ON THE FACTS OF EARTHQUAKE PHZNOMENA. 55 
may be expected to exceed that of a heavy thunder-storm, and may, guoad 
this particular part of earthquake phenomena, realize the dreams of the older 
philosophers, who thought an earthquake was a thunder-storm under ground. 
In this then I believe is to be found the usual source of the flame or flash, 
seen suddenly to appear and vanish at the mouth of the rent, and the iden- 
tification of the supposed flame or flash with electricity in an analogous case, 
was made by Sir W. Hamilton, who alludes to its violent disturbance always 
in the cloud above the crater of a volcano in eruption, though he suggests 
no origin for such disturbance in the case of fissures opening, from which he 
had satisfied himself by subsequent examination, that there was no evidence . 
of flame or volcanic exhalations of any sort having issued from their mouths. 
And as to the smoke which has even been described by some authors as dust, 
I fancy it has been none other in almost all cases, if not in all. 
Eye-witnesses of the falling of towns and cities by earthquakes describe 
the volume of dust that rises from the shattered buildings as instantly ob- 
scuring the scene of desolation from view; thus Catania in 1692 disappeared 
in an instant in a cloud of dust; and any one who has seen a large blast fired 
in a quarry of hard rock will remember the dust that rises through and over 
the falling and shattered masses. There can thus be no doubt that the rend- 
ing of a mountain mass of rock must be attended with similar volumes of 
dust, and that the same must attend the fracture of earthy materials, such 
as the clay of the plains of Calabria or that of Lisbon, which from the dry- 
ness and heat of both climates must be for many feet down in a friable con- 
dition. But there is another cause yet for the cloud of steam, or dust and 
steam, even when the walls of the fissure may be perfectly wet. By the 
sudden yawning of one of these vast chasms a void space is instantly opened, 
into which of course the surrounding atmosphere immediately rushes; a 
partial vacuum is thus for the moment produced just above the mouth of the 
crevasse ; the great mass of air suddenly rarified or expanded has its capacity 
for heat increased, its sensible temperature is therefure as suddenly lowered, 
and a deposition of vapour, in form of a great cloud, takes place above the 
érevasse, which is greater or less in proportion as the dew-point is higher or 
lower at the time and place. 
Conversely, if the crevasse be wet and suddenly opened, to a considerable 
depth, the temperature of its sides and of the water dripping from them being 
that due to the depth, and therefore above that of the air at the surface, 
will instantly fill it with steam or vapour ; this will rise and mingle with the 
air above in the form of steam clouds at every breath of wind that enters 
the chasin and disturbs its repose, will be slowly driven out by the descent 
of the colder surrounding air of the surface, or may be wholly expelled if 
the crevasse close again, as it often does; and these sources of change of 
state in the air and vaporization of water, gr condensation thereof, are them- 
selves powerful causes of electrical disturbance. 
Whether therefore the formation through which a fissure or crevasse is 
éloven during an earthquake be hard or soft, dry or wet, on the mountain or 
in the plains, whether it be due directly to the earth-wave or shock, or se- 
condarily to stibsidence or slipping, I conceive that there is abundant evidence 
of sufficient meteorological and electrical disturbance to account for the 
elouds of steam or supposed smoke, flashes or sudden flame, and dust, so often 
mentioned as occurring far from volcanic active centres. 
Smoke in the true and ordinary sense of this word, it may be remarked 
in passing, has never been observed by any competent authority actually 
issuing from even any volcanic vent. The gaseous products are almost 
wholly vapour of Water, holding some acids, as SO? and Cl+ H, in suspension, 
