( 4«4 > 
that were they found of equal Size with the black 
Cryftals, and of a white Water (which 1 imagine may 
be) their Hardnefs and Weight (in both which they 
exceed any other Foflil) would probably make them 
preferable to the Diamond. However, as the deeper 
Colours of thefe Cryftals feems to arife from a greater 
Proportion of Iron in their Compofition, which they 
throw off in an Iron Slag upon Fufion, and which 
changes by proper Degrees of Heat into a Crocus, 
thereby changing the Colour of the Cryftal to a 
brighter Red j fo the white Tin Oar is certainly to be 
efteemed both richeft and beft, as moft free from Iron. 
Paper the thirteenth contains a Piece of the Load 
contained in Paper the feventh, in which the Cryftals 
are of a brighter red, from its being heated red-hot. 
{Thefe Specimens were all produced before the Soci- 
ety , and are fince prefented to Sir Hans Sloane, 
*P ref dent.) 
Thefe Cryftals feem to be the heavieft Bodies the 
Earth produces, except Quick (liver and aftual Metals. 
Their fpecifick Gravity is to Water as 90 ~ to 10 • to 
Rock Cryftal in Water as 90 i to 2 6 ; to Diamond as 
90 i to 34; and to pure malleable Tin, as found by 
repeated Trials, as 90 i to 78 ; from whence appears 
the Poffibility of what fome Miners affirm, viz,. That 
a cubical Inch of fome Tin Oars, will yield more than 
a cubical Inch of Metal. 
Having already taken Notice that the Cryftals of 
Tin are fometimes fo fmall as to efcape the Eye, and 
fo difteminated in the Load as not to make above 
| oc th, or * 000 Part of the Load, one would naturally 
imagine it an endlefs Labour to cleanfe the Oar from 
fuch a vaft Difproportion of Rubbiffi. But the great 
3 fpecifick 
