iston, and consequently the
acid, which manifests itself as aerial acid, must be produced.”[21]
This is reversing what may be termed the true explanation on the basis
of the phlogistic theory. For Scheele supposes that oxygen contains
phlogiston, and by losing it, yields carbon dioxide. On the other hand,
the consistent explanation would be that carbon is carbonic acid plus
phlogiston, and that when it burns it loses phlogiston and becomes
carbonic acid again. We see how confused the phlogistic ideas became
after the discovery of oxygen, and how ripe the time was for Lavoisier
to formulate the views which are now universally accepted.

In the concluding sections of his treatise Scheele describes
experiments which prove the solubility of “fire-air” in water; he
mentions a convenient test for free oxygen in solution, viz. a mixture
of ferrous sulphate and lime, which turns dark green, and finally
rust-, when added to water containing oxygen; and he shows that
water is deprived of oxygen by the presence of a leech, kept in it for
two days.

It is impossible not to recognise in Scheele one of the most acute
intellects and able experimenters whom the world has ever seen. And
although we cannot but feel surprise that his discoveries did not lead
him to take the step of renouncing the hypothesis of phlogiston, it
must be borne in mind that the doctrine was surrounded with the halo of
old age, and sanctioned by many names of great repute in their time.
We shall see later that Cavendish, one of the greatest of English
chemists, on weighing the rival theories, decided in favour of the
phlogistic hypothesis. The actual escape of flame, a visible entity,
from a burning substance, may have had much to do with this decision;
and the uncertainty concerning the nature of heat, and the doubt
whether it was not a form of imponderable matter, may have led both
Scheele and Cavendish to retain the older views. It was Lavoisier who
first dared to throw off the shackles of tradition; and this he did
before oxygen had been discove