_STRUCTURE AND PROPERTIES OF ICE._
Being desirous of examining how the interior of a mass of ice is
affected by a beam of radiant heat sent through it, I availed myself
of the sunny weather of September and October 1857. The sunbeams,
condensed by a lens, were sent in various directions through slabs of
ice. The path of every beam was observed to be instantly studded with
lustrous spots, which increased in magnitude and number as the action
continued. On examining the spots more closely, they were found to be
flattened spheroids, and around each of them the ice was so liquefied
as to form a beautiful flower-shaped figure possessing six petals. From
this number there was no deviation. At first the edges of the liquid
leaves were unindented; but a continuance of the action usually caused
the edges to become serrated like those of ferns. When the ice was
caused to move across the beam, or when the beam was caused to traverse
different portions of the ice in succession, the sudden generation and
crowding together of these liquid flowers, with their central spots
shining with more than metallic brilliancy, was exceedingly beautiful.
In almost all cases the flowers were formed in planes parallel to the
surface of freezing; it mattered not whether the beam traversed the
ice parallel to this surface or perpendicular to it. Some apparent
exceptions to this rule were found, which will form the subject of
future investigation.
The general appearance of the shining spots at the centres of the
flowers was that of the bubbles of air entrapped in the ice; to examine
whether they contained air or not, portions of ice containing them
were immersed in warm water. When the ice surrounding the cavities had
completely melted, the latter instantly collapsed, and no trace of air
rose to the surface of the water. A vacuum, therefore, had been formed
at the centre of each spot, due, doubtless, to the well-known fact that
the volume of water in each flower was less than that of the ice, by
the melting of which the flower was produced.
The associated air-and-water cells, found in such numbers in the ice
of glaciers, and also observed in lake ice, were next examined. Two
hypotheses have been started to account for these cells. One attributes
them to the absorption of the sun’s heat by the air of the bubbles,
and the consequent melting of the ice which surrounds them. The other
hypothesis supposes that the liquid in the cells never has been
frozen, but has continued in the liquid condition from the _névé_ or
origin of the glacier downwards. Now if the water in the cells be due
to the melting of the ice, the associated air must be _rarefied_,
because the volume of the liquid is less than that of the ice which
produced it; whereas if the air be simply that entrapped in the snow of
the _névé_, it will not be thus rarefied. Here, then, we have a test as
to whether the water-cells have been produced by the melting of the ice.
Portions of ice containing these compound cells were immersed in hot
water, the ice around the cavities being thus gradually melted away.
When a liquid connexion was established between the bubble and the
atmosphere, the former collapsed to a smaller bubble. In many cases the
residual bubble did not reach the hundredth part of the magnitude of
the primitive one. There was no exception to this rule, and it proves
that the water of these particular cavities, at all events, is really
due to the melting of the adjacent ice.
But how was the ice surrounding the bubbles melted? The hypothesis
that the melting is due to the absorption of the solar rays by the air
of the bubbles is that of M. Agassiz, which has been reproduced and
subscribed to by the Messrs. Schlagintweit, and accepted generally as
the true one. Let us pursue it to its consequences.
Comparing equal _weights_ of air and water, experiment proves that to
raise a given weight of water one degree in temperature, as much heat
would be needed as would raise the same weight of air four degrees.
Comparing equal _volumes_ of air and water, the water is known to be
770 times heavier than the air; consequently, for a given volume of air
to raise an equal volume of water one degree in temperature, it must
part with 770 × 4 = 3080 degrees.
Now the quantity of heat necessary to melt a given weight of ice would
raise the same weight of water 142.6 Fahr. degrees in temperature.
Hence to produce, by the melting of ice, an amount of water equal to
itself in bulk, a bubble of air must yield up 3080 × 142.6, or upwards
of four hundred thousand degrees Fahrenheit.
This is the amount of heat which, according to the hypothesis of M.
Agassiz and the Messrs. Schlagintweit, is absorbed by the bubble of air
under the eyes of the observer. That is to say, the air is capable of
absorbing an amount of heat which, had it not been communicated to the
surrounding ice, would raise the bubble to a temperature 160 times that
of fused cast iron. Did air possess this enormous power of absorption
it would not be without inconvenience for the animal and vegetable life
of our planet.
The fact is, that a bubble of air at the earth’s surface is unable,
in the slightest appreciable degree, to absorb the sun’s rays; for
those rays before they reach the earth have been perfectly sifted by
their passage through the atmosphere. I made the following experiment
illustrative of this point: The rays from an electric lamp were
condensed by a lens, and the concentrated beam sent through the bulb
of a differential thermometer. The heat of the beam was intense; still
not the slightest effect was produced upon the thermometer. In fact,
all the rays that _air_ could absorb had been absorbed before the
thermometer was reached, while the rays that glass could absorb had
been _absorbed by the lens_. The heat consequently passed through the
thin glass envelope of the thermometer, and the air within it, without
imparting the slightest sensible heat to either.
The liquid bubbles observed in lake ice, and those which occur in the
deeper portions of glacier ice, are produced by heat which has been
_conducted_ through the substance without melting it. Regarding heat as
a mode of motion, it seems natural to infer, that inasmuch as within
the mass each molecule is controlled in its motion by the surrounding
molecules, the liberty of liquidity must be attained by the molecules
at the surface of ice before the molecules in the interior can attain
this liberty. But if a cavity exist in the interior, the molecules
surrounding that cavity are in a condition similar to those at the
surface; and they may be liberated by an amount of motion which has
been transmitted through the ice without prejudice to its solidity. The
conception is helped when we call to mind the transmission of motion
through a series of elastic balls, by which the last ball of the series
is detached, while the others do not suffer visible separation. It
may indeed be proved, by actual experiment, that the interior portion
of a mass of ice can be liquefied by an amount of heat which has been
conducted through the exterior portions without melting them.
Now precisely the converse of this takes place when two pieces of
ice, at 32° Fahr., with moist surfaces, are brought into contact.
Superficial portions are by this act transferred to the centre where a
temperature of 32° is not quite sufficient to produce liquefaction. The
motion of liquidity which the surfaces possessed before contact is now
checked, and the pieces of ice freeze together. This appears to furnish
a satisfactory explanation of all the cases of this nature which have
hitherto been observed.
The particles of a crushed mass of ice at 32°, or a ball of moist snow,
may, it is now well known, be squeezed into slabs or cups of ice. That
moisture is necessary here, and that the same agent is necessary in
the conversion of snow into glacier ice, was proved by the following
experiment. A ball of ice was cooled in a bath of solid carbonic acid
and ether, and thus rendered perfectly dry. Placed in a suitable mould,
and subjected to hydraulic pressure, the ball was crushed; but the
crushed fragments remained as _white and opaque_ as those of crushed
glass. The particles, while thus dry, could not be squeezed so as to
form pellucid ice, which is so easily obtained when the compressed mass
is at a temperature of 32° Fahr.