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seem as if all the dephlogisticated air could not
be recovered from them by heat alone. In like
manner, according to this hypothesis, the rationale
of the production of dephlogisticated air from red
precipitate is, that during the solution of the
quicksilver in the acid and the subsequent cal-
cination, the acid is decompounded, and quits part
of its dephlogisticated air to the quicksilver, whence
it comes over in the form of nitrous air, and leaves
the quicksilver behind united to dephlogisticated
air, which, by a further increase of heat, is driven
off, while the quicksilver resumes its metallic form.
In procuring dephlogisticated air from nitre, the
acid is also decompounded ; but with this differ-
ence, that it suffers some of its dephlogisticated air
to escape, while it remains united to the alkali
itself in the form of phlogisticated nitrous acid.
As to the production of dephlogisticated air from
plants, it may be said that vegetable substances


consist chiefly of three different bases, one of
which [hydrogen], when united to dephlogisticated
air, forms water ; another [carbon] fixed air ; and
the third phlogisticated air [nitrogen] ; and that,
by means of vegetation, each of these substances
are decomposed, and yield their dephlogisticated
air ; and that, in burning, they again acquire
dephlogisticated air, and are restored to their
pristine form.

" It seems, therefore, from what has been
said, as if the phenomena of nature might be
explained very well on this principle, without the
help of phlogiston ; and indeed, as adding de-
phlogisticated air to a body comes to the same
thing as depriving it of its phlogiston and adding
water to it, and as there are perhaps no bodies
destitute of water, and as I know no way by
which phlogiston may be transferred from one
body to another, without leaving it uncertain
whether water is not at the same time transferred,
it will be very difficult to determine by experiment
which of these opinions is the truest ; but as the
commonly -received principle of phlogiston explains
all phenomena, at least as well as Mr. Lavoisier's,
I have adhered to that."


" Another thing which Mr. Lavoisier endeavours
to prove is that dephlogisticated air is the acidify-
ing principle. From what has been explained, it
appears that this is no more than saying that acids
lose their acidity by uniting to phlogiston, which,
with regard to the nitrous, vitriolic, phosphoric,
and arsenical acids, is certainly true." " But as to
the marine acid and acid of tartar, it does not
appear that they are capable of loosing their acidity
by any union with phlogiston."

Here Cavendish does not consider the question
of gain of weight on loss of phlogiston, or if he does,
he must ascribe it to simultaneous entry of water.
And experimental research at that time was not
far enough advanced to enable him to decide finally
as to the truth of this hypothesis.

In his next memoir, read before the Koyal
Society on June 2nd, 1785, Cavendish relates
experiments on the passage of electric sparks
through air, the experiment having first been tried
by Priestley. Priestley says : l " Lastly, the same
effect [i.e. the diminution of the volume of
common air], I find, is produced by the electric

1 Experiments and Observations on Different Kinds of Air, vol. i. p.
181, and vol. ii. p. 238. Second edition, 1776.


spark, though I had no expectation of this event
when I made the experiment." And again : " At
the time of my former publication, I had found
that taking the electric spark in given quantities
of several kinds of air had a very remarkable effect
on them ; that it diminished common air and made
it noxious, making it deposit its fixed air exactly
like any phlogistic process ; from whence I con-
cluded that the electric matter either is or contains

Cavendish had mentioned this process casually
as one of the methods of phlogisticating air ; in
beginning his second paper he says : " I now find
that though I was right in supposing the phlogisti-
cation of the air does not proceed from phlogiston
communicated to it by the electric spark, and
that no part of the air is converted into fixed
air, yet that the real cause of the diminution
is very different from what I suspected, and
depends upon the conversion of phlogisticated air
into nitrous acid." The apparatus he used was
very simple. It consisted of a glass siphon filled
with mercury, each leg dipping into a glass likewise
containing mercury ; the air was admitted by a gas-
pipette into the bend of the siphon, and on con-


necting the mercury in one of the glasses with a
ball placed near the prime conductor of an
electric machine, and the other with earth,
sparks could be made to pass from the mercury in
one limb to that in the other.

