i 3 6 NATURAL HISTORY 
mifinformed yields not only arfenick and fulphur, but a powder 
very near, if not equal to, and the fame as ultramarine. 
In the year 1750 I had a fample of the fame plate-mundic, 
abundantly mixed with fpeltre, from a work in Gwynier parifh. 
It may therefore be well worth while to enquire farther into this 
mundic, whether ultramarine may be procured from it in any an- 
fwerable quantity ; and, fecondly, whether it may ferve the pur- 
poles, or increale the powers and quantity of fpeltre ; and, in con- 
tinual fearching, other ufes, not now forefeen, may probably occur. 
sect. xvi. 
Mundic 
concretions 
formed at 
different 
times, and 
are ftill 
forming. 
Cryftals and mundics are frequently found in the fame beds, and 
by the infpe&ion of feveral fpecimens, we may reft allured, that fome 
mundics were indurated before the cryftals, as appears by the plain 
imprellion made in the cryftals which adhered to them, and from 
which we may ealily feparate them ; and other fpecimens will as 
readily convince us that they were indurated fince the cryftals, being 
formed into cappings and incruftation s upon the culpldes of the 
Cornifh cryftals, from which they have vilibly received the hexago- 
nal imprellion. Further : There is great reafcn to believe, that 
mundics are perpetually forming (as is probably tl cafe Oi ail ores) 
new combinations where they have proper room, h rty, nidus, and 
fubjedts to fix upon ; for in the fragment of a Cornilh cryftal 
brought me, in the year 1752, from a mine which had layn idle 
about thirty years, I perceived an incruftation of granulated befprink- 
led mundic beginning to coat the cryftal in the fratfture : now, 
there is all reafon to believe, that this cryftal was broke in the co- 
lumnar part when the mine was worked laft, which is betwixt thirty 
and forty years ago, fo that this mundic incruftation muft have 
fixed itfelf on this fra&ure fince that term. The following accident 
confirmed the fuppofition : Having laid by fome mundics in a 
drawer, and coming about two years after to examine them, I 
found feveral glebes of yellow-mundic, which were feparated when 
I put them by, flicking clofe together ; one glebe had picked up a 
bit of blue vitriol, a grain of lead, a grain of copper, and a grain 
of cryftal ; and the yellow-mundic had alfo fhot round about, and 
clolely embraced a piece of the plate-mundic : hence we fee that 
the mineral principles are always active, and forming new concre- 
tions ; and likely this activity is in proportion to the mineral and 
metallic falts which the foflil contains : here the yellow-mundic 
was moft atftive, as conftfting of more fait than the plate-mundic ; 
but the brown-mundic has more fait ftill than the yellow, and will 
divide and fall in pieces commonly in any moift place, and fhoot 
0 By a letter from Dr. J. Andrew from Leyden, 1738. 
forth 
