12 Dr. G. T. Prior — Progress of Mineralogy. 



new problems presented. The importance of the accurate determination, 

 of the chemical composition of minerals, as insisted upon by Berzelius 

 and Rammelsberg, was too obvious to be long neglected; and in 

 1850 J. D. Dana, in the third edition of his System of Mineralogy, 

 the textbook which has since become the standard work of reference 

 for English-speaking mineralogists, rejected the natural history 

 classification, "with its classes, orders, genera, and Latin names," 

 as set forth in the first edition, and replaced it by a chemical one. 

 As Dana remarked, however, the natural history classification had 

 some advantages in "displaying many of the natural groups which 

 chemistry was slow to recognize", for example, the felspars. 

 A reconciliation of the purely chemical and the natural history 

 classification was effected by Gustav Kose, who in 1852 published 

 his crystallo-chemical system under which crystalline form was 

 given almost as much importance as chemical composition in the 

 distribution of minerals into groups. This was the system which 

 was adopted by Story-Maskelyne for the re-arrangement of the 

 mineral collection of the British Museum. The general acceptance 

 of such a system, in which minerals were classified, in the first place 

 by chemical composition, and in the second place by crystallographic 

 characters so as to bring together members of isomorphous groups, 

 marked a distinct advance in the science of mineralogy. 



It was in the very year in which the Geological Magazine first 

 appeared that Tschermak published his discovery of the nature of 

 the felspar group. The fact that minerals with such apparently 

 dissimilar chemical compositions as albite and anorthite should 

 crystallize together in all proportions threw a new light upon the 

 principle of isomorphism, and helped to pave the way towards the 

 modern view that it is the arrangement in space of the atoms 

 themselves that determines the crystal structure. 



During the period under review, indeed, the great problem which 

 has exercised the minds of mineralogists has been the determination 

 of the connexion between the chemical composition, the crystalline 

 form, and the physical (more especially optical) characters of 

 minerals. 



The close interrelation between optical and crystallographic 

 characters had been to a large extent already recognized by the 

 work of. Brewster, who had shown how different types of crystals 

 differed in their behaviour towards polarized light. The gradual 

 accumulation of knowledge which led to the distribution of crystals 

 into groups or systems, each distinguished from the others by the 

 optical characters of its members, is well brought out in the 

 Introduction to the Study of Minerals, one of the British Museum 

 Guidebooks, first published in 1884, for which geologists desirous 

 of gaining some knowledge of minerals owed a debt of gratitude to 

 Sir Lazarus Fletcher, the successor to Story-Maskelyne in the Keepen- 

 ship of the Mineral Department. The exact determination of the optical 

 characters of rock-forming minerals such as the felspars, pyroxenes, 

 and amphiboles, which has been rendered possible, first by the 

 researches of Des Cloizeaux and Tschermak, and later by the 

 improvements in apparatus and methods effected by more modern 



