xi 



admirable sagacity with which they had been determined by Berzelius. 



Mineralogists and chemists had long been occupied with researches on the 

 relation between chemical composition and crystalline form ; they had 

 discovered a number of important facts bearing upon the subject, but no 

 one had discovered the basis upon which the phenomena rested. Fuchs 

 had already observed that some of the constituents of a mineral might be 

 replaced by others without any change of form, and had called these con- 

 stituents vicarious, but by adducing the sesquioxide of iron and lime as 

 vicarious constituents in Gehlenite, he showed that the true explanation 

 had eluded his grasp. Fuchs had moreover remarked the close resemblance 

 of the mineral sulphates to one another, as well as that of the rhombohedral 

 carbonates. He also showed that strontianite was not rhombohedral as 

 Hauy supposed, but prismatic, and that it resembled Aragonite in form. 

 The small percentage of strontian detected in Aragonite by Stromeyer was 

 regarded by Fuchs as the cause of the resemblance of the forms of the 

 two minerals, as the very small quantity of carbonate of lime in chalybite 

 had been supposed the cause of its resemblance to calcite. The only con- 

 clusion which Fuchs drew from the resemblances of these minerals was, that 

 certain substances possess such an overpowering force of crystallization, 

 that, even when present in small quantity, they constrain other substances 

 to assume their form. 



In November 1821 Mitscherlich returned to Berlin, was elected a Mem- 

 ber of the Academy of Sciences and appointed Professor extraordinary 

 in the University, and remained in that position till 1825, when he became 

 Professor in ordinary. In the summer of 1822 he gave his first lecture on 

 Chemistry to a large audience. He also continued his researches on isomor- 

 phism, and those which he had commenced in Stockholm, especially those 

 which bore upon the artificial formation of minerals. He exhibited to the 

 Academy a collection of about forty crystallized substances, which he had 

 found in the slag-heaps surrounding the copper-smelting furnaces of 

 Fahlun during a visit he paid to that place in 1820, in the company of 

 Berzelius. Of these, however, he described only two, a silicate of prot- 

 oxide of iron isomorphous with olivine, and a mica, the composition of 

 which approximates closely to that of a black mica of Siberia. He re- 

 sumed these researches along with Berthier in the winter of 1823 and 

 1824, which he passed in Paris, and by fusing the mineral constituents 

 together in proper proportions, succeeded in producing diopside, idocrase, 

 and garnet. 



In the course of his examination of the phosphates and arseniates he 

 had observed that the acid phosphate of soda crystallizes in two totally 

 different forms, both of which belong to the prismatic system, but cannot 

 be referred to the same parameters. From this he inferred that the 

 ultimate atoms of crystallized bodies by change of circumstances may ad- 

 mit of a change in their arrangement, and hazarded the opinion that, as 

 Aragonite resembles strontianite and cerussite in form, and calcite re- 



