A History of Science, vol 4 | Page 8

pure oxygen and
hydrogen, however, Cavendish found that pure water was formed,
leaving slight traces of any other, substance which might not be
interpreted as being Chemical impurities. There was only one possible
explanation of this phenomenon--that hydrogen and oxygen, when
combined, form water.
"By experiments with the globe it appeared," wrote Cavendish, "that
when inflammable and common air are exploded in a proper proportion,
almost all the inflammable air, and near one-fifth the common air, lose
their elasticity and are condensed into dew. And by this experiment it
appears that this dew is plain water, and consequently that almost all
the inflammable air is turned into pure water.
"In order to examine the nature of the matter condensed on firing a
mixture of dephlogisticated and inflammable air, I took a glass globe,
holding 8800 grain measures, furnished with a brass cock and an
apparatus for firing by electricity. This globe was well exhausted by an
air-pump, and then filled with a mixture of inflammable and
dephlogisticated air by shutting the cock, fastening the bent glass tube
into its mouth, and letting up the end of it into a glass jar inverted into
water and containing a mixture of 19,500 grain measures of
dephlogisticated air, and 37,000 of inflammable air; so that, upon
opening the cock, some of this mixed air rushed through the bent tube
and filled the globe. The cock was then shut and the included air fired
by electricity, by means of which almost all of it lost its elasticity (was
condensed into water vapors). The cock was then again opened so as to

let in more of the same air to supply the place of that destroyed by the
explosion, which was again fired, and the operation continued till
almost the whole of the mixture was let into the globe and exploded.
By this means, though the globe held not more than a sixth part of the
mixture, almost the whole of it was exploded therein without any fresh
exhaustion of the globe."
At first this condensed matter was "acid to the taste and contained two
grains of nitre," but Cavendish, suspecting that this was due to
impurities, tried another experiment that proved conclusively that his
opinions were correct. "I therefore made another experiment," he says,
"with some more of the same air from plants in which the proportion of
inflammable air was greater, so that the burnt air was almost
completely phlogisticated, its standard being one-tenth. The condensed
liquor was then not at all acid, but seemed pure water."
From these experiments he concludes "that when a mixture of
inflammable and dephlogisticated air is exploded, in such proportions
that the burnt air is not much phlogisticated, the condensed liquor
contains a little acid which is always of the nitrous kind, whatever
substance the dephlogisticated 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."[2]
These same experiments, which were undertaken to discover the
composition of water, led him to discover also the composition of nitric
acid. He had observed that, in the combustion of hydrogen gas with
common air, the water was slightly tinged with acid, but that this was
not the case when pure oxygen gas was used. Acting upon this
observation, he devised an experiment to determine the nature of this
acid. He constructed an apparatus whereby an electric spark was passed
through a vessel containing common air. After this process had been
carried on for several weeks a small amount of liquid was formed. This
liquid combined with a solution of potash to form common nitre, which
"detonated with charcoal, sparkled when paper impregnated with it was
burned, and gave out nitrous fumes when sulphuric acid was poured on

it." In other words, the liquid was shown to be nitric acid. Now, since
nothing but pure air had been used in the initial experiment, and since
air is composed of nitrogen and oxygen, there seemed no room to doubt
that nitric acid is a combination of nitrogen and oxygen.
This discovery of the nature of nitric acid seems to have been about the
last work of importance that Cavendish did in the field of chemistry,
although almost to the hour of his death he was constantly occupied
with scientific observations. Even in the last moments of his life this
habit asserted itself, according to Lord Brougham. "He died on March
10, 1810, after a short illness, probably the first, as well as the last,
which he ever suffered. His habit of curious observation continued to
the end. He was desirous of marking the progress of the disease and the
gradual extinction of the vital powers. With these ends in view,