CHEMICAL COMPOSITION. 21 
of other alumina and iron minerals. In the analysis of a commercial 
corunduiu the percentage of alumina found does not indicate the per- 
centage of corundum in the sample, for many other minerals, as 
spinel, amphibole, pyroxene, chloi'ite, garnet, etc., that are in the 
sample contain more or less alumina, which would also be included in 
the percentage obtained. To determine, therefore, the percentage of 
rorinidum in a sam])le, it nnist be directly separated from the rest of 
the minerals that are a part of the sample. A number of bad errors 
and erroneous statements have been made regarding commercial 
corundum by estimating the quantity of corundum by the percentage 
of alumina ol)tained in the chemical analysis, which rated the sample 
altogether too high. This mistake has been made in regard to the 
emery from near Peekskill, N. Y., described on pages 137-138. In 
this emery there is sometimes a larger per cent of spinel than of corun- 
dum, but in the chemical analysis of the commercial product the 
percentage of alumina would not be very much below a good emery, 
for the mineral spinel contains nearly 72 per cent of alumina. The 
abrasive efficiency of such an emery Avould be considerably lower 
tlian that of a true emery, as the cutting quality of the spinel is much 
loAver than that of the corundum. 
DETERMINATION OF PERCENTAGE OF CORUNDUM IN AN ORE. 
The material is crushed in an iron mortar so as to go through a 
14-mesh sieve and sampled. Two grains of the material are taken 
and treated with concentrated hydrochloric acid on the water bath 
for two hours. The residue is then filtered off, dried, and fused 
one-half hour with 6 grams of sodium-carbonate mixture (two part ; 
NaX^0;5, one part KoCO.,) in a platinum crucible over an ordinary 
P)unsen I)urner. The fused mass is digested with hot water, and the 
solution is decanted through a filter. The residue is treated with a 
large excess of dilute hydrochloric acid, and this solution is decanted 
thi-ough the same filter. The filter paper is dried and ignited in a 
])latinum dish, the residue from treatment Avith hydrochloric acid is 
added to the dish, and the whole treated Avith an excess of concen- 
trated hydi'ofluoric acid. The excess of hydrofluoric acid is evapo- 
rated off on the water bath, the residue is treated with hot w^ater, and 
is finally transferred to an ashless filter paper. This i)a]>er and its 
contents are dried and then transferred to a weighed ])latiniun 
crucible. The paper is removed by ignition; the crucible is cooled 
and weighed. The increase in weight (residue) is calculated as pure 
corundum. 
