Measurement and Cleavage of certain Primitive Crystals. 261 



pyramid, varying from 78° 35' to 78° 48'. One elongated crystal 

 similar to fig. 10, gave P' on the opposed plane over the summit 

 103° 40'. All these crystals reflected with uncommon brilliancy. 

 One of eight crystals, sufficiently bright to afford clear reflections on 

 each of the four elongated planes P P (fig. 10.) and more nearly 

 approaching to agreement than any of the others, gave on the two 

 obtuse angles 101° 12' and 101° 15', and on the two acute 78° 40' 

 and 78° 43'. Another among them gave one incidence of 78° S5\ 

 and another of 78° 48' on the two acute angles. 



1 am induced to be thus particular, not only on account of the 

 vast difference between the measurements given by Haiiy and 

 those obtained by means of the reflecting goniometer, but also be- 

 cause it is scarcely possible to find another in the whole range of 

 mineral substances, which seems to unite more decidedly the cha- 

 racters of perfect crystallization and brilliant surface. It seemed 

 therefore pre-eminently adapted to the use of the reflecting gonio- 

 meter. The result however proves that the sulphate of lead is one 

 among many minerals, on even the perfect reflections of whose na- 

 tural planes no reliance can be placed without numerous coinciding 

 results ; if indeed it would be possible to obtain them at all. 



In the endeavour to cleave this substance parallel with its natural 

 joints, I was at first greatly foiled by its extreme brittleness, which 

 without great care causes its fracture to assume the conchoidal 

 form. The directions for finding the joints given by Haiiy, are 

 quoted in his own words ; if I comprehend them they are not 

 accurate, but they are not expressed with his usual perspicuity. 



As the elongated planes present the largest surfaces, it was an 

 inducement to attempt a cleavage parallel with them in the first 

 instance, on the presumption of their being planes of the primitive 

 octohedron ; but after destroying a large number of crystals, I was 



Vol. iv. 2 l 



