212 T. S. Hunt on the Constitution 
ai the fraction, is like the 5th, and differs from the Ist by 
The ne of Rozena is SizOz1(R ris)O1c=Si20 
nt ‘Riri 3)Os6, so de we have the four following homologous 
genera, the last two having each two species. 
Sis Os 0, Mi6O, 6=(Siso, ras, R3)O1 06. 
Si20Os0, Ms1sOas=(sico, rae, Riz )Oros 
Si20O6 0, Ms 00s o=(Sie 0, Tao, Ri 0)O1 10, and (Sic o,f4 sRs 0: 10. 
Siz 00. + MiriOss (sea, ras, Rz)Onie, ‘and (sieo, rasRa)Orts, 
The variations in the calculated volumes of the tourmalines 
are greater than we have found in some other isomorphous 
species. Instead of taking the equivalent weight and ‘greatest 
density of each variety, Rammelsberg has only calculated the vol- 
ume of each group, from a mean of the equivalents and densities, 
a method which cannot be expected to give accurate results. 
The following monoclinic crystals have been indicated by 
Mr. Dana, as isomorphous ; ige glauber-salt, spodumene, py- 
roxene, acmite and hornblende.* 
The sulphates being bibasic, glauber salt j is S2Na:Os, 20HO= 
322°4, which, witha density of 1-469, gives a volume of 219, 
or doubling the formula 438. Borax is 2B0s, NaO, 10HO, which 
multiplied by four gives with a density of 1:73, an equivalent 
volume of 441: 
Spodumene is ‘represented by (Nat Lif), Sie +441 Sig, which mul- 
tiplied by two and expanded, becomes sie oOv o(al2sLisNa2)Ose, 
with an equivalent weight of 1455-5 and a density of 3-18 giving 
a volume of 457. The formula of white pyroxene, dion 1S 
sis2Os2(Ca:sMgis)Oz6 which with a density of 3-24, gives 
also the volume 457. Hedenbergite belongs to the same type, 
and is sis2O052(CaisFe12Mg)O26 with a density of 3-5 and a 
volume of 462. The aluminous augite, hudsonite, is represented 
by (sis sali 0)Os 8, (Fei 6Caz Mn)O2s, 4 atid witha density of 3° ‘A6 
gives a volume of 448. Wollastonite, a lime pyroxene, approxi- 
matively isomorphous with the above species, is s044Ca22O022, 
and with a density of 2-9 has a volume of 446:1 
In the different varieties of augite or nate the oxygen of 
the silica, or of the silica and alumina combined, is to the oxyge” 
of ae protoxyd bases as 2 : 1, while in * homblendes the ratio 
isas 9:4. There are however many ex of minerals with 
the ‘eaten of pyroxene and the eile Be horoblonde, while 
there are hornblendes whose analyses give the augite ratio 0 
2: Carinthite with the form of a hornblende, pertains to the 
same type as hudsonite; Dana’s formula equalst (siz sals)O2z8, 
> e.CasMg-)Or, with a density 3-127, and a volume of 258°, 
which gives ae the formula (sis al) Ox s, (Fe:CasMgi+)O2: 4 
ane of 442: 
* This Journal, [2] vol. i ix, p. ot — p. 429, also vol. x, P- 119. 
+ This Journal, [2] vol. ix, p. 2 
