OF ARTS AND SCIENCES. 



487 



angles were ; but on examination it was found that the lines, and prism 

 faces as well, could all be referred to a cube with twin members on all 

 the trigonal axes. 



The face of such a twin is represented by the diagram between the 

 two figures of the crystal, and the upper figure was actually drawn 



Fig. 6. Coahuila, Mexico. 



from the cube diagram by means of a parallel ruler, and it will at 

 once be seen from the close coincidence of the two figures that the 

 upper one must be correct, the lower one only varying within the 

 limit of error of a mechanical drawing. 



On examining the Hauptmannsdorf iron, which has long been known 

 to break with a cubic fracture, a cube face under the microscope ap- 



