Opticks 1717 - Up

Property We have in the RefraEtion of Tfland
Cryital, defcribed firfE by Eraj~us B~rthohe,
and afterwards more exaAly by ~-~llyeniu.r, in
his Book ‘De la Lw~niere. This Cryital is a pellucid
@file Stone, clear’as Water or Cryftal of
the Rock, and without Colour ; enduring a red
Heat without lofing its tranfparency, and in a
very i%rong Heat calcining wi.thout F&on.
Steep’d a Day or two in U’atef, it lofes its natural
Polifll. Being rubb’d on Cloth, it attraCts
pieces of Straws and other light things, like Ambar
or Glafs ; and with A~ZU firtti it makes an
Ebullition. If Seems to be a fort of Talk, and
is found in form of an,:oblique Paralleloyiped,
with iix yarallelo,vram Sides and eight foblid An7
gles. The obtufc Angles of the Parallelograms
are each of them IOI .Degrees and 5-2 Minutes ;
tl~c acute ones 78 Degrees and 8 Minutes.. Two
of the folid Angles oppofite to one another, as
C and E , are compa&d each of
them with three of thefe ok&e y7:t $~~‘~-
Angles ) and each of the other A L ’
fix with one obtufe and two acute ones. It
cleaves eafily in Planes parallel to any of its
Sides, and got in any other Planes. It cleaves
with a glory polite Surface not perpeCt!y plane9
but with @me little unevennek It is eafily
fcratch’d, and b reafon of its foftnefs it takes
a Polish very di fTl ‘cultly. It: poliflles better up--
,on poliMd Looking-glafs than upon Metal, and
perhaps better upon Pitch, Leather. or Parchment.
Afterwards it muit: be rybb’d with a
little Oil or W,hitc of an Egg, to fill Up its
-Ij~r~tc~es 1. ; whereby it will becc?nlSj: very tra&
parent: