i xiii 1 
i3om, and never more than two diferent kinds 
found mixed together in one body ; whereas ia 
the mineral kingdom it is very common, though 
it will neverthelefs always remain concealed 
from e ery one, however penetrating, %vho has 
not employed himfelf in the compounding or de-^ 
compounding fuch bodies^ as far as the prefent 
knowledge of thefe matters will permit. 
So much may be faid in general concerning 
this Eflay ; but now I ought more particularly 
to inform my readers of the motives why I have 
now and then deviated from the orders and dif- 
tinflions hitherto ufed. 
Earths and Stones are comprehended in one 
clafs, becaufe i. they conlift of the fame prin- 
ciples ^ 2. they are by turns converted from one 
into the other, infomuch that an earth may 
in length of time become as hard as a ftone, 
and vice verfa ; nor can the true difference be- 
tween a ftone and an earth be pofitively pointed 
out by the degrees of hardnefs or foftnefs \ for 
where is it that the common chalk iinifhes, and 
the lime-ftone begins in the Englifh ftrata ? 
and how is a clay, whether in water or not, to 
be diftinguifhed from the foft and unftuous 
foap-rock, or Smedis. 
The divifion of earths into Vitrefcentes and 
Apyri is here omitted, fince all of them are, in 
a due degree of heat, found equally apt, either 
per fe^ or by means of fome natural or artificial 
mixture, to be reduced to glafs equally as well 
as thofe hitherto called Vitrefcentes ; which aro 
l^^early the ipoft refradory in the fire, and ought 
tQ 
