IOWA ACADEMY OF SCIENCE 
203 
with acids may sometimes be necessary to remove all the silica. 
The different portions are transferred with the filter ash to a 
weighed platinum crucible, and heated with a Bunsen burner and 
finally with a blast lamp for ten minutes, when it is cooled and 
weighed. The purity of the silica may be tested with a few drops 
of dilute sulphuric acid and 10 cc. to 15 cc. of dilute hydrofluoric 
acid, and evaporating to dryness by which the silica is volatilized. 
Quartz is especially hard to volatilize and several additions of 
hydrofluoric acid may be necessary. 1 
Instead of the fusion with sodium carbonate, the fine rock pow- 
der can be dissolved in a porcelain evaporating dish, covered with 
a watch glass and evaporated to a fine dry powder by constant 
stirring with a glass rod flattened on one end. The powder is 
moistened with concentrated hydrochloric acid, left on the water 
bath for a few moments and dilute hydrochloric acid and water 
are added. The insoluble residue is usually silica or clay, and the 
purity should be tested with sulphuric and hydrofluoric acids. The 
purity test may also be made by digesting the insoluble residue 
for one hour in a platinum dish on the water bath with a moder- 
ately concentrated solution of sodium carbonate. 
By simply dissolving the rock powder in pure dilute hydrochloric 
acid and filtering off the insoluble residue, without evaporating the 
substance to dryness, the silica is often very satisfactorily deter- 
mined as will be shown later. Kortright 2 calls attention to the 
adhering of silica to the surface of a porcelain dish, when it has 
been used to evaporate the substance to dryness, causing a rough 
coating on the glaze. This can not be removed even by a “police- 
man” but may be recovered by warming the dish on the steam bath 
with ammonium hydrate. This phenomenon had been noted by 
Hillebrand 3 and also by Morse. 4 To obviate this difficulty, it is 
recommended that platinum evaporating dishes be used instead of 
porcelain. In a large number of silica determinations made with 
evaporating dishes of porcelain, we have failed to notice the adher- 
ence referred to. 
SILICA IN A SILICATE. 
In the treatment of a silicate six or eight times as much sodium 
carbonate must be used as of the rock powder taken. All is thor- 
1. Hillebrand Bulletin 305 U. S. Geol. Surv., page 174. 
2. Chem. News 95.9, Jan. 4, 1907. 
3. Journal Am. Chem. Soc. 25, 1192. 
4. Exercises in Quant. Analy . , page 311. 
