26 MEMOIR ON EMERY. 



are the hardest. I will not insist on the slight difference 

 between the hardness of the sapphire and ruby, having made 

 only one experiment upon each of these minerals. 



The two varieties of corundum are so evidently united by 

 their system of crystallization that I would not undertake to 

 separate them on account of the presence of water in one 

 of them, and that in variable quantity; nevertheless, the fact 

 is important, as it explains to a certain extent their differences 

 in structure and hardness. I would remark that great pains 

 were used to ascertain whether the water might not be due to 

 the presence of diaspore or some other hydrate of alumina; 

 but after the most careful and repeated examinations this has 

 been decided in the negative. 



HYDRARGILLITE. 



Hydrargillite is rarely met with. I have one specimen with 

 this mineral forming the external coating of a crystal of corun- 

 dum, and also a hexagonal prism of the same mineral. It was 

 not analyzed, but its physical properties and its reactions under 

 the blowpipe served to prove its identity with this mineral. 

 The specimen in my possession comes from Gumuch-dagh. 



DIASPORE. 



This mineral up to the present time has not occupied a very 

 important position in mineralogy, and has been found only in 

 two or three localities. In the course of this article I hope to 

 show that it plays a somewhat important part in the emery 

 and corundum formations. Before my attention was drawn to 

 the minerals, first discovered by M. Lelievre, it was studied by 

 M.Dufrenoy on that coming from Siberia, and by M. Haidinger 

 on the diaspore of Schemnitz. Before going farther I would 

 remark that the gangue of the latter, which has been described 

 as analogous to steatite, was found by me not to be such, but a 

 hydrated silicate of alumina, similar to one found with the 

 emery of JSTaxos* 



To the localities of diaspore already known I have to add 



* The gangue of the Schemnitz diaspore has been examined by Hutzel- 

 mann (see Pogg. Ann., LXXVIII, 575), who makes it to contain three dis- 

 tinct hydrates of alumina; but this fact can not be considered as sufficiently 

 established. One of these hydrates is named Dillnite, and another is near 

 Agalmatolite. 



