saltpetre, in which the potash is replaced by some unknown earth.
And in speculating on the manner in which saltpetre is formed,
he enunciates the hypothesis, "that nitre is, formed by a real
_decomposition of the air itself_, the _bases_ that are presented to
it having, in such circumstances, a nearer affinity with the spirit
of nitre than that kind of earth with which it is united in the
atmosphere." [11]

It would have been hard for the most ingenious person to have wandered
farther from the truth than Priestley does in this hypothesis; and,
though Lavoisier undoubtedly treated Priestley very ill, and pretended
to have discovered dephlogisticated air, or oxygen, as he called it,
independently, we can almost forgive him when we reflect how different
were the ideas which the great French chemist attached to the body
which Priestley discovered.

They are like two navigators of whom the first sees a new country, but
takes clouds for mountains and mirage for lowlands; while the second
determines its length and breadth, and lays down on a chart its exact
place, so that, thenceforth, it serves as a guide to his successors,
and becomes a secure outpost whence new explorations may be pushed.

Nevertheless, as Priestley himself somewhere remarks, the first object
of physical science is to ascertain facts, and the service which he
rendered to chemistry by the definite establishment of a large number
of new and fundamentally important facts, is such as to entitle him to
a very high place among the fathers of chemical science.

It is difficult to say whether Priestley's philosophical, political,
or theological views were most responsible for the bitter hatred which