perpetuated, and the vibratory motion of light supported throughout the solar system. But this doctrine concerning the circulation of phlogiston is not to be introduced on such an occasion as the present, till it has been first proved to take place in such matter as we can make experiments upon; in which it hath not yet appeared that the inflammable principle, when once destroyed, is ever restored again in the common course of things. Ashes will never imbibe phlogiston, and become inflammable with long keeping; neither doth it appear that phlogiston as such ever survives the operation of the fire, because the purest alcohol, or perfectly rectified spirit, when dissipated by the action of fire, turns chiefly to an insipid water, which, if kept for a thousand years, would never turn into spirit to be again inflamed.

The circulating principle, however, in some sense or other, must be embraced, as necessary to keep up the stock of matter at the sun but if the matter of light is progressive, this expedient of the regeneration of phlogiston (allowing it to be agreeable to nature) would be found very insufficient. Therefore it will be most easy and natural to believe, as the ancients did, that the sun,

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while he supplies the world with light, is himself supplied with the same matter circulating backward to his orb in some other condition.

This principle, if admitted, will be attended with the following advantages: first, it will consist with the progressive motion of light, the source of which will be inexhaustible, because there can be no exhaustion where there is circulation. Secondly, it will agree best with that analogy which is so remarkable in the other departments of nature. The blood hath a progressive motion in the bodies of all animals; water hath a progressive motion in the earth and seas; wind hath a progressive motion in the atmosphere; and all these are kept up by the grand principle of circulation: the blood returns into its own source; the water returns to the ocean from whence it came; the air circulates with contrary currents in the atmosphere; the equatorial and polar parts are always supplying each other reciprocally, to restore an equilibrium: so that if the matter of the sun returns into itself, there will be nothing singular in the case; it will be found to do as all other matter does: and that philosophy will ever be the

best,

best, which creates the fewest principles, and explains nature in such a way as to make it consistent with itself. Thirdly, the fire of the heavens, (as the same rule of analogy obliges us to suppose,) will then be understood to burn on the same principle as other fires do upon earth, with a constant supply of elementary matter. And lastly, we shall have this farther advantage, which indeed is the greatest of all, that there will be a double motion perpetual in the system, the first and greatest of all secondary causes, accommodated in every respect to account for the revolutions of the heavenly bodies, which no other hypothesis yet invented ever hath, or ever will be able to do, with any appearance of consistency and probability.

On what Conditions there may be Motion in a Plenum.

It has been made a grand question in philosophy, whether bodies can move in a space which is filled with matter, commonly called a plenum? But this question cannot be answered, without first considering the condition of the matter, and stating the circumstances of the moving body. If the matter,

so filling any space, is in a fluid condition, so that the parts can slide freely over one another, they will be able to move in different or even contrary directions at the same time; and while the place of the whole mass continues the same, the place of the parts which compose it may be changed every moment. Such a sort of intestine motion arises naturally, and continues long undiminished, in a liquor, by that act of fermentation which arises from a mixture of heterogeneous principles, (such as air, fire, water, oil, salt, and earth,) in the same mass. The fulness of the space is therefore no objection to a free motion of the parts of any fluid mass amongst themselves; neither is it an objection to the motion of any solid body in such a fluid medium. If a vessel is filled with water, and closely stopped, any solid body that floats in it will move freely from one side to the other, or from the top to the bottom; because the parts of the fluid, which are displaced before, fall into the space behind, as the body leaves it. So fast as the body proceeds, just so fast do the parts of the fluid recede; so that there is neither impediment nor vacuity.

Suppose a circular groove upon any hori zontal surface, and so many spherules laid

in this groove, as just to touch one another completely, and fill up the circle. The fulness of the space is no objection to their motion: they will move for eyer in the direction of the circle; because, as fast as one of the spherules is moved, so fast does the next before it make room for it, and the next behind supply the place of it. The same is true in every similar case: there may be fulness of matter, and yet there may be motion, provided there is a circulation amongst the parts.

If we examine into the circumstances of any solid body moving in a fluid medium, it is easy to foresee what will happen to it. When the body is moved by any artificial force or effort of violence, contrary to the nature of the medium in which it moves; the parts of the medium, in recovering their natural state, will resist the motion of the body, till the equilibrium is restored, and the body is at rest. Such, therefore, are the circumstances of all violent motion, that it is soon destroyed by resistance; though the times in which it is destroyed will differ exceedingly among themselves, according to the fluidity, tenacity, density, or subtilty of the medium in which it happens. But if the motion

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