PROFESSOR. HEDDLE ON THE MINERALOGY OF SCOTLAND. 201 
—that one silica saturated the sesquioxides, another the protoxides, while an 
opal silica took up any non-basic water ? Should such a conclusion prove to be 
correct, we may hope to explain how a certain quantity of silica, and no more, 
may be replaced, as in hornblende, by alumina. — 
- Even, however, as regards the question of the convertibility of one modifica- 
tion into another, we do not find a conformity of opinion. 
Of our methods of examining into the condition and the purity of our silicas, 
two only may be said to be in general use—evaporation with hydrofluoric acid, 
and boiling in a saturated solution of sodium carbonate: the first, from the 
difficulty of procuring a sufficiently pure acid, and from the destructive nature 
of its fumes, being little employed. 
- RAMMELSBERG, making use of the latter test of purity, holds that pure silica . 
is only soluble in boiling solution of sodium carbonate, if it has been gently 
heated; and that the same silica is rendered more or less insoluble in the same 
solution, if continuously Azghly heated ; as it is thereby converted into tridy- 
mite. The application of this view would be that the very great difference in 
the solubilities. of the silicas got from various minerals by different analysts is 
due to different degrees, and undue continuance of heating. 
The late Mr Wattace Youne, on the other hand, remarks—“In testing 
the residual silicic acid by boiling with sodium carbonate solution, a great 
deal of trouble was experienced frequently by its sparing solubility in that 
reagent. I have not as yet been able to find out why the silicic acid 
‘should be quite soluble in some cases and not so in others, when subsequent 
examination showed it to be equally pure in both cases. For instance, in one 
specimen of analcime, which had been decomposed with hydrochloric acid, the 
silicic acid was almost. insoluble in sodium carbonate solution; prolonged 
hoiling and using different proportions of water, &c., did not seem to have any 
effect. On being fused with alkaline carbonate .and separating as usual, it was 
found to be. pure, and was now quite soluble in the sodium carbonate solution. 
Another specimen of analcime decomposed by acid gave 55°56 per cent. 
of silicic acid very sparingly soluble; a fresh portion of the mineral fused 
‘with alkaline carbonate gave 55°52 per cent. of silicic acid which dissolved 
‘in sodium carbonate, and. gave a clear solution. The difference of time and 
intensity of heat employed in the ignition of the silicic acid did not seem to 
affect its solubility.” 
And Mr Youne makes the following remark—most pertinent to the 
analysis of rocks :—“ The occasional sparing solubility of silicic acid comes 
to be of great importance in an analysis of mixed silicates only partially 
soluble in acids, when, after the acid treatment, the residue is boiled in sodium 
carbonate solution to extract the separated silicic acid, washed and dried, and 
the insoluble silicates fused with alkaline carbonate as usual. The silicic 
VOL. XXXVIII. PART I. 3G 
