hillebeand.] SILICA, SEPARATION FROM ALUMINA, ETC. 53 
increases the amount of silica carried into the filtrate. Ii does not 
appear from Gilbert's paper that the blast-furnace slags, on which he 
experimented, contained titanium, phosphorus, or iron in appreciable 
amounts. Basic magnesian rocks usually do, and in such cases ii is 
probable that the employment of a drying temperature of L20 would 
materially add to the large impurity always to be expected with the 
silica. In other cases he confirms the earlier belief that drying tem- 
peratures higher than that of the water bath increase the amount of 
insoluble impurity, chiefly alumina, in the silica, and that this amount 
can not be reduced by long digestion with hydrochloric acid. Further, 
he confirms Lindo's statement that evaporation with sulphuric arid till 
the appearance of white fumes gives a higher result in silica than with 
hydrochloric acid. But for general rock analysis the use of sulphuric 
acid at this stage must be rejected utterly. 
Blasting for twenty to thirty minutes 1 is necessary to expel all 
moisture from the silica, and it is then not hygroscopic. Its weight 
should alwa} T s be corrected for impurities, which are never absent, by 
evaporating with hydrofluoric and sulphuric acids and again blasting. 
If toward the end of evaporation with these acids, when the hydro- 
fluoric acid has been driven off and the sulphates begin to appear in 
solid form, the residue has a peculiar milky or enamel-like appearance, 
it maybe taken as evidence of much phosphorus and titanium. This 
appearance is possibly due to zirconium with the phosphorus and 
titanium, 3 and is so unusual and striking that it is worth while calling 
attention to it. With basic rocks very rich in titanium and phospho- 
rus the residue may amount to 2 or even 3 per cent of the rock. 
The subsequent precipitate of alumina, etc., is usually ignited in 
the crucible containing the residue from the silica. 
It might be supposed that this residue would contain most of the 
barium of those rocks carrying that element, together with sulphur or 
sulphates, but the reverse is true as a rule. Only when there is a con- 
siderable excess of S0 3 over the BaO will much of the latter be found 
there, and in the vast majority of cases there is none at all. Should 
some be present, its removal and estimation at this stage is not neces- 
sary, as it can be more conveniently recovered later, together with the 
silica accompanying the alumina, etc., precipitate (p. 5(1). 
The separation of silica in rocks containing fluorine has been touched 
upon in commenting on the boric-oxide and sodium-carbonate methods 
of fusion, 3 and will be considered further under the head of Fluorine 
(P- 103). 
•It must be borne well in mind that some platinum crucibles lose weight steadily and very appre 
ciably on long blasting.nol onlywhennew but even after long use. When a crucible suffers from 
this defect the rate oi loss should i>e ascertained from time to timeand allowance made accordingly, 
or else the weight of the crucible should be taken after and nol before ignition of the precipitate. 
(Sec on this subject Hall,Jour.Am.Chem.Soc.,Vol.XXII,p. 194, Li .) 
2 See first footnote, p. 57. 
:! See p. 17 and footnote. ]> 50. 
