54 SOME PKINCIPLES AND METHODS OF ROCK ANALYSIS, [bull. 176 
Platinum infiltrates. — The filtrates from the silica alwa3 T s contain 
notable amounts of platinum. This arises in very small degree from 
the crucible fusion, in a larger one indirectly from the action of 
hydrochloric acid on manganate, vanadate, and sometimes ehromate 
of sodium, and, if much iron is present, in no small degree from the 
reduction of ferric chloride to ferrous by the platinum of the dish. 
This last reaction is little known, apparently, but is mentioned in 
Gmelin-Kraut, 1 and can be readily demonstrated by evaporation of 
ferric chloride in platinum. 
The removal of this platinum before precipitating alumina and iron 
is not necessary (but see second footnote, p. 57), and to do so involves 
the reoxidation of all iron and subsequent boiling to remove or destroy 
the excess of oxidizing agent, together with the expenditure of much 
valuable time. The iron is already oxidized by the fusion, and needs 
no further help in that direction. Nevertheless, if time is not a prime 
object, its removal by hydrogen sulphide is to be recommended. In 
the following descriptions, however, it is assumed that the platinum 
has not been gotten rid of at this stage. 
VII. METALS PRECIPITABLE BY HYDROGEN SULPHIDE. 
The presence in appreciable amounts of metals precipitable by hydro- 
gen sulphide, except perhaps copper, is of such infrequent occurrence 
in most rocks that discussion is unnecessary in their connection. In 
case it is necessary to precipitate them at this stage, however, it is 
always well to bear in mind that a little titanium may be thrown down 
along with them. Separations of the silica should be made in porce- 
lain, to eliminate platinum, or, better still, the quantitative estimation 
of these metals should be made in a separate portion of the rock broken 
up by the action of hydrofluoric and sulphuric acids. 
VIII. ALUMINUM. TOTAL IRON. 
INDIRECT METHOD FOR ALUMINUM. 
The common practice in this laboratory is to find alumina by difference, 
after deducting from the precipitate produced by ammonia or sodium 
acetate the sum of all other oxides this precipitate may contain. Of 
these, only ferric oxide, titanic oxide, and the trace of silica are deter- 
mined in this portion (see also second footnote, p. 57), those of phos- 
phorus, vanadium, chromium, and zirconium being looked for in other 
portions of the rock powder. This throws upon the alumina all errors 
involved in their separate determinations; but these may balance, and 
1 Annrg. Chem., Vol. Ill, p. 359. Sixth revised edition. 
