his Treatise on Air and
Fire, 1 Scheele defines air. It is that fluid invisible
substance which we continually breathe ; which

1 The accurate translation of Scheele's Treatise published by the
Alembic Club (William F. Clay, 1894) has been made use of here.



in DISCOVERY OF DEPHLOGISTICATED AIR 89

surrounds the whole surface of the earth, is very
elastic, and possesses weight. "It is always filled
with an astonishing quantity of all kinds of exhala-
tions, which are so finely divided in it that they are
scarcely visible, even in the sun's rays." 1 It also
contains another elastic substance resembling air,
termed aerial acid by Bergman (identical with
Black's fixed air). Since atmospheric air has not
been completely converted into fixed air by admix-
ture of foreign materials, " I hope I do not err if I
assume as many kinds of air as experiment reveals
to me. For when I have collected an elastic fluid,
and observe concerning it that its expansive power
is increased by heat and diminished by cold, while
it still uniformly retains its elastic fluidity, but also
discover in it properties and behaviour different
from those of common air, then I consider myself
justified in believing that this is a peculiar kind of
air. I say that air thus collected must retain its
elasticity even in the greatest cold, because other-
wise an innumerable multitude of varieties of air
would have to be assumed, since it is very probable
that all substances can be converted by excessive
heat into a vapour resembling air." 2
1 4. 2 5.



90 THE GASES OF THE ATMOSPHERE CHAP.

After defining the properties characteristic of
air, namely, its power of supporting combustion, its
diminution by one third or one quarter during the
combustion of any substance which does not pro-
duce any fluid resembling air, its insolubility in
water, its power of supporting life, and the fact of
its being favourable to the growth of plants, Scheele
demonstrates that air must consist of at least two
elastic fluids. This he proves by exposing it to
" liver of sulphur " (polysulphide of potassium),
when six parts out of twenty were absorbed. He
obtained the same result by employing a solution of
sulphur in caustic potash, and also by polysulphide
of calcium, prepared by boiling lime-water with
sulphur, and by means of yellow sulphide of
ammonium. Nitric oxide, " the nitrous air which
arises on the dissolution of metals in nitrous acid,"
produces a similar contraction, and so also do
oil of turpentine and "drying oils" in general.
Dippel's animal oil, obtained by distilling bones,
and ferrous hydroxide, produced from " vitriol of
iron" and " caustic ley," or ferrous sulphate and
caustic potash, may also be used as absorbents ; as
may also iron filings moistened with water, a solu-
tion of iron in vinegar, and a solution of cuprous



in DISCOVERY OF DEPHLOGISTICATED AIR 91

chloride. " In none of the foregoing kinds of air
can a candle burn or the smallest spark glow."

He accounts for these results by the theory
that all such absorbents contain phlogiston, which
is attracted by the air, and, combining with it,
diminishes its bulk. The alkalies and lime attract
the vitriolic acid of the sulphides used, and the
air attracts the phlogiston. " But whether the
phlogiston which was lost by the substances was
still present in the air left behind in the bottle,
or whether the air which was lost had united
and fixed itself with the materials, such as liver
of sulphur, oils, etc., are questions of import-
ance." 1 The conclusion that such air, which had
received phlogiston and had contracted in volume,
ought to be specifically heavier than common air
was, however, rudely dissipated by experiment.
The air must therefore contain two fluids, one of
which does not manifest the least attraction for
phlogiston, while the other is peculiarly disposed to
such attraction. " But where this latter kind of air
has gone to, after it has united with the inflammable
substance, is a question which must be decided by
further experiments, and not by conjectures." 2

1 16. 2 16.



