the sulphate of magnesia at Monte della Guardia . 297 
in width; Others, which are now filled with water, or de- 
stroyed by the crumbling down of the rock, were described 
to me as of still greater length. The ore, thus obtained, is 
broken down into small pieces, and roasted in an open pit, or 
kiln, about 20 feet in depth ; with the alternation of wood, so 
disposed as most effectually to aid the combustion. This 
process of roasting continues generally for ten days. The 
kiln being sufficiently cooled, the ore is removed thence, and 
disposed in large heaps, underneath a shed in the vicinity. 
Here it remains for several months fully exposed to the air, 
and occasionally moistened with water thrown upon it, to aid 
the chemical changes by which the salt is found. An efflo- 
rescence of sulphate of magnesia soon commences, and gra- 
dually proceeds, so as to cover with minute crystals the sur- 
face of the ore, which, during this time, crumbles down into 
very small fragments. 
This is the usual commencement of the process, but there 
are varieties of the ore, more easily decomposable, which re- 
quire less roasting, or may even be submitted to the after 
processes, without any application of heat. Other varieties, 
on the contrary, require more roasting, to prepare them for 
the proper efflorescence of the salt, which efflorescence itself 
varies in rapidity, according to the particular description of 
the ore. 
The materials, brought into this strata, are then lixiviated ; 
and the liquor containing in solution the sulphates of mag- 
nesia, of copper, and of iron, is filtered ; in passing it through 
layers of sand and straw, disposed in large wooden vessels^ 
These processes repeated as often as may be necessary, the 
next object is the separation from the liquor, of the metallic 
