Dr. Holland on the manufMtiire of 
sulphates. When the sulphate of copper is perceived to be 
abundant, the metal is precipitated by refuse of iron, intro- 
duced into the liquor. The sulphate of iron, and any remain- 
ing sulphate of copper, are decomposed by the addition of 
lime, in the state of milk of lime, which causes a precipitate 
of sulphate of lime, together with the metallic oxides. 
The lime employed for this decomposition, is obtained 
from the magnesian limestone, already described as belong- 
ing to this district. 1 hat of Monte del Gazzo is preferred, 
both as nearer to the spot, and possibly from its containing 
a larger proportion of magnesia, than the other magnesian 
limestones of this coast, about 1 6 per cent., as appears from 
analysis made of it. It appears probable, that a certain por- 
tion of sulphate of magnesia is obtained from the use of this 
limestone, in decomposing the metallic salts, and the process 
of manufacture thereby rendered somewhat more productive. 
The quantity of lime employed may equal about T ~_ part 
of the weight of the pyritic ore. 
The metals being thus separated from the saline liquor, it 
is filtered anew, and then evaporated to a certain point in a 
large copper boiler. It is subsequently let off into small 
glazed earthen vessels where the crystallization of the salt 
takes place. M. Mojon mentions the circumstance of its 
being needful frequently to disturb the crystallization, to 
satisfy the prejudice of the purchasers, who consider it essen- 
tial that the salt should be delivered to them in the form of 
small spicular crystals. 
The produce of sulphate of magnesia varies of course ac- 
cording to the quality of the ore. In general, perhaps, it may 
be stated at of the weight of the material employed. The 
