independently and without ever having heard of the previous
discovery by Priestley. In this book, also, he shows that air is
composed chiefly of oxygen and nitrogen gas.

Early in his experimental career Scheele undertook the solution
of the composition of black oxide of manganese, a substance that
had long puzzled the chemists. He not only succeeded in this,
but incidentally in the course of this series of experiments he
discovered oxygen, baryta, and chlorine, the last of far greater
importance, at least commercially, than the real object of his
search. In speaking of the experiment in which the discovery was
made he says:

"When marine (hydrochloric) acid stood over manganese in the cold
it acquired a dark reddish-brown color. As manganese does not
give any colorless solution without uniting with phlogiston
[probably meaning hydrogen], it follows that marine acid can
dissolve it without this principle. But such a solution has a
blue or red color. The color is here more brown than red, the
reason being that the very finest portions of the manganese,
which do not sink so easily, swim in the red solution; for
without these fine particles the solution is red, and red mixed
with black is brown. The manganese has here attached itself so
loosely to acidum salis that the water can precipitate it, and
this precipitate behaves like ordinary manganese. When, now, the
mixture of manganese and spiritus salis was set to digest, there
arose an effervescence and smell of aqua regis."[6]

The "effervescence" he refers to was chlorine, which he proceeded
to confine in a suitable vessel and examine more fully. He