TRANSITION TO CLASSIFICATORY SCIENCE. 205 



not better preserved by supposing that ammonia, as 

 a compound of hydrogen and another principle, is 

 " a type of the composition of the metals." 



Our history, which is the history of what we 

 know, has little to do with such conjectures. There 

 are, however, some not unimportant principles 

 which bear upon them, and which, as they are 

 usually employed, belong to the science which next 

 comes under our review, Mineralogy. 



