448 Mr. Henry on the Natural Hi/lory, idc. 
On the Natural History and Origin of 
Magnesian Earth, •particularly as connected 
with thofe of Sea Salt, and of Nitre ; with 
Observations onfome of the Chemical Pro¬ 
perties of that Earth, which have been y 
hitherto , either unknown , or undetermined. By 
Thomas Henry, F. R. S. &V. 
AT UR A LISTS and Chemifts formerly 
^ divided the different kinds of earths into 
five genera, viz. the Siliceous, or Verifiable, the 
Calcareous, the Gypfeous, the Argillaceous and 
the Talcky. But recent difcoveries having proved 
the Gypfeous, to be a compound of Calcareous 
earth and vitriolic acid, and the Talcky, to con- 
fid of a mixture of Argillaceous with other earths, 
the genera were reduced to three. M. Baume 
has even attempted a farther reduction ; * and 
has endeavoured to prove, that two of thefe are 
merely modifications of the other: argillaceous 
earth being, according to him, a vitriolic fait, 
* Baume, Chymie Experimentale et Rationee, vol I. 
far la Terre. 
formed 
