XVII. ] CONSTITUTION AND VOLUME OF MINERAL SPECIES. 443 



sulphates (M 2 2 )n. S 2 8 , while cancrinite, and perhaps some 

 scapolites, are [may perhaps be] basic carbonates. All other 

 silicates -are reducible to the same type as the spinels, n(M 2 2 ), 



the formula of silica itself being written siO Boric, 



titanic, tantalic, and niobic acids are reduced to the same for- 

 mula as silica." 



" Homoeomorphous species have similar equivalent volumes, 

 so that the density in species thus related enables us to deter- 

 mine their comparative equivalent weights, and to fix their 

 positions in a homologous series. The proportion between the 

 silica and the other oxides may vary greatly in related species, 

 while the characters of the genus or the order are preserved, 

 This is illustrated in hornblende, diopside, and aluminous 

 pyroxenes like hudsonite. .... The triclinic feldspars, of 

 which albite and anorthite are the representatives, furnish an- 

 other example." [These, it was shown, might be reduced to a 

 common formula M^O^, to which petalite was also referred, 

 while orthoclase was described as belonging to a homologous 

 genus, MgoOgo, represented by (si 45 al 12 K 3 )0 60 , with the re- 

 mark that although this formula agrees with a large num- 

 ber of analyses, there are those which appear to show more 

 alkali. Petalite was (si 51 al 10 Li 3 )0 64 , with a density of 2.45 

 and an equivalent volume of 401.5. The formulas then as- 

 signed to the two feldspars first named were, respectively : ] 



Density. Eq. vol. 



Anorthite . . (si 32 al 04 Ca 8 )0 64 . . 2.76 . . 405.0. 

 Albite . . . (si 48 al w Na 4 )0 64 . . 2.62 . . 402.4. 



" Between anorthite and albite may be placed vosgite, labra- 

 dorite, andesin. and oligoclase, whose composition and densi- 

 ties are such that they all enter into the same general formula 

 with them, and have the same equivalent volume. The results 

 of their analysis are by no means constant, and it is probable 

 that many, if not all of them may be variable mixtures of 

 albite and anorthite. Such crystalline mixtures are very com- 

 mon ; thus in the alums, aluminium, iron, and chromium, and 

 potassium and ammonium may replace one another in indefinite 

 proportions Heintz has shown by fractional precipita- 



