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Updated: May 4, 2025
On the other hand, Priestley found that air "phlogisticated" by combustion or respiration could be "dephlogisticated," or have the properties of pure common air restored to it, by the action of green plants in sunshine.
In respiration, part of this air, passing the membranes of the lungs, unites with the blood and imparts to it its florid color, while the remainder, uniting with phlogiston exhaled from venous blood, forms mixed air. It is dephlogisticated air combined with water that enables fishes to live in it."
Who can tell but that in time this pure air may become a fashionable article in luxury?... Perhaps we may from these experiments see that though pure dephlogisticated air might be useful as a medicine, it might not be so proper for us in the usual healthy state of the body." This suggestion as to the possible usefulness of oxygen as a medicine was prophetic.
Priestley obtained the same sort of air from red lead, but, as he says himself, he remained in ignorance of the properties of this new kind of air for seven months, or until March 1775, when he found that the new air behaved with "nitrous gas" in the same way as the dephlogisticated part of common air does; but that, instead of being diminished to four-fifths, it almost completely vanished, and, therefore, showed itself to be "between five and six times as good as the best common air I have ever met with."
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.
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.
Professor Snail adjusted his field-glass and examined the rampart critically. Finally he said: "'The fact that it is not diaphanous convinces me that it is a dense vapor formed by the calorification of ascending moisture dephlogisticated by refraction. A few endiometrical experiments would confirm this, but it is not necessary. The thing is obvious."
From these experiments he concludes "that when a mixture of inflammable and dephlogisticated air is exploded, in such proportions that the burnt air is not much phlogisticated, the condensed liquor contains a little acid which is always of the nitrous kind, whatever substance the dephlogisticated air is procured from; but if the proportion be such that the burnt air is almost entirely phlogisticated, the condensed liquor is not at all acid, but seems pure water, without any addition whatever."
He laid the foundations of gas analysis; he discovered the complementary actions of animal and vegetable life upon the constituents of the atmosphere; and, finally, he crowned his work, this day one hundred years ago, by the discovery of that "pure dephlogisticated air" to which the French chemists subsequently gave the name of oxygen.
Neon, helium, liquid air, liquid hydrogen these would seem strange terms to the men who on discovering oxygen and nitrogen named them "dephlogisticated air" and "phlogisti-cated air" respectively.
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