180 Prof. J. D. Dana on a Connexion between 



MnO 2 (pyrolusite), is orthorhombic, and approximately isomor- 

 phous with orthorhombic TiO 2 (Brookite), the former having for 

 the angles of the prism I, and the domes l-i/l-t, respectively 

 93° 40', 104° 22', 107° 54' ; and the latter for the corresponding 

 angles 93° 16', 96° 46', 99° 50'. MnO 2 in the tetragonal state 

 is unknown except when it is in combination with 2 MnO, as in 

 Hausmannite. The protoxide of manganese, MnO, it may be 

 remarked, is isometric, like MgO, it having been obtained artifi- 

 cially in octahedrons and cubo-octahedrons by Deville. 



Among artificial compounds, there are the following tetragonal 

 species all conforming to the principle stated : — 



KF + HF; BeS + 4aq; NiS + 7aq; Ni*S + 6aq; NiSe+7aq; 



ZnSe-f-7aq; (K,H) 3 ?; (Am,H) 3 F?(K,H) 3 As; (Am, H) 3 As; 



CaG + 2aq;(Am + H)B 2 + 3aq;NaO,I0 7 ;KCl + CuCl + 2aq; 



AmCl + CuCl + 2aq; AgS + 2NH 3 , Ag'0r + 2NH 3 , 2KAc 



(acetic acid) + Cu6 4- 12aq; CaAc -f CuAc + 8aq, KAc + 2CAc 



+ 2aq, AgAc + 2©Ac-f-2aq; (JCa + §H) 3 T (tartaric acid, 



containing 50) + SbT -f- 7aq. 



Omitting a few complex organic compounds, these are all the 

 tetragonal species in the two volumes of Bammelsberg's ' Crys- 



tallographic Chemistry/ excepting AgCl, Hg 2 Cl. Other ex- 

 amples might be mentioned, but the above are fully sufficient. 



The correspondence with the law for tetragonal species is so 

 general that we may reasonably believe that the apparent excep- 

 tions, where the composition and crystallization are correctly 

 given, may be brought into conformity to it by an application of 

 one or the other of the following principles. 



a. The principle of polymer ism. — Hg 2 Cl is Hg 2 Cl 2 in the new 

 system of chemistry; and if the whole is doubled, it becomes 

 Hg 4 Cl 4 j which is probably the true formula of this species in 

 the tetragonal state, the only crystalline state yet known. 



b. Part of the ingredients may be only accessory, or subordinate 

 to a dominant part which determines the crystallization. — Water 

 is commonly admitted to be present in this way in most of the 

 compounds in which it occurs ; although essential to the species, 

 it is subordinate, crystallogenically at least, to the rest. Water 

 is now believed to be not the only substance that may play the 



being 11 9° 22', and : 1 (which might as well be \-i) in Hausmannite being 

 121° 3'. Braunite is much nearer cassiterite, rutile, and zircon, the corre- 

 sponding angles for on a pyramidal plane in these four species being re- 

 spectively 135° 26', 136° 26', 137° 40', 137° 50'. Thus the anatase and 

 rutile form of TiO 2 are severally represented by Hausmannite and Braunite. 



