X PKEFACK. 



amined and described ; consequently it is less advis- 

 able to receive at once newly discovered minerals 

 into the system, when we see that even those deter- 

 minations, which were usually considered most firmly 

 established, have frequently been found, on more 

 accurate examination, to be erroneous. The species 

 contained in this appendix must be viewed exactly 

 in the same light as the plantce mcertce sedis in the 

 natural system in Botany (not in artificial systems, 

 which cannot admit of any appendix), which are 

 not included in any of the systematic unities, not- 

 withstanding the advantage that the examination of 

 one, or of two individuals at the most, should here 

 be sufficient for knowing the species to its full ex- 

 tent. 



The properties of the minerals contained in the 

 second Appendix are such, that we cannot expect 

 that they will ever form particular species, since 

 they are not susceptible of a natural-historical deter- 

 mination. Some minerals of this description have 

 been enumerated in the Observations annexed to 

 those species, to the decomposition of which they 

 owe their existence, as, for instance, Porcelain-Earth, 

 which is noticed under the head of Prismatic Feld- 

 spar. 



The copper-plates, which have been extremely well 

 executed by Mr MILLER, are intended not merely to 

 represent the figures quoted in the general descrip- 



