46 Prof. Maskelyne aad Dr. Lang's Miner alogical Notes. 



sharow, in which the planes of even well-developed crystals have 

 undergone an alteration in their relative position. 



One crystal of this kind was found to be a twin, fig. 2. The 

 two individuals crossed one another nearly at right angles, and 

 the faces a and v both were in one plane. The twin face is there- 

 fore parallel to (01 1), as for this twin face the two individuals 

 cross at angles of 94° 8' and 85° 52'. The crystal was so small 

 that it was impossible to measure angles formed by faces of the 

 two individuals. 



The crystals on which the prism (0 1 1) predominates are com- 

 binations of the faces ab cnhuz (figs. 3, 4) ; the face c being 

 always more developed than b, and the face z occurring only 

 very seldom. None of these faces are very brilliant, and the 

 worst of it is that the face u is always curved, so that it was 

 found impossible to get an angle of the prism for measurement 

 except by a rough approximation. 



I found ° a 



na = 2o lo 



ha = 44 appr. 



su = 35 „ 



We see that the crystals of this kind are composed of faces 

 which, with the exception of the face c, are new. As the angles 

 could not be measured with great accuracy, it might be supposed 

 that the face h is not a new face, but in fact identical with m. 

 But this is contradicted by the results obtained from the optical 

 properties exhibited by the crystal ; for in the polarizing appa- 

 ratus one finds that the axis of greatest optical elasticity must 

 be parallel to the edge ah, and therefore this edge must be parallel 

 to the axis b ; for the observations of Descloizeaux give the fol- 

 lowing symbol as representing the optical orientation, 



c6 a; 



+ 



that is to say, the axis of smallest optical elasticity (which is at 

 the same time the first mean line) is parallel to the greatest 

 crystallographic parameter, the axis of greatest elasticity parallel 

 to the mean, and the axis of mean elasticity parallel to the small- 

 est parameter. 



5. Aerolitics. J5yN. S. Maskelyne. 



The branch of science that treats of Meteorites has acquired 

 sufficient importance to justify our giving it a special name, and 

 I therefore propose for it the denomination with which this article 

 is headed. Many reasons conspire to render this study of u aero- 

 litics " one of increasing interest, and to make it highly desirable 

 that collections of meteorites should exist to illustrate it, as com- 



