of Magnefum Earth . 451 
i. quantity of calcareous matter as to dilguife 
its diftinguifhing features, and even make it 
aflume fome of the character!fries of calcareous 
earth. But the accurate experiments of Mefirs. 
Black and Margraaf, foon removed the obftacles, 
which impeded our more perfect acquaintance 
with its nature, and they procured it pure from 
the mixture of other earths; in which ftate they 
found it fcr different, from what it had been re- 
prefented, that it was declared to be an earth, 
Jui generis. But though Dr. Black went thus 
far, he did not proceed to fet it at the head of 
a diftinct genus; but placed it, with all thofe 
earths, which he has claffed together, under the 
general generic head of Ahjorbent Earths , of which 
he was content to make this a fpecies. It has 
fince obtained a more honourable rank, and forms 
a feparate genus. 
M. Fourcrov, who follows the arrangement of 
his predecefior M. Bouquet, has placed calca¬ 
reous, ponderous, and magneffan earths among 
the falts. There fhould feem to be ftrong ob¬ 
jections to this arrangement, even with refpeft 
to the two former ; though their being fapid, 
and foluble in water, may give fome counte¬ 
nance to it. But the infipidity and total infolu- 
bility of pure magnefia, together with its not 
being fuffble, by the ftrongeft heat we can apply, 
as will prefently be friewn, are proofs of its being 
a real earth. 
G g 2 Though 
