of white Glass, which cools much quicker then green Glass, and is thereby made much brittler) acquire a very _porous_ and very brittle _texture_: so that if with the point of a Needle or Bodkin, the inside of any of them be rubbed prety hard, and then laid on a Table, it will, within a very little while, break into many pieces with a brisk noise, and throw the parts above a span asunder on the Table: Now though the pieces are not so small as those of a _fulminating_ drop, yet they as plainly shew, that the outward parts of the Glass have a great _Conatus_ to fly asunder, were they not held together by the _tenacity_ of the parts of the inward Surface: for we see as soon as those parts are crazed by hard rubbing, and thereby their tenacity spoiled, the springiness of the more outward parts quickly makes a divulsion, and the broken pieces will, if the concave Surface of them be further scratcht with a Diamond, fly again into smaller pieces. From which preceding considerations it will follow Sixthly, That the sudden flying asunder of the parts as soon as this Arch is any where disordered or broken, proceeds from the springing of the parts; which, indeavouring to _extricate_ themselves as soon as they get the liberty, they perform it with such a quickness, that they throw one another away with very great violence: for the Particles that compose the Crust have a _Conatus_ to lye further from one another, and therefore as soon as the external parts are loosened they dart themselves outward with great violence, just as so many Springs would do, if they were detained and fastened to the body, as soon as they should be suddenly loosened; and the internal parts drawing inward, they contract so violently; that they rebound back again and fly into multitude of small shivers or sands. Now though they appear not, either to the naked Eye, or the _Microscope_, yet I am very apt to think there may be abundance of small flaws or cracks, which, by reason the strong reflecting Air is not got between the _contiguous_ parts, appear not. And that this may be so, I argue from this, that I have very often been able to make a crack or flaw, in some convenient pieces of Glass, to appear and disappear at pleasure, according as by pressing together, or pulling asunder the contiguous parts, I excluded or admitted the strong reflecting Air between the parts: And it is very probable, that there may be some Body, that is either very rarified Air, or something _analogous_ to it, which fills the bubbles of these drops; which I argue, first, from the roundness of them, and next, from the vivid reflection of Light which they exhibite: Now though I doubt not, but that the Air in them is very much rarified, yet that there is some in them, to such as well consider this Experiment of the disappearing of a crack upon the _extruding_ of the Air, I suppose it will seem more then probable.