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phenomena. The opponents of the new doctrines
Priestley chief among them did their best to
disprove the view that water was a compound of
oxygen and hydrogen. But in vain. Many of
Lavoisier's opponents had to admit the justice of
his views; and in 1787 De Morveau, Berthollet,
and Fourcroy joined Lavoisier in reconstructing the
nomenclature of chemistry on a new basis, which is
substantially that in use at the present day. Black,
too, was a convert, but Priestley and Cavendish
remained true to their old faith, and one of
Priestley's last acts was to publish a defence of the
phlogistic theory. We shall see later how Caven-
dish carefully considered the rival theories, and what
reasons induced him to cast his vote for the older one.



Ii6 THE GASES OF THE ATMOSPHERE CHAP.

Among the numerous memoirs which Lavoisier
communicated to the Academy during the ten
years between 1772 and 1782, one still remains
to be mentioned. It was published as early as
1777, but it must be remembered that many of
these memoirs were antedated. It referred to the
respiration of animals ; and Lavoisier concluded,
on the ground that the phenomena of respiration
are essentially similar to those of combustion and
calcination, that the only portion of the air which
supports animal life is the oxygen. The azote or
nitrogen is inhaled along with the oxygen, but
is exhaled unaltered. The oxygen, however, is
gradually converted into carbonic acid ; and when
a certain amount, but by no means the whole,
has been thus changed, the air becomes unfit for
respiration. If the carbonic acid is withdrawn
by means of lime-water or caustic alkali, the
residue is air poor in oxygen, and the azote is
the same as that left after the calcination of
metals, or the burning of a candle, in air.

At the time of his impeachment Lavoisier was
engaged in experiments on perspiration, along with
Se'guin. He had nearly finished his experimental
work, but had drawn up no account of it. His



in OVERTHROW OF THE PHLOGISTIC THEORY 117

request that his life might be prolonged until
he had compiled a statement of his results was
refused ; but Se*guin, who was fortunately spared,
undertook the task. The facts collected do not,
however, bear directly on our subject, and shall
not be further alluded to here.

This account of Lavoisier's researches would be
incomplete without a reference to his text-book
of chemistry, Traite elementaire de Chimie, in
which his views are stated in order, and with
great clearness. The nomenclature current at the
time was so cumbrous that it was almost, if not
quite, impossible for the supporters of the new
theory to express their meaning in an intelligible
manner. De Morveau had suggested a nomen-
clature for salts ; Black, too, had invented one ; but
neither of these systems was adapted to represent the
new views. It was partly with the object of avoiding
such embarrassment that Lavoisier wrote his Treatise.

He begins with a clear statement of what is
generally termed "the states of matter" solid
liquid, and gaseous and points out that solids
and liquids are almost all capable of change into
the aeriform state by the addition of " caloric."
Proceeding next to the consideration of the



ii8 THE GASES OF THE ATMOSPHERE CHAP.

nature of air, he shows that it must necessarily
contain all those gases capable of existence at
the ordinary temperature ; and he explains how
water -vapour must be one of them, seeing that
even though water is a liquid at the ordinary
temperature, it is capable, like many other liquids,
of existing as vapour, when mixed with other
gases. He next treats of the analysis of air, and
describes his classical experiment of heating four
ounces of mercury for twelve days in a retort
communicating with a bell-shaped receiver, stand-
ing in a mercury trough. Having marked the
initial height of the air in the jar by means of
a piece of gummed paper, he found that, after
twelve days' heating close to the boiling-point,
the air had diminished in volume by about one-
sixth, and that the mercury had become covered
with a red deposit of mercurius calcinatus per se,
which, when collected, weighed 45 grains. The
residual air in the retort and in the jar was incap-
able of supporting life or combustion ; but the red
precipitate, when heated, lost 3| grains of its
weight, yielding 41|- grains of metallic mercury,
while it evolved 7 or 8 cubic inches of oxygen,
capable of supporting the combustion of a candle



HI OVERTHROW OF THE PHLOGISTIC THEORY 119

vividly, and of causing charcoal to burn with a
crackling noise, throwing out sparks. Oxygen was
thus successfully separated from air, and obtained
from it in a pure condition for the first time, in a
single series of operations.

