﻿136 E. 8. Dana — Crystallization of Gold. 



are also prominent those of the trisoctahedron 3-3 (m), usually 

 strongly striated ; a hexoctahedron, one set of whose edges are 

 apparently truncated by the trisoctahedron and traces of a 

 second hexoctahedron. The determination of this first men- 

 tioned hexoctahedron, as will be seen, requires exact measure- 

 ments, and on all the larger crystals the planes are not bright 

 enough to be used on the goniometer. A single very small 

 crystal (J mm across) was found, however, on which these faces 

 were brilliant though excessively minute. On this the three 

 angles of the hexoctahedron were measured (with the usual 

 compound goniometer), as also the inclination of a face upon 

 the adjacent octahedral and trisoctahedral faces. Two inde- 

 pendent measurements for different faces were obtained in each 

 case. These measurements are liable to an error of from 5' to 

 10 / in consequence of the want of sharpness of the images 

 obtained, but they are accurate enough to prove beyond all 

 question that the true symbol is 18-f- (18*10*1). This will be 

 seen from the following comparison of measured and calculated 

 angles : 



Edge A. Edge B. Edge C. 





18-10-1 ^18-1-10 



18-1-10 ^18-1-1C 



> 18-10-1 „ 10-18-1 



Meas. 

 Calc. 



.... 35° 50', 36° 9' 



35° 58' 



5° 43', 5° 

 5° 34' 



41' 



31° 53', 32° 1' 

 31° 51' 





X 



18-10-1^111 





xm 

 18-10-1^311 





Meas 35° 



44', 35° 45' 





18° 2', 18° 10' 



18° 4J' 



It will be seen that the agreement between measured and 

 calculated angles is remarkably close, considering the nature of 

 the faces. The goniometer also showed, independently of the 

 calculations, that the trisoctahedral planes do not actually 

 truncate the edges of the hexoctahedron, although the planes 

 18*10*1, 311, 10*18*1 do fall very nearly in a zone. 



The positive determination of the form is of some interest, 

 both because it is so common in the California crystals, as will 

 be noted later, and because it was observed by Rose on speci-' 

 mens from the Ural many years ago. In his paper, already 

 referred to, he remarks upon the occurrence of a hexoctahe- 

 dron in conjunction with the dodecahedron, octahedron, tris- 

 octahedron 3-3 and the hexoctahedron 4-2; he gives several 

 figures, one of which has been copied in Dana's Mineralogy 

 (f. 53, p. 3). The crystals examined by Rose * afforded only 

 rough measurements, and to the symbol obtained 19-f^- (19* 

 11*1) he consequently did not attach very great importance. 



*Rose uses the letters t for this hexoctahedron and n for 4-2; the writer, 

 following Miller, restricts the letter m to 3-3, t to 4-2, and uses x for the hexocta- 

 hedron 18-f. 