92 THE GASES OF THE ATMOSPHERE CHAP.

To decide this question, Scheele burned in air
substances such as phosphorus, which do not pro-
duce by their combustion any kind of "air." The
result was that the air lost 9 volumes out of an
original 30, or about one-third of its bulk. A
flame of hydrogen burning in air caused it to lose
one-fifth of its volume. On burning a candle,
some spirits of wine, or some charcoal, in a con-
fined quantity of air, very little, if any, diminu-
tion of volume was noticed; but on shaking the
air with milk of lime, contraction ensued, but
not to the same extent as when phosphorus was
burnt in it. This greatly puzzled Scheele ; we
now know that such combustibles are not able to
remove all the oxygen, but that they are extin-
guished when only a portion of each has entered
into combination. Here, again, however, his memory
comes to his help, for he says, "It is known that
one part of aerial acid mixed with ten parts of
ordinary air extinguishes fire ; and there are here
in addition, expanded by the heat of the flame
and surrounding the latter, the watery vapours
produced by the destruction of those oily sub-
stances. It is these two elastic fluids, separating
themselves from such a flame, which present no



in DISCOVERY OF DEPHLOGISTICATED AIR 93

small hindrance to the fire which would otherwise
burn much longer, especially since there is here
no current of air by means of which they can
be driven away from the flame. When the aerial
acid is separated from this air by milk of lime,
then a candle can burn in it again, though only for
a very short time." Thus the question was
correctly solved. Scheele's acumen led him at
once to make experiments admirably adapted to
discover the true reason ; he was not turned aside
by any imaginary difficulties, but went straight to
the point. He next burned sulphur in confined
air, and found little alteration of volume, but on
shaking with clear lime-water, absorption took
place, and one-sixth of the air was removed. " The
lime-water was not in the least precipitated in this
case, an indication that sulphur gives out no aerial
acid during its combustion, but another substance
resembling air ; this is the volatile acid of sulphur,
which occupies again the empty space produced by
the union of the inflammable substance with air." 2
The next set of experiments were devised " to
prove that ordinary air, consisting of two kinds of
elastic fluids, can be compounded again, after these

1 22. 2 23.



94 THE GASES OF THE ATMOSPHERE CHAP.

have been separated from one another by means of
phlogiston."

" I have already stated that I was not able to
find again the lost air. One might indeed object
that the lost air remains in the residual air which
can no more unite with phlogiston ; for, since I
have found that it is lighter than ordinary air, it
might be believed that the phlogiston, united
with this air, makes it lighter, as appears to be
known already from other experiments. But since
phlogiston is a substance, which always pre-
supposes some weight, I much doubt whether such
hypothesis has any foundation." 1 He had formerly
conjectured that hydrogen, the "air" obtained by
the action of vitriol on zinc, might be phlogiston ;
" still, other experiments are contrary to this."

Scheele next directs attention to acid of nitre,
and points out that when prepared in absence of
organic material, it is nearly colourless ; but that
if phlogiston be given to it, it becomes red. At
the end of a distillation of pure nitre with pure
sulphuric acid, however, red fumes are produced :
"Where does the acid now obtain its phlogiston?
There is the difficulty."



in DISCOVERY OF DEPHLOGISTICATED AIR 95

He collected some of this "red air" in a
bladder containing milk of lime, to prevent its
corrosive action ; and having tried whether the
resulting gas, which was now no longer red, would
support combustion, " the candle began to burn with
a large flame, whereby it gave out such a bright
light that it was sufficient to dazzle the eyes. I
mixed one part of this air with three parts of that
air in which fire would not burn ; I had here an
air which was like the ordinary air in every
respect. Since this air is necessarily required for
the origination of fire, and makes up about the
third part of our common air, I shall call it after
this, for the sake of shortness, Fire-air ; but the
other air, which is not in the least serviceable for
the fiery phenomena, I shall designate after this
with the name already known, Vitiated air." l How
history repeats itself! Here is Scheele, in 1772,
reproducing Mayow's name " fire-air particles " for
the same substance of which Mayow had inferred
the existence a century before, and which he had
pointed out as being present in the acid of nitre
as well as in common air.

This air is not a " dry acid of nitre converted

1 29.



96 THE GASES OF THE ATMOSPHERE CHAP.

into elastic vapours," for it does not produce
nitre with alkalies ; moreover, it can be prepared
from substances which have nothing in common
with nitre, no compound of nitre having been used
during their preparation. Scheele next describes
experiments proving that " fire-air" is produced


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