[ -zs ]

till only the external part of the glass was changed, I haveoften observed, that on hastily raising the fire, the un-changed glass has melted and run out, leaving a cavity inthe middle with a crust of porcelain on each'fide.

When the porcelain has been so far baked, that thefibrous texture has disappeared, and a coarse granulatedone come in its place, it proves far softer than before :it now neither strikes fire with steel, nor cuts glass jbut is itself cut with ease both by common glass andby the file.

With this imperfection, it acquires an advantage ofgreater resistance to fire : the longer the cementation wascontinued, the fusibility seemed always to be more andmore diminished. A piece of the concave bottom of acommon green bottle, which had its fibres only in partchanged into grains, stood the melting of a lump ofbrass, of about two ounces, without anywise altering itsfigure, or suffering any other apparent change, than thatthe thin edges were rounded off, and covered in someparts with a green glazing, which seemed to have exudedfrom the mass. Some pieces of the fame cemented glassbeing put into a small crucible, into which another wasinverted and closely luted, and the whole urged for twohours or more in a sea-coal fire vehemently excited bybellows, the pieces melted together into a very spongymass, of an almost pearly whiteness and some brightness,intermixed in different parts with a green glass, exactlyresembling the glass employed, and which probably wasno other than a part of it, that had escaped unchanged inthe cementation, though not distinguishable by the eye,till thus spued out from the less fusible porcelain, and col-lected in its cavities. Pieces which had acquired through-out a sine bright grain, were likewise in an intense fire,made to melt or soften into lumps, which generally proved

spongy: