4;o Mr. Henry on the Natural Plijloiy, &c. 
earths, may be admitted to form a new gentm 
And, on thefe grounds, two new genera have 
lately been added to the other three. The bafis 
of fome fpars, * * * § which had been generally deem-^ 
ed to be calcareous earth, has been proved 
to differ from it, in its affinities and fome other 
properties; and has been admitted to form a 
diftindt genus, under the title of Barytic or Pon¬ 
derous Earth : and another kind of earth, which 
though rarely, if ever, found pure in nature, y<"t 
abounds, in great quantities, in various forms of 
combination, from which it may be feparated by 
chemical means, has been allowed to form a 
fifth genus, under the appellation of Muriatic , 
or, Magnefian Earth. 
This earth was alfo, for fome time, con¬ 
founded with calcareous earth, of which it was 
fuppofed to be a modification ; for by the earlier 
modes of obtaining it, it was mixed with luch 
* x. Marmor Metallicum, Croftjl. 182. Gypfum Spa- 
thofurn, Spathum Fufibile, Margraf. &c. Terra ponde- 
rofa vitriolata. Bergman. Sciagrapbia Ponderous Spar. 
2. Marmor Metallicum, Cronjledt Min. § 18. B. Calk 
or Cauk. 
3. Gypfum cryftallifatum capillare, Cronjledt. Min. 
§ 19. B. Radiated Cauk. 
4. This earth has been alfo found of a fparry appearance, 
and combined with aerial acid only ; Terra ponderofa aerata. 
See Dr. Withering’s Experiments and Qbferaiations on the 
Terra Ponderofa. Phil, Tranf. vol LXX 1 V. p. 293. 
a quantity 
