276 Mr. L. Fletcher's Crystallography} Notes. 



vations with reference to Daniell's and other cells do not 

 appear to me to lead to any certain conclusion. 



Similar indirect methods have to be employed for measuring 

 the intensity of a current in absolute measure; the factor of 

 reduction of the tangent-galvanometer employed, for example, 

 has to be determined. This problem has been considerably 

 facilitated by the recent investigations of F. Kohlrausch and 

 Mascart on the electrochemical equivalent of silver, which, 

 however, exhibit small deviations among themselves. There 

 is here consequently a rich field for accurate investigation, for 

 which we have already a series of valuable preliminary inves- 

 tigations. All these labours, however, can only give a final 

 result sufficient for our present purpose when they are exe- 

 cuted upon some common and well organized plan, and are 

 carried out with the most perfect experimental means. 



XXXI. Crystallographic Notes. By L. Fletcher, M.A., of 

 the Mineral Department, British Museum; late Fellow of 

 University College, Oxford*. 



[Plate VI.] 



X. On Twins of Copper Pyrites. 



rpHE memoir of Haidingerf on the Crystallisations of 

 J- Copper Pyrites, published so long ago as the year 1822, 

 was so exhaustive and withal so simple in its character that 

 little seemed to be left to tempt the crystallographer to devote 

 further study to this mineral; and in fact, with the exception 

 of the papers of Sadebeck, whose early death all interested in 

 the progress of mineralogy must so deeply deplore, and the 

 confirmatory data in Groth's Catalogue of the Collection of 

 the University of Strassburg, we have still no other informa- 

 tion at our disposal. A study of these memoirs and of the 

 various text-books of mineralogy leaves upon one's mind such 

 a feeling of doubt as to the true statement of one of the laws 

 of twin-growth, and that (as will be explained later) a law 

 almost, if not quite, unique in the domain of crystallography, 

 that, at the suggestion of Professor Maskelyne, the collection 

 of copper pyrites in this Museum has been examined with a 

 view to a possible settlement of the difficulty. 



To get a clear idea of the present position, it is necessary to 

 trace the history of this particular law from its first statement 

 down to the present time. 



In the above memoir of 1822, the twin-growths of copper 



* Read before the Crystallological Society, June 3, 1882. 

 t Memoirs of the Wernerian Society, vol. iv. p. 1, 1822. 



