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Updated: May 25, 2025
Stahl, the author of this theory, asserted that there is a principle of inflammability, to which he gave the name phlogiston, having the quality of uniting with substances. Thus, when what we now term a metallic oxide was united to it, a metal was produced; and, if the phlogiston were withdrawn, the metal passed back into its earthy or oxidized state.
And now at last fire itself, the last of the four "elements" and the keystone to the phlogiston arch, was shown to be nothing more than one of the manifestations of the new element, oxygen, and not "phlogiston" or any other intangible substance.
Arguing thus, Priestley, of course, named the new aeriform substance dephlogisticated air, and thought of it as ordinary air deprived of some, or it might be all, of its phlogiston. The breathing of animals and the burning of substances were supposed to load the atmosphere with phlogiston.
It was from this aspect that he investigated the process of oxidation. What had already been observed by a few others, though without being taken seriously by them, he found confirmed that, contrary to the phlogiston theory, matter does not lose weight through oxidation but gains weight.
This circumstance is expressed in the common language by saying that a body burns ill or with difficulty." It needed the genius of such a man as Lavoisier to complete the refutation of the false but firmly grounded phlogiston theory, and against such a book as his Elements of Chemistry the feeble weapons of the supporters of the phlogiston theory were hurled in vain.
Priestley thought of properties in much the same way as the alchemists thought of them, as wrappings, or coverings of an essential something, from which they can be removed and around which they can again be placed. The protean principle of phlogiston was always at hand, and, by skilful management, was ready to adapt itself to any facts.
From first to last, he was a stiff adherent of the phlogiston doctrine which was prevalent when his studies commenced; and, by a curious irony of fate, the man who by the discovery of what he called "dephlogisticated air" furnished the essential datum for the true theory of combustion, of respiration, and of the composition of water, to the end of his days fought against the inevitable corollaries from his own labours.
The alchemists were dreamers rather than doers after all, and though it is the fashion to laud the doers it is often the dreamers that see most clearly. As the flame leapt upward from the burning wood they saw in it a rare, pure, ethereal substance which they called Phlogiston.
Priestley did not, however, go so far as this; he was content to suppose that in some way, which he did not explain, the process of calcination resulted in the loss of phlogiston by the mercury, and the gain, by the dephlogisticated mercury, of the property of yielding exceedingly pure or dephlogisticated air when it was heated very strongly.
Our author has written upon the subject of phlogiston; one would suppose that he should be well acquainted with inflammable bodies at least; let us see then what he has to observe upon that subject.
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