OF CORNWALL. 123 
fome are yellow, brown, cloudy, opake white, green, purple, 
black, fome freckled with little fpecks of various colours and mag- 
nitudes, every cryftal being either pure and pellucid, or receiving its 
tinge from the mineral juices and earths adjacent. Thefe hones, it is 
obfervable, are generally more clear and tranfparent at the point 
than at the root. I have had two famples of cryftalline cufpides en- 
tirely black, and have feen no more, they being very rare. This 
fort is taken notice of by Linnaeus Syft. Nat. page 167, and is, I 
think, his Nitrum quai'tzofum nigrum , or Morion \ 
The more clear they are, the heavier; of which the reafon feems ; SE ct. vii. 
to be, that in clear ones there is more hone in any given quantity, Weight, 
than in opake ones, which latter confifting of earthy and mineral 
impurities mixed with the ftony, muft therefore under any equal 
furface include lefs ftone than the purer ones, and this obfervation 
is confirmed by the firperior weight of the true diamond to that of 
other ftones. The weight of our Cornifh diamonds to water, I find 
at a medium, as ten and a half is to four. 
The fame reafon that makes the pure more heavy than thofe sect.viii. 
which are otherwife, makes them alfo harder ; the more ftony fimi- Hardnefs. 
lar matter there is, the clofer is the connexion of parts; whereas 
the cohefion is greatly weakened, and the body becomes more fri- 
able by means of the earthy parts which intrude themfelves among 
the ftony ; the clearer therefore our Cornifti cryftals are, the better 
their points will cut glafs (though not fo free, or fo deep as the true 
diamonds) and the better they will bear engraving for feals. 
The texture of thefe figur’d cryftals is various; fome are of an sect.ix.j 
uniform texture, of one colour and confiftence throughout : this Texture - 
was the cafe of Fig. v. vi. xxiv. and many others here exhibited. 
Some fpring as from a center, or one common line, as Fig. v. vi. 
vii. xxxiii ; fome have hexagonal fheaths defcribed one within 
another, as Fig. xxx. a very remarkable ftrudture, and not eafily 
accounted for ; fome learned men imagine that they are different 
incruftations, applied fucceflively at different times, one without 
the other, and fuch indeed at firft fight they feem to be, but if we 
recur to the original formation of thefe bodies (as in order to dif- 
cover truth we muft) it will be very difficult, not to fay impoftible, 
for us to conceive any pofition or direction in which the middlemoft 
fhaft could lie, fo as that three incruftations of fuch an equal thick- 
nefs fhould form round it, neither will the laws of gravitation and 
1 Vide Morion. 
projection 