In Lavoisier we see a master mind, not only
capable of devising and executing beautiful ex-
periments, but of assimilating those of others, and
deducing from them their true meaning. Although
his additions to the known chemical compounds
were few in number, and cannot be compared
with those of Scheele or of Priestley, yet his
reasoning in disproof of the phlogistic theory was so
accurate and so exact that it rapidly secured con-
viction. With the exceptions already mentioned,
almost all the eminent chemists of the day
accepted his conclusions ; and one Kirwan who
had written a formal treatise in defence of the
phlogistic theory, was so fair-minded that after
his work had been translated into French and
published with comments, he acknowledged that
the old theory was dead, and that truth had
conquered.

It will be interesting now to trace Cavendish's
part in developing the history of the discovery



120 THE GASES OF THE ATMOSPHERE CHAP, m

of the constituents of air, and to note his argu-
ments in favour of the phlogistic theory. Although
Cavendish never publicly acknowledged its in-
sufficiency, yet he had ceased to occupy himself
with chemical problems at the time when its
adoption was universal, and his true opinions
have never been recorded.



CHAPTER IV

1 c PHLOGISTIC ATED AIR" INVESTIGATED BY CAVEN-
DISH HIS DISCOVERY OF THE COMPOSITION
OF WATER

WHILE Lavoisier was engaged in experiments
on oxygen, Cavendish, too, was devoting his
attention to the constituents of air, but in a some-
what different manner. His early experiments led
him to the discovery of the composition of water ;
and it has already been pointed out how necessary
a knowledge of the true nature of hydrogen is
to the understanding of the phenomena of com-
bustion. His second paper deals with the inactive
constituent of air the mephitic portion, now known
as nitrogen or azote. But before considering these,
a sketch of his life will prove of interest.

The Honourable Henry Cavendish was a very
singular man, retiring and uncommunicative to
a degree ; hence little is known of his early life.



121



122 THE GASES OF THE ATMOSPHERE CHAP.

He was the elder son of Lord Charles Cavendish,
who was the third son of the second Duke of
Devonshire. His only brother, Frederick, was also
an eccentric, but a very benevolent man, and
the two brothers, though they seldom met, lived
on excellent terms with each other. Henry
Cavendish was born at Nice in October 1731.
His mother died when he was two years old.
Nothing is known of his childhood and youth,
save that he attended Hackney School from 1742
to 1749, and that he went to Cambridge in the end
of 1749, and remained till 1753 without taking
a degree. After leaving Cambridge it is supposed
that he lived in London for ten years. It is known
that his allowance from his father amounted to
500 a year, and that his rooms were a set of
stables fitted up for his accommodation. It is
probable that this was his own choice, and that
he made use of them chiefly as a laboratory and
a workshop. Although at his father's death and
by the legacy of an aunt he acquired a large
fortune, he never spent more than a fraction of it.
He left more than a million sterling to his relative,
Lord George Cavendish ; but they saw each other
only once a year, and the interview seldom lasted



iv PHLOGISTICATED AIR 123

more than ten minutes. The writer of his obituary
notice, M. Biot, epigrammatically said : " II etait
le plus riche de tous les savans, et le plus savant
de tous les riches."

He was a regular attendant at the meetings
of the Royal Society, of which he was made a
Fellow in 1760, and was a constant diner at the
Royal Society Club. It is said that he used to
talk to his neighbour at table so long as others
did not join in the conversation ; but if the con-
versation took a general turn, he was silent.

His death took place in February 1810, and
was as solitary as his life. It is related by his
servant that Cavendish, on feeling his end ap-
proaching, dismissed him from the room, telling
him to come back in half an hour. He disobeyed
instructions, and, being anxious, found some pre-
text to enter the room. Cavendish ordered him
away in a voice of displeasure, and on returning
the man found his master dead.

Such a life demands our pity ; yet, if an object of
human life is to give pleasure to its possessor, we can
hardly say that Cavendish's was a failure. Ordinary
mortals have a craving for the sympathy of their
fellows ; Cavendish appears to have been devoid



