280 Mr. L. Fletcher's Crystallographic Note*. 



positive tetrahedron of the other, the composition-plane is 

 perpendicular to the twin-plane. The tetrahedron-faces there- 

 fore must really fall into a plane; and I hope that crystals 

 may vet be found which will leave the matter beyond doubt." 



From this we conclude : — first, that Sadebeck had followed 

 Naumann, assuming his explanation to be that of Haidinger ; 

 secondly, that, owing to the practical difficulty of distinguish- 

 ing between growths according to the two laws, he could come 

 to no decision from simple examination of the specimens ; 

 and, thirdly, that he declared in favour ^of Haidinger's view 

 merely that the law might not be at variance with a second 

 law, which was true in certain cases, but of which the general 

 application had not been proved. 



Apparently before 1876 there was another change of view 

 on the part of Sadebeck ; for on page 82 of Eose and Sade- 

 beck's ' Crystallography '*, where this law is briefly referred 

 to, we read that "the individuals have a face of the form 

 {101} for composition-plane ;" and a footnote gives a refer- 

 ence to the first paper of Sadebeck, without stating whether 

 or not he had since obtained that evidence of the incorrect- 

 ness of Haidinger's explanation which was confessedly want- 

 ing so late as the time of publication of the second. The 

 omission of any reference to the difficulty may have arisen 

 from unwillingness to perplex the student of an elementary 

 text-book. 



The next mention of the law is made in Groth's Catalogue of 

 the Strassburg Collection (1878). We there find that, " as for 

 the regular growths of copper pyrites, the results of Sadebeck 

 are quite confirmed by the specimens in the Strassburg collec- 

 tion ;" and further on we read that these particular growths 

 are symmetric twins about a plane of the form {101}. In 

 other words, Sadebeck's first explanation, or that of Naumann, 

 is accepted. There is no reference to the difficulty in which 

 Sadebeck had found himself placed; indeed it is quite possible 

 that the later explanation of Sadebeck, agreeing with that of 

 Haidinger, had escaped notice owing to its having appeared 

 in a paper dealing with a more general subject and having no 

 reference to copper pyrites in its title : in any case no mea- 

 surements are recorded which render it possible to distinguish 

 between the two statements of the law. 



In the latter part of the same year, according to a paper on 

 haplohedral hemihedry f, either Sadebeck was in a state of 

 doubt as to which is the correct explanation, or else he con- 

 sidered both correct ; for Ave read as follows : — " If, on the 

 * Rose and Sadebeck's Element* der KrystaUographie, 1876. 

 f "Ueber geneigtflachige Heniiedrie," Zeit. d, deutsch. qeolog. Gesellseh. 

 p. HOI, vol. xxx. 1878. 



