[ xili ] 



dom, and never more than two different kinds 

 found mixed together in one body ; whereas in 

 the mineral kingdom it is very common, though 

 it will neverthelefs always remain concealed 

 from every one, however penetrating, who has 

 not employed himfelf in the compounding or de-> 

 compounding fitch bodies, as far as the prefent 

 knowledge of thefe matters will permit. 



So much may be faid in general concerning 

 this Effay ; but now I ought more particularly 

 to inform my readers of the motives why I have 

 now and then deviated from the orders and dis- 

 tinctions hitherto uted. 



Earths and Stones are comprehended in one 

 clai's, becaufe i. they confift 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 r cerfa : 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 finifhes, and 

 the lime-done begins in the Englifh ftrata ? 

 and hovV is a clay, whether in water or not, to 

 be diftinguifhed from the foft and undtuous 

 foap-rock, or Sme&is. 



The divifion of earths into Vitrefcentes and 

 Apyri is here emitted, 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 are 

 nearly the moft refra&ory in the fire^ and ought 



to 



