328 The American Geologist. May, i89i 
corresponding union with metals, sulphur in the form of sulphide 
of silicon, then, in the ti'eatment with HF1 during the solution 
of the Si0 2 , the SiS g must on becoming free, combine with H 2 
to form Si0 2 and H 2 S. In fact, when a half grain of the ma- 
terial already examined, a mixture of melanophlogite crystals of 
a specific gravity 2, 044 with pulverized opal, was overflowed with 
HF1 in a platinum dish, H 2 S was produced, which made itself 
apparent as such not only by the odor but also by the reaction 
with lead-paper. This is, in my judgment, a characteristic re- 
action for the presence of SiS 2 , for it appeared also in red heat, as 
in the destruction of organic substance. Still one might be in 
doubt whether there is a mechanical mixture or a chemical union. 
The noticeable constancy of the composition in all analyses hith- 
erto made indicates a chemical union. If we take the mean of 
all the analyses hitherto, that is Si0 2 =91.69 and S0 3 =5.85, as 
foundation, we can from them calculate the formula SiS 2 +42SiO^ 
(Si i3 84 S 2 ). From Friedel's formula, SO 3 +20SiO 2 , is derived 
in like manner the formula SiS 2 4-39Si0 2 . 
If it be desired to determine the sulphur directly, dissolve sil- 
ver-oxide in excess of hydrofluoric acid, and after adding some 
water to the pulverized melanophlogite treat it with this liquid 
until the mineral has entirely disappeared. The result is a black 
precipitate of Ag 2 S which can be filtered out and analyzed. In 
this manner I obtained from 0.4993 grains of melanophlogite, of 
a specific gravity of 2.044, 0.071 grains of Ag 2 S, corresponding 
to 1.84 per cent, of sulphur (or 4. 5 per cent, of S0 3 ), whereas I 
had before obtained 3. 82 per cent, of S0 3 from the solution with 
Na CO . This number was then somewhat too low. After red 
heat I obtained with the silver solution 1.28 of S (3.2 per cent, 
of SO ). With the same solution of AgFl in HF1 I obtained 
from very thin scales of melanophlogite on crystals of sulphur 
0. 58 per cent. S in the test with silver solution, and in the de- 
composition with saltpeter and soda I obtained 0.56 per cent, of S. 
As to how the variation in density of melanophlogite, as 
pointed out by me, may be explained, I cannot yet state, owing 
to insufficiency of material, as well as the question of pseudo- 
morphs. On the other hand it appears to me possible that the 
acquiring of a black color in the presence of heat can be attributed 
to the formation of iron-sulphide, as melanophlogite always con- 
tains some iron. 
Giessen, March S, 1891. 
