Through Magic Glasses and Other Lectures

at I have to look round and remember that it was before any of you were born, in order to persuade myself that

ut puffs of smoke, with a booming noise, for several days, but we thought nothing of that, for it had been common enough for slight eruptions to take place at intervals ever since the great eruption of 1867. This cloud, however, was far larger and wider-spread than usual, and as we were looking at it we saw a thin red line begin some way down the side of the mounta

g.

. Ves

n eruption by the a

n followed a long ascent of about two hours and a half through the dark night. Silently and carefully we travelled on over the broad masses of slaggy lava of former years, along which a narrow horse-p

l rugged plain not far from the spot, we alighted from our horses, which were growing frightened with the glare, and walk

oud was broken and we could see the dark sky, the stars looked white as silver in contrast. We could now trace clearly the outline of the summit towering abo

of the lava flow, my heart bounded with excitement. I could not stay more than a few seconds, for the gases and vapour choked me; but for that short time it felt like a dream to be standing close to a river of molten rock, which a few hours before had been lying deep in the bowels of the earth. Glancing upwards to where this river issued from the cone in the mo

ich the Observatory stands there is, or was, a steep cliff, and over this the lava stream, now densely black, fell in its way to the valley below, and as it fell it broke i

e to cross over to these cones, and I could only marvel at the mass of ashes of which the top of the mountain was composed, and plunge a stick into an old lava stream to see how hot it still remained below. Peaceful and quiet as the mountain seemed then, I c

g.

vius in 1864. (

in-side. The truth is, that though we know now a good deal about volcanoes themselves, we know very little about the mighty cauldrons deep down in the earth from which they come. Our deepest

s have been active not long before. Now if there were an ocean of melted rock at a certain distance down below the crust all over the globe, there could scarcely be such a great difference between one place and another, and for this and many other reasons geologists are inc

the coast of the great oceans or of some enormous inland sea or lake. But at present all we can say is, that in certain parts of the globe there must be from time to time great masses of rock heated till they are white-hot, and having white-hot water ming

nnot you picture to yourselves how when this happened the rock would swell into a liquid state, and how the water would force its way upwards into cracks and fissures expanding into steam as it went. Then would be heard strange rumbling noises underground, as all these heavily oppressed white-hot substances upheaved and rent the crust above them. And a

in the Azores, and after remaining a short time was washed away by the waves. In the same way Graham's Island appeared off the coast of Sicily in 18

kes had alarmed the people, and at last one day the sea drew back from the shore and the ground sank about fourteen feet, and then on the night of Sunday, September 29, 1538, it was hurled up again, and steam, fiery gases, stones, and mud burst forth, driving away the frightened people from the village of Puzzuoli about two miles distant. For a whol

with trees and vines in the time of the Greeks till in the year A.D. 79 occurred the terrific outburst which destroyed Herculaneum and Pompeii, shattering old Vesuvius to pieces, so that only the cliffs on the northwest remain and are called Somma (see Fig. 37), while the new Vesuvius has grow

the mountain cools and hardens, shutting in once more the seething mass below. This was the state of the crater when I saw it in 1864, though small streams still flowed out of two minute cones;

d the history of a lava-flow has been made very clear during the last few years, chiefly by the help of the microscope and chemical experiments. If we imagin

g.

ection of an a

lar clouds d, d. e, Rain-shower from escaped vapour. f, f, Shower of blocks, cooled bombs, stones, and ashes falli

ck is fused, only it would have floating in it great blocks of solid rock, and rounded stones called bombs which have been formed from pieces of half-melted rock whirled in air and falling back into the crater, together with clinkers or scori?, dust and sand, all torn off and gr

es, dust, and scori?. Meanwhile the lava surging below was forcing a passage g for itself in a weak part of the mountain-side and, just at the time when our attention was called to Vesuvius, the violent pressure from below rent open a mouth or crater at h, so that the lava began to flow down the mountain in a steady stream. This, relieving the funnel, enabled the huge steam bubbles d, d to r

ir dry, and the steam floated peacefully away. The next night, however, when many people hurried down from Rome to see the sight they were woefully

on its slopes, and that on the side of the mountain behind the Hermitage a new lava stream was added to the many which have flowed there of late years. Wha

.[1] In fact, there are so many holes and bubbles in it that it is very light and floats on the top of the heavier lava below, falling over it on to the mountain-side when it comes to the end of the stream. Still, however, the great mass moves on, so that the stream slides over these fallen cli

g.

lava-flow.

ture. 2, Middle portion where crystals form. 3, Slaggy c

om the mountain. But you must not stand still or your shoes would soon be burnt, and if you break the crust with a stick you will at once see the r

lower part of the stream. In the clinkers above, where the cooling goes on very rapidly, the lavas formed are semi-transparent and look much like common bottle-glass. In fa

g.

es of crystallites and microliths which a

2

he first beginnings of crystals forming out of the ground-mass of glassy lava as it cools down. They are not real crystals, but the first step toward them, and by a careful examination of glassy lavas which have cooled at different rates, they ha

ths.[3] Under the next microscope you can see the microliths much more

g.

the microscope, showing well-dev

lowing and carrying them along in streams. You will notice that each one has its greatest length in the direction of the lines, just as pieces of stick are carried along

grained, and the magnifying-glass shows many crystals in it, as well as a number of microliths. For this lava, covered by the crust abo

such as porphyry and granite made up entirely of crystals we must look inside the mountain where

ries, so that we can see the inside. The dykes laid bare in the cliffs of Somma are old fissures filled with molten rock which has cooled down, and they show us many stony lavas; and Mr. Judd tells us of one beautiful example of a ruined volcano which composes the whole island of Mull in the Hebrides, where such dykes can be traced right back to a centre. This centre must once have been a mass of melted matter far down in the eart

top, hurrying the crystallites and microliths along with it-down through the volcano to the depths of the earth, where the perfect

g.

or Granite, drawn

in the depths when these crystals were formed. A few years ago I could not have answered you, but now chemists, and esp

, Labrador felspar, and augite. This they proposed to make artificially, so they took proper quantities of silica, alumina, oxide of iron, lime, potash, and soda, and putting them in a crucible, melted them by keeping them at a white heat. Then they lowered the temperature to an orange-heat, that is a heat sufficient to melt steel. They kept this heat for forty-eight hours, after which they took out some of the mixture and, letting it cool, examined a slice under the microscope. Within it they found crys

t crystals and beginnings of crystals grew up to form the rock which is found in Vesuvius. And what is still more interesting, they gr

at the beginning of this new study, and there is plenty for you boys to do by and by when you grow up. Many experiments have failed as yet to imitate certain rocks, and it is re

ds curving round them. These crystals have clearly not been formed in the glass while the lava was flowing, first because they are too large to have grown up so rapidly, and secondly because they are broken at the edges in places and sometimes partly melted. Th

flows, for a thoroughly melted lava shoots along at a great pace and often travels several miles in a very short time; but an i

g.

rystals brought up from inside the volcano in the fluid lava. The dark

when the microliths were formed after it was poured out above ground. And as we know that different substances form their crystals at very different temperatures, it is not surprising that some

e these, all of which have their history; but this would lead us too far. We must be content for the present with having roughly traced a flow of lava from the depths below, where

ires, and garnets, and many others which Nature forms far away out of our sight. All these depend, though indirectly, upon the strange effects of underground heat, and if you have once formed a picture in your minds of what must have been going on before that magnificent lava stream crept down the mountain-side and added its small contribu

e surface of such a stream see t

n lines is called flui

little; li

white; tep