Xll 



sembles dolomite, chalybite, and diallogite, it is possible for the substances 

 isomorphous with Aragonite to crystallize in the form of calcite, and the 

 substances isomorphous with calcite to ciystalhze in the form of Aragonite, 

 and so greatly enlarge each group of isomorphous bodies. This opinion 

 was looked upon with great distrust by chemists and mineralogists. All 

 the examples he had brought forward were taken from compound bodies, 

 which possibly might have contained admixtures which analysis had failed 

 to detect, and the substances assumed to have the same composition might 

 after all be different. These doubts were suggested by the analyses of 

 Aragonite, which had been pronounced by some of the most eminent che- 

 mists of the time to be pure carbonate of lime ; then Stromeyer detected 

 strontia in it, which, notwithstanding that its amount was very small, and 

 different in Aragonite from different localities, was immediately regarded as 

 the cause of the difference of its form from that of calcite ; lastly, Buch- 

 holtz proved the existence of a variety of Aragonite absolutely free from 

 any admixture of strontia, to which, therefore, the difference of form could 

 not by any possibility be due. At this conjuncture Mitscherlich made the 

 remarkable discovery that sulphur also takes different forms under different 

 circumstances. The crystals obtained from solutions belong to the pris- 

 matic system, and are identical in form with those which occur in nature ; 

 but when sulphur is fused and allowed to cool, with proper management 

 distinct crystals are obtained, but they are entirely different from the 

 former, inasmuch as they belong to the oblique system. This observation 

 was of great importance, because sulphur being a simple substance crystal- 

 lizable at pleasure in either of its two forms, the difference of form could 

 not be attributed to a difference of composition. He had already proved 

 that the acid phosphate of soda and carbonate of lime possessed the same 

 property of crystallization in two different forms, which he now considered 

 as appertaining to all simple substances and their chemical combinations, 

 and to which he gave the name of dimorphism. He regarded it, more- 

 over, as affording an explanation of the fact that bodies possessing analo- 

 gous chemical constitutions are not always isomorphous. The memoir on 

 the dimorphism of sulphur was presented to the Academy on the 26th of 

 July 1826. 



It was found that the forms of isomorphous substances are not absolutelv 

 identical, except, of course, when they belong to the cubic system, but 

 exhibit some differences, showing that the chemical nature of the substance 

 is not altogether without influence on the form. In order to determine 

 the difference between the angles of isomorphous bodies with greater 

 accuracy than was attainable by the use of the ordinary Wollaston's gonio- 

 meter, he caused a goniometer to be constructed by Pistor, provided with 

 four verniers, each reading to 10", and with a telescope magnifying twenty 

 times for viewing the reflexions of the signal in the faces of the crystal. 

 With this instrument, in the summer of 1823 he began to measure the 

 angles of calcite from Iceland, and was surprised to find differences iu the 



