P R E F A C E. xi 



or other of the conftituent parts, as is parti- 

 cularly obfervable in zeolytes, which differ 

 exceedingly in the proportion of water which 

 enters into them as one of their conftituent 

 principles: alfo in the fparry or calcareous 

 iron ore, whofe proportion of calcareous 

 earth is fubjeft to great variation ; befides all 

 this, various fubftances fpecifically different* 

 poffefs very nearly the fame fpecific gravity. 



Neverthelefs I am far from afferting, that 

 the confideration of mere external properties 

 is intirely ufelefs ; on the contrary, I am per- 

 fuaded that from the confideration of thefe 

 alone, a very probable conje&ure may be 

 drawn in moft cafes concerning the nature of 

 foffils, whofe fpecific properties are already 

 known by analyfis, but this conje&ure muft 

 be formed by an experienced eye : for it is 

 not poffible by any defcription, to convey an 

 adequate idea of thofe minute differences, or 

 as I may fay, the phyfiognomies of foffils, by 

 which alone they are diftinguifhable. On this 

 account 1 have been very fuccinft in this par- 

 ticular, rather hoping to help, than expelling 

 to form the coup (fceil. Thofe who love to 

 amufe themfelves with a detailed defcription 

 of thefe external forms, may confult with ad- 

 vantage Mr. Rome de Lijle's improved edition 

 of his Cryftallographie *, though $aily expe- 



* Or the improved edition of Cronfe^ which Mr. Magel* 

 Ian promifes foou to publifh. 



rience 



