54 II L. Wells and S. L. Penjield—Gerhardtite, etc. 



curate measurement. Many trials were made before crystals 

 suitable for measurement were obtained and a few crystals only 

 were selected, which although small were very perfect. Many 

 of the crystals have a different habit from that given in fig. 2, 

 being terminated at the extremity of the b axis by the pris- 

 matic faces only, but none were found which admitted of 

 measurement. 



The cleavage is basal, perfect, a second cleavage is parallel to 

 a (100). The crystals are brittle but do not bend like those of 

 the natural nitrate. 



Under the polarizing microscope the crystals show an extinc- 

 tion parallel to the b axis. In convergent light the axes are 

 seen to lie in the plane of symmetry. One axis is visible in the 

 field of the microscope, inclined about 40° to a normal to the basal 

 plane, the other axis is outside of the field of the instrument. 

 The bisectrix lies in the obtuse secant; its inclination could not 

 be determined. One of the largest tabular crystals showed one 

 axis in the axial angle apparatus, the other was totally reflected. 

 In the solution of Hgl a in Kl (n= 1*703 for yellow, 1*722 for 

 green) both axes could be seen showing a marked inclined dis- 

 persion p<jv, the axial angle being 



2H = 59° 22' for yellow. 

 2H=r 63° 50' for green. 



Double-refraction, negative. 



Pleochroism as in the orthorhombic crystals : 1 1 b green. 



± b blue. 



The crystals of the natural and artificial compound are very 

 much alike, making a very interesting case of dimorphism. 

 The points of similarity are repeated below. 



Orthorhombic, natural, &: b: c = 0-921 75 : 1 : 1-1562 



/3= 90 

 Cleavage 001 and 100 



Munoclinic, artificial, d:b:c= 0-9190 : 1 : 1-1402 



/S=85° 27' 

 Cleavage 001 and 100. 



Plane of the optic axes in the brachypinacoid in the former 

 and in the corresponding plane of symmetry in the latter. 

 Dispersion almost of the same amount and p<j> in both cases. 

 Double-refraction negative, and pleochroism similar with almost 

 identical shades of color in both. 



Chemical a?ialysis : 

 I. 1-2373 grams yielded -1388 H 2 and -8202 CuO. 

 II. -5314 gram yielded -0615 H 2 0, -3519 CuO, and 25-4 cc. dry N 

 at 12-9° and 764 mm. (corrected). 



