162 Mr. Brooke on Isomorphism. 



alike, and they were ascertained therefore not to be strictly 

 isomorphous. The sulphates are right rhombic prisms, and 

 a corresponding dihedral angle of each afforded the following 

 measurements: 



Sulphate of barytes 101° 42' 



strontian 104 



lead 103 42 



It became necessary therefore that the doctrine of isomor- 

 phism, in the strict sense of the term, should as a general prin- 

 ciple be abandoned ; and it is not unreasonable to conclude 

 that the crystals wliich suggested the theory, and which ap- 

 pear to measure alike, may really differ in some small quantity 

 which the goniometer does not detect. 



But although the doctrine of isomorphism, or absolute 

 identity of form, cannot be supported, it has been said that 

 the forms in each respective case belong to the same system of 

 crystallization, and they have therefore been termed plesio- 

 morphous by Mr. Miller, of Cambridge, in a paper on some 

 artificial crystals, read to the Cambridge Philosophical So- 

 ciety, in March 1830; and if even the class of primary form 

 can be indicated with certainty by the chemical composition 

 of a crystallized body, a benefit will so far have been con- 

 ferred on science by the theory of Mr. Mitscherlich. 



But the doctrine of isomorphism has been carried much 

 further by Mr. Rose, and other eminent chemists on the con- 

 tinent, and has given rise to a theory, from which some check 

 to the progress of mineralogical chemistry is greatly to be 

 apprehended. 



It is well known to mineralogists, that there is a consider- 

 able diversity in the chemical composition of some minerals 

 which have been regarded as belonging to the same species. 

 Among the most remarkable examples are amphibole, pyrox- 

 ene and garnet. 



Before the theory of isomorphism was proposed, it was 

 conceived that amphibole was composed of some invariable 

 elements in definite proportions, although, from the disagree- 

 ment in the analysis of different specimens, neither the nature 

 nor the proportions of those elements had been accurately 

 determined. 



It was also supposed that the differences in the actual com- 

 position of different specimens were occasioned by the acci- 

 dental mixture of extraneous matter with the real constituent 

 elements of the mineral. This view is taken by Haliy, and 

 is illustrated in reference to amphibole as follows : 



Analysis 



