Measurement and Cleavage of certain Primitive Crystals. 251 



use of the reflecting goniometer ; these agree in the incidence of 

 P on P as 136° 47', given by Haiiy as 137° 10'. These crystals 

 are very small, and as they differ from each other, and exhibit the 

 planes of some modifications not hitherto described, I have not 

 found courage to run the hazard of sacrificing them, incidental to 

 the attempt to cleave them in the direction of* their natural joints. 



Specular Iron, 



Fig. 3. 



This substance, a variety of the fer oligiste of Haiiy, may be 

 split in the direction of its natural joints when held in the hand, 

 by means of the pincers, care being taken to place their edges 

 parallel with the primitive planes, which are generally observable 

 on the edges of the crystals from Elba. In this manner I have 

 procured six fragments, one of them the primitive rhomboide, 

 which is slightly acute, nearly complete, and all of them having 

 two or more planes sufficiently brilliant for the use of the reflecting 

 goniometer, and all afford the measurements of 86° 10' or 93° 50', 

 some of them both ; the former being 59' less, the latter 59' more 

 than the measurements obtained by Haiiy on the natural planes by 

 means of the common goniometer. The perfect agreements afforded 

 by the fragments, have so far satisfied me that the results are the 

 true value of the angles of the primitive rhomboide, that I have 

 not attempted to measure its angles by means of the reflections to 

 be observed on the natural planes, which are often very brilliant; 

 for experience has confirmed me in the opinion that as the natural 

 planes do not often yield results agreeing amongst themselves, they 

 cannot be relied on with the same confidence as the planes obtained 



