ON THE PRISMATIC STRUCTURE OF THE ICE IN GLACIÈRES.
It was natural to suppose that the prismatic structure which I found so
very general in the glacières was the result of some cause or causes
coming into operation after the first formation of the ice. On this
point M. Thury's visit to the Glacière of S. Georges in the spring of
1852 affords valuable information, for at that time the coating of ice
on the wall, evidently newly formed, did not present the _structure
aréolaire_ which he had observed in his summer visit to the cave. He
suggests that, since ice is less coherent at a temperature of 32°
F.--which is approximately the temperature of the ice-caves during
several months of the year--than when exposed to a greater degree of
cold, its molecules will then become free to assume a fresh system of
arrangement.[196] On the other hand, Professor Faraday has found that
ice formed under a temperature some degrees below the ordinary freezing
point has a well-marked crystalline structure.[197] M. Thury suggests
also, as a possibility, what I have found to be the case, by frequent
observations, that the prismatic ice has greater power of resisting heat
than ordinary ice; and on this supposition he accounts for the fact of
hollow stalactites being found in the Cavern of S. Georges.[198] At the
commencement of the hot season, the atmospheric temperature of the
glacières rises gradually; and when it has almost reached 32° F., the
prismatic change takes place in the ice, extending to a limited depth
below the surface. The central parts of the stalactites retain their
ordinary structure, and are after a time exposed to a general
temperature rather above than below the freezing point; and thus they
come to melt, the water escaping either by accidental fissures between
some of the prisms, or by the extremity of the stalactite, or by some
part of the surface which has chanced to escape the prismatic
arrangement, and has itself melted under increased temperature.[199]
M. Héricart de Thury describes the peculiar structure of the ice which
he found in the Glacière of the Foire de Fondeurle.[200] He found that
the crystallised portions were very distinctly marked, displaying for
the most part a six-sided arrangement; and in the interior of a hollow
stalactite he found numerous needles of ice perfectly crystallised, the
crystals being some triangular and some six-sided. He was unable to
detect any perfect pyramid.[201] I have already quoted Olafsen's
observations on the polygonal lining which he saw on the surface of the
ice in the Surtshellir. The French Encyclopædia [202] relates that M.
Hassenfratz saw ice served up at table at Chambéry which broke into
hexagonal prisms; and when he was shown the ice-houses where it was
stored, he found considerable blocks of ice containing hexahedral prisms
terminated by corresponding pyramids.
In vol. xv. (New Series) of the American Journal of Science,[203] an
extract is given from a letter describing the 'Ice Spring' in the Rocky
Mountains, which the mountaineers consider to be one of the curiosities
of the great trail from the States to Oregon and California. It is
situated in a low marshy 'swale' to the right of the Sweetwater river,
and about forty miles from the South Pass. The ground is filled with
springs; and about 18 inches below the turf lies a smooth and horizontal
sheet of ice, which remains the year round, protected by the soil and
grass above it. On July 12th, 1849, it was from 2 to 4 inches thick; but
one of the guides stated that he had seen it a foot deep. It was
perfectly clear, and disposed in hexagonal prisms, separating readily at
the natural joints. The ice had a slightly saline taste,[204] the ground
above it being impregnated with salt, and the water near tasting of
sulphur. The upper surface of the stratum of ice was perfectly smooth.
In Poggendorff's _Annalen_ (1841, Erganzsband, 517-19,--Boué, an old
offender in that way, says 1842) there is an account of ice being
found in the Westerwald, near the village of Frickhofen at the foot of
the _Dornburg_, among basaltic débris about 500 feet above the
sea.[205] Commencing at a depth of 2 feet below the surface, the ice
reaches from 20 to 22 feet farther down, where the loose stones give
place to dry sand. The ice is in thin layers on the stones, and is
deposited in the form of clear and regular hexagonal crystals. The
lateral extent through which this phenomenon obtains is from 40 to 50
feet each way, and is greater in winter than in summer. As in other
cases that have been noticed in basaltic débris, the snow which falls
upon the surface here is speedily melted. The _Allgemeine Zeitung_
(1840, No. 309), from which the account in Poggendorff is taken,
suggested that the melted snow-water which would thus run down among
the interstices would readily freeze below the surface, while the
heavy cold air of winter would be stored up at the lower levels, and
the poor conducting powers of basaltic rock[206] would favour its
permanence through the summer. The temperature of the cold current
which was perceptible in the parts of the mass of débris where the ice
existed was 1° R. (34°·25 F.). Nothing but a few lichens grow on the
surface of the débris.