124 THE GASES OF THE ATMOSPHERE CHAP.

of any such sensation. Indeed, his experiments
were in many cases not published until long after
they had been made. He appears to have carried
on his work for his own information, and to have
been indifferent to the impression which his labours
made on his fellow-men. Yet his inquiries cover
a more extensive field than those of almost any
other man of science. They begin with experiments
on arsenic, by which he endeavoured to determine
the difference between the element arsenic and its
two oxides. He held that arsenic acid was more
thoroughly " deprived of phlogiston " than arsenious
acid (i.e. more highly oxidised) ; and on the same
occasion he studied the effect of the addition of air
to nitric oxide, produced by the action of nitric acid
on the element arsenic and on arsenious oxide. His
next experiments related to heat ; and had he pub-
lished them, he would doubtless have anticipated
Black in his discovery of latent heat. His paper on
4 'Factitious Airs," published in the Philosophical
Transactions for 1766, deals with the properties
of hydrogen, carbon dioxide, and the gases pro-
duced by the destructive distillation of organic
substances. As we shall see later, he supposed
that hydrogen, generated by the action of acids



iv PHLOGISTICATED AIR 125

on metals, came out of the metal, and was an un-
known principle in combination with phlogiston, if
indeed it was not phlogiston itself ; and this idea is
not absurd, for many metals, and indeed a very large
number of minerals, evolve hydrogen when heated,
the gas having been " occluded " in their pores.

In 1772 he communicated privately to Dr.
Priestley the results of a series of experiments
dealing with nitrogen. To prepare it he passed air
repeatedly over red-hot charcoal, and absorbed the
resulting carbon dioxide in potash. The residue was
nitrogen. His description of it is " The specific
gravity of this air was found to differ very little
from that of common air; of the two it seemed
rather lighter. It extinguished flame, and ren-
dered common air unfit for making bodies burn in
the same manner as fixed air, but in a less degree,
as a candle which burned about 80 seconds in pure
common air, and which went out immediately in
common air mixed with ^ths of fixed air, burned
about 26 seconds in common air mixed with the
same proportion of this burnt air." 1 He named it,
as usual, " mephitic air," and it is certain that,
although Cavendish did not publish his results, his

1 Brit. Assoc. Report, 1839, p. 64.



126 THE GASES OF THE ATMOSPHERE CHAP.

discovery was not later in date than Rutherford's.
Dealing next with the phenomena observed when
that curious fish, the torpedo, produces shocks, he
ascribed them to the discharge of electricity, and
he was the first to distinguish between intensity, or
potential, and quantity of electricity, a distinction
now familiar to all.

It was in 1777 that he commenced his beautiful
" Experiments on Air," the first account of which
was published in 1783. They led to the discovery
of the constant quantitative composition of the
atmosphere, of the compound nature of water, and
of the composition of nitric acid, and pointed the
way to the recent discovery of argon.

In determining the composition of the atmo-
sphere Cavendish made use of nitric oxide in
presence of water, as a means of removing oxygen.
This process, originally devised by Mayow, was
rediscovered by Priestley, who employed it to
ascertain the "goodness" of various samples of
air ; in Cavendish's hands it became an accurate
quantitative method. The title of his paper, pub-
lished in the Philosophical Transactions for 1783,
is "Of a new Eudiometer." The term " eudio-
meter," signifiying "measurer of goodness," was



iv PHLOGISTICATED AIR 127

devised when it was supposed that ordinary air
presented considerable variations in its power of
supporting respiration and combustion, according
to the seasons, and according to the place from
which it was collected. Dr. Ingenhousz had found
a greater absorption when air from near the sea-
coast was tested by Priestley's method with nitric
oxide, than when town-air was employed ; and he
ascribed the salubrious nature of sea-air to its
being richer in " vital air." The Abbe* Fontana,
too, had made similar experiments, and had come
to similar conclusions. Cavendish modified Fon-
tana's apparatus, rendering it capable of giving
more accurate results ; and during the last half of
the year 1781 he analysed the air collected on
sixty days, some fine, some wet, and some foggy.
He also collected air from different localities, some-
times at Marlborough Street, sometimes at Ken-
sington, which was then a country village. The
results of his analyses establish as the com-
position of air, freed from carbon dioxide by
potash :

7 9 '16 per cent of phlogisticated air (nitrogen).

20*84 per cent of dephlogisticated air (oxygen).

This result does not differ materially from those



128 THE GASES OF THE ATMOSPHERE CHAP.

obtained by the best modern analyses, which give,
within very small variations :

79*04 per cent of nitrogen, argon, etc.,

20 '9 6 per cent of oxygen,

after absorption of carbon dioxide, ammonia, and
water-vapour.

