( 8oa ) 
of Air, cafually intercepted in the vifcous matter of the coin 
raining Bodies, (which I fuppofedjUpon good grounds, to have 
been once foniewhat ^MJ before it came to be hard ; adding, 
that His Majefiy^ who was Critical in thcfe matters^might pro- 
bably have the Curiofity, I had, to have the worft of them 
broken, to be fatisfied what kind of bodies the included were. 
Hereupon, to content me, one of them was open'd , and that 
which had appeared a Feurl was found to be but a Cavity , that 
contained no groffer fubftance than Air* And I have by me a 
well (hap d piece of G/a/of a good thicknefs, v^'nh ^n jiereal 
iMle in the middle, which by fome qualities, particularly its 
P^^r-like flbape and vivid refleflion, docs not ill refemblea 
fair, though not OrUnt^FtAtL But in fuch like Obfervations, 
the Pofition of the Eye,and that wherein the Body receives the 
beams of Light, may be very condderable. For I have by me 
a fmall Stone (with which I have puzled the Skilful JemOer of 
a great Prince to determine what kind of Gem\i is) that being 
laid flat upon ones hand, or a piece of Paper, and lookt on di- 
reflly downwards, looks almoft like a piece of common Glaf^ 
and is tranfparent : But if theEyebefo placed , that the Inci- 
dent beams of Light, by whofe Refleftion *cis feen, fall with a 
convenient degree of obliquity upon the Stone, it makes an 
exceeding pretty fliew, fometimes appearing like a fine 
and fometimes rw)t very unlike an Orient Pearl. 
It may not be altogether impertinent , and at Ieaft,for the 
Novelty of the way of Trial, it will not probably be 
Exp.xvT. unpleafing,if I here mention sn Attempt to try,\v he- 
ther, when the Rays of light rebound from bubbles 
inviron'd with an uniform Solid body (which cafe is fomcwbat 
differing from that of Bubbles look'd upon in an exhauftcd 
JReceiver,) the Rcfleftion be only, or almoft only, from thegroP 
fer Particles of the Air, and not alfo from fome Subtile matter 
harboured, as well as they, is the fame Caviries ? But to bring 
this queftion to Tria!,fcemed difficult enough^ becaufe it isfo, 
to include very^ rarified Air in a confiftent body, diaphamud e- 
nough to let its refleftion be eafijy obferved. To compafs this, 
> in tk ufef. I thought upQO thc following Expedient. We niade^ 
»f Experiment., according, to the eafip direftion * elfewhere given, 
mjofophji^ . ^f^^ otheii^^ purppfcs,) a competent quantity of a 
E^Jhm or G^w^ fubftance 5 that looked like high coloured 
