252 Mr. William Phillips on the 



by cleaving crystals in the direction of their natural joints, which 

 almost always agree ; when they do not, the cause may always be 

 discovered by the observer. 



Diopside. 

 Fig. 6. 



The crystals of this substance in my collection are not brilliant 

 enough on the natural planes to give perfect reflections. On apply- 

 ing the pincers to one of them parallel with the planes of its prism, 

 I found that it did not yield in that direction, but in that of its 

 diagonals. The only three fragments submitted to the reflecting 

 goniometer agree in affording the measurement of M on M, 87° 5' ; 

 being 37' less than that obtained by Haiiy on the natural planes. 

 Two of these fragments also yield the complementary incidence of 

 92° 55' \ being the angle of M on the adjacent plane over the edge A. 



The diopside is considered by Haiiy to be a variety of the py- 

 roxene. In the attempt to cleave the latter substance, I have not been 

 able to overcome the difficulties it presents. One crystal yielded to 

 the equal pressure of the edges of the pincers, but did not present 

 brilliant surfaces in more than one direction. It may therefore be 

 true of the pyroxene as of many other minerals, that its cleavage is 

 more difficult in one direction than another ; but the circumstance 

 just mentioned may perhaps in this instance be attributable to the 

 heat which this crystal had the appearance of having undergone. 

 Two others, of considerable external lustre, fell into powder under 

 the pressure of the pincers. Two crystals presenting clear reflections 

 on the natural planes, gave the incidence of M on M, one 86° 55', 

 the other 87° 5'. Two others, also brilliant, gave the value of M 

 on the adjacent plane over the edge A, each 93°. 



