77 



All the figures are microscopic, and have been examined 

 under perpendicularly incident light, produced by means of a 

 glass prism placed above the objective. On account of the 

 smallness, as well of the figures as of the crystals themselves, 

 it has been impossible to decide the position of the faces, and 

 as, moreover, the figures are bounded by highly rounded faces, 

 they do not in the goniometer produce distinct reflexes, but 

 only quite faint lights, not even gathered in distinct zones, but 

 widely spread. In the folloving remarks therefore only the out- 

 line of the etching figures will be mentioned, as it appears very 

 distinctly by means of the perpendicularly incident light. 



a. Etching Figures produced by Acids. 



As well inorganic as organic acids in very different degrees 

 of concentration were used for the etching. The figures are 

 in all cases of about the same form ; according to the strength 

 and concentration of the acids the duration of the etching has 

 to be very much varied. To get the crystal etched to a proper 

 degree, so that not the whole plane is destroyed, the time may 

 at the different acids be as follows : 



cone. HF in 5 — 10 seconds, 



cone. HCl, HNO.^, or H^SO^ in 5 — 15 minutes. 

 10— 20pCt. » « » in 1—2 days. 



cone, oxalic acid in 1 — 2 days. 



cone, tartric acid in 1 — 2 weeks. 

 I have not succeeded in producing figures by acetic acid. As 

 the etching figures produced by the different acids resemble 

 each other very much, I shall in the following speak of each 

 separate crystal face; only the most common faces, those oc- 

 curring in type 1, have been examined. 



è{010} is among all the faces the one which is most 

 highly corroded by acids, so that often the whole area is 

 covered with figures, while this is only more or less the case 

 with the other faces. The form of the figures is always com- 



