452 Mr. Henry on the Natural Hi/lory, &c. 
Though magnefian earth is rarely found pure, 
in nature, it is contained, in a faline form, in all 
nitre heaps, from the mother ley of which fair, 
it was formerly extrafted ; in the water of feveral 
medicinal fprings, and even in our common 
wells. But the fea is the great fource from 
which we obtain it; in the waters of which, it 
is united to the marine acid, in which form it 
remains, after the cryftallifation of the fea fait, 
and is afterwards feparated from the marine, and 
united to the vitriolic, acid, by a particular pro- 
cefs. It is then called Sal Catharticus Amarus , 
and, from this fait, the magnefian earth may 
be precipitated. One hundred parts of the fait 
yield, on precipitation by a mild alkali, forty- 
two of magnefia, united with aerial acid. If 
a cauftic alkali be employed, the product 
amounts to only twenty-five parts. But a por¬ 
tion of this lad may be driven off by calcination, 
and confifts of water, and, perhaps, Jome aerial 
acid, for it is difficult to procure the alkali quite 
free from that gas. The Magnefia is then laid 
to b t pure. One hundred parts of fal catharticus 
amarus contain only nineteen of pure Magnefia, 
and according to Bergman, thirty-three of vi¬ 
triolic acid, and forty-eight of water. But 
Mr. Kirvvan, who reckons only on the real 
acid, makes the proportion of acid twenty-four, 
and of water fifty-feven, parts. 
Sir 
