80 



frosting ; or, as the Count expresses it, the lustre of those 

 coloured metal plates known by the name of foil, and are 

 most splendid when the light falls on the broad planes. The 

 edges are more opaque, partly from the contrary direction 

 of the crystal, and partly from the strias in the direction of 

 the laminae. Fig. 2. is a general group of crystals. Fig. 3. 

 Shows a variety in my possession of a yellower tint *. 



The lower geometrical figures show , according to Count 

 Bournon's measurement^ that if the inclined sides were to 

 be increased by a regular set of decreasing plates placed 

 upon the surface till they formed an equilateral triangle, 

 they would become oblique octaedrons, (see right hand 

 figure •) and if they further continued on these planes till 

 they were lost, they would produce a rhomboidal prism, 

 which, as it seems to agree with the fragments, may be the 

 primitive form. I should have observed that it not only 

 splits into laminae on the broad planes, but that it also rea- 

 dily does so with the side facets. Its fracture is sometimes 

 irregularly conchoidal and glassy. Spec. grav. 2,548. Mr, 

 Chenevix found it to contain oxide of copper 58, arsenic 

 acid 21, water 21. 



* These two are somewhat magnified. 



