_ PROFESSOR HEDDLE ON THE MINERALOGY OF SCOTLAND. 205 
during the long continued heating in the beakers; a portion of their substance 
is dissolved, mingling with the contents, it is evident that a silica originally sepa- 
rated pure, may, by the faults of the process adopted, be rendered impure. 
We cannot hope to execute correct analyses of silicates CO. we employ vessels 
of a material other than glass. 
The above results support the finding of RAMMELSBERG, that a high heat 
converts silica soluble in the test solution into an insoluble ee but they 
go to show more than this. 
First, That different varieties or states of silica are so converted in very 
different amounts ; second, That so far as experiments go, it is the more finely- 
divided ‘silica which suffers the greatest amount of change; and thirdly, That 
the insoluble powder left after boiling in the test-solution should never be con-. 
fidently set down as being pure silica, or tridymite; seeing that it may contain 
as much as 6 per cent. of its weight of alumina: so that in every case where 
_ the analysis is intended to be founded on for the deduction of a ne, this 
so-called tridymite should be fluxed, and itself analysed. 
But, though thus finding, with RAMMELSBERG, that the continuance of a high 
_heat converts a soluble silica into an insoluble, I am unable to accept this as 
_ the explanation of the very varying amounts of insoluble silica obtained from 
minerals by different analysts ;—an explanation altogether different is required. 
It is unquestionably the fact that the more carefully every part of the pro- 
“cess connected with the separation of the silica is performed, the greater will 
be the proportion of it soluble; but I find that even the same mineral, when 
‘treated more than once in identically the same way, may yield very different 
quantities of insoluble silica ; the silica, therefore, must come out of the mineral 
in different states, from modifications of the treatment, so slight as to: be un- 
avoidable in ordinary manipulation. 
The treatment to which the silicas obtained in 1 my analyses were subjected 
“was the following :— 
After thorough drying in a filter-drying bath, the silica, removed from the 
filter and placed in a platinum crucible, was very gradually brought down into 
the flame of a Bunsen burner during 20 minutes, and was then heated in that 
flame for 40 minutes longer. After weighing, it was boiled for four hours in 6 
cubic inches of a saturated solution of sodium sesquicarbonate ; thrown on a 
filter, thoroughly washed, re-heated, and the insoluble portion weighed. Should 
there have been anything the least suspicious in its appearance or colour, 
either when hot or cold, it was re-fused and examined. 
Of the “‘silicas” of 181 minerals so examined, the average percentage of 
VOL. XXVIII. PARTI, - ; 3H 
