The First Part.

I can shew you some pieces of Glass which I saw flow down from an Earthen Crucible purposely Expos’d for a good while, with Silver in it, to a very vehement Fire. And some that deal much in the Fusion of Metals Informe me, that the melting of a great part of a Crucible into Glass is no great Wonder in their Furnaces. I remember, I have Observ’d too in the Melting of great Quantities of Iron out of the Oar, by the Help of store of Charcoal (for they Affirm that Sea-Coal will not yield a Flame strong enough) that by the prodigious Vehemence of the Fire, Excited by vast Bellows (made to play by great Wheels turn’d about by Water) part of the Materials Expos’d to it was, instead of being Analyz’d, Colliquated, and turn’d into a Dark, Solid and very Ponderous Glass, and that in such Quantity, that in some places I have seen the very High-wayes, neer such Iron-works, mended with Heaps of such Lumps of Glasse, instead of Stones and Gravel. And I have also Observ’d, that some kind of Fire-stone it Self, having been employ’d in Furnaces wherein it was expos’d to very strong and lasting Fires, has had all its Fixt Parts so Wrought on by the Fire, as to be Perfectly Vitrifi’d, which I have try’d by Forcing from it Pretty large Pieces of Perfect and Transparent Glass. And lest You might think, Eleutherius, that the Question’d Definition of Heat may be Demonstrated, by the Definition which is wont to be given and Acquiesc’d in, of its contrary Quality, Cold, whose property is taught to be tam Homogenea, quam Heterogenea congregare; Give me leave to represent to You, that neither is this Definition unquestionable; for not to Mention the Exceptions, which a Logician, as such, may Take at it, I Consider that the Union of Heterogeneous Bodies which is Suppos’d to be the Genuine Production of Cold, is not Perform’d by every Degree of Cold. For we see for Instance that in the Urine of Healthy Men, when the Liquor has been Suffer’d a while to stand, the Cold makes a Separation of the Thinner Part from the Grosser, which Subsides to the Bottom, and Growes Opacous there; whereas if the Urinal be Warme, these Parts readily Mingle again, and the whole Liquor becomes Transparent as before. And when, by