ffiLLEBKAND.] SPECIAL OPERATIONS. 109 
is present in quantity, however, this test must not be accepted al once. 
as final, for some, if not all, olivines are much more soluble in nitric 
acid of the above strength than Professor Pirsson was led to believe 
from his original tests. If, therefore, on evaporation of the filtrate, 
much iron is indicated, the gelatinization may well be due to olivine 
alone or in part, and then the quantitative relation of silica to iron plus 
magnesium should be ascertained. It must also be borne in mind that 
any other very soluble silicates present will be more or less affected, 
and that apatite is largely or wholly dissolved. It is possible that still 
more dilute nitric, or perhaps some other, acid may exert a slighter 
solvent action on olivine without being appreciably less effective in 
dissolving nepheline, etc. In combination with a quantitative analysis 
of the extract the method is perhaps susceptible of a wider application 
than the particular case for which it was first used. It is well worth 
further study. 
ESTIMATION OF SOLUBLE SILICA. 
Very often in treatment by acids silica is separated in gelatinous or 
granular form mixed with the unattacked minerals, and it becomes 
necessary to remove or estimate this silica, or else to discriminate 
between soluble and insoluble silica already existing together. Usu- 
ally a boiling solution of sodium carbonate has been employed for this 
purpose, though the caustic alkalies have found advocates. 
Lunge & Millberg 1 have lately conclusively shown that quartz is 
not nearly so insoluble in solutions of the caustic alkalies as has been 
supposed, but that given a sufficient degree of subdivision it can be 
brought wholly into solution; that it is impossible to secure correct 
separation of quartz and opaline silica by the use of either caustic or 
carbonated alkalies; and that digestion on the water bath for 15 min- 
utes with 5 per cent solution of sodium carbonate is the only way to 
secure exaet separation of unignited precipitated silica from quartz, 
and then only when the finest flour has been removed by levigation. 
The authors say: 
If, however, no more of such flour is present than is produced in the ordinary 
operations of powdering and sifting through cloth of the finest mesh, the error aris- 
ing from the above mentioned treatment is so slight that it can generally be neglected; 
it reaches 0.1 to at the most 0.2 per cent of the total silica, by which amount the 
quartz will appear too low, the amorphous silica too high. 
The above authors also show, however, that the solvent action of 
the caustic alkalies on quartz becomes very apparent only when the 
material has been reduced to such an utterly impalpable degree of 
fineness as is practically never reached in the preparation of samples 
for rock analysis. For this reason I have no hesitation in recommend- 
ing the employment of a dilute solution of sodium hydroxide when 
1 Zeitschr. fur angewandte Chemie, L897, pp. 393, 425. 
