To this hypothesis it may be objected, that, if diminished air be air saturated with phlogiston, it ought to be inflammable. But this by no means follows; since its inflammability may depend upon some particular mode of combination, or degree of affinity, with which we are not acquainted. Besides, inflammable air seems to consist of some other principle, or to have some other constituent part, besides phlogiston and common air, as is probable from that remarkable deposit, which, as I have observed, is made by inflammable air, both from iron and zinc.
It is not improbable, however, but that a greater degree of heat may inflame that air which extinguishes a common candle, if it could be conveniently applied. Air that is inflammable, I observe, extinguishes red-hot wood; and indeed inflammable substances can only be those which, in a certain degree of heat, have a less affinity with the phlogiston they contain, than the air, or some other contiguous substance, has with it; so that the phlogiston only quits one substance, with which it was before combined, and enters another, with which it may be combined in a very different manner. This substance, however, whether it be air or any thing else, being now fully saturated with phlogiston, and not being able to take any more, in the same circumstances, must necessarily extinguish fire, and put a stop to the ignition of all other bodies, that is, to the farther escape of phlogiston from them.
That plants restore noxious air, by imbibing the phlogiston with which it is loaded, is very agreeable to the conjectures of Dr. Franklin, made many years ago, and expressed in the following extract from the last edition of his Letters, p. 346.
"I have been inclined to think that the fluid fire, as well as the fluid air, is attracted by plants in their growth, and becomes consolidated with the other materials of which they are formed, and makes a great part of their substance; that, when they come to be digested, and to suffer in the vessels a kind of fermentation, part of the fire, as well as part of the air, recovers its fluid active state again, and diffuses itself in the body, digesting and separating it; that the fire so re-produced, by digestion and separation, continually leaving the body, its place is supplied by fresh quantities, arising from the continual separation; that whatever quickens the motion of the fluids in an animal, quickens the separation, and re-produces more of the fire, as exercise; that all the fire emitted by wood, and other combustibles, when burning, existed in them before in a solid state, being only discovered when separating; that some fossils, as sulphur, sea-coal, &c. contain a great deal of solid fire; and that, in short, what escapes and is dissipated in the burning of bodies, besides water and earth, is generally the air and fire, that before made parts of the solid."
FOOTNOTES:
[8] I conclude from the experiments of M. Lavoisier, which were made with a much better burning lens than I had an opportunity of making use of, that there was no real calcination of the metals, though they were made to fume in inflammable or nitrous air; because he was not able to produce more than a slight degree of calcination in any given quantity of common air.