The product obtained by passing sparks through
air in this manner turned litmus red, and gave rise
to no cloud in lime-water, while the air was reduced
to two-thirds of its original volume; nor did the lime-
water give a precipitate on introducing some fixed
air, thus showing that it had been saturated by an
acid. It was found, too, that " soap-lees," or solution
of caustic potash, if present, diminished the volume
more rapidly than did lime-water; and repeated trials
proved that " when five parts of pure dephlogisti-
cated air were mixed with three parts of common air,
almost the whole of the air was made to disappear."
The nitrate of potassium thus produced caused paper
soaked in it and dried to deflagrate ; and it con-
tained no suphuric acid, " There is no reason to
think that any other acid entered into it except
the nitrous." But it gave a precipitate with silver
nitrate ; and Cavendish, suspecting that this was
silver nitrite, prepared some potassium nitrite by
heating the nitrate ; on comparing the white


precipitate which this solution gave with silver
nitrate with that obtained from his " soap-lees," he
found them identical. There was therefore no
" muriatic acid " present, which would have yielded
chloride of silver, of appearance somewhat similar
to the nitrite.

As it had previously been shown to be probable
that phlogisticated air is nitrous air united with
phlogiston, and that nitrous air is nitric acid united
with phlogiston, " we may safely conclude that in
the present experiments the phlogisticated air was
enabled, by means of the electric spark, to unite to,
or form a chemical combination with, the dephlo-
gisticated air, and was thereby reduced to nitrous
acid, which united to the soap-lees and formed a
solution of nitre ; for in these experiments the two
airs actually disappeared, and nitrous acid was
actually formed in their room." " A further con-
firmation of the above-mentioned opinion is that,
as far as I can perceive, no diminution of air is
produced when the electric spark is passed either
through pure dephlogisticated air or through
perfectly phlogisticated air, which indicates a
necessity of a combination of these two airs to
produce the acid. Moreover, it was found in the


last experiment that the quantity of nitre procured
was the same that the soap-lees would have pro-
duced if saturated with nitrous acid ; which shows
that the production of the nitre was not owing
to any decomposition of the soap-lees."

Nothing more clearly shows the care with which
Cavendish reasoned than these last quotations. No
loophole is left unstopped ; every precaution is
taken to make the proof as faultless as it is
possible for a proof to be.

But this was not enough. It was necessary for
Cavendish to show that, so far as he could ascer-
tain it experimentally, all the phlogisticated air
was capable of combining with dephlogisticated air
to form nitre. This he next proceeded to do.

" As far as the experiments hitherto published
extend, we scarcely know more of the phlogisti-
cated part of our atmosphere than that it is not
diminished by lime-water, caustic alkalies, or
nitrous air ; that it is unfit to support fire or
maintain life in animals ; and that its specific
gravity is not much less than that of common air ;
so that though the nitrous acid, by being united to
phlogiston, is converted into air possessed of these
properties, and consequently, though it was reason-


able to suppose that part at least of the phlogisti-
cated air of the atmosphere consists of this acid
united to phlogiston, yet it might fairly be doubted
whether the whole is of this kind, or whether there
are not in reality many different substances con-
founded together by us under the name of phlo-
gisticated air. I therefore made an experiment to
determine whether the whole of a given portion of
the phlogisticated air of the atmosphere could be
reduced to nitrous acid, or whether there was not
a part of a different nature from the rest, which
would refuse to undergo that change. The fore-
going experiments, indeed, in some measure decided
this point, as much the greatest part of the air let
up into the tube lost its elasticity ; yet, as some
remained unabsorbed, it did not appear for certain
whether that was of the same nature as the rest
or not. For this purpose I diminished a similar
mixture of dephlogisticated and common air in the
same manner as before, till it was reduced to a
small part of its original bulk. I then, in order to
deconipound as much as I could of the phlogisti-
cated air which remained in the tube, added some
dephlogisticated air to it, and continued the spark
until no further diminution took place. Having


by these means condensed as much as I could of
the phlogisticated air, I let up some solution of liver
of sulphur to absorb the dephlogisticated air ; after
which only a small bubble of air remained unab-
sorbed, which certainly was not more than xiu^ n OI>
the bulk of the phlogisticated air let up into the
tube ; so that, if there is any part of the phlogisti-
cated air of our atmosphere which differs from the
rest, and cannot be reduced to nitrous acid, we may
safely conclude that it is not more than x|~oth part
of the whole." We shall afterwards see that this
is a marvellously close estimate. There is actually
^jth part of the supposed nitrogen of the air which
will not combine with oxygen when sparked with
it in presence of potash.

