Mayow proved that the air wherein a substance is burnt, or an animal breathes, diminishes in volume during the burning, or the breathing.

He tried, without much success, to restore to air that part of it which disappears when combustion, or respiration, proceeds in it.
What happens when a substance is burnt in the air?
The alchemists answered this question by asserting that the substance is separated or analysed into things simpler than itself.

Boyle said: the process is not necessarily a simplification; it may be, and certainly sometimes is, the formation of something more complicated than the original substance, and when this happens, the process often consists in the fixation of "the matter of fire" by the burning substance.

Rey said: calcination, of a metal at anyrate, probably consists in the fixation of particles of air by the substance which is calcined.

Mayow answered the question by asserting, on the ground of the results of his experiments, that the substance which is being calcined lays hold of a particular constituent of the air, not the air as a whole.
Now, it is evident that if Mayow's answer was a true description of the process of calcination, or combustion, it should be possible to separate the calcined substance into two different things, one of which would be the thing which was calcined, and the other would be that constituent of the air which had united with the burning, or calcining, substance.