the sulphate of magnesia at Motile della Guardia. 299 
ore remaining after the first filtration, is usually roasted and 
lixiviated a second time, to obtain the portion of salt which it 
is still capable of affording. 
It is almost unnecessary to make any remarks on the 
theory of the process just described. The sulphuric acid, 
formed by the action of heat, air, and water upon the pyritic 
ore, combines with the metals, and with the magnesian earth, 
which the ore contains. I am not aware of any experiments 
that have been made, to ascertain the proportion of salt which 
may be derived from the use of the magnesian limestone. 
Probably, however, it is extremely small ; and it would 
seem very doubtful, from the nature of the process, whether 
any improvement to the manufacture can be expected from 
this source. The effect of employing more of the limestone 
than is strictly necessary, would be, that the lime, rather 
than the magnesia, would unite with the sulphuric acid ; and 
if the addition of the mixed earth were in excess, the lime 
alone would have effect in the decomposition ; and sulphate 
of lime, with the metallic oxides, would be the new products 
obtained. 
M. Mojon has proposed a variation in the manufacture at 
Monte della Guardia ; depending on the different tendency of 
the sulphate of magnesia, and of the metallic sulphates to- 
wards crystallization. The principal object of the variation 
would be to save the sulphate of iron, now lost by decompo- 
sition ; and to increase the value of the produce of copper by 
obtaining it in the state of sulphate. This change in the pro- 
cess has not hitherto been adopted ; and probably might 
be found to be attended with several difficulties in practice. 
The buildings connected with the establishment at Monte 
mdcccxvi. R r 
