188 Mr. G. P. Scrope on the Formation of Craters, 
was subjected to the action of 24 ounces of iodine in presence of 
an excess of phosphorus, as in the preparation of iodide of zthyle. 
Reaction ensued, and the decanted liquid gave a distillate which 
was heavier than water; but on redistilling, this distillate showed 
no fixed boiling-point ; and on rectification, two liquids were iso- 
lated, having the respective boiling-points and all the properties 
of iodide of zthyle and iodide of amyle. Now, according to the 
two latter of the above formule, we should in the one case expect 
a body having the composition C™*™ H™*=+1J, and in the other 
a body having the composition C?™+» H?™+™+1J_ The forma- 
tion of two distinct iodides is incompatible with both of these, 
and supports, if it does not prove, the correctness of the formula 
C2m F2m+1 O, C22 H22+10, 
Owens College, Manchester, 
August 1857. 
XXIII. On the Formation of Craters, and the Nature of the 
Liquidity of Lavas. By G. Pouterr Scrorz, Esg., M.P., 
F.R.S., F.G.S. 
[Concluded from p. 142.] 
I, (JHE Nature of the Liquidity of Lavas.—So much for that 
branch of my subject,—the formation of cones and 
craters. I wish now to ask attention to some circumstances 
respecting the mode of emission and nature of the lavas that 
proceed from them. I have already spoken of the comparatively 
tranquil manner in which some lava-streams are seen to well out 
from the flank of a voleano, or its summit, and the probability 
that differences in the liquidity or viscosity of the heated matter 
at the time of its efflux may occasion corresponding differences 
in the character of the phenomena. Observation confirms this 
expectation ; and it has been remarked, that the very liquid and 
vitrified lavas, such as those of Kilauea and Bourbon, are poured 
out more or less tranquilly without any very violent explosions, 
their imprisoned vapours evidently escapmg with comparative 
ease, while the more viscous and ultimately stony lavas, possess- 
ing a minor degree of liquidity, and consequently not allowing 
so easy a passage to the vapours that rise through, and struggle 
to escape from them, are protruded with fiercer explosive bursts, 
and the ejection of far greater quantities of scoria and other 
fragmentary matters. 
This observation, coupled with other reasons to which I shall 
presently advert, led me to an opinion expressed in the works pre- 
viously referred to, that the ordinary crystalline or granular lavas 
(making exception of the vitreous varieties), although at a white 
heat at the moment of their emission from a volcanic vent, are 
