81 



(ml 



b. Etching figures produced by alkalies. 



Alkalies have not by far so great an influence on the ilvaite 

 as the acids; nevertheless they are able to produce etching 

 figures, showing very characteristic deviations as to form from 

 the preceding ones. 



An attempt to use the alkalies in a melted state in order 

 to produce a stronger effect, did not succeed. If a crystal of 

 ilvaite is immersed into melting NaOH, or Na^CO^, all the 

 faces are immediately corroded to a very high degree, but no 

 regular figures are produced. By using a mixture of the men- 

 tioned substances and melting NaNO^, which latter substance 

 does not itself corrode the crystal, no effect is seen for more 

 or fewer minutes, according as the mixture contains more or 

 less saltpeter; but in the moment the crystal begins to be cor- 

 roded, all the faces are at once destroyed, quite irregularly 

 grooved and furrowed. 



If, on the other hand, the crystal 

 is for some weeks immersed in a 

 concentrated dissolution of NaOH, a 

 few small and faint figures are pro- 

 duced. Only on the prismatic faces 

 s(l20} and ä{210} these figures have 

 a more regular form; the majority of 

 the other faces are not at all cor- 

 roded; in a few, as «'{30l} and /"{04l} 

 small, roundish, and very indistinct 

 figures may be found. 



5(120} is provided with rather 

 numerous figures ; they are always 

 very small and faint, generally deve- 

 loped as shown in the figure. 



(Fig. 28) The most characteristic 

 feature is that the figures are always 



XXV. 



(101) 



(301) 



(12»; 



(301 



(111) 



(010) 



Fig. 28. Etching figures 

 on {120} 

 produced by alkalies. 

 6 