But there still remained, in Cavendish's opinion,
one point unproved. It was still conceivable that the
potash might contain some "inflammable matter"
which would diminish the air on sparking, and there-
fore oxygen nearly pure was sparked in presence of
potash; but only a very small diminution of volume
occurred, owing probably to some nitrogen present
as an impurity in the oxygen. Water was sub-
stituted for potash with the same result ; but if
litmus was added to the water the colour was dis-


charged, and lime-water introduced into the tube
gave a cloud, showing that "the litmus, if not
burnt, was at least decompounded, so as to lose
entirely its purple colour and to yield fixed air ; so
that, though soap-lees cannot be decompounded by
the process, yet the solution of litmus can, and so
very likely might the solutions of many other com-
bustible substances."

Such are the chemical researches of Cavendish.
Of all experimenters on the subject he was un-
doubtedly the greatest, though Mayow and Scheele
were near rivals. But his researches were so com-
plete that it is scarcely possible to criticise. He
was not content with partial results : every point
was proved and re-proved, and every possibility
of erroneous conclusion was allowed for. It is
curious that he did not employ the balance to
check his results. Had he done so he could not
have remained an adherent of the phlogistic theory.
Although, as we have seen, he was perfectly
acquainted with the method in which his results
were interpreted by Lavoisier, he chose the old
well-trodden path leading to the wilderness of dis-
torted facts. Lavoisier tried to repeat Cavendish's
experiments, but without success ; but an account


is to be found in the last part of his Experiments
on Air, published in 1788, of the successful re-
petition by a Committee of the Koyal Society of
the conversion of nitrogen into nitric acid by the
electric spark in presence of oxygen and potash.

His remaining papers deal with meteorological
and astronomical subjects. One, published in
1790, refers to the height of a remarkable aurora
seen in 1784; another to the civil year of the
Hindoos ; and another to a method for reducing
lunar distances. And in 1798 his famous memoir
on the density of the earth appeared. It would be
quite beyond the province of this book to enter
into any detail regarding it ; but it may be re-
marked in passing that the method consisted in
measuring, by means of a torsion balance, the
attraction of one leaden ball for another, and that
recent experiments, made with the utmost refine-
ment, have barely altered the number which he
obtained, 5*4, to 5*527.

His last paper, on an improvement in a machine
for dividing astronomical instruments, was published
in 1809, the year before his death.

Nothing has been said as yet regarding the rival
claims of Watt to the discovery of the composition


of water, and little need be said. The discovery
was made by both in 1784, yet Cavendish visited
Watt at Birmingham, in 1785, and was apparently
on the best of terms with him ; and Watt, as proved
by Cavendish's diary, showed him many of his
devices connected with the steam-engine. There
can be no doubt that Watt had also discovered
that when hydrogen and oxygen are exploded
together water is the sole product, but he coupled
the phenomenon with views involving the material
nature of heat, or caloric, as it was then called,
which Cavendish repudiated.

Cavendish's later work was carried out in a villa
at Clapham, which was fitted as a laboratory, work-
shop, and observatory, but he had a town-house near
the British Museum, at the corner of Gower Street
and Montague Place. He had also a library in
Dean Street, Soho, which was available for any
scientific man who chose to present himself. So
singular were Cavendish's habits that when he
wished a book he went to this house and borrowed
it as from a public library, giving a receipt for it.

Of all men, Cavendish was probably the most
singular, but there can be no question of his extra-
ordinary genius.



