motions downward. But, besides these motions, there is motion in a circle, which is unnatural to these elements, but which is a more perfect motion than the other, because a circle is a perfect line, and a straight line is not; and there must be something to which this motion is natural. From this it is evident that there is some essence of body different from those of the four elements, more divine than those and superior to them. If things which move in a circle move contrary to nature, it is marvelous or rather absurd that this, the unnatural motion, should alone be continuous and eternal; for unnatural motions decay speedily. And so from all this we must collect that, besides the four elements which we have here and about us, there is another removed far off, and the more excellent in proportion as it is more distant from us." This element was called the quinta essentia by Latin writers, and the word quintessence in our own language frequently brings to mind this singular conception, which, although so absurd to us, held for ages a wonderful control over the human mind.
It is not, however, our purpose to trace the influence of the dynamical conceptions of Aristotle on the development of physical science, interesting and instructive as such a study would be. We are here dealing only with the conception of an element or principle of material bodies, also involved in this reasoning; and it is obvious that this early conception of an element was not that of a definite substance—as we now understand the word substance—that is, something subsistens per se—but rather that of the essentia or substantia which were supposed to underlie the external attributes of bodies, and of which these last were merely accidents. Earth was the underlying principle of all solid bodies, whose multifarious forms were as familiar to Aristotle as to us. So all liquid bodies were forms of water, and all aëriform bodies manifestations of the all-diffusive air; and the ancients, at times even more acute than ourselves, made distinctions between conditions, both of water and air, which we know are not essential.
We know that flame is simply intensely heated gas rising in a denser atmosphere; but it was perfectly natural that the ancients should regard such a startling effect as a manifestation of a fourth condition of matter still lighter and more subtile than air, and the conception of fire as a fundamental principle of nature once formed, the phenomena of combustion appeared to them as direct evidences of the escape of this principle of fire from the burning bodies.
The famous theory of phlogiston, advanced by Becher and Stahl during the seventeenth century, was simply a development of these views without any essential change. Phlogiston was merely a new name for the fourth element of Aristotle. As by Aristotle all combustible bodies were assumed to hold the principle of fire.
