(f Magnejian Earth. 449 
formed by the union of vitriolic acid with fili- 
ceous earth ; and the calcareous, a combination 
of the fame earth, with certain portions of air 
and water, effedted by the operation of various 
marine animals: and he fuppofes, that when 
calcareous earth is deprived of air and water, it 
will return to its primitive date, viz. that of fili- 
ceous earth. 
This theory is, perhaps, rather fanciful, than 
juft. The operations of Nature, it mull be 
allowed, are generally Ample—but we may fim- 
plefy too far; and, in forming fyftems, we fhould 
not fuffer our imaginations to carry us beyond 
thofe bounds, which our fenfes, and experiment, 
warrant. 
Calcareous earth, indeed, in the form in which 
we commonly find it, is a compound, confiding 
of earth, air, and water; and is not cpnfidered 
as pure, till it be deprived, by fire, of the two 
laft elements. It is then, properly, pure calcare¬ 
ous earth ; but it does not appear that longer cal¬ 
cination, though it may dived it of fome of the 
properties of calcareous , will ever reduce it to the 
nature of filiceous earth. 
Every earth with which we meet, and which, 
v/hen feparated from thofe acids, or other 
accidental bodies, that are combined with it, 
refids every power we poffefs, to produce any 
farther decompofition, and yet differs in all, or 
any of its properties, from the other known 
^ 0L * G g earths. 
