332 S. L. Penjield — Crystallization of Serderite. 



Some of the measured angles are recorded in the following 

 table, where it will be observed that the agreement with tin' 

 calculated is usually very satisfactory, thus indicating that the 

 axial ratio is nearly correct. Where the variation is consider- 

 able the faces were always either rounded or otherwise poorly 

 adapted for measurement. 



Measured. Calculated. Measured. Calculated. 



c - a, 001 ~ 100 = 89° 54'* m a tt, TlO ~ 331 = 22° 23' 22 ; 30' 



c ~ e, 001^302 = 45 25* n Alt, 331 a 331 = 59 3 



b a v, 010 a 031 = 37 57* c a k, 001 a 122 = 28 35* 



c A »i. 001 a 110 = . 89° 55' k a k, 122 a 122= 44 34 



to a to, 110 a 110 = 64 25 64 29 r ~ k, 112 a 122 =11 3 10 35* 



b aTO, 010 a 110 = 57 47 57 45* c a t, 001 a 032 =32 46 32 40" 



c a r, 001 a 11/2 = 21 52 21 49 /a £ 032 ~ 032 =65 36 65 20 



r a r, 112 a 112 = 22 43 22 52* & a s, 010 a 061 =21 17 21 18 



c ~ q, 001 a 332 = 50 8 50 11 a ~ v, 100 a 031 = 89 55 89 56 



q a q, 332 a 332 = 48 20 48 22 a a w, 100 a 3-12-4=72 34 72 35 



c ~ n, 001 a331 = 67 7 67 20 c~w, 001 a 3-12-4= 54 2 



n a«, 331 a 331 = 58 56 58 59 w a m>, 342-4 a 3-_12'-4= 97 36 



b a n, 010 a 331 = 60 30* 60 304 c a z, 001 a 394 = 47 26 



c a It, 001 a 331 = 67 29 5 a z, 010 a 394 = 49 .. 49 22* 



Optical properties — Sections parallel to the clinopinacoid 

 show inclined extinction. With yellow light, Na flame, the 

 axis of greatest elasticity, which is the acute bisectrix, was 

 found to make an angle of + 2|° with the clino axis, hence, as 

 the axial inclination /3 is approximately 90°, c is inclined 2^° to 

 o in the acute angle ft. The plane of the optical axes is the 

 clinopinacoid and the double refraction is negative and rather 

 strong. For the determination of the mean index of refrac- 

 tion a prism was prepared with its edge parallel to the ortho 

 axis. Owing to the character of the material the prism was 

 small and its optical orientation only approximately correct. 

 The results of the measurements are given in tabular form 

 below. Two plates were prepared, one at right angles to the 

 acute bisectrix, the other parallel to the base, which is approxi- 

 mately at right angles to the obtuse bisectrix. The first of 

 these was so small that the divergence of the optical axes could 

 not be observed in air but was determined in a-monobrom- 

 naphthaline {ny = 1-6572). The results are as follows, all 

 made with yellow light, lS~a flame : 



From prism : a = 21° 58', 6 = 14° 16', hen>e fi = 1-632 

 2H« = 70° 44' hence 2Va = 71° 59' 

 2Ho = 105 1 hence 2Vo = 107 21 



179° 20' 

 Theory, 180 00 



Also from 2H« and 2H<9 there were calculated for compari- 

 son 2Va = 72° 12' and /9 = 1-628. The first of these agrees 



