145 



measure of the crystallized petalite from Elba. I have not, 

 however, succeeded in finding any analogy between the crystallo- 

 graphical constants of the two minerals. On the other hand 

 there exists a partial agreement between the axial relations of 

 leucosphenite and eudidymite. This appears distinctly if 

 the rt-axis of the former mineral be multiplied by three; one 

 then obtains 



for leucosphenite: а\Ъ\с = 1,7439:1 : 0,8501; ß = 93° 23' 

 - eudidymite: а:Ъ:с = 1,71075:1:1,10712; ß = 93°45V2'. 



The angle ß of the two minerals agrees very closely, and 

 so is also the case with the «-axes. The angles in the vertical 

 zone are for both very near 60°, i. e. they are both in a sense 

 pseudo- hexagonal. With both minerals twinning occurs with 

 the third pinacoid as composition-face. If the chemical formula 

 of eudidymite is written 



BeSiO^'HNaSiO^O^, 



then this mineral may be regarded as partially derived frøm 

 the same silicic acid, 



which must be assumed to enter into the composition of 

 leucosphenite. The two minerals cannot, it is true, be said to 

 be isomorphous in the proper sense of the word , but it is an 

 indisputable fact that there exists a remarkable analogy between 

 them crystallographically as well as chemically. 



Before the blowpipe leucosphenite decrepitates and in 

 the forceps fuses with some difficulty to a dark globule. In 

 the salt of phosphorus bead a skeleton of sihca is obtained; 

 on cooling the bead becomes opalescent. The mineral is de- 

 composed by hydrofluoric acid, but not acted upon by other 

 acids. 



The common leucosphenite has been met with only at the 

 locaUty No. 2. On digging in this place a rather considerable 

 xiiv. 10 



