PROCEEDINGS OF THE PERTHSHIRE SOCIETY OF NATURAL SCIENCE, 
229 
consist of pumice of an extraordinarily light kind. They 
were what he terms overblown, i.e., the ordinary air- 
bubbles that are in pumice were much extended, and 
blown so thin that they appeared to have been burst into 
fragments resembling pieces of broken watch-glasses. 
They are (says he) of extreme tenuity, being about 
1-15,000th of an inch in thickness, and there can be no 
doubt that they are capable of floating a long time in the 
atmosphere in currents. That volcanic dust can be sus¬ 
pended in the atmosphere, and carried great distances, is 
further shewn by the fact that snow which fell at Phila¬ 
delphia in America about two years ago, on being melted 
and evaporated, was found to contain undoubted volcanic 
particles; and it was believed that these might have come 
from Krakatoa, a distance of 10,000 miles. It was, how¬ 
ever, still more probable that they had come from Alaska, 
in the N.W. corner of America, where a great eruption 
occurred on the previous summer. Whilst, then, there 
are many well - authenticated cases which shew that 
volcanic matter in inconceivably minute particles and 
filaments may be projected far up into the atmosphere, 
and may float in it for a long time, and be carried to great 
distances, when we come to apply this fact to the explana¬ 
tion of the glow, and seek from it a satisfactory reason for 
its long continuance, its extraordinary brilliancy, and the 
immense area over which it extended, we are met with 
many and formidable difficulties. 
Not the least of these is the consideration that, to explain 
the phenomenon, the volcanic particles must be supposed 
to have been driven to the very highest regions of the 
atmosphere, and to have floated there for many, many 
months. To suppose them to have an elevation of ten or 
fifteen miles would not at all account for the appearances 
presented, since the refractions through those of them 
which were transparent, and the reflection from those 
which were opaque, would not at such an elevation have 
extended the glow to the time after sunset to which we 
know it did extend. They must have had an elevation of 
30 or 40 miles from the earth’s surface to explain the elon¬ 
gation of time and the other phenomena witnessed. But 
at 30 or 40 miles from the earth’s surface the atmosphere is 
so thin and rare, so nearly approaching a vacuum, that it is 
utterly inconceivable that any solid matter, however 
attenuated, could remain for weeks and months sus¬ 
pended in it. 
Besides, how can we conceive of a force sufficient to 
project atoms so minute to such regions ? Terrific though 
the forces of a volcano are, consider for a moment what the 
theory under consideration requires them to do. They 
must propel atoms of matter so minute that only a high- 
power microscope can make them visible, through 30 miles, 
or the greater part of the atmosphere of the earth,—and 
that, too, in direct opposition to the action of gravity,—a 
feat which no initial velocity we can conceive of could 
accomplish with particles so minute. Had the particles 
been each the size of a man’s fist and a couple of pounds in 
weight, they might have been driven the distance, or even 
beyond it. An illustration of what we mean may be taken 
thus:—A piece of ordnance, if loaded with a full charge of 
powder and a heavy projectile, will carry 12 or 15 miles ; 
whilst the same gun, with the same charge of powder, will 
not drive a mass of snipe-dust, of the same weight as the 
projectile, more than a few hundred yards. 
No doubt the action of an active volcano is considerably 
different from that of a gun, since over the vent of the vol¬ 
cano there may be conceived to be an enormous column of 
heated air rushing up through the more solid atmosphere, 
up which chimney (if we may so regard it) there rushes 
prodigious volumes of heated air, and dust, and steam, and 
smoke. In this way it is quite conceivable that the minute 
particles found on the deck of the Arabella, and in the 
snow at Philadelphia, were carried high up into the 
atmosphere, and floated by its currents many hundred 
miles; but that they should have been so elevated beyond 
6 or 7 miles is quite impossible. Beyond the scirrhus 
clouds 5 or 6 miles from the earth, we have reason to be¬ 
lieve that the currents in the atmosphere are very faint 
indeed; and at 15 or 20 miles they altogether cease to exist. 
Volcanic particles, therefore, though they found their way 
to such a region, would change their longitude and latitude 
with extreme slowness. Now, if volcanic dust from 
Krakatoa occasioned the glow, it must in some unaccount¬ 
able way have found its way through the higher regions of 
the atmosphere, all round the globe, in the shortest space of 
time, since the glow suddenly and simultaneously, and with 
equal brilliancy, appeared in all the countries of the earth. 
But, formidable though the difficulty to the dust theory 
which we have just stated is, there is another and much 
more formidable one, which stares us in the face, viz., the 
quantity of dust which would be required to produce the 
phenomena. The idea of its being ejected from a single 
volcanic eruption, however great that eruption may have 
been, has only to be stated in order that it may be dis¬ 
carded. The area of the earth’s surface is about 200 
millions of square miles, and the atmosphere over this 
immense area to be filled with minute particles, in its 
higher regions, so as to produce the phenomena of the 
glow, must have had not one Krakatoa in eruption for a 
few days, but many volcanoes belching forth their con¬ 
tents for many months. 
