In like manner, let the subject of enquiry be the substance of the moon, to determine whether it be rare, flamy, or aerial, as many of the ancient philosophers conceived; or solid and dense, as Gilbert, with many of the moderns, and some of the ancients, contend. The reasons of the latter opinion depend chiefly upon this, that the moon reflects the of the sun, and that light appears to be reflected by none but solid bodies. And, therefore, if there are any, those may be reckoned crucial instances, with regard to this subject, which demonstrate that reflection may be made by a rare body, as flame is, provided it be sufficiently thick.

rays

And, doubtless, one cause of the twilight, among others, is the reflection of the sun's rays from the upper part of the air. We sometimes see the rays of the sun reflected in clear evenings, from the edges of dewy clouds, with a more resplendent brightness than that afforded by the body of the moon; and yet it is not certain that those clouds are collected into a dense body of water. So likewise we see that the dark air behind a window by night reflects the light of a candle, as well as a dense body.

Experiment also should be made of transmitting the sun's rays through a hole, upon a dusky or blue flame, for the free, open, and uncon

fined rays of the sun, falling upon obscure flames, seem to deaden them, and make them appear rather like white fumes than flames*. And these are instances that occur for the present, to shew the nature and use of the crucial instances with regard to this subject, though, doubtless, better may be found for the purpose. But let it always be observed, that a reflection from flame is not to be expected, unless the flame be of some depth, thickness, or body; for otherwise it inclines to transparency. But this is to be held certain, that light in an uniform substance, is always either received and transmitted or reflected +.

Again; let the nature sought be projectile motion, as the motion of a dart, an arrow, a bullet, &c. through the air. This motion the schools according to their custom, have slightly passed over, esteeming it sufficient to distinguish it by name of violent motion, from that they call natural; and for the first impulse, satisfying themselves with this, that two bodies cannot be in the same place, otherwise a penetration of dimensions would ensue, without at all regarding the continued progress of this motion.

See Sir Isaac Newton's Optics, passim; and Dr. Hook's Lectures of Light.

+ See Mr. Boyle of Colours, and Sir Isaac Newton's Optics.

The cross-way here lies thus. This motion is either caused by the air propelling and gathering itself behind the projectile, as the water does behind a ship, and the winds behind the chaff which they blow away, &c. or else by the parts of the body not sustaining the impulse, but urging forward to relax themselves, by succession, from the impelling force. Fracasto

rius, and nearly all those who have made any subtile enquiry into this motion, take the first path; nor can it be questioned, but the air has some share herein, though, doubtless, the other motion is a true one, as appears from numerous experiments.

But, among the rest, this may be a crucial instance to the purpose; that a piece of stubborn iron plate, wire, a quil, or the like, being bent in the middle by the fingers, will, when let go, spontaneously fly back. Now, it is plain, that this motion cannot be attributed to the air, collecting itself behind the body; because the ' origin of the motion is in the middle of the plate, wire, or quill, and not in the extremities or ends*.

Again; let the nature sought be the rapid and powerful expansion of gunpowder into flame, whereby such vast masses of building, are over

* See the essay upon Violent and Projectile Motion.

TO

turned, and such great weights thrown to a considerable distance, as we see in the springing of mines, the firing of mortars, &c. The double road here lies thus. This motion is either excited by the mere appetite of the body to dilate itself after it is set on fire, or by a mixed appetite of the crude spirit of the nitre, which, with great rapidity, avoids and flies from fire; and violently bursts out from the midst thereof, as from a prison. The schools and the vulgar opinion here consider only the former appetite; for men have thought they philosophized notably, in asserting flame to be endowed by the form of the element, with a necessary quality of possessing a larger space, than the same body possessed when it had the form of gunpowder, and that this motion must from thence ensue.

But here they observe not, that, though this be true, upon supposition the flame is ready generated, yet the generation of flame may be hindered by a mass of matter able to suppress and suffocate it, so that the thing may not be reduced to the necessity they speak of. Indeed, as to the necessity of the expansion, and the consequent explosion, or discharge of the bullet, or obstructing body, when the flame is generated, they judge rightly; but this necessity is plainly avoided, if the solid body suppress or prevent the flame, before it is generated. And

we see that flame, especially in its first genere ration, is soft and gentle, requiring a cavity wherein to play and exert itself; whence such a violence cannot be attributed to flame of itself.

I

Without doubt, the generation of this flatu+ lent flame, or, as it were, fiery wind, arises from a conflict of two bodies, that have very different natures; the one being highly inflammable, which is the nature powerful in sulphur; the other highly impatient of flame, which is the case in the crude spirit of the nitre: whence a wonderful conflict arises; the sulphur immediately catching all the flame it is capable of, and the spirit of nitre bursting forth with all its violence, at the same time dilating itself, as air, water, and all crude bodies do, when affected by heat, and whilst it breaks forth, and flies off every way, it blows up the flame of the sulphur, as it were with internal bellows; the wil- low-coal, in the composition, serving for little more than to incorporate and commodiously unite the sulphur and salt-petre together*.

+

But the crucial instances upon this subject might be of two kinds; the one with regard to those bodies which are most inflammable, ash

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See the Essay upon the Cause of the Motion of Ex plosion in guns and gunpowder. Jamnă to the gd zabwcq