Crystalline Form and Chemical Constitution. 181 



part of indifferentism in compounds ; and many formulae have 

 of late been written by chemists admitting this. Apophyllite 



is a tetragonal species consisting of R + 2Si + 2H. Making the 

 water basic, there is still no conformity to the type of either the 

 unisilicates or bisilicates, the oxygen-ratio for the bases and silica 

 being 3:4. If half the water be regarded as basic, and the for- 

 mula be written (R, H)*Si + HSI, it is made to consist of a do- 

 minant part which is a unisilicate analogous to the tetragonal 

 species meionite, mellilite, &c, and a bisilicate which is a kind 

 of opal or water-glass, well known to be a " colloid," or uncrys- 

 tallizable, and which therefore might well have no effect toward 

 modifying the crystallization as determined by the other part. 



2. Hexagonal species, and the number 3. — Hexagonal species 

 have been stated to occur among the sesquioxides (as Fe 2 3 , 

 Al 2 3 , Cr 2 3 ), the bisilicates (as in beryl, eudialyte, dioptase, 

 pyrosmahte, chabazite, Gmelinite), and the carbonates (in cal- 

 cite, and the allied species), in which compounds the number of 

 atoms of oxygen is 3 or a multiple of 3. Other examples are : — 



Pyrargyrite and Proustite, 3AgS -f- (Sb, As) 2 S 3 , in which the 



number of atoms of sulphur is 6; Gibbsite, All! 3 ; al unite, 



KS + 3A1S + 6aq ; apatite, 3Ca 3 'P + CaCl ; Coquimbite, 



FeS 3 + 9aq; A1 2 C1 3 + 12aq; MgS + 6aq; A1S 3 + 27aq; 



SrOS 2 5 -f-4aq, and the corresponding salt of lime and of 



lead; KN; NaN ; (Ca+H 2 )g; AgO, C10 7 + 4H ; 3NaCl-f 



IrCl 3 + 24aq; KC1 + 2MgCl + 12aq; MgCl + PtCl 2 +6aq. 



The exceptions to the principle are to be accounted for in the 

 same manner as those under the tetragonal system. Alongside 

 of the hexagonal sesquioxides, Fe 2 O 3 , Al 2 O 3 , Cr 2 O 3 , there is 

 the hexagonal protoxide ZnO, similar in angle. Applying the 

 principle of polymerism and writing the formula Zn 3 O 3 , it then 

 has, like the sesquioxides, 3 of 0. This view of the protoxide 

 is abundantly illustrated and sustained among the silicates. For 

 the constitution of the larger part of them (garnet, scapolite, 

 epidote, &c.) is based on the mutual replacement of 1 of sesqui- 

 oxides (R 2 3 ), and 3 of protoxides (3R0); and this mutual 

 replacement signifies isomorphism of R 2 O 3 and R 3 3 . Again, 

 graphite, or hexagonal carbon, has been shown to have its atomic 

 weight nearly three times as great as that of ordinary carbon ; 

 and it is altogether probable, therefore, that in this hexagonal 

 state carbon is C 3 , in accordance with the principle in view. 

 Hexagonally crystallized water, on the same ground, is not HO, 

 but H 3 O 3 . ZnS occurs both in isometric and hexagonal forms ; 



