(744) 
there was adifcourftat London pf a certain pot of Glafs-mettal 
which brake in theGlafs-houfe at Wooliige s in the bottom of 
which was fouad a quantity of Opal-ghk. And although the ve- 
ry perfons, who had compounded it, endeavoured to repeat that 
accidental Experiment, yet they could never bring it to pafs, as 
Iwasmfosm'd by a perfon concern^ in it. Thelaft week I was 
two daies a* Harlem on purpofe to fee the Experiment of the 
making of this counterfeited <?^/-glafs which is there done by 
Rule. It is very lively, I contefs , and as I guefs, performed on- 
]y by the degrees of heat, producing the Colors $ of which de- 
grees I Jiatfe.by me feveral, I took notice of, with fome cu- 
riofity,in the operation. When the Compofition is thoroughly 
melted, they take out fome on thrpointofsn Iron-rod, which 
being cool'd either in the Air or Water, is colourlefs and pellucid- 
but being put into the mouth of the furqace on the fame rod' 
and there tuni'd by the hand for a little fp/ace, hath its little bo- 
dies fovarioufly pofited in feveral parts of the fame piece, as that 
the light falling on them, being varioufly modified thereby, re- 
prefents thofe feveral Colours, that are feen in the true opal. 
Whether it be the greatett, or leaft degree of heat, that rendeis 
it a white opaque Body., I have let flip, but this I know, (which 
feems remarkable) that the colours of it may be deftroy'd and re- 
ftor'd, according to the various motions (I fuppofe) of its particles 
by heat. 
They alfo make there the Jmethyfl and Safbir and have re- 
cover -d the hundred years lofs of incorporating Red-glafs 5 and 
have fome mettal, that isefteem'd to equal Chryftalm hardnefs 
as well as colour.. To give you an account of their Mill to grind 
andiEngins topoliih Looking-glaflTes, will be needlefs: lonely 
add, that they can diamond or fquare their looking-glaflesln their 
Grinding-mill. 
Some AnimaiverftonS) mitten in a Letter by Dr. John 
Wallis,*/? a printed Paper , enutul'd Refponfio Fran* 
cifci du Laurens adEpiftolam D. Wallifii ad CI, \r 
rr^Tp^ Hs Oldenburgium fcriptam . * 
Epiftofem tuam, Vir Ckn{fme> quae DuLamnfti impreffam chartam 
M** 1 (quar 