In the following year, 1784, Cavendish pub-
lished the first of his great memoirs, entitled Ex-
periments on Air. His experiments were made
principally " with a view to find out the cause of
the diminution which common air is well known
to suffer by all the various ways in which it is
phlogisticated, and to discover what becomes of
the air thus lost or condensed."

Cavendish chose processes for " phlogisticating "
air in the course of which no fixed air should be pro-
duced. He therefore avoided the use of animal and
vegetable materials, and confined himself to com-
bustibles, such as sulphur or phosphorus, to the
calcination of metals, the explosion of inflammable
air, and the admixture of nitrous air. He adds as a
suggestion, " Perhaps it may be supposed that I
ought to add to these the electric spark ; but I
think it much more likely that the phlogistication
of the air, and production of fixed air, in this



iv PHLOGISTICATED AIR 129

process is owing to the burning of some inflam-
mable matter in the apparatus." We shall see
later what magnificent results arose from this last
mode of " phlogisticating " air.

He begins with an account of a repetition of an
experiment of Mr. Warltire's, related by Priestley,
in which a mixture of hydrogen and air was ex-
ploded in a copper vessel, with the result that they
observed a loss of a few grains in weight ; it is also
stated by Warltire that if the explosion took place
in a glass vessel, it became dewy, " which confirmed
an opinion he had long entertained, that common
air deposits its moisture by phlogistication." But
Cavendish, using a glass vessel of much greater
capacity than Warltire's, could remark no change
of weight; and he concluded that 423 measures of
hydrogen, or " inflammable air" as he named it, are
"nearly sufficient to completely phlogisticate 1000
of common air, and that the bulk of the air remain-
ing after the explosion is then very little more than
f ths of the common air employed ; so that, as
common air cannot be reduced to a much less bulk
than that, by any method of phlogistication, we
may safely conclude that, when they are mixed in
this proportion and exploded, almost all the in-



130 THE GASES OF THE ATMOSPHERE CHAP.

flammable air, and about ^th part of the common
air, lose their elasticity, and are condensed into the
dew which lines the glass.

" The better to examine the nature of this
'dew/ 500,000 grain measures of inflammable air
were burnt with about 2^ times that quantity of
common air, and the burnt air made to pass through
a glass cylinder 8 feet long and about f of an inch
in diameter, in order to deposit the dew." "By
this means upwards of 135 grains of water were
condensed in the cylinder, which had no taste or
smell, and which left no sensible sediment when
evaporated to dryness, neither did it yield any
pungent smell during the evaporation ; in short, it
seemed pure water." " And by this experiment it
appears that this dew is plain water, and conse-
quently that almost all the inflammable and about
Jth of the common air are turned into pure
water."

But on firing little by little a mixture of " de-
phlogisticated air" or oxygen, obtained from red
precipitate (that is, mercuric oxide prepared by
heating the nitrate), with twice its volume of " in-
flammable air" or hydrogen, the resulting water
was acid to the taste, and on evaporation with



iv PHLOGISTICATED AIR 131

alkali gave a small quantity about 2 grains of
nitre. Cavendish suspected that the acid came
from the nitrate of mercury in his red precipitate,
and, to test this, procured his oxygen from other
sources from red-lead and sulphuric acid, and from
the leaves of plants but still with the same result :
nitric acid was formed. Kepeating the experiment
so as to have present an excess of hydrogen, he
found that no acid was produced.

"From the foregoing experiments it appears
that when a mixture of inflammable and dephlo-
gisticated air is exploded in such proportion that
the burnt air is not much phlogisticated, the con-
densed liquor contains a little acid, which is always
of the nitrous kind, whatever substance the de-
phi ogisticated air is procured from ; but if the
proportion be such that the burnt air is almost
entirely phlogisticated, the condensed liquor is not
at all acid, but seems pure water, without any
addition whatever ; and as, when they are mixed
in that proportion, very little air remains after the
explosion, almost the whole being condensed, it
follows that almost the whole of the inflammable
and dephlogisticated air is converted into pure
water." The quantity of uncombined gas was so



132 THE GASES OF THE ATMOSPHERE CHAP.

small that it must be regarded as an impurity.
" There can be little doubt that it proceeds only
from the impurities mixed with the dephlogisticated
and inflammable air, and consequently that if those
airs could be obtained perfectly pure, the whole
would be condensed."

