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,
that he might not be disturbed, he desired to be left alone. His
servant, returning sooner than he had wished, was ordered again
to leave the chamber of death, and when be came back a second