128 Mr. G. P. Scrope on the Formation of Craters, 
remained in the form of a somewhat bulky black precipitate of 
sulphide of aluminium at the bottom of the crucible. This preci- 
pitate was washed by decantation until tolerably free from alkali, 
the presence of a small quantity of which seemed to prevent its 
decomposition by water. In proportion, however, as the alkali is 
removed, the addition of water causes an evolution of sulphur- 
etted hydrogen, a grayish-white precipitate of the hydrated ses- 
quioxide of alumina being formed. 
The original black precipitate, when exposed to the air on a 
filter, is converted in a very short time almost entirely into the 
oxide of aluminium. The sulphide, when heated in the air, 
ignites, sometimes burning quietly, at others giving off bright 
scintillations much resembling those of the metal, in both cases 
sulphurous acid being formed. 
The alumina used in the above experiments was prepared by 
igniting alum, previously deprived of its water of crystallization ; 
but alumina in almost every state appears to be attacked by the 
sulphide of sodium, even emery-powder and pipe-clay being, the 
first wholly, the second partially deoxidized by it. 
XIII. On the Formation of Craters, and the Nature of the 
Liquidity of Lavas. By G. Poututr Scrorz, Esg., M.P., 
F.R.S., F.G.S.* 
INTRODUCTION. 
ik is now some thirty years since I published two works+ 
upon the Phenomena of Volcanoes, Active and Extinct. I 
described in them, as accurately as I could, by pen and pencil, 
what I had observed during a residence of some duration among 
the voleanic districts of France and Italy; and explained, in 
considerable detail, the laws which, from those observations, I 
believed to regulate the remarkable developments of subter- 
ranean energies usually called voleanic, which have played so 
important a part in the construction of the superficial crust of 
our planet. 
The general principle on which I proceeded in the theoretical 
portion of these works was the same which had been previously 
employed by Hutton and Playfair, and was subsequently adopted, 
with signal success, by Sir Charles Lyell,—namely, to refer, so 
far as is possible, appearances the origin of which has not been 
witnessed, to such causes as are seen or known to produce ana- 
logous appearances in the present day,—instead of resorting for 
the purpose to imaginary hypotheses. 
* From the Journal of the Geological Society, vol. xii. p. 326. 
t * Considerations on Volcanoes,” &c., 1825-6. “ On the Geology of 
Central France,” &c., 1826-7. 