These are, I think, all the references I have met with to the prismatic
structure of subterranean ice. But there is an interesting account in
Poggendorff 's _Annalen_,[207] by a private teacher in Jena, of the
crystalline appearance of ice under slow thaw near that town. In the
winter of 1840, the Saale was frozen, and the ice remained unbroken till
the middle of January, when the thermometer rose suddenly, and the
river in consequence overflowed the lower grounds, and carried large
masses of ice on to the fields, where it was left when the water
subsided. On the 20th of January the thermometer fell again, and
remained below the freezing point till the 12th of February: some of the
ice did not disappear till the following month.
When the ice had lain a short time, cracks appeared on the surface
exposed to the sun, and spread like a network from the edges towards the
centre of the surface. At first there was no regularity in the
connection of these lines, and the several meshes were of very different
sizes. After a time, the larger meshes split up into smaller, and the
system of network was found to penetrate below the surface, the cracks
deepening into furrows, which descended perpendicularly from the
surface, and divided the ice into long thin rhomboidal pillars. The
surface-end of some of these pillars was strongly marked with right
lines parallel to one of the sides of the mesh, and it was found that
there was a tendency in the ice to split down planes through these lines
and parallel to the corresponding side-plane. Parallel to the original
surface of the mass of ice, the pillars broke off evenly. The
side-planes had a rounded, wrinkled appearance; and their mutual
inclinations--as far as could be determined--were from 105° to 115°, and
from 66° to 75°. When these ice-pillars were examined by means of
polarised light, they were found to possess a feeble double-refracting
power.
The writer of the article in Poggendorff suggests a question which he
was not sure how to answer:--Is this appearance in correspondence with
the original formation of the ice, or does it only appear under slow
thaw?
It is worthy of remark, that from the 1st to the 11th of February the
thermometer was never higher than 22°·8 F., and during that time fell as
low as 21° below zero, i.e. 43° below the freezing point.
Professor Tyndall has informed me that in the winters of 1849, 1850,
1851, he found the banks of a river in Germany loaded with massive
layers of drift-ice, in a state of thaw, and was struck by the fact that
every layer displayed the prismatic structure described above, the axes
of the prisms being at right angles to the surfaces of freezing. It may
be, he adds, that this structure is in the first place determined by the
act of freezing, but it does not develop itself until the ice thaws.
M. Hassenfratz observed an appearance in ice on the Danube at
Vienna[208] corresponding to that described at Jena. He gives no
information as to the state of the weather or the temperature at the
time, nor any of the circumstances under which the ice came under his
notice. One of the masses of ice which he describes was crystallised in
prisms of various numbers of sides: of these prisms the greater part
were hexahedral and irregular. Another mass was composed of prisms in
the form of truncated pyramids; and in another he found quadrilateral
and octahedral prisms, the former splitting parallel to the faces, and
also truncated pyramids with five and six sides. He adds, that he had
frequently seen in the upper valleys tufts of ice growing, as it were,
out of the ground, and striated externally, but had never succeeded in
discovering any internal organisation, until one evening in a time of
thaw, when he found by means of a microscope that the striated tufts of
ice had assumed the same structure on a small scale as that which he had
observed on the Danube.
