T. 8. Hunt on Octahedral Oligist Iron. 371 
written Fe 8.0, Al #20, are isomorphous with Fe20, Mg20 and 
Zn20.- The analyses of many species of silicious minerals prove 
ceedingly variable. Thus in the epidotes while the first ratio is 
one of equality, the eond ranges from 2:1 to 1:1, and yet 
vitae varieties have all the same - erystalline form, and spect 
ehar 
M. Kop pp, it is well known, has shown that the expression obs 
tained by or oes 8 the atomic —— of different substances: ‘by. “ 
endes to 
With whieh — they correspon. inp pean ks eat as Dana has shown, 
in atomic volume. The analyses of fi rolite, Le a , sillimanite and kyanite, all 
s, show betw 
ind to 
n an examination of the ana _ es 0 aluminous pyroxenes and hornble 
will be found that on adding the oxen of the alumina to that of ss vailica,t the sum 
bears to that of the oxygen of the protoxyd bases, me relation as exists in 
Ose Vari Ne con ina. Silica represente SiQgs then 
chemically equivalent to Al2 O03 l4 Og in the notation here adopted); but this 
is equal t Os or 8(al2 0), and six atoms of then equivalent to 
a of silicon in SiOg, or al2 to Si if we write ’ ca ts Hy peculiar func- 
on of alu is then connected with a change of equivalent, and is a case in poi 
to illustrate our idea con et i + change of chemical ilabota, 
ater errant ” ich aluminium in this combination sustains to the basic alumi: 
nalogous to that which Mng and Cr2s sia to Mng mng and cr2 m their 
faites combinations, ‘when ens their basic ya ers they bece alent to S, 
anganates and chro a the homologous ane of the sulphates. Cra 
enero ace to SH ( O4: ut Cr2 crs is the equivalent of S_precisely 
als becomes the eq ; 
silica in these aluminous snianenls may then be replaced to a varying 
by alumina, and if from a comparison of the chemical analyses of epidotes, ate 
¢., with their crystalline forms and densities, it is s at al ay 
the protox ses, it is equally established upon the same grounds, that 1 
“cd minerals, alumina takes the place of silica. In the one case al2 
tba, * njattts volume of Mg20, and in the other, 3al2 0 sustains ‘the same 
relations to 
If we maar ‘alumina and silica in their uncombined states, we find that 
phire has an atomic volume of 12;9, that of quartz being 17°; as great Gifierences as 
bers of an acknowledged isomorphous series as between cal- 
: 4 : ir . °.20/. ‘ ifference ibe 
consi as 
iis tho eat dw not 
a ott this kind bet and 
wl Jounal (3 vol. ix, p, 220. 
