PROFESSOR HEDDLE ON THE MINERALOGY OF SCOTLAND. 203 
traces of alumina and lime ; the high heat insoluble silica contained 1:27 per 
cent. of ferric oxide, ‘83 of lime, ‘83 of magnesia, and traces of alumina. 
‘The low heat insoluble silica from the anthophyllite contained only traces 
of alumina; the high heat insoluble “silica” contained 21:76 per cent. of 
alumina, 19 per cent. of lime, and ‘44 per cent. of magnesia. 
The low heat insoluble silica from the andesine contained no impurity ; that 
from the high heat contained 1°34 per cent. of alumina, 1°17 per cent. of lime, 
and °18 per cent. of magnesia. 7 | 
Over-heating during granulation, therefore, largely increases the amount of 
insoluble silica ; and this is due to the chlorides being decomposed, and the silica 
re-entering into combination with the bases present in the mineral. 
3. What are the Conditions or Modifications in which “ Silica” is separated 
during an ESOS 
where are— Ast, The flocculent or semi- gelatinous ; 2d, the hyaline granular ; 
3d, the white opaque granular ; as seen from minerals which have been fused 
with alkaline carbonates. Minerals which have been decomposed by acid show 
only the first two. 
, Whatever be the reason, the “ silica” obtained from minerals decomposed 
by acid is, after dehydration by heat, as a rule, much less soluble’ than that 
_ obtained from those which have been fluxed ; the acid would appear to ‘decom- 
pose by soaking out ‘the bases while it combined with them, leaving the silica 
behind in the. crystalloidal state; the fusion with the alkaline carbonate | 
converts the crystalloidal silica into its colloidal and more soluble form ; 
though an after-heating may reconvert a certain portion. 
The silica obtained in an insoluble state, or rendered insoluble by unduly 
continued dehydration of the first and second varieties, is of about equal purity. 
It cannot be set down as pure silica, for I find that it generally contains traces of 
alumina, sometimes as much as 5:9 per cent., sometimes also traces of lime. 
‘Should it be even slightly opaque or dull white, its purity is doubtful. 
Singularly enough, the most impure insoluble “silica” obtained by a proper 
method of granulation which I have examined * was considered suspicious from 
its being unusually light and slimy. It was got from a margarodite, the silica 
of which yielded 5°68 per cent. of the suspicious powder. This powder gave in 
100 parts—silica, not yet absolutely soluble, 69°27 per cent., alumina Parity 
lime 2°01, magnesia 1° 61. 
Of the above noted “silicas,” the first is impalpable to the touch of a glass - 
rod; the second is erystalline gritty ; the third is hard, crunching only on the 
application of an amount of force which might fracture the containing beaker. 
* T except, of course the cases where titanic acid is present. 
