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Updated: May 25, 2025
To the very last, after all his friends had capitulated, Priestley kept up the fight. From America he sent out his last defy to the enemy, in 1800, in a brochure entitled "The Doctrine of Phlogiston Upheld," etc. In the mind of its author it was little less than a paean of victory; but all the world beside knew that it was the swan-song of the doctrine of phlogiston.
As this new air thus appeared to be completely free from phlogiston, Priestley called it "dephlogisticated air." What was the nature of this air? Priestley's view, in fact, is that atmospheric air is a kind of saltpetre, in which the potash is replaced by some unknown earth.
This theory reveals the great uncertainty into which man's thinking about the world of the senses had arrived at that time. Clinging to ideas inherited from antiquity, man's consciousness was already so far restricted to the forming of pure matter-bound concepts that he was tempted to conceive heat as a material element. To this heat-substance the name 'phlogiston' was given.
But far from finding that phlogiston or any other substance had been driven off, he found that something had been taken on: that the metal "absorbed air," and that the increased weight of the metal corresponded to the amount of air "absorbed." Meanwhile he was within grasp of two great discoveries, that of oxygen and of the composition of the air, which Priestley made some two years later.
By these experiments the theory of Phlogiston was destroyed; and with its destruction, the whole alchemical apparatus of Principles and Elements, Essences and Qualities, Souls and Spirits, disappeared.
Yet it may not be altogether amiss to reflect that the physicist of to-day is no more certain of his ether than was his predecessor of the eighteenth century of the existence of certain alleged substances which he called phlogiston, caloric, corpuscles of light, and magnetic and electric fluids.
It was spreading, apparently just under the phlogiston layer, reflecting back the glare. And the wind was caused by the great rising column of superheated gases over the sun. He staggered on, while the sand gave way slowly to patches of green. With the sun gone and the sky falling into complete shreds, this world was certainly doomed.
The foundation for such a theory was finally laid, as we shall see presently, by the work of Black, Priestley, Cavendish, and Lavoisier, in the eighteenth century, but the phlogiston theory cannot be said to have finally succumbed until the opening years of the nineteenth century.
Even with much heat, it could not be done. For see you, the upper air is filled with phlogiston, which no man can breathe. Also, the phlogiston has negative weight, as every school child must know. Your liquid sky would sink through it, since negative weight must in truth be lighter than no weight, while nothing else would rise through the layer.
It led away from the idea of a "spirit" in metals that could not be seen, felt, or appreciated by any of the senses, and substituted for it a principle which, although a falsely conceived one, was still much more tangible than the "spirit," since it could be seen and felt as free phlogiston and weighed and measured as combined phlogiston.
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