distilled is heated and converted into vapour in one vessel, and
chilled and reconverted into liquid in another. What has just been
stated renders it plain that the earth and its atmosphere constitute
a vast distilling apparatus in which the equatorial ocean plays the
part of the boiler, and the chill regions of the poles the part of
the condenser. In this process of distillation _heat_ plays quite as
necessary a part as _cold_, and before Bishop Heber could speak of
"Greenland's icy mountains," the equatorial ocean had to be warmed by
the sun. We shall have more to say upon this question afterwards.
Illustrative Experiments.
65. I have said that when heated, air expands. If you wish to verify
this for yourself, proceed thus. Take an empty flask, stop it by a
cork; pass through the cork a narrow glass tube. By heating the tube
in a spirit-lamp you can bend it downwards, so that when the flask is
standing upright the open end of the narrow tube may dip into water.
Now cause the flame of your spirit-lamp to play against the flask.
The flame heats the glass, the glass heats the air; the air expands,
is driven through the narrow tube, and issues in a storm of bubbles
from the water.
66. Were the heated air unconfined, it would rise in the heavier
cold air. Allow a sunbeam or any other intense light to fall upon a
white wall or screen in a dark room. Bring a heated poker, a candle,
or a gas flame underneath the beam. An ascending current rises from
the heated body through the beam, and the action of the air upon the
light is such as to render the wreathing and waving of the current
strikingly visible upon the screen. When the air is hot enough, and
therefore light enough, if entrapped in a paper bag it carries the
bag upwards, and you have the Fire-balloon.
67. Fold two sheets of paper into two cones and suspend them with
their closed points upwards from the end of a delicate balance. See
that the cones balance each other. Then place for a moment the flame
of a spirit-lamp beneath the open base of one of them; the hot air
ascends from the lamp and instantly tosses upwards the cone above it.
68. Into an inverted glass shade introduce a little smoke. Let the
air come to rest, and then simply place your hand at the open mouth
of the shade. Mimic hurricanes are produced by the air warmed by the
hand, which are strikingly visible when the smoke is illuminated by a
strong light.
69. The heating of the tropical air by the sun is _indirect_. The
solar beams have scarcely any power to heat the air through which
they pass; but they heat the land and ocean, and these communicate
their heat to the air in contact with them. The air and vapour start
upwards charged with the heat thus communicated.
§ 7. _Tropical Rains._
70. But long before the air and vapour from the equator reach the
poles, precipitation occurs. Wherever a humid warm wind mixes with a
cold dry one, rain falls. Indeed the heaviest rains occur at those
places where the sun is vertically overhead. We must enquire a little
more closely into their origin.
71. Fill a bladder about two-thirds full of air at the sea level,
and take it to the summit of Mont Blanc. As you ascend, the bladder
becomes more and more distended; at the top of the mountain it is
fully distended, and has evidently to bear a pressure from within.
Returning to the sea level you find that the tightness disappears,
the bladder finally appearing as flaccid as at first.
72. The reason is plain. At the sea level the air within the
bladder has to bear the pressure of the whole atmosphere, being
thereby squeezed into a comparatively small volume. In ascending
the mountain, you leave more and more of the atmosphere behind; the
pressure becomes less and less, and by its expansive force the air
within the bladder swells as the outside pressure is diminished. At
the top of the mountain the expansion is quite sufficient to render
the bladder tight, the pressure within being then actually greater
than the pressure without. By means of an air-pump we can show the
expansion of a balloon partly filled with air, when the external
pressure has been in part removed.
73. But why do I dwell upon this? Simply to make plain to you that
the _unconfined air_, heated at the earth's surface, and ascending by
its lightness, must expand more and more the higher it rises in the
atmosphere.
