Sheparcfs Treatise on Mineralogy. 171 



to the species, and to which is affixed in separate paragraphs and 

 usually in smaller type a variety of other information drawn frora 

 other sciences and here collated, not because it belongs to mine- 

 ralogy, but for the convenience of the student, who would oth- 

 erwise be obliged to refer for it to numerous, independent works. 



You observe in the next place, that "chemical characters alone 

 are perhaps as insufficient as characters purely physical, in the de- 

 termination of minerals." It is perhaps impossible to settle this 

 point from the history of the science, since it has scarcely ever 

 been attempted ; the practice rather having been to employ both 

 sets of characters together, sometimes leaning more to one side 

 and sometimes more to the other. But to myself, it has always 

 appeared that the uncertainty has been just in proportion to the 

 preponderance allowed to the chemical side ; while if even a 

 parity in point of conclusiveness between the two methods could 

 be established, I should yet claim that the pure method should 

 be adopted in preference to the mixed, not merely on the score 

 of logic, but also because it neither presupposes a knowledge of 

 chemistry, nor leads the student to a harmful neglect of the nat- 

 ural properties of minerals. 



You observe that " to the chemist, rutile and anatase, Silliman- 

 ite and kyanite, white iron pyrites and iron pyrites, carbonate of 

 lime and arragonite, garnet and idocrase, are identical substances." 

 It is true that in the present state of chemical science we are 

 obliged to regard these sets as identical, unless the new research 

 of Glocker* relative to one of the supposed varieties of white 

 iron pyrites should lead to a single exception in the list ; but be 

 that as it may, all allow these substances to be different in mine- 

 ralogy, in virtue of the non-identity of their natural properties, 

 and not as you remark by the laws of dimorphism ; for the prin- 

 ciples of this modern branch of physics do not establish it as ab- 

 solutely certain that among many dimorphous bodies any such 

 change of hardness and gravity attends that of form also, as 

 would necessarily lead to the establishment of distinct species, in 

 the case of dimorphous minerals. 



But not only do you object to the principle on which the prac- 

 tical or determinative part of the work is founded ; you allege 

 also, that the tables " are too brief to enable the learner, who is 

 unacquainted with the science to determine a species with facil- 



* Pogg. Ann., LV, 489. 



