7 8 SOME PRINCIPLES AND METHODS OF ROCK ANALYSIS, [bull. 176. 
residue in hydrochloric acid, separate silica as usual, precipitate alu- 
mina, etc. , by ammonia, dissolve the precipitate again in hydrochloric 
acid, evaporate, and proceed as by the former method, which in most 
cases would undoubtedly give better results than this one. 
XVI. PHOSPHORUS. 
It is sometimes possible to extract all phosphorus from a rock by 
simple digestion with nitric acid, but quite as often, if not oftener, 
this fails; hence the necessity for resorting to one of the longer methods 
of extraction detailed below. 
PROCEDURE WHEN MATERIAL IS AMPLE. 
When material is ample it is best to use one portion for phosphorus 
only and to proceed as follows: 
Fuse with sodium carbonate, separate silica by a single evaporation 
with nitric acid, treat the ignited silica with hydrofluoric and a drop 
or two of sulphuric acids, evaporate to expel hydrofluoric acid, bring 
the small residue into solution by boiling with nitric acid and add it 
to the main portion, in which, after addition of considerable ammo- 
nium nitrate, precipitate the phosphorus by molybdate solution. 
The turbidity often observed on dissolving the precipitated and 
washed phospho-molybdatc in ammonia is due to a compound of phos- 
phorus. If the addition of a small fragment of a crystal of citric or 
tartaric acid fails to dissolve it, this should always be re-fused with 
sodium carbonate, extracted with water and the filtrate otherwise 
treated as above, in order to secure the phosphorus in it. 
According to Gooch and Austin, 1 in order to secure a magnesium- 
ammonium phosphate of normal composition, the procedure at this 
point should be as follows: To the phosphate solution, containing not 
more than 5 to 10 per cent of ammonium chloride and a slight excess 
of magnesia mixture, a little ammonia is added, and the precipitate is 
washed in due time with weak ammonia water. In general, however, 
as these conditions can seldom be fulfilled, they recommend to decant 
the supernatant liquid through the filter which is later to receive the 
precipitate, to dissolve this in as little hydrochloric acid as possible, to 
reprecipitate by dilute ammonia without further addition of magnesia 
mixture, and to wash finally with weakl} r ammoniacal water. Excess 
of ammonia, of ammonium salts, and of precipitant are all objection- 
able. In rock analysis the second precipitation will seldom be neces- 
sary. For ignition, etc., of the precipitate, see this subject under 
Magnesium (p. 66.) 
'Am. Jour. Sci., 4th series, Vol. VII, p. 187, 1899; Zeitschr. fur anorg. Chemie, Vol XX, p. 121, 
1899; Chemical News, Vol. LXXIX, pp. 233, 244, 255, 1899. 