The next paragraph is interesting. "During
the last summer also [of 1781] a friend of mine
[Sir Charles Blagden ; see p. 112] gave some
account of them [these experiments] to Mr.
Lavoisier, as well as of the conclusion drawn
from them, that dephlogisticated air is only water
deprived of phlogiston ; but at that time, so far
was Mr. Lavoisier from thinking any such opinion
warranted, that, till he was prevailed upon to repeat
the experiment himself, he found some difficulty
in believing that nearly the whole of the two airs
could be converted into water."

And next comes an important deduction.
" Phlogisticated air appears to be nothing else than
the nitrous acid united to phlogiston ; for when
nitre is deflagrated with charcoal, the acid is almost
entirely converted into this kind of air." This is
the first statement of the true relation between
nitrogen and nitric acid ; we should now state the



iv PHLOGISTICATED AIR 133

matter by the expression, "Nitrogen is nothing
else than nitric acid deprived of oxygen." And
the further deduction is made that "it is well
known that nitrous acid is also converted by phlo-
gistication into nitrous air, in which respect there
seems a considerable analogy between that and
the vitriolic acid; for this acid, when united to
a smaller proportion of phlogiston, forms the
volatile sulphurous acid and vitriolic acid air, both
of which, by exposure to the atmosphere, lose their
phlogiston, though not very fast, and are turned
back into the vitriolic acid ; but when united to a
greater proportion of phlogiston, it forms sulphur,
which shows no signs of acidity." " In like
manner the nitrous acid, united to a certain
quantity of phlogiston, forms nitrous acid and
nitrous air, which readily quit their phlogiston to
common air ; but when united to a different, in all
probability a larger quantity, it forms phlogisticated
air, which shows no signs of acidity, and is still less
disposed to part with its phlogiston than sulphur."
But the origin of the acid in water made from
inflammable and dephlogisticated air was still un-
explained. To settle this point Cavendish added
to an explosive mixture of oxygen and hydrogen a



134 THE GASES OF THE ATMOSPHERE CHAP.

tenth of its volume of nitrogen, and found that the
water was much more strongly acid ; and if hydro-
gen was much in excess, a still greater amount
of nitric acid was produced. After relating these
experiments he proceeds :

"From what has been said there seems the
utmost reason to think that dephlogisticated air
is only water deprived of its phlogiston, and that
inflammable air, as was before said, is either phlo-
gisticated water or else pure phlogiston, but in all
probability the former." In a footnote he gives
his reason for the choice, viz. that it requires a red-
heat to cause hydrogen and oxygen to combine,
while nitrous air combines with oxygen at the
ordinary temperature ; now, if hydrogen were pure
phlogiston, one would expect it to combine more
readily than nitrous gas, which has been shown to
be a compound of nitric acid with phlogiston. It
seems inexplicable that dephlogisticated air should
refuse to unite at the ordinary temperature with
pure phlogiston, when it is able to extract it from
substances with which it has an affinity. Hence
it is unlikely that hydrogen is phlogiston itself.

And a few paragraphs farther on Cavendish
very nearly discards the phlogistic theory by this



iv PHLOGISTICATED AIR 135

statement : " Instead of saying air is phlogisticated
or dephlogisticated by any means, it would be more
strictly just to say, it is deprived of, or receives, an
addition of dephlogisticated air; but as the other
expression is convenient, and can scarcely be con-
sidered as improper, I shall still frequently make
use of it in the remainder of this paper."

And now we come to the consideration of
Lavoisier's new theory, and its rejection in favour
of the old one of phlogiston. It is curious to follow
the reasoning which made such an exceptionally
acute thinker as Cavendish deliberately reject the
true explanation. Cavendish first states his results
in Lavoisier's terms :

" According to this hypothesis, we must suppose
that water consists of inflammable air united to
dephlogisticated air ; that nitrous air, vitriolic acid
air (sulphur dioxide), and the phosphoric acid are
also combinations of phlogisticated air, sulphur, and
phosphorus with dephlogisticated air ; and that the
two former, by a further addition of the same
substance, are reduced to the common nitrous and
vitriolic acids ; that the metallic calces consist of
the metals themselves united to the same sub-
stance, commonly, however, with a mixture of



136 'THE GASES OF THE ATMOSPHERE CHAP.

fixed air ; that on exposing the calces of the
perfect metals to sufficient heat, all the dephlo-
gisticated air is driven off, and the calces are
restored to their metallic form ; but as the calces
of the imperfect metals are vitrified by heat,
instead of recovering the metallic form, it should


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