106 SOME PKINCIPLES AND METHODS OF ROCK ANALYSIS, [bull. 176. 
The sulphur of sulphides may sometimes be correctly determined 
by extraction with aqua regia or some other powerful oxidizer, but 
not always; so that it is better by far to fuse with sulphur-free 
sodium carbonate and a little niter over the Bunsen burner, and for 
a few moments over the blast, fitting the crucible into a hole in 
asbestos board (Lunge) to prevent access of sulphur from the flame. 
After thorough disintegration of the fusion in water, to which a 
drop or two of alcohol has been added for the purpose of reducing 
manganese, the solution is filtered and the residue washed with a 
dilute solution of sodium carbonate. In the filtrate (100-250 cm. 3 in 
bulk) the sulphur is precipitated at boiling heat Iry barium chloride 
in excess after slightly acidifying by hydrochloric acid. Evaporation 
to dryness first with acid, in order to eliminate silica, is needless, for 
in the above bulk of solution there will almost never be the least sep- 
aration of silica with the barium sulphate. It is well that this is so, 
for evaporation on the water bath heated by gas to remove silica 
would in many cases involve an error fully equal to the sulphur pres- 
ent by contamination from the sulphur of the gas burned. 
Owing to the small amount of sulphur in rocks, special purification 
of the barium sulphate obtained is hardly ever needful, especially as it 
has been precipitated in absence of iron. Should there be fear of a 
trace of silica being present, it can be removed by a drop of hydrofluoric 
and sulphuric acids before weighing the barium sulphate. 
This, of course, gives the total sulphur in the rock. If soluble sul- 
phates and sulphides as well as insoluble sulphates and sulphides are 
present together, the sulphur of the first is found in solution after 
extraction by hydrochloric acid in a carbon-dioxide atmosphere, and 
that of the decomposable sulphides by collecting the hydrogen sulphide 
evolved. 1 In the residue the sulphur of the insoluble sulphides can be 
estimated, or from the total sulphur found in another portion its amount 
can be calculated. The error involved in the above estimation of the 
sulphur of soluble sulphides, due to the possible reducing effect of 
hydrogen sulphide on ferric salts, is probably negligible. Most of 
the hydrogen sulphide would be expelled before any such action could 
take place and probably before the ferric salts were largely attacked, 
but of course the small proportion of sulphur set free as such from ])jr- 
rhotite would escape estimation and introduce further uncertainty. In 
general, it would be safe enough to assume the composition Fe 7 S 8 for 
pyrrhotite. However carefully all these separate determinations may 
be carried out, the final figures for ferrous and ferric oxides can hardly 
be regarded as more than approximations when much sulphide is 
present. (See pp. 94-95.) 
1 With pyrrhotite a small fraction of its sulphur — one-eighth if the formula Fe 7 S 8 is adopted — is 
liberated as free sulphur and not as hydrogen sulphide. 