74. And now I have to introduce to you a new fact, towards the
statement of which I have been working for some time. It is this:
_The ascending air is chilled by its expansion._ Indeed this chilling
is one source of the coldness of the higher atmospheric regions. And
now fix your eye upon those mixed currents of air and aqueous vapour
which rise from the warm tropical ocean. They start with plenty of
heat to preserve the vapour as vapour; but as they rise they come
into regions already chilled, and they are still further chilled by
their own expansion. The consequence might be foreseen. The load of
vapour is in great part precipitated, dense clouds are formed, their
particles coalesce to rain-drops, which descend daily in gushes so
profuse that the word "torrential" is used to express the copiousness
of the rainfall. I could show you this chilling by expansion, and
also the consequent precipitation of clouds.
75. Thus long before the air from the equator reaches the poles
its vapour is in great part removed from it, having redescended to
the earth as rain. Still a good quantity of the vapour is carried
forward, which yields hail, rain, and snow in northern and southern
lands.
Illustrative Experiments.
76. I have said that the air is chilled during its expansion. Prove
this, if you like, thus. With a condensing syringe, you can force air
into an iron box furnished with a stopcock, to which the syringe is
screwed. Do so till the density of the air within the box is doubled
or trebled. Immediately after this condensation, both the box and
the air within it are warm, and can be proved to be so by a proper
thermometer. Simply turn the cock and allow the compressed air to
stream into the atmosphere. The current, if allowed to strike the
thermometer, visibly chills it; and with other instruments the chill
may be made more evident still. Even the hand feels the chill of the
expanding air.
77. Throw a strong light, a concentrated sunbeam for example, across
the issuing current; if the compressed air be ordinary humid air, you
see the precipitation of a little cloud by the chill accompanying the
expansion. This cloud-formation may, however, be better illustrated
in the following way:--
78. In a darkened room send a strong beam of light through a glass
tube three feet long and three inches wide, stopped at its ends by
glass plates. Connect the tube by means of a stopcock with a vessel
of about one-fourth its capacity, from which the air has been removed
by an air-pump. The exhausted cylinder of the pump itself will answer
capitally. Fill the glass tube with humid air; then simply turn on
the stopcock which connects it with the exhausted vessel. Having
more room the air expands, cold accompanies the expansion, and, as a
consequence, a dense and brilliant cloud immediately fills the tube.
If the experiment be made for yourself alone you may see the cloud
in ordinary daylight; indeed, the brisk exhaustion of any receiver
filled with humid air is known to produce this condensation.
79. Other vapours than that of water may be thus precipitated,
some of them yielding clouds of intense brilliancy, and displaying
iridescences, such as are sometimes, but not frequently, seen in the
clouds floating over the Alps.
80. In science what is true for the small is true for the large. Thus
by combining the conditions observed on a large scale in nature we
obtain on a small scale the phenomena of atmospheric clouds.
§ 8. _Mountain Condensers._
81. To complete our view of the process of atmospheric precipitation
we must take into account the action of mountains. Imagine a
south-west wind blowing across the Atlantic towards Ireland. In
its passage it charges itself with aqueous vapour. In the south of
Ireland it encounters the mountains of Kerry: the highest of these
is Magillicuddy's Reeks, near Killarney. Now the lowest stratum of
this Atlantic wind is that which is most fully charged with vapour.
When it encounters the base of the Kerry mountains it is tilted up
and flows bodily over them. Its load of vapour is therefore carried
to a height, it expands on reaching the height, it is chilled in
consequence of the expansion, and comes down in copious showers
of rain. From this, in fact, arises the luxuriant vegetation of
Killarney; to this, indeed, the lakes owe their water supply. The
cold crests of the mountains also aid in the work of condensation.
82. Note the consequence. There is a town called Cahirciveen to
the south-west of Magillicuddy's Reeks, at which observations of
the rainfall have been made, and a good distance farther to the
north-east, right in the course of the south-west wind, there is
another town, called Portarlington, at which observations of rainfall
have also been made. But before the wind reaches the latter station
it has passed over the mountains of Kerry and left a great portion of
its moisture behind it. What is the result? At Cahirciveen, as shown
by Dr. Lloyd, the rainfall amounts to 59 inches in a year, while at
Portarlington it is only 21 inches.
