50.2 Prof. Kobell on Asterism and 



gonal of the face ; on the basal surface a regular three- rayed 

 star is also seen, whose rays are directed to the junction edges 

 with the rhombohedron of cleavage. Sometimes a fourth ray 

 goes through the star, dividing the angle of 120°. 



The phenomenon presented by dolomite is like that of calcite 

 when a cleavage-piece is corroded with hydrochloric acid, by 

 leaving it for a few days in the acid, or accelerating the action 

 by heating. But the reflected image is different from that of 

 calcite, in that the angle a between the rays is notably obtuser, 

 and that the ray r is very short, and not, as in calcite, turned 

 towards the lateral edge, but towards the vertical edge. In this 

 corrosion, dull and striated bevelled edges are observed on the 

 vertical faces. 



With nitric acid I only obtained distorted pictures ; even with 

 rough polishing I could not obtain the band of light as with calcite. 



In the magnesite of Snarum in Norway, when a cleavage piece 

 is boiled for some time in hydrochloric acid, the image of reflexion 

 islikethatof dolomite — though the angle between a is still greater, 

 and the ray r very short but also turned towards the vertical 

 solid angle. 



Siderite (from Nassau), when boiled in hydrochloric acid, 

 behaved like dolomite. 



In the rhombic system I observed in the basal faces of tolerably 

 large crystals of tartrate of soda and potash, which had been 

 moistened with water and then rubbed over with a dry cloth, the 

 beautiful reflected figure 12, which by frequent corrosion under- 

 goes many changes, and, on looking through it, appears like fig. 

 13. When I used water instead of hydrochloric acid, fig. 12 

 gave place to an indefinite rhombic spot of light, but it imme- 

 diately appeared again when the surface was rubbed with a moist 

 cloth. 



When a prism of nitroprusside of sodium (the combination of 

 the rhombic prism of 105° 10' with the macro- and brachydia- 

 gonal spaces) is very slightly corroded with water, the figures of 

 reflexion are shown as fig. 14 represents them. At a certain 

 angle the rays upon the p faces of the rhombic prism are obtained 

 tolerably large, and only a three-rayed star appears ; the crosses 

 belong to the macro- and bra chy diagonal faces. 



Chloride of potassium and bismuth gives, when corroded by 

 breathing, an oblique cross on the basal face towards the sides of 

 the rhombus of this face, and also a band of light towards the 

 long diagonal ; under the same circumstances there is in chloride 

 of barium a band of light towards the short diagonal of the ordi- 

 nary rhombic plates ; with further corrosion, spots of light show 

 themselves near it, but no band in the direction of the long 

 diagonal. 



