596 



DR MACVICAR ON THE LAW OF THE VOLUMES OF AERIFORMS 



which the most complete is, as usual, the compound dodecatom, which closely 

 written is,— 



Calcite or Marble . G = ('^-»°«^^ii = M+%144 | ^^^ _ ^.^^ 



oACi o X 324 J ^ 



Limestone, as chemically conceived, seems a singular combination. But if we 

 could dedouble alumina and peroxide of iron, as we can dedouble limestone into 

 a protoxide, and a deutoxide, that singularity would disappear. Thus, Avritten 

 according to the law of symmetry or union by difference, carbonate of lime is 

 OCaOCO similar in structure to OAIOAIO, and OFeOFeO, &c. ; and for mineralo- 

 gical notation it might be written CaC like Jv, and W. 



The fully differentiated phosphate (see type ^) gives. 



Bone Phosphate P Ca P (Ca)io P Ca P = 4 (PO^SCaO). 



the usual compound dodecatom occupying 8 volumes. Written with the usual 

 formula, we obtain for this substance. 



^ ^ (P053CaO)i2 _ 71 + 84 xl2 _^,^rj^ And with poles marked 3-35. Mean 3-11. Expt. 3-1 . 3-2. 

 2AC^ " 



2x324 



Purely magnesia minerals are rare, but as a mineral constituent, none is more 

 interesting or important than magnesia. 



Theory. Walmsted. 



14Si02 

 Olivine F S F (Mg S Mg) j , F S F = 24MgO 



4FeO 

 324 



420 



40-2 



40-1 ^ 



480 



45-9 



44-2 1 , 



144 



13-8 



15-3 J ^ 



)1044( 



3-22. 



Expt. 3-3 



from 

 Somma. 



3-4. 



It is not the symmetrical or septary element MgSiMg, however, but the 



quinary combinations MgSi and CaSi, i.e. the sesquiform, so great a favourite with 

 nature, that are the great mineral constituents. They seem generally to form 

 into dodecatoms, the poles marked by the application of molecule to molecule, or 

 by the omission of an atom of the basic element, or by substitution of Fe for Mg 

 or Ca. Of these, the simplest symmetrical structure is that in which 2Fe serve 

 to unite 3 dodecatoms, the central consisting of CaS, and the lateral of MgS. 



(MgS \^f (CaS )i2F (Mg S)^2 omitting from the six poles (Mg + Mg) (Ca + Ca) (Mg + Mg); 



or written out, — 





S(MgSXoS,F, 



S(CaS),„S,F,S(MgS)ioS. 



Hornblende . 



( 36Si02 



J 20MgO 



• • ) lOCaO 



( 2FeO 



G = 2x324 



Theory. Bonsdort. 



1080 59-0 59-7 \ 



400 21-8 2M / 



280 15-3 14-3 r 



72 3-9 3-9 j 



;i832( 2-82 Expt. 2-9 



from Taberg. 



3-4. 