83. Again, you may sometimes descend from the Alps when the fall of
rain and snow is heavy and incessant, into Italy, and find the sky
over the plains of Lombardy blue and cloudless, the wind at the same
time _blowing over the plain towards the Alps_. Below the wind is hot
enough to keep its vapour in a perfectly transparent state; but it
meets the mountains, is tilted up, expanded, and chilled. The cold of
the higher summits also helps the chill. The consequence is that the
vapour is precipitated as rain or snow, thus producing bad weather
upon the heights, while the plains below, flooded with the same air,
enjoy the aspect of the unclouded summer sun. Clouds blowing _from_
the Alps are also sometimes dissolved over the plains of Lombardy.
84. In connection with the formation of clouds by mountains, one
particularly instructive effect may be here noticed. You frequently
see a streamer of cloud many hundred yards in length drawn out from
an Alpine peak. Its steadiness appears perfect, though a strong
wind may be blowing at the same time over the mountain head. Why
is the cloud not blown away? It is blown away; its permanence is
only apparent. At one end it is incessantly dissolved, at the other
end it is incessantly renewed: supply and consumption being thus
equalized, the cloud appears as changeless as the mountain to which
it seems to cling. When the red sun of the evening shines upon these
cloud-streamers they resemble vast torches with their flames blown
through the air.
§ 9. _Architecture of Snow._
85. We now resemble persons who have climbed a difficult peak,
and thereby earned the enjoyment of a wide prospect. Having made
ourselves masters of the conditions necessary to the production of
mountain snow, we are able to take a comprehensive and intelligent
view of the phenomena of glaciers.
86. A few words are still necessary as to the formation of snow. The
molecules and atoms of all substances, when allowed free play, build
themselves into definite and, for the most part, beautiful forms
called crystals. Iron, copper, gold, silver, lead, sulphur, when
melted and permitted to cool gradually, all show this crystallizing
power. The metal bismuth shows it in a particularly striking manner,
and when properly fused and solidified, self-built crystals of great
size and beauty are formed of this metal.
87. If you dissolve saltpetre in water, and allow the solution to
evaporate slowly, you may obtain large crystals, for no portion of
the salt is converted into vapour. The water of our atmosphere is
fresh though it is derived from the salt sea. Sugar dissolved in
water, and permitted to evaporate, yields crystals of sugar-candy.
Alum readily crystallizes in the same way. Flints dissolved, as they
sometimes are in nature, and permitted to crystallize, yield the
prisms and pyramids of rock crystal. Chalk dissolved and crystallized
yields Iceland spar. The diamond is crystallized carbon. All our
precious stones, the ruby, sapphire, beryl, topaz, emerald, are all
examples of this crystallizing power.
88. You have heard of the force of gravitation, and you know that
it consists of an attraction of every particle of matter for every
other particle. You know that planets and moons are held in their
orbits by this attraction. But gravitation is a very simple affair
compared to the force, or rather forces, of crystallization. For here
the ultimate particles of matter, inconceivably small as they are,
show themselves possessed of attractive and repellent poles, by the
mutual action of which the shape and structure of the crystal are
determined. In the solid condition the attracting poles are rigidly
locked together; but if sufficient heat be applied the bond of union
is dissolved, and in the state of fusion the poles are pushed so far
asunder as to be practically out of each other's range. The natural
tendency of the molecules to build themselves together is thus
neutralized.
89. This is the case with water, which as a liquid is to all
appearance formless. When sufficiently cooled the molecules are
brought within the play of the crystallizing force, and they then
arrange themselves in forms of indescribable beauty. When snow
is produced in calm air, the icy particles build themselves into
beautiful stellar shapes, each star possessing six rays. There is no
deviation from this type, though in other respects the appearances
of the snow-stars are infinitely various. In the polar regions these
exquisite forms were observed by Dr. Scoresby, who gave numerous
drawings of them. I have observed them in mid-winter filling the air,
and loading the slopes of the Alps. But in England they are also to
be seen, and no words of mine could convey so vivid an impression of
their beauty as the annexed drawings of a few of them, executed at
Greenwich by Mr. Glaisher.
