According to Diogenes Laertius and Hierocles, Pythagoras affirmed, that the air which invests our earth is impure and mixed; but that the air above it is essentially pure and healthful. He calls it “free ether, emancipated from all gross matter, a celestial substance that fills all space, and penetrates at will the pores of all bodies.”
Aristotle, explaining Pythagoras’s opinion of ether, ascribes the same also to Anaxagoras. Aristotle himself, in another place, understands by ether, a fifth element pure and unalterable, of an active and vital nature, but entirely different from air and fire.
Empedocles, one of the most celebrated disciples of Pythagoras, is quoted by Plutarch, and St. Clemens Alexandrinus, as admitting an ethereal substance, which filled all space, and contained in it all the bodies of the universe, and which he calls by the names of Titan and Jupiter.
Plato distinguishes air into two kinds, the one gross and filled with vapours, which is what we breathe; the other “more refined, called ether, in which the celestial bodies are immerged, and where they roll.”
The nature of air was not less known to the ancients than that of ether. They regarded it as a general “menstruum,” containing all the volatile parts of every thing in nature, which being variously agitated, and differently combined, produced meteors, tempests, and all the other changes we experience. They also were acquainted with its weight, though the experiments transmitted to us, relative to this, are but few. Aristotle speaks of “a vessel filled with air as weighing more than one quite empty.” Treating of respiration, he reports the opinion of Empedocles, who ascribes the cause of it “to the weight of the air, which by its pressure insinuates itself with force” into the lungs. Plutarch, in the same terms, expresses the sentiments of Asclepiades. He represents him, among other things, as saying, that “the external air by its weight opens its way with force into the breast.” Heron of Alexandria ascribes effects to the elasticity of the air, which show that he perfectly understood that property of it.
Seneca also knew its weight, spring, and elasticity. He describes “the constant effort it makes to expand itself when it is compressed;” and he affirms, that “it has the property of condensing itself, and forcing its way through all obstacles that oppose its passage.”
It is still more surprising, however, that Ctesibius, “upon the principle of the air’s elasticity,” invented Wind-guns, which we look upon as a modern contrivance. Philo of Byzantium gives a very full and exact description of that curious machine, planned upon the property of the air’s being capable of condensation, and so constructed as to manage and direct the force of that element, in such a manner as to carry stones with rapidity to the greatest distance.
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