OF NATURAL PHILOSOPHY. 295 



discovered, and that the primitive formations, as 

 they are called in geology, which bear no marks of 

 having been produced by the destruction of others, 

 are also remarkable for the beauty and distinctness 

 of character of their minerals. 



(331.) The great difficulty which has been expe- 

 rienced in attempts to classify mineral substances 

 by their chemical constituents has arisen from the 

 observed presence, in some specimens of minerals 

 bearing that general resemblance in other respects 

 as well as agreement in form which would seem to 

 entitle them to be considered as alike, of ingredients 

 foreign to the usual composition of the species, and 

 that occasionally in so large a proportion as to ren- 

 der it unjustifiable to refer their occurrence to ac- 

 cidental impurities. These cases, as well as some 

 anomalies observed in the classification of minerals 

 by their crystalline forms, which seemed to show 

 that the same substance might occasionally appear 

 under two distinct forms, as well as some remark- 

 able coincidences between the forms of substances 

 quite distinct from each other in a chemical point of 

 view, have within a recent period given rise to a 

 branch of the science of crystallography of a very 

 curious and important nature. The isomorphism 

 of certain groups of chemical elements has already 

 afforded us an example illustrative of the manner in 

 which inductions sometimes receive unexpected 

 verifications (see 180.). The laws and relations 

 thus brought to light are among the most curious 

 and interesting parts of modern science, and 

 seem likely in their further developement to af- 

 ford ample scope for the exercise of chemical and 

 u 4 



