The principle of phlogiston could be recognised by the senses as it was in the act of escaping from a substance; and the other principle of combustible things was scarcely a Principle in the alchemical sense, for, in the case of metals at any rate, it remained when the things which had contained it were burnt, and could be seen, handled, and weighed.

To say that metals are composed of phlogiston and an earthy substance, was to express facts in such a language that the expression might be made the basis of experimental inquiry; it was very different from the assertion that metals are produced by the spiritual actions of the three Principles, salt, mercury and sulphur, the first of which is not salt, the second is not mercury, and the third is not sulphur.
The followers of Stahl often spoke of metals as composed of phlogiston and an _element_ of an earthy character; this expression also was an advance, from the hazy notion of _Element_ in purely alchemical writings, towards accuracy and fulness of description.

An element was now something which could he seen and experimented with; it was no longer a semi-spiritual existence which could not be grasped by the senses.
The phlogistic theory regarded the calcination of a metal as the separation of it into two things, unlike the metal, and unlike each other; one of these things was phlogiston, the other was an earth-like residue.

The theory thought of the re-formation of a metal from its calx, that is, the earthy substance which remains after combustion, as the combination of two things to produce one, apparently homogeneous, substance.

Metals appeared to the phlogisteans, as they appeared to the alchemists, to be composite substances.