tions, or moving with some different affection. Air will move different ways at the same time, if it is of different densities, or moves with two different affections, as are those of wind, and the vibratory progression of sound. The etherial medium, which is finer than air, is subject to all those different affections which produce gravity, magnetism, elasticity, cohesion, electricity, heat, and illumination. How far these affections may arise from the different densities of the medium, or the different magnitude of its parts, should be considered. Sir Isaac Newton was of opinion, that even the light of the sun itself is in several conditions, as consisting of parts differing in magnitude; and thence he deduces the different refrangibilities of the coloured rays.

10. That all undecaying motion hath this property by the constitution of nature, that it will renew itself upon its own principles, without any foreign impulsion; which doctrine necessarily excludes the artificial force of projection from having any share in the lasting motions of the world. The bodies in the heavens are moved by such causes, as would renew their progress, if it were possible for them to be stopped; otherwise their

motion is not maintained upon physical principles, agreeable to what we observe in the other parts of nature; but by miracle, in contradiction to the common course of things. Nothing is here objected to the schemes of motion deduced from the properties of curve lines, because geometrical evidence, without physical evidence, will prove nothing in this subject. The powers assumed to account for motion, are indeed the proper objects of our consideration; therefore it has been remarked on the centrifugal force, or force in the tangent, that it can be of no use as a power, because it is no more than a consequence of that motion. which is already established without it, and, as such, cannot be brought in to account for it with any appearance of reason.

11. That the velocities of the planetary motions being vastly exaggerated by being compared with diminutive bodies, should be estimated according to the scale of their own magnitudes; whence it may be more easily conceived how a small force of the elements (comparatively speaking). may suffice to carry those vast bodies in their orbits, and turn them upon their axes.

12. That in order to account for the last

ing motions of the heavenly bodies, it is most agreeable to reason and observation, to admit a general principle of circulation in all fluid matter; which circulation certainly prevails in such parts of nature as are more immediately subjected to our examination; and may well be extended to the sun itself, and the elementary matter in the celestial spaces.

13. That the parts of any fluid medium may move freely among themselves, though they constitute a plenum; and that solid bodies may not only move in such a medium, but preserve their motion undiminished, if their motion conspires with the motion of the medium.

14. Lastly, that the resistance which bodies meet with when they are projected or moved by any act of violence, does by no means infer the necessity of a vacuum to other bodies, which are moved on other principles, according to the common course of

nature.

No person, who is at all versed in philosophical disquisitions, need be reminded, that this subject of natural motion is both subtile and difficult; hard to be explained, because it is hard to be understood: therefore a can

did reader will not be too hasty in judging my expressions, when even my conceptions, which those expressions are intended to open, may be supposed in some instances to have fallen very far below the subject.

These, however, so far as we can reach them at present, are the properties and conditions of motion: which being previously considered, we shall proceed next to the elements; and shall endeavour to shew how they are distinguished in nature, and consequently how they ought to be divided in philosophy.

159

DISCOURSE III.

On the Nature and Uses of the
Elements.

WE give the name of elements to those

simple bodies of which others are composed, and into which they are again resolved. An element, with respect to itself, is a substance so simple that it cannot be resolved into any other with respect to other things, the elements are those materials, into some or other of which, all compound bodies are resolvable.

The ancients generally agreed that the elements are four in number; earth, water, air, and fire*: and though the chemists have often disputed this doctrine, it does not appear that they have been able to establish any in its stead that will stand the test of examination; or about which they can generally

* Στοιχεια μὲν καλεμεν, γήν, ὕδωρ, πυρ, αερα αρχας δε λεγόμεν, δια το εδεν εχειν πρότερον εξ ξ γενναται. Stob. Εcl. Phys, ch. 13.