WITH the advent of Lavoisier's system of repre-
senting the phenomena of combustion, and the
expression in his terms of the various changes
resulting in air when metals are oxidised, and when
carbonaceous substances burn, the investigation
of air was abandoned. It was no longer regarded
as a mysterious element, possessed of " chaotic "
properties, but was held to be a mixture of oxygen,
nitrogen, and small quantities of carbon dioxide and
water vapour, together with a trace of ammonia.
More exact determinations of the proportion be-
tween its oxygen and so-called nitrogen than Caven-
dish had made by the nitric-oxide method were
carried out in 1804 by Gay-Lussac and Humboldt,
by explosion with measured quantities of hydrogen,
according to the method suggested by Volta ; and

they concluded, from a large number of analyses



made on specimens collected in all weathers and
from various localities, that 100 volumes of air
contained 21 volumes of oxygen and 79 volumes of
nitrogen. These experiments, too, led Gay-Lussac
to the conviction that oxygen and hydrogen unite
to form water in the exact proportion of one
volume of the former to two volumes of the latter ;
and he published, some years later, accounts of
numerous experiments of the same kind, as the
result of which he found that, when two gases com-
bine or react with each other, they do so in some
simple number of volumes ; for example, one to
one, one to two, or one to three.

The almost constant relation between the
volumes of oxygen and nitrogen in air made it
appear not unlikely, in the opinion of some, that
air was a compound, and not a mixture ; for the law
of combination in definite proportions had by this
time been enunciated by Professor Thomas Thom-
son, Dalton's intimate friend. But between the
numbers 21 and 79 there exists no such simple ratio;
and, moreover, on artificially producing air by mixing
oxygen and nitrogen, there are none of the usual
phenomena which characterise the formation of a
compound : there is no rise or fall of temperature,


nor does the product differ in any way in properties
from the constituents. And in 1846 Bunsen showed
that the proportion between oxygen and nitrogen is
not a constant one, but that the oxygen varies be-
tween 20*97 and 20'84 ; the experimental error did
not exceed 0*03 volume, while the difference found
amounted to 0*13 volume. Regnault, Angus Smith,
A. K. Leeds, and von Jolly confirmed these re-
sults at later dates, from analyses of air collected
from all parts of the world.

That air contains ammonia was first observed
by Scheele. He found that the stopper of a bottle
containing muriatic acid, when exposed to air,
became covered with a film or deposit which he
recognised to be sal ammoniac, or ammonium

The amount of ammonia in atmospheric air is,
however, exceedingly small, and it is best detected
in rain-water, which dissolves it ; thus the air is
considerably poorer in ammonia after a shower.
The ammonia, small though its proportion is,
plays a great part, although not an exclusive one,
in yielding to plants their supply of nitrogen. The
rain, percolating through the soil, leaves the am-
monia behind, in some form of combination ; and


it is then attacked by the nitrifying ferments and
converted into nitrates, from which the plants
derive the nitrogen which forms part of their sub-
stance, in combination with carbon, oxygen, and

Ihere are also traces of nitric and nitrous acids
in a.r, which are apparently in combination with
ammonia. While the ammonia has been found to
vary between 0*1 and 100 volumes per million
volunes of air the latter number refers to Man-
chester streets nitrous and nitric acids are present
in stil smaller amounts ; and in spite of the wide-
spread opinion that ozone is contained in air, its
occurience is still a matter of dispute. That some
powerul oxidising agent such as ozone or hydrogen
peroxide is present appears certain ; but the char-
acteristic test for ozone the formation of peroxide of
silver m exposure of metallic silver to its influence
has never been successful. On the other hand,
a small quantity of hydrogen dioxide also, like
water, a compound of oxygen and hydrogen, but
one containing more oxygen than water appears
to be ilmost constantly present in air. Its amount
is als) extremely minute : it does not exceed
one pert per million. Its presence in air was dis-


covered by Schonbein. The atmosphere further
contains dust, some of which appears to consist
largely of metallic iron, which is conjectured to be
of extraterrestrial origin minute meteorites in feet
and also the spores of micro-organisms ; but tlese
spores, however important from a biological or a
sanitary point of view, hardly come within the
scope of the chemical composition of air. They
serve to emphasise the conjectures of Boyle ard of
Scheele that air may contain " corpuscles " rf all
sorts, some in the form of dry exhalations, vhile
other innumerable particles may be sent out from
the celestial luminaries. But it has been bund
that air contains corpuscles of another natur3, the
consideration of which will come later.

