374 PROCEEDINGS OF THE NATIONAL MUSEUM. vol.54. 



Two habits are represented among the crystals. The most abun- 

 dant habit, shown in idealized diagram in figure 2. is acicular, the 

 crystals averaging 0.1 mm. in diameter and 5 mm. in length. Two 

 prisms are well developed, the second-order one being usualh" domi- 

 nant, both showing slight curvature and vertical striation. These 

 needles are mostly terminated simply b}" a basal plane, but occa- 

 sionally pyramid faces are present. The other habit, represented by 

 a very few crystals, is similar to that figured by Dana for the min- 

 eral, a single stout prism terminated by pyramids of the same order, 

 the prism being horizontftlly striated like quartz, as shown in figure 1. 

 It might be supposed that two different minerals are represented, but 

 the angles of both types proved to be identical.^ 



As the average of a number of good measurements the p angle 

 of the principal pyramid was found to be 40° 02', the value adopted 

 b}' Professor Golclschmidt in his " Winkeltabellen " ; the figure of 

 Haidinger (39° 50'). cited by Dana, being undoubtedly in error. 

 This makes the axial ratio c=0.7275 in the orientation usually 

 adopted foi- hexagonal minerals in this country (G^). equivalent to 

 f=1.260 in Professor Golclschmidt's (G^) position. The angles of 

 the other forms shown in the figures correspond to this ratio. Not 

 only are all of the forms heretofore reported on this mineral present, 

 but two new forms, which are named a (symbol 3032) and z (3031) 

 are also developed, the first on a crystal of the acicular habit, the 

 other on a prismatic one : 



p„ calculated, 51° 34', observed 51° 20' ±20'. 

 p^ calculated, 68° 45', observed 69° 00' ±20'. 



It was thought best to confirm the optical and crystallographic 

 identification of the mineral as mimetite by chemical tests. Removal 

 of sufficient material for a complete and accurate analysis would 

 have destroyed the specimen, but 0.0060 gram of acicular crystals 

 were picked out of inconspicuous cavities and analyzed as fully as 

 possible. The mineral is readily soluble in cold dilute nitric acid, 

 and from such a solution the chlorine was precipitated hj silver 

 nitrate, and after removal of excess silver and evaporation the lead 

 was precipitated by hj^drochloric acid and alcohol, the precipitates 

 being collected and weighed on a small Gooch crucible. Part of 

 the arsenic was volatilized by the evaporation, but hydrogen sulfide 

 precipitated the remainder, and after removal of the excess of the 

 reagent and evaporation with nitric acid ammonium molybdate 

 failed to yield a precipitate, showing the absence of phosphorus, 



I The United States National Museum equipment not including a Goldschmidt two- 

 circle goniometer, all measurements of crystal angles described in this paper have been 

 made on the one in the Geophysical Laboratory of the Carnegie Institution, and thanks 

 are herewith extended to Messrs. Wright and Merwin, of that laboratory, for their kind- 

 ness in placing this instrument at the writer's disposal. 



