of Magnefian Earth. 455 
collected them under one view, as it may be 
ufeful to Tome artifts, particularly thofe con¬ 
cerned in the potteries, to know what earths 
and dones contain it, and in what proportions. 
Magnefia, as prepared for the fhops, would be 
too expenfive for the purpofes of manufactures, 
which may perhaps, often, be equally anfwered 
by ufing it in thefe combined forms. 
It may feem extraordinary, that a fubftance, 
fo plentifully diftufed in a date of combination, 
fhould fo rarely occur pure. * But the cafe is the 
fame, as the celebrated Bergman has obferved, 
with Clay, which is always found, in nature, com¬ 
bined with other fubdances. f Siliceous earth 
is indeed, fometimes, found pure in the form of 
Rock Cryjtal ; but Bergman detected fmall por¬ 
tions of both argillaceous and calcareous earth, 
in the fpecimens which he examined. Ponde¬ 
rous earth is generally combined with vitriolic 
acid, and the mod pure date, in which it has 
been obferved, is its combination with aerial 
* I have been allured by Mr. Fabroni, that Magnefia 
has been found pure, in the Pyrenees. This, however, 
was, probably, not pure, in the prefent acceptation of the 
term ; but pure from any other admixture, than that of 
aerial acid, for which magnefian earth has a ftrong 
attra&ion. 
f Clay, commonly fo called, does not, in general, con¬ 
tain more than from twenty-five to forty-eight parts, in 
jne hundred of pure argillaceous earth. 
Gg 4 acid, 
