before they were so exceeding hard, that they could not be broken without much difficulty; and upon their breaking the whole drop would fly in pieces with very great violence. The Reason of which last seems to be, that the leisurely heating and cooling of the parts does not only wast some part of the Glass it self, but ranges all the parts into a better order, and gives each Particle an opportunity of relaxing its self, and consequently neither will the parts hold so strongly together as before, nor be so difficult to be broken: The parts now more easily yielding, nor will the other parts fly in pieces, because the parts have no bended Springs. The relaxation also in the temper of hardned Steel, and hammered Metals, by nealing them in the fire, seems to proceed from much the same cause. For both by quenching suddenly such Metals as have vitrifed parts interspers'd, as Steel has, and by hammering of other kinds that do not so much abound with them, as Silver, Brass, &c. the parts are put into and detained in a bended posture, which by the agitation of Heat are shaken, and loosened, and suffered to unbend themselves.
Observ. VIII.Of the fiery Sparks struck from a Flint or Steel.
IT is a very common Experiment, by striking with a Flint against a Steel,Schem. 5. to make certain fiery and shining Sparks to fly out from between those two compressing Bodies. About eight years since, upon casually reading the Explication of this odd Phænomenon, by the most Ingenious Des Cartes, I had a great desire to be satisfied, what that Substance was that gave such a shining and bright Light: And to that end I spread a sheet of white Paper, and on it, observing the place where several of these Sparks seemed to vanish, I found certain very small, black, but glittering Spots of a movable Substance, each of which examining with my Microscope, I found to be a small round Globule; some of which, as they looked prety small, so did they from their Surface yield a very bright and strong reflection on that side which was next the Light; and each look'd almost like a prety bright Iron-Ball, whose Surface was prety regular, such as is represented by the Figure A. In this I could perceive the Image of the Window prety well, or of a Stick, which I moved up and down between the Light and it. Others I found, which were, as to the bulk of the Ball, prety regularly round, but the Surface of them, as it was not very smooth, but rough, and more irregular, so was the reflection from it more faint and confused. Such were the Surfaces of B. C. D. and E. Some of these I found cleft or cracked, as C, others quite broken in two and hollow, as D. which seemed to be half the hollow shell of a Granado, broken irregularly in pieces. Several others I found of other shapes; but that which is represented by E, I observed to be a very big Spark of fire, which went out upon one side of the Flint that I struck fire withall, to
