MINE R A L O G Y, 
i29 
S E C T. CCXLY. 
'Observation's on Arsenic. 
Such ores as confift. of arfenic united folely with 
iron, or with iron and fulphur, cannot be employ^ 
ed to any other ufe than to the preparation of ar- 
fenical .products *, for which reafon they ought to 
be ranged among the arfenic ores. Some have 
indeed denied this difference ^between the arfenical 
pyritse *, but it is however neceffary to make fomie 
difference, with Tefpe‘61 to the prefence or abfence 
of fuiphur, although the greateft quantity of arfe- 
nic is got from the calcination of the , cobalt ores, 
and that the true arfenical pyritae do not deferve to 
be feparately employed. 
Although it is difficult to reduce the arfenic by 
way of precipitation, one cannot for that reafon 
deny it to be of a metallic nature ; for the famie 
way of reafoning might have been ufed againf: the 
zink in the .calamine, before the method to extract 
that femi-metal in its metallic ftate, now known, 
was difcovered : 'But thofe who know that metals 
only can be mixed with metals, fo as to preferve 
the folidity and fome ductility in the compound, 
and who at the fame time are ignorant of any me- 
tallic earth, which cannot be reduced to its me- 
tallic ilate again, could never entertain fiich 
-notions. 
It is indeed true, that fuiphur, in regard to the 
brittlenefs which it .produces in 'metals, is of no 
worfe effed than arfenic j but this laft may by it- 
felf, and mixed only with a pure phlogifton, be 
fubiimed into a metallic form, which is more plain- 
ly feen in the Scherbencobolt (Sedt. ccxxxix.)* I 
cafily perceive that it may be objected by thofe, 
Q 3 who 
