3^2 M. Berxelius on Isomorphism a$ [Hat, 



crystallizing power of its constituent parts ; but at the very time 

 when the results of accurate chemical analyses began an uncer- 

 tain contest with that principle of Haliy's school, which consi- 

 ders that tioo bodies of different composition can never liave the 

 same crystalline form, unless it belong to limited forms {formes 

 limites), the question was all at once decided by the opportune 

 and unexpected discovery of M. Mitscherhch, according to 

 which, bodies composed of different elements, but of an equal 

 number of atoms similarly combined, assume the same crystalline 

 form. MM. Rose, BonsdorfF, and Trolle-Wachtmeister, have 

 already profited by the light which this discovery has thrown on 

 mineralogy, and proved that the species called pyroxene, amphi- 

 bole, and garnet, contain a great number of different compounds, 

 formed in an analogous manner ; so that if a species, according 

 to the received definition, be composed of combinations similar 

 in their elements and in their proportions, the three crystalline 

 forms above-mentioned must contain a great number of mineral- 

 ogical species ; for most of the pyroxenes, araphiboles, and 

 garnets from different localities, differ in the number and pro- 

 portion of their elements, although those elements are combined 

 in the same manner. However, there certainly is no mineralogist 

 who would not be shocked at the idea of making a particular 

 species of every amphibole or garnet differently composed, and 

 yet, on the other hand, they cannot with any greater propriety 

 be considered as identical. What, therefore, is to be done ? 



I do not believe that our knowledge is yet sufficiently advanced 

 to enable us to give a satisfactory answer to this question ; and 

 hence arises the difficulty of a first attempt to treat mineralogy 

 according to chemical principles. If, on one hand, it be true 

 that two garnets, for instance, having no other common element 

 but silica, cannot be considered as of the same species, it is not 

 less so on the other, that they may differ in an infinity of ways ; 

 and, as we must not assume as identical what is not so, nor 

 establish endless varieties, we have to seek between these two 

 extremes that just medium which is by no means easily found. 

 A mean, however, we must adopt, but on the condition of aban- 

 doning it for a better, when the further progress of the science 

 shall have enabled us to do so. 



It is now clear, therefore, that the hitherto generally received 

 definition of a mineralogical species, the same elements combined 

 in the same proportions, whether we add Huiiy's expression, with 

 the same limited crystalline forms, or not, can no longer be 

 admitted in every case in which isomorphous substitutions inter- 

 fere ; and, till a generally applicable principle shall have been 

 discovered, we must adopt a particular mode of viewing those 

 cases. On one hand, the crystalline form, on the other, the 

 formula of composition mark them as a group of combinations, 

 which, by their greater or less mutual conformity, perfectly 



