[ 357 ] 



XL VII. Supplementary Note to Professor Des Cloizeaux's Me- 

 moir on Ilumite (Phil. Mag. [V.] vol. ii.). By Professor 

 A. Des CLoizEAuxf. 



SINCE the communication of this memoir to the Crystallo- 

 logical Society, I have resumed the examination of the 

 optical character of a certain number of crystals of the three 

 types of Vesuvian humite which I received from Professor 

 Scacchi. The results obtained are similar to those which I 

 have already published ; but the crystals of type II. are espe- 

 cially remarkable for the great number of bands twinned round 

 the normal to the base, to which the bands are parallel. In 

 consequence of this twinning, a face a 1 , for example, is formed 

 very frequently by a portion of a 1 occupying its proper position, 

 and by a portion of o* belonging to a twin band. The same 

 coincidence occurs with the corresponding faces a* and o*y 

 «2 and A 1 , b 1 , and cfe, &c. Now, no reentering angle being ob- 

 served on these compound faces, their incidences on the base 

 should be equal. It is precisely what one remarks in the cal- 

 culated angles contained in the following Table, and which 

 have been obtained by substituting for two of the elemental 

 angles, which I have borrowed from Scacchi, the following : 



*ph 1 = 109° V (vom Rath) instead of 108° 58' (Scacchi). 

 V* =125° 50' „ „ 125° 52' „ 



b : a : c : : 419058 : 907949 : 69SQ66. 



pe 2 141 50 



pa 2 114 58 



pb 1 135 19 



pe 1 122 28 



p0 95 19 



pdA 135 19 



pa 1 135 57 



^^ 125 2 



pb% 125 50 



po\ 135 57 



po B 125 2 



*pdb 125 50 



pa* 119 52 



pa 3 103 9 



pb* 113 26 



po% 119 52 



g l a z 135 41 



pdi 113 26 



pah 109 1 



V h z 135 41 



pm 98 13 



*ph l 109 1 







XLVIII. Note on the Laiv of Twinning and Hemihedrism of 

 Leucophane. By Emile Bektrand, Parish . 



THE crystallography of leucophane has been treated of by 

 Professor Des Cloizeaux in his Manuel de Mindralogie, by 

 Mr. Greg (Phil. Mag. [IV.], vol. ix. p. 510), and by Professor 



t Communicated by the Crvstallological Society, having been read 

 April 12, 1877. 



