hillebrand.] FLUORINE, SULPHUR. 105 
hot nitric acid and tested for phosphorus by ammonium molybdate 
solution. If phosphate is absent the impurity may have been silica or 
calcium silicate— which of these it would be difficult to decide, in the 
former case the fluorine might be safely deduced from that of the sul- 
phate, but not in the latter. If the rock were rich in sulphur it might 
happen that calcium sulphate would be thrown down with the fluoride, 
but this should be removed by thorough washing. If not, and it were 
certainly the only impurity present, the fluorine could be calculated, 
after conversion of the fluoride into sulphate, by the formula: 
CaS0 4 — CaF 2 :2F:: Dill', between impure CaSO- and CaF 2 : x. 
It is an exceptional case when there is exact agreement between the 
weight of fluoride and sulphate, and with the small amounts usually 
met in rocks the error may be an appreciable one in percentage of 
fluorine, though of no great significance otherwise. 
There is no qualitative test which will reveal with certainty the 
presence of fluorine in rocks. Heating the powder before the blow- 
pipe with sodium metaphosphate on a piece of curved platinum foil 
inserted into one end of a glass tube, or in a bulb tube, is not to be 
relied on in all cases. While as little as one-tenth of 1 per cent of 
fluorine can sometimes be thus detected with ease, much larger amounts 
in another class of rocks may fail to show. 
XXIV. SULPHUR. 
Before proceeding to the estimation of sulphur, its condition, if pres- 
ent, should be ascertained. 
Evolution of hydrogen sulphide on boiling with hydrochloric acid is 
evidence of a soluble sulphide, usually pyrrhotite, but possibly lazurite. 
Extraction of magnetic particles reacting for sulphur shows pyrrhotite 
to have been in part at least the source of the hydrogen sulphide. A 
reaction for sulphuric acid in the filtered solution indicates a soluble 
sulphate, usually nOselite or hauynite. If the residue, when well 
washed and treated with aqua regia or hydrochloric acid and bromine, 
gives more sulphuric acid, the probable presence of pyrite is shown. 
Should this solution likewise show arsenic, the sulphide may be arsen- 
op} T rite, which, however, is of very rare occurrence in igneous rocks, 
if, indeed, it is ever found there. 
For the quantitative extraction of the sulphur of soluble sulphates, 
simple boiling with hydrochloric acid suffices, which should be done in 
an atmosphere of carbonic acid if pyrites or other oxidizable sul- 
phides are present, and should be finished as quickly as possible in 
order to minimize the error resulting from oxidation to sulphuric acid 
of the sulphur of sulphides, if present, by any ferric salts that may 
have been dissolved. 
