Brush and Dana—Danburite from Russell, N.Y. 117 
the lower end of the blue, the axial angle must be for the ordi- 
nary temperature exactly 90°. It would be easy to calculate 
the wave-length of the rays answering this condition, but since 
the sections employed were not faultless the angles are not 
bse at accurate and hence the calculation would have but little 
va 
Oviically danburite does not agree very closely with topaz, 
for with the latter species the axes lie in the brachydiagonal, 
the vertical axis coinciding with the acute bisectrix ; the axial 
angle is also quite different. It is interesting to note, owever, 
that the mean indices of refraction are not far apart; thus for 
the D line in the spectrum, we have 
B, ton sae = 1637 
Topaz = 16138 
Chemical composition.—The quantitative ee examina- 
tion of the mineral was made by Mr. W. J. Comstock, of the 
Sheffield Laboratory, to whom we wish here to yen our 
grateful acknowledgments for the following analyses: 
: Il. Mea: 
48:16 48°30 pact ee 48°23 
Boron trioxide is AF ras 26°67 27718 26°93 
i 23-26 98-92... one pren 23°24 
Alumina* 0°48 0°46 pean aca 36° AT 
Ignition 0°64 0°63 speeds eae 63 
* With trace Fe.Qs, 99°50 
The mineral was decomposed by fusion with sodium carbon- 
ate for the silica and bases in Nos. I and II, and the boric acid 
in Nos. III and IV was obtained by Stromeyer’s method as 
potassium boro-fluoride. A further decomposition was effec- 
ted with fluohydric acid to make special examination for 
alkalies, which gave a negative result. 
In view of the close homceomorphism of our mineral with 
topaz, we requested Mr. Comstock to make special examina- 
tion for shang but the result proved the absence of this and 
allied elem 
Mr. CothanX analyses offer a remarkable confirmation 
of the analyses of Smith and Brush* of the Danbury mineral, 
the mean of whic ve 
SiO, BO; AlO;, Fe,0; Mn,0, CaO MgO ign. 
48°15 = 27-15 0-30 056 2237 040 050 = 9943 
The quantivalents ratio from the mean of Comstock’s analy- 
ses gives for SiO,: B,O,: CaO = 8-04::38°88::4°14 and that 
from the analyses of ‘Smith and Brush is 8-02: 73°89: fl 
There can be no question that the true theo retical ratio 
2:1:1, This leads to the formula reread accepted, that 
* This Journal, I, xvi, 365, 1853, 
