Mr. L. Fletcher's Cvystallographic Notes. 211 



pyrites were assigned by Haidinger to three distinct laws. In 

 the first kind, the twin-plane, or plane of rotation, is a face of 

 the octahedron {111}, and the composition-plane, or plane 

 of junction, is generally parallel, but sometimes perpendicular, 

 to the twin-plane : these growths correspond to the blende 

 twins of the Cubic system. In the second, the twin-plane is a 

 face of the octahedron {101}, and the composition-plane is 

 perpendicular to the twin-plane. In the third, the twin-plane is 

 a face of the prism {110}, and the individuals are interpene- 

 trant. The second law is that with respect to which uncer- 

 tainty has arisen; and it is with this law that we have now to 

 deal. 



In the original memoir of 1822 the growth is so clearly 

 described, and the law so distinctly expressed, that it is impos- 

 sible to read the memoir and to mistake its meaning. Three 

 years later, however, Haidinger published in the ' Edinburgh 

 Journal of Science ' a series of papers " On the Regular Com- 

 position of Crystallised Bodies," copiously illustrated with 

 figures which have since found their way into almost every 

 manual of the science ; and in its natural place in the tetra- 

 gonal (or pyramidal) system he describes once more, though 

 briefly, the particular growth of copper pyrites which we are 

 about to consider. There is little doubt that this secondary 

 description has been the cause of serious misunderstanding. 



On page 68 of vol. iii. (1825) we read as follows : — " Regular 

 composition often also takes place in this species parallel to a 

 plane of P — 1{1 1}, or perpendicular to the terminal edges 

 of P {1 1 1}. There are particularly two varieties of this case 

 which in the present place deserve our attention. The indi- 

 viduals are either joined in pairs, or one central individual is 

 surrounded by four others, added in the direction of all the 

 edges of P. The product of the first, in the fundamental 

 pyramid, would be fig. 30 [similar to fig. 6]. This has not 

 yet been observed ; but it will serve for explaining fig. 31 , a 

 variety of the form P-^{0 01}, P-2{112}, Pjlll}, 



^2 P {3 2}, and ^— P + 1{3 3 2}; from the mines of the 



district of Siegen in Prussia. This and several other interest- 

 ing varieties of forms from the same locality I have described 

 on another occasion {Mem. Wern. Soc. vol. iv. part 1, p. 1, 

 1822), from specimens in the possession of Mr. Sack, of Bonn." 

 Taken by itself, this explanation might at first sight suggest 

 the interpretation which seems to have been placed upon it by 

 some crystallographers — namely, that there are twin-growths 

 of copper pyrites in which the plane of composition is parallel 

 to a plane of the octahedron {101}. 