Up to within the last few years it , was su|posed
that the constituents of air had all been discovered.
But Lord Kayleigh and Sir William Kamsay found
that the supposed nitrogen of the air is in jeality
a mixture of nitrogen with a new gaseous element,
to which they give the name " argon," on account
of its chemical inactivity (apyov, idle, inactive).

In his presidental address to Section A )f the
British Association at Southampton in 1882 Lord
Rayleigh alluded to an investigation which le had


begun on the densities of hydrogen and oxygen,
relatively to each other. The object of the research
was to discover whether the atomic weights of
these gases, determinable from their densities and
from the proportions by volume in which they com-
bine, was actually as 1 to 16, or whether some
fractional number was necessary to express the
weight of an atom of oxygen relatively to that
of hydrogen. In 1888 his first account of the
determination was published in the Proceedings
of the Eoyal Society. In 1889 he published a
continuation of his first paper, and in 1892 he
gave his final results ; the number obtained
was 15*882 for the atomic weight of oxygen,
calculated from its density, hydrogen being taken
as 1. In 1893 further experiments on densities
were published, 1 those of oxygen and nitrogen
being specially considered with reference to the
density of air. He found the weights of one litre
of oxygen, nitrogen, and air, at normal temperature
and pressure to be

Oxygen .... 1*42952 grams
"Nitrogen". . . .1-25718
Air 1-29327

1 Proc. Roy. Soc. vol. liii. p. 134.


A simple calculation leads to the composition
of purified air. The percentage of oxygen must be
20-941, and that of "nitrogen" 79'059, in order to
give a mixture of which the weight of a litre is
1 '29327. Now this corresponds with the results of
the best analyses, quoted on p. 1 28. And the accuracy
of these determinations of density is confirmed
by this means, as well as by results of other
experiments made by Leduc, von Jolly, and

But Lord Rayleigh was not content to prepare
his gases by one process only. The oxygen, of
which the mean value of the weight of a litre is
given above, was prepared in three different ways : by
the electrolysis of water, by heating chlorates, and
by heating potassium permanganate. The results
showed that the only difference which could be
detected, and that an extremely minute one, must
be attributed to experimental error. The actual
weights of the contents of his globe were

Electrolysis, May 1892 . . *, 2 '62 72 grams

,. - 2-6271

Heating chlorates, May 1892. . 2*6269

June . 2-6269

Heatin g permanganate, January 1893 2*6271

These numbers are subject to a deduction of


0*00056, due to the fact that when the globe was
empty of air, its capacity was somewhat reduced,
owing to the external pressure of the atmosphere.

It was next deemed necessary to test whether
nitrogen was homogeneous by preparing it too by
several different methods. In the same paper Lord
Rayleigh (p. 146) mentions that nitrogen, prepared
from ammonia, its compound with hydrogen, is
somewhat lighter than " atmospheric nitrogen,"
the deficiency in weight amounting to about 1
part in 200. Now it is evident from inspection
of the numbers quoted above that the accuracy
of the density determination may be trusted to
within 1 part in 10,000, and that the balance would
detect a discrepancy one-fiftieth of that observed
in the densities of " atmospheric " and " chemical "
nitrogen. In a letter to Nature, Lord Rayleigh
asked for suggestions from chemists as to the
reason of this curious anomaly, but his letter went
without reply. He himself was inclined to believe
that the difference was due to the decomposition of
some of the ordinary molecules of nitrogen, usually
believed to consist of two atoms in union with
each other, in molecules consisting of one atom
and as it is held that equal numbers of molecules


inhabit the same volume, temperature and pressure
being equal, if the total number of molecules in his
globe were increased by the splitting of some double
atom molecules into single-atom molecules, the effect
would be that, owing to an admixture of some lighter
molecules, the density would be somewhat reduced.
But two other suppositions were entertained as
possible. The oxygen might have been imperfectly

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Online LibraryWilliam RamsayThe gases of the atmosphere, the history of their discovery → online text (page 8 of 16)