Oe aa 
PROFESSOR HEDDLE ON THE MINERALOGY OF SCOTLAND, 199 
whole iron present is now reduced by zinc, and again titrated by perman- 
ganate—giving a second determination of the ¢ota/ iron, as a check upon that 
already made on the mixed solution of iron and alumina. 
We have now ascertained the absolute amount of ferrous oxide, and this 
calculated to ferric oxide, and as such deducted from the total ferric oxide, as 
ascertained after the original peroxidation, gives the absolute amount of ferric 
oxide existing as such in the mineral. 
A biank result from numerous blank experiments, and a closely accordant 
result in numerous experiments on the same mineral, repeatedly instituted, 
showed that this process is to be relied on. 
As regards silicates, at least, its expedition leaves nothing to be desired. 
I have been constrained to prefer it to EARLy’s very similar process, because 
I have repeatedly failed in procuring hydrofluoric acid free from iron; and I 
find that the substitution of the ammonium fluoride for fluorspar is not satisfac- 
tory. With the former salt, the reaction is so energetic, that there is a risk of 
loss of substance by projection from the crucible, while the duration of the 
action is not sufficient for the complete decomposition of the substance 
operated on. Ammonium fluoride, moreover, is frequently contaminated with 
iron; could it be obtained pure, its employment in the above process would 
enable us to determine alkalies by a method superior to any in use; and were 
we in possession of the permanganate of ammonium we might also in the same 
operation—z.2., in the same quantity of material—determine likewise the 
amount of both the oxides of iron. 
4. Determination of Alkalies.—In all my more recent analyses I have employed 
LAURENCE SmitTH’s process; of this I think very highly, perhaps as highly as 
does its author, though I am not prepared to admit the extreme expeditious- 
ness which he assigns to it. From the frit or fusion in by far the greater 
number of cases forming a solid mass which dissolves extremely slowly if at all in 
water, and from the consumpt of time in the getting down so much lime, I now 
invariably dissolve the fusion in acid. When this is done, I find that a deter- 
mination may be made in about a day, if little attention is paid to anything 
else. I also hold that it is absolutely necessary jinely to pound the mineral— 
more so, indeed, than for a fusion with an alkaline carbonate. I use baryta- 
water whenever there is much magnesia present; and I find that it requires 
watchfulness and unusual care to prevent a slight loss of sodium chloride, in 
getting rid of so large a quantity of ammonium salts. 
5. The “ Silica” of these Analyses. What is it? and in what state did tt 
exist in the mineral?—Probably no one who has made many analyses of 
silicates, and examined the “silica,” will confidently say that it is silica. 
But to begin at the beginning, what is silica? We actually know eight 
silicas; and we speculate upon several “condensed silicas.” The silicas 