A Frenchman who was present in the room in which the Chemical Section of
the British Association met at Bath, and heard a paper which I read
there on this prismatic structure, suggested that it was probably
something akin to the rhomboidal form assumed by dried mud; and I have
since been struck by the great resemblance to it, as far as the surface
goes, which the pits of mud left by the coprolite-workers near Cambridge
offer, of course on a very large scale. This led me to suppose that the
intense dryness which would naturally be the result of the action of
some weeks or months of great cold upon subterranean ice might be one of
the causes of its assuming this form, and the observations at Jena would
rather confirm than contradict this view: competent authorities,
however, seem inclined to believe that warmth, and not cold, is the
producing cause.[209]
Professor Tyndall found, in the course of his experiments on the discs
and flowers produced in the interior of a mass of ice by sending a warm
ray through the mass, that the pieces of ice were in some cases
traversed by hazy surfaces of discontinuity, which divided the
apparently continuous mass into irregular prismatic segments. The
intersections of the bounding surfaces of these segments with the
surface of the slab of ice formed a very irregular network of
lines.[210] I am inclined, however, to think that the irregularity in
these cases proved to be so much greater than that observed in the
glacières, that this interior prismatic subdivision must be referred to
some different cause.
FOOTNOTES:
[Footnote 196: The continued extrication of latent heat by ice, as it is
cooled a few degrees below 32° F., appears to indicate a molecular
change subsequent to the first freezing.--_Phil. Trans._, as quoted in
the next note.]
[Footnote 197: See the paper 'On Liquid Diffusion as applied to
Analysis,' by the Master of the Mint (_Phil. Trans._ 1861, p. 222).]
[Footnote 198: Compare the description of one of the hollow stalagmites I
explored in the Schafloch, p. 145.]
[Footnote 199: Professor Tyndall has pointed out that, owing to the want
of perfect homogeneity, some parts of a block of ice exposed to a
temperature of 32° F. will melt, while others remain solid _(Phil.
Trans_. 1858, p. 214). He also arrived at the conclusion (p. 219) that
heat could be conducted through the substance of a mass, and melt
portions of the interior, without visible prejudice to the solidity of
the other parts of the mass.]
[Footnote 200: _Journal des Mines_, xxxiii. 157. See also an English
translation of his account in the second volume of the _Edinburgh
Journal of Science_.]
[Footnote 201: It is to be hoped that the accuracy of his scientific
descriptions exceeds that of his topographical information; for he
states that the glacière is two leagues from Valence, whereas it cost me
six hours' drive on a level road, and five and a half hours' walking and
climbing, to reach it from that town.]
[Footnote 202: Branch _Physique_, article _Glace_]
[Footnote 203: P. 146 (an. 1853).]
[Footnote 204: Dr. Lister experimented on sea-water in December 1684
(_Ph. Trans_, xiv. 836), and found that though it took two nights to
freeze, it was much harder when once frozen than common ice, lasting for
three-quarters of an hour under a heat which melted 100 times its bulk
of common ice at once. It was marked with oblong squares, and had a salt
taste. Ice formed from water with an admixture of sulphuric acid is said
to assume a crystalline appearance.]
[Footnote 205: See also a pamphlet entitled _Das unterirdische Eisfeld
bei der Dornburg am Südlichen Fusse des Westerwaldes_, by Thomä of
Wiesbaden (32 pages, with a map of the district), published in 1841.]
[Footnote 206: But see page 262.]
[Footnote 207: lv. (an 1842), 472.]
[Footnote 208: _Journal de Physique_, xxvi. (an 1785), 34.]
[Footnote 209: In looking through some early volumes of the
_Philosophical Transactions_, I found an 'Extract of a letter written by
Mr. Muraltus of Zurich (September 1668), concerning the Icy and
Chrystallin Mountains of Helvetia, called the Gletscher, English'd out
of Latin' (_Phil. Trans._ iv. 982), which at first looked something like
an assertion of the prismatic structure of ice on a large scale. The
English version is as follows:--'The snow melted by the heat of the
summer, other snow being faln within a little while after, and hardened
into ice, which by little and little in a long tract of time depurating
itself turns into a stone, not yielding in hardness and clearness to
chrystall. Such stones closely joyned and compacted together compose a
whole mountain, and that a very firm one; though in summer-time the
country-people have observed it to burst asunder with great cracking,
thunder-like.']
[Footnote 210: See the woodcut illustrating Professor Tyndall's remarks
in the 148th volume of the _Philosophical Transactions_ (1858, p. 214).]
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