VIEWS ON MINERAL SPECIES. 443 



predict the composition of some of the less fully investigated 

 minerals ; the recent analysis of Leadhillite, for example, 

 by Pirsson and Wells agrees absolutely with the composi- 

 ion which Groth had in 1889 assigned to this mineral in 

 order to express it in the simplest possible manner as a 

 basic sulphate and carbonate. 



NEW SYSTEMS OF CLASSIFICATION. 



From what has been said it is clear that according to 

 modern views the true " individuals " which should con- 

 stitute the units of mineralogy, whether they be called 

 species or not, are all those simple compounds which by 

 their intermixtures and mutual replacements give rise to all 

 the known minerals ; they are as it were the letters of the 

 mineralogical alphabet, though they are not all found in 

 nature as minerals. 



Their study is in reality part of a much more compre- 

 hensive system which shall take into account the characters 

 and relationships of all known crystallisable substances 

 whether minerals or laboratory products ; the latter fill gaps 

 in the system which can never be complete without them. 



(In such a system non-crystallisable substances occupy 

 a position by themselves, and one of entirely subordinate 

 importance as regards mineralogy ; it may be doubted 

 whether more than one such substance, namely opal, occurs 

 as a natural inorganic mineral ; earthy minerals we are 

 accustomed to regard merely as amorphous varieties which 

 bear the same relation to possible crystalline compounds 

 that an amorphous precipitate of barium sulphate bears to 

 the mineral Barytes.) 



The study of the simple crystalline individuals which 

 are the units of Mineralogy and of Chemistry can only be 

 carried on by the help of their morphological and physical 

 relationships ; the important contribution which minerals 

 make to this study is the large number of suggestive 

 isomorphous mixtures which are to be found in the Mineral 

 Kingdom. Upon these is based a system of mineralogical 

 classification which is far more important than any attempt 

 to frame a rigid definition of species. 