90. It is worth pausing to think what wonderful work is going on in
the atmosphere during the formation and descent of every snow-shower:
what building power is brought into play! and how imperfect seem the
productions of human minds and hands when compared with those formed
by the blind forces of nature!
91. But who ventures to call the forces of nature blind? In reality,
when we speak thus we are describing our own condition. The blindness
is ours; and what we really ought to say, and to confess, is that our
powers are absolutely unable to comprehend either the origin or the
end of the operations of nature.
92. But while we thus acknowledge our limits, there is also reason
for wonder at the extent to which science has mastered the system
of nature. From age to age, and from generation to generation, fact
has been added to fact, and law to law, the true method and order
of the Universe being thereby more and more revealed. In doing this
science has encountered and overthrown various forms of superstition
and deceit, of credulity and imposture. But the world continually
produces weak persons and wicked persons; and as long as they
continue to exist side by side, as they do in this our day, very
debasing beliefs will also continue to infest the world.
§ 10. _Atomic Poles._
93. "What did I mean when, a few moments ago (88), I spoke of
attracting and repellent poles?" Let me try to answer this question.
You know that astronomers and geographers speak of the earth's poles,
and you have also heard of magnetic poles, the poles of a magnet
being the points at which the attraction and repulsion of the magnet
are as it were concentrated.
[Illustration: SNOW CRYSTALS.]
94. Every magnet possesses, two such poles; and if iron filings be
scattered over a magnet, each particle becomes also endowed with
two poles. Suppose such particles devoid of weight and floating in
our atmosphere, what must occur when they come near each other?
Manifestly the repellent poles will retreat from each other, while
the attractive poles will approach and finally lock themselves
together. And supposing the particles, instead of a single pair, to
possess several pairs of poles arranged at definite points over their
surfaces; you can then picture them, in obedience to their mutual
attractions and repulsions, building themselves together to form
masses of definite shape and structure.
95. Imagine the molecules of water in calm cold air to be gifted
with poles of this description, which compel the particles to lay
themselves together in a definite order, and you have before your
mind's eye the unseen architecture which finally produces the
visible and beautiful crystals of the snow. Thus our first notions
and conceptions of poles are obtained from the sight of our eyes
in looking at the effects of magnetism; and we then transfer these
notions and conceptions to particles which no eye has ever seen. The
power by which we thus picture to ourselves effects beyond the range
of the senses is what philosophers call the Imagination, and in the
effort of the mind to seize upon the unseen architecture of crystals,
we have an example of the "scientific use" of this faculty. Without
imagination we might have _critical_ power, but not _creative_ power
in science.
§ 11. _Architecture of Lake Ice._
96. We have thus made ourselves acquainted with the beautiful
snow-flowers self-constructed by the molecules of water in calm cold
air. Do the molecules show this architectural power when ordinary
water is frozen? What, for example, is the structure of the ice over
which we skate in winter? Quite as wonderful as the flowers of the
snow. The observation is rare, if not new, but I have seen in water
slowly freezing six-rayed ice-stars formed, and floating free on the
surface. A six-rayed star, moreover, is typical of the construction
of all our lake ice. It is built up of such forms wonderfully
interlaced.
97. Take a slab of lake ice and place it in the path of a
concentrated sunbeam. Watch the track of the beam through the ice.
Part of the beam is stopped, part of it goes through; the former
produces internal liquefaction, the latter has no effect whatever
upon the ice. But the liquefaction is not uniformly diffused. From
separate spots of the ice little shining points are seen to sparkle
forth. Every one of those points is surrounded by a beautiful liquid
flower with six petals.
98. Ice and water are so optically alike that unless the light fall
properly upon these flowers you cannot see them. But what is the
central spot? A vacuum. Ice swims on water because, bulk for bulk,
it is lighter than water; so that when ice is melted it shrinks in
size. Can the liquid flowers then occupy the whole space of the ice
melted? Plainly no. A little empty space is formed with the flowers,
and this space, or rather its surface, shines in the sun with the
lustre of burnished silver.
