1820.] to Professor Gilbert. 331 



man to delineate 13 varieties of crystals. I shall state some of 

 it here. 



The essential constituents of the schorl family are not confined 

 to boron. They are 



1. Electropositive. 



a. Either an alkaline earth or alkali. Among these the oxide 

 of zinc is to be ranked; perhaps even oxide of iron or manganese 

 may be substituted ; or all of these may exist together. 



0. Alumina. (It is essential even in boracite, which probably 

 contains some per cents, more than is at present known.) It 

 has still to be pointed out in anatase. 



2. Electronegative. 



a. Silica, or rather quartz, for which oxide of titanium is 

 sometimes substituted. 



h. Boron. (The appearances which the schorl family exhibit 

 when thrown into red-hot saltpetre, demonstrate that, boracite, 

 &c. contains a combustible substance (boron), not a product of 

 combustion {boracic acid) ; although this last is the substance 

 obtained by analysis. It is a product, not an educt. Boron is 

 still to be found in anatase and andalusite. 



The substances upon which the system of crystallization, and 

 consequently the species depend, are 



1. In boracite (tetrahedral schorl) magnesia. Boracite is 

 therefore a magnesian schorl. 



2. In tourmaline (trigonal schorl) the quartz substance. It is 

 therefore siliceous, or quartz schorl. 



3. In anatase (tetragonal schorl) titanium. It is therefore a 

 titanium schorl. 



4. In andalusite (rhombic schorl) a substance not yet disco- 

 vered experimentally ; very probably fluorine. 



5. In axinite (rhomboidal schorl) boron. (This substance 

 therefore, is not merely essential as a generic, but still more 

 essential as a specific substance ; and I first ascertained the 

 necessity of its presence in this mineral from theory.) It is 

 consequently a boronic schorl. 



Boron is indeed an essential constituent of datholite ; but as 

 in it the hardness of schorl is wanting ; so likewise is the consti- 

 tuent which gives hardness ; namely alumina. 



You perceive that I have not reposed upon any authority. In 

 all systematic disputes in the dominions of nature, the sole and 

 ultimate arbiter is nature herself. And in the mineral kingdom 

 nature has expressed here classes and orders by the external 

 characters. In the schorls we perceive the same lustre, the 

 same colours, the same play of light, the same polarity of crys- 

 tallization (determined in my treatise), the same hardness, the 

 same specific gravity, from 2*9 to 3*3 (and if that of anatase be 

 correct, as high as 3-8), the same electricity, &c. Hence it is 

 evident how completely nature has given us the means of classi- 



