The phlogiston in a combustible substance was supposed to be so intimately associated with something else that our senses cannot perceive it; nevertheless, the theory said, it is there; we can see only the escaping phlogiston, we can perceive only the phlogiston which is set free from its combination with other things.

The theory thought of phlogiston as imprisoned in the thing which can be burnt, and as itself forming part of the prison; that the prisoner should be set free, the walls of the prison had to be removed; the freeing of the prisoner destroyed the prison.

As escaping, or free, phlogiston was called fire, or flame, so the phlogiston in a combustible substance was sometimes called combined fire, or flame in the state of combination.

A peculiarity of the strange thing called phlogiston was that it preferred to be concealed in something, hidden, imprisoned, combined; free phlogiston* was supposed to be always ready to become combined phlogiston.
*Transcriber's Note: Original "phlogstion".
The phlogistic theory said that what remains when a substance has been burnt is the original substance deprived of phlogiston; and, therefore, to restore the phlogiston to the product of burning is to re-form the combustible substance.

But how is such a restoration of phlogiston to be accomplished?
Evidently by heating the burnt thing with something which is very ready to burn.