Page:Philosophical Transactions - Volume 003.djvu/121

 there was a discourse at London of a certain pot of Glass-mettal, which brake in the Glass-house at Woolidge; in the bottom of which was found a quantity of Opal-glass. And although the very persons, who had compounded it, endeavoured to repeat that accidental Experiment, yet they could never bring it to pass, as I was inform'd by a person concern'd in it. The last week I was two daies at Harlem on purpose to see the Experiment of the making of this counterfeited Opal-glass which is there done by Rule. It is very lively, I confess, and as I guess, perform'd only by the degrees of heat, producing the Colors; of which degrees I have by me several, I took notice of with some curiosity, in the operation. When the Composition is thoroughly melted, they take out some on the point of an Iron-rod, which being cool'd either in the Air or Water, is colourless and pellucid; but being put into the mouth of the furnace on the same rod, and there turn'd by the hand for a little space, hath its little bodies so variously posited in several parts of the same piece, as that the light falling on them, being variously modified thereby, represents those several Colours, that are seen in the true Opal. Whether it be the greatest, or least degree of heat, that renders it a white opaque Body, l have let slip; but this I know (which seems remarkable) that the colours of it may be destroy'd and restor'd, according to the various motions (I suppose) of its particles by heat.

They also make there the Amethyst and Saphir; and have recover'd the hundred years loss of incorporating Red-glass; and have some mettal, that is esteem'd to equal Chrystal in hardness as well as colour. To give you an account of their Mill to grind, and Engins to polish Looking-glasses, will be needless: I onely add, that they can diamond or square their looking-glasses in their Grinding-mill.

Epistolam tuam, Vir Clarissime, quæ Du Laurensii impressam chartam Rh