of the Sulphato-tri-Carbonate of Lead. 229 



masses, the faces of Pi? -f- 1 or e are considerably increased, while 



Tp 



those of g or P disappear, the whole will assume, very near- 



* 



ly, the form of an acute rhombohedron, whose apices and lateral 

 solid angles are truncated. The incidence of e upon e' is 

 := 72 39', almost the same as the angle given by Mr BROOKE 

 for the terminal edge of his acute rhombohedron. Even 

 in crystals most perfectly formed,, it is very easy to overlook 

 -the small salient angle of 179 10' upon the faces RST, sup- 

 posed perpendicular to the axis of the rhombohedron ; but this 

 composition is often so intricate, particularly in larger crystals, 

 that it sometimes becomes difficult to point out the direction 

 and extent of each separate individual, though the existence of 

 the composition is indicated by small, salient and re-entering 

 angles, and proves, with the highest degree of evidence, that the 

 forms of Axotomous Lead-baryte are not Rhombohedral but He- 

 miprismatic. 



Although the observation of the optical properties of mine- 

 rals can never supersede the study of their regular forms, yet 

 the preceding examination of the forms of Axotomous Lead- 

 baryte, affords an ample proof that they may be highly useful in 

 guiding us through the latter, particularly if these regular forms 

 nearly coincide with certain limits. The crystallographic re- 

 searches relative to this species are attended with considerable 

 difficulties, since the angles approach in every instance within 

 one degree to the limits of 120 and 90, and the regular com- 

 position very often hinders the crystals from being observed on 

 all sides, while the inclination of the optical axes of no polarisa- 

 tion upon each other is very considerable, and easily ascertained. 



The inferences drawn from Dr BREWSTER'S general law, 

 respecting the existence of two polarising axes in crystallised sub- 



