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The above values show that the two species are closely homoemorphous. 
The mean index of refraction (/3) for danburite is found to be — 
(3 = 1-634. Red (Li) 
= 1-637. Yellow (Na) 
= 1-646. Blue (Cu S0 4 ) 
In the case of the two minerals the mean indices of refraction are not far 
apart ; thus for the D line in the spectrum we have — 
/ 3 Danburite = 1*637 
Topaz = 1-6138 
The means of several analyses showed the mineral to have the composition 
given below 
Silicic acid 
. 48-23 
Boracic acid 
. 26-93 
Lime 
. 23-24 
Alumina . 
. 0-47 
Lost by ignition 
. 0-63 
99-50 
These numbers agree closely with those found by Smith and Brush when 
examining the Danbury mineral, and indicate the formula Ca 2 Si 0 4 + B 4 
Si 3 0 12 as that of the mineral. There does not appear to be any immediate 
relation between danburite and topaz in chemical composition, which, con- 
sidering the similarity in crystalline form, is rather remarkable. The 
mineral is slightly acted upon by hydrochloric acid, sufficiently to give the 
reaction of boracic acid with turmeric paper. When previously ignited to 
the point of fusion the mineral gelatinizes with acid. The crystallographic 
results obtained in the case of the Russell specimens do not accord with 
those found with the specimens from Danbury, Conn. ; and this is accounted 
for by the crystalline specimens in the latter locality being imbedded in 
felspar , 1 where apparent planes, at best of a problematical nature, certainly 
did not represent the crystalline form of the species/ 
The Copper present in Coal . — An examination by Stolba of specimens of 
coal chiefly from Bohemia shows the invariable presence of a small quantity 
of copper. ( Sitzber . hohm. Gesellschaft der Wiss., April, 1880.) The ashes of 
the coals, carefully prepared, always showed a strong copper reaction ; the 
pure coal itself contained a trace only of copper ; the pyrites accompanying 
it gave a strong reaction. In fact the strength of the reaction appears to go 
hand in hand with the amount of pyrites present in the coal. The never- 
failing copper of the coal, determines the amount of copper present in iron 
prepared with such coal or coke. The copper present in the coal with 
which we heat our ovens can be shown by the following simple method. 
When the coal is burnt and ceases to give a flame, and only the so-called 
glow is to be observed, a spoonful of pure salt is to be thrown upon it, and 
stirred about with a tongs or stick of wood. Immediately the azure-blue flames 
of carbonic oxide containing copper chloride are produced, and the appear- 
ance lasts some time. Coal which contains much pyrites exhibits the colour 
with great intensity and in great beauty. This, doubtless, is the cause of the 
colour which is so familiar to most English people, and for which many 
explanations have been proposed. 
