PREFACE. XI 



gebraical system of notation. Nor have I thought it best 

 to quote the corrected angles in place of such as were ob- 

 tained by observation, except in those crystals depending 

 upon forms of invariable dimensions. In other cases, it 

 would no doubt have been safe to have employed such an- 

 gles in the great majority of instances, yet the slight varia- 

 tions occurring in die constancy of angles in the individuals 

 of several species, still leaves us in doubt what dimensions 

 to assume as the most free from error by which to correct 

 the others. Until this confusion is removed, the student 

 will not suffer much inconvenience by adopting the present 

 somewhat circuitous method of becoming acquainted with 

 crystals. And besides, to have adopted the mathematical 

 treatment of crystals alluded to, would have prevented a 

 large number of persons from understanding the subject, 

 to whom in its present popular fo/rn, it is perfectly intel- 

 ligible. 



In the preparation of this work it has been necessary to 

 examine a great number of doubtful minerals, as well as 

 newly proposed species with which the Scientific Journals 

 abound. It may therefore be expected that something 

 should be said in this place concerning the disposition which 

 has been made of these materials. But a single new spe- 

 cies, the Microlite, has been proposed; though a number of 

 observations have been made, calculated to place the spe- 

 cific claims of a few minerals before proposed in a strong- 

 er light. 



If, however, my investigations have not led me to in- 

 crease the number of species, they have on the other 

 hand, compelled me to treat a number heretofore regarded 

 distinct, as varieties only of older species, examples of 