99. In all cases the flowers are formed parallel to the surface of
freezing. They are formed when the sun shines upon the ice of every
lake; sometimes in myriads, and so small as to require a magnifying
glass to see them. They are always attainable, but their beauty is
often marred by internal defects of the ice. Even one portion of the
same piece of ice may show them exquisitely, while the second portion
shows them imperfectly.
100. Annexed is a very imperfect sketch of these beautiful figures.
101. Here we have a reversal of the process of crystallization. The
searching solar beam is delicate enough to take the molecules down
without deranging the order of their architecture. Try the experiment
for yourself with a pocket-lens on a sunny day. You will not find the
flowers confused; they all lie parallel to the surface of freezing.
In this exquisite way every bit of the ice over which our skaters
glide in winter is put together.
102. I said in (97) that a portion of the sunbeam was stopped by the
ice and liquefied it. What is this portion? The dark heat of the
sun. The great body of the light waves and even a portion of the dark
ones, pass through the ice without losing any of their heating power.
When properly concentrated on combustible bodies, even after having
passed through the ice, their burning power becomes manifest.
[Illustration: LIQUID FLOWERS IN LAKE ICE.]
103. And the ice itself may be employed to concentrate them. With an
ice-lens in the polar regions Dr. Scoresby has often concentrated the
sun's rays so as to make them burn wood, fire gunpowder, and melt
lead; thus proving that the heating power is retained by the rays,
even after they have passed through so cold a substance.
104. By rendering the rays of the electric lamp parallel, and then
sending them through a lens of ice, we obtain all the effects which
Dr. Scoresby obtained with the rays of the sun.
§ 12. _The Source of the Arveiron. Ice Pinnacles, Towers, and Chasms
of the Glacier des Bois. Passage to the Montanvert._
105. Our preparatory studies are for the present ended, and thus
informed, let us approach the Alps. Through the village of Chamouni,
in Savoy, a river rushes which is called the Arve. Let us trace this
river backwards from Chamouni. At a little distance from the village
the river forks; one of its branches still continues to be called
the Arve, the other is the Arveiron. Following this latter we come
to what is called the "source of the Arveiron"--a short hour's walk
from Chamouni. Here, as in the case of the Rhone already referred
to, you are fronted by a huge mass of ice, the end of a glacier, and
from an arch in the ice the Arveiron issues. Do not trust the arch in
summer. Its roof falls at intervals with a startling crash, and would
infallibly crush any person on whom it might fall.
106. We must now be observant. Looking about us here, we find in
front of the ice curious heaps and ridges of débris, which are more
or less concentric. These are the _terminal moraines_ of the glacier.
We shall examine them subsequently.
107. We now turn to the left, and ascend the slope beside the
glacier. As we ascend we get a better view, and find that the ice
here fills a narrow valley. We come upon another singular ridge,
not of fresh débris, like those lower down, but covered in part
with trees, and appearing to be literally as "old as the hills." It
tells a wonderful tale. We soon satisfy ourselves that the ridge is
an ancient moraine, and at once conclude that the glacier, at some
former period of its existence, was vastly larger than it is now.
This old moraine stretches right across the main valley, and abuts
against the mountains at the opposite side.
[Illustration: SOURCE OF THE ARVEIRON.]
108. Having passed the terminal portion of the glacier, which is
covered with stones and rubbish, we find ourselves beside a very
wonderful exhibition of ice. The glacier descends a steep gorge, and
in doing so is riven and broken in the most extraordinary manner.
Here are towers, and pinnacles, and fantastic shapes wrought out by
the action of the weather, which put one in mind of rude sculpture.
Annexed is a sketch of an ice-pinnacle. From deep chasms in the
glacier issues a delicate shimmer of blue light. At times we hear a
sound like thunder, which arises either from the falling of a tower
of ice, or from the tumble of a huge stone into a chasm. The glacier
maintains this wild and chaotic character for some time; and the best
iceman would find himself defeated in any attempt to get along it.
[Illustration: ICE-PINNACLE